RFC 2615

PPP over SONET/SDH

📅 published June 1999, from ietf

RFC 2615: PPP over SONET/SDH 🎀

Network Working Group 💖
Authors: A. Malis (Ascend Communications, Inc.) and W. Simpson (DayDreamer) 👯
Category: Standards Track 🔥
Date: June 1999 📅
Obsoletes: RFC 1619 🙈

okay bestie, this one is the spec for sending PPP over SONET/SDH circuits, and it literally replaced RFC 1619 💅 ✨

Status of This Memo 📝

okay so this document is an Internet standards track protocol for the Internet community 🫶 It's also asking for discussion and suggestions for improvements, so speak up 🗣️ For the standardization state and status, check the current edition of the "Internet Official Protocol Standards" (STD 1) 📚 Distribution of this memo is unlimited 🌈

Copyright Notice 💜

Abstract 🦋

The Point-to-Point Protocol (PPP) [1] is the standard way to carry multi-protocol datagrams over point-to-point links 🧚 This document explains how to use PPP over Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) circuits 🍓

this doc replaces and obsoletes RFC 1619, so see section 7 for the summary of what changed from RFC 1619 🫶

Table of Contents 🌸

1.     Introduction ..........................................    2
2.     Physical Layer Requirements ...........................    3
3.     Framing ...............................................    4
4.     X**43 + 1 Scrambler Description .......................    4
5.     Configuration Details .................................    6
6.     Security Considerations ...............................    6
7.     Changes from RFC 1619 .................................    7
8.     Intellectual Property .................................    7
9.     Acknowledgments .......................................    8
10.    References ............................................    8
11.    Authors' Addresses ....................................    9
12.    Full Copyright Statement ..............................   10

1. Introduction ✨

PPP was built as a standard way to talk over point-to-point links 💖 It first got deployed over short local lines, leased lines, and plain-old-telephone-service (POTS) using modems 📞 As new packet services and faster lines showed up, PPP slid right into those environments too, and it's still easy to deploy there 🚀

this spec is mainly about using the PPP encapsulation over SONET/SDH links 🎀 SONET/SDH is by definition a point-to-point circuit, so PPP is a total match for these links 💅

the differences between SONET and SDH, aside from the terminology, are pretty minor 🙄 For PPP encapsulation over SONET/SDH, they're either inconsequential or irrelevant 🫠

For the convenience of the reader, here are the equivalent terms 🧚

    SONET                   SDH
---------------------------------------------
SPE                         VC
STS-SPE                     Higher Order VC (VC-3/4/4-Nc)
STS-1 frame                 STM-0 frame (rarely used)
STS-1-SPE                   VC-3
STS-1 payload               C-3
STS-3c frame                STM-1 frame, AU-4
STS-3c-SPE                  VC-4
STS-3c payload              C-4
STS-12c/48c/192c frame      STM-4/16/64 frame, AU-4-4c/16c/64c
STS-12c/48c/192c-SPE        VC-4-4c/16c/64c
STS-12c/48c/192c payload    C-4-4c/16c/64c

The only SONET/SDH SPE/VCs that are currently supported are these 👑

    SONET                   SDH
----------------------------------------
STS-3c-SPE                  VC-4
STS-12c-SPE                 VC-4-4c
STS-48c-SPE                 VC-4-16c
STS-192c-SPE                VC-4-64c

The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED, SHALL, SHALL NOT, SHOULD, and SHOULD NOT are to be interpreted as defined in [6] 🔥

2. Physical Layer Requirements 🌈

PPP treats SONET/SDH transport as octet oriented synchronous links 💜 SONET/SDH links are full-duplex by definition, so data can flow both ways at once 🦋

Interface Format 💅

PPP in HDLC-like framing shows up to the physical layer as an octet interface 🎀 There's no provision for sub-octets to be supplied or accepted [3][5], so it's all bytes, all the time 🙈

The octet stream gets mapped into the SONET STS-SPE/SDH Higher Order VC, and the octet boundaries line up with the SONET STS-SPE/SDH Higher Order VC octet boundaries ✨

Scrambling happens while data is being inserted into the SONET STS-SPE/SDH Higher Order VC 🧚 It gives you adequate transparency and protects against potential security threats (see Section 6) 🔒 For backwards compatibility with RFC 1619 (STS-3c-SPE/VC-4 only), the scrambler MAY have an on/off capability, where it's bypassed entirely when it's in the off mode 🙃 If that capability is provided, the default MUST be set to scrambling enabled 💖

For PPP over SONET/SDH, the whole SONET/SDH payload gets scrambled. That means the SONET STS-SPE/SDH Higher Order VC minus the path overhead and any fixed stuff, using a self-synchronous scrambler with the polynomial X**43 + 1 🫶 Section 4 has the full scrambler breakdown 🍓

The proper order of operations is 💅

When transmitting:

IP -> PPP -> FCS generation -> Byte stuffing -> Scrambling  ->
SONET/SDH framing

When receiving:

SONET/SDH framing -> Descrambling -> Byte destuffing -> FCS
detection -> PPP -> IP

The Path Signal Label (C2) tells you what's inside the SONET STS-SPE/SDH Higher Order VC 🦋 The value 22 (16 hex) means PPP with X^43 + 1 scrambling [4] ✨

For compatibility with RFC 1619 (STS-3c-SPE/VC-4 only), if scrambling is configured to be off, the Path Signal Label uses the value 207 (CF hex) to mean PPP without scrambling 🙈

The Multiframe Indicator (H4) is unused, and it MUST be zero 🔥

Control Signals 🌸

PPP doesn't require control signals 🙄 When they're available, using them can unlock greater functionality and performance 🚀 The implications are discussed in [2] 📚

3. Framing 🧚

The framing for octet-synchronous links is described in "PPP in HDLC-like Framing" [2] 💜

The PPP frames get located by row within the SONET STS-SPE/SDH Higher Order VC payload 🎀 Because frames are variable in length, they're allowed to cross SONET STS-SPE/SDH Higher Order VC boundaries, which is giving chaotic-but-valid energy 🌈

4. `X**43 + 1` Scrambler Description 🧚

the X**43 + 1 scrambler transmitter and receiver work like this ✨

Transmitter schematic:

                                           Unscrambled Data
                                                  |
                                                  v
     +-------------------------------------+    +---+
  +->|     --> 43 bit shift register -->   |--->|xor|
  |  +-------------------------------------+    +---+
  |                                               |
  +-----------------------------------------------+
                                                  |
                                                  v
                                            Scrambled Data

Receiver schematic:

                                            Scrambled Data
                                                  |
  +-----------------------------------------------+
  |                                               |
  |                                               v
  |  +-------------------------------------+    +---+
  +->|     --> 43 bit shift register -->   |--->|xor|
     +-------------------------------------+    +---+
                                                  |
                                                  v
                                          Unscrambled Data

Note: the HDLC FCS is calculated least significant bit first, as shown here 👀 <- <- <- <- A B C D (so the FCS calculator is fed like this: A[0], A[1], ... A[7], B[0], B[1], etc...). The scrambler works the opposite way, most significant bit first, as shown here: -> -> -> -> A B C D so the scrambler is fed like this: A[7], A[6], ... A[0], B[7], B[6], etc... 🫠

The scrambler runs continuously through the bytes of the SONET STS-SPE/SDH Higher Order VC 🌊 It skips the bytes of SONET Path Overhead and any fixed stuff (see Figure 20 of ANSI T1.105 [3] or Figure 10-17 of ITU G.707 [5]) 📚 The scrambling state at the start of a SONET STS-SPE/SDH Higher Order VC is the state left at the end of the previous one 💖 So the scrambler runs nonstop and never gets reset per frame ✨ The initial seed is picked randomly by the transmitter to help operational security (see Section 6) 🔒 That means the first 43 transmitted bits after startup or reframe operation won't descramble correctly, so this is a known quirk and not a bug 🙈

7. Changes from RFC 1619 🎀

okay so bestie, the BIG glow-up from RFC 1619 was adding payload scrambling when you shove those HDLC-like framed PPP packets into the SONET STS-SPE/SDH Higher Order VC 💅✨. Everything else is supporting cast, but that scrambling is the main character 👑.

Here's the other stuff that changed 🦋

The terminology got updated to match how ANSI and ITU-T talk, so it's giving official 📚

The applicability is now specifically restricted to these pairs 💖

    SONET                   SDH
----------------------------------------
STS-3c-SPE                  VC-4
STS-12c-SPE                 VC-4-4c
STS-48c-SPE                 VC-4-16c
STS-192c-SPE                VC-4-64c

The Path Signal Label (C2) is set to 22 (16 hex) when you're using X^43 + 1 scrambling 🧚

The 32-bit FCS is REQUIRED except for operation with STS-3c-SPE/VC-4, where the 16-bit FCS is allowed 🌸 (but the 32-bit FCS is still RECOMMENDED, so don't skip it lol 🙈)

The Security Considerations section was added, and yes it matters 🔥

Wait, I have to fix that last bit, because the key words need their exact force 😭 Here is the actual version with the requirement levels kept as written:

The 32-bit FCS is REQUIRED except for operation with STS-3c-SPE/VC-4, in which case the 16-bit FCS is allowed (but the 32-bit FCS is still RECOMMENDED).

8. Intellectual Property 🌈

The IETF takes no position on whether any intellectual property or other rights claimed to apply to the technology in this document are valid or in scope. It also doesn't say whether any license under those rights might or might not be available, and it doesn't claim to have tried to find any such rights 🧐. If you want info on the IETF's procedures for rights in standards-track and standards-related documents, check BCP-11 📚. Copies of claimed rights that were submitted for publication, any promised licenses, or the results of an attempt to get a general license or permission to use those proprietary rights by implementers or users of this specification can be obtained from the IETF Secretariat 💌.

The IETF invites any interested party to bring to its attention any copyrights, patents, patent applications, or other proprietary rights that might cover technology required to practice this standard 🫶. Send that info to the IETF Executive Director 💜.

9. Acknowledgments 💖

The scrambler description came from J. Manchester, S. Davida, B. Doshi, and J. Anderson of Lucent Technologies, R. Broberg of Cisco Systems, and Peter Lothberg of Sprint Corporation 🌟. The security analysis came from Iain Verigin of PMC-Sierra and Larry McAdams of Cisco Systems 🔐. The authors also want to thank the members of the IETF's Point-to-Point Protocol Extensions Working Group for all their suggestions and improvements to the text 🙌.

10. References 📖

[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, Daydreamer, July 1994.

[2] Simpson, W., Editor, "PPP in HDLC-like Framing", STD 51, RFC 1662, Daydreamer, July 1994.

[3] American National Standards Institute, "Synchronous Optical Network (SONET) - Basic Description including Multiplex Structure, Rates and Formats," ANSI T1.105-1995.

[4] American National Standards Institute, "Synchronous Optical Network (SONET)--Payload Mappings," T1.105.02-1998.

[5] ITU Recommendation G.707, "Network Node Interface For The Synchronous Digital Hierarchy", 1996.

[6] Bradner, S., "Key words for use in RFCs to indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

11. Authors' Addresses 🎀

Andrew G. Malis Ascend Communications, Inc. 1 Robbins Road Westford, MA 01810 USA

Phone: +1 978 952 7414 EMail: malis@ascend.com

William Allen Simpson DayDreamer Computer Systems Consulting Services 1384 Fontaine Madison Heights, Michigan 48071

EMail: wsimpson@GreenDragon.com

12. Full Copyright Statement ✨

This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English 🌍.

The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns 💜.

This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE 💅.

Acknowledgement

Funding for the RFC Editor function is currently provided by the Internet Society 🦋.

📦 where this page comes from

ietf: rfc2615 1999-06
💖 girlified by claude-haiku-5-5 on 2026-10-16: a language model's rewrite of the original, under its terms