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0 pointschetanahuja11y ago0 comments

Completely agree with your righteous rage about TCP over bad connections. I allowed myself a bitter chuckle to the GP's "networks are generally pretty reliable" thing.

Minor disagreement though:

"Edit: the only downside of UDP VPNs is that stateful firewalls can have extremely short timeouts of UDP "connections" (e.g. 30 seconds!)"

Plenty of mobile networks will timeout inactive TCP connections in less than 30 seconds. TCP keepalives are an absolute requirement on long running mobile connections.... e.g. Google maintained GCM connections on Android for notifications... a simple packet capture will show you the frequency of keepalives there... and it's almost always more frequent than per 30 seconds.

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3 comments · 2 top-level

zrm11y ago· 1 in thread

Timeouts that short violate RFCs. Established TCP connections can't be abandoned unless idle for two hours four minutes (RFC5382 REQ-5) and even UDP timeouts normally have to be at least two minutes (RFC4787 REQ-5).

If people are violating the RFCs then applications that detect it should probably start notifying users exactly why their battery life is suffering.

chias11y ago

I see a lot of "should"s. When push comes to shove, though, a notification is not going to change the behavior of the network.

There's a lot of complaints I have about how some networks (especially mobile ones) deviate from RFCs and break specifications. A 30 second timeout is not nearly at the top.

agwa11y ago

> Plenty of mobile networks will timeout inactive TCP connections in less than 30 seconds.

Good grief - that is awful, but sadly believable. Do you happen to know who does that? A couple years ago I tested AT&T's 3G and found that the TCP timeout was 30 minutes, versus 30 seconds for UDP. I'd love to know numbers for other carriers.

Edit: Found an interesting paper from 2011 that tested 73 cellular carriers worldwide and found only 4 with TCP timeouts less than 5 minutes. The majority had timeouts greater than 30 minutes, and 21 had a timeout in the 5-30 minute range. Some of my faith in humanity has been restored. http://www.cs.ucr.edu/~zhiyunq/pub/sigcomm11_netpiculet.pdf (see page 8, table 5)

Edit: A paper from 2012 which measured Verizon and Sprint at 30 minutes, and AT&T at 3 minutes (my tests are more recent, so perhaps AT&T wised up?) http://www.cs.umass.edu/~yungchih/publication/12_mtcp_4g_tec... (see page 11, table VI)

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