network protocols handbook.pdf

183Protocols GuideWAN - PPP ProtocolsProtocol NameMPPP: MultiLink Point toPoint Protocol (MultiPPP)Protocol DescriptionMultiLink Point to Point Protocol (MultiPPP or MPPP) is a methodfor splitting, recombining and sequencing datagrams acrossmultiple logical data links. PPP MultiLink (MP) protocol is basedon an LCP option negotiation that permits a system to indicateto its peer that it is capable of combining multiple physical linksinto a “bundle”. Multilink is negotiated during the initial LCP optionnegotiation. A system indicates to its peer that it is willing todo multilink by sending the multilink option as part of the initialLCP option negotiation.Once multilink has been successfully negotiated, the sendingsystem is free to send PDUs encapsulated and/or fragmentedwith the multilink header. To establish communications over apoint-to-point link, each end of the PPP link must first send LCPpackets to configure the data link during Link Establishmentphase. After the link has been established, there is an Authenticationphase in which the Authentication protocols can be usedto determine identifiers associated with each system connectedby the link.Multilink coordinates multiple independent links between a fixedpair of systems, providing a virtual link with greater bandwidththan any of the constituent members. The aggregate link, orbundle, is named by the pair of identifiers for two systems connectedby the multiple links. A system identifier may includeinformation provided by PPP Authentication and informationprovided by LCP negotiation. The bundled links can be differentphysical links, as in multiple async lines, but may also beinstances of multiplexed links, such as ISDN, X.25 or FrameRelay. The links can be of different kinds, such as pairing dialupasync links with leased synchronous links.Multilink operation is moduled as a virtual PPP link-layer entitywhere packets received over different physical link-layer entitiesare identified as belonging to a separate PPP network protocol(the Multilink Protocol, or MP) and recombined and sequencedaccording to information present in a multilink fragmentationheader. All packets received over links identified as belonging tothe multilink arrangement are presented to the same networklayerprotocol processing machine, whether they have multilinkheaders or not.Network protocol packets to be transmitted using the multilinkprocedure are first encapsulated (but not framed) according tonormal PPP procedures, and large packets are broken up intomultiple segments sized appropriately for the multiple physicallinks. A new PPP header consisting of the Multilink ProtocolIdentifier, and the Multilink header is inserted before each section.Protocol StructureThe header of MP can either be in Long Sequence Number Fragmentformat or Short Sequence Number Fragment Format.Long Sequence Number Fragment Format1 2 3 4 5 6 7 8 32bitB E 0 0 0 0 0 0 Sequence NumberSequence Number (L)Fragment Data…Short Sequence Number Fragment Format1 2 3 4 16bitB E 0 0 Sequence NumberFragment Data…• Address, Control – Compressed fields for the Address,the control field and the Protocol ID.• PID (L), PID (H) - Protocol Identifier for PPP MultiLink;this is 0x00-0x3d• B(egining) Bit - One bit field set to 1 on the first fragmentderived from a PPP packet and set to 0 for allother fragments from the same PPP packet.• E(nd) Bit - A one bit field set to 1 on the last fragmentand set to 0 for all other fragments.• 00 - Reserved field with the value of 0• Sequence Number - The sequence field is a 24 bit or12 bit number that is incremented for every fragmenttransmitted.• Fragment Data - The data itself.Related protocolsPPP, PPPoE, PPPoA, SLIP, CHAP, HDLC, LCP, NCPSponsor SourceMP (MultiPPP) is defined by IETF (http://www.ietf.org) .Referencehttp://www.javvin.com/protocol/rfc1990.pdfThe PPP Multilink Protocol (MP).http://www.javvin.com/protocol/rfc1661.pdfThe Point-to-Point Protocol (PPP)