PPP over SONET/SDH

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

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].

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.

   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

   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.

5.  Configuration Details

   Other than the FCS length (see below), the standard LCP sync
   configuration defaults apply to SONET/SDH links.

   The following Configuration Options are RECOMMENDED for STS-3c-
   SPE/VC-4:

      Magic Number
      No Address and Control Field Compression
      No Protocol Field Compression

   For operation at STS-12c-SPE/VC-4-4c and above, Address and Control
   Field Compression and Protocol Field Compression are NOT RECOMMENDED.
   The Magic Number option remains RECOMMENDED.

   Regarding the FCS length, with one exception, the 32-bit FCS MUST be
   used for all SONET/SDH rates. For STS-3c-SPE/VC-4 only, the 16-bit
   FCS MAY be used, although the 32-bit FCS is RECOMMENDED.  The FCS
   length is set by provisioning and is not negotiated.

6.  Security Considerations

   The major change from RFC 1619 is the addition of payload scrambling
   when inserting the HDLC-like framed PPP packets into the SONET STS-
   SPE/SDH Higher Order VC.  RFC 1619 was operationally found to permit
   malicious users to generate packets with bit patterns that could
   create SONET/SDH-layer low-transition-density synchronization
   problems, emulation of the SDH set-reset scrambler pattern, and
   replication of the STM-N frame alignment word.

   The use of the x^43 + 1 self-synchronous scrambler was introduced to
   alleviate these potential security problems.  Predicting the output
   of the scrambler requires knowledge of the 43-bit state of the
   transmitter as the scrambling of a known input is begun.  This
   requires knowledge of both the initial 43-bit state of the scrambler
   when it started and every byte of data scrambled by the device since
   it was started.  The odds of guessing correctly are 1/2**43, with the
   additional probability of 1/127 that a correct guess will leave the
   frame properly aligned in the SONET/SDH payload, which results in a
   probability of 9e-16 against being able to deliberately cause
   SONET/SDH-layer problems.  This seems reasonably secure for this
   application.

   This scrambler is also used when transmitting ATM over SONET/SDH, and
   public network carriers have considerable experience with its use.

   A known security issue is bandwidth reduction by intentional
   transmission of characters or sequences requiring transparency
   processing by including flag and/or escape characters in user data. A
   user may cause up to a 100% increase in the bandwidth required for
   transmitting his or her packets by filling the packet with flag
   and/or escape characters.

7.  Changes from RFC 1619

   As mentioned in the previous section, the major change from RFC 1619
   was the addition of payload scrambling when inserting the HDLC-like
   framed PPP packets into the SONET STS-SPE/SDH Higher Order VC.  Other
   changes were:

   The terminology was updated to better match that used by ANSI and
   ITU-T.

   The specification's applicability is now specifically restricted to:

       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 using X^43 + 1
   scrambling.

   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).

   The Security Considerations section was added.

8.  Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors 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 or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

9.  Acknowledgments

   The scrambler description was provided by 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 was provided by Iain Verigin of PMC-Sierra and Larry McAdams
   of Cisco Systems.  The authors would also like to thank the members
   of the IETF's Point-to-Point Protocol Extensions Working Group for
   their many 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

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

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