Slow SCP through network

@stw:

With that, I ran a couple of traceroutes, and I'm getting different routes every time:

/srv# traceroute node.in.europe
traceroute to node.in.europe (87.230.ooo.ooo), 30 hops max, 60 byte packets
 1  a1.7.1243.static.theplanet.com (67.18.7.161)  0.551 ms  0.658 ms  0.645 ms
 2  xe-2-0-0.car03.dllstx2.networklayer.com (67.18.7.89)  0.178 ms  0.206 ms  0.191 ms
 3  po101.dsr02.dllstx2.networklayer.com (70.87.254.77)  0.582 ms  0.661 ms  0.611 ms
 4  te4-3.dsr02.dllstx3.networklayer.com (70.87.255.129)  0.768 ms  0.760 ms te3-2.dsr02.dllstx3.networklayer.com (70.87.253.133)  0.812 ms
 5  ae17.bbr02.eq01.dal03.networklayer.com (173.192.18.230)  50.695 ms  50.724 ms ae17.bbr01.eq01.dal03.networklayer.com (173.192.18.226)  0.477 ms
 6  dls-bb1-link.telia.net (213.248.102.173)  0.490 ms  0.548 ms  0.534 ms
 7  ash-bb1-link.telia.net (213.155.133.178)  60.992 ms  60.413 ms ae2-20G.scr2.DAL1.gblx.net (67.16.141.237)  5.790 ms
 8  ldn-bb1-link.telia.net (80.91.246.69)  109.003 ms * po6.ar4.AMS2.gblx.net (67.17.107.174)  124.616 ms
 9  ldn-b5-link.telia.net (80.91.248.216)  109.141 ms  109.125 ms *
10  * * *
11  xe-0-0-1.dr-master.r1.cgn3.hosteurope.de (176.28.4.14)  130.149 ms xe-0-2-0.cr-merak.fra2.hosteurope.de (176.28.4.2)  123.437 ms xe-0-0-1.dr-master.r1.cgn3.hosteurope.de (176.28.4.14)  128.728 ms
12  xe-2-2-0.cr-pollux.cgn3.hosteurope.de (80.237.129.169)  128.345 ms  128.334 ms  128.287 ms

I can repeat the traceroute, and it'll be different hosts everytime, however the dropouts are usually close to the Germany node, in the telia.net network.

Looks like the varying routes start already in the networklayer.com network, as the traffic sometimes seem to go out through telia.net and sometimes through gblx.net? (Based on the varying hosts at the same hopcount in the trace above.)

Speculation follows:

Seems like it's networklayer.com that for whatever reason switch back and forth between these two… Which quite possibly is because the route to whichever one they prefer is flapping or something to that regard.

It's unclear if your problem related to which path is used, ie if one is actually notably better than the other, or if the problem is the actual switching back and forth.

@stw:

I don't want to bug Linode's staff too much with support tickets
And yet Linode's Accounting Dept bugs me every month wanting payment.

You pay for service - don't be afraid to use it. Worse that can happen is they say it's not a hardware/infrastructure problem so you're on your own.

I would lean towards a network thing, as well. scp's progress indicator is based on when the network stack accepts the data, not necessarily when it is actually delivered. (Try scping a file from home to somewhere else… it sits on 100% like its a Windows OS installation.) This smells a lot like inconvenient packet loss.

Try laying down a ping while doing the scp, or maybe even mtr. Whatever is causing the routing to change is probably also dropping packets for a few seconds.

First of all, thanks everyone for your insights.

@hawk7000:

Looks like the varying routes start already in the networklayer.com network, as the traffic sometimes seem to go out through telia.net and sometimes through gblx.net? (Based on the varying hosts at the same hopcount in the trace above.)

That's true - I noticed some of the replies came from different hosts, but I didn't see a pattern in there. I agree with you, networklayer.com is probably switching routes between telia.net and gblx.net - which makes it hard to tell what network (telia.net or gblx.net) makes SCP take this long - or whether it's the switching altogether that causes it.

It still puzzles me that sometimes SCP finishes within seconds, and sometimes only after minutes have passed - yet the routes change within a traceroute. I'd assume that at some point, the speed of SCP would pick up if the route changes, or at least that it remains consistently low if route flapping itself is the issue - but instead the speed is either consistently slow, or consistently fast during a transfer.

@hoopycat:

This smells a lot like inconvenient packet loss.

Try laying down a ping while doing the scp, or maybe even mtr. Whatever is causing the routing to change is probably also dropping packets for a few seconds.

That's a good idea - it's funny how I use traceroute and all, and then forget using one of the most basic tools. I guess I neglected that because I assumed that commercial connections would not possibly have packet loss, and that it'd be a problem constrained to poor ADSL lines.

Anyway, you were correct; I ran a ping during the scp, and it would have a packet loss of around 20%.

For a 25 second SCP, I got

--- node.in.europe ping statistics ---
26 packets transmitted, 20 received, 23% packet loss, time 25026ms
rtt min/avg/max/mdev = 127.911/129.334/131.692/0.947 ms

and for a 2 minute SCP I got

--- node.in.europe ping statistics ---
140 packets transmitted, 114 received, 18% packet loss, time 139113ms
rtt min/avg/max/mdev = 126.246/128.959/133.576/1.009 ms

None of the replies were delivered out of order. During the "fast" scp (8 seconds) I had no packet loss. I tries to run SCP over a different port (1235) in an attempt to see whether port 1234 would be throttled, but I get the same figures.

So, thanks to you I realized that the problem is (a pretty significant?) packet loss, even though I am not sure, and probably can't find out, whether it's congestion caused or caused by the route switching. With that, is there anything I can do? Is this something Linode (or the German hoster) have any influence over? As in, could (and would) Linode choose a different route, or is it out of their hands anyway (in which case I wouldn't bother asking), because it's no longer in their network?

Telia.net is in Stockholm, networklayer.com's whois information is proxied (Domains By Proxy), which strikes me a bit as odd, seeing hiding contact information is something I would only expect individuals to do. Still, who would have more influence over the route - Linode or the provider in Germany?

Oh wow. TCP performance tends to decay pretty hard beyond about 5% packet loss, so the fact that it is working at all ought to be a pleasant surprise. Keep in mind that a lot of this is hidden from you because of a very large transmit buffer (see the Send-Q column on netstat -nt)… what looks smooth and constant from scp's perspective is very likely fits-and-starts under the hood. (scp really crams a lot into the sendq.)

This is probably worth tickets from both ends. In general, contact the party/parties with whom you have a business relationship. Neither Softlayer (theplanet.com, networklayer.com) nor Global Crossing nor Telia will deal with you directly, so start from the ends. (This is also handy because, in all but the most trivial cases, the return path will be totally different than the forward path, and packet loss could occur on either with similar effect. Did the packet get lost on the way there, or did the acknowledgement get lost on the way here?)

@hoopycat:

… (This is also handy because, in all but the most trivial cases, the return path will be totally different than the forward path, and packet loss could occur on either with similar effect. Did the packet get lost on the way there, or did the acknowledgement get lost on the way here?)
…Use 2ping and find out!

@hoopycat:

(scp really crams a lot into the sendq.)
If you use the -l (lower-case L) option, it seems to prevent scp from doing that. I've found it to be useful for getting more honest status reports from scp when transferring small files (which scp would normally just report 100% completion on immediately, as it's dumped the entire file into the send queue).

Hi Folks, I have a found a solution for this, but when using the windows app of WinSCP. Still, maybe it will give you a direction for the Linux scp command (although I did not find there, or for the ssh command, any matching attribute to tweak):

I found this - https://winscp.net/forum/viewtopic.php?t=25705, meaning the issue is at the SCP/SSH level.
So I disabled "Connection -> Optimize connection buffer size" in the WinSCP GUI location of Site Manager > Select the needed site > Edit > Advanced > Connection pane".

This changed my download speed (from the Linode server to my PC) to reach 50 Mbps as max speed but the average was about 35-40, while without this change it was about 12-13 Mbps.
https://winscp.net/eng/docs/ui_login_connection

FYI.