Protocols other than TCP: UDP and ICMP
Copyright (C) 1987, Charles L. Hedrick. Anyone may reproduce this
document, in whole or in part, provided that: (1) any copy or
republication of the entire document must show Rutgers University as
the source, and must include this notice; and (2) any other use of
this material must reference this manual and Rutgers University, and
the fact that the material is copyright by Charles Hedrick and is used
by permission.
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
than 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.
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Steven E. Newton /
<snewton@oac.hsc.uth.tmc.edu> / 1-20-94
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