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only knows the address of the source and the destination of
the packet, and it makes its best effort to deliver the packet
to its destination.
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.
The header also contains a sequence number that is used to
ensure the packets are received in the correct order. The
packets are not numbered, but rather the octets the packets
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VH-2402-L3 Management Guide 151
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