





Network Working Group                                     C. Richards
Request for Comments: 2125                          Shiva Corporation
Category: Standards Track                                    K. Smith
                                          Ascend Communications, Inc.
                                                           March 1997


              The PPP Bandwidth Allocation Protocol (BAP)
          The PPP Bandwidth Allocation Control Protocol (BACP)

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

   This document proposes a method to manage the dynamic bandwidth
   allocation of implementations supporting the PPP multilink protocol
   [2].  This is done by defining the Bandwidth Allocation Protocol
   (BAP), as well as its associated control protocol, the Bandwidth
   Allocation Control Protocol (BACP).  BAP can be used to manage the
   number of links in a multilink bundle.  BAP defines datagrams to co-
   ordinate adding and removing individual links in a multilink bundle,
   as well as specifying which peer is responsible for which decisions
   regarding managing bandwidth during a multilink connection.

Table of Contents

   1.     Introduction ..........................................    2
      1.1       Specification of Requirements ...................    2
      1.2       Terminology .....................................    3
   2.     New LCP Configuration Option ..........................    3
      2.1       Link Discriminator ..............................    3
   3.     BACP Operation ........................................    4
   4.     BACP Configuration Options ............................    5
      4.1       Favored-Peer ....................................    5
   5.     BAP Operation .........................................    7
      5.1       Link Management .................................    7
      5.2       Bandwidth Management ............................    8
      5.3       BAP Packets .....................................    8
      5.4       Race Conditions .................................    9
      5.5       BAP Datagram Format .............................    9
         5.5.1  Call-Request ....................................   12
         5.5.2  Call-Response ...................................   12



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         5.5.3  Callback-Request ................................   13
         5.5.4  Callback-Response ...............................   13
         5.5.5  Link-Drop-Query-Request .........................   13
         5.5.6  Link-Drop-Query-Response ........................   13
         5.5.7  Call-Status-Indication ..........................   14
         5.5.8  Call-Status-Response ............................   14
   6.     BAP Datagram Options ..................................   14
      6.1       Link-Type .......................................   15
      6.2       Phone-Delta .....................................   17
         6.2.1  Phone-Delta Sub-Options .........................   18
      6.3       No-Phone-Number-Needed ..........................   19
      6.4       Reason ..........................................   20
      6.5       Link-Discriminator ..............................   21
      6.6       Call-Status .....................................   21
   Appendix - List of BAP datagrams and associated fields .......   23
   ACKNOWLEDEMENTS ..............................................   23
   SECURITY CONSIDERATIONS ......................................   23
   REFERENCES ...................................................   24
   CHAIR'S ADDRESS ..............................................   24
   EDITORS'S ADDRESSES ..........................................   24

1.  Introduction

   As PPP multilink implementations become increasingly common, there is
   a greater need for some conformity in how to manage bandwidth over
   such links. BACP and BAP provide a flexible yet robust way of
   managing bandwidth between 2 peers.  BAP does this by defining Call-
   Control packets and a protocol that allows peers to co-ordinate the
   actual bandwidth allocation and de-allocation.  Phone number deltas
   may be passed in the Call-Control packets to minimize the end user's
   configuration.

1.1.  Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

   MUST      This word, or the adjective "required", means that the
             definition is an absolute requirement of the specification.

   MUST NOT  This phrase means that the definition is an absolute
             prohibition of the specification.

   SHOULD    This word, or the adjective "recommended", means that there
             may exist valid reasons in particular circumstances to
             ignore this item, but the full implications must be
             understood and carefully weighed before choosing a
             different course.



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   MAY       This word, or the adjective "optional", means that this
             item is one of an allowed set of alternatives.  An
             implementation which does not include this option MUST be
             prepared to interoperate with another implementation which
             does include the option.

1.2.  Terminology

   This document frequently uses the following terms:

   peer      The other end of the point-to-point link

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

   BOD (bandwidth on demand)
         BOD refers to the ability of a system to allocate and remove
         links in a multilink system to change the bandwidth of a
         multilink bundle.  This may be done in response to changing
         line conditions and it also may be done in response to changing
         resource conditions.  In either case, changing bandwidth
         dynamically during a multilink connection is referred to as
         BOD.

2.  New LCP Configuration Option

   Implementations MUST implement LCP as defined in [1].  LCP MUST be in
   the Network-Layer Protocol phase before BACP can be negotiated.

2.1.  Link Discriminator

   Description

      This LCP Configuration Option is used to declare a unique
      discriminator for the link that the option is sent over.  This
      option MUST be negotiated by LCP on every link.  BAP uses the link
      discriminator to differentiate the various links in a multilink
      bundle. Each link in a multilink bundle MUST have a unique
      discriminator.  The discriminator is independent for each peer, so
      each link may have 2 different LCP Link Discriminator values, one
      for each peer. When the Link Discriminator is sent in a BAP
      packet, the transmitter sends the Link Discriminator Option value
      received from its peer in the peer's LCP Configure Request packet.




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   A summary of the Link Discriminator LCP 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    |       Link Discriminator      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      23 for Link Discriminator option.

   Length

      4

   Link Discriminator

      The Link Discriminator field is 2 octets in Length, and it
      contains a unique identifier used to indicate a particular link in
      a multilink bundle.  The Link Discriminator for a link MUST be
      unique among the Link Discriminators assigned by this endpoint for
      this bundle.  The Link Discriminator MAY be assigned in a
      sequential, monotonically increasing manner.

3.  BACP Operation

   BACP uses the same packet exchange mechanism as the Link Control
   Protocol defined in [1].  BACP packets MUST NOT be exchanged until
   PPP has reached the Network-Layer Protocol phase.  BACP packets
   received before this phase is reached should be silently discarded.

   BACP is negotiated once per multilink bundle.  If BACP is negotiated
   on any of the links in a multilink bundle, it is opened for all of
   the links in the bundle.

   The Bandwidth Allocation Control Protocol is exactly the same as the
   Link Control Protocol [1] with the following exceptions:

      Data Link Layer Protocol Field

         Exactly one BACP packet is encapsulated in the Information
         field of PPP Data Link Layer frames where the Protocol field
         indicates Type hex c02b (Bandwidth Allocation Control
         Protocol).





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      Code field

         Only Codes 1 through 7 (Configure-Request, Configure-Ack,
         Configure-Nak, Configure-Reject, Terminate-Request, Terminate-
         Ack and Code-Reject) are used.  Other Codes should be treated
         as unrecognized and should result in Code-Rejects.

   Configuration Option Types

         BACP has a distinct set of Configuration Options, which are
         defined in the next section.

4.  BACP Configuration Options

   BACP Configuration Options allow negotiation of desirable BACP
   parameters.  These options are used in Config-Request, Config-Ack,
   Config-Nak, and Config-Reject packets.  BACP uses the same
   Configuration Option format defined for LCP [1], with a seperate set
   of Options.

   Current values of BACP Configuration Options are assigned as follows:

      1     Favored-Peer

4.1.  Favored-Peer

   Description

      This Configuration Option is used to determine which peer is
      favored in the event of a race condition in which 2 peers
      simultaneously transmit the same BAP request.  Each peer
      negotiates a 4 octet magic number, which is successfully
      negotiated when the 2 Magic-Numbers are different.  The favored
      peer is the peer that transmits the lowest Magic-Number in its
      Favored-Peer Configuration Option.

      The Favored-Peer Configuration Option MUST be implemented.














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      BACP will usually be negotiated after only one link of a multilink
      bundle has reached the Network-Layer Protocol phase. In this
      situation, it is acceptable for the peer that initiated the
      connection to use a Magic-Number of 1, and the peer that responded
      to the connection to use a Magic-Number of 0xFFFFFFFF.  If a
      multilink bundle has been established with links that were
      originated by each peer, or if it is not clear which peer has
      initiated a link (on a leased line, for example), then a random
      number MUST be used for the Magic-Number.  Refer to the
      description of the LCP Magic-Number Configuration Option in [1]
      for an explanation of how to create a useful random number.

      When a Configure-Request is received with a Favored-Peer
      Configuration Option, the received Magic-Number is compared with
      the Magic-Number of the last Configure-Request sent to the peer.
      If the two Magic-Numbers are different, then the Favored-Peer
      negotiation has been successful, and the Favored-Peer Option
      SHOULD be acknowledged.  If the two Magic-Numbers are equal, a
      Configure-Nak MUST be sent specifying a different Magic-Number
      value.  A new Configure-Request SHOULD NOT be sent to the peer
      until normal processing would cause it to be sent (that is, until
      a Configure-Nak is received or the Restart timer runs out).

   A summary of the Favored-Peer 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

      1 for Favored-Peer

   Length

      6

   Magic-Number

      The Magic-Number field is four octets, and indicates a number
      which is very likely to be unique to one end of the link.  A
      Magic-Number of zero is illegal and MUST always be Nak'd.




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5.  BAP Operation

5.1.  Link Management

   BAP defines packets, parameters and negotiation procedures to allow
   two endpoints to negotiate gracefully adding and dropping links from
   a multilink bundle.  An implementation can:

      o Request permission to add a Link to a bundle (Call-Request)

      o Request that the peer add a link to a bundle via a callback
        (Callback-Request)

      o Negotiate with the peer to drop a link from a bundle (this
        implies that the peer can refuse) (Link-Drop-Query-Request)

   After BACP reaches the opened state, either peer MAY request that
   another link be added to the bundle by sending a BAP Call- or
   Callback-Request packet.  A Call-Request packet is sent if the
   implementation wishes to originate the call for the new link, and a
   Callback-Request packet is sent if the implementation wishes its peer
   to originate the call for the new link.  The implementation receiving
   a Call- or Callback-Request MUST respond with a Call- or Callback-
   Response with a valid Response Code.

   After BACP reaches the opened state, either peer MAY request that a
   link be dropped from the bundle.  A BAP Link-Drop-Query-Request
   packet is sent to the peer to negotiate dropping a link.  The peer
   MUST respond with a Link-Drop-Query-Response.   If the peer is
   agreeable to dropping the link the implementation MUST issue an LCP
   Terminate-Request to initiate dropping the link.

   If an implementation wishes to force dropping a link without
   negotiation, it should simply send an LCP Terminate-Request packet on
   the link (without sending any BAP Link-Drop-Query-Request).

   After an LCP Terminate-Request is sent an implementation SHOULD stop
   transmitting data packets on that link, but still continue to receive
   and process data packets normally until receipt of a Terminate-Ack
   from the peer.  The receiver of an LCP Terminate-Request SHOULD stop
   transmitting packets before issuing the Terminate-Ack.  This
   procedure will insure that no data is lost in either direction.









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5.2.  Bandwidth Management

   BAP allows two peer implementations to manage the bandwidth available
   to the protocols using the multilink bundle by negotiating when to
   add and drop links (See Link Management).  Use of the negotiation
   features of BAP makes it unnecessary to require a 'common' algorithm
   for determining when to add and remove links in a multilink bundle.

   BOD decisions can be based on link utilization.  A BAP implementation
   may monitor its transmit traffic, both transmit and receive traffic,
   or choose not to monitor traffic in either direction.  If a server
   system implements bi-directional monitoring, it will allow BOD
   operation with a client that does not monitor traffic in either
   direction, which will minimize the end-user's configuration.  When an
   implementation decides that it is time to remove a link due to
   traffic monitoring, it MUST transmit a Link-Drop-Query-Request to
   inquire if the peer agrees to drop a link from the current multilink
   bundle.  When an implementation receives a Link-Drop-Query-Request,
   it SHOULD base its response on the traffic it is monitoring.  It MUST
   NOT base its response solely on its receive data heuristics.

   The operation of the Link-Drop-Query-Request and -Response datagrams
   causes a link in a multilink bundle to be left up as long as either
   implementation that is monitoring link utilization determines that it
   is necessary.

   BOD decisions can also be based on the resources (e.g., physical
   port, B-channel, etc.) available to an implementation.  For example,
   an implementation might remove a link from a multilink bundle to
   answer an incoming voice call, or might add a link when a line
   becomes free due to the termination of a separate PPP call on another
   port.  An implementation MUST use an LCP Terminate-Request to remove
   a link due to a resource condition.

5.3.  BAP Packets

   All of the BAP Request and Indication packets require a Response
   packet in response before taking any action.

   An implementation MUST set a timer when sending a Request or
   Indication packet. The value of this timer SHOULD depend on the type
   and speed of the link or links in use.  Upon expiration of this
   timer, the implementation MUST retransmit the request or indication,
   with an identical identification number.  This procedure will insure
   that the peer receives the proper request or indication even if a
   packet is lost during transmission.  If a response packet is lost the
   peer will realize that this is not a new request or indication
   packet.



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   If the number of retransmissions exceeds the number supported by the
   implementation for this packet, the implementation MAY take
   appropriate recovery action. For example, if no response to a Link-
   Drop-Query-Request is received after 2 retransmissions, an
   implementation MAY initiate dropping the link by sending an LCP
   Terminate-Request for that link.

   Since BAP packets help determine the amount of bandwidth available to
   an implementation, PPP SHOULD give them priority over other data
   packets when transmitting.  This will help insure the prompt addition
   and removal of links in a multilink bundle.  This is especially
   important when adding links to a bundle due to bandwidth constraints.

5.4.  Race Conditions

   In order to resolve race conditions, an implementation MUST implement
   the BACP Favored-Peer Configuration Option.

   A race condition can occur if both implementations send a Call-
   Request, Callback-Request or Link-Drop-Query-Request at the same
   time.  These race conditions should be solved as follows:

      If each implementation sends a Call-Request or Callback-Request at
      the same time, the implementation with the lowest BACP Favored-
      Peer Magic-Number value SHOULD be favored.

      If each implementation sends a Link-Drop-Query-Request at the same
      time, the same scheme SHOULD be used as for Call-Requests.

5.5.  BAP Datagram Format

   Description

      Before any BAP packets may be communicated, PPP MUST reach the
      Network-Layer Protocol phase, and BACP MUST reach the opened
      state.

      Exactly one BAP packet is encapsulated in the Information field of
      PPP Data Link Layer frames where the Protocol field indicates type
      hex c02d (Bandwidth Allocation Protocol).

      Because ISDN Terminal Adapters sometimes are used to do multilink
      with a non-multilink aware client, BAP datagrams MUST NOT be
      compressed or encrypted.  Otherwise, the ISDN TA may not be able
      to properly intercept BAP datagrams needed to control the
      multilink connection.  This refers to compression of the whole
      datagram; Address-and-Control-Field-Compression and Protocol-
      Field-Compression are allowed if properly negotiated.



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      The maximum length of a BAP packet transmitted over a PPP link is
      the same as the maximum length of the Information field of a PPP
      data link layer frame.

      Bandwidth Allocation Protocol datagrams can be catagorized as
      either Request, Indication or Response packets.  Every Request and
      Indication datagram has a corresponding Response packet.  Request
      and Indication datagrams have a slightly different format from
      Response datagrams, as the Response datagrams include a Response
      Code octet.

      All of the BAP datagrams MUST be supported by an implementation.
      However, that does not mean an implementation must support all BAP
      datagram actions.  An implementation MAY send a Request-Rej to a
      Request that it does not implement.

   A summary of the Bandwidth Allocation Protocol datagram Request and
   Indication 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Data ...
   +-+-+-+-+-+-+-+-+

   A summary of the Bandwidth Allocation Protocol datagram Response
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Response Code |    Data ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      The Type field is one octet and identifies the type of BAP
      datagram packet.  Datagram types are defined as follows.  This
      field is coded in binary coded hexadecimal.






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         01       Call-Request
         02       Call-Response
         03       Callback-Request
         04       Callback-Response
         05       Link-Drop-Query-Request
         06       Link-Drop-Query-Response
         07       Call-Status-Indication
         08       Call-Status-Response

      The various types of BAP datagrams are explained in the following
      sections.

   Identifier

      The Identifier field is one octet and is binary coded.  It aids in
      matching Requests and Indications with Responses.  Call-Status-
      Indication packets MUST use the same Identifier as was used by the
      original Call-Request or Callback-Request that was used to
      initiate the call.  All other Request or Indication packets MUST
      use a unique Identifier for each new Request or Indication.  All
      Response packets MUST use the same Identifier as the Identifier in
      the Request or Indication packet being responded to.  When re-
      transmitting a request or indication, the Identifier MUST be the
      same as the Identifier used on the previous transmission of the
      request or indication.

   Length

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

   Response Code

      The Response Code is only present in Response datagrams.  It is
      binary coded and can have the following values:

         00000000        Request-Ack
         00000001        Request-Nak
         00000010        Request-Rej
         00000011        Request-Full-Nak

      The Request-Ack Response Code is sent to indicate that the Request
      or Indication command is valid and was successfully received by an
      implementation. The Request-Nak Response Code is sent to indicate
      that the Request command was received, but an implementation does



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      not want the requested action performed at this time.  If a
      Response containing a Request-Nak Response Code is received, the
      original Request MAY be retried after an implementation determines
      that sufficient time has elapsed.  The Request-Rej Response Code
      is sent to indicate that the Request command received by an
      implementation is not implemented (i.e., if reception of a
      particular request type is not supported by the peer.) The
      Request-Full-Nak Response Code is sent to indicate that the
      Request command was received, but an implementation does not want
      the requested action performed.  The Request-Full-Nak is used to
      indicate that an implementation has reached the maximum (for a
      Call- or Callback-Request) or the minimum (for a Link-Drop-Query-
      Request) bandwidth configured or available for this multilink
      bundle.  If a Response containing a Request-Full-Nak Response Code
      is received, the original Request SHOULD NOT be retried until the
      total bandwidth of the multilink bundle has changed.

   Data

      The Data field is variable in length, and will usually contain the
      list of zero or more BAP Options that the sender desires to
      transmit. The format of BAP Options is described in a later
      chapter.

5.5.1.  Call-Request

   Before originating a call to add another link to a multilink bundle,
   an implementation MUST transmit a Call-Request packet.  This will
   inform the receiver of the request to add another link to the bundle
   and give the receiver a chance to inform the implementation of the
   phone number of a free port that can be called.

   The options field MUST include the Link-Type option.  The options
   field MAY include the No-Phone-Number and/or the Reason options.

   Upon reception of a Call-Request, a Call-Response datagram MUST be
   transmitted.

5.5.2.  Call-Response

   An implementation MUST transmit a Call-Response datagram in response
   to a received Call-Request datagram.  If the Call-Request is
   acceptable, the Call-Response MUST have a Response Code of Request-
   Ack.  The Phone-Delta option MUST be included in a Call-Response
   packet with a Response Code of Request-Ack unless the Call-Request
   included the No-Phone-Number option. The options field MAY include
   the Reason and/or Link-Type options.




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5.5.3.  Callback-Request

   An implementation that wants its peer to originate another link to
   add to the multilink bundle MUST transmit a Callback-Request packet
   to its peer.  This will inform the receiver of the request to add
   another link to the bundle along with the number to be called.

   The options field MUST include the Link-Type and Phone-Delta options.
   The Reason option MAY also be included.

   Upon reception of a Callback-Request, a Callback-Response datagram
   MUST be transmitted.

5.5.4.  Callback-Response

   An implementation MUST transmit a Callback-Response datagram in
   response to a received Callback-Request datagram.  If the Callback-
   Request is acceptable, the Callback-Response MUST have a Response
   Code of Request-Ack.  A Callback-Response packet MAY include the
   Link-Type option.

5.5.5.  Link-Drop-Query-Request

   An implementation that determines that a link is no longer needed and
   wishes to negotiate dropping it (e.g., based on a throughput BOD
   decision), MUST transmit a Link-Drop-Query-Request packet. The
   options field MUST include the Link-Discriminator option (containing
   the receiver's Link-Discriminator), and MAY include the Reason
   option.

   Upon reception of a Link-Drop-Query-Request, an implementation MUST
   transmit a Link-Drop-Query-Response datagram.  The Response-Code will
   be Request-Ack if it agrees to drop the link; if it does not agree to
   drop the link the Response-Code will be Request-Nak or Request-Full-
   Nak.  After the receipt of a Link-Drop-Query-Response with a Response
   Code of Request-Ack, the transmitter of the Link-Drop-Query-Request
   MUST initiate tear down of the indicated link by sending an LCP
   Terminate-Request packet on the designated link.

5.5.6.  Link-Drop-Query-Response

   An implementation transmits a Link-Drop-Query-Response datagram in
   response to a received Link-Drop-Query-Request datagram.  If the
   implementation agrees (e.g., based on its throughput BOD algorithm)
   to reduce the bandwidth of the multilink bundle, then the Response
   Code MUST be set to Request-Ack.





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   The Reason option MAY be included in the Link-Drop-Query-Response
   packet.

   The Link-Drop-Query-Request datagram MUST be supported, as well as
   the underlying implementation to respond to it.  This means that a
   Link-Drop-Query-Response with a Response Code of Request-Rej MUST NOT
   be transmitted in response to a Link-Drop-Query-Request.

5.5.7.  Call-Status-Indication

   After an implementation attempts to add a link to a bundle as the
   result of a Call-Request or a Callback-Request, it MUST send a Call-
   Status-Indication packet to its peer to indicate if the attempt to
   add the link succeeded or failed.  One Indication MUST be sent for
   each attempt made. For each Call-Status-Indication packet transmitted
   with the Call-Status Option Action octet set to Retry, a subsequent
   Call-Status-Indication packet MUST be sent to indicate the success or
   failure of the retry.  The Call-Status option MUST be included to
   inform the receiver of the status of the attempt to add a link and
   the action the implementation will take in case of failure.  The
   reason option MAY also be included in the Call-Status-Indication
   packet.

   Upon reception of a Call-Status-Indication packet which indicates a
   failure, an implementation may log the failure and reason code.  Upon
   reception of any Call-Status-Indication packet, a Call-Status-
   Response datagram MUST be transmitted.

5.5.8.  Call-Status-Response

   An implementation transmits a Call-Status-Response datagram in
   response to a received Call-Status-Indication datagram.  The Response
   Code field MUST be set to Request-Ack in this packet.  The Reason
   option MAY be included in this packet.

6.  BAP Datagram Options

   BAP Datagram Options are used in various BAP packets.  Their use in
   various packets is as defined below.  The format of these options
   loosely follows the formatting conventions of LCP Configuration
   Options.  When there are multiple BAP Options in one BAP packet, the
   options MAY be transmitted in any order.









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   A summary of the BAP Option format is shown below.  The fields are
   transmitted from left to right.

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


   Type

      The type field is one octet, and indicates the type of the BAP
      Datagram Option.  This field is binary coded Hexadecimal.  The
      following options are currently defined:

         01   Link-Type
         02   Phone-Delta
         03   No-Phone-Number-Needed
         04   Reason
         05   Link-Discriminator
         06   Call-Status


   Length

      The Length field is one octet, and indicates the length of this
      BAP Option including the Type, Length, and Data fields.

   Data

      The Data field is zero or more octets, and contains information
      specific to the BAP Option.  The format and length of the Data
      field is determined by the Type and Length fields.

6.1.  Link-Type

   Description

      This option indicates the general type of link indicated for the
      operation being performed.  This option does not indicate a
      specific link type, rather it gives some general characteristics
      of the desired link type.  This option MAY be used along with
      other knowledge (i.e., the type of the other link(s) in the bundle
      or user configuration) to determine the type of link desired to be
      used in the operation.  It MUST be included in a Call- or
      Callback-Request, and MAY be included in a Call- or Callback-
      Response.



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   A summary of the Link-Type BAP 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     |       Link Speed (kbps)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Link Type    |
   +-+-+-+-+-+-+-+-+

   Type

      01 for Link-Type.

   Length

      The Length field is one octet, and indicates the length of this
      BAP Option including the Type, Length and Link Type fields.

   Link Speed

      The Link Speed field is 2 octets, and indicates the requested
      speed of the desired link in kilobits per second.  This field is
      coded as 2 binary coded hexadecimal octets, with the most
      significant octet sent first.

   Link Type

      The Link Type field is a bit mask.  It is 1 octet in length.  Bit
      0 of the Link Type field corresponds to bit 39 of the Link-Type
      BAP Option as described above.  If a bit is set, it indicates
      support of the corresponding link type.  If the link indicated is
      different than the supported link types, no bit will be set.
      Otherwise, at least one bit MUST be set.  If an implementation
      supports more than one link type, more than one bit MAY be set.

         Bit     Link type
         ---     -------------
          0      ISDN
          1      X.25
          2      analog
          3      switched digital (non-ISDN)
          4      ISDN data over voice
          5-7    reserved

      If the Length field contains more bits than are defined by this
      specification, then any bits that are not defined should be



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      ignored.  In order to allow for future expansion of this field, it
      is important to properly support receiving a Link Type field
      longer than what is defined by this specification.  If the Length
      field is shorter than the number of bits defined, then the
      implementation should set all bits not received to 0.

6.2.  Phone-Delta

   Description

      The BAP Phone-Delta Option is used by an implementation to give
      its peer the information needed to make a call.  Due to the
      difficulty of determining which dialing prefixes (if any) are
      necessary to dial a given phone number/national destination
      code/country code combination, the phone number to be dialed will
      be based on a previously known number.  This MAY be the original
      number used to establish the first link of the multilink bundle, a
      number configured by the user, the phone number used to make a
      callback connection, or a number determined in some other way.
      The Phone-Delta Option will consist of a Subscriber-Number Sub-
      Option along with a Unique-Digits Sub-Option that indicates how
      many of the digits of the Subscriber-Number are unique among the
      ports in use, previously used, and to be used in the multilink
      bundle.  There is also an optional Phone-Number-Sub-Address Sub-
      Option.

      An implementation MAY include more than one Phone-Delta option in
      a response.  This indicates that there is more than one phone
      number that can be used for the requested operation.  The Phone-
      Delta option MUST appear in a Callback-Request.  It also MUST
      appear in a Call-Response with a Response Code set to Request-Ack
      if the Call-Request did not contain the No-Phone-Number option.
      It MAY be included in the Call-Status-Indication packet.

   A summary of the Phone-Delta BAP 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     |Sub-Option Type| Sub-Option Len|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Sub-Option...
   +-+-+-+-+-+-+-+-+

   Type

      02 for Phone-Delta.



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   Length

      The Length field is one octet, and indicates the length of this
      BAP Option including the Type, Length, and Sub-Option fields.

   Sub-Option Type

      The following Sub-Option Types are defined for the Phone-Delta
      option.

          01   Unique-Digits
          02   Subscriber-Number
          03   Phone-Number-Sub-Address

   Sub-Option Length

      The Sub-Option Length field is one octet, and indicates the length
      of this BAP Sub-Option including the Sub-Option Type, Sub-Option
      Length, and Sub-Option fields.

6.2.1.  Phone-Delta Sub-Options

   Unique-Digits

      The Unique-Digits Sub-Option field consists of one octet that is a
      count of the number of rightmost digits of the Subscriber-Number
      that are different from the set of phone numbers of the ports used
      in this multilink connection.  (For example, if the first port of
      a multilink bundle has a phone number of 123456789, and an
      implementation wanted its peer to call a port with a phone number
      of 123456888, the Unique-Digits octet would be 3.) If the Phone-
      Number-Sub-Address Sub-Option is present, the Unique-Digits Sub-
      Option MUST NOT include any of the Sub Address digits in its count
      of different rightmost digits.

      This field is required.

   Subscriber-Number

      This field is the phone number of the port that should be called
      by the peer. Any digits that precede the rightmost unique digits
      of the Subscriber-Number are provided for informational purposes
      only, and do not need to be included in this field.  This field is
      an ASCII string and MUST contain only ASCII characters indicating
      valid phone number digits.  This field is required.






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   Phone-Number-Sub-Address

      This field is the sub address of the port to be called by the
      peer.  This sub-option SHOULD only be used for an ISDN call. This
      field is an ASCII string and only contains valid phone number
      digits. This field is optional.

6.3.  No-Phone-Number-Needed

   Description

      The No-Phone-Number option indicates that the calling
      implementation is already configured with the phone number of its
      multilink peer and the answering implementation MUST NOT include
      the Phone Number option in the response.  This may be for security
      reasons, for configuration reasons, or for any other reason.

      This option MAY be used in a Call-Request packet.

   A summary of the No-Phone-Number BAP 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

      03 for No-Phone-Number.

   Length

      2
















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6.4.  Reason

   Description

      This option is used to indicate a reason for the Request or
      Response.  It is meant to be used for informational purposes only.
      This option MAY be used in any BAP packet.

   A summary of the Reason BAP 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     |         Reason String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      04 for Reason.

   Length

      The Length field is one octet, and indicates the length of this
      BAP Option including the Type, Length and Reason String fields.

   Reason String

      This is an ASCII string.  The content of the field is
      implementation dependent.  An implementation MAY ignore the Reason
      String field.




















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6.5.  Link-Discriminator

   Description

      The Link-Discriminator option MUST be used in a Link-Drop-Query-
      Request datagram.  This option is used to inform the receiver of a
      Link-Drop-Query-Request of which link will be dropped.

   A summary of the Link-Discriminator BAP 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     |       Link Discriminator      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      05 for Link-Discriminator

   Length

      4

   Link Discriminator

      The Link Discriminator field is 2 octets in length.  It contains
      the Link Discriminator that was contained in the LCP Link-
      Discriminator Configuration Option sent by the receiver of the
      packet containing the Link Discriminator.

6.6.  Call-Status

   Description

      The Call-Status option MUST be used in a Call-Status-Indication
      datagram.  This option is used to inform the receiver of the
      Call-Status-Indication datagram of the status of the completed
      call attempt, as well as a possible action that will be taken (if
      the call failed).










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   A summary of the Call-Status BAP 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     |     Status    |     Action    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      06 for Call-Status.

   Length

      4

   Status

      The Status field is 1 octet in length.  If the call was
      successful, the value MUST be set to 0.  A non-zero value
      indicates a call failure.  A value of 255 indicates a non-specific
      failure, and a more specific call status MAY be indicated by using
      the same number as the Q.931 cause value (i.e., 1 is unassigned
      number, 17 is user busy, etc.)

   Action

   The Action octet indicates what action the calling implementation is
   taking after a failed call.  If the call was sucessful, the Action
   octet MUST be set to 0.

   The Action octet can have the following values:

      0 - No retry
      1 - Retry















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Appendix

List of BAP datagrams and associated fields.

datagram                      mandatory fields           allowed options
--------                      -----------------          ---------------
Call-Request                  Link-Type                  No-Phone-Number
Call-Response                                            Phone-Delta
                                                         Link-Type
Callback-Request              Link-Type
                              Phone-Delta
Callback-Response                                        Link-Type
Link-Drop-Query-Request       Link-Discriminator
Link-Drop-Query-Response
Call-Status-Indication        Call-Status                Phone-Delta
Call-Status-Response

   The Reason option is allowed to be included with any BAP datagram.

History of BACP

   The first version of BACP was written by Craig Richards of Shiva
   Corporation.  This version was enhanced and improved by the MPCP
   Working Group, a collaborative effort of 3Com, Ascend, Bay Networks,
   Cisco, Microsoft, Shiva, US Robotics and Xylogics.

Acknowledgements

   Kevin Smith of Ascend for his contributions based on his work on the
   MP+ Specification.  Gerry Meyer and Robert Myhill of Shiva for their
   early comments and improvements.  Andy Nicholson of Microsoft for his
   improvements to the bandwidth management scheme.  Dana Blair and Andy
   Valencia of Cisco, Cheng Chen and Dan Brennan of 3Com for their good
   ideas as part of the MPCP Working Group. All of the members of the
   MPCP working group for their ability to work with their competitors
   with enthusiasm to produce a better protocol for the industry.

Security Considerations

   Security issues are not discussed in this memo.











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References

[1]   Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
      51, RFC 1661, Daydreamer, July 1994.

[2]   Sklower, Lloyd, McGregor, Carr & Coradetti, "The PPP Multilink
      Protocol", RFC 1990,  University of California, Berkeley, Lloyd
      Internetworking, Newbridge Networks Corporation, Sidewalk
      Software, August 1996.

Chair's Address

   The working group can be contacted via the current chair:

      Karl Fox
      Ascend Communications
      3518 Riverside Drive, Suite 101
      Columbus, Ohio  43221

      (614)451-1883

      EMail: karl@ascend.com

Editors' Addresses

      Craig Richards
      Shiva Corporation
      28 Crosby Drive
      Bedford, MA  01730
      VOICE   +1 617 270 8419
      FAX     +1 617 270 8599

      EMail: crich@us.shiva.com


      Kevin Smith
      Ascend Communications, Inc.
      1275 Harbor Bay Parkway
      Alameda, CA  94501
      CA

      EMail: kevin@ascend.com









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