Ccna routing and Switching Complete Study Guide

UDP Segment Format

Figure 3.13 clearly illustrates UDP’s markedly lean overhead as compared to TCP’s hungry 

requirements. Look at the figure carefully—can you see that UDP doesn’t use windowing 

or provide for acknowledgments in the UDP header?

f I g u r e   3 .13     UDP segment

16-bit destination port

16-bit source port

Bit 31

Bit 15

Bit 0

Bit 16

16-bit checksum

16-bit length

Data

8 bytes

It’s important for you to understand what each field in the UDP segment is:

Source port    Port number of the application on the host sending the data

Destination port    Port number of the application requested on the destination host

Length    Length of UDP header and UDP data

Checksum    Checksum of both the UDP header and UDP data fields

Data    Upper-layer data

UDP, like TCP, doesn’t trust the lower layers and runs its own CRC. Remember that the 

Frame Check Sequence (FCS) is the field that houses the CRC, which is why you can see the 

FCS information.

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TCP/IP and the DoD Model 

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The following shows a UDP segment caught on a network analyzer:

UDP - User Datagram Protocol

 Source Port:      1085

 Destination Port: 5136

 Length:           41

 Checksum:         0x7a3c

 UDP Data Area:

 ..Z......00 01 5a 96 00 01 00 00 00 00 00 11 0000 00

...C..2._C._C  2e 03 00 43 02 1e 32 0a 00 0a 00 80 43 00 80

Frame Check Sequence: 0x00000000

Notice that low overhead! Try to find the sequence number, ack number, and window 

size in the UDP segment. You can’t because they just aren’t there!