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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts 97
The information required for IP to do its job is contained in a
series of octets added to the beginning of the packet called
headers. A header contains a few octets of data added to the
packet by the protocol in order to keep track of it.
Other protocols on other network devices can add and extract
their own headers to and from packets as they cross networks.
This is analogous to putting data into an envelope and sending
the envelope to a higher-level protocol, and having the higher-
level protocol put the entire envelope into it’s own, larger
envelope. This process is referred to as encapsulation.
Many levels of encapsulation are required for a packet to cross
the Internet.
Packet Headers
TCP
Most data transmissions are much longer that a single packet.
The data must then be divided up among a series of packets.
These packets must be transmitted, received and then
reassembled into the original data. TCP handles these
functions.
TCP must know how large a packet the network can process.
To do this, the TCP protocols at each end of a connection state
how large a packet they can handle and the smaller of the two
is selected.
The TCP header contains at least 20 octets. The source and
destination TCP port numbers are the most important fields.
These specify the connection between two TCP protocols on two
network devices.