It seems that there is a per TCP connection speed limit on Cable. I have a box at Linode in London and when transferring files via SSH the rate is slow ~200kB/s. First I thought it was just a slow route, but if I start two SSH session, running in side by side, I can have two parallel downloads each running at the ~200kB/s speed, so they are summing to something nearer ~400kB/s. Why can't I get ~400kB/s in the one connection when I can pull that cumulatively over two separate SSH sessions?
I can demo this as Linode has a speed test page that give you a file download https://www.linode.com/speedtest/
Using wget I can get http://speedtest.london.linode.com/100MB-london.bin in 9m 13s the file is 104857600B so average is 185kB/s
Then using wget in two separate terminals in parallel (I added ?1 and ?2 to the end of each URL so it didn't come from the transparent proxy) and I got 9m 44s for 175kB/s and 9m 45s for 175kB/s. If it was a bandwidth constrained connection I would have expected something like 19m 30s to complete the 200MB, but that is not the case.
Trying some of the other Linode speed test files I get the same results for Japan and Fremont. The speed is different but there seems to be a per connection limit and running in parallel doesn't seem to affect other download.
This isn't an issue at the Linode end as I pull the same file down to a box in the US 1m 40s 1MB/s so the Linode end can spit the content out fast enough on its speed test files.
So is there a per connection limit? Is there anyway to work around this so I can just have the one connection operating at full speed?
Traceroute for completeness
<snip>:~$ traceroute speedtest.london.linode.com
traceroute to speedtest.london.linode.com (22.214.171.124), 30 hops max, 60 byte packets
1 <snip> 0.591 ms 0.579 ms 0.725 ms
2 * * *
3 lo0.internet.ivpn.pe25.telstraclear.net (126.96.36.199) 67.656 ms 67.729 ms 67.711 ms
4 ie2-g-0-0-0.telstraclear.net (188.8.131.52) 68.069 ms 68.070 ms 68.066 ms
5 * * *
6 unknown.telstraglobal.net (184.108.40.206) 68.177 ms 65.703 ms 65.807 ms
7 i-0-0-4-0.tlot-core01.bx.telstraglobal.net (220.127.116.11) 156.907 ms 242.681 ms 243.022 ms
8 i-0-0-0-3.eqla01.bi.telstraglobal.net (18.104.22.168) 242.474 ms 241.824 ms 241.969 ms
9 63-218-51-149.static.pccwglobal.net (22.214.171.124) 283.869 ms 283.768 ms 283.598 ms
10 telecity.ge9-9.br02.ldn01.pccwbtn.net (126.96.36.199) 438.956 ms 394.823 ms 422.172 ms
11 188.8.131.52 (184.108.40.206) 422.297 ms 422.202 ms 422.249 ms
12 speedtest.london.linode.com (220.127.116.11) 421.653 ms 411.505 ms 411.487 ms