[asterisk-ss7] Link inactivity detection with dahdi based MTP2

[Marcelo Pacheco]

Hi Matt and asterisk-ss7 members,

Looking at the dahdi-linux code, there seems to be an important feature
lacking on the dahdi mtp2 feature:

Should the mtp channel go down, without the E1/T1 span go down (no
alarms), there's no notification sent to the application (usually asterisk).


I suggest an inactivity timer be implemented on dahdi, so if no new mtp2
messages get received over a 100ms period, a HDLC Abort message (or any
other specific message) gets send to userland. This way should libss7
receive say, 2 messages in close proximity it will consider the link
dead and initiate realignment.

Additionally should this error condition be raised to userland, upon
receiving a valid repeated message, it would be sent to userland to
indicate things are back to normal.

Without this, technically libss7 isn't compliant with a few of Q.7xx
specs. I've seen real world situations where lack of this feature would
cause serious problems.

Specially this would be a serious issue with E1 links between Asterisk
and a TDM switch, where the signalling link is switched to a STP using a
semi permanent call. Should the STP or the link between the TDM switch
and the STP die, the only available indication would be that the
signalling link would go dead (with perhaps one HDLC Abort or CRC error
being detected, should the link go down preciselly between FISUs, then
no error at all might be detected). This is very common in Brazil, the
largest carriers use this layout exclusively on interconnects.


Matt, what do you think ? Do you have a better suggestion than an Abort
message ?

Regards,

Marcelo Pacheco