CCNA Cisco Certified Network Associate Study Guide - FTP Server

116 Chapter 3 � Internet Protocol
In the preceding trace, a “syn” packet is sent to the destination device. The
syn sequence is telling the remote destination device that it wants to create a
session.
TCP Session: Syn Packet Acknowledgment
The next trace shows an acknowledgment to the syn packet. Notice the “Ack
is valid,” which means the source port was accepted and the device agreed to
create a virtual circuit with the originating host.
TCP - Transport Control Protocol
Source Port: 80 World Wide Web HTTP
Destination Port: 1144
Sequence Number: 2873580788
Ack Number: 9356571
Offset: 6
Reserved: %000000
Code: %010010
Ack is valid
Synch Sequence
Window: 8576
Checksum: 0x5F85
Urgent Pointer: 0
TCP Options:
Option Type: 2 Maximum Segment Size
Length: 4
MSS: 1460
No More HTTP Data
Frame Check Sequence: 0x6E203132
Notice that the response from the server shows the source is 80 and the destination
is the 1144 sent from the originating host.
The Internet Layer Protocols
There are two main reasons for the Internet layer’s existence: routing, and
providing a single network interface to the upper layers.
None of the upper- or lower-layer protocols have any functions relating to
routing. The complex and important task of routing is the job of the Internet