At the Internet layer in our TCP/IP protocol stack, the only information avail- able is the address of the remote node. No other information is available to the protocol, and none is needed. However, without additional information like a port number, your receiving node is limited to conducting a single network communication at any one time. Since modern operating systems allow multi- ple applications to run simultaneously, you must be able to address multiple applications on the receiving node simultaneously, instead of just one. If you consider that each networked application can “listen” on one or more ports, you can see that by using an IP address and a port, you can communicate with multiple applications simultaneously, up to any limits imposed by the operat- ing system and protocol stack.
The User Datagram Protocol (UDP)
In the TCP/IP protocol stack, there are two protocols that provide a mechanism that allows applications to communicate with other applications using ports. One is the Transmission Control Protocol (TCP), which we will discuss in the next section, and the other is the User Datagram Protocol (UDP). UDP makes no guarantee of packet delivery. UDP datagrams can be lost, can arrive out of sequence, can be copied many times, and can be sent faster than the receiving node can process them. Thus, an application that uses UDP takes full responsibility for controlling message loss, reliability, sequencing, and loss of connection. This can be both an advantage and a disadvantage to developers, for while UDP is a lightweight protocol that can be used quickly, the additional application overhead needed to thoroughly manage packet transfer is often overlooked or poorly implemented.
UDP datagrams have a simple format. Like other datagrams, UDP datagrams consist of a header and a data payload. The header is divided into four fields, each 16 bits in size. These fields specify the source port, the destination port, the length of the datagram, and a checksum.
The source port is optional. If used, it specifies the port to which replies should be sent. If unused, it should be set to zero. The length field is the total number of octets in the datagram itself, header and data. The minimum value for length is 8, which is the length of the header by itself.
The checksum value is also optional, and if unused should be set to zero. Even though it’s optional, however, it should be used, since IP doesn’t compute a checksum on the data portion of its own datagram. Thus, without a UDP checksum, there’s no other way to check for header integrity on the receiving node. To compute the checksum, UDP uses a pseudo-header, which is prepended to the datagram, followed by an octet of zeros, which is appended, to get an exact multiple of 16 bits. The entire object, pseudo-header and all, is then used to compute the checksum.
The octet used for padding is not transmitted with the UDP datagram, nor is the pseudo-header, and neither is counted when computing the length of the datagram.
Network Services Using UDP
A number of network services use UDP and have reserved ports. A list of some of the more popular services is shown below:
Published on Thu 21 March 2002 by Daisy Batty in Networking with tag(s): udp