Fun how this is more or less the contrary of what I think, so I'll add my point of view as a counter argument. IMHO if you need a transport layer, you are 99.9% of cases well served by TCP and should not consider UDP at all if not as a last resort to implement your transport protocol. Instead UDP is the way to go when you don't need a transport layer at all, and you can do with trivial ack / resend strategy (DNS is the obvious example), or just best-effort transfers where packet loss is not an issue at all (think a thermometer sending readings every second). In those cases the TCP three way handshake would be an overkill, and the benefits of TCP are minimal because of the use case.
> Instead UDP is the way to go when you don't need a transport layer at all, and you can do with trivial ack / resend strategy (DNS is the obvious example), or just best-effort transfers where packet loss is not an issue at all (think a thermometer sending readings every second)
That depends I guess. As somebody who works mostly with cloud telephony, SIP (signaling) and RTP (audio) are almost always carried over UDP. Funny thing is that you'll see more occurrences of one-way audio or call drops because of people implementing the application level protocol like SIP wrong rather than the transport layer letting you down.
That said, I would any day prefer a private MPLS network rather than the public internet.
Though I can understand why somebody who writes a fast nosql database prefers TCP. Also, thanks for redis, we use it a lot and love it! :)
SIP signalling over UDP was a bad idea, especially with the mandatory TCP failover. SIP's actually a spectacular clusterfuck of a design overall anyways. It's actually impossible to implement SIP in an unambiguous manner on today's networks because even the parsing rules are so bad, that multiple, popular, stacks have incompatible ways of dealing with things.
UDP for RTP makes sense because transmitting packets is pointless. In case of packetloss, the end user has to interpolate the result either via software or in their head.
This private MPLS network... it wouldn't happen to run over the exact same equipment that'd be handling your IP traffic, would it?
At the same time, beware of "helpful" lower layer protocols.
Mobile wireless stuff (EDGE/3G) is going to great lengths to avoid dropping packets, so if you get conditions right (moving train in the areas with poor coverage is one place where you can easily see this), you can get the packets "reliably delivered" in 20 seconds and more.
TCP has been designed in such a way that it interprets packet drops as a sign of "congestion" (which was typically true in ye olden days of purely wired networking), and it will start sending less data in response.
Whereas in wireless networking, occasional packet drops are just a fact of life and are not indicative of competing flows trying to share the channel. So it actually makes [some] sense that wireless protocols try to compensate for the behaviour of the transport protocol used by 90% of all data: TCP.
The one case where you don't really want TCP semantics, is when you want SCTP/RTP semantics: basically, user datagrams decomposed into frames and then priority-multiplexed over a reliable circuit, then buffered back into in-order messages. This is, it turns out, what web browsers, game clients, VoIP services, and a lot of other things want—to the point that if we're going to have one kernel-implemented protocol and everything else has to live on top of UDP, I wish the one kernel protocol was SCTP. (Because one-channel SCTP really looks a lot like TCP.)
Cheaper SCTP isn't quite enough—one of the major problems with lots of little TCP connections is that they all have their own backpressure, so you get things like worker processes that share a remote flapping their window sizes up and down in response to one-anothers' requests. Ideally, backpressure would occur at the IP layer (host-to-host rather than port-to-port) but SCTP gives you the ability to have a set of streams that share a backpressure "group" on both the local and remote ends.
