Re: downloads start fast, then slow down

From: /dev/null (dev.null_at_BeginThread.com)
Date: 11/17/04


Date: Wed, 17 Nov 2004 15:42:52 GMT


> I don't think that this is the problem because when I use my FTP
> client, I get download speeds of >300 kB/s. Also, I used to (last
> week) get these speeds using my browser as well (for the entire
> download). Any other ideas?

There are a lot of things that can affect tcp speeds from one week to the
next. With tcp it dynamically slows transmissions down according to the
"apparent" speed across the entire connection. IIRC (and it's been a while,
someone feel free to correct me) there are basically two ways:

1. A router along the way hits a bottleneck (it has more data to send
across a link than that link can handle at that moment), it sends back icmp
packets to the sources of the data going over that link saying "slow down".
Usually those sources will then slow the rate down and will not speed back
up for the existing connections.

2. The transmitting end waits for tcp confirmation packets that say "the
remote end got packet X". After so many packets have been sent without a
confirmation it can stop transmission awaiting those confirmation packets
(which will suddenly look like a huge drop in speed) or it can begin to
retransmit some of the "stale" packets, which again appears to be a drop in
speed because the other end probably already has those packets.

Since you are noticing that your ftp doesn't notice this but your browser
does I'd say it's the responsiveness of the browser to the transmission. If
it doesn't respond fast enough in the beginning the transmitting end decides
to slow down and doesn't speed back up.