rfc1504.txt
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Point-to-Point Tunneling In point-to-point tunneling, two remote AppleTalk local area networks (LANs) connected to half-routers communicate with one another over a point-to-point link. A point-to-point link may consist of modems communicating over a standard telephone line or a leased line, such as a T1 line. Figure 2-14 shows an example of point-to-point tunneling. <<Figure 2-14 Point-to-point tunneling>> Generally, exterior routers use null domain identifiers on point-to- point links, because there is no IP address to be administrated and the opposite end of the tunnel is already uniquely identified. However, an exterior router may use other domain-identifier formats. Point-to-Point Protocol The Point-to-Point Protocol (PPP) is a data-link-layer protocol that provides a standard method of encapsulating and decapsulatingOppenheimer [Page 17] RFC 1504 Appletalk Update-Based Routing Protocol August 1993 network-layer protocol information, and transmitting that information over point-to-point links. PPP includes an extensible Link Control Protocol (LCP) and a suite of Network Control Protocols (NCPs) that configure, enable, and disable various network-layer protocols. The AppleTalk Control Protocol (ATCP) is a PPP NCP for AppleTalk protocols. ATCP configures, enables, and disables the AppleTalk network-layer protocol DDP on the half-router at each end of a point-to-point link. ATCP also specifies the protocol that a half- router uses to propagate routing information-for example, AURP. When using AURP for routing-information propagation, a half-router uses a specific PPP protocol type to identify AURP routing-information packets-that is, packets preceded by a domain header. PPP provides separate channels for AppleTalk data packets and AppleTalk routing- information packets. Thus, a half-router can use DDP encapsulation to send AppleTalk data packets without including their domain headers. When using AURP, a half-router should accept both AppleTalk data packets that are preceded by domain headers and DDP-encapsulated packets. NOTE: The Request for Comments (RFC) 1378, "The PPP AppleTalk Control Protocol (ATCP)," provides a detailed specification of ATCP, as well as information about using PPP to send AppleTalk data.3. PROPAGATING ROUTING INFORMATION WITH THE APPLETALK UPDATE-BASED ROUTING PROTOCOL This chapter describes the required elements of AURP. It provides detailed information about using the AppleTalk Update-based Routing Protocol (AURP) to propagate routing information between AppleTalk exterior routers connected through a foreign network or over a point-to-point link, and includes information about the AURP architectural model one-way connections exchanging routing information updating routing information notifying other exterior routers that an exterior router is going down obtaining zone information packet formatsOppenheimer [Page 18] RFC 1504 Appletalk Update-Based Routing Protocol August 1993 error codes AURP Architectural Model AURP provides the functionality of the Routing Table Maintenance Protocol (RTMP) and the Zone Information Protocol (ZIP) while eliminating most of the routing traffic generated by these protocols. Figure 3-1 shows the architectural model for AURP. <<Figure 3-1 AURP architectural model>> Generally, an AppleTalk router uses RTMP and ZIP to maintain routing information, and sends RTMP data packets, ZIP Queries, and ZIP Replies out its ports. However, if one of the router's ports is connected to an AppleTalk tunnel, the architectural model for the router's central routing module becomes more complex. Logically, the central routing module in an exterior router communicates RTMP and ZIP information to an RTMP/ZIP-to-AURP conversion module, which sends AURP data packets out the tunneling port. RTMP/ZIP-to-AURP Conversion Module The RTMP/ZIP-to-AURP conversion module maintains split-horizoned routing-table information and network number-to-zone name mappings for each exterior router on the tunnel-that is, a copy of the routing information for each exterior router's local internet. Figure 3-2 shows the architectural components of the RTMP/ZIP-to-AURP conversion module. <<Figure 3-2 RTMP/ZIP-to-AURP conversion module architecture>> The AURP module of the conversion module obtains routing information from the other exterior routers on the tunnel, then periodically updates the routing-table information and the mappings in the conversion module. The RTMP module passes this routing-table information to the exterior router's central routing module. Logically, the RTMP module generates an RTMP data packet for each exterior router on the tunnel every ten seconds-the RTMP retransmission time-then passes the packet to the central routing module. The RTMP/ZIP-to-AURP conversion module also maintains a split- horizoned copy of the routing information maintained by the exterior router in which it resides. Logically, the conversion module obtains the routing information from RTMP data packets and ZIP Replies sent by the exterior router's central routing module, then updates the routing information in the conversion module.Oppenheimer [Page 19] RFC 1504 Appletalk Update-Based Routing Protocol August 1993 The AURP module exports routing information about its local AppleTalk internet to other exterior routers on the tunnel. AURP Transport Layering AURP can propagate routing information between exterior routers using a simple, reliable transport based on an underlying datagram service-such as the default transport-layer service for AURP, AURP-Tr. See the section "AURP-Tr," later in this chapter, for more information. a more complex transport-layer service-such as TCP Figure 3-3 shows the AURP transport-layering model. <<Figure 3-3 AURP transport-layering model>> Maintaining Current Routing Information With AURP AURP allows exterior routers to maintain current routing information for other exterior routers on a tunnel by supporting the reliable, initial exchange of split-horizoned routing information - that is, the routing information for an exterior router's local internet reliable updates to that information whenever it changes If an internet topology does not change, AURP generates significantly less routing traffic than RTMP and ZIP. Thus, an administrator can connect very large AppleTalk internets through a tunnel, and the resulting internet generates little or no routing traffic on the tunnel. When an exterior router discovers another exterior router on the tunnel-that is, a peer exterior router-it can request that exterior router to send its routing information. In a reliable, initial exchange of split-horizoned routing information, the peer exterior router returns its network-number list. The peer exterior router also returns each connected network's zone information in an unsequenced series of zone-information packets. If the exterior router requesting the routing information does not receive complete zone information for a network, it must retransmit requests for zone information until it receives the information. Once an exterior router requesting routing information from a peer exterior router has received that exterior router's network-numberOppenheimer [Page 20] RFC 1504 Appletalk Update-Based Routing Protocol August 1993 list and complete zone information, it typically requests the peer exterior router to notify it of any changes to that routing information. The peer exterior router then provides the requesting exterior router with reliable updates to its routing information- however, it sends no other routing information. Notifying Other Exterior Routers of Events If an exterior router has requested notification of changes in another exterior router's split-horizoned routing information, that exterior router must notify the requesting exterior router of any event that changes its routing information. Thus, an exterior router must send updated routing information to the requesting exterior router whenever any of the following events occur: the addition of a new, exported network-that is, a network that is not hidden-to the exterior router's local internet and, consequently, to its routing table a change in the path to an exported network that causes the exterior router to access that network through its local internet rather than through a tunneling port the removal of an exported network from the exterior router's routing table because a network in the exterior router's local internet has gone down a change in the path to an exported network that causes the exterior router to access that network through a tunneling port rather than through its local internet a change in the distance to an exported network a change to a zone name in the zone list of an exported network- an event not currently supported by ZIP or the current version of AURP the exterior router goes down or is shut down Routing-information updates allow an exterior router to maintain accurate, split-horizoned routing information for a peer exterior router on a tunnel. AURP-Tr AURP-Tr, the default transport-layer service for AURP, provides a simple, reliable transport that is based on an underlying datagram service. When using AURP-Tr, only one sequenced transaction can beOppenheimer [Page 21] RFC 1504 Appletalk Update-Based Routing Protocol August 1993 outstanding, or unacknowledged, at a time-greatly simplifying the implementation of AURP, without limiting its functionality. One-Way Connections A one-way connection is an asymmetrical link between a data sender and a data receiver that are using AURP-Tr, in which an exterior router functioning as a data sender sends a sequenced, reliable, unidirectional data stream to an exterior router functioning as a data receiver. An exterior router can send routing information over a one-way connection as sequenced data transaction data Sequenced data is data sent in sequence by the data sender and delivered reliably to the data receiver. Typically, the sending of sequenced data is unprovoked-that is, it is not requested by a data receiver. However, a data receiver can request sequenced data. Figure 3-4 shows sequenced data being sent across a one-way connection. <<Figure 3-4 Sequenced data on a one-way connection>> Transaction data-also referred to as out-of-band data-is data sent unsequenced by the data sender through a linked request/response transaction that is initiated by the data receiver. The data receiver can use a one-way connection to request transaction data from the data sender. If the data receiver does not receive a response, it must retransmit its request. Figure 3-5 shows a one-way connection on which the data receiver requests transaction data from the data sender. <<Figure 3-5 Request for transaction data on a one-way connection>> Generally, communication between two exterior routers is bidirectional-that is, two one-way connections exist between the exterior routers, with each exterior router acting as the data sender on one connection and the data receiver on the other. Thus, each exterior router can send its routing information to the other. Initial Information Exchange When an AppleTalk exterior router discovers another exterior router on the tunnel, it uses the underlying transport-layer service to open a connection with that exterior router. When using AURP-Tr, an exterior router opens this connection as a one-way connection.Oppenheimer [Page 22] RFC 1504 Appletalk Update-Based Routing Protocol August 1993 Open Request Packet⌨️ 快捷键说明
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