rfc1976.txt

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Network Working Group                                       K. Schneider
Request for Comments: 1976                                    S. Venters
Category: Informational                                     ADTRAN, Inc.
                                                             August 1996


  PPP for Data Compression in Data Circuit-Terminating Equipment (DCE)

Status of This Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  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.  PPP
   defines an extensible Link Control Protocol, and proposes a family of
   Network Control Protocols for establishing and configuring different
   network-layer protocols.

   The PPP Serial Data Transport Protocol (PPP-SDTP) [2] provides a
   standard method for encapsulating and transporting serial data over a
   PPP link.  The PPP Compression Control Protocol [3] provides a
   standard method for selecting and using a compression protocol to
   provide for data compression on a PPP link.

   This document defines a specific set of parameters for these
   protocols and an LCP extension to define a standard way of using PPP
   for data compression of serial data in Data Circuit-Terminating
   Equipment (DCE).

Table of Contents

     1.     Introduction ..........................................    2
        1.1       Specification of Requirements ...................    2
        1.2       Terminology .....................................    3
     2.     Modes of Operation ....................................    3
     3.     PPP Link Control Protocol Extension ...................    4
     4.     Required PPP Elements .................................    4
        4.1       Required Configuration Options and Parameters ...    5
     5.     Mode-1 Requirements ...................................    6
        5.1       Detailed Mode-1 Example .........................    7
     6.     Initial Handshake Operation ...........................    8
     7.     Security ..............................................    9
     8.     References ............................................    9
     CHAIR'S ADDRESS ..............................................    9



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RFC 1976                        PPP DCE                      August 1996


     AUTHORS' ADDRESSES ...........................................   10

1.  Introduction

   This document is a product of the TR30.1 ad hoc committee on
   compression of synchronous data.  It represents a component of a
   proposal to use PPP to provide compression of synchronous serial data
   in DSU/CSUs.

   PPP [1] has three main components:

      1. A method for encapsulating multi-protocol datagrams.

      2. A Link Control Protocol (LCP) for establishing, configuring,
         and testing the data-link connection.

      3. A family of Network Control Protocols for establishing and
         configuring different network-layer protocols.

   In addition to providing support for multi-protocol datagrams, the
   Point-to-Point Protocol (PPP) [1] has defined an effective and robust
   negotiating mechanism that can be used on point to point links.  When
   used in conjunction with the PPP Compression Control Protocol [3] and
   one of the PPP Compression Protocols [4], PPP provides an
   interoperable method of employing data compression on a point-to-
   point link.

   The PPP Serial Data Transport Protocol (PPP-SDTP) and the PPP Serial
   Data Control Protocol (PPP-SDCP) [2] have been developed to allow
   serial data to be carried within a PPP packet.  PPP-SDTP uses a
   terminal adaption header based on that of ITU-T Recommendation V.120
   [5].

1.1.  Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

   MUST      This word, or the adjective "required", means that the
             definition is an absolute requirement of the specification.

   MUST NOT  This phrase means that the definition is an absolute
             prohibition of the specification.

   SHOULD    This word, or the adjective "recommended", means that there
             may exist valid reasons in particular circumstances to
             ignore this item, but the full implications must be
             understood and carefully weighed before choosing a



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RFC 1976                        PPP DCE                      August 1996


             different course.

   MAY       This word, or the adjective "optional", means that this
             item is one of an allowed set of alternatives.  An
             implementation which does not include this option MUST be
             prepared to interoperate with another implementation which
             does include the option.

1.2.  Terminology

   This document frequently uses the following terms:

   datagram  The unit of transmission in the network layer (such as IP).
             A datagram may be encapsulated in one or more packets
             passed to the data link layer.

   frame     The unit of transmission at the data link layer.  A frame
             may include a header and/or a trailer, along with some
             number of units of data.

   packet    The basic unit of encapsulation, which is passed across the
             interface between the network layer and the data link
             layer.  A packet is usually mapped to a frame; the
             exceptions are when data link layer fragmentation is being
             performed, or when multiple packets are incorporated into a
             single frame.

   peer      The other end of the point-to-point link.

   silently discard
             This means the implementation discards the packet without
             further processing.  The implementation SHOULD provide the
             capability of logging the error, including the contents of
             the silently discarded packet, and SHOULD record the event
             in a statistics counter.

2.  Modes of Operation

   This document provides for three modes of operation for DCE devices:
   Mode-0 represents transparent operation; Mode-1 a simplified,
   predefined compression format; and Mode-2, a full PPP implementation
   providing compression of serial data.

   Mode-0 represents the operating mode of currently deployed DCEs that
   operate in transparent mode, without any DCE-to-DCE protocols.
   Mode-1 devices implement data compression upon detecting an initial
   handshake, which is implemented via an newly defined LCP
   Configuration Option called the DCE-Identifier.  The DCE-Identifier



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RFC 1976                        PPP DCE                      August 1996


   is used both to distinguish DCE devices from PPP bridges and routers,
   and to all Mode-1 and Mode-2 DCE devices to interoperate, via its
   Mode parameter.  In Mode-1, the parameters of operation are not
   negotiable.  Mode-2 devices implement the full LCP state machine and
   are therefore capable of negotiating alternatives to the default set
   of paramaters and options.  Mode-2 devices must also support Mode-1
   operation, and fall-back to such operation when connected to a Mode-1
   device.  The mechanism of the Mode-1/Mode-2 handshake is given in
   Section 7.

3.  PPP Link Control Protocol Extension

   The use of PPP in Compression DCE requires the use of an additional
   LCP Configuration Option:

      25  DCE-Identifier


   DCE-Identifier

      The presence of this option indicates that the system sending it
      is Data Circuit-Terminating Equipment (DCE) that desires to
      establish a serial data compression link.

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

      Type

         25

      Length

         3

      Mode

         1   Mode-1 (No Additional Negotiation)
         2   Mode-2 (Full PPP Negotiation and State Machine)

4.  Required PPP Elements

   Unlike PPP's native bridge/router environment, the minimum
   requirement for use of PPP in DCE equipment is not simply
   interoperability, but rather interoperability with effective data



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RFC 1976                        PPP DCE                      August 1996


   compression.  With this in mind, it is desirable to specify a minimum
   PPP feature set, that is somewhat different than that of a normal PPP
   bridge/router requirement.

   This different feature set includes: support for compression, support
   of a single default compression algorithm, support of Protocol-Field
   compression, support of Address-and-Control-Field-Compression,
   support of PPP-SDTP, etc.

   The minimum feature set includes the following protocols:

      PPP Link Control Protocol (Mode-1 must include a subset) [1]
      PPP in HDLC-like Framing [6]
      PPP Compression Control Protocol (Mode-2 only) [3]
      PPP LZS-DCP Compression Protocol [4]
      PPP Serial Data Transport Protocol [2]
      PPP Serial Data Control Protocol (Mode-2 only) [2]

   The Stacker-LZS algorithm from Stac Electronics was chosen as the
   default compression algorithm for DCE devices.  This decision was
   made by the TR30.1 ad hoc based on licensing issues (agreeing to
   non-discriminatory and reasonable terms), compression ratios, its
   efficient use of processor and memory resources, and speed
   scalability.  A compression protocol incorporating in-band
   synchronization signaling was defined for the Stacker algorithm and
   selected for the default compression protocol.  This protocol is
   known as the PPP LZS-DCP Compression Protocol [4].  Although the PPP
   LZS-DCP Compression Protocol specifies a number of formats and
   history management options, only the default format with a single
   history must be implemented.

4.1.  Required Configuration Options and Parameters

   To ensure that implementations are able to interoperate with
   effective data compression, a required set of Configuration Options
   are specified.  These Options are assumed in Mode-1, and are
   negotiated in Mode-2, using the standard PPP negotiation state
   machine.  (Mode-1 uses a fixed packet format with a predetermined set
   of values for LCP, LZS-DCP, and SDTP Configuration
   Options/parameters.  The required values listed in this section are
   the predefined values.)

   The following LCP Configuration Options [7] MUST be supported:

      Maximum-Receive-Unit                 1550
      Address/Control-Field-Compression    Yes
      Protocol-Field-Compression           Yes




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