A.1 Encapsulation of OSPF packets

OSPF runs directly over the Internet Protocol's network layer. OSPF packets are therefore encapsulated solely by IP and local data-link headers.

OSPF does not define a way to fragment its protocol packets, and depends on IP fragmentation when transmitting packets larger than the network MTU. The OSPF packet types that are likely to be large (Database Description Packets, Link State Request, Link State Update, and Link State Acknowledgment packets) can usually be split into several separate protocol packets, without loss of functionality. This is recommended; IP fragmentation should be avoided whenever possible. Using this reasoning, an attempt should be made to limit the sizes of packets sent over virtual links to 576 bytes. However, if necessary, the length of OSPF packets can be up to 65,535 bytes (including the IP header).

The other important features of OSPF's IP encapsulation are:

• Use of IP multicast.

  Some OSPF messages are multicast, when sent over multi-access networks. Two distinct IP multicast addresses are used. Packets sent to these multicast addresses should never be forwarded; they are meant to travel a single hop only. To ensure that these packets will not travel multiple hops, their IP TTL must be set to 1.

  AllSPFRouters

  This multicast address has been assigned the value 224.0.0.5. All routers running OSPF should be prepared to receive packets sent to this address. Hello packets are always sent to this destination. Also, certain OSPF protocol packets are sent to this address during the flooding procedure.

  AllDRouters

  This multicast address has been assigned the value 224.0.0.6. Both the Designated Router and Backup Designated Router must be prepared to receive packets destined to this address. Certain OSPF protocol packets are sent to this address during the flooding procedure.

• OSPF is IP protocol number 89.

  This number has been registered with the Network Information Center. IP protocol number assignments are documented in [RFC 1340].

• Routing protocol packets are sent with IP TOS of 0.

  The OSPF protocol supports TOS-based routing. Routes to any particular destination may vary based on TOS. However, all OSPF routing protocol packets are sent using the normal service TOS value of binary 0000 defined in [RFC 1349].

• Routing protocol packets are sent with IP precedence set to Internetwork Control.

  OSPF protocol packets should be given precedence over regular IP data traffic, in both sending and receiving. Setting the IP precedence field in the IP header to Internetwork Control [RFC 791] may help implement this objective.