Ccna routing and Switching Complete Study Guide

TCP Segment Format

Since the upper layers just send a data stream to the protocols in the Transport layers, I’ll 

use Figure 3.12 to demonstrate how TCP segments a data stream and prepares it for the 

Internet layer. When the Internet layer receives the data stream, it routes the segments as 

packets through an internetwork. The segments are handed to the receiving host’s Host-

to-Host layer protocol, which rebuilds the data stream for the upper-layer applications or 

protocols.

f I g u r e   3 .12     TCP segment format

4-bit

header

length

Reserved

Flags

16-bit window size

16-bit destination port

16-bit source port

16-bit urgent pointer

16-bit TCP checksum

32-bit sequence number

32-bit acknowledgment number

Options

Data

Figure 3.12 shows the TCP segment format and shows the different fields within the 

TCP header. This isn’t important to memorize for the Cisco exam objectives, but you need 

to understand it well because it’s really good foundational information.

The TCP header is 20 bytes long, or up to 24 bytes with options. You need to under-

stand what each field in the TCP segment is in order to build a strong educational 

foundation:

Source port    This is the port number of the application on the host sending the data, 

which I’ll talk about more thoroughly a little later in this chapter.