The UDP Pseudo Header
The UDP pseudo header associates the UDP message with its IP header. UDP adds the UDP pseudo header to the beginning of the UDP message only for the checksum calculation; it is not sent as part of the UDP message. The UDP pseudo header assures the receiver that a routing or fragmentation process did not improperly modify key fields in the IP header.
Figure 9-4 shows the UDP pseudo header.
The UDP pseudo header consists of the Source IP Address field, the Destination IP Address field, an Unused field set to 0, the Protocol field for UDP (17 or 0x11), and the UDP Length field. When sending a UDP message, UDP is aware of all of these values. When receiving a UDP message, IP indicates all of these values to UDP.
Figure 9-4. The ...
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