4.3. Routing protocol packets

The OSPF protocol runs directly over IP, using IP protocol 89. OSPF does not provide any explicit fragmentation/reassembly support. When fragmentation is necessary, IP fragmentation/reassembly is used. OSPF protocol packets have been designed so that large protocol packets can generally be split into several smaller protocol packets. This practice is recommended; IP fragmentation should be avoided whenever possible.

Routing protocol packets should always be sent with the IP TOS field set to 0. If at all possible, routing protocol packets should be given preference over regular IP data traffic, both when being sent and received. As an aid to accomplishing this, OSPF protocol packets should have their IP precedence field set to the value Internetwork Control (see [RFC 791]).

All OSPF protocol packets share a common protocol header that is described in Appendix A. The OSPF packet types are listed below in Table 8. Their formats are also described in Appendix A.

             Type   Packet  name           Protocol  function
             __________________________________________________________
             1      Hello                  Discover/maintain  neighbors
             2      Database Description   Summarize database contents
             3      Link State Request     Database download
             4      Link State Update      Database update
             5      Link State Ack         Flooding acknowledgment

                            Table 8: OSPF packet types.

OSPF's Hello protocol uses Hello packets to discover and maintain neighbor relationships. The Database Description and Link State Request packets are used in the forming of adjacencies. OSPF's reliable update mechanism is implemented by the Link State Update and Link State Acknowledgment packets.

Each Link State Update packet carries a set of new link state advertisements one hop further away from their point of origination. A single Link State Update packet may contain the link state advertisements of several routers. Each advertisement is tagged with the ID of the originating router and a checksum of its link state contents. The five different types of OSPF link state advertisements are listed below in Table 9.

As mentioned above, OSPF routing packets (with the exception of Hellos) are sent only over adjacencies. Note that this means that all OSPF protocol packets travel a single IP hop, except those that are sent over virtual adjacencies. The IP source address of an OSPF protocol packet is one end of a router adjacency, and the IP destination address is either the other end of the adjacency or an IP multicast address.

       LS     Advertisement      Advertisement description
       type   name
       _________________________________________________________
       1      Router links       Originated by all routers.
              advertisements     This advertisement describes
                                 the collected states of the
                                 router's interfaces to an
                                 area. Flooded throughout a
                                 single area only.
       _________________________________________________________
       2      Network links      Originated for multi-access
              advertisements     networks by the Designated
                                 Router. This advertisement
                                 contains the list of routers
                                 connected to the network.
                                 Flooded throughout a single
                                 area only.
       _________________________________________________________
       3,4    Summary link       Originated by area border
              advertisements     routers, and flooded through-
                                 out the advertisement's
                                 associated area. Each summary
                                 link advertisement describes
                                 a route to a destination out-
                                 side the area, yet still inside
                                 the AS (i.e., an inter-area
                                 route). Type 3 advertisements
                                 describe routes to networks.
                                 Type 4 advertisements describe
                                 routes to AS boundary routers.
       _________________________________________________________
       5      AS external link   Originated by AS boundary
              advertisements     routers, and flooded through-
                                 out the AS. Each AS external
                                 link advertisement describes
                                 a route to a destination in
                                 another Autonomous System.
                                 Default routes for the AS can
                                 also be described by AS
                                 external link advertisements.

                Table 9: OSPF link state advertisements.