RFC 3373 (rfc3373) - Page 2 of 9
Three-Way Handshake for Intermediate System to Intermediate System (IS-IS) Point-to-Point Adjacencies
Alternative Format: Original Text Document
RFC 3373 Three-Way Handshake for IS-IS September 2002 adjacencies on point-to-point links. The protocol does not operate correctly if these subnetwork requirements for point-to-point links are not met. The basic mechanism defined in the standard is that each side declares the other side to be reachable if a Hello packet is heard from it. Once this occurs, each side then sends a Complete Sequence Number PDU (CSNP) to trigger database synchronization. Three failure modes are known. First, if the link goes down and then comes back up, or one of the systems restarts, and the CSNP packet is lost, and the network has a cut set of one through the link, the link state databases on either side of the link will not synchronize for a full LSP refresh period (up to eighteen hours). A second, more serious failure, is that if the link fails in only one direction, the failure will only be detected by one of the systems. Normally only one of the two systems will announce the adjacency in its link state packets, and the SPF algorithm will thus ignore the link. However, if there are two parallel links between systems and one of them fails in one direction, SPF will still calculate paths between the two systems, and the system that does not notice the failure will attempt to pass traffic down the failed link (in the direction that does not work). The third issue is that on some physical layers, the interconnectivity between endpoints can change without causing a link-layer-down condition. In this case, a system may receive packets that are actually destined for a different system (or a different link on the same system). The receiving system may end up thinking that it has an adjacency with the remote system when in fact the remote system is adjacent with a third system. The solution proposed here ensures correct operation of the protocol over unreliable point-to-point links. As part of the solution to the three-way handshaking issue, a method is defined to remove the limitation of 255 point-to-point interfaces imposed by IS-IS [1]. This method is more robust than the ad hoc methods currently in use. 3. Overview of Extensions 3.1 Handshaking The intent is to provide a three-way handshake for point-to-point adjacency establishment in a backward compatible fashion. This is done by providing an optional mechanism that allows each system to report its adjacency three-way state; this allows a system to only declare an adjacency to be up if it knows that the other system is receiving its IS-IS Hello (IIH) packets. Katz & Saluja Informational
