Connected: An Internet Encyclopedia
8.2. Receiving protocol packets

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8.2. Receiving protocol packets

8.2. Receiving protocol packets

Whenever a protocol packet is received by the router it is marked with the interface it was received on. For routers that have virtual links configured, it may not be immediately obvious which interface to associate the packet with. For example, consider the Router RT11 depicted in Figure 6. If RT11 receives an OSPF protocol packet on its interface to Network N8, it may want to associate the packet with the interface to Area 2, or with the virtual link to Router RT10 (which is part of the backbone). In the following, we assume that the packet is initially associated with the non-virtual link.[3]

In order for the packet to be accepted at the IP level, it must pass a number of tests, even before the packet is passed to OSPF for processing:

Next, the OSPF packet header is verified. The fields specified in the header must match those configured for the receiving interface. If they do not, the packet should be discarded:

Next, the packet must be authenticated. This depends on the AuType specified (see Appendix D). The authentication procedure may use an Authentication key, which can be configured on a per-interface basis. If the authentication fails, the packet should be discarded.

If the packet type is Hello, it should then be further processed by the Hello Protocol (see Section 10.5). All other packet types are sent/received only on adjacencies. This means that the packet must have been sent by one of the router's active neighbors. If the receiving interface is a multi-access network (either broadcast or non-broadcast) the sender is identified by the IP source address found in the packet's IP header. If the receiving interface is a point-to-point link or a virtual link, the sender is identified by the Router ID (source router) found in the packet's OSPF header. The data structure associated with the receiving interface contains the list of active neighbors. Packets not matching any active neighbor are discarded.

At this point all received protocol packets are associated with an active neighbor. For the further input processing of specific packet types, consult the sections listed in Table 11.

              Type   Packet name            detailed section (receive)
              1      Hello                  Section 10.5
              2      Database description   Section 10.6
              3      Link state request     Section 10.7
              4      Link state update      Section 13
              5      Link state ack         Section 13.7

            Table 11: Sections describing OSPF protocol packet reception.

Next: 9. The Interface Data Structure

Connected: An Internet Encyclopedia
8.2. Receiving protocol packets