D-Link DES-3326SR User Manual page 85

DES-3326SR Layer 3 Fast Ethernet Switch User's Guide
communication between the two computers, this is done by
TCP/IP. TCP/IP handles the actual sending and receiving of
the packets that make up the e-mail exchange.
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.
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