RFC 2701

Network Working Group                                         G. Malkin
Request for Comments: 2701                              Nortel Networks
Category: Informational                                  September 1999

                            Nortel Networks
          Multi-link Multi-node PPP Bundle Discovery Protocol

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.


   This document specifies a standard way for Multi-link PPP to operate
   across multiple nodes.  Both the mechanism by which the Bundle Head
   is discovered and the PPP fragment encapsulation are specified.


   I would like to thank Joe Frazier for filling in some of the details
   and reviewing this document.

1.  Introduction

   Multi-link PPP [MP] allows a dial-in user to open multiple PPP
   connections to a given host.  In general, this is done on an on-
   demand basis.  That is, a secondary link, or multiple secondary
   links, are established when the data load on the primary link, and
   any previously established secondary links, nears capacity.  As the
   load decreases, the secondary link(s) may be disconnected.

   Many dial-in hosts which support multi-link PPP dial the same phone
   number for all links.  This implies that there exists a rotary at the
   Point Of Presence (POP) which routes incoming calls to a bank of
   modems.  These may be physically independent modems connected to
   Remote Access Server (RAS) and a rotary of analog phone lines, or a
   RAS with internal modems connected to analog lines or a T1/E1 or
   T3/E3 channel.  In any case, a given RAS can only handle just so many
   simultaneous connections.  A typical POP may need to support hundreds
   of connections, but no RAS today can handle that many.  This creates
   a problem when a user's primary PPP connection is established to one

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RFC 2701                          MMP                     September 1999

   RAS in a POP and a secondary connection is established to another.
   This may occur because the first RAS has no available modems, or
   because incoming calls are assigned to ports in a round-robin
   fashion, for example, and the second call is simply assigned to
   another RAS.

   The solution to this problem is to provide a mechanism by which a RAS
   can determine if a Multi-link PPP connection is a primary or
   secondary and, if a secondary, where the Bundle Head (the process
   within a RAS which reassembles the PPP fragments transmitted over the
   primary and secondary links) resides.  If the Bundle Head resides on
   a different RAS, a protocol must be used to transfer the PPP
   fragments to the RAS containing the Bundle Head so that the PPP frame
   can be reassembled.

   Section 2 of this document specifies the Discovery Mechanism.
   Section 3 specifies the Transfer Protocol.  Section 4 specifies the
   configuration parameters needed for the Discovery Protocol.

2.  Bundle Head Discovery Mechanism

   When a user dials into a RAS and negotiates Multi-link PPP (MP)
   during the Link Control Protocol (LCP) phase, the RAS must determine
   which one of the following three cases exists:

   1- This is the primary (first) link of the MP connection.  In this
      case, the RAS should create the Bundle Head.

   2- This is a secondary link of the MP connection and the Bundle Head
      resides on this RAS.  In this case, the RAS should add the link to
      the Bundle (standard MP).

   3- This is a secondary link of the MP connection and the Bundle Head
      resides on a different RAS.  In this case, the RAS should
      establish a path (see section 3) to the RAS that has the Bundle
      Head, and use that path to transfer MP fragments.

   In operation, a RAS will make the determination for case 2 first
   (because it is the easiest and requires no communication with other
   RASes.  If the Bundle Head is not local, the Discovery Protocol is
   used to determine where the Bundle Head is, if it exists at all.

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2.1 Packet Format

   See "IANA Considerations" (section 6) for UDP port number assignment.

   A Discovery Message has the following format:

      | type |length| random ID  | hash | endpoint ID  |


   type - 2 octets

      Message type: 1-query, 2-response.

   length - 2 octets

      The length (in octets) of the endpoint ID.

   Random ID - 4 octets

      A random identifier generated by the RAS used to resolve
      contention.  See "Contention Handling" (section 2.4) for the use
      of this field.

   hash - 2 octets

      The unsigned sum (modulo 2^16) of the unsigned octets of the
      Endpoint ID.  A value of zero indicates that no hash has been
      generated.  See "Endpoint Identifier Matching" (section 2.2) for
      the use of this field.

   endpoint ID - variable length

      The endpoint identifier of the connection.  From the discovery
      protocol's point of view, this is an opaque value.  However, to
      ensure multi-vendor interoperability, the format of this field
      must be defined.  The descriptions of, and legal values for, the
      fields in the endpoint ID are defined in [MP].

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RFC 2701                          MMP                     September 1999

         |remote|remote|remote|local |local |local |user  | user |
         |EPD   |EPD   |EPD   |EPD   |EPD   |EPD   |name  | name |
         |class |length|data  |class |length|data  |length| data |

         EPD = EndPoint Descriminator.
         remote = dial-in host.
         local = RAS.
         class and length fields are 1-octet in length.
         data fields are of variable (including zero) length.

   The MP protocol requires that the RASes all have the same Local EPD.
   For MMP, this implies that a RAS may not use its IP or Ethernet
   address as an EPD.  This also implies that all RASes on a rotary must
   have the same EPD.  RASes on different rotaries may share different
   EPDs.  The Local EPD is included in the endpoint identifier to ensure
   that RASes on different rotaries, but sharing a common Ethernet, will
   not join a particular discovery if the Remote EPDs just happen to be
   the same.

   Except for unicast Response Messages, all messages are sent to the
   multicast address specified in "IANA Considerations".  If a system
   cannot send multicast messages, the limited broadcast address
   ( should be used.

2.2 Endpoint Identifier Matching

   Comparing Endpoint IDs can be time consuming.  First, the classes of
   the EPDs must be determined, then the values compared.  These
   comparisons might be fast arithmetic compares or slow octet-wise
   compares of 20-octet long values.  To improve performance, because
   the protocol is time-driven, the hash field may be used for a fast

   When a Bundle Head is created, the hash is created and stored along
   with the Endpoint ID.  When a Query or Response Message is generated,
   the hash is created and stored in the message.  When a RAS receives a
   message, it can do a quick comparison of the hash in the message to
   the hashes in its tables.  If a hash does not match, the Endpoint ID
   cannot match.  However, if a hash does match, the Endpoint IDs must
   be properly compared to verify the match.

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RFC 2701                          MMP                     September 1999

   Obviously, there is a cost associated with creating the hashes, but
   they are created only once per message and once for each Bundle Head
   creation.  However, the comparisons occur multiple times in multiple
   RASes for each new secondary connection.  Therefore, there is a net
   savings in processing.

2.3 Protocol Operation

   Throughout this section, configurable variables are specified by
   their names (e.g., ROBUSTNESS refers to the number of transmits).

   The Discovery Protocol begins by multicasting ROBUSTNESS Query
   Messages at QUERY_INTERVAL intervals.  If no Response Message for
   that Request is received within QUERY_INTERVAL of the last broadcast
   (a total time of ROBUSTNESS * QUERY_INTERVAL), the RAS assumes that
   this is the primary link and begins to build the Bundle Head.  It
   then sends a multicast Response Message (in case another link comes
   up after the time-out but before the Bundle Head is built).  If a
   Response Message is received (i.e., a Bundle Head exists on another
   RAS), no additional Query Messages are sent and the RAS establishes a
   path to the RAS containing the Bundle Head.

   If a RAS receives a Query Message for an MP connection for which it
   has the Bundle Head, it sends a unicast Response Message to the
   querier.  Note that no repetition of the Response Message is
   necessary because, if it is lost, the querier's next query message
   will trigger a new Response Message.

2.4 Contention Handling

   If, while sending Query Messages, a Query Message for the same MP
   connection is received, it indicates that the Dial-in Node has
   brought up multiple links simultaneously.  The resolution to this
   contention is to elect the bundle head.  To do this, each RAS waits
   until all Query Messages are sent (ROBUSTNESS * QUERY_INTERVAL).  At
   that time, the RAS with the lowest Random ID becomes the Bundle Head.
   If two or more RASes have the same Random ID, the RAS with the lowest
   IP address becomes the Bundle Head.  That RAS then sends TWO Response
   Messages, with a QUERY_INTERVAL interval, and indicates to the MP
   process that a Bundle Head should be formed.  When the other RAS(es)
   receive the Response Message, they cease broadcasting (if they
   haven't already sent ROBUSTNESS Query Messages), stop listening for
   additional Response Messages, and indicate to their respective MP
   processes where the Bundle Head resides.

   Note that a RAS generates a Random ID for each connection and uses
   that value for all Query and Response messages associated with that
   connection.  The same Random ID must not be reused until it can be

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RFC 2701                          MMP                     September 1999

   guaranteed that another RAS will not mistake the message for an old
   message from a previous connection.  For this reason, it is
   recommended that the Random ID be either monotonically increasing or
   a clock value (either time since boot or time of day).

2.5 MP Operation

   MP must use the following algorithm to ensure that there are no
   windows of vulnerability during which multiple Bundle Heads might be
   created for the same MP connection.

   When an MP link is negotiated, MP first checks to see if it already
   has the Bundle Head for this connection (i.e., is this a secondary
   link).  If it does, it should attach to it and not initiate a
   discovery.  As an optimization, if MP does not have a Bundle Head for
   this connection, but does have a existing secondary link for it, MP
   should attach to the known Bundle Head without initiating discovery.

   If MP knows of no Bundle Head for this connection, it should initiate
   a discovery.  If the discovery should locate a Bundle Head, it should
   attach to the indicated bundle head.  If no Bundle Head is found, MP
   should create a Bundle Head.

   When a RAS determines that it is to become the Bundle Head for a
   connection, it should establish the Bundle Head as quickly as
   possible so Query Messages for that connection from other RASes will
   be recognized.  If a RAS indicates that it will become the Bundle
   Head, but delays establishment of it, other RASes may time out on
   their discovery and begin to establish additional Bundle Heads of
   their own.

3.  Transfer Protocol

   The Layer 2 Tunneling Protocol (L2TP) [L2TP] will be used to transfer
   PPP fragments from a RAS containing a secondary link to the RAS
   containing the Bundle Head.  By specifying the use of an existing
   protocol, it is neither necessary to create nor implement a new

4.  Configuration

   There are two required configuration switches and one conditional
   configuration switch.  None of the switches are optional.

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RFC 2701                          MMP                     September 1999

4.1 Robustness - required

   This switch sets the number of transmits (repetitions) for Query
   Messages.  It may be set between 1 and 15.  The default is 3.  Be
   aware that lower settings may create windows of vulnerability.
   Higher settings may cause MP timeouts, but may be needed on very
   lossy or congested networks.

4.2 Query Interval - required

   This switch sets the interval between Query Messages and the interval
   between multicast Response Messages.  It should be calibrated in
   deciseconds (1/10 second) and may be set between 1 and 15.  The
   default is 1.  Be aware that higher settings may cause MP timeouts,
   but may be needed on very slow systems/networks.

4.3 TTL - conditional

   This switch sets the IP Time-To-Live (TTL) of all Discovery packets.
   For systems which are using the limited broadcast address, this
   switch should not be implemented and the TTL should be set to 1.  The
   default value should be 1.

5.  Security Considerations

   No security is designed into the Discovery Mechanism.  When not
   forwarding multicast packets (or when using the limited broadcast
   address), the discovery packets are restricted to a single LAN.  If
   the LAN is physically secure, there is no need for software security.
   If the multicast packets are forwarded, but the range is limited to a
   small, physically secure network (e.g., a POP), there is still no
   need for software security.  If the discovery packets are allowed to
   cross an internet (and this is NOT recommended for timing reasons),
   authentication of RASes may be done with IPSEC.  For increased
   security on a LAN, or in a POP, IPSEC may still be used.

   L2TP security is discussed in [L2TP].

6.  IANA Considerations

   UDP port number: 581
   Multicast address:

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RFC 2701                          MMP                     September 1999

7.  References

   [MP]    Sklower, K., Lloyd, B., McGregor, G., Carr, D. and
           T. Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990,
           August 1996.

   [L2TP]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.
           and B. Palter, "Layer Two Tunneling Protocol "L2TP"",  RFC
           2661, August 1999.

Author's Address

   Gary Scott Malkin
   Nortel Networks
   11 Elizabeth Drive
   Chelmsford, MA  01824-4111

   Phone: +1 (978) 250-3635
   Email: gmalkin@nortelnetworks.com

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RFC 2701                          MMP                     September 1999

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