CCNA Cisco Certified Network Associate Study Guide - FTP Server

108 Chapter 3 � Internet Protocol
two TCP layers also agree on the amount of information that’s going to be
sent before the recipient’s TCP sends back an acknowledgment. With everything
agreed upon in advance, the path is paved for reliable communication
to take place.
TCP is a full-duplex, connection-oriented, reliable, accurate protocol, and
establishing all these terms and conditions, in addition to error checking, is
no small task. TCP is very complicated and, not surprisingly, costly in terms
of network overhead. Since today’s networks are much more reliable than
those of yore, this added reliability is often unnecessary.
TCP Segment Format
Since the upper layers just send a data stream to the protocols in the Transport
layers, we’ll demonstrate how TCP segments a data stream and prepares
it for the Network layer. The Network layer then routes the segments
as packets through an internetwork. The packets are handed to the receiving
host’s Transport layer protocol, which rebuilds the data stream to hand to
the upper-layer applications or protocols.
Figure 3.3 shows the TCP segment format. The figure shows the different
fields within the TCP header.
FIGURE 3.3 TCP segment format
Bit 0 Bit 15
Header
length (4)
Source port (16) Destination port (16)
Reserved (6)
Checksum (16)
Sequence number (32)
Acknowledgment number (32)
Code bits (6)
Options (0 or 32 if any)
Data (varies)
Bit 16 Bit 31
Window (16)
Urgent (16)
20 bytes