[sr-dev] new feature: option to remove contacts based on sip nat keepalives

[Daniel-Constantin Mierla]

Hello,

over the weekend I committed the code that allows to remove contacts 
from location table if the device does not respond to several attempts 
of SIP nat keepalives (usually OPTIONS requests).

The feature works only for SIP over UDP, being the only transport for 
which nathelper module sends keepalives (or sip ping requests as written 
in some docs).

Also, the lack of keepalive reply is tracked only for location records 
stored in memory (so it does not work with db only mode for usrloc). 
This is because the keepalives are sent stateless, not using tm module, 
to avoid overloading of timers and shared memory when dealing with large 
subscriber base -- therefore the implementation is very lightweight in 
using resources, targeting of course scalability. Detection of replies 
for keepalives is done based on from tag which carries the internal 
unique id of the location contact (which is available from v3.3).

In order to enable this feature you have to set the keepalive_timeout 
parameter of nathelper module -- that's the only addition you have to 
add to a config that sends SIP nat keepalives. Internally, the modules 
store the timestamp of the last SIP keepalive reply (nathelper detects 
the right reply and calls usrloc API for storing the timestamp in 
contact structure in memory).

http://kamailio.org/docs/modules/devel/modules_k/nathelper.html#id2535244

Before sending a new keepalive to a contact, it is checking if the last 
keepalive reply is older than keepalive_timeout value and if yes, will 
lower the expires to 10 seconds (if the expires is higher). There is no 
special handling of keepalive reply codes, all of them will update the 
last keepalive timestamp.

This feature should be useful in mobile networks, to avoid stacking lot 
of invalid contacts which could result in lot of branches causing many 
retransmissions.

There is plant to go for tcp/tls as well and detect when the connection 
is down, but looks like no spare time in the near future. However, right 
now using set_forward_no_connect() will avoid trying to create new 
connections for natted uses whose registration connection is down.

Testing and feedback will be welcome.

Cheers,
Daniel

-- 
Daniel-Constantin Mierla - http://www.asipto.com
http://twitter.com/#!/miconda - http://www.linkedin.com/in/miconda
Kamailio Advanced Training, Seattle, USA, Sep 23-26, 2012 - http://asipto.com/u/katu
Kamailio Practical Workshop, Netherlands, Sep 10-12, 2012 - http://asipto.com/u/kpw