rfc1662.txt

pptp第二层隧道模块

Network Working Group                                 W. Simpson, Editor
Request for Comments: 1662                                    Daydreamer
STD: 51                                                        July 1994
Obsoletes: 1549
Category: Standards Track


                        PPP in HDLC-like Framing


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 HDLC-like framing for PPP
   encapsulated packets.


Table of Contents


     1.     Introduction ..........................................    1
        1.1       Specification of Requirements ...................    2
        1.2       Terminology .....................................    2

     2.     Physical Layer Requirements ...........................    3

     3.     The Data Link Layer ...................................    4
        3.1       Frame Format ....................................    5
        3.2       Modification of the Basic Frame .................    7

     4.     Octet-stuffed framing .................................    8
        4.1       Flag Sequence ...................................    8
        4.2       Transparency ....................................    8
        4.3       Invalid Frames ..................................    9
        4.4       Time Fill .......................................    9
           4.4.1  Octet-synchronous ...............................    9
           4.4.2  Asynchronous ....................................    9
        4.5       Transmission Considerations .....................   10
           4.5.1  Octet-synchronous ...............................   10
           4.5.2  Asynchronous ....................................   10


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     5.     Bit-stuffed framing ...................................   11
        5.1       Flag Sequence ...................................   11
        5.2       Transparency ....................................   11
        5.3       Invalid Frames ..................................   11
        5.4       Time Fill .......................................   11
        5.5       Transmission Considerations .....................   12

     6.     Asynchronous to Synchronous Conversion ................   13

     7.     Additional LCP Configuration Options ..................   14
        7.1       Async-Control-Character-Map (ACCM) ..............   14

     APPENDICES ...................................................   17
     A.     Recommended LCP Options ...............................   17
     B.     Automatic Recognition of PPP Frames ...................   17
     C.     Fast Frame Check Sequence (FCS) Implementation ........   18
        C.1       FCS table generator .............................   18
        C.2       16-bit FCS Computation Method ...................   19
        C.3       32-bit FCS Computation Method ...................   21

     SECURITY CONSIDERATIONS ......................................   24
     REFERENCES ...................................................   24
     ACKNOWLEDGEMENTS .............................................   25
     CHAIR'S ADDRESS ..............................................   25
     EDITOR'S ADDRESS .............................................   25




1.  Introduction

   This specification provides for framing over both bit-oriented and
   octet-oriented synchronous links, and asynchronous links with 8 bits
   of data and no parity.  These links MUST be full-duplex, but MAY be
   either dedicated or circuit-switched.

   An escape mechanism is specified to allow control data such as
   XON/XOFF to be transmitted transparently over the link, and to remove
   spurious control data which may be injected into the link by
   intervening hardware and software.

   Some protocols expect error free transmission, and either provide
   error detection only on a conditional basis, or do not provide it at
   all.  PPP uses the HDLC Frame Check Sequence for error detection.
   This is commonly available in hardware implementations, and a
   software implementation is provided.






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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
             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
             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.


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2.  Physical Layer Requirements

   PPP is capable of operating across most DTE/DCE interfaces (such as,
   EIA RS-232-E, EIA RS-422, and CCITT V.35).  The only absolute
   requirement imposed by PPP is the provision of a full-duplex circuit,
   either dedicated or circuit-switched, which can operate in either an
   asynchronous (start/stop), bit-synchronous, or octet-synchronous
   mode, transparent to PPP Data Link Layer frames.

   Interface Format

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

   Transmission Rate

      PPP does not impose any restrictions regarding transmission rate,
      other than that of the particular DTE/DCE interface.

   Control Signals

      PPP does not require the use of control signals, such as Request
      To Send (RTS), Clear To Send (CTS), Data Carrier Detect (DCD), and
      Data Terminal Ready (DTR).

      When available, using such signals can allow greater functionality
      and performance.  In particular, such signals SHOULD be used to
      signal the Up and Down events in the LCP Option Negotiation
      Automaton [1].  When such signals are not available, the
      implementation MUST signal the Up event to LCP upon
      initialization, and SHOULD NOT signal the Down event.

      Because signalling is not required, the physical layer MAY be
      decoupled from the data link layer, hiding the transient details
      of the physical transport.  This has implications for mobility in
      cellular radio networks, and other rapidly switching links.

      When moving from cell to cell within the same zone, an
      implementation MAY choose to treat the entire zone as a single
      link, even though transmission is switched among several
      frequencies.  The link is considered to be with the central
      control unit for the zone, rather than the individual cell
      transceivers.  However, the link SHOULD re-establish its
      configuration whenever the link is switched to a different
      administration.

      Due to the bursty nature of data traffic, some implementations
      have choosen to disconnect the physical layer during periods of



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      inactivity, and reconnect when traffic resumes, without informing
      the data link layer.  Robust implementations should avoid using
      this trick over-zealously, since the price for decreased setup
      latency is decreased security.  Implementations SHOULD signal the
      Down event whenever "significant time" has elapsed since the link
      was disconnected.  The value for "significant time" is a matter of
      considerable debate, and is based on the tariffs, call setup
      times, and security concerns of the installation.



3.  The Data Link Layer

   PPP uses the principles described in ISO 3309-1979 HDLC frame
   structure, most recently the fourth edition 3309:1991 [2], which
   specifies modifications to allow HDLC use in asynchronous
   environments.

   The PPP control procedures use the Control field encodings described
   in ISO 4335-1979 HDLC elements of procedures, most recently the
   fourth edition 4335:1991 [4].

      This should not be construed to indicate that every feature of the
      above recommendations are included in PPP.  Each feature included
      is explicitly described in the following sections.

   To remain consistent with standard Internet practice, and avoid
   confusion for people used to reading RFCs, all binary numbers in the
   following descriptions are in Most Significant Bit to Least
   Significant Bit order, reading from left to right, unless otherwise
   indicated.  Note that this is contrary to standard ISO and CCITT
   practice which orders bits as transmitted (network bit order).  Keep
   this in mind when comparing this document with the international
   standards documents.

















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3.1.  Frame Format

   A summary of the PPP HDLC-like frame structure is shown below.  This
   figure does not include bits inserted for synchronization (such as
   start and stop bits for asynchronous links), nor any bits or octets
   inserted for transparency.  The fields are transmitted from left to
   right.

           +----------+----------+----------+
           |   Flag   | Address  | Control  |
           | 01111110 | 11111111 | 00000011 |
           +----------+----------+----------+
           +----------+-------------+---------+
           | Protocol | Information | Padding |
           | 8/16 bits|      *      |    *    |
           +----------+-------------+---------+
           +----------+----------+-----------------
           |   FCS    |   Flag   | Inter-frame Fill
           |16/32 bits| 01111110 | or next Address
           +----------+----------+-----------------

   The Protocol, Information and Padding fields are described in the
   Point-to-Point Protocol Encapsulation [1].

   Flag Sequence

      Each frame begins and ends with a Flag Sequence, which is the
      binary sequence 01111110 (hexadecimal 0x7e).  All implementations
      continuously check for this flag, which is used for frame
      synchronization.

      Only one Flag Sequence is required between two frames.  Two
      consecutive Flag Sequences constitute an empty frame, which is
      silently discarded, and not counted as a FCS error.

   Address Field

      The Address field is a single octet, which contains the binary
      sequence 11111111 (hexadecimal 0xff), the All-Stations address.
      Individual station addresses are not assigned.  The All-Stations
      address MUST always be recognized and received.

      The use of other address lengths and values may be defined at a
      later time, or by prior agreement.  Frames with unrecognized
      Addresses SHOULD be silently discarded.






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   Control Field

      The Control field is a single octet, which contains the binary
      sequence 00000011 (hexadecimal 0x03), the Unnumbered Information
      (UI) command with the Poll/Final (P/F) bit set to zero.

      The use of other Control field values may be defined at a later
      time, or by prior agreement.  Frames with unrecognized Control
      field values SHOULD be silently discarded.

   Frame Check Sequence (FCS) Field

      The Frame Check Sequence field defaults to 16 bits (two octets).
      The FCS is transmitted least significant octet first, which
      contains the coefficient of the highest term.

      A 32-bit (four octet) FCS is also defined.  Its use may be
      negotiated as described in "PPP LCP Extensions" [5].