rfc1504.txt
来自「中、英文RFC文档大全打包下载完全版 .」· 文本 代码 · 共 1,548 行 · 第 1/5 页
TXT
1,548 行
You can implement many of these optional features on routers that use either AURP or RTMP (Routing Table Maintenance Protocol) for routing-information propagation. Figure 1-1 shows how the three major components of AURP interact. <<Figure 1-1 Major components of AURP>> Wide Area Routing Enhancements Provided by AURP AURP provides AppleTalk Phase 2-compatible routing for large wide area networks (WANs). Key wide area routing enhancements provided by AURP include: tunneling through TCP/IP internets and other foreign network systems point-to-point tunneling basic security-including device hiding and network hiding remapping of remote network numbers to resolve numbering conflictsOppenheimer [Page 6] RFC 1504 Appletalk Update-Based Routing Protocol August 1993 internet clustering to minimize routing traffic and routing- information storage requirements hop-count reduction to allow the creation of larger internets improved use of alternate paths through hop-count weighting and the designation of backup paths2. WIDE AREA APPLETALK CONNECTIVITY This chapter describes the wide area connectivity capabilities provided by the AppleTalk Update-based Routing Protocol (AURP), including: AppleTalk tunneling tunneling through TCP/IP internets tunneling over point-to-point links AppleTalk Tunneling Tunneling allows a network administrator to connect two or more native internets through a foreign network system to form a large wide area network (WAN). For example, an AppleTalk WAN might consist of two or more native AppleTalk internets connected through a tunnel built on a TCP/IP internet. In such an AppleTalk WAN, native internets use AppleTalk protocols, while the foreign network system uses a different protocol family. A tunnel connecting AppleTalk internets functions as a single, virtual data link between the internets. A tunnel can be either a foreign network system or a point-to-point link. Figure 2-1 shows an AppleTalk tunnel. <<Figure 2-1 AppleTalk tunnel>> There are two types of tunnels: dual-endpoint tunnels, which have only two routers on a tunnel-for example, point-to-point tunnels multiple-endpoint tunnels-herein referred to as multipoint tunnels- which have two or more routers on a tunnel AURP implements multipoint tunneling by providing mechanisms for data encapsulation and the propagation of routing information to specific routers.Oppenheimer [Page 7] RFC 1504 Appletalk Update-Based Routing Protocol August 1993 Exterior Routers An AppleTalk router with a port that connects an AppleTalk internet to a tunnel is an exterior router. An exterior router always sends split-horizoned routing information to the other exterior routers on a multipoint tunnel. That is, an exterior router on a multipoint tunnel sends routing information for only its local internet to other exterior routers on that tunnel. An exterior router never exports routing information obtained from other exterior routers on the tunnel, because the exterior routers communicate their own routing information to one another. As shown in Figure 2-2, the absence or presence of redundant paths, or loops, across a tunnel changes the way an exterior router defines its local internet. For more information about redundant paths, see the section "Redundant Paths" in Chapter 4. If no loops exist across a tunnel, an exterior router's local internet comprises all networks connected directly or indirectly to other ports on the exterior router. When loops exist across a tunnel, an exterior router's local internet comprises only those networks for which the next internet router is not across a tunnel. Using this definition of a local internet, two exterior routers' local internets might overlap if loops existed across a tunnel. For more information about routing loops, see the section "Routing Loops" in Chapter 4. <<Figure 2-2 An exterior router's local internet>> An exterior router functions as an AppleTalk router within its local internet and as an end node in the foreign network system connecting AppleTalk internets. An exterior router uses RTMP to communicate routing information to its local internet, and uses AURP and the network-layer protocol of the tunnel's underlying foreign network system to communicate with other exterior routers connected to the tunnel. An exterior router encapsulates AppleTalk data packets using the headers required by the foreign network system, then forwards the packets to another exterior router connected to the tunnel. FORWARDING DATA: When forwarding AppleTalk data packets across a multipoint tunnel, an exterior router encapsulates the AppleTalk data packets in the packets of the tunnel's underlying foreign network system by adding the headers required by that network system adds an AURP-specific header-called a domain header-immediately preceding each AppleTalk data packetOppenheimer [Page 8] RFC 1504 Appletalk Update-Based Routing Protocol August 1993 A domain header contains additional addressing information-including a source domain identifier and destination domain identifier. For more information about domain headers, see the sections "AppleTalk Data-Packet Format" and "AppleTalk Data-Packet Format for IP Tunneling" later in this chapter. For detailed information about domain identifiers, see the section "Domain Identifiers" later in this chapter. Before forwarding a data packet to a network in another exterior router's local internet, an exterior router must obtain the foreign- protocol address of the exterior router that is the next internet router in the path to the packet's destination network. The exterior router then sends the packet to that exterior router's foreign- protocol address using the network-layer protocol of the foreign network system. The exterior router need not know anything further about how the packet traverses this virtual data link. Once the destination exterior router receives the packet, it removes the headers required by the foreign network system and the domain header, then forwards the packet to its destination in the local AppleTalk internet. If the length of an AppleTalk data packet in bytes is greater than that of the data field of a foreign-protocol packet, a forwarding exterior router must fragment the AppleTalk data packet into multiple foreign-protocol packets, then forward these packets to their destination. Once the destination exterior router receives all of the fragments that make up the AppleTalk data packet, it reassembles the packet. CONNECTING MULTIPLE TUNNELS TO AN EXTERIOR ROUTER: An exterior router can also connect two or more multipoint tunnels. As shown in Figure 2-3, when an exterior router connects more than one multipoint tunnel, the tunnels can be built on any of the following: the same foreign network system different foreign network systems similar, but distinct foreign network systems <<Figure 2-3 Connecting multiple tunnels to an exterior router>> Whether the tunnels connected to an exterior router are built on similar or different foreign network systems, each tunnel acts as an independent, virtual data link. As shown in Figure 2-4, an exterior router connected to multiple tunnels functions logically as though it were two or more exterior routers connected to the same AppleTalkOppenheimer [Page 9] RFC 1504 Appletalk Update-Based Routing Protocol August 1993 network, with each exterior router connected to a different tunnel. <<Figure 2-4 An exterior router connected to multiple tunnels>> Fully Connected and Partially Connected Tunnels An AppleTalk multipoint tunnel functions as a virtual data link. AURP assumes full connectivity across a multipoint tunnel-that is, all exterior routers on such a tunnel can communicate with one another. An exterior router always sends split-horizoned routing information to other exterior routers on a multipoint tunnel. That is, an exterior router on a multipoint tunnel sends routing information for only its local internet to other exterior routers on that tunnel. An exterior router never exports routing information obtained from other exterior routers on the tunnel, because exterior routers communicate their routing information to one another. If all exterior routers connected to a multipoint tunnel are aware of and can send packets to one another, that tunnel is fully connected. If some of the exterior routers on a multipoint tunnel are not aware of one another, the tunnel is only partially connected. Figure 2-5 shows examples of a fully connected tunnel, a partially connected tunnel, and two fully connected tunnels. <<Figure 2-5 Fully connected and partially connected tunnels>> In the second example shown in Figure 2-5, the network administrator may have connected the tunnel partially for one of these reasons: to prevent the local internets connected to exterior routers A and C from communicating with one another, while providing full connectivity between the local internets connected to exterior router B and the local internets connected to both exterior routers A and C because local internets connected to exterior routers A and C need access only to local internets connected to exterior router B-not to each other's local internets because exterior routers A and C-which should be aware of one another-were misconfigured Generally, an exterior router cannot determine whether a multipoint tunnel is fully connected or partially connected. In the second example in Figure 2-5, exterior router B does not know whether exterior routers A and C are aware of one another. However, exteriorOppenheimer [Page 10] RFC 1504 Appletalk Update-Based Routing Protocol August 1993 router B must assume that the tunnel is fully connected, and that exterior routers A and C can exchange routing information. An exterior router should never forward routing information received from other exterior routers back across the tunnel. It should always send split-horizoned routing information to other exterior routers. If connecting exterior routers A and C directly would be either expensive or slow, a network administrator could instead establish two independent multipoint tunnels-one connecting exterior routers A and B, another connecting exterior routers B and C-as shown in the third example in Figure 2-5. Exterior routers A and C could then establish connectivity by routing all data packets forwarded by one to the other through exterior router B. Hiding Local Networks From Tunnels When configuring a tunneling port on an exterior router, a network administrator can provide network-level security to a network in the exterior router's local internet by hiding that network. Hiding a specific network in the exterior router's local internet prevents internets across a multipoint tunnel from becoming aware of the presence of that network. When the exterior router exchanges routing information with other exterior routers connected to the tunnel, it exports no information about any hidden networks to the exterior routers from which the networks are hidden. An administrator can specify that certain networks in the exterior router's local internet be hidden from a specific exterior router connected to the tunnel or from all exterior routers on the tunnel. Nodes on the local internet of an exterior router from which a network is hidden cannot access that network. Neither the zones on a hidden network nor the names of devices in those zones appear in the Chooser on computers connected to such an internet. When a network is hidden, its nodes are also unable to access internets from which the network is hidden. If a node on a hidden network sends a packet across a tunnel to a node on an internet from which it is hidden, even if the packet arrives at its destination, the receiving node cannot respond. The exterior router connected to the receiving node's internet does not know the return path to the node on the hidden network. Thus, it appears to the node on the hidden network that the node to which it sent the packet is inaccessible. ADVANTAGES AND DISADVANTAGES OF NETWORK HIDING: Network hiding provides the following advantages: On large, global WANs, a network administrator can configure network-level security for an organization's internets.Oppenheimer [Page 11] RFC 1504 Appletalk Update-Based Routing Protocol August 1993⌨️ 快捷键说明
复制代码Ctrl + C
搜索代码Ctrl + F
全屏模式F11
增大字号Ctrl + =
减小字号Ctrl + -
显示快捷键?