MultiLink Point to Point Protocol (MultiPPP or MP) is a method for splitting, recombining and sequencing datagrams across multiple logical data links. PPP MultiLink (MP) protocol 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". 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.

Once multilink has been successfully negotiated, the sending system is free to send PDUs encapsulated and/or fragmented with the multilink header. 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, there is an Authentication phase in which the Authentication protocols can be used to determine identifiers associated with each system connected by the link.

Mulitlink coordinates 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 can be of different kinds, such as pairing dialup async links with leased synchronous links.

Multilink operation is moduled as a virtual PPP link-layer entity where 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.

Network Protocol packets to be transmitted using the multilink procedure 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.

Protocol Structure

The header of MP can either be in Long Sequence Number Fragment format or Short Sequence Number Fragment Format.

Long Sequence Number Fragment Format: