network protocols handbook.pdf

63Protocols GuideTCP/IP - Network Layer ProtocolsNetwork Layer ProtocolsRouting ProtocalsProtocol NameBGP (BGP-4): Border GatewayProtocolProtocol DescriptionThe Border Gateway Protocol (BGP), runs over TCP and is aninter-Autonomous System routing protocol. BGP is the only protocolthat is designed to deal with a network of the Internet’ssize, and the only protocol that can deal well with having multipleconnections to unrelated routing domains. It is built on experiencegained with EGP. The primary function of a BGP systemis to exchange network reachability information with otherBGP systems. This network reachability information includesinformation on the list of Autonomous Systems (ASs) that reachabilityinformation traverses. This information is sufficient to constructa graph of AS connectivity from which routing loops maybe pruned and some policy decisions at the AS level may beenforced.BGP-4 provides a new set of mechanisms for supporting classlessinterdomain routing (CIDR). These mechanisms includesupport for advertising an IP prefix and eliminate the concept ofnetwork “class” within BGP. BGP-4 also introduces mechanismswhich allow aggregation of routes, including aggregation of ASpaths. These changes provide support for the proposed supernettingscheme.Protocol StructureMarkerLengthTypeMarker (16 bytes)Length(2 bytes)Type(1 byte)Message containing a value predictable by thereceiver of the message.The length of the message including the header.The message type. Possible messages are:Open, Update, Notification, KeepAlive.Sponsor SourceBGP is defined by IETF (http://www.ietf.org) RFC1771.Referencehttp://www.javvin.com/protocol/rfc1771.pdfA Border Gateway Protocol 4 (BGP-4)http://www.javvin.com/protocol/rfc1772.pdfApplication of the Border Gateway Protocol in the Internethttp://www.javvin.com/protocol/rfc1773.pdfExperience with the BGP-4 protocolhttp://www.javvin.com/protocol/rfc1774.pdfBGP-4 Protocol AnalysisAfter a transport protocol connection is established, thefirst message sent by each side is an OPENmessage. If the OPEN message is acceptable,a KEEPALIVE message confirming the OPENis sent back. Once the OPEN is confirmed, UP-DATE, KEEPALIVE, and NOTIFICATION messagesmay be exchanged. The format of eachtype of messages could be found in the referencedocuments.Related protocolsIP, TCP, EGP