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rfc:rfc1548

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

                 The Point-to-Point Protocol (PPP)

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 The Point-to-Point Protocol (PPP) provides a standard method for
 transporting multi-protocol datagrams over point-to-point links.  PPP
 is comprised of three main components:
    1. A method for encapsulating multi-protocol datagrams.
    2. A Link Control Protocol (LCP) for establishing, configuring,
       and testing the data-link connection.
    3. A family of Network Control Protocols (NCPs) for establishing
       and configuring different network-layer protocols.
 This document defines the PPP organization and methodology, and the
 PPP encapsulation, together with an extensible option negotiation
 mechanism which is able to negotiate a rich assortment of
 configuration parameters and provides additional management
 functions.  The PPP Link Control Protocol (LCP) is described in terms
 of this mechanism.
 This document is the product of the Point-to-Point Protocol Working
 Group of the Internet Engineering Task Force (IETF).  Comments should
 be submitted to the ietf-ppp@ucdavis.edu mailing list.

Simpson [Page 1] RFC 1548 The Point-to-Point Protocol December 1993

Table of Contents

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

Simpson [Page 2] RFC 1548 The Point-to-Point Protocol December 1993

1. Introduction

 Encapsulation
    The PPP encapsulation provides for multiplexing of different
    network-layer protocols simultaneously over the same link.  It is
    intended that PPP provide a common solution for easy connection of
    a wide variety of hosts, bridges and routers [1].
    The PPP encapsulation has been carefully designed to retain
    compatibility with most commonly used supporting hardware.
    Only 8 additional octets are necessary to form the encapsulation
    when used with the default HDLC framing.  In environments where
    bandwidth is at a premium, the encapsulation and framing may be
    shortened to 2 or 4 octets.
    To support high speed implementations, the default encapsulation
    uses only simple fields, only one of which needs to be examined
    for demultiplexing.  The default header and information fields
    fall on 32-bit boundaries, and the trailer may be padded to an
    arbitrary boundary.
  Link Control Protocol
    In order to be sufficiently versatile to be portable to a wide
    variety of environments, PPP provides a Link Control Protocol
    (LCP).  The LCP is used to automatically agree upon the
    encapsulation format options, handle varying limits on sizes of
    packets, authenticate the identity of its peer on the link,
    determine when a link is functioning properly and when it is
    defunct, detect a looped-back link and other common
    misconfiguration errors, and terminate the link.
  Network Control Protocols
    Point-to-Point links tend to exacerbate many problems with the
    current family of network protocols.  For instance, assignment and
    management of IP addresses, which is a problem even in LAN
    environments, is especially difficult over circuit-switched
    point-to-point links (such as dial-up modem servers).  These
    problems are handled by a family of Network Control Protocols
    (NCPs), which each manage the specific needs required by their
    respective network-layer protocols.  These NCPs are defined in
    companion documents.

Simpson [Page 3] RFC 1548 The Point-to-Point Protocol December 1993

  Configuration
    It is intended that PPP links be easy to configure.  By design,
    the standard defaults handle all common configurations.  The
    implementor can specify improvements to the default configuration,
    which are automatically communicated to the peer without operator
    intervention.  Finally, the operator may explicitly configure
    options for the link which enable the link to operate in
    environments where it would otherwise be impossible.
    This self-configuration is implemented through an extensible
    option negotiation mechanism, wherein each end of the link
    describes to the other its capabilities and requirements.
    Although the option negotiation mechanism described in this
    document is specified in terms of the Link Control Protocol (LCP),
    the same facilities are designed to be used by other control
    protocols, especially the family of NCPs.

1.1 Specification of Requirements

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

Simpson [Page 4] RFC 1548 The Point-to-Point Protocol December 1993

1.2 Terminology

    This document frequently uses the following terms:
  datagram
    The unit of transmission in the network layer (such as IP).  A
    datagram may be encapsulated in one or more packets passed to the
    data link layer.
  frame
    The unit of transmission at the data link layer.  A frame may
    include a header and/or a trailer, along with some number of units
    of data.
  packet
    The basic unit of encapsulation, which is passed across the
    interface between the network layer and the data link layer.  A
    packet is usually mapped to a frame; the exceptions are when data
    link layer fragmentation is being performed, or when multiple
    packets are incorporated into a single frame.
  peer
    The other end of the point-to-point link.
  silently discard
    This means the implementation discards the packet without further
    processing.  The implementation SHOULD provide the capability of
    logging the error, including the contents of the silently
    discarded packet, and SHOULD record the event in a statistics
    counter.

2. PPP Encapsulation

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

Simpson [Page 5] RFC 1548 The Point-to-Point Protocol December 1993

 A summary of the PPP encapsulation is shown below.  The fields are
 transmitted from left to right.
            +----------+-------------+---------+
            | Protocol | Information | Padding |
            | 16 bits  |      *      |    *    |
            +----------+-------------+---------+
  Protocol Field
    The Protocol field is two octets and its value identifies the
    datagram encapsulated in the Information field of the packet.  The
    field is transmitted and received most significant octet first.
    The structure of this field is consistent with the ISO 3309
    extension mechanism for address fields.  All Protocols MUST be
    odd; the least significant bit of the least significant octet MUST
    equal "1".  Also, all Protocols MUST be assigned such that the
    least significant bit of the most significant octet equals "0".
    Frames received which don't comply with these rules MUST be
    treated as having an unrecognized Protocol.
    Protocol field values in the "0***" to "3***" range identify the
    network-layer protocol of specific packets, and values in the
    "8***" to "b***" range identify packets belonging to the
    associated Network Control Protocols (NCPs), if any.
    Protocol field values in the "4***" to "7***" range are used for
    protocols with low volume traffic which have no associated NCP.
    Protocol field values in the "c***" to "f***" range identify
    packets as link-layer Control Protocols (such as LCP).
    Up-to-date values of the Protocol field are specified in the most
    recent "Assigned Numbers" RFC [2].  Current values are assigned as
    follows:
         Value (in hex)  Protocol Name
         0001            Padding Protocol
         0003 to 001f    reserved (transparency inefficient)
         0021            Internet Protocol
         0023            OSI Network Layer
         0025            Xerox NS IDP
         0027            DECnet Phase IV
         0029            Appletalk
         002b            Novell IPX
         002d            Van Jacobson Compressed TCP/IP
         002f            Van Jacobson Uncompressed TCP/IP

Simpson [Page 6] RFC 1548 The Point-to-Point Protocol December 1993

         0031            Bridging PDU
         0033            Stream Protocol (ST-II)
         0035            Banyan Vines
         0037            unused
         0039            AppleTalk EDDP
         003b            AppleTalk SmartBuffered
         003d            Multi-Link
         005d            reserved (compression inefficient)
         00cf            reserved (PPP NLPID)
         00fd            1st choice compression
         00ff            reserved (compression inefficient)
         0201            802.1d Hello Packets
         0203            IBM Source Routing BPDU
         0231            Luxcom
         0233            Sigma Network Systems
         8021            Internet Protocol Control Protocol
         8023            OSI Network Layer Control Protocol
         8025            Xerox NS IDP Control Protocol
         8027            DECnet Phase IV Control Protocol
         8029            Appletalk Control Protocol
         802b            Novell IPX Control Protocol
         802d            Reserved
         802f            Reserved
         8031            Bridging NCP
         8033            Stream Protocol Control Protocol
         8035            Banyan Vines Control Protocol
         8037            unused
         8039            Reserved
         803b            Reserved
         803d            Multi-Link Control Protocol
         80fd            Compression Control Protocol
         80ff            Reserved
         c021            Link Control Protocol
         c023            Password Authentication Protocol
         c025            Link Quality Report
         c223            Challenge Handshake Authentication Protocol
    Developers of new protocols MUST obtain a number from the Internet
    Assigned Numbers Authority (IANA), at IANA@isi.edu.
  Information Field
    The Information field is zero or more octets.  The Information
    field contains the datagram for the protocol specified in the
    Protocol field.

Simpson [Page 7] RFC 1548 The Point-to-Point Protocol December 1993

    The maximum length for the Information field, including Padding,
    is termed the Maximum Receive Unit (MRU), which defaults to 1500
    octets.  By negotiation, consenting PPP implementations may use
    other values for the MRU.
  Padding
    On transmission, the Information field MAY be padded with an
    arbitrary number of octets up to the MRU.  It is the
    responsibility of each protocol to distinguish padding octets from
    real information.

3. PPP Link Operation

3.1 Overview

 In order to establish communications over a point-to-point link, each
 end of the PPP link MUST first send LCP packets to configure and test
 the data link.  After the link has been established, the peer MAY be
 authenticated.  Then, PPP MUST send NCP packets to choose and
 configure one or more network-layer protocols.  Once each of the
 chosen network-layer protocols has been configured, datagrams from
 each network-layer protocol can be sent over the link.
 The link will remain configured for communications until explicit LCP
 or NCP packets close the link down, or until some external event
 occurs (an inactivity timer expires or network administrator
 intervention).

3.2 Phase Diagram

 In the process of configuring, maintaining and terminating the
 point-to-point link, the PPP link goes through several distinct
 phases:
 +------+        +-----------+           +--------------+
 |      | UP     |           | OPENED    |              | SUCCESS/NONE
 | Dead |------->| Establish |---------->| Authenticate |--+
 |      |        |           |           |              |  |
 +------+        +-----------+           +--------------+  |
    ^          FAIL |                   FAIL |             |
    +<--------------+             +----------+             |
    |                             |                        |
    |            +-----------+    |           +---------+  |
    |       DOWN |           |    |   CLOSING |         |  |
    +------------| Terminate |<---+<----------| Network |<-+
                 |           |                |         |
                 +-----------+                +---------+

Simpson [Page 8] RFC 1548 The Point-to-Point Protocol December 1993

3.3 Link Dead (physical-layer not ready)

 The link necessarily begins and ends with this phase.  When an
 external event (such as carrier detection or network administrator
 configuration) indicates that the physical-layer is ready to be used,
 PPP will proceed to the Link Establishment phase.
 During this phase, the LCP automaton (described below) will be in the
 Initial or Starting states.  The transition to the Link Establishment
 phase will signal an Up event to the automaton.
  Implementation Note:
    Typically, a link will return to this phase automatically after
    the disconnection of a modem.  In the case of a hard-wired line,
    this phase may be extremely short -- merely long enough to detect
    the presence of the device.

3.4 Link Establishment Phase

 The Link Control Protocol (LCP) is used to establish the connection
 through an exchange of Configure packets.  This exchange is complete,
 and the LCP Opened state entered, once a Configure-Ack packet
 (described below) has been both sent and received.
 All Configuration Options are assumed to be at default values unless
 altered by the configuration exchange.  See the section on LCP
 Configuration Options for further discussion.
 It is important to note that only Configuration Options which are
 independent of particular network-layer protocols are configured by
 LCP.  Configuration of individual network-layer protocols is handled
 by separate Network Control Protocols (NCPs) during the Network-Layer
 Protocol phase.
 Any non-LCP packets received during this phase MUST be silently
 discarded.

3.5 Authentication Phase

 On some links it may be desirable to require a peer to authenticate
 itself before allowing network-layer protocol packets to be
 exchanged.
 By default, authentication is not mandatory.  If an implementation
 desires that the peer authenticate with some specific authentication
 protocol, then it MUST negotiate the use of that authentication
 protocol during Link Establishment phase.

Simpson [Page 9] RFC 1548 The Point-to-Point Protocol December 1993

 Authentication SHOULD take place as soon as possible after link
 establishment.  However, link quality determination MAY occur
 concurrently.  An implementation MUST NOT allow the exchange of link
 quality determination packets to delay authentication indefinitely.
 Advancement from the Authentication phase to the Network-Layer
 Protocol phase MUST NOT occur until authentication has completed,
 using the negotiated authentication protocol.  If authentication
 fails, PPP SHOULD proceed instead to the Link Termination phase.
 Any Network Control Protocol or network-layer protocol packets
 received during this phase MUST be silently discarded.

3.6 Network-Layer Protocol Phase

 Once PPP has finished the previous phases, each network-layer
 protocol (such as IP, IPX, or AppleTalk) MUST be separately
 configured by the appropriate Network Control Protocol (NCP).
 Each NCP MAY be Opened and Closed at any time.
  Implementation Note:
    Because an implementation may initially use a significant amount
    of time for link quality determination, implementations SHOULD
    avoid fixed timeouts when waiting for their peers to configure a
    NCP.
    After a NCP has reached the Opened state, PPP will carry the
    corresponding network-layer protocol packets.  Any network-layer
    protocol packets received when the corresponding NCP is not in the
    Opened state MUST be silently discarded.
  Implementation Note:
    There is an exception to the preceding paragraphs, due to the
    availability of the LCP Protocol-Reject (described below).  While
    LCP is in the Opened state, any protocol packet which is
    unsupported by the implementation MUST be returned in a Protocol-
    Reject.  Only protocols which are supported are silently
    discarded.
    During this phase, link traffic consists of any possible
    combination of LCP, NCP, and network-layer protocol packets.

3.7 Link Termination Phase

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

Simpson [Page 10] RFC 1548 The Point-to-Point Protocol December 1993

 the loss of carrier, authentication failure, link quality failure,
 the expiration of an idle-period timer, or the administrative closing
 of the link.  LCP is used to close the link through an exchange of
 Terminate packets.  When the link is closing, PPP informs the
 network-layer protocols so that they may take appropriate action.
 After the exchange of Terminate packets, the implementation SHOULD
 signal the physical-layer to disconnect in order to enforce the
 termination of the link, particularly in the case of an
 authentication failure.  The sender of the Terminate-Request SHOULD
 disconnect after receiving a Terminate-Ack, or after the Restart
 counter expires.  The receiver of a Terminate-Request SHOULD wait for
 the peer to disconnect, and MUST NOT disconnect until at least one
 Restart time has passed after sending a Terminate-Ack.  PPP SHOULD
 proceed to the Link Dead phase.
 Any non-LCP packets received during this phase MUST be silently
 discarded.
  Implementation Note:
    The closing of the link by LCP is sufficient.  There is no need
    for each NCP to send a flurry of Terminate packets.  Conversely,
    the fact that one NCP has Closed is not sufficient reason to cause
    the termination of the PPP link, even if that NCP was the only NCP
    currently in the Opened state.

4. The Option Negotiation Automaton

 The finite-state automaton is defined by events, actions and state
 transitions.  Events include reception of external commands such as
 Open and Close, expiration of the Restart timer, and reception of
 packets from a peer.  Actions include the starting of the Restart
 timer and transmission of packets to the peer.
 Some types of packets -- Configure-Naks and Configure-Rejects, or
 Code-Rejects and Protocol-Rejects, or Echo-Requests, Echo-Replies and
 Discard-Requests -- are not differentiated in the automaton
 descriptions.  As will be described later, these packets do indeed
 serve different functions.  However, they always cause the same
 transitions.

Events Actions

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

Simpson [Page 11] RFC 1548 The Point-to-Point Protocol December 1993

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

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

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

RUC = Receive-Unknown-Code scj = Send-Code-Reject RXJ+ = Receive-Code-Reject (permitted)

  or Receive-Protocol-Reject

RXJ- = Receive-Code-Reject (catastrophic)

  or Receive-Protocol-Reject

RXR = Receive-Echo-Request ser = Send-Echo-Reply

  or Receive-Echo-Reply
  or Receive-Discard-Request

4.1 State Diagram

 The simplified state diagram which follows describes the sequence of
 events for reaching agreement on Configuration Options (opening the
 PPP link) and for later termination of the link.
 This diagram is not a complete representation of the automaton.
 Implementation MUST be done by consulting the actual state transition
 table.
 Events are in upper case.  Actions are in lower case.  For these
 purposes, the state machine is initially in the Closed state.  Once
 the Opened state has been reached, both ends of the link have met the
 requirement of having both sent and received a Configure-Ack packet.

Simpson [Page 12] RFC 1548 The Point-to-Point Protocol December 1993

               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 13] RFC 1548 The Point-to-Point Protocol December 1993

4.2 State Transition Table

The complete state transition table follows.  States are indicated
horizontally, and events are read vertically.  State transitions and
actions are represented in the form action/new-state.  Multiple
actions are separated by commas, and may continue on succeeding lines
as space requires; multiple actions may be implemented in any
convenient order.  The state may be followed by a letter, which
indicates an explanatory footnote.  The dash ('-') indicates an
illegal transition.
       | State
       |    0         1         2         3         4         5
 Events| Initial   Starting  Closed    Stopped   Closing   Stopping
 ------+-----------------------------------------------------------
  Up   |    2     irc,scr/6     -         -         -         -
  Down |    -         -         0       tls/1       0         1
  Open |  tls/1       1     irc,scr/6     3r        5r        5r
  Close|    0         0         2         2         4         4
       |
   TO+ |    -         -         -         -       str/4     str/5
   TO- |    -         -         -         -       tlf/2     tlf/3
       |
  RCR+ |    -         -       sta/2 irc,scr,sca/8   4         5
  RCR- |    -         -       sta/2 irc,scr,scn/6   4         5
  RCA  |    -         -       sta/2     sta/3       4         5
  RCN  |    -         -       sta/2     sta/3       4         5
       |
  RTR  |    -         -       sta/2     sta/3     sta/4     sta/5
  RTA  |    -         -         2         3       tlf/2     tlf/3
       |
  RUC  |    -         -       scj/2     scj/3     scj/4     scj/5
  RXJ+ |    -         -         2         3         4         5
  RXJ- |    -         -       tlf/2     tlf/3     tlf/2     tlf/3
       |
  RXR  |    -         -         2         3         4         5

Simpson [Page 14] RFC 1548 The Point-to-Point Protocol December 1993

          | State
          |    6         7         8           9
    Events| Req-Sent  Ack-Rcvd  Ack-Sent    Opened
    ------+-----------------------------------------
     Up   |    -         -         -           -
     Down |    1         1         1         tld/1
     Open |    6         7         8           9r
     Close|irc,str/4 irc,str/4 irc,str/4 tld,irc,str/4
          |
      TO+ |  scr/6     scr/6     scr/8         -
      TO- |  tlf/3p    tlf/3p    tlf/3p        -
          |
     RCR+ |  sca/8   sca,tlu/9   sca/8   tld,scr,sca/8
     RCR- |  scn/6     scn/7     scn/6   tld,scr,scn/6
     RCA  |  irc/7     scr/6x  irc,tlu/9   tld,scr/6x
     RCN  |irc,scr/6   scr/6x  irc,scr/8   tld,scr/6x
          |
     RTR  |  sta/6     sta/6     sta/6   tld,zrc,sta/5
     RTA  |    6         6         8       tld,scr/6
          |
     RUC  |  scj/6     scj/7     scj/8       scj/9
     RXJ+ |    6         6         8           9
     RXJ- |  tlf/3     tlf/3     tlf/3   tld,irc,str/5
          |
     RXR  |    6         7         8         ser/9
 The states in which the Restart timer is running are identifiable by
 the presence of TO events.  Only the Send-Configure-Request, Send-
 Terminate-Request and Zero-Restart-Counter actions start or re-start
 the Restart timer.  The Restart timer is stopped when transitioning
 from any state where the timer is running to a state where the timer
 is not running.
    [p]   Passive option; see Stopped state discussion.
    [r]   Restart option; see Open event discussion.
    [x]   Crossed connection; see RCA event discussion.

4.3 A Day in the Life

 Here is an example of how a typical implementation might use the
 automaton to implement LCP in a dial-up environment:
  1. The Network Access Server is powered on (Initial state, Link Dead

phase).

  1. A configuration file indicates that a particular link is to be

Simpson [Page 15] RFC 1548 The Point-to-Point Protocol December 1993

    used for PPP access (Open: tls/Starting).  The This-Layer-Started
    event turns on DTR to a modem, readying it for accepting calls.
  1. An incoming call is answered. The modem CD triggers configuration

negotiation (Up: irc,scr/Req-Sent, Link Establishment phase).

  1. A Configure-Request is received, which is acknowleged (RCR+:

sca/Ack-Sent).

  1. The Request is acknowleged (RCA: irc,tlu/Opened). The This-

Layer-Up event starts authentication and quality monitoring

    protocols (Authentication phase).
  1. When authentication and quality monitoring are satisfied, they

send an Up event to start the available NCPs (Network-Layer

    Protocol phase).
  1. Later, the peer is finished, and closes the link. A Terminate-

Request arrives (RTR: tld,zrc,sta/Stopping, Termination phase).

    The This-Layer-Down action sends the Down event to any NCPs, while
    the Terminate-Ack is sent.  The Zero-Restart-Counter action causes
    the link to wait for the peer to process the Terminate-Ack, with
    no retries.
  1. When the Restart Timer times out (TO-: tlf/Stopped), the This-

Layer-Finished action signals the modem to hang up by dropping

    DTR.
  1. When the CD from the modem drops (Down: tls/Starting), the This-

Layer-Started action raises DTR again, readying it for the next

    call (returning to the Link Dead phase).

4.4 States

 Following is a more detailed description of each automaton state.
  Initial
    In the Initial state, the lower layer is unavailable (Down), and
    no Open has occurred.  The Restart timer is not running in the
    Initial state.
  Starting
    The Starting state is the Open counterpart to the Initial state.
    An administrative Open has been initiated, but the lower layer is
    still unavailable (Down).  The Restart timer is not running in the
    Starting state.

Simpson [Page 16] RFC 1548 The Point-to-Point Protocol December 1993

    When the lower layer becomes available (Up), a Configure-Request
    is sent.
  Closed
    In the Closed state, the link is available (Up), but no Open has
    occurred.  The Restart timer is not running in the Closed state.
    Upon reception of Configure-Request packets, a Terminate-Ack is
    sent.  Terminate-Acks are silently discarded to avoid creating a
    loop.
  Stopped
    The Stopped state is the Open counterpart to the Closed state.  It
    is entered when the automaton is waiting for a Down event after
    the This-Layer-Finished action, or after sending a Terminate-Ack.
    The Restart timer is not running in the Stopped state.
    Upon reception of Configure-Request packets, an appropriate
    response is sent.  Upon reception of other packets, a Terminate-
    Ack is sent.  Terminate-Acks are silently discarded to avoid
    creating a loop.
  Rationale:
    The Stopped state is a junction state for link termination, link
    configuration failure, and other automaton failure modes.  These
    potentially separate states have been combined.
    There is a race condition between the Down event response (from
    the This-Layer-Finished action) and the Receive-Configure- Request
    event.  When a Configure-Request arrives before the Down event,
    the Down event will supercede by returning the automaton to the
    Starting state.  This prevents attack by repetition.
  Implementation Option:
    After the peer fails to respond to Configure-Requests, an
    implementation MAY wait passively for the peer to send Configure-
    Requests.  In this case, the This-Layer-Finished action is not
    used for the TO- event in states Req-Sent, Ack- Rcvd and Ack-Sent.
    This option is useful for dedicated circuits, or circuits which
    have no status signals available, but SHOULD NOT be used for
    switched circuits.

Simpson [Page 17] RFC 1548 The Point-to-Point Protocol December 1993

  Closing
    In the Closing state, an attempt is made to terminate the
    connection.  A Terminate-Request has been sent and the Restart
    timer is running, but a Terminate-Ack has not yet been received.
    Upon reception of a Terminate-Ack, the Closed state is entered.
    Upon the expiration of the Restart timer, a new Terminate-Request
    is transmitted and the Restart timer is restarted.  After the
    Restart timer has expired Max-Terminate times, this action may be
    skipped, and the Closed state may be entered.
  Stopping
    The Stopping state is the Open counterpart to the Closing state.
    A Terminate-Request has been sent and the Restart timer is
    running, but a Terminate-Ack has not yet been received.
  Rationale:
    The Stopping state provides a well defined opportunity to
    terminate a link before allowing new traffic.  After the link has
    terminated, a new configuration may occur via the Stopped or
    Starting states.
  Request-Sent
    In the Request-Sent state an attempt is made to configure the
    connection.  A Configure-Request has been sent and the Restart
    timer is running, but a Configure-Ack has not yet been received
    nor has one been sent.
  Ack-Received
    In the Ack-Received state, a Configure-Request has been sent and a
    Configure-Ack has been received.  The Restart timer is still
    running since a Configure-Ack has not yet been sent.
  Ack-Sent
    In the Ack-Sent state, a Configure-Request and a Configure-Ack
    have both been sent but a Configure-Ack has not yet been received.
    The Restart timer is always running in the Ack-Sent state.
  Opened
    In the Opened state, a Configure-Ack has been both sent and
    received.  The Restart timer is not running in the Opened state.

Simpson [Page 18] RFC 1548 The Point-to-Point Protocol December 1993

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

4.5 Events

 Transitions and actions in the automaton are caused by events.
  Up
    The Up event occurs when a lower layer indicates that it is ready
    to carry packets.
    Typically, this event is used by a modem handling or calling
    process, or by some other coupling of the PPP link to the physical
    media, to signal LCP that the link is entering Link Establishment
    phase.
    It also can be used by LCP to signal each NCP that the link is
    entering Network-Layer Protocol phase.  That is, the This-Layer-Up
    action from LCP triggers the Up event in the NCP.
  Down
    The Down event occurs when a lower layer indicates that it is no
    longer ready to carry packets.
    Typically, this event is used by a modem handling or calling
    process, or by some other coupling of the PPP link to the physical
    media, to signal LCP that the link is entering Link Dead phase.
    It also can be used by LCP to signal each NCP that the link is
    leaving Network-Layer Protocol phase.  That is, the This-Layer-
    Down action from LCP triggers the Down event in the NCP.
  Open
    The Open event indicates that the link is administratively
    available for traffic; that is, the network administrator (human
    or program) has indicated that the link is allowed to be Opened.
    When this event occurs, and the link is not in the Opened state,
    the automaton attempts to send configuration packets to the peer.
    If the automaton is not able to begin configuration (the lower
    layer is Down, or a previous Close event has not completed), the
    establishment of the link is automatically delayed.

Simpson [Page 19] RFC 1548 The Point-to-Point Protocol December 1993

    When a Terminate-Request is received, or other events occur which
    cause the link to become unavailable, the automaton will progress
    to a state where the link is ready to re-open.  No additional
    administrative intervention is necessary.
  Implementation Option:
    Experience has shown that users will execute an additional Open
    command when they want to renegotiate the link.  This might
    indicate that new values are to be negotiated.
    Since this is not the meaning of the Open event, it is suggested
    that when an Open user command is executed in the Opened, Closing,
    Stopping, or Stopped states, the implementation issue a Down
    event, immediately followed by an Up event.  This will cause the
    renegotiation of the link, without any harmful side effects.
  Close
    The Close event indicates that the link is not available for
    traffic; that is, the network administrator (human or program) has
    indicated that the link is not allowed to be Opened.  When this
    event occurs, and the link is not in the Closed state, the
    automaton attempts to terminate the connection.  Futher attempts
    to re-configure the link are denied until a new Open event occurs.
  Implementation Note:
    When authentication fails, the link SHOULD be terminated, to
    prevent attack by repetition and denial of service to other users.
    Since the link is administratively available (by definition), this
    can be accomplished by simulating a Close event to the LCP,
    immediately followed by an Open event.
    The Close followed by an Open will cause an orderly termination of
    the link, by progressing from the Closing to the Stopping state,
    and the This-Layer-Finished action can disconnect the link.  The
    automaton waits in the Stopped or Starting states for the next
    connection attempt.
  Timeout (TO+,TO-)
    This event indicates the expiration of the Restart timer.  The
    Restart timer is used to time responses to Configure-Request and
    Terminate-Request packets.
    The TO+ event indicates that the Restart counter continues to be
    greater than zero, which triggers the corresponding Configure-

Simpson [Page 20] RFC 1548 The Point-to-Point Protocol December 1993

    Request or Terminate-Request packet to be retransmitted.
    The TO- event indicates that the Restart counter is not greater
    than zero, and no more packets need to be retransmitted.
  Receive-Configure-Request (RCR+,RCR-)
    This event occurs when a Configure-Request packet is received from
    the peer.  The Configure-Request packet indicates the desire to
    open a connection and may specify Configuration Options.  The
    Configure-Request packet is more fully described in a later
    section.
    The RCR+ event indicates that the Configure-Request was
    acceptable, and triggers the transmission of a corresponding
    Configure-Ack.
    The RCR- event indicates that the Configure-Request was
    unacceptable, and triggers the transmission of a corresponding
    Configure-Nak or Configure-Reject.
  Implementation Note:
    These events may occur on a connection which is already in the
    Opened state.  The implementation MUST be prepared to immediately
    renegotiate the Configuration Options.
  Receive-Configure-Ack (RCA)
    The Receive-Configure-Ack event occurs when a valid Configure-Ack
    packet is received from the peer.  The Configure-Ack packet is a
    positive response to a Configure-Request packet.  An out of
    sequence or otherwise invalid packet is silently discarded.
  Implementation Note:
    Since the correct packet has already been received before reaching
    the Ack-Rcvd or Opened states, it is extremely unlikely that
    another such packet will arrive.  As specified, all invalid
    Ack/Nak/Rej packets are silently discarded, and do not affect the
    transitions of the automaton.
    However, it is not impossible that a correctly formed packet will
    arrive through a coincidentally-timed cross-connection.  It is
    more likely to be the result of an implementation error.  At the
    very least, this occurance SHOULD be logged.

Simpson [Page 21] RFC 1548 The Point-to-Point Protocol December 1993

  Receive-Configure-Nak/Rej (RCN)
    This event occurs when a valid Configure-Nak or Configure-Reject
    packet is received from the peer.  The Configure-Nak and
    Configure-Reject packets are negative responses to a Configure-
    Request packet.  An out of sequence or otherwise invalid packet is
    silently discarded.
  Implementation Note:
    Although the Configure-Nak and Configure-Reject cause the same
    state transition in the automaton, these packets have
    significantly different effects on the Configuration Options sent
    in the resulting Configure-Request packet.
  Receive-Terminate-Request (RTR)
    The Receive-Terminate-Request event occurs when a Terminate-
    Request packet is received.  The Terminate-Request packet
    indicates the desire of the peer to close the connection.
  Implementation Note:
    This event is not identical to the Close event (see above), and
    does not override the Open commands of the local network
    administrator.  The implementation MUST be prepared to receive a
    new Configure-Request without network administrator intervention.
  Receive-Terminate-Ack (RTA)
    The Receive-Terminate-Ack event occurs when a Terminate-Ack packet
    is received from the peer.  The Terminate-Ack packet is usually a
    response to a Terminate-Request packet.  The Terminate-Ack packet
    may also indicate that the peer is in Closed or Stopped states,
    and serves to re-synchronize the link configuration.
  Receive-Unknown-Code (RUC)
    The Receive-Unknown-Code event occurs when an un-interpretable
    packet is received from the peer.  A Code-Reject packet is sent in
    response.
  Receive-Code-Reject, Receive-Protocol-Reject (RXJ+,RXJ-)
    This event occurs when a Code-Reject or a Protocol-Reject packet
    is received from the peer.
    The RXJ+ event arises when the rejected value is acceptable, such

Simpson [Page 22] RFC 1548 The Point-to-Point Protocol December 1993

    as a Code-Reject of an extended code, or a Protocol-Reject of a
    NCP.  These are within the scope of normal operation.  The
    implementation MUST stop sending the offending packet type.
    The RXJ- event arises when the rejected value is catastrophic,
    such as a Code-Reject of Configure-Request, or a Protocol-Reject
    of LCP!  This event communicates an unrecoverable error that
    terminates the connection.
  Receive-Echo-Request, Receive-Echo-Reply, Receive-Discard-Request
  (RXR)
  This event occurs when an Echo-Request, Echo-Reply or Discard-
  Request packet is received from the peer.  The Echo-Reply packet is
  a response to a Echo-Request packet.  There is no reply to an Echo-
  Reply or Discard-Request packet.

4.6 Actions

 Actions in the automaton are caused by events and typically indicate
 the transmission of packets and/or the starting or stopping of the
 Restart timer.
  Illegal-Event (-)
    This indicates an event that cannot occur in a properly
    implemented automaton.  The implementation has an internal error,
    which should be reported and logged.  No transition is taken, and
    the implementation SHOULD NOT reset or freeze.
  This-Layer-Up (tlu)
    This action indicates to the upper layers that the automaton is
    entering the Opened state.
    Typically, this action is used by the LCP to signal the Up event
    to a NCP, Authentication Protocol, or Link Quality Protocol, or
    MAY be used by a NCP to indicate that the link is available for
    its network layer traffic.
  This-Layer-Down (tld)
    This action indicates to the upper layers that the automaton is
    leaving the Opened state.
    Typically, this action is used by the LCP to signal the Down event
    to a NCP, Authentication Protocol, or Link Quality Protocol, or
    MAY be used by a NCP to indicate that the link is no longer

Simpson [Page 23] RFC 1548 The Point-to-Point Protocol December 1993

    available for its network layer traffic.
  This-Layer-Started (tls)
    This action indicates to the lower layers that the automaton is
    entering the Starting state, and the lower layer is needed for the
    link.  The lower layer SHOULD respond with an Up event when the
    lower layer is available.
  Implementation Note:
    This results of this action are highly implementation dependent.
    The transitions where this event is indicated are defined
    according to a message passing architecture, rather than a
    signalling architecture.  If the action is desired to control
    specific signals (such as DTR), other transitions for the action
    are likely to be required (Open in Closed, RCR in Stopped).
  This-Layer-Finished (tlf)
    This action indicates to the lower layers that the automaton is
    entering the Stopped or Closed states, and the lower layer is no
    longer needed for the link.  The lower layer SHOULD respond with a
    Down event when the lower layer has terminated.
    Typically, this action MAY be used by the LCP to advance to the
    Link Dead phase, or MAY be used by a NCP to indicate to the LCP
    that the link may terminate when there are no other NCPs open.
  Implementation Note:
    This results of this action are highly implementation dependent.
    The transitions where this event is indicated are defined
    according to a message passing architecture, rather than a
    signalling architecture.  If the action is desired to control
    specific signals (such as DTR), other transitions for the action
    are likely to be required (Close in Starting, Down in Closing).
  Initialize-Restart-Counter (irc)
    This action sets the Restart counter to the appropriate value
    (Max-Terminate or Max-Configure).  The counter is decremented for
    each transmission, including the first.

Simpson [Page 24] RFC 1548 The Point-to-Point Protocol December 1993

  Implementation Note:
    In addition to setting the Restart counter, the implementation
    MUST set the timeout period to the initial value when Restart
    timer backoff is used.
  Zero-Restart-Counter (zrc)
    This action sets the Restart counter to zero.
  Implementation Note:
    This action enables the FSA to pause before proceeding to the
    desired final state, allowing traffic to be processed by the peer.
    In addition to zeroing the Restart counter, the implementation
    MUST set the timeout period to an appropriate value.
  Send-Configure-Request (scr)
    The Send-Configure-Request action transmits a Configure-Request
    packet.  This indicates the desire to open a connection with a
    specified set of Configuration Options.  The Restart timer is
    started when the Configure-Request packet is transmitted, to guard
    against packet loss.  The Restart counter is decremented each time
    a Configure-Request is sent.
  Send-Configure-Ack (sca)
    The Send-Configure-Ack action transmits a Configure-Ack packet.
    This acknowledges the reception of a Configure-Request packet with
    an acceptable set of Configuration Options.
  Send-Configure-Nak (scn)
    The Send-Configure-Nak action transmits a Configure-Nak or
    Configure-Reject packet, as appropriate.  This negative response
    reports the reception of a Configure-Request packet with an
    unacceptable set of Configuration Options.  Configure-Nak packets
    are used to refuse a Configuration Option value, and to suggest a
    new, acceptable value.  Configure-Reject packets are used to
    refuse all negotiation about a Configuration Option, typically
    because it is not recognized or implemented.  The use of
    Configure-Nak versus Configure-Reject is more fully described in
    the section on LCP Packet Formats.
  Send-Terminate-Request (str)
    The Send-Terminate-Request action transmits a Terminate-Request

Simpson [Page 25] RFC 1548 The Point-to-Point Protocol December 1993

    packet.  This indicates the desire to close a connection.  The
    Restart timer is started when the Terminate-Request packet is
    transmitted, to guard against packet loss.  The Restart counter is
    decremented each time a Terminate-Request is sent.
  Send-Terminate-Ack (sta)
    The Send-Terminate-Ack action transmits a Terminate-Ack packet.
    This acknowledges the reception of a Terminate-Request packet or
    otherwise serves to synchronize the state machines.
  Send-Code-Reject (scj)
    The Send-Code-Reject action transmits a Code-Reject packet.  This
    indicates the reception of an unknown type of packet.
  Send-Echo-Reply (ser)
    The Send-Echo-Reply action transmits an Echo-Reply packet.  This
    acknowledges the reception of an Echo-Request packet.

4.7 Loop Avoidance

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

4.8 Counters and Timers

  Restart Timer
    There is one special timer used by the automaton.  The Restart
    timer is used to time transmissions of Configure-Request and
    Terminate- Request packets.  Expiration of the Restart timer
    causes a Timeout event, and retransmission of the corresponding
    Configure-Request or Terminate-Request packet.  The Restart timer
    MUST be configurable, but SHOULD default to three (3) seconds.
  Implementation Note:
    The Restart timer SHOULD be based on the speed of the link.  The
    default value is designed for low speed (2,400 to 9,600 bps), high

Simpson [Page 26] RFC 1548 The Point-to-Point Protocol December 1993

    switching latency links (typical telephone lines).  Higher speed
    links, or links with low switching latency, SHOULD have
    correspondingly faster retransmission times.
    Instead of a constant value, the Restart timer MAY begin at an
    initial small value and increase to the configured final value.
    Each successive value less than the final value SHOULD be at least
    twice the previous value.  The initial value SHOULD be large
    enough to account for the size of the packets, twice the round
    trip time for transmission at the link speed, and at least an
    additional 100 milliseconds to allow the peer to process the
    packets before responding.  Some circuits add another 200
    milliseconds of satellite delay.  Round trip times for modems
    operating at 14,400 bps have been measured in the range of 160 to
    more than 600 milliseconds.
  Max-Terminate
    There is one required restart counter for Terminate-Requests.
    Max- Terminate indicates the number of Terminate-Request packets
    sent without receiving a Terminate-Ack before assuming that the
    peer is unable to respond.  Max-Terminate MUST be configurable,
    but SHOULD default to two (2) transmissions.
  Max-Configure
    A similar counter is recommended for Configure-Requests.  Max-
    Configure indicates the number of Configure-Request packets sent
    without receiving a valid Configure-Ack, Configure-Nak or
    Configure- Reject before assuming that the peer is unable to
    respond.  Max- Configure MUST be configurable, but SHOULD default
    to ten (10) transmissions.
  Max-Failure
    A related counter is recommended for Configure-Nak.  Max-Failure
    indicates the number of Configure-Nak packets sent without sending
    a Configure-Ack before assuming that configuration is not
    converging.  Any further Configure-Nak packets are converted to
    Configure-Reject packets.  Max-Failure MUST be configurable, but
    SHOULD default to ten (10) transmissions.

5. LCP Packet Formats

 There are three classes of LCP packets:
    1. Link Configuration packets used to establish and configure a
       link (Configure-Request, Configure-Ack, Configure-Nak and

Simpson [Page 27] RFC 1548 The Point-to-Point Protocol December 1993

       Configure-Reject).
    2. Link Termination packets used to terminate a link (Terminate-
       Request and Terminate-Ack).
    3. Link Maintenance packets used to manage and debug a link
       (Code-Reject, Protocol-Reject, Echo-Request, Echo-Reply, and
       Discard-Request).
 This document describes Version 1 of the Link Control Protocol.  In
 the interest of simplicity, there is no version field in the LCP
 packet.  If a new version of LCP is necessary in the future, the
 intention is that a new PPP Protocol field value will be used to
 differentiate Version 1 LCP from all other versions.  A correctly
 functioning Version 1 LCP implementation will always respond to
 unknown Protocols (including other versions) with an easily
 recognizable Version 1 packet, thus providing a deterministic
 fallback mechanism for implementations of other versions.
 Regardless of which Configuration Options are enabled, all LCP Link
 Configuration, Link Termination, and Code-Reject packets (codes 1
 through 7) are always sent as if no Configuration Options were
 enabled.  This ensures that such LCP packets are always recognizable
 even when one end of the link mistakenly believes the link to be
 open.
  Implementation Note:
    In particular, the Async-Control-Character-Map (ACCM) default for
    the type of link is used, and no address, control, or protocol
    field compression is allowed.
    Exactly one LCP packet is encapsulated in the PPP Information
    field, where the PPP Protocol field indicates type hex c021 (Link
    Control Protocol).
 A summary of the Link Control Protocol packet format is shown below.
 The fields are transmitted from left to right.
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Code      |  Identifier   |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Data ...
+-+-+-+-+

Simpson [Page 28] RFC 1548 The Point-to-Point Protocol December 1993

 Code
    The Code field is one octet and identifies the kind of LCP packet.
    When a packet is received with an invalid Code field, a Code-
    Reject packet is transmitted.
    Up-to-date values of the LCP Code field are specified in the most
    recent "Assigned Numbers" RFC [2].  This specification concerns
    the following values:
          1       Configure-Request
          2       Configure-Ack
          3       Configure-Nak
          4       Configure-Reject
          5       Terminate-Request
          6       Terminate-Ack
          7       Code-Reject
          8       Protocol-Reject
          9       Echo-Request
          10      Echo-Reply
          11      Discard-Request
  Identifier
    The Identifier field is one octet and aids in matching requests
    and replies.  When a packet is received with an invalid Identifier
    field, the packet is silently discarded.
  Length
    The Length field is two octets and indicates the length of the LCP
    packet including the Code, Identifier, Length and Data fields.
    Octets outside the range of the Length field are treated as
    padding and are ignored on reception.  When a packet is received
    with an invalid Length field, the packet is silently discarded.
  Data
    The Data field is zero or more octets as indicated by the Length
    field.  The format of the Data field is determined by the Code
    field.

5.1 Configure-Request

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

Simpson [Page 29] RFC 1548 The Point-to-Point Protocol December 1993

    Options field filled with any desired changes to the link
    defaults.  Configuration Options SHOULD NOT be included with
    default values.
    Upon reception of a Configure-Request, an appropriate reply MUST
    be transmitted.
 A summary of the Configure-Request packet format is shown below.  The
 fields are transmitted from left to right.
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Code      |  Identifier   |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
 Code
    1 for Configure-Request.
 Identifier
    The Identifier field MUST be changed whenever the content of the
    Options field changes, and whenever a valid reply has been
    received for a previous request.  For retransmissions, the
    Identifier MAY remain unchanged.
 Options
    The options field is variable in length and contains the list of
    zero or more Configuration Options that the sender desires to
    negotiate.  All Configuration Options are always negotiated
    simultaneously.  The format of Configuration Options is further
    described in a later section.

5.2 Configure-Ack

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

Simpson [Page 30] RFC 1548 The Point-to-Point Protocol December 1993

    On reception of a Configure-Ack, the Identifier field MUST match
    that of the last transmitted Configure-Request.  Additionally, the
    Configuration Options in a Configure-Ack MUST exactly match those
    of the last transmitted Configure-Request.  Invalid packets are
    silently discarded.
 A summary of the Configure-Ack packet format is shown below.  The
 fields are transmitted from left to right.
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Code      |  Identifier   |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
 Code
    2 for Configure-Ack.
 Identifier
    The Identifier field is a copy of the Identifier field of the
    Configure-Request which caused this Configure-Ack.
 Options
    The Options field is variable in length and contains the list of
    zero or more Configuration Options that the sender is
    acknowledging.  All Configuration Options are always acknowledged
    simultaneously.

5.3 Configure-Nak

 Description
    If every element of the received Configuration Options is
    recognizable but some values are not acceptable, then the
    implementation MUST transmit a LCP packet with the Code field set
    to 3 (Configure-Nak), the Identifier field copied from the
    received Configure-Request, and the Options field filled with only
    the unacceptable Configuration Options from the Configure-Request.
    All acceptable Configuration Options are filtered out of the
    Configure-Nak, but otherwise the Configuration Options from the
    Configure-Request MUST NOT be reordered.
    Options which have no value fields (boolean options) MUST use the

Simpson [Page 31] RFC 1548 The Point-to-Point Protocol December 1993

    Configure-Reject reply instead.
    Each Configuration Option which is allowed only a single instance
    MUST be modified to a value acceptable to the Configure-Nak
    sender.  The default value MAY be used, when this differs from the
    requested value.
    When a particular type of Configuration Option can be listed more
    than once with different values, the Configure-Nak MUST include a
    list of all values for that option which are acceptable to the
    Configure-Nak sender.  This includes acceptable values that were
    present in the Configure-Request.
    Finally, an implementation may be configured to request the
    negotiation of a specific Configuration Option.  If that option is
    not listed, then that option MAY be appended to the list of Nak'd
    Configuration Options in order to prompt the peer to include that
    option in its next Configure-Request packet.  Any value fields for
    the option MUST indicate values acceptable to the Configure-Nak
    sender.
    On reception of a Configure-Nak, the Identifier field MUST match
    that of the last transmitted Configure-Request.  Invalid packets
    are silently discarded.
    Reception of a valid Configure-Nak indicates that a new
    Configure-Request MAY be sent with the Configuration Options
    modified as specified in the Configure-Nak.  When multiple
    instances of a Configuration Option are present, the peer SHOULD
    select a single value to include in its next Configure-Request
    packet.
    Some Configuration Options have a variable length.  Since the
    Nak'd Option has been modified by the peer, the implementation
    MUST be able to handle an Option length which is different from
    the original Configure-Request.
 A summary of the Configure-Nak packet format is shown below.  The
 fields are transmitted from left to right.
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Code      |  Identifier   |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+

Simpson [Page 32] RFC 1548 The Point-to-Point Protocol December 1993

  Code
    3 for Configure-Nak.
  Identifier
    The Identifier field is a copy of the Identifier field of the
    Configure-Request which caused this Configure-Nak.
  Options
    The Options field is variable in length and contains the list of
    zero or more Configuration Options that the sender is Nak'ing.
    All Configuration Options are always Nak'd simultaneously.

5.4 Configure-Reject

 Description
    If some Configuration Options received in a Configure-Request are
    not recognizable or are not acceptable for negotiation (as
    configured by a network administrator), then the implementation
    MUST transmit a LCP packet with the Code field set to 4
    (Configure-Reject), the Identifier field copied from the received
    Configure-Request, and the Options field filled with only the
    unacceptable Configuration Options from the Configure-Request.
    All recognizable and negotiable Configuration Options are filtered
    out of the Configure-Reject, but otherwise the Configuration
    Options MUST NOT be reordered or modified in any way.
    On reception of a Configure-Reject, the Identifier field MUST
    match that of the last transmitted Configure-Request.
    Additionally, the Configuration Options in a Configure-Reject MUST
    be a proper subset of those in the last transmitted Configure-
    Request.  Invalid packets are silently discarded.
    Reception of a valid Configure-Reject indicates that a new
    Configure-Request SHOULD be sent which does not include any of the
    Configuration Options listed in the Configure-Reject.
 A summary of the Configure-Reject packet format is shown below.  The
 fields are transmitted from left to right.

Simpson [Page 33] RFC 1548 The Point-to-Point Protocol December 1993

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Code      |  Identifier   |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
  Code
    4 for Configure-Reject.
  Identifier
    The Identifier field is a copy of the Identifier field of the
    Configure-Request which caused this Configure-Reject.
  Options
    The Options field is variable in length and contains the list of
    zero or more Configuration Options that the sender is rejecting.
    All Configuration Options are always rejected simultaneously.

5.5 Terminate-Request and Terminate-Ack

 Description
    LCP includes Terminate-Request and Terminate-Ack Codes in order to
    provide a mechanism for closing a connection.
    A LCP implementation wishing to close a connection SHOULD transmit
    a LCP packet with the Code field set to 5 (Terminate-Request), and
    the Data field filled with any desired data.  Terminate-Request
    packets SHOULD continue to be sent until Terminate-Ack is
    received, the lower layer indicates that it has gone down, or a
    sufficiently large number have been transmitted such that the peer
    is down with reasonable certainty.
    Upon reception of a Terminate-Request, a LCP packet MUST be
    transmitted with the Code field set to 6 (Terminate-Ack), the
    Identifier field copied from the Terminate-Request packet, and the
    Data field filled with any desired data.
    Reception of an unelicited Terminate-Ack indicates that the peer
    is in the Closed or Stopped states, or is otherwise in need of
    re-negotiation.
 A summary of the Terminate-Request and Terminate-Ack packet formats

Simpson [Page 34] RFC 1548 The Point-to-Point Protocol December 1993

 is shown below.  The fields are transmitted from left to right.
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Code      |  Identifier   |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Data ...
+-+-+-+-+
  Code
    5 for Terminate-Request;
    6 for Terminate-Ack.
  Identifier
    On transmission, the Identifier field MUST be changed whenever the
    content of the Data field changes, and whenever a valid reply has
    been received for a previous request.  For retransmissions, the
    Identifier MAY remain unchanged.  On reception, the Identifier
    field of the Terminate-Request is copied into the Identifier field
    of the Terminate-Ack packet.
  Data
    The Data field is zero or more octets and contains uninterpreted
    data for use by the sender.  The data may consist of any binary
    value and may be of any length from zero to the peer's established
    MRU minus four.

5.6 Code-Reject

 Description
    Reception of a LCP packet with an unknown Code indicates that one
    of the communicating LCP implementations is faulty or incomplete.
    This error MUST be reported back to the sender of the unknown Code
    by transmitting a LCP packet with the Code field set to 7 (Code-
    Reject), and the inducing packet copied to the Rejected-
    Information field.
    Upon reception of a Code-Reject, the implementation SHOULD report
    the error, since it is unlikely that the situation can be
    rectified automatically.

Simpson [Page 35] RFC 1548 The Point-to-Point Protocol December 1993

 A summary of the Code-Reject packet format is shown below.  The
 fields are transmitted from left to right.
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Code      |  Identifier   |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rejected-Packet ...
+-+-+-+-+-+-+-+-+
  Code
    7 for Code-Reject.
  Identifier
    The Identifier field MUST be changed for each Code-Reject sent.
  Rejected-Information
    The Rejected-Information field contains a copy of the LCP packet
    which is being rejected.  It begins with the Information field,
    and does not include any Data Link Layer headers nor an FCS.  The
    Rejected-Information MUST be truncated to comply with the peer's
    established MRU.

5.7 Protocol-Reject

  Description
    Reception of a PPP packet with an unknown Protocol field indicates
    that the peer is attempting to use a protocol which is
    unsupported.  This usually occurs when the peer attempts to
    configure a new protocol.  If the LCP state machine is in the
    Opened state, then this error MUST be reported back to the peer by
    transmitting a LCP packet with the Code field set to 8 (Protocol-
    Reject), the Rejected-Protocol field set to the received Protocol,
    and the inducing packet copied to the Rejected-Information field.
    Upon reception of a Protocol-Reject, the implementation MUST stop
    sending packets of the indicated protocol at the earliest
    opportunity.
    Protocol-Reject packets can only be sent in the LCP Opened state.
    Protocol-Reject packets received in any state other than the LCP
    Opened state SHOULD be silently discarded.

Simpson [Page 36] RFC 1548 The Point-to-Point Protocol December 1993

 A summary of the Protocol-Reject packet format is shown below.  The
 fields are transmitted from left to right.
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Code      |  Identifier   |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Rejected-Protocol       |      Rejected-Information ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Code
    8 for Protocol-Reject.
  Identifier
    The Identifier field MUST be changed for each Protocol-Reject
    sent.
  Rejected-Protocol
    The Rejected-Protocol field is two octets and contains the PPP
    Protocol field of the packet which is being rejected.
  Rejected-Information
    The Rejected-Information field contains a copy of the packet which
    is being rejected.  It begins with the Information field, and does
    not include any Data Link Layer headers nor an FCS.  The
    Rejected-Information MUST be truncated to comply with the peer's
    established MRU.

5.8 Echo-Request and Echo-Reply

 Description
    LCP includes Echo-Request and Echo-Reply Codes in order to provide
    a Data Link Layer loopback mechanism for use in exercising both
    directions of the link.  This is useful as an aid in debugging,
    link quality determination, performance testing, and for numerous
    other functions.
    An Echo-Request sender transmits a LCP packet with the Code field
    set to 9 (Echo-Request), the Identifier field set, the local
    Magic-Number (if any) inserted, and the Data field filled with any
    desired data, but not exceeding the peer's established MRU minus
    eight.

Simpson [Page 37] RFC 1548 The Point-to-Point Protocol December 1993

    Upon reception of an Echo-Request, a LCP packet MUST be
    transmitted with the Code field set to 10 (Echo-Reply), the
    Identifier field copied from the received Echo-Request, the local
    Magic-Number (if any) inserted, and the Data field copied from the
    Echo-Request, truncating as necessary to avoid exceeding the
    peer's established MRU.
    Echo-Request and Echo-Reply packets may only be sent in the LCP
    Opened state.  Echo-Request and Echo-Reply packets received in any
    state other than the LCP Opened state SHOULD be silently
    discarded.
 A summary of the Echo-Request and Echo-Reply packet formats is shown
 below.  The fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         Magic-Number                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Data ...
 +-+-+-+-+
  Code
    9 for Echo-Request;
    10 for Echo-Reply.
  Identifier
    On transmission, the Identifier field MUST be changed whenever the
    content of the Data field changes, and whenever a valid reply has
    been received for a previous request.  For retransmissions, the
    Identifier MAY remain unchanged.
    On reception, the Identifier field of the Echo-Request is copied
    into the Identifier field of the Echo-Reply packet.
  Magic-Number
    The Magic-Number field is four octets and aids in detecting links
    which are in the looped-back condition.  Until the Magic-Number
    Configuration Option has been successfully negotiated, the Magic-
    Number MUST be transmitted as zero.  See the Magic-Number
    Configuration Option for further explanation.

Simpson [Page 38] RFC 1548 The Point-to-Point Protocol December 1993

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

5.9 Discard-Request

 Description
    LCP includes a Discard-Request Code in order to provide a Data
    Link Layer sink mechanism for use in exercising the local to
    remote direction of the link.  This is useful as an aid in
    debugging, performance testing, and for numerous other functions.
    The sender transmits a LCP packet with the Code field set to 11
    (Discard-Request), the Identifier field set, the local Magic-
    Number (if any) inserted, and the Data field filled with any
    desired data, but not exceeding the peer's established MRU minus
    eight.
    Discard-Request packets may only be sent in the LCP Opened state.
    On reception, the receiver MUST simply throw away any Discard-
    Request that it receives.
 A summary of the Discard-Request packet format is shown below.  The
 fields are transmitted from left to right.
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Code      |  Identifier   |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Magic-Number                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Data ...
+-+-+-+-+
  Code
    11 for Discard-Request.
  Identifier
    The Identifier field MUST be changed for each Discard-Request
    sent.

Simpson [Page 39] RFC 1548 The Point-to-Point Protocol December 1993

  Magic-Number
    The Magic-Number field is four octets and aids in detecting links
    which are in the looped-back condition.  Until the Magic-Number
    Configuration Option has been successfully negotiated, the Magic-
    Number MUST be transmitted as zero.  See the Magic-Number
    Configuration Option for further explanation.
  Data
    The Data field is zero or more octets and contains uninterpreted
    data for use by the sender.  The data may consist of any binary
    value and may be of any length from zero to the peer's established
    MRU minus four.

6. LCP Configuration Options

 LCP Configuration Options allow negotiation of modifications to the
 default characteristics of a point-to-point link.  If a Configuration
 Option is not included in a Configure-Request packet, the default
 value for that Configuration Option is assumed.
 Some Configuration Options MAY be listed more than once.  The effect
 of this is Configuration Option specific, and is specified by each
 such Configuration Option description.  (None of the Configuration
 Options in this specification can be listed more than once.)
 The end of the list of Configuration Options is indicated by the
 length of the LCP packet.
 Unless otherwise specified, all Configuration Options apply in a
 half-duplex fashion; typically, in the receive direction of the link
 from the point of view of the Configure-Request sender.
 A summary of the Configuration Option format is shown below.  The
 fields are transmitted from left to right.
         0                   1
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |     Type      |    Length     |    Data ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Type
    The Type field is one octet and indicates the type of
    Configuration Option.  Up-to-date values of the LCP Option Type
    field are specified in the most recent "Assigned Numbers" RFC [2].

Simpson [Page 40] RFC 1548 The Point-to-Point Protocol December 1993

    This specification concerns the following values:
             1       Maximum-Receive-Unit
             2       Async-Control-Character-Map
             3       Authentication-Protocol
             4       Quality-Protocol
             5       Magic-Number
             6       RESERVED
             7       Protocol-Field-Compression
             8       Address-and-Control-Field-Compression
  Length
    The Length field is one octet and indicates the length of this
    Configuration Option including the Type, Length and Data fields.
    If a negotiable Configuration Option is received in a Configure-
    Request but with an invalid Length, a Configure-Nak SHOULD be
    transmitted which includes the desired Configuration Option with
    an appropriate Length and Data.
  Data
    The Data field is zero or more octets and information specific to
    the Configuration Option.  The format and length of the Data field
    is determined by the Type and Length fields.

6.1 Maximum-Receive-Unit

 Description
    This Configuration Option may be sent to inform the peer that the
    implementation can receive larger packets, or to request that the
    peer send smaller packets.
    The default value is 1500 octets.  If smaller packets are
    requested, an implementation MUST still be able to receive the
    full 1500 octet information field in case link synchronization is
    lost.
  Implementation Note:
    This option is used to indicate an implementation capability.  The
    peer is not required to maximize the use of the capacity.  For
    example, when a MRU is indicated which is 2048 octets, the peer is
    not required to send any packet with 2048 octets.  The peer need
    not Configure-Nak to indicate that it will only send smaller
    packets, since the implementation will always require support for
    at least 1500 octets.

Simpson [Page 41] RFC 1548 The Point-to-Point Protocol December 1993

 A summary of the Maximum-Receive-Unit Configuration Option format is
 shown below.  The fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |      Maximum-Receive-Unit     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Type
    1
  Length
    4
  Maximum-Receive-Unit
    The Maximum-Receive-Unit field is two octets, and specifies the
    maximum number of octets in the Information and Padding fields.
    It does not include the framing, Protocol field, FCS, nor any
    transparency bits or bytes.

6.2 Async-Control-Character-Map

 Description
    This Configuration Option provides a method to negotiate the use
    of control character transparency on asynchronous links.
    For asynchronous links, the default value is 0xffffffff, which
    causes all octets less than 0x20 to be mapped into an appropriate
    two octet sequence.  For most other links, the default value is 0,
    since there is no need for mapping.
    However, it is rarely necessary to map all control characters, and
    often it is unnecessary to map any control characters.  The
    Configuration Option is used to inform the peer which control
    characters MUST remain mapped when the peer sends them.
    The peer MAY still send any other octets in mapped format, if it
    is necessary because of constraints known to the peer.  The peer
    SHOULD Configure-Nak with the logical union of the sets of mapped
    octets, so that when such octets are spuriously introduced they
    can be ignored on receipt.

Simpson [Page 42] RFC 1548 The Point-to-Point Protocol December 1993

 A summary of the Async-Control-Character-Map Configuration Option
 format is shown below.  The fields are transmitted from left to
 right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |  Async-Control-Character-Map
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         ACCM (cont)           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Type
    2
  Length
    6
  Async-Control-Character-Map
    The Async-Control-Character-Map field is four octets and indicates
    the set of control characters to be mapped.  The map is sent most
    significant octet first.
    Each numbered bit corresponds to the octet of the same value.  If
    the bit is cleared to zero, then that octet need not be mapped.
    If the bit is set to one, then that octet MUST remain mapped.  For
    example, if bit 19 is set to zero, then the ASCII control
    character 19 (DC3, Control-S) MAY be sent in the clear.
       Note: The least significant bit of the least significant octet
       (the final octet transmitted) is numbered bit 0, and would map
       to the ASCII control character NUL.

6.3 Authentication-Protocol

 Description
    On some links it may be desirable to require a peer to
    authenticate itself before allowing network-layer protocol packets
    to be exchanged.
    This Configuration Option provides a method to negotiate the use
    of a specific authentication protocol.  By default, authentication
    is not required.

Simpson [Page 43] RFC 1548 The Point-to-Point Protocol December 1993

    An implementation MUST NOT include multiple Authentication-
    Protocol Configuration Options in its Configure-Request packets.
    Instead, it SHOULD attempt to configure the most desirable
    protocol first.  If that protocol is Configure-Nak'd, then the
    implementation SHOULD attempt the next most desirable protocol in
    the next Configure-Request.
    If an implementation sends a Configure-Ack with this Configuration
    Option, then it is agreeing to authenticate with the specified
    protocol.  An implementation receiving a Configure-Ack with this
    Configuration Option SHOULD expect the peer to authenticate with
    the acknowledged protocol.
    There is no requirement that authentication be full duplex or that
    the same protocol be used in both directions.  It is perfectly
    acceptable for different protocols to be used in each direction.
    This will, of course, depend on the specific protocols negotiated.
 A summary of the Authentication-Protocol Configuration Option format
 is shown below.  The fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |     Authentication-Protocol   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Data ...
 +-+-+-+-+
  Type
    3
  Length
    >= 4
  Authentication-Protocol
    The Authentication-Protocol field is two octets and indicates the
    authentication protocol desired.  Values for this field are always
    the same as the PPP Protocol field values for that same
    authentication protocol.
    Up-to-date values of the Authentication-Protocol field are
    specified in the most recent "Assigned Numbers" RFC [2].  Current
    values are assigned as follows:

Simpson [Page 44] RFC 1548 The Point-to-Point Protocol December 1993

      Value (in hex)    Protocol
      c023              Password Authentication Protocol
      c223              Challenge Handshake Authentication Protocol
  Data
    The Data field is zero or more octets and contains additional data
    as determined by the particular protocol.

6.4 Quality-Protocol

  Description
    On some links it may be desirable to determine when, and how
    often, the link is dropping data.  This process is called link
    quality monitoring.
    This Configuration Option provides a method to negotiate the use
    of a specific protocol for link quality monitoring.  By default,
    link quality monitoring is disabled.
    There is no requirement that quality monitoring be full duplex or
    that the same protocol be used in both directions.  It is
    perfectly acceptable for different protocols to be used in each
    direction.  This will, of course, depend on the specific protocols
    negotiated.
 A summary of the Quality-Protocol Configuration Option format is
 shown below.  The fields are transmitted from left to right.
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |        Quality-Protocol       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Data ...
+-+-+-+-+
  Type
    4
  Length
    >= 4

Simpson [Page 45] RFC 1548 The Point-to-Point Protocol December 1993

  Quality-Protocol
    The Quality-Protocol field is two octets and indicates the link
    quality monitoring protocol desired.  Values for this field are
    always the same as the PPP Protocol field values for that same
    monitoring protocol.
    Up-to-date values of the Quality-Protocol field are specified in
    the most recent "Assigned Numbers" RFC [2].  Current values are
    assigned as follows:
             Value (in hex)          Protocol
             c025                    Link Quality Report
  Data
    The Data field is zero or more octets and contains additional data
    as determined by the particular protocol.

6.5 Magic-Number

 Description
    This Configuration Option provides a method to detect looped-back
    links and other Data Link Layer anomalies.  This Configuration
    Option MAY be required by some other Configuration Options such as
    the Quality-Protocol Configuration Option.  By default, the
    Magic-Number is not negotiated, and zero is inserted where a
    Magic-Number might otherwise be used.
    Before this Configuration Option is requested, an implementation
    MUST choose its Magic-Number.  It is recommended that the Magic-
    Number be chosen in the most random manner possible in order to
    guarantee with very high probability that an implementation will
    arrive at a unique number.  A good way to choose a unique random
    number is to start with an unique seed.  Suggested sources of
    uniqueness include machine serial numbers, other network hardware
    addresses, time-of-day clocks, etc.  Particularly good random
    number seeds are precise measurements of the inter-arrival time of
    physical events such as packet reception on other connected
    networks, server response time, or the typing rate of a human
    user.  It is also suggested that as many sources as possible be
    used simultaneously.
    When a Configure-Request is received with a Magic-Number
    Configuration Option, the received Magic-Number is compared with
    the Magic-Number of the last Configure-Request sent to the peer.

Simpson [Page 46] RFC 1548 The Point-to-Point Protocol December 1993

    If the two Magic-Numbers are different, then the link is not
    looped-back, and the Magic-Number SHOULD be acknowledged.  If the
    two Magic-Numbers are equal, then it is possible, but not certain,
    that the link is looped-back and that this Configure-Request is
    actually the one last sent.  To determine this, a Configure-Nak
    MUST be sent specifying a different Magic-Number value.  A new
    Configure-Request SHOULD NOT be sent to the peer until normal
    processing would cause it to be sent (that is, until a Configure-
    Nak is received or the Restart timer runs out).
    Reception of a Configure-Nak with a Magic-Number different from
    that of the last Configure-Nak sent to the peer proves that a link
    is not looped-back, and indicates a unique Magic-Number.  If the
    Magic-Number is equal to the one sent in the last Configure-Nak,
    the possibility of a looped-back link is increased, and a new
    Magic-Number MUST be chosen.  In either case, a new Configure-
    Request SHOULD be sent with the new Magic-Number.
    If the link is indeed looped-back, this sequence (transmit
    Configure-Request, receive Configure-Request, transmit Configure-
    Nak, receive Configure-Nak) will repeat over and over again.  If
    the link is not looped-back, this sequence might occur a few
    times, but it is extremely unlikely to occur repeatedly.  More
    likely, the Magic-Numbers chosen at either end will quickly
    diverge, terminating the sequence.  The following table shows the
    probability of collisions assuming that both ends of the link
    select Magic-Numbers with a perfectly uniform distribution:
             Number of Collisions        Probability
             --------------------   ---------------------
                     1              1/2**32    = 2.3 E-10
                     2              1/2**32**2 = 5.4 E-20
                     3              1/2**32**3 = 1.3 E-29
    Good sources of uniqueness or randomness are required for this
    divergence to occur.  If a good source of uniqueness cannot be
    found, it is recommended that this Configuration Option not be
    enabled; Configure-Requests with the option SHOULD NOT be
    transmitted and any Magic-Number Configuration Options which the
    peer sends SHOULD be either acknowledged or rejected.  In this
    case, loop-backs cannot be reliably detected by the
    implementation, although they may still be detectable by the peer.
    If an implementation does transmit a Configure-Request with a
    Magic-Number Configuration Option, then it MUST NOT respond with a
    Configure-Reject if it receives a Configure-Request with a Magic-
    Number Configuration Option.  That is, if an implementation
    desires to use Magic Numbers, then it MUST also allow its peer to

Simpson [Page 47] RFC 1548 The Point-to-Point Protocol December 1993

    do so.  If an implementation does receive a Configure-Reject in
    response to a Configure-Request, it can only mean that the link is
    not looped-back, and that its peer will not be using Magic-
    Numbers.  In this case, an implementation SHOULD act as if the
    negotiation had been successful (as if it had instead received a
    Configure-Ack).
    The Magic-Number also may be used to detect looped-back links
    during normal operation as well as during Configuration Option
    negotiation.  All LCP Echo-Request, Echo-Reply, and Discard-
    Request packets have a Magic-Number field.  If Magic-Number has
    been successfully negotiated, an implementation MUST transmit
    these packets with the Magic-Number field set to its negotiated
    Magic-Number.
    The Magic-Number field of these packets SHOULD be inspected on
    reception.  All received Magic-Number fields MUST be equal to
    either zero or the peer's unique Magic-Number, depending on
    whether or not the peer negotiated a Magic-Number.  Reception of a
    Magic-Number field equal to the negotiated local Magic-Number
    indicates a looped-back link.  Reception of a Magic- Number other
    than the negotiated local Magic-Number or the peer's negotiated
    Magic-Number, or zero if the peer didn't negotiate one, indicates
    a link which has been (mis)configured for communications with a
    different peer.
    Procedures for recovery from either case are unspecified and may
    vary from implementation to implementation.  A somewhat
    pessimistic procedure is to assume a LCP Down event.  A further
    Open event will begin the process of re-establishing the link,
    which can't complete until the loop-back condition is terminated
    and Magic-Numbers are successfully negotiated.  A more optimistic
    procedure (in the case of a loop-back) is to begin transmitting
    LCP Echo-Request packets until an appropriate Echo-Reply is
    received, indicating a termination of the loop-back condition.
 A summary of the Magic-Number Configuration Option format is shown
 below.  The fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |          Magic-Number
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Magic-Number (cont)       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Simpson [Page 48] RFC 1548 The Point-to-Point Protocol December 1993

  Type
    5
  Length
    6
  Magic-Number
    The Magic-Number field is four octets and indicates a number which
    is very likely to be unique to one end of the link.  A Magic-
    Number of zero is illegal and MUST always be Nak'd, if it is not
    Rejected outright.

6.6 Protocol-Field-Compression

 Description
    This Configuration Option provides a method to negotiate the
    compression of the PPP Protocol field.  By default, all
    implementations MUST transmit packets with two octet PPP Protocol
    fields.
    PPP Protocol field numbers are chosen such that some values may be
    compressed into a single octet form which is clearly
    distinguishable from the two octet form.  This Configuration
    Option is sent to inform the peer that the implementation can
    receive such single octet Protocol fields.
    As previously mentioned, the Protocol field uses an extension
    mechanism consistent with the ISO 3309 extension mechanism for the
    Address field; the Least Significant Bit (LSB) of each octet is
    used to indicate extension of the Protocol field.  A binary "0" as
    the LSB indicates that the Protocol field continues with the
    following octet.  The presence of a binary "1" as the LSB marks
    the last octet of the Protocol field.  Notice that any number of
    "0" octets may be prepended to the field, and will still indicate
    the same value (consider the two binary representations for 3,
    00000011 and 00000000 00000011).
    When using low speed links, it is desirable to conserve bandwidth
    by sending as little redundant data as possible.  The Protocol-
    Field-Compression Configuration Option allows a trade-off between
    implementation simplicity and bandwidth efficiency.  If
    successfully negotiated, the ISO 3309 extension mechanism may be
    used to compress the Protocol field to one octet instead of two.
    The large majority of packets are compressible since data

Simpson [Page 49] RFC 1548 The Point-to-Point Protocol December 1993

    protocols are typically assigned with Protocol field values less
    than 256.
    Compressed Protocol fields MUST NOT be transmitted unless this
    Configuration Option has been negotiated.  When negotiated, PPP
    implementations MUST accept PPP packets with either double-octet
    or single-octet Protocol fields, and MUST NOT distinguish between
    them.
    The Protocol field is never compressed when sending any LCP
    packet.  This rule guarantees unambiguous recognition of LCP
    packets.
    When a Protocol field is compressed, the Data Link Layer FCS field
    is calculated on the compressed frame, not the original
    uncompressed frame.
 A summary of the Protocol-Field-Compression Configuration Option
 format is shown below.  The fields are transmitted from left to
 right.
              0                   1
              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |     Type      |    Length     |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Type
    7
  Length
    2

6.7 Address-and-Control-Field-Compression

 Description
    This Configuration Option provides a method to negotiate the
    compression of the Data Link Layer Address and Control fields.  By
    default, all implementations MUST transmit frames with Address and
    Control fields appropriate to the link framing.
    Since these fields usually have constant values for point-to-point
    links, they are easily compressed.  This Configuration Option is
    sent to inform the peer that the implementation can receive
    compressed Address and Control fields.

Simpson [Page 50] RFC 1548 The Point-to-Point Protocol December 1993

    If a compressed frame is received when Address-and-Control-Field-
    Compression has not been negotiated, the implementation MAY
    silently discard the frame.
    The Address and Control fields MUST NOT be compressed when sending
    any LCP packet.  This rule guarantees unambiguous recognition of
    LCP packets.
    When the Address and Control fields are compressed, the Data Link
    Layer FCS field is calculated on the compressed frame, not the
    original uncompressed frame.
 A summary of the Address-and-Control-Field-Compression configuration
 option format is shown below.  The fields are transmitted from left
 to right.
              0                   1
              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |     Type      |    Length     |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Type
    8
  Length
    2

A. LCP Recommended Options

 The following Configurations Options are recommended:
    SYNC LINES
    Magic Number Link Quality Monitoring No Address and Control Field
    Compression No Protocol Field Compression
    ASYNC LINES
    Async Control Character Map Magic Number Address and Control Field
    Compression Protocol Field Compression

Security Considerations

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

Simpson [Page 51] RFC 1548 The Point-to-Point Protocol December 1993

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

References

  [1] Perkins, D., "Requirements for an Internet Standard
      Point-to-Point Protocol", RFC 1547, December 1993.
  [2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
      USC/Information Sciences Institute, July 1992.

Acknowledgments

 Much of the text in this document is taken from the WG Requirements,
 and RFCs 1171 & 1172, by Drew Perkins of Carnegie Mellon University,
 and by Russ Hobby of the University of California at Davis.
 Many people spent significant time helping to develop the Point-to-
 Point Protocol.  The complete list of people is too numerous to list,
 but the following people deserve special thanks: Rick Adams (UUNET),
 Ken Adelman (TGV), Fred Baker (ACC), Mike Ballard (Telebit), Craig
 Fox (Network Systems), Karl Fox (Morning Star Technologies), Phill
 Gross (AN&S), former WG chair Russ Hobby (UC Davis), David Kaufman
 (Proteon), former WG chair Steve Knowles (FTP Software), former WG
 chair Brian Lloyd (L&A), John LoVerso (Xylogics), Bill Melohn (Sun
 Microsystems), Mike Patton (MIT), former WG chair Drew Perkins
 (Fore), Greg Satz (cisco systems), John Shriver (Proteon), Vernon
 Schryver (Silicon Graphics), and Asher Waldfogel (Wellfleet).
 The "Day in the Life" example was instigated by Kory Hamzeh (Avatar).
 In this version, improvements in wording were also provided by Scott
 Ginsburg, Mark Moraes, and Timon Sloan, as they worked on
 implementations.
 Special thanks to Morning Star Technologies for providing computing
 resources and network access support for writing this specification.

Chair's Address

 The working group can be contacted via the current chair:
    Fred Baker
    Advanced Computer Communications
    315 Bollay Drive
    Santa Barbara, California, 93111
    EMail: fbaker@acc.com

Simpson [Page 52] RFC 1548 The Point-to-Point Protocol December 1993

Editor's Address

 Questions about this memo can also be directed to:
    William Allen Simpson
    Daydreamer
    Computer Systems Consulting Services
    1384 Fontaine
    Madison Heights, Michigan  48071
    EMail: Bill.Simpson@um.cc.umich.edu

Simpson [Page 53]

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