|MP, PPP Multilink Protocol|
|Type:||PPP network layer protocol.|
|Working groups:||pppext, Point-to-Point Protocol Extensions.|
MP was designed to combine multiple physical connections into a bundle.
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.
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:
- The system offering the option is capable of combining multiple physical links into one logical link;
- 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;
- 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 multilink header.
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.
RFC 2686, page 1:
A companion document describes an architecture for providing integrated services over low-bitrate links, such as modem lines, ISDN B-channels, and sub-T1 links. The main components of the architecture are: a real-time encapsulation format for asynchronous and synchronous low-bitrate links, a header compression architecture optimized for real-time flows, elements of negotiation protocols used between routers (or between hosts and routers), and announcement protocols used by applications to allow this negotiation to take place.
This document proposes the fragment-oriented solution for the real- time encapsulation format part of the architecture. The general approach is to start from the PPP Multilink fragmentation protocol and provide a small number of extensions to add functionality and reduce the overhead.
|PPP header||MP header||Data :::|
MP header (Long Sequence Number Fragment Format):
MP header (Short Sequence Number Fragment Format):
|B||E||0||0||Sequence Number||Data :::|
B, Beginning Fragment.
Indicates this is the first fragment derived from a PPP packet.
E, Ending Fragment.
Indicates this is the last fragment derived from a PPP packet.
12 or 24 bits.
This field 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 LCP configuration option 18 (Multilink Short Sequence Number Header Format).
The Bearer channel, which is the primary channel for an ISDN connection.
The computer or peripheral device has a temporary telephone connection by modem through the public telecommunications network to a computer network.
(RFC 1983) A temporary, as opposed to dedicated, connection between machines established over a phone line (analog or ISDN).
ISDN, Integrated Services Digital Network.
(RFC 1983) An emerging technology which is beginning to be offered by the telephone carriers of the world. ISDN combines voice and digital network services in a single medium, making it possible to offer customers digital data services as well as voice connections through a single "wire." The standards that define ISDN are specified by CCITT.
Leased Synchronous Link.
The computer or peripheral device has a dedicated telephone connection through a private telecommunications carrier to a computer network.
Multiple signals (analog or digital) are divided into data and voice when in transmission.
[RFC 1990] The PPP Multilink Protocol (MP).
[RFC 2686] The Multi-Class Extension to Multi-Link PPP.
[RFC 1717] The PPP Multilink Protocol (MP).