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Chapter 3
Internet Protocols
UDP doesn't use windowing or provide for acknowledgments represented
within it?
F I G U R E 3 . 4
UDP segment
It's important for you to understand what each field in the UDP segment
is. The UDP segment contains the following fields:
Source port Port number of the host sending the data.
Destination port
Port number of the application requested on the
destination host.
Length of the segment Length of UDP header and UDP data.
CRC 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.
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 00
00 00
...C..2...._C._C 2e 03 00 43 02 1e 32 0a 00 0a 00 80 43
00 80
Frame Check Sequence: 0x00000000
Bit 0
Bit 15
Source port (16)
Destination port (16)
Length (16)
Checksum (16)
Data (if any)
Bit 16
Bit 31
8 bytes
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