Other IP Protocols

Protocols other than TCP: UDP and ICMP
So far, we have described only connections that use TCP. Recall that TCP is responsible
for breaking up messages into datagrams, and reassembling them properly. However in
many applications, we have
messages that will always fit in a single datagram. An example is name lookup. When a
user attempts to make a connection to another system, he will generally specify the
system by name, rather than Internet
address. His system has to translate that name to an address before it can do anything.
Generally, only a few systems have the database used to translate names to addresses. So
the user's system will want to send a query to one of the systems that has the database.
This query is going to be very short. It will certainly fit in one datagram. So will the
answer. Thus it seems silly to use TCP. Of course TCP does
more than just break things up into datagrams. It also makes sure that the data arrives,
resending datagrams where necessary. But for a question that fits in a single datagram,
we don't need all the
complexity of TCP to do this. If we don't get an answer after a few seconds, we can just
ask again. For applications like this, there are alternatives to TCP.
The most common alternative is UDP ("user datagram protocol"). UDP is designed for
applications where you don't need to put sequences of datagrams together. It fits into the
system much like TCP. There is a
UDP header. The network software puts the UDP header on the front of your data, just as
it would put a TCP header on the front of your data. Then UDP sends the data to IP,
which adds the IP header, putting
UDP's protocol number in the protocol field instead of TCP's protocol number. However
UDP doesn't do as much as TCP does. It doesn't split data into multiple datagrams. It
doesn't keep track of what it has
sent so it can resend if necessary. About all that UDP provides is port numbers, so that
several programs can use UDP at once. UDP port numbers are used just like TCP port
numbers. There are well-known port
numbers for servers that use UDP. Note that the UDP header is shorter than a TCP
header. It still has source and destination port numbers, and a checksum, but that's about
it. No sequence number, since it is not needed. UDP is used by the protocols that handle
name lookups (see IEN 116, RFC 882, and RFC 883), and a number of similar protocols.
Another alternative protocol is ICMP ("Internet Control Message Protocol"). ICMP is
used for error messages, and other messages intended for the TCP/IP software itself,
rather than any particular
user program. For example, if you attempt to connect to a host, your system may get back
an ICMP message saying "host unreachable". ICMP can also be used to find out some
information about the network. See RFC 792 for details of ICMP. ICMP is similar to
UDP, in that it handles messages that fit in one datagram. However it is even simpler tha
UDP. It doesn't even have port numbers in its header. Since all ICMP messages are
interpreted by the network software itself, no port numbers are needed to say where a
ICMP message is supposed to go.