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I was searching to know: "Does uploading and downloading at the same time reduce download speed?" and I found this:

Should uploading a file on DSL kill the download speed?

but it is related to TCP connections, not UDP.

I know torrent uses UDP connection. Then does allowing torrent upload reduce torrent download speed?

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Whether UDP or TCP is used, to have a reliable transfer, packets have to be acknowledged. (In the case of UDP, the application itself must generate the acks since the protocol doesn't.) Thus the same reasoning applies as to the other question you referred to.

Saturating the "up" part of your Internet connection slows downloading because packets coming "down" have to be acknowledged by your computer and those acknowledgements have to go out via the up link. If your computer falls too far behind in acks because the up link is choked with data, the sender will slow down and start retransmitting the unacknowledged packets. This will appear to you as a decrease in download speed.

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actually, udp is connectionless and does not send ack packets. –  Rex Mar 29 '12 at 3:36
    
If you're going to transfer files reliably over UDP you'll have to use some kind of acknowledgment. But the questioner is probably using a UDP based torrent tracker while still doing the file transfers with TCP. –  Kyle Jones Mar 29 '12 at 4:43
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If you are transferring over UDP, it is not reliable (by design) so there is no acknowledgement. Torrent transfer will generally uses TCP (tcp/6881-tcp/6889) which will send ack packets as it is TCP. But your answer is unclear and implies UDP will have reliable transfer and ack's which is strictly a function of TCP and never on UDP. –  Rex Mar 29 '12 at 14:45
    
@Rex Can you answer the question descriptive ? –  hamed Mar 29 '12 at 23:40
    
@Rex I understand the difference between TCP and UDP, but it doesn't matter here. No matter which protocol is used, there are going to be ack packets, even if they must be generated by the application itself, otherwise you can't have reliable file transfer. And since those packets have to exist, there must be uplink bandwidth available for them or the download will stall. I've edited the answer to indicate that UDP itself doesn't send ack packets. –  Kyle Jones Mar 30 '12 at 0:04
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It will affect your download speeds, yes - especially since you are on DSL. If your network is maxing out it's outbound bandwidth with packets to peers (sharing), you won't have enough spare outbound bandwidth to ask for more pieces of the torrent you are receiving.

Adjust your outbound bandwidth to about two-thirds what your link can handle, and you should be golden.

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