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rfc:rfc1331

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.

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

Table of Contents

   1.     Introduction ..........................................    1
      1.1       Specification of Requirements ...................    3
      1.2       Terminology .....................................    3
   2.     Physical Layer Requirements ...........................    4
   3.     The Data Link Layer ...................................    5
      3.1       Frame Format ....................................    6
   4.     PPP Link Operation ....................................   10
      4.1       Overview ........................................   10
      4.2       Phase Diagram ...................................   10
      4.3       Link Dead (physical-layer not ready) ............   10
      4.4       Link Establishment Phase ........................   11
      4.5       Authentication Phase ............................   11
      4.6       Network-Layer Protocol Phase ....................   12
      4.7       Link Termination Phase ..........................   12
   5.     The Option Negotiation Automaton ......................   14
      5.1       State Diagram ...................................   15
      5.2       State Transition Table ..........................   16
      5.3       States ..........................................   18
      5.4       Events ..........................................   20
      5.5       Actions .........................................   24
      5.6       Loop Avoidance ..................................   26
      5.7       Counters and Timers .............................   27
   6.     LCP Packet Formats ....................................   28
      6.1       Configure-Request ...............................   30
      6.2       Configure-Ack ...................................   31
      6.3       Configure-Nak ...................................   32
      6.4       Configure-Reject ................................   33
      6.5       Terminate-Request and Terminate-Ack .............   35
      6.6       Code-Reject .....................................   36
      6.7       Protocol-Reject .................................   38
      6.8       Echo-Request and Echo-Reply .....................   39
      6.9       Discard-Request .................................   40
   7.     LCP Configuration Options .............................   42
      7.1       Format ..........................................   43
      7.2       Maximum-Receive-Unit ............................   44
      7.3       Async-Control-Character-Map .....................   45
      7.4       Authentication-Protocol .........................   47
      7.5       Quality-Protocol ................................   49
      7.6       Magic-Number ....................................   51

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

      7.7       Protocol-Field-Compression ......................   54
      7.8       Address-and-Control-Field-Compression ...........   56
   APPENDICES ...................................................   58
   A.     Asynchronous HDLC .....................................   58
   B.     Fast Frame Check Sequence (FCS) Implementation ........   61
      B.1       FCS Computation Method ..........................   61
      B.2       Fast FCS table generator ........................   63
   C.     LCP Recommended Options ...............................   64
   SECURITY CONSIDERATIONS ......................................   65
   REFERENCES ...................................................   65
   ACKNOWLEDGEMENTS .............................................   66
   CHAIR'S ADDRESS ..............................................   66
   AUTHOR'S ADDRESS .............................................   66

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

1. Introduction

 Motivation
    In the last few years, the Internet has seen explosive growth in
    the number of hosts supporting TCP/IP.  The vast majority of these
    hosts are connected to Local Area Networks (LANs) of various
    types, Ethernet being the most common.  Most of the other hosts
    are connected through Wide Area Networks (WANs) such as X.25 style
    Public Data Networks (PDNs).  Relatively few of these hosts are
    connected with simple point-to-point (i.e., serial) links.  Yet,
    point-to-point links are among the oldest methods of data
    communications and almost every host supports point-to-point
    connections.  For example, asynchronous RS-232-C [1] interfaces
    are essentially ubiquitous.
 Encapsulation
    One reason for the small number of point-to-point IP links is the
    lack of a standard encapsulation protocol.  There are plenty of
    non-standard (and at least one de facto standard) encapsulation
    protocols available, but there is not one which has been agreed
    upon as an Internet Standard.  By contrast, standard encapsulation
    schemes do exist for the transmission of datagrams over most
    popular LANs.
    PPP provides an encapsulation protocol over both bit-oriented
    synchronous links and asynchronous links with 8 bits of data and
    no parity.  These links MUST be full-duplex, but MAY be either
    dedicated or circuit-switched.  PPP uses HDLC as a basis for the
    encapsulation.
    PPP has been carefully designed to retain compatibility with most
    commonly used supporting hardware.  In addition, an escape
    mechanism is specified to allow control data such as XON/XOFF to
    be transmitted transparently over the link, and to remove spurious
    control data which may be injected into the link by intervening
    hardware and software.
    The PPP encapsulation also provides for multiplexing of different
    network-layer protocols simultaneously over the same link.  It is
    intended that PPP provide a common solution for easy connection of
    a wide variety of hosts, bridges and routers.
    Some protocols expect error free transmission, and either provide
    error detection only on a conditional basis, or do not provide it
    at all.  PPP uses the HDLC Frame Check Sequence for error
    detection.  This is commonly available in hardware

Simpson [Page 1] RFC 1331 Point-to-Point Protocol May 1992

    implementations, and a software implementation is provided.
    By default, only 8 additional octets are necessary to form the
    encapsulation.  In environments where bandwidth is at a premium,
    the encapsulation may be shortened to as few as 2 octets.  To
    support high speed hardware implementations, PPP provides that the
    default encapsulation header and information fields fall on 32-bit
    boundaries, and allows the trailer to be padded to an arbitrary
    boundary.
 Link Control Protocol
    More importantly, the Point-to-Point Protocol defines more than
    just an encapsulation scheme.  In order to be sufficiently
    versatile to be portable to a wide variety of environments, PPP
    provides a Link Control Protocol (LCP).  The LCP is used to
    automatically agree upon the encapsulation format options, handle
    varying limits on sizes of packets, authenticate the identity of
    its peer on the link, determine when a link is functioning
    properly and when it is defunct, detect a looped-back link and
    other common misconfiguration errors, and terminate the link.
 Network Control Protocols
    Point-to-Point links tend to exacerbate many problems with the
    current family of network protocols.  For instance, assignment and
    management of IP addresses, which is a problem even in LAN
    environments, is especially difficult over circuit-switched
    point-to-point links (such as dial-up modem servers).  These
    problems are handled by a family of Network Control Protocols
    (NCPs), which each manage the specific needs required by their
    respective network-layer protocols.  These NCPs are defined in
    other documents.
 Configuration
    It is intended that PPP be easy to configure.  By design, the
    standard defaults should handle all common configurations.  The
    implementor may specify improvements to the default configuration,
    which are automatically communicated to the peer without operator
    intervention.  Finally, the operator may explicitly configure
    options for the link which enable the link to operate in
    environments where it would otherwise be impossible.
    This self-configuration is implemented through an extensible
    option negotiation mechanism, wherein each end of the link
    describes to the other its capabilities and requirements.
    Although the option negotiation mechanism described in this

Simpson [Page 2] RFC 1331 Point-to-Point Protocol May 1992

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

1.1. Specification of Requirements

 In this document, several words are used to signify the requirements
 of the specification.  These words are often capitalized.
 MUST
    This word, or the adjective "required", means that the definition
    is an absolute requirement of the specification.
 MUST NOT
    This phrase means that the definition is an absolute prohibition
    of the specification.
 SHOULD
    This word, or the adjective "recommended", means that there may
    exist valid reasons in particular circumstances to ignore this
    item, but the full implications should be understood and carefully
    weighed before choosing a different course.
 MAY
    This word, or the adjective "optional", means that this item is
    one of an allowed set of alternatives.  An implementation which
    does not include this option MUST be prepared to interoperate with
    another implementation which does include the option.

1.2. Terminology

 This document frequently uses the following terms:
 peer
    The other end of the point-to-point link.
 silently discard
    This means the implementation discards the packet without further
    processing.  The implementation SHOULD provide the capability of
    logging the error, including the contents of the silently
    discarded packet, and SHOULD record the event in a statistics
    counter.

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

2. Physical Layer Requirements

 The Point-to-Point Protocol is capable of operating across any
 DTE/DCE interface (e.g., EIA RS-232-C, EIA RS-422, EIA RS-423 and
 CCITT V.35).  The only absolute requirement imposed by PPP is the
 provision of a full-duplex circuit, either dedicated or circuit-
 switched, which can operate in either an asynchronous (start/stop) or
 synchronous bit-serial mode, transparent to PPP Data Link Layer
 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).

Simpson [Page 4] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 5] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 6] RFC 1331 Point-to-Point Protocol May 1992

 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

Simpson [Page 7] RFC 1331 Point-to-Point Protocol May 1992

 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.

Simpson [Page 8] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 9] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 10] RFC 1331 Point-to-Point Protocol May 1992

 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.

Simpson [Page 11] RFC 1331 Point-to-Point Protocol May 1992

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

Simpson [Page 12] RFC 1331 Point-to-Point Protocol May 1992

 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.

Simpson [Page 13] RFC 1331 Point-to-Point Protocol May 1992

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

Simpson [Page 14] RFC 1331 Point-to-Point Protocol May 1992

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-
 |        |   +------------------------------->+<-------+   |
 |        |                                                 |
 +<-------+<------------------------------------------------+

Simpson [Page 15] RFC 1331 Point-to-Point Protocol May 1992

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

Simpson [Page 16] RFC 1331 Point-to-Point Protocol May 1992

    | 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.

Simpson [Page 17] RFC 1331 Point-to-Point Protocol May 1992

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

Simpson [Page 18] RFC 1331 Point-to-Point Protocol May 1992

       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

Simpson [Page 19] RFC 1331 Point-to-Point Protocol May 1992

    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

Simpson [Page 20] RFC 1331 Point-to-Point Protocol May 1992

    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

Simpson [Page 21] RFC 1331 Point-to-Point Protocol May 1992

    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.

Simpson [Page 22] RFC 1331 Point-to-Point Protocol May 1992

    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

Simpson [Page 23] RFC 1331 Point-to-Point Protocol May 1992

    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.

Simpson [Page 24] RFC 1331 Point-to-Point Protocol May 1992

    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.

Simpson [Page 25] RFC 1331 Point-to-Point Protocol May 1992

 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.

Simpson [Page 26] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 27] RFC 1331 Point-to-Point Protocol May 1992

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 ...
 +-+-+-+-+

Simpson [Page 28] RFC 1331 Point-to-Point Protocol May 1992

 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.

Simpson [Page 29] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 30] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 31] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 32] RFC 1331 Point-to-Point Protocol May 1992

 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.

Simpson [Page 33] RFC 1331 Point-to-Point Protocol May 1992

    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.

Simpson [Page 34] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 35] RFC 1331 Point-to-Point Protocol May 1992

 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.

Simpson [Page 36] RFC 1331 Point-to-Point Protocol May 1992

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

Simpson [Page 37] RFC 1331 Point-to-Point Protocol May 1992

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

Simpson [Page 38] RFC 1331 Point-to-Point Protocol May 1992

    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 ...
 +-+-+-+-+

Simpson [Page 39] RFC 1331 Point-to-Point Protocol May 1992

 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.

Simpson [Page 40] RFC 1331 Point-to-Point Protocol May 1992

 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.

Simpson [Page 41] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 42] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 43] RFC 1331 Point-to-Point Protocol May 1992

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

Simpson [Page 44] RFC 1331 Point-to-Point Protocol May 1992

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

Simpson [Page 45] RFC 1331 Point-to-Point Protocol May 1992

 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).

Simpson [Page 46] RFC 1331 Point-to-Point Protocol May 1992

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 ...
 +-+-+-+-+

Simpson [Page 47] RFC 1331 Point-to-Point Protocol May 1992

 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.

Simpson [Page 48] RFC 1331 Point-to-Point Protocol May 1992

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:

Simpson [Page 49] RFC 1331 Point-to-Point Protocol May 1992

       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

Simpson [Page 50] RFC 1331 Point-to-Point Protocol May 1992

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

Simpson [Page 51] RFC 1331 Point-to-Point Protocol May 1992

    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

Simpson [Page 52] RFC 1331 Point-to-Point Protocol May 1992

    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.

Simpson [Page 53] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 54] RFC 1331 Point-to-Point Protocol May 1992

    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.

Simpson [Page 55] RFC 1331 Point-to-Point Protocol May 1992

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     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Simpson [Page 56] RFC 1331 Point-to-Point Protocol May 1992

 Type
    8
 Length
    2
 Default
    Not compressed.

Simpson [Page 57] RFC 1331 Point-to-Point Protocol May 1992

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.

Simpson [Page 58] RFC 1331 Point-to-Point Protocol May 1992

    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

Simpson [Page 59] RFC 1331 Point-to-Point Protocol May 1992

       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.

Simpson [Page 60] RFC 1331 Point-to-Point Protocol May 1992

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,

Simpson [Page 61] RFC 1331 Point-to-Point Protocol May 1992

    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);
 }

Simpson [Page 62] RFC 1331 Point-to-Point Protocol May 1992

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");
 }

Simpson [Page 63] RFC 1331 Point-to-Point Protocol May 1992

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

Simpson [Page 64] RFC 1331 Point-to-Point Protocol May 1992

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",

Simpson [Page 65] RFC 1331 Point-to-Point Protocol May 1992

       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

Simpson [Page 66]

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