A.3.1 The OSPF packet header

Every OSPF packet starts with a common 24 byte header. This header contains all the necessary information to determine whether the packet should be accepted for further processing. This determination is described in Section 8.2 of the specification.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |     Type      |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version #

The OSPF version number. This specification documents version 2 of the protocol.

Type

The OSPF packet types are as follows. The format of each of these packet types is described in a succeeding section.

                          Type   Description
                          ________________________________
                          1      Hello
                          2      Database Description
                          3      Link State Request
                          4      Link State Update
                          5      Link State Acknowledgment

Packet length

The length of the protocol packet in bytes. This length includes the standard OSPF header.

Router ID

The Router ID of the packet's source. In OSPF, the source and destination of a routing protocol packet are the two ends of an (potential) adjacency.

Area ID

A 32 bit number identifying the area that this packet belongs to. All OSPF packets are associated with a single area. Most travel a single hop only. Packets travelling over a virtual link are labelled with the backbone Area ID of 0.0.0.0.

Checksum

The standard IP checksum of the entire contents of the packet, starting with the OSPF packet header but excluding the 64-bit authentication field. This checksum is calculated as the 16-bit one's complement of the one's complement sum of all the 16-bit words in the packet, excepting the authentication field. If the packet's length is not an integral number of 16-bit words, the packet is padded with a byte of zero before checksumming.

AuType

Identifies the authentication scheme to be used for the packet. Authentication is discussed in Appendix D of the specification. Consult Appendix D for a list of the currently defined authentication types.

Authentication

A 64-bit field for use by the authentication scheme.