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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.


 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.


 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
 least the likelihood of this is greatly reduced.

Sklower, Lloyd, McGregor & Carr [Page 2] RFC 1717 PPP Multilink November 1994

1.2. Functional Description

 The method discussed here is similar to the multilink protocol
 described in ISO 7776 [4], but offers the additional ability to split
 and recombine packets, thereby reducing latency, and potentially
 increase the effective maximum receive unit (MRU).  Furthermore,
 there is no requirement here for acknowledged-mode operation on the
 link layer, although that is optionally permitted.
 Multilink is based on an LCP option negotiation that permits a system
 to indicate to its peer that it is capable of combining multiple
 physical links into a "bundle".  Only under exceptional conditions
 would a given pair of systems require the operation of more than one
 bundle connecting them.
 Multilink is negotiated during the initial LCP option negotiation.  A
 system indicates to its peer that it is willing to do multilink by
 sending the multilink option as part of the initial LCP option
 negotiation.  This negotiation indicates three things:
 1.   The system offering the option is capable of combining
      multiple physical links into one logical link;
 2.   The system is capable of receiving upper layer protocol data
      units (PDU) fragmented using the multilink header (described
      later) and reassembling the fragments back into the original
      PDU for processing;
 3.   The system is capable of receiving PDUs of size N octets
      where N is specified as part of the option even if N is larger
      than the maximum receive unit (MRU) for a single physical
 Once multilink has been successfully negotiated, the sending system
 is free to send PDUs encapsulated and/or fragmented with the
 multilink header.

1.3. Conventions

 The following language conventions are used in the items of
 specification in this document:
 o    MUST, SHALL or MANDATORY -- the item is an absolute requirement
      of the specification.
 o    SHOULD or RECOMMENDED -- the item should generally be followed
      for all but exceptional circumstances.

Sklower, Lloyd, McGregor & Carr [Page 3] RFC 1717 PPP Multilink November 1994

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

2. General Overview

 In order to establish communications over a point-to-point link, each
 end of the PPP 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 in which the
 authentication protocols can be used to determine identifiers
 associated with each system connected by the link.
 The goal of multilink operation is to coordinate multiple independent
 links between a fixed pair of systems, providing a virtual link with
 greater bandwidth than any of the constituent members.  The aggregate
 link, or bundle, is named by the pair of identifiers for two systems
 connected by the multiple links.  A system identifier may include
 information provided by PPP Authentication [3] and information
 provided by LCP negotiation.  The bundled links can be different
 physical links, as in multiple async lines, but may also be instances
 of multiplexed links, such as ISDN, X.25 or Frame Relay.  The links
 may also be of different kinds, such as pairing dialup async links
 with leased synchronous links.
 We suggest that multilink operation can be modeled as a virtual PPP
 link-layer entity wherein packets received over different physical
 link-layer entities are identified as belonging to a separate PPP
 network protocol (the Multilink Protocol, or MP) and recombined and
 sequenced according to information present in a multilink
 fragmentation header.  All packets received over links identified as
 belonging to the multilink arrangement are presented to the same
 network-layer protocol processing machine, whether they have
 multilink headers or not.
 The packets to be transmitted using the multilink procedure are
 encapsulated according to the rules for PPP where the following
 options would have been manually configured:
      o  No async control character Map
      o  No Magic Number
      o  No Link Quality Monitoring
      o  Address and Control Field Compression
      o  Protocol Field Compression
      o  No Compound Frames
      o  No Self-Describing-Padding

Sklower, Lloyd, McGregor & Carr [Page 4] RFC 1717 PPP Multilink November 1994

 Of course, individual links are permitted to have different settings
 for these options.  As described below, member links SHOULD negotiate
 Self-Describing-Padding, even though pre-fragmented packets MUST NOT
 be padded.
 LCP negotiations are not permitted on the bundle itself.  An
 implementation MUST NOT transmit LCP Configure-Request, -Reject,
 -Ack, -Nak, Terminate-Request or -Ack packets via the multilink
 procedure, and an implementation receiving them MUST silently discard
 them.  (By "silently discard" we mean to not generate any PPP packets
 in response; an implementation is free to generate a log entry
 registering the reception of the unexpected packet).  By contrast,
 other LCP packets having control functions not associated with
 changing the defaults for the bundle itself are permitted.  An
 implementation MAY transmit LCP Code-Reject, Protocol-Reject, Echo-
 Request, Echo-Reply and Discard-Request Packets.
 The effective MRU for the logical-link entity is negotiated via an
 LCP option.  It is irrelevant whether Network Control Protocol
 packets are encapsulated in multilink headers or not, or even over
 which link they are sent, once that link identifies itself as
 belonging to a multilink arrangement.
 Note that network protocols that are not sent using multilink headers
 cannot be sequenced.  (And consequently will be delivered in any
 convenient way).
 For example, consider the case in Figure 1.  Link 1 has negotiated
 network layers NL 1, NL 2, and MP between two systems.  The two
 systems then negotiate MP over Link 2.
 Frames received on link 1 are demultiplexed at the data link layer
 according the PPP network protocol identifier and can be sent to NL
 1, NL 2, or MP.  Link 2 will accept frames with all network protocol
 identifiers that Link 1 does.
 Frames received by MP are further demultiplexed at the network layer
 according to the PPP network protocol identifier and sent to NL 1 or
 NL 2.  Any frames received by MP for any other network layer
 protocols are rejected using the normal protocol reject mechanism.

Sklower, Lloyd, McGregor & Carr [Page 5] RFC 1717 PPP Multilink November 1994

                    Figure 1.  Multilink Overview.
   Network Layer
                  ______           ______
                 /      \         /      \
                |  NL 1  |       |  NL 2  |
                 \______/         \______/
                   | | |             | | |
                   | | +-------------o-o-o-+
                   | +------+  +-----+ | | |
                   |        |  |       | | |
                   | +------o--o-------+ + |
                   | |      |__|_        | |
                   | |     /      \      | |
                   | |    |  MLCP  | <--- Link Layer
                   | |     \______/    Demultiplexing
                   | |        |          | |
                   | |        |          | |
                   | |        | <--- Virtual Link
                   | |        |          | |
                   | |        |          | |
                   | |        |          | |
                   | |        +          | |
                ___|_|        |       ___|_|
               /      \       |      /      \
              |   LCP  |------+-----|  LCP   | <--- Link Layer
               \______/              \______/       Demultiplexing
                  |                      |
                  |                      |
                Link 1                 Link 2

3. Packet Formats

 In this section we describe the layout of individual fragments, which
 are the "packets" in the Multilink Protocol.  Network Protocol
 packets are first encapsulated (but not framed) according to normal
 PPP procedures, and large packets are broken up into multiple
 segments sized appropriately for the multiple physical links.  A new
 PPP header consisting of the Multilink Protocol Identifier, and the
 Multilink header is inserted before each section.  (Thus the first
 fragment of a multilink packet in PPP will have two headers, one for
 the fragment, followed by the header for the packet itself).

Sklower, Lloyd, McGregor & Carr [Page 6] RFC 1717 PPP Multilink November 1994

 Systems implementing the multilink procedure are not required to
 fragment small packets.  There is also no requirement that the
 segments be of equal sizes, or that packets must be broken up at all.
 A possible strategy for contending with member links of differing
 transmission rates would be to divide the packets into segments
 proportion to the transmission rates.  Another strategy might be to
 divide them into many equal fragments and distribute multiple
 fragments per link, the numbers being proportional to the relative
 speeds of the links.
 PPP multilink fragments are encapsulated using the protocol
 identifier 0x00-0x3d.  Following the protocol identifier is a four
 byte header containing a sequence number, and two one bit fields
 indicating that the fragment begins a packet or terminates a packet.
 After negotiation of an additional PPP LCP option, the four byte
 header may be optionally replaced by a two byte header with only a 12
 bit sequence space.  Address & Control and Protocol ID compression
 are assumed to be in effect.  Individual fragments will, therefore,
 have the following format:
           Figure 2:  Long Sequence Number Fragment Format.
 PPP Header:  | Address 0xff  | Control 0x03  |
              | PID(H)  0x00  | PID(L)  0x3d  |
 MP Header:   |B|E|0|0|0|0|0|0|sequence number|
              |      sequence number (L)      |
              |        fragment data          |
              |               .               |
              |               .               |
              |               .               |
 PPP FCS:     |              FCS              |

Sklower, Lloyd, McGregor & Carr [Page 7] RFC 1717 PPP Multilink November 1994

           Figure 3:  Short Sequence Number Fragment Format.
 PPP Header:  | Address 0xff  | Control 0x03  |
              | PID(H)  0x00  | PID(L)  0x3d  |
 MP Header:   |B|E|0|0|    sequence number    |
              |    fragment data              |
              |               .               |
              |               .               |
              |               .               |
 PPP FCS:     |              FCS              |
 The (B)eginning fragment bit is a one bit field set to 1 on the first
 fragment derived from a PPP packet and set to 0 for all other
 fragments from the same PPP packet.
 The (E)nding fragment bit is a one bit field set to 1 on the last
 fragment and set to 0 for all other fragments.  A fragment may have
 both the (B)eginning and (E)nding fragment bits set to 1.
 The sequence field is a 24 bit or 12 bit number that is incremented
 for every fragment transmitted.  By default, the sequence field is 24
 bits long, but can be negotiated to be only 12 bits with an LCP
 configuration option described below.
 Between the (E)nding fragment bit and the sequence number is a
 reserved field, whose use is not currently defined, which MUST be set
 to zero.  It is 2 bits long when the use of short sequence numbers
 has been negotiated, 6 bits otherwise.
 In this multilink protocol, a single reassembly structure is
 associated with the bundle.  The multilink headers are interpreted in
 the context of this structure.
 The FCS field shown in the diagram is inherited from the normal
 framing mechanism from the member link on which the packet is
 transmitted.  There is no separate FCS applied to the reconstituted
 packet as a whole if transmitted in more than one fragment.

Sklower, Lloyd, McGregor & Carr [Page 8] RFC 1717 PPP Multilink November 1994

3.1. Padding Considerations

 Systems that support the multilink protocol SHOULD implement Self-
 Describing-Padding.  A system that implements self-describing-padding
 by definition will either include the padding option in its initial
 LCP Configure-Requests, or (to avoid the delay of a Configure-Reject)
 include the padding option after receiving a NAK containing the
 A system that must pad its own transmissions but does not use Self-
 Describing-Padding when not using multilink, MAY continue to not use
 Self-Describing-Padding if it ensures by careful choice of fragment
 lengths that only (E)nding fragments of packets are padded.  A system
 MUST NOT add padding to any packet that cannot be recognized as
 padded by the peer.  Non-terminal fragments MUST NOT be padded with
 trailing material by any other method than Self-Describing-Padding.
 A system MUST ensure that Self-Describing-Padding as described in RFC
 1570 [11] is negotiated on the individual link before transmitting
 any multilink data packets if it might pad non-terminal fragments or
 if it would use network or compression protocols that are vulnerable
 to padding, as described in RFC 1570.  If necessary, the system that
 adds padding MUST use LCP Configure-NAK's to elicit a Configure-
 Request for Self-Describing-Padding from the peer.
 Note that LCP Configure-Requests can be sent at any time on any link,
 and that the peer will always respond with a Configure-Request of its
 own.  A system that pads its transmissions but uses no protocols
 other than multilink that are vulnerable to padding MAY delay
 ensuring that the peer has Configure-Requested Self-Describing-
 Padding until it seems desireable to negotiate the use of Multilink
 itself.  This permits the interoperability of a system that pads with
 older peers that support neither Multilink nor Self-Describing-

4. Trading Buffer Space Against Fragment Loss

 In a multilink procedure one channel may be delayed with respect to
 the other channels in the bundle.  This can lead to fragments being
 received out of order, thus increasing the difficulty in detecting
 the loss of a fragment.  The task of estimating the amount of space
 required for buffering on the receiver becomes more complex because
 of this.  In this section we discuss a technique for declaring that a
 fragment is lost, with the intent of minimizing the buffer space
 required, yet minimizing the number of avoidable packet losses.

Sklower, Lloyd, McGregor & Carr [Page 9] RFC 1717 PPP Multilink November 1994

4.1. Detecting Fragment Loss

 On each member link in a bundle, the sender MUST transmit fragments
 with strictly increasing sequence numbers (modulo the size of the
 sequence space).  This requirement supports a strategy for the
 receiver to detect lost fragments based on comparing sequence
 numbers.  The sequence number is not reset upon each new PPP packet,
 and a sequence number is consumed even for those fragments which
 contain an entire PPP packet, i.e., one in which both the (B)eginning
 and (E)nding bits are set.
 An implementation MUST set the sequence number of the first fragment
 transmited on a newly-constructed bundle to zero.  (Joining a
 secondary link to an exisiting bundle is invisible to the protocol,
 and an implementation MUST NOT reset the sequence number space in
 this situation).
 The receiver keeps track of the incoming sequence numbers on each
 link in a bundle and maintains the current minimum of the most
 recently received sequence number over all the member links in the
 bundle (call this M).  The receiver detects the end of a packet when
 it receives a fragment bearing the (E)nding bit.  Reassembly of the
 packet is complete if all sequence numbers up to that fragment have
 been received.
 A lost fragment is detected when M advances past the sequence number
 of a fragment bearing an (E)nding bit of a packet which has not been
 completely reassembled (i.e., not all the sequence numbers between
 the fragment bearing the (B)eginning bit and the fragment bearing the
 (E)nding bit have been received).  This is because of the increasing
 sequence number rule over the bundle.
 An implementation MUST assume that if a fragment bears a (B)eginning
 bit, that the previously numbered fragment bore an (E)nding bit.
 Thus if a packet is lost bearing the (E)nding bit, and the packet
 whose fragment number is M contains a (B)eginning bit, the
 implementation MUST discard fragments for all unassembled packets
 through M-1, but SHOULD NOT discard the fragment bearing the new
 (B)eginning bit on this basis alone.
 The detection of a lost fragment causes the receiver to discard all
 fragments up to M.  If the fragment with sequence number M has the
 (B)eginning bit set then the receiver starts reassembling the new
 packet, otherwise the receiver resynchronizes on the next fragment
 bearing the (B)eginning bit.  All fragments received while the
 receiver is attempting to resynchronize not bearing the (B)eginning
 bit SHOULD be discarded.

Sklower, Lloyd, McGregor & Carr [Page 10] RFC 1717 PPP Multilink November 1994

 Fragments may be lost due to corruption of individual packets or
 catastrophic loss of the link (which may occur only in one
 direction).  This version of the multilink protocol mandates no
 specific procedures for the detection of failed links.  The PPP link
 quality management facility, or the periodic issuance of LCP echo-
 requests could be used to achieve this.
 Senders SHOULD avoid keeping any member links idle to maximize early
 detection of lost fragments by the receiver, since the value of M is
 not incremented on idle links.  Senders SHOULD rotate traffic among
 the member links if there isn't sufficient traffic to overflow the
 capacity of one link to avoid idle links.
 Loss of the final fragment of a transmission can cause the receiver
 to stall until new packets arrive.  The likelihood of this may be
 decreased by sending a null fragment on each member link in a bundle
 that would otherwise become idle immediately after having transmitted
 a fragment bearing the (E)nding bit, where a null fragment is one
 consisting only of a multilink header bearing both the (B)egin and
 (E)nding bits (i.e., having no payload).  Implementations concerned
 about either wasting bandwidth or per packet costs are not required
 to send null fragments and may elect to defer sending them until a
 timer expires, with the marginally increased possibility of lengthier
 stalls in the receiver.  The receiver SHOULD implement some type of
 link idle timer to guard against indefinite stalls.
 The increasing sequence per link rule prohibits the reallocation of
 fragments queued up behind a failing link to a working one, a
 practice which is not unusual for implementations of ISO multilink
 over LAPB [4].

4.2. Buffer Space Requirements

 There is no amount of buffering that will guarantee correct detection
 of fragment loss, since an adversarial peer may withhold a fragment
 on one channel and send arbitrary amounts on the others.  For the
 usual case where all channels are transmitting, you can show that
 there is a minimum amount below which you could not correctly detect
 packet loss.  The amount depends on the relative delay between the
 channels, (D[channel-i,channel-j]), the data rate of each channel,
 R[c], the maximum fragment size permitted on each channel, F[c], and
 the total amount of buffering the transmitter has allocated amongst
 the channels.
 When using PPP, the delay between channels could be estimated by
 using LCP echo request and echo reply packets.  (In the case of links
 of different transmission rates, the round trip times should be
 adjusted to take this into account.)  The slippage for each channel

Sklower, Lloyd, McGregor & Carr [Page 11] RFC 1717 PPP Multilink November 1994

 is defined as the bandwidth times the delay for that channel relative
 to the channel with the longest delay, S[c] = R[c] * D[c,c-worst].
 (S[c-worst] will be zero, of course!)
 A situation which would exacerbate sequence number skew would be one
 in which there is extremely bursty traffic (almost allowing all
 channels to drain), and then where the transmitter would first queue
 up as many consecutively numbered packets on one link as it could,
 then queue up the next batch on a second link, and so on.  Since
 transmitters must be able to buffer at least a maximum- sized
 fragment for each link (and will usually buffer up at least two) A
 receiver that allocates any less than S[1] + S[2] + ... + S[N] + F[1]
 + ... + F[N], will be at risk for incorrectly assuming packet loss,
 and therefore, SHOULD allocate at least twice that.

5. PPP Link Control Protocol Extensions

 If reliable multilink operation is desired, PPP Reliable Transmission
 [6] (essentially the use of ISO LAPB) MUST be negotiated prior to the
 use of the Multilink Protocol on each member link.
 Whether or not reliable delivery is employed over member links, an
 implementation MUST present a signal to the NCP's running over the
 multilink arrangement that a loss has occurred.
 Compression may be used separately on each member link, or run over
 the bundle (as a logical group link).  The use of multiple
 compression streams under the bundle (i.e., on each link separately)
 is indicated by running the Compression Control Protocol [5] but with
 an alternative PPP protocol ID.

5.1. Configuration Option Types

 The Multilink Protocol introduces the use of additional LCP
 Configuration Options:
      o  Multilink Maximum Received Reconstructed Unit
      o  Multilink Short Sequence Number Header Format
      o  Endpoint Discriminator

Sklower, Lloyd, McGregor & Carr [Page 12] RFC 1717 PPP Multilink November 1994

5.1.1. Multilink MRRU LCP option

                 Figure 4:  Multilink MRRU LCP option
  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 = 17   |   Length = 4  | Max-Receive-Reconstructed-Unit|
 The presence of this option indicates that the system sending it
 implements the PPP Multilink Protocol, and unless rejected, will
 construe all packets receive on this link as being able to be
 processed by a common protocol machine with any other packets
 received from the same peer on any other link on which this option
 has been accepted.  A system MUST NOT accept the Multilink MRRU LCP
 Option if it is not willing to symmetrically have the packets it
 sends interpreted in the same fashion.
 This option also advises the peer that the implementation will be
 able to reconstruct a PPP packet whose payload will contain the
 number of bytes as Max-Receive-Reconstructed-Unit.
 A system MAY indicate the desire to conduct multilink operation
 solely by use of the Multilink Short Sequence Number Header Format
 LCP option (discussed next); the default value for MRRU option is
 1600 bytes if not otherwise explicitly negotiated.
 Note: this option corresponds to what would have been the MRU of the
 bundle when conceptualized as a PPP-like entity.

5.1.2. Short Sequence Number Header Format Option

         Figure 5:  Short Sequence Number Header Format Option
  0                   1
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
 |   Type = 18   |  Length = 2   |
 This option advises the peer that the implementation wishes to
 receive fragments with short, 12 bit sequence numbers.  By default
 sequence, numbers are 24 bits long.  When this option is received, an
 implementation MUST either transmit all subsequent multilink packets
 on all links of the bundle with 12 bit sequence numbers or
 configure-NAK or configure-Reject the option.

Sklower, Lloyd, McGregor & Carr [Page 13] RFC 1717 PPP Multilink November 1994

 An implementation wishing to transmit multilink fragments with short
 sequence numbers MAY include the multilink short sequence number in a
 configure-NAK to ask that the peer respond with a request to receive
 short sequence numbers.  The peer is not compelled to respond with
 the option.

5.1.3. Endpoint Discriminator Option

               Figure 7:  Endpoint Discriminator Option
  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 = 19   |     Length    |    Class      |  Address ...
 The Endpoint Discriminator Option represents identification of the
 system transmitting the packet.  This option advises a system that
 the peer on this link could be the same as the peer on another
 existing link.  If the option distinguishes this peer from all
 others, a new bundle MUST be established from the link being
 negotiated.  If this option matches the class and address of some
 other peer of an existing link, the new link MUST be joined to the
 bundle containing the link to the matching peer or MUST establish a
 new bundle, depending on the decision tree shown in (1) through (4)
 To securely join an existing bundle, a PPP authentication protocol
 [3] must be used to obtain authenticated information from the peer to
 prevent a hostile peer from joining an existing bundle by presenting
 a falsified discriminator option.
 This option is not required for multilink operation.  If a system
 does not receive either of the Multilink MRRU or Short Sequence
 options, but does receive the Endpoint Discriminator Option, and
 there is no manual configuration providing outside information, the
 implementation MUST NOT assume that multilink operation is being
 requested on this basis alone.
 As there is also no requirement for authentication, there are four
 sets of scenarios:
 (1)  No authentication, no discriminator:
      All new links MUST be joined to one bundle.
 (2)  Discriminator, no authentication:
      Discriminator match -> MUST join matching bundle,
      discriminator mismatch -> MUST establish new bundle.

Sklower, Lloyd, McGregor & Carr [Page 14] RFC 1717 PPP Multilink November 1994

 (3)  No discriminator, authentication:
      Authenticated match -> MUST join matching bundle,
      authenticated mismatch -> MUST establish new bundle.
 (4)  Discriminator, authentication:
      Discriminator match and authenticated match -> MUST join bundle,
      discriminator mismatch -> MUST establish new bundle,
      authenticated mismatch -> MUST establish new bundle.
 The option contains a Class which selects an identifier address space
 and an Address which selects a unique identifier within the class
 address space.
 This identifier is expected to refer to the mechanical equipment
 associated with the transmitting system.  For some classes,
 uniqueness of the identifier is global and is not bounded by the
 scope of a particular administrative domain.  Within each class,
 uniqueness of address values is controlled by a class dependent
 policy for assigning values.
 Each endpoint may chose an identifier class without restriction.
 Since the objective is to detect mismatches between endpoints
 erroneously assumed to be alike, mismatch on class alone is
 sufficient.  Although no one class is recommended, classes which have
 universally unique values are preferred.
 This option is not required to be supported either by the system or
 the peer.  If the option is not present in a Configure-Request, the
 system MUST NOT generate a Configure-Nak of this option, instead it
 SHOULD behave as if it had received the option with Class = 0,
 Address = 0.  If a system receives a Configure-Nak or Configure-
 Reject of this option, it MUST remove it from any additional
 The size is determined from the Length field of the element.  For
 some classes, the length is fixed, for others the length is variable.
 The option is invalid if the Length field indicates a size below the
 minimum for the class.
 An implementation MAY use the Endpoint Discriminator to locate
 administration or authentication records in a local database.  Such
 use of this option is incidental to its purpose and is deprecated
 when a PPP Authentication protocol [3] can be used instead.  Since
 some classes permit the peer to generate random or locally assigned
 address values, use of this option as a database key requires prior
 agreement between peer administrators.

Sklower, Lloyd, McGregor & Carr [Page 15] RFC 1717 PPP Multilink November 1994

 The specification of the subfields are:
      19 = for Endpoint Discriminator
      3 + length of Address
      The Class field is one octet and indicates the identifier
      address space.  The most up-to-date values of the LCP Endpoint
      Discriminator Class field are specified in the most recent
      "Assigned Numbers" RFC [7].  Current values are assigned as
      0    Null Class
      1    Locally Assigned Address
      2    Internet Protocol (IP) Address
      3    IEEE 802.1 Globally Assigned MAC Address
      4    PPP Magic-Number Block
      5    Public Switched Network Directory Number
      The Address field is one or more octets and indicates the
      identifier address within the selected class.  The length and
      content depend on the value of the Class as follows:
      Class 0 - Null Class
           Maximum Length: 0
           This class is the default value if the option is not
           present in a received Configure-Request.

Sklower, Lloyd, McGregor & Carr [Page 16] RFC 1717 PPP Multilink November 1994

      Class 1 - Locally Assigned Address
           Maximum Length: 20
           This class is defined to permit a local assignment in the
           case where use of one of the globally unique classes is not
           possible.  Use of a device serial number is suggested.  The
           use of this class is deprecated since uniqueness is not
      Class 2 - Internet Protocol (IP) Address
           Fixed Length: 4
           An address in this class contains an IP host address as
           defined in [8].
      Class 3 - IEEE 802.1 Globally Assigned MAC Address
           Fixed Length: 6
           An address in this class contains an IEEE 802.1 MAC address
           in canonical (802.3) format [9].  The address MUST have the
           global/local assignment bit clear and MUST have the
           multicast/specific bit clear.  Locally assigned MAC
           addresses should be represented using Class 1.
      Class 4 - PPP Magic-Number Block
           Maximum Length: 20
           This is not an address but a block of 1 to 5 concatenated
           32 bit PPP Magic-Numbers as defined in [2].  This class
           provides for automatic generation of a value likely but not
           guaranteed to be unique.  The same block MUST be used by an
           endpoint continuously during any period in which at least
           one link is in the LCP Open state.  The use of this class
           is deprecated.

Sklower, Lloyd, McGregor & Carr [Page 17] RFC 1717 PPP Multilink November 1994

           Note that PPP Magic-Numbers are used in [2] to detect
           unexpected loopbacks of a link from an endpoint to itself.
           There is a small probability that two distinct endpoints
           will generate matching magic-numbers.  This probability is
           geometrically reduced when the LCP negotiation is repeated
           in search of the desired mismatch, if a peer can generate
           uncorrelated magic-numbers.
           As used here, magic-numbers are used to determine if two
           links are in fact from the same peer endpoint or from two
           distinct endpoints.  The numbers always match when there is
           one endpoint.  There is a small probability that the
           numbers will match even if there are two endpoints.  To
           achieve the same confidence that there is not a false match
           as for LCP loopback detection, several uncorrelated magic-
           numbers can be combined in one block.
      Class 5 - Public Switched Network Directory Number
           Maximum Length: 15
           An address in this class contains an octet sequence as
           defined by I.331 (E.164) representing an international
           telephone directory number suitable for use to access the
           endpoint via the public switched telephone network [10].

6. Closing Member links

 Member links may be terminated according to normal PPP LCP procedures
 using LCP terminate-request and terminate-ack packets on that member
 link.  Since it is assumed that member links usually do not reorder
 packets, receipt of a terminate ack is sufficient to assume that any
 multilink protocol packets ahead of it are at no special risk of
 Receipt of an LCP terminate-request on one link does not conclude the
 procedure on the remaining links.
 So long as any member links in the bundle are active, the PPP state
 for the bundle persists as a separate entity.
 If the multilink procedure is used in conjunction with PPP reliable
 transmission, and a member link is not closed gracefully, the
 implementation should expect to receive packets which violate the
 increasing sequence number rule.

Sklower, Lloyd, McGregor & Carr [Page 18] RFC 1717 PPP Multilink November 1994

7. Interaction with Other Protocols

 In the common case, LCP, and the Authentication Control Protocol
 would be negotiated  over each member link.  The Network Protocols
 themselves and associated control exchanges would normally have been
 conducted once, on the bundle.
 In some instances it may be desirable for some Network Protocols to
 be exempted from sequencing requirements, and if the MRU sizes of the
 link did not cause fragmentation, those protocols could be sent
 directly over the member links.
 Although explicitly discouraged above, if there were several member
 links connecting two implementations, and independent sequencing of
 two protocol sets were desired, but blocking of one by the other was
 not, one could describe two multilink procedures by assigning
 multiple endpoint identifiers to a given system.  Each member link,
 however, would only belong to one bundle.  One could think of a
 physical router as housing two logically separate implementations,
 each of which is independently configured.
 A simpler solution would be to have one link refuse to join the
 bundle, by sending a Configure-Reject in response to the Multilink
 LCP option.

8. Security Considerations

 Operation of this protocol is no more and no less secure than
 operation of the PPP authentication protocols [3].  The reader is
 directed there for further discussion.

Sklower, Lloyd, McGregor & Carr [Page 19] RFC 1717 PPP Multilink November 1994

9. References

 [1] Leifer, D., Sheldon, S., and B. Gorsline "A Subnetwork Control
     Protocol for ISDN Circuit-Switching", University of Michigan
     (unpublished), March 1991.
 [2] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
     RFC 1661, Daydreamer, July 1994.
 [3] Lloyd, B., and W. Simpson, "PPP Authentication Protocols", RFC
     1334, Lloyd Internetworking, Daydreamer, October 1992.
 [4] International Organisation for Standardization, "HDLC -
     Description of the X.25 LAPB-Compatible DTE Data Link
     Procedures", International Standard 7776, 1988
 [5] Rand, D., "The PPP Compression Control Protocol (CCP)", PPP
     Extensions Working Group, Work in Progress.
 [6] Rand, D., "PPP Reliable Transmission", PPP Extensions Working
     Group, Work in Progress.
 [7] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
     USC/Information Sciences Institute, October 1994.
 [8] Postel, J., Editor, "Internet Protocol - DARPA Internet Program
     Protocol Specification", STD 5, RFC 791, USC/Information Sciences
     Institute, September 1981.
 [9] Institute of Electrical and Electronics Engineers, Inc., "IEEE
     Local and Metropolitan Area Networks: Overview and Architecture",
     IEEE Std. 802-1990, 1990.
[10] The International Telegraph and Telephone Consultative Committee
     (CCITT), "Numbering Plan for the ISDN Area", Recommendation I.331
     (E.164), 1988.
[11] Simpson, W., Editor, "PPP LCP Extensions", RFC 1570, Daydreamer,
     January 1994.

Sklower, Lloyd, McGregor & Carr [Page 20] RFC 1717 PPP Multilink November 1994

10. Authors' Addresses

 Keith Sklower
 Computer Science Department
 384 Soda Hall, Mail Stop 1776
 University of California
 Berkeley, CA 94720-1776
 Phone:  (510) 642-9587
 EMail:  sklower@CS.Berkeley.EDU
 Brian Lloyd
 Lloyd Internetworking
 3031 Alhambra Drive
 Cameron Park, CA 95682
 Phone: (916) 676-1147
 Glenn McGregor
 Lloyd Internetworking
 3031 Alhambra Drive
 Cameron Park, CA 95682
 Phone: (916) 676-1147
 Dave Carr
 Newbridge Networks Corporation
 600 March Road
 P.O. Box 13600
 Kanata, Ontario,
 Canada, K2K 2E6
 Phone:  (613) 591-3600
 EMail:  dcarr@Newbridge.COM

Sklower, Lloyd, McGregor & Carr [Page 21]

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