[asterisk-users] Timeout in IAX vs SIP

[Yuan LIU]

When Asterisk dials an IAX destination with no registration, it very quickly 
comes to the conclusion that it can't make the call
    -- Executing [500 at default:2] Dial("Zap/1-1", 
"IAX2/guest at misery.digium.com/s at default") in new stack
    -- Called guest at misery.digium.com/s at default
[Jan 29 21:43:15] NOTICE[1957]: chan_iax2.c:2686 __auto_congest: 
Auto-congesting call due to slow response
    -- IAX2/216.207.245.8:4569-1 is circuit-busy
    -- Hungup 'IAX2/216.207.245.8:4569-1'
  == Everyone is busy/congested at this time (1:0/1/0)
But if Asterisk Dials a SIP destination it doesn't have a registration, it 
waits for a very long time before giving up.

What is the difference?  Does IAX use TCP instead of UDP?  Is there some way 
to change timeout value in SIP attempt so it gives up in a reasonable time?

Yuan Liu