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.These include:46 TCP/IP Tutorial and Technical Overview The use of a single IP prefix for the domain.External routers must have anentry for the organization that lies partly or wholly outside the normalhierarchy.Where a domain is multi-homed but all of the attached TRDsthemselves are topologically nearby, it would be appropriate for thedomain's IP prefix to include those bits common to all of the attachedTRDs.For example, if all of the TRDs were wholly within the UnitedStates, an IP prefix implying an exclusively North American domain wouldbe appropriate. The use of one IP prefix for each attached TRD, with hosts in the domainhaving IP addresses containing the IP prefix of the most appropriate TRD.The organization appears to be a set of routing domains. Assigning an IP prefix from one of the attached TRDs.This TRD becomesa default TRD for the domain but other domains can explicitly route by oneof the alternative TRDs. The use of IP prefixes to refer to sets of multi-homed domains having theTRD attachments.For example, there may be an IP prefix to refer tosingle-homed domains attached to network A, one to refer to single-homeddomains attached to network B and one to refer to dual-homed domainsattached to networks A and B.Each of these has various advantages, disadvantages and side effects.Forexample, the first approach tends to result in inbound traffic entering the targetdomain closer to the sending host than the second approach, and therefore alarger proportion of the network costs are incurred by the receivingorganization.Because multi-homed domains can vary greatly in character and none of theabove schemes is suitable for all such domains, there is no single policy that isbest and RFC 1518 does not specify any rules for choosing between them.2.1.7.1 CIDR ImplementationThe implementation of CIDR in the Internet is primarily based on Border GatewayProtocol Version 4 (see 3.4.2,  Border Gateway Protocol (BGP-4) on page 135).The implementation strategy, described in RFC 1520  Exchanging RoutingInformation Across Provider Boundaries in the CIDR Environment involves a stagedprocess through the routing hierarchy beginning with backbone routers.Networkservice providers are divided into four types:Type 1Those that cannot employ any default inter-domain routing.Type 2Those that use default inter-domain routing but require explicit routes for asubstantial proportion of the assigned IP network numbers.Type 3Those that use default inter-domain routing and supplement it with a smallnumber of explicit routes.Type 4Those that perform all inter-domain routing using only default routes.The CIDR implementation involves an implementation beginning with the Type 1network providers, then the Type 2 and finally the Type 3 ones.CIDR has alreadybeen widely deployed in the backbone and over 9,000 class-based routes havebeen replaced by approximately 2,000 CIDR-based routes.Chapter 2.Internetworking and Transport Layer Protocols 472.1.8 IP DatagramThe unit of transfer of a data packet in TCP/IP is called an IP datagram.It is madeup of a header containing information for IP and data that is only relevant to thehigher level protocols.header database IP datagram.physical network header IP datagram as dataencapsulated within the physical network's frame3376\3376F203Figure 19.IP - Format of a Base IP DatagramIP can handle fragmentation and re-assembly of IP datagrams.The maximumlength of an IP datagram is 65,535 bytes (or octets).There is also a requirementfor all TCP/IP hosts to support IP datagrams of size up to 576 bytes withoutfragmentation.Fragments of a datagram all have a header, basically copied from the originaldatagram, and data following it.They are treated as normal IP datagrams whilebeing transported to their destination.Note, however, that if one of the fragmentsgets lost, the complete datagram is considered lost since IP does not provide anyacknowledgment mechanism, so the remaining fragments will simply be discardedby the destination host.2.1.8.1 IP Datagram FormatThe IP datagram header is a minimum of 20 bytes long:48 TCP/IP Tutorial and Technical Overview1 1 2 30 4 8 6 9 4 1ServiceVERS HLENTotal LengthTypeFragmentID FLGOffsetHeaderTTL ProtocolChecksumSource IP AddressDestination IP AddressIP Options PaddingData.Figure 20.IP - Format of an IP Datagram HeaderWhere:VERSThe version of the IP protocol.The current version is 4 [ Pobierz całość w formacie PDF ]