RFC 2171
MAPOS - Multiple Access Protocol over SONET/SDH Version 1
RFC 2171 🦋 MAPOS - Multiple Access Protocol over SONET/SDH Version 1
- Stream: Network Working Group 🌸
- Authors: K. Murakami and M. Maruyama, NTT Laboratories 💅
- Category: Informational (so it is not a standards track doc, bestie 👀)
- Date: June 1997
- Obsoletes / Updates: this title block doesn't list anything it obsoletes or updates 🧚
Status of this Memo ✨
Okay so this memo gives information to the Internet community. It does not specify an Internet standard of any kind, so don't go treating it like one 🙈 Distribution of this memo is unlimited, so share it with everyone 💖
Authors' Note 🫶
Real talk, this memo documents a multiple access protocol for sending network-protocol datagrams over SONET/SDH. The datagrams get wrapped in High-Level Data Link Control (HDLC) frames. Now the important part: this document is NOT the product of an IETF working group, and it is NOT a standards track document. It also hasn't necessarily gotten the massive, in-depth community review that standards track docs get, so keep that in mind 💀
Abstract 🌈
This document describes MAPOS, which stands for Multiple Access Protocol over SONET/SDH, and it's all about transmitting network-protocol datagrams over SONET/SDH 🦋 It focuses on the core protocol. Other documents listed in the bibliography can be referenced alongside this one to support and provide services for higher-layer protocols, so there's a whole squad of helpers 👑
1. Introduction 💖
1.1 SONET/SDH
Okay bestie, the Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) [1][2][3][4] family is a set of ITU-T standard protocols. Their whole vibe is giving common, simple, and flexible interface for broadband optical fiber transmission systems ✨ They allow direct octet-synchronous multiplexing of lower rate tributaries, and telephone carriers around the world have deployed SONET/SDH-compliant transmission systems like crazy 🌍
This document defines MAPOS, which is a method for transmitting HDLC frames over SONET/SDH. The main thing is that SONET/SDH is an inherently point-to-point link, and MAPOS gives it multiple access capability. That's how you build a seamless networking environment that uses SONET/SDH as the transmission medium for both LAN and WAN. Literally iconic 🔥
1.2 Possible Configurations
Now for the layouts 🎀 MAPOS provides multiple access in a broadcast and multicast-capable switched LAN environment that runs on SONET/SDH lines as the transmission media. The diagrams below show the possible configurations. In (a), two end nodes are connected directly to each other. In (b), you get a star-topology "SONET-LAN" where many end nodes plug into one HDLC frame switch. The frame switch forwards packets between nodes and gives you the multiple access capability. In (c), several frame switches link together to make a switching cluster, which is so main character energy 👑
+------+ +------+
| Node +--------------------------------+ Node |
+------+ +------+
(a) Point-to-Point configuration
+------+ +---------------+
| Node +--------------------------------+ |
+------+ | |
| |
+------+ | |
| Node +--------------------------------+ |
+------+ | |
| Frame Switch |
+------+ | |
| Node +--------------------------------+ |
+------+ | |
| |
+------+ | |
| Node +--------------------------------+ |
+------+ +---------------+
(b) Point-to-Multipoint configuration
+--------+ +--------+
| Frame +----------------------+ Frame |
| Switch +--------+ +--------+ Switch |
+--+-----+ +-+----+-+ +--------+
| | Frame | +--------+
+--+-----+ | Switch | +--------+ | Frame |
| Frame | +-----+--+ | Frame +------+ Switch |
| Switch | +---------+ Switch | ++-------+
+-------++ +--------+ |
|________________________________________|
(c) Switching cluster configuration
Figure 1. Possible configurationsEach port on a switch has a unique identifier within that switch. A node connected to a switch port must inherit the address of the port. So the node address is equal to the port identifier, and it's unique within the switch 🧚
In a switch cluster, a node address gets subnetted 💅 The high-order bits (the part where the corresponding bits in the "subnet mask" are 1) indicate the switch address. The remaining low-order bits indicate the unique node address within the switch. Put those two fields together and you get a unique address for a given node 🌸
Either way, the address may be configured manually into a node interface, or it may be assigned automatically by the address assignment mechanism described in [5] ✨
Also, note that any two components may be connected either directly or through a long-haul SONET/SDH leased line 💜
1.3 Packet Transmission
The protocol is connection-less 🦋 When a node wishes to talk to another node, it just fills in the destination address of an HDLC frame, places that frame in one or more SONET/SDH payloads, and sends it over a SONET/SDH link. No setup call needed, bestie, it's that easy 💖
The switch forwards the frame to its destination based on the destination address. In a switch cluster, the frame may be forwarded by multiple switches before it's eventually delivered to the specified node. Broadcast and multicast are also supported 📡 Frames with an invalid destination address are silently discarded 🙈
Like Ethernet, the multiple access capability comes from a switch or a switch cluster. MAPOS is a link layer protocol, so it doesn't care about upper-layer protocols. It can support any network layer protocol, IP included 🌈 MAPOS IPv4 support is described in [6] 🎀
2. Physical Layer 🧚
This protocol treats the underlying end-to-end SONET/SDH transmission link as if it were a plain, transparent channel. It sends HDLC frames inside SONET/SDH payloads and expects them to arrive at the other end unaltered 💅
Each node and switch SHOULD terminate SONET/SDH overhead, such as section overhead, line overhead, and path overhead, according to the SONET/SDH specification. Unfortunately, SONET and SDH overhead interpretations are not identical. On top of that, some SONET/SDH implementations use some overhead bytes in proprietary ways 💀
The details of the interpretation are beyond the scope of this document. Appendix A covers some of the biggest differences among SONET, SDH, and their implementations, the ones that often cause interoperability problems. Implementors of SONET/SDH interfaces are strongly encouraged to be aware of these differences and to provide workaround options in their products 🫶
3. Data Link Layer 🌸
3.1 HDLC Frame Format
MAPOS uses the same HDLC-like framing as PPP-over-SONET, described in RFC-1662[7] ✨ Figure 2 shows the frame format. Logical Link Control (LLC) and Sublayer/Sub-Network Access Protocol (SNAP) are not used here. The frame does not include the bytes for transparency. The fields are transmitted from left to right 👑
+----------+----------+----------+----------+
| | | | |
| Flag | Address | Control | Protocol |
| 01111110 | 8bits | 00000011 | 16 bits |
+----------+----------+----------+----------+
+-------------+------------+----------+-----------
| | | | Inter-frame
| Information | FCS | Flag | fill or next
| | 16/32 bits | 01111110 | address
+-------------+------------+----------+------------
Figure 2. Frame formatFlag Sequence
Flag sequence is used for frame synchronization 🔥 Each frame begins and ends with a flag sequence of 01111110 (0x7E). If one frame immediately follows another, one flag sequence may be treated as both the end of the preceding frame and the beginning of the one right after it. When the line is idle, the flag sequence is to be transmitted continuously on the line 🦋
Address
The address field holds the destination HDLC address. A switch forwards the frame based on this field, and it's 8 bits wide 🎀 The LSB marks the end of this field and MUST always be 1. The MSB tells you whether the frame is unicast or multicast. An MSB of 0 means unicast, and the remaining six bits give the destination node address. An MSB of 1 means multicast, and the remaining six bits give the group address. The address 11111111 (0xFF) means the frame is a broadcast frame. The address 00000001 (0x01) is reserved to identify the control processor inside a switch. Frames with an invalid address SHOULD be silently discarded 💅
+-------------+-+
| | | | | | | | |
| | node addr |1|
+-+-----------+-+
^ ^
| |
| +------- EA bit (always 1)
|
1 : broadcast, multicast
0 : unicast
Figure 3 Address formatControl
The control field contains a single octet, 00000011 (0x03). In HDLC nomenclature, that means the frame is an Unnumbered Information (UI) frame with the Poll/Final (P/F) bit set to zero 💜 Frames with any other control field values SHOULD be silently discarded 🙈
Protocol
The protocol field tells you which protocol the datagram in the information field belongs to 🌈 It conforms to the ISO 3309 extension mechanism. The values for this field can be found in the most recent "Assigned Numbers" [8] and "MAPOS Version 1 Assigned Numbers" [9] ✨
Information
The information field contains the datagram for the protocol named in the protocol field. Its length may vary, but it SHALL NOT exceed 65,280 (64K - 256) octets 📦
Frame Check Sequence (FCS)
By default, the frame check sequence (FCS) field is 16 bits long. Optionally, a 32-bit FCS MAY be used instead 🧚 The FCS is calculated over all bits of the address, control, protocol, and information fields, prior to escape conversions. The least significant octet of the result is transmitted first, since it contains the coefficient of the highest term 🔥
Inter-frame fill
A sending station MUST continuously transmit the flag sequence as inter-frame fill after the FCS field. The inter-frame flag sequences MUST be silently discarded by the receiving station 💖
When an under-run occurs during DMA in the sending station, it MUST abort the frame transfer and continuously send the flag sequence to indicate the error 🚨
3.2. Octet-Synchronous Framing 🎀
okay bestie, so MAPOS treats SONET/SDH as a byte-oriented synchronous link, which is why it uses an octet stuffing procedure 💅 SONET/SDH already gives transparency, so the Async-Control-Character-Map (ACCM) is not used at all. no cap. Here's how HDLC frames get mapped into the SONET/SDH payload ✨
Each HDLC frame is separated from another frame by one or more flag sequences, 01111110 (0x7E). There's an escape sequence that escapes the flag sequence and itself 🦋 Before sending the frame, but after the FCS computation, every 01111110 (0x7E) that isn't a flag is to be converted to 01111101 01011110 (0x7D 0x5E), and 01111101 (0x7D) is to be converted to 01111101 01011101 (0x7D 0x5D). When a frame is received, this conversion must be reversed prior to FCS computation. the reversing part is so important, periodt 😭
4. Further Reading 🧚
to fully use the MAPOS protocol, it's useful to check out other documents[5][6][9][10] alongside this one 📚🌸
5. Security Considerations 🔐
Security issues are not discussed in this memo. 💀 bestie, this is the one section where the memo just says nothing, so don't go assuming anything is covered here 🙈
References
[1] CCITT Recommendation G.707: Synchronous Digital Hierarchy Bit Rates (1990).
[2] CCITT Recommendation G.708: Network Node Interface for Synchronous Digital Hierarchy (1990).
[3] CCITT Recommendation G.709: Synchronous Multiplexing Structure (1990).
[4] American National Standard for Telecommunications - Digital Hierarchy - Optical Interface Rates and Formats Specification, ANSI T1.105-1991.
[5] Murakami, K. and M. Maruyama, "A MAPOS version 1 Extension - Node Switch Protocol," RFC2173, June, 1997.
[6] Murakami, K. and M. Maruyama, "IPv4 over MAPOS Version 1," RFC2176, June, 1997.
[7] Simpson, W., editor, "PPP in HDLC-like Framing," RFC1662, July 1994.
[8] IANA, "IANA-Assignments," http://www.iana.org/iana/assignments.html
[9] Maruyama, M. and K. Murakami, "MAPOS Version 1 Assigned Numbers," RFC2172, June 1997.
[10] Murakami, K. and M. Maruyama, "A MAPOS version 1 Extension - Switch Switch Protocol," RFC2174, June, 1997.
Acknowledgements 🫶
the authors want to give a huge shoutout to John P. Mullaney, Clark Bremer, Masayuki Kobayashi, Paul Francis, Toshiaki Yoshida, and Takahiro Sajima for their contributions and thoughtful suggestions 💖 the real MVPs, obsessed
Author's Address 💌
Ken Murakami
NTT Software Laboratories
3-9-11, Midori-cho
Musashino-shi
Tokyo-180, Japan
E-mail: murakami@ntt-20.ecl.net
Mitsuru Maruyama
NTT Software Laboratories
3-9-11, Midori-cho
Musashino-shi
Tokyo-180, Japan
E-mail: mitsuru@ntt-20.ecl.netAPPENDIX A. Differences among SONET, SDH, and their Implementations 👑
okay, last stop! this appendix briefly covers the big differences between SONET (an ANSI standard), SDH (an ITU-T standard), and how each one actually gets implemented 🌈
AU pointer (H1, H2, H3) ✨
The AU pointer is made up of bytes H1, H2, and H3. Bits 5 and 6 of the H1 byte are called "SS bits," and they're used to indicate the offset into the payload where the beginning of a SPE is located. (Quick note: "SPE" is a SONET term, and SDH calls it "VC.") For OC-3c, SONET sets the SS bits of the second and third H1 bytes to 0, whereas SDH sets them to 10 for AU-4 and 01 for AU-31. The SS bits may be ignored at the receiving station, but some transmission systems discard SONET/SDH frames with SS bits they don't expect. the sending station should be aware of this and include a configuration option to handle it 🙈
Z1 and Z2 🦋
The Z bytes are reserved in SONET/SDH. Some transmission systems, however, use them in a proprietary way. SONET uses Z1 for Line Error Monitoring. NTT, a carrier in Japan, used Z1 for Automatic Protection Switching (APS.) 💅
DCC Bytes 🍓
The D bytes are called the Data Communication channel (DCC), and they're defined for maintenance and operations. However, some carriers and vendors use them in a proprietary way. For example, NTT's STM-1 UNI uses the D4, D5, and D6 bytes to transfer section and path maintenance information. the proprietary stuff is giving chaos, but that's the spec 😭🫶
📦 where this page comes from
- ietf: rfc2171 1997-06
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms