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Versions: 00 01 02 03 04 05 RFC 2125

PPP Working Group                                         Craig Richards
Internet Draft                                         Shiva Corporation
expires September 1996                                       Kevin Smith
                                             Ascend Communications, Inc.
                                                              March 1996


              The PPP Bandwidth Allocation Protocol (BAP)
          The PPP Bandwidth Allocation Control Protocol (BACP)
                     draft-ietf-pppext-bacp-02.txt


Status of this Memo

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

   Distribution of this memo is unlimited.

   Internet Drafts are working documents of the Internet Engineering
   Task Force (IETF), its Areas, and its Working Groups.  Note that
   other groups may also distribute working documents as Internet
   Drafts.

   Internet Drafts are draft documents valid for a maximum of six
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   Please check the 1id-abstracts.txt listing contained in the
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   status of any Internet Draft.

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.




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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. Interoperable implementations of PPP multilink have had
   problems such as thrashing, when links are repeatedly brought up and
   torn down in a short amount of time.  BACP and BAP provide a means of
   solving problems associated with interoperable thrashing
   implementations, they also 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 numbers 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.

   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



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             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 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, it is the
      peer's Link Discriminator which is sent.

   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.






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   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 SHOULD 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 8071 (Bandwidth Allocation Control
         Protocol).

      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 does not have any Configuration Options.  Any options
         SHOULD not be sent, and any options received SHOULD be rejected
         using a Config-Reject packet.





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4.  BAP Operation

4.1.  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 Drop-Link-Query-Request
   packet is sent to the peer to negotiate dropping a link.  The peer
   MUST respond with a Drop-Link-Query-Response.   If the peer is
   agreeable to dropping the link the implementation should then 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 Drop-Link-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 peer SHOULD stop transmitting packets before
   issuing the Terminate-Ack.  This procedure will insure that no data
   is lost in either direction.

   A BAP Link-Drop-Query-Request is used to inquire if the peer agrees
   to drop a link from the current multilink bundle. Origination of a
   Link-Drop-Query-Request is an optional part of BAP.  A system
   implementing BAP MUST support the receipt of Link-Drop-Query-Request
   from its peer.  When an implementation wants to negotiate dropping a



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   link, it MUST transmit a Link-Drop-Query-Request.  When an
   implementation receives a Link-Drop-Query-Request, it MUST base its
   response on its transmit heuristics or on its configuration (e.g., if
   the request would cause the number of active links to fall below the
   allowable minimum number of links configured for the active multilink
   bundle).  If the implementation receiving a Link-Drop-Query-Request
   is not monitoring its transmit data and is not configured otherwise,
   it MUST accept the request to drop a link.

4.2.  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).  The Bandwidth Allocation
   Protocol does not include an algorithm for determining when to add
   and remove links in a multilink bundle. These algorithms are left to
   the implementors. It is not necessary to include such an algorithm in
   the protocol, and including it may limit the abilities of
   implementations to work optimally.  Leaving out the bandwidth on
   demand algorithm also improves chances for interoperability and makes
   the protocol more flexible.

   This section defines two important Types of BOD: Resource BOD and
   Throughput BOD.  This does not preclude other Types of BOD from being
   implemented (e.g., Time of Day).  The rest of this specification
   refers to actions to take when implementing these types of BOD.

   Throughput BOD refers to BOD decisions made based on the amount of
   data being sent, received or queued to be sent over a multilink
   bundle. An example of this is when a link is added to a bundle due to
   a large file being transferred across the bundle.

   Resource BOD refers to BOD decisions 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.  A system implementing BACP/BAP MAY support  neither,
   either, both or some other Type of Bandwidth On Demand.  Regardless
   of the Type of BOD implemented, if it uses BAP it must use the
   procedures in this specification for adding and dropping links.

   Resource BOD support is an optional part of BAP. An implementation
   does not have to locally make resource based BOD decisions as part of
   BAP.  However, any system implementing BAP MUST support resource BOD
   management by its peer.  An implementation MUST use an LCP
   Terminate-Request to remove a link due to Resource BOD.




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   Throughput BOD support is an optional part of BAP.  An implementation
   does not have to locally make throughput based BOD decisions as part
   of BAP.  However, any system implementing BAP must handle throughput
   BOD management by its peer (i.e., receipt of Link-Drop-Query-
   Request). When an implementation decides that it's time to remove a
   link due to a throughput BOD decision, an implementation 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 MUST base its
   response on its transmit heuristics (if it implements Throughput BOD)
   or on its configuration (e.g., if the request would cause the number
   of active links to fall below the allowable minimum number of links
   configured for the active multilink bundle).  If the implementation
   receiving a Link-Drop-Query-Request is not monitoring its transmit
   data and is not configured otherwise, it MUST accept the request to
   drop a link.  It MUST NOT base its response on its receive data
   heuristics. By making the decision to respond to a Link-Drop-Query-
   Request based on transmit heuristics only, BAP maximizes
   interoperability of various types of throughput BOD implementations.

   A BAP implementation may monitor its transmit traffic, both transmit
   and receive traffic, or choose not to monitor traffic in either
   direction.  Server systems SHOULD implement bi-directional monitoring
   and single-user dialin clients MAY implement any type of monitoring.
   This will allow client BOD implementations that require minimal end-
   user configuration.


4.3.  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 same request or
   indication, with the identical identification number unless the
   implementation has exceeded the maximum number of retransmissions it
   supports for this packet.  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.

   This procedure will insure that the peer receives the proper request
   or indication even if a packet is lost during transmission.  If a



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   Response packet is lost, this retransmission scheme will insure that
   the original Request or Indication will be retransmitted with the
   same identification number, so the peer will realize that this is not
   a new request or indication packet.

   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.

4.4.  Race Conditions

   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 username
      value SHOULD be favored.  A username value is compared by
      converting the ASCII bytes of the username used by the peer during
      authentication into hexadecimal octets to create a number.  If
      both implementations are using the same username, then the lowest
      Multilink Endpoint Discriminator SHOULD be favored.  Once again,
      each Multilink Endpoint Discrininators is first converted into
      hexadecimal octets starting with the Class octet, before being
      compared.  This means that the favored peer's request SHOULD be
      acked by its peer, and the unfavored peer's request SHOULD be
      naked by the favored peer.

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

4.5.  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 0071 (Bandwidth Allocation Protocol).

      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.



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      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.

   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, as well as which
      links are added or removed.  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 that
      failed. 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



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      implementation. The Request-Nak Response Code is sent to indicate
      that the Request command was received, but an implementation does
      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 receiving a callback
      is not implemented 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.

4.5.1.  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 peer of the request to add another link to the bundle and
   give the peer 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.

4.5.2.  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; otherwise the Response Code MUST be Request-Nak or Request-
   Full-Nak. The Phone-Number option MUST be included in a Call-Response
   packet with a Response Code of Request-Ack unless the Call-Request



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

4.5.3.  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 peer of the request to add another
   link to the bundle and will also inform the peer of the number to be
   called.

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

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

4.5.4.  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.

4.5.5.  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, 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.

4.5.6.  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



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   Code MUST be set to Request-Ack. Otherwise, the Response Code MUST be
   set to either Request-Nak or Request-Full-Nak.

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

4.5.7.  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.  The Call-Status option MUST be
   included to inform the peer of the status of the call and the action
   the implementation will take in case of call failure.  The reason
   option MAY also be included in the Call-Status-Indication packet.

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

4.5.8.  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.


5.  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.

   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 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+







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   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-Number
         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.

5.1.  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.

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










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    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 32 of the Link-Type
      BAP Option as described above.  If a bit is set, it indicates
      support of the corresponding link type (more than one bit MAY be
      set):

         Bit     Link type
         ---     -------------
          0      ISDN
          1      X.25
          2      analog
          3-7    reserved

      If the Length field contains more bits than are defined by this
      specification, then any bits that are not defined should be
      ignored.  If the Length field is shorter than the number of bits
      defined, then the implementation should set all bits not received
      to 0.





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5.2.  Phone-Number

   Description

      This option is used to transmit an implementation's phone number
      to its peer.  For example, this phone number could be either the
      phone number of a hunt group for this device, or the specific
      phone number of a free port on this device. An implementation MAY
      include more than one Phone-Number option in a response.  This
      indicates that there is more than one phone number that can be
      used for the requested operation.  The Phone-Number option MUST
      appear in a Callback-Request if the Response Code is set to
      Request-Ack.  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-Number 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-Number.

   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-Options Types are defined for the Phone-Number
      option.

          01   Unique Digits
          02   Subscriber Number
          03   Phone Number Sub Address





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5.2.1.  Phone-Number Sub-Options

   Unique Digits

      This byte is a count of the number of unique digits in the phone
      number.  The Unique Digits byte indicates the number of rightmost
      digits of the complete phone number that are different from port
      to port on the given device.  (For example, if all phone numbers
      on a given device are 617/555-89XX, the Unique Digits byte is 2,
      if all phone numbers are 617/55X-XXXX, then the Unique Digits byte
      will be 5).  This field is required.

      The purpose of the unique digits sub-option is to aid the
      originating implementation in phone number parsing.  With this
      information, the implementation that originates a call does not
      have to know which combination of access codes, country codes,
      dialing codes, area codes and extension numbers are necessary.  It
      just takes the digits contained in the original phone number
      dialed, and replaces the rightmost unique digits with the
      rightmost unique digits of the new phone number. For example, if
      the original number dialed is 9,16175512345, and the new phone
      number has an area code of 617, a phone number of 5598765, and a
      unique digits value of 5, then the number to be dialed will be
      created by replacing the rightmost 5 digits of the original number
      (12345) with the rightmost 5 digits of the new number (98765),
      which results in a new phone number to be dialed of 9,16175598765.

   Subscriber Number

      This field is the phone number of the port that should be called
      by the peer.  It MAY include the National Destination and Country
      Codes.  This field is an ASCII string and MUST contain only ASCII
      characters 0-9 inclusive (valid phone number digits).  This field
      is required.

   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.

5.3.  No-Phone-Number-Needed

   Description

      The No-Phone-Number option indicates that the calling
      implementation is already configured with the phone number of its



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      multilink peer and the peer 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

5.4.  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.






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   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.

5.5.  Link-Discriminator

   Description

      The Link-Discriminator option MUST be used in a Link-Drop-Query-
      Request datagram.  This option is used to inform the peer 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 peer.

5.6.  Call-Status

   Description

      The Call-Status option MUST be used in a Call-Status-Indication



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      datagram.  This option is used to inform the peer of the status of
      the completed call attempt, as well as a possible action that will
      be taken (if the call failed).

   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 - Will retry same number
      2 - Will retry next number in list









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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-Number
                                                            Link-Type
   Callback-Request              Link-Type                  Phone-Number
   Callback-Response                                        Link-Type
   Link-Drop-Query-Request       Link-Discriminator
   Link-Drop-Query-Response
   Call-Status-Indication        Call-Status                Phone-Number
   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.



References

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



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   [2]   Sklower, Lloyd, McGregor & Carr, "The PPP Multilink Protocol",
         RFC 1717,  PPP Extensions Working Group, Work in Progress.

Chair's Address

   The working group can be contacted via the current chair:

      Fred Baker
      Cisco Systems
      519 Lado Drive
      Santa Barbara, California 93111
      VOICE   +1 408 526 4257
      FAX     +1 805 681-0115

      EMail: fred@cisco.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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                           Table of Contents


     1.     Introduction ..........................................    1
        1.1       Specification of Requirements ...................    1
        1.2       Terminology .....................................    1

     2.     New LCP Configuration Option ..........................    2
        2.1       Link Discriminator ..............................    2

     3.     BACP Operation ........................................    3

     4.     BAP Operation .........................................    4
        4.1       Link Management .................................    4
        4.2       Bandwidth Management ............................    5
        4.3       BAP Packets .....................................    6
        4.4       Race Conditions .................................    7
        4.5       BAP Datagram Format .............................    7
           4.5.1  Call-Request ....................................   10
           4.5.2  Call-Response ...................................   10
           4.5.3  Callback-Request ................................   11
           4.5.4  Callback-Response ...............................   11
           4.5.5  Link-Drop-Query-Request .........................   11
           4.5.6  Link-Drop-Query-Response ........................   11
           4.5.7  Call-Status-Indication ..........................   12
           4.5.8  Call-Status-Response ............................   12

     5.     BAP Datagram Options ..................................   12
        5.1       Link-Type .......................................   13
        5.2       Phone-Number ....................................   15
           5.2.1  Phone-Number Sub-Options ........................   16
        5.3       No-Phone-Number-Needed ..........................   16
        5.4       Reason ..........................................   17
        5.5       Link-Discriminator ..............................   18
        5.6       Call-Status .....................................   18

     Appendix - List of BAP datagrams and associated fields .......   20

     ACKNOWLEDEMENTS ..............................................   20

     SECURITY CONSIDERATIONS ......................................   20

     REFERENCES ...................................................   20

     CHAIR'S ADDRESS ..............................................   21

     EDITORS'S ADDRESSES ..........................................   21




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