D-Link manual DES-3326S Layer 3 Fast Ethernet Switch User’s Guide

DES-3326S Layer 3 Fast Ethernet Switch User’s Guide

TCP/IP is a layered set of protocols. An example, such as sending e-mail, can illustrate this. There is first a protocol for sending and receiving e-mail. This protocol defines a set of commands to identify the sender, the recipient, and the content of the e-mail. The e-mail protocol will not handle the actual 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.

96 Switch Management and Operating Concepts