Network Working Group K. Sklower
Request for Comments: 1990 University of California, Berkeley
Obsoletes: 1717 B. Lloyd
Category: Standards Track G. McGregor
Newbridge Networks Corporation
August 1996 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  options and protocols.
The differences between the current PPP Multilink specification (RFC
1717) and this memo are explained in Section 11. Any system
implementing the additional restrictions required by this memo will
be backwards compatible with conforming RFC 1717 implementations.
The authors specifically wish to thank Fred Baker of ACC, Craig Fox
of Network Systems, Gerry Meyer of Spider Systems, 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 ................................................ 21.1. Motivation ................................................ 21.2. Functional Description .................................... 31.3. Conventions ............................................... 42. General Overview ............................................ 43. Packet Formats .............................................. 73.1. Padding Considerations .................................... 104. Trading Buffer Space Against Fragment Loss .................. 104.1. Detecting Fragment Loss ................................... 114.2. Buffer Space Requirements ................................. 125. PPP Link Control Protocol Extensions ........................ 135.1. Configuration Option Types ................................ 135.1.1. Multilink MRRU LCP option ............................... 145.1.2. Short Sequence Number Header Format Option .............. 155.1.3. Endpoint Discriminator Option ........................... 156. Initiating use of Multilink Headers ......................... 197. Closing Member links ........................................ 208. Interaction with Other Protocols ............................ 209. Security Considerations ..................................... 2110. References ................................................. 2111. Differences from RFC 1717 .................................. 2211.1. Negotiating Multilink, per se ............................ 2211.2. Initial Sequence Number defined .......................... 2211.3. Default Value of the MRRU ................................ 2211.4. Config-Nak of EID prohibited ............................. 2211.5. Uniformity of Sequence Space ............................. 2211.6. Commencing and Abating use of Multilink Headers .......... 2311.7. Manual Configuration and Bundle Assignment ............... 2312. Authors' Addresses ......................................... 241. Introduction
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., ).
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.
1.2. Functional Description
The method discussed here is similar to the multilink protocol
described in ISO 7776 , 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
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 link.
Once multilink has been successfully negotiated, the sending system
is free to send PDUs encapsulated and/or fragmented with the
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.
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  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
According to the rules specified in RFC1661, this means that an
implementation MUST accept reassembled packets with and without
leading zeroes present in the Protocol Field of the reassembled
packet. Although it is explicitly forbidden below to include the
Address and Control fields (usually, the two bytes FF 03) in the
material to be fragmented, it is a good defensive programming
practice to accept the packet anyway, ignoring the two bytes if
present, as that is what RFC1661 specifies.
As a courtesy to implementations that perform better when certain
alignment obtains, it is suggested that a determination be made when
a bundle is created on whether to transmit leading zeroes by
examining whether PFC has been negotiated on the first link admitted
into a bundle. This determination should be kept in force so long as
a bundle persists.
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. Since the Protocol Field Compression mode on the member
link allows a sending system to include a leading byte of zero or not
at its discretion, this is an alternative mechanism for generating
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
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.
Figure 1. Multilink Overview.
/ \ / \
| 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.
Although it would otherwise be permitted by the PPP spec,
implementations MUST NOT include the Address and Control Field in the
logical entity to be fragmented. 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).
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 |
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.
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  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.
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
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
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. Any sequence number so
detected is assumed to correspond to a fragment which has been lost.
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, whose sequence number was deduced
to be U, causes the receiver to discard all fragments up to the
lowest numbered fragment with an ending bit (possibly deduced)
greater than or equal to U. However, the quantity M may jump into
the middle of a chain of packets which can be successful completed.
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 .
4.2. 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
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
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 + S + ... + S[N] + F
+ ... + 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
 (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  but with
an alternative PPP protocol ID.
5.1. Configuration Option Types
The Multilink Protocol introduces the use of additional LCP
o Multilink Maximum Received Reconstructed Unit
o Multilink Short Sequence Number Header Format
o Endpoint Discriminator
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 LCP option indicates that the system sending it
implements the PPP Multilink Protocol. If not rejected, the system
will construe all packets received 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.
The Max-Receive-Reconstructed unit field is two octets, and specifies
the maximum number of octets in the Information fields of reassembled
packets. A system MUST be able to receive the full 1500 octet
Information field of any reassembled PPP packet although it MAY
attempt to negotiate a smaller, or larger value. The number 1500
here comes from the specification for the MRU LCP option in PPP; if
this requirement is changed in a future version of RFC 1661, the same
rules will apply here.
A system MUST include the LCP MRRU option in every LCP negotiation
intended to instantiate a bundle or to join an existing bundle. If
the LCP MRRU option is offered on a link which is intended to join an
existing bundle, a system MUST offer the same Max-Receive-
Reconstruct-Unit value previously negotiated for the bundle.
A system MUST NOT send any multilink packets on any link unless its
peer has offered the MMRU LCP option and the system has configure-
Ack'ed it during the most recent LCP negotiation on that link. A
system MAY include the MMRU LCP option in a configure-NAK, if its
peer has not offered it (until, according to PPP rules, the peer
Note: the MRRU value conveyed im this option corresponds to the MRU
of the bundle when conceptualized as a PPP entity; but the rules for
the Multilink MRRU option are different from the LCP MRU option, as
some value MUST be offered in every LCP negotiation, and that
confirmation of this option is required prior to multilink
5.1.2. Short Sequence Number Header Format Option
Figure 5: Short Sequence Number Header Format Option
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. When a peer
system configure-Ack's this option, it MUST transmit all multilink
packets on all links of the bundle with 12 bit sequence numbers or
configure-Reject the option. If 12 bit sequence numbers are desired,
this option MUST be negotiated when the bundle is instantiated, and
MUST be explicitly included in every LCP configure request offered by
a system when the system intends to include that link in an existing
bundle using 12 bit sequence numbers. If this option is never
negotiated during the life of a bundle, sequence numbers are 24 bits
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
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
 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 the Multilink MRRU option, 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, unless
there is manual configuration to the contrary.
It is also permissible to have more than one manually
configured bundle connecting two given systems.
(2) Discriminator, no authentication:
Discriminator match -> MUST join matching bundle,
discriminator mismatch -> MUST establish new bundle.
(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
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 for any
reason; 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
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  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.
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 . 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.
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 .
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 . 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 . 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
Note that PPP Magic-Numbers are used in  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
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 .
6. Initiating use of Multilink Headers
When the use of the Multilink protocol has been negotiated on a link
(say Y), and the link is being added to a bundle which currently
contains a single existing link (say X), a system MUST transmit a
Multilink-encapsulated packet on X before transmitting any Multilink-
encapsulated packets on Y.
Since links may be added and removed from a bundle without destroying
the state associated with it, the fragment should be assigned the
appropriate (next) fragment number. As noted earlier, the first
fragment transmitted in the life of a bundle is assigned fragment
7. 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. However, if the there
is a unique link in the bundle, and all the other links were closed
gracefully (with Terminate-Ack), an implementation MAY cease using
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.
8. 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
9. Security Considerations
Operation of this protocol is no more and no less secure than
operation of the PPP authentication protocols . The reader is
directed there for further discussion.
 Leifer, D., Sheldon, S., and B. Gorsline, "A Subnetwork Control
Protocol for ISDN Circuit-Switching", University of Michigan
(unpublished), March 1991.
 Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, Daydreamer, July 1994.
 Lloyd, B., and W. Simpson, "PPP Authentication Protocols", RFC
1334, Lloyd Internetworking, Daydreamer, October 1992.
 International Organisation for Standardization, "HDLC -
Description of the X.25 LAPB-Compatible DTE Data Link
Procedures", International Standard 7776, 1988
 Rand, D., "The PPP Compression Control Protocol (CCP)", PPP
Extensions Working Group, RFC 1962, June 1996.
 Rand, D., "PPP Reliable Transmission", RFC 1663, Novell, July
 Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
USC/Information Sciences Institute, October 1994.
 Postel, J., Editor, "Internet Protocol - DARPA Internet Program
Protocol Specification", STD 5, RFC 791, USC/Information Sciences
Institute, September 1981.
 Institute of Electrical and Electronics Engineers, Inc., "IEEE
Local and Metropolitan Area Networks: Overview and Architecture",
IEEE Std. 802-1990, 1990.
 The International Telegraph and Telephone Consultative Committee
(CCITT), "Numbering Plan for the ISDN Area", Recommendation I.331
 Simpson, W., Editor, "PPP LCP Extensions", RFC 1570, Daydreamer,
11. Differences from RFC 1717
This section documents differences from RFC 1717. There are
restrictions placed on implementations that were absent in RFC 1717;
systems obeying these restrictions are fully interoperable with RFC
1717 - compliant systems.
11.1. Negotiating Multilink, per se
RFC 1717 permitted either the use of the Short Sequence Number Header
Format (SSNHF) or the Maximum Reconstructed Receive Unit (MRRU)
options by themselves to indicate the intent to negotiate multilink.
This specification forbids the use of the SSNHF option by itself; but
does permit the specific of both options together. Any
implementation which otherwise conforms to rfc1717 and also obeys
this restriction will interoperate with any RFC 1717 implementation.
11.2. Initial Sequence Number defined
This specification requires that the first sequence number
transmitted after the virtual link has reached to open state be 0.
11.3. Default Value of the MRRU
This specfication removes the default value for the MRRU, (since it
must always be negotiated with some value), and specifies that an
implementation must be support an MRRU with same value as the default
MRU size for PPP.
11.4. Config-Nak of EID prohibited
This specification forbids the config-Naking of an EID for any
11.5. Uniformity of Sequence Space
This specification requires that the same sequence format be employed
on all links in a bundle.