10.5. Receiving Hello Packets

This section explains the detailed processing of a received Hello Packet. (See Section A.3.2 for the format of Hello packets.) The generic input processing of OSPF packets will have checked the validity of the IP header and the OSPF packet header. Next, the values of the Network Mask, HelloInterval, and RouterDeadInterval fields in the received Hello packet must be checked against the values configured for the receiving interface. Any mismatch causes processing to stop and the packet to be dropped. In other words, the above fields are really describing the attached network's configuration. However, there is one exception to the above rule: on point-to-point networks and on virtual links, the Network Mask in the received Hello Packet should be ignored.

The receiving interface attaches to a single OSPF area (this could be the backbone). The setting of the E-bit found in the Hello Packet's Options field must match this area's ExternalRoutingCapability. If AS external advertisements are not flooded into/throughout the area (i.e, the area is a "stub") the E-bit must be clear in received Hello Packets, otherwise the E-bit must be set. A mismatch causes processing to stop and the packet to be dropped. The setting of the rest of the bits in the Hello Packet's Options field should be ignored.

At this point, an attempt is made to match the source of the Hello Packet to one of the receiving interface's neighbors. If the receiving interface is a multi-access network (either broadcast or non-broadcast) the source is identified by the IP source address found in the Hello's IP header. If the receiving interface is a point-to-point link or a virtual link, the source is identified by the Router ID found in the Hello's OSPF packet header. The interface's current list of neighbors is contained in the interface's data structure. If a matching neighbor structure cannot be found, (i.e., this is the first time the neighbor has been detected), one is created. The initial state of a newly created neighbor is set to Down.

When receiving an Hello Packet from a neighbor on a multi-access network (broadcast or non-broadcast), set the neighbor structure's Neighbor ID equal to the Router ID found in the packet's OSPF header. When receiving an Hello on a point-to- point network (but not on a virtual link) set the neighbor structure's Neighbor IP address to the packet's IP source address.

Now the rest of the Hello Packet is examined, generating events to be given to the neighbor and interface state machines. These state machines are specified either to be executed or scheduled (see Section 4.4). For example, by specifying below that the neighbor state machine be executed in line, several neighbor state transitions may be effected by a single received Hello:

• Each Hello Packet causes the neighbor state machine to be executed with the event HelloReceived.

• Then the list of neighbors contained in the Hello Packet is examined. If the router itself appears in this list, the neighbor state machine should be executed with the event 2- WayReceived. Otherwise, the neighbor state machine should be executed with the event 1-WayReceived, and the processing of the packet stops.

• Next, the Hello Packet's Router Priority field is examined. If this field is different than the one previously received from the neighbor, the receiving interface's state machine is scheduled with the event NeighborChange. In any case, the Router Priority field in the neighbor data structure should be updated accordingly.

• Next the Designated Router field in the Hello Packet is examined. If the neighbor is both declaring itself to be Designated Router (Designated Router field = Neighbor IP address) and the Backup Designated Router field in the packet is equal to 0.0.0.0 and the receiving interface is in state Waiting, the receiving interface's state machine is scheduled with the event BackupSeen. Otherwise, if the neighbor is declaring itself to be Designated Router and it had not previously, or the neighbor is not declaring itself Designated Router where it had previously, the receiving interface's state machine is scheduled with the event NeighborChange. In any case, the Neighbors' Designated Router item in the neighbor structure is updated accordingly.

• Finally, the Backup Designated Router field in the Hello Packet is examined. If the neighbor is declaring itself to be Backup Designated Router (Backup Designated Router field = Neighbor IP address) and the receiving interface is in state Waiting, the receiving interface's state machine is scheduled with the event BackupSeen. Otherwise, if the neighbor is declaring itself to be Backup Designated Router and it had not previously, or the neighbor is not declaring itself Backup Designated Router where it had previously, the receiving interface's state machine is scheduled with the event NeighborChange. In any case, the Neighbor's Backup Designated Router item in the neighbor structure is updated accordingly.

On non-broadcast multi-access networks, receipt of an Hello Packet may also cause an Hello Packet to be sent back to the neighbor in response. See Section 9.5.1 for more details.