Network Working Group                                         W. Simpson
Request for Comments: 1619                                    Daydreamer
Category: Standards Track                                       May 1994


                           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.



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 Heirarchy (SDH) circuits.

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



Applicability

   This specification is intended for those implementations which desire
   to use the PPP encapsulation over high speed private point-to-point
   links, such as intra-campus single-mode fiber which may already be
   installed and unused.  Because the PPP encapsulation has relatively
   low overhead, it is anticipated that significantly higher throughput
   can be attained compared to other SONET/SDH payload mappings, at a
   significantly lower cost for line termination equipment.

                           Table of Contents


     1.     Introduction ..........................................    1

     2.     Physical Layer Requirements ...........................    1

     3.     Framing ...............................................    2

     4.     Configuration Details .................................    3

     SECURITY CONSIDERATIONS ......................................    3

     REFERENCES ...................................................    3

     ACKNOWLEDGEMENTS .............................................    3

     CHAIR'S ADDRESS ..............................................    4

     AUTHOR'S ADDRESS .............................................    4

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.

   The Synchronous Optical Network (SONET) [3] is an octet-synchronous
   multiplex scheme that defines a family of standard rates and formats.
   Despite the name, it is not limited to optical links.  Electrical
   specifications have been defined for single-mode fiber, multi-mode
   fiber, and CATV 75 ohm coaxial cable.  The transmission rates are
   integral multiples of 51.840 Mbps, which may be used to carry T3/E3
   bit-synchronous signals.  The allowed multiples are currently
   specified as

           STS-1    51.840         STS-18    933.120
           STS-3   155.520         STS-24  1,244.160
           STS-9   466.560         STS-36  1,866.240
           STS-12  622.080         STS-48  2,488.320


   The CCITT Synchronous Digital Heirarchy (SDH) defines a subset of
   SONET transmission rates beginning at 155.520 Mbps [5].

           SONET           SDH equivalent
           STS-3c          STM-1
           STS-12c         STM-4
           STS-48c         STM-16




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 presents an octet interface to the physical layer.  There is
      no provision for sub-octets to be supplied or accepted.

      The octet stream is mapped into the SONET/SDH Synchronous Payload
      Envelope (SPE), with the octet boundaries aligned with the SPE
      octet boundaries.

      No scrambling is needed during insertion into the SPE.

      The Path Signal Label (C2) is intended to indicate the contents of
      the SPE.  The experimental value of 207 (cf hex) is used to
      indicate PPP.

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

   Transmission Rate

      The basic rate for PPP over SONET/SDH is that of STS-3c/STM-1 at
      155.520 Mbps.  The available information bandwidth is 149.760
      Mbps, which is the STS-3c/STM-1 SPE with section, line and path
      overhead removed.  This is the same super-rate mapping that is
      used for ATM and FDDI [4].

      Lower signal rates MUST use the Virtual Tributary (VT) mechanism
      of SONET/SDH.  This maps existing signals up to T3/E3 rates
      asynchronously into the SPE, or uses available clocks for bit-
      synchronous and byte-synchronous mapping.

      Higher signal rates SHOULD conform to the SDH STM series, rather
      than the SONET STS series, as equipment becomes available.  The
      STM series progresses in powers of 4 (instead of 3), and employs
      fewer steps, which is likely to simplify multiplexing and
      integration.

   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
   Framing" [2].

   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.

4.  Configuration Details

   The standard LCP sync 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
      32-bit FCS




Security Considerations

   Security issues are not discussed in this memo.



References

   [1]   Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", RFC
         1548, Daydreamer, December 1993.

   [2]   Simpson, W., Editor, "PPP in HDLC Framing", RFC 1549, 
         Daydreamer, December 1993.

   [3]   "American National Standard for Telecommunications - Digital
         Hierarchy - Optical Interface Rates and Formats Specification",
         ANSI T1.105-1991.

   [4]   "American National Standard for Telecommunications -
         Synchronous Optical Network (SONET) Payload Mappings", ANSI
         T1.105.02-1993 draft.

   [5]   CCITT Recommendation G.707, "Synchronous Digital Hierarchy Bit
         Rates", June 1992.

Acknowledgments

   PPP over SONET was first proposed by Craig Partridge (BBN).  Some
   information was obtained from the good folks at Bellcore.

   Technical assistance and information was also provided by Victor
   Demjanenko (SUNY Buffalo).

   Special thanks to Morning Star Technologies for providing computing
   resources and network access support for writing this specification.



Chair's Address

   The working group can be contacted via the current chair:

      Fred Baker
      Advanced Computer Communications
      315 Bollay Drive
      Santa Barbara, California  93117

      EMail: fbaker@acc.com



Author's Address

   Questions about this memo can also be directed to:

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

      EMail: Bill.Simpson@um.cc.umich.edu
             bsimpson@MorningStar.com