The advantages of TCP over UDP are quite clear, as TCP guarantees that the sent data actually arrives, that it arrives in order and that there are no duplicates, while UDP provides none of these guarantees (only offers "best effort" policy). Unlike TCP, UDP does not provide any flow and congestion control.

On the surface, an unreliable transport protocol like UDP may not seem very worthwhile or desirable. But in fact, UDP can be very useful in certain situations, and it enjoys one key advantage over TCP - speed. The reliability features built into TCP can be expensive in terms of overhead at execution time.

Therefore, many applications find UDP well-suited for their needs, for the following reasons:

While 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 feature is most useful to applications which exchange sporadic and low-volume data.

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. 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. TCP needs more header fields in order to guarantee reliability, while UDP, that gives no guarantees, does not.

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.

Lower latency

With TCP, if a packet is lost, but the next packet makes it through, the kernel will withhold that packet until the earlier packet can be re-sent. This is because TCP is a guaranteed, in-order, stream protocol. This means that "fresh" data will sit in the kernel, becoming "stale", while waiting for the TCP timeout to be retransmitted (a minimum of 3 seconds for a lost packet). This is why UDP is usually better for games, voice conferencing, and other low-latency applications.

Broadcast and multicast

Being a connection-oriented protocol, TCP does not support broadcast and multicast. Therefore, applications that require this kind of service will have to use UDP as a transport protocol.

Application flexibility

The fact that UDP lacks built-in reliability mechanisms can be considered an advantage from the application designer's point of view. 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.

For example, 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. In fact, 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.

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