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Obsoleted by: 1331, 1332 PROPOSED STANDARD

Network Working Group                                         D. Perkins
Request for Comments: 1172                                           CMU
                                                                R. Hobby
                                                                UC Davis
                                                               July 1990



    The Point-to-Point Protocol (PPP) Initial Configuration Options



Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community.

   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.

   This proposal is the product of the Point-to-Point Protocol Working
   Group of the Internet Engineering Task Force (IETF).  Comments on
   this memo should be submitted to the IETF Point-to-Point Protocol
   Working Group chair.

   Distribution of this memo is unlimited.

Abstract

   The Point-to-Point Protocol (PPP) provides a method for transmitting
   datagrams over serial point-to-point links.  PPP is composed of

      1) a method for encapsulating datagrams over serial links,
      2) an extensible Link Control Protocol (LCP), and
      3) a family of Network Control Protocols (NCP) for establishing
      and configuring different network-layer protocols.

   The PPP encapsulating scheme, the basic LCP, and an NCP for
   controlling and establishing the Internet Protocol (IP) (called the
   IP Control Protocol, IPCP) are defined in The Point-to-Point Protocol
   (PPP) [1].

   This document defines the intial options used by the LCP and IPCP. It
   also defines a method of Link Quality Monitoring and a simple
   authentication scheme.






Perkins & Hobby                                                 [Page i]

RFC 1172                  PPP Initial Options                  July 1990


                           Table of Contents


     1.     Introduction ..........................................    1

     2.     Link Control Protocol (LCP) Configuration Options .....    1
        2.1       Maximum-Receive-Unit ............................    2
        2.2       Async-Control-Character-Map .....................    3
        2.3       Authentication-Type .............................    5
        2.4       Magic-Number ....................................    7
        2.5       Link-Quality-Monitoring .........................   10
        2.6       Protocol-Field-Compression ......................   11
        2.7       Address-and-Control-Field-Compression ...........   13

     3.     Link Quality Monitoring ...............................   15
        3.1       Design Motivation ...............................   15
        3.2       Design Overview .................................   15
        3.3       Processes .......................................   16
        3.4       Counters ........................................   18
        3.5       Measurements, Calculations, State Variables .....   19
        3.6       Link-Quality-Report Packet Format ...............   21
        3.7       Policy Suggestions ..............................   25
        3.8       Example .........................................   25

     4.     Password Authentication Protocol ......................   27
        4.1       Packet Format ...................................   27
        4.2       Authenticate ....................................   29
        4.3       Authenticate-Ack ................................   31
        4.4       Authenticate-Nak ................................   32

     5.     IP Control Protocol (IPCP) Configuration Options ......   33
        5.1       IP-Addresses ....................................   34
        5.2       Compression-Type ................................   36

     REFERENCES ...................................................   37

     SECURITY CONSIDERATIONS ......................................   37

     AUTHOR'S ADDRESS .............................................   37












Perkins & Hobby                                                [Page ii]

RFC 1172                  PPP Initial Options                  July 1990


1.  Introduction

   The Point-to-Point Protocol (PPP) [1] proposes a standard method of
   encapsulating IP datagrams, and other Network Layer protocol
   information, over point-to-point links.  PPP also proposes an
   extensible Option Negotiation Protocol.  [1] specifies only the
   protocol itself; the initial set of Configuration Options are
   described in this document.  These Configuration Options allow MTUs
   to be changed, IP addresses to be dynamically assigned, header
   compression to be enabled, and much more.

   This memo is divided into several sections.  Section 2 describes
   Configuration Options for the Link Control Protocol. Section 3
   specifies the use of the Link Quality Monitoring option. Section 4
   defines a simple Password Authentication Protocol. Finally, Section 5
   specifies Configuration Options for the IP Control Protocol.

2.  Link Control Protocol (LCP) Configuration Options

   As described in [1], LCP Configuration Options allow modifications to
   the standard characteristics of a point-to-point link to be
   negotiated.  Negotiable modifications proposed in this document
   include such things as the maximum receive unit, async control
   character mapping, the link authentication method, etc.

   The initial proposed values for the LCP Configuration Option Type
   field (see [1]) are assigned as follows:

      1       Maximum-Receive-Unit
      2       Async-Control-Character-Map
      3       Authentication-Type
      4       NOT ASSIGNED
      5       Magic-Number
      6       Link-Quality-Monitoring
      7       Protocol-Field-Compression
      8       Address-and-Control-Field-Compression















Perkins & Hobby                                                 [Page 1]

RFC 1172                  PPP Initial Options                  July 1990


2.1.  Maximum-Receive-Unit

   Description

      This Configuration Option provides a way to negotiate the maximum
      packet size used across one direction of a link.  By default, all
      implementations must be able to receive frames with 1500 octets of
      Information.

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

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

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

   Type

      1

   Length

      4

   Maximum-Receive-Unit

      The Maximum-Receive-Unit field is two octets and indicates the new
      maximum receive unit.  The Maximum-Receive-Unit covers only the
      Data Link Layer Information field but not the header, trailer or
      any transparency bits or bytes.

   Default

      1500









Perkins & Hobby                                                 [Page 2]

RFC 1172                  PPP Initial Options                  July 1990


2.2.  Async-Control-Character-Map

   Description

      This Configuration Option provides a way to negotiate the use of
      control character mapping on asynchronous links.  By default, PPP
      maps all control characters into an appropriate two character
      sequence.  However, it is rarely necessary to map all control
      characters and often times it is unnecessary to map any
      characters.  A PPP implementation may use this Configuration
      Option to inform the remote end which control characters must
      remain mapped and which control characters need not remain mapped
      when the remote end sends them.  The remote end may still send
      these control characters in mapped format if it is necessary
      because of constraints at its (the remote) end.  This option does
      not solve problems for communications links that can send only 7-
      bit characters or that can not send all non-control characters.

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

   A summary of the Async-Control-Character-Map Configuration Option
   format is shown below.  The fields are transmitted from left to
   right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |  Async-Control-Character-Map
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             (cont)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      2



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RFC 1172                  PPP Initial Options                  July 1990


   Length

      6

   Async-Control-Character-Map

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

   Default

      All ones (0xffffffff).

































Perkins & Hobby                                                 [Page 4]

RFC 1172                  PPP Initial Options                  July 1990


2.3.  Authentication-Type

   Description

      On some links it may be desirable to require a peer to
      authenticate itself before allowing Network Layer protocol data to
      be exchanged.  This Configuration Option provides a way to
      negotiate the use of a specific authentication protocol.  By
      default, authentication is not necessary.  If an implementation
      requires that the remote end authenticate with some specific
      authentication protocol, then it should negotiate the use of that
      authentication protocol with this Configuration Option.

      Successful negotiation of the Authentication-Type option adds an
      additional Authentication phase to the Link Control Protocol.
      This phase is after the Link Quality Determination phase, and
      before the Network Layer Protocol Configuration Negotiation phase.
      Advancement from the Authentication phase to the Network Layer
      Protocol Configuration Negotiation phase may not occur until the
      peer is successfully authenticated using the negotiated
      authentication protocol.

      An implementation may allow the remote end to pick from more than
      one authentication protocol. To achieve this, it may include
      multiple Authentication-Type Configuration Options in its
      Configure-Request packets.  An implementation receiving a
      Configure-Request specifying multiple Authentication-Types may
      accept at most one of the negotiable authentication protocols and
      should send a Configure-Reject specifying all of the other
      specified authentication protocols.

      It is recommended that each PPP implementation support
      configuration of authentication parameters at least on a per-
      interface basis, if not a per peer entity basis.  The parameters
      should specify which authetication techniques are minimally
      required as a prerequisite to establishment of a PPP connection,
      either for the specified interface or for the specified peer
      entity.  Such configuration facilities are necessary to prevent an
      attacker from negotiating a reduced security authentication
      protocol, or no authentication at all, in an attempt to circumvent
      this authentication facility.

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




Perkins & Hobby                                                 [Page 5]

RFC 1172                  PPP Initial Options                  July 1990


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

      This document defines a simple Password Authentication Protocol in
      Section 4.  Development of other more secure protocols is
      encouraged.

   A summary of the Authentication-Type 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-Type       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Data ...
   +-+-+-+-+

   Type

      3

   Length

      >= 4

   Authentication-Type

      The Authentication-Type field is two octets and indicates the type
      of authentication protocol desired.  Values for the
      Authentication-Type are always the same as the PPP Data Link Layer
      Protocol field values for that same authentication protocol.  The
      most up-to-date values of the Authentication-Type field are
      specified in "Assigned Numbers" [2].  Initial values are assigned
      as follows:

         Value (in hex)          Protocol

         c023                    Password Authentication Protocol

   Data

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




Perkins & Hobby                                                 [Page 6]

RFC 1172                  PPP Initial Options                  July 1990


   Default

      No authentication protocol necessary.


2.4.  Magic-Number

   Description

      This Configuration Option provides a way to detect looped-back
      links and other Data Link Layer anomalies.  This Configuration
      Option may be required by some other Configuration Options such as
      the Link-Quality-Monitoring Configuration Option.

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

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

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



Perkins & Hobby                                                 [Page 7]

RFC 1172                  PPP Initial Options                  July 1990


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

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

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

      If an implementation does transmit a Configure-Request with a
      Magic-Number Configuration Option, then it MUST NOT respond with a
      Configure-Reject if its peer also transmits a Configure-Request
      with a Magic-Number Configuration Option.  That is, if an
      implementation desires to use Magic Numbers, then it MUST also
      allow its peer to do so.  If an implementation does receive a
      Configure-Reject in response to a Configure-Request, it can only
      mean that the link is not looped-back, and that its peer will not
      be using Magic-Numbers.  In this case, an implementation may 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 Echo-Request, Echo-Reply, Discard-Request, and
      Link-Quality-Report LCP packets have a Magic-Number field which
      MUST normally be transmitted as zero, and MUST normally be ignored
      on reception.  However, once a Magic-Number has been successfully
      negotiated, an LCP implementation MUST begin transmitting these
      packets with the Magic-Number field set to its negotiated Magic-
      Number.  Additionally, the Magic-Number field of these packets may
      be inspected on reception. All received Magic-Number fields should
      be equal to either zero or the peer's unique Magic-Number,



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RFC 1172                  PPP Initial Options                  July 1990


      depending on whether or not the peer negotiated one.  Reception of
      a Magic-Number field equal to the negotiated local Magic-Number
      indicates a looped-back link.  Reception of a Magic-Number other
      than the negotiated local Magic-Number or or the peer's negotiated
      Magic-Number, or zero if the peer didn't negotiate one, indicates
      a link which has been (mis)configured for communications with a
      different peer.

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

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |          Magic-Number
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Magic-Number (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      5

   Length

      6

   Magic-Number

      The Magic-Number field is four octets and indicates a number which
      is very likely to be unique to one end of the link.  A Magic-
      Number of zero is illegal and must not be sent.

   Default

      None.



Perkins & Hobby                                                 [Page 9]

RFC 1172                  PPP Initial Options                  July 1990


2.5.  Link-Quality-Monitoring

   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 and is implemented by periodically transmitting
      Link-Quality-Report packets as described in Section 3.  The Link-
      Quality-Monitoring Configuration Option provides a way to enable
      the use of Link-Quality-Report packets, and also to negotiate the
      rate at which they are transmitted.  By default, Link Quality
      Monitoring and the use of Link-Quality-Report packets is disabled.

   A summary of the Link-Quality-Monitoring 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     |        Reporting-Period
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Reporting-Period (cont)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      6

   Length

      6

   Reporting-Period

      The Reporting-Period field is four octets and indicates the
      maximum time in micro-seconds that the remote end should wait
      between transmission of LCP Link-Quality-Report packets.  A value
      of zero is illegal and should always be nak'd or rejected.  An LCP
      implementation is always free to transmit LCP Link-Quality-Report
      packets at a faster rate than that which was requested by, and
      acknowledged to, the remote end.

   Default

      None






Perkins & Hobby                                                [Page 10]

RFC 1172                  PPP Initial Options                  July 1990


2.6.  Protocol-Field-Compression

   Description

      This Configuration Option provides a way to negotiate the
      compression of the Data Link Layer Protocol field.  By default,
      all implementations must transmit standard PPP frames with two
      octet Protocol fields. However, PPP Protocol field numbers are
      chosen such that some values may be compressed into a single octet
      form which is clearly distinguishable from the two octet form.
      This Configuration Option may be sent to inform the remote end
      that you can receive compressed single octet Protocol fields.
      Compressed Protocol fields may not be transmitted unless this
      Configuration Option has been received.

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

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

      However, when using low speed links, it is desirable to conserve
      bandwidth by sending as little redundant data as possible.  The
      Protocol Compression Configuration Option allows a trade-off
      between implementation simplicity and bandwidth efficiency.  If
      successfully negotiated, the ISO 3309 extension mechanism may be
      used to compress the Protocol field to one octet instead of two.
      The large majority of frames are compressible since data protocols
      are typically assigned with Protocol field values less than 256.

      To guarantee unambiguous recognition of LCP packets, the Protocol
      field must never be compressed when sending any LCP packet.  In
      addition, PPP implementations must continue to be robust and MUST
      accept PPP frames with double-octet, as well as single-octet,
      Protocol fields, and MUST NOT distinguish between them.

      When a Protocol field is compressed, the Data Link Layer FCS field



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RFC 1172                  PPP Initial Options                  July 1990


      is calculated on the compressed frame, not the original
      uncompressed frame.

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

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

   Type

      7

   Length

      2

   Default

      Disabled.



























Perkins & Hobby                                                [Page 12]

RFC 1172                  PPP Initial Options                  July 1990


2.7.  Address-and-Control-Field-Compression

   Description

      This Configuration Option provides a way to negotiate the
      compression of the Data Link Layer Address and Control fields.  By
      default all implementations must transmit frames with Address and
      Control fields and must use the hexadecimal values 0xff and 0x03
      respectively.  Since these fields have constant values, they are
      easily compressed.  this Configuration Option may be used to
      inform the remote end that you can receive compressed Address and
      Control fields.

      Compressed Address and Control fields are formed by simply
      omitting them in all non-ambiguous cases.  Ambiguous frames may
      not be compressed.  Ambiguous cases result when the two octets
      following the Address and Control fields have values that could be
      interpreted as valid Address and Control fields (i.e., 0xff,
      0x03).  This can happen when Protocol-Field-Compression is enabled
      and the Protocol field is compressed to one octet.  If the
      Protocol value is 0xff, and the first octet of the Information
      field is 0x03, the result is ambiguous and the Address and Control
      fields must not be compressed on transmission.

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

      One additional case in which the Address and Control fields must
      never be compressed is 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     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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RFC 1172                  PPP Initial Options                  July 1990


   Type

      8

   Length

      2

   Default

      Not compressed.








































Perkins & Hobby                                                [Page 14]

RFC 1172                  PPP Initial Options                  July 1990


3.  Link Quality Monitoring

   Data communications links are rarely perfect. Packets can be dropped
   or corrupted for various reasons (line noise, equipment failure,
   buffer overruns, etc.).  Sometimes, it is desirable to determine
   when, and how often, the link is dropping data.  Routers, for
   example, may want to temporarily allow another route to take
   precedence.  An implementation may also have the option of
   disconnecting and switching to an alternate link.  The process of
   determining data loss is called "Link Quality Monitoring".

3.1.  Design Motivation

   There are many different ways to measure link quality, and even more
   ways to react to it.  Rather than specifying a single scheme, Link
   Quality Monitoring is divided into a "mechanism" and a "policy".  PPP
   fully specifies the "mechanism" for Link Quality Monitoring by
   defining the LCP Link-Quality-Report (LQR) packet and specifying a
   procedure for its use.  PPP does NOT specify a Link Quality
   Monitoring "policy" -- how to judge link quality or what to do when
   it is inadequate.  That is left as an implementation decision, and
   can be different at each end of the link.  Implementations are
   allowed, and even encouraged, to experiment with various link quality
   policies.  The Link Quality Monitoring mechanism specification
   insures that two implementations with different policies may
   communicate and interoperate.

   To allow flexible policies to be implemented, the PPP Link Quality
   Monitoring mechanism measures data loss in units of packets, octets,
   and Link-Quality-Reports.  Each measurement is made separately for
   each half of the link, both inbound and outbound.  All measurements
   are communicated to both ends of the link so that each end of the
   link can implement its own link quality policy for both its outbound
   and inbound links.

   Finally, the Link Quality Monitoring protocol is designed to be
   implementable on many different kinds of systems. Although it may be
   common to implement PPP (and especially Link Quality Monitoring) as a
   single software process, multi-process implementations with hardware
   support are also envisioned. The PPP Link Quality Monitoring
   mechanism provides for this by careful definition of the Link-
   Quality-Report packet format, and by specifiying reference points for
   all data transmission and reception measurements.

3.2.  Design Overview

   Each Link Quality Monitoring implementation maintains counts of the
   number of packets and octets transmitted and successfully received,



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RFC 1172                  PPP Initial Options                  July 1990


   and periodically transmits this information to its peer in a Link-
   Quality-Report packet.  These packets contain three sections: a
   Header section, a Counters section, and a Measurements section.

   The Header section of the packet consists of the normal LCP Link
   Maintenance packet header including Code, Identifier, Length and
   Magic-Number fields.

   The Counters section of the packet consists of four counters, and
   provides the information necessary to measure the quality of the
   link.  The LQR transmitter fills in two of these counters: Out-Tx-
   Packets-Ctr and Out-Tx-Octets-Ctr (described later).  The LQR
   receiver fills in the two remaining counters: In-Rx-Packets-Ctr and
   In-Rx-Octets-Ctr (described later).  These counters are similar to
   sequence numbers; they are constantly increasing to give a "relative"
   indication of the number of packets and octets communicated across
   the outbound link.  By comparing the values in successive Link-
   Quality-Reports, an LQR receiver can compute the "absolute" number of
   packets and octets communicated across its inbound link. Comparing
   these absolute numbers then gives an indication of an inbound link's
   quality.  Relative numbers, rather than absolute, are transmitted
   because they greatly simplify link synchronization; an implementation
   merely waits to receive two LQR packets.

   The Measurements section of the packet consists of six state
   variables: In-Tx-LQRs, Last-In-Id, In-Tx-Packets, In-Tx-Octets, In-
   Rx-Packets, and In-Rx-Octets (described later).  This section allows
   an implementation to report inbound link quality measurements to its
   peer (for which the report will instead indicate outbound link
   quality) by transmitting the absolute, rather than relative, number
   of LQRs, packets, and octets communicated across the inbound link.
   These values are calculated by observing the Counters section of the
   Link-Quality-Report packets received on the inbound link.  Absolute
   numbers may be used in this section without synchronization problems
   because it is necessary to receive only one LQR packet to have valid
   information.

   Link Quality Monitoring is described in more detail in the following
   sections.  First, a description of the processes comprising the Link
   Quality Monitoring mechanism is presented.  This is followed by the
   packet and byte counters maintained; the measurements, calculations,
   and state variables used; the format of the Link-Quality-Report
   packet; some policy suggestions; and, finally, an example link
   quality calculation.

3.3.  Processes

   The PPP Link Quality Monitoring mechanism is described using a



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RFC 1172                  PPP Initial Options                  July 1990


   "logical process" model. As shown below, there are five logical
   processes duplicated at each end of the duplex link.

   +---------+   +-------+   +----+ Outbound
   |         |-->|  Mux  |-->| Tx |=========>
   | Link-   |   +-------+   +----+
   | Manager |
   |         |   +-------+   +----+ Inbound
   |         |<--| Demux |<--| Rx |<=========
   +---------+   +-------+   +----+

   Link-Manager

      The Link-Manager process transmits and receives Link-Quality-
      Reports, and implements the desired link quality policy.  LQR
      packets are transmitted at a constant rate, which is negotiated by
      the LCP Link-Quality-Monitoring Configuration Option.  The Link-
      Manager process fills in only the Header and Measurements sections
      of the packet; the Counters section of the packet is filled in by
      the Tx and Rx processes.

   Mux

      The Mux process multiplexes packets from the various protocols
      (e.g., LCP, IP, XNS, etc.) into a single, sequential, and
      prioritized stream of packets.  Link-Quality-Report packets MUST
      be given the highest possible priority to insure that link quality
      information is communicated in a timely manner.

   Tx

      The Tx process maintains the counters Out-Tx-Packets-Ctr and Out-
      Tx-Octets-Ctr which are used to measure the amount of data which
      is transmitted on the outbound link.  When Tx processes a Link-
      Quality-Report packet, it inserts the values of these counters
      into the Counters section of the packet.  Because these counters
      represent relative, rather than absolute, values, the question of
      when to update the counters, before or after they are inserted
      into a Link-Quality-Report packet, is left as an implementation
      decision. However, an implementation MUST make this decision the
      same way every time.  The Tx process MUST follow the Mux process
      so that packets are counted in the order transmitted to the link.

   Rx

      The Rx process maintains the counters In-Rx-Packets-Ctr and In-
      Rx-Octets-Ctr which are used to measure the amount of data which
      is received by the inbound link.  When Rx processes a Link-



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RFC 1172                  PPP Initial Options                  July 1990


      Quality-Report packet, it inserts the values of these counters
      into the Counters section of the packet.  Again, the question of
      when to update the counters, before or after they are inserted
      into a Link-Quality-Report packet, is left as an implementation
      decision which MUST be made consistently the same way.

   Demux

      The Demux process demultiplexes packets for the various protocols.
      The Demux process MUST follow the Rx process so that packets are
      counted in the order received from the link.

3.4.  Counters

   In order to fill in the Counters section of a Link-Quality-Report
   packet, Link Quality Monitoring requires the implementation of one
   8-bit unsigned, and four 32-bit unsigned, monotonically increasing
   counters.  These counters may be reset to any initial value before
   the first Link-Quality-Report is transmitted, but MUST NOT be reset
   again until LCP has left the Open state.  Counters wrap to zero when
   their maximum value is reached (for 32 bit counters: 0xffffffff + 1 =
   0).

   Out-Identifier-Ctr

      Out-Identifier-Ctr is an 8-bit counter maintained by the Link-
      Manager process which increases by one for each transmitted Link-
      Quality-Report packet.

   Out-Tx-Packets-Ctr

      Out-Tx-Packets-Ctr is a 32-bit counter maintained by the Tx
      process which increases by one for each transmitted Data Link
      Layer packet.

   Out-Tx-Octets-Ctr

      Out-Tx-Octets-Ctr is a 32-bit counter maintained by the Tx process
      which increases by one for each octet in a transmitted Data Link
      Layer packet.  All octets which are included in the FCS
      calculation MUST be counted, as should the FCS octets themselves.
      All other octets MUST NOT be counted.

   In-Rx-Packets-Ctr

      In-Rx-Packets-Ctr is a 32-bit counter maintained by the Rx process
      which increases by one for each successfully received Data Link
      Layer packet.  Packets with incorrect FCS fields or other problems



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RFC 1172                  PPP Initial Options                  July 1990


      MUST not be counted.

   In-Rx-Octets-Ctr

      In-Rx-Octets-Ctr is a 32-bit counter maintained by the Rx process
      which increases by one for each octet in a successfully received
      Data Link Layer packet.  All octets which are included in an FCS
      calculation MUST be counted, as should the FCS octets themselves.
      All other octets MUST NOT be counted.

3.5.  Measurements, Calculations, State Variables

   In order to fill in the Measurements section of a Link-Quality-Report
   packet, Link Quality Monitoring requires the Link-Manager process to
   make a number of calculations and keep a number of state variables.
   These calculations are made, and these state variables updated, each
   time a Link-Quality-Report packet is received from the inbound link.

   In-Tx-LQRs

      In-Tx-LQRs is an 8-bit state variable which indicates the number
      of Link-Quality-Report packets which the peer had to transmit in
      order for the local end to receive exactly one LQR.  In-Tx-LQRs
      defines the length of the "period" over which In-Tx-Packets, In-
      Tx-Octets, In-Rx-Packets, and In-Rx-Octets were measured.  In-Tx-
      LQRs is calculated by subtracting Last-In-Id from the received
      Identifier.  If more than 255 LQRs in a row are lost, In-Tx-LQRs
      will be ambiguous since the Identifier field and all state
      variables based on it are only 8 bits.  It is assumed that the
      Link Quality Monitoring policy will be robust enough to handle
      this case (it should probably close down the link long before this
      happens).

   Last-In-Id

      Last-In-Id is an 8-bit state variable which stores the value of
      the last received Identifier.  Last-In-Id should be updated after
      In-Tx-LQRs has been calculated.

   In-Tx-Packets

      In-Tx-Packets is a 32-bit state variable which indicates the
      number of packets which were transmitted on the inbound link
      during the last period.  In-Tx-Packets is calculated by
      subtracting Last-Out-Tx-Packets-Ctr from the received Out-Tx-
      Packets-Ctr.





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RFC 1172                  PPP Initial Options                  July 1990


   Last-Out-Tx-Packets-Ctr

      Last-Out-Tx-Packets-Ctr is a 32-bit state variable which stores
      the value of the last received Out-Tx-Packets-Ctr.  Last-Out-Tx-
      Packets-Ctr should be updated after In-Tx-Packets has been
      calculated.

   In-Tx-Octets

      In-Tx-Octets is a 32-bit state variable which indicates the number
      of octets which were transmitted on the inbound link during the
      last period.  In-Tx-Octets is calculated by subtracting Last-Out-
      Tx-Octets-Ctr from the received Out-Tx-Octets-Ctr.

   Last-Out-Tx-Octets-Ctr

      Last-Out-Tx-Octets-Ctr is a 32-bit state variable which stores the
      value of the last received Out-Tx-Octets-Ctr.  Last-Out-Tx-
      Octets-Ctr should be updated after In-Tx-Octets has been
      calculated.

   In-Rx-Packets

      In-Rx-Packets is a 32-bit state variable which indicates the
      number of packets which were received on the inbound link during
      the last period.  In-Rx-Packets is calculated by subtracting
      Last-In-Rx-Packets-Ctr from the received In-Rx-Packets-Ctr.

   Last-In-Rx-Packets-Ctr

      Last-In-Rx-Packets-Ctr is a 32-bit state variable which stores the
      value of the last received In-Rx-Packets-Ctr.  Last-In-Rx-
      Packets-Ctr should be updated after In-Rx-Packets has been
      calculated.

   In-Rx-Octets

      In-Rx-Octets is a 32-bit state variable which indicates the number
      of octets which were received on the inbound link during the last
      period.  In-Rx-Octets is calculated by subtracting Last-In-Rx-
      Octets-Ctr from the received In-Rx-Octets-Ctr.

   Last-In-Rx-Octets-Ctr

      Last-In-Rx-Octets-Ctr is a 32-bit state variable which stores the
      value of the last received In-Rx-Octets-Ctr.  Last-In-Rx-Octets-
      Ctr should be updated after In-Rx-Octets has been calculated.




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RFC 1172                  PPP Initial Options                  July 1990


   Measurements-Valid

      Measurements-Valid is a 1-bit boolean state variable which
      indicates whether or not the In-Tx-Packets, In-Tx-Octets, In-Rx-
      Packets, and In-Rx-Octets state variables contain valid
      measurements.  These measurements cannot be considered valid until
      two or more Link-Quality-Report packets have been received on the
      inbound link.  This bit should be reset when LCP reaches the Open
      state and should be set after the receipt of exactly two LQRs.

3.6.  Link-Quality-Report Packet Format

   A Summary of the Link-Quality-Report packet format is shown below.
   The fields are transmitted from left to right.  The Code, Identifier,
   Length, and Magic-Number fields make up the normal LCP Link
   Maintenance packet header; the In-Tx-LQRS, Last-In-Id, V, In-Tx-
   Packets, In-Tx-Octets, In-Rx-Packets, In-Rx-Octets fields contain
   digested absolute measurements; and the Out-Tx-Packets-Ctr, Out-Tx-
   Octets-Ctr, In-Rx-Packets-Ctr, and In-Rx-Octets-Ctr fields contain
   raw relative counts.  Note that as transmitted over the link, this
   packet format does not include the In-Rx-Packets-Ctr and In-Rx-
   Octets-Ctr fields which are logically appended to the packet by the
   Rx process after reception on the inbound link.




























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RFC 1172                  PPP Initial Options                  July 1990


    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                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  In-Tx-LQRs   |   Last-In-Id  |           Reserved          |V|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         In-Tx-Packets                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         In-Tx-Octets                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         In-Rx-Packets                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         In-Rx-Octets                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Out-Tx-Packets-Ctr                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Out-Tx-Octets-Ctr                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /
   /
   /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        In-Rx-Packets-Ctr                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        In-Rx-Octets-Ctr                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code

      12 for Link-Quality-Report.

   Identifier

      The Identifier field is one octet and indicates the sequence
      number for this Link-Quality-Report. The Identifier field is
      copied from the Out-Identifier-Ctr counter on transmission.  On
      reception, the Identifier field is used to calculate In-Tx-LQRs
      and is then stored in Last-In-Id.

      The Link-Quality-Report Identifier sequence number space MUST be
      separate from that of all other LCP packets; for example,
      transmission of an LCP Echo-Request must not cause the Out-
      Identifier-Ctr counter to be incremented.





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RFC 1172                  PPP Initial Options                  July 1990


   Length

      The Length field is two octets and indicates the length of the LQM
      packet including the Code, Identifier, Length and all defined
      fields. Octets outside the range of the length field should be
      treated as Data Link Layer padding and should be ignored on
      reception.  In order for the correct In-Tx-Octets and In-Rx-Octets
      values to be calculated, Link-Quality-Reports MUST be consistently
      transmitted with the same amount of padding.

   Magic-Number

      The Magic-Number field is four octets and aids in detecting
      looped-back links.  Unless modified by a Configuration Option, the
      Magic-Number MUST always be transmitted as zero and MUST always be
      ignored on reception. If Magic-Numbers have been negotiated,
      incoming LQM packets should be checked to make sure that the local
      end is not seeing its own Magic-Number and thus a looped-back
      link.

   In-Tx-LQRs

      The In-Tx-LQRs field is one octet and indicates the number of
      periods covered by the Measurements section of this Link-Quality-
      Report.  The In-Tx-LQRs field is copied from the In-Tx-LQRs state
      variable on transmission.

   Last-In-Id

      The Prev-In-Id field is one octet and indicates the age of the
      Measurements section of this Link-Quality-Report. The Last-In-Id
      field is copied from the Last-In-Id field on transmission.  On
      reception, the Last-In-Id field may be compared with the Out-
      Identifier-Ctr to determine how many, if any, outbound Link-
      Quality-Reports have been lost.

   V

      The V field is 1 bit and indicates whether or not the Measurements
      section of this Link-Quality-Report is valid.  The V field is
      copied from the Measurements-Valid state variable on transmission.
      If the V field is not set to 1, then the In-Tx-LQRs, Last-In-Id,
      In-Tx-Packets, In-Tx-Octets, In-Rx-Packets and In-Rx-Octets fields
      should be ignored.

   Reserved

      The Reserved field is 15 bits and is intended to pad the remaining



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RFC 1172                  PPP Initial Options                  July 1990


      packet fields to even four-octet boundaries for the convenience of
      hardware implementations. The Reserved field should always be
      transmitted as zero and ignored on reception.

   In-Tx-Packets

      The In-Tx-Packets field is four octets and indicates the number of
      packets transmitted on the inbound link of the Link-Quality-Report
      transmitter during the last measured period.  The In-Tx-Packets
      field is copied from the In-Tx-Packets state variable on
      transmission.

   In-Tx-Octets

      The In-Tx-Octets field is four octets and indicates the number of
      octets transmitted on the inbound link of the Link-Quality-Report
      transmitter during the last measured period.  The In-Tx-Octets
      field is copied from the In-Tx-Octets state variable on
      transmission.

   In-Rx-Packets

      The In-Rx-Packets field is four octets and indicates the number of
      packets received on the inbound link of the Link-Quality-Report
      transmitter during the last measured period.  The In-Rx-Packets
      field is copied from the In-Rx-Packets state variable on
      transmission.

   In-Rx-Octets

      The In-Rx-Octets field is four octets and indicates the number of
      octets received on the inbound link of the Link-Quality-Report
      transmitter during the last measured period.  The In-Rx-Octets
      field is copied from the In-Rx-Octets state variable on
      transmission.

   Out-Tx-Packets

      The Out-Tx-Packets field is four octets and is used to calculate
      the number of packets transmitted on the outbound link of the
      Link-Quality-Report transmitter during a period.  The Out-Tx-
      Packets field is copied from the Out-Tx-Packets-Ctr counter on
      transmission.

   Out-Tx-Octets

      The Out-Tx-Octets field is four octets and is used to calculate
      the number of octets transmitted on the outbound link of the



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RFC 1172                  PPP Initial Options                  July 1990


      Link-Quality-Report transmitter during a period.  The Out-Tx-
      Octets field is copied from the Out-Tx-Octets-Ctr counter on
      transmission.

   In-Rx-Packets

      The In-Rx-Packets field is four octets and is used to calculate
      the number of packets received on the inbound link of the Link-
      Quality-Report receiver during a period.  The In-Rx-Packets field
      is copied from the In-Rx-Packets-Ctr counter on reception.  The
      In-Rx-Packets is not shown because it is not actually transmitted
      over the link.  Rather, it is logically appended (in an
      implementation dependent manner) to the packet by the
      implementation's Rx process.

   In-Rx-Octets

      The In-Rx-Octets field is four octets and is used to calculate the
      number of octets  received on the inbound link of the Link-
      Quality-Report receiver during a period.  The In-Rx-Octets field
      is copied from the In-Rx-Octets-Ctr counter on reception.  The
      In-Rx-Octets is not shown because it is not actually transmitted
      over the link.  Rather, it is logically appended (in an
      implementation dependent manner) to the packet by the
      implementation's Rx process.

3.7.  Policy Suggestions

   Link-Quality-Report packets provide a mechanism to determine the link
   quality, but it is up to each implementation to decide when the link
   is usable.  It is recommended that this policy implement some amount
   of hysteresis so that the link does not bounce up and down.  A
   particularly good policy is to use a K out of N algorithm.  In such
   an algorithm, there must be K successes out of the last N periods for
   the link to be considered of good quality.

   Procedures for recovery from poor quality links are unspecified and
   may vary from implementation to implementation.  A suggested approach
   is to immediately close all other Network-Layer protocols (i.e.,
   cause IPCP to transmit a Terminate-Req), but to continue transmitting
   Link-Quality-Reports.  Once the link quality again reaches an
   acceptable level, Network-Layer protocols can be reconfigured.

3.8.  Example

   An example may be helpful.  Assume that Link-Manager implementation A
   transmits a Link-Quality-Report which is received by Link-Manager
   implementation B at time t0 with the following values:



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RFC 1172                  PPP Initial Options                  July 1990


      Out-Tx-Packets    5
      Out-Tx-Octets   100
      In-Rx-Packets     3
      In-Rx-Octets     70

   Assume that A then transmits 20 IP packets with 200 octets, of which
   15 packets and 150 octets are received by B.  At time t1, A transmits
   another LQR which is received by B as follows:

      Out-Tx-Packets   26 (5 old, plus 20 IP, plus 1 LQR)
      Out-Tx-Octets   342 (42 for LQR)
      In-Rx-Packets    19
      In-Rx-Octets    262

   Implementation B can now calculate the number of packets and octets
   transmitted, received and lost on its inbound link as follows:

      In-Tx-Packets   =  26 -   5 =  21
      In-Tx-Octets    = 342 - 100 = 242
      In-Rx-Packets   =  10 -   3 =  16
      In-Rx-Octets    = 262 -  70 = 192
      In-Lost-Packets =  21 -  16 =   5
      In-Lost-Octets  = 242 - 192 =  50

   After doing these calculations, B evaluates the measurements in what
   ever way its implemented policy specifies.  Also, the next time that
   B transmits an LQR to A, it will report these values in the
   Measurements section, thereby allowing A to evaluate these same
   measurements.






















Perkins & Hobby                                                [Page 26]

RFC 1172                  PPP Initial Options                  July 1990


4.  Password Authentication Protocol

   The Password Authentication Protocol (PAP) may be used to
   authenticate a peer by verifying the identity of the remote end of
   the link.  Use of the PAP must first be negotiated using the LCP
   Authentication-Type Configuration Option.  Successful negotiation
   adds an additional Authentication phase to the Link Control Protocol,
   after the Link Quality Determination phase, and before the Network
   Layer Protocol Configuration Negotiation phase.  PAP packets received
   before the Authentication phase is reached should be silently
   discarded.  The Authentication phase is exited once an Authenticate-
   Ack packet is sent or received.

   PAP is intended for use primarily by hosts and routers that connect
   via switched circuits or dial-up lines to a PPP network server.  The
   server can then use the identification of the connecting host or
   router in the selection of options for network layer negotiations or
   failing authentication, drop the connection.

   Note that PAP is not a strong authentication method.  Passwords are
   passed over the circuit in the clear and there is no protection from
   repeated trial and error attacks.  Work is currently underway on more
   secure authentication methods for PPP and other protocols.  It is
   strongly recommended to switch to these methods when they become
   available.


4.1.  Packet Format

   Exactly one Password Authentication Protocol packet is encapsulated
   in the Information field of PPP Data Link Layer frames where the
   protocol field indicates type hex c023 (Password Authentication
   Protocol).  A summary of the Password Authentication 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 type of PAP packet.
      PAP Codes are assigned as follows:



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RFC 1172                  PPP Initial Options                  July 1990


         1       Authenticate
         2       Authenticate-Ack
         3       Authenticate-Nak

   Identifier

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

   Length

      The Length field is two octets and indicates the length of the PAP
      packet including the Code, Identifier, Length and Data fields.
      Octets outside the range of the Length field should be treated as
      Data Link Layer padding and should be ignored on reception.

   Data

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































Perkins & Hobby                                                [Page 28]

RFC 1172                  PPP Initial Options                  July 1990


4.2.  Authenticate

   Description

      The Authenticate packet is used to begin the Password
      Authentication Protocol.  An implementation having sent a LCP
      Configure-Ack packet with an Authentication-Type Configuration
      Option further specifying the Password Authentication Protocol
      must send an Authenticate packet during the Authentication phase.
      An implementation receiving a Configure-Ack with said
      Configuration Option should expect the remote end to send an
      Authenticate packet during this phase.

      An Authenticate packet is sent with the Code field set to 1
      (Authenticate) and the Peer-ID and Password fields filled as
      desired.

      Upon reception of an Authenticate, some type of Authenticate reply
      MUST be transmitted.

   A summary of the Authenticate 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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Peer-ID Length|  Peer-Id ...
   +-+-+-+-+-+-+-+-+-+-+-+-+
   | Passwd-Length |  Password  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code

      1 for Authenticate.

   Identifier

      The Identifier field is one octet and aids in matching requests
      and replies.  The Identifier field should be changed each time a
      Authenticate is transmitted which is different from the preceding
      request.

   Peer-ID-Length

      The Peer-ID-Length field is one octet and indicates the length of
      the Peer-ID field



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RFC 1172                  PPP Initial Options                  July 1990


   Peer-ID

      The Peer-ID field is zero or more octets and indicates the name of
      the peer to be authenticated.

   Passwd-Length

      The Passwd-Length field is one octet and indicates the length of
      the Password field

   Password

      The Password field is zero or more octets and indicates the
      password to be used for authentication.





































Perkins & Hobby                                                [Page 30]

RFC 1172                  PPP Initial Options                  July 1990


4.3.  Authenticate-Ack

   Description

      If the Peer-ID/Password pair received in an Authenticate is both
      recognizable and acceptable, then a PAP implementation should
      transmit a PAP packet with the Code field set to 2 (Authenticate-
      Ack), the Identifier field copied from the received Authenticate,
      and the Message field optionally filled with an ASCII message.

   A summary of the Authenticate-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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Msg-Length   |  Message  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

      2 for Authenticate-Ack.

   Identifier

      The Identifier field is one octet and aids in matching requests
      and replies.  The Identifier field MUST be copied from the
      Identifier field of the Authenticate which caused this
      Authenticate-Ack.

   Msg-Length

      The Msg-Length field is one octet and indicates the length of the
      Message field

   Message

      The Message field is zero or more octets and indicates an ASCII
      message.










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RFC 1172                  PPP Initial Options                  July 1990


4.4.  Authenticate-Nak

   Description

      If the Peer-ID/Password pair received in a Authenticate is not
      recognizable or acceptable, then a PAP implementation should
      transmit a PAP packet with the Code field set to 3 (Authenticate-
      Nak), the Identifier field copied from the received Authenticate,
      and the Message field optionally filled with an ASCII message.

   A summary of the Authenticate-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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Msg-Length   |  Message  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

      3 for Authenticate-Nak.

   Identifier

      The Identifier field is one octet and aids in matching requests
      and replies.  The Identifier field MUST be copied from the
      Identifier field of the Authenticate which caused this
      Authenticate-Nak.

   Msg-Length

      The Msg-Length field is one octet and indicates the length of the
      Message field

   Message

      The Message field is zero or more octets and indicates an ASCII
      message.










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RFC 1172                  PPP Initial Options                  July 1990


5.  IP Control Protocol (IPCP) Configuration Options

IPCP Configuration Options allow negotiatiation of desirable Internet
Protocol parameters.  Negotiable modifications proposed in this document
include IP addresses and compression protocols.

The initial proposed values for the IPCP Configuration Option Type field
(see [1]) are assigned as follows:

   1       IP-Addresses
   2       Compression-Type








































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RFC 1172                  PPP Initial Options                  July 1990


5.1.  IP-Addresses

   Description

      This Configuration Option provides a way to negotiate the IP
      addresses to be used on each end of the link.  By default, no IP
      addresses are assigned to either end.  An address specified as
      zero shall be interpreted as requesting the remote end to specify
      the address.  If an implementation allows the assignment of
      multiple IP addresses, then it may include multiple IP Address
      Configuration Options in its Configure-Request packets.  An
      implementation receiving a Configure-Request specifying multiple
      IP Address Configuration Options may send a Configure-Reject
      specifying one or more of the specified IP Addresses.  An
      implementation which desires that no IP addresses be assigned
      (such as a "half-gateway") may reject all IP Address Configuration
      Options.

   A summary of the IP-Addresses 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     |     Source-IP-Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Source-IP-Address (cont)      |  Destination-IP-Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Destination-IP-Address (cont)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      1

   Length

      10

   Source-IP-Address

      The four octet Source-IP-Address is the desired local address of
      the sender of a Configure-Request.  In a Configure-Ack,
      Configure-Nak or Configure-Reject, the Source-IP-Address is the
      remote address of the sender, and is thus a local address with
      respect to the Configuration Option receiver.





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RFC 1172                  PPP Initial Options                  July 1990


   Destination-IP-Address

      The four octet Destination-IP-Address is the remote address with
      respect to the sender of a Configure-Request.  In a Configure-Ack,
      Configure-Nak or Configure-Reject, the Destination-IP-Address is
      the local address of the sender, and is thus a remote address with
      respect to the Configuration Option receiver.

   Default

      No IP addresses assigned.








































Perkins & Hobby                                                [Page 35]

RFC 1172                  PPP Initial Options                  July 1990


5.2.  Compression-Type

   Description

      This Configuration Option provides a way to negotiate the use of a
      specific compression protocol.  By default, compression is not
      enabled.

   A summary of the Compression-Type 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     |       Compression-Type        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Data ...
   +-+-+-+-+

   Type

      2

   Length

      >= 4

   Compression-Type

      The Compression-Type field is two octets and indicates the type of
      compression protocol desired.  Values for the Compression-Type are
      always the same as the PPP Data Link Layer Protocol field values
      for that same compression protocol.  The most up-to-date values of
      the Compression-Type field are specified in "Assigned Numbers"
      [2].  Initial values are assigned as follows:

         Value (in hex)          Protocol

         0037                    Van Jacobson Compressed TCP/IP

   Data

      The Data field is zero or more octets and contains additional data
      as determined by the compression protocol indicated in the
      Compression-Type field.






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RFC 1172                  PPP Initial Options                  July 1990


   Default

      No compression protocol enabled.


References

   [1]   Perkins, D., "The Point-to-Point Protocol for the Transmission
         of Multi-Protocol of Datagrams Over Point-to-Point Links", RFC
         1171, July, 1990.

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


Security Considerations

   Security issues are discussed in Section 2.3.


Author's Address

   This proposal is the product of the Point-to-Point Protocol Working
   Group of the Internet Engineering Task Force (IETF). The working
   group can be contacted via the chair:

      Russ Hobby
      UC Davis
      Computing Services
      Davis, CA 95616

      Phone: (916) 752-0236

      EMail: rdhobby@ucdavis.edu

   Questions about this memo can also be directed to:

      Drew D. Perkins
      Carnegie Mellon University
      Networking and Communications
      Pittsburgh, PA 15213

      Phone: (412) 268-8576

      EMail: ddp@andrew.cmu.edu






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RFC 1172                  PPP Initial Options                  July 1990


Acknowledgments

   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: Ken Adelman (TGV),
   Craig Fox (NSC), Phill Gross (NRI), Russ Hobby (UC Davis), David
   Kaufman (Proteon), John LoVerso (Xylogics), Bill Melohn (Sun
   Microsystems), Mike Patton (MIT), Drew Perkins (CMU), Greg Satz
   (cisco systems) and Asher Waldfogel (Wellfleet).










































Perkins & Hobby                                                [Page 38]


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