What do you use when you need reliable UDP?

Question

If you have a situation where a TCP connection is potentially too slow and a UDP 'connection' is potentially too unreliable what do you use? There are various standard reliable UDP protocols out there, what experiences do you have with them?

Please discuss one protocol per reply and if someone else has already mentioned the one you use then consider voting them up and using a comment to elaborate if required.

I'm interested in the various options here, of which TCP is at one end of the scale and UDP is at the other. Various reliable UDP options are available and each brings some elements of TCP to UDP.

I know that often TCP is the correct choice but having a list of the alternatives is often useful in helping one come to that conclusion. Things like Enet, RUDP, etc that are built on UDP have various pros and cons, have you used them, what are your experiences?

For the avoidance of doubt there is no more information, this is a hypothetical question and one that I hoped would elicit a list of responses that detailed the various options and alternatives available to someone who needs to make a decision.

Answer 1

Did you consider compressing your data ?

As stated above, we lack information about the exact nature of your problem, but compressing the data to transport them could help.

Answer 2

Protocol DCCP, standardized in RFC 4340, "Datagram Congestion Control Protocol" may be what you are looking for.

It seems implemented in Linux.

Answer 3

ENET - http://enet.bespin.org/

I've worked with ENET as a reliable UDP protocol and written an asynchronous sockets friendly version for a client of mine who is using it in their servers. It works quite nicely but I don't like the overhead that the peer to peer ping adds to otherwise idle connections; when you have lots of connections pinging all of them regularly is a lot of busy work.

ENET gives you the option to send multiple 'channels' of data and for the data sent to be unreliable, reliable or sequenced. It also includes the aforementioned peer to peer ping which acts as a keep alive.

Answer 4

As others have pointed out, your question is very general, and whether or not something is 'faster' than TCP depends a lot on the type of application.

TCP is generally as fast as it gets for reliable streaming of data from one host to another. However, if your application does a lot of small bursts of traffic and waiting for responses, UDP may be more appropriate to minimize latency.

There is an easy middle ground. Nagle's algorithm is the part of TCP that helps ensure that the sender doesn't overwhelm the receiver of a large stream of data, resulting in congestion and packet loss.

If you need the reliable, in-order delivery of TCP, and also the fast response of UDP, and don't need to worry about congestion from sending large streams of data, you can disable Nagle's algorithm:

int opt = -1;
if (setsockopt(sock_fd, IPPROTO_TCP, TCP_NODELAY, (char *)&opt, sizeof(opt)))
  printf("Error disabling Nagle's algorithm.\n");

Answer 5

The best way to achieve reliability using UDP is to build the reliability in the application program itself( for example, by adding acknowledgment and retransmission mechanisms)

Answer 6

RUDP. Many socket servers for games implement something similar.