I'm a dial-up user. For the first time ever, I have the same computer attached to two projects. A while after I activated (after suspending for several days) that computer's network activity, there was a period when the BOINC client was downloading new work for one project while still uploading results from the other project.

What I noticed was that on this occasion, the download speed (instantaneous byte transfer rate) was reported (by boincmgr) as very SMALL while the upload speed was reported in the range I expected. As far as I know, my communications line is set up for full-duplex -- meaning that it ought to be capable of "normal" byte transfer speeds in both the "up" direction and the "down" direction -- simultaneously.

Was it the BOINC client that "throttled" my download speed in that situation ?
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What I noticed was that on this occasion, the download speed (instantaneous byte transfer rate) was reported (by boincmgr) as very SMALL while the upload speed was reported in the range I expected. As far as I know, my communications line is set up for full-duplex -- meaning that it ought to be capable of "normal" byte transfer speeds in both the "up" direction and the "down" direction -- simultaneously.

Was it the BOINC client that "throttled" my download speed in that situation?

Actually this is quite normal for any TCP/IP communication over relatively slow line.

Basically, when an application transfers data over TCP/IP (eg. downloading from a HTTP server which is the case when fetching new work from BOINC server), majority of data flows from server towards client. But there are some packets frowing from client towards server - so called acknowledgement (ACK) packets. This is required by TCP/IP protocol.
While amount of data transferred for these ACK packets is not large they still need to be transfered. Only some (relatively small) amount of data is actually transmited by sending party. THen it waits for ACK packets to arrive and after that some more data gets transmitted. The purpose is that if some data doesn't arrive at receiving party, receiver uses ACK packets to inform sender about that and missing packets get re-transmitted.

In your case you have two simultaneous transfers: one download and one upload. The uploading stream fills the IP buffers - which is noemally good as this enables you to maximize throughput. But those tiny ACK packets get jammed and are then transmitted with noticable delay. This causes delay at sending party - in your particular case at BOINC server. All of it eventually reduces overall throughput.

Said in few words: upload kills download - and this is specially true for any asymetrical links.

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Metod ...