Connectionless Transport: UDP

The Internet makes two transport protocols available to its applications, UDP and TCP. In this section we take a close look at UDP: how it works and what it does. The reader is encouraged to refer back to material in Section 2.1, which includes an overview of the UDP service model, and to the material in Section 2.7, which discusses socket programming overUDP.

To motivate our discussion about UDP, suppose you were interested in designing a no-frills, bare-bones transport protocol. How might you go about doing this? You might first consider using a vacuous transport protocol. In particular, on the sending side, you might consider taking the messages from the application process and passing them directly to the network layer; and on the receiving side, you might consider taking the messages arriving from the network layer and passing them directly to the application process. But as we learned in the previous section, we have to do a little more than nothing. At the very least, the transport layer must provide a multiplexing/demultiplexing service in order to pass data between the network layer and the correct application.

UDP, defined in [RFC 768], does just about as little as a transport protocol can. Aside from the multiplexing/demultiplexing function and some light error checking, it adds nothing to IP. In fact, if the application developer chooses UDP instead of TCP, then the application is talking almost directly with IP. UDP takes messages from application process, attaches source and destination port number fields for the multiplexing/demultiplexing service, adds two other fields of minor importance, and passes the resulting "segment" to the network layer. The network layer encapsulates the segment into an IP datagram and then makes a best-effort attempt to deliver the segment to the receiving host. If the segment arrives at the receiving host, UDP uses the port numbers and the IP source and destination addresses to deliver the data in the segment to the correct application process. Note that with UDP there is no handshaking between sending and receiving transport-layer entities before sending a segment. For this reason, UDP is said to be connectionless.

DNS is an example of an application-layer protocol that uses UDP. When the DNS application (see section 2.5) in a host wants to make a query, it constructs a DNS query message and passes the message to a UDP socket (see Section 2.7). Without performing any handshaking, UDP adds a header fields to the message and passes the resulting segment to the network layer. The network layer encapsulates the UDP segment into a datagram and sends the datagram to a name server. The DNS application at the querying host then waits for a reply to its query. If it doesn't receive a reply (possibly because UDP lost the query or the reply), it either tries sending the query to another nameserver, or it informs the invoking application that it can't get a reply. We mention that the DNS specification permits DNS to run over TCP instead of UDP; in practice, however, DNS almost always runs over UDP.

Now you might be wondering why an application developer would ever choose to build an application over UDP rather than over TCP. Isn't TCP always preferable to UDP since TCPprovides a reliable data transfer service and UDP does not? The answer is no, as many applications are better suited for UDP for the following reasons:

• No connection establishment. As we shall discuss in Section 3.5, TCP uses a three-way handshake before it starts to transfer data. UDP just blasts away without any formal preliminaries. Thus UDP does not introduce any delay to establish a connection. This is probably the principle reason why DNS runs over UDP rather than TCP – DNS would be much slower if it ran over TCP. HTTP uses TCP rather than UDP, since reliability is critical for Web pages with text. But, as we briefly discussed in Section 2.2, the TCPconnection establishment delay in HTTP is an important contributor to the world wide wait.
• No connection state. TCP maintains connection state in the end systems. This connection state includes receive and send buffers, congestion control parameters, and sequence and acknowledgment number parameters. We will see in Section 3.5 that this state information is needed to implement TCP's reliable data transfer service and to provide congestion control. UDP, on the other hand, does not maintain connection state and does not track any of these parameters. For this reason, a server devoted to a particular application can typically support many more active clients when the application runs over UDP rather than TCP.
• Small segment header overhead. The TCP segment has 20 bytes of header overhead in every segment, whereas UDP only has 8 bytes of overhead.
• Unregulated send rate. TCP has a congestion control mechanism that throttles the sender when one or more links between sender and receiver becomes excessively congested. This throttling can have a severe impact on real-time applications, which can tolerate some packet loss but require a minimum send rate. On the other hand, the speed at which UDP sends data is only constrained by the rate at which the application generates data, the capabilities of the source (CPU, clock rate, etc.) and the access bandwidth to the Internet. We should keep in mind, however, that the receiving host does not necessarily receive all the data – when the network is congested, a significant fraction of the UDP-transmitted data could be lost due to router buffer overflow. Thus, the receive rate is limited by network congestion even if the sending rate is not constrained.

Table 3.1-1 lists popular Internet applications and the transport protocols that they use. As we expect, e-mail, remote terminal access, the Web and file transfer run over TCP – these applications need the reliable data transfer service of TCP. Nevertheless, many important applications run over UDP rather TCP. UDP is used for RIP routing table updates (see Chapter 4 on the network layer), because the updates are sent periodically, so that lost updates are replaced by more up-to-date updates. UDP is used to carry network management (SNMP – see Chapter 8) data. UDP is preferred to TCP in this case, since network management must often run when the network is in a stressed state – precisely when reliable, congestion-controlled data transfer is difficult to achieve. Also, as we mentioned earlier, DNS runs over UDP, thereby avoiding TCP's connection establishment delays.

As shown in Figure 3.1-1, UDP is also commonly used today with multimedia applications, such as Internet phone, real-time video conferencing, and streaming of stored audio and video. We shall take a close look at these applications in Chapter 6. We just mention now that all of these applications can tolerate a small fraction of packet loss, so that reliable data transfer is not absolutely critical for the success of the application. Furthermore, interactive real-time applications, such as Internet phone and video conferencing, react very poorly to TCP's congestion control. For these reasons, developers of multimedia applications often choose to run the applications over UDP instead of TCP. Finally, because TCP cannot be employed with multicast, multicast applications run over UDP.

Although commonly done today, running multimedia applications over UDP is controversial to say the least. As we mentioned above, UDP lacks any form of congestion control. But congestion control is needed to prevent the network from entering a congested state in which very little useful work is done. If everyone were to start streaming high bit-rate video without using any congestion control, there would be so much packet overflow at routers that no one would see anything. Thus, the lack of congestion control in UDP is a potentially serious problem. Many researchers have proposed new mechanisms to force all sources, including UDP sources, to perform adaptive congestion control [Mahdavi].

Before discussing the UDP segment structure, we mention that it is possible for an application to have reliable data transfer when using UDP. This can be done if reliability is built into the application itself (e.g., by adding acknowledgement and retransmission mechanisms, such as those we shall study in the next section). But this a non-trivial task that would keep an application developer busy debugging for a long time. Nevertheless, building reliability directly into the application allows the application to have its cake and eat it too – that is, application processes can communicate reliably without being constrained by the transmission rate constraints imposed by TCP's congestion control mechanism. Application-level reliability also allows an application to tailor its own application-specific form of error control. An interactive real-time may occasionally choose to retransmit a lost message, provided that round trip network delays are small enough to avoid adding significant playout delays [Papadopoulos 1996].

Many of today's proprietary streaming applications do just this – they run over UDP, but they have built acknowledgements and retransmissions into the application in order reduce packet loss.

UDP Segment Structure

The UDP segment structure, shown in Figure 3.3-2, is defined in [RFC 768].

Figure 3.3-2: UDP segment structure

The application data occupies the data field of the UDP datagram. For example, for DNS, the data field contains either a query message or a response message. For a streaming audio application, audio samples fill the data field. The UDP header has only four fields, each consisting of four bytes. As discussed in the previous section, the port numbers allow the destination host to pass the application data to the correct process running on that host (i.e., perform the demultiplexing function). The checksum is used by the receiving host to check if errors have been introduced into the segment during the course of its transmission from source to destination. (Basic principles of error detection are described in Section 5.2.).

UDP Checksum

The UDP checksum provides for error detection. UDP at the sender side performs the one's complement of the sum of all the 16-bit words in the segment. This result is put in the checksum field of the UDP segment. (In truth, the checksum is also calculated over a few of the fields in the IP header in addition to the UDP segment. But we ignore this detail in order to see the forest through the trees.) When the segment arrives (if it arrives!) at the receiving host, all 16-bit words are added together, including the checksum. If this sum equals 1111111111111111, then the segment has no detected errors. If one of the bits is a zero, then we know that errors have been introduced into the segment.

Here we give a simple example of the checksum calculation. You can find details about efficient implementation of the calculation in the [RFC 1071]. As an example, suppose that we have the following three 16-bit words:

0110011001100110

0101010101010101

0000111100001111

The sum of first of these 16-bit words is:

0110011001100110

0101010101010101

----------------

1011101110111011

Adding the third word to the above sum gives

1011101110111011

0000111100001111

----------------

1100101011001010

The 1's complement is obtained by converting all the 0s to 1s and converting all the 1s to 0s. Thus the 1's complement of the sum 1100101011001010 is 0011010100110101, which becomes the checksum. At the receiver, all four 16-bit words are added, including the checksum. If no errors are introduced into the segment, then clearly the sum at the receiver will be 1111111111111111. If one of the bits is a zero, then we know that errors have been introduced into the segment. In section 5.1, we'll see that the Internet checksum is not foolproof – even if the sum equals 111111111111111, it is still possible that there are undetected errors in the segment. For this reason, a number of protocols use more sophisticated error detection techniques than simple checksumming.

You may wonder why UDP provides a checksum in the first place, as many link-layer protocols (including the popular Ethernet protocol) also provide error checking? The reason is that there is no guarantee that all the links between source and destination provide error checking – one of the links may use a protocol that does not provide error checking. Because IP is supposed to run over just about any layer-2 protocol, it is useful for the transport layer to provide error checking as a safety measure. Although UDP provides error checking, it does not do anything to recover from an error. Some implementations of UDP simply discard the damaged segment; others pass the damaged segment to the application with a warning.

That wraps up our discussion of UDP. We will soon see that TCP offers reliable data transfer to its applications as well as other services that UDP doesn't offer. Naturally, TCP is also more complex than UDP. Before discussing TCP, however, it will be useful to step back and first discuss the underlying principles of reliable data transfer, which we do in the subsequent section. We will then explore TCP in Section 3.5, where we will see that TCP has it foundations in these underlying principles.