RFC 1771
A Border Gateway Protocol 4 (BGP-4)
A Border Gateway Protocol 4 (BGP-4) 🦋
(part 1 of 12 🧩)
bestie, this is the RFC that helps the big networks of the Internet talk to each other 🌐 and it's giving serious main character energy. Here's the tea on the title block: it's RFC 1771, from the Network Working Group. The editors are Y. Rekhter (T.J. Watson Research Center, IBM Corp.) and T. Li (cisco Systems). It's a Standards Track doc from March 1995, and it obsoletes RFC 1654, so this one is the upgrade 💅
Status of this Memo
okay so this is an Internet standards track protocol for the Internet community ✨ It also asks for discussion and suggestions for improvements 💖 If you want to know where this protocol stands in terms of standardization, check the current edition of the "Internet Official Protocol Standards" (STD 1) 📚 And distribution of this memo is unlimited, so share it with your whole group chat 🫶
Abstract
this doc, along with its companion doc "Application of the Border Gateway Protocol in the Internet", defines an inter-autonomous system routing protocol for the Internet 🌸
1. Acknowledgements
first, the receipts 🧾 This doc was originally published as RFC 1267 in October 1991, and it was written by Kirk Lougheed (cisco Systems) and Yakov Rekhter (IBM) 💜
we'd like to thank Guy Almes (ANS), Len Bosack (cisco Systems), and Jeffrey C. Honig (Cornell University) for their contributions to the earlier version ✨
we'd also like to explicitly thank Bob Braden (ISI) for reviewing the earlier version, and for his constructive and valuable comments 🙈
then there's Bob Hinden, the Director for Routing of the Internet Engineering Steering Group, plus the team of reviewers he gathered to look over the previous version (BGP-2) 👑 That team was Deborah Estrin, Milo Medin, John Moy, Radia Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya. They were a whole vibe of toughness, professionalism, and courtesy all at once 🔥
this updated version is the product of the IETF IDR Working Group, with Yakov Rekhter and Tony Li as editors 🎀 Some sections borrowed heavily from IDRP [7], which is the OSI counterpart of BGP. Credit for that goes to the ANSI X3S3.3 group, chaired by Lyman Chapin (BBN), and to Charles Kunzinger (IBM Corp.), who was the IDRP editor within that group 🧚 We'd also like to thank Mike Craren (Proteon, Inc.), Dimitry Haskin (Bay Networks, Inc.), John Krawczyk (Bay Networks, Inc.), and Paul Traina (cisco Systems) for their insightful comments 🦋
we'd specially like to acknowledge the numerous contributions from Dennis Ferguson (MCI) 🌈
and the work of Yakov Rekhter was supported in part by the National Science Foundation under Grant Number NCR-9219216 🍓
2. Introduction
okay so BGP is an inter-Autonomous System routing protocol 🌐 It's built on what people learned from EGP, which is defined in RFC 904 [1], and from EGP usage in the NSFNET Backbone, described in RFC 1092 [2] and RFC 1093 [3] 💅
the main job of a BGP speaking system is to swap network reachability information with other BGP systems ✨ That info includes the list of Autonomous Systems (ASs) that the reachability information goes through. That list is enough to build a graph of AS connectivity, which is how routing loops can get pruned and some policy decisions at the AS level can be enforced 🫶
BGP-4 brings a fresh set of mechanisms for classless interdomain routing 💖 These include support for advertising an IP prefix, and they get rid of the concept of network "class" within BGP entirely 🙈 BGP-4 also adds mechanisms that let routes be aggregated, including aggregation of AS paths. These changes support the proposed supernetting scheme [8, 9] 🔥
now, if you want to figure out which policies you can enforce with BGP, you have to focus on one rule: a BGP speaker only advertises to its peers (the other BGP speakers it talks to) in neighboring ASs the routes that it itself uses 💅 That rule matches the "hop-by-hop" routing paradigm that's used pretty much everywhere on the current Internet 🌸 Some policies just can't be supported by "hop-by-hop", so they need techniques like source routing to enforce them ✨ For example, BGP doesn't let one AS send traffic to a neighboring AS while intending that traffic take a different route from the one taken by traffic that started in that neighboring AS 💀 BGP can support any policy that fits the "hop-by-hop" paradigm, though. And since the current Internet only uses "hop-by-hop", and BGP can support any policy that fits it, BGP is highly applicable as an inter-AS routing protocol for today's Internet 👑
a more complete rundown of which policies can and can't be enforced with BGP is outside this document's scope, but go check the companion document about BGP usage [5] 🦋
BGP runs over a reliable transport protocol 🌈 That means it doesn't have to implement explicit update fragmentation, retransmission, acknowledgement, or sequencing, which is so relieving 😭 Any authentication scheme that the transport protocol uses can be used in addition to BGP's own authentication mechanisms 🔐 The error notification mechanism in BGP assumes the transport protocol supports a "graceful" close. In other words, all outstanding data gets delivered before the connection is closed 🫶
BGP uses TCP [4] as its transport protocol 💜 TCP meets BGP's transport requirements and shows up in basically every commercial router and host 🧚 In the descriptions that follow, the phrase "transport protocol connection" can be read as a TCP connection. BGP uses TCP port 179 to set up its connections 🎀
this document uses the term `Autonomous System' (AS) all the way through 🌸 The classic definition of an Autonomous System is a set of routers under a single technical administration. They use an interior gateway protocol and common metrics to route packets inside the AS, and an exterior gateway protocol to route packets to other ASs 📚 Since that classic definition came about, it's become really common for a single AS to run several interior gateway protocols, and sometimes several sets of metrics inside the AS too 🌈 So the term Autonomous System is there to stress one thing: even when multiple IGPs and metrics are in use, the administration of an AS still looks to other ASs like it has a single coherent interior routing plan, and it presents a consistent picture of which destinations are reachable through it ✨
the planned use of BGP in the Internet environment, including topology, how BGP and IGPs interact, and how routing policy rules get enforced, is covered in a companion document [5] 💖 This document is the first in a series planned to explore different aspects of BGP application 🦋 And if you have comments, send them to the BGP mailing list at bgp@ans.net 💌
3. Summary of Operation
so two systems form a transport protocol connection with each other 🫶 They swap messages to open and confirm the connection parameters 💅 The first data to flow is the entire BGP routing table 🌸 After that, incremental updates get sent whenever the routing tables change ✨ BGP doesn't need a periodic refresh of the whole BGP routing table, which means a BGP speaker must keep the current version of the entire BGP routing tables of all its peers for as long as the connection lasts 🧠 KeepAlive messages get sent periodically to make sure the connection is still alive 💓 Notification messages get sent when there are errors or special conditions 🚨 And if a connection runs into an error condition, a notification message is sent and then the connection gets closed 💀
the hosts running the Border Gateway Protocol don't have to be routers 🌷 A non-routing host could trade routing information with routers via EGP, or even via an interior routing protocol. That non-routing host could then use BGP to exchange routing information with a border router in another Autonomous System 🧚 What all this implies, and what it could be used for, is left for further study 🔭
if a particular AS has several BGP speakers and it's providing transit service for other ASs, then you have to be careful to keep a consistent view of routing within the AS 👑 The interior routing protocol gives a consistent view of the AS's interior routes ✨ A consistent view of the routes outside the AS can come from having all the BGP speakers within the AS keep direct BGP connections with each other 🫶 Using a common set of policies, the BGP speakers agree on which border routers will act as exit/entry points for particular destinations outside the AS 🦋 That info then gets passed to the AS's internal routers, possibly through the interior routing protocol 🌈 Be careful to make sure the interior routers have all been updated with transit information before the BGP speakers tell other ASs that transit service is being provided 💅
connections between BGP speakers in different ASs are called "external" links 💜 BGP connections between BGP speakers in the same AS are called "internal" links 🌸 In the same way, a peer in a different AS is an external peer, and a peer in the same AS may be described as an internal peer 🎀
3.1 Routes: Advertisement and Storage
okay this part is the heart of it 💖 For this protocol, a route is a unit of information that pairs a destination with the attributes of a path to that destination ✨
- Routes get advertised between a pair of BGP speakers in UPDATE messages 🌐 The destination is the systems whose IP addresses are reported in the Network Layer Reachability Information (NLRI) field, and the path is the info reported in the path attributes fields of that same UPDATE message 💌
- Routes are stored in the Routing Information Bases (RIBs), which are the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out 🧠 Routes that will be advertised to other BGP speakers must be present in the Adj-RIBs-Out. Routes that the local BGP speaker will use must be present in the Loc-RIB, and the next hop for each of those routes must be present in the local BGP speaker's forwarding information base. And routes received from other BGP speakers are present in the Adj-RIBs-In 🔥
if a BGP speaker decides to advertise a route, it may add to or modify the route's path attributes before it advertises it to a peer 🦋
BGP gives speakers ways to tell their peer that a previously advertised route is no longer available for use 🫶 There are three methods a given BGP speaker can use to say a route has been withdrawn from service 👇
a) The IP prefix that expresses the destinations for a previously advertised route can be advertised in the WITHDRAWN ROUTES field of the UPDATE message. That marks the associated route as no longer available for use 💀
b) A replacement route with the same Network Layer Reachability Information can be advertised, or 🌸
c) The BGP speaker to BGP speaker connection can be closed. That implicitly removes from service all the routes the pair of speakers had advertised to each other 😭
that's the end of part 1, bestie, so keep your eyes peeled for part 2 🌈💖
3.2 Routing Information Bases 🗂️
okay so the Routing Information Base (RIB) inside a BGP speaker is made up of three distinct parts 💅 let's get into it:
- a) Adj-RIBs-In: these store the routing info learned from inbound UPDATE messages 📥. Their contents are basically the routes that are available as an input to the Decision Process 🧠
- b) Loc-RIB: this one holds the local routing info that the BGP speaker has selected by running its local policies over whatever's in its Adj-RIBs-In 🎯 very main character energy
- c) Adj-RIBs-Out: these store the info the local BGP speaker has picked for advertisement to its peers 📤. That info rides along in the local speaker's UPDATE messages and gets advertised to those peers 🦋
ngl here's the quick recap 💖 the Adj-RIBs-In hold the unprocessed routing info that peers advertised to the local BGP speaker 📬, the Loc-RIB holds the routes the local speaker's Decision Process has selected 👑, and the Adj-RIBs-Out organize the routes for advertisement to specific peers using the local speaker's UPDATE messages 🌸
Although the conceptual model separates Adj-RIBs-In, Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an implementation must maintain three separate copies of the routing information 🙈 The choice of implementation (for example, 3 copies of the information vs 1 copy with pointers) is not constrained by the protocol. so basically, you do you 💅
4. Message Formats ✨
this section is where we break down the message formats BGP uses 🧚
Messages are sent over a reliable transport protocol connection 🫶 A message is processed only after it is entirely received. The maximum message size is 4096 octets. All implementations are required to support this maximum message size 🔥 The smallest message that may be sent consists of a BGP header without a data portion, or 19 octets.
4.1 Message Header Format 🎀
every message has a fixed-size header 💖 There may or may not be a data portion following the header, depending on the message type. Here's how the fields are laid out:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ +
| Marker |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Marker: this 16-octet field holds a value that the receiver of the message can predict 🔮 If the Type of the message is OPEN, or if the OPEN message carries no Authentication Information (as an Optional Parameter), then the Marker must be all ones. Otherwise, the value of the marker can be predicted by some computation specified as part of the authentication mechanism (which is specified as part of the Authentication Information) used. The Marker can be used to detect loss of synchronization between a pair of BGP peers, and to authenticate incoming BGP messages 🛡️
Length: this 2-octet unsigned integer is the total length of the message, including the header, in octets 📏 Thus, e.g., it lets you locate the (Marker field of the) next message in the transport-level stream 🧭 The value of the Length field must always be at least 19 and no greater than 4096, and may be further constrained, depending on the message type. No "padding" of extra data after the message is allowed, so the Length field must have the smallest value required given the rest of the message 🙅♀️
Type: this 1-octet unsigned integer is the type code of the message 🏷️ The following type codes are defined:
1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE4.2 OPEN Message Format 👑
After a transport protocol connection is established, the first message each side sends is an OPEN message 🤝 If the OPEN message is acceptable, a KEEPALIVE message confirming the OPEN gets sent back. Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION messages may be exchanged 💜
In addition to the fixed-size BGP header, the OPEN message contains the following fields:
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
+-+-+-+-+-+-+-+-+
| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Autonomous System |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt Parm Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Optional Parameters |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Version: this 1-octet unsigned integer is the protocol version number of the message. The current BGP version number is 4 🔢
My Autonomous System: this 2-octet unsigned integer is the Autonomous System number of the sender 🌐
Hold Time: this 2-octet unsigned integer is the number of seconds the sender proposes for the value of the Hold Timer ⏳ Upon receipt of an OPEN message, a BGP speaker MUST calculate the value of the Hold Timer by using the smaller of its configured Hold Time and the Hold Time received in the OPEN message. The Hold Time MUST be either zero or at least three seconds. An implementation may reject connections on the basis of the Hold Time 🚪 The calculated value is the maximum number of seconds that may elapse between the receipt of successive KEEPALIVE, and/or UPDATE messages by the sender.
BGP Identifier: this 4-octet unsigned integer is the BGP Identifier of the sender 🪪 A given BGP speaker sets the value of its BGP Identifier to an IP address assigned to that BGP speaker. The value of the BGP Identifier is determined on startup and is the same for every local interface and every BGP peer ✨
Optional Parameters Length: this 1-octet unsigned integer is the total length of the Optional Parameters field in octets. If the value of this field is zero, no Optional Parameters are present 🙃
Optional Parameters: this field may contain a list of optional parameters, where each parameter is encoded as a <Parameter Type, Parameter Length, Parameter Value> triplet 🎁
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
| Parm. Type | Parm. Length | Parameter Value (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...Parameter Type is a one octet field that unambiguously identifies individual parameters 🏷️ Parameter Length is a one octet field that contains the length of the Parameter Value field in octets. Parameter Value is a variable length field that is interpreted according to the value of the Parameter Type field.
This document defines the following Optional Parameters:
- a) Authentication Information (Parameter Type 1) 🔐: this optional parameter may be used to authenticate a BGP peer. The Parameter Value field contains a 1-octet Authentication Code followed by a variable length Authentication Data.
0 1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+
| Auth. Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authentication Data |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Authentication Code: this 1-octet unsigned integer indicates the authentication mechanism being used 🗝️ Whenever an authentication mechanism is specified for use within BGP, three things must be included in the specification:
- the value of the Authentication Code which indicates use of the mechanism,
- the form and meaning of the Authentication Data, and
- the algorithm for computing values of Marker fields.
Note that a separate authentication mechanism may be used in establishing the transport level connection 👀
Authentication Data: the form and meaning of this field is a variable-length field that depends on the Authentication Code 🧩
The minimum length of the OPEN message is 29 octets (including message header) 📐
4.3 UPDATE Message Format 🦋
UPDATE messages are used to transfer routing information between BGP peers 💌 The information in the UPDATE packet can be used to construct a graph describing the relationships of the various Autonomous Systems. By applying rules to be discussed, routing information loops and some other anomalies may be detected and removed from inter-AS routing 🕸️
An UPDATE message is used to advertise a single feasible route to a peer, or to withdraw multiple unfeasible routes from service (see 3.1). An UPDATE message may simultaneously advertise a feasible route and withdraw multiple unfeasible routes from service. The UPDATE message always includes the fixed-size BGP header, and can optionally include the other fields as shown below 🌸
+-----------------------------------------------------+
| Unfeasible Routes Length (2 octets) |
+-----------------------------------------------------+
| Withdrawn Routes (variable) |
+-----------------------------------------------------+
| Total Path Attribute Length (2 octets) |
+-----------------------------------------------------+
| Path Attributes (variable) |
+-----------------------------------------------------+
| Network Layer Reachability Information (variable) |
+-----------------------------------------------------+Unfeasible Routes Length:
Okay so this 2-octets unsigned integer tells you the total length of the Withdrawn Routes field in octets 💖 Its value MUST allow the length of the Network Layer Reachability Information field to be determined, and I'll show you how that works below ✨
A value of 0 means no routes are being withdrawn from service, and the WITHDRAWN ROUTES field is not present in this UPDATE message 🦋
Withdrawn Routes:
This is a variable length field with a list of IP address prefixes for the routes being withdrawn from service 🌸 Each prefix is encoded as a 2-tuple of the form <length, prefix>, and it looks like this:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+Here's what each piece does:
a) Length:
The Length field gives the length in bits of the IP address prefix. A length of zero means a prefix that matches all IP addresses (and the prefix itself is zero octets) 🌈
b) Prefix:
The Prefix field holds the IP address prefixes, followed by enough trailing bits to make the end of the field land on an octet boundary. Note that the value of those trailing bits is irrelevant 🙈
Total Path Attribute Length:
This 2-octets unsigned integer tells you the total length of the Path Attributes field in octets 🍓 Its value MUST allow the length of the Network Layer Reachability field to be determined, as explained below.
A value of 0 means no Network Layer Reachability Information field is present in this UPDATE message 💅
Path Attributes:
Every UPDATE has a variable length sequence of path attributes ✨ Each one is a triple <attribute type, attribute length, attribute value>, and the length is variable.
Attribute Type is a two-octet field made of the Attribute Flags octet followed by the Attribute Type Code octet 💜
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Now the flag bits, which are the juiciest part 👑
- High-order bit (bit 0), the Optional bit: it says whether the attribute is optional (set to 1) or well-known (set to 0).
- Second high-order bit (bit 1), the Transitive bit: for an optional attribute, it says whether that attribute is transitive (set to 1) or non-transitive (set to 0). For well-known attributes, the Transitive bit MUST be set to 1. (See Section 5 for a discussion of transitive attributes.)
- Third high-order bit (bit 2), the Partial bit: for an optional transitive attribute, it says whether the info is partial (set to 1) or complete (set to 0). For well-known attributes and for optional non-transitive attributes, the Partial bit MUST be set to 0.
- Fourth high-order bit (bit 3), the Extended Length bit: it says whether the Attribute Length is one octet (set to 0) or two octets (set to 1). Extended Length MAY be used only if the attribute value is longer than 255 octets.
- Lower-order four bits: these are unused. They MUST be zero (and MUST be ignored when received).
The Attribute Type Code octet holds the Attribute Type Code. The currently defined codes are discussed in Section 5 🦋
If the Extended Length bit is set to 0, the third octet of the Path Attribute holds the length of the attribute data in octets. If the Extended Length bit is set to 1, then the third and fourth octets hold that length instead 🌸
The remaining octets are the attribute value, and they get interpreted based on the Attribute Flags and the Attribute Type Code. The supported Attribute Type Codes, their values and uses are these:
a) ORIGIN (Type Code 1):
ORIGIN is a well-known mandatory attribute that tells you where the path info came from 🧚 The data octet can be one of these values:
Value Meaning
0 IGP - Network Layer Reachability Information
is interior to the originating AS
1 EGP - Network Layer Reachability Information
learned via EGP
2 INCOMPLETE - Network Layer Reachability
Information learned by some other meansIts usage is defined in 5.1.1 💖
b) AS_PATH (Type Code 2):
AS_PATH is a well-known mandatory attribute made up of a sequence of AS path segments. Each segment is a triple <path segment type, path segment length, path segment value> 🛣️
The path segment type is a 1-octet field with these values:
Value Segment Type
1 AS_SET: unordered set of ASs a route in the
UPDATE message has traversed
2 AS_SEQUENCE: ordered set of ASs a route in
the UPDATE message has traversedThe path segment length is a 1-octet field holding the number of ASs in the path segment value field. The path segment value field has one or more AS numbers, and each one is a 2-octets long field 🍓
Usage of this attribute is defined in 5.1.2 ✨
c) NEXT_HOP (Type Code 3):
This is a well-known mandatory attribute that gives the IP address of the border router to use as the next hop to the destinations listed in the Network Layer Reachability field of the UPDATE message 🌈
Its usage is defined in 5.1.3 💅
d) MULTI_EXIT_DISC (Type Code 4):
This is an optional non-transitive attribute holding a four octet non-negative integer. The value of this attribute MAY be used by a BGP speaker's decision process to tell apart multiple exit points to a neighboring autonomous system 🦋
Its usage is defined in 5.1.4 🎀
e) LOCAL_PREF (Type Code 5):
LOCAL_PREF is a well-known discretionary attribute holding a four octet non-negative integer. A BGP speaker uses it to tell other BGP speakers in its own autonomous system how much it prefers an advertised route, and it's basically the originating speaker's degree of preference 👑 Usage of this attribute is described in 5.1.5.
f) ATOMIC_AGGREGATE (Type Code 6)
ATOMIC_AGGREGATE is a well-known discretionary attribute of length 0 🫶 A BGP speaker uses it to tell other BGP speakers that the local system picked a less specific route without picking a more specific route that's included in it. Usage of this attribute is described in 5.1.6.
g) AGGREGATOR (Type Code 7)
AGGREGATOR is an optional transitive attribute of length 6 🧚 It holds the last AS number that formed the aggregate route (encoded as 2 octets), followed by the IP address of the BGP speaker that formed the aggregate route (encoded as 4 octets). Usage of this attribute is described in 5.1.7.
Network Layer Reachability Information:
This variable length field holds a list of IP address prefixes 💜 The length in octets of the Network Layer Reachability Information is not encoded explicitly, but you can calculate it like this:
UPDATE message Length - 23 - Total Path Attributes Length -
Unfeasible Routes LengthWhere UPDATE message Length is the value encoded in the fixed-size BGP header, and Total Path Attribute Length and Unfeasible Routes Length are the values encoded in the variable part of the UPDATE message. The 23 is the combined length of the fixed-size BGP header, the Total Path Attribute Length field and the Unfeasible Routes Length field 🔥
Reachability information is encoded as one or more 2-tuples of the form <length, prefix>, and here's the layout:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+Here's what each piece does:
a) Length:
The Length field gives the length in bits of the IP address prefix. A length of zero means a prefix that matches all IP addresses (and the prefix itself is zero octets) 🌸
b) Prefix:
The Prefix field holds the IP address prefixes, followed by enough trailing bits to make the end of the field land on an octet boundary. Note that the value of the trailing bits is irrelevant 🙈
Length check time! ✨ The minimum length of the UPDATE message is 23 octets: 19 octets for the fixed header, plus 2 octets for the Unfeasible Routes Length, plus 2 octets for the Total Path Attribute Length. In that minimum case, the Unfeasible Routes Length is 0 and the Total Path Attribute Length is 0 💖
An UPDATE message can advertise at most one route, and that route can be described by several path attributes. All path attributes in a given UPDATE message apply to the destinations carried in its Network Layer Reachability Information field 🦋
An UPDATE message can list multiple routes to be withdrawn from service. Each route is identified by its destination (expressed as an IP prefix), which uniquely pins down the route in the context of the BGP speaker - BGP speaker connection it was previously advertised on 🌈
An UPDATE message MAY advertise only routes to be withdrawn from service. In that case it will not include path attributes or Network Layer Reachability Information. Conversely, it MAY advertise only a feasible route, in which case the WITHDRAWN ROUTES field need not be present 💀
4.4 KEEPALIVE Message Format 🦋
okay so BGP doesn't lean on any transport-protocol keep-alive thing to check if peers are still around 🫠 instead, peers just swap KEEPALIVE messages often enough that the Hold Timer never gets the chance to expire 💅 a reasonable max gap between them is one third of the Hold Time interval ✨
KEEPALIVE messages MUST NOT be sent more frequently than one per second, so no spamming 🙅♀️ an implementation MAY tweak how often it sends them based on the Hold Time interval 🎀
if the negotiated Hold Time interval is zero, then periodic KEEPALIVE messages MUST NOT be sent 🙈
a KEEPALIVE is just the message header and nothing else, so it's 19 octets total 🌸
4.5 NOTIFICATION Message Format 🚨
a NOTIFICATION gets sent whenever an error condition is detected 🙀 and the BGP connection gets closed right after, immediately, no chill 💀
the connection is slammed shut the moment the NOTIFICATION goes out
besides the fixed-size BGP header, the NOTIFICATION message has these fields:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error code | Error subcode | Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Error Code 🔢 this 1-octet unsigned integer tells you what type of NOTIFICATION it is. these Error Codes are defined:
Error Code Symbolic Name Reference
1 Message Header Error Section 6.1
2 OPEN Message Error Section 6.2
3 UPDATE Message Error Section 6.3
4 Hold Timer Expired Section 6.5
5 Finite State Machine Error Section 6.6
6 Cease Section 6.7Error Subcode 🔍 another 1-octet unsigned integer, giving more specific info about what went wrong. each Error Code can have one or more Error Subcodes attached to it 💖
if there's no suitable Error Subcode defined, then a zero (Unspecific) value goes in the Error Subcode field 🫶
Message Header Error subcodes:
1 - Connection Not Synchronized.
2 - Bad Message Length.
3 - Bad Message Type.OPEN Message Error subcodes:
1 - Unsupported Version Number.
2 - Bad Peer AS.
3 - Bad BGP Identifier. '
4 - Unsupported Optional Parameter.
5 - Authentication Failure.
6 - Unacceptable Hold Time.UPDATE Message Error subcodes:
1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Attribute.
4 - Attribute Flags Error.
5 - Attribute Length Error.
6 - Invalid ORIGIN Attribute
7 - AS Routing Loop.
8 - Invalid NEXT_HOP Attribute.
9 - Optional Attribute Error.
10 - Invalid Network Field.
11 - Malformed AS_PATH.Data 📦 this variable-length field helps diagnose why the NOTIFICATION happened. what's in it depends on the Error Code and Error Subcode. see Section 6 below for the full tea ☕
also, the length of the Data field can be worked out from the message Length field with this formula:
Message Length = 21 + Data Lengththe minimum length of a NOTIFICATION message is 21 octets, message header included 🧚
5. Path Attributes ✨
this section is all about the path attributes of the UPDATE message 💜
path attributes fall into four categories, bestie:
- Well-known mandatory.
- Well-known discretionary.
- Optional transitive.
- Optional non-transitive.
well-known attributes MUST be recognized by all BGP implementations 👑 some of them are mandatory and have to be in every UPDATE message, and others are discretionary, so they may or may not show up in a given UPDATE message 🌈
all well-known attributes must be passed along to other BGP peers (after proper updating, if necessary) 🦋
on top of the well-known ones, each path can carry one or more optional attributes 🎀 it's not required or expected that every BGP implementation supports every optional attribute, no pressure 🙃 how an unrecognized optional attribute gets handled depends on the Transitive bit in the attribute flags octet 💅 paths with unrecognized transitive optional attributes should be accepted. if you accept a path with an unrecognized transitive optional attribute and pass it along, that attribute has to go along with the path to other BGP peers with the Partial bit in the Attribute Flags octet set to 1 🫶 if you accept a path with a recognized transitive optional attribute and pass it along, and some previous AS already set the Partial bit to 1, the current AS does NOT set it back to 0 🙅♀️ unrecognized non-transitive optional attributes must be quietly ignored and NOT passed along to other BGP peers 🤫
new transitive optional attributes can get attached to the path by the originator, or by any other AS in the path ✨ if the originator didn't attach them, the Partial bit in the Attribute Flags octet is set to 1 🌸 the rules for attaching new non-transitive optional attributes depend on the specific attribute, and the documentation for each new non-transitive optional attribute is expected to spell those rules out (the description of the MULTI_EXIT_DISC attribute gives an example) 📖 all optional attributes, transitive and non-transitive, may be updated by ASs in the path (if appropriate) 🔥
the sender of an UPDATE message should order path attributes in ascending order of attribute type 🧁 the receiver of an UPDATE message must be prepared to handle path attributes that are out of order 🫠
the same attribute cannot appear more than once within the Path Attributes field of a particular UPDATE message 🚫
5.1 Path Attribute Usage 💅
the usage of each BGP path attribute is described in the clauses below 🍓
5.1.1 ORIGIN
ORIGIN is a well-known mandatory attribute 👑 it shall be generated by the autonomous system that originates the associated routing information 🌱 it shall also be included in the UPDATE messages of all BGP speakers that choose to pass this info along to other BGP speakers 🦋
5.1.2 AS_PATH
AS_PATH is a well-known mandatory attribute 🛣️ it shows the autonomous systems that the routing information in this UPDATE message has travelled through ✨ the components of this list can be AS_SETs or AS_SEQUENCEs 🧩
when a BGP speaker propagates a route it learned from another BGP speaker's UPDATE message, it shall modify the route's AS_PATH attribute based on where the BGP speaker it's sending the route to is located 💖
a) when a given BGP speaker advertises the route to another BGP speaker located in its own autonomous system, the advertising speaker shall not modify the AS_PATH attribute associated with the route 🫶
b) when a given BGP speaker advertises the route to a BGP speaker located in a neighboring autonomous system, the advertising speaker shall update the AS_PATH attribute like this:
- if the first path segment of the AS_PATH is of type AS_SEQUENCE, the local system shall prepend its own AS number as the last element of the sequence (put it in the leftmost position) 🌟
- if the first path segment of the AS_PATH is of type AS_SET, the local system shall prepend a new path segment of type AS_SEQUENCE to the AS_PATH, including its own AS number in that segment 🎀
when a BGP speaker originates a route then:
a) the originating speaker shall include its own AS number in the AS_PATH attribute of all UPDATE messages sent to BGP speakers located in neighboring autonomous systems. (in this case, the AS number of the originating speaker's autonomous system will be the only entry in the AS_PATH attribute) 🌈
b) the originating speaker shall include an empty AS_PATH attribute in all UPDATE messages sent to BGP speakers located in its own autonomous system. (an empty AS_PATH attribute is one whose length field contains the value zero) 🧚
5.1.3 NEXT_HOP 🏠
okay so NEXT_HOP is the attribute that tells everyone which border router's IP address to use as the next hop to the destinations listed in the UPDATE message 💅. If a border router is in the same AS as its peer, then that peer is an internal border router 🎀. If not, it's an external border router 🦋.
A BGP speaker can advertise any internal border router as the next hop, provided that the interface linked to that router's IP address (the one given in NEXT_HOP) shares a common subnet with both the local and remote BGP speakers 🧚. A BGP speaker can also advertise any external border router as the next hop, as long as that IP address was learned from one of the speaker's peers and the interface tied to it (again, as given in NEXT_HOP) shares a common subnet with the local and remote BGP speakers 🌸. And a BGP speaker needs to be able to support disabling advertisement of external border routers 🙈.
A BGP speaker must never advertise an address of a peer to that peer as a NEXT_HOP, for a route the speaker is originating. A BGP speaker must never install a route with itself as the next hop. no notes, these are hard rules 💀
When a BGP speaker advertises the route to a BGP speaker located in its own autonomous system, the advertising speaker shall not modify the NEXT_HOP attribute associated with the route 🔁. When a BGP speaker receives the route via an internal link, it may forward packets to the NEXT_HOP address if the address contained in the attribute is on a common subnet with the local and remote BGP speakers ✨.
5.1.4 MULTI_EXIT_DISC 🚪
the MULTI_EXIT_DISC attribute may be used on external (inter-AS) links to discriminate among multiple exit or entry points to the same neighboring AS 💜. Its value is a four octet unsigned number, which is called a metric 🔢. All other factors being equal, the exit or entry point with the lower metric should be preferred 📉.
If it's received over external links, the MULTI_EXIT_DISC attribute may be propagated over internal links to other BGP speakers within the same AS 🏡. But it is never propagated to other BGP speakers in neighboring AS's 🙅♀️. Stay in your lane, MED 😤
5.1.5 LOCAL_PREF 👑
LOCAL_PREF is a well-known discretionary attribute that shall be included in all UPDATE messages that a given BGP speaker sends to the other BGP speakers located in its own autonomous system 🫶. A BGP speaker shall calculate the degree of preference for each external route and include that degree of preference when advertising a route to its internal peers 🧮. The higher degree of preference should be preferred 🔝. A BGP speaker shall use the degree of preference learned via LOCAL_PREF in its decision process (see section 9.1.1) 🧠.
A BGP speaker shall not include this attribute in UPDATE messages that it sends to BGP speakers located in a neighboring autonomous system 🚫. If it is contained in an UPDATE message received from a BGP speaker that is not located in the same autonomous system as the receiving speaker, then this attribute shall be ignored by the receiving speaker 🙈.
5.1.6 ATOMIC_AGGREGATE 🧩
ATOMIC_AGGREGATE is a well-known discretionary attribute 💅. Picture this: a BGP speaker gets a set of overlapping routes from one of its peers (see 9.1.4) and picks the less specific route without selecting the more specific one 👀. Then the local system shall attach the ATOMIC_AGGREGATE attribute to the route when propagating it to other BGP speakers, if that attribute is not already present in the received less specific route 📦.
A BGP speaker that receives a route with the ATOMIC_AGGREGATE attribute shall not remove the attribute from the route when propagating it to other speakers 🔒. It also shall not make any NLRI of that route more specific (as defined in 9.1.4) when advertising this route to other BGP speakers 🙅♀️. And honestly, it needs to be cognizant of the fact that the actual path to destinations, as specified in the NLRI of the route, may traverse ASs that are not listed in the AS_PATH attribute. It can have the loop-free property and still take a detour 🗺️😭
5.1.7 AGGREGATOR 🌈
AGGREGATOR is an optional transitive attribute, which may be included in updates that are formed by aggregation (see Section 9.2.4.2) 🎁. A BGP speaker which performs route aggregation may add the AGGREGATOR attribute, and that attribute shall contain its own AS number and IP address 🆔✨
6. BGP Error Handling 🚨
This section describes actions to be taken when errors are detected while processing BGP messages 👀.
When any of the conditions described here are detected, a NOTIFICATION message with the indicated Error Code, Error Subcode, and Data fields is sent, and the BGP connection is closed 🛑. If no Error Subcode is specified, then a zero must be used 0️⃣.
The phrase "the BGP connection is closed" means that the transport protocol connection has been closed and that all resources for that BGP connection have been deallocated 🧹. Routing table entries associated with the remote peer are marked as invalid 🙃. The fact that the routes have become invalid is passed to other BGP peers before the routes are deleted from the system 📣.
Unless specified explicitly, the Data field of the NOTIFICATION message that is sent to indicate an error is empty 🫙.
6.1 Message Header error handling 📨
All errors detected while processing the Message Header are indicated by sending the NOTIFICATION message with Error Code Message Header Error 🚩. The Error Subcode elaborates on the specific nature of the error 🔍.
The expected value of the Marker field of the message header is all ones if the message type is OPEN 1️⃣. The expected value of the Marker field for all other types of BGP messages is determined based on the presence of the Authentication Information Optional Parameter in the BGP OPEN message, and the actual authentication mechanism (if the Authentication Information in the BGP OPEN message is present) 🔐. If the Marker field of the message header is not the expected one, then a synchronization error has occurred and the Error Subcode is set to Connection Not Synchronized 😵💫.
If the Length field of the message header is less than 19 or greater than 4096, or if the Length field of an OPEN message is less than the minimum length of the OPEN message, or if the Length field of an UPDATE message is less than the minimum length of the UPDATE message, or if the Length field of a KEEPALIVE message is not equal to 19, or if the Length field of a NOTIFICATION message is less than the minimum length of the NOTIFICATION message, then the Error Subcode is set to Bad Message Length 📏. The Data field contains the erroneous Length field 🧾.
If the Type field of the message header is not recognized, then the Error Subcode is set to Bad Message Type 🤷♀️. The Data field contains the erroneous Type field 🧾.
6.2 OPEN message error handling 🎀
All errors detected while processing the OPEN message are indicated by sending the NOTIFICATION message with Error Code OPEN Message Error 🚩. The Error Subcode elaborates on the specific nature of the error 🔍.
If the version number contained in the Version field of the received OPEN message is not supported, then the Error Subcode is set to Unsupported Version Number 🕰️. The Data field is a 2-octet unsigned integer, which indicates the largest locally supported version number less than the version the remote BGP peer bid (as indicated in the received OPEN message) 🔢.
If the Autonomous System field of the OPEN message is unacceptable, then the Error Subcode is set to Bad Peer AS 🙅♀️. The determination of acceptable Autonomous System numbers is outside the scope of this protocol 🤫.
If the Hold Time field of the OPEN message is unacceptable, then the Error Subcode MUST be set to Unacceptable Hold Time ⏳. An implementation MUST reject Hold Time values of one or two seconds ✋. An implementation MAY reject any proposed Hold Time 🫣. An implementation which accepts a Hold Time MUST use the negotiated value for the Hold Time 🤝.
If the BGP Identifier field of the OPEN message is syntactically incorrect, then the Error Subcode is set to Bad BGP Identifier 🆔. Syntactic correctness means that the BGP Identifier field represents a valid IP host address 🌐.
If one of the Optional Parameters in the OPEN message is not recognized, then the Error Subcode is set to Unsupported Optional Parameters 🧐.
If the OPEN message carries Authentication Information (as an Optional Parameter), then the corresponding authentication procedure is invoked 🔑. If the authentication procedure (based on Authentication Code and Authentication Data) fails, then the Error Subcode is set to Authentication Failure 🚫🔐
6.3. UPDATE message error handling. 💅
okay bestie, so any error we find while processing an UPDATE message gets reported by sending a NOTIFICATION message with Error Code UPDATE Message Error 😭. The error subcode is the part that spills the tea on exactly what went wrong 🫶.
Error checking for an UPDATE message starts with the path attributes 🎀. If the Unfeasible Routes Length or Total Attribute Length is way too big (aka if Unfeasible Routes Length + Total Attribute Length + 23 is more than the message Length), the Error Subcode is Malformed Attribute List 💀.
If any recognized attribute has Attribute Flags that clash with its Attribute Type Code, the Error Subcode is Attribute Flags Error. The Data field holds the erroneous attribute (type, length and value) 🧁.
If any recognized attribute has an Attribute Length that doesn't match the expected length (based on its attribute type code), the Error Subcode is Attribute Length Error. The Data field holds the erroneous attribute (type, length and value) 📏.
If any of the mandatory well-known attributes are missing, the Error Subcode is Missing Well-known Attribute. The Data field holds the Attribute Type Code of the missing one 🔍.
If any of the mandatory well-known attributes aren't recognized, the Error Subcode is Unrecognized Well-known Attribute. The Data field holds the unrecognized attribute (type, length and value) 🙈.
If the ORIGIN attribute has an undefined value, the Error Subcode is Invalid Origin Attribute. The Data field holds the unrecognized attribute (type, length and value) 🌈.
If the NEXT_HOP attribute field is syntactically incorrect, the Error Subcode is Invalid NEXT_HOP Attribute. The Data field holds the incorrect attribute (type, length and value). Syntactic correctness means that NEXT_HOP represents a valid IP host address 🧚.
Semantic correctness only applies to the external BGP links. It means the interface tied to the IP address in NEXT_HOP shares a common subnet with the receiving BGP speaker, and that address isn't the receiving BGP speaker's own IP address 🦋.
If NEXT_HOP is semantically incorrect, the error should be logged and the route should be ignored. In this case, no NOTIFICATION message should be sent.
okay so the key thing here: a semantically wrong NEXT_HOP gets logged and the route gets dropped, but no NOTIFICATION goes out 🙅♀️✨
The AS_PATH attribute gets checked for syntactic correctness. If the path is syntactically incorrect, the Error Subcode is Malformed AS_PATH 🍓.
If an optional attribute is recognized, its value gets checked. If an error shows up, the attribute gets discarded and the Error Subcode is Optional Attribute Error. The Data field holds the attribute (type, length and value) 🫧.
If any attribute appears more than once in the UPDATE message, the Error Subcode is Malformed Attribute List 👑.
The NLRI field in the UPDATE message gets checked for syntactic validity. If the field is syntactically incorrect, the Error Subcode is Invalid Network Field 💔.
6.4. NOTIFICATION message error handling. 🎀
If a peer sends a NOTIFICATION message and there's an error inside it, sadly there's no way to report that error with another NOTIFICATION message 😩. Any such error, like an unrecognized Error Code or Error Subcode, should be noticed, logged locally, and brought to the attention of the peer's administration. How to actually do that is outside the scope of this doc, so it's a whole other story 🤷♀️.
6.5. Hold Timer Expired error handling. ⏰
If a system doesn't receive successive KEEPALIVE and/or UPDATE and/or NOTIFICATION messages within the period in the Hold Time field of the OPEN message, then the NOTIFICATION message with the Hold Timer Expired Error Code must be sent and the BGP connection closed. no excuses, bestie 💅.
6.6. Finite State Machine error handling. 🧠
Any error the BGP Finite State Machine detects (like getting an unexpected event) gets reported by sending a NOTIFICATION message with Error Code Finite State Machine Error 🌸.
6.7. Cease. 🫶
Without any fatal errors (the ones indicated in this section), a BGP peer may choose to close its BGP connection at any time by sending a NOTIFICATION message with Error Code Cease ✨. However, the Cease NOTIFICATION message must not be used when a fatal error indicated by this section does exist. 🚫
6.8. Connection collision detection. 💥
If a pair of BGP speakers try to establish a TCP connection to each other at the same time, two parallel connections between them might totally form. We call this a connection collision 😭. Clearly, one of those connections has to go 🙅♀️.
Based on the value of the BGP Identifier, there's a convention for figuring out which connection to keep when a collision happens. The convention is to compare the BGP Identifiers of the peers involved and keep only the connection started by the BGP speaker with the higher-valued BGP Identifier 👑.
Upon receiving an OPEN message, the local system must examine all of its connections that are in the OpenConfirm state. A BGP speaker may also examine connections in an OpenSent state if it knows the peer's BGP Identifier by some means outside the protocol. If, among these connections, there's a connection to a remote BGP speaker whose BGP Identifier equals the one in the OPEN message, then the local system runs this collision resolution procedure 🔍:
- The local BGP Identifier gets compared to the BGP Identifier of the remote system (as specified in the OPEN message).
- If the local BGP Identifier is less than the remote one, the local system closes the BGP connection that already exists (the one already in the
OpenConfirmstate), and accepts the BGP connection initiated by the remote system 🦋. - Otherwise, the local system closes the newly created BGP connection (the one tied to the newly received OPEN message), and keeps using the existing one (the one already in the
OpenConfirmstate) 🌈.
Comparing BGP Identifiers is done by treating them as (4-octet long) unsigned integers. 🔢
A connection collision with an existing BGP connection that is in Established states causes an unconditional closing of the newly created connection 🙅. Note that a connection collision can't be detected with connections that are in Idle, Connect, or Active states 🧚.
Closing the BGP connection (the result of the collision resolution procedure) is done by sending a NOTIFICATION message with Error Code Cease 💜.
7. BGP Version Negotiation. 🌈
BGP speakers can negotiate the protocol version by making multiple attempts to open a BGP connection, starting with the highest version number each one supports. If an open attempt fails with Error Code OPEN Message Error and Error Subcode Unsupported Version Number, the BGP speaker has access to: the version number it tried, the version number its peer tried, the version number its peer passed in the NOTIFICATION message, and the version numbers it supports 🧁. If the two peers support one or more common versions, this helps them quickly figure out the highest common version. So to support BGP version negotiation, future versions of BGP must retain the format of the OPEN and NOTIFICATION messages. 👑✨
8. BGP Finite State machine ✨
okay bestie, this section is where BGP's whole brain lives: the Finite State Machine (FSM) 🧠💅 here's the quick tour of what BGP does in each state. the condensed version of the FSM is in Appendix 1 🎀
BGP starts out in the Idle state 🌸
Idle state
Idle is giving closed-door energy 🚪 BGP refuses all incoming BGP connections, and no resources get allocated to the peer 🙅♀️ when the Start event shows up (triggered by either the system or an operator), the local system initializes all its BGP resources, starts the ConnectRetry timer, kicks off a transport connection to the other BGP peer while listening for any connection the remote BGP peer starts, and moves to Connect ✨ the exact value of the ConnectRetry timer is a local matter, but it SHOULD be sufficiently large to allow TCP initialization 🧸
if a BGP speaker detects an error, it shuts the connection down and goes back to Idle 💀 getting out of Idle needs a Start event. if that event is generated automatically, persistent BGP errors can cause persistent flapping of the speaker 😭 to avoid that mess, it is RECOMMENDED that Start events SHOULD NOT be generated immediately for a peer that was previously moved to Idle because of an error. for that kind of peer, the time between consecutive Start events (if they're generated automatically) SHALL exponentially increase. the initial timer value SHALL be 60 seconds, and the time SHALL be doubled for each consecutive retry 🔁
any other event received in Idle gets ignored 🙈
Connect state
Connect is the waiting room 🪑 BGP is waiting for the transport protocol connection to be completed.
if the transport protocol connection succeeds, the local system clears the ConnectRetry timer, completes initialization, sends an OPEN message to its peer, and moves to OpenSent 🎉
if the transport protocol connect fails (e.g., retransmission timeout), the local system restarts the ConnectRetry timer, keeps listening for a connection the remote BGP peer starts, and changes its state to Active 🥀
when the ConnectRetry timer expires, the local system restarts it, initiates a transport connection to the other BGP peer, keeps listening for a connection the remote BGP peer starts, and stays in Connect 🔄
Start event is ignored in the Active state. 🙃
for any other event (triggered by either system or operator), the local system releases all BGP resources tied to this connection and moves to Idle 🧹
Active state
Active is BGP out here hustling for a peer by starting a transport protocol connection 🏃♀️
if the transport protocol connection succeeds, the local system clears the ConnectRetry timer, completes initialization, sends an OPEN message to its peer, sets its Hold Timer to a large value, and moves to OpenSent 💖 a Hold Timer value of 4 minutes is suggested ⏱️
when the ConnectRetry timer expires, the local system restarts it, initiates a transport connection to the other BGP peer, keeps listening for a connection the remote BGP peer starts, and changes its state to Connect 🔁
if the local system notices that a remote peer is trying to set up a BGP connection, and that peer's IP address isn't an expected one, the local system restarts the ConnectRetry timer, rejects the attempted connection, keeps listening for a connection from the remote BGP peer, and stays in Active 🚫
Start event is ignored in the Active state. 🙃
for any other event (triggered by either system or operator), the local system releases all BGP resources tied to this connection and moves to Idle 🧹
OpenSent state
OpenSent is where BGP waits for an OPEN message from its peer 👀 when one arrives, all its fields get checked for correctness. if the BGP message header checking or OPEN message checking detects an error (see Section 6.2), or a connection collision (see Section 6.8) is found, the local system sends a NOTIFICATION message and moves to Idle 🚨
if there are no errors in the OPEN message, BGP sends a KEEPALIVE message and sets a KeepAlive timer ✅ the Hold Timer, which was originally set to a large value (see above), gets replaced with the negotiated Hold Time value (see section 4.2). if the negotiated Hold Time value is zero, the Hold Time timer and KeepAlive timers don't get started. if the value of the Autonomous System field is the same as the local Autonomous System number, the connection is "internal"; otherwise, it's "external". (This will affect UPDATE processing as described below.) finally, the state changes to OpenConfirm 🥳
if a disconnect notification comes from the underlying transport protocol, the local system closes the BGP connection, restarts the ConnectRetry timer, keeps listening for a connection the remote BGP peer starts, and goes into Active 🔌
if the Hold Timer expires, the local system sends a NOTIFICATION message with error code Hold Timer Expired and moves to Idle ⏰
in response to the Stop event (triggered by either system or operator), the local system sends a NOTIFICATION message with Error Code Cease and moves to Idle 🛑
Start event is ignored in the OpenSent state. 🙃
in response to any other event, the local system sends a NOTIFICATION message with Error Code Finite State Machine Error and moves to Idle 💀
whenever BGP changes its state from OpenSent to Idle, it closes the BGP (and transport-level) connection and releases all resources tied to that connection 🧹
OpenConfirm state
OpenConfirm is the "are we really doing this?" moment 💜 BGP waits for a KEEPALIVE or NOTIFICATION message.
if the local system receives a KEEPALIVE message, it changes its state to Established 🎉
if the Hold Timer expires before a KEEPALIVE message is received, the local system sends a NOTIFICATION message with error code Hold Timer Expired and moves to Idle ⏰
if the local system receives a NOTIFICATION message, it moves to Idle 😔
if the KeepAlive timer expires, the local system sends a KEEPALIVE message and restarts its KeepAlive timer 💓
if a disconnect notification is received from the underlying transport protocol, the local system changes its state to Idle 🔌
in response to the Stop event (triggered by either system or operator), the local system sends a NOTIFICATION message with Error Code Cease and moves to Idle 🛑
Start event is ignored in the OpenConfirm state. 🙃
in response to any other event, the local system sends a NOTIFICATION message with Error Code Finite State Machine Error and moves to Idle 💀
whenever BGP changes its state from OpenConfirm to Idle, it closes the BGP (and transport-level) connection and releases all resources tied to that connection 🧹
Established state
Established is the main character era 👑 BGP can exchange UPDATE, NOTIFICATION, and KEEPALIVE messages with its peer 🥰
if the local system receives an UPDATE or KEEPALIVE message, it restarts its Hold Timer, as long as the negotiated Hold Time value is non-zero ⏳
if the local system receives a NOTIFICATION message, it changes its state to Idle 😭
if the local system receives an UPDATE message and the UPDATE message error handling procedure (see Section 6.3) detects an error, the local system sends a NOTIFICATION message and changes its state to Idle 🚨
if a disconnect notification is received from the underlying transport protocol, the local system changes its state to Idle 🔌
if the Hold Timer expires, the local system sends a NOTIFICATION message with Error Code Hold Timer Expired and changes its state to Idle ⏰
if the KeepAlive timer expires, the local system sends a KEEPALIVE message and restarts its KeepAlive timer 💓
each time the local system sends a KEEPALIVE or UPDATE message, it restarts its KeepAlive timer, unless the negotiated Hold Time value is zero 🔁
in response to the Stop event (initiated by either system or operator), the local system sends a NOTIFICATION message with Error Code Cease and changes its state to Idle 🛑
Start event is ignored in the Established state. 🙃
in response to any other event, the local system sends a NOTIFICATION message with Error Code Finite State Machine Error and changes its state to Idle 💀
whenever BGP changes its state from Established to Idle, it closes the BGP (and transport-level) connection, releases all resources tied to that connection, and deletes all routes derived from that connection 🗑️✨
9. UPDATE Message Handling ✨
Okay so an UPDATE message may be received only in the Established state 🫶 When one shows up, every single field gets checked for validity, exactly as laid out in Section 6.3. Don't skip that part, bestie 💅
If an optional non-transitive attribute is unrecognized, it gets quietly ignored 🙈 But if an optional transitive attribute is unrecognized, the Partial bit (the third high-order bit) in the attribute flags octet gets set to 1, and the attribute is kept around for propagation to other BGP speakers 🦋 That's giving "pass it along, don't drop it" energy.
If an optional attribute is recognized and has a valid value, then depending on what type of attribute it is, it gets processed locally, retained, and updated (if necessary) for possible propagation to other BGP speakers 💜
If the UPDATE message contains a non-empty WITHDRAWN ROUTES field, the previously advertised routes whose destinations (expressed as IP prefixes) are contained in that field shall be removed from the Adj-RIB-In. This BGP speaker shall run its Decision Process, since the previously advertised route is no longer available for use 🍓 Bye bye, route 👋
If the UPDATE message contains a feasible route, it shall be placed in the appropriate Adj-RIB-In, and the following additional actions shall be taken 🔥
- i) If its Network Layer Reachability Information (NLRI) is identical to the one of a route currently stored in the Adj-RIB-In, then the new route shall replace the older route in the Adj-RIB-In, thus implicitly withdrawing the older route from service. The BGP speaker shall run its Decision Process, since the older route is no longer available for use. It's a full glow-up swap 💖
- ii) If the new route is an overlapping route that is included (see 9.1.4) in an earlier route contained in the Adj-RIB-In, the BGP speaker shall run its Decision Process, since the more specific route has implicitly made a portion of the less specific route unavailable for use. The little one got eaten, periodt 🍽️
- iii) If the new route has identical path attributes to an earlier route contained in the Adj-RIB-In, and is more specific (see 9.1.4) than the earlier route, no further actions are necessary ✨ Easy, no notes.
- iv) If the new route has NLRI that is not present in any of the routes currently stored in the Adj-RIB-In, then the new route shall be placed in the Adj-RIB-In. The BGP speaker shall run its Decision Process 🧚
- v) If the new route is an overlapping route that is less specific (see 9.1.4) than an earlier route contained in the Adj-RIB-In, the BGP speaker shall run its Decision Process on the set of destinations described only by the less specific route 🌸
9.1 Decision Process
Okay, the Decision Process is the big brain of it all 🧠 It selects routes for subsequent advertisement by applying the policies in the local Policy Information Base (PIB) to the routes stored in its Adj-RIB-In. The output is the set of routes that will be advertised to all peers, and the selected routes get stored in the local speaker's Adj-RIB-Out 🎀
The selection process is formalized by defining a function that takes the attribute of a given route as an argument and returns a non-negative integer denoting the degree of preference for the route. The function that calculates that degree of preference shall not use as its inputs any of the following: the existence of other routes, the non-existence of other routes, or the path attributes of other routes. Route selection then means applying the degree of preference function to each feasible route one by one, and then picking the one with the highest degree of preference 👑
The Decision Process operates on routes contained in each Adj-RIB-In, and it is responsible for:
- selection of routes to be advertised to BGP speakers located in the local speaker's autonomous system
- selection of routes to be advertised to BGP speakers located in neighboring autonomous systems
- route aggregation and route information reduction
The Decision Process takes place in three distinct phases, each triggered by a different event 🌈
- a) Phase 1 is responsible for calculating the degree of preference for each route received from a BGP speaker located in a neighboring autonomous system, and for advertising to the other BGP speakers in the local autonomous system the routes that have the highest degree of preference for each distinct destination.
- b) Phase 2 is invoked on completion of Phase 1. It is responsible for choosing the best route out of all those available for each distinct destination, and for installing each chosen route into the appropriate Loc-RIB.
- c) Phase 3 is invoked after the Loc-RIB has been modified. It is responsible for disseminating routes in the Loc-RIB to each peer located in a neighboring autonomous system, according to the policies contained in the PIB. Route aggregation and information reduction can optionally be performed within this phase 🦋
9.1.1 Phase 1: Calculation of Degree of Preference
Phase 1 is the first step in the glow-up 💅 The Phase 1 decision function shall be invoked whenever the local BGP speaker receives an UPDATE message from a peer located in a neighboring autonomous system that advertises a new route, a replacement route, or a withdrawn route.
The Phase 1 decision function is a separate process, and it completes when it has no further work to do ⭐
The Phase 1 decision function shall lock an Adj-RIB-In prior to operating on any route contained within it, and shall unlock it after operating on all new or unfeasible routes contained within it 🔒
For each newly received or replacement feasible route, the local BGP speaker shall determine a degree of preference. If the route is learned from a BGP speaker in the local autonomous system, either the value of the LOCAL_PREF attribute shall be taken as the degree of preference, or the local system shall compute the degree of preference of the route based on preconfigured policy information. If the route is learned from a BGP speaker in a neighboring autonomous system, then the degree of preference shall be computed based on preconfigured policy information. The exact nature of this policy information and the computation involved is a local matter 🧁 The local speaker shall then run the internal update process of 9.2.1 to select and advertise the most preferable route 💖
9.1.2 Phase 2: Route Selection
Phase 2 is the main character moment, bestie 👑 The Phase 2 decision function shall be invoked on completion of Phase 1. The Phase 2 function is a separate process, and it completes when it has no further work to do. It shall consider all routes that are present in the Adj-RIBs-In, including the ones received from BGP speakers located in its own autonomous system and the ones received from BGP speakers located in neighboring autonomous systems 🌟
The Phase 2 decision function shall be blocked from running while the Phase 3 decision function is in process. The Phase 2 function shall lock all Adj-RIBs-In prior to commencing its function, and shall unlock them on completion 🔐
If the NEXT_HOP attribute of a BGP route depicts an address to which the local BGP speaker doesn't have a route in its Loc-RIB, the BGP route SHOULD be excluded from the Phase 2 decision function. Not a vibe, so it gets left out 🙈
For each set of destinations for which a feasible route exists in the Adj-RIBs-In, the local BGP speaker shall identify the route that has:
- a) the highest degree of preference of any route to the same set of destinations, or
- b) is the only route to that destination, or
- c) is selected as a result of the Phase 2 tie breaking rules specified in 9.1.2.1.
The local speaker SHALL then install that route in the Loc-RIB, replacing any route to the same destination that is currently being held in the Loc-RIB 💌 The local speaker MUST determine the immediate next hop to the address depicted by the NEXT_HOP attribute of the selected route by performing a lookup in the IGP and selecting one of the possible paths in the IGP. This immediate next hop MUST be used when installing the selected route in the Loc-RIB. If the route to the address depicted by the NEXT_HOP attribute changes such that the immediate next hop changes, route selection should be recalculated as specified above 🔄 Stay on top of it, babe.
Unfeasible routes shall be removed from the Loc-RIB, and corresponding unfeasible routes shall then be removed from the Adj-RIBs-In 🗑️ Clean sweep, no cap 🧹✨
9.1.2.1. Breaking Ties (Phase 2) 💅
okay so, a BGP speaker can have a bunch of routes to the same destination in its Adj-RIBs-In that all have the same degree of preference 🎀. But the local speaker can only pick ONE of them for the Loc-RIB 😭. So it looks at every equally preferable route, whether it came from a BGP speaker in a neighboring autonomous system or from another BGP speaker in its own autonomous system ✨.
The tie-breaking procedure here assumes that every BGP speaker in an autonomous system can work out the cost of a path (interior distance) to the address shown in the NEXT_HOP attribute of each candidate route 🧚. Ties SHALL be broken according to this algorithm:
- a) If the local system is configured to take
MULTI_EXIT_DISCinto account, and the candidate routes differ in theirMULTI_EXIT_DISCattribute, pick the route with the lowest value ofMULTI_EXIT_DISC🔥. - b) Otherwise, pick the route with the lowest cost (interior distance) to the entity shown in its
NEXT_HOPattribute. If several routes have the same cost, the tie SHALL be broken like this:- if at least one of the candidate routes was advertised by a BGP speaker in a neighboring autonomous system, pick the route advertised by the neighboring-AS BGP speaker whose BGP Identifier is the lowest among all the other BGP speakers in neighboring autonomous systems;
- otherwise, pick the route advertised by the BGP speaker whose BGP Identifier is the lowest.
9.1.3. Phase 3: Route Dissemination 🦋
The Phase 3 decision function SHALL be invoked when Phase 2 is complete, or when any of these events happen:
- a) when routes in a Loc-RIB to local destinations have changed
- b) when locally generated routes learned by means outside of BGP have changed
- c) when a new BGP speaker - BGP speaker connection has been established
The Phase 3 function is its own separate process, and it wraps up once it has no further work to do 🫶. The Phase 3 Routing Decision function SHALL be blocked from running while the Phase 2 decision function is in process.
All routes in the Loc-RIB SHALL be processed into a corresponding entry in the associated Adj-RIBs-Out. Route aggregation and information reduction techniques (see 9.2.4.1) MAY optionally be applied ✨.
For the benefit of future support of inter-AS multicast capabilities, a BGP speaker that participates in inter-AS multicast routing SHALL advertise a route it receives from one of its external peers, and if it installs that route in its Loc-RIB, it SHALL advertise it back to the peer it was received from. For a BGP speaker that does not participate in inter-AS multicast routing, such an advertisement is OPTIONAL. When doing such an advertisement, the NEXT_HOP attribute SHOULD be set to the address of the peer. An implementation MAY also optimize such an advertisement by truncating information in the AS_PATH attribute so it includes only its own AS number and that of the peer that advertised the route (such truncation requires the ORIGIN attribute to be set to INCOMPLETE). On top of that, an implementation is not REQUIRED to pass optional or discretionary path attributes with such an advertisement 💜.
When the updating of the Adj-RIBs-Out and the Forwarding Information Base (FIB) is complete, the local BGP speaker SHALL run the external update process of 9.2.2 🚀.
9.1.4. Overlapping Routes 🌈
A BGP speaker MAY transmit routes with overlapping Network Layer Reachability Information (NLRI) to another BGP speaker. NLRI overlap happens when a set of destinations is identified in non-matching multiple routes. Since BGP encodes NLRI using IP prefixes, overlap will always show subset relationships. A route describing a smaller set of destinations (a longer prefix) is said to be more specific than a route describing a larger set of destinations (a shorter prefix). Likewise, a route describing a larger set of destinations (a shorter prefix) is said to be less specific than a route describing a smaller set of destinations (a longer prefix) 🧁.
The precedence relationship effectively splits a less specific route into two parts:
- a set of destinations described only by the less specific route, and
- a set of destinations described by the overlap of the less specific and the more specific routes
When overlapping routes are present in the same Adj-RIB-In, the more specific route SHALL take precedence, going in order from more specific to least specific 👑.
The set of destinations described by the overlap is a portion of the less specific route that is feasible but is not currently in use. If a more specific route is withdrawn later on, the set of destinations described by the overlap will still be reachable using the less specific route.
If a BGP speaker receives overlapping routes, the Decision Process SHALL take into account the semantics of those overlapping routes. In particular, if a BGP speaker accepts the less specific route while rejecting the more specific route from the same peer, then the destinations represented by the overlap may not forward along the ASs listed in the AS_PATH attribute of that route. So a BGP speaker has these choices 😭:
- a) Install both the less and the more specific routes
- b) Install the more specific route only
- c) Install the non-overlapping part of the less specific route only (that implies de-aggregation)
- d) Aggregate the two routes and install the aggregated route
- e) Install the less specific route only
- f) Install neither route
If a BGP speaker chooses e), then it SHOULD add the ATOMIC_AGGREGATE attribute to the route. A route that carries the ATOMIC_AGGREGATE attribute can not be de-aggregated. That means the NLRI of this route can not be made more specific. Forwarding along such a route does not guarantee that IP packets will actually traverse only the ASs listed in the AS_PATH attribute of the route.
If a BGP speaker chooses a), it MUST NOT advertise the more general route without the more specific route. Please don't skip this one, bestie 🙈⚠️
9.2. Update-Send Process 📬
The Update-Send process is responsible for advertising UPDATE messages to all peers. For example, it distributes the routes chosen by the Decision Process to other BGP speakers that may be located in either the same autonomous system or a neighboring autonomous system. Rules for information exchange between BGP speakers located in different autonomous systems are given in 9.2.2, and rules for information exchange between BGP speakers located in the same autonomous system are given in 9.2.1 🎀.
Distribution of routing information between a set of BGP speakers, all located in the same autonomous system, is called internal distribution.
9.2.1. Internal Updates 🫧
The Internal update process is all about distributing routing information to BGP speakers located in the local speaker's autonomous system.
When a BGP speaker receives an UPDATE message from another BGP speaker located in its own autonomous system, the receiving BGP speaker SHALL NOT re-distribute the routing information contained in that UPDATE message to other BGP speakers located in its own autonomous system 🙅♀️.
When a BGP speaker receives a new route from a BGP speaker in a neighboring autonomous system, it SHALL advertise that route to all other BGP speakers in its autonomous system by means of an UPDATE message if any of the following conditions occur:
- 1) the degree of preference assigned to the newly received route by the local BGP speaker is higher than the degree of preference that the local speaker has assigned to other routes received from BGP speakers in neighboring autonomous systems, or
- 2) there are no other routes that have been received from BGP speakers in neighboring autonomous systems, or
- 3) the newly received route is selected as a result of breaking a tie between several routes which have the highest degree of preference, and the same destination (the tie-breaking procedure is specified in 9.2.1.1).
When a BGP speaker receives an UPDATE message with a non-empty WITHDRAWN ROUTES field, it SHALL remove from its Adj-RIB-In all routes whose destinations were carried in this field (as IP prefixes). The speaker SHALL take the following additional steps 🥺:
- 1) if the corresponding feasible route had not been previously advertised, then no further action is necessary
- 2) if the corresponding feasible route had been previously advertised, then:
- i) if a new route is selected for advertisement that has the same Network Layer Reachability Information as the unfeasible routes, then the local BGP speaker SHALL advertise the replacement route
- ii) if a replacement route is not available for advertisement, then the BGP speaker SHALL include the destinations of the unfeasible route (in form of IP prefixes) in the
WITHDRAWN ROUTESfield of anUPDATEmessage, and SHALL send this message to each peer to whom it had previously advertised the corresponding feasible route
All feasible routes that are advertised SHALL be placed in the appropriate Adj-RIBs-Out, and all unfeasible routes that are advertised SHALL be removed from the Adj-RIBs-Out 🧹.
9.2.1.1. Breaking Ties (Internal Updates) 🍓
If a local BGP speaker has connections to several BGP speakers in neighboring autonomous systems, there will be multiple Adj-RIBs-In associated with these peers. These Adj-RIBs-In might contain several equally preferable routes to the same destination, all of which were advertised by BGP speakers located in neighboring autonomous systems. The local BGP speaker SHALL select one of these routes according to the following rules 💖:
- a) If the candidate routes differ only in their
NEXT_HOPandMULTI_EXIT_DISCattributes, and the local system is configured to take theMULTI_EXIT_DISCattribute into account, select the route that has the lowest value of theMULTI_EXIT_DISCattribute. - b) If the local system can ascertain the cost of a path to the entity depicted by the
NEXT_HOPattribute of the candidate route, select the route with the lowest cost. - c) In all other cases, select the route that was advertised by the BGP speaker whose BGP Identifier has the lowest value.
9.2.2. External Updates 🌸
The external update process is all about distributing routing information to BGP speakers located in neighboring autonomous systems. As part of the Phase 3 route selection process, the BGP speaker has updated its Adj-RIBs-Out and its Forwarding Table. All newly installed routes, and all newly unfeasible routes for which there is no replacement route, SHALL be advertised to BGP speakers located in neighboring autonomous systems by means of an UPDATE message 📣.
Any routes in the Loc-RIB marked as unfeasible SHALL be removed. Changes to the reachable destinations within its own autonomous system SHALL also be advertised in an UPDATE message 🦋✨
9.2.3 Controlling Routing Traffic Overhead ✨
okay so BGP is lowkey obsessed with not flooding the network 🌊. It limits how much routing traffic (aka UPDATE messages 📨) gets sent, because that protects two things: the link bandwidth needed to advertise those UPDATE messages, and the processing power the Decision Process needs to digest everything inside them 🧠. Main character efficiency 💅
9.2.3.1 Frequency of Route Advertisement 🦋
The parameter MinRouteAdvertisementInterval decides the minimum amount of time that MUST elapse between advertising routes to a particular destination from a single BGP speaker. This rate limit works per destination, even though the value of MinRouteAdvertisementInterval gets set per BGP peer 🎀
Two UPDATE messages sent from a single BGP speaker that advertise feasible routes to some common set of destinations, received from BGP speakers in neighboring autonomous systems, MUST be separated by at least MinRouteAdvertisementInterval. Honestly, the only way to hit that exactly is to keep a separate timer for every common set of destinations, and that's unwarranted overhead 🙄 no thank you. Any technique that makes sure the gap between two such UPDATE messages is at least MinRouteAdvertisementInterval, and also keeps a constant upper bound on that gap, is totally acceptable ✅
Since fast convergence is needed inside an autonomous system, this procedure does not apply to routes received from other BGP speakers in the same autonomous system ⚡ To avoid long-lived black holes 🕳️, the procedure also doesn't apply to the explicit withdrawal of unfeasible routes (that is, routes whose destinations, expressed as IP prefixes, are listed in the WITHDRAWN ROUTES field of an UPDATE message).
This procedure doesn't limit the rate of route selection, only the rate of route advertisement. So if new routes get selected multiple times while we're waiting for MinRouteAdvertisementInterval to expire, the last route selected SHALL be advertised at the end of MinRouteAdvertisementInterval 🏁
9.2.3.2 Frequency of Route Origination 💌
The parameter MinASOriginationInterval determines the minimum amount of time that MUST elapse between successive advertisements of UPDATE messages that report changes within the advertising BGP speaker's own autonomous systems.
9.2.3.3 Jitter 🎲
To minimize the likelihood that the distribution of BGP messages from a given BGP speaker will contain peaks 📈, jitter SHOULD be applied to the timers associated with MinASOriginationInterval, Keepalive, and MinRouteAdvertisementInterval. A given BGP speaker SHALL apply the same jitter to each of these quantities no matter which destinations the updates are going to. In other words, jitter does not get applied on a "per peer" basis 🙅♀️
The amount of jitter SHALL be determined by multiplying the base value of the appropriate timer by a random factor that's uniformly distributed in the range from 0.75 to 1.0 🎰
9.2.4 Efficient Organization of Routing Information 🗂️
Once a BGP speaker has picked the routing information it's going to advertise, it MAY take advantage of several methods to organize that information efficiently 🧚
9.2.4.1 Information Reduction 🧃
Information reduction may imply a reduction in granularity of policy control. Once information gets collapsed, the same policies will apply to all destinations and paths in the equivalence class, so it's a tradeoff 🤏
The Decision Process MAY optionally reduce the amount of information it places in the Adj-RIBs-Out using any of the following methods:
a) Network Layer Reachability Information (NLRI):
Destination IP addresses can be represented as IP address prefixes. When the address structure lines up with the systems under control of an autonomous system administrator, it'll be possible to shrink the NLRI carried in the UPDATE messages 📦
b) AS_PATHs:
AS path information can be represented as ordered AS_SEQUENCEs or unordered AS_SETs. AS_SETs are used in the route aggregation algorithm described in 9.2.4.2 🫶 They reduce the size of the AS_PATH information by listing each AS number only once, no matter how many times it may have shown up in the multiple AS_PATHs that got aggregated.
An AS_SET implies that the destinations listed in the NLRI can be reached through paths that traverse at least some of the constituent autonomous systems. AS_SETs give enough info to avoid routing information looping 🔁 but their use may prune potentially feasible paths, since those paths aren't listed individually anymore like they are in AS_SEQUENCEs. In practice this isn't likely to be a problem, though! Once an IP packet reaches the edge of a group of autonomous systems, the BGP speaker there is likely to have more detailed path information and can tell individual paths to destinations apart 👀
9.2.4.2 Aggregating Routing Information 🧩
Aggregation is the process of combining the characteristics of several different routes so that a single route can be advertised. Aggregation can happen as part of the decision process, to reduce the amount of routing information that will be placed in the Adj-RIBs-Out 💖
Aggregation reduces the amount of information a BGP speaker has to store and swap with other BGP speakers. Routes can be aggregated by applying the following procedure separately to path attributes of the same type, and to the Network Layer Reachability Information.
Routes that have the following attributes SHALL NOT be aggregated unless the corresponding attributes of each route are identical: MULTI_EXIT_DISC, NEXT_HOP. 🚫
Path attributes that have different type codes can not be aggregated together. Path attributes of the same type code MAY be aggregated, following these rules:
ORIGIN attribute: If at least one route among the aggregated routes has ORIGIN with the value INCOMPLETE, then the aggregated route MUST have the ORIGIN attribute with the value INCOMPLETE. Otherwise, if at least one route among the aggregated routes has ORIGIN with the value EGP, then the aggregated route MUST have the origin attribute with the value EGP. In all other case, the value of the ORIGIN attribute of the aggregated route is INTERNAL 🏷️
AS_PATH attribute: If the routes to be aggregated have identical AS_PATH attributes, then the aggregated route has the same AS_PATH attribute as each individual route 🪞
For the purpose of aggregating AS_PATH attributes, we model each AS inside the AS_PATH attribute as a tuple <type, value>. The "type" tells us what kind of path segment the AS belongs to (e.g. AS_SEQUENCE, AS_SET), and the "value" is the AS number 🔢 If the routes to be aggregated have different AS_PATH attributes, then the aggregated AS_PATH attribute SHALL satisfy all of the following conditions:
- all tuples of the type AS_SEQUENCE in the aggregated AS_PATH SHALL appear in all of the AS_PATHs in the initial set of routes to be aggregated.
- all tuples of the type AS_SET in the aggregated AS_PATH SHALL appear in at least one of the AS_PATHs in the initial set (they MAY appear as either AS_SET or AS_SEQUENCE types).
- for any tuple X of the type AS_SEQUENCE in the aggregated AS_PATH which precedes tuple Y in the aggregated AS_PATH, X precedes Y in each AS_PATH in the initial set which contains Y, regardless of the type of Y.
- No tuple with the same value SHALL appear more than once in the aggregated AS_PATH, regardless of the tuple's type.
An implementation MAY choose any algorithm that conforms to these rules. At a minimum, a conformant implementation SHALL be able to perform the following algorithm, which meets all of the above conditions 🧮
- determine the longest leading sequence of tuples (as defined above) common to all the AS_PATH attributes of the routes to be aggregated. Make this sequence the leading sequence of the aggregated AS_PATH attribute.
- set the type of the rest of the tuples from the AS_PATH attributes of the routes to be aggregated to AS_SET, and append them to the aggregated AS_PATH attribute.
- if the aggregated AS_PATH has more than one tuple with the same value (regardless of the tuple's type), eliminate all but one such tuple by deleting tuples of the type AS_SET from the aggregated AS_PATH attribute.
Appendix 6, section 6.8 presents another algorithm that satisfies the conditions and allows for more complex policy configurations 📚
ATOMIC_AGGREGATE: If at least one of the routes to be aggregated has the ATOMIC_AGGREGATE path attribute, then the aggregated route SHALL have this attribute as well ⚛️
AGGREGATOR: All AGGREGATOR attributes of all routes to be aggregated SHOULD be ignored 🙈
9.3 Route Selection Criteria 🏆
Generally speaking, additional rules for comparing routes among several alternatives are outside the scope of this document. There are two exceptions:
- If the local AS appears in the AS path of the new route being considered, then that new route cannot be viewed as better than any other route. If such a route were ever used, a routing loop would result 🔄💀
- In order to achieve successful distributed operation, only routes with a likelihood of stability can be chosen. So an AS MUST avoid using unstable routes, and it MUST NOT make rapid spontaneous changes to its choice of route. Quantifying the terms "unstable" and "rapid" in the previous sentence will take experience, but the principle is crystal clear 💎
9.4 Originating BGP routes 🌱
A BGP speaker MAY originate BGP routes by injecting routing information acquired by some other means (e.g. via an IGP) into BGP. A BGP speaker that originates BGP routes SHALL assign the degree of preference to these routes by passing them through the Decision Process (see Section 9.1). These routes MAY also be distributed to other BGP speakers within the local AS as part of the Internal update process (see Section 9.2.1) 📣
Whether non-BGP acquired routes get distributed within an AS via BGP depends on the environment within the AS (e.g. type of IGP). That decision SHOULD be controlled via configuration ⚙️🫶
Appendix 1. BGP FSM State Transitions and Actions ✨
Okay bestie, this appendix is the full tea on how the BGP FSM hops between states when BGP events happen 🦋. Everything here assumes the negotiated Hold Time value is non-zero. Here are the states and events you need to know:
BGP States:
1 - Idle
2 - Connect
3 - Active
4 - OpenSent
5 - OpenConfirm
6 - Established
BGP Events:
1 - BGP Start
2 - BGP Stop
3 - BGP Transport connection open
4 - BGP Transport connection closed
5 - BGP Transport connection open failed
6 - BGP Transport fatal error
7 - ConnectRetry timer expired
8 - Hold Timer expired
9 - KeepAlive timer expired
10 - Receive OPEN message
11 - Receive KEEPALIVE message
12 - Receive UPDATE messages
13 - Receive NOTIFICATION messageNext up is the big table 💅. It maps out each state transition and the actions that fire off with it. Main character energy, no cap.
Event Actions Message Sent Next State
--------------------------------------------------------------------
Idle (1)
1 Initialize resources none 2
Start ConnectRetry timer
Initiate a transport connection
others none none 1
Connect(2)
1 none none 2
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Restart ConnectRetry timer none 3
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1
Active (3)
1 none none 3
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Close connection 3
Restart ConnectRetry timer
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1
OpenSent(4)
1 none none 4
4 Close transport connection none 3
Restart ConnectRetry timer
6 Release resources none 1
10 Process OPEN is OK KEEPALIVE 5
Process OPEN failed NOTIFICATION 1
others Close transport connection NOTIFICATION 1
Release resources
OpenConfirm (5)
1 none none 5
4 Release resources none 1
6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 5
11 Complete initialization none 6
Restart Hold Timer
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources
Established (6)
1 none none 6
4 Release resources none 1
6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 6
11 Restart Hold Timer KEEPALIVE 6
12 Process UPDATE is OK UPDATE 6
Process UPDATE failed NOTIFICATION 1
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources
---------------------------------------------------------------------Last but not least, here's the condensed version 🌸, for when you just want the vibes at a glance:
Events| Idle | Connect | Active | OpenSent | OpenConfirm | Estab
| (1) | (2) | (3) | (4) | (5) | (6)
|--------------------------------------------------------------
1 | 2 | 2 | 3 | 4 | 5 | 6
| | | | | |
2 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
3 | 1 | 4 | 4 | 1 | 1 | 1
| | | | | |
4 | 1 | 1 | 1 | 3 | 1 | 1
| | | | | |
5 | 1 | 3 | 3 | 1 | 1 | 1
| | | | | |
6 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
7 | 1 | 2 | 2 | 1 | 1 | 1
| | | | | |
8 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
9 | 1 | 1 | 1 | 1 | 5 | 6
| | | | | |
10 | 1 | 1 | 1 | 1 or 5 | 1 | 1
| | | | | |
11 | 1 | 1 | 1 | 1 | 6 | 6
| | | | | |
12 | 1 | 1 | 1 | 1 | 1 | 1 or 6
| | | | | |
13 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
---------------------------------------------------------------Appendix 2. Comparison with RFC1267 💖
Okay so BGP-4 can run in a world where a whole bunch of reachable destinations get squished into one single IP prefix 🎀. Network classes and subnetting? Not in BGP-4's vocabulary, bestie. To make that work, BGP-4 changes the semantics and encoding tied to the AS_PATH attribute, and it adds new text defining what IP prefixes mean. That's what lets it support the proposed supernetting scheme [9].
To make configuration less of a headache, this version adds LOCAL_PREF, which helps with route selection procedures ✨.
INTER_AS_METRIC got a glow-up and is now MULTI_EXIT_DISC 🦋. There's also a brand-new ATOMIC_AGGREGATE attribute 🧚, which makes sure certain aggregates don't get de-aggregated. And AGGREGATOR can be tacked onto aggregated routes so everyone can see which AS, and which BGP speaker inside that AS, did the aggregating 👑.
To keep Hold Timers symmetric, the Hold Time now gets negotiated per connection 🫶. And zero Hold Times? Now supported, periodt 💅.
Appendix 3. Comparison with RFC 1163 🌈
Everything in Appendix 2 is included, plus the following tea 🍓.
To catch and recover from BGP connection collisions, a new field called BGP Identifier got added to the OPEN message 🔥. The procedure for detecting and recovering from collision is spelled out in new text in Section 6.8.
This version also doesn't make the border router passed in the NEXT_HOP path attribute live in the same Autonomous System as the BGP Speaker anymore 🌸.
The new doc additionally optimizes and simplifies how info about previously reachable routes gets exchanged ✨.
Appendix 4. Comparison with RFC 1105 🎀
Everything from Appendices 2 and 3, plus this 💅.
The RFC1105 Finite State Machine got some minor tweaks so it could play nicely with the TCP user interface in 4.3 BSD 🧚.
Those Up/Down/Horizontal relations from RFC1105? Gone from the protocol, no cap 🫶.
Here's what changed in the message format from RFC1105 💅:
- The
Hold Timefield got moved out of the BGP header and into theOPENmessage 💖. - The
versionfield also left the BGP header for theOPENmessage 🦋. - The
Link Typefield is gone from theOPENmessage 🙈. - The
OPEN CONFIRMmessage is eliminated, and its job is now covered by the implicit confirmation from theKEEPALIVEmessage ✨. - The
UPDATEmessage format got a big overhaul 😭. New fields were added to support multiple path attributes. - The
Markerfield got expanded, and its role is broader now, so it can support authentication 🔥.
Quick naming lesson 🤓: quite often, BGP as specified in RFC 1105 gets called BGP-1, BGP as specified in RFC 1163 is BGP-2, BGP as specified in RFC1267 is BGP-3, and BGP as specified in this document is BGP-4. Iconic, honestly.
Appendix 5. TCP options that may be used with BGP ✨
If your local system's TCP user interface supports the TCP PUSH function, then each BGP message should be transmitted with the PUSH flag set 🍓. Setting the PUSH flag forces BGP messages to go out promptly to the receiver.
If your local system's TCP user interface lets you set precedence for a TCP connection, then the BGP transport connection should be opened with precedence set to the Internetwork Control (110) value 🌸 (see also [6]).
Appendix 6. Implementation Recommendations 🧚
Okay, this section is where the implementation recommendations live 🫶. This section presents some implementation recommendations.
6.1. Multiple Networks Per Message ✨
Okay bestie, so BGP lets you cram multiple address prefixes into ONE message as long as they share the same AS path and next-hop gateway 🎀. Making use of this is highly recommended, no cap. Why? Because with just one prefix per message, the receiver's overhead goes up a lot 😭. It's not only the extra messages to receive. Scanning the routing table for updates to BGP peers and other routing protocols (and sending the messages for them) also happens again for every single message 💀.
Here's one way to build messages with lots of prefixes per AS path and gateway, even when your routing table isn't organized by AS path 💅. As you scan the table, process each address prefix. For its AS path and gateway, allocate a message if one doesn't exist yet, and if it does exist, just append the new prefix to it. If the message doesn't have room for the new prefix, transmit it, allocate a fresh message, and insert the prefix into that one. Once the whole table has been scanned, send every allocated message and release the resources 🧚. Max compression happens when all the destinations covered by the prefixes share a gateway and common path attributes, which means many prefixes can fit into one 4096-byte message. Iconic 👑.
When you're peering with a BGP implementation that does not compress multiple prefixes into one message 🙈, it may be necessary to take steps to reduce the overhead from the flood of data you get when a peer is acquired or a significant network topology change happens. One method is to limit the rate of updates 🦋. This gets rid of the redundant scanning of the routing table for flash updates to BGP peers and other routing protocols. The catch is that it increases the propagation latency of routing information, so it's a bit of a trade-off 😌. If you pick a minimum flash update interval that's not much greater than the time it takes to process the multiple messages, this latency should be minimized. An even better method would be to read all received messages before sending updates 🌈.
6.2. Processing Messages on a Stream Protocol 🌸
BGP uses TCP as its transport, and TCP is a stream 🌊. That means all the data for received messages doesn't necessarily show up at the same time. This can make it really annoying to process the data as messages, especially on systems like BSD Unix, where you can't tell how much data has been received but not yet processed 😩.
One method you can use in this situation is to first try to read just the message header. For the KEEPALIVE message type, the header is a complete message. For other message types, the header should first be verified, especially the total length 🔍. If all the checks succeed, the specified length minus the size of the message header is how much data is left to read ✨. An implementation that would "hang" the routing information process while trying to read from a peer could set up a message buffer (4096 bytes) per peer, and fill it with data as it becomes available until a complete message has been received 💜.
6.3. Reducing route flapping 🦋
To avoid excessive route flapping, a BGP speaker that needs to withdraw a destination and also send an update about a more specific or less specific route shall combine them into the same UPDATE message 🎀. Bundle it up, babe.
6.4. BGP Timers ⏰
BGP employs five timers: ConnectRetry, Hold Time, KeepAlive, MinASOriginationInterval, and MinRouteAdvertisementInterval. Here are the suggested values:
ConnectRetry: 120 secondsHold Time: 90 secondsKeepAlive: 30 secondsMinASOriginationInterval: 15 secondsMinRouteAdvertisementInterval: 30 seconds
An implementation of BGP MUST allow these timers to be configurable 🔥. Non-negotiable, bestie.
6.5. Path attribute ordering 💖
Implementations that combine update messages the way we talked about above in 6.1 may prefer to see all path attributes presented in a known order 🧁. That lets them quickly spot sets of attributes from different update messages that are semantically identical. To make that easier, it's a useful optimization to order the path attributes by type code. This optimization is entirely optional 💅.
6.6. AS_SET sorting 🌷
Another useful optimization is to sort the AS numbers found in an AS_SET. It's a clean way to simplify things when you're dealing with that situation 🧠. This optimization is entirely optional too, so no pressure 🫶.
6.7. Control over version negotiation 🎀
Since BGP-4 can carry aggregated routes that can't be properly represented in BGP-3, an implementation that supports BGP-4 and another BGP version should provide the capability to only speak BGP-4 on a per-peer basis 🌈. That way you stay in control, one peer at a time.
6.8. Complex AS_PATH aggregation 👑
An implementation that chooses to provide a path aggregation algorithm which keeps significant amounts of path information may wish to use the following procedure 🧩. Let's get into it!
For the purpose of aggregating AS_PATH attributes of two routes, we model each AS as a tuple <type, value>, where "type" identifies a type of the path segment the AS belongs to (e.g. AS_SEQUENCE, AS_SET), and "value" is the AS number. Two ASs are said to be the same if their corresponding <type, value> tuples are the same 🫧.
The algorithm to aggregate two AS_PATH attributes works as follows 🔍:
a) Identify the same ASs (as defined above) within each AS_PATH attribute that are in the same relative order within both AS_PATH attributes. Two ASs, X and Y, are said to be in the same order if either:
- X precedes Y in both
AS_PATHattributes, or - Y precedes X in both
AS_PATHattributes.
b) The aggregated AS_PATH attribute consists of ASs identified in (a) in exactly the same order as they appear in the AS_PATH attributes to be aggregated. If two consecutive ASs identified in (a) do not immediately follow each other in both of the AS_PATH attributes to be aggregated, then the intervening ASs (ASs that are between the two consecutive ASs that are the same) in both attributes are combined into an AS_SET path segment that consists of the intervening ASs from both AS_PATH attributes. This segment is then placed in between the two consecutive ASs identified in (a) of the aggregated attribute. If two consecutive ASs identified in (a) immediately follow each other in one attribute, but do not follow in another, then the intervening ASs of the latter are combined into an AS_SET path segment. This segment is then placed in between the two consecutive ASs identified in (a) of the aggregated attribute 🧵.
If as a result of the above procedure a given AS number appears more than once within the aggregated AS_PATH attribute, all, but the last instance (rightmost occurrence) of that AS number should be removed from the aggregated AS_PATH attribute 🌙.
References 📚
[1] Mills, D., "Exterior Gateway Protocol Formal Specification", RFC 904, BBN, April 1984.
[2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET Backbone", RFC 1092, T.J. Watson Research Center, February 1989.
[3] Braun, H-W., "The NSFNET Routing Architecture", RFC 1093, MERIT/NSFNET Project, February 1989.
[4] Postel, J., "Transmission Control Protocol - DARPA Internet Program Protocol Specification", STD 7, RFC 793, DARPA, September 1981.
[5] Rekhter, Y., and P. Gross, "Application of the Border Gateway Protocol in the Internet", RFC 1772, T.J. Watson Research Center, IBM Corp., MCI, March 1995.
[6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol Specification", STD 5, RFC 791, DARPA, September 1981.
[7] "Information Processing Systems - Telecommunications and Information Exchange between Systems - Protocol for Exchange of Inter-domain Routeing Information among Intermediate Systems to Support Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993
[8] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy", RFC 1519, BARRNet, cisco, MERIT, OARnet, September 1993
[9] Rekhter, Y., Li, T., "An Architecture for IP Address Allocation with CIDR", RFC 1518, T.J. Watson Research Center, cisco, September 1993
Security Considerations 🔒
Security issues are not discussed in this document. Full stop, no cap, so this one's a must-know gap for anyone implementing it 💀.
Editors' Addresses 📮
Yakov Rekhter
T.J. Watson Research Center IBM Corporation
P.O. Box 704, Office H3-D40
Yorktown Heights, NY 10598
Phone: +1 914 784 7361
EMail: yakov@watson.ibm.comTony Li
cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
EMail: tli@cisco.com📦 where this page comes from
- ietf: rfc1771 1995-03
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms