Packet Headers - D-Link DES-3250TG User Manual

DES-3250TG Layer 2 Fast Ethernet Switch User's Guide

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
contain are. If there are 100 octets of data in each packet, the
first packet is numbered 0, the second 100, the third 200, etc.
To insure that the data in a packet is received uncorrupted,
TCP adds the binary value of all the octets in the packet and
writes the sum in the checksum field. The receiving TCP
recalculates the checksum and if the numbers are different, the
packet is dropped.
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