CCNA Cisco Certified Network Associate Study Guide - FTP Server

112 Chapter 3 � Internet Protocol
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
Notice the low overhead! Try to find the sequence number, ack number,
and window size. You will notice that these are absent from the UDP segment.
Key Concepts of Host-to-Host Protocols
Since we have seen both a connection-oriented (TCP) and connectionless
(UDP) protocol in action, it would be good to summarize the two here. The
following list highlights some of the key concepts that you should keep in
mind regarding these two protocols.
TCP UDP
Sequenced Unsequenced
Reliable Unreliable
Connection-oriented Connectionless
Virtual circuit Low overhead