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Stupid (?) Network Question: Is ADSL Full Duplex?

On Thu, 9 Jul 2009, Kent Borg wrote:

> I have residential ADSL service from Covad.  I am not a regular bit
> torrent user, but have used it a little recently.
> Odd discovery: My download rate seems to be antagonistic with my upload
> rate.  Using the bit torrent client "Transmission" under Ubuntu if I
> leave the upload rate unlimited my download rate drops terribly.  If I
> limit the upload rate to my download rate goes up!  Why?
> I thought the bit torrent protocol rewarded those who supply bits by
> letting them download bits faster, so if ADSL (asymmetric as it is) were
> full duplex I should get my fastest download rate by not limiting my
> upload rate, yet limiting to 15 Kbps seems to maximize my download rate.
> Does this possibly have something to do with the NAT traversal stuff?
> (I haven't opened a reverse port for bit torrent.)  Is my Linux box
> doing some stupid choking on these bits?  Is my DSL modem/router buggy?
> Is Covad doing something strange?  Have I hit some bug in the bit
> torrent protocol or other implementations?  Is it a problem with doing
> Wifi as my last link to my notebook?

Sometimes, limiting your upload rate will increase your download rate. 
This is especially true for asymmetric connections such as cable and ADSL, 
where the outbound bandwidth is much smaller than the inbound bandwidth. 
If you are seeing very high upload rates and low download rates, this is 
probably the case. The reason this happens is due to the nature of TCP/IP 
-- every packet received must be acknowledged with a small outbound 
packet. If the outbound link is saturated with BitTorrent data, the 
latency of these TCP/IP ACKs will rise, causing poor efficiency.


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