PPP LCP Extensions

Network Working Group                                 W. Simpson, Editor
Request for Comments: 1570                                    Daydreamer
Updates: 1548                                               January 1994
Category: Standards Track


                           PPP LCP Extensions

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) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.  PPP
   defines an extensible Link Control Protocol (LCP) for establishing,
   configuring, and testing the data-link connection.  This document
   defines several additional LCP features which have been suggested
   over the past few years.

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

Table of Contents

     1.     Additional LCP Packets ................................    1
        1.1       Identification ..................................    1
        1.2       Time-Remaining ..................................    3
     2.     Additional LCP Configuration Options ..................    6
        2.1       FCS-Alternatives ................................    6
           2.1.1  LCP considerations ..............................    7
           2.1.2  Null FCS ........................................    8
        2.2       Self-Describing-Padding .........................    9
        2.3       Callback ........................................   11
        2.4       Compound-Frames .................................   12
           2.4.1  LCP considerations ..............................   14
     APPENDICES ...................................................   15
     A.     Fast Frame Check Sequence (FCS) Implementation ........   15
        A.1       32-bit FCS Computation Method ...................   15
     SECURITY CONSIDERATIONS ......................................   17
     REFERENCES ...................................................   17
     ACKNOWLEDGEMENTS .............................................   18
     CHAIR'S ADDRESS ..............................................   18
     EDITOR'S ADDRESS .............................................   18

1.  Additional LCP Packets

   The Packet format and basic facilities are already defined for LCP
   [1].

   Up-to-date values of the LCP Code field are specified in the most
   recent "Assigned Numbers" RFC [2].  This specification concerns the
   following values:

      12      Identification
      13      Time-Remaining




1.1.  Identification

   Description

      This Code provides a method for an implementation to identify
      itself to its peer.  This Code might be used for many diverse
      purposes, such as link troubleshooting, license enforcement, etc.

      Identification is a Link Maintenance packet.  Identification
      packets MAY be sent at any time, including before LCP has reached
      the Opened state.

      The sender transmits a LCP packet with the Code field set to 12
      (Identification), the Identifier field set, the local Magic-Number
      (if any) inserted, and the Message field filled with any desired
      data, but not exceeding the default MRU minus eight.

      Receipt of an Identification packet causes the RXR or RUC event.
      There is no response to the Identification packet.

      Receipt of a Code-Reject for the Identification packet SHOULD
      generate the RXJ+ (permitted) event.

      Rationale:

         This feature is defined as part of LCP, rather than as a
         separate PPP Protocol, in order that its benefits may be
         available during the earliest possible stage of the Link
         Establishment phase.  It allows an operator to learn the
         identification of the peer even when negotiation is not
         converging.  Non-LCP packets cannot be sent during the Link
         Establishment phase.

         This feature is defined as a separate LCP Code, rather than a
         Configuration-Option, so that the peer need not include it with
         other items in configuration packet exchanges, and handle
         "corrected" values or "rejection", since its generation is both
         rare and in one direction.  It is recommended that
         Identification packets be sent whenever a Configure-Reject is
         sent or received, as a final message when negotiation fails to
         converge, and when LCP reaches the Opened state.

   A summary of the Identification 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                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Message ...
   +-+-+-+-+-+-+-+-+


   Code

      12 for Identification

   Identifier

      The Identifier field MUST be changed for each Identification sent.

   Length

      >= 8

   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.

   Message

      The Message field is zero or more octets, and its contents are
      implementation dependent.  It is intended to be human readable,
      and MUST NOT affect operation of the protocol.  It is recommended
      that the message contain displayable ASCII characters 32 through
      126 decimal.  Mechanisms for extension to other character sets are
      the topic of future research.  The size is determined from the
      Length field.

      Implementation Note:

         The Message will usually contain such things as the sender's
         hardware type, PPP software revision level, and PPP product
         serial number, MIB information such as link speed and interface
         name, and any other information that the sender thinks might be
         useful in debugging connections.  The format is likely to be
         different for each implementor, so that those doing serial
         number tracking can validate their numbers.  A robust
         implementation SHOULD treat the Message as displayable text,
         and SHOULD be able to receive and display a very long Message.



1.2.  Time-Remaining

   Description

      This Code provides a mechanism for notifying the peer of the time
      remaining in this session.

      The nature of this information is advisory only.  It is intended
      that only one side of the connection will send this packet
      (generally a "network access server").  The session is actually
      concluded by the Terminate-Request packet.

      Time-Remaining is a Link Maintenance packet.  Time-Remaining
      packets may only be sent in the LCP Opened state.

      The sender transmits a LCP packet with the Code field set to 13
      (Time-Remaining), the Identifier field set, the local Magic-Number
      (if any) inserted, and the Message field filled with any desired
      data, but not exceeding the peer's established MRU minus twelve.

      Receipt of an Time-Remaining packet causes the RXR or RUC event.
      There is no response to the Time-Remaining packet.

      Receipt of a Code-Reject for the Time-Remaining packet SHOULD
      generate the RXJ+ (permitted) event.

      Rationale:

         This notification is defined as a separate LCP Code, rather
         than a Configuration-Option, in order that changes and warning
         messages may occur dynamically during the session, and that the
         information might be determined after Authentication has
         occurred.  Typically, this packet is sent when the link enters
         Network-Layer Protocol phase, and at regular intervals
         throughout the session, particularly near the end of the
         session.

   A summary of the Time-Remaining 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                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Seconds-Remaining                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Message ...
   +-+-+-+-+-+-+-+-+


   Code

      13 for Time-Remaining

   Identifier

      The Identifier field MUST be changed for each Time-Remaining sent.

   Length

      >= 12

   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.

   Seconds-Remaining

      The Seconds-Remaining field is four octets and indicates the
      number of integral seconds remaining in this session.  This 32 bit
      unsigned value is sent most significant octet first.  A value of
      0xffffffff (all ones) represents no timeout, or "forever".

   Message

      The Message field is zero or more octets, and its contents are
      implementation dependent.  It is intended to be human readable,
      and MUST NOT affect operation of the protocol.  It is recommended
      that the message contain displayable ASCII characters 32 through
      126 decimal.  Mechanisms for extension to other character sets are
      the topic of future research.  The size is determined from the
      Length field.

2.  Additional LCP Configuration Options

   The Configuration Option format and basic options are already defined
   for LCP [1].

   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:

      9       FCS-Alternatives
      10      Self-Describing-Padding
      13      Callback
      15      Compound-Frames




2.1.  FCS-Alternatives

   Description

      This Configuration Option provides a method for an implementation
      to specify another FCS format to be sent by the peer, or to
      negotiate away the FCS altogether.

      This option is negotiated separately in each direction.  However,
      it is not required that an implementation be capable of
      concurrently generating a different FCS on each side of the link.

      The negotiated FCS values take effect only during Authentication
      and Network-Layer Protocol phases.  Frames sent during any other
      phase MUST contain the default FCS.

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

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


   Type

      9
   Length

      3

   Options

      This field is one octet, and is comprised of the "logical or" of
      the following values:

          1   Null FCS
          2   CCITT 16-bit FCS
          4   CCITT 32-bit FCS


   Implementation Note:

      For most PPP HDLC framed links, the default FCS is the CCITT 16-
      bit FCS.  Some framing techniques and high speed links may use
      another format as the default FCS.


2.1.1.  LCP considerations

   The link can be subject to loss of state, and the LCP can re-
   negotiate at any time.  When the LCP begins renegotiation or
   termination, it is recommended that the LCP Configure-Request or
   Terminate-Request packet be sent with the last negotiated FCS, then
   change to the default FCS, and a duplicate LCP packet is sent with
   the default FCS.  The Identifier field SHOULD NOT be incremented for
   each such duplicate packet.

   On receipt of a LCP Configure-Request or Terminate-Request packet,
   the implementation MUST change to the default FCS for both
   transmission and reception.  If a Request packet is received which
   contains a duplicate Identifier field, a new reply MUST be generated.

   Implementation Notes:

      The need to send two packets is only necessary after the
      Alternative-FCS has already been negotiated.  It need not occur
      during state transitions when there is a natural indication that
      the default FCS is in effect, such as the Down and Up events.  It
      is necessary to send two packets in the Ack-Sent and Opened
      states, since the peer could mistakenly believe that the link has
      Opened.

      It is possible to send a single 48-bit FCS which is a combination
      of the 16-bit and 32-bit FCS.  This may be sent instead of sending
      the two packets described above.  We have not standardized this
      procedure because of intellectual property concerns.  If such a
      48-bit FCS is used, it MUST only be used for LCP packets.


2.1.2.  Null FCS

   The Null FCS SHOULD only be used for those network-layer and
   transport protocols which have an end-to-end checksum available, such
   as TCP/IP, or UDP/IP with the checksum enabled.  That is, the Null
   FCS option SHOULD be negotiated together with another non-null FCS
   option in a heterogeneous environment.

   When a configuration (LCP or NCP) or authentication packet is sent,
   the FCS MUST be included.  When a configuration (LCP or NCP) or
   authentication packet is received, the FCS MUST be verified.

   There are several cases to be considered:

   Null FCS alone

      The sender generates the FCS for those frames which require the
      FCS before sending the frame.

      When a frame is received, it is not necessary to check the FCS
      before demultiplexing.  Any FCS is treated as padding.

      Receipt of an Authentication or Control packet would be discovered
      after passing the frame to the demultiplexer.  Verification of the
      FCS can easily be accomplished using one of the software
      algorithms defined in "PPP in HDLC Framing" [3] (16-bit FCS) and
      Appendix A (32-bit FCS).

   Null FCS with another FCS, using software

      This is similar to the above case.

      Those packets which are required to have the FCS (Authentication,
      Control, or Network-Protocols lacking a checksum) are checked
      using software after demultiplexing.  Packets which fail the FCS
      test are discarded as usual.

   Null FCS with another FCS, using hardware

      A flag is passed with the frame, indicating whether or not it has
      passed the hardware FCS check.  The incorrect FCS MUST be passed
      with the rest of the data.  The frame MUST NOT be discarded until
      after demultiplexing, and only those frames that require the FCS
      are discarded.

   All three FCS forms (Null, 16 and 32) may be used concurrently on
   different frames when using software.  That is probably not possible
   with most current hardware.



2.2.  Self-Describing-Padding

   Description

      This Configuration Option provides a method for an implementation
      to indicate to the peer that it understands self-describing pads
      when padding is added at the end of the PPP Information field.

      This option is most likely to be used when some protocols, such as
      network-layer or compression protocols, are configured which
      require detection and removal of any trailing padding.  Such
      special protocols are identified in their respective documents.

      If the option is Rejected, the peer MUST NOT add any padding to
      the identified special protocols, but MAY add padding to other
      protocols.

      If the option is Ack'd, the peer MUST follow the procedures for
      adding self-describing pads, but only to the specifically
      identified protocols.  The peer is not required to add any padding
      to other protocols.

      Implementation Notes:

         This is defined so that the Reject handles either case where
         the peer does not generate self-describing pads.  When the peer
         never generates padding, it may safely Reject the option.  When
         the peer does not understand the option, it also will not
         successfully configure a special protocol which requires
         elimination of pads.

         While some senders might only be capable of adding padding to
         every protocol or not adding padding to any protocol, by design
         the receiver need not examine those protocols which do not need
         the padding stripped.

         To avoid unnecessary configuration handshakes, an
         implementation which generates padding, and has a protocol
         configured which requires the padding to be known, SHOULD
         include this Option in its Configure-Request, and SHOULD
         Configure-Nak with this Option when it is not present in the
         peer's Request.

      Each octet of self-describing pad contains the index of that
      octet.  The first pad octet MUST contain the value one (1), which
      indicates the Padding Protocol to the Compound-Frames option.
      After removing the FCS, the final pad octet indicates the number
      of pad octets to remove.  For example, three pad octets would
      contain the values 1, 2, 3.

      The Maximum-Pad-Value (MPV) is also negotiated.  Only the values 1
      through MPV are used.  When no padding would otherwise be
      required, but the final octet of the PPP Information field
      contains the value 1 through MPV, at least one self-describing pad
      octet MUST be added to the frame.  If the final octet is greater
      than MPV, no additional padding is required.

      Implementation Notes:

         If any of the pad octets contain an incorrect index value, the
         entire frame SHOULD be silently discarded.  This is intended to
         prevent confusion with the FCS-Alternatives option, but might
         not be necessary in robust implementations.

         Since this option is intended to support compression protocols,
         the Maximum-Pad-Value is specified to limit the likelihood that
         a frame may actually become longer.

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

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


   Type

      10

   Length

      3
   Maximum

      This field specifies the largest number of padding octets which
      may be added to the frame.  The value may range from 1 to 255, but
      values of 2, 4, or 8 are most likely.



2.3.  Callback

   Description

      This Configuration Option provides a method for an implementation
      to request a dial-up peer to call back.  This option might be used
      for many diverse purposes, such as savings on toll charges.

      When Callback is successfully negotiated, and authentication is
      complete, the Authentication phase proceeds directly to the
      Termination phase, and the link is disconnected.

      Then, the peer re-establishes the link, without negotiating
      Callback.

      Implementation Notes:

         A peer which agrees to this option SHOULD request the
         Authentication-Protocol Configuration Option.  The user
         information learned during authentication can be used to
         determine the user location, or to limit a user to certain
         locations, or merely to determine whom to bill for the service.

         Authentication SHOULD be requested in turn by the
         implementation when it is called back, if mutual authentication
         is desired.

   A summary of the Callback 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     |   Operation   |  Message ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type

      13
   Length

      >= 3

   Operation

      The Operation field is one octet and indicates the contents of the
      Message field.

      0       location is determined by user authentication

      1       Dialing string, the format and contents of which assumes
              configuration knowledge of the specific device which is
              making the callback.

      2       Location identifier, which may or may not be human
              readable, to be used together with the authentication
              information for a database lookup to determine the
              callback location.

      3       E.164 number.

      4       Distinguished name.

   Message

      The Message field is zero or more octets, and its general contents
      are determined by the Operation field.  The actual format of the
      information is site or application specific, and a robust
      implementation SHOULD support the field as undistinguished octets.
      The size is determined from the Length field.

      It is intended that only an authorized user will have correct site
      specific information to make use of the Callback.  The
      codification of the range of allowed usage of this field is
      outside the scope of this specification.



2.4.  Compound-Frames

   Description

      This Configuration Option provides a method for an implementation
      to send multiple PPP encapsulated packets within the same frame.
      This option might be used for many diverse purposes, such as
      savings on toll charges.

      Only those PPP Protocols which have determinate lengths or
      integral length fields may be aggregated into a compound frame.

      When Compound-Frames is successfully negotiated, the sender MAY
      add additional packets to the same frame.  Each packet is
      immediately followed by another Protocol field, with its attendant
      datagram.

      When padding is added to the end of the Information field, the
      procedure described in Self-Describing-Padding is used.
      Therefore, this option MUST be negotiated together with the Self-
      Describing-Padding option.

      If the FCS-Alternatives option has been negotiated, self
      describing padding MUST always be added.  That is, the final
      packet MUST be followed by a series of octets, the first of which
      contains the value one (1).

      On receipt, the first Protocol field is examined, and the packet
      is processed as usual.  For those datagrams which have a
      determinate length, the remainder of the frame is returned to the
      demultiplexor.  Each succeeding Protocol field is processed as a
      separate packet.  This processing is complete when a packet is
      processed which does not have a determinate length, when the
      remainder of the frame is empty, or when the Protocol field is
      determined to have a value of one (1).

      The PPP Protocol value of one (1) is reserved as the Padding
      Protocol.  Any following octets are removed as padding.

   A summary of the Compound-Frames 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

      15

   Length

      2

2.4.1.  LCP considerations

   During initial negotiation, the Compound-Frames option can be used to
   minimize the negotiation latency, by reducing the number of frames
   exchanged.

   The first LCP Configure-Request packet is sent as usual in a single
   frame, including the Self-Describing-Padding and Compound-Frames
   options.

   The peer SHOULD respond with a Configure-Ack, followed in a compound
   frame by its LCP Configure-Request, and any NCP Configure-Requests
   desired.

   Upon receipt, the local implementation SHOULD process the Configure-
   Ack as usual.  Since the peer has agreed to send compound frames, the
   implementation MUST examine the remainder of the frame for additional
   packets.  If the peer also specified the Self-Describing-Padding and
   Compound-Frames options in its Configure-Request, the local
   implementation SHOULD retain its Configure-Ack, and further NCP
   configuration packets SHOULD be added to the return frame.

   Together with the peer's final return frame, the minimum number of
   frames to complete configuration is 4.

A.  Fast Frame Check Sequence (FCS) Implementation

A.1.  32-bit FCS Computation Method

   The following code provides a table lookup computation for
   calculating the 32-bit Frame Check Sequence as data arrives at the
   interface.

   /*
    * u32 represents an unsigned 32-bit number.  Adjust the typedef for
    * your hardware.
    */
   typedef unsigned long u32;

   static u32 fcstab_32[256] =
      {
      0x00000000, 0x77073096, 0xee0e612c, 0x990951ba,
      0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3,
      0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
      0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91,
      0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
      0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
      0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec,
      0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5,
      0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
      0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
      0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940,
      0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
      0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116,
      0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f,
      0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
      0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
      0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a,
      0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
      0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818,
      0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
      0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
      0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457,
      0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c,
      0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
      0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
      0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
      0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
      0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9,
      0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086,
      0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
      0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4,
      0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad,
      0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
      0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683,
      0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
      0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
      0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe,
      0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7,
      0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
      0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
      0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252,
      0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
      0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60,
      0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79,
      0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
      0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f,
      0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04,
      0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
      0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a,
      0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
      0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
      0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21,
      0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e,
      0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
      0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
      0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45,
      0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
      0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db,
      0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0,
      0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
      0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6,
      0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf,
      0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
      0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
      };

   #define PPPINITFCS32  0xffffffff   /* Initial FCS value */
   #define PPPGOODFCS32  0xdebb20e3   /* Good final FCS value */

   /*
    * Calculate a new FCS given the current FCS and the new data.
    */
   u32 pppfcs32(fcs, cp, len)
       register u32 fcs;
       register unsigned char *cp;
       register int len;
       {
       ASSERT(sizeof (u32) == 4);
       ASSERT(((u32) -1) > 0);
       while (len--)

           fcs = (((fcs) >> 8) ^ fcstab_32[((fcs) ^ (*cp++)) & 0xff]);

       return (fcs);
       }

   /*
    * How to use the fcs
    */
   tryfcs32(cp, len)
       register unsigned char *cp;
       register int len;
   {
       u32 trialfcs;

       /* add on output */
       trialfcs = pppfcs32( PPPINITFCS32, cp, len );
       trialfcs ^= 0xffffffff;             /* complement */
       cp[len] = (trialfcs & 0x00ff);      /* Least significant byte first */
       cp[len+1] = ((trialfcs >>= 8) & 0x00ff);
       cp[len+2] = ((trialfcs >>= 8) & 0x00ff);
       cp[len+3] = ((trialfcs >> 8) & 0x00ff);

       /* check on input */
       trialfcs = pppfcs32( PPPINITFCS32, cp, len + 4 );
       if ( trialfcs == PPPGOODFCS32 )
           printf("Good FCS\n");
   }


Security Considerations

   Security issues are briefly discussed in sections concerning the
   Callback Configuration Option.


References

   [1]   Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", RFC
         1548, Daydreamer, December 1993.

   [2]   Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
         RFC 1340, USC/Information Sciences Institute, July 1992.

   [3]   Simpson, W., Editor, "PPP in HDLC Framing", RFC 1549,
         Daydreamer, December 1993.

Acknowledgments

   The Identification feature was suggested by Bob Sutterfield (Morning
   Star Technologies).

   The Time-Remaining feature was suggested by Brad Parker (FCR).

   Some of the original text for FCS-Alternatives was provided by Arthur
   Harvey (then of DEC).  The Null FCS was requested by Peter Honeyman
   (UMich).  The 32-bit FCS example code was provided by Karl Fox
   (Morning Star Technologies).

   Self-Describing-Padding was suggested and named by Fred Baker (ACC).

   Compound-Frames was suggested by Keith Sklower (Berkeley).

   Special thanks to Morning Star Technologies for providing computing
   resources and network access support for writing this specification.


Chair's Address

   The working group can be contacted via the current chair:

      Fred Baker
      Advanced Computer Communications
      315 Bollay Drive
      Santa Barbara, California  93117

      EMail: 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

      EMail: Bill.Simpson@um.cc.umich.edu
             bsimpson@MorningStar.com