Well-Known Sockets And The Application Layer - D-Link DES-3326 User Manual

DES-3326 Layer 3 Fast Ethernet Switch User's Guide
header and looks at its protocol field. If the protocol field is TCP,
the packet is sent to TCP. TCP then looks at the sequence number
and uses this number and other data from the headers to
reassemble the data into the original file.
Well-Known Sockets and the Application
Layer
Application protocols run 'on top of' TCP/IP. When an application
wants to send data or a message, it gives the data to TCP. Because
TCP and IP take care of the networking details, the application can
look at the network connection as a simple data stream.
To transfer a file across a network using the File Transfer Protocol
(FTP), a connection must first be established. The computer
requesting the file transfer must connect specifically to the FTP
server on the computer that has the file.
This is accomplished using sockets. A socket is a pair of TCP port
numbers used to establish a connection from one computer to
another. TCP uses these port numbers to keep track of
connections. Specific port numbers are assigned to applications
that wait for requests. These port numbers are referred to as 'well-
known' ports.
TCP will open a connection to the FTP server using some random
port number, 1234 for example, on the local computer. TCP will
specify port 21 for the FTP server. Port 21 is the well-known port
number for FTP servers. Note that there are two different FTP
programs running in this example – an FTP client that requests
the file to be transferred, and an FTP server that sends the file to
the FTP client. The FTP server accepts commands from the client,
so the FTP client must know how to connect to the server (must
know the TCP port number) in order to send commands. The FTP
Server can use any TCP port number to send the file, so long as it
is sent as part of the connection setup.
Switch Management and Operating Concepts
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