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Chapter 3
Internet Protocols
Urgent pointer Indicates the end of urgent data.
Option Sets the maximum TCP segment size to either 0 or 32 bits, if any.
Data Handed down to the TCP protocol at the Transport layer, which
includes the upper-layer headers.
Let's take a look at a TCP segment copied from a network analyzer:
TCP - Transport Control Protocol
Source Port: 5973
Destination Port: 23
Sequence Number: 1456389907
Ack Number: 1242056456
Offset: 5
Reserved: %000000
Code: %011000
Ack is valid
Push Request
Window: 61320
Checksum: 0x61a6
Urgent Pointer: 0
No TCP Options
TCP Data Area:
vL.5.
+.5.+.5.+.5 76 4c 19 35 11 2b 19 35 11 2b 19 35 11
2b 19 35
+. 11 2b 19
Frame Check Sequence: 0x0d00000f
Did you notice that everything I talked about above is in the segment? As
you can see from the number of fields in the header, TCP creates a lot of
overhead. Application developers may opt for efficiency over reliability to
save overhead, and so, User Datagram Protocol was also defined at the
Transport layer as an alternative.
User Datagram Protocol (UDP)
If you were to compare User Datagram Protocol (UDP) with TCP, the former
is basically the scaled-down economy model that's sometimes referred to as a
thin protocol. Like a thin person on a park bench, a thin protocol doesn't take
up a lot of room--or in this case, much bandwidth on a network.
UDP doesn't offer all the bells and whistles of TCP either, but it does do
a fabulous job of transporting information that doesn't require reliable
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