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UDP ACK Question

3 posts in this topic


I had a question on how to send an ACK.

So, I am working on a UDP Server, so from a high level can someone please tell me if my logic is correct:

Assuming 1 server, 2 players (possibly more than 2 players)

1) Player 1 sends data packs 1,2,3,4,5 to server (waits for ACK) for broadcast to players who joined
2) Server sends to rest of players, in this case Player 2.
3) Player 2 receives packets 3,4,1 ...
-- Does player 2 send an ACK to the server?
-----If so, what if those never reach player 1?
-- Does player 2 send an ACK directly back to Player1 somehow?
-----Does Player1 send the IP along?

4) Player 1 keeps sending packets it does not get ACK for after X time interval. In this case, it would resend packets 2, 5.

This would be for a out-of-order guaranteed packet delivery.


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If you need guaranteed delivery, you need to send the other end some information about what you have already gotten.
You will also need to be able to discard multiples of the same packet, so having a window of packet numbers you know may be in flight, and a window of packets where you know you've received them all (and thus can discard any duplicates and just send an ack to the other end) is useful.
Note that you can't "guarantee" delivery -- if the other end's network goes down, you will not be able to get any packet through.

For more information, see the "two generals problem" on Wikipedia :-)

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Is this 'broadcast'  a replicate send to the player list, or a real packet 'broadcast' which usually will only work within the local subnet (LAN party mode)??


If its a actual broadcast why not have Player 1 do the broadcast itself and cut out the middleman ?


If its packet replication then unless the server has a bigger pipe or is used because of punchthrough issues or IP priivacy then again Player 1 culd be sending its own replicated set of packets to each of the other players.


With a server in the middle (for whatever reason) Players 1s ACKs are between it and that Server and the forwarding/broadcast send is between the server and all the other players.  Otherwise it too many delays and failure points and IP notifications etc..


Years ago I did a guaranteed UDP with a sliding window system and bitmapped ACK history  on every packet sent the opposite direction

With packets indexed, if you see an ACK on the bitmap and there are previous ones (lower index #) not ACKED you can be pretty certain you will need to resend those unACKd packets again without a tiemout wait




Also remember that some data requires timely reception (like constant movement positions) and if lost will be replaced eventually (possibly quite soon) by replacement info.   Stuff like that and Application controllable throttling  is one of the reasons for customizing using UDP instead of just using TCP


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Some additional thougths:

- If you don't use unreliable packets, you might want to use TCP.
- You don't need to wait for ack for unreliable packets. If you use UDP, you should bandwidth constrain your game to some particular "min spec" and then just send when you need to send.
- Acks are only needed for logically reliable messages that are part of internal channels/streams that you build on top of the unreliable UDP delivery.
- Acks are on a per-node-pair level. Player1:Server is one such pair. Server:Player2 is another such pair. The two are independent.
- You will need to buffer any reliable messages on the sending end until you get an ack from the receiving end.

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