Why is UDP + a software reliable ordering system faster than TCP?

Solution 1:

General/Specialisation

TCP is a general purpose reliable system
UDP +whatever is a special purpose reliable system.

Specialized things are usually better than general purpose things for the thing they are specialized.

Stream / Message

TCP is stream-based
UDP is message-based

Sending discrete gaming information maps usually better to a message-based paradigm. Sending it through a stream is possible but horribly ineffective. If you want to reliably send a huge amount of data (File transfer), TCP is quite effective. That's why Bit-torrent use UDP for control messages and TCP for data sending.

Solution 2:

We switched from reliable to unreliable in "league of legends" about a year ago because of several advantages which have since proven to be true:

1) Old information becomes irrelevant. If I send a health packet and it doesn't arrive... I don't want to have to wait for that same health packet to resend when I know its changed.

2) Order is sometimes not necessary. If I'm sending different messages to different systems it may not be necessary to get those messages in order. I don't force the client to wait for in-order messages.

3) Unreliable doesn't get backed up with messages... ie waiting for acknowledgements which means you can resolve loss spikes much more quickly.

4) You can control resends when necessarily more efficiently. Such as repacking something that didn't send into another packet. (TCP does repack but you can do it more efficiently with knowledge about how your program works.)

5) Flow control of message such as throwing away messages that are less relevant when the network suddenly spikes. The network system can choose not to resend less relevant messages when you have a loss spike. With TCP you'd still have a queue of messages that are trying to resend which may be lower priority.

6) Smaller header packet... don't really need to say much about that.

Solution 3:

There's much more of a difference between UDP and TCP than just reliability and sequencing:

At the heart of the matter is the fact that UDP is connectionless while TCP is connected. This simple difference leads to a host of other differences that I'm not going to be able to reasonbly summarize here. You can read the analysis below for much more detail.

TCP - UDP Comparative Analysis