The PPP Multilink Protocol (MP)
RFC 1717

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Network Working Group                                         K. Sklower
Request for Comments: 1717            University of California, Berkeley
Category: Standards Track                                       B. Lloyd
                                                             G. McGregor
                                                   Lloyd Internetworking
                                                                 D. Carr
                                          Newbridge Networks Corporation
                                                           November 1994

                    The PPP Multilink Protocol (MP)

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 for splitting, recombining and
   sequencing datagrams across multiple logical data links.  This work
   was originally motivated by the desire to exploit multiple bearer
   channels in ISDN, but is equally applicable to any situation in which
   multiple PPP links connect two systems, including async links.  This
   is accomplished by means of new PPP [2] options and protocols.

Acknowledgements

   The authors specifically wish to thank Fred Baker of ACC, Craig Fox
   of Network Systems, Gerry Meyer of Spider Systems, Tom Coradetti of
   Digiboard (for the Endpoint Discriminator option), Dan Brennan of
   Penril Datability Networks, Vernon Schryver of SGI (for the
   comprehensive discussion of padding), and the members of the IP over
   Large Public Data Networks and PPP Extensions working groups, for
   much useful discussion on the subject.

Table of Contents

   1. Introduction ................................................    2
   1.1. Motivation ................................................    2
   1.2. Functional Description ....................................    3
   1.3. Conventions ...............................................    3
   2. General Overview ............................................    4
   3. Packet Formats ..............................................    6
   3.1. Padding Considerations ....................................    9

Sklower, Lloyd, McGregor & Carr                                 [Page 1]
RFC 1717                     PPP Multilink                 November 1994

   4. Trading Buffer Space Against Fragment Loss ..................    9
   4.1. Detecting Fragment Loss ...................................   10
   4.2. Buffer Space Requirements .................................   11
   5. PPP Link Control Protocol Extensions ........................   12
   5.1. Configuration Option Types ................................   12
   5.1.1. Multilink MRRU LCP option ...............................   13
   5.1.2. Short Sequence Number Header Format Option ..............   13
   5.1.3. Endpoint Discriminator Option ...........................   14
   6. Closing Member links ........................................   18
   7. Interaction with Other Protocols ............................   19
   8. Security Considerations .....................................   19
   9. References ..................................................   20
   10. Authors' Addresses .........................................   21

1.  Introduction

1.1.  Motivation

   Basic Rate and Primary Rate ISDN both offer the possibility of
   opening multiple simultaneous channels between systems, giving users
   additional bandwidth on demand (for additional cost).  Previous
   proposals for the transmission of internet protocols over ISDN have
   stated as a goal the ability to make use of this capability, (e.g.,
   Leifer et al., [1]).

   There are proposals being advanced for providing synchronization
   between multiple streams at the bit level (the BONDING proposals);
   such features are not as yet widely deployed, and may require
   additional hardware for end system.  Thus, it may be useful to have a
   purely software solution, or at least an interim measure.

   There are other instances where bandwidth on demand can be exploited,
   such as using a dialup async line at 28,800 baud to back up a leased
   synchronous line, or opening additional X.25 SVCs where the window
   size is limited to two by international agreement.

   The simplest possible algorithms of alternating packets between
   channels on a space available basis (which might be called the Bank
   Teller's algorithm) may have undesirable side effects due to
   reordering of packets.

   By means of a four-byte sequencing header, and simple synchronization
   rules, one can split packets among parallel virtual circuits between
   systems in such a way that packets do not become reordered, or at