The UDP Header

The UDP header is a fixed-length size of 8 bytes consisting of four fields, as Figure 9-2 shows.

The structure of the UDP header

Figure 9-2. The structure of the UDP header

The fields in the UDP header are defined as follows:

  • Source Port. A 2-byte field that identifies the source Application Layer protocol sending the UDP message. The use of a source port is optional and, when not used, is set to 0. IP multicast traffic, such as videocasts sent using UDP, can use 0 because no reply to the video traffic is expected. Typical Application Layer protocols use the source port of an incoming UDP message as the destination port for replies. The combination of the IP header’s ...

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