How accurate is TIMEOUT(absolute)?

Asterisk Asterisk Dialplan
1

I use asterisk 1:15.2.1~dfsg-1+deb10u2 from the debian buster distribution.
I run the chan_sip.so module.

in my dialplan I have this:

exten => 7001,1,Set(TIMEOUT(absolute)=10)
exten => 7001,n,Answer()
exten => 7001,n,Set(i=1)
exten => 7001,n,While(True)
exten => 7001,n,SayNumber(${i})
exten => 7001,n,Wait(1)
exten => 7001,n,Set(i=$[${i} + 1])
exten => 7001,n,EndWhile()
exten => 7001,n,Hangup()

and it works. But …

how accurate is TIMEOUT(absolute)=10 ?
I have seen I can also configure TIMEOUT(absolute)=9.995 Should it make a difference ?

The source e.g: funcs/func_timeout.c that uses function ast_tvdiff_ms to set ast_channel_whentohangup lets me assume it is millisecond accurate. Is this true ?

Can I then expect millisecond accuracy ?

Does the TIMEOUT use the time between the “SIP OK” after “SIP INVITE” and the SIP BYE ?

In my tcpdump pcap files I see times between 10.006 sec to 10.025 sec if I check “SIP OK” to “SIP BYE”.

Is there a better way to make sure a call is exactly 10sec long (no millisec longer) ?

2

Things take time to process both on your own system, in Asterisk, across the network, and anything remote. It’s not possible to have it be exactly 10 seconds.

3

Measured where? Speed of light considerations apply, for a start, but there will be much larger network propagation delays, and you may lose packets.

4

For me it would be enough if I could see that my asterisk machine would send the “BYE” exactly 10sec after the “SIP OK”. So network does not matter for this local tcpdump.

5

It’s really still not possible.

6

thank you for answer. I will only measure locally. You are right I can lose packets. But I will only do a local tcpdump. I only want to see that my asterisk after it gets the “SIP OK” sends a BYE exact after 10sec.

7

So does it make a difference in the internal processing of asterisk if I configure one of the 3 lines:
TIMEOUT(absolute)=9.995
TIMEOUT(absolute)=10
TIMEOUT(absolute)=10.005 ?

Or does asterisk do some rounding ?

8

I can’t think why anyone would care enough to check the code. Also, the machine architecture will affect the resolution, as I think will some OS configuration options.

I rather suspect the OS only guarantees at least a specified delay.

9

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