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RFC 1331 - The Point-to-Point Protocol (PPP) for the Transmissio


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

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

Status of this Memo

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

Abstract

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

      1. A method for encapsulating datagrams over serial links.

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

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

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

   This RFC is a product of the Point-to-Point Protocol Working Group of
   the Internet Engineering Task Force (IETF).  Comments on this memo
   should be submitted to the ietf-ppp@ucdavis.edu mailing list.

Table of Contents

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

1.  Introduction

   Motivation

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

   Encapsulation

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

      PPP provides an encapsulation protocol over both bit-oriented
      synchronous links and asynchronous links with 8 bits of data and
      no parity.  These links MUST be full-duplex, but MAY be either
      dedicated or circuit-switched.  PPP uses HDLC as a basis for the
      encapsulation.

      PPP has been carefully designed to retain compatibility with most
      commonly used supporting hardware.  In addition, an escape
      mechanism is specified to allow control data such as XON/XOFF to
      be transmitted transparently over the link, and to remove spurious
      control data which may be injected into the link by intervening
      hardware and software.

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

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

      implementations, and a software implementation is provided.

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

   Link Control Protocol

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

   Network Control Protocols

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

   Configuration

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

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

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

1.1.  Specification of Requirements

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

   MUST

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

   MUST NOT

      This phrase means that the definition is an absolute prohibition
      of the specification.

   SHOULD

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

   MAY

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

1.2.  Terminology

   This document frequently uses the following terms:

   peer

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

   silently discard

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

2.  Physical Layer Requirements

   The Point-to-Point Protocol is capable of operating across any
   DTE/DCE interface (e.g., EIA RS-232-C, EIA RS-422, EIA RS-423 and
   CCITT V.35).  The only absolute requirement imposed by PPP is the
   provision of a full-duplex circuit, either dedicated or circuit-
   switched, which can operate in either an asynchronous (start/stop) or
   synchronous bit-serial mode, transparent to PPP Data Link Layer
   frames.  PPP does not impose any restrictions regarding transmission
   rate, other than those imposed by the particular DTE/DCE interface in
   use.

   PPP does not require any particular synchronous encoding, such as FM,
   NRZ, or NRZI.

   Implementation Note:

      NRZ is currently most widely available, and on that basis is
      recommended as a default.  When configuration of the encoding is
      allowed, NRZI is recommended as an alternative, because of its
      relative immunity to signal inversion configuration errors.

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

   Implementation Note:

      When available, using such signals can allow greater functionality
      and performance.  In particular, such signals SHOULD be used to
      signal the Up and Down events in the Option Negotiation Automaton
      (described below).

3.  The Data Link Layer

   The Point-to-Point Protocol uses the principles, terminology, and
   frame structure of the International Organization For
   Standardization's (ISO) High-level Data Link Control (HDLC)
   procedures (ISO 3309-1979 [2]), as modified by ISO 3309:1984/PDAD1
   "Addendum 1: Start/stop transmission" [5].  ISO 3309-1979 specifies
   the HDLC frame structure for use in synchronous environments.  ISO
   3309:1984/PDAD1 specifies proposed modifications to ISO 3309-1979 to
   allow its use in asynchronous environments.

   The PPP control procedures use the definitions and Control field
   encodings standardized in ISO 4335-1979 [3] and ISO 4335-
   1979/Addendum 1-1979 [4].  The PPP frame structure is also consistent
   with CCITT Recommendation X.25 LAPB [6], since that too is based on
   HDLC.

   The purpose of this memo is not to document what is already
   standardized in ISO 3309.  We assume that the reader is already
   familiar with HDLC, or has access to a copy of [2] or [6].  Instead,
   this paper attempts to give a concise summary and point out specific
   options and features used by PPP.  Since "Addendum 1: Start/stop
   transmission", is not yet standardized and widely available, it is
   summarized in Appendix A.

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

3.1.  Frame Format

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

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

   Inter-frame Time Fill

   For asynchronous links, inter-frame time fill SHOULD be accomplished
   in the same manner as inter-octet time fill, by transmitting
   continuous "1" bits (mark-hold state).

   For synchronous links, the Flag Sequence SHOULD be transmitted during
   inter-frame time fill.  There is no provision for inter-octet time
   fill.

   Implementation Note:

      Mark idle (continuous ones) SHOULD NOT be used for idle
      synchronous inter-frame time fill.  However, certain types of
      circuit-switched links require the use of mark idle, particularly
      those that calculate accounting based on bit activity.  When mark
      idle is used on a synchronous link, the implementation MUST ensure
      at least 15 consecutive "1" bits between Flags, and that the Flag
      Sequence is generated at the beginning and end of a frame.

Flag Sequence

   The Flag Sequence is a single octet and indicates the beginning or
   end of a frame.  The Flag Sequence consists of the binary sequence
   01111110 (hexadecimal 0x7e).

   The Flag is a frame separator.  Only one Flag is required between two
   frames.  Two consecutive Flags constitute an empty frame, which is
   ignored.

   Implementation Note:

      The "shared zero mode" Flag Sequence "011111101111110" SHOULD NOT
      be used.  When not avoidable, such an implementation MUST ensure
      that the first Flag Sequence detected (the end of the frame) is
      promptly communicated to the link layer.

Address Field

   The Address field is a single octet and contains the binary sequence
   11111111 (hexadecimal 0xff), the All-Stations address.  PPP does not
   assign individual station addresses.  The All-Stations address MUST
   always be recognized and received.  The use of other address lengths
   and values may be defined at a later time, or by prior agreement.
   Frames with unrecognized Addresses SHOULD be silently discarded, and
   reported through the normal network management facility.

Control Field

   The Control field is a single octet and contains the binary sequence
   00000011 (hexadecimal 0x03), the Unnumbered Information (UI) command
   with the P/F bit set to zero.  Frames with other Control field values
   SHOULD be silently discarded.

Protocol Field

   The Protocol field is two octets and its value identifies the
   protocol encapsulated in the Information field of the frame.

   This Protocol field is defined by PPP and is not a field defined by
   HDLC.  However, the Protocol 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 considered as
   having an unrecognized Protocol, and handled as specified by the LCP.
   The Protocol field is transmitted and received most significant octet
   first.

   Protocol field values in the "0---" to "3---" range identify the
   network-layer protocol of specific datagrams, and values in the "8--
   -" to "b---" range identify datagrams 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

   datagrams as link-layer Control Protocols (such as LCP).

   The most up-to-date values of the Protocol field are specified in the
   most recent "Assigned Numbers" RFC [11].  Current values are assigned
   as follows:

      Value (in hex)  Protocol Name

      0001 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            reserved (until 1993)
      00ff            reserved (compression inefficient)

      0201            802.1d Hello Packets
      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

      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 end of the Information field is found by locating the
   closing Flag Sequence and allowing two octets for the Frame Check
   Sequence field.  The default maximum length of the Information field
   is 1500 octets.  By negotiation, consenting PPP implementations may
   use other values for the maximum Information field length.

   On transmission, the Information field may be padded with an
   arbitrary number of octets up to the maximum length.  It is the
   responsibility of each protocol to disambiguate padding octets from
   real information.

Frame Check Sequence (FCS) Field

   The Frame Check Sequence field is normally 16 bits (two octets).  The
   use of other FCS lengths may be defined at a later time, or by prior
   agreement.

   The FCS field is calculated over all bits of the Address, Control,
   Protocol and Information fields not including any start and stop bits
   (asynchronous) and any bits (synchronous) or octets (asynchronous)
   inserted for transparency.  This does not include the Flag Sequences
   or the FCS field itself.  The FCS is transmitted with the coefficient
   of the highest term first.

      Note: When octets are received which are flagged in the Async-
      Control-Character-Map, they are discarded before calculating the
      FCS.  See the description in Appendix A.

   For more information on the specification of the FCS, see ISO 3309
   [2] or CCITT X.25 [6].

      Note: A fast, table-driven implementation of the 16-bit FCS
      algorithm is shown in Appendix B.  This implementation is based on
      [7], [8], and [9].

Modifications to the Basic Frame Format

   The Link Control Protocol can negotiate modifications to the standard
   PPP frame structure.  However, modified frames will always be clearly
   distinguishable from standard frames.

4.  PPP Link Operation

4.1.  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).

4.2.  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 |<-+
                   |           |                |         |
                   +-----------+                +---------+

4.3.  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.

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

   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.

4.5.  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 necessary.  If an implementation
   requires 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 the peer is successfully
   authenticated using the negotiated authentication protocol.  In the
   event of failure to authenticate, PPP SHOULD proceed instead to the
   Link Termination phase.

4.6.  Network-Layer Protocol Phase

   Once PPP has finished the previous phases, each network-layer
   protocol (such as IP) 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 SHOULD be silently discarded.

   During this phase, link traffic consists of any possible combinations
   of LCP, NCP, and network-layer protocol packets.  Any NCP or
   network-layer protocol packets received during any other phase SHOULD
   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 supported protocols are silently discarded.

4.7.  Link Termination Phase

   PPP may terminate the link at any time.  This will usually be done at
   the request of a human user, but might happen because of a physical
   event such as the loss of carrier, authentication failure, link
   quality failure, or the expiration of an idle-period timer.

   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.

   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 a NCP has Closed is not sufficient reason to cause
      the termination of the PPP link, even if that NCP was the only
      currently NCP in the Opened state.

5.  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-Start
   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
                                             -  = illegal action

5.1.  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-
   |        |   +------------------------------->+<-------+   |
   |        |                                                 |
   +<-------+<------------------------------------------------+

5.2.  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.  The state may be followed by a letter, which
   indicates an explanatory footnote.

   Rationale:

      In previous versions of this table, a simplified non-deterministic
      finite-state automaton was used, with considerable detailed
      information specified in the semantics.  This lead to
      interoperability problems from differing interpretations.

      This table functions similarly to the previous versions, with the
      up/down flags expanded to explicit states, and the active/passive
      paradigm eliminated.  It is believed that this table interoperates
      with previous versions better than those versions themselves.

      | 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   tld,scj,scr/6
 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 SHOULD be 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.

5.3.  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.

5.4.  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 to signal LCP
      that the link is entering Link Establishment phase, or used to
      signal a NCP that the link is entering Network-Layer Protocol
      phase.

   Down

      The Down event occurs when a lower layer indicates that it is no
      longer ready to carry packets.  Typically, this event is used to
      signal LCP that the link is entering Link Dead phase, or used to
      signal a NCP that the link is leaving Network-Layer Protocol
      phase.

   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 should be necessary.

      Implementation Note:

         Experience has shown that users will execute an additional Open
         command when they want to renegotiate the link.  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.

   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.

5.5.  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 SHOULD NOT occur.  The implementation
      probably has an internal error.

   This-Layer-Up (tlu)

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

      Typically, this action MAY be 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 traffic.

   This-Layer-Down (tld)

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

      Typically, this action MAY be 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 traffic.

   This-Layer-Start (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.

      This action is highly implementation dependent.

   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.

      This action is highly implementation dependent.

   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.

   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.  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.

5.6.  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.

5.7.  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 MAY 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 (19,200 bps or less), high
      switching latency links (typical telephone lines).  Higher speed
      links, or links with low switching latency, SHOULD have
      correspondingly faster retransmission times.

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.

6.  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 Data Link Layer 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 in the full, standard form, as if no
   Configuration Options were enabled.  This ensures that LCP
   Configure-Request packets are always recognizable even when one end
   of the link mistakenly believes the link to be open.

   Exactly one Link Control Protocol packet is encapsulated in the
   Information field of PPP Data Link Layer frames where the 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.

      The most up-to-date values of the LCP Code field are specified in
      the most recent "Assigned Numbers" RFC [11].  Current values are
      assigned as follows:

         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
         12      RESERVED

   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 should be treated as
      Data Link Layer padding and should be 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.

6.1.  Configure-Request

   Description

      A LCP 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 default
      link Configuration Options.

      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 SHOULD be changed on each transmission.  On
      reception, the Identifier field should be copied into the
      Identifier field of the appropriate reply packet.

   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.

6.2.  Configure-Ack

   Description

      If every Configuration Option received in a Configure-Request is
      both recognizable and acceptable, then a LCP implementation should
      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.

6.3.  Configure-Nak

   Description

      If every element of the received Configuration Options is
      recognizable but some are not acceptable, then a LCP
      implementation should 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.

      Each of the Nak'd Configuration Options MUST be modified to a
      value acceptable to the Configure-Nak sender.  Options which have
      no value fields (boolean options) use the Configure-Reject reply
      instead.

      Finally, an implementation may be configured to request the
      negotiation of a specific option.  If that option is not listed,
      then that option may be appended to the list of Nak'd
      Configuration Options in order to request the peer to list 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.

      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.

6.4.  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 a LCP implementation
      should 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.

6.5.  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

      The Identifier field is one octet and aids in matching requests
      and replies.

   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
      maximum Information field length minus four.

6.6.  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 is one octet and is for use by the
      transmitter.

   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 PPP Data Link Layer headers nor the FCS.

      The Rejected-Information MUST be truncated to comply with the
      peer's established maximum Information field length.

6.7.  Protocol-Reject

   Description

      Reception of a PPP frame with an unknown Data Link Layer Protocol
      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, a LCP implementation SHOULD
      stop transmitting frames of the indicated protocol.

      Protocol-Reject packets may 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 is one octet and is for use by the
      transmitter.

   Rejected-Protocol

      The Rejected-Protocol field is two octets and contains the
      Protocol of the Data Link Layer frame which is being rejected.

   Rejected-Information

      The Rejected-Information field contains a copy from the frame

      which is being rejected.  It begins with the Information field,
      and does not include any PPP Data Link Layer headers nor the FCS.
      The Rejected-Information MUST be truncated to comply with the
      peer's established maximum Information field length.

6.8.  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 inserted, and the Data field filled with any desired
      data, up to but not exceeding the peer's established maximum
      Information field length 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 inserted, and the Data field copied from the Echo-
      Request, truncating as necessary to avoid exceeding the peer's
      established maximum Information field length.

      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

      The Identifier field is one octet and aids in matching Echo-
      Requests and Echo-Replies.

   Magic-Number

      The Magic-Number field is four octets and aids in detecting links
      which are in the looped-back condition.  Unless modified by a
      Configuration Option, the Magic-Number MUST be transmitted as zero
      and MUST be ignored on reception.  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
      maximum Information field length minus eight.

6.9.  Discard-Request

   Description

      LCP includes a Discard-Request Code in order to provide a Data
      Link Layer data 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.

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

      A discard receiver MUST simply throw away an Discard-Request that
      it receives.

      Discard-Request packets may only be sent in the LCP Opened state.

   A summary of the Discard-Request 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

      11 for Discard-Request.

   Identifier

      The Identifier field is one octet and is for use by the Discard-
      Request transmitter.

   Magic-Number

      The Magic-Number field is four octets and aids in detecting links
      which are in the looped-back condition.  Unless modified by a
      configuration option, the Magic-Number MUST be transmitted as zero
      and MUST be ignored on reception.  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
      maximum Information field length minus four.

7.  LCP Configuration Options

   LCP Configuration Options allow modifications to the standard
   characteristics of a point-to-point link to be negotiated.
   Negotiable modifications include such things as the maximum receive
   unit, async control character mapping, the link authentication
   method, etc.  If a Configuration Option is not included in a
   Configure-Request packet, the default value for that Configuration
   Option is assumed.

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

   Unless otherwise specified, each Configuration Option is not listed
   more than once in a Configuration Options list.  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.

   Also unless otherwise specified, all Configuration Options apply in a
   half-duplex fashion.  When negotiated, they apply to only one
   direction of the link, typically in the receive direction when
   interpreted from the point of view of the Configure-Request sender.

7.1.  Format

   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.  The most up-to-date values of the LCP
      Option Type field are specified in the most recent "Assigned
      Numbers" RFC [11].  Current values are assigned as follows:

         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 indicates the value or
      other information for this Configuration Option.  The format and
      length of the Data field is determined by the Type and Length
      fields.

7.2.  Maximum-Receive-Unit

   Description

      This Configuration Option may be sent to inform the peer that the
      implementation can receive larger frames, or to request that the
      peer send smaller frames.  If smaller frames are requested, an
      implementation MUST still be able to receive 1500 octet frames in
      case link synchronization is lost.

   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 indicates the new
      maximum receive unit.  The Maximum-Receive-Unit covers only the
      Data Link Layer Information field.  It does not include the
      header, padding, FCS, nor any transparency bits or bytes.

   Default

      1500

7.3.  Async-Control-Character-Map

   Description

      This Configuration Option provides a way to negotiate the use of
      control character mapping on asynchronous links.  By default, PPP
      maps all control characters into an appropriate two character
      sequence.  However, it is rarely necessary to map all control
      characters and often it is unnecessary to map any characters.  A
      PPP implementation may use this Configuration Option to inform the
      peer which control characters must remain mapped and which control
      characters need not remain mapped when the peer sends them.  The
      peer may still send these control characters in mapped format if
      it is necessary because of constraints at the peer.

      There may be some use of synchronous-to-asynchronous converters
      (some built into modems) in Point-to-Point links resulting in a
      synchronous PPP implementation on one end of a link and an
      asynchronous implementation on the other.  It is the
      responsibility of the converter to do all mapping conversions
      during operation.  To enable this functionality, synchronous PPP
      implementations MUST always accept a Async-Control-Character-Map
      Configuration Option (it MUST always respond to an LCP Configure-
      Request specifying this Configuration Option with an LCP
      Configure-Ack).  However, acceptance of this Configuration Option
      does not imply that the synchronous implementation will do any
      character mapping, since synchronous PPP uses bit-stuffing rather
      than character-stuffing.  Instead, all such character mapping will
      be performed by the asynchronous-to-synchronous converter.

   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 new async control character map.  The map is encoded in big-
      endian fashion where each numbered bit corresponds to the ASCII
      control character of the same value.  If the bit is cleared to
      zero, then that ASCII control character need not be mapped.  If
      the bit is set to one, then that ASCII control character must
      remain mapped.  E.g., if bit 19 is set to zero, then the ASCII
      control character 19 (DC3, Control-S) may be sent in the clear.

         Note: The bit ordering of the map is as described in section
         3.1, Most Significant Bit to Least Significant Bit.  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.

   Default

      All ones (0xffffffff).

7.4.  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 way to
      negotiate the use of a specific authentication protocol.  By
      default, authentication is not necessary.

      An implementation SHOULD 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 Rejected, then the
      implementation could attempt the next most desirable protocol in
      the next Configure-Request.

      An implementation receiving a Configure-Request specifying
      Authentication-Protocols MAY choose at most one of the negotiable
      authentication protocols and MUST send a Configure-Reject
      including the other specified authentication protocols.  The
      implementation MAY reject all of the proposed authentication
      protocols.

      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 Data Link Layer Protocol field values for that
      same authentication protocol.

      The most up-to-date values of the Authentication-Protocol field
      are specified in the most recent "Assigned Numbers" RFC [11].
      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.

Default

   No authentication protocol necessary.

7.5.  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 way 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 Data Link Layer Protocol field values
      for that same monitoring protocol.

      The most up-to-date values of the Quality-Protocol field are
      specified in the most recent "Assigned Numbers" RFC [11].  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.

   Default

      None

7.6.  Magic-Number

   Description

      This Configuration Option provides a way 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 Monitoring-Protocol Configuration Option.

      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
      should 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 (i.e., 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 should 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 its peer also transmits 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 which MUST normally be
      zero, and MUST normally be ignored on reception.  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 one.

      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.

   Default

      None.

7.7.  Protocol-Field-Compression

   Description

      This Configuration Option provides a way to negotiate the
      compression of the Data Link Layer Protocol field.  By default,
      all implementations MUST transmit standard PPP frames with two
      octet Protocol fields.  However, 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.
      Compressed Protocol fields MUST NOT be transmitted unless this
      Configuration Option has been negotiated.

      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 representations for 3, 00000011
      and 00000000 00000011).

      In the interest of simplicity, the standard PPP frame uses this
      fact and always sends Protocol fields with a two octet
      representation.  Protocol field values less than 256 (decimal) are
      prepended with a single zero octet even though transmission of
      this, the zero and most significant octet, is unnecessary.

      However, when using low speed links, it is desirable to conserve
      bandwidth by sending as little redundant data as possible.  The
      Protocol 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 frames are compressible since data protocols
      are typically assigned with Protocol field values less than 256.

      In addition, PPP implementations must continue to be robust and
      MUST accept PPP frames 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

   Default

      Disabled.

7.8.  Address-and-Control-Field-Compression

   Description

      This Configuration Option provides a way 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 and MUST use the hexadecimal values 0xff and 0x03
      respectively.  Since these fields have constant values, they are
      easily compressed.  This Configuration Option is sent to inform
      the peer that the implementation can receive compressed Address
      and Control fields.

      Compressed Address and Control fields are formed by simply
      omitting them.  By definition the first octet of a two octet
      Protocol field will never be 0xff, and the Protocol field value
      0x00ff is not allowed (reserved) to avoid ambiguity.

      On reception, the Address and Control fields are decompressed by
      examining the first two octets.  If they contain the values 0xff
      and 0x03, they are assumed to be the Address and Control fields.
      If not, it is assumed that the fields were compressed and were not
      transmitted.

      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

   Default

      Not compressed.

A.  Asynchronous HDLC

   This appendix summarizes the modifications to ISO 3309-1979 proposed
   in ISO 3309:1984/PDAD1, as applied in the Point-to-Point Protocol.
   These modifications allow HDLC to be used with 8-bit asynchronous
   links.

   Transmission Considerations

      All octets are transmitted with one start bit, eight bits of data,
      and one stop bit.  There is no provision in either PPP or ISO
      3309:1984/PDAD1 for seven bit asynchronous links.

   Flag Sequence

      The Flag Sequence is a single octet and indicates the beginning or
      end of a frame.  The Flag Sequence consists of the binary sequence
      01111110 (hexadecimal 0x7e).

   Transparency

      On asynchronous links, a character stuffing procedure is used.
      The Control Escape octet is defined as binary 01111101
      (hexadecimal 0x7d) where the bit positions are numbered 87654321
      (not 76543210, BEWARE).

      After FCS computation, the transmitter examines the entire frame
      between the two Flag Sequences.  Each Flag Sequence, Control
      Escape octet and octet with value less than hexadecimal 0x20 which
      is flagged in the Remote Async-Control-Character-Map is replaced
      by a two octet sequence consisting of the Control Escape octet and
      the original octet with bit 6 complemented (i.e., exclusive-or'd
      with hexadecimal 0x20).

      Prior to FCS computation, the receiver examines the entire frame
      between the two Flag Sequences.  Each octet with value less than
      hexadecimal 0x20 is checked.  If it is flagged in the Local
      Async-Control-Character-Map, it is simply removed (it may have
      been inserted by intervening data communications equipment).  For
      each Control Escape octet, that octet is also removed, but bit 6
      of the following octet is complemented.  A Control Escape octet
      immediately preceding the closing Flag Sequence indicates an
      invalid frame.

         Note: The inclusion of all octets less than hexadecimal 0x20
         allows all ASCII control characters [10] excluding DEL (Delete)
         to be transparently communicated through almost all known data
         communications equipment.

      The transmitter may also send octets with value in the range 0x40
      through 0xff (except 0x5e) in Control Escape format.  Since these
      octet values are not negotiable, this does not solve the problem
      of receivers which cannot handle all non-control characters.
      Also, since the technique does not affect the 8th bit, this does
      not solve problems for communications links that can send only 7-
      bit characters.

      A few examples may make this more clear.  Packet data is
      transmitted on the link as follows:

         0x7e is encoded as 0x7d, 0x5e.
         0x7d is encoded as 0x7d, 0x5d.
         0x01 is encoded as 0x7d, 0x21.

      Some modems with software flow control may intercept outgoing DC1
      and DC3 ignoring the 8th (parity) bit.  This data would be
      transmitted on the link as follows:

         0x11 is encoded as 0x7d, 0x31.
         0x13 is encoded as 0x7d, 0x33.
         0x91 is encoded as 0x7d, 0xb1.
         0x93 is encoded as 0x7d, 0xb3.

   Aborting a Transmission

      On asynchronous links, frames may be aborted by transmitting a "0"
      stop bit where a "1" bit is expected (framing error) or by
      transmitting a Control Escape octet followed immediately by a
      closing Flag Sequence.

   Time Fill

      On asynchronous links, inter-octet and inter-frame time fill MUST
      be accomplished by transmitting continuous "1" bits (mark-hold
      state).

         Note: On asynchronous links, inter-frame time fill can be
         viewed as extended inter-octet time fill.  Doing so can save
         one octet for every frame, decreasing delay and increasing
         bandwidth.  This is possible since a Flag Sequence may serve as
         both a frame close and a frame begin.  After having received
         any frame, an idle receiver will always be in a frame begin
         state.

         Robust transmitters should avoid using this trick over-
         zealously since the price for decreased delay is decreased
         reliability.  Noisy links may cause the receiver to receive

         garbage characters and interpret them as part of an incoming
         frame.  If the transmitter does not transmit a new opening Flag
         Sequence before sending the next frame, then that frame will be
         appended to the noise characters causing an invalid frame (with
         high reliability).  Transmitters should avoid this by
         transmitting an open Flag Sequence whenever "appreciable time"
         has elapsed since the prior closing Flag Sequence.  It is
         suggested that implementations will achieve the best results by
         always sending an opening Flag Sequence if the new frame is not
         back-to-back with the last.  The maximum value for "appreciable
         time" is likely to be no greater than the typing rate of a slow
         to average typist, say 1 second.

B.  Fast Frame Check Sequence (FCS) Implementation

B.1.  FCS Computation Method

   The following code provides a table lookup computation for
   calculating the Frame Check Sequence as data arrives at the
   interface.  This implementation is based on [7], [8], and [9].  The
   table is created by the code in section B.2.

   /*
    * u16 represents an unsigned 16-bit number.  Adjust the typedef for
    * your hardware.
    */
   typedef unsigned short u16;

   /*
    * FCS lookup table as calculated by the table generator in section
    * B.2.
    */
   static u16 fcstab[256] = {
      0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
      0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
      0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
      0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
      0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
      0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
      0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
      0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
      0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
      0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
      0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
      0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
      0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
      0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
      0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
      0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
      0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
      0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
      0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
      0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
      0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
      0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
      0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
      0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
      0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
      0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
      0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,

      0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
      0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
      0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
      0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
      0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
   };

   #define PPPINITFCS      0xffff  /* Initial FCS value */
   #define PPPGOODFCS      0xf0b8  /* Good final FCS value */

   /*
    * Calculate a new fcs given the current fcs and the new data.
    */
   u16 pppfcs(fcs, cp, len)
       register u16 fcs;
       register unsigned char *cp;
       register int len;
   {
       ASSERT(sizeof (u16) == 2);
       ASSERT(((u16) -1) > 0);
       while (len--)
           fcs = (fcs >> 8) ^ fcstab[(fcs ^ *cp++) & 0xff];

       return (fcs);
   }

B.2.  Fast FCS table generator

   The following code creates the lookup table used to calculate the
   FCS.

   /*
    * Generate a FCS table for the HDLC FCS.
    *
    * Drew D. Perkins at Carnegie Mellon University.
    *
    * Code liberally borrowed from Mohsen Banan and D. Hugh Redelmeier.
    */

   /*
    * The HDLC polynomial: x**0 + x**5 + x**12 + x**16 (0x8408).
    */
   #define P       0x8408

   main()
   {
       register unsigned int b, v;
       register int i;

       printf("typedef unsigned short u16;\n");
       printf("static u16 fcstab[256] = {");
       for (b = 0; ; ) {
           if (b % 8 == 0)
               printf("\n");

           v = b;
           for (i = 8; i--; )
               v = v & 1 ? (v >> 1) ^ P : v >> 1;

           printf("0x%04x", v & 0xFFFF);
           if (++b == 256)
               break;
           printf(",");
       }
       printf("\n};\n");
   }

C.  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, and the Authentication-Protocol Configuration
   Option.  Further discussion is planned in a separate document
   entitled PPP Authentication Protocols.

References

   [1]   Electronic Industries Association, EIA Standard RS-232-C,
         "Interface Between Data Terminal Equipment and Data
         Communications Equipment Employing Serial Binary Data
         Interchange", August 1969.

   [2]   International Organization For Standardization, ISO Standard
         3309-1979, "Data communication - High-level data link control
         procedures - Frame structure", 1979.

   [3]   International Organization For Standardization, ISO Standard
         4335-1979, "Data communication - High-level data link control
         procedures - Elements of procedures", 1979.

   [4]   International Organization For Standardization, ISO Standard
         4335-1979/Addendum 1, "Data communication - High-level data
         link control procedures - Elements of procedures - Addendum 1",
         1979.

   [5]   International Organization For Standardization, Proposed Draft
         International Standard ISO 3309:1983/PDAD1, "Information
         processing systems - Data communication - High-level data link
         control procedures - Frame structure - Addendum 1: Start/stop
         transmission", 1984.

   [6]   International Telecommunication Union, CCITT Recommendation
         X.25, "Interface Between Data Terminal Equipment (DTE) and Data
         Circuit Terminating Equipment (DCE) for Terminals Operating in
         the Packet Mode on Public Data Networks", CCITT Red Book,
         Volume VIII, Fascicle VIII.3, Rec. X.25., October 1984.

   [7]   Perez, "Byte-wise CRC Calculations", IEEE Micro, June, 1983.

   [8]   Morse, G., "Calculating CRC's by Bits and Bytes", Byte,
         September 1986.

   [9]   LeVan, J., "A Fast CRC", Byte, November 1987.

   [10]  American National Standards Institute, ANSI X3.4-1977,
         "American National Standard Code for Information Interchange",

         1977.

   [11]  Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1060,
         USC/Information Sciences Institute, March 1990.

Acknowledgments

   Much of the text in this document is taken from the WG Requirements
   (unpublished), 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 (NSC), Karl Fox (Morning Star Technologies), Phill Gross (NRI),
   former WG chair Russ Hobby (UC Davis), David Kaufman (Proteon),
   former WG chair Steve Knowles (FTP Software), John LoVerso
   (Xylogics), Bill Melohn (Sun Microsystems), Mike Patton (MIT), former
   WG chair Drew Perkins (CMU), Greg Satz (cisco systems) and Asher
   Waldfogel (Wellfleet).

Chair's Address

   The working group can be contacted via the current chair:

      Brian Lloyd
      Lloyd & Associates
      3420 Sudbury Road
      Cameron Park, California 95682

      Phone: (916) 676-1147

      EMail: brian@ray.lloyd.com

Author's Address

   Questions about this memo can also be directed to:

      William Allen Simpson
      Daydreamer
      Computer Systems Consulting Services
      P O Box 6205
      East Lansing, MI  48826-6025

      EMail: bsimpson@ray.lloyd.com

 

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