TCP connection hangs on Debian

Question

Hi,

Some tcp connections hang sometimes and I can't find the problem. When Maven is downloading libraries from maven repo, sometimes it blocks in one of the downloads, making the entire build hang. It is difficult to reproduce as it only happens rarely, but I was able to capture it on wireshark (with tshark).

This is the end of the build output:

[INFO] Unable to find resource 'org.apache.maven.doxia:doxia-module-xhtml:jar:1.0' in repository unicore.eu (http://unicore-dev.zam.kfa-juelich.de/maven)
Downloading: http://unicore-dev.zam.kfa-juelich.de/maven/org/apache/maven/doxia/doxia-module-xhtml/1.0/doxia-module-xhtml-1.0.jar
28K downloaded  (maven-invoker-2.0.11.jar)
[INFO] Unable to find resource 'org.apache.maven.doxia:doxia-module-xhtml:jar:1.0' in repository unicore.eu (http://unicore-dev.zam.kfa-juelich.de/maven)
Downloading: http://repo1.maven.org/maven2/org/apache/maven/doxia/doxia-module-xhtml/1.0/doxia-module-xhtml-1.0.jar
21K downloaded  (doxia-module-xhtml-1.0.jar)

It hangs here. It is in this state for some days without progress. With ss, I can see the problematic connection:

ESTAB   0    0    ::ffff:128.142.136.87:57230 ::ffff:68.232.34.223:www  
cubic wscale:0,7 rto:216 rtt:16/8 ato:40 cwnd:10 send 7.3Mbps rcv_rtt:16 rcv_space:37960

Now the wireshark output. I stopped capturing after a while - I know I should have continued. https://www.cloudshark.org/captures/36aa46f0f901

Thanks for any hints!

Answer 1

From a first glance at the tracefile, it looks like host repo1.maven.org is overloaded. Look at the time gaps between frames 94 and 95 and also between frames 114 and 115. Your client has acknowledged all data, still the server takes a long time to send new data. What is keeping it?

It might be on purpose to throttle bandwidth, although I would have expected a more granular control in that case. Or maybe there is a proxy in between that downloads chunks and then quickly passes the data down to you?