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

RUDP - Reliable User Datagram Protocol

This provides:

• Acknowledgment of received packets
• Windowing and congestion control
• Retransmission of lost packets
• Overbuffering (Faster than real-time streaming)

It seems slightly more configurable with regards to keep alives then ENet but it doesn't give you as many options (i.e. all data is reliable and sequenced not just the bits that you decide should be). It looks fairly straight forward to implement.

Answer 2

It's difficult to answer this question without some additional information on the domain of the problem. For example, what volume of data are you using? How often? What is the nature of the data? (eg. is it unique, one off data? Or is it a stream of sample data? etc.) What platform are you developing for? (eg. desktop/server/embedded) To determine what you mean by "too slow", what network medium are you using?

But in (very!) general terms I think you're going to have to try really hard to beat tcp for speed, unless you can make some hard assumptions about the data that you're trying to send.

For example, if the data that you're trying to send is such that you can tolerate the loss of a single packet (eg. regularly sampled data where the sampling rate is many times higher than the bandwidth of the signal) then you can probably sacrifice some reliability of transmission by ensuring that you can detect data corruption (eg. through the use of a good crc)

But if you cannot tolerate the loss of a single packet, then you're going to have to start introducing the types of techniques for reliability that tcp already has. And, without putting in a reasonable amount of work, you may find that you're starting to build those elements into a user-space solution with all of the inherent speed issues to go with it.

Answer 3

What about SCTP. It's a standard protocol by the IETF (RFC 4960)

It has chunking capability which could help for speed.

Update: a comparison between TCP and SCTP shows that the performances are comparable unless two interfaces can be used.

Update: a nice introductory article.

Answer 4

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?

The key word in your sentence is 'potentially'. I think you really need to prove to yourself that TCP is, in fact, too slow for your needs if you need reliability in your protocol.

If you want to get reliability out of UDP then you're basically going to be re-implementing some of TCP's features on top of UDP which will probably make things slower than just using TCP in the first place.

Answer 5

May be RFC 5405, "Unicast UDP Usage Guidelines for Application Designers" will be useful for you.

Answer 6

We have some defense industry customers that use UDT (UDP-based Data Transfer) (see http://udt.sourceforge.net/) and are very happy with it. I see that is has a friendly BSD license as well.