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Versions: 00                                                            
INTERNET-DRAFT                                             Thomas Narten
                                                          August 7, 1998

        PPP Over SONET Applicability Statement for Historic Status


Status of this Memo

   This document is an Internet-Draft. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups. Note that other groups may also distribute
   working documents as Internet-Drafts.

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   Distribution of this memo is unlimited.

   This Internet Draft expires February, 1999.

1.  Introduction

   PPP over Sonet [RFC 1619] uses "HDLC-like framing" as defined in [RFC
   1662]. These documents were developed to address a void in existing
   SONET standards at that time.

   T1.X1 is the ANSI accredited T1 Technical Subcommitee responsible for
   developing SONET standards, including SONET rates, formats and
   payload mappings. Its output is also brought into the ITU, for
   example as in ITU-T Recommendation G.707 [G.707].

   At the December, 1997 IETF in Washington DC, a BOF was held to
   discuss a potential weakness in previous mappings. Specifically, long
   packets (where "long" can include those carried by PPP) containing a
   special bit pattern can cause a SONET reciever to declare a "loss of

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INTERNET-DRAFT                                            August 7, 1998

   signal". The minutes of the BOF are included in Appendix A.

   T1.X1 has recently defined a mapping for HDLC over SONET [T1.105.02,
   T1.105] that addresses the potential weakness described above.  The
   work done by T1.X1 on SONET supersedes past efforts that have taken
   place within the IETF.  The recommendation of this applicability
   statement is that all PPP over SONET implementations use the
   combination of RFC 1662 plus "Packet Over Sonet/SDH" [POS]. The first
   document describes how to encapsulate PPP in HDLC; the second
   document describes how to encapsulate HDLC in SONET. We also
   recommend that RFC 1619 be reclassified as Historic.

2.  References

     [RFC 1619] Simpson, W., "PPP over SONET/SDH", RFC 1619, May 1994.

     [RFC 1662] Simpson, W., "PPP in HDLC-like Framing", RFC 1662, July

     [T1.105.02] ANSI T1.105.02 "Synchronous Optical Network (SONET) -
             Payload Mappings"

     [T1.105] ANSI T1.105 "Synchronous Optical Network (SONET) - Basic
             Description including Multiplex Structure, Rates and

     [G.707] CCITT Recommendation G.707, "Synchronous Digital Hierarchy
             Bit Rates", 1998 (??? - XXX).
     [POS] Narten, T., "Packet over Sonet/SDH", draft-narten-packet-

3.  Author's Address

   Thomas Narten
   IBM Corporation
   3039 Cornwallis Ave.
   PO Box 12195 - BRQA/502
   Research Triangle Park, NC 27709-2195

   Phone: 919-254-7798
   EMail: narten@raleigh.ibm.com

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INTERNET-DRAFT                                            August 7, 1998

   APPENDIX A: Minutes of the PPP Over Sonet/SDH Bof

   Minutes of the PPP Over Sonet/SDH Bof 10 December 1997

   Chair: Mike O'Dell

   Scribe: Frank Kastenholz

    0. Introduction and Agenda-Bashing
    1. Statement of the Problem
    2. Stop gaps for the near-term
    3. Long-term Solution

   Mike O'Dell opened the meeting, presenting the agenda. There was no

   Peter Lothberg gave a brief talk on what the problem was:

        If a SONET receiver detects a "long" string of 0s ("no light")
        then the receiver will declare a "loss of signal".  Of course,
        someone may, legitimately, wish to send a large number of 0s;
        this should not cause an LOS.

        In order to prevent this, SONET uses a 'scrambler' which
        attempts to ensure transmission of a sufficient number of 1s so
        that the receiver will not declare a false LOS.  This scrambler
        works well for rather short data blocks. However, for long
        blocks (such as PPP Packets), it is possible to devise packets
        that, after scrambling, show up on the fiber as a long string of
        0s (i.e. no light). Of course, it is also possible that such a
        packet could be generated in the normal course of events (i.e.
        non-maliciously and non-conciously)

        Furthermore, an active attack could be launched against the
        SONET infrastructure from anyplace in the Internet.

        This is not a real operational issue right now. The current
        error rates on SONET links (including otherwise unexplained LOS)
        is well within the nominal error rates expected for SONET.
        However, since this issue was brought up, there have been some
        reports of attacks being made.

   Peter then presented a stop-gap to deal with this issue in the short

        A new scrambler (the "ATM Scrambler") would be inserted between
        the HDLC Byte-Stuffing and the normal sonet payload scrambler.

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INTERNET-DRAFT                                            August 7, 1998

        This has the property that it can be done in software in the
        near term and migrated into hardware later on. The ATM Scrambler
        is a 1+X**43 scrambler. This scrambler is well known and easy to
        implement, is believed to have a sufficient number of states to
        protect the SONET networks. On the down-side, it does introduce
        error multiplication (because of the feed-back into the shift-
        register) and may interact with the CRC-16 of PPP Packets in
        ways to miss some errors.

        The error multiplication was deemed not to be a significant
        issue since once a packet was "errored", it really wasn't all
        that big a deal if it had n bits of error or 2n bits or...

        The interaction with CRC-16 can be alleviated in several ways:
           - Packet header consistancy checks (eg, the packet length and
             the IP header's Total Length field musth be consistant)
           - Transport error detection
           - Use of CRC-32

             Karl Fox suggested that the PPP/SONET specification be
             modified to require that CRC32 be used all the time (i.e.
             not be negotiated, just "always used").

   Mike O'Dell then offered as a long-term solution that ANSI T1.X1 do
   an addendum to T1.105.02 (?) to specify "HDLC Over Sonet". T1.X1
   would formally adopt the ATM scrambler as a part of this

   General Discussion then ensued.

   Bill Simpson recommended that the solution be simple byte stuffing in
   order to prevent the known bad bit patterns from appearing.

   There were many comments back and forth on the relative merits of
   byte stuffing vs 1+X**43 scrambling (which was easier, which was more
   robust, implementing in hardware vs software, and so forth).

   A "sense-of-the-BOF" was taken via the time-honored IETF Hum:
      - The IETF should pursue the 1+X**43 scrambler solution offered by
        Peter Lothberg for the near-term.
      - The byte-stuffing technique would not be pursued.
      - The long-term solution would be an ANSI T1.X1 "HDLC-over-SONET
        Mapping" which would use the 1+X**43 scrambler.

   Discussion then turned to the SONET Payload C2 byte. Should a new
   code be allocated for the "new"      (i.e. with 1+X**43 scrambling)
   payload format?
      - The current standard (RFC1619) specifies an experimental code

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INTERNET-DRAFT                                            August 7, 1998

        point of 207 (0xcf) for the C2 byte.
      - A new code-point would allow receivers to differentiate new-
        from old-format payloads.
      - It was not clear whether all implementations generated or
        checked the C2-byte code-point, so putting in a new code point
        might not be a real win.

   The recommendation of the BOF was that the IETF would not pursue a
   new code-point and would use the old one for "new-format" packets.
   However, if and when ANSI T1.X1 recommends a code-point for their
   mapping, that number should be adopted "with all deliberate haste."

   ANSI T1X1 expects to make a decision on this in January 1998.  Their
   results will be announced to the PPP Mailing List.

   Peter Lothberg then made a brief presentation for "Bytes Over Sonet",
   a proposed new standard, with the following suggested features:
      - No byte stuffing, length-fields would be used.
      - A Combined scrambler/CRC
      - Combined multiple STM-1s
      - Some kind of TDM
   A mailing list will be set up: bos@stupi.se. Subscription requests
   should be sent to bos-request@stupi.se

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