D-Link DES-3326 User Manual page 97

DES-3326 Layer 3 Fast Ethernet Switch User's Guide
TCP makes sure the e-mail commands and messages are received
by the appropriate computers. It keeps track of what is sent and
what is received, and retransmits any packets that are lost or
dropped. TCP also handles the division of large messages into
several Ethernet packets, and makes sure these packets are
received and reassembled in the correct order.
Because these functions are required by a large number of
applications, they are grouped into a single protocol, rather than
being the part of the specifications for just sending e-mail. TCP is
then a library of routines that application software can use when
reliable network communications are required.
IP is also a library of routines, but with a more general set of
functions. IP handles the routing of packets from the source to the
destination. This may require the packets to traverse many
different networks. IP can route packets through the necessary
gateways and provides the functions required for any user on one
network to communicate with any user on another connected
network.
The communication interface between TCP and IP is relatively
simple. When IP received a packet, it does not know how this
packet is related to others it has sent (or received) or even which
connection the packet is part of. IP 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
Switch Management and Operating Concepts
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