Udp And Icmp - Enterasys VH-2402-L3 Management Manual

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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