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.routed wasalready remarkably open, especially compared to Xerox's RIP and other proprietary RIP-like variants.The IETF's open standard added only minor modifications to routed.These enhancements were designedto provide backward compatibility with other known RIP-like and routed-like protocols.RFC 1058 SpecificationsRFC 1058 specified an open standard RIP.This RIP, like its proprietary ancestors, was designed as asimple distance-vector routing protocol.It was specifically designed for use as an Interior GatewayProtocol (IGP) in small, simple networks.RIP Packet FormatRIP uses a special packet to collect and share information about distances to known internetworkeddestinations.This packet contains the following structure:A 1-octet command fieldA 1-octet version number fieldA 2-octet zero fieldA 2-octet Address Family Identifier (AFI) fieldAnother 2-octet zero fieldA 4-octet Internetworking Address (i.e., IP Address) fieldA 4-octet zero fieldAnother 4-octet zero fieldA 4-octet metric fieldFigure 8-2 illustrates the RIP packet format.Figure 8-2: The RIP packet format.Note Examining the RIP packet structure reveals the lack of a conventional data field.RIP is a routingprotocol, not a routed protocol.As such, application data is not passed in a RIP packet.Such data mustuse a routed protocol suite, such as TCP/IP, regardless of which routing protocol was used to calculate itsroute.http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2908.htm (5 of 31) [02/02/2001 11.38.23]RIPFigure 8-2 illustrates a RIP packet with routing information fields for just a single destination.RIPpackets can support up to 25 occurrences of the AFI, IP Address, and Metric fields within a singlepacket.This enables one RIP packet to be used to update multiple entries in other routers' routing tables.RIP packets that contain multiple routing entries just repeat the packet structure from the AFI through theMetric field, including all Zero fields.The repeated structures are appended to the end of the structuredepicted in Figure 8-2.Figure 8-3 shows a RIP packet with two table entries.Figure 8-3: The RIP packet format with two table entries.The IP Address field can contain either the address of its originator or a series of IP addresses that theoriginator has in its routing table.Request packets contain a single entry and include the originator'saddress.Response packets can include up to 25 entries of a RIP router's routing table.The overall size limitation of a RIP packet is 512 octets.Therefore, in larger RIP networks, a request fora full routing table update may require the transmission of several RIP packets.No provisions were madefor the resequencing of the packets on arrival at their destination; individual routing table entries are notsplit among RIP packets.The contents of any given RIP packet are complete unto themselves, eventhough they may only be a subset of a complete routing table.The recipient node is free to process theupdates as the packets are received without having to resequence them.A RIP router may contain 100 entries in its routing table, for example.Sharing its routing informationwith other RIP nodes would require four RIP packets, each one containing 25 entries.If a receiving nodereceived packet number 4 first (containing entries numbered 86 through 100), it could just update thatportion of its routing table first.There are no sequential dependencies.This allows RIP packets to beforwarded without the overheads of a fully featured transport protocol such as TCP.The Command FieldThe Command field indicates whether the RIP packet was generated as a request or as a response to arequest.The same frame structure is used for both occurrences:A request packet asks a router to send all, or part, of its routing table.A response packet contains routing table entries that are to be shared with other RIP nodes in thenetwork.A response packet can be generated either in response to a request or as an unsolicitedupdate.The Version Number FieldThe Version Number field contains the version of RIP that was used to generate the RIP packet.http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2908.htm (6 of 31) [02/02/2001 11.38.23]RIPAlthough RIP is an open standard routing protocol, it is not frozen in time.The architects of RFC 1058anticipated this and provided a field that RIP nodes could use to specifically identify the newest versionthat they conform to.RIP has been treated to updates over the years, and these updates are reflected in aversion number.To date, only two version numbers have been assigned---numbers 1 and 2.The versionof RIP described in this chapter uses version number 1.RIP 2, explored in the next chapter, uses versionnumber 2 in this field.The Zero FieldsThe numerous Zero fields embedded in each RIP packet are silent testimony to the proliferation ofRIP-like protocols before RFC 1058.Most of the Zero fields were contrived as a means of providingbackward compatibility with older RIP-like protocols, without supporting all their proprietary features.Two such obsolete mechanisms are traceon and traceoff, for example.These mechanisms wereabandoned by RFC 1058, yet the open standard RIP needed to be backward compatible with theproprietary RIP-like protocols that did support them.Therefore, RFC 1058 preserved their space in thepacket but required this space to always be set to zeros.Packets that are received with these fields set tosomething other than zeroes are just discarded.Not all the Zero fields originated in this manner.At least one of the Zero fields was reserved for anunspecified future use.Reserving a field for an unspecified future use is what usually happens when acommittee cannot reach consensus on how to use that space.This is particularly true in cases, such asRIP, where backward compatibility required conformance to a predetermined header size.Therefore,reserving a field for future use accomplishes two things:It creates future flexibility for the protocol.It provides a noncommittal means of creating consensus where none previously existed.The AFI FieldThe AFI field specifies the address family that is represented by the IP Address field [ Pobierz całość w formacie PDF ]