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24-port NWay Ethernet Switch User’s Guide
Switch Management and Operating Concepts 89
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.