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PPP over Simple Data Link (SDL) using SONET/SDH with ATM-like framing

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 2823.
Authors James D. Carlson , Paul Langner , James Manchester , Enrique J. Hernandez-Valencia
Last updated 2013-03-02 (Latest revision 1999-10-19)
RFC stream Internet Engineering Task Force (IETF)
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IESG IESG state RFC 2823 (Experimental)
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PPP Working Group                                             J. Carlson
Internet Draft                                       IronBridge Networks
Expires April 2000                                            P. Langner
                              Lucent Technologies Microelectronics Group
                                                E. J. Hernandez-Valencia
                                                     Lucent Technologies
                                                           J. Manchester
                                                     Lucent Technologies
                                                            October 1999

                    PPP over Simple Data Link (SDL)
                 using SONET/SDH with ATM-like framing

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   To view the list Internet-Draft Shadow Directories, see

   This document is the product of the Point-to-Point Protocol
   Extensions Working Group of the Internet Engineering Task Force
   (IETF).  Comments should be submitted to the
   mailing list.

   Distribution of this memo is unlimited.

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   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links, and
   RFCs 1662 [2] and 2615 [3] provide a means to carry PPP over
   Synchronous Optical Network (SONET) [4] and Synchronous Digital
   Hierarchy (SDH) [5] circuits.  This document extends these standards
   to include a new encapsulation for PPP called Simple Data Link (SDL)
   [6].  SDL provides a very low overhead alternative to HDLC-like
   encapsulation, and can also be used on SONET/SDH links.


   This specification is intended for those implementations that use PPP
   over high speed point-to-point circuits, both with so-called "dark
   fiber" and over public telecommunications networks.  Because this
   enhanced PPP encapsulation has very low overhead and good hardware
   scaling characteristics, it is anticipated that significantly higher
   throughput can be attained when compared to other possible SONET/SDH
   payload mappings, and at a significantly lower cost for line
   termination equipment.

   SDL is defined over other media types and for other data link
   protocols, but this specification covers only the use of PPP over SDL
   on SONET/SDH.

   The use of SDL requires the presentation of packet length information
   in the SDL header.  Thus, hardware implementing SDL must have access
   to the packet length when generating the header, and where a router's
   input link does not have this information (that is, for non-SDL input
   links), the router may be required to buffer the entire packet before
   transmission.  "Worm-hole" routing is thus at least problematic with
   SDL, unless the input links are also SDL.  This, however, does not
   appear to be a great disadvantage on modern routers due to the
   general requirement of length information in other parts of the
   system, notably in queuing and congestion control strategies such as
   Weighted Fair Queuing [7] and Random Early Detect [8].

   This document is not a replacement for the existing HDLC-like framing
   mandated by RFC 2615 [3].  Instead, the authors intend to gain
   implementation experience with this technique for operational and
   performance evaluation purposes, and would like to hear from others
   either considering or using the protocol as described in this
   document.  Please see Section 15 of this document for contact

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Table of Contents

   1.  Introduction ...............................................    4
   2.  Compliance .................................................    4
   3.  Physical Layer Requirements ................................    5
   3.1.  Payload Types ............................................    5
   3.2.  Control Signals ..........................................    6
   3.3.  Synchronization Modes ....................................    7
   3.4.  Simple-Data-Link LCP Option ..............................    7
   3.5.  Framing ..................................................    8
   3.6.  Framing Example ..........................................   10
   3.7.  Synchronization Procedure ................................   11
   3.8.  Scrambler Operation ......................................   11
   3.9.  CRC Generation ...........................................   12
   3.10.  Error Correction ........................................   13
   4.  Performance Analysis .......................................   14
   4.1.  Mean Time To Frame (MTTF) ................................   14
   4.2.  Mean Time To Synchronization (MTTS) ......................   15
   4.3.  Probability of False Frame (PFF) .........................   16
   4.4.  Probability of False Synchronization (PFS) ...............   16
   4.5.  Probability of Loss of Frame (PLF) .......................   16
   5.  The Special Messages .......................................   16
   5.1.  Scrambler State ..........................................   17
   5.2.  A/B Message ..............................................   17
   6.  The Set-Reset Scrambler Option .............................   17
   6.1.  The Killer Packet Problem ................................   17
   6.2.  SDL Set-Reset Scrambler ..................................   18
   6.3.  SDL Scrambler Synchronization ............................   18
   6.4.  SDL Scrambler Operation ..................................   19
   7.  Configuration Details ......................................   21
   7.1.  Default LCP Configuration ................................   21
   7.2.  Modification of the Standard Frame Format ................   21
   8.  Implementation Details .....................................   22
   8.1.  CRC Generation ...........................................   22
   8.2.  Error Correction Tables ..................................   24
   9.  Security Considerations ....................................   25
   10.  References ................................................   26
   11.  Acknowledgments ...........................................   27
   12.  Working Group and Chair Address ...........................   27
   13.  Intellectual Property Notices .............................   27
   14.  Copyright Notice ..........................................   28
   15.  Authors' Addresses ........................................   29

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1.  Introduction

   The Path Signal Label (SONET/SDH overhead byte named C2; referred to
   as PSL in this document) is intended to indicate the type of data
   carried on the path.  This data, in turn, is referred to as the SONET
   Synchronous Payload Envelope (SPE) or SDH Administrative Unit Group
   (AUG).  The experimental PSL value of decimal 207 (CF hex) is
   currently [3] used to indicate that the SPE contains PPP framed using
   RFC 1662 Octet Synchronous (O-S) framing and transmission without
   scrambling, and the value 22 (16 hex) is used to indicated PPP framed
   using O-S framing and transmission with ATM-style X^43+1 scrambling.

   This document describes a method to enable the use of SDL framing for
   PPP over SONET/SDH, and describes the framing technique and require-
   ments for PPP.  While O-S framing on SONET/SDH has a fixed seven
   octet overhead per frame plus a worst-case overhead of 100% of all
   data octets transmitted, SDL has a fixed eight octet per frame over-
   head with zero data overhead.  Unlike O-S framing, SDL also provides
   positive indication of link synchronization.

   Note:  This document describes two new SONET/SDH Path Signal Label
   (PSL) values, 23 (17 hex) and 24 (18 hex).  These values have not
   been allocated by any applicable standards body.  A joint contribu-
   tion has been made to ANSI subcommittee T1X1.5 requesting the assign-
   ment of 17 hex as the PSL for an SDL over SONET mapping with the pro-
   posed self synchronous scrambler, and the assignment of 18 hex as the
   PSL for an SDL over SONET mapping with the proposed set-reset scram-
   bler in recommendation T1.105.

2.  Compliance

   In this document, the words that are used to define the significance
   of each particular requirement are capitalized.

   These words are:

   *    "MUST"

        This word means that the item is an absolute requirement of the

   *    "MUST NOT"

        This phrase means that the item is an absolute prohibition of the

   *    "SHOULD"

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        This word means that there may exist valid reasons in particular
        circumstances to ignore this item, but the full implications
        should be understood and the case carefully weighed before
        choosing a different course.

   *    "SHOULD NOT"

        This phrase means that there may exist valid reasons in
        particular circumstances to apply this item, but the full
        implications should be understood and the case carefully weighed
        before choosing a different course.

   *    "MAY"

        This word means that this item is truly optional.  One vendor may
        choose to include the item because a particular marketplace
        requires it or because it enhances the product, for example;
        another vendor may omit the same item.

   An implementation is not compliant if it fails to satisfy one or more
   of the MUST or MUST NOT requirements for this protocol.  An implemen-
   tation that satisfies all of the MUST, MUST NOT, SHOULD, and SHOULD
   NOT requirements for this protocol is said to be "unconditionally
   compliant."  One that satisfies all the MUST and MUST NOT require-
   ments but not all the SHOULD or SHOULD NOT requirements is said to be
   "conditionally compliant."

3.  Physical Layer Requirements

   PPP treats SONET/SDH transport as octet-oriented synchronous links.
   No provision is made to transmit partial octets.  Also, SONET/SDH
   links are full-duplex by definition.

3.1.  Payload Types

   Only synchronous payloads STS-1 and higher are considered in this
   document.  Lower speed synchronous, such as VT1.5-SPE/VC-11, and
   plesiochronous payload mappings, such as T1 and T3, are defined for
   SONET/SDH and for the SDL algorithm itself, but, since HDLC-like
   framing is defined for PPP on those media, PPP over SDL is not

   SDL is separately defined as a PPP transport for use on raw fiber
   without SONET/SDH framing for use as an alternative to bit-
   synchronous HDLC.  Please see the separate work-in-progress for

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3.2.  Control Signals

   The PPP over SONET/SDH mapping allows the use of the PSL as a control
   signal.  Not all equipment, however, is capable of setting or detect-
   ing this value, and any use must take this into account.  Equipment
   employing only SDL MUST be capable of transmitting PSL with value 23,
   and MAY also be capable of transmitting PSL with value 24, but need
   not be capable of detecting the peer's value or capable of changing
   its own value.

   There are two methods to enable SDL, an LCP-negotiated method and a
   prior-arrangement method.  The former allows for easier configuration
   and compatibility with existing equipment, while the latter allows
   general use with separate SONET/SDH transmission equipment with PSL
   limitations.  Both types of implementations will freely interoperate
   given the procedures below.

   LCP-negotiated systems MUST be capable of changing their transmitted
   PSL value and detecting the peer's value.  Equipment without these
   features MUST NOT support LCP negotiation of SDL.

   When SDL is negotiated by LCP, LCP negotiation MUST be started with
   the PSL value initially set to 22 or 207 and the corresponding non-
   SDL O-S PPP encapsulation MUST be used.  The SDL LCP option is then
   placed in the LCP Configure-Request messages transmitted.  On recep-
   tion of LCP Configure-Request with an SDL LCP option or when the
   peer's transmitted PSL value is received as 23 (or 24), the implemen-
   tation MUST shut down LCP by sending a Down event to its state
   machine, then switch its transmitted PSL value to 23 (or 24), switch
   encapsulation mode to SDL, wait for SDL synchronization, and then
   restart LCP by sending an Up event into LCP.  Otherwise, if the peer
   does not transmit PSL value 23 (or 24) and it does not include the
   SDL LCP option in its LCP Configure-Request messages, then operation
   using non-SDL O-S PPP encapsulation continues.  If the received PSL
   value subsequently received reverts from 23 (or 24) to any other
   value, then this is treated as a Down event into the LCP state
   machine, and an Up event MUST be generated if the new value is recog-
   nized as a valid PPP framing mode.

   When SDL is enabled by prior arrangement, the PSL SHOULD be transmit-
   ted as 23 (or 24).  Any other value may also be used by prior exter-
   nal arrangement with the peer, although the values 22 and 207 are
   discouraged.  (Such use is enforced by an administrator, and is out-
   side the scope of this specification.)  When SDL is enabled by prior
   arrangement, the SDL LCP option SHOULD NOT be negotiated by the

   An implementation-specific configuration option SHOULD exist to

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   enable the use of prior-arrangement versus LCP-negotiated modes.
   This option SHOULD be presented to an administrator, and SHOULD
   default to LCP-negotiated if the hardware permits.  Otherwise, if the
   hardware implementation precludes non-SDL modes of operation, then it
   MUST default to prior-arrangement mode.

   The LCP-negotiated method of operation is compatible with the current
   version of G.783 [12].  This method may not be compatible, however,
   with some non-intrusive SDH path monitoring equipment based on
   obsolete versions of G.783.  The change in PSL value indicated by the
   LCP negotiation method will cause this equipment to declare an alarm
   condition on the path.  For this reason, the prior-arrangement method
   MUST be used on any SDH network that is using such monitoring equip-

3.3.  Synchronization Modes

   Unlike O-S encapsulation, SDL provides a positive indication that it
   has achieved synchronization with the peer.  An SDL PPP implementa-
   tion MUST provide a means to temporarily suspend PPP data transmis-
   sion (both user data and negotiation traffic) if synchronization loss
   is detected.  An SDL PPP implementation SHOULD also provide a confi-
   gurable timer that is started when SDL is initialized and restarted
   on the loss of synchronization, and is terminated when link synchron-
   ization is achieved.  If this timer expires, implementation-dependent
   action should be taken to report the hardware failure.

3.4.  Simple-Data-Link LCP Option

   A new LCP Configuration Option is used to request Simple Data Link
   (SDL) [6] operation for the PPP link.

   A summary of the Simple-Data-Link Configuration Option format for the
   Link Control Protocol (LCP) is shown below.  The fields are transmit-
   ted from left to right.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   |     Type      |    Length     |



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   This option is used only as a hint to the peer that SDL over
   SONET/SDH operation is preferred by the sender.  If the current
   encapsulation mode is not SDL, then the only appropriate response to
   reception of this option by an SDL speaker is to then switch the
   encapsulation mode to SDL (as detailed in the section above) and res-
   tart LCP.  Non SDL-speakers SHOULD instead send LCP Configure-Reject
   for the option.

   If either LCP Configure-Nak or LCP Configure-Reject is received for
   this option, then the next transmitted LCP Configure-Request MUST NOT
   include this option.  If LCP Configure-Ack with this option is
   received, it MUST NOT be treated as a request to switch into SDL
   mode.  If the received LCP Configure-Request message does not contain
   an SDL LCP option, an implementation MUST NOT send an unsolicited
   Configure-Nak for the option.

   (An implementation of SDL that is already in SDL framing mode and
   receives this option in an LCP Configure-Request message MAY, both
   for clarity and for convergence reasons, elect to send LCP
   Configure-Ack.  It MUST NOT restart LCP nor change framing modes in
   this case.)

3.5.  Framing

   The PPP frames are located by row within the SPE payload.  Because
   frames are variable in length, the frames are allowed to cross SPE
   boundaries.  Bytes marked as "overhead" or "fixed stuff" in SONET/SDH
   documentation for concatenated streams are not used as payload bytes.

   With reference to the Lucent SDL specification [6] when SDL framing
   for PPP is employed, the SDL "Datagram Offset" feature is set to the
   value 4.  This corresponds to the fixed overhead value 4 in the
   description below.  The "A" and "B" messages are never used.  These
   optional features of SDL are not described in this document, but are
   rather described in Lucent's SDL specification.

   Fixing the Datagram Offset value described in the Lucent documenta-
   tion to 4 allows a PPP MRU/MTU up to 65536 using SDL.

   SDL framing is in general accomplished by the use of a four octet
   header on the packet.  This fixed-length header allows the use of a
   simple framer to detect synchronization as described in section 3.7.
   For use with PPP, this fixed-length header precedes each PPP/HDLC

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   packet as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |         Packet Length         |          Header CRC           |
   |     PPP packet (beginning with address and control fields)    |
   |                             .....                             |
   |                            SDL CRC                            |

   The four octet length header is DC balanced by exclusive-OR (also
   known as "modulo 2 addition") with the hex value B6AB31E0.  This is
   the maximum transition, minimum sidelobe, Barker-like sequence of
   length 32.  No other scrambling is done on the header itself.

   Packet Length is an unsigned 16 bit number in network byte order.
   Unlike the PPP FCS, the Header CRC is a CRC-16 generated with initial
   value zero and transmitted in network byte order.  The PPP packet is
   scrambled, begins with the address and control fields, and may be any
   integral octet length (i.e., it is not padded unless the Self
   Describing Padding option is used).  The Packet CRC is also scram-
   bled, and has a mode-dependent length (described below), and is
   located only on an octet boundary; no alignment of this field may be

   When the Packet Length value is 4 or greater, the distance in octets
   between one message header and the next in SDL is the sum of 8 plus
   the Packet Length field.  The value 8 represents a fixed overhead of
   4 octets plus the fixed length of the Packet CRC field.  When the
   Packet Length is 0, the distance to the next header is 4 octets.
   This is the idle fill header.  When the Packet Length is 1 to 3, the
   distance to the next header is 12 octets.  These headers are used for
   special SDL messages used only with optional scrambling and manage-
   ment modes.  See section 5 for details of the messages.

   General SDL, like PPP, allows the use of no CRC, ITU-T CRC-16, or
   ITU-T CRC-32 for the packet data.  However, because the Packet Length
   field does not include the CRC length, synchronization cannot be
   maintained if the CRC type is changed per RFC 1570 [9], because
   frame-to-frame distance is, as described above, calculated including
   the CRC length.  Thus, this PPP over SDL specification fixes the CRC
   type to CRC-32 (four octets), and all SDL implementations MUST reject
   any LCP FCS Alternatives Option [9] requested by the peer when in SDL

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   PPP over SDL implementations MAY allow a configuration option to set
   different CRC types for use by prior arrangement.  Any such configur-
   able option MUST default to CRC-32, and MUST NOT include LCP negotia-
   tion of FCS Alternatives.

   Setting the SDL Datagram Offset value to 4 accounts for the 4 octet
   SDL header overhead.  With the SDL Datagram Offset set to 4, the
   value placed in the Packet Length field is exactly the length in
   octets of the PPP frame itself, including the address and control
   fields but not including the CRC field (the RFC 1662 PPP FCS field is
   not used with SDL).  Note again that the Datagram Offset is just an
   arithmetic value; it does not occupy bits in the message itself.

   Because Packet Lengths below 4 are reserved, the Packet Length MUST
   be 4 or greater for any legal PPP packet.  PPP packets with fewer
   octets, which are not possible without address/control or protocol
   field compression, MUST be padded to length 4 for SDL.

   Inter-packet time fill is accomplished by sending the four octet
   length header with the Packet Length set to zero.  No provision is
   made for intra-packet time fill.

   By default, an independent, self-synchronous x^43+1 scrambler is used
   on the data portion of the message including the 32 bit CRC.  This is
   done in exactly the same manner as with the ATM x^43+1 scrambler on
   an ATM channel.  The scrambler is not clocked when SDL header bits
   are transmitted.  Thus, the data scrambling MAY be implemented in an
   entirely independent manner from the SDL framing, and the data stream
   may be prescrambled before insertion of SDL framing marks.

   Optionally, by prior arrangement, SDL links MAY use a set-reset
   scrambler as described in section 6.  If this option is provided, it
   MUST be configurable by the administrator, and the option MUST
   default to the self-synchronous scrambler.

3.6.  Framing Example

   To help clarify this structure, the following example may be helpful.
   First we have an LCP Configure-Request message that we wish to
   transmit over SDL:

       FF 03 C0 21 01 01 00 04

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   Next, we create an SDL header for the length of this packet (8
   octets), a header CRC, and an SDL CRC.

       00 08 81 08 FF 03 C0 21 01 01 00 04 D1 F5 21 5E

   Finally, we DC-balance the header with the barker-like sequence:

       B6 A3 B0 E8 FF 03 C0 21 01 01 00 04 D1 F5 21 5E

   Note that the final length of the message is 8 (original message
   length) plus 4 (fixed datagram offset value) plus 4 (fixed CRC
   length), or 16 octets.

3.7.  Synchronization Procedure

   The link synchronization procedure is similar to the I.432 section ATM HEC delineation procedure [10], except that the SDL mes-
   sages are variable length.  The machine starts in HUNT state until a
   four octet sequence in the data stream with a valid CRC-16 is found.
   (Note that the CRC-16 single-bit error correction technique described
   in section 3.10 is not employed until the machine is in in SYNCH
   state.  The header must have no bit errors in order to leave HUNT
   state.)  Such a valid sequence is a candidate SDL header.  On finding
   the valid sequence, the machine enters PRESYNCH state.  Any one
   invalid SDL header in PRESYNCH state returns the link to HUNT state.

   If a second valid SDL header is seen after entering PRESYNCH state,
   then the link enters SYNCH state and PPP transmission is enabled.  If
   an invalid SDL header is detected, then the link is returned to HUNT
   state without enabling PPP transmission.

   Once the link enters SYNCH state, the SDL header single bit error
   correction logic is enabled (see section 3.10).  Any unrecoverable
   header CRC error returns the link to HUNT state, disables PPP
   transmission, and disables the error correction logic.

3.8.  Scrambler Operation

   The transmit and receive scramblers are shift registers with 43
   stages that MAY be initialized to all-ones when the link is initial-
   ized.  Synchronization is maintained by the data itself.

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               Transmit                            Receive

    |                                  |
    v                                  |
    XOR<-------------------------+     +->D0-+->D1-> ... ->D41->D42-+
    |                            |     |                            |
    +->D0-+->D1-> ... ->D41->D42-+     XOR<-------------------------+
    |                                  |
    v                                  v
    OUT (TO SDL FRAMER)                DATA-STREAM (TO PPP)

   Each XOR is an exclusive-or gate; also known as a modulo-2 adder.
   Each Dn block is a D-type flip-flop clocked on the appropriate data

   The scrambler is clocked once after transmission or reception of each
   bit of payload and before the next bit is applied as input.  Bits
   within an octet are, per SONET/SDH practice, transmitted and received

3.9.  CRC Generation

   The CRC-16 and CRC-32 generator polynomials used by SDL are the ITU-T
   polynomials [11].  These are:



   The SDL Header CRC and the CRC-16 used for each of the three special
   messages (scrambler state, message A, and message B; see section 5)
   are all generated using an initial remainder value of 0000 hex.

   The optional CRC-16 on the payload data (this mode is not used with
   PPP over SDL except by prior arrangement) uses the initial remainder
   value of FFFF hex for calculation and the bits are complemented
   before transmission.  The final CRC remainder, however, is transmit-
   ted in network byte order, unlike the regular PPP FCS.  If the CRC-16
   algorithm is run over all of the octets including the appended CRC
   itself, then the remainder value on intact packets will always be
   E2F0 hex.  Alternatively, an implementation may stop CRC calculation
   before processing the appended CRC itself, and do a direct com-

   The CRC-32 on the payload data (used for PPP over SDL) uses the ini-
   tial remainder value of FFFFFFFF hex for calculation and the bits are

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   complemented before transmission.  The CRC, however, is transmitted
   in network byte order, most significant bit first, unlike the
   optional PPP 32 bit FCS, which is transmitted in reverse order.  The
   remainder value on intact packets when the appended CRC value is
   included in the calculation is 38FB2284.

   C code to generate these CRCs is found in section 8.1.

3.10.  Error Correction

   The error correction technique is based on the use of a Galois number
   field, as with the ATM HEC correction.  In a Galois number field,
   f(a+b) = f(a) + f(b).  Since the CRC-16 used for SDL forms such a
   field, we can state that CRC(message+error) = CRC(message) +
   CRC(error).  Since the CRC-16 remainder of a properly formed message
   is always zero, this means that, for the N distinct "error" strings
   corresponding to a single bit error, there are N distinct CRC(error)
   values, where N is the number of bits in the message.

   A table look-up is thus applied to the CRC-16 residue after calcula-
   tion over the four octet SDL header to correct bit errors in the
   header and to detect multiple bit errors.  For the optional set-reset
   scrambler, a table look-up is similarly applied to the CRC-16 residue
   after calculation over the eight octet scrambler state message to
   correct bit errors and to detect multiple bit errors.  (This second
   correction is also used for the special SDL A and B messages, which
   are not used for PPP over SDL.)

   Note:  No error correction is performed for the payload.

   Note:  This error correction technique is used only when the link has
   entered SYNCH state.  While in HUNT or PRESYNCH state, error correc-
   tion should not be performed, and only messages with syndrome 0000
   are accepted.  If the calculated syndrome does not appear in this
   table, then an unrecoverable error has occurred.  Any such error in
   the SDL header will return the link to HUNT state.

   Since the CRC calculation is started with zero, the two tables can be
   merged.  The four octet table is merely the last 32 entries of the
   eight octet table.

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   Eight octet (64 bit) single bit error syndrome table (in hexade-

     FD81 F6D0 7B68 3DB4 1EDA 0F6D 8FA6 47D3
     ABF9 DDEC 6EF6 377B 93AD C1C6 60E3 B861
     D420 6A10 3508 1A84 0D42 06A1 8B40 45A0
     22D0 1168 08B4 045A 022D 8906 4483 AA51
     DD38 6E9C 374E 1BA7 85C3 CAF1 ED68 76B4
     3B5A 1DAD 86C6 4363 A9A1 DCC0 6E60 3730
     1B98 0DCC 06E6 0373 89A9 CCC4 6662 3331
     9188 48C4 2462 1231 8108 4084 2042 1021

   Thus, if the syndrome 6EF6 is seen on an eight octet message, then
   the third bit (hex 20) of the second octet is in error.  Similarly,
   if 48C4 is seen on an eight octet message, then the second bit (hex
   40) in the eighth octet is in error.  For a four octet message, the
   same two syndromes would indicate a multiple bit error for 6EF6, and
   a single bit error in the second bit of the fourth octet for 48C4.

   Note that eight octet messages are used only for the optional set-
   reset scrambling mode, described in section 6.

   Corresponding C code to generate this table is found in section 8.2.

4.  Performance Analysis

   There are five general statistics that are important for framing
   algorithms.  These are:

     MTTF   Mean time to frame
     MTTS   Mean time to synchronization
     PFF    Probability of false frame
     PFS    Probability of false synchronization
     PLF    Probability of loss of frame

   The following sections summarize each of these statistics for SDL.
   Details and mathematic development can be found in the Lucent SDL
   documentation [6].

4.1.  Mean Time To Frame (MTTF)

   This metric measures the amount of time required to establish correct
   framing in the input data.  This may be measured in any convenient
   units, such as seconds or bytes.  For SDL, the relevant measurement
   is in packets, since fragments of packets are not useful.

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   In order to calculate MTTF, we must first determine how often the
   frame detection state machine is "unavailable" because it failed to
   detect the next incoming SDL frame in the data stream.

   Since the probability of a false header detection using CRC-16 in
   random data is 2^-16 and this rate is large compared to the allowable
   packet size, it is worthwhile to run multiple parallel frame-
   detection state machines.  Each machine starts with a different can-
   didate framing point in order to reduce the probability of falsely
   detecting user data as a valid frame header.

   The results for this calculation, given maximal 64KB packets and
   slightly larger than Internet average 354 byte packets, are:

     Number of  Unavailability  Unavailability
      Framers    64KB packets   354 byte pkts
         1         3.679E-1        5.373E-3
         2         3.083E-2        1.710E-6
         3         2.965E-3        9.712E-10
         4         2.532E-4        4.653E-13

   Using these values, MTTF can be calculated as a function of the Bit
   Error Rate (BER).  These plots show a characteristically flat region
   for all BERs up to a knee, beyond which the begins to rise sharply.
   In all cases, this knee point has been found to occur at a BER of
   approximately 1E-4, which is several orders of magnitude above that
   observed on existing SONET/SDH links.  The flat rate values are sum-
   marized as:

     Number of  Flat region   Flat region
      Framers   64KB packets   354 bytes
         1         3.58          1.52
         2         1.595         1.5
         3         1.52          1.5
         4         1.5           1.5

   Thus, for common packet sizes in an implementation with two parallel
   framers using links with a BER of 1E-4 or better, the MTTF is approx-
   imately 1.5 packets.  This is also the optimal time, since it
   represents initiating framing at an average point half-way into one
   packet, and achieving good framing after seeing exactly one correctly
   framed packet.

4.2.  Mean Time To Synchronization (MTTS)

   The MTTS for SDL with a self-synchronous scrambler is the same as the
   MTTF, or 1.5 packets.

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   The MTTS for SDL using the optional set-reset scrambler is one half
   of the scrambling state transmission interval (in packets) plus the
   MTTF.  For insertion at the default rate of one per eight packets,
   the MTTS is 5.5 packets.

   (The probability of receiving a bad scrambling state transmission
   should also be included in this calculation.  The probability of ran-
   dom corruption of this short message is shown in the SDL document [6]
   to be small enough that it can be neglected for this calculation.)

4.3.  Probability of False Frame (PFF)

   The PFF is 2.328E-10 (2^-32), since false framing requires two con-
   secutive headers with falsely correct CRC-16.

4.4.  Probability of False Synchronization (PFS)

   The PFS for SDL with the self-synchronous scrambler is the same as
   the PFF, or 2.328E-10 (2^-32).

   The PFS for SDL with the set-reset scrambler is 5.421E-20 (2^-64),
   and is calculated as the PFF above multiplied by the probability of a
   falsely detected scrambler state message, which itself contains two
   independent CRC-16 calculations.

4.5.  Probability of Loss of Frame (PLF)

   The PLF is a function of the BER, and for SDL is approximately the
   square of the BER multiplied by 500, which is the probability of two
   or more bit errors occurring within the 32 bit SDL header.  Thus, at
   a BER of 1E-5, the PLF is 5E-8.

5.  The Special Messages

   When the SDL Packet Length field has any value between 0000 and 0003,
   the message following the header has a special, pre-defined length.
   The 0 value is a time-fill on an idle link, and no other data fol-
   lows.  The next octet on the link is the first octet of the next SDL

   The values 1 through 3 are defined in the following subsections.
   These special messages each consist of a six octet data portion fol-
   lowed by another CRC-16 over that data portion, as with the SDL
   header, and this CRC is used for single bit error correction.

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5.1.  Scrambler State

   The special value of 1 for Packet Length is reserved to transfer the
   scrambler state from the transmitter to the receiver for the optional
   set-reset scrambler.  In this case, the SDL header is followed by six
   octets (48 bits) of scrambler state.  Neither the scrambler state nor
   the CRC are scrambled.

5.2.  A/B Message

   The special values of 2 and 3 for Packet Length are reserved for "A"
   and "B" messages, which are also six octets in length followed by two
   octets of CRC-16.  Each of these eight octets are scrambled.  No use
   for these messages with PPP SDL is defined.  These messages are
   reserved for use by link maintenance protocols, in a manner analogous
   to ATM's OAM cells.

6.  The Set-Reset Scrambler Option

   PPP over SDL uses a self-synchronous scrambler.  SDL implementations
   MAY also employ a set-reset scrambler to avoid some of the possible
   inherent problems with self-synchronous scramblers.

6.1.  The Killer Packet Problem

   Scrambling in general solves two problems.  First, SONET and SDH
   interfaces require a minimum density of bit transitions in order to
   maintain hardware clock recovery.  Since data streams frequently con-
   tain long runs of all zeros or all ones, scrambling the bits using a
   pseudo-random number sequence breaks up these patters.  Second, all
   link-layer synchronization mechanisms rely on detecting long-range
   patterns in the received data to detect framing.

   Self-synchronous scramblers are an easy way to partially avoid these
   problems.  One problem that is inherent with self-synchronous, how-
   ever, is that long user packets from malicious sites can make use of
   the known properties of these scramblers to inject either long
   strings of zeros or other synchronization-destroying patterns into
   the link.  For public networks, where the data presented to the net-
   work is usually multiplexed (interleaved) with multiple unrelated
   streams, the clocking problem does not pose a significant threat to
   the public network.  It does, however, pose a threat to the PPP-
   speaking device, and it poses a threat to long lines that are unchan-

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   Such carefully constructed packets are called "killer packets."

6.2.  SDL Set-Reset Scrambler

   An alternative to the self-synchronous scrambler is the externally
   synchronized or "set-reset" scrambler.  This is a free-running scram-
   bler that is not affected by the patterns in the user data, and
   therefore minimizes the possibility that a malicious user could
   present data to the network that mimics an undesirable data pattern.

   The option set-reset scrambler defined for SDL is an
   x^48+x^28+x^27+x+1 independent scrambler initialized to all ones when
   the link enters PRESYNCH state and reinitialized if the value ever
   becomes all zero bits.  As with the self-synchronous scrambler, all
   octets in the PPP packet data following the SDL header through the
   final packet CRC are scrambled.

   This mode MAY be detected automatically.  If a scrambler state mes-
   sage is received (as described in the following section), an SDL
   implementation that includes the set-reset scrambler option may
   switch from self-synchronous into set-reset mode automatically.  An
   SDL implementation that does not include the set-reset scrambler MUST
   NOT send scrambler state messages.

6.3.  SDL Scrambler Synchronization

   As described in the previous section, the special value of 1 for
   Packet Length is reserved to transfer the scrambler state from the
   transmitter to the receiver.  In this case, the SDL header is fol-
   lowed by six octets (48 bits) of scrambler state plus two octets of
   CRC-16 over the scrambler state.  None of these eight octets are

   SDL synchronization consists of two components, link and scrambler
   synchronization.  Both must be completed before PPP data flows on the

   If a valid SDL header is seen in PRESYNCH state, then the link enters
   SYNCH state, and the scrambler synchronization sequence is started.
   If an invalid SDL header is detected, then the link is returned to
   HUNT state, and PPP transmission is suspended.

   When scrambler synchronization is started, a scrambler state message
   is sent (Packet Length set to 1 and six octets of scrambler state in
   network byte order follow the SDL header).  When a scrambler syn-
   chronization message is received from the peer, PPP transmission is

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   Scrambler state messages are periodically transmitted to keep the
   peers in synchronization.  A period of once per eight transmitted
   packets is suggested, and it SHOULD be configurable.  Excessive
   packet CRC errors detected indicates an extended loss of synchroniza-
   tion and should trigger link resynchronization.

   On reception of a scrambler state message, an SDL implementation MUST
   compare the received 48 bits of state with the receiver's scrambler
   state.  If any of these bits differ, then a synchronization slip
   error is declared.  After such an error, the next valid scrambler
   state message received MUST be loaded into the receiver's scrambler,
   and the error condition is then cleared.

6.4.  SDL Scrambler Operation

   The transmit and receive scramblers are shift registers with 48
   stages that are initialized to all-ones when the link is initialized.
   Each is refilled with all one bits if the value in the shift register
   ever becomes all zeros.  This scrambler is not reset at the beginning
   of each frame, as is the SONET/SDH X^7+X^6+1 scrambler, nor is it
   modified by the transmitted data, as is the ATM self-synchronous
   scrambler.  Instead it is kept in synchronization using special SDL

    |     ^                 ^      ^                  |
    |     |                 |      |                  |
    +->D0-+->D1-> ... ->D26-+->D27-+->D28-> ... ->D47-+

   Each XOR is an exclusive-or gate; also known as a modulo-2 adder.
   Each Dn block is a D-type flip-flop clocked on the appropriate data

   The scrambler is clocked once after transmission of each bit of SDL
   data, whether or not the transmitted bit is scrambled.  When scram-
   bling is enabled for a given octet, the OUT bit is exclusive-ored
   with the raw data bit to produce the transmitted bit.  Bits within an
   octet are transmitted MSB-first.

   Reception of scrambled data is identical to transmission.  Each
   received bit is exclusive-ored with the output of the separate
   receive data scrambler.

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   To generate a scrambler state message, the contents of D47 through D0
   are snapshot at the point where the first scrambler state bit is
   sent.  D47 is transmitted as the first bit of the output.  The first
   octet transmitted contains D47 through D40, the second octet D39
   through D32, and the sixth octet D7 through D0.

   The receiver of a scrambler state message MUST first run the CRC-16
   check and correct algorithm over this message.  If the CRC-16 message
   check detects multiple bit errors, then the message is dropped and is
   not processed further.

   Otherwise, it then should compare the contents of the entire receive
   scrambler state D47:D0 with the corrected message.  (By pipelining
   the receiver with multiple clock stages between SDL Header error-
   correction block and the descrambling block, the receive descrambler
   will be on the correct clock boundary when the message arrives at the
   descrambler.  This means that the decoded scrambler state can be
   treated as immediately available at the beginning of the D47 clock
   cycle into the receive scrambler.)

   If any of the received scrambler state bits is different from the
   corresponding shift register bit, then a soft error flag is set.  If
   the flag was already set when this occurs, then a synchronization
   slip error is declared.  This error SHOULD be counted and reported
   through implementation-defined network management procedures.  When
   the receiver has this soft error flag set, any scrambler state mes-
   sage that passes the CRC-16 message check without multiple bit errors
   is clocked directly into the receiver's state register after the com-
   parison is done, and the soft error flag is then cleared.  Otherwise,
   while uncorrectable scrambler state messages are received, the soft
   error flag state is maintained.

   (The intent of this mechanism is to reduce the likelihood that a
   falsely corrected scrambler state message with multiple bit errors
   can corrupt the running scrambler state.)

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7.  Configuration Details

7.1.  Default LCP Configuration

   The LCP synchronous configuration defaults apply to SONET/SDH links.

   The following Configuration Options are recommended:

      Magic Number
      No Address and Control Field Compression
      No Protocol Field Compression
      No FCS alternatives (32-bit FCS default)

   This configuration means that PPP over SDL generally presents a 32-
   bit aligned datagram to the network layer.  With the address, con-
   trol, and protocol field intact, the PPP overhead on each packet is
   four octets.  If the SDL framer presents the SDL packet header to the
   PPP input handling in order to communicate the packet length (the
   Lucent implementation does not do this, but other hardware implemen-
   tations may), this header is also four octets, and alignment is

7.2.  Modification of the Standard Frame Format

   Since SDL does take the place of HDLC as a transport for PPP, it is
   at least tempting to remove the HDLC-derived overhead.  This is not
   done for PPP over SDL in order to preserve the message alignment and
   to allow for the future possibility interworking with other services
   (e.g., Frame Relay).

   By prior external arrangement or via LCP negotiation, any two SDL
   implementations MAY agree to omit the address and control fields or
   implement protocol field compression on a link.  Such use is not
   described by this document and MUST NOT be the default on any SDL

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8.  Implementation Details

8.1.  CRC Generation

   The following unoptimized code generates proper CRC-16 and CRC-32
   values for SDL messages.  Note that the polynomial bits are numbered
   in big-endian order for SDL CRCs; bit 0 is the MSB.

     typedef unsigned char u8;
     typedef unsigned short u16;
     typedef unsigned long u32;

     #define POLY16  0x1021
     #define POLY32  0x04C11DB7

     crc16(u16 crcval, u8 cval)
         int i;

         crcval ^= cval << 8;
         for (i = 8; i--; )
             crcval = crcval & 0x8000 ? (crcval << 1) ^ POLY16 :
                 crcval << 1;
         return crcval;

     crc32(u32 crcval, u8 cval)
         int i;

         crcval ^= cval << 24;
         for (i = 8; i--; )
             crcval = crcval & 0x80000000 ? (crcval << 1) ^ POLY32 :
                 crcval << 1;
         return crcval;

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     crc16_special(u8 *buffer, int len)
         u16 crc;

         crc = 0;
         while (--len >= 0)
             crc = crc16(crc,*buffer++);
         return crc;

     crc16_payload(u8 *buffer, int len)
         u16 crc;

         crc = 0xFFFF;
         while (--len >= 0)
             crc = crc16(crc,*buffer++);
         return crc ^ 0xFFFF;

     crc32_payload(u8 *buffer, int len)
         u32 crc;

         crc = 0xFFFFFFFFul;
         while (--len >= 0)
             crc = crc32(crc,*buffer++);
         return crc ^ 0xFFFFFFFFul;

     make_sdl_header(int packet_length, u8 *buffer)
         u16 crc;

         buffer[0] = (packet_length >> 8) & 0xFF;
         buffer[1] = packet_length & 0xFF;
         crc = crc16_special(buffer,2);
         buffer[0] ^= 0xB6;
         buffer[1] ^= 0xAB;
         buffer[2] = ((crc >> 8) & 0xFF) ^ 0x31;
         buffer[3] = (crc & 0xFF) ^ 0xE0;

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8.2.  Error Correction Tables

   To generate the error correction table, the following implementation
   may be used.  It creates a table called sdl_error_position, which is
   indexed on CRC residue value.  The tables can be used to determine if
   no error exists (table entry is equal to FE hex), one correctable
   error exists (table entry is zero-based index to errored bit with MSB
   of first octet being 0), or more than one error exists, and error is
   uncorrectable (table entry is FF hex).  To use for eight octet mes-
   sages, the bit index from this table is used directly.  To use for
   four octet messages, the index is treated as an unrecoverable error
   if it is below 32, and as bit index plus 32 if it is above 32.

   The program also prints out the error syndrome table shown in section
   3.10.  This may be used as part of a "switch" statement in a hardware

       u8 sdl_error_position[65536];

       /* Calculate new CRC from old^(byte<<8) */
       crc16_t8(u16 crcval)
           u16 f1,f2,f3;

           f1 = (crcval>>8) | (crcval<<8);
           f2 = (crcval>>12) | (crcval&0xF000) | ((crcval>>7)&0x01E0);
           f3 = ((crcval>>3) & 0x1FE0) ^ ((crcval<<4) & 0xF000);
           return f1^f2^f3;

       generate_error_table(u8 *bptab, int nbytes)
           u16 crc;
           int i, j, k;

           /* Marker for no error */
           bptab[0] = 0xFE;

           /* Marker for >1 error */
           for (i = 1; i < 65536; i++ )
               bptab[i] = 0xFF;

           /* Mark all single bit error cases. */
           printf("Error syndrome table:\n");
           for (i = 0; i < nbytes; i++) {
               putchar(' ');

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               for (j = 0; j < 8; j++) {
                   crc = 0;
                   for (k = 0; k < i; k++)
                         crc = crc16_t8(crc);
                   crc = crc16_t8(crc ^ (0x8000>>j));
                   for (k++; k < nbytes; k++)
                         crc = crc16_t8(crc);
                   bptab[crc] = (i * 8) + j;
                   printf(" %04X",crc);

       main(int argc, char **argv)
           u8 buffer[8] = {
           u16 crc;
           int i;


           /* Run sample message through check routine. */
           crc = 0;
           for (i = 0; i < 8; i++)
               crc = crc16_t8(crc ^ (buffer[i]<<8));

           /* Output is 0000 64 -- no error encountered. */
           printf("\nError test:  CRC %04X, bit position %d\n",

9.  Security Considerations

   The reliability of public SONET/SDH networks depends on well-behaved
   traffic that does not disrupt the synchronous data recovery mechan-
   isms.  This document describes framing and scrambling options that
   are used to ensure the distribution of transmitted data such that
   SONET/SDH design assumptions are not likely to be violated.

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10.  References

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

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

      [3]   Malis, A. and W. Simpson, "PPP over SONET/SDH," RFC 2615,
            June 1999.

      [4]   "American National Standard for Telecommunications -
            Synchronous Optical Network (SONET) Payload Mappings," ANSI

      [5]   ITU-T Recommendation G.707, "Network Node Interface for the
            Synchronous Digital Hierarchy (SDH)," March 1996.

      [6]   Doshi, B., Dravida, S., Hernandez-Valencia, E., Matragi, W.,
            Qureshi, M.,  Anderson, J., Manchester, J.,"A Simple Data
            Link Protocol for High Speed Packet Networks", Bell Labs
            Technical Journal, pp. 85-104, Vol.4 No.1, January-March

      [7]   Demers, A., S. Keshav, and S. Shenker, "Analysis and
            simulation of a fair queueing algorithm," ACM SIGCOMM volume
            19 number 4, pp. 1-12, September 1989.

      [8]   Floyd, S. and V. Jacobson, "Random Early Detection Gateways
            for Congestion Avoidance," IEEE/ACM Transactions on
            Networking, August 1993.

      [9]   Simpson, W., Editor, "PPP LCP Extensions," RFC 1570,
            Daydreamer, January 1994.

      [10]  ITU-T Recommendation I.432.1, "B-ISDN User-Network Interface
            - Physical Layer Specification: General Characteristics,"
            February 1999.

      [11]  ITU-T Recommendation V.41, "Code-independent error-control
            system," November 1989.

      [12]  ITU-T Recommendation G.783, "Characteristics of synchronous
            digital hierarchy (SDH) equipment functional blocks," April

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11.  Acknowledgments

   PPP over SONET was first proposed by Craig Partridge (BBN) and is
   documented by Andrew Malis and William Simpson as RFC 2615.

   Much of the material in this document was supplied by Lucent.

   Other length-prefixed forms of framing for PPP have gone before SDL,
   such as William Simpson's "PPP in Ether-like Framing" expired draft.

12.  Working Group and Chair Address

   The working group can be contacted via the mailing list (ietf-; send mail to to subscribe),
   or via the current chair:

     Karl Fox
     Extant, Inc.
     3496 Snouffer Road, Suite 100
     Columbus, Ohio 43235


13.  Intellectual Property Notices

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to per-
   tain 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

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14.  Copyright Notice

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

   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 develop-
   ing 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

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15.  Authors' Addresses

     James Carlson
     IronBridge Networks
     55 Hayden Avenue
     Lexington MA  02421-7996
     Phone:  +1 781 372 8132
     Fax:    +1 781 372 8090

     Paul Langner
     Lucent Technologies Microelectronics Group
     555 Union Boulevard
     Allentown PA  18103-1286

     Enrique J. Hernandez-Valencia
     Lucent Technologies
     101 Crawford Corners Rd.
     Holmdel NJ  07733-3030

     James Manchester
     Lucent Technologies
     101 Crawford Corners Rd.
     Holmdel NJ  07733-3030

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