UDP上可靠层需要哪些机制？

Question

I've been working on writing my own networking engine for my own game development side projects. This requires the options of having unreliable, reliable, and ordered reliable messages. I have not, however, been able to identify all of the mechanisms necessary for reliable and ordered reliable protocols.

What are the required mechanisms for a reliable layer over UDP? Additional details are appreciated.

So far, I gather that these are requirements:

• Acknowledge received messages with a sequence number.
• Resend unacknowledged messages after a retransmission time expires.
• Track round trip times for each destination in order to calculate an appropriate retransmission time.
• Identify and remove duplicate packets.
• Handle overflowing sequence numbers looping around.

This has influenced my architecture to have reliable message headers with sequences and timestamps, acknowledge messages that echo a received sequence and timestamp, a system for tracking appropriate retransmission times based on address, and a thread that a) receives messages and queues them for user receipt, b) acknowledges reliable messages, and c) retransmits unacknowledged messages with expired retransmission timers.

NOTE: Reliable UDP is not the same as TCP. Even ordered reliable UDP is not the same as TCP. I am not secretly unaware that I really want TCP. Also, before someone plays the semantics games, yes... reliable UDP is an "oxymoron". This is a layer over UDP that makes for reliable delivery.

Answer 1

You might like to take a look at the answers to this question: What do you use when you need reliable UDP?

I'd add 'flow control' to your list. You want to be able to control the amount of data you're sending on a particular link depending on the round trip time's you're getting or you'll flood the link and just be throwing datagrams away.

Answer 2

Note that depending on the overall protocol, it might be possible to dispense with retransmission timers. See, for example, the Quake 3 network protocol .

In Q3 reliable packets are simply sent until an ack is seen.

Answer 3

Why are you trying to re-invent TCP? It provides all of the features you originally stated, and has been show to work well.

EDIT - Since your comments show that you have additional requirements not originally stated, you should consider whether a hybrid model using multiple sockets would be better than trying to fulfill all of those criteria in a single application-layer protocol.

Actually it seems that what you really need is SCTP .

SCTP supports:

1. message based (rather than byte stream) transmissions
2. multiple streams over a single netsock socket
3. ordered or unordered receipt of packets

... message ordering is optional in SCTP; a receiving application may choose to process messages in the order they are received instead of the order they were sent