Datasheet

“main” (Installation and Administration) — 2004/6/25 — 13:29 — page 417 — #443

Linux in the Network

For every layer to serve its designated function, additional information re-

garding each layer must be saved in the data packet. This takes place in the

header of the packet. Every layer attaches a small block of data, called the

protocol header, to the front of each emerging packet. A sample TCP/IP

data packet traveling over an ethernet cable is illustrated in Figure 21.2.

Figure 21.2: TCP/IP Ethernet Packet

The proof sum is located at the end of the packet, not at the beginning. This

simpliﬁes things for the network hardware. The largest amount of usage

data possible in one packet is 1460 bytes in an ethernet network.

When an application sends data over the network, the data passes through

each layer, all implemented in the Linux kernel except layer 1 (network

card). Each layer is responsible for preparing the data so it can be passed to

the next layer below. The lowest layer is ultimately responsible for sending

the data. The entire procedure is reversed when data is received. Like the

layers of an onion, in each layer the protocol headers are removed from the

transported data. Finally, layer 4 is responsible for making the data avail-

able for use by the applications at the destination. In this manner, one layer

only communicates with the layer directly above or below it. For applica-

tions, it is irrelevant whether data is transmitted via a 100 MBit/s FDDI

network or via a 56-kbit/s modem line. Likewise, it is irrelevant for the

data line which kind of data is transmitted, as long as packets are in the

correct format.

417SUSE LINUX Enterprise Server