Microsoft Windows XP Unleashed

FIGURE 22.6 In a ring topology, the central MAU organizes the network nodes in a ring.

Talking the Talk: Networking Protocols

In diplomacy, protocol defines the rules and formalities that ensure smooth communica-

tions between nations and cultures. A networking protocol performs a similar function:

It’s a set of standards that define how information is exchanged between two systems

across a network connection.

For example, consider what appears to be a simple procedure: exchanging a file between

two networked computers. I mentioned earlier that files and all other network transmis-

sions are broken down into packets. Because a large file might consist of hundreds or

even thousands of packets, there has to be some way of coordinating how all this infor-

mation is sent and received. Here are just a few of the questions that must be answered

for even the simplest file transfer to succeed:

• Which computer is sending the packets?

• Where are the packets supposed to go?

• What is the structure of each packet? How big is the header? How big is each data

field inside the header? What order are the data fields in? What kinds of addresses

Talking the Talk: Networking Protocols 597

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MAU

are being used? What kind of error checking mechanism is being used? Where does

the data start?

• How many packets are in the transfer?

• In what order should the packets be reassembled?

• What happens if a packet arrives damaged?

• How long should the destination computer wait for a packet to arrive?

• What happens if a packet hasn’t arrived after the allotted time?

• How does the source computer know that the destination computer has received a

particular packet and, eventually, the entire file?

NOTE

If you’d like to see some examples of packet structure, refer to Chapter 17, “Implementing

TCP/IP for Internet Connections.” That chapter shows you the structure for both IP and TCP

packets.

As you can see, it takes an incredible amount of give-and-take to coordinate any kind of

network interaction. The inherent complexity of this process means that if the source and

destination systems are even slightly out of sync, the file either will arrive corrupted or

won’t arrive at all. Network protocols are designed to ensure that this doesn’t happen.

The protocol specifies in no uncertain terms all the details of any kind of network trans-

fer. Generally speaking, protocols fall into two categories: transport protocols and

network protocols.

With a transport protocol (also called a connection-oriented protocol), a virtual commu-

nications channel is established between two nodes, and the protocol uses this channel to

send packets between the nodes. Because the source and destination are defined in

advance, the packets need not contain full address information. The constant link

between the two nodes provides the protocol with an efficient path for exchanging

messages, so this type of communications method is useful for applications that require a

long-term connection (such as a network monitoring program). However, some overhead

is involved in both setting up and closing the channel, so this method isn’t suitable for

short-lived communications.

With a network protocol (also called a connectionless protocol), no communications

channel is established between nodes. Instead, the protocol builds each packet with all

the information required for the network to deliver each packet and for the destination

node to reassemble the packets into the original file. These self-contained independent

packets are called datagrams. The protocol then ships out the packets without notifying

or negotiating with the destination node. All the network has to do is transmit the

packets to the destination or to some intermediate stop along the way. This method

requires a bit more packet overhead, but it’s efficient for short bursts because there’s no

need to set up or shut down a channel between nodes.

CHAPTER 22 Setting Up and Accessing a Small Network598