Cable Forum - View Single Post - Customer Services & Capped Upload Speeds (NTL Reading)

djb61 · 12-04-2007, 18:51

The reason the download suffers when you max out your upstream is all to do with ACK packets. The way TCP/IP works is that when you receive download packets you send an ACK packet (very small as it contains no data) back to the host you are downloading from to confirm you received the data.
When you max out your upstream these ACK packets take longer to get sent back as they wait in the queue with all other outgoing data, this causes the remote host to either throttle or worse case resend data thinking you haven't received it, resulting in a slowing down of download speed either way.

The solution is either to simply throttle your uploading application below your upstream limit to leave room for ACKs to be sent or to use some kind of traffic shaping/QoS software to give ACKs higher priority so they don't have to wait in the queue. I do the latter on the FreeBSD machine my cable modem is connected to and it works well, I can max out upstream with marginal affect to my downstream. I'm guessing however that you are using Windows and I can't suggest any software to do the same there, but undoubtably there is some, maybe someone else has some suggestions.

12-04-2007, 18:51	#4
djb61 Inactive Join Date: Aug 2006 Posts: 10	Re: Customer Services & Capped Upload Speeds (NTL Reading) The reason the download suffers when you max out your upstream is all to do with ACK packets. The way TCP/IP works is that when you receive download packets you send an ACK packet (very small as it contains no data) back to the host you are downloading from to confirm you received the data. When you max out your upstream these ACK packets take longer to get sent back as they wait in the queue with all other outgoing data, this causes the remote host to either throttle or worse case resend data thinking you haven't received it, resulting in a slowing down of download speed either way. The solution is either to simply throttle your uploading application below your upstream limit to leave room for ACKs to be sent or to use some kind of traffic shaping/QoS software to give ACKs higher priority so they don't have to wait in the queue. I do the latter on the FreeBSD machine my cable modem is connected to and it works well, I can max out upstream with marginal affect to my downstream. I'm guessing however that you are using Windows and I can't suggest any software to do the same there, but undoubtably there is some, maybe someone else has some suggestions.