Amer Networks SS3GR24i - Manual (Page 47)

SS3GR24i/SS3GR48i Layer3 Stackable Gigabit Switch User’s Guide
FTP transfers actually involve two different connections. The
connection begins by the FTP sending commands to send a
particular file. Once the commands are sent, a second connection is
opened for the actual data transfer. Although it is possible to send
data on the same connection, it is very convenient for the FTP
client to be able to continue to send commands (such as ‘stop
sending this file’).
UDP and ICMP
There are many applications that do not require long messages
that cannot fit into a single packet. Looking up computer names is
an example. Users wanting to make connections to other
computers will usually use a name rather than the computer’s IP
or MAC address. The user’s computer must be able to determine
the remote computer’s address before a connection can be made. A
designated computer on the network will contain a database of
computer names and their corresponding IP and MAC addresses.
The user’s computer will send a query to the name database
computer, and the database computer will send a response. Both
the query and the response are very short. There is no need to
divide the query or response between multiple packets, so the
complexity of TCP is not required. If there is no response to the
query after a period of time, the query can simply be resent.
The User Datagram Protocol (UDP) is designed for
communications that do not require division among multiple
packets and subsequent reassembly. UDP does not keep track of
what is sent.
UDP uses port numbers in a way that is directly analogous to TCP.
There are well-known UDP port numbers for servers that use
UDP.
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