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rfc:rfc1661

Network Working Group W. Simpson, Editor Request for Comments: 1661 Daydreamer STD: 51 July 1994 Obsoletes: 1548 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.

Table of Contents

   1.     Introduction ..........................................    1
      1.1       Specification of Requirements ...................    2
      1.2       Terminology .....................................    3
   2.     PPP Encapsulation .....................................    4

Simpson [Page i] RFC 1661 Point-to-Point Protocol July 1994

   3.     PPP Link Operation ....................................    6
      3.1       Overview ........................................    6
      3.2       Phase Diagram ...................................    6
      3.3       Link Dead (physical-layer not ready) ............    7
      3.4       Link Establishment Phase ........................    7
      3.5       Authentication Phase ............................    8
      3.6       Network-Layer Protocol Phase ....................    8
      3.7       Link Termination Phase ..........................    9
   4.     The Option Negotiation Automaton ......................   11
      4.1       State Transition Table ..........................   12
      4.2       States ..........................................   14
      4.3       Events ..........................................   16
      4.4       Actions .........................................   21
      4.5       Loop Avoidance ..................................   23
      4.6       Counters and Timers .............................   24
   5.     LCP Packet Formats ....................................   26
      5.1       Configure-Request ...............................   28
      5.2       Configure-Ack ...................................   29
      5.3       Configure-Nak ...................................   30
      5.4       Configure-Reject ................................   31
      5.5       Terminate-Request and Terminate-Ack .............   33
      5.6       Code-Reject .....................................   34
      5.7       Protocol-Reject .................................   35
      5.8       Echo-Request and Echo-Reply .....................   36
      5.9       Discard-Request .................................   37
   6.     LCP Configuration Options .............................   39
      6.1       Maximum-Receive-Unit (MRU) ......................   41
      6.2       Authentication-Protocol .........................   42
      6.3       Quality-Protocol ................................   43
      6.4       Magic-Number ....................................   45
      6.5       Protocol-Field-Compression (PFC) ................   48
      6.6       Address-and-Control-Field-Compression (ACFC)
   SECURITY CONSIDERATIONS ......................................   51
   REFERENCES ...................................................   51
   ACKNOWLEDGEMENTS .............................................   51
   CHAIR'S ADDRESS ..............................................   52
   EDITOR'S ADDRESS .............................................   52

Simpson [Page ii] RFC 1661 Point-to-Point Protocol July 1994

1. Introduction

 The Point-to-Point Protocol is designed for simple links which
 transport packets between two peers.  These links provide full-duplex
 simultaneous bi-directional operation, and are assumed to deliver
 packets in order.  It is intended that PPP provide a common solution
 for easy connection of a wide variety of hosts, bridges and routers
 [1].
 Encapsulation
    The PPP encapsulation provides for multiplexing of different
    network-layer protocols simultaneously over the same link.  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 within the default HDLC-like 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, detect a looped-back link and other common
    misconfiguration errors, and terminate the link.  Other optional
    facilities provided are authentication of the identity of its peer
    on the link, and determination when a link is functioning properly
    and when it is failing.
 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

Simpson [Page 1] RFC 1661 Point-to-Point Protocol July 1994

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

1.1. Specification of Requirements

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

Simpson [Page 2] RFC 1661 Point-to-Point Protocol July 1994

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

Simpson [Page 3] RFC 1661 Point-to-Point Protocol July 1994

2. PPP Encapsulation

 The PPP encapsulation is used to disambiguate multiprotocol
 datagrams.  This encapsulation requires framing to indicate the
 beginning and end of the encapsulation.  Methods of providing framing
 are specified in companion documents.
 A summary of the PPP encapsulation is shown below.  The fields are
 transmitted from left to right.
         +----------+-------------+---------+
         | Protocol | Information | Padding |
         | 8/16 bits|      *      |    *    |
         +----------+-------------+---------+
 Protocol Field
    The Protocol field is one or 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).

Simpson [Page 4] RFC 1661 Point-to-Point Protocol July 1994

    Up-to-date values of the Protocol field are specified in the most
    recent "Assigned Numbers" RFC [2].  This specification reserves
    the following values:
    Value (in hex)  Protocol Name
    0001            Padding Protocol
    0003 to 001f    reserved (transparency inefficient)
    007d            reserved (Control Escape)
    00cf            reserved (PPP NLPID)
    00ff            reserved (compression inefficient)
    8001 to 801f    unused
    807d            unused
    80cf            unused
    80ff            unused
    c021            Link Control Protocol
    c023            Password Authentication Protocol
    c025            Link Quality Report
    c223            Challenge Handshake Authentication Protocol
    Developers of new protocols MUST obtain a number from the Internet
    Assigned Numbers Authority (IANA), at IANA@isi.edu.
 Information Field
    The Information field is zero or more octets.  The Information
    field contains the datagram for the protocol specified in the
    Protocol field.
    The maximum length for the Information field, including Padding,
    but not including the Protocol field, 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.

Simpson [Page 5] RFC 1661 Point-to-Point Protocol July 1994

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 which are specified in the following simplified state diagram:
 +------+        +-----------+           +--------------+
 |      | UP     |           | OPENED    |              | SUCCESS/NONE
 | Dead |------->| Establish |---------->| Authenticate |--+
 |      |        |           |           |              |  |
 +------+        +-----------+           +--------------+  |
    ^               |                        |             |
    |          FAIL |                   FAIL |             |
    +<--------------+             +----------+             |
    |                             |                        |
    |            +-----------+    |           +---------+  |
    |       DOWN |           |    |   CLOSING |         |  |
    +------------| Terminate |<---+<----------| Network |<-+
                 |           |                |         |
                 +-----------+                +---------+
 Not all transitions are specified in this diagram.  The following
 semantics MUST be followed.

Simpson [Page 6] RFC 1661 Point-to-Point Protocol July 1994

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 later) will be in the
 Initial or Starting states.  The transition to the Link Establishment
 phase will signal an Up event to the LCP 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 link,
    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 later) has been both sent and received.
 All Configuration Options are assumed to be at default values unless
 altered by the configuration exchange.  See the chapter 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.
 The receipt of the LCP Configure-Request causes a return to the Link
 Establishment phase from the Network-Layer Protocol phase or
 Authentication phase.

Simpson [Page 7] RFC 1661 Point-to-Point Protocol July 1994

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 request the use of that authentication
 protocol during Link Establishment phase.
 Authentication SHOULD take place as soon as possible after link
 establishment.  However, link quality determination MAY occur
 concurrently.  An implementation MUST NOT allow the exchange of link
 quality determination packets to delay authentication indefinitely.
 Advancement from the Authentication phase to the Network-Layer
 Protocol phase MUST NOT occur until authentication has completed.  If
 authentication fails, the authenticator SHOULD proceed instead to the
 Link Termination phase.
 Only Link Control Protocol, authentication protocol, and link quality
 monitoring packets are allowed during this phase.  All other packets
 received during this phase MUST be silently discarded.
 Implementation Notes:
    An implementation SHOULD NOT fail authentication simply due to
    timeout or lack of response.  The authentication SHOULD allow some
    method of retransmission, and proceed to the Link Termination
    phase only after a number of authentication attempts has been
    exceeded.
    The implementation responsible for commencing Link Termination
    phase is the implementation which has refused authentication to
    its peer.

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.

Simpson [Page 8] RFC 1661 Point-to-Point Protocol July 1994

 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 supported
 network-layer protocol packets received when the corresponding NCP is
 not in the Opened state MUST be silently discarded.
 Implementation Note:
    While LCP is in the Opened state, any protocol packet which is
    unsupported by the implementation MUST be returned in a Protocol-
    Reject (described later).  Only protocols which are supported are
    silently discarded.
 During this phase, link traffic consists of any possible combination
 of LCP, NCP, and network-layer protocol packets.

3.7. Link Termination Phase

 PPP can terminate the link at any time.  This might happen because of
 the loss of carrier, authentication failure, link quality failure,
 the expiration of an idle-period timer, or the administrative closing
 of the link.
 LCP is used to close the link through an exchange of Terminate
 packets.  When the link is closing, PPP informs the network-layer
 protocols so that they may take appropriate action.
 After the exchange of Terminate packets, the implementation SHOULD
 signal the physical-layer to disconnect in order to enforce the
 termination of the link, particularly in the case of an
 authentication failure.  The sender of the Terminate-Request SHOULD
 disconnect after receiving a Terminate-Ack, or after the Restart
 counter expires.  The receiver of a Terminate-Request SHOULD wait for
 the peer to disconnect, and MUST NOT disconnect until at least one
 Restart time has passed after sending a Terminate-Ack.  PPP SHOULD
 proceed to the Link Dead phase.
 Any non-LCP packets received during this phase MUST be silently
 discarded.

Simpson [Page 9] RFC 1661 Point-to-Point Protocol July 1994

 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.

Simpson [Page 10] RFC 1661 Point-to-Point Protocol July 1994

4. The Option Negotiation Automaton

 The finite-state automaton is defined by events, actions and state
 transitions.  Events include reception of external commands such as
 Open and Close, expiration of the Restart timer, and reception of
 packets from a peer.  Actions include the starting of the Restart
 timer and transmission of packets to the peer.
 Some types of packets -- Configure-Naks and Configure-Rejects, or
 Code-Rejects and Protocol-Rejects, or Echo-Requests, Echo-Replies and
 Discard-Requests -- are not differentiated in the automaton
 descriptions.  As will be described later, these packets do indeed
 serve different functions.  However, they always cause the same
 transitions.
 Events                                   Actions
 Up   = lower layer is Up                 tlu = This-Layer-Up
 Down = lower layer is Down               tld = This-Layer-Down
 Open = administrative Open               tls = This-Layer-Started
 Close= administrative Close              tlf = This-Layer-Finished
 TO+  = Timeout with counter > 0          irc = Initialize-Restart-Count
 TO-  = Timeout with counter expired      zrc = Zero-Restart-Count
 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

Simpson [Page 11] RFC 1661 Point-to-Point Protocol July 1994

4.1. 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 tlf/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 12] RFC 1661 Point-to-Point Protocol July 1994

    | 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-Count 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.
 The events and actions are defined according to a message passing
 architecture, rather than a signalling architecture.  If an action is
 desired to control specific signals (such as DTR), additional actions
 are likely to be required.
 [p]   Passive option; see Stopped state discussion.
 [r]   Restart option; see Open event discussion.
 [x]   Crossed connection; see RCA event discussion.

Simpson [Page 13] RFC 1661 Point-to-Point Protocol July 1994

4.2. States

 Following is a more detailed description of each automaton state.
 Initial
    In the Initial state, the lower layer is unavailable (Down), and
    no Open has occurred.  The Restart timer is not running in the
    Initial state.
 Starting
    The Starting state is the Open counterpart to the Initial state.
    An administrative Open has been initiated, but the lower layer is
    still unavailable (Down).  The Restart timer is not running in the
    Starting state.
    When the lower layer becomes available (Up), a Configure-Request
    is sent.
 Closed
    In the Closed state, the link is available (Up), but no Open has
    occurred.  The Restart timer is not running in the Closed state.
    Upon reception of Configure-Request packets, a Terminate-Ack is
    sent.  Terminate-Acks are silently discarded to avoid creating a
    loop.
 Stopped
    The Stopped state is the Open counterpart to the Closed state.  It
    is entered when the automaton is waiting for a Down event after
    the This-Layer-Finished action, or after sending a Terminate-Ack.
    The Restart timer is not running in the Stopped state.
    Upon reception of Configure-Request packets, an appropriate
    response is sent.  Upon reception of other packets, a Terminate-
    Ack is sent.  Terminate-Acks are silently discarded to avoid
    creating a loop.
    Rationale:
       The Stopped state is a junction state for link termination,
       link configuration failure, and other automaton failure modes.
       These potentially separate states have been combined.
       There is a race condition between the Down event response (from

Simpson [Page 14] RFC 1661 Point-to-Point Protocol July 1994

       the This-Layer-Finished action) and the Receive-Configure-
       Request event.  When a Configure-Request arrives before the
       Down event, the Down event will supercede by returning the
       automaton to the Starting state.  This prevents attack by
       repetition.
    Implementation Option:
       After the peer fails to respond to Configure-Requests, an
       implementation MAY wait passively for the peer to send
       Configure-Requests.  In this case, the This-Layer-Finished
       action is not used for the TO- event in states Req-Sent, Ack-
       Rcvd and Ack-Sent.
       This option is useful for dedicated circuits, or circuits which
       have no status signals available, but SHOULD NOT be used for
       switched circuits.
 Closing
    In the Closing state, an attempt is made to terminate the
    connection.  A Terminate-Request has been sent and the Restart
    timer is running, but a Terminate-Ack has not yet been received.
    Upon reception of a Terminate-Ack, the Closed state is entered.
    Upon the expiration of the Restart timer, a new Terminate-Request
    is transmitted, and the Restart timer is restarted.  After the
    Restart timer has expired Max-Terminate times, the Closed state is
    entered.
 Stopping
    The Stopping state is the Open counterpart to the Closing state.
    A Terminate-Request has been sent and the Restart timer is
    running, but a Terminate-Ack has not yet been received.
    Rationale:
       The Stopping state provides a well defined opportunity to
       terminate a link before allowing new traffic.  After the link
       has terminated, a new configuration may occur via the Stopped
       or Starting states.
 Request-Sent
    In the Request-Sent state an attempt is made to configure the
    connection.  A Configure-Request has been sent and the Restart
    timer is running, but a Configure-Ack has not yet been received

Simpson [Page 15] RFC 1661 Point-to-Point Protocol July 1994

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

4.3. Events

 Transitions and actions in the automaton are caused by events.
 Up
    This 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
    This event occurs when a lower layer indicates that it is no

Simpson [Page 16] RFC 1661 Point-to-Point Protocol July 1994

    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
    This event indicates that the link is administratively available
    for traffic; that is, the network administrator (human or program)
    has indicated that the link is allowed to be Opened.  When this
    event occurs, and the link is not in the Opened state, the
    automaton attempts to send configuration packets to the peer.
    If the automaton is not able to begin configuration (the lower
    layer is Down, or a previous Close event has not completed), the
    establishment of the link is automatically delayed.
    When a Terminate-Request is received, or other events occur which
    cause the link to become unavailable, the automaton will progress
    to a state where the link is ready to re-open.  No additional
    administrative intervention is necessary.
    Implementation Option:
       Experience has shown that users will execute an additional Open
       command when they want to renegotiate the link.  This might
       indicate that new values are to be negotiated.
       Since this is not the meaning of the Open event, it is
       suggested that when an Open user command is executed in the
       Opened, Closing, Stopping, or Stopped states, the
       implementation issue a Down event, immediately followed by an
       Up event.  Care must be taken that an intervening Down event
       cannot occur from another source.
       The Down followed by an Up will cause an orderly renegotiation
       of the link, by progressing through the Starting to the
       Request-Sent state.  This will cause the renegotiation of the
       link, without any harmful side effects.
 Close
    This event indicates that the link is not available for traffic;

Simpson [Page 17] RFC 1661 Point-to-Point Protocol July 1994

    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.  Care
       must be taken that an intervening Close event cannot occur from
       another source.
       The Close followed by an Open will cause an orderly termination
       of the link, by progressing through the Closing to the Stopping
       state, and the This-Layer-Finished action can disconnect the
       link.  The automaton waits in the Stopped or Starting states
       for the next connection attempt.
 Timeout (TO+,TO-)
    This event indicates the expiration of the Restart timer.  The
    Restart timer is used to time responses to Configure-Request and
    Terminate-Request packets.
    The TO+ event indicates that the Restart counter continues to be
    greater than zero, which triggers the corresponding Configure-
    Request or Terminate-Request packet to be retransmitted.
    The TO- event indicates that the Restart counter is not greater
    than zero, and no more packets need to be retransmitted.
 Receive-Configure-Request (RCR+,RCR-)
    This event occurs when a Configure-Request packet is received from
    the peer.  The Configure-Request packet indicates the desire to
    open a connection and may specify Configuration Options.  The
    Configure-Request packet is more fully described in a later
    section.
    The RCR+ event indicates that the Configure-Request was
    acceptable, and triggers the transmission of a corresponding
    Configure-Ack.
    The RCR- event indicates that the Configure-Request was

Simpson [Page 18] RFC 1661 Point-to-Point Protocol July 1994

    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)
    This event occurs when a valid Configure-Ack packet is received
    from the peer.  The Configure-Ack packet is a positive response to
    a Configure-Request packet.  An out of sequence or otherwise
    invalid packet is silently discarded.
    Implementation Note:
       Since the correct packet has already been received before
       reaching the Ack-Rcvd or Opened states, it is extremely
       unlikely that another such packet will arrive.  As specified,
       all invalid Ack/Nak/Rej packets are silently discarded, and do
       not affect the transitions of the automaton.
       However, it is not impossible that a correctly formed packet
       will arrive through a coincidentally-timed cross-connection.
       It is more likely to be the result of an implementation error.
       At the very least, this occurance SHOULD be logged.
 Receive-Configure-Nak/Rej (RCN)
    This event occurs when a valid Configure-Nak or Configure-Reject
    packet is received from the peer.  The Configure-Nak and
    Configure-Reject packets are negative responses to a Configure-
    Request packet.  An out of sequence or otherwise invalid packet is
    silently discarded.
    Implementation Note:
       Although the Configure-Nak and Configure-Reject cause the same
       state transition in the automaton, these packets have
       significantly different effects on the Configuration Options
       sent in the resulting Configure-Request packet.
 Receive-Terminate-Request (RTR)
    This event occurs when a Terminate-Request packet is received.
    The Terminate-Request packet indicates the desire of the peer to

Simpson [Page 19] RFC 1661 Point-to-Point Protocol July 1994

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

Simpson [Page 20] RFC 1661 Point-to-Point Protocol July 1994

4.4. Actions

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

Simpson [Page 21] RFC 1661 Point-to-Point Protocol July 1994

    Typically, this action MAY be used by the LCP to advance to the
    Link Dead phase, or MAY be used by a NCP to indicate to the LCP
    that the link may terminate when there are no other NCPs open.
    This results of this action are highly implementation dependent.
 Initialize-Restart-Count (irc)
    This action sets the Restart counter to the appropriate value
    (Max-Terminate or Max-Configure).  The counter is decremented for
    each transmission, including the first.
    Implementation Note:
       In addition to setting the Restart counter, the implementation
       MUST set the timeout period to the initial value when Restart
       timer backoff is used.
 Zero-Restart-Count (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)
    A Configure-Request packet is transmitted.  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)
    A Configure-Ack packet is transmitted.  This acknowledges the
    reception of a Configure-Request packet with an acceptable set of
    Configuration Options.
 Send-Configure-Nak (scn)
    A Configure-Nak or Configure-Reject packet is transmitted, as
    appropriate.  This negative response reports the reception of a

Simpson [Page 22] RFC 1661 Point-to-Point Protocol July 1994

    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 chapter on LCP Packet Formats.
 Send-Terminate-Request (str)
    A Terminate-Request packet is transmitted.  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)
    A Terminate-Ack packet is transmitted.  This acknowledges the
    reception of a Terminate-Request packet or otherwise serves to
    synchronize the automatons.
 Send-Code-Reject (scj)
    A Code-Reject packet is transmitted.  This indicates the reception
    of an unknown type of packet.
 Send-Echo-Reply (ser)
    An Echo-Reply packet is transmitted.  This acknowledges the
    reception of an Echo-Request packet.

4.5. Loop Avoidance

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

Simpson [Page 23] RFC 1661 Point-to-Point Protocol July 1994

4.6. Counters and Timers

 Restart Timer
    There is one special timer used by the automaton.  The Restart
    timer is used to time transmissions of Configure-Request and
    Terminate-Request packets.  Expiration of the Restart timer causes
    a Timeout event, and retransmission of the corresponding
    Configure-Request or Terminate-Request packet.  The Restart timer
    MUST be configurable, but SHOULD default to three (3) seconds.
    Implementation Note:
       The Restart timer SHOULD be based on the speed of the link.
       The default value is designed for low speed (2,400 to 9,600
       bps), high switching latency links (typical telephone lines).
       Higher speed links, or links with low switching latency, SHOULD
       have correspondingly faster retransmission times.
       Instead of a constant value, the Restart timer MAY begin at an
       initial small value and increase to the configured final value.
       Each successive value less than the final value SHOULD be at
       least twice the previous value.  The initial value SHOULD be
       large enough to account for the size of the packets, twice the
       round trip time for transmission at the link speed, and at
       least an additional 100 milliseconds to allow the peer to
       process the packets before responding.  Some circuits add
       another 200 milliseconds of satellite delay.  Round trip times
       for modems operating at 14,400 bps have been measured in the
       range of 160 to more than 600 milliseconds.
 Max-Terminate
    There is one required restart counter for Terminate-Requests.
    Max-Terminate indicates the number of Terminate-Request packets
    sent without receiving a Terminate-Ack before assuming that the
    peer is unable to respond.  Max-Terminate MUST be configurable,
    but SHOULD default to two (2) transmissions.
 Max-Configure
    A similar counter is recommended for Configure-Requests.  Max-
    Configure indicates the number of Configure-Request packets sent
    without receiving a valid Configure-Ack, Configure-Nak or
    Configure-Reject before assuming that the peer is unable to
    respond.  Max-Configure MUST be configurable, but SHOULD default
    to ten (10) transmissions.

Simpson [Page 24] RFC 1661 Point-to-Point Protocol July 1994

 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 for peer requested
    options are converted to Configure-Reject packets, and locally
    desired options are no longer appended.  Max-Failure MUST be
    configurable, but SHOULD default to five (5) transmissions.

Simpson [Page 25] RFC 1661 Point-to-Point Protocol July 1994

5. LCP Packet Formats

 There are three classes of LCP packets:
    1. Link Configuration packets used to establish and configure a
       link (Configure-Request, Configure-Ack, Configure-Nak and
       Configure-Reject).
    2. Link Termination packets used to terminate a link (Terminate-
       Request and Terminate-Ack).
    3. Link Maintenance packets used to manage and debug a link
       (Code-Reject, Protocol-Reject, Echo-Request, Echo-Reply, and
       Discard-Request).
 In the interest of simplicity, there is no version field in the LCP
 packet.  A correctly functioning LCP implementation will always
 respond to unknown Protocols and Codes with an easily recognizable
 LCP 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
 negotiated.  In particular, each Configuration Option specifies a
 default value.  This ensures that such LCP packets are always
 recognizable, even when one end of the link mistakenly believes the
 link to be open.
 Exactly one LCP packet is encapsulated in the PPP Information field,
 where the PPP Protocol field indicates type hex c021 (Link Control
 Protocol).
 A summary of the Link Control Protocol packet format is shown below.
 The fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Data ...
 +-+-+-+-+
 Code
    The Code field is one octet, and identifies the kind of LCP

Simpson [Page 26] RFC 1661 Point-to-Point Protocol July 1994

    packet.  When a packet is received with an unknown 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 document 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 without affecting the
    automaton.
 Length
    The Length field is two octets, and indicates the length of the
    LCP packet, including the Code, Identifier, Length and Data
    fields.  The Length MUST NOT exceed the MRU of the link.
    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
    without affecting the automaton.
 Data
    The Data field is zero or more octets, as indicated by the Length
    field.  The format of the Data field is determined by the Code
    field.

Simpson [Page 27] RFC 1661 Point-to-Point Protocol July 1994

5.1. Configure-Request

 Description
    An implementation wishing to open a connection MUST transmit a
    Configure-Request.  The Options field is 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 contents 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 chapter.

Simpson [Page 28] RFC 1661 Point-to-Point Protocol July 1994

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 Configure-Ack.  The acknowledged
    Configuration Options MUST NOT be reordered or modified in any
    way.
    On reception of a Configure-Ack, the Identifier field MUST match
    that of the last transmitted Configure-Request.  Additionally, the
    Configuration Options in a Configure-Ack MUST exactly match those
    of the last transmitted Configure-Request.  Invalid packets are
    silently discarded.
 A summary of the Configure-Ack packet format is shown below.  The
 fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Options ...
 +-+-+-+-+
 Code
    2 for Configure-Ack.
 Identifier
    The Identifier field is a copy of the Identifier field of the
    Configure-Request which caused this Configure-Ack.
 Options
    The Options field is variable in length, and contains the list of
    zero or more Configuration Options that the sender is
    acknowledging.  All Configuration Options are always acknowledged
    simultaneously.

Simpson [Page 29] RFC 1661 Point-to-Point Protocol July 1994

5.3. Configure-Nak

 Description
    If every instance of the received Configuration Options is
    recognizable, but some values are not acceptable, then the
    implementation MUST transmit a Configure-Nak.  The Options field
    is filled with only the unacceptable Configuration Options from
    the Configure-Request.  All acceptable Configuration Options are
    filtered out of the Configure-Nak, but otherwise the Configuration
    Options from the Configure-Request MUST NOT be reordered.
    Options which have no value fields (boolean options) MUST use the
    Configure-Reject reply instead.
    Each Configuration Option which is allowed only a single instance
    MUST be modified to a value acceptable to the Configure-Nak
    sender.  The default value MAY be used, when this differs from the
    requested value.
    When a particular type of Configuration Option can be listed more
    than once with different values, the Configure-Nak MUST include a
    list of all values for that option which are acceptable to the
    Configure-Nak sender.  This includes acceptable values that were
    present in the Configure-Request.
    Finally, an implementation may be configured to request the
    negotiation of a specific Configuration Option.  If that option is
    not listed, then that option MAY be appended to the list of Nak'd
    Configuration Options, in order to prompt the peer to include that
    option in its next Configure-Request packet.  Any value fields for
    the option MUST indicate values acceptable to the Configure-Nak
    sender.
    On reception of a Configure-Nak, the Identifier field MUST match
    that of the last transmitted Configure-Request.  Invalid packets
    are silently discarded.
    Reception of a valid Configure-Nak indicates that when a new
    Configure-Request is sent, the Configuration Options MAY be
    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

Simpson [Page 30] RFC 1661 Point-to-Point Protocol July 1994

    the original Configure-Request.
 A summary of the Configure-Nak packet format is shown below.  The
 fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Options ...
 +-+-+-+-+
 Code
    3 for Configure-Nak.
 Identifier
    The Identifier field is a copy of the Identifier field of the
    Configure-Request which caused this Configure-Nak.
 Options
    The Options field is variable in length, and contains the list of
    zero or more Configuration Options that the sender is Nak'ing.
    All Configuration Options are always Nak'd simultaneously.

5.4. Configure-Reject

 Description
    If some Configuration Options received in a Configure-Request are
    not recognizable or are not acceptable for negotiation (as
    configured by a network administrator), then the implementation
    MUST transmit a Configure-Reject.  The Options field is 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

Simpson [Page 31] RFC 1661 Point-to-Point Protocol July 1994

    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 when a new
    Configure-Request is sent, it MUST NOT include any of the
    Configuration Options listed in the Configure-Reject.
 A summary of the Configure-Reject packet format is shown below.  The
 fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Options ...
 +-+-+-+-+
 Code
    4 for Configure-Reject.
 Identifier
    The Identifier field is a copy of the Identifier field of the
    Configure-Request which caused this Configure-Reject.
 Options
    The Options field is variable in length, and contains the list of
    zero or more Configuration Options that the sender is rejecting.
    All Configuration Options are always rejected simultaneously.

Simpson [Page 32] RFC 1661 Point-to-Point Protocol July 1994

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.
    An implementation wishing to close a connection SHOULD transmit a
    Terminate-Request.  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 Terminate-Ack MUST be
    transmitted.
    Reception of an unelicited Terminate-Ack indicates that the peer
    is in the Closed or Stopped states, or is otherwise in need of
    re-negotiation.
 A summary of the Terminate-Request and Terminate-Ack packet formats
 is shown below.  The fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Data ...
 +-+-+-+-+
 Code
    5 for Terminate-Request;
    6 for Terminate-Ack.
 Identifier
    On transmission, the Identifier field MUST be changed whenever the
    content of the Data field changes, and whenever a valid reply has
    been received for a previous request.  For retransmissions, the
    Identifier MAY remain unchanged.
    On reception, the Identifier field of the Terminate-Request is
    copied into the Identifier field of the Terminate-Ack packet.

Simpson [Page 33] RFC 1661 Point-to-Point Protocol July 1994

 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.  The end of the field is indicated by the Length.

5.6. Code-Reject

 Description
    Reception of a LCP packet with an unknown Code indicates that the
    peer is operating with a different version.  This MUST be reported
    back to the sender of the unknown Code by transmitting a Code-
    Reject.
    Upon reception of the Code-Reject of a code which is fundamental
    to this version of the protocol, the implementation SHOULD report
    the problem and drop the connection, since it is unlikely that the
    situation can be rectified automatically.
 A summary of the Code-Reject packet format is shown below.  The
 fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Rejected-Packet ...
 +-+-+-+-+-+-+-+-+
 Code
    7 for Code-Reject.
 Identifier
    The Identifier field MUST be changed for each Code-Reject sent.
 Rejected-Packet
    The Rejected-Packet 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-Packet MUST be truncated to comply with the peer's

Simpson [Page 34] RFC 1661 Point-to-Point Protocol July 1994

    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 automaton is in the Opened
    state, then this MUST be reported back to the peer by transmitting
    a Protocol-Reject.
    Upon reception of a Protocol-Reject, the implementation MUST stop
    sending packets of the indicated protocol at the earliest
    opportunity.
    Protocol-Reject packets can only be sent in the LCP Opened state.
    Protocol-Reject packets received in any state other than the LCP
    Opened state SHOULD be silently discarded.
 A summary of the Protocol-Reject packet format is shown below.  The
 fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Rejected-Protocol       |      Rejected-Information ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Code
    8 for Protocol-Reject.
 Identifier
    The Identifier field MUST be changed for each Protocol-Reject
    sent.
 Rejected-Protocol
    The Rejected-Protocol field is two octets, and contains the PPP
    Protocol field of the packet which is being rejected.

Simpson [Page 35] RFC 1661 Point-to-Point Protocol July 1994

 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.
    Upon reception of an Echo-Request in the LCP Opened state, an
    Echo-Reply MUST be transmitted.
    Echo-Request and Echo-Reply packets MUST 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.

Simpson [Page 36] RFC 1661 Point-to-Point Protocol July 1994

 Identifier
    On transmission, the Identifier field MUST be changed whenever the
    content of the Data field changes, and whenever a valid reply has
    been received for a previous request.  For retransmissions, the
    Identifier MAY remain unchanged.
    On reception, the Identifier field of the Echo-Request is copied
    into the Identifier field of the Echo-Reply packet.
 Magic-Number
    The Magic-Number field is four octets, and aids in detecting links
    which are in the looped-back condition.  Until the Magic-Number
    Configuration Option has been successfully negotiated, the Magic-
    Number MUST be transmitted as zero.  See the Magic-Number
    Configuration Option for further explanation.
 Data
    The Data field is zero or more octets, and contains uninterpreted
    data for use by the sender.  The data may consist of any binary
    value.  The end of the field is indicated by the Length.

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.
    Discard-Request packets MUST only be sent in the LCP Opened state.
    On reception, the receiver MUST silently discard any Discard-
    Request that it receives.

Simpson [Page 37] RFC 1661 Point-to-Point Protocol July 1994

 A summary of the Discard-Request packet format is shown below.  The
 fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         Magic-Number                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Data ...
 +-+-+-+-+
 Code
    11 for Discard-Request.
 Identifier
    The Identifier field MUST be changed for each Discard-Request
    sent.
 Magic-Number
    The Magic-Number field is four octets, and aids in detecting links
    which are in the looped-back condition.  Until the Magic-Number
    Configuration Option has been successfully negotiated, the Magic-
    Number MUST be transmitted as zero.  See the Magic-Number
    Configuration Option for further explanation.
 Data
    The Data field is zero or more octets, and contains uninterpreted
    data for use by the sender.  The data may consist of any binary
    value.  The end of the field is indicated by the Length.

Simpson [Page 38] RFC 1661 Point-to-Point Protocol July 1994

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 field 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.
 Design Philosophy
    The options indicate additional capabilities or requirements of
    the implementation that is requesting the option.  An
    implementation which does not understand any option SHOULD
    interoperate with one which implements every option.
    A default is specified for each option which allows the link to
    correctly function without negotiation of the option, although
    perhaps with less than optimal performance.
    Except where explicitly specified, acknowledgement of an option
    does not require the peer to take any additional action other than
    the default.
    It is not necessary to send the default values for the options in
    a Configure-Request.
 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 ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Simpson [Page 39] RFC 1661 Point-to-Point Protocol July 1994

 Type
    The Type field is one octet, and indicates the type of
    Configuration Option.  Up-to-date values of the LCP Option Type
    field are specified in the most recent "Assigned Numbers" RFC [2].
    This document concerns the following values:
       0       RESERVED
       1       Maximum-Receive-Unit
       3       Authentication-Protocol
       4       Quality-Protocol
       5       Magic-Number
       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 or unrecognized 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 contains information
    specific to the Configuration Option.  The format and length of
    the Data field is determined by the Type and Length fields.
    When the Data field is indicated by the Length to extend beyond
    the end of the Information field, the entire packet is silently
    discarded without affecting the automaton.

Simpson [Page 40] RFC 1661 Point-to-Point Protocol July 1994

6.1. Maximum-Receive-Unit (MRU)

 Description
    This Configuration Option may be sent to inform the peer that the
    implementation can receive larger packets, or to request that the
    peer send smaller packets.
    The default value is 1500 octets.  If smaller packets are
    requested, an implementation MUST still be able to receive the
    full 1500 octet information field in case link synchronization is
    lost.
    Implementation Note:
       This option is used to indicate an implementation capability.
       The peer is not required to maximize the use of the capacity.
       For example, when a MRU is indicated which is 2048 octets, the
       peer is not required to send any packet with 2048 octets.  The
       peer need not Configure-Nak to indicate that it will only send
       smaller packets, since the implementation will always require
       support for at least 1500 octets.
 A summary of the Maximum-Receive-Unit Configuration Option format is
 shown below.  The fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |      Maximum-Receive-Unit     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
    1
 Length
    4
 Maximum-Receive-Unit
    The Maximum-Receive-Unit field is two octets, and specifies the
    maximum number of octets in the Information and Padding fields.
    It does not include the framing, Protocol field, FCS, nor any
    transparency bits or bytes.

Simpson [Page 41] RFC 1661 Point-to-Point Protocol July 1994

6.2. 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 protocol for authentication.  By default,
    authentication is not required.
    An implementation MUST NOT include multiple Authentication-
    Protocol Configuration Options in its Configure-Request packets.
    Instead, it SHOULD attempt to configure the most desirable
    protocol first.  If that protocol is Configure-Nak'd, then the
    implementation SHOULD attempt the next most desirable protocol in
    the next Configure-Request.
    The implementation sending the Configure-Request is indicating
    that it expects authentication from its peer.  If an
    implementation sends a Configure-Ack, then it is agreeing to
    authenticate with the specified protocol.  An implementation
    receiving a Configure-Ack 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

Simpson [Page 42] RFC 1661 Point-to-Point Protocol July 1994

 Length
    >= 4
 Authentication-Protocol
    The Authentication-Protocol field is two octets, and indicates the
    authentication protocol desired.  Values for this field are always
    the same as the PPP Protocol field values for that same
    authentication protocol.
    Up-to-date values of the Authentication-Protocol field are
    specified in the most recent "Assigned Numbers" RFC [2].  Current
    values are assigned as follows:
    Value (in hex)  Protocol
    c023            Password Authentication Protocol
    c223            Challenge Handshake Authentication Protocol
 Data
    The Data field is zero or more octets, and contains additional
    data as determined by the particular protocol.

6.3. 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.
    The implementation sending the Configure-Request is indicating
    that it expects to receive monitoring information from its peer.
    If an implementation sends a Configure-Ack, then it is agreeing to
    send the specified protocol.  An implementation receiving a
    Configure-Ack SHOULD expect the peer to send the acknowledged
    protocol.
    There is no requirement that quality monitoring be full-duplex or

Simpson [Page 43] RFC 1661 Point-to-Point Protocol July 1994

    that the same protocol be used in both directions.  It is
    perfectly acceptable for different protocols to be used in each
    direction.  This will, of course, depend on the specific protocols
    negotiated.
 A summary of the Quality-Protocol Configuration Option format is
 shown below.  The fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |        Quality-Protocol       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Data ...
 +-+-+-+-+
 Type
    4
 Length
    >= 4
 Quality-Protocol
    The Quality-Protocol field is two octets, and indicates the link
    quality monitoring protocol desired.  Values for this field are
    always the same as the PPP Protocol field values for that same
    monitoring protocol.
    Up-to-date values of the Quality-Protocol field are specified in
    the most recent "Assigned Numbers" RFC [2].  Current values are
    assigned as follows:
    Value (in hex)  Protocol
    c025            Link Quality Report
 Data
    The Data field is zero or more octets, and contains additional
    data as determined by the particular protocol.

Simpson [Page 44] RFC 1661 Point-to-Point Protocol July 1994

6.4. 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 a unique seed.  Suggested sources of
    uniqueness include machine serial numbers, other network hardware
    addresses, time-of-day clocks, etc.  Particularly good random
    number seeds are precise measurements of the inter-arrival time of
    physical events such as packet reception on other connected
    networks, server response time, or the typing rate of a human
    user.  It is also suggested that as many sources as possible be
    used simultaneously.
    When a Configure-Request is received with a Magic-Number
    Configuration Option, the received Magic-Number is compared with
    the Magic-Number of the last Configure-Request sent to the peer.
    If the two Magic-Numbers are different, then the link is not
    looped-back, and the Magic-Number SHOULD be acknowledged.  If the
    two Magic-Numbers are equal, then it is possible, but not certain,
    that the link is looped-back and that this Configure-Request is
    actually the one last sent.  To determine this, a Configure-Nak
    MUST be sent specifying a different Magic-Number value.  A new
    Configure-Request SHOULD NOT be sent to the peer until normal
    processing would cause it to be sent (that is, until a Configure-
    Nak is received or the Restart timer runs out).
    Reception of a Configure-Nak with a Magic-Number different from
    that of the last Configure-Nak sent to the peer proves that a link
    is not looped-back, and indicates a unique Magic-Number.  If the
    Magic-Number is equal to the one sent in the last Configure-Nak,
    the possibility of a looped-back link is increased, and a new
    Magic-Number MUST be chosen.  In either case, a new Configure-
    Request SHOULD be sent with the new Magic-Number.
    If the link is indeed looped-back, this sequence (transmit
    Configure-Request, receive Configure-Request, transmit Configure-

Simpson [Page 45] RFC 1661 Point-to-Point Protocol July 1994

    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, looped-back links 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 when it receives a Configure-Request with a
    Magic-Number Configuration Option.  That is, if an implementation
    desires to use Magic Numbers, then it MUST also allow its peer to
    do so.  If an implementation does receive a Configure-Reject in
    response to a Configure-Request, it can only mean that the link is
    not looped-back, and that its peer will not be using Magic-
    Numbers.  In this case, an implementation SHOULD act as if the
    negotiation had been successful (as if it had instead received a
    Configure-Ack).
    The Magic-Number also may be used to detect looped-back links
    during normal operation, as well as during Configuration Option
    negotiation.  All LCP Echo-Request, Echo-Reply, and Discard-
    Request packets have a Magic-Number field.  If Magic-Number has
    been successfully negotiated, an implementation MUST transmit
    these packets with the Magic-Number field set to its negotiated
    Magic-Number.
    The Magic-Number field of these packets SHOULD be inspected on
    reception.  All received Magic-Number fields MUST be equal to
    either zero or the peer's unique Magic-Number, depending on
    whether or not the peer negotiated a Magic-Number.

Simpson [Page 46] RFC 1661 Point-to-Point Protocol July 1994

    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, 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 looped-back condition is
    terminated, and Magic-Numbers are successfully negotiated.  A more
    optimistic procedure (in the case of a looped-back link) is to
    begin transmitting LCP Echo-Request packets until an appropriate
    Echo-Reply is received, indicating a termination of the looped-
    back condition.
 A summary of the Magic-Number Configuration Option format is shown
 below.  The fields are transmitted from left to right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |          Magic-Number
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Magic-Number (cont)       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
    5
 Length
    6
 Magic-Number
    The Magic-Number field is four octets, and indicates a number
    which is very likely to be unique to one end of the link.  A
    Magic-Number of zero is illegal and MUST always be Nak'd, if it is
    not Rejected outright.

Simpson [Page 47] RFC 1661 Point-to-Point Protocol July 1994

6.5. Protocol-Field-Compression (PFC)

 Description
    This Configuration Option provides a method to negotiate the
    compression of the PPP Protocol field.  By default, all
    implementations MUST transmit packets with two octet PPP Protocol
    fields.
    PPP Protocol field numbers are chosen such that some values may be
    compressed into a single octet form which is clearly
    distinguishable from the two octet form.  This Configuration
    Option is sent to inform the peer that the implementation can
    receive such single octet Protocol fields.
    As previously mentioned, the Protocol field uses an extension
    mechanism consistent with the ISO 3309 extension mechanism for the
    Address field; the Least Significant Bit (LSB) of each octet is
    used to indicate extension of the Protocol field.  A binary "0" as
    the LSB indicates that the Protocol field continues with the
    following octet.  The presence of a binary "1" as the LSB marks
    the last octet of the Protocol field.  Notice that any number of
    "0" octets may be prepended to the field, and will still indicate
    the same value (consider the two binary representations for 3,
    00000011 and 00000000 00000011).
    When using low speed links, it is desirable to conserve bandwidth
    by sending as little redundant data as possible.  The Protocol-
    Field-Compression Configuration Option allows a trade-off between
    implementation simplicity and bandwidth efficiency.  If
    successfully negotiated, the ISO 3309 extension mechanism may be
    used to compress the Protocol field to one octet instead of two.
    The large majority of packets are compressible since data
    protocols are typically assigned with Protocol field values less
    than 256.
    Compressed Protocol fields MUST NOT be transmitted unless this
    Configuration Option has been negotiated.  When negotiated, PPP
    implementations MUST accept PPP packets with either double-octet
    or single-octet Protocol fields, and MUST NOT distinguish between
    them.
    The Protocol field is never compressed when sending any LCP
    packet.  This rule guarantees unambiguous recognition of LCP
    packets.
    When a Protocol field is compressed, the Data Link Layer FCS field
    is calculated on the compressed frame, not the original

Simpson [Page 48] RFC 1661 Point-to-Point Protocol July 1994

    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

Simpson [Page 49] RFC 1661 Point-to-Point Protocol July 1994

6.6. Address-and-Control-Field-Compression (ACFC)

 Description
    This Configuration Option provides a method to negotiate the
    compression of the Data Link Layer Address and Control fields.  By
    default, all implementations MUST transmit frames with Address and
    Control fields appropriate to the link framing.
    Since these fields usually have constant values for point-to-point
    links, they are easily compressed.  This Configuration Option is
    sent to inform the peer that the implementation can receive
    compressed Address and Control fields.
    If a compressed frame is received when Address-and-Control-Field-
    Compression has not been negotiated, the implementation MAY
    silently discard the frame.
    The Address and Control fields MUST NOT be compressed when sending
    any LCP packet.  This rule guarantees unambiguous recognition of
    LCP packets.
    When the Address and Control fields are compressed, the Data Link
    Layer FCS field is calculated on the compressed frame, not the
    original uncompressed frame.
 A summary of the Address-and-Control-Field-Compression configuration
 option format is shown below.  The fields are transmitted from left
 to right.
  0                   1
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
    8
 Length
    2

Simpson [Page 50] RFC 1661 Point-to-Point Protocol July 1994

Security Considerations

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

References

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

Acknowledgements

 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@merit.edu mailing list.
 Much of the text in this document is taken from the working group
 requirements [1]; and RFCs 1171 & 1172, by Drew Perkins while at
 Carnegie Mellon University, and by Russ Hobby of the University of
 California at Davis.
 William Simpson was principally responsible for introducing
 consistent terminology and philosophy, and the re-design of the phase
 and negotiation state machines.
 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, Ken
 Adelman, Fred Baker, Mike Ballard, Craig Fox, Karl Fox, Phill Gross,
 Kory Hamzeh, former WG chair Russ Hobby, David Kaufman, former WG
 chair Steve Knowles, Mark Lewis, former WG chair Brian Lloyd, John
 LoVerso, Bill Melohn, Mike Patton, former WG chair Drew Perkins, Greg
 Satz, John Shriver, Vernon Schryver, and Asher Waldfogel.
 Special thanks to Morning Star Technologies for providing computing
 resources and network access support for writing this specification.

Simpson [Page 51] RFC 1661 Point-to-Point Protocol July 1994

Chair's Address

 The working group can be contacted via the current chair:
    Fred Baker
    Advanced Computer Communications
    315 Bollay Drive
    Santa Barbara, California  93117
    fbaker@acc.com

Editor's Address

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

Simpson [Page 52]

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