rfc1934.txt

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Network Working Group                                           K. Smith
Request For Comments: 1934                         Ascend Communications
Category: Informational                                       April 1996


                 Ascend's Multilink Protocol Plus (MP+)

Status of This Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This document proposes an extension to the PPP Multilink Protocol
   (MP) [1]. Multilink Protocol Plus (MP+) is a new control protocol for
   managing multiple data links that are bundled by MP.

Table Of Contents

1.      Introduction.................................................2
        1.1     Functional Description...............................2
        1.2     Conventions..........................................3
2.      General Overview.............................................3
        2.1     Operation............................................4
3.      MP+ Frame Formats............................................4
        3.1     Error Control (EC) Layer.............................6
                3.1.1   Error Control State Machine..................7
        3.2     Multilink Plus Control Messages......................9
        3.3     Multilink Plus Message Formats......................10
                3.3.1   VERSION_EXCHANGE_REQ Message Format.........10
                3.3.2   VERSION_EXCHANGE_RSP Message Format.........12
                3.3.3   ADD_REQ Message Format......................13
                3.3.4   ADD_RSP Message Format......................15
                3.3.5   ADD_COMPLETE Message Format.................16
                3.3.6   REMOVE_REQ Message Format...................17
                3.3.7   REMOVE_RSP Message Format...................17
                3.3.8   REMOVE_COMPLETE Message Format..............18
                3.3.9   CLOSE_REQ Message Format....................19
                3.3.10  CLOSE_RSP Message Format....................19
                3.3.11  REMOTE_MGMT_REQ Message Format..............20
                3.3.12  REMOTE_MGMT_RSP Message Format..............20
                3.3.13  REMOTE_MGMT_RX_REQ Message Format...........21
                3.3.14  REMOTE_MGMT_TX_REQ Message Format...........22
                3.3.15  REMOTE_MGMT_TX_RSP Message Format...........22
                3.3.16  CLEAR_REQ Message Format....................23
        3.4     Events..............................................23



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RFC 1934                Multilink Protocol Plus               April 1996


        3.5     State Machine.......................................25
                3.5.1   States......................................25
                3.5.2   Common Actions..............................26
                3.5.3   MP+STATE_INITIAL state machine..............32
                3.5.4   MP+STATE_IDLE state machine.................35
                3.5.5   MP+STATE_ADD state machine..................37
                3.5.6   MP+STATE_REMOVE state machine...............41
                3.5.7   MP+STATE_CLOSE state machine................44
4.      PPP LCP Extensions..........................................46
5.      Security Considerations.....................................47
6.      References..................................................47
7.      Author's Address............................................47

1. Introduction

   The PPP Multilink Protocol (MP), is a set of features that provide
   inverse multiplexing at the packet/fragment level by bundling
   multiple independent links between a fixed pair of systems, providing
   a virtual link with greater bandwidth than any of the constituent
   members.

   Once multiple channels have been established  MP is responsible for
   managing channel use to insure in-sequence delivery of user packets.

   MP+ is an extension to MP that adds an inband control channel to
   provide a new level of session management and control.

   MP+ also allows remote device management of (unconfigured) systems.
   This feature allows a network operations center to dial into an
   unconfigured system and remotely manage it, before ethernet
   interface, IP address, and other LCP and system configuration
   information is entered.  (This does require local configuration of
   the WAN interfaces to the extent required to answer an incoming
   call).

1.1 Functional Description

   The features of MP+ include:

   * Ability to negotiate to add and subtract channels when bandwidth
     needs change.

   * Phone number management so calling stations need not know every
     possible number; answering stations can manage their own resources.

   * A simple remote management interface.





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RFC 1934                Multilink Protocol Plus               April 1996


   To perform the above functions  MP+ is split into a call management
   layer and a reliable delivery layer.  The call management layer is
   the source and sink of  MP+ control messages.  The reliable delivery
   layer adds a simple acknowledge and retry mechanism.

   MP+ only takes network bandwidth when in the process of performing a
   user request, e.g. adding and subtracting bandwidth.

   NOTE: Neither MP, or MP+ define the process that makes the bandwidth
   requirement determination. That is outside the scope of either of
   these protocols and will likely be implementation dependent.

1.2  Conventions

   The following language conventions are used in the items of
   specification in this document:

      MUST,  SHALL or  MANDATORY -- the item is an absolute requirement
      of the specification.

      SHOULD or RECOMMENDED --   the item should generally be followed
      for all but exceptional circumstances.

      MAY or OPTIONAL --  the item is truly optional and may be followed
      or ignored according to the needs of the implementor.

2.  General Overview

   PPP
      In order to establish communications over a point-to-point link,
      each end of the PPP [2] link must first send LCP packets to
      configure the data link during  link establishment  phase.  After
      the link has been established, PPP provides for an  authentication
      phase.

   MP The goal of multilink operation is to  bundle  multiple
      independent links between a fixed pair of systems, providing a
      virtual link with greater bandwidth than any of the constituent
      members.

   MP+ MP+ is also negotiated during initial LCP option negotiation.  A
      system indicates to its peer that it is willing to do MP+ by
      sending the  MP+ option as part of the initial LCP option
      negotiation.  The  MP+  option MUST NOT be negotiated unless MP is
      also negotiated. When used, MP+ adds a virtual unit-to-unit
      control channel.

   A peer may elect to:



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RFC 1934                Multilink Protocol Plus               April 1996


   Acknowledge both the MP and MP+ options, indicating that both MP and
   MP+ will be used.

   Acknowledge the MP option and reject the MP+ option. Operation will
   fall back to MP.

   Reject both options.  Standard PPP will be used for this connection.

2.1. Operation

   Standard PPP
      In standard PPP the LCP negotiation phase is followed by an
      optional authentication phase, and then one or more NCPs are
      initiated.

   PPP with MP The LCP negotiation phase and authentication phase are
      identical to standard PPP.  The ability to initiate an MP
      aggregate data link is indicated by sending an MP option - as
      described in [1].

   PPP with MP and MP+ When MP+ is negotiated at LCP startup, the same
      procedures are followed as when MP is negotiated alone.  The MP+
      LCP option is negotiated to indicate the ability to use the MP+
      feature.The first connection between endpoints causes the MP+
      process to be started for the connection.

3.  MP+ Frame Formats

                    +---------------+---------------+
       PPP Header:  | Address 0xff  | Control 0x03  |
                    +---------------+---------------+
                    | PID(H)  0x00  | PID(L)  0x73  |
                    +-+-+-+-+-------+---------------+
       MP Header:   |1|1|0|0|0|0|0|1| seq # high    |
                    +-+-+-+-+-------+---------------+
                    |   sequence number low bits    |
                    +---------------+---------------+
                    |         control data          |
                    |               .               |
                    |               .               |
                    |               .               |
                    +---------------+---------------+
       PPP FCS:     |              FCS              |
                    +---------------+---------------+

   Figure 1: Multilink Plus Frame Format (long sequence number format)





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RFC 1934                Multilink Protocol Plus               April 1996


                    +---------------+---------------+
       PPP Header:  | Address 0xff  | Control 0x03  |
                    +---------------+---------------+
                    | PID(H)  0x00  | PID(L)  0x73  |
                    +-+-+-+-+-------+---------------+
       MP Header:   |1|1|0|1|  sequence number      |
                    +-+-+-+-+-------+---------------+
                    |         control data          |
                    |               .               |
                    |               .               |
                    |               .               |
                    +---------------+---------------+
       PPP FCS:     |              FCS              |
                    +---------------+---------------+

   Figure 2: Multilink Plus Frame Format (short sequence number format)

   MP+ frames use a similar structure to MP fragments.

   The MP+ assigned PID is designated 00 73.

   MP+ control uses the following two rules:

  - MP+ control frames have their own sequence number space,
    controlled by MP+.

  - MP+ control frames MUST NOT be fragmented.

NOTE: Implementations of this protocol prior to the date of submission
      of this specification to the IETF use the same PID as MP, but
      sets the LSB of the reserved bits in the MP header to 1 - this
      is how the MP+ packets are discriminated from MP fragments.
      So the header of the MP+ packet looks like:

                00 3d c1 ......

      As compared to an MP packet that looks like:

                00 3d c0 ......  or
                00 3d 80 ......  or
                00 3d 40 ......










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RFC 1934                Multilink Protocol Plus               April 1996


3.1. Error Control (EC) Layer (MP+ control only)

   The error control layer that runs over the virtual inband channel is
   as simple as it can get, while handling the possibility of errors on
   the line.

   An assumption is made that errors are infrequent, and that at the
   same time messages are rarely, if ever, dropped on the floor. The
   implication of this is that "timing out" on retransmission of
   messages does no harm.  If a message cannot get through, then it
   simply is retried some number of times.  After giving up, the only
   recourse is to notify the call management layer (of MP) that the
   session has died.

                    +---------------+---------------+
       PPP Header:  | Address 0xff  | Control 0x03  |
                    +---------------+---------------+
                    | PID(H)  0x00  | PID(L)  0x73  |
                    +-+-+-+-+-------+---------------+
       MP+ Header:  |1|1|0|0|0|0|0|1| seq # high    |
                    +-+-+-+-+-------+---------------+
                    |   sequence number low bits    |
                    +---------------+---------------+
        EC Header:  | Error Control Message Type    |
                    |   32 bits reserved            |
                    +---------------+---------------+
         MP+ Data:  |    MP+ Message                |  May not be
                    |                               |  present.

   Figure 3: MP+ control message format (shown long sequence number
             format)

  Error Control Message Type:

   1      DATA_MSG:         This message contains MP+ data transferred
                            between the peers.

   2      ACK_MSG:          An acknowledgement of a previous data message.

   When set to DATA_MSG, the remainder of the frame contains an MP+
   Control message.

   When set to ACK_MSG, the remainder of the frame consists only of the
   PPP Frame Check Sum (FCS).







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RFC 1934                Multilink Protocol Plus               April 1996


3.1.1. Error Control State Machine

   This layer is controlled by a simple state machine.  There are three
   states:

   Stopped        There is no connection between peers.

   Idle           There is a connection between peers;
                  no unacknowledged messages pending.

   Pending        There is a connection between peers;
                  awaiting an acknowledgement to the
                  last message sent.

   Messages from the call management layer are queued for transmission
   whenever the link is in the  pending  state.  For simplicity, only
   one outstanding message may be in the link at any given time.  The
   entire procedure is defined in table 1.

Event                           State
______________________________________________________________________
                                Stopped    Idle         Pending
======================================================================
Start                           1,Idle     -,*          -,*
______________________________________________________________________
Received ACK_MSG                **         2,Start      5,Idle|Pending
current tx sequence number
______________________________________________________________________
Received ACK_MSG                **         -,*          -,*
last tx sequence number
______________________________________________________________________
Received ACK_MSG                **         2,Start      2,Start
other tx sequence number
______________________________________________________________________
Received DATA_MSG               **         6,*          6,*
current rx sequence number
______________________________________________________________________
Received DATA_MSG               **         7,*          7,*
previous rx sequence number
______________________________________________________________________
Received DATA_MSG               **         2,Start      2,Start
other rx sequence number
______________________________________________________________________
Receive Invalid Frame           **         2,Start      2,Start