rfc1963.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                       K. Schneider
Request for Comments: 1963                                    S. Venters
Category: Informational                                     ADTRAN, Inc.
                                                             August 1996


               PPP Serial Data Transport Protocol (SDTP)

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.

   This document describes a new Network level protocol (from the PPP
   point of view), PPP Serial Data Transport Protocol, that provides
   encapsulation and an associated control protocol for transporting
   serial data streams over a PPP link.  This protocol was developed for
   the purpose of using PPP's many features to provide a standard method
   for synchronous data compression.  The encapsulation uses a header
   structure based on that of the ITU-T Recommendation V.120 [2].

Table of Contents

     1.     Introduction ..........................................    2
     2.     SDTP Packets ..........................................    3
        2.1       Padding .........................................    4
        2.2       Packet Formats ..................................    4
     3.     Serial Data Control Protocol ..........................   11
     4.     SDCP Configuration Option Format ......................   12
        4.1       Packet-Format ...................................   13
        4.2       Header-Type .....................................   13
        4.3       Length-Field-Present ............................   14
        4.4       Multi-Port ......................................   14
        4.5       Transport-Mode ..................................   15
        4.6       Maximum-Frame-Size ..............................   16
        4.7       Allow-Odd-Frames ................................   16
        4.8       FCS-Type ........................................   17
        4.9       Flow-Expiration-Time ............................   18
     SECURITY CONSIDERATIONS ......................................   19



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RFC 1963                        PPP SDTP                     August 1996


     REFERENCES ...................................................   19
     CHAIR'S ADDRESS ..............................................   20
     AUTHORS' ADDRESSES ...........................................   20

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 data in
   DSU/CSUs.

   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 [4-10], PPP provides an
   interoperable method of employing data compression on a point-to-
   point link.

   This document provides a PPP encapsulation for serial data,
   specifying a transport protocol, PPP Serial Data Transport Protocol
   (PPP-SDTP), and an associated control protocol, PPP Serial Data
   Control Protocol (PPP-SDCP).  When these protocols are added to above
   mentioned PPP protocols, PPP can be used to provide compression of
   serial data on a point-to-point link.

   This first edition of PPP-SDTP/SDCP covers HDLC-like synchronous
   serial data and asynchronous serial data.  It does this by using a
   terminal adaption header based on that of ITU-T Recommendation V.120
   [2].  Support may be added in the future for other synchronous
   protocols as the marketplace demands.

   The V.120 terminal adaption header allows transported data frames to
   be split over several packets, supports the transport of DTE port
   idle and error information, and optionally supports the transport of
   DTE control state information.

   In addition to the V.120 Header, fields can be added to the packet
   format through negotiation to provide support for features not
   included in the V.120 header.  The extra fields are: a Length Field,
   which is used to distinguish packets in compound frames, and a Port
   field, which is used to provide multi-port multiplexing capability.
   The protocol also allows reserved bits in the V.120 header to be used
   to transport non-octet aligned frames and to provide a flow control
   mechanism.






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RFC 1963                        PPP SDTP                     August 1996


   To provide these features, PPP-SDTP permits a single frame format to
   be selected from several possible formats by using PPP-SDCP
   negotiation.  The terminal adaption header can be either fixed length
   or variable length, to allow either simplicity or flexibility.

   The default frame format places the terminal adaption header at the
   end of the packet.  This permits optimal transmitter timelines when
   user frames are segmented and compression is also used in conjunction
   with this protocol.

2.  SDTP Packets

   Before any SDTP packets may be communicated, PPP must reach the
   Network-Layer Protocol phase, and the SDTP Control Protocol must
   reach the Opened state.

   By default, exactly one SDTP packet is encapsulated in the PPP
   Information field, where the PPP Protocol field indicates type hex
   0049 (PPP-SDTP).  If the Length-Field-Present Configuration Option
   and the LCP Compound-Frames Configuration Option are successfully
   negotiated, multiple SDTP packets may be placed in the PPP
   Information field, and they are distinguished by the presence of
   Length fields in each packet.

   The maximum length of the SDTP datagram transmitted over a PPP link
   is limited only by the negotiated Maximum-Frame-Size and the maximum
   length of the Information field of a PPP encapsulated packet.  Note
   that if compression is used on the PPP link, this the maximum length
   of the SDTP datagram  may be larger or smaller than the maximum
   length of the Information field of a PPP encapsulated packet,
   depending on the particular compression algorithm and protocol used.

   ITU-T Recommendation V.120 [2] defines an adaption header that is
   used with its asynchronous and synchronous modes of operation.  SDTP
   packets include this header as a Header field to provide the protocol
   adaption function.  Using negotiation, additional fields can be added
   to the packet to provide sequencing and multiplexing capability
   within SDTP. SDTP also has an option of using the reserved bits of
   the header to provide a flow control mechanism and support for
   transporting non-octet aligned data frames.

   The default SDTP packet format is designed to allow the efficient use
   of the protocol's segmentation feature when combined with a PPP
   Compression Protocol [4-10].  This format is a little different from
   other PPP NCP's in that data is read from both ends of the packet.
   The Header field is placed at the end of the SDTP packet, with the
   order of the octets reversed.  This somewhat unique format has been
   selected to allow optimal transmitter timelines when compression is



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RFC 1963                        PPP SDTP                     August 1996


   used and transported data frames are split into multiple SDTP
   packets.  In such a situation, the Header field contains the
   information about whether the data is split into multiple packets or
   not, so if it is located at the end of a packet, the decision can be
   made after observing the compressed size of the packet.  The Header
   field can then simply be run through the compressor after the
   decision has been made.

   When the Header field is placed before the data, as in the optional
   packet format, the transmitter must make the decision about whether
   to split a frame over multiple packets without knowing about the
   compressibility of the frame.  Therefore the optional format is
   designed to be used when transported frames are not split into
   multiple SDTP packets or where SDTP is not coupled with compression.
   It is believed that this format may be useful for some hardware
   implementations.

2.1.  Padding

   If padding is used, SDTP packets require the use of the Length Field
   or the previous negotiation of the LCP Self-Describing-Padding
   Configuration Option [11].

2.2.  Packet Formats

   The default SDTP packet format is shown below. The fields are
   transmitted from left to right.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        PPP Protocol ID        |    Transported Data ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Header -  H   |
   +-+-+-+-+-+-+-+-+

   The two complete frame formats are shown below:  Header-Last and
   Header-First.  Header-Last is the default packet format. The
   additional fields provided support for:  Control State Information
   (CS), multiple packets and multi-port multiplexing.  Again, the
   fields are transmitted from left to right.  Descriptions of the
   fields follow the packet formats.









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RFC 1963                        PPP SDTP                     August 1996


   Header-Last

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        PPP Protocol ID        |          (Length)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    (Port)     |  Transported Data / (Odd-Pad) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Header - (CS) :       H       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Header-First

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        PPP Protocol ID        |          (Length)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    (Port)     | Header  -  H  :     (CS)      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Transported Data / (Odd-Pad) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   PPP Protocol ID

      The PPP Protocol ID field is described in the Point-to-Point
      Protocol Encapsulation [1].

      When the SDTP Protocol is successfully negotiated by the SDTP
      Control Protocol (SDCP), the value is 0049 hex.  This value may be
      compressed to one octet when Protocol-Field-Compression is
      negotiated, or if one of the PPP compression protocols [4-10] is
      used.

   Length

      The optional Length field is present in every SDTP packet upon
      successful negotiation of the Length-Field-Present Configuration
      Option.

      The value of the Length field is the combined lengths of the
      Length, Port (if present), Header, Transmitted Data, and Odd-Pad
      (if present) fields in octets.

      The length of the Length field defaults to one octet.  Valid
      lengths are from 2 to 255 octets, since each packet must include



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RFC 1963                        PPP SDTP                     August 1996


      at least a one octet Header field.

      If desired, the length field can be negotiated to be two octets in
      length.  In that case, valid lengths are from 2 to 65535 octets,
      and the field is transmitted most significant octet first.

      In either case, a length of 0 means that the combined length is
      the same as the length of the remainder of the PPP Information
      Field.

   Port

      The optional Port field is present in every SDTP packet upon
      successful negotiation of the Multi-Port Option.

      The length of the Port field is one octet. Valid Port numbers are
      0 to 254.  Port number 255 is reserved for control purposes (see
      section on flow control).

   Header

      The Header field is the terminal adaption header from ITU-T
      Recommendation V.120.  As specified in that document, it contains
      up to two octets: The terminal adaption header octet (H), and the
      optional header extension for control state information (CS).
      SDTP only supports the protocol sensitive operation of V.120; bit
      transparent operation is not supported.  The descriptions of the
      header bits provided below are derived from the descriptions
      provided in Recommendation V.120.  In addition to the bit
      definitions of V.120, SDTP optionally permits the use of reserved
      bits to be used for flow control and to provide support for non-
      octet aligned frames.

      The length of the Header field is either one or two octets, and is
      determined by the value of the E bit in the first octet.  By
      default, the E-bit must be set in the H octet and the CS octet is
      not present.  A Configuration Option may be negotiated to allow
      the use of the CS octet, or even to require its presence in every
      packet.












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RFC 1963                        PPP SDTP                     August 1996


      H (V.120 Terminal Adaption Header)

         The format of the first octet of the Header field is shown
         below:

            0     1     2     3     4     5     6     7
         +-----+-----+-----+-----+-----+-----+-----+-----+
         |  E  | BR  | Res | FC  | C2  | C1  |  B  |  F  |
         +-----+-----+-----+-----+-----+-----+-----+-----+

         E - Extension Bit

            The E bit is the extension bit.  If set to 0, it indicates
            that the Control-2 field is present.

         BR - Break / HDLC Idle Bit

            In asynchronous mode, the BR bit indicates the invocation of
            the BREAK function by the DTE.  A value of 1 indicates
            BREAK.

            In synchronous HDLC mode, the BR bit is used to indicate
            that DTE port is receiving HDLC idle condition.  A value of
            1 indicates this idle condition.

         Res - Reserved


            This bit is reserved and MUST be set to 0.  (This is a
            reserved bit in V.120.)


         FC - Flow Control