rfc2615.txt

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Network Working Group                                          A. Malis
Request for Comments: 2615                  Ascend Communications, Inc.
Obsoletes: 1619                                              W. Simpson
Category: Standards Track                                    DayDreamer
                                                              June 1999


                           PPP over SONET/SDH

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.
   This document describes the use of PPP over Synchronous Optical
   Network (SONET) and Synchronous Digital Hierarchy (SDH) circuits.

   This document replaces and obsoletes RFC 1619.  See section 7 for a
   summary of the changes 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






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RFC 2615                  PPP over SONET/SDH                   June 1999


1.  Introduction

   PPP was designed as a standard method of communicating over
   point-to-point links.  Initial deployment has been over short local
   lines, leased lines, and plain-old-telephone-service (POTS) using
   modems.  As new packet services and higher speed lines are introduced,
   PPP is easily deployed in these environments as well.

   This specification is primarily concerned with the use of the PPP
   encapsulation over SONET/SDH links.  Since SONET/SDH is by definition
   a point-to-point circuit, PPP is well suited to use over these links.

   Real differences between SONET and SDH (other than terminology) are
   minor; for the purposes of encapsulation of PPP over SONET/SDH, they
   are inconsequential or irrelevant.

   For the convenience of the reader, we list the equivalent terms below:

       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 currently supported SONET/SDH SPE/VCs are the following:

       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].







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RFC 2615                  PPP over SONET/SDH                   June 1999


2.  Physical Layer Requirements

   PPP treats SONET/SDH transport as octet oriented synchronous links.
   SONET/SDH links are full-duplex by definition.

   Interface Format

      PPP in HDLC-like framing presents an octet interface to the
      physical layer.  There is no provision for sub-octets to be
      supplied or accepted [3][5].

      The octet stream is mapped into the SONET STS-SPE/SDH Higher Order
      VC, with the octet boundaries aligned with the SONET STS-SPE/SDH
      Higher Order VC octet boundaries.

      Scrambling is performed during insertion into the SONET STS-
      SPE/SDH Higher Order VC to provide adequate transparency and
      protect 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 the scrambler is
      bypassed entirely when it is in the off mode.  If this capability
      is provided, the default MUST be set to scrambling enabled.

      For PPP over SONET/SDH, the entire SONET/SDH payload (SONET STS-
      SPE/SDH Higher Order VC minus the path overhead and any fixed
      stuff) is scrambled using a self-synchronous scrambler of
      polynomial X**43 + 1.  See Section 4 for the description of the
      scrambler.

      The proper order of operation 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) indicates the contents of the SONET STS-
   SPE/SDH Higher Order VC.  The value of 22 (16 hex) is used to
   indicate PPP with X^43 + 1 scrambling [4].

   For compatibility with RFC 1619 (STS-3c-SPE/VC-4 only), if scrambling
   has been configured to be off, then the value 207 (CF hex) is used
   for the Path Signal Label to indicate PPP without scrambling.



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RFC 2615                  PPP over SONET/SDH                   June 1999


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

   Control Signals

      PPP does not require the use of control signals.  When available,
      using such signals can allow greater functionality and
      performance.  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 are located by row within the SONET STS-SPE/SDH Higher
   Order VC payload.  Because frames are variable in length, the frames
   are allowed to cross SONET STS-SPE/SDH Higher Order VC boundaries.

4.  X**43 + 1 Scrambler Description

   The X**43 + 1 scrambler transmitter and receiver operation are as
   follows:

   Transmitter schematic:

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
















Malis & Simpson             Standards Track                     [Page 4]

RFC 2615                  PPP over SONET/SDH                   June 1999


   Receiver schematic:

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


   Note: While the HDLC FCS is calculated least significant bit first as
   shown:

              <-  <-  <-  <-
              A   B   C   D

   (that is, the FCS calculator is fed as follows: A[0], A[1], ... A[7],
   B[0], B[1], etc...),  scrambling is done in the opposite manner, most
   significant bit first, as shown:

               ->  ->  ->  ->
               A   B   C   D.

   That is, the scrambler is fed as follows: A[7], A[6], ... A[0], B[7],
   B[6], etc...

   The scrambler operates continuously through the bytes of the SONET
   STS-SPE/SDH Higher Order VC, bypassing 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 beginning of a SONET
   STS-SPE/SDH Higher Order VC is the state at the end of the previous
   SONET STS-SPE/SDH Higher Order VC.  Thus, the scrambler runs
   continuously and is not reset per frame. The initial seed is randomly
   chosen by transmitter to improve operational security (see Section
   6).  Consequently, the first 43 transmitted bits following startup or
   reframe operation will not be descrambled correctly.










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