Novell LINUX ENTERPRISE SERVER 11 - ADMINISTRATION Administration Manual page 215

Figure 17.1 Simplified Layer Model for TCP/IP
Host sun
Application Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
The diagram provides one or two examples for each layer. The layers are ordered ac-
cording to abstraction levels. The lowest layer is very close to the hardware. The upper-
most layer, however, is almost a complete abstraction from the hardware. Every layer
has its own special function. The special functions of each layer are mostly implicit in
their description. The data link and physical layers represent the physical network used,
such as ethernet.
Almost all hardware protocols work on a packet-oriented basis. The data to transmit is
collected into packets (it cannot be sent all at once). The maximum size of a TCP/IP
packet is approximately 64 KB. Packets are normally quite smaller, as the network
hardware can be a limiting factor. The maximum size of a data packet on an ethernet
is about fifteen hundred bytes. The size of a TCP/IP packet is limited to this amount
when the data is sent over an ethernet. If more data is transferred, more data packets
need to be sent by the operating system.
For the layers to serve their designated functions, additional information regarding 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 17.2, "TCP/IP Ethernet Packet"
Applications
TCP, UDP
IP
Ethernet, FDDI, ISDN
Cable, Fiberglass
Data Transfer
Host earth
Application Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
(page 202). The proof sum is
Basic Networking 201