RFC 9293
Transmission Control Protocol (TCP)
RFC 9293: Transmission Control Protocol (TCP) 🎀
okay bestie, this is the big one 💅 RFC 9293 is the TCP spec, and it's the glow-up of the classic RFC 793 (the OG from 1981 🧚). The editor is W. Eddy, Ed. from MTI Systems, and it's published by the IETF as an Internet Standards Track doc, STD 7, dated August 2022 📅. It is obsoleting RFC 793, 879, 2873, 6093, 6429, 6528, and 6691 (basically all the scattered patches got merged into one place, iconic 🫶). It is also updating RFC 1011, 1122, and 5961. The ISSN is 2070-1721 🌸.
Abstract ✨
okay so this doc specifies TCP, which is a super important transport-layer protocol in the Internet protocol stack 🦋. It has kept evolving for decades as the Internet grew, and a bunch of changes have been made to the RFC 793 version of TCP over time, but they were only ever documented in a messy, piecemeal way 😭. This document gathers all of those changes and brings them together with the protocol spec from RFC 793, so it's like one clean source of truth 💖.
It obsoletes RFC 793, plus RFCs 879, 2873, 6093, 6429, 6528, and 6691, which had all updated bits of RFC 793. It updates RFCs 1011 and 1122, and it should be treated as a replacement for the parts of those documents that deal with TCP requirements 🔥. It also updates RFC 5961 by adding a small clarification about reset handling in the SYN-RECEIVED state. And the TCP header control bits from RFC 793 got updated based on RFC 3168 ✨.
Status of This Memo 🌈
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It went through public review and got approved for publication by the Internet Engineering Steering Group (IESG). For more on Internet Standards, check out Section 2 of RFC 7841 🎀.
Info about the current status of this document, any errata, and how to give feedback on it can be found at https://www.rfc-editor.org/info/rfc9293 🔗.
Copyright Notice 💜
Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please read these documents carefully, because they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions, and they are provided without warranty as described in the Revised BSD License 🫶.
This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English 🙈.
Table of Contents 🍓
1. Purpose and Scope
2. Introduction
2.1. Requirements Language
2.2. Key TCP Concepts
3. Functional Specification
3.1. Header Format
3.2. Specific Option Definitions
3.2.1. Other Common Options
3.2.2. Experimental TCP Options
3.3. TCP Terminology Overview
3.3.1. Key Connection State Variables
3.3.2. State Machine Overview
3.4. Sequence Numbers
3.4.1. Initial Sequence Number Selection
3.4.2. Knowing When to Keep Quiet
3.4.3. The TCP Quiet Time Concept
3.5. Establishing a Connection
3.5.1. Half-Open Connections and Other Anomalies
3.5.2. Reset Generation
3.5.3. Reset Processing
3.6. Closing a Connection
3.6.1. Half-Closed Connections
3.7. Segmentation
3.7.1. Maximum Segment Size Option
3.7.2. Path MTU Discovery
3.7.3. Interfaces with Variable MTU Values
3.7.4. Nagle Algorithm
3.7.5. IPv6 Jumbograms
3.8. Data Communication
3.8.1. Retransmission Timeout
3.8.2. TCP Congestion Control
3.8.3. TCP Connection Failures
3.8.4. TCP Keep-Alives
3.8.5. The Communication of Urgent Information
3.8.6. Managing the Window
3.9. Interfaces
3.9.1. User/TCP Interface
3.9.2. TCP/Lower-Level Interface
3.10. Event Processing
3.10.1. OPEN Call
3.10.2. SEND Call
3.10.3. RECEIVE Call
3.10.4. CLOSE Call
3.10.5. ABORT Call
3.10.6. STATUS Call
3.10.7. SEGMENT ARRIVES
3.10.8. Timeouts
4. Glossary
5. Changes from RFC 793
6. IANA Considerations
7. Security and Privacy Considerations
8. References
8.1. Normative References
8.2. Informative References
Appendix A. Other Implementation Notes
A.1. IP Security Compartment and Precedence
A.1.1. Precedence
A.1.2. MLS Systems
A.2. Sequence Number Validation
A.3. Nagle Modification
A.4. Low Watermark Settings
Appendix B. TCP Requirement Summary
Acknowledgments
Author's Address1. Purpose and Scope 🧚
In 1981, RFC 793 [16] dropped and documented the Transmission Control Protocol (TCP), replacing the earlier published specs for TCP 👑.
Ever since, TCP has been widely implemented, and it's been used as the transport protocol for tons of applications across the Internet 💅.
For several decades, RFC 793 plus a bunch of other documents have teamed up to act as the core spec for TCP [49] 🫶. Over time, a lot of errata got filed against RFC 793. There have also been deficiencies found and fixed in security, performance, and plenty of other areas. The enhancements piled up across many separate documents, and they were never gathered into one comprehensive update to the base spec 😭.
So the goal of this document is to bring together all of the IETF Standards Track changes and other clarifications that have been made to the base TCP functional spec (RFC 793), and to unify them into one updated version of the spec ✨.
Some companion documents are referenced for important algorithms that TCP uses (for example, congestion control), but they haven't been completely included here. This is a deliberate choice 🦋: this base spec can be used with multiple extra algorithms that get developed and plugged in separately. This document focuses on the common basis that all TCP implementations must support in order to interoperate. Some TCP features have become super complicated on their own (advanced loss recovery and congestion control, for example), so future companion documents may try to bring those together in the same way 🌸.
RFC 793 and other updates also include informative and descriptive text that helps readers understand the design and operation of the protocol, on top of the protocol spec that describes the TCP segment format, the generation and processing rules, and what's supposed to be implemented in code 🍓. This document doesn't try to change or update that informative text. It only updates the normative protocol spec. Where it makes sense, it keeps references to the docs that have the important explanations and rationale 💜.
This document is meant to be useful both for checking existing TCP implementations for conformance, and for writing brand new implementations 🎀.
2. Introduction 💖
RFC 793 has a discussion of the TCP design goals, and it gives examples of how it works, including connection establishment, connection termination, and packet retransmission to repair losses 🌈.
This document describes the basic functionality expected in modern TCP implementations, and it replaces the protocol spec in RFC 793. It doesn't copy or try to update the introduction and philosophy content in Sections 1 and 2 of RFC 793. Other documents are referenced for explanations of the theory of operation, the rationale, and detailed discussion of design decisions. This document only focuses on the normative behavior of the protocol 🫶.
The "TCP Roadmap" [49] is a more extensive guide to the RFCs that define TCP and describe various important algorithms. It has sections on strongly encouraged enhancements that improve performance and other aspects of TCP beyond the basic operation in this document 🔥. For example, implementing congestion control (e.g., [8]) is a TCP requirement, but it's a complex topic on its own, so it isn't described in detail here. There are lots of options and possibilities that don't affect basic interoperability 🙈. Similarly, most TCP implementations today include the high-performance extensions in [47], but those aren't strictly required or discussed in this document. Multipath considerations for TCP are also specified separately in [59] 🦋.
A list of changes from RFC 793 is in Section 5 ✨.
2.1. Requirements Language 👑
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [3] [12] when, and only when, they appear in all capitals, as shown here 💅.
Each use of RFC 2119 keywords in the document is individually labeled and referenced in Appendix B, which summarizes implementation requirements 🌸.
Sentences using "MUST" are labeled as "MUST-X", with X being a numeric identifier so the requirement is easy to find when it gets referenced from Appendix B 🎀.
In the same way, sentences using "SHOULD" are labeled "SHLD-X", sentences using "MAY" are labeled "MAY-X", and sentences using "RECOMMENDED" are labeled "REC-X" 🧚.
For the purposes of this labeling, "SHOULD NOT" and "MUST NOT" are labeled the same way as "SHOULD" and "MUST" instances 💜.
2.2. Key TCP Concepts 🦋
TCP gives applications a reliable, in-order, byte-stream service 💖.
The application byte-stream travels over the network in TCP segments, and each TCP segment gets sent as an Internet Protocol (IP) datagram 🌈.
TCP reliability comes from detecting packet losses (through sequence numbers) and errors (through per-segment checksums), plus correcting them with retransmission 🫶.
TCP supports unicast delivery of data. There are anycast applications that can successfully use TCP without any modifications, though there's some risk of instability because lower-layer forwarding behavior can change [46] 🙈.
TCP is connection oriented, though it doesn't inherently include a liveness detection capability 🔥.
Data can flow both ways over TCP connections, though applications are free to send data only one way if they want 🌸.
TCP uses port numbers to identify application services and to multiplex distinct flows between hosts 🍓.
You can find a more detailed description of TCP features compared to other transport protocols in Section 3.1 of [52]. Further description of the motivations for developing TCP and its role in the Internet protocol stack is in Section 2 of [16] and in earlier versions of the TCP specification ✨.
3. Functional Specification 🎀
3.1. Header Format ✨
okay so TCP segments get sent as internet datagrams 📦 The IP header comes first and carries a bunch of info, including the source and destination host addresses [1] [13]. The TCP header follows the IP headers and holds the stuff that's specific to TCP 💜 This split means other host-level protocols can exist too! Early on, the IP header fields were actually part of TCP, so this layout is a glow-up 🦋
This document describes TCP, which uses TCP headers. Obsessed with the clarity 🎀
A TCP header, followed by any user data in the segment, is formatted like this, using the style from [66] 🌸
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data | |C|E|U|A|P|R|S|F| |
| Offset| Rsrvd |W|C|R|C|S|S|Y|I| Window |
| | |R|E|G|K|H|T|N|N| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Urgent Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Options] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
: Data :
: |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note that one tick mark represents one bit position.
Figure 1: TCP Header Formatwhere:
Source Port: 16 bits 🔌 The source port number.
Destination Port: 16 bits 🎯 The destination port number.
Sequence Number: 32 bits 🔢 The sequence number of the first data octet in this segment (except when the SYN flag is set). If SYN is set, the sequence number is the initial sequence number (ISN) and the first data octet is ISN+1. Main character energy 👑
Acknowledgment Number: 32 bits 🫶 If the ACK control bit is set, this field contains the value of the next sequence number the sender of the segment is expecting to receive. Once a connection is established, this is always sent. Literally always 💖
Data Offset (DOffset): 4 bits 📏 The number of 32-bit words in the TCP header. This indicates where the data begins. The TCP header (even one including options) is an integer multiple of 32 bits long. Lowkey the cleanest math ever 🧚
Reserved (Rsrvd): 4 bits 🌙 A set of control bits reserved for future use. Must be zero in generated segments and must be ignored in received segments if the corresponding future features are not implemented by the sending or receiving host. So the rule is: send zeros, and ignore it if you don't know the feature yet 🙈
Control bits: The control bits are also known as "flags". Assignment is managed by IANA from the "TCP Header Flags" registry [62]. The currently assigned control bits are CWR, ECE, URG, ACK, PSH, RST, SYN, and FIN. Eight little flags doing the most 💅
- CWR: 1 bit Congestion Window Reduced (see [6]). 🌊
- ECE: 1 bit ECN-Echo (see [6]). 📣
- URG: 1 bit Urgent pointer field is significant. 🚨
- ACK: 1 bit Acknowledgment field is significant. ✅
- PSH: 1 bit Push function (see the Send Call description in Section 3.9.1). 🚀
- RST: 1 bit Reset the connection. 💀
- SYN: 1 bit Synchronize sequence numbers. 🤝
- FIN: 1 bit No more data from sender. Bye bye 👋
Window: 16 bits 🪟 The number of data octets beginning with the one indicated in the acknowledgment field that the sender of this segment is willing to accept. The value is shifted when the window scaling extension is used [47].
The window size MUST be treated as an unsigned number, or else large window sizes will appear like negative windows and TCP will not work (MUST-1). It is RECOMMENDED that implementations will reserve 32-bit fields for the send and receive window sizes in the connection record and do all window computations with 32 bits (REC-1). Big windows, no drama 🌈
Checksum: 16 bits 🧮 The checksum field is the 16-bit ones' complement of the ones' complement sum of all 16-bit words in the header and text. The checksum computation needs to ensure the 16-bit alignment of the data being summed. If a segment contains an odd number of header and text octets, alignment can be achieved by padding the last octet with zeros on its right to form a 16-bit word for checksum purposes. The pad is not transmitted as part of the segment. While computing the checksum, the checksum field itself is replaced with zeros. The math is giving perfectionist 🫧
The checksum also covers a pseudo-header (Figure 2) conceptually prefixed to the TCP header. The pseudo-header is 96 bits for IPv4 and 320 bits for IPv6. Including the pseudo-header in the checksum gives the TCP connection protection against misrouted segments. This information is carried in IP headers and is transferred across the TCP/network interface in the arguments or results of calls by the TCP implementation on the IP layer. Protection against misrouted segments? Iconic 🛡️
+--------+--------+--------+--------+
| Source Address |
+--------+--------+--------+--------+
| Destination Address |
+--------+--------+--------+--------+
| zero | PTCL | TCP Length |
+--------+--------+--------+--------+
Figure 2: IPv4 Pseudo-headerPseudo-header components for IPv4:
- Source Address: the IPv4 source address in network byte order 🌐
- Destination Address: the IPv4 destination address in network byte order 📍
- zero: bits set to zero 0️⃣
- PTCL: the protocol number from the IP header 🏷️
- TCP Length: the TCP header length plus the data length in octets (this is not an explicitly transmitted quantity but is computed), and it does not count the 12 octets of the pseudo-header. It's computed, not sent, so no extra bytes on the wire 🤫
For IPv6, the pseudo-header is defined in Section 8.1 of RFC 8200 [13] and contains the IPv6 Source Address and Destination Address, an Upper-Layer Packet Length (a 32-bit value otherwise equivalent to TCP Length in the IPv4 pseudo-header), three bytes of zero padding, and a Next Header value, which differs from the IPv6 header value if there are extension headers present between IPv6 and TCP. IPv6 gets its own version of the pseudo-header, and it's a whole vibe 🌟
The TCP checksum is never optional. The sender MUST generate it (MUST-2) and the receiver MUST check it (MUST-3). No skipping this one, bestie 🙅♀️🔥
Urgent Pointer: 16 bits 🆘 This field communicates the current value of the urgent pointer as a positive offset from the sequence number in this segment. The urgent pointer points to the sequence number of the octet following the urgent data. This field is only to be interpreted in segments with the URG control bit set. Only read it when URG is set, periodt 💅
Options: [TCP Option]; size(Options) == (DOffset-5)*32; present only when DOffset > 5. Note that this size expression also includes any padding trailing the actual options present. This one's a whole math moment 🧠
Options may occupy space at the end of the TCP header and are a multiple of 8 bits in length. All options are included in the checksum. An option may begin on any octet boundary. There are two cases for the format of an option:
- Case 1: A single octet of option-kind.
- Case 2: An octet of option-kind (Kind), an octet of option-length, and the actual option-data octets.
The option-length counts the two octets of option-kind and option-length as well as the option-data octets.
Note that the list of options may be shorter than the Data Offset field might imply. The content of the header beyond the End of Option List Option MUST be header padding of zeros (MUST-69). 🫠 Zeros after the end, no exceptions!
The list of all currently defined options is managed by IANA [62], and each option is defined in other RFCs, as indicated there. That set includes experimental options that can be extended to support multiple concurrent usages [45]. Options living in their own RFCs is so organized 🗂️
A given TCP implementation can support any currently defined options, but the following options MUST be supported (MUST-4 -- note Maximum Segment Size Option support is also part of MUST-14 in Section 3.7.1):
+======+========+============================+
| Kind | Length | Meaning |
+======+========+============================+
| 0 | - | End of Option List Option. |
+------+--------+----------------------------+
| 1 | - | No-Operation. |
+------+--------+----------------------------+
| 2 | 4 | Maximum Segment Size. |
+------+--------+----------------------------+
Table 1: Mandatory Option SetThese options are specified in detail in Section 3.2. ✨
A TCP implementation MUST be able to receive a TCP Option in any segment (MUST-5). Every implementation has to be able to receive options, no matter which segment they show up in 🎁
A TCP implementation MUST (MUST-6) ignore without error any TCP Option it does not implement, assuming that the option has a length field. All TCP Options except End of Option List Option (EOL) and No-Operation (NOP) MUST have length fields, including all future options (MUST-68). TCP implementations MUST be prepared to handle an illegal option length (e.g., zero); a suggested procedure is to reset the connection and log the error cause (MUST-7). So if you don't recognise an option, just ignore it quietly 🤫, and if the length is illegal, reset and log it 🚫
Note: There is ongoing work to extend the space available for TCP Options, such as [65]. More room for options is coming, and I'm here for it 🌸
Data: variable length 🍓 User data carried by the TCP segment. The actual tea, the whole reason the segment exists 🫶
3.2. Specific Option Definitions ✨
Okay bestie, the mandatory option set is honestly a small crew 🫶. A TCP Option in it is one of three things: an End of Option List Option, a No-Operation Option, or a Maximum Segment Size Option 💖.
First up is the End of Option List Option 🎀. It's formatted like this:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+Here's the breakdown:
Kind: 1 byte, withKind == 0.
This option code marks the end of the option list 🔚. It might not line up with the end of the TCP header according to the Data Offset field, so keep that in mind. It's used at the end of all the options, not after each one, and you only need it if the options wouldn't otherwise end right where the TCP header ends 🙈.
Next, the No-Operation Option 🦋:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| 1 |
+-+-+-+-+-+-+-+-+Kind: 1 byte, withKind == 1.
This one can sit between options, for example to line up the start of the next option on a word boundary ✨. Senders aren't guaranteed to use it, so receivers MUST be prepared to process options even if they don't start on a word boundary (MUST-64). Receivers have to be chill about it, no exceptions 💅.
Last one is the Maximum Segment Size Option 👑:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Length | Maximum Segment Size (MSS) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Kind: 1 byte, withKind == 2.
If this option is there, it tells you the maximum receive segment size at the TCP endpoint that sent the segment 📏. That value is capped by the IP reassembly limit 🧢. This field may be sent in the initial connection request (so, in segments with the SYN control bit set) and MUST NOT be sent in other segments (MUST-65). Iconic restraint 😭. If the option isn't used, any segment size is allowed 🌈. There's a fuller explanation in Section 3.7.1 if you want the deep dive 🧚.
Length: 1 byte, withLength == 4.
This is the length of the option in bytes 📐.
Maximum Segment Size (MSS): 2 bytes.
This is the maximum receive segment size at the TCP endpoint that sent this segment 💜.
3.2.1. Other Common Options
Other RFCs define some more popular options that are recommended for implementation if you want high performance 🚀. They aren't required for basic TCP interoperability, though 🙃. These are the TCP Selective Acknowledgment (SACK) Option [22] [26], the TCP Timestamp (TS) Option [47], and the TCP Window Scale (WS) Option [47] 🍓.
3.2.2. Experimental TCP Options
Experimental TCP Option values are defined in [30], and [45] tells you the current recommended usage for those experimental values 🧪.
3.3. TCP Terminology Overview 📚
This section is your starter pack of key terms 🎒. You'll need them for the detailed protocol stuff in the rest of the doc. There's also a glossary of terms in Section 4 if you get lost 🗺️.
3.3.1. Key Connection State Variables
Before we get into how a TCP implementation actually operates, we need some proper vocab 🤓. Keeping a TCP connection going means keeping state for several variables. We picture all of them living in a connection record called a Transmission Control Block, or TCB 🗂️. Stored in the TCB are things like the local and remote IP addresses and port numbers 🏠, the IP security level and compartment of the connection (see Appendix A.1) 🔐, pointers to the user's send and receive buffers 📦, and pointers to the retransmit queue and the current segment 🔁. On top of that, a bunch of variables about the send and receive sequence numbers also get stored in the TCB 🔢. The send and receive sequence variables are listed below 👇.
+==========+=====================================================+
| Variable | Description |
+==========+=====================================================+
| SND.UNA | send unacknowledged |
+----------+-----------------------------------------------------+
| SND.NXT | send next |
+----------+-----------------------------------------------------+
| SND.WND | send window |
+----------+-----------------------------------------------------+
| SND.UP | send urgent pointer |
+----------+-----------------------------------------------------+
| SND.WL1 | segment sequence number used for last window update |
+----------+-----------------------------------------------------+
| SND.WL2 | segment acknowledgment number used for last window |
| | update |
+----------+-----------------------------------------------------+
| ISS | initial send sequence number |
+----------+-----------------------------------------------------+
Table 2: Send Sequence Variables
+==========+=================================+
| Variable | Description |
+==========+=================================+
| RCV.NXT | receive next |
+----------+---------------------------------+
| RCV.WND | receive window |
+----------+---------------------------------+
| RCV.UP | receive urgent pointer |
+----------+---------------------------------+
| IRS | initial receive sequence number |
+----------+---------------------------------+
Table 3: Receive Sequence VariablesThese diagrams can help you connect some of those variables to the sequence space 🧩.
1 2 3 4
----------|----------|----------|----------
SND.UNA SND.NXT SND.UNA
+SND.WND
1 - old sequence numbers that have been acknowledged
2 - sequence numbers of unacknowledged data
3 - sequence numbers allowed for new data transmission
4 - future sequence numbers that are not yet allowed
Figure 3: Send Sequence SpaceThe send window is the part of the sequence space labeled 3 in Figure 3 📤.
1 2 3
----------|----------|----------
RCV.NXT RCV.NXT
+RCV.WND
1 - old sequence numbers that have been acknowledged
2 - sequence numbers allowed for new reception
3 - future sequence numbers that are not yet allowed
Figure 4: Receive Sequence SpaceThe receive window is the part of the sequence space labeled 2 in Figure 4 📥.
There are also some variables that get used a lot in the discussion, and they take their values from the fields of the current segment 🎯. Here they are 💖.
+==========+===============================+
| Variable | Description |
+==========+===============================+
| SEG.SEQ | segment sequence number |
+----------+-------------------------------+
| SEG.ACK | segment acknowledgment number |
+----------+-------------------------------+
| SEG.LEN | segment length |
+----------+-------------------------------+
| SEG.WND | segment window |
+----------+-------------------------------+
| SEG.UP | segment urgent pointer |
+----------+-------------------------------+
Table 4: Current Segment Variables3.3.2. State Machine Overview ✨
okay so a connection goes through a bunch of different states across its whole lifetime 🦋 the states are LISTEN, SYN-SENT, SYN-RECEIVED, ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT, and the fictional state CLOSED. CLOSED is fictional bc it's literally the state where there's no TCB, so there's no connection at all 👻 here's the quick rundown:
- LISTEN 👂 waiting for a connection request from any remote TCP peer and port.
- SYN-SENT 📤 waiting for a matching connection request after having sent a connection request.
- SYN-RECEIVED 🤝 waiting for a confirming connection request acknowledgment after having both received and sent a connection request.
- ESTABLISHED 💖 an open connection, where data received can be delivered to the user. This is the normal state for the data transfer phase of the connection.
- FIN-WAIT-1 👋 waiting for a connection termination request from the remote TCP peer, or an acknowledgment of the connection termination request previously sent.
- FIN-WAIT-2 🫶 waiting for a connection termination request from the remote TCP peer.
- CLOSE-WAIT 🥺 waiting for a connection termination request from the local user.
- CLOSING 😵💫 waiting for a connection termination request acknowledgment from the remote TCP peer.
- LAST-ACK 🏁 waiting for an acknowledgment of the connection termination request previously sent to the remote TCP peer. (this termination request sent to the remote TCP peer already included an acknowledgment of the termination request sent from the remote TCP peer).
- TIME-WAIT ⏳ waiting for enough time to pass to be sure the remote TCP peer received the acknowledgment of its connection termination request, and to avoid new connections being impacted by delayed segments from previous connections.
- CLOSED 🫥 no connection state at all.
a TCP connection moves from one state to another in response to events 🌸 the events are the user calls OPEN, SEND, RECEIVE, CLOSE, ABORT, and STATUS; the incoming segments, especially the ones carrying the SYN, ACK, RST, and FIN flags; and timeouts.
the OPEN call says whether connection establishment is going to be actively pursued, or passively waited for 🎀
a passive OPEN means the process wants to accept incoming connection requests, which is the opposite of an active OPEN, where it's the one starting the connection 🧚
the state diagram in Figure 5 only shows state changes, along with the events that cause them and the resulting actions 💅 it doesn't cover error conditions or actions that aren't tied to a state change. a later section goes into more detail on how the TCP implementation reacts to events. also, some state names are abbreviated or hyphenated differently in the diagram than they are everywhere else in the doc, so don't be confused 🙈
NOTA BENE: this diagram is only a summary and must not be taken as the total specification. Many details are not included. 💀
+---------+ ---------\ active OPEN
| CLOSED | \ -----------
+---------+<---------\ \ create TCB
| ^ \ \ snd SYN
passive OPEN | | CLOSE \ \
------------ | | ---------- \ \
create TCB | | delete TCB \ \
V | \ \
rcv RST (note 1) +---------+ CLOSE | \
-------------------->| LISTEN | ---------- | |
/ +---------+ delete TCB | |
/ rcv SYN | | SEND | |
/ ----------- | | ------- | V
+--------+ snd SYN,ACK / \ snd SYN +--------+
| |<----------------- ------------------>| |
| SYN | rcv SYN | SYN |
| RCVD |<-----------------------------------------------| SENT |
| | snd SYN,ACK | |
| |------------------ -------------------| |
+--------+ rcv ACK of SYN \ / rcv SYN,ACK +--------+
| -------------- | | -----------
| x | | snd ACK
| V V
| CLOSE +---------+
| ------- | ESTAB |
| snd FIN +---------+
| CLOSE | | rcv FIN
V ------- | | -------
+---------+ snd FIN / \ snd ACK +---------+
| FIN |<---------------- ------------------>| CLOSE |
| WAIT-1 |------------------ | WAIT |
+---------+ rcv FIN \ +---------+
| rcv ACK of FIN ------- | CLOSE |
| -------------- snd ACK | ------- |
V x V snd FIN V
+---------+ +---------+ +---------+
|FINWAIT-2| | CLOSING | | LAST-ACK|
+---------+ +---------+ +---------+
| rcv ACK of FIN | rcv ACK of FIN |
| rcv FIN -------------- | Timeout=2MSL -------------- |
| ------- x V ------------ x V
\ snd ACK +---------+delete TCB +---------+
-------------------->|TIME-WAIT|------------------->| CLOSED |
+---------+ +---------+
Figure 5: TCP Connection State Diagramthe notes for Figure 5 are next 📝
Note 1: the move from SYN-RECEIVED to LISTEN on receiving a RST only happens if you got to SYN-RECEIVED after a passive OPEN 🫧
Note 2: the figure leaves out a transition from FIN-WAIT-1 to TIME-WAIT, for when a FIN comes in and the local FIN also gets acknowledged 🙃
Note 3: a RST can be sent from any state, with a matching transition to TIME-WAIT (see [70] for the rationale) ✨ these transitions aren't drawn out, bc the diagram would be super hard to read. likewise, getting a RST from any state leads to LISTEN or CLOSED, but that's left off the diagram too for legibility 👑
3.4. Sequence Numbers ✨
okay so the big idea here is that every single octet of data on a TCP connection gets its own sequence number 🔢🫶 bestie, that means every octet can be acknowledged too! The ack system is cumulative, which is so iconic, because an ack for sequence number X basically says "I've got everything up to but not including X" 💅 and that makes duplicate detection super straightforward when stuff gets retransmitted 🔁✨ Inside a segment, the first data octet right after the header is the lowest numbered, and the rest just count up consecutively 🎀
ngl the sequence number space is finite, even though it's huge 🌸 it runs from 0 to 2^32 - 1, so ALL the arithmetic has to be done modulo 2^32 🧮💖 unsigned math keeps things in order as they wrap from 2^32 - 1 back to 0, but computer modulo math is kinda tricky 🙈 so be extra careful when you write the comparisons, periodt 💅 the symbol "=<" means "less than or equal" (modulo 2^32) ✨
the comparisons TCP has to do all the time are basically:
(a) figuring out whether an ack is talking about a sequence number that was sent but isn't acked yet 🦋
(b) figuring out whether every sequence number in a segment has been acked, so it can be yanked off the retransmission queue 🧹
(c) figuring out whether an incoming segment has sequence numbers that are expected, aka it "overlaps" the receive window 🪟
when you send data, you're gonna get acks back, and these are the variables you need to process them 👑
SND.UNA = oldest unacknowledged sequence number
SND.NXT = next sequence number to be sent
SEG.ACK = acknowledgment from the receiving TCP peer (next
sequence number expected by the receiving TCP peer)
SEG.SEQ = first sequence number of a segment
SEG.LEN = the number of octets occupied by the data in the segment
(counting SYN and FIN)
SEG.SEQ+SEG.LEN-1 = last sequence number of a segmenta brand-new ack (called an "acceptable ack") is one where this inequality holds 💜
SND.UNA < SEG.ACK =< SND.NXTa segment on the retransmission queue counts as fully acknowledged if its sequence number plus its length is less than or equal to the ack value in the incoming segment 🌈✨
when data comes IN, these are the comparisons you need 🫶
RCV.NXT = next sequence number expected on an incoming segment,
and is the left or lower edge of the receive window
RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming
segment, and is the right or upper edge of the receive window
SEG.SEQ = first sequence number occupied by the incoming segment
SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming
segmenta segment is judged to be sitting in valid receive sequence space if either of these is true 🦋
RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WNDor
RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WNDso the first check looks at whether the START of the segment lands in the window, and the second checks whether the END does 🪟 if it passes either one, the segment has data in the window, no cap 💖
ok but it's a LITTLE more complicated than that 🙈 because of zero windows and zero-length segments, there are four cases for whether an incoming segment is acceptable 💀
+=========+=========+======================================+
| Segment | Receive | Test |
| Length | Window | |
+=========+=========+======================================+
| 0 | 0 | SEG.SEQ = RCV.NXT |
+---------+---------+--------------------------------------+
| 0 | >0 | RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND |
+---------+---------+--------------------------------------+
| >0 | 0 | not acceptable |
+---------+---------+--------------------------------------+
| >0 | >0 | RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND |
| | | |
| | | or |
| | | |
| | | RCV.NXT =< SEG.SEQ+SEG.LEN-1 < |
| | | RCV.NXT+RCV.WND |
+---------+---------+--------------------------------------+
Table 5: Segment Acceptability Testsnote that when the receive window is zero, NO segments should be acceptable except ACK segments 🙅♀️ which means a TCP implementation can keep a zero receive window while still sending data and getting ACKs back, lowkey wild 🦋 and a TCP receiver MUST process the RST and URG fields of all incoming segments, even when the receive window is zero (MUST-66) 🚨👑
we've also used the numbering scheme to protect some control info 🛡️✨ control flags get implicitly included in the sequence space, so they can be retransmitted and acked without any mix-ups (basically, one and only one copy of the control gets acted on) 💖 control info isn't physically carried in the segment data space, so we need rules for implicitly assigning sequence numbers to it 🧚 the SYN and FIN are the only controls that need this protection, and they're only used when a connection is opening and closing 🌙 for sequence number purposes, the SYN counts as happening BEFORE the first actual data octet of its segment, and the FIN counts as happening AFTER the last actual data octet 🎀 SEG.LEN includes both data and the sequence-space-occupying controls 💅 and when a SYN is present, SEG.SEQ is the sequence number of the SYN 🫶
3.4.1. Initial Sequence Number Selection 🎀
okay so a connection is defined by a pair of sockets, and connections can be reused 🔁 each new instance is called an "incarnation" of the connection 🦋 the problem is: "how does the TCP implementation tell duplicate segments from previous incarnations apart?" 😭 this gets extra real if the connection is opened and closed super fast, or if it breaks with memory loss and then gets reestablished 💀 to help with this, the TIME-WAIT state limits how fast connections can be reused, and the initial sequence number selection (described below) adds even more protection against confusion about which incarnation an incoming packet belongs to 🌸
to avoid confusion, we have to make sure segments from one incarnation can't get used while the same sequence numbers might still be floating around the network from an earlier one 🌊 we want this to hold even if a TCP endpoint totally forgets which sequence numbers it's been using 🙈 when new connections get created, an initial sequence number (ISN) generator picks a new 32-bit ISN ✨ and there are security issues if an off-path attacker can predict or guess ISN values [42] 🚨 literally a whole vibe to protect against 😤
TCP initial sequence numbers come from a number sequence that keeps increasing until it wraps, loosely known as a "clock" ⏰ this clock is a 32-bit counter that typically ticks at least once about every 4 microseconds 🕐 it's NOT assumed to be realtime or precise, and it doesn't need to persist across reboots 🔄 the clock part is there so that, within the Maximum Segment Lifetime (MSL), generated ISNs stay unique, since it cycles about every 4.55 hours, which is way longer than the MSL 🥹💖 also, for modern high-speed networks where a connection could start and quickly advance sequence numbers so they overlap within the MSL, it's recommended to implement the Timestamp Option, which comes up later in Section 3.4.3 🧚🌈
A TCP implementation MUST use the above type of "clock" for clock-driven selection of initial sequence numbers (MUST-8) 🫶 and SHOULD generate its initial sequence numbers with this expression 👑
ISN = M + F(localip, localport, remoteip, remoteport, secretkey)where M is the 4 microsecond timer, and F() is a pseudorandom function (PRF) of the connection's identifying parameters ("localip, localport, remoteip, remoteport") and a secret key ("secretkey") (SHLD-1) ✨ F() MUST NOT be computable from the outside (MUST-9) 🔐 otherwise an attacker could still guess sequence numbers from the ISN used for some other connection 😱 the PRF could be implemented as a cryptographic hash of the concatenation of the TCP connection parameters and some secret data 🔑 for discussion of picking a specific hash algorithm and managing the secret key data, see Section 3 of [42] 📚
for each connection there's a send sequence number and a receive sequence number 💕 the initial send sequence number (ISS) is chosen by the data-sending TCP peer, and the initial receive sequence number (IRS) is learned during the connection-establishing procedure 🦋
for a connection to be established or initialized, the two TCP peers have to synchronize on each other's initial sequence numbers 🤝 this happens in an exchange of connection-establishing segments carrying a control bit called "SYN" (for synchronize) plus the initial sequence numbers 💫 segments carrying the SYN bit are also called "SYNs" for short 🌟 so the solution needs a suitable mechanism for picking an ISN AND a slightly involved handshake to swap ISNs 🙃
the synchronization requires each side to send its own ISN and receive a confirmation of it in an ack from the remote peer 🔁 each side also has to receive the remote peer's ISN and send a confirming ack back 💅
1) A --> B SYN my sequence number is X
2) A <-- B ACK your sequence number is X
3) A <-- B SYN my sequence number is Y
4) A --> B ACK your sequence number is Ybecause steps 2 and 3 can be combined in a single message, this is called the three-way (or three message) handshake (3WHS) 🤝✨ (bestie, that's the 3WHS!! we're getting so close to the good stuff 😭🎀)
a 3WHS is necessary because sequence numbers aren't tied to a global clock in the network, and TCP implementations may pick their ISNs in different ways 🌍 the receiver of the first SYN has no way of knowing whether the segment is an old one unless it remembers the last sequence number used on the connection (which isn't always possible) 🙈 so it has to ask the sender to verify the SYN 🫶 the three-way handshake and the perks of a clock-driven ISN scheme are discussed in [69] 📖💜
3.4.2. Knowing When to Keep Quiet 🌙
okay bestie, so there's this theoretical problem 🙈 where data could get corrupted if old segments hanging around in the network get mixed up with new ones after a host reboots, but only if the same port numbers and sequence space get reused. The "quiet time" concept is how this gets handled. We're including the discussion for situations where it might matter 💅, even though it's not felt to be necessary in most current implementations. This was more of a thing earlier in TCP's history. On today's Internet, the error-prone conditions are sufficiently unlikely that it's safe to ignore ✨
So why is it negligible now? Three reasons, and they're iconic: (a) ISS and ephemeral port randomization have reduced the likelihood of reusing port numbers and sequence numbers after reboots, (b) the effective MSL of the Internet has declined as links have become faster, and (c) reboots often take longer than an MSL anyways 😭
To be sure that a TCP implementation doesn't create a segment with a sequence number that an old segment still in the network could duplicate, the TCP endpoint must keep quiet for an MSL before assigning any sequence numbers when it starts up or recovers from a situation where its memory of sequence numbers in use was lost. For this specification, the MSL is taken to be 2 minutes. This is an engineering choice and may be changed if experience shows it's desirable to do so 🧚 Also, if a TCP endpoint gets reinitialized in some sense but still remembers the sequence numbers in use, then it need not wait at all. It just must make sure to use sequence numbers larger than those recently used. Main character glow-up 💖
3.4.3. The TCP Quiet Time Concept 🫶
Hosts that for any reason lose knowledge of the last sequence numbers transmitted on each active (i.e., not closed) connection shall delay emitting any TCP segments for at least the agreed MSL in the internet system that the host is a part of. Below, we explain why this spec says so. TCP implementers may violate the "quiet time" restriction, but only at the risk of causing some old data to be accepted as new, or new data to be rejected as old duplicated data, by some receivers in the internet system 🙃
TCP endpoints consume sequence number space each time a segment is formed and entered into the network output queue at a source host. The duplicate detection and sequencing algorithm in TCP relies on the unique binding of segment data to sequence space, to the extent that sequence numbers won't cycle through all 2^32 values before the segment data bound to those sequence numbers has been delivered and acknowledged by the receiver and all duplicate copies of the segments have "drained" from the internet. Without that assumption, two distinct TCP segments could conceivably be assigned the same or overlapping sequence numbers, which would confuse the receiver about which data is new and which is old. Remember that each segment is bound to as many consecutive sequence numbers as there are octets of data and SYN or FIN flags in the segment 🍓
Under normal conditions, TCP implementations keep track of the next sequence number to emit and the oldest one awaiting acknowledgment, so they can avoid mistakenly reusing a sequence number before its first use has been acknowledged. That alone doesn't guarantee that old duplicate data drains from the net, so the sequence space has been made large to reduce the probability that a wandering duplicate causes trouble when it arrives. At 2 megabits/sec., it takes 4.5 hours to use up 2^32 octets of sequence space. Since the maximum segment lifetime in the net isn't likely to exceed a few tens of seconds, this is deemed ample protection for foreseeable nets, even if data rates escalate to 10s of megabits/sec. At 100 megabits/sec., the cycle time is 5.4 minutes, which may be a little short but is still within reason. Much higher data rates are possible today, with implications described in the final paragraph of this subsection 🌈
The basic duplicate detection and sequencing algorithm in TCP can be defeated, though, if a source TCP endpoint has no memory of the sequence numbers it last used on a given connection. For example, if the TCP implementation started all connections with sequence number 0, then after the host reboots, a TCP peer might re-form an earlier connection (possibly after half-open connection resolution) and emit packets with sequence numbers identical to or overlapping with packets still in the network that were emitted on an earlier incarnation of the same connection. Without knowledge of the sequence numbers used on a particular connection, the TCP specification recommends that the source delay for MSL seconds before emitting segments on the connection, so segments from the earlier incarnation have time to drain from the system 🦋
Even hosts that can remember the time of day and use it to select initial sequence numbers aren't immune to this problem (i.e., even if time of day is used to select an initial sequence number for each new connection incarnation) 🙈
Let's say a connection is opened starting with sequence number S. The connection isn't used much, and eventually the initial sequence number function (ISN(t)) takes on a value equal to the sequence number, say S1, of the last segment this TCP endpoint sent on that particular connection. Now the host reboots at that exact instant and establishes a new incarnation of the connection. The chosen initial sequence number is S1 = ISN(t) -- last used sequence number on old incarnation of connection! If recovery happens quickly enough, old duplicates in the net bearing sequence numbers near S1 may arrive and be treated as new packets by the receiver of the new incarnation 😱
The problem is that the recovering host may not know how long it was down between rebooting, and it doesn't know whether old duplicates from earlier connection incarnations are still in the system 🫠
One way to deal with this is to deliberately delay emitting segments for one MSL after recovering from a reboot. This is the "quiet time" specification. Hosts that prefer to avoid waiting, and are willing to risk possible confusion of old and new packets at a given destination, may choose not to wait for the "quiet time". Implementers may give TCP users the ability to select, on a connection-by-connection basis, whether to wait after a reboot, or they may informally implement the "quiet time" for all connections. Obviously, even where a user selects to "wait", this isn't necessary after the host has been "up" for at least MSL seconds 💜
To summarize: every segment emitted occupies one or more sequence numbers in the sequence space, and the numbers occupied by a segment are "busy" or "in use" until MSL seconds have passed. Upon rebooting, a block of space-time is occupied by the octets and SYN or FIN flags of any potentially still in-flight segments. If a new connection starts too soon and uses any of the sequence numbers in the space-time footprint of those potentially still in-flight segments from the previous connection incarnation, there's a potential sequence number overlap area that could confuse the receiver 🌸
High-performance cases have shorter cycle times than the megabits-per-second rates the base TCP design described above considers. At 1 Gbps, the cycle time is 34 seconds, it's only 3 seconds at 10 Gbps, and around a third of a second at 100 Gbps. In these higher-performance cases, TCP Timestamp Options and Protection Against Wrapped Sequences (PAWS) [47] provide the needed capability to detect and discard old duplicates. That's the plot twist, and it's giving very "problem solved" energy 👑✨
3.5. Establishing a Connection ✨
okay so the "three-way handshake" 🤝 is the procedure TCP uses to set up a connection. Usually one TCP peer starts it and another one answers, but it's also totally fine if two peers start it at the exact same time. That's called simultaneous open 👯. When it happens, each peer gets a SYN segment with no acknowledgment after it has already sent its own SYN. Old duplicate SYNs can sometimes make it look like a simultaneous open is happening when it isn't 🙈, and that's where proper use of "reset" segments comes in and clears up the confusion 💅.
Next up are some examples of connection initiation 🎀. These examples don't show connection synchronization using data-carrying segments, but that's still perfectly legit ✨, as long as the receiving TCP endpoint doesn't hand the data to its user until it's clear the data is valid. For example, the data can sit in a buffer at the receiver until the connection reaches the ESTABLISHED state, since the three-way handshake makes false connections less likely 🫶. It's a trade-off between memory and messages, because providing info for this checking takes one or the other 🧠.
The simplest 3WHS is in Figure 6 🦋. Here's how to read the figures 👀. Each line has a number so we can refer to it. Right arrows (-->) mean a TCP segment is leaving TCP Peer A for TCP Peer B, or arriving at B from A. Left arrows (<--) mean the reverse. Ellipses (...) mean a segment is still floating around in the network (delayed) 🌫️. Comments go in parentheses. The TCP connection states show what state each peer is in AFTER the segment leaves or arrives (the segment's contents are in the middle of each line). The segment contents are abbreviated, so you only see the sequence number, control flags, and ACK field. Other stuff like window, addresses, lengths, and text is left out so it's easier to read 💖.
TCP Peer A TCP Peer B
1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED
3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED
5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED
Figure 6: Basic Three-Way Handshake for Connection SynchronizationSo on line 2 of Figure 6, TCP Peer A starts things off by sending a SYN segment that says it'll use sequence numbers beginning with 100 🌱. On line 3, TCP Peer B sends its own SYN and acknowledges the SYN it got from TCP Peer A 🙌. Notice that the acknowledgment field says TCP Peer B is now waiting to hear sequence 101, which acknowledges the SYN that took up sequence 100 💜.
On line 4, TCP Peer A replies with an empty segment that has an ACK for TCP Peer B's SYN 💌. Then on line 5, TCP Peer A sends some data 📦. Here's the fun bit 😭: the line 5 segment has the same sequence number as the line 4 one, and that's because the ACK doesn't take up any sequence number space. If it did, we'd end up ACKing ACKs, and that's a whole mess nobody wants 💀.
Simultaneous initiation is only a little more complicated, and you can see it in Figure 7 🎭. Each TCP peer's connection state cycles from CLOSED to SYN-SENT to SYN-RECEIVED to ESTABLISHED 🔄.
TCP Peer A TCP Peer B
1. CLOSED CLOSED
2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
3. SYN-RECEIVED <-- <SEQ=300><CTL=SYN> <-- SYN-SENT
4. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED
5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
7. ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED
Figure 7: Simultaneous Connection SynchronizationA TCP implementation MUST support simultaneous open attempts (MUST-10). This one's a must-have, bestie 🚨.
Also, a TCP implementation MUST keep track of whether a connection has reached the SYN-RECEIVED state because of a passive OPEN or an active OPEN (MUST-11). Keeping that straight matters 🧾.
The main reason for the three-way handshake is to stop old duplicate connection initiations from causing chaos 🌀. To handle that, there's a special control message called reset 🔁. If the receiving TCP peer is in a non-synchronized state (SYN-SENT or SYN-RECEIVED), it goes back to LISTEN when it gets an acceptable reset 🙃. If the TCP peer is in one of the synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, or TIME-WAIT), it aborts the connection and tells its user 🛑. We'll talk about that second case under "half-open" connections below 👀.
TCP Peer A TCP Peer B
1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
3. (duplicate) ... <SEQ=90><CTL=SYN> --> SYN-RECEIVED
4. SYN-SENT <-- <SEQ=300><ACK=91><CTL=SYN,ACK> <-- SYN-RECEIVED
5. SYN-SENT --> <SEQ=91><CTL=RST> --> LISTEN
6. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED
7. ESTABLISHED <-- <SEQ=400><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
8. ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK> --> ESTABLISHED
Figure 8: Recovery from Old Duplicate SYNFor a simple example of recovering from old duplicates, check out Figure 8 🕵️♀️. On line 3, an old duplicate SYN shows up at TCP Peer B. B can't tell it's old, so it responds like normal (line 4) 🤷♀️. TCP Peer A notices the ACK field is wrong and sends back a RST (reset), with its SEQ field picked so the segment looks believable 🧐. When TCP Peer B gets that RST, it goes back to LISTEN 🫡. Then when the original SYN finally arrives on line 6, the synchronization just goes through normally 🎉. If the line 6 SYN had shown up before the RST, things could've gotten more complicated, with RSTs flying in both directions 🌪️.
3.5.1. Half-Open Connections and Other Anomalies
okay this is where things get spicy 🌶️. An established connection is called "half-open" if one TCP peer has closed or aborted it on its end without the other one knowing 👻. It's also half-open if the two ends have become out of sync because of a failure or reboot that wiped out memory 🧠💥. These connections will automatically reset if anyone tries to send data in either direction. Still, half-open connections are expected to be pretty unusual 🌙.
If the connection no longer exists at site A, then when the user at site B tries to send data on it, the site B TCP endpoint will receive a reset control message 📨. That message tells site B's TCP endpoint that something is wrong, and it's expected to abort the connection 🚫.
Now picture two user processes, A and B, chatting away when a failure or reboot wipes out the memory of A's TCP implementation 😱. Depending on the operating system running A's TCP implementation, there's probably some kind of error recovery mechanism 🛠️. Once the TCP endpoint is back up, A will probably start again from the very beginning or from a recovery point 🔄. So A will likely try to OPEN the connection again, or try to SEND on the connection it still thinks is open 🤞. In that second case, A gets the error message "connection not open" from its own local TCP implementation 🙃. To try to set up the connection, A's TCP implementation sends a segment containing SYN 📤. This whole situation leads to Figure 9 🎬. After TCP Peer A reboots, the user tries to reopen the connection, but meanwhile TCP Peer B still thinks the connection is open 😤.
TCP Peer A TCP Peer B
1. (REBOOT) (send 300,receive 100)
2. CLOSED ESTABLISHED
3. SYN-SENT --> <SEQ=400><CTL=SYN> --> (??)
4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK> <-- ESTABLISHED
5. SYN-SENT --> <SEQ=100><CTL=RST> --> (Abort!!)
6. SYN-SENT CLOSED
7. SYN-SENT --> <SEQ=400><CTL=SYN> -->
Figure 9: Half-Open Connection DiscoveryWhen the SYN arrives on line 3, TCP Peer B is in a synchronized state and the incoming segment is outside the window 🪟. So B replies with an acknowledgment that says what sequence number it expects to hear next (ACK 100) 📬. TCP Peer A sees that this segment doesn't acknowledge anything it sent. Since A isn't synchronized, it sends a reset (RST) because it has spotted a half-open connection 🕵️. TCP Peer B aborts on line 5 💥. TCP Peer A keeps trying to set up the connection, and at that point the whole problem is just the basic three-way handshake from Figure 6 🔁.
There's also an interesting alternative case 🧩. That happens when TCP Peer A reboots and TCP Peer B tries to send data on what it thinks is a synchronized connection. Figure 10 shows this one 📊. Here, the data that arrives at TCP Peer A from TCP Peer B on line 2 is unacceptable because there's no such connection anymore, so TCP Peer A sends a RST 🚨. The RST is acceptable, so TCP Peer B processes it and aborts the connection 🫠.
TCP Peer A TCP Peer B
1. (REBOOT) (send 300,receive 100)
2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED
3. --> <SEQ=100><CTL=RST> --> (ABORT!!)
Figure 10: Active Side Causes Half-Open Connection DiscoveryIn Figure 11, two TCP peers, A and B, both have passive connections waiting for a SYN 🌷. An old duplicate shows up at TCP Peer B on line 2 and wakes it up 🫨. B sends back a SYN-ACK on line 3, and that causes TCP A to generate a RST, because the ACK on line 3 isn't acceptable 🙅♀️. TCP Peer B accepts the reset and goes back to its passive LISTEN state 😌.
TCP Peer A TCP Peer B
1. LISTEN LISTEN
2. ... <SEQ=Z><CTL=SYN> --> SYN-RECEIVED
3. (??) <-- <SEQ=X><ACK=Z+1><CTL=SYN,ACK> <-- SYN-RECEIVED
4. --> <SEQ=Z+1><CTL=RST> --> (return to LISTEN!)
5. LISTEN LISTEN
Figure 11: Old Duplicate SYN Initiates a Reset on Two Passive SocketsThere are a bunch of other cases that can happen too 🌈. They're all covered by the following rules for RST generation and processing 💜.
3.5.2. Reset Generation ✨
Okay bestie, so a TCP user or app can send a reset on a connection at any time 💅, but the protocol also generates resets itself when certain error conditions pop up (more on that below). The side that issues a reset should enter the TIME-WAIT state, because that generally helps reduce the load on busy servers, for the reasons explained in [70] 🌸.
Here's the main rule: a reset (RST) gets sent whenever a segment shows up that apparently isn't meant for the current connection. But a reset must not be sent if it's not clear that this is the case 🙈.
There are three groups of states, so let's go through them 🦋
- If the connection does not exist (CLOSED), then a reset is sent in response to any incoming segment except another reset. This is how a SYN that doesn't match an existing connection gets rejected 🚫.
If the incoming segment has the ACK bit set, the reset takes its sequence number from the ACK field of the segment. Otherwise, the reset has sequence number zero and its ACK field is set to the sum of the sequence number and segment length of the incoming segment. The connection remains in the CLOSED state.
- If the connection is in any non-synchronized state (LISTEN, SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges something not yet sent (the segment carries an unacceptable ACK), or if an incoming segment has a security level or compartment (Appendix A.1) that does not exactly match the level and compartment requested for the connection, a reset is sent.
If the incoming segment has an ACK field, the reset takes its sequence number from the ACK field of the segment. Otherwise, the reset has sequence number zero and the ACK field is set to the sum of the sequence number and segment length of the incoming segment. The connection remains in the same state.
- If the connection is in a synchronized state (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), any unacceptable segment (out-of-window sequence number or unacceptable acknowledgment number) must be responded to with an empty acknowledgment segment (without any user data) containing the current send sequence number and an acknowledgment indicating the next sequence number expected to be received, and the connection remains in the same state.
If an incoming segment has a security level or compartment that does not exactly match the level and compartment requested for the connection, a reset is sent and the connection goes to the CLOSED state. The reset takes its sequence number from the ACK field of the incoming segment. 🔥
3.5.3. Reset Processing 🧹
In all states except SYN-SENT, all reset (RST) segments get validated by checking their SEQ fields. A reset is valid if its sequence number is in the window. In the SYN-SENT state (a RST received in response to an initial SYN), the RST is acceptable if the ACK field acknowledges the SYN.
The receiver of a RST validates it first, and then changes state. If the receiver was in the LISTEN state, it ignores it. If the receiver was in SYN-RECEIVED state and had previously been in the LISTEN state, then the receiver returns to the LISTEN state. Otherwise, the receiver aborts the connection and goes to the CLOSED state. If the receiver was in any other state, it aborts the connection, advises the user, and goes to the CLOSED state 👑
TCP implementations SHOULD allow a received RST segment to include data (SHLD-2). It has been suggested that a RST segment could contain diagnostic data that explains the cause of the RST. No standard has yet been established for such data 🤷♀️
3.6. Closing a Connection 🎀
CLOSE is an operation meaning "I have no more data to send." 💌 Closing a full-duplex connection is kinda tricky, of course, since it may not be obvious how to treat the receiving side of the connection. So we chose to treat CLOSE in a simplex fashion. The user who CLOSEs may continue to RECEIVE until the TCP receiver is told that the remote peer has CLOSED also. That means a program could initiate several SENDs followed by a CLOSE, and then keep RECEIVEing until it's signaled that a RECEIVE failed because the remote peer has CLOSED. The TCP implementation will signal a user, even if no RECEIVEs are outstanding, that the remote peer has closed, so the user can terminate their side gracefully ✨
A TCP implementation will reliably deliver all buffers SENT before the connection was CLOSED, so a user who expects no data in return only needs to wait to hear that the connection was CLOSED successfully to know that all their data was received at the destination TCP endpoint. Users must keep reading connections they close for sending until the TCP implementation indicates there is no more data 📖
There are essentially three cases:
- The user initiates by telling the TCP implementation to CLOSE the connection (TCP Peer A in Figure 12).
- The remote TCP endpoint initiates by sending a FIN control signal (TCP Peer B in Figure 12).
- Both users CLOSE simultaneously (Figure 13).
Case 1: Local user initiates the close 🚪
In this case, a FIN segment can be constructed and placed on the outgoing segment queue. No further SENDs from the user will be accepted by the TCP implementation, and it enters the FIN-WAIT-1 state. RECEIVEs are allowed in this state. All segments preceding and including FIN will be retransmitted until acknowledged. When the other TCP peer has both acknowledged the FIN and sent a FIN of its own, the first TCP peer can ACK this FIN. Note that a TCP endpoint receiving a FIN will ACK but not send its own FIN until its user has CLOSED the connection also 🫶
Case 2: TCP endpoint receives a FIN from the network 📬
If an unsolicited FIN arrives from the network, the receiving TCP endpoint can ACK it and tell the user that the connection is closing. The user will respond with a CLOSE, upon which the TCP endpoint can send a FIN to the other TCP peer after sending any remaining data. The TCP endpoint then waits until its own FIN is acknowledged, whereupon it deletes the connection. If an ACK is not forthcoming, after the user timeout the connection is aborted and the user is told 😬
Case 3: Both users close simultaneously 💥
A simultaneous CLOSE by users at both ends of a connection causes FIN segments to be exchanged (Figure 13). When all segments preceding the FINs have been processed and acknowledged, each TCP peer can ACK the FIN it has received. Both will, upon receiving these ACKs, delete the connection 🌈
TCP Peer A TCP Peer B
1. ESTABLISHED ESTABLISHED
2. (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT
3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT
4. (Close)
TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK
5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED
6. (2 MSL)
CLOSED
Figure 12: Normal Close Sequence TCP Peer A TCP Peer B
1. ESTABLISHED ESTABLISHED
2. (Close) (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> ... FIN-WAIT-1
<-- <SEQ=300><ACK=100><CTL=FIN,ACK> <--
... <SEQ=100><ACK=300><CTL=FIN,ACK> -->
3. CLOSING --> <SEQ=101><ACK=301><CTL=ACK> ... CLOSING
<-- <SEQ=301><ACK=101><CTL=ACK> <--
... <SEQ=101><ACK=301><CTL=ACK> -->
4. TIME-WAIT TIME-WAIT
(2 MSL) (2 MSL)
CLOSED CLOSED
Figure 13: Simultaneous Close SequenceA TCP connection may terminate in two ways: (1) the normal TCP close sequence using a FIN handshake (Figure 12), and (2) an "abort" in which one or more RST segments are sent and the connection state is immediately discarded. If the local TCP connection is closed by the remote side due to a FIN or RST received from the remote side, then the local application MUST be informed whether it closed normally or was aborted (MUST-12). 🚨
3.6.1. Half-Closed Connections 🌓
The normal TCP close sequence delivers buffered data reliably in both directions. Since the two directions of a TCP connection are closed independently, it is possible for a connection to be "half closed", i.e., closed in only one direction. A host is permitted to keep sending data in the open direction on a half-closed connection 🦋
A host MAY implement a "half-duplex" TCP close sequence, so that an application that has called CLOSE cannot continue to read data from the connection (MAY-1). If such a host issues a CLOSE call while received data is still pending in the TCP connection, or if new data is received after CLOSE is called, its TCP implementation SHOULD send a RST to show that data was lost (SHLD-3). See [23], Section 2.17 for discussion 🧐
When a connection is closed actively, it MUST linger in the TIME-WAIT state for a time 2xMSL (Maximum Segment Lifetime) (MUST-13). However, it MAY accept a new SYN from the remote TCP endpoint to reopen the connection directly from TIME-WAIT state (MAY-2), if it:
(1) assigns its initial sequence number for the new connection to be larger than the largest sequence number it used on the previous connection incarnation, and
(2) returns to TIME-WAIT state if the SYN turns out to be an old duplicate.
When the TCP Timestamp Options are available, an improved algorithm is described in [40] in order to support higher connection establishment rates. This algorithm for reducing TIME-WAIT is a Best Current Practice that SHOULD be implemented, since Timestamp Options are commonly used, and using them to reduce TIME-WAIT provides benefits for busy Internet servers (SHLD-4). 💜
3.7. Segmentation ✨
Okay so "segmentation" is the thing TCP does when it takes a stream of bytes from the sending app and chops it up into TCP segments 🔪 Bestie, these segments often do NOT line up one-to-one with the app's send (or socket write) calls 🙈 Apps can write at the granularity of messages from the upper-layer protocol, but TCP guarantees no correlation between the boundaries of the segments sent and received and the boundaries of the read or write buffers of user application data. 💅
In some specific protocols, like Remote Direct Memory Access (RDMA) using Direct Data Placement (DDP) and Marker PDU Aligned Framing (MPA) [34], you can get performance optimizations when you control how TCP segments line up with application data units. MPA even has a specific mechanism for detecting and verifying that relationship between TCP segments and application message data structures, but that's specific to apps like RDMA 🎀 In general, multiple goals influence how big the segments a TCP implementation creates should be 📏
Goals driving the sending of larger segments include:
- Reducing the number of packets in flight within the network.
- Increasing processing efficiency and potential performance by enabling a smaller number of interrupts and inter-layer interactions.
- Limiting the overhead of TCP headers.
Note that the performance benefits of sending larger segments may decrease as the size increases, and there may be boundaries where advantages are reversed 🙃 For instance, on some implementation architectures, 1025 bytes within a segment could lead to worse performance than 1024 bytes, due purely to data alignment on copy operations. Not fair, but that's just how it goes 💀
Goals driving the sending of smaller segments include:
- Avoiding sending a TCP segment that would result in an IP datagram larger than the smallest MTU along an IP network path, because this results in either packet loss or packet fragmentation. Making matters worse, some firewalls or middleboxes may drop fragmented packets or ICMP messages related to fragmentation. 😭
- Preventing delays to the application data stream, especially when TCP is waiting on the application to generate more data, or when the application is waiting on an event or input from its peer in order to generate more data. ⏳
- Enabling "fate sharing" between TCP segments and lower-layer data units (e.g., below IP, for links with cell or frame sizes smaller than the IP MTU). 🦋
Towards meeting these competing sets of goals, TCP includes several mechanisms, including the Maximum Segment Size Option, Path MTU Discovery, the Nagle algorithm, and support for IPv6 Jumbograms, as discussed in the following subsections. It's a whole juggling act, fr fr 🤹♀️
3.7.1. Maximum Segment Size Option
Okay, the MSS Option is a must-have. TCP endpoints MUST implement both sending and receiving the MSS Option (MUST-14). 👑
TCP implementations SHOULD send an MSS Option in every SYN segment when its receive MSS differs from the default 536 for IPv4 or 1220 for IPv6 (SHLD-5), and MAY send it always (MAY-3). 🌸
If an MSS Option is not received at connection setup, TCP implementations MUST assume a default send MSS of 536 (576 - 40) for IPv4 or 1220 (1280 - 60) for IPv6 (MUST-15). 🧚
The maximum size of a segment that a TCP endpoint really sends, the "effective send MSS", MUST be the smaller (MUST-16) of two things: the send MSS (which reflects the available reassembly buffer size at the remote host, the EMTU_R [19]) and the largest transmission size permitted by the IP layer (EMTU_S [19]). Here's the math:
Eff.snd.MSS = min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsizewhere:
- SendMSS is the MSS value received from the remote host, or the default 536 for IPv4 or 1220 for IPv6, if no MSS Option is received.
- MMS_S is the maximum size for a transport-layer message that TCP may send.
- TCPhdrsize is the size of the fixed TCP header and any options. This is 20 in the (rare) case that no options are present, but it may be larger if TCP Options are to be sent. Note that some options might not be included on all segments, but that for each segment sent, the sender should adjust the data length accordingly, within the Eff.snd.MSS.
- IPoptionsize is the size of any IPv4 options or IPv6 extension headers associated with a TCP connection. Note that some options or extension headers might not be included on all packets, but that for each segment sent, the sender should adjust the data length accordingly, within the Eff.snd.MSS.
The MSS value to be sent in an MSS Option should be equal to the effective MTU minus the fixed IP and TCP headers. By ignoring both IP and TCP Options when calculating the value for the MSS Option, if there are any IP or TCP Options to be sent in a packet, then the sender must decrease the size of the TCP data accordingly. RFC 6691 [43] discusses this in greater detail. 🤓
The MSS value to be sent in an MSS Option must be less than or equal to:
MMS_R - 20where MMS_R is the maximum size for a transport-layer message that can be received (and reassembled at the IP layer) (MUST-67). TCP obtains MMS_R and MMS_S from the IP layer; see the generic call GET_MAXSIZES in Section 3.4 of RFC 1122. These are defined in terms of their IP MTU equivalents, EMTU_R and EMTU_S [19].
⚠️ When TCP is used in a situation where either the IP or TCP headers are not fixed, the sender must reduce the amount of TCP data in any given packet by the number of octets used by the IP and TCP options. This has been a point of confusion historically, as explained in RFC 6691, Section 3.1. Read that one, bestie 📚
3.7.2. Path MTU Discovery
A TCP implementation may be aware of the MTU on directly connected links, but it will rarely have insight about MTUs across an entire network path 🗺️ For IPv4, RFC 1122 recommends an IP-layer default effective MTU of less than or equal to 576 for destinations not directly connected, and for IPv6 this would be 1280. Using these fixed values limits TCP connection performance and efficiency, so it's not it, sis 🙅♀️ Instead, implementing Path MTU Discovery (PMTUD) and Packetization Layer Path MTU Discovery (PLPMTUD) is strongly recommended so TCP can improve its segmentation decisions. Both PMTUD and PLPMTUD help TCP choose segment sizes that avoid both on-path (for IPv4) and source fragmentation (IPv4 and IPv6). ✨
PMTUD for IPv4 [2] or IPv6 [14] works together across TCP, IP, and ICMP. It relies both on avoiding source fragmentation and on setting the IPv4 DF (don't fragment) flag, the latter to inhibit on-path fragmentation. It also relies on ICMP errors from routers along the path whenever a segment is too large to traverse a link. Several adjustments to a TCP implementation with PMTUD are described in RFC 2923 to deal with problems experienced in practice [27]. 🛠️ PLPMTUD [31] is a Standards Track improvement to PMTUD. It relaxes the requirement for ICMP support across a path and improves performance in cases where ICMP is not consistently conveyed, but it still tries to avoid source fragmentation. The mechanisms in all four of these RFCs are recommended to be included in TCP implementations. 🫶
The TCP MSS Option specifies an upper bound for the size of packets that can be received (see [43]). Hence, setting the value in the MSS Option too small can impact the ability of PMTUD or PLPMTUD to find a larger path MTU. 🥲 RFC 1191 discusses this implication of many older TCP implementations setting the TCP MSS to 536 (corresponding to the IPv4 576 byte default MTU) for non-local destinations, rather than deriving it from the MTUs of connected interfaces as recommended. Not the move 🙈
3.7.3. Interfaces with Variable MTU Values
The effective MTU can sometimes vary, as when used with variable compression, e.g., RObust Header Compression (ROHC) [37]. It's tempting for a TCP implementation to advertise the largest possible MSS, to support the most efficient use of compressed payloads 🤏 Unfortunately, some compression schemes occasionally need to transmit full headers (and thus smaller payloads) to resynchronize state at their endpoint compressors/decompressors. If the largest MTU is used to calculate the value to advertise in the MSS Option, TCP retransmission may interfere with compressor resynchronization. 😵💫
As a result, when the effective MTU of an interface varies packet-to-packet, TCP implementations SHOULD use the smallest effective MTU of the interface to calculate the value to advertise in the MSS Option (SHLD-6). Smallest one wins, periodt 💜
3.7.4. Nagle Algorithm
The "Nagle algorithm" was described in RFC 896 [17] and was recommended in RFC 1122 [19] to mitigate an early problem of too many small packets being generated. 🐣 It has been implemented in most current TCP code bases, sometimes with minor variations (see Appendix A.3).
If there is unacknowledged data (i.e., SND.NXT > SND.UNA), then the sending TCP endpoint buffers all user data (regardless of the PSH bit) until the outstanding data has been acknowledged, or until the TCP endpoint can send a full-sized segment (Eff.snd.MSS bytes). 🧺
A TCP implementation SHOULD implement the Nagle algorithm to coalesce short segments (SHLD-7). However, there MUST be a way for an application to disable the Nagle algorithm on an individual connection (MUST-17). In all cases, sending data is also subject to the limitation imposed by the slow start algorithm [8]. So the app always gets a say, which is iconic 🫡
Since there can be problematic interactions between the Nagle algorithm and delayed acknowledgments, some implementations use minor variations of the Nagle algorithm, such as the one described in Appendix A.3. 🙃
3.7.5. IPv6 Jumbograms
In order to support TCP over IPv6 Jumbograms, implementations need to be able to send TCP segments larger than the 64-KB limit that the MSS Option can convey. RFC 2675 [24] defines that an MSS value of 65,535 bytes is to be treated as infinity, and Path MTU Discovery [14] is used to determine the actual MSS. Big segments, big energy 🐋
The Jumbo Payload Option need not be implemented or understood by IPv6 nodes that do not support attachment to links with an MTU greater than 65,575 [24], and the present IPv6 Node Requirements does not include support for Jumbograms [55]. Totally optional, no pressure 🌈
3.8. Data Communication ✨
Okay, the connection is established, so now the real fun begins 💌. Data gets communicated by swapping segments back and forth. Segments can go missing because of errors (a checksum test failure) or network congestion, so TCP uses retransmission to make sure every single segment gets delivered 🫶. Duplicate segments can also show up, thanks to the network or to TCP retransmission. Remember the sequence numbers section (Section 3.4)? TCP runs certain tests on the sequence and acknowledgment numbers in each segment to check that they're acceptable 🧐.
The sender keeps track of the next sequence number to use in the variable SND.NXT. The receiver keeps track of the next sequence number it expects in the variable RCV.NXT. The sender also tracks the oldest unacknowledged sequence number in the variable SND.UNA. If the data flow is momentarily idle and everything sent has been acknowledged, then all three variables will be equal 🌸.
When the sender creates a segment and transmits it, it advances SND.NXT. When the receiver accepts a segment, it advances RCV.NXT and sends an acknowledgment. When the data sender gets an acknowledgment, it advances SND.UNA. How far apart these variables are is a measure of the delay in the communication ⏳. The amount each variable moves forward is the length of the data plus the SYN or FIN flags in the segment. Also, once you're in the ESTABLISHED state, all segments need to carry current acknowledgment information, no exceptions 💅.
The CLOSE user call implies a push function (see Section 3.9.1), and so does the FIN control flag in an incoming segment 🦋.
3.8.1. Retransmission Timeout
Networks are super variable, especially when they make up an internetwork system, and TCP connections get used in a wide range of ways 🌈. Because of that, the retransmission timeout (RTO) has to be dynamically determined.
The RTO MUST be computed according to the algorithm in [10], including Karn's algorithm for taking RTT samples (MUST-18) 👑.
RFC 793 has an early example procedure for computing the RTO, based on work mentioned in IEN 177 [71]. That got replaced by the algorithm described in RFC 1122, which was later updated in RFC 2988 and then again in RFC 6298 🍓.
RFC 1122 allows that if a retransmitted packet is identical to the original packet (which implies not only that the data boundaries haven't changed, but also that none of the headers have changed), then the same IPv4 Identification field MAY be used (see Section 3.2.1.5 of RFC 1122) (MAY-4) ✨. The same IP Identification field may be reused anyways, since it's only meaningful when a datagram is fragmented [44]. TCP implementations should not rely on or typically interact with this IPv4 header field in any way 🙈. It's not a reasonable way to indicate duplicate sent segments, nor to identify duplicate received segments, so don't even try 💀.
3.8.2. TCP Congestion Control
RFC 2914 [5] explains why congestion control matters for the Internet 🌍.
RFC 1122 required implementation of Van Jacobson's congestion control algorithms, slow start and congestion avoidance, along with exponential backoff for successive RTO values for the same segment 🔥. RFC 2581 gave an IETF Standards Track description of slow start and congestion avoidance, along with fast retransmit and fast recovery. RFC 5681 is the current description of these algorithms and the current Standards Track specification with guidelines for TCP congestion control 💜. RFC 6298 describes exponential backoff of RTO values, including keeping the backed-off value until a subsequent segment with new data has been sent and acknowledged without retransmission ⭐.
A TCP endpoint MUST implement the basic congestion control algorithms slow start, congestion avoidance, and exponential backoff of RTO to avoid creating congestion collapse conditions (MUST-19) 🚦. RFC 5681 and RFC 6298 describe the basic algorithms on the IETF Standards Track that are broadly applicable. Plenty of other suitable algorithms exist and have been widely used, and many TCP implementations support a set of alternative algorithms that can be configured on the endpoint 🧁. An endpoint MAY implement such alternative algorithms, as long as they're conformant with the TCP specifications from the IETF Standards Track described in RFC 2914, RFC 5033 [7], and RFC 8961 [15] (MAY-18) 🎀.
Explicit Congestion Notification (ECN) was defined in RFC 3168. It's an IETF Standards Track enhancement with lots of benefits [51] 🌟.
A TCP endpoint SHOULD implement ECN as described in RFC 3168 (SHLD-8) 🫧.
3.8.3. TCP Connection Failures
When a TCP endpoint keeps retransmitting the same segment over and over, that's a sign something's wrong with the remote host or the internetwork path 😬. The failure might be short or long. The following procedure MUST be used to handle excessive retransmissions of data segments (MUST-20) 📋:
(a) There are two thresholds, R1 and R2, that measure how much retransmission has happened for the same segment. R1 and R2 might be measured in time units or as a count of retransmissions (with the current RTO and corresponding backoffs as a conversion factor, if needed) 🧮.
(b) When the number of transmissions of the same segment reaches or exceeds threshold R1, pass negative advice (see Section 3.3.1.4 of [19]) to the IP layer, to trigger dead-gateway diagnosis 🚨.
(c) When the number of transmissions of the same segment reaches a threshold R2 that's greater than R1, close the connection 🔒.
(d) An application MUST (MUST-21) be able to set the value for R2 for a particular connection. For example, an interactive application might set R2 to "infinity", which gives the user control over when to disconnect 👩💻.
(e) TCP implementations SHOULD inform the application of the delivery problem (unless the application has disabled that information; see the "Asynchronous Reports" section (Section 3.9.1.8)) when R1 is reached and before R2 (SHLD-9). This lets a remote login application tell the user, for example 💬.
The value of R1 SHOULD correspond to at least 3 retransmissions, at the current RTO (SHLD-10). The value of R2 SHOULD correspond to at least 100 seconds (SHLD-11) ⏱️.
An attempt to open a TCP connection could fail because of excessive retransmissions of the SYN segment, or because of receiving a RST segment or an ICMP Port Unreachable 🚫. SYN retransmissions MUST be handled in the general way just described for data retransmissions, including notifying the application layer 📣.
However, the values of R1 and R2 may be different for SYN and data segments. In particular, R2 for a SYN segment MUST be set large enough to provide retransmission of the segment for at least 3 minutes (MUST-23) ⌛. The application can close the connection (i.e., give up on the open attempt) sooner, of course 🙋♀️.
3.8.4. TCP Keep-Alives
A TCP connection is said to be "idle" if, for some long amount of time, no incoming segments have been received and there's no new or unacknowledged data to send 😴.
Implementers MAY include "keep-alives" in their TCP implementations (MAY-5), although this practice isn't universally accepted 🤷♀️. Some TCP implementations have included a keep-alive mechanism anyway. To confirm that an idle connection is still active, these implementations send a probe segment designed to elicit a response from the TCP peer 🔍. Such a segment generally contains SEG.SEQ = SND.NXT-1 and may or may not contain one garbage octet of data 🗑️. If keep-alives are included, the application MUST be able to turn them on or off for each TCP connection (MUST-24), and they MUST default to off (MUST-25) 🔘.
Keep-alive packets MUST only be sent when no sent data is outstanding, and no data or acknowledgment packets have been received for the connection within an interval (MUST-26). This interval MUST be configurable (MUST-27) and MUST default to no less than two hours (MUST-28) 🕑.
It's extremely important to remember that ACK segments containing no data are not reliably transmitted by TCP 🚨. Because of that, if a keep-alive mechanism is implemented, it MUST NOT interpret failure to respond to any specific probe as a dead connection (MUST-29) 🛑.
An implementation SHOULD send a keep-alive segment with no data (SHLD-12). However, it MAY be configurable to send a keep-alive segment containing one garbage octet (MAY-6), for compatibility with erroneous TCP implementations 🧩.
3.8.5. The Communication of Urgent Information
Because of implementation differences and middlebox interactions, new applications SHOULD NOT employ the TCP urgent mechanism (SHLD-13) 🙅♀️. However, TCP implementations MUST still include support for the urgent mechanism (MUST-30). Information on how some TCP implementations interpret the urgent pointer can be found in RFC 6093 [39] 📖.
The objective of the TCP urgent mechanism is to let the sending user stimulate the receiving user to accept some urgent data, and to let the receiving TCP endpoint indicate to the receiving user when all the currently known urgent data has been received by the user 📣.
This mechanism lets a point in the data stream be designated as the end of urgent information. Whenever this point is ahead of the receive sequence number (RCV.NXT) at the receiving TCP endpoint, the TCP implementation must tell the user to go into "urgent mode". When the receive sequence number catches up to the urgent pointer, the TCP implementation must tell the user to go into "normal mode". If the urgent pointer is updated while the user is in "urgent mode", the update will be invisible to the user 👀.
The method uses an urgent field carried in all segments transmitted. The URG control flag indicates that the urgent field is meaningful and must be added to the segment sequence number to yield the urgent pointer. If that flag is absent, there's no urgent data outstanding 🚩.
To send an urgent indication, the user must also send at least one data octet 🔔. If the sending user also indicates a push, timely delivery of the urgent information to the destination process gets enhanced ⚡. Note that because changes in the urgent pointer correspond to data being written by a sending application, the urgent pointer cannot "recede" in the sequence space. Still, a TCP receiver should be robust to invalid urgent pointer values 🛡️.
A TCP implementation MUST support a sequence of urgent data of any length (MUST-31) [19] 💪.
The urgent pointer MUST point to the sequence number of the octet following the urgent data (MUST-62) 🎯.
A TCP implementation MUST (MUST-32) inform the application layer asynchronously whenever it receives an urgent pointer and there was previously no pending urgent data, or whenever the urgent pointer advances in the data stream. The TCP implementation MUST (MUST-33) provide a way for the application to learn how much urgent data remains to be read from the connection, or at least to determine whether more urgent data remains to be read [19] 🫶✨.
3.8.6. Managing the Window 🪟
okay so the window in each segment tells you the range of sequence numbers the data receiver is ready to accept right now 💖 and it's basically tied to how much buffer space that connection has free ✨
the sending side packs its data into segments that fit the current window 📦 and it's allowed to repackage segments sitting on the retransmission queue 🎀 not required, but it can be helpful 💅
with one-way data flow, the window info rides on ACK segments that all have the same sequence number, so there's no way to reorder them if they show up out of order 😭 not a huge disaster, but it means the window can sometimes run on old reports from the receiver 🙈 the fix is to act on window info from segments carrying the highest acknowledgment number (aka an acknowledgment number equal to or greater than the highest one received before) 👑
a big window encourages transmissions 🚀 but if more data shows up than can be accepted, it gets tossed 💀 which means excessive retransmissions and extra load on the network and both endpoints 😩 a tiny window can restrict sending so much that you get a round-trip delay between each new segment 🐢
the mechanisms let an endpoint advertise a large window and then later advertise a much smaller one without having accepted that much data 😬 this so-called "shrinking the window" is strongly discouraged 🚫 the robustness principle [19] says peers won't shrink the window themselves, but they will be prepared for other peers doing it 🫶
here's the real talk: a TCP receiver SHOULD NOT shrink the window, i.e., move the right window edge to the left (SHLD-14) 🙅♀️ however, the sending TCP peer MUST be robust against window shrinking, which might make the "usable window" (see Section 3.8.6.2.1) go negative (MUST-34) 🛡️
if that happens, the sender SHOULD NOT send new data (SHLD-15), but it SHOULD retransmit normally the old unacknowledged data between SND.UNA and SND.UNA+SND.WND (SHLD-16) 🔁 the sender MAY also retransmit old data beyond SND.UNA+SND.WND (MAY-7), but it SHOULD NOT time out the connection just because data past the right window edge isn't acknowledged (SHLD-17) 🥹 and if the window shrinks to zero, the TCP implementation MUST probe it in the standard way (described below) (MUST-35) 🔍
3.8.6.1. Zero-Window Probing 🧪
the sending TCP peer must regularly transmit at least one octet of new data (if there's any available), or retransmit to the receiving peer even when the send window is zero 🫧 this is called "probing" the window, and it's what makes sure that when either side has a zero window, the reopening gets reliably reported to the other side 💌 other documents call this Zero-Window Probing (ZWP) 🪄
probing of zero (offered) windows MUST be supported (MUST-36) 💪 a TCP implementation MAY keep its offered receive window closed indefinitely (MAY-8) 🌙 but as long as the receiving peer keeps sending acknowledgments in response to the probe segments, the sending TCP peer MUST allow the connection to stay open (MUST-37) 🌸 this is what lets TCP handle stuff like the "printer ran out of paper" situation described in Section 4.2.2.17 of [19] 🖨️ the behavior is subject to the implementation's resource management concerns, as noted in [41] ⚠️
when the receiving TCP peer has a zero window and a segment arrives, it must still send an acknowledgment showing its next expected sequence number and current window (zero) 📬
the transmitting host SHOULD send the first zero-window probe once a zero window has existed for the retransmission timeout period (SHLD-29) (Section 3.8.1) ⏰ and it SHOULD increase the interval between successive probes exponentially (SHLD-30) 📈
3.8.6.2. Silly Window Syndrome Avoidance 🙃
the "Silly Window Syndrome" (SWS) is a stable pattern of small incremental window movements that leads to extremely poor TCP performance 😵💫 the algorithms to avoid it are described below for both the sending side and the receiving side 🧠 RFC 1122 has way more detail on the SWS problem 📚 and get this: the Nagle algorithm and the sender's SWS avoidance algorithm play complementary roles 🤝 Nagle discourages sending tiny segments when the data to send grows in small increments, while SWS avoidance discourages small segments caused by the right window edge advancing in small increments 🎯
3.8.6.2.1. Sender's Algorithm -- When to Send Data 📤
a TCP implementation MUST include a SWS avoidance algorithm in the sender (MUST-38) ✅
the Nagle algorithm from Section 3.7.4 also describes how to coalesce short segments 🧩
the sender's SWS avoidance is trickier than the receiver's, because the sender doesn't know the receiver's total buffer space (RCV.BUFF) directly 🫣 an approach that works well is for the sender to calculate Max(SND.WND), the largest send window it has seen so far on the connection, and use that as an estimate of RCV.BUFF 📏 but it can only be an estimate, since the receiver may reduce the size of RCV.BUFF at any time 😬 to avoid the resulting deadlock, there needs to be a timeout that forces transmission of data and overrides the SWS avoidance algorithm 🔓 in practice, that timeout should seldom occur 🙏
the "usable window" is:
U = SND.UNA + SND.WND - SND.NXTwhich is the offered window minus the data sent but not yet acknowledged 🧮 if D is the amount of data queued in the sending TCP endpoint but not yet sent, then the following set of rules is recommended 💡
Send data:
(1) if a maximum-sized segment can be sent, i.e., if:
min(D,U) >= Eff.snd.MSS;
(2) or if the data is pushed and all queued data can be sent now,
i.e., if:
[SND.NXT = SND.UNA and] PUSHed and D <= U
(the bracketed condition is imposed by the Nagle algorithm);
(3) or if at least a fraction Fs of the maximum window can be sent,
i.e., if:
[SND.NXT = SND.UNA and]
min(D,U) >= Fs * Max(SND.WND);
(4) or if the override timeout occurs.Fs is a fraction whose recommended value is 1/2 ➗ the override timeout should be in the range 0.1 - 1.0 seconds ⏱️ it may be convenient to combine this timer with the timer used to probe zero windows (Section 3.8.6.1) 🫶
3.8.6.2.2. Receiver's Algorithm -- When to Send a Window Update 📥
a TCP implementation MUST include a SWS avoidance algorithm in the receiver (MUST-39) 🛡️
the receiver's SWS avoidance algorithm decides when the right window edge may be advanced, which is customarily called "updating the window" 🪟 this algorithm teams up with the delayed ACK algorithm (Section 3.8.6.3) to determine when an ACK segment containing the current window will actually be sent to the receiver 💌
the fix on the receiver side is to avoid advancing the right window edge RCV.NXT+RCV.WND in small increments, even when data comes in from the network in small segments 🐾
say the total receive buffer space is RCV.BUFF 🫙 at any given moment, RCV.USER octets of that total might be tied up with data that's been received and acknowledged but that the user process hasn't consumed yet 🍽️ when the connection is quiet, RCV.WND = RCV.BUFF and RCV.USER = 0 😌
keeping the right window edge fixed while data arrives and gets acknowledged means the receiver has to offer less than its full buffer space 🧠 so the receiver must specify a RCV.WND that keeps RCV.NXT+RCV.WND constant as RCV.NXT increases ✨ that means the total buffer space RCV.BUFF is generally split into three parts 🍕
|<------- RCV.BUFF ---------------->|
1 2 3
----|---------|------------------|------|----
RCV.NXT ^
(Fixed)
1 - RCV.USER = data received but not yet consumed;
2 - RCV.WND = space advertised to sender;
3 - Reduction = space available but not yet
advertised.the suggested receiver SWS avoidance algorithm is to keep RCV.NXT+RCV.WND fixed until the reduction satisfies:
RCV.BUFF - RCV.USER - RCV.WND >=
min( Fr * RCV.BUFF, Eff.snd.MSS )where Fr is a fraction whose recommended value is 1/2, and Eff.snd.MSS is the effective send MSS for the connection (see Section 3.7.1) 🧮 once that inequality is satisfied, RCV.WND is set to RCV.BUFF-RCV.USER 🎉
the big picture here: this algorithm generally advances RCV.WND in increments of Eff.snd.MSS (for realistic receive buffers, Eff.snd.MSS < RCV.BUFF/2) 📈 also, the receiver has to use its own Eff.snd.MSS, assuming it's the same as the sender's 🤞
3.8.6.3. Delayed Acknowledgments -- When to Send an ACK Segment 🕰️
a host receiving a stream of TCP data segments can boost efficiency for both the network and the hosts by sending fewer than one ACK (acknowledgment) segment per data segment received 🚄 this is known as a "delayed ACK" 🫠
a TCP endpoint SHOULD implement a delayed ACK (SHLD-18) 🌿 but an ACK should not be excessively delayed, and in particular the delay MUST be less than 0.5 seconds (MUST-40) ⏳ an ACK SHOULD be generated for at least every second full-sized segment or 2*RMSS bytes of new data, where RMSS is the MSS specified by the TCP endpoint receiving the segments to be acknowledged (or the default value if none is specified) (SHLD-19) 📦 excessive ACK delays can mess up the round-trip timing and packet "clocking" algorithms 😵 for a more complete discussion of delayed ACK behavior, see Section 4.2 of RFC 5681 [8], which includes recommendations to immediately acknowledge out-of-order segments, segments above a gap in sequence space, or segments that fill all or part of a gap, so loss recovery speeds up 🏃♀️💨
there are also several current practices that lead to even fewer ACKs, including generic receive offload (GRO) [72], ACK compression, and ACK decimation [28] 🌀
3.9. Interfaces 🔌
okay so there are of course two interfaces we care about: the user/TCP interface and the TCP/lower-level interface 🫶 we have a pretty elaborate model of the user/TCP interface 💅 but the interface to the lower-level protocol module is left unspecified here, since the lower-level protocol's own specification will spell it out in detail 📝 for the case where the lower level is IP, we note some of the parameter values that TCP implementations might use 🌸
3.9.1. User/TCP Interface 🎀
ngl, the following description of user commands to the TCP implementation is, like, at best fictional 🙈, because every operating system has its own vibe and its own facilities. So we have to warn you that different TCP implementations may have different user interfaces. BUT all TCP implementations must provide a certain minimum set of services so they can all support the same protocol hierarchy 💅. This section specifies the functional interfaces required of all TCP implementations.
Section 3.1 of [53] also identifies primitives provided by TCP and could be used as an additional reference for implementers 📚.
The following sections functionally characterize a user/TCP interface. The notation is similar to most procedure or function calls in high-level languages, but this usage is not meant to rule out trap-type service calls ✨.
The user commands below specify the basic functions the TCP implementation must perform to support interprocess communication. Individual implementations must define their own exact format, and they may provide combinations or subsets of the basic functions in single calls. In particular, some implementations may wish to automatically OPEN a connection on the first SEND or RECEIVE issued by the user for a given connection 🦋.
In providing interprocess communication facilities, the TCP implementation must not only accept commands, but must also return information to the processes it serves. The latter consists of:
- (a) general information about a connection (e.g., interrupts, remote close, binding of unspecified remote socket).
- (b) replies to specific user commands indicating success or various types of failure.
3.9.1.1. Open 🚪
Format:
Format: OPEN (local port, remote socket, active/passive [, timeout]
[, Diffserv field] [, security/compartment] [, local IP address] [,
options]) -> local connection nameIf the active/passive flag is set to passive, then this is a call to LISTEN for an incoming connection, so it's basically giving "waiting by the door" energy 🧚. A passive OPEN may have either a fully specified remote socket to wait for a particular connection, or an unspecified remote socket to wait for any call. A fully specified passive call can be made active by the subsequent execution of a SEND.
A transmission control block (TCB) is created and partially filled in with data from the OPEN command parameters 🧱.
Every passive OPEN call either creates a new connection record in LISTEN state, or it returns an error; it MUST NOT affect any previously created connection record (MUST-41). Protect those old records like they're your fave hoodie 🧥.
A TCP implementation that supports multiple concurrent connections MUST provide an OPEN call that will functionally allow an application to LISTEN on a port while a connection block with the same local port is in SYN-SENT or SYN-RECEIVED state (MUST-42). Multitasking queen behavior 👑.
On an active OPEN command, the TCP endpoint will begin the procedure to synchronize (i.e., establish) the connection at once 🚀.
The timeout, if present, lets the caller set up a timeout for all data submitted to TCP. If data is not successfully delivered to the destination within the timeout period, the TCP endpoint will abort the connection. The present global default is five minutes ⏰.
The TCP implementation or some component of the operating system will verify the user's authority to open a connection with the specified Diffserv field value or security/compartment. The absence of a Diffserv field value or security/compartment specification in the OPEN call indicates the default values must be used. So no bouncer, no entry 🚫.
TCP will accept incoming requests as matching only if the security/compartment information is exactly the same as that requested in the OPEN call. No close enough, it has to be an exact match 🔐.
The Diffserv field value indicated by the user only impacts outgoing packets, may be altered en route through the network, and has no direct bearing or relation to received packets 🌐.
A local connection name will be returned to the user by the TCP implementation. The local connection name can then be used as a shorthand term for the connection defined by the <local socket, remote socket> pair 🏷️.
The optional "local IP address" parameter MUST be supported to allow the specification of the local IP address (MUST-43). This is the move for apps that need to pick which local IP address gets used when multihoming is present 🌸.
A passive OPEN call with a specified "local IP address" parameter will await an incoming connection request to that address. If the parameter is unspecified, a passive OPEN will await an incoming connection request to any local IP address, and then bind the local IP address of the connection to the particular address that is used 👀.
For an active OPEN call, a specified "local IP address" parameter will be used for opening the connection. If the parameter is unspecified, the host will choose an appropriate local IP address (see RFC 1122, Section 3.3.4.2) 💅.
If an application on a multihomed host does not specify the local IP address when actively opening a TCP connection, then the TCP implementation MUST ask the IP layer to select a local IP address before sending the (first) SYN (MUST-44). See the function GET_SRCADDR() in Section 3.4 of RFC 1122 🧠.
At all other times, a previous segment has either been sent or received on this connection, and TCP implementations MUST use the same local address that was used in those previous segments (MUST-45). Consistency is everything, bestie 🫶.
A TCP implementation MUST reject as an error a local OPEN call for an invalid remote IP address (e.g., a broadcast or multicast address) (MUST-46). Absolutely not, that address is giving nothing 💀.
3.9.1.2. Send 📨
Format:
Format: SEND (local connection name, buffer address, byte count,
URGENT flag [, PUSH flag] [, timeout])This call causes the data contained in the indicated user buffer to be sent on the indicated connection. If the connection has not been opened, the SEND is considered an error. Some implementations may allow users to SEND first, in which case an automatic OPEN would be done. For example, this might be one way for application data to be included in SYN segments 🌈. If the calling process is not authorized to use this connection, an error is returned.
A TCP endpoint MAY implement PUSH flags on SEND calls (MAY-15). If PUSH flags are not implemented, then the sending TCP peer: (1) MUST NOT buffer data indefinitely (MUST-60), and (2) MUST set the PSH bit in the last buffered segment (i.e., when there is no more queued data to be sent) (MUST-61). The remaining description below assumes the PUSH flag is supported on SEND calls 🍓.
If the PUSH flag is set, the application intends the data to be transmitted promptly to the receiver, and the PSH bit will be set in the last TCP segment created from the buffer 📣.
The PSH bit is not a record marker and is independent of segment boundaries. The transmitter SHOULD collapse successive bits when it packetizes data, to send the largest possible segment (SHLD-27). Not a record marker, not a border, just vibes 🫧.
If the PUSH flag is not set, the data may be combined with data from subsequent SENDs for transmission efficiency. When an application issues a series of SEND calls without setting the PUSH flag, the TCP implementation MAY aggregate the data internally without sending it (MAY-16). Note that when the Nagle algorithm is in use, TCP implementations may buffer the data before sending, without regard to the PUSH flag (see Section 3.7.4) 🧃.
An application program is logically required to set the PUSH flag in a SEND call whenever it needs to force delivery of the data to avoid a communication deadlock. However, a TCP implementation SHOULD send a maximum-sized segment whenever possible (SHLD-28) to improve performance (see Section 3.8.6.2.1) 🚀.
New applications SHOULD NOT set the URGENT flag [39] due to implementation differences and middlebox issues (SHLD-13). Hard pass on URGENT for new apps 🙅♀️.
If the URGENT flag is set, segments sent to the destination TCP peer will have the urgent pointer set. The receiving TCP peer will signal the urgent condition to the receiving process if the urgent pointer indicates that data preceding the urgent pointer has not been consumed by the receiving process. The purpose of the URGENT flag is to stimulate the receiver to process the urgent data, and to indicate to the receiver when all the currently known urgent data has been received. The number of times the sending user's TCP implementation signals urgent will not necessarily be equal to the number of times the receiving user will be notified of the presence of urgent data 🚨.
If no remote socket was specified in the OPEN, but the connection is established (e.g., because a LISTENing connection has become specific due to a remote segment arriving for the local socket), then the designated buffer is sent to the implied remote socket. Users who make use of OPEN with an unspecified remote socket can make use of SEND without ever explicitly knowing the remote socket address 🎯.
However, if a SEND is attempted before the remote socket becomes specified, an error will be returned. Users can use the STATUS call to determine the status of the connection. Some TCP implementations may notify the user when an unspecified socket is bound 🔔.
If a timeout is specified, the current user timeout for this connection is changed to the new one ⏱️.
In the simplest implementation, SEND would not return control to the sending process until either the transmission was complete or the timeout had been exceeded. However, this simple method is both subject to deadlocks (for example, both sides of the connection might try to do SENDs before doing any RECEIVEs) and offers poor performance, so it is not recommended. Nobody wants a standstill, periodt 💀.
A more sophisticated implementation would return immediately to allow the process to run concurrently with network I/O, and, furthermore, to allow multiple SENDs to be in progress. Multiple SENDs are served in first come, first served order, so the TCP endpoint will queue those it cannot service immediately 🎟️.
We have implicitly assumed an asynchronous user interface in which a SEND later elicits some kind of SIGNAL or pseudo-interrupt from the serving TCP endpoint. An alternative is to return a response immediately. For instance, SENDs might return immediate local acknowledgment, even if the segment sent had not been acknowledged by the distant TCP endpoint. We could optimistically assume eventual success. If we are wrong, the connection will close anyway due to the timeout. In implementations of this kind (synchronous), there will still be some asynchronous signals, but these will deal with the connection itself, and not with specific segments or buffers 🤞.
In order for the process to distinguish among error or success indications for different SENDs, it might be appropriate for the buffer address to be returned along with the coded response to the SEND request. TCP-to-user signals are discussed below, indicating the information that should be returned to the calling process 📬.
3.9.1.3. Receive 📥
Format: RECEIVE (local connection name, buffer address, byte count)
-> byte count, URGENT flag [, PUSH flag]okay so RECEIVE is how you grab a receiving buffer tied to a connection 🫶 It allocates that buffer for the connection you name. If there was no OPEN before it, or the calling process isn't authorized to use that connection, you get an error. no cap, that's a hard no 🙅♀️
The simplest version would just not hand control back until the buffer is full or something goes wrong, but ngl that setup is super prone to deadlocks 💀 A more sophisticated version lets several RECEIVEs be outstanding at once, and they get filled as segments show up. That gives you more throughput ✨ but you need a fancier, possibly asynchronous, way to tell the program that a PUSH was seen or a buffer got filled. it's giving complicated, but it's worth it 💅
A TCP receiver MAY pass a received PSH bit to the application layer via the PUSH flag in the interface (MAY-17), but it is not required (this was clarified in RFC 1122, Section 4.2.2.2). The rest of the RECEIVE stuff below assumes that passing the PUSH indication is supported 🎀
If enough data arrive to fill the buffer before a PUSH is seen, the PUSH flag won't be set in the response to the RECEIVE. The buffer just gets filled with as much data as it can hold 🍓 If a PUSH shows up before the buffer is full, the buffer gets returned partially filled and PUSH gets indicated. main character energy for PUSH 👑
If there's urgent data, the user gets informed as soon as it arrives via a TCP-to-user signal, so the receiving user should be in "urgent mode". If the URGENT flag is on, there's still more urgent data coming. If the URGENT flag is off, this RECEIVE call has returned all the urgent data, and the user can now leave "urgent mode" 🦋 Also, data after the urgent pointer (non-urgent data) can't be delivered in the same buffer as the urgent data before it, unless the boundary is clearly marked for the user. Don't mix them up, bestie 🙈
To tell apart several outstanding RECEIVEs, and to handle the case where a buffer isn't completely filled, the return code comes with both a buffer pointer and a byte count that shows the actual length of the data received 🧚
Other RECEIVE designs might have the TCP endpoint allocate the buffer storage, or the TCP endpoint might share a ring buffer with the user 💜
3.9.1.4. Close 🚪
Format: CLOSE (local connection name)CLOSE makes the named connection close. If the connection isn't open, or the calling process isn't authorized to use it, you get an error. Closing is meant to be graceful 🌸 Outstanding SENDs keep getting transmitted (and retransmitted), as flow control allows, until they've all been serviced. So it should be acceptable to make several SEND calls, follow them with a CLOSE, and expect all the data to reach the destination. It should also be clear that users should keep doing RECEIVE on CLOSING connections, since the remote peer might still be sending the last of its data 🫶
Here's the key vibe: CLOSE means "I have no more to send," but it does not mean "I will not receive any more." 💅 It can happen (if the user-level protocol isn't well thought out) that the closing side can't get rid of all its data before timing out. In that case, CLOSE turns into ABORT and the closing TCP peer gives up 😭
The user can CLOSE the connection at any time on their own initiative, or in response to various prompts from the TCP implementation (e.g., remote close executed, transmission timeout exceeded, destination inaccessible) 🌈
Because closing a connection needs communication with the remote TCP peer, connections may stay in the closing state for a short time. Trying to reopen the connection before the TCP peer replies to the CLOSE command will result in error responses. not me trying to reopen too early 🙃
Close also implies push function. ✨
3.9.1.5. Status 📊
Format: STATUS (local connection name) -> status dataSTATUS is an implementation-dependent user command, and it could be excluded without adverse effect 🤷♀️ The info returned would typically come from the TCB associated with the connection.
This command returns a data block with the following information:
- local socket,
- remote socket,
- local connection name,
- receive window,
- send window,
- connection state,
- number of buffers awaiting acknowledgment,
- number of buffers pending receipt,
- urgent state,
- Diffserv field value,
- security/compartment, and
- transmission timeout.
Depending on the state of the connection, or on the implementation itself, some of this info may not be available or meaningful 💀 If the calling process isn't authorized to use this connection, you get an error. This is what stops unauthorized processes from snooping on a connection's info 🔒
3.9.1.6. Abort 🛑
Format: ABORT (local connection name)ABORT makes all pending SENDs and RECEIVES get aborted, the TCB gets removed, and a special RST message gets sent to the remote TCP peer of the connection 💥 Depending on the implementation, users might get abort indications for each outstanding SEND or RECEIVE, or they might just get an ABORT-acknowledgment. it's giving dramatic exit 🎬
3.9.1.7. Flush 🧹
Some TCP implementations have included a FLUSH call. It empties the TCP send queue of any data the user has issued SEND calls for but that's still to the right of the current send window. In other words, it flushes as much queued send data as possible without losing sequence number synchronization ✨ The FLUSH call MAY be implemented (MAY-14).
3.9.1.8. Asynchronous Reports 📣
There MUST be a mechanism for reporting soft TCP error conditions to the application (MUST-47). Generically, we assume this takes the form of an application-supplied ERROR_REPORT routine that may be upcalled asynchronously from the transport layer:
ERROR_REPORT(local connection name, reason, subreason)The exact encoding of the reason and subreason parameters isn't specified here. However, the conditions reported asynchronously to the application MUST include:
- ICMP error message arrived (see Section 3.9.2.2 for description of handling each ICMP message type, since some message types need to be suppressed from generating reports to the application)
- Excessive retransmissions (see Section 3.8.3)
- Urgent pointer advance (see Section 3.8.5)
That said, an application program that doesn't want to receive such ERROR_REPORT calls SHOULD be able to effectively disable these calls (SHLD-20) 🙌 flexibility is everything 💖
3.9.1.9. Set Differentiated Services Field (IPv4 TOS or IPv6 Traffic Class) 🏷️
The application layer MUST be able to specify the Differentiated Services field for segments sent on a connection (MUST-48). The Differentiated Services field includes the 6-bit Differentiated Services Codepoint (DSCP) value. It is not required, but the application SHOULD be able to change the Differentiated Services field during the connection lifetime (SHLD-21). TCP implementations SHOULD pass the current Differentiated Services field value without change to the IP layer when they send segments on the connection (SHLD-22) 🌟
The Differentiated Services field is specified independently in each direction of the connection, so the receiver application specifies the Differentiated Services field used for ACK segments 🦋
TCP implementations MAY pass the most recently received Differentiated Services field up to the application (MAY-9). 🧚
3.9.2. TCP/Lower-Level Interface 🧱
The TCP endpoint calls on a lower-level protocol module to actually send and receive info over a network. The two current standard Internet Protocol (IP) versions layered below TCP are IPv4 [1] and IPv6 [13] 🌐
If the lower-level protocol is IPv4, it provides arguments for a type of service (used within the Differentiated Services field) and for a time to live. TCP uses these settings for those parameters:
- Diffserv field: The IP header value for the Diffserv field is given by the user. This includes the bits of the Diffserv Codepoint (DSCP).
- Time to Live (TTL): The TTL value used to send TCP segments MUST be configurable (MUST-49).
- Note that RFC 793 specified one minute (60 seconds) as a constant for the TTL because the assumed maximum segment lifetime was two minutes. This was meant to explicitly ask that a segment be destroyed if it couldn't be delivered by the internet system within one minute. RFC 1122 updated RFC 793 to require that the TTL be configurable ⏰
- Note that the Diffserv field is permitted to change during a connection (Section 4.2.4.2 of RFC 1122). However, the application interface might not support this ability, and the application doesn't have knowledge of individual TCP segments, so this can only be done at a coarse granularity, at best. This limitation is further discussed in RFC 7657 (Sections 5.1, 5.3, and 6) [50]. Generally, an application SHOULD NOT change the Diffserv field value during the course of a connection (SHLD-23) 🙅♀️
Any lower-level protocol will have to provide the source address, destination address, and protocol fields, and some way to determine the "TCP length." That's both to provide the functional equivalent service of IP and to be used in the TCP checksum 🧮
When received options are passed up to TCP from the IP layer, a TCP implementation MUST ignore options that it does not understand (MUST-50). Iconic boundary behavior 💅
A TCP implementation MAY support the Timestamp (MAY-10) and Record Route (MAY-11) Options. ✨
3.9.2.1. Source Routing 🗺️
If the lower level is IP (or another protocol that provides this feature) and source routing is used, the interface must allow the route information to be communicated. This is especially important so that the source and destination addresses used in the TCP checksum are the originating source and ultimate destination. It's also important to preserve the return route to answer connection requests 🔁
An application MUST be able to specify a source route when it actively opens a TCP connection (MUST-51), and this MUST take precedence over a source route received in a datagram (MUST-52) 👑
When a TCP connection is OPENed passively and a packet arrives with a completed IP Source Route Option (containing a return route), TCP implementations MUST save the return route and use it for all segments sent on this connection (MUST-53). If a different source route arrives in a later segment, the later definition SHOULD override the earlier one (SHLD-24) 🫶
3.9.2.2. ICMP Messages ✨
okay bestie, so TCP implementations MUST act on an ICMP error message that gets passed up from the IP layer 🙌 and they have to send it to the connection that actually created the error (MUST-54). 🕵️♀️ The demultiplexing info you need is hiding in the IP header inside that ICMP message.
And yes, this applies to ICMPv6 too, not just IPv4 ICMP 🌈
[35] talks about specific ICMP and ICMPv6 messages that count as either "soft" or "hard" errors, and each one can get a different response. Here's how each class gets treated, rn 👇
- Source Quench
- TCP implementations MUST silently discard any received ICMP Source Quench messages (MUST-55). See [11] for discussion. 🫶
- Soft Errors
- For IPv4 ICMP, these include: Destination Unreachable -- codes 0, 1, 5; Time Exceeded -- codes 0, 1; and Parameter Problem.
- For ICMPv6, these include: Destination Unreachable -- codes 0, 3; Time Exceeded -- codes 0, 1; and Parameter Problem -- codes 0, 1, 2.
- Since these Unreachable messages indicate soft error conditions, a TCP implementation MUST NOT abort the connection (MUST-56), and it SHOULD make the information available to the application (SHLD-25). 💖
- Hard Errors
- For ICMP these include Destination Unreachable -- codes 2-4.
- These are hard error conditions, so TCP implementations SHOULD abort the connection (SHLD-26). [35] notes that some implementations do not abort connections when an ICMP hard error is received for a connection that is in any of the synchronized states. 😭
Note that [35], Section 4 describes widespread implementation behavior that treats soft errors as hard errors during connection establishment. It's giving chaos, honestly 💀
3.9.2.3. Source Address Validation 🛡️
RFC 1122 requires addresses to be validated in incoming SYN packets:
An incoming SYN with an invalid source address MUST be ignored either by TCP or by the IP layer [(MUST-63)] (see Section 3.2.1.3). A TCP implementation MUST silently discard an incoming SYN segment that is addressed to a broadcast or multicast address [(MUST-57)].
This stops connection state and replies from getting generated by mistake 🙅♀️ and implementers should note that this guidance applies to all incoming segments, not just SYNs, as RFC 1122 specifically says. main character energy for validation 👑
3.10. Event Processing 🦋
The processing in this section is an example of one possible implementation. Other implementations may have slightly different processing sequences, but they should differ from those in this section only in detail, not in substance. 💅
The activity of the TCP endpoint can be characterized as responding to events. The events that occur can be cast into three categories: user calls, arriving segments, and timeouts. This section describes the processing the TCP endpoint does in response to each of the events. In many cases, the processing required depends on the state of the connection. 🌸
Events that occur:
User Calls
OPEN
SEND
RECEIVE
CLOSE
ABORT
STATUS
Arriving Segments
SEGMENT ARRIVES
Timeouts
USER TIMEOUT
RETRANSMISSION TIMEOUT
TIME-WAIT TIMEOUTThe model of the TCP/user interface is that user commands receive an immediate return and possibly a delayed response via an event or pseudo-interrupt. In the following descriptions, the term "signal" means cause a delayed response. 🔔
Error responses in this document are identified by character strings. For example, user commands referencing connections that do not exist receive "error: connection not open". 🙈
Please note in the following that all arithmetic on sequence numbers, acknowledgment numbers, windows, et cetera, is modulo 2^32 (the size of the sequence number space). Also note that =< means less than or equal to (modulo 2^32). Math bestie, you gotta be precise 🧮
A natural way to think about processing incoming segments is to imagine that they are first tested for proper sequence number (i.e., that their contents lie in the range of the expected "receive window" in the sequence number space) and then that they are generally queued and processed in sequence number order. ✨
When a segment overlaps other already received segments, we reconstruct the segment to contain just the new data and adjust the header fields to be consistent. 🧩
Note that if no state change is mentioned, the TCP connection stays in the same state. 🎀
3.10.1. OPEN Call 🚪
CLOSED STATE (i.e., TCB does not exist)
- Create a new transmission control block (TCB) to hold connection state information. Fill in local socket identifier, remote socket, Diffserv field, security/compartment, and user timeout information. Note that some parts of the remote socket may be unspecified in a passive OPEN and are to be filled in by the parameters of the incoming SYN segment. Verify the security and Diffserv value requested are allowed for this user, if not, return
"error: Diffserv value not allowed"or"error: security/compartment not allowed". If passive, enter the LISTEN state and return. If active and the remote socket is unspecified, return"error: remote socket unspecified"; if active and the remote socket is specified, issue a SYN segment. An initial send sequence number (ISS) is selected. A SYN segment of the form<SEQ=ISS><CTL=SYN>is sent. Set SND.UNA to ISS, SND.NXT to ISS+1, enter SYN-SENT state, and return. 🌟 - If the caller does not have access to the local socket specified, return
"error: connection illegal for this process". If there is no room to create a new connection, return"error: insufficient resources". 🙅♀️
LISTEN STATE
- If the OPEN call is active and the remote socket is specified, then change the connection from passive to active, select an ISS. Send a SYN segment, set SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data associated with SEND may be sent with SYN segment or queued for transmission after entering ESTABLISHED state. The urgent bit if requested in the command must be sent with the data segments sent as a result of this command. If there is no room to queue the request, respond with
"error: insufficient resources". If the remote socket was not specified, then return"error: remote socket unspecified". 🚀
SYN-SENT STATE
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
- Return
"error: connection already exists". 😤
3.10.2. SEND Call 📨
CLOSED STATE (i.e., TCB does not exist)
- If the user does not have access to such a connection, then return
"error: connection illegal for this process". - Otherwise, return
"error: connection does not exist".
LISTEN STATE
- If the remote socket is specified, then change the connection from passive to active, select an ISS. Send a SYN segment, set SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data associated with SEND may be sent with SYN segment or queued for transmission after entering ESTABLISHED state. The urgent bit if requested in the command must be sent with the data segments sent as a result of this command. If there is no room to queue the request, respond with
"error: insufficient resources". If the remote socket was not specified, then return"error: remote socket unspecified". 🚀
SYN-SENT STATE
SYN-RECEIVED STATE
- Queue the data for transmission after entering ESTABLISHED state. If no space to queue, respond with
"error: insufficient resources". 📦
ESTABLISHED STATE
CLOSE-WAIT STATE
- Segmentize the buffer and send it with a piggybacked acknowledgment (acknowledgment value = RCV.NXT). If there is insufficient space to remember this buffer, simply return
"error: insufficient resources". 🍓 - If the URGENT flag is set, then SND.UP <- SND.NXT and set the urgent pointer in the outgoing segments. 🚨
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
- Return
"error: connection closing"and do not service request. 🙅♀️
3.10.3. RECEIVE Call 📥
CLOSED STATE (i.e., TCB does not exist)
- If the user does not have access to such a connection, return
"error: connection illegal for this process". - Otherwise, return
"error: connection does not exist".
LISTEN STATE
SYN-SENT STATE
SYN-RECEIVED STATE
- Queue for processing after entering ESTABLISHED state. If there is no room to queue this request, respond with
"error: insufficient resources". 🧺
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
- If insufficient incoming segments are queued to satisfy the request, queue the request. If there is no queue space to remember the RECEIVE, respond with
"error: insufficient resources". - Reassemble queued incoming segments into receive buffer and return to user. Mark "push seen" (PUSH) if this is the case. 💌
- If RCV.UP is in advance of the data currently being passed to the user, notify the user of the presence of urgent data. 🔔
- When the TCP endpoint takes responsibility for delivering data to the user, that fact must be communicated to the sender via an acknowledgment. The formation of such an acknowledgment is described below in the discussion of processing an incoming segment. 🫶
CLOSE-WAIT STATE
- Since the remote side has already sent FIN, RECEIVEs must be satisfied by data already on hand, but not yet delivered to the user. If no text is awaiting delivery, the RECEIVE will get an
"error: connection closing"response. Otherwise, any remaining data can be used to satisfy the RECEIVE. 🥀
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
- Return
"error: connection closing". 🙃
3.10.4. CLOSE Call 🔒
CLOSED STATE (i.e., TCB does not exist)
- If the user does not have access to such a connection, return
"error: connection illegal for this process". - Otherwise, return
"error: connection does not exist".
LISTEN STATE
- Any outstanding RECEIVEs are returned with
"error: closing"responses. Delete TCB, enter CLOSED state, and return. 👋
SYN-SENT STATE
- Delete the TCB and return
"error: closing"responses to any queued SENDs, or RECEIVEs. 🗑️
SYN-RECEIVED STATE
- If no SENDs have been issued and there is no pending data to send, then form a FIN segment and send it, and enter FIN-WAIT-1 state; otherwise, queue for processing after entering ESTABLISHED state. 🏁
ESTABLISHED STATE
- Queue this until all preceding SENDs have been segmentized, then form a FIN segment and send it. In any case, enter FIN-WAIT-1 state. 🌙
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
- Strictly speaking, this is an error and should receive an
"error: connection closing"response. An"ok"response would be acceptable, too, as long as a second FIN is not emitted (the first FIN may be retransmitted, though). 🤔
CLOSE-WAIT STATE
- Queue this request until all preceding SENDs have been segmentized; then send a FIN segment, enter LAST-ACK state. 🫠
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
- Respond with
"error: connection closing". 💜
3.10.5. ABORT Call 💅
okay so ABORT is the "nope, we're done, delete everything" button 💀 what happens depends on which state the connection is in, bestie 🫶
CLOSED STATE (i.e., TCB does not exist) 🌸
- If the user should not have access to such a connection, return
"error: connection illegal for this process". - Otherwise, return
"error: connection does not exist".
LISTEN STATE 🎀
- Any outstanding RECEIVEs should be returned with
"error: connection reset"responses. Delete TCB, enter CLOSED state, and return.
SYN-SENT STATE 🦋
- All queued SENDs and RECEIVEs should be given
"connection reset"notification. Delete the TCB, enter CLOSED state, and return.
SYN-RECEIVED STATE, ESTABLISHED STATE, FIN-WAIT-1 STATE, FIN-WAIT-2 STATE, CLOSE-WAIT STATE 😭
- Send a reset segment:
<SEQ=SND.NXT><CTL=RST>- All queued SENDs and RECEIVEs should be given
"connection reset"notification; all segments queued for transmission (except for the RST formed above) or retransmission should be flushed. Delete the TCB, enter CLOSED state, and return.
CLOSING STATE, LAST-ACK STATE, TIME-WAIT STATE 🌈
- Respond with
"ok"and delete the TCB, enter CLOSED state, and return.
3.10.6. STATUS Call 🔥
this one's giving "just tell me where we're at" energy 🧚 you ask, and it hands back the state plus the TCB pointer ✨
CLOSED STATE (i.e., TCB does not exist) 💜
- If the user should not have access to such a connection, return
"error: connection illegal for this process". - Otherwise, return
"error: connection does not exist".
LISTEN STATE 🍓
- Return
"state = LISTEN"and the TCB pointer.
SYN-SENT STATE 🌸
- Return
"state = SYN-SENT"and the TCB pointer.
SYN-RECEIVED STATE 🦋
- Return
"state = SYN-RECEIVED"and the TCB pointer.
ESTABLISHED STATE 👑
- Return
"state = ESTABLISHED"and the TCB pointer.
FIN-WAIT-1 STATE 🫶
- Return
"state = FIN-WAIT-1"and the TCB pointer.
FIN-WAIT-2 STATE ⭐
- Return
"state = FIN-WAIT-2"and the TCB pointer.
CLOSE-WAIT STATE 🎀
- Return
"state = CLOSE-WAIT"and the TCB pointer.
CLOSING STATE 🙈
- Return
"state = CLOSING"and the TCB pointer.
LAST-ACK STATE 💀
- Return
"state = LAST-ACK"and the TCB pointer.
TIME-WAIT STATE 🌈
- Return
"state = TIME-WAIT"and the TCB pointer.
3.10.7. SEGMENT ARRIVES 🚀
this is the big one, bestie 😭 every time a segment shows up, the state decides how we handle it. so let's go state by state 💅
3.10.7.1. CLOSED STATE 🌸
if the state is CLOSED (aka the TCB doesn't exist), then all data in the incoming segment gets discarded 🗑️ an incoming segment that has a RST gets discarded too. an incoming segment that doesn't have a RST causes a RST to get sent back in response. the acknowledgment and sequence field values get picked so that the reset is acceptable to the TCP endpoint that sent the offending segment, which is honestly so considerate 🫶
if the ACK bit is off, sequence number zero is used:
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>if the ACK bit is on:
<SEQ=SEG.ACK><CTL=RST>Return.
3.10.7.2. LISTEN STATE 🎀
if the state is LISTEN, then we check things in order, and it's giving methodical 🧠
First, check for a RST:
- An incoming RST segment could not be valid, since it could not have been sent in response to anything sent by this incarnation of the connection. An incoming RST should be ignored. Return.
Second, check for an ACK:
- Any acknowledgment is bad if it arrives on a connection still in the LISTEN state. An acceptable reset segment should be formed for any arriving ACK-bearing segment. The RST should be formatted as follows:
<SEQ=SEG.ACK><CTL=RST>- Return.
Third, check for a SYN:
- If the SYN bit is set, check the security. If the security/compartment on the incoming segment does not exactly match the security/compartment in the TCB, then send a reset and return:
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>- Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ, and any other control or text should be queued for processing later. ISS should be selected and a SYN segment sent of the form:
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>- SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection state should be changed to SYN-RECEIVED. Note that any other incoming control or data (combined with SYN) will be processed in the SYN-RECEIVED state, but processing of SYN and ACK should not be repeated. If the listen was not fully specified (i.e., the remote socket was not fully specified), then the unspecified fields should be filled in now.
Fourth, other data or control:
- This should not be reached. Drop the segment and return. Any other control or data-bearing segment (not containing SYN) must have an ACK and thus would have been discarded by the ACK processing in the second step, unless it was first discarded by RST checking in the first step.
3.10.7.3. SYN-SENT STATE 🦋
if the state is SYN-SENT, then here we go 🎢
First, check the ACK bit:
- If the ACK bit is set,
- If
SEG.ACK =< ISSorSEG.ACK > SND.NXT, send a reset (unless the RST bit is set, if so drop the segment and return):
- If
<SEQ=SEG.ACK><CTL=RST>- and discard the segment. Return.
- If
SND.UNA < SEG.ACK =< SND.NXT, then the ACK is acceptable. Some deployed TCP code has used the checkSEG.ACK == SND.NXT(using "==" rather than "=<"), but this is not appropriate when the stack is capable of sending data on the SYN because the TCP peer may not accept and acknowledge all of the data on the SYN.
Second, check the RST bit:
- If the RST bit is set,
- A potential blind reset attack is described in RFC 5961 [9]. The mitigation described in that document has specific applicability explained therein, and is not a substitute for cryptographic protection (e.g., IPsec or TCP-AO). A TCP implementation that supports the mitigation described in RFC 5961 SHOULD first check that the sequence number exactly matches RCV.NXT prior to executing the action in the next paragraph. 🚨
- If the ACK was acceptable, then signal to the user
"error: connection reset", drop the segment, enter CLOSED state, delete TCB, and return. Otherwise (no ACK), drop the segment and return.
Third, check the security:
- If the security/compartment in the segment does not exactly match the security/compartment in the TCB, send a reset:
- If there is an ACK:
<SEQ=SEG.ACK><CTL=RST>- Otherwise:
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>- If a reset was sent, discard the segment and return.
Fourth, check the SYN bit:
- This step should be reached only if the ACK is ok, or there is no ACK, and the segment did not contain a RST.
- If the SYN bit is on and the security/compartment is acceptable, then RCV.NXT is set to SEG.SEQ+1, IRS is set to SEG.SEQ. SND.UNA should be advanced to equal SEG.ACK (if there is an ACK), and any segments on the retransmission queue that are thereby acknowledged should be removed.
- If SND.UNA > ISS (our SYN has been ACKed), change the connection state to ESTABLISHED, form an ACK segment:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>- and send it. Data or controls that were queued for transmission MAY be included. Some TCP implementations suppress sending this segment when the received segment contains data that will anyways generate an acknowledgment in the later processing steps, saving this extra acknowledgment of the SYN from being sent. If there are other controls or text in the segment, then continue processing at the sixth step under Section 3.10.7.4 where the URG bit is checked; otherwise, return.
- Otherwise, enter SYN-RECEIVED, form a SYN,ACK segment:
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>- and send it. Set the variables:
SND.WND <- SEG.WND
SND.WL1 <- SEG.SEQ
SND.WL2 <- SEG.ACKIf there are other controls or text in the segment, queue them for processing after the ESTABLISHED state has been reached, return.
- Note that it is legal to send and receive application data on SYN segments (this is the "text in the segment" mentioned above). There has been significant misinformation and misunderstanding of this topic historically. Some firewalls and security devices consider this suspicious. However, the capability was used in T/TCP [21] and is used in TCP Fast Open (TFO) [48], so is important for implementations and network devices to permit. 🌟
Fifth, if neither of the SYN or RST bits is set, then drop the segment and return. 💜
3.10.7.4. Other States 🌈
otherwise, we do the checks below 💅
First, check sequence number:
- SYN-RECEIVED STATE, ESTABLISHED STATE, FIN-WAIT-1 STATE, FIN-WAIT-2 STATE, CLOSE-WAIT STATE, CLOSING STATE, LAST-ACK STATE, TIME-WAIT STATE
- Segments are processed in sequence. Initial tests on arrival are used to discard old duplicates, but further processing is done in SEG.SEQ order. If a segment's contents straddle the boundary between old and new, only the new parts are processed.
- In general, the processing of received segments MUST be implemented to aggregate ACK segments whenever possible (MUST-58). For example, if the TCP endpoint is processing a series of queued segments, it MUST process them all before sending any ACK segments (MUST-59).
- There are four cases for the acceptability test for an incoming segment:
+=========+=========+======================================+
| Segment | Receive | Test |
| Length | Window | |
+=========+=========+======================================+
| 0 | 0 | SEG.SEQ = RCV.NXT |
+---------+---------+--------------------------------------+
| 0 | >0 | RCV.NXT =< SEG.SEQ < |
| | | RCV.NXT+RCV.WND |
+---------+---------+--------------------------------------+
| >0 | 0 | not acceptable |
+---------+---------+--------------------------------------+
| >0 | >0 | RCV.NXT =< SEG.SEQ < |
| | | RCV.NXT+RCV.WND |
| | | |
| | | or |
| | | |
| | | RCV.NXT =< SEG.SEQ+SEG.LEN-1 |
| | | < RCV.NXT+RCV.WND |
+---------+---------+--------------------------------------+
Table 6: Segment Acceptability Tests- In implementing sequence number validation as described here, please note Appendix A.2.
- If the RCV.WND is zero, no segments will be acceptable, but special allowance should be made to accept valid ACKs, URGs, and RSTs.
- If an incoming segment is not acceptable, an acknowledgment should be sent in reply (unless the RST bit is set, if so drop the segment and return):
Okay so once that challenge ACK is on its way, the segment is getting dropped and we return, no drama 💅
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>- After sending the acknowledgment, drop the unacceptable segment and return. Bye bye, segment 👋
- 🕰️ Heads up for TIME-WAIT: there's an improved algorithm in [40] for handling incoming SYN segments that uses timestamps instead of the sequence number check we just did. When the improved algorithm is implemented, the logic above is not applicable for incoming SYN segments with Timestamp Options, received on a connection in the TIME-WAIT state. It's giving "different rules for different vibes" 🦋
- In the following it's assumed that the segment is the idealized segment that begins at RCV.NXT and does not exceed the window. You could tailor actual segments to fit this assumption by trimming off any portions that lie outside the window (including SYN and FIN) and only processing further if the segment then begins at RCV.NXT. Segments with higher beginning sequence numbers SHOULD be held for later processing (SHLD-31). Main character energy: wait your turn 🎀
Second, check the RST bit:
- RFC 5961 [9], Section 3 describes a potential blind reset attack and optional mitigation approach. This does not provide a cryptographic protection (e.g., as in IPsec or TCP-AO) but can be applicable in situations described in RFC 5961. For stacks implementing the protection described in RFC 5961, the three checks below apply; otherwise, processing for these states is indicated further below. 🔐
- If the RST bit is set and the sequence number is outside the current receive window, silently drop the segment. 🫥
- If the RST bit is set and the sequence number exactly matches the next expected sequence number (RCV.NXT), then TCP endpoints MUST reset the connection in the manner prescribed below according to the connection state. Okay this is the big one ✨
- If the RST bit is set and the sequence number does not exactly match the next expected sequence value, yet is within the current receive window, TCP endpoints MUST send an acknowledgment (challenge ACK):
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>After sending the challenge ACK, TCP endpoints MUST drop the unacceptable segment and stop processing the incoming packet further. Note that RFC 5961 and Errata ID 4772 [99] contain additional considerations for ACK throttling in an implementation. Throttling the ACKs, bestie, don't spam 🙈
- SYN-RECEIVED STATE
- If the RST bit is set,
- If this connection was initiated with a passive OPEN (i.e., came from the LISTEN state), then return this connection to LISTEN state and return. The user need not be informed. If this connection was initiated with an active OPEN (i.e., came from SYN-SENT state), then the connection was refused; signal the user "connection refused". In either case, the retransmission queue should be flushed. And in the active OPEN case, enter the CLOSED state and delete the TCB, and return. 🚪
- If the RST bit is set,
- ESTABLISHED STATE
- FIN-WAIT-1 STATE
- FIN-WAIT-2 STATE
- CLOSE-WAIT STATE
- If the RST bit is set, then any outstanding RECEIVEs and SEND should receive "reset" responses. All segment queues should be flushed. Users should also receive an unsolicited general "connection reset" signal. Enter the CLOSED state, delete the TCB, and return. 💀
- CLOSING STATE
- LAST-ACK STATE
- TIME-WAIT STATE
- If the RST bit is set, then enter the CLOSED state, delete the TCB, and return. Bye, connection 🫠
Third, check security:
- SYN-RECEIVED STATE
- If the security/compartment in the segment does not exactly match the security/compartment in the TCB, then send a reset and return. 🛡️
- ESTABLISHED STATE
- FIN-WAIT-1 STATE
- FIN-WAIT-2 STATE
- CLOSE-WAIT STATE
- CLOSING STATE
- LAST-ACK STATE
- TIME-WAIT STATE
- If the security/compartment in the segment does not exactly match the security/compartment in the TCB, then send a reset; any outstanding RECEIVEs and SEND should receive "reset" responses. All segment queues should be flushed. Users should also receive an unsolicited general "connection reset" signal. Enter the CLOSED state, delete the TCB, and return. 🚨
- Note this check is placed following the sequence check to prevent a segment from an old connection between these port numbers with a different security from causing an abort of the current connection. The order matters, iykyk 💜
Fourth, check the SYN bit:
- SYN-RECEIVED STATE
- If the connection was initiated with a passive OPEN, then return this connection to the LISTEN state and return. Otherwise, handle per the directions for synchronized states below. 🎯
- ESTABLISHED STATE
- FIN-WAIT-1 STATE
- FIN-WAIT-2 STATE
- CLOSE-WAIT STATE
- CLOSING STATE
- LAST-ACK STATE
- TIME-WAIT STATE
- If the SYN bit is set in these synchronized states, it may be either a legitimate new connection attempt (e.g., in the case of TIME-WAIT), an error where the connection should be reset, or the result of an attack attempt, as described in RFC 5961 [9]. For the TIME-WAIT state, new connections can be accepted if the Timestamp Option is used and meets expectations (per [40]). For all other cases, RFC 5961 provides a mitigation with applicability to some situations, though there are also alternatives that offer cryptographic protection (see Section 7). RFC 5961 recommends that in these synchronized states, if the SYN bit is set, irrespective of the sequence number, TCP endpoints MUST send a "challenge ACK" to the remote peer:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>- After sending the acknowledgment, TCP implementations MUST drop the unacceptable segment and stop processing further. Note that RFC 5961 and Errata ID 4772 [99] contain additional ACK throttling notes for an implementation. 🧊
- For implementations that do not follow RFC 5961, the original behavior described in RFC 793 follows in this paragraph. If the SYN is in the window it is an error: send a reset, any outstanding RECEIVEs and SEND should receive "reset" responses, all segment queues should be flushed, the user should also receive an unsolicited general "connection reset" signal, enter the CLOSED state, delete the TCB, and return. That's the old-school way, lowkey iconic 📜
- If the SYN is not in the window, this step would not be reached and an ACK would have been sent in the first step (sequence number check). ✅
Fifth, check the ACK field:
- if the ACK bit is off, drop the segment and return 🙅♀️
- if the ACK bit is on,
- RFC 5961 [9], Section 5 describes a potential blind data injection attack, and mitigation that implementations MAY choose to include (MAY-12). TCP stacks that implement RFC 5961 MUST add an input check that the ACK value is acceptable only if it is in the range of ((SND.UNA - MAX.SND.WND) =< SEG.ACK =< SND.NXT). All incoming segments whose ACK value doesn't satisfy the above condition MUST be discarded and an ACK sent back. The new state variable MAX.SND.WND is defined as the largest window that the local sender has ever received from its peer (subject to window scaling) or may be hard-coded to a maximum permissible window value. When the ACK value is acceptable, the per-state processing below applies:
- SYN-RECEIVED STATE
- If SND.UNA < SEG.ACK =< SND.NXT, then enter ESTABLISHED state and continue processing with the variables below set to:
SND.WND <- SEG.WND
SND.WL1 <- SEG.SEQ
SND.WL2 <- SEG.ACK- If the segment acknowledgment is not acceptable, form a reset segment
<SEQ=SEG.ACK><CTL=RST>- and send it. 📮
- ESTABLISHED STATE
- If SND.UNA < SEG.ACK =< SND.NXT, then set SND.UNA <- SEG.ACK. Any segments on the retransmission queue that are thereby entirely acknowledged are removed. Users should receive positive acknowledgments for buffers that have been SENT and fully acknowledged (i.e., SEND buffer should be returned with "ok" response). If the ACK is a duplicate (SEG.ACK =< SND.UNA), it can be ignored. If the ACK acks something not yet sent (SEG.ACK > SND.NXT), then send an ACK, drop the segment, and return. 🥳
- If SND.UNA =< SEG.ACK =< SND.NXT, the send window should be updated. If (SND.WL1 < SEG.SEQ or (SND.WL1 = SEG.SEQ and SND.WL2 =< SEG.ACK)), set SND.WND <- SEG.WND, set SND.WL1 <- SEG.SEQ, and set SND.WL2 <- SEG.ACK.
- Note that SND.WND is an offset from SND.UNA, that SND.WL1 records the sequence number of the last segment used to update SND.WND, and that SND.WL2 records the acknowledgment number of the last segment used to update SND.WND. The check here prevents using old segments to update the window. Old segments stay out of the window, period 🚫
- FIN-WAIT-1 STATE
- In addition to the processing for the ESTABLISHED state, if the FIN segment is now acknowledged, then enter FIN-WAIT-2 and continue processing in that state. 👋
- FIN-WAIT-2 STATE
- In addition to the processing for the ESTABLISHED state, if the retransmission queue is empty, the user's CLOSE can be acknowledged ("ok") but do not delete the TCB. 🌸
- CLOSE-WAIT STATE
- Do the same processing as for the ESTABLISHED state. 🔁
- CLOSING STATE
- In addition to the processing for the ESTABLISHED state, if the ACK acknowledges our FIN, then enter the TIME-WAIT state; otherwise, ignore the segment. 🤷♀️
- LAST-ACK STATE
- The only thing that can arrive in this state is an acknowledgment of our FIN. If our FIN is now acknowledged, delete the TCB, enter the CLOSED state, and return. Closure arc complete 🎬
- TIME-WAIT STATE
- The only thing that can arrive in this state is a retransmission of the remote FIN. Acknowledge it, and restart the 2 MSL timeout. ⏳
Sixth, check the URG bit:
- ESTABLISHED STATE
- FIN-WAIT-1 STATE
- FIN-WAIT-2 STATE
- okay so if the URG bit is set 🚨, we do
RCV.UP <- max(RCV.UP,SEG.UP), which keeps whichever urgent pointer is further ahead 💅. then we tell the user the remote side has urgent data, but only if the urgent pointer (RCV.UP) is in advance of the data already consumed. if the user was already told (or is still in "urgent mode") for this same continuous run of urgent data, don't ping them again 🙈 - CLOSE-WAIT STATE
- CLOSING STATE
- LAST-ACK STATE
- TIME-WAIT STATE
- this should literally never happen since a FIN has already come in from the remote side, so just ignore the URG 🙃
- okay seventh, we process the segment text 📝
- ESTABLISHED STATE
- FIN-WAIT-1 STATE
- FIN-WAIT-2 STATE
- once we're in ESTABLISHED, we can actually deliver segment data into the user's RECEIVE buffers 📦 data gets moved in until the buffer is full or the segment is empty. if the segment empties and carries a PUSH flag, the user gets told a PUSH came in when the buffer is handed back 🫶
- once the TCP endpoint takes responsibility for delivering the data to the user, it must also acknowledge the receipt of the data ✅
- once the TCP endpoint takes responsibility for the data, it moves RCV.NXT forward over the data it accepted and adjusts RCV.WND to match the current buffer availability. the total of RCV.NXT and RCV.WND should not be reduced 📈
- a TCP implementation MAY send an ACK segment acknowledging RCV.NXT when a valid segment arrives that is in the window but not at the left window edge (MAY-13) 🦋
- please note the window management suggestions in Section 3.8 💖
- send an acknowledgment of the form:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>- this acknowledgment should be piggybacked on a segment being transmitted if possible without incurring undue delay 🎀
- CLOSE-WAIT STATE
- CLOSING STATE
- LAST-ACK STATE
- TIME-WAIT STATE
- this should not occur since a FIN has been received from the remote side. ignore the segment text 🙈
- okay eighth, we check the FIN bit 👀
- do not process the FIN if the state is CLOSED, LISTEN, or SYN-SENT, since the SEG.SEQ cannot be validated. drop the segment and return 🚫
- if the FIN bit is set, signal the user "connection closing" and return any pending RECEIVEs with the same message, advance RCV.NXT over the FIN, and send an acknowledgment for the FIN. note that FIN implies PUSH for any segment text not yet delivered to the user 💜
- SYN-RECEIVED STATE
- ESTABLISHED STATE
- enter the CLOSE-WAIT state.
- FIN-WAIT-1 STATE
- if our FIN has been ACKed (perhaps in this segment), then enter TIME-WAIT, start the time-wait timer, and turn off the other timers. otherwise, enter the CLOSING state. ⏰
- FIN-WAIT-2 STATE
- enter the TIME-WAIT state. start the time-wait timer, and turn off the other timers ⏳
- CLOSE-WAIT STATE
- remain in the CLOSE-WAIT state.
- CLOSING STATE
- remain in the CLOSING state.
- LAST-ACK STATE
- remain in the LAST-ACK state.
- TIME-WAIT STATE
- remain in the TIME-WAIT state. restart the 2 MSL time-wait timeout 🔁
and return. 🌸
3.10.8. Timeouts
USER TIMEOUT
- for any state, if the user timeout expires, flush all queues and signal the user "error: connection aborted due to user timeout" in general and for any outstanding calls. then delete the TCB, enter the CLOSED state, and return 💀
RETRANSMISSION TIMEOUT
- for any state, if the retransmission timeout expires on a segment in the retransmission queue, send the segment at the front of the retransmission queue again, reinitialize the retransmission timer, and return 🔄
TIME-WAIT TIMEOUT
- if the time-wait timeout expires on a connection, delete the TCB, enter the CLOSED state, and return ✨
4. Glossary ✨
okay bestie, this is the vocab list 📖💖 every one of these terms shows up all over the spec, so let's lock them in
ACK 🫶 A control bit (acknowledge) occupying no sequence space, which indicates that the acknowledgment field of this segment specifies the next sequence number the sender of this segment is expecting to receive, hence acknowledging receipt of all previous sequence numbers.
connection 🦋 A logical communication path identified by a pair of sockets.
datagram 📬 A message sent in a packet-switched computer communications network.
Destination Address 🎯 The network-layer address of the endpoint intended to receive a segment.
FIN 👋 A control bit (finis) occupying one sequence number, which indicates that the sender will send no more data or control occupying sequence space.
flush 🧹 To remove all of the contents (data or segments) from a store (buffer or queue).
fragment 🧩 A portion of a logical unit of data. In particular, an internet fragment is a portion of an internet datagram.
header 🏷️ Control information at the beginning of a message, segment, fragment, packet, or block of data.
host 💻 A computer. In particular, a source or destination of messages from the point of view of the communication network.
Identification 🔖 An Internet Protocol field. This identifying value assigned by the sender aids in assembling the fragments of a datagram.
internet address 🌐 A network-layer address.
internet datagram 📦 A unit of data exchanged between internet hosts, together with the internet header that allows the datagram to be routed from source to destination.
internet fragment 🧃 A portion of the data of an internet datagram with an internet header.
IP 🌍 Internet Protocol. See [1] and [13].
IRS 🌱 The Initial Receive Sequence number. The first sequence number used by the sender on a connection.
ISN 🎲 The Initial Sequence Number. The first sequence number used on a connection (either ISS or IRS). Selected in a way that is unique within a given period of time and is unpredictable to attackers. this one is giving security queen energy 👑
ISS 🌱 The Initial Send Sequence number. The first sequence number used by the sender on a connection.
left sequence ⬅️ This is the next sequence number to be acknowledged by the data-receiving TCP endpoint (or the lowest currently unacknowledged sequence number) and is sometimes referred to as the left edge of the send window.
module 🧱 An implementation, usually in software, of a protocol or other procedure.
MSL ⏳ Maximum Segment Lifetime, the time a TCP segment can exist in the internetwork system. Arbitrarily defined to be 2 minutes.
octet 🍓 An eight-bit byte.
Options 🎛️ An Option field may contain several options, and each option may be several octets in length.
packet 📮 A package of data with a header that may or may not be logically complete. More often a physical packaging than a logical packaging of data.
port 🚪 The portion of a connection identifier used for demultiplexing connections at an endpoint.
process 🏃 A program in execution. A source or destination of data from the point of view of the TCP endpoint or other host-to-host protocol.
PUSH 🚀 A control bit occupying no sequence space, indicating that this segment contains data that must be pushed through to the receiving user.
RCV.NXT 📥 receive next sequence number
RCV.UP 📥 receive urgent pointer
RCV.WND 📥 receive window
receive next sequence number 📥 This is the next sequence number the local TCP endpoint is expecting to receive.
receive window 🪟 This represents the sequence numbers the local (receiving) TCP endpoint is willing to receive. Thus, the local TCP endpoint considers that segments overlapping the range RCV.NXT to RCV.NXT + RCV.WND - 1 carry acceptable data or control. Segments containing sequence numbers entirely outside this range are considered duplicates or injection attacks and discarded. 🛡️
RST 🚫 A control bit (reset), occupying no sequence space, indicating that the receiver should delete the connection without further interaction. The receiver can determine, based on the sequence number and acknowledgment fields of the incoming segment, whether it should honor the reset command or ignore it. In no case does receipt of a segment containing RST give rise to a RST in response. 💅
SEG.ACK 📨 segment acknowledgment
SEG.LEN 📏 segment length
SEG.SEQ 🔢 segment sequence
SEG.UP 📍 segment urgent pointer field
SEG.WND 🪟 segment window field
segment 🎀 A logical unit of data. In particular, a TCP segment is the unit of data transferred between a pair of TCP modules.
segment acknowledgment 📨 The sequence number in the acknowledgment field of the arriving segment.
segment length 📏 The amount of sequence number space occupied by a segment, including any controls that occupy sequence space.
segment sequence 🔢 The number in the sequence field of the arriving segment.
send sequence 📤 This is the next sequence number the local (sending) TCP endpoint will use on the connection. It is initially selected from an initial sequence number curve (ISN) and is incremented for each octet of data or sequenced control transmitted.
send window 🪟 This represents the sequence numbers that the remote (receiving) TCP endpoint is willing to receive. It is the value of the window field specified in segments from the remote (data-receiving) TCP endpoint. The range of new sequence numbers that may be emitted by a TCP implementation lies between SND.NXT and SND.UNA + SND.WND - 1. (Retransmissions of sequence numbers between SND.UNA and SND.NXT are expected, of course.) 🔁
SND.NXT 📤 send sequence
SND.UNA ⬅️ left sequence
SND.UP 📍 send urgent pointer
SND.WL1 🕰️ segment sequence number at last window update
SND.WL2 🕰️ segment acknowledgment number at last window update
SND.WND 🪟 send window
socket (or socket number, or socket address, or socket identifier) 🔌 An address that specifically includes a port identifier, that is, the concatenation of an Internet Address with a TCP port.
Source Address 📍 The network-layer address of the sending endpoint.
SYN 🤝 A control bit in the incoming segment, occupying one sequence number, used at the initiation of a connection to indicate where the sequence numbering will start.
TCB 🗂️ Transmission control block, the data structure that records the state of a connection.
TCP 👑 Transmission Control Protocol: a host-to-host protocol for reliable communication in internetwork environments.
TOS 🕰️ Type of Service, an obsoleted IPv4 field. The same header bits currently are used for the Differentiated Services field [4] containing the Differentiated Services Codepoint (DSCP) value and the 2-bit ECN codepoint [6].
Type of Service 🔁 See "TOS".
URG 🚨 A control bit (urgent), occupying no sequence space, used to indicate that the receiving user should be notified to do urgent processing as long as there is data to be consumed with sequence numbers less than the value indicated by the urgent pointer.
urgent pointer 📍 A control field meaningful only when the URG bit is on. This field communicates the value of the urgent pointer that indicates the data octet associated with the sending user's urgent call.
5. Changes from RFC 793 💖
okay so this section is the glow-up story 🌸 it explains what changed between the old TCP spec and this one
This document obsoletes RFC 793 as well as RFCs 6093 and 6528, which updated 793. In all cases, only the normative protocol specification and requirements have been incorporated into this document, and some informational text with background and rationale may not have been carried in. The informational content of those documents is still valuable in learning about and understanding TCP, and they are valid Informational references, even though their normative content has been incorporated into this document. 📚
The main body of this document was adapted from RFC 793's Section 3, titled "FUNCTIONAL SPECIFICATION", with an attempt to keep formatting and layout as close as possible. 🎀
The collection of applicable RFC errata that have been reported and either accepted or held for an update to RFC 793 were incorporated (Errata IDs: 573 [73], 574 [74], 700 [75], 701 [76], 1283 [77], 1561 [78], 1562 [79], 1564 [80], 1571 [81], 1572 [82], 2297 [83], 2298 [84], 2748 [85], 2749 [86], 2934 [87], 3213 [88], 3300 [89], 3301 [90], 6222 [91]). Some errata were not applicable due to other changes (Errata IDs: 572 [92], 575 [93], 1565 [94], 1569 [95], 2296 [96], 3305 [97], 3602 [98]). 🩹
Changes to the specification of the urgent pointer described in RFCs 1011, 1122, and 6093 were incorporated. See RFC 6093 for detailed discussion of why these changes were necessary. 📍
The discussion of the RTO from RFC 793 was updated to refer to RFC 6298. The text on the RTO in RFC 1122 originally replaced the text in RFC 793; however, RFC 2988 should have updated RFC 1122 and has subsequently been obsoleted by RFC 6298. ⏱️
RFC 1011 [18] contains a number of comments about RFC 793, including some needed changes to the TCP specification. These are expanded in RFC 1122, which contains a collection of other changes and clarifications to RFC 793. The normative items impacting the protocol have been incorporated here, though some historically useful implementation advice and informative discussion from RFC 1122 is not included here. The present document, which is now the TCP specification rather than RFC 793, updates RFC 1011, and the comments noted in RFC 1011 have been incorporated. 🧠
RFC 1122 contains more than just TCP requirements, so this document can't obsolete RFC 1122 entirely. It is only marked as "updating" RFC 1122; however, it should be understood to effectively obsolete all of the material on TCP found in RFC 1122. 🙈
The more secure initial sequence number generation algorithm from RFC 6528 was incorporated. See RFC 6528 for discussion of the attacks that this mitigates, as well as advice on selecting PRF algorithms and managing secret key data. 🔐
A note based on RFC 6429 was added to explicitly clarify that system resource management concerns allow connection resources to be reclaimed. RFC 6429 is obsoleted in the sense that the clarification it describes has been reflected within this base TCP specification. ♻️
The description of congestion control implementation was added based on the set of documents that are IETF BCP or Standards Track on the topic and the current state of common implementations. 🌈
6. IANA Considerations 📋
okay so this section is all about the "Transmission Control Protocol (TCP) Header Flags" registry, and IANA made several changes to it 💅
RFC 3168 originally created this registry, but it only filled it in with the new bits defined in RFC 3168 🙈 It kinda forgot about the other bits that were already described in RFC 793 and other docs. Bit 7 has since also been updated by RFC 8311 [54] ✨
The "Bit" column got renamed to "Bit Offset" 🎀 because it shows each flag's offset within the 16-bit aligned view of the TCP header in Figure 1. The bits in offsets 0 through 3 are the TCP segment Data Offset field, so they're not header flags 🧚
IANA also added a brand new column for "Assignment Notes" 📝 Here's what they assigned:
+========+===================+===========+====================+
| Bit | Name | Reference | Assignment Notes |
| Offset | | | |
+========+===================+===========+====================+
| 4 | Reserved for | RFC 9293 | |
| | future use | | |
+--------+-------------------+-----------+--------------------+
| 5 | Reserved for | RFC 9293 | |
| | future use | | |
+--------+-------------------+-----------+--------------------+
| 6 | Reserved for | RFC 9293 | |
| | future use | | |
+--------+-------------------+-----------+--------------------+
| 7 | Reserved for | RFC 8311 | Previously used by |
| | future use | | Historic RFC 3540 |
| | | | as NS (Nonce Sum). |
+--------+-------------------+-----------+--------------------+
| 8 | CWR (Congestion | RFC 3168 | |
| | Window Reduced) | | |
+--------+-------------------+-----------+--------------------+
| 9 | ECE (ECN-Echo) | RFC 3168 | |
+--------+-------------------+-----------+--------------------+
| 10 | Urgent pointer | RFC 9293 | |
| | field is | | |
| | significant (URG) | | |
+--------+-------------------+-----------+--------------------+
| 11 | Acknowledgment | RFC 9293 | |
| | field is | | |
| | significant (ACK) | | |
+--------+-------------------+-----------+--------------------+
| 12 | Push function | RFC 9293 | |
| | (PSH) | | |
+--------+-------------------+-----------+--------------------+
| 13 | Reset the | RFC 9293 | |
| | connection (RST) | | |
+--------+-------------------+-----------+--------------------+
| 14 | Synchronize | RFC 9293 | |
| | sequence numbers | | |
| | (SYN) | | |
+--------+-------------------+-----------+--------------------+
| 15 | No more data from | RFC 9293 | |
| | sender (FIN) | | |
+--------+-------------------+-----------+--------------------+
Table 7: TCP Header FlagsThe "TCP Header Flags" registry has also been moved into a subregistry under the global "Transmission Control Protocol (TCP) Parameters" registry <https://www.iana.org/assignments/tcp-parameters/> 🌈
The registry's Registration Procedure is still Standards Action 🔥 but the Reference now points to this document, and the Note has been removed 🫶
7. Security and Privacy Considerations 🔐
okay bestie, this part is serious 😭 TCP's design only has rudimentary security features. They make connections and application data transfer more robust and reliable, but TCP has no built-in cryptography for confidentiality, authentication, or any other typical security function 💀
Non-cryptographic enhancements (e.g., [9]) have been developed to make TCP connections more robust against particular types of attacks, but they're limited in what they protect (e.g., see Section 1.1 of [9]). So applications usually lean on lower-layer protocols (e.g., IPsec) and upper-layer protocols (e.g., TLS) to get security and privacy for TCP connections and the application data inside them 🛡️ Methods based on TCP Options have been developed too, to support some security capabilities ✨
If you want to fully protect confidentiality, integrity, and authentication for TCP connections (including their control flags), IPsec is the only current effective method 👑 For integrity protection and authentication there's the TCP Authentication Option (TCP-AO) [38], with a proposed extension that would also provide confidentiality for the segment payload 🔒 Other methods in this section may protect the payload's confidentiality or integrity, but for the TCP header they only cover a subset of the fields (e.g., tcpcrypt [57]) or none at all (e.g., TLS) 😬 Other security features added to TCP (e.g., ISN generation, sequence number checks, and others) can only partially hinder attacks 🙈
Applications with long-lived TCP flows have been vulnerable to attacks that exploit how control flags are processed, as described in earlier TCP specifications [33] 💀 TCP-MD5 was a commonly implemented TCP Option for authentication on some of these connections, but it had flaws and is now deprecated 🥀 TCP-AO protects long-lived TCP connections from these attacks and has superior properties to TCP-MD5 💜 It does not provide any privacy for application data or for the TCP headers 🙅♀️
The "tcpcrypt" [57] experimental extension to TCP can cryptographically protect connection data 🦋 Metadata about the TCP flow is still visible, but the application stream is well protected. Within the TCP header, only the urgent pointer and the FIN flag are protected through tcpcrypt 🫣
The TCP Roadmap [49] has notes about several RFCs related to TCP security 🗺️ Many of the enhancements from those RFCs are already built into this document, including ISN generation, mitigating blind in-window attacks, and improving handling of soft errors and ICMP packets 🌸 All of these are discussed in more detail in the RFCs that originally described the changes to earlier TCP specifications. Also, see RFC 6093 [39] for security considerations related to the urgent pointer field, which also discourages new applications from using the urgent pointer ⚠️
TCP is often used for bulk transfer flows, so some attacks can abuse the TCP congestion control logic 🌊 One example is "ACK-division" attacks. Updates to the TCP congestion control specifications include mechanisms like Appropriate Byte Counting (ABC) [29], which act as mitigations for these attacks 🧯
Other attacks try to exhaust the resources of a TCP server 🏋️ Examples include SYN flooding [32] and wasting resources on non-progressing connections [41]. Operating systems commonly implement mitigations for these, and some common defenses also use proxies, stateful firewalls, and other technologies outside the end-host TCP implementation 🏰
The concept of a protocol's "wire image" is described in RFC 8546 [56] 👀 It explains how TCP's cleartext headers expose more metadata to nodes on the path than is strictly needed to route packets to their destination. On-path adversaries may be able to leverage this metadata 🕵️♀️ Lessons learned from TCP in this respect have been applied in the design of newer transports like QUIC [60] ✨ The IETF also documented, in RFC 9065 [61], considerations for future TCP extensions and other transports, based partly on experiences with TCP and its extensions 📚 There are also IAB recommendations in RFC 8558 [58] and [67] 🎀
There are also methods of "fingerprinting" that can be used to infer the host TCP implementation (operating system) version or platform information 🔍 They collect observations of several aspects, such as the options present in segments, the ordering of options, the specific behaviors in the case of various conditions, packet timing, packet sizing, and other aspects of the protocol that are left for an implementer to determine. Those observations can then be used to identify information about the host and its implementation 📸
Since ICMP message processing can also interact with TCP connections, there's potential for ICMP-based attacks against TCP connections 🌪️ These are discussed in RFC 5927 [100], along with mitigations that have been implemented 🛠️
8. References 📚
8.1. Normative References 📚✨
okay so these are the normative refs, aka the docs you literally need to have read before the rest makes sense 🤓💅
- [1] Postel, J., "Internet Protocol", STD 5, RFC 791, DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>. - [2] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>. - [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. - [4] Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>. - [5] Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>. - [6] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>. - [7] Floyd, S. and M. Allman, "Specifying New Congestion Control Algorithms", BCP 133, RFC 5033, DOI 10.17487/RFC5033, August 2007,
<https://www.rfc-editor.org/info/rfc5033>. - [8] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>. - [9] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's Robustness to Blind In-Window Attacks", RFC 5961, DOI 10.17487/RFC5961, August 2010,
<https://www.rfc-editor.org/info/rfc5961>. - [10] Paxson, V., Allman, M., Chu, J., and M. Sargent, "Computing TCP's Retransmission Timer", RFC 6298, DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>. - [11] Gont, F., "Deprecation of ICMP Source Quench Messages", RFC 6633, DOI 10.17487/RFC6633, May 2012,
<https://www.rfc-editor.org/info/rfc6633>. - [12] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>. - [13] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>. - [14] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., "Path MTU Discovery for IP version 6", STD 87, RFC 8201, DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>. - [15] Allman, M., "Requirements for Time-Based Loss Detection", BCP 233, RFC 8961, DOI 10.17487/RFC8961, November 2020,
<https://www.rfc-editor.org/info/rfc8961>.
8.2. Informative References 🧚🌸
these ones are the extra reading, the deep-dive lore that's good to know but not required to get the basics 💜
- [16] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>. - [17] Nagle, J., "Congestion Control in IP/TCP Internetworks", RFC 896, DOI 10.17487/RFC0896, January 1984,
<https://www.rfc-editor.org/info/rfc896>. - [18] Reynolds, J. and J. Postel, "Official Internet protocols", RFC 1011, DOI 10.17487/RFC1011, May 1987,
<https://www.rfc-editor.org/info/rfc1011>. - [19] Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>. - [20] Almquist, P., "Type of Service in the Internet Protocol Suite", RFC 1349, DOI 10.17487/RFC1349, July 1992,
<https://www.rfc-editor.org/info/rfc1349>. - [21] Braden, R., "T/TCP -- TCP Extensions for Transactions Functional Specification", RFC 1644, DOI 10.17487/RFC1644, July 1994,
<https://www.rfc-editor.org/info/rfc1644>. - [22] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, DOI 10.17487/RFC2018, October 1996,
<https://www.rfc-editor.org/info/rfc2018>. - [23] Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner, J., Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known TCP Implementation Problems", RFC 2525, DOI 10.17487/RFC2525, March 1999,
<https://www.rfc-editor.org/info/rfc2525>. - [24] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms", RFC 2675, DOI 10.17487/RFC2675, August 1999,
<https://www.rfc-editor.org/info/rfc2675>. - [25] Xiao, X., Hannan, A., Paxson, V., and E. Crabbe, "TCP Processing of the IPv4 Precedence Field", RFC 2873, DOI 10.17487/RFC2873, June 2000,
<https://www.rfc-editor.org/info/rfc2873>. - [26] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An Extension to the Selective Acknowledgement (SACK) Option for TCP", RFC 2883, DOI 10.17487/RFC2883, July 2000,
<https://www.rfc-editor.org/info/rfc2883>. - [27] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, DOI 10.17487/RFC2923, September 2000,
<https://www.rfc-editor.org/info/rfc2923>. - [28] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M. Sooriyabandara, "TCP Performance Implications of Network Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449, December 2002,
<https://www.rfc-editor.org/info/rfc3449>. - [29] Allman, M., "TCP Congestion Control with Appropriate Byte Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February 2003,
<https://www.rfc-editor.org/info/rfc3465>. - [30] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4, ICMPv6, UDP, and TCP Headers", RFC 4727, DOI 10.17487/RFC4727, November 2006,
<https://www.rfc-editor.org/info/rfc4727>. - [31] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>. - [32] Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>. - [33] Touch, J., "Defending TCP Against Spoofing Attacks", RFC 4953, DOI 10.17487/RFC4953, July 2007,
<https://www.rfc-editor.org/info/rfc4953>. - [34] Culley, P., Elzur, U., Recio, R., Bailey, S., and J. Carrier, "Marker PDU Aligned Framing for TCP Specification", RFC 5044, DOI 10.17487/RFC5044, October 2007,
<https://www.rfc-editor.org/info/rfc5044>. - [35] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461, DOI 10.17487/RFC5461, February 2009,
<https://www.rfc-editor.org/info/rfc5461>. - [36] StJohns, M., Atkinson, R., and G. Thomas, "Common Architecture Label IPv6 Security Option (CALIPSO)", RFC 5570, DOI 10.17487/RFC5570, July 2009,
<https://www.rfc-editor.org/info/rfc5570>. - [37] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust Header Compression (ROHC) Framework", RFC 5795, DOI 10.17487/RFC5795, March 2010,
<https://www.rfc-editor.org/info/rfc5795>. - [38] Touch, J., Mankin, A., and R. Bonica, "The TCP Authentication Option", RFC 5925, DOI 10.17487/RFC5925, June 2010,
<https://www.rfc-editor.org/info/rfc5925>. - [39] Gont, F. and A. Yourtchenko, "On the Implementation of the TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093, January 2011,
<https://www.rfc-editor.org/info/rfc6093>. - [40] Gont, F., "Reducing the TIME-WAIT State Using TCP Timestamps", BCP 159, RFC 6191, DOI 10.17487/RFC6191, April 2011,
<https://www.rfc-editor.org/info/rfc6191>. - [41] Bashyam, M., Jethanandani, M., and A. Ramaiah, "TCP Sender Clarification for Persist Condition", RFC 6429, DOI 10.17487/RFC6429, December 2011,
<https://www.rfc-editor.org/info/rfc6429>. - [42] Gont, F. and S. Bellovin, "Defending against Sequence Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February 2012,
<https://www.rfc-editor.org/info/rfc6528>. - [43] Borman, D., "TCP Options and Maximum Segment Size (MSS)", RFC 6691, DOI 10.17487/RFC6691, July 2012,
<https://www.rfc-editor.org/info/rfc6691>. - [44] Touch, J., "Updated Specification of the IPv4 ID Field", RFC 6864, DOI 10.17487/RFC6864, February 2013,
<https://www.rfc-editor.org/info/rfc6864>. - [45] Touch, J., "Shared Use of Experimental TCP Options", RFC 6994, DOI 10.17487/RFC6994, August 2013,
<https://www.rfc-editor.org/info/rfc6994>. - [46] McPherson, D., Oran, D., Thaler, D., and E. Osterweil, "Architectural Considerations of IP Anycast", RFC 7094, DOI 10.17487/RFC7094, January 2014,
<https://www.rfc-editor.org/info/rfc7094>. - [47] Borman, D., Braden, B., Jacobson, V., and R. Scheffenegger, Ed., "TCP Extensions for High Performance", RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>. - [48] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>. - [49] Duke, M., Braden, R., Eddy, W., Blanton, E., and A. Zimmermann, "A Roadmap for Transmission Control Protocol (TCP) Specification Documents", RFC 7414, DOI 10.17487/RFC7414, February 2015,
<https://www.rfc-editor.org/info/rfc7414>. - [50] Black, D., Ed. and P. Jones, "Differentiated Services (Diffserv) and Real-Time Communication", RFC 7657, DOI 10.17487/RFC7657, November 2015,
<https://www.rfc-editor.org/info/rfc7657>. - [51] Fairhurst, G. and M. Welzl, "The Benefits of Using Explicit Congestion Notification (ECN)", RFC 8087, DOI 10.17487/RFC8087, March 2017,
<https://www.rfc-editor.org/info/rfc8087>. - [52] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind, Ed., "Services Provided by IETF Transport Protocols and Congestion Control Mechanisms", RFC 8095, DOI 10.17487/RFC8095, March 2017,
<https://www.rfc-editor.org/info/rfc8095>. - [53] Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of Transport Features Provided by IETF Transport Protocols", RFC 8303, DOI 10.17487/RFC8303, February 2018,
<https://www.rfc-editor.org/info/rfc8303>.
Appendix A. Other Implementation Notes 🌸
This section is bestie-level extra credit 🎀 It has additional notes and references on TCP implementation decisions that are currently not a part of the RFC series or included within the TCP standard. Implementers can consider these items, but there was not yet a consensus to include them in the standard 💅
A.1. IP Security Compartment and Precedence 🔐
Okay so the IPv4 specification [1] has a precedence value in the Type of Service (TOS) field, which is now obsoleted 🥀 It got modified in [20] and then obsoleted by the definition of Differentiated Services (Diffserv) [4]. Setting and conveying TOS between the network layer, the TCP implementation, and applications is obsolete, and Diffserv replaces it in the current TCP specification ✨
RFC 793 required checking the IP security compartment and precedence on incoming TCP segments for consistency within a connection and with application requests. Each of these aspects of IP has become outdated, without specific updates to RFC 793 😭 The precedence issues were fixed by [25], which is Standards Track, so this present TCP specification includes those changes 🫶 However, the state of IP security options that Multi-Level Secure (MLS) systems may use is not as clear in the IETF right now 🙈
Resetting connections when incoming packets don't meet the expected security compartment or precedence expectations has been recognized as a possible attack vector [63] 💀 There has also been discussion about amending the TCP specification so connections aren't aborted due to nonmatching IP security compartment and Diffserv codepoint values 🦋
A.1.1. Precedence 👑
In Diffserv, the former precedence values are treated as Class Selector codepoints, and the Diffserv architecture describes methods for compatible treatment 🧚 The RFC TCP specification defined by RFCs 793 and 1122 included logic that tried to have connections use the highest precedence requested by either endpoint application, and to keep the precedence consistent throughout a connection. This logic came from the obsolete TOS and doesn't apply to Diffserv, so it should not be included in TCP implementations 🙅♀️ Changes to Diffserv values within a connection are discouraged, though 🍓 For discussion of this, see RFC 7657 (Sections 5.1, 5.3, and 6) [50].
The obsoleted TOS processing rules in TCP assumed that precedence values were bidirectional (or symmetric) on a connection, but the Diffserv architecture is asymmetric 🌈 Problems with the old TCP logic in this regard were described in [25], and the solution described there is to ignore IP precedence in TCP ✨ Since RFC 2873 is a Standards Track document (although it isn't marked as updating RFC 793), current implementations are expected to be robust in these conditions 💜 Note that the Diffserv field value used in each direction is part of the interface between TCP and the network layer, and the values in use can be indicated both ways between TCP and the application 🫶
[54] Black, D., "Relaxing Restrictions on Explicit Congestion Notification (ECN) Experimentation", RFC 8311, DOI 10.17487/RFC8311, January 2018, https://www.rfc-editor.org/info/rfc8311.
[55] Chown, T., Loughney, J., and T. Winters, "IPv6 Node Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504, January 2019, https://www.rfc-editor.org/info/rfc8504.
[56] Trammell, B. and M. Kuehlewind, "The Wire Image of a Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April 2019, https://www.rfc-editor.org/info/rfc8546.
[57] Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack, Q., and E. Smith, "Cryptographic Protection of TCP Streams (tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019, https://www.rfc-editor.org/info/rfc8548.
[58] Hardie, T., Ed., "Transport Protocol Path Signals", RFC 8558, DOI 10.17487/RFC8558, April 2019, https://www.rfc-editor.org/info/rfc8558.
[59] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C. Paasch, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March 2020, https://www.rfc-editor.org/info/rfc8684.
[60] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, May 2021, https://www.rfc-editor.org/info/rfc9000.
[61] Fairhurst, G. and C. Perkins, "Considerations around Transport Header Confidentiality, Network Operations, and the Evolution of Internet Transport Protocols", RFC 9065, DOI 10.17487/RFC9065, July 2021, https://www.rfc-editor.org/info/rfc9065.
[62] IANA, "Transmission Control Protocol (TCP) Parameters", https://www.iana.org/assignments/tcp-parameters/.
[63] Gont, F., "Processing of IP Security/Compartment and Precedence Information by TCP", Work in Progress, Internet-Draft, draft-gont-tcpm-tcp-seccomp-prec-00, 29 March 2012, https://datatracker.ietf.org/doc/html/draft-gont-tcpm-tcp-seccomp-prec-00.
[64] Gont, F. and D. Borman, "On the Validation of TCP Sequence Numbers", Work in Progress, Internet-Draft, draft-gont-tcpm-tcp-seq-validation-04, 11 March 2019, https://datatracker.ietf.org/doc/html/draft-gont-tcpm-tcp-seq-validation-04.
[65] Touch, J. and W. M. Eddy, "TCP Extended Data Offset Option", Work in Progress, Internet-Draft, draft-ietf-tcpm-tcp-edo-12, 15 April 2022, https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-tcp-edo-12.
[66] McQuistin, S., Band, V., Jacob, D., and C. Perkins, "Describing Protocol Data Units with Augmented Packet Header Diagrams", Work in Progress, Internet-Draft, draft-mcquistin-augmented-ascii-diagrams-10, 7 March 2022, https://datatracker.ietf.org/doc/html/draft-mcquistin-augmented-ascii-diagrams-10.
[67] Thomson, M. and T. Pauly, "Long-Term Viability of Protocol Extension Mechanisms", RFC 9170, DOI 10.17487/RFC9170, December 2021, https://www.rfc-editor.org/info/rfc9170.
[68] Minshall, G., "A Suggested Modification to Nagle's Algorithm", Work in Progress, Internet-Draft, draft-minshall-nagle-01, 18 June 1999, https://datatracker.ietf.org/doc/html/draft-minshall-nagle-01.
[69] Dalal, Y. and C. Sunshine, "Connection Management in Transport Protocols", Computer Networks, Vol. 2, No. 6, pp. 454-473, DOI 10.1016/0376-5075(78)90053-3, December 1978, https://doi.org/10.1016/0376-5075(78)90053-3.
[70] Faber, T., Touch, J., and W. Yui, "The TIME-WAIT state in TCP and Its Effect on Busy Servers", Proceedings of IEEE INFOCOM, pp. 1573-1583, DOI 10.1109/INFCOM.1999.752180, March 1999, https://doi.org/10.1109/INFCOM.1999.752180.
[71] Postel, J., "Comments on Action Items from the January Meeting", IEN 177, March 1981, https://www.rfc-editor.org/ien/ien177.txt.
[72] "Segmentation Offloads", The Linux Kernel Documentation, https://www.kernel.org/doc/html/latest/networking/segmentation-offloads.html.
[73] RFC Errata, Erratum ID 573, RFC 793, https://www.rfc-editor.org/errata/eid573.
[74] RFC Errata, Erratum ID 574, RFC 793, https://www.rfc-editor.org/errata/eid574.
[75] RFC Errata, Erratum ID 700, RFC 793, https://www.rfc-editor.org/errata/eid700.
[76] RFC Errata, Erratum ID 701, RFC 793, https://www.rfc-editor.org/errata/eid701.
[77] RFC Errata, Erratum ID 1283, RFC 793, https://www.rfc-editor.org/errata/eid1283.
[78] RFC Errata, Erratum ID 1561, RFC 793, https://www.rfc-editor.org/errata/eid1561.
[79] RFC Errata, Erratum ID 1562, RFC 793, https://www.rfc-editor.org/errata/eid1562.
[80] RFC Errata, Erratum ID 1564, RFC 793, https://www.rfc-editor.org/errata/eid1564.
[81] RFC Errata, Erratum ID 1571, RFC 793, https://www.rfc-editor.org/errata/eid1571.
[82] RFC Errata, Erratum ID 1572, RFC 793, https://www.rfc-editor.org/errata/eid1572.
[83] RFC Errata, Erratum ID 2297, RFC 793, https://www.rfc-editor.org/errata/eid2297.
[84] RFC Errata, Erratum ID 2298, RFC 793, https://www.rfc-editor.org/errata/eid2298.
[85] RFC Errata, Erratum ID 2748, RFC 793, https://www.rfc-editor.org/errata/eid2748.
[86] RFC Errata, Erratum ID 2749, RFC 793, https://www.rfc-editor.org/errata/eid2749.
[87] RFC Errata, Erratum ID 2934, RFC 793, https://www.rfc-editor.org/errata/eid2934.
[88] RFC Errata, Erratum ID 3213, RFC 793, https://www.rfc-editor.org/errata/eid3213.
[89] RFC Errata, Erratum ID 3300, RFC 793, https://www.rfc-editor.org/errata/eid3300.
[90] RFC Errata, Erratum ID 3301, RFC 793, https://www.rfc-editor.org/errata/eid3301.
[91] RFC Errata, Erratum ID 6222, RFC 793, https://www.rfc-editor.org/errata/eid6222.
[92] RFC Errata, Erratum ID 572, RFC 793, https://www.rfc-editor.org/errata/eid572.
[93] RFC Errata, Erratum ID 575, RFC 793, https://www.rfc-editor.org/errata/eid575.
[94] RFC Errata, Erratum ID 1565, RFC 793, https://www.rfc-editor.org/errata/eid1565.
[95] RFC Errata, Erratum ID 1569, RFC 793, https://www.rfc-editor.org/errata/eid1569.
[96] RFC Errata, Erratum ID 2296, RFC 793, https://www.rfc-editor.org/errata/eid2296.
[97] RFC Errata, Erratum ID 3305, RFC 793, https://www.rfc-editor.org/errata/eid3305.
[98] RFC Errata, Erratum ID 3602, RFC 793, https://www.rfc-editor.org/errata/eid3602.
[99] RFC Errata, Erratum ID 4772, RFC 5961, https://www.rfc-editor.org/errata/eid4772.
[100] Gont, F., "ICMP Attacks against TCP", RFC 5927, DOI 10.17487/RFC5927, July 2010, https://www.rfc-editor.org/info/rfc5927.
A.1.2. MLS Systems 🔐
okay so the IP Security Option (IPSO) and its compartment, the one from [1], got a glow-up in RFC 1038, and then RFC 1038 got replaced by RFC 1108 🫶 the Commercial IP Security Option (CIPSO) is defined in FIPS-188, which NIST withdrew in 2015, but some vendors and operating systems still support it 💅 RFC 1108 is now Historic, but RFC 791 itself has never been updated to remove the IP Security Option, so it's still technically sitting there 🙈 for IPv6 there's a similar option called the Common Architecture Label IPv6 Security Option (CALIPSO), defined in [36] ✨ RFC 793 has logic that brings the IP security/compartment information into how TCP segments get treated 🧚
references to the IP "security/compartment" in this document may be relevant for Multi-Level Secure (MLS) system implementers 🎀 but if you're not building an MLS system, you can ignore them, which matches how things actually run on the Internet 💜 see Appendix A.1 for more of the tea ☕
also, RFC 5570 describes some MLS networking scenarios where IPSO, CIPSO, or CALIPSO may be used 👑 in those special cases, TCP implementers should see Section 7.3.1 of RFC 5570 and follow the guidance in that document 🔥
A.2. Sequence Number Validation 🌸
so there are cases where the TCP sequence number validation rules can stop ACK fields from getting processed 😭 that can lead to connection issues, as described in [64] 🦋 that doc covers problems in conditions of simultaneous open, self-connects, simultaneous close, and simultaneous window probes, which is a lot of chaos bestie 💀 it also describes possible changes to the TCP specification that would fix this by expanding the acceptable sequence numbers 🌈
honestly these conditions rarely happen on the Internet 🫠 common operating systems use different alternative mitigations, and the standard hasn't been updated yet to pick one of them 🙃 but implementers should still consider the problems described in [64] 💖
A.3. Nagle Modification 🍓
in common operating systems, both the Nagle algorithm and delayed acknowledgments are implemented and turned on by default ✨ TCP is used by tons of apps with a request-response style of communication, and the combo of Nagle plus delayed acknowledgments can tank application performance 😩 a modification to the Nagle algorithm is described in [68] that makes things better for these apps 🫶
this modification is already in some common operating systems and doesn't mess with TCP interoperability 💅 plus, lots of apps just turn Nagle off, since that's generally supported through a socket option 🙈 the TCP standard hasn't been updated to include this Nagle modification, but implementers may find it beneficial to consider 🌟
A.4. Low Watermark Settings 💧
some operating system kernel TCP implementations have socket options that let you set how many bytes sit in the buffer before the socket layer passes sent data to TCP (SO_SNDLOWAT), or passes received data up to the application (SO_RCVLOWAT) 🎀
another socket option, TCP_NOTSENT_LOWAT, controls the amount of unsent bytes in the write queue 🧚 this can help a sending TCP application avoid piling up huge amounts of buffered data (and the latency that comes with it) 😤 for example, this might be useful for apps multiplexing data from multiple upper-level streams onto one connection, especially when those streams are a mix of interactive/real-time and bulk data transfer 🔥
Appendix B. TCP Requirement Summary 📋
this section is adapted from RFC 1122 🦋
heads up: there is no requirement related to PLPMTUD in this list, but PLPMTUD is recommended 👀
+=================+=========+======+========+=====+========+======+
| Feature | ReqID | MUST | SHOULD | MAY | SHOULD | MUST |
| | | | | | NOT | NOT |
+=================+=========+======+========+=====+========+======+
| PUSH flag |
+=================+=========+======+========+=====+========+======+
| Aggregate or | MAY-16 | | | X | | |
| queue un-pushed | | | | | | |
| data | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Sender collapse | SHLD-27 | | X | | | |
| successive PSH | | | | | | |
| bits | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| SEND call can | MAY-15 | | | X | | |
| specify PUSH | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * If cannot: | MUST-60 | | | | | X |
| sender | | | | | | |
| buffer | | | | | | |
| indefinitely | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * If cannot: | MUST-61 | X | | | | |
| PSH last | | | | | | |
| segment | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Notify | MAY-17 | | | X | | |
| receiving ALP^1 | | | | | | |
| of PSH | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Send max size | SHLD-28 | | X | | | |
| segment when | | | | | | |
| possible | | | | | | |
+=================+=========+======+========+=====+========+======+
| Window |
+=================+=========+======+========+=====+========+======+
| Treat as | MUST-1 | X | | | | |
| unsigned number | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Handle as | REC-1 | | X | | | |
| 32-bit number | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Shrink window | SHLD-14 | | | | X | |
| from right | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * Send new | SHLD-15 | | | | X | |
| data when | | | | | | |
| window | | | | | | |
| shrinks | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * Retransmit | SHLD-16 | | X | | | |
| old unacked | | | | | | |
| data within | | | | | | |
| window | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * Time out | SHLD-17 | | | | X | |
| conn for | | | | | | |
| data past | | | | | | |
| right edge | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Robust against | MUST-34 | X | | | | |
| shrinking | | | | | | |
| window | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Receiver's | MAY-8 | | | X | | |
| window closed | | | | | | |
| indefinitely | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Use standard | MUST-35 | X | | | | |
| probing logic | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Sender probe | MUST-36 | X | | | | |
| zero window | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * First probe | SHLD-29 | | X | | | |
| after RTO | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * Exponential | SHLD-30 | | X | | | |
| backoff | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Allow window | MUST-37 | X | | | | |
| stay zero | | | | | | |
| indefinitely | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Retransmit old | MAY-7 | | | X | | |
| data beyond | | | | | | |
| SND.UNA+SND.WND | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Process RST and | MUST-66 | X | | | | |
| URG even with | | | | | | |
| zero window | | | | | | |
+=================+=========+======+========+=====+========+======+
| Urgent Data |
+=================+=========+======+========+=====+========+======+
| Include support | MUST-30 | X | | | | |
| for urgent | | | | | | |
| pointer | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Pointer | MUST-62 | X | | | | |
| indicates first | | | | | | |
| non-urgent | | | | | | |
| octet | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Arbitrary | MUST-31 | X | | | | |
| length urgent | | | | | | |
| data sequence | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Inform ALP^1 | MUST-32 | X | | | | |
| asynchronously | | | | | | |
| of urgent data | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| ALP^1 can learn | MUST-33 | X | | | | |
| if/how much | | | | | | |
| urgent data Q'd | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| ALP employ the | SHLD-13 | | | | X | |
| urgent | | | | | | |
| mechanism | | | | | | |
+=================+=========+======+========+=====+========+======+
| TCP Options |
+=================+=========+======+========+=====+========+======+
| Support the | MUST-4 | X | | | | |
| mandatory | | | | | | |
| option set | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Receive TCP | MUST-5 | X | | | | |
| Option in any | | | | | | |
| segment | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Ignore | MUST-6 | X | | | | |
| unsupported | | | | | | |
| options | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Include length | MUST-68 | X | | | | |
| for all options | | | | | | |
| except EOL+NOP | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Cope with | MUST-7 | X | | | | |
| illegal option | | | | | | |
| length | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Process options | MUST-64 | X | | | | |
| regardless of | | | | | | |
| word alignment | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Implement | MUST-14 | X | | | | |
| sending & | | | | | | |
| receiving MSS | | | | | | |
| Option | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| IPv4 Send MSS | SHLD-5 | | X | | | |
| Option unless | | | | | | |
| 536 | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| IPv6 Send MSS | SHLD-5 | | X | | | |
| Option unless | | | | | | |
| 1220 | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Send MSS Option | MAY-3 | | | X | | |
| always | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| IPv4 Send-MSS | MUST-15 | X | | | | |
| default is 536 | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| IPv6 Send-MSS | MUST-15 | X | | | | |
| default is 1220 | | | | | | |okay the table keeps going bestie 🎀 we've got the PUSH flag, window, urgent data and the whole TCP Options crew so far, and there's more of this checklist coming next 💅✨
Okay bestie, the checklist keeps going, and it's still giving organized 🎀 Here's the next chunk of the table, sorted by topic like a color-coded planner ✨
+-----------------+---------+------+--------+-----+--------+------+
| Calculate | MUST-16 | X | | | | |
| effective send | | | | | | |
| seg size | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| MSS accounts | SHLD-6 | | X | | | |
| for varying MTU | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| MSS not sent on | MUST-65 | | | | | X |
| non-SYN | | | | | | |
| segments | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| MSS value based | MUST-67 | X | | | | |
| on MMS_R | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Pad with zero | MUST-69 | X | | | | |
+=================+=========+======+========+=====+========+======+
| TCP Checksums |
+=================+=========+======+========+=====+========+======+
| Sender compute | MUST-2 | X | | | | |
| checksum | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Receiver check | MUST-3 | X | | | | |
| checksum | | | | | | |
+=================+=========+======+========+=====+========+======+
| ISN Selection |
+=================+=========+======+========+=====+========+======+
| Include a | MUST-8 | X | | | | |
| clock-driven | | | | | | |
| ISN generator | | | | | | |
| component | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Secure ISN | SHLD-1 | | X | | | |
| generator with | | | | | | |
| a PRF component | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| PRF computable | MUST-9 | | | | | X |
| from outside | | | | | | |
| the host | | | | | | |
+=================+=========+======+========+=====+========+======+
| Opening Connections |
+=================+=========+======+========+=====+========+======+
| Support | MUST-10 | X | | | | |
| simultaneous | | | | | | |
| open attempts | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| SYN-RECEIVED | MUST-11 | X | | | | |
| remembers last | | | | | | |
| state | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Passive OPEN | MUST-41 | | | | | X |
| call interfere | | | | | | |
| with others | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Function: | MUST-42 | X | | | | |
| simultaneously | | | | | | |
| LISTENs for | | | | | | |
| same port | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Ask IP for src | MUST-44 | X | | | | |
| address for SYN | | | | | | |
| if necessary | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * Otherwise, | MUST-45 | X | | | | |
| use local | | | | | | |
| addr of | | | | | | |
| connection | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| OPEN to | MUST-46 | | | | | X |
| broadcast/ | | | | | | |
| multicast IP | | | | | | |
| address | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Silently | MUST-57 | X | | | | |
| discard seg to | | | | | | |
| bcast/mcast | | | | | | |
| addr | | | | | | |
+=================+=========+======+========+=====+========+======+
| Closing Connections |
+=================+=========+======+========+=====+========+======+
| RST can contain | SHLD-2 | | X | | | |
| data | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Inform | MUST-12 | X | | | | |
| application of | | | | | | |
| aborted conn | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Half-duplex | MAY-1 | | | X | | |
| close | | | | | | |
| connections | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * Send RST to | SHLD-3 | | X | | | |
| indicate | | | | | | |
| data lost | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| In TIME-WAIT | MUST-13 | X | | | | |
| state for 2MSL | | | | | | |
| seconds | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * Accept SYN | MAY-2 | | | X | | |
| from TIME- | | | | | | |
| WAIT state | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * Use | SHLD-4 | | X | | | |
| Timestamps | | | | | | |
| to reduce | | | | | | |
| TIME-WAIT | | | | | | |
+=================+=========+======+========+=====+========+======+
| Retransmissions |
+=================+=========+======+========+=====+========+======+
| Implement | MUST-19 | X | | | | |
| exponential | | | | | | |
| backoff, slow | | | | | | |
| start, and | | | | | | |
| congestion | | | | | | |
| avoidance | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Retransmit with | MAY-4 | | | X | | |
| same IP | | | | | | |
| identity | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Karn's | MUST-18 | X | | | | |
| algorithm | | | | | | |
+=================+=========+======+========+=====+========+======+
| Generating ACKs |
+=================+=========+======+========+=====+========+======+
| Aggregate | MUST-58 | X | | | | |
| whenever | | | | | | |
| possible | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Queue out-of- | SHLD-31 | | X | | | |
| order segments | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Process all Q'd | MUST-59 | X | | | | |
| before send ACK | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Send ACK for | MAY-13 | | | X | | |
| out-of-order | | | | | | |
| segment | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Delayed ACKs | SHLD-18 | | X | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * Delay < 0.5 | MUST-40 | X | | | | |
| seconds | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * Every 2nd | SHLD-19 | | X | | | |
| full-sized | | | | | | |
| segment or | | | | | | |
| 2*RMSS ACK'd | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Receiver SWS- | MUST-39 | X | | | | |
| Avoidance | | | | | | |
| Algorithm | | | | | | |
+=================+=========+======+========+=====+========+======+
| Sending Data |
+=================+=========+======+========+=====+========+======+
| Configurable | MUST-49 | X | | | | |
| TTL | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Sender SWS- | MUST-38 | X | | | | |
| Avoidance | | | | | | |
| Algorithm | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Nagle algorithm | SHLD-7 | | X | | | |
+-----------------+---------+------+--------+-----+--------+------+
| * Application | MUST-17 | X | | | | || can disable | ||||||
|---|---|---|---|---|---|---|
| Nagle | ||||||
| algorithm |
+=================+=========+======+========+=====+========+======+
| Connection Failures |
+=================+=========+======+========+=====+========+======+
| Negative advice | MUST-20 | X | ||||
|---|---|---|---|---|---|---|
| to IP on R1 | ||||||
| retransmissions |
+-----------------+---------+------+--------+-----+--------+------+
| Close | MUST-20 | X | ||||
|---|---|---|---|---|---|---|
| connection on | ||||||
| R2 | ||||||
| retransmissions |
+-----------------+---------+------+--------+-----+--------+------+
| ALP^1 can set | MUST-21 | X | ||||
|---|---|---|---|---|---|---|
| R2 |
+-----------------+---------+------+--------+-----+--------+------+
| Inform ALP of | SHLD-9 | X | ||||
|---|---|---|---|---|---|---|
| R1<=retxs<R2 |
+-----------------+---------+------+--------+-----+--------+------+
| Recommended | SHLD-10 | X | ||||
|---|---|---|---|---|---|---|
| value for R1 |
+-----------------+---------+------+--------+-----+--------+------+
| Recommended | SHLD-11 | X | ||||
|---|---|---|---|---|---|---|
| value for R2 |
+-----------------+---------+------+--------+-----+--------+------+
| Same mechanism | MUST-22 | X | ||||
|---|---|---|---|---|---|---|
| for SYNs |
+-----------------+---------+------+--------+-----+--------+------+
| * R2 at least | MUST-23 | X | ||||
|---|---|---|---|---|---|---|
| 3 minutes | ||||||
| for SYN |
+=================+=========+======+========+=====+========+======+
| Send Keep-alive Packets |
+=================+=========+======+========+=====+========+======+
| Send Keep-alive | MAY-5 | X | ||||
|---|---|---|---|---|---|---|
| Packets: |
+-----------------+---------+------+--------+-----+--------+------+
| * Application | MUST-24 | X | ||||
|---|---|---|---|---|---|---|
| can request |
+-----------------+---------+------+--------+-----+--------+------+
| * Default is | MUST-25 | X | ||||
|---|---|---|---|---|---|---|
| "off" |
+-----------------+---------+------+--------+-----+--------+------+
| * Only send if | MUST-26 | X | ||||
|---|---|---|---|---|---|---|
| idle for | ||||||
| interval |
+-----------------+---------+------+--------+-----+--------+------+
| * Interval | MUST-27 | X | ||||
|---|---|---|---|---|---|---|
| configurable |
+-----------------+---------+------+--------+-----+--------+------+
| * Default at | MUST-28 | X | ||||
|---|---|---|---|---|---|---|
| least 2 hrs. |
+-----------------+---------+------+--------+-----+--------+------+
| * Tolerant of | MUST-29 | X | ||||
|---|---|---|---|---|---|---|
| lost ACKs |
+-----------------+---------+------+--------+-----+--------+------+
| * Send with no | SHLD-12 | X | ||||
|---|---|---|---|---|---|---|
| data |
+-----------------+---------+------+--------+-----+--------+------+
| * Configurable | MAY-6 | X | ||||
|---|---|---|---|---|---|---|
| to send | ||||||
| garbage | ||||||
| octet |
+=================+=========+======+========+=====+========+======+
| IP Options |
+=================+=========+======+========+=====+========+======+
| Ignore options | MUST-50 | X | ||||
|---|---|---|---|---|---|---|
| TCP doesn't | ||||||
| understand |
+-----------------+---------+------+--------+-----+--------+------+
| Timestamp | MAY-10 | X | ||||
|---|---|---|---|---|---|---|
| support |
+-----------------+---------+------+--------+-----+--------+------+
| Record Route | MAY-11 | X | ||||
|---|---|---|---|---|---|---|
| support |
+-----------------+---------+------+--------+-----+--------+------+
| Source Route: |
+-----------------+---------+------+--------+-----+--------+------+
| * ALP^1 can | MUST-51 | X | ||||
|---|---|---|---|---|---|---|
| specify |
+-----------------+---------+------+--------+-----+--------+------+
| * Overrides | MUST-52 | X | ||||
|---|---|---|---|---|---|---|
| src route | ||||||
| in | ||||||
| datagram |
+-----------------+---------+------+--------+-----+--------+------+
| * Build return | MUST-53 | X | ||||
|---|---|---|---|---|---|---|
| route from | ||||||
| src route |
+-----------------+---------+------+--------+-----+--------+------+
| * Later src | SHLD-24 | X | ||||
|---|---|---|---|---|---|---|
| route | ||||||
| overrides |
+=================+=========+======+========+=====+========+======+
| Receiving ICMP Messages from IP |
+=================+=========+======+========+=====+========+======+
| Receiving ICMP | MUST-54 | X | ||||
|---|---|---|---|---|---|---|
| messages from | ||||||
| IP |
+-----------------+---------+------+--------+-----+--------+------+
| * Dest Unreach | SHLD-25 | X | ||||
|---|---|---|---|---|---|---|
| (0,1,5) => | ||||||
| inform ALP |
+-----------------+---------+------+--------+-----+--------+------+
| * Abort on | MUST-56 | X | ||||
|---|---|---|---|---|---|---|
| Dest Unreach | ||||||
| (0,1,5) |
+-----------------+---------+------+--------+-----+--------+------+
| * Dest Unreach | SHLD-26 | X | ||||
|---|---|---|---|---|---|---|
| (2-4) => | ||||||
| abort conn |
+-----------------+---------+------+--------+-----+--------+------+
| * Source | MUST-55 | X | ||||
|---|---|---|---|---|---|---|
| Quench => | ||||||
| silent | ||||||
| discard |
+-----------------+---------+------+--------+-----+--------+------+
| * Abort on | MUST-56 | X | ||||
|---|---|---|---|---|---|---|
| Time | ||||||
| Exceeded |
+-----------------+---------+------+--------+-----+--------+------+
| * Abort on | MUST-56 | X | ||||
|---|---|---|---|---|---|---|
| Param | ||||||
| Problem |
+=================+=========+======+========+=====+========+======+
| Address Validation |
+=================+=========+======+========+=====+========+======+
| Reject OPEN | MUST-46 | X | ||||
|---|---|---|---|---|---|---|
| call to invalid | ||||||
| IP address |
+-----------------+---------+------+--------+-----+--------+------+
| Reject SYN from | MUST-63 | X | ||||
|---|---|---|---|---|---|---|
| invalid IP | ||||||
| address |
+-----------------+---------+------+--------+-----+--------+------+
| Silently | MUST-57 | X | ||||
|---|---|---|---|---|---|---|
| discard SYN to | ||||||
| bcast/mcast | ||||||
| addr |
+=================+=========+======+========+=====+========+======+
| TCP/ALP Interface Services |
+=================+=========+======+========+=====+========+======+
| Error Report | MUST-47 | X | ||||
|---|---|---|---|---|---|---|
| mechanism |
+-----------------+---------+------+--------+-----+--------+------+
| ALP can disable | SHLD-20 | X | ||||
|---|---|---|---|---|---|---|
| Error Report | ||||||
| Routine |
+-----------------+---------+------+--------+-----+--------+------+
| ALP can specify | MUST-48 | X | ||||
|---|---|---|---|---|---|---|
| Diffserv field | ||||||
| for sending |
+-----------------+---------+------+--------+-----+--------+------+
| * Passed | SHLD-22 | X | ||||
|---|---|---|---|---|---|---|
| unchanged to | ||||||
| IP |
+-----------------+---------+------+--------+-----+--------+------+
| ALP can change | SHLD-21 | X | ||||
|---|---|---|---|---|---|---|
| Diffserv field | ||||||
| during | ||||||
| connection |
+-----------------+---------+------+--------+-----+--------+------+
| ALP generally | SHLD-23 | X | ||||
|---|---|---|---|---|---|---|
| changing |
| Diffserv during | | | | | | |
| conn. | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| Pass received | MAY-9 | | | X | | |
| Diffserv field | | | | | | |
| up to ALP | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
| FLUSH call | MAY-14 | | | X | | |
+-----------------+---------+------+--------+-----+--------+------+
| Optional local | MUST-43 | X | | | | |
| IP addr param | | | | | | |
| in OPEN | | | | | | |
+=================+=========+======+========+=====+========+======+
| RFC 5961 Support |
+=================+=========+======+========+=====+========+======+
| Implement data | MAY-12 | | | X | | |
| injection | | | | | | |
| protection | | | | | | |
+=================+=========+======+========+=====+========+======+
| Explicit Congestion Notification |
+=================+=========+======+========+=====+========+======+
| Support ECN | SHLD-8 | | X | | | |
+=================+=========+======+========+=====+========+======+
| Alternative Congestion Control |
+=================+=========+======+========+=====+========+======+
| Implement | MAY-18 | | | X | | |
| alternative | | | | | | |
| conformant | | | | | | |
| algorithm(s) | | | | | | |
+-----------------+---------+------+--------+-----+--------+------+
Table 8: TCP Requirements SummaryFOOTNOTES: (1) "ALP" means Application-Layer Program. 🧚
Acknowledgments 🎀
okay so this document is basically a glow-up of RFC 793, and that one was edited by Jon Postel 💅 honestly his work was so good that TCP lasted three whole decades before anyone felt the need to revise it. iconic behavior, no cap. 👑
Andre Oppermann was a contributor and helped edit the first revision of this document. 🫶
we also have to thank the IETF TCPM working group chairs who helped out while this was being worked on:
- Michael Scharf
- Yoshifumi Nishida
- Pasi Sarolahti
- Michael Tüxen
during the discussions on the TCPM mailing list, in working group meetings, and through area reviews, a bunch of people sent in helpful comments, critiques, and reviews. they're listed alphabetically by last name, and the list is giving main character energy 🌸: Praveen Balasubramanian, David Borman, Mohamed Boucadair, Bob Briscoe, Neal Cardwell, Yuchung Cheng, Martin Duke, Francis Dupont, Ted Faber, Gorry Fairhurst, Fernando Gont, Rodney Grimes, Yi Huang, Rahul Jadhav, Markku Kojo, Mike Kosek, Juhamatti Kuusisaari, Kevin Lahey, Kevin Mason, Matt Mathis, Stephen McQuistin, Jonathan Morton, Matt Olson, Tommy Pauly, Tom Petch, Hagen Paul Pfeifer, Kyle Rose, Anthony Sabatini, Michael Scharf, Greg Skinner, Joe Touch, Michael Tüxen, Reji Varghese, Bernie Volz, Tim Wicinski, Lloyd Wood, and Alex Zimmermann. 💖
Joe Touch gave extra help clarifying the description of segment size parameters and the PMTUD/PLPMTUD recommendations. ✨ Markku Kojo helped put together the text in the section on TCP Congestion Control. 🦋
this document also includes content from errata that were reported by (listed chronologically): Yin Shuming, Bob Braden, Morris M. Keesan, Pei-chun Cheng, Constantin Hagemeier, Vishwas Manral, Mykyta Yevstifeyev, EungJun Yi, Botong Huang, Charles Deng, Merlin Buge. errata heroes unite 🌈🙈
Author's Address 💌
Wesley M. Eddy (editor)
MTI Systems
United States of America
Email: wes@mti-systems.comWesley M. Eddy was the editor, and that's the end of the doc. periodt. 💜😭
📦 where this page comes from
- ietf: rfc9293 2022-08
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms