Overview of Computer Networks

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by vivi74uk

3.1.5 Session Layer

This layer is concerned with management of a session, i.e. the duration of a connection between two network

nodes. The word connection here does not mean something physical, but rather refers to an agreement

between two nodes that some chunks of data with some relation to each other will be exchanged for some

time. Actually, TCP does this in some senses, as does the socket interface to TCP, which is very much like

the interfaces for reading or writing a file (described in more detail later).

3.1.6 Presentation Layer

This layer deals with such matters as translating between character codes, if the source uses one and the

destination the other. In the old days, this could mean ASCII at one end and EBCDIC on the other end.

Today, though, it could mean for example two different coding systems for Chinese characters, Big 5 and

GB.

3.1.7 Application Layer

You can write programs at the application layer yourself, and of course you use many programs written by

others, such as ftp, Web browsers, e-mail utilities, and so on.

3.2 How the Layers Interact

The Physical Layer is obviously implemented in hardware. So is the Data Layer, in the sense that the NIC

will handle this layer and is hardware.8 These days the Network Layer is also usually hardware; as noted

above, Ethernet hubs are now common in the home, and include routing capabilities. The Transport Layer

and above are usually implemented in software, actually as part of the OS.

Information is communicated from one layer to the next.9 For instance, think of the file-transfer example

presented earlier. The file-transfer program, say ftp, works in the Application Layer. It will call socket

service functions in the Session Layer, such as the socket() function, which opens a network connection in

a manner very similar to the open() function which opens a file. These functions will in turn call functions

7The "users" here are the application programs being run, e.g. ftp at saturn and ftpd (the FTP server) at holstein.

8More technically, firmware-software stored in ROM.

9In some cases a layer is "skipped." At the Session Layer, a raw socket can be opened, in which the socket communicates

directly with IP in the Network Layer.

for TCP operations in the Transport Layer, which will themselves call functions for IP operations at the

Network layer. The latter will then-say we are on an Ethernet-issue machine instructions (e.g. IN and

OUT in the case of Intel CPUs) to the Ethernet NIC, which will use the Data Link and Physical Layers to

put frames out onto the LAN. At any given layer, a function passes the message in a packet of bits to the

next lower layer. The packet grows larger at each layer, because each layer adds more information.

Say we are using the put command in ftp to copy a file named zyx to the destination machine. When we do

this, ftp calls the write() function to send data,10 A typical call to write() will contain the actual data to be