rfc1331.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                                         W. Simpson
Request for Comments: 1331                                    Daydreamer
Obsoletes: RFCs 1171, 1172                                      May 1992



                   The Point-to-Point Protocol (PPP)
                                for the
                Transmission of Multi-protocol Datagrams
                       over Point-to-Point Links


Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Abstract

   The Point-to-Point Protocol (PPP) provides a method for transmitting
   datagrams over serial point-to-point links.  PPP is comprised of
   three main components:

      1. A method for encapsulating datagrams over serial links.

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

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

   This document defines the PPP encapsulation scheme, together with the
   PPP Link Control Protocol (LCP), an extensible option negotiation
   protocol which is able to negotiate a rich assortment of
   configuration parameters and provides additional management
   functions.

   This RFC is a product of the Point-to-Point Protocol Working Group of
   the Internet Engineering Task Force (IETF).  Comments on this memo
   should be submitted to the [e-mail] mailing list.








Simpson                                                         [Page i]

RFC 1331                Point-to-Point Protocol                 May 1992


Table of Contents


     1.     Introduction ..........................................    1
        1.1       Specification of Requirements ...................    3
        1.2       Terminology .....................................    3

     2.     Physical Layer Requirements ...........................    4

     3.     The Data Link Layer ...................................    5
        3.1       Frame Format ....................................    6

     4.     PPP Link Operation ....................................   10
        4.1       Overview ........................................   10
        4.2       Phase Diagram ...................................   10
        4.3       Link Dead (physical-layer not ready) ............   10
        4.4       Link Establishment Phase ........................   11
        4.5       Authentication Phase ............................   11
        4.6       Network-Layer Protocol Phase ....................   12
        4.7       Link Termination Phase ..........................   12

     5.     The Option Negotiation Automaton ......................   14
        5.1       State Diagram ...................................   15
        5.2       State Transition Table ..........................   16
        5.3       States ..........................................   18
        5.4       Events ..........................................   20
        5.5       Actions .........................................   24
        5.6       Loop Avoidance ..................................   26
        5.7       Counters and Timers .............................   27

     6.     LCP Packet Formats ....................................   28
        6.1       Configure-Request ...............................   30
        6.2       Configure-Ack ...................................   31
        6.3       Configure-Nak ...................................   32
        6.4       Configure-Reject ................................   33
        6.5       Terminate-Request and Terminate-Ack .............   35
        6.6       Code-Reject .....................................   36
        6.7       Protocol-Reject .................................   38
        6.8       Echo-Request and Echo-Reply .....................   39
        6.9       Discard-Request .................................   40

     7.     LCP Configuration Options .............................   42
        7.1       Format ..........................................   43
        7.2       Maximum-Receive-Unit ............................   44
        7.3       Async-Control-Character-Map .....................   45
        7.4       Authentication-Protocol .........................   47
        7.5       Quality-Protocol ................................   49
        7.6       Magic-Number ....................................   51



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RFC 1331                Point-to-Point Protocol                 May 1992


        7.7       Protocol-Field-Compression ......................   54
        7.8       Address-and-Control-Field-Compression ...........   56

     APPENDICES ...................................................   58

     A.     Asynchronous HDLC .....................................   58

     B.     Fast Frame Check Sequence (FCS) Implementation ........   61
        B.1       FCS Computation Method ..........................   61
        B.2       Fast FCS table generator ........................   63

     C.     LCP Recommended Options ...............................   64

     SECURITY CONSIDERATIONS ......................................   65

     REFERENCES ...................................................   65

     ACKNOWLEDGEMENTS .............................................   66

     CHAIR'S ADDRESS ..............................................   66

     AUTHOR'S ADDRESS .............................................   66





























Simpson                                                       [Page iii]

RFC 1331                Point-to-Point Protocol                 May 1992


1.  Introduction

   Motivation

      In the last few years, the Internet has seen explosive growth in
      the number of hosts supporting TCP/IP.  The vast majority of these
      hosts are connected to Local Area Networks (LANs) of various
      types, Ethernet being the most common.  Most of the other hosts
      are connected through Wide Area Networks (WANs) such as X.25 style
      Public Data Networks (PDNs).  Relatively few of these hosts are
      connected with simple point-to-point (i.e., serial) links.  Yet,
      point-to-point links are among the oldest methods of data
      communications and almost every host supports point-to-point
      connections.  For example, asynchronous RS-232-C [1] interfaces
      are essentially ubiquitous.

   Encapsulation

      One reason for the small number of point-to-point IP links is the
      lack of a standard encapsulation protocol.  There are plenty of
      non-standard (and at least one de facto standard) encapsulation
      protocols available, but there is not one which has been agreed
      upon as an Internet Standard.  By contrast, standard encapsulation
      schemes do exist for the transmission of datagrams over most
      popular LANs.

      PPP provides an encapsulation protocol over both bit-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.  PPP uses HDLC as a basis for the
      encapsulation.

      PPP has been carefully designed to retain compatibility with most
      commonly used supporting hardware.  In addition, 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.

      The PPP encapsulation also provides for multiplexing of different
      network-layer protocols simultaneously over the same link.  It is
      intended that PPP provide a common solution for easy connection of
      a wide variety of hosts, bridges and routers.

      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



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RFC 1331                Point-to-Point Protocol                 May 1992


      implementations, and a software implementation is provided.

      By default, only 8 additional octets are necessary to form the
      encapsulation.  In environments where bandwidth is at a premium,
      the encapsulation may be shortened to as few as 2 octets.  To
      support high speed hardware implementations, PPP provides that the
      default encapsulation header and information fields fall on 32-bit
      boundaries, and allows the trailer to be padded to an arbitrary
      boundary.

   Link Control Protocol

      More importantly, the Point-to-Point Protocol defines more than
      just an encapsulation scheme.  In order to be sufficiently
      versatile to be portable to a wide variety of environments, PPP
      provides a Link Control Protocol (LCP).  The LCP is used to
      automatically agree upon the encapsulation format options, handle
      varying limits on sizes of packets, authenticate the identity of
      its peer on the link, determine when a link is functioning
      properly and when it is defunct, detect a looped-back link and
      other common misconfiguration errors, and terminate the link.

   Network Control Protocols

      Point-to-Point links tend to exacerbate many problems with the
      current family of network protocols.  For instance, assignment and
      management of IP addresses, which is a problem even in LAN
      environments, is especially difficult over circuit-switched
      point-to-point links (such as dial-up modem servers).  These
      problems are handled by a family of Network Control Protocols
      (NCPs), which each manage the specific needs required by their
      respective network-layer protocols.  These NCPs are defined in
      other documents.

   Configuration

      It is intended that PPP be easy to configure.  By design, the
      standard defaults should handle all common configurations.  The
      implementor may specify improvements to the default configuration,
      which are automatically communicated to the peer without operator
      intervention.  Finally, the operator may explicitly configure
      options for the link which enable the link to operate in
      environments where it would otherwise be impossible.

      This self-configuration is implemented through an extensible
      option negotiation mechanism, wherein each end of the link
      describes to the other its capabilities and requirements.
      Although the option negotiation mechanism described in this



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RFC 1331                Point-to-Point Protocol                 May 1992


      document is specified in terms of the Link Control Protocol (LCP),
      the same facilities may be used by the Internet Protocol Control
      Protocol (IPCP) and others in the family of NCPs.

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 should 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:

   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.



Simpson                                                         [Page 3]

RFC 1331                Point-to-Point Protocol                 May 1992


2.  Physical Layer Requirements

   The Point-to-Point Protocol is capable of operating across any
   DTE/DCE interface (e.g., EIA RS-232-C, EIA RS-422, EIA RS-423 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) or
   synchronous bit-serial mode, transparent to PPP Data Link Layer