[asterisk-users] Asterisk freezes with Fixup failed on channel SIP/...<MASQ>

[Udo Schacht-Wiegand]

On a production system, running 1.4.17 (compiled from bristuff-0.4.0-test6-xr1) we had this strange issue two times in the last
weeks:

[2009-01-13 13:58:30] WARNING[1213] channel.c: Fixup failed on channel SIP/2332-081d0108<MASQ>, strange things may happen.
[2009-01-13 13:58:30] WARNING[1213] channel.c: Hangup failed!  Strange things may happen!
[2009-01-13 13:58:30] WARNING[1213] channel.c: Failed to perform masquerade
[2009-01-13 13:58:30] WARNING[1213] channel.c: Channel 'SIP/2332-081d0108' may not have been hung up properly

and:

[2009-01-23 14:27:17] WARNING[21528] channel.c: Fixup failed on channel SIP/2332-083c3778<MASQ>, strange things may happen.
[2009-01-23 14:27:17] WARNING[21528] channel.c: Hangup failed!  Strange things may happen!
[2009-01-23 14:27:17] WARNING[21528] channel.c: Failed to perform masquerade
[2009-01-23 14:27:17] WARNING[21528] channel.c: Channel 'SIP/2332-083c3778' may not have been hung up properly

Both times all SIP channels got stuck and the CLI became inresponsive. Calls continued for a while, but new SIP calls could not be
established.

On the second time this happended, all SIP phones could not subscribe to the Asterisk any longer and a few minutes later the log
filled with:

[2009-01-23 14:43:21] ERROR[22319] chan_sip.c: Call to peer '2333' rejected due to usage limit of 10

On the CLI one could see, that there were 100s of (rejected) calls to this SIP phones.

The phones that show up in the ERROR messages are in a group call made by a 
Dial(Local/...&Local.../&Local/...) construct. But other SIP phones were affected as well. It seemed like the whole chan_sip module
became stuck. I also could not "unload chan_sip.so", but can't remeber the exact error message it gave.

The only thing that was left was to restart Asterisk. 

Can someone give me some clue what the 'Fixup failed ...' and 'masquerade' warnings actually mean?

Any help appreciated.
Udo