TCP/IP Tutorial and Technical Overview - IBM Redbooks

The UDP datagram has an 8-byte header, as described in Figure 4-2 on
page 148.
Source Port
Length
Figure 4-2 UDP: Datagram format
Where:
Source Port Indicates the port of the sending process. It is the port to
which replies are addressed.
Destination Port Specifies the port of the destination process on the
destination host.
Length The length (in bytes) of this user datagram, including the
header.
Checksum An optional 16-bit one's complement of the one's
complement sum of a pseudo-IP header, the UDP
header, and the UDP data. In Figure 4-3, we see a
pseudo-IP header. It contains the source and destination
IP addresses, the protocol, and the UDP length.
Figure 4-3 UDP: Pseudo-IP header
The pseudo-IP header effectively extends the checksum to include the
original (unfragmented) IP datagram.
148 TCP/IP Tutorial and Technical Overview
Data...
Source IP address
Destination IP address
Destination Port
Checksum
Zero Protocol TCP Length