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RFC 1548 - The Point-to-Point Protocol (PPP)


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Network Working Group                                         W. Simpson
Request for Comments: 1548                                    Daydreamer
Obsoletes: RFC 1331                                        December 1993
Category: Standards Track

                   The Point-to-Point Protocol (PPP)

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) provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.  PPP
   is comprised of 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 (NCPs) for establishing
         and configuring different network-layer protocols.

   This document defines the PPP organization and methodology, and the
   PPP encapsulation, together with an extensible option negotiation
   mechanism which is able to negotiate a rich assortment of
   configuration parameters and provides additional management
   functions.  The PPP Link Control Protocol (LCP) is described in terms
   of this mechanism.

   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@ucdavis.edu mailing list.

Table of Contents

   1.   Introduction ................................................3
   1.1  Specification of Requirements ...............................4
   1.2  Terminology .................................................5
   2.   PPP Encapsulation ...........................................5
   3.   PPP Link Operation ..........................................8
   3.1  Overview ....................................................8
   3.2  Phase Diagram ...............................................8
   3.3  Link Dead (physical-layer not ready) ........................9
   3.4  Link Establishment Phase ....................................9
   3.5  Authentication Phase ........................................9
   3.6  Network-Layer Protocol Phase ................................10
   3.7  Link Termination Phase ......................................10
   4.   The Option Negotiation Automaton ............................11
   4.1  State Diagram ...............................................12
   4.2  State Transition Table ......................................14
   4.3  A Day in the Life ...........................................15
   4.4  States ......................................................16
   4.5  Events ......................................................19
   4.6  Actions .....................................................23
   4.7  Loop Avoidance ..............................................26
   4.8  Counters and Timers .........................................26
   5.   LCP Packet Formats ..........................................27
   5.1  Configure-Request ...........................................29
   5.2  Configure-Ack ...............................................30
   5.3  Configure-Nak ...............................................31
   5.4  Configure-Reject ............................................33
   5.5  Terminate-Request and Terminate-Ack .........................34
   5.6  Code-Reject .................................................35
   5.7  Protocol-Reject .............................................36
   5.8  Echo-Request and Echo-Reply .................................37
   5.9  Discard-Request .............................................39
   6.   LCP Configuration Options ...................................40
   6.1  Maximum-Receive-Unit ........................................41
   6.2  Async-Control-Character-Map .................................42
   6.3  Authentication-Protocol .....................................43
   6.4  Quality-Protocol ............................................45
   6.5  Magic-Number ................................................46
   6.6  Protocol-Field-Compression ..................................49
   6.7  Address-and-Control-Field-Compression .......................50
   APPENDIX A. LCP Recommended Options ..............................51
   SECURITY CONSIDERATIONS ..........................................51
   REFERENCES .......................................................52
   ACKNOWLEDGEMENTS .................................................52
   CHAIR'S ADDRESS ..................................................52
   EDITOR'S ADDRESS .................................................53

1. Introduction

   Encapsulation

      The PPP encapsulation 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 [1].

      The PPP encapsulation has been carefully designed to retain
      compatibility with most commonly used supporting hardware.

      Only 8 additional octets are necessary to form the encapsulation
      when used with the default HDLC framing.  In environments where
      bandwidth is at a premium, the encapsulation and framing may be
      shortened to 2 or 4 octets.

      To support high speed implementations, the default encapsulation
      uses only simple fields, only one of which needs to be examined
      for demultiplexing.  The default header and information fields
      fall on 32-bit boundaries, and the trailer may be padded to an
      arbitrary boundary.

    Link Control Protocol

      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
      companion documents.

    Configuration

      It is intended that PPP links be easy to configure.  By design,
      the standard defaults handle all common configurations.  The
      implementor can 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
      document is specified in terms of the Link Control Protocol (LCP),
      the same facilities are designed to be used by other control
      protocols, especially 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 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

      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. PPP Encapsulation

   The PPP encapsulation is used to disambiguate multiprotocol
   datagrams.  This encapsulation requires framing to indicate the
   beginning and end of the encapsulation.  Methods of providing framing
   are specified in companion documents.

   A summary of the PPP encapsulation is shown below.  The fields are
   transmitted from left to right.

              +----------+-------------+---------+
              | Protocol | Information | Padding |
              | 16 bits  |      *      |    *    |
              +----------+-------------+---------+

    Protocol Field

      The Protocol field is two octets and its value identifies the
      datagram encapsulated in the Information field of the packet.  The
      field is transmitted and received most significant octet first.

      The structure of this field is consistent with the ISO 3309
      extension mechanism for address fields.  All Protocols MUST be
      odd; the least significant bit of the least significant octet MUST
      equal "1".  Also, all Protocols MUST be assigned such that the
      least significant bit of the most significant octet equals "0".
      Frames received which don't comply with these rules MUST be
      treated as having an unrecognized Protocol.

      Protocol field values in the "0***" to "3***" range identify the
      network-layer protocol of specific packets, and values in the
      "8***" to "b***" range identify packets belonging to the
      associated Network Control Protocols (NCPs), if any.

      Protocol field values in the "4***" to "7***" range are used for
      protocols with low volume traffic which have no associated NCP.
      Protocol field values in the "c***" to "f***" range identify
      packets as link-layer Control Protocols (such as LCP).

      Up-to-date values of the Protocol field are specified in the most
      recent "Assigned Numbers" RFC [2].  Current values are assigned as
      follows:

           Value (in hex)  Protocol Name

           0001            Padding Protocol
           0003 to 001f    reserved (transparency inefficient)
           0021            Internet Protocol
           0023            OSI Network Layer
           0025            Xerox NS IDP
           0027            DECnet Phase IV
           0029            Appletalk
           002b            Novell IPX
           002d            Van Jacobson Compressed TCP/IP
           002f            Van Jacobson Uncompressed TCP/IP

           0031            Bridging PDU
           0033            Stream Protocol (ST-II)
           0035            Banyan Vines
           0037            unused
           0039            AppleTalk EDDP
           003b            AppleTalk SmartBuffered
           003d            Multi-Link
           005d            reserved (compression inefficient)
           00cf            reserved (PPP NLPID)
           00fd            1st choice compression
           00ff            reserved (compression inefficient)

           0201            802.1d Hello Packets
           0203            IBM Source Routing BPDU
           0231            Luxcom
           0233            Sigma Network Systems

           8021            Internet Protocol Control Protocol
           8023            OSI Network Layer Control Protocol
           8025            Xerox NS IDP Control Protocol
           8027            DECnet Phase IV Control Protocol
           8029            Appletalk Control Protocol
           802b            Novell IPX Control Protocol
           802d            Reserved
           802f            Reserved
           8031            Bridging NCP
           8033            Stream Protocol Control Protocol
           8035            Banyan Vines Control Protocol
           8037            unused
           8039            Reserved
           803b            Reserved
           803d            Multi-Link Control Protocol
           80fd            Compression Control Protocol
           80ff            Reserved

           c021            Link Control Protocol
           c023            Password Authentication Protocol
           c025            Link Quality Report
           c223            Challenge Handshake Authentication Protocol

      Developers of new protocols MUST obtain a number from the Internet
      Assigned Numbers Authority (IANA), at IANA@isi.edu.

    Information Field

      The Information field is zero or more octets.  The Information
      field contains the datagram for the protocol specified in the
      Protocol field.

      The maximum length for the Information field, including Padding,
      is termed the Maximum Receive Unit (MRU), which defaults to 1500
      octets.  By negotiation, consenting PPP implementations may use
      other values for the MRU.

    Padding

      On transmission, the Information field MAY be padded with an
      arbitrary number of octets up to the MRU.  It is the
      responsibility of each protocol to distinguish padding octets from
      real information.

3.  PPP Link Operation

3.1 Overview

   In order to establish communications over a point-to-point link, each
   end of the PPP link MUST first send LCP packets to configure and test
   the data link.  After the link has been established, the peer MAY be
   authenticated.  Then, PPP MUST send NCP packets to choose and
   configure one or more network-layer protocols.  Once each of the
   chosen network-layer protocols has been configured, datagrams from
   each network-layer protocol can be sent over the link.

   The link will remain configured for communications until explicit LCP
   or NCP packets close the link down, or until some external event
   occurs (an inactivity timer expires or network administrator
   intervention).

3.2 Phase Diagram

   In the process of configuring, maintaining and terminating the
   point-to-point link, the PPP link goes through several distinct
   phases:

   +------+        +-----------+           +--------------+
   |      | UP     |           | OPENED    |              | SUCCESS/NONE
   | Dead |------->| Establish |---------->| Authenticate |--+
   |      |        |           |           |              |  |
   +------+        +-----------+           +--------------+  |
      ^          FAIL |                   FAIL |             |
      +<--------------+             +----------+             |
      |                             |                        |
      |            +-----------+    |           +---------+  |
      |       DOWN |           |    |   CLOSING |         |  |
      +------------| Terminate |<---+<----------| Network |<-+
                   |           |                |         |
                   +-----------+                +---------+

3.3 Link Dead (physical-layer not ready)

   The link necessarily begins and ends with this phase.  When an
   external event (such as carrier detection or network administrator
   configuration) indicates that the physical-layer is ready to be used,
   PPP will proceed to the Link Establishment phase.

   During this phase, the LCP automaton (described below) will be in the
   Initial or Starting states.  The transition to the Link Establishment
   phase will signal an Up event to the automaton.

    Implementation Note:

      Typically, a link will return to this phase automatically after
      the disconnection of a modem.  In the case of a hard-wired line,
      this phase may be extremely short -- merely long enough to detect
      the presence of the device.

3.4 Link Establishment Phase

   The Link Control Protocol (LCP) is used to establish the connection
   through an exchange of Configure packets.  This exchange is complete,
   and the LCP Opened state entered, once a Configure-Ack packet
   (described below) has been both sent and received.

   All Configuration Options are assumed to be at default values unless
   altered by the configuration exchange.  See the section on LCP
   Configuration Options for further discussion.

   It is important to note that only Configuration Options which are
   independent of particular network-layer protocols are configured by
   LCP.  Configuration of individual network-layer protocols is handled
   by separate Network Control Protocols (NCPs) during the Network-Layer
   Protocol phase.

   Any non-LCP packets received during this phase MUST be silently
   discarded.

3.5 Authentication Phase

   On some links it may be desirable to require a peer to authenticate
   itself before allowing network-layer protocol packets to be
   exchanged.

   By default, authentication is not mandatory.  If an implementation
   desires that the peer authenticate with some specific authentication
   protocol, then it MUST negotiate the use of that authentication
   protocol during Link Establishment phase.

   Authentication SHOULD take place as soon as possible after link
   establishment.  However, link quality determination MAY occur
   concurrently.  An implementation MUST NOT allow the exchange of link
   quality determination packets to delay authentication indefinitely.

   Advancement from the Authentication phase to the Network-Layer
   Protocol phase MUST NOT occur until authentication has completed,
   using the negotiated authentication protocol.  If authentication
   fails, PPP SHOULD proceed instead to the Link Termination phase.

   Any Network Control Protocol or network-layer protocol packets
   received during this phase MUST be silently discarded.

3.6 Network-Layer Protocol Phase

   Once PPP has finished the previous phases, each network-layer
   protocol (such as IP, IPX, or AppleTalk) MUST be separately
   configured by the appropriate Network Control Protocol (NCP).

   Each NCP MAY be Opened and Closed at any time.

    Implementation Note:

      Because an implementation may initially use a significant amount
      of time for link quality determination, implementations SHOULD
      avoid fixed timeouts when waiting for their peers to configure a
      NCP.

      After a NCP has reached the Opened state, PPP will carry the
      corresponding network-layer protocol packets.  Any network-layer
      protocol packets received when the corresponding NCP is not in the
      Opened state MUST be silently discarded.

    Implementation Note:

      There is an exception to the preceding paragraphs, due to the
      availability of the LCP Protocol-Reject (described below).  While
      LCP is in the Opened state, any protocol packet which is
      unsupported by the implementation MUST be returned in a Protocol-
      Reject.  Only protocols which are supported are silently
      discarded.

      During this phase, link traffic consists of any possible
      combination of LCP, NCP, and network-layer protocol packets.

3.7 Link Termination Phase

   PPP can terminate the link at any time.  This might happen because of

   the loss of carrier, authentication failure, link quality failure,
   the expiration of an idle-period timer, or the administrative closing
   of the link.  LCP is used to close the link through an exchange of
   Terminate packets.  When the link is closing, PPP informs the
   network-layer protocols so that they may take appropriate action.

   After the exchange of Terminate packets, the implementation SHOULD
   signal the physical-layer to disconnect in order to enforce the
   termination of the link, particularly in the case of an
   authentication failure.  The sender of the Terminate-Request SHOULD
   disconnect after receiving a Terminate-Ack, or after the Restart
   counter expires.  The receiver of a Terminate-Request SHOULD wait for
   the peer to disconnect, and MUST NOT disconnect until at least one
   Restart time has passed after sending a Terminate-Ack.  PPP SHOULD
   proceed to the Link Dead phase.

   Any non-LCP packets received during this phase MUST be silently
   discarded.

    Implementation Note:

      The closing of the link by LCP is sufficient.  There is no need
      for each NCP to send a flurry of Terminate packets.  Conversely,
      the fact that one NCP has Closed is not sufficient reason to cause
      the termination of the PPP link, even if that NCP was the only NCP
      currently in the Opened state.

4. The Option Negotiation Automaton

   The finite-state automaton is defined by events, actions and state
   transitions.  Events include reception of external commands such as
   Open and Close, expiration of the Restart timer, and reception of
   packets from a peer.  Actions include the starting of the Restart
   timer and transmission of packets to the peer.

   Some types of packets -- Configure-Naks and Configure-Rejects, or
   Code-Rejects and Protocol-Rejects, or Echo-Requests, Echo-Replies and
   Discard-Requests -- are not differentiated in the automaton
   descriptions.  As will be described later, these packets do indeed
   serve different functions.  However, they always cause the same
   transitions.

Events                                  Actions

Up   = lower layer is Up                tlu = This-Layer-Up
Down = lower layer is Down              tld = This-Layer-Down
Open = administrative Open              tls = This-Layer-Started
Close= administrative Close             tlf = This-Layer-Finished

TO+  = Timeout with counter > 0         irc = Initialize-Restart-Counter
TO-  = Timeout with counter expired     zrc = Zero-Restart-Counter

RCR+ = Receive-Configure-Request (Good) scr = Send-Configure-Request
RCR- = Receive-Configure-Request (Bad)
RCA  = Receive-Configure-Ack            sca = Send-Configure-Ack
RCN  = Receive-Configure-Nak/Rej        scn = Send-Configure-Nak/Rej

RTR  = Receive-Terminate-Request        str = Send-Terminate-Request
RTA  = Receive-Terminate-Ack            sta = Send-Terminate-Ack

RUC  = Receive-Unknown-Code             scj = Send-Code-Reject
RXJ+ = Receive-Code-Reject (permitted)
    or Receive-Protocol-Reject
RXJ- = Receive-Code-Reject (catastrophic)
    or Receive-Protocol-Reject
RXR  = Receive-Echo-Request             ser = Send-Echo-Reply
    or Receive-Echo-Reply
    or Receive-Discard-Request

4.1 State Diagram

   The simplified state diagram which follows describes the sequence of
   events for reaching agreement on Configuration Options (opening the
   PPP link) and for later termination of the link.

   This diagram is not a complete representation of the automaton.
   Implementation MUST be done by consulting the actual state transition
   table.

   Events are in upper case.  Actions are in lower case.  For these
   purposes, the state machine is initially in the Closed state.  Once
   the Opened state has been reached, both ends of the link have met the
   requirement of having both sent and received a Configure-Ack packet.

                 RCR                    TO+
               +--sta-->+             +------->+
               |        |             |        |
         +-------+      |   RTA +-------+      | Close +-------+
         |       |<-----+<------|       |<-str-+<------|       |
         |Closed |              |Closing|              |Opened |
         |       | Open         |       |              |       |
         |       |------+       |       |              |       |
         +-------+      |       +-------+              +-------+
                        |                                ^
                        |                                |
                        |         +-sca----------------->+
                        |         |                      ^
                RCN,TO+ V    RCR+ |     RCR-         RCA |    RCN,TO+
               +------->+         |   +------->+         |   +--scr-->+
               |        |         |   |        |         |   |        |
         +-------+      |   TO+ +-------+      |       +-------+      |
         |       |<-scr-+<------|       |<-scn-+       |       |<-----+
         | Req-  |              | Ack-  |              | Ack-  |
         | Sent  | RCA          | Rcvd  |              | Sent  |
  +-scn->|       |------------->|       |       +-sca->|       |
  |      +-------+              +-------+       |      +-------+
  |   RCR- |   | RCR+                           |   RCR+ |   | RCR-
  |        |   +------------------------------->+<-------+   |
  |        |                                                 |
  +<-------+<------------------------------------------------+

4.2 State Transition Table

  The complete state transition table follows.  States are indicated
  horizontally, and events are read vertically.  State transitions and
  actions are represented in the form action/new-state.  Multiple
  actions are separated by commas, and may continue on succeeding lines
  as space requires; multiple actions may be implemented in any
  convenient order.  The state may be followed by a letter, which
  indicates an explanatory footnote.  The dash ('-') indicates an
  illegal transition.

         | State
         |    0         1         2         3         4         5
   Events| Initial   Starting  Closed    Stopped   Closing   Stopping
   ------+-----------------------------------------------------------
    Up   |    2     irc,scr/6     -         -         -         -
    Down |    -         -         0       tls/1       0         1
    Open |  tls/1       1     irc,scr/6     3r        5r        5r
    Close|    0         0         2         2         4         4
         |
     TO+ |    -         -         -         -       str/4     str/5
     TO- |    -         -         -         -       tlf/2     tlf/3
         |
    RCR+ |    -         -       sta/2 irc,scr,sca/8   4         5
    RCR- |    -         -       sta/2 irc,scr,scn/6   4         5
    RCA  |    -         -       sta/2     sta/3       4         5
    RCN  |    -         -       sta/2     sta/3       4         5
         |
    RTR  |    -         -       sta/2     sta/3     sta/4     sta/5
    RTA  |    -         -         2         3       tlf/2     tlf/3
         |
    RUC  |    -         -       scj/2     scj/3     scj/4     scj/5
    RXJ+ |    -         -         2         3         4         5
    RXJ- |    -         -       tlf/2     tlf/3     tlf/2     tlf/3
         |
    RXR  |    -         -         2         3         4         5

            | State
            |    6         7         8           9
      Events| Req-Sent  Ack-Rcvd  Ack-Sent    Opened
      ------+-----------------------------------------
       Up   |    -         -         -           -
       Down |    1         1         1         tld/1
       Open |    6         7         8           9r
       Close|irc,str/4 irc,str/4 irc,str/4 tld,irc,str/4
            |
        TO+ |  scr/6     scr/6     scr/8         -
        TO- |  tlf/3p    tlf/3p    tlf/3p        -
            |
       RCR+ |  sca/8   sca,tlu/9   sca/8   tld,scr,sca/8
       RCR- |  scn/6     scn/7     scn/6   tld,scr,scn/6
       RCA  |  irc/7     scr/6x  irc,tlu/9   tld,scr/6x
       RCN  |irc,scr/6   scr/6x  irc,scr/8   tld,scr/6x
            |
       RTR  |  sta/6     sta/6     sta/6   tld,zrc,sta/5
       RTA  |    6         6         8       tld,scr/6
            |
       RUC  |  scj/6     scj/7     scj/8       scj/9
       RXJ+ |    6         6         8           9
       RXJ- |  tlf/3     tlf/3     tlf/3   tld,irc,str/5
            |
       RXR  |    6         7         8         ser/9

   The states in which the Restart timer is running are identifiable by
   the presence of TO events.  Only the Send-Configure-Request, Send-
   Terminate-Request and Zero-Restart-Counter actions start or re-start
   the Restart timer.  The Restart timer is stopped when transitioning
   from any state where the timer is running to a state where the timer
   is not running.

      [p]   Passive option; see Stopped state discussion.

      [r]   Restart option; see Open event discussion.

      [x]   Crossed connection; see RCA event discussion.

4.3 A Day in the Life

   Here is an example of how a typical implementation might use the
   automaton to implement LCP in a dial-up environment:

   -  The Network Access Server is powered on (Initial state, Link Dead
      phase).

   -  A configuration file indicates that a particular link is to be

      used for PPP access (Open: tls/Starting).  The This-Layer-Started
      event turns on DTR to a modem, readying it for accepting calls.

   -  An incoming call is answered.  The modem CD triggers configuration
      negotiation (Up: irc,scr/Req-Sent, Link Establishment phase).

   -  A Configure-Request is received, which is acknowleged (RCR+:
      sca/Ack-Sent).

   -  The Request is acknowleged (RCA: irc,tlu/Opened).  The This-
      Layer-Up event starts authentication and quality monitoring
      protocols (Authentication phase).

   -  When authentication and quality monitoring are satisfied, they
      send an Up event to start the available NCPs (Network-Layer
      Protocol phase).

   -  Later, the peer is finished, and closes the link.  A Terminate-
      Request arrives (RTR: tld,zrc,sta/Stopping, Termination phase).
      The This-Layer-Down action sends the Down event to any NCPs, while
      the Terminate-Ack is sent.  The Zero-Restart-Counter action causes
      the link to wait for the peer to process the Terminate-Ack, with
      no retries.

   -  When the Restart Timer times out (TO-: tlf/Stopped), the This-
      Layer-Finished action signals the modem to hang up by dropping
      DTR.

   -  When the CD from the modem drops (Down: tls/Starting), the This-
      Layer-Started action raises DTR again, readying it for the next
      call (returning to the Link Dead phase).

4.4 States

   Following is a more detailed description of each automaton state.

    Initial

      In the Initial state, the lower layer is unavailable (Down), and
      no Open has occurred.  The Restart timer is not running in the
      Initial state.

    Starting

      The Starting state is the Open counterpart to the Initial state.
      An administrative Open has been initiated, but the lower layer is
      still unavailable (Down).  The Restart timer is not running in the
      Starting state.

      When the lower layer becomes available (Up), a Configure-Request
      is sent.

    Closed

      In the Closed state, the link is available (Up), but no Open has
      occurred.  The Restart timer is not running in the Closed state.

      Upon reception of Configure-Request packets, a Terminate-Ack is
      sent.  Terminate-Acks are silently discarded to avoid creating a
      loop.

    Stopped

      The Stopped state is the Open counterpart to the Closed state.  It
      is entered when the automaton is waiting for a Down event after
      the This-Layer-Finished action, or after sending a Terminate-Ack.
      The Restart timer is not running in the Stopped state.

      Upon reception of Configure-Request packets, an appropriate
      response is sent.  Upon reception of other packets, a Terminate-
      Ack is sent.  Terminate-Acks are silently discarded to avoid
      creating a loop.

    Rationale:

      The Stopped state is a junction state for link termination, link
      configuration failure, and other automaton failure modes.  These
      potentially separate states have been combined.

      There is a race condition between the Down event response (from
      the This-Layer-Finished action) and the Receive-Configure- Request
      event.  When a Configure-Request arrives before the Down event,
      the Down event will supercede by returning the automaton to the
      Starting state.  This prevents attack by repetition.

    Implementation Option:

      After the peer fails to respond to Configure-Requests, an
      implementation MAY wait passively for the peer to send Configure-
      Requests.  In this case, the This-Layer-Finished action is not
      used for the TO- event in states Req-Sent, Ack- Rcvd and Ack-Sent.

      This option is useful for dedicated circuits, or circuits which
      have no status signals available, but SHOULD NOT be used for
      switched circuits.

    Closing

      In the Closing state, an attempt is made to terminate the
      connection.  A Terminate-Request has been sent and the Restart
      timer is running, but a Terminate-Ack has not yet been received.

      Upon reception of a Terminate-Ack, the Closed state is entered.
      Upon the expiration of the Restart timer, a new Terminate-Request
      is transmitted and the Restart timer is restarted.  After the
      Restart timer has expired Max-Terminate times, this action may be
      skipped, and the Closed state may be entered.

    Stopping

      The Stopping state is the Open counterpart to the Closing state.
      A Terminate-Request has been sent and the Restart timer is
      running, but a Terminate-Ack has not yet been received.

    Rationale:

      The Stopping state provides a well defined opportunity to
      terminate a link before allowing new traffic.  After the link has
      terminated, a new configuration may occur via the Stopped or
      Starting states.

    Request-Sent

      In the Request-Sent state an attempt is made to configure the
      connection.  A Configure-Request has been sent and the Restart
      timer is running, but a Configure-Ack has not yet been received
      nor has one been sent.

    Ack-Received

      In the Ack-Received state, a Configure-Request has been sent and a
      Configure-Ack has been received.  The Restart timer is still
      running since a Configure-Ack has not yet been sent.

    Ack-Sent

      In the Ack-Sent state, a Configure-Request and a Configure-Ack
      have both been sent but a Configure-Ack has not yet been received.
      The Restart timer is always running in the Ack-Sent state.

    Opened

      In the Opened state, a Configure-Ack has been both sent and
      received.  The Restart timer is not running in the Opened state.

      When entering the Opened state, the implementation SHOULD signal
      the upper layers that it is now Up.  Conversely, when leaving the
      Opened state, the implementation SHOULD signal the upper layers
      that it is now Down.

4.5 Events

   Transitions and actions in the automaton are caused by events.

    Up

      The Up event occurs when a lower layer indicates that it is ready
      to carry packets.

      Typically, this event is used by a modem handling or calling
      process, or by some other coupling of the PPP link to the physical
      media, to signal LCP that the link is entering Link Establishment
      phase.

      It also can be used by LCP to signal each NCP that the link is
      entering Network-Layer Protocol phase.  That is, the This-Layer-Up
      action from LCP triggers the Up event in the NCP.

    Down

      The Down event occurs when a lower layer indicates that it is no
      longer ready to carry packets.

      Typically, this event is used by a modem handling or calling
      process, or by some other coupling of the PPP link to the physical
      media, to signal LCP that the link is entering Link Dead phase.

      It also can be used by LCP to signal each NCP that the link is
      leaving Network-Layer Protocol phase.  That is, the This-Layer-
      Down action from LCP triggers the Down event in the NCP.

    Open

      The Open event indicates that the link is administratively
      available for traffic; that is, the network administrator (human
      or program) has indicated that the link is allowed to be Opened.
      When this event occurs, and the link is not in the Opened state,
      the automaton attempts to send configuration packets to the peer.

      If the automaton is not able to begin configuration (the lower
      layer is Down, or a previous Close event has not completed), the
      establishment of the link is automatically delayed.

      When a Terminate-Request is received, or other events occur which
      cause the link to become unavailable, the automaton will progress
      to a state where the link is ready to re-open.  No additional
      administrative intervention is necessary.

    Implementation Option:

      Experience has shown that users will execute an additional Open
      command when they want to renegotiate the link.  This might
      indicate that new values are to be negotiated.

      Since this is not the meaning of the Open event, it is suggested
      that when an Open user command is executed in the Opened, Closing,
      Stopping, or Stopped states, the implementation issue a Down
      event, immediately followed by an Up event.  This will cause the
      renegotiation of the link, without any harmful side effects.

    Close

      The Close event indicates that the link is not available for
      traffic; that is, the network administrator (human or program) has
      indicated that the link is not allowed to be Opened.  When this
      event occurs, and the link is not in the Closed state, the
      automaton attempts to terminate the connection.  Futher attempts
      to re-configure the link are denied until a new Open event occurs.

    Implementation Note:

      When authentication fails, the link SHOULD be terminated, to
      prevent attack by repetition and denial of service to other users.
      Since the link is administratively available (by definition), this
      can be accomplished by simulating a Close event to the LCP,
      immediately followed by an Open event.

      The Close followed by an Open will cause an orderly termination of
      the link, by progressing from the Closing to the Stopping state,
      and the This-Layer-Finished action can disconnect the link.  The
      automaton waits in the Stopped or Starting states for the next
      connection attempt.

    Timeout (TO+,TO-)

      This event indicates the expiration of the Restart timer.  The
      Restart timer is used to time responses to Configure-Request and
      Terminate-Request packets.

      The TO+ event indicates that the Restart counter continues to be
      greater than zero, which triggers the corresponding Configure-

      Request or Terminate-Request packet to be retransmitted.

      The TO- event indicates that the Restart counter is not greater
      than zero, and no more packets need to be retransmitted.

    Receive-Configure-Request (RCR+,RCR-)

      This event occurs when a Configure-Request packet is received from
      the peer.  The Configure-Request packet indicates the desire to
      open a connection and may specify Configuration Options.  The
      Configure-Request packet is more fully described in a later
      section.

      The RCR+ event indicates that the Configure-Request was
      acceptable, and triggers the transmission of a corresponding
      Configure-Ack.

      The RCR- event indicates that the Configure-Request was
      unacceptable, and triggers the transmission of a corresponding
      Configure-Nak or Configure-Reject.

    Implementation Note:

      These events may occur on a connection which is already in the
      Opened state.  The implementation MUST be prepared to immediately
      renegotiate the Configuration Options.

    Receive-Configure-Ack (RCA)

      The Receive-Configure-Ack event occurs when a valid Configure-Ack
      packet is received from the peer.  The Configure-Ack packet is a
      positive response to a Configure-Request packet.  An out of
      sequence or otherwise invalid packet is silently discarded.

    Implementation Note:

      Since the correct packet has already been received before reaching
      the Ack-Rcvd or Opened states, it is extremely unlikely that
      another such packet will arrive.  As specified, all invalid
      Ack/Nak/Rej packets are silently discarded, and do not affect the
      transitions of the automaton.

      However, it is not impossible that a correctly formed packet will
      arrive through a coincidentally-timed cross-connection.  It is
      more likely to be the result of an implementation error.  At the
      very least, this occurance SHOULD be logged.

    Receive-Configure-Nak/Rej (RCN)

      This event occurs when a valid Configure-Nak or Configure-Reject
      packet is received from the peer.  The Configure-Nak and
      Configure-Reject packets are negative responses to a Configure-
      Request packet.  An out of sequence or otherwise invalid packet is
      silently discarded.

    Implementation Note:

      Although the Configure-Nak and Configure-Reject cause the same
      state transition in the automaton, these packets have
      significantly different effects on the Configuration Options sent
      in the resulting Configure-Request packet.

    Receive-Terminate-Request (RTR)

      The Receive-Terminate-Request event occurs when a Terminate-
      Request packet is received.  The Terminate-Request packet
      indicates the desire of the peer to close the connection.

    Implementation Note:

      This event is not identical to the Close event (see above), and
      does not override the Open commands of the local network
      administrator.  The implementation MUST be prepared to receive a
      new Configure-Request without network administrator intervention.

    Receive-Terminate-Ack (RTA)

      The Receive-Terminate-Ack event occurs when a Terminate-Ack packet
      is received from the peer.  The Terminate-Ack packet is usually a
      response to a Terminate-Request packet.  The Terminate-Ack packet
      may also indicate that the peer is in Closed or Stopped states,
      and serves to re-synchronize the link configuration.

    Receive-Unknown-Code (RUC)

      The Receive-Unknown-Code event occurs when an un-interpretable
      packet is received from the peer.  A Code-Reject packet is sent in
      response.

    Receive-Code-Reject, Receive-Protocol-Reject (RXJ+,RXJ-)

      This event occurs when a Code-Reject or a Protocol-Reject packet
      is received from the peer.

      The RXJ+ event arises when the rejected value is acceptable, such

      as a Code-Reject of an extended code, or a Protocol-Reject of a
      NCP.  These are within the scope of normal operation.  The
      implementation MUST stop sending the offending packet type.

      The RXJ- event arises when the rejected value is catastrophic,
      such as a Code-Reject of Configure-Request, or a Protocol-Reject
      of LCP!  This event communicates an unrecoverable error that
      terminates the connection.

    Receive-Echo-Request, Receive-Echo-Reply, Receive-Discard-Request
    (RXR)

    This event occurs when an Echo-Request, Echo-Reply or Discard-
    Request packet is received from the peer.  The Echo-Reply packet is
    a response to a Echo-Request packet.  There is no reply to an Echo-
    Reply or Discard-Request packet.

4.6 Actions

   Actions in the automaton are caused by events and typically indicate
   the transmission of packets and/or the starting or stopping of the
   Restart timer.

    Illegal-Event (-)

      This indicates an event that cannot occur in a properly
      implemented automaton.  The implementation has an internal error,
      which should be reported and logged.  No transition is taken, and
      the implementation SHOULD NOT reset or freeze.

    This-Layer-Up (tlu)

      This action indicates to the upper layers that the automaton is
      entering the Opened state.

      Typically, this action is used by the LCP to signal the Up event
      to a NCP, Authentication Protocol, or Link Quality Protocol, or
      MAY be used by a NCP to indicate that the link is available for
      its network layer traffic.

    This-Layer-Down (tld)

      This action indicates to the upper layers that the automaton is
      leaving the Opened state.

      Typically, this action is used by the LCP to signal the Down event
      to a NCP, Authentication Protocol, or Link Quality Protocol, or
      MAY be used by a NCP to indicate that the link is no longer

      available for its network layer traffic.

    This-Layer-Started (tls)

      This action indicates to the lower layers that the automaton is
      entering the Starting state, and the lower layer is needed for the
      link.  The lower layer SHOULD respond with an Up event when the
      lower layer is available.

    Implementation Note:

      This results of this action are highly implementation dependent.

      The transitions where this event is indicated are defined
      according to a message passing architecture, rather than a
      signalling architecture.  If the action is desired to control
      specific signals (such as DTR), other transitions for the action
      are likely to be required (Open in Closed, RCR in Stopped).

    This-Layer-Finished (tlf)

      This action indicates to the lower layers that the automaton is
      entering the Stopped or Closed states, and the lower layer is no
      longer needed for the link.  The lower layer SHOULD respond with a
      Down event when the lower layer has terminated.

      Typically, this action MAY be used by the LCP to advance to the
      Link Dead phase, or MAY be used by a NCP to indicate to the LCP
      that the link may terminate when there are no other NCPs open.

    Implementation Note:

      This results of this action are highly implementation dependent.

      The transitions where this event is indicated are defined
      according to a message passing architecture, rather than a
      signalling architecture.  If the action is desired to control
      specific signals (such as DTR), other transitions for the action
      are likely to be required (Close in Starting, Down in Closing).

    Initialize-Restart-Counter (irc)

      This action sets the Restart counter to the appropriate value
      (Max-Terminate or Max-Configure).  The counter is decremented for
      each transmission, including the first.

    Implementation Note:

      In addition to setting the Restart counter, the implementation
      MUST set the timeout period to the initial value when Restart
      timer backoff is used.

    Zero-Restart-Counter (zrc)

      This action sets the Restart counter to zero.

    Implementation Note:

      This action enables the FSA to pause before proceeding to the
      desired final state, allowing traffic to be processed by the peer.
      In addition to zeroing the Restart counter, the implementation
      MUST set the timeout period to an appropriate value.

    Send-Configure-Request (scr)

      The Send-Configure-Request action transmits a Configure-Request
      packet.  This indicates the desire to open a connection with a
      specified set of Configuration Options.  The Restart timer is
      started when the Configure-Request packet is transmitted, to guard
      against packet loss.  The Restart counter is decremented each time
      a Configure-Request is sent.

    Send-Configure-Ack (sca)

      The Send-Configure-Ack action transmits a Configure-Ack packet.
      This acknowledges the reception of a Configure-Request packet with
      an acceptable set of Configuration Options.

    Send-Configure-Nak (scn)

      The Send-Configure-Nak action transmits a Configure-Nak or
      Configure-Reject packet, as appropriate.  This negative response
      reports the reception of a Configure-Request packet with an
      unacceptable set of Configuration Options.  Configure-Nak packets
      are used to refuse a Configuration Option value, and to suggest a
      new, acceptable value.  Configure-Reject packets are used to
      refuse all negotiation about a Configuration Option, typically
      because it is not recognized or implemented.  The use of
      Configure-Nak versus Configure-Reject is more fully described in
      the section on LCP Packet Formats.

    Send-Terminate-Request (str)

      The Send-Terminate-Request action transmits a Terminate-Request

      packet.  This indicates the desire to close a connection.  The
      Restart timer is started when the Terminate-Request packet is
      transmitted, to guard against packet loss.  The Restart counter is
      decremented each time a Terminate-Request is sent.

    Send-Terminate-Ack (sta)

      The Send-Terminate-Ack action transmits a Terminate-Ack packet.
      This acknowledges the reception of a Terminate-Request packet or
      otherwise serves to synchronize the state machines.

    Send-Code-Reject (scj)

      The Send-Code-Reject action transmits a Code-Reject packet.  This
      indicates the reception of an unknown type of packet.

    Send-Echo-Reply (ser)

      The Send-Echo-Reply action transmits an Echo-Reply packet.  This
      acknowledges the reception of an Echo-Request packet.

4.7 Loop Avoidance

   The protocol makes a reasonable attempt at avoiding Configuration
   Option negotiation loops.  However, the protocol does NOT guarantee
   that loops will not happen.  As with any negotiation, it is possible
   to configure two PPP implementations with conflicting policies that
   will never converge.  It is also possible to configure policies which
   do converge, but which take significant time to do so.  Implementors
   should keep this in mind and SHOULD implement loop detection
   mechanisms or higher level timeouts.

4.8 Counters and Timers

    Restart Timer

      There is one special timer used by the automaton.  The Restart
      timer is used to time transmissions of Configure-Request and
      Terminate- Request packets.  Expiration of the Restart timer
      causes a Timeout event, and retransmission of the corresponding
      Configure-Request or Terminate-Request packet.  The Restart timer
      MUST be configurable, but SHOULD default to three (3) seconds.

    Implementation Note:

      The Restart timer SHOULD be based on the speed of the link.  The
      default value is designed for low speed (2,400 to 9,600 bps), high

      switching latency links (typical telephone lines).  Higher speed
      links, or links with low switching latency, SHOULD have
      correspondingly faster retransmission times.

      Instead of a constant value, the Restart timer MAY begin at an
      initial small value and increase to the configured final value.
      Each successive value less than the final value SHOULD be at least
      twice the previous value.  The initial value SHOULD be large
      enough to account for the size of the packets, twice the round
      trip time for transmission at the link speed, and at least an
      additional 100 milliseconds to allow the peer to process the
      packets before responding.  Some circuits add another 200
      milliseconds of satellite delay.  Round trip times for modems
      operating at 14,400 bps have been measured in the range of 160 to
      more than 600 milliseconds.

    Max-Terminate

      There is one required restart counter for Terminate-Requests.
      Max- Terminate indicates the number of Terminate-Request packets
      sent without receiving a Terminate-Ack before assuming that the
      peer is unable to respond.  Max-Terminate MUST be configurable,
      but SHOULD default to two (2) transmissions.

    Max-Configure

      A similar counter is recommended for Configure-Requests.  Max-
      Configure indicates the number of Configure-Request packets sent
      without receiving a valid Configure-Ack, Configure-Nak or
      Configure- Reject before assuming that the peer is unable to
      respond.  Max- Configure MUST be configurable, but SHOULD default
      to ten (10) transmissions.

    Max-Failure

      A related counter is recommended for Configure-Nak.  Max-Failure
      indicates the number of Configure-Nak packets sent without sending
      a Configure-Ack before assuming that configuration is not
      converging.  Any further Configure-Nak packets are converted to
      Configure-Reject packets.  Max-Failure MUST be configurable, but
      SHOULD default to ten (10) transmissions.

5. LCP Packet Formats

   There are three classes of LCP packets:

      1. Link Configuration packets used to establish and configure a
         link (Configure-Request, Configure-Ack, Configure-Nak and

         Configure-Reject).

      2. Link Termination packets used to terminate a link (Terminate-
         Request and Terminate-Ack).

      3. Link Maintenance packets used to manage and debug a link
         (Code-Reject, Protocol-Reject, Echo-Request, Echo-Reply, and
         Discard-Request).

   This document describes Version 1 of the Link Control Protocol.  In
   the interest of simplicity, there is no version field in the LCP
   packet.  If a new version of LCP is necessary in the future, the
   intention is that a new PPP Protocol field value will be used to
   differentiate Version 1 LCP from all other versions.  A correctly
   functioning Version 1 LCP implementation will always respond to
   unknown Protocols (including other versions) with an easily
   recognizable Version 1 packet, thus providing a deterministic
   fallback mechanism for implementations of other versions.

   Regardless of which Configuration Options are enabled, all LCP Link
   Configuration, Link Termination, and Code-Reject packets (codes 1
   through 7) are always sent as if no Configuration Options were
   enabled.  This ensures that such LCP packets are always recognizable
   even when one end of the link mistakenly believes the link to be
   open.

    Implementation Note:

      In particular, the Async-Control-Character-Map (ACCM) default for
      the type of link is used, and no address, control, or protocol
      field compression is allowed.

      Exactly one LCP packet is encapsulated in the PPP Information
      field, where the PPP Protocol field indicates type hex c021 (Link
      Control Protocol).

   A summary of the Link Control Protocol 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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Code      |  Identifier   |            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |    Data ...
  +-+-+-+-+

   Code

      The Code field is one octet and identifies the kind of LCP packet.
      When a packet is received with an invalid Code field, a Code-
      Reject packet is transmitted.

      Up-to-date values of the LCP Code field are specified in the most
      recent "Assigned Numbers" RFC [2].  This specification concerns
      the following values:

            1       Configure-Request
            2       Configure-Ack
            3       Configure-Nak
            4       Configure-Reject
            5       Terminate-Request
            6       Terminate-Ack
            7       Code-Reject
            8       Protocol-Reject
            9       Echo-Request
            10      Echo-Reply
            11      Discard-Request

    Identifier

      The Identifier field is one octet and aids in matching requests
      and replies.  When a packet is received with an invalid Identifier
      field, the packet is silently discarded.

    Length

      The Length field is two octets and indicates the length of the LCP
      packet including the Code, Identifier, Length and Data fields.
      Octets outside the range of the Length field are treated as
      padding and are ignored on reception.  When a packet is received
      with an invalid Length field, the packet is silently discarded.

    Data

      The Data field is zero or more octets as indicated by the Length
      field.  The format of the Data field is determined by the Code
      field.

5.1 Configure-Request

    Description

      An implementation wishing to open a connection MUST transmit a LCP
      packet with the Code field set to 1 (Configure-Request), and the

      Options field filled with any desired changes to the link
      defaults.  Configuration Options SHOULD NOT be included with
      default values.

      Upon reception of a Configure-Request, an appropriate reply MUST
      be transmitted.

   A summary of the Configure-Request 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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Code      |  Identifier   |            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Options ...
  +-+-+-+-+

   Code

      1 for Configure-Request.

   Identifier

      The Identifier field MUST be changed whenever the content of the
      Options field changes, and whenever a valid reply has been
      received for a previous request.  For retransmissions, the
      Identifier MAY remain unchanged.

   Options

      The options field is variable in length and contains the list of
      zero or more Configuration Options that the sender desires to
      negotiate.  All Configuration Options are always negotiated
      simultaneously.  The format of Configuration Options is further
      described in a later section.

5.2 Configure-Ack

   Description

      If every Configuration Option received in a Configure-Request is
      recognizable and all values are acceptable, then the
      implementation MUST transmit a LCP packet with the Code field set
      to 2 (Configure-Ack), the Identifier field copied from the
      received Configure-Request, and the Options field copied from the
      received Configure-Request.  The acknowledged Configuration
      Options MUST NOT be reordered or modified in any way.

      On reception of a Configure-Ack, the Identifier field MUST match
      that of the last transmitted Configure-Request.  Additionally, the
      Configuration Options in a Configure-Ack MUST exactly match those
      of the last transmitted Configure-Request.  Invalid packets are
      silently discarded.

   A summary of the Configure-Ack 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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Code      |  Identifier   |            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Options ...
  +-+-+-+-+

   Code

      2 for Configure-Ack.

   Identifier

      The Identifier field is a copy of the Identifier field of the
      Configure-Request which caused this Configure-Ack.

   Options

      The Options field is variable in length and contains the list of
      zero or more Configuration Options that the sender is
      acknowledging.  All Configuration Options are always acknowledged
      simultaneously.

5.3 Configure-Nak

   Description

      If every element of the received Configuration Options is
      recognizable but some values are not acceptable, then the
      implementation MUST transmit a LCP packet with the Code field set
      to 3 (Configure-Nak), the Identifier field copied from the
      received Configure-Request, and the Options field filled with only
      the unacceptable Configuration Options from the Configure-Request.
      All acceptable Configuration Options are filtered out of the
      Configure-Nak, but otherwise the Configuration Options from the
      Configure-Request MUST NOT be reordered.

      Options which have no value fields (boolean options) MUST use the

      Configure-Reject reply instead.

      Each Configuration Option which is allowed only a single instance
      MUST be modified to a value acceptable to the Configure-Nak
      sender.  The default value MAY be used, when this differs from the
      requested value.

      When a particular type of Configuration Option can be listed more
      than once with different values, the Configure-Nak MUST include a
      list of all values for that option which are acceptable to the
      Configure-Nak sender.  This includes acceptable values that were
      present in the Configure-Request.

      Finally, an implementation may be configured to request the
      negotiation of a specific Configuration Option.  If that option is
      not listed, then that option MAY be appended to the list of Nak'd
      Configuration Options in order to prompt the peer to include that
      option in its next Configure-Request packet.  Any value fields for
      the option MUST indicate values acceptable to the Configure-Nak
      sender.

      On reception of a Configure-Nak, the Identifier field MUST match
      that of the last transmitted Configure-Request.  Invalid packets
      are silently discarded.

      Reception of a valid Configure-Nak indicates that a new
      Configure-Request MAY be sent with the Configuration Options
      modified as specified in the Configure-Nak.  When multiple
      instances of a Configuration Option are present, the peer SHOULD
      select a single value to include in its next Configure-Request
      packet.

      Some Configuration Options have a variable length.  Since the
      Nak'd Option has been modified by the peer, the implementation
      MUST be able to handle an Option length which is different from
      the original Configure-Request.

   A summary of the Configure-Nak 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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Code      |  Identifier   |            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Options ...
  +-+-+-+-+

    Code

      3 for Configure-Nak.

    Identifier

      The Identifier field is a copy of the Identifier field of the
      Configure-Request which caused this Configure-Nak.

    Options

      The Options field is variable in length and contains the list of
      zero or more Configuration Options that the sender is Nak'ing.
      All Configuration Options are always Nak'd simultaneously.

5.4 Configure-Reject

   Description

      If some Configuration Options received in a Configure-Request are
      not recognizable or are not acceptable for negotiation (as
      configured by a network administrator), then the implementation
      MUST transmit a LCP packet with the Code field set to 4
      (Configure-Reject), the Identifier field copied from the received
      Configure-Request, and the Options field filled with only the
      unacceptable Configuration Options from the Configure-Request.
      All recognizable and negotiable Configuration Options are filtered
      out of the Configure-Reject, but otherwise the Configuration
      Options MUST NOT be reordered or modified in any way.

      On reception of a Configure-Reject, the Identifier field MUST
      match that of the last transmitted Configure-Request.
      Additionally, the Configuration Options in a Configure-Reject MUST
      be a proper subset of those in the last transmitted Configure-
      Request.  Invalid packets are silently discarded.

      Reception of a valid Configure-Reject indicates that a new
      Configure-Request SHOULD be sent which does not include any of the
      Configuration Options listed in the Configure-Reject.

   A summary of the Configure-Reject 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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Code      |  Identifier   |            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Options ...
  +-+-+-+-+

    Code

      4 for Configure-Reject.

    Identifier

      The Identifier field is a copy of the Identifier field of the
      Configure-Request which caused this Configure-Reject.

    Options

      The Options field is variable in length and contains the list of
      zero or more Configuration Options that the sender is rejecting.
      All Configuration Options are always rejected simultaneously.

5.5 Terminate-Request and Terminate-Ack

   Description

      LCP includes Terminate-Request and Terminate-Ack Codes in order to
      provide a mechanism for closing a connection.

      A LCP implementation wishing to close a connection SHOULD transmit
      a LCP packet with the Code field set to 5 (Terminate-Request), and
      the Data field filled with any desired data.  Terminate-Request
      packets SHOULD continue to be sent until Terminate-Ack is
      received, the lower layer indicates that it has gone down, or a
      sufficiently large number have been transmitted such that the peer
      is down with reasonable certainty.

      Upon reception of a Terminate-Request, a LCP packet MUST be
      transmitted with the Code field set to 6 (Terminate-Ack), the
      Identifier field copied from the Terminate-Request packet, and the
      Data field filled with any desired data.

      Reception of an unelicited Terminate-Ack indicates that the peer
      is in the Closed or Stopped states, or is otherwise in need of
      re-negotiation.

   A summary of the Terminate-Request and Terminate-Ack packet formats

   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Code      |  Identifier   |            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |    Data ...
  +-+-+-+-+

    Code

      5 for Terminate-Request;

      6 for Terminate-Ack.

    Identifier

      On transmission, the Identifier field MUST be changed whenever the
      content of the Data field changes, and whenever a valid reply has
      been received for a previous request.  For retransmissions, the
      Identifier MAY remain unchanged.  On reception, the Identifier
      field of the Terminate-Request is copied into the Identifier field
      of the Terminate-Ack packet.

    Data

      The Data field is zero or more octets and contains uninterpreted
      data for use by the sender.  The data may consist of any binary
      value and may be of any length from zero to the peer's established
      MRU minus four.

5.6 Code-Reject

   Description

      Reception of a LCP packet with an unknown Code indicates that one
      of the communicating LCP implementations is faulty or incomplete.
      This error MUST be reported back to the sender of the unknown Code
      by transmitting a LCP packet with the Code field set to 7 (Code-
      Reject), and the inducing packet copied to the Rejected-
      Information field.

      Upon reception of a Code-Reject, the implementation SHOULD report
      the error, since it is unlikely that the situation can be
      rectified automatically.

   A summary of the Code-Reject 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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Code      |  Identifier   |            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Rejected-Packet ...
  +-+-+-+-+-+-+-+-+

    Code

      7 for Code-Reject.

    Identifier

      The Identifier field MUST be changed for each Code-Reject sent.

    Rejected-Information

      The Rejected-Information field contains a copy of the LCP packet
      which is being rejected.  It begins with the Information field,
      and does not include any Data Link Layer headers nor an FCS.  The
      Rejected-Information MUST be truncated to comply with the peer's
      established MRU.

5.7 Protocol-Reject

    Description

      Reception of a PPP packet with an unknown Protocol field indicates
      that the peer is attempting to use a protocol which is
      unsupported.  This usually occurs when the peer attempts to
      configure a new protocol.  If the LCP state machine is in the
      Opened state, then this error MUST be reported back to the peer by
      transmitting a LCP packet with the Code field set to 8 (Protocol-
      Reject), the Rejected-Protocol field set to the received Protocol,
      and the inducing packet copied to the Rejected-Information field.

      Upon reception of a Protocol-Reject, the implementation MUST stop
      sending packets of the indicated protocol at the earliest
      opportunity.

      Protocol-Reject packets can only be sent in the LCP Opened state.
      Protocol-Reject packets received in any state other than the LCP
      Opened state SHOULD be silently discarded.

   A summary of the Protocol-Reject 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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Code      |  Identifier   |            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       Rejected-Protocol       |      Rejected-Information ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Code

      8 for Protocol-Reject.

    Identifier

      The Identifier field MUST be changed for each Protocol-Reject
      sent.

    Rejected-Protocol

      The Rejected-Protocol field is two octets and contains the PPP
      Protocol field of the packet which is being rejected.

    Rejected-Information

      The Rejected-Information field contains a copy of the packet which
      is being rejected.  It begins with the Information field, and does
      not include any Data Link Layer headers nor an FCS.  The
      Rejected-Information MUST be truncated to comply with the peer's
      established MRU.

5.8 Echo-Request and Echo-Reply

   Description

      LCP includes Echo-Request and Echo-Reply Codes in order to provide
      a Data Link Layer loopback mechanism for use in exercising both
      directions of the link.  This is useful as an aid in debugging,
      link quality determination, performance testing, and for numerous
      other functions.

      An Echo-Request sender transmits a LCP packet with the Code field
      set to 9 (Echo-Request), the Identifier field set, the local
      Magic-Number (if any) inserted, and the Data field filled with any
      desired data, but not exceeding the peer's established MRU minus
      eight.

      Upon reception of an Echo-Request, a LCP packet MUST be
      transmitted with the Code field set to 10 (Echo-Reply), the
      Identifier field copied from the received Echo-Request, the local
      Magic-Number (if any) inserted, and the Data field copied from the
      Echo-Request, truncating as necessary to avoid exceeding the
      peer's established MRU.

      Echo-Request and Echo-Reply packets may only be sent in the LCP
      Opened state.  Echo-Request and Echo-Reply packets received in any
      state other than the LCP Opened state SHOULD be silently
      discarded.

   A summary of the Echo-Request and Echo-Reply packet formats 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Magic-Number                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Data ...
   +-+-+-+-+

    Code

      9 for Echo-Request;

      10 for Echo-Reply.

    Identifier

      On transmission, the Identifier field MUST be changed whenever the
      content of the Data field changes, and whenever a valid reply has
      been received for a previous request.  For retransmissions, the
      Identifier MAY remain unchanged.

      On reception, the Identifier field of the Echo-Request is copied
      into the Identifier field of the Echo-Reply packet.

    Magic-Number

      The Magic-Number field is four octets and aids in detecting links
      which are in the looped-back condition.  Until the Magic-Number
      Configuration Option has been successfully negotiated, the Magic-
      Number MUST be transmitted as zero.  See the Magic-Number
      Configuration Option for further explanation.

    Data

      The Data field is zero or more octets and contains uninterpreted
      data for use by the sender.  The data may consist of any binary
      value and may be of any length from zero to the peer's established
      MRU minus eight.

5.9 Discard-Request

   Description

      LCP includes a Discard-Request Code in order to provide a Data
      Link Layer sink mechanism for use in exercising the local to
      remote direction of the link.  This is useful as an aid in
      debugging, performance testing, and for numerous other functions.

      The sender transmits a LCP packet with the Code field set to 11
      (Discard-Request), the Identifier field set, the local Magic-
      Number (if any) inserted, and the Data field filled with any
      desired data, but not exceeding the peer's established MRU minus
      eight.

      Discard-Request packets may only be sent in the LCP Opened state.
      On reception, the receiver MUST simply throw away any Discard-
      Request that it receives.

   A summary of the Discard-Request 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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Code      |  Identifier   |            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         Magic-Number                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |    Data ...
  +-+-+-+-+

    Code

      11 for Discard-Request.

    Identifier

      The Identifier field MUST be changed for each Discard-Request
      sent.

    Magic-Number

      The Magic-Number field is four octets and aids in detecting links
      which are in the looped-back condition.  Until the Magic-Number
      Configuration Option has been successfully negotiated, the Magic-
      Number MUST be transmitted as zero.  See the Magic-Number
      Configuration Option for further explanation.

    Data

      The Data field is zero or more octets and contains uninterpreted
      data for use by the sender.  The data may consist of any binary
      value and may be of any length from zero to the peer's established
      MRU minus four.

6.  LCP Configuration Options

   LCP Configuration Options allow negotiation of modifications to the
   default characteristics of a point-to-point link.  If a Configuration
   Option is not included in a Configure-Request packet, the default
   value for that Configuration Option is assumed.

   Some Configuration Options MAY be listed more than once.  The effect
   of this is Configuration Option specific, and is specified by each
   such Configuration Option description.  (None of the Configuration
   Options in this specification can be listed more than once.)

   The end of the list of Configuration Options is indicated by the
   length of the LCP packet.

   Unless otherwise specified, all Configuration Options apply in a
   half-duplex fashion; typically, in the receive direction of the link
   from the point of view of the Configure-Request sender.

   A summary of the Configuration Option format is shown below.  The
   fields are transmitted from left to right.

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

    Type

      The Type field is one octet and indicates the type of
      Configuration Option.  Up-to-date values of the LCP Option Type
      field are specified in the most recent "Assigned Numbers" RFC [2].

      This specification concerns the following values:

               1       Maximum-Receive-Unit
               2       Async-Control-Character-Map
               3       Authentication-Protocol
               4       Quality-Protocol
               5       Magic-Number
               6       RESERVED
               7       Protocol-Field-Compression
               8       Address-and-Control-Field-Compression

    Length

      The Length field is one octet and indicates the length of this
      Configuration Option including the Type, Length and Data fields.
      If a negotiable Configuration Option is received in a Configure-
      Request but with an invalid Length, a Configure-Nak SHOULD be
      transmitted which includes the desired Configuration Option with
      an appropriate Length and Data.

    Data

      The Data field is zero or more octets and information specific to
      the Configuration Option.  The format and length of the Data field
      is determined by the Type and Length fields.

6.1 Maximum-Receive-Unit

   Description

      This Configuration Option may be sent to inform the peer that the
      implementation can receive larger packets, or to request that the
      peer send smaller packets.

      The default value is 1500 octets.  If smaller packets are
      requested, an implementation MUST still be able to receive the
      full 1500 octet information field in case link synchronization is
      lost.

    Implementation Note:

      This option is used to indicate an implementation capability.  The
      peer is not required to maximize the use of the capacity.  For
      example, when a MRU is indicated which is 2048 octets, the peer is
      not required to send any packet with 2048 octets.  The peer need
      not Configure-Nak to indicate that it will only send smaller
      packets, since the implementation will always require support for
      at least 1500 octets.

   A summary of the Maximum-Receive-Unit Configuration Option 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |      Maximum-Receive-Unit     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Type

      1

    Length

      4

    Maximum-Receive-Unit

      The Maximum-Receive-Unit field is two octets, and specifies the
      maximum number of octets in the Information and Padding fields.
      It does not include the framing, Protocol field, FCS, nor any
      transparency bits or bytes.

6.2 Async-Control-Character-Map

   Description

      This Configuration Option provides a method to negotiate the use
      of control character transparency on asynchronous links.

      For asynchronous links, the default value is 0xffffffff, which
      causes all octets less than 0x20 to be mapped into an appropriate
      two octet sequence.  For most other links, the default value is 0,
      since there is no need for mapping.

      However, it is rarely necessary to map all control characters, and
      often it is unnecessary to map any control characters.  The
      Configuration Option is used to inform the peer which control
      characters MUST remain mapped when the peer sends them.

      The peer MAY still send any other octets in mapped format, if it
      is necessary because of constraints known to the peer.  The peer
      SHOULD Configure-Nak with the logical union of the sets of mapped
      octets, so that when such octets are spuriously introduced they
      can be ignored on receipt.

   A summary of the Async-Control-Character-Map Configuration Option
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |  Async-Control-Character-Map
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         ACCM (cont)           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Type

      2

    Length

      6

    Async-Control-Character-Map

      The Async-Control-Character-Map field is four octets and indicates
      the set of control characters to be mapped.  The map is sent most
      significant octet first.

      Each numbered bit corresponds to the octet of the same value.  If
      the bit is cleared to zero, then that octet need not be mapped.
      If the bit is set to one, then that octet MUST remain mapped.  For
      example, if bit 19 is set to zero, then the ASCII control
      character 19 (DC3, Control-S) MAY be sent in the clear.

         Note: The least significant bit of the least significant octet
         (the final octet transmitted) is numbered bit 0, and would map
         to the ASCII control character NUL.

6.3 Authentication-Protocol

   Description

      On some links it may be desirable to require a peer to
      authenticate itself before allowing network-layer protocol packets
      to be exchanged.

      This Configuration Option provides a method to negotiate the use
      of a specific authentication protocol.  By default, authentication
      is not required.

      An implementation MUST NOT include multiple Authentication-
      Protocol Configuration Options in its Configure-Request packets.
      Instead, it SHOULD attempt to configure the most desirable
      protocol first.  If that protocol is Configure-Nak'd, then the
      implementation SHOULD attempt the next most desirable protocol in
      the next Configure-Request.

      If an implementation sends a Configure-Ack with this Configuration
      Option, then it is agreeing to authenticate with the specified
      protocol.  An implementation receiving a Configure-Ack with this
      Configuration Option SHOULD expect the peer to authenticate with
      the acknowledged protocol.

      There is no requirement that authentication be full duplex or that
      the same protocol be used in both directions.  It is perfectly
      acceptable for different protocols to be used in each direction.
      This will, of course, depend on the specific protocols negotiated.

   A summary of the Authentication-Protocol Configuration Option 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     Authentication-Protocol   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Data ...
   +-+-+-+-+

    Type

      3

    Length

      >= 4

    Authentication-Protocol

      The Authentication-Protocol field is two octets and indicates the
      authentication protocol desired.  Values for this field are always
      the same as the PPP Protocol field values for that same
      authentication protocol.

      Up-to-date values of the Authentication-Protocol field are
      specified in the most recent "Assigned Numbers" RFC [2].  Current
      values are assigned as follows:

        Value (in hex)    Protocol

        c023              Password Authentication Protocol
        c223              Challenge Handshake Authentication Protocol

    Data

      The Data field is zero or more octets and contains additional data
      as determined by the particular protocol.

6.4 Quality-Protocol

    Description

      On some links it may be desirable to determine when, and how
      often, the link is dropping data.  This process is called link
      quality monitoring.

      This Configuration Option provides a method to negotiate the use
      of a specific protocol for link quality monitoring.  By default,
      link quality monitoring is disabled.

      There is no requirement that quality monitoring be full duplex or
      that the same protocol be used in both directions.  It is
      perfectly acceptable for different protocols to be used in each
      direction.  This will, of course, depend on the specific protocols
      negotiated.

   A summary of the Quality-Protocol Configuration Option 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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Type      |    Length     |        Quality-Protocol       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |    Data ...
  +-+-+-+-+

    Type

      4

    Length

      >= 4

    Quality-Protocol

      The Quality-Protocol field is two octets and indicates the link
      quality monitoring protocol desired.  Values for this field are
      always the same as the PPP Protocol field values for that same
      monitoring protocol.

      Up-to-date values of the Quality-Protocol field are specified in
      the most recent "Assigned Numbers" RFC [2].  Current values are
      assigned as follows:

               Value (in hex)          Protocol

               c025                    Link Quality Report

    Data

      The Data field is zero or more octets and contains additional data
      as determined by the particular protocol.

6.5 Magic-Number

   Description

      This Configuration Option provides a method to detect looped-back
      links and other Data Link Layer anomalies.  This Configuration
      Option MAY be required by some other Configuration Options such as
      the Quality-Protocol Configuration Option.  By default, the
      Magic-Number is not negotiated, and zero is inserted where a
      Magic-Number might otherwise be used.

      Before this Configuration Option is requested, an implementation
      MUST choose its Magic-Number.  It is recommended that the Magic-
      Number be chosen in the most random manner possible in order to
      guarantee with very high probability that an implementation will
      arrive at a unique number.  A good way to choose a unique random
      number is to start with an unique seed.  Suggested sources of
      uniqueness include machine serial numbers, other network hardware
      addresses, time-of-day clocks, etc.  Particularly good random
      number seeds are precise measurements of the inter-arrival time of
      physical events such as packet reception on other connected
      networks, server response time, or the typing rate of a human
      user.  It is also suggested that as many sources as possible be
      used simultaneously.

      When a Configure-Request is received with a Magic-Number
      Configuration Option, the received Magic-Number is compared with
      the Magic-Number of the last Configure-Request sent to the peer.

      If the two Magic-Numbers are different, then the link is not
      looped-back, and the Magic-Number SHOULD be acknowledged.  If the
      two Magic-Numbers are equal, then it is possible, but not certain,
      that the link is looped-back and that this Configure-Request is
      actually the one last sent.  To determine this, a Configure-Nak
      MUST be sent specifying a different Magic-Number value.  A new
      Configure-Request SHOULD NOT be sent to the peer until normal
      processing would cause it to be sent (that is, until a Configure-
      Nak is received or the Restart timer runs out).

      Reception of a Configure-Nak with a Magic-Number different from
      that of the last Configure-Nak sent to the peer proves that a link
      is not looped-back, and indicates a unique Magic-Number.  If the
      Magic-Number is equal to the one sent in the last Configure-Nak,
      the possibility of a looped-back link is increased, and a new
      Magic-Number MUST be chosen.  In either case, a new Configure-
      Request SHOULD be sent with the new Magic-Number.

      If the link is indeed looped-back, this sequence (transmit
      Configure-Request, receive Configure-Request, transmit Configure-
      Nak, receive Configure-Nak) will repeat over and over again.  If
      the link is not looped-back, this sequence might occur a few
      times, but it is extremely unlikely to occur repeatedly.  More
      likely, the Magic-Numbers chosen at either end will quickly
      diverge, terminating the sequence.  The following table shows the
      probability of collisions assuming that both ends of the link
      select Magic-Numbers with a perfectly uniform distribution:

               Number of Collisions        Probability
               --------------------   ---------------------
                       1              1/2**32    = 2.3 E-10
                       2              1/2**32**2 = 5.4 E-20
                       3              1/2**32**3 = 1.3 E-29

      Good sources of uniqueness or randomness are required for this
      divergence to occur.  If a good source of uniqueness cannot be
      found, it is recommended that this Configuration Option not be
      enabled; Configure-Requests with the option SHOULD NOT be
      transmitted and any Magic-Number Configuration Options which the
      peer sends SHOULD be either acknowledged or rejected.  In this
      case, loop-backs cannot be reliably detected by the
      implementation, although they may still be detectable by the peer.

      If an implementation does transmit a Configure-Request with a
      Magic-Number Configuration Option, then it MUST NOT respond with a
      Configure-Reject if it receives a Configure-Request with a Magic-
      Number Configuration Option.  That is, if an implementation
      desires to use Magic Numbers, then it MUST also allow its peer to

      do so.  If an implementation does receive a Configure-Reject in
      response to a Configure-Request, it can only mean that the link is
      not looped-back, and that its peer will not be using Magic-
      Numbers.  In this case, an implementation SHOULD act as if the
      negotiation had been successful (as if it had instead received a
      Configure-Ack).

      The Magic-Number also may be used to detect looped-back links
      during normal operation as well as during Configuration Option
      negotiation.  All LCP Echo-Request, Echo-Reply, and Discard-
      Request packets have a Magic-Number field.  If Magic-Number has
      been successfully negotiated, an implementation MUST transmit
      these packets with the Magic-Number field set to its negotiated
      Magic-Number.

      The Magic-Number field of these packets SHOULD be inspected on
      reception.  All received Magic-Number fields MUST be equal to
      either zero or the peer's unique Magic-Number, depending on
      whether or not the peer negotiated a Magic-Number.  Reception of a
      Magic-Number field equal to the negotiated local Magic-Number
      indicates a looped-back link.  Reception of a Magic- Number other
      than the negotiated local Magic-Number or the peer's negotiated
      Magic-Number, or zero if the peer didn't negotiate one, indicates
      a link which has been (mis)configured for communications with a
      different peer.

      Procedures for recovery from either case are unspecified and may
      vary from implementation to implementation.  A somewhat
      pessimistic procedure is to assume a LCP Down event.  A further
      Open event will begin the process of re-establishing the link,
      which can't complete until the loop-back condition is terminated
      and Magic-Numbers are successfully negotiated.  A more optimistic
      procedure (in the case of a loop-back) is to begin transmitting
      LCP Echo-Request packets until an appropriate Echo-Reply is
      received, indicating a termination of the loop-back condition.

   A summary of the Magic-Number Configuration Option 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |          Magic-Number
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Magic-Number (cont)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Type

      5

    Length

      6

    Magic-Number

      The Magic-Number field is four octets and indicates a number which
      is very likely to be unique to one end of the link.  A Magic-
      Number of zero is illegal and MUST always be Nak'd, if it is not
      Rejected outright.

6.6 Protocol-Field-Compression

   Description

      This Configuration Option provides a method to negotiate the
      compression of the PPP Protocol field.  By default, all
      implementations MUST transmit packets with two octet PPP Protocol
      fields.

      PPP Protocol field numbers are chosen such that some values may be
      compressed into a single octet form which is clearly
      distinguishable from the two octet form.  This Configuration
      Option is sent to inform the peer that the implementation can
      receive such single octet Protocol fields.

      As previously mentioned, the Protocol field uses an extension
      mechanism consistent with the ISO 3309 extension mechanism for the
      Address field; the Least Significant Bit (LSB) of each octet is
      used to indicate extension of the Protocol field.  A binary "0" as
      the LSB indicates that the Protocol field continues with the
      following octet.  The presence of a binary "1" as the LSB marks
      the last octet of the Protocol field.  Notice that any number of
      "0" octets may be prepended to the field, and will still indicate
      the same value (consider the two binary representations for 3,
      00000011 and 00000000 00000011).

      When using low speed links, it is desirable to conserve bandwidth
      by sending as little redundant data as possible.  The Protocol-
      Field-Compression Configuration Option allows a trade-off between
      implementation simplicity and bandwidth efficiency.  If
      successfully negotiated, the ISO 3309 extension mechanism may be
      used to compress the Protocol field to one octet instead of two.
      The large majority of packets are compressible since data

      protocols are typically assigned with Protocol field values less
      than 256.

      Compressed Protocol fields MUST NOT be transmitted unless this
      Configuration Option has been negotiated.  When negotiated, PPP
      implementations MUST accept PPP packets with either double-octet
      or single-octet Protocol fields, and MUST NOT distinguish between
      them.

      The Protocol field is never compressed when sending any LCP
      packet.  This rule guarantees unambiguous recognition of LCP
      packets.

      When a Protocol field is compressed, the Data Link Layer FCS field
      is calculated on the compressed frame, not the original
      uncompressed frame.

   A summary of the Protocol-Field-Compression Configuration Option
   format is shown below.  The fields are transmitted from left to
   right.

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

    Type

      7

    Length

      2

6.7 Address-and-Control-Field-Compression

   Description

      This Configuration Option provides a method to negotiate the
      compression of the Data Link Layer Address and Control fields.  By
      default, all implementations MUST transmit frames with Address and
      Control fields appropriate to the link framing.

      Since these fields usually have constant values for point-to-point
      links, they are easily compressed.  This Configuration Option is
      sent to inform the peer that the implementation can receive
      compressed Address and Control fields.

      If a compressed frame is received when Address-and-Control-Field-
      Compression has not been negotiated, the implementation MAY
      silently discard the frame.

      The Address and Control fields MUST NOT be compressed when sending
      any LCP packet.  This rule guarantees unambiguous recognition of
      LCP packets.

      When the Address and Control fields are compressed, the Data Link
      Layer FCS field is calculated on the compressed frame, not the
      original uncompressed frame.

   A summary of the Address-and-Control-Field-Compression configuration
   option format is shown below.  The fields are transmitted from left
   to right.

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

    Type

      8

    Length

      2

A. LCP Recommended Options

   The following Configurations Options are recommended:

      SYNC LINES

      Magic Number Link Quality Monitoring No Address and Control Field
      Compression No Protocol Field Compression

      ASYNC LINES

      Async Control Character Map Magic Number Address and Control Field
      Compression Protocol Field Compression

Security Considerations

   Security issues are briefly discussed in sections concerning the
   Authentication Phase, the Close event, and the Authentication-

   Protocol Configuration Option.  Further discussion is in a companion
   document entitled PPP Authentication Protocols.

References

    [1] Perkins, D., "Requirements for an Internet Standard
        Point-to-Point Protocol", RFC 1547, December 1993.

    [2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
        USC/Information Sciences Institute, July 1992.

Acknowledgments

   Much of the text in this document is taken from the WG Requirements,
   and RFCs 1171 & 1172, by Drew Perkins of Carnegie Mellon University,
   and by Russ Hobby of the University of California at Davis.

   Many people spent significant time helping to develop the Point-to-
   Point Protocol.  The complete list of people is too numerous to list,
   but the following people deserve special thanks: Rick Adams (UUNET),
   Ken Adelman (TGV), Fred Baker (ACC), Mike Ballard (Telebit), Craig
   Fox (Network Systems), Karl Fox (Morning Star Technologies), Phill
   Gross (AN&S), former WG chair Russ Hobby (UC Davis), David Kaufman
   (Proteon), former WG chair Steve Knowles (FTP Software), former WG
   chair Brian Lloyd (L&A), John LoVerso (Xylogics), Bill Melohn (Sun
   Microsystems), Mike Patton (MIT), former WG chair Drew Perkins
   (Fore), Greg Satz (cisco systems), John Shriver (Proteon), Vernon
   Schryver (Silicon Graphics), and Asher Waldfogel (Wellfleet).

   The "Day in the Life" example was instigated by Kory Hamzeh (Avatar).
   In this version, improvements in wording were also provided by Scott
   Ginsburg, Mark Moraes, and Timon Sloan, as they worked on
   implementations.

   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, 93111

      EMail: fbaker@acc.com

Editor'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

 

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