Summary:ASTERISK-13736: Never reply 503, use 500 instead (don't break DNS SRV failover)
Reporter:Iñaki Baz Castillo (ibc)Labels:
Date Opened:2009-03-12 06:18:22Date Closed:2011-06-07 14:03:17
Versions:Frequency of
Description:As reported in bug ASTERISK-13728, Asterisk shouldn't reply 503 to the client since a 503 forces a client to contact an alternate server when it's not needed at all (Asterisk is still alive and working).

RFC 3261:
21.5.4 503 Service Unavailable
   A client (proxy or UAC) receiving a 503 (Service Unavailable) SHOULD
   attempt to forward the request to an alternate server. It SHOULD NOT
   forward any other requests to that server for the duration specified
   in the Retry-After header field, if present.

As mmichelson tells in ASTERISK-13728:

 "It appears there are several response codes which Asterisk translates into
  an AST_CONTROL_CONGESTION frame and then queues onto the channel. When the
  other side reads this AST_CONTROL_CONGESTION frame, if it is a SIP channel,
  it will send a 503."

I propose to replace ALL the 503 replies in chan_sip.c to 500. Asterisk should *never* reply a 503 except in the case Asterisk *does* know it is not working properly, and AFAIK there is no way in Asterisk to know it.
Replying 503 breaks SIP DNS SRV failover (RFC 3263) so 500 would be a more appropiate response code.

For example, OpenSer/Kamailio/OpenSIPS proxy converts a 503 from downstream into a 500 upstream. If a proxy/PBX replies 503 to a client it is the same as saying "don't use me, try other server".

Asterisk should reply 503 in some cases, for example after a Dial through a E1/T1 trunk with no avaliable channels. Then, the caller would receive the 503 and try other server. In this case a 503 is correct (so probably an agressive "sed 's/503/500/g'" in all chan_sip.c would break it).

Is it feasible? I think Asterisk shouldn't convert so many response codes into AST_CONTROL_CONGESTION, but use something as AST_INTERNAL_ERROR which is translated into "SIP 500" while AST_CONTROL_CONGESTION is translated into "SIP 503" (as now).
Comments:By: Iñaki Baz Castillo (ibc) 2009-03-12 06:57:57

Other correct case in which Asterisk should reply 503 in when exceeding the maximun number of channels defined in asterisk.conf ("maxcalls" option).

By: Mark Michelson (mmichelson) 2009-03-13 15:50:22

I can think of a couple of other cases where a 503 would be appropriate:

1. If there is some sort of internal failure while processing a message (such as a memory allocation failure or failure to masquerade channels) then a 503 seems appropriate.

2. If Asterisk is in the process of shutting down, sending a 503 would be appropriate.

By: Iñaki Baz Castillo (ibc) 2009-03-16 03:51:07

Yes, these are two good cases to reply 503.

By: David Vossel (dvossel) 2009-08-25 11:28:10

I understand the issue, and I understand where your concern is coming from, but changing all 503 errors 500 errors in chan_sip is not a good idea... If it was only SIP that we were concerned about this might be different, but its not.  SIP talks to ISDN and other channels, and in many cases a congestion frame should be translated into a 503 error and sent out.  There is a dialplan solution for this however.  If you understand your setup enough to know a 503 error should not be forwarded, changing the hangup cause in the dialplan, ${HANGUPCAUSE}, from AST_CAUSE_CONGESTION or AST_CAUSE_SWITCH_CONGESTION to AST_CAUSE_FAILURE will convert the 503 to a 500 error.