Novell LINUX ENTERPRISE SERVER 10 - INSTALLATION AND ADMINISTRATION 11-05-2007 Installation Manual page 564

located at the end of the packet, not at the beginning. This simplifies things for the
network hardware.
Figure 30.2 TCP/IP Ethernet Packet
Usage Data (maximum 1460 bytes)
TCP (Layer 4) Protocol Header (approx. 20 bytes)
IP (Layer 3) Protocol Header (approx. 20 bytes)
Ethernet (Layer 2) Protocol Header (approx. 14 bytes) + Checksum (2 bytes)
When an application sends data over the network, the data passes through each layer,
all implemented in the Linux kernel except the physical layer. Each layer is responsible
for preparing the data so it can be passed to the next layer. 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, the transport layer is responsible for making the data available
for use by the applications at the destination. In this manner, one layer only communi-
cates with the layer directly above or below it. For applications, 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.
546 Installation and Administration