4.1 OSPF Overview
4.1.3 OSPF packet formats
OSPF is a routing protocol designed specifically for the TCP/IP environment. Another noteworthy point about OSPF is that it does not perform fragmentation and reassembly; these functions are performed at the IP layer. Every OSPF packet shares a common 24-byte protocol header, as shown in the graphic. Routing protocol packets are sent with a type of service of zero. There are a variety of OSPF packet types. However, each type shares a common OSPF header. The structure of these 24-byte OSPF headers is as follows:
  • Version --- This portion of the header is one byte long and indicates what version of OSPF is running (Note: the current version of OSPF is Version 2).
  • Type --- The second byte of the header identifies the type of OSPF packet. There are five OSPF packet types, as indicated in Figure .
  • Packet Length --- A two byte field that indicates the entire length of the packet.
  • Router ID --- A four byte field used to identify the router originating the OSPF packet.
  • Area ID --- A four byte field used to identify the area to which the packet belongs.
  • Checksum --- A two byte checksum field used to check the message for errors. Good packets are retained and damaged packets are discarded.
  • Authentication Type --- A two byte field that identifies the form of identification that is used on this packet.
  • Authentication --- An eight byte field used for authentication data that allows the receiver to authenticate the sender.