[PJSUA][ACC] Service-Route header not cleared about pjproject HOT 5 CLOSED

@TheQue42 , your explanation makes sense.

Based on the above description, I have tried to create a PR to fix the issue in #3823.

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So I've been running some more tests; the issues are:

• Service-Route headers previously added are not removed if a subsequent response doesn't contain Service-Route headers. That is because of the check here:

  pjproject/pjsip/src/pjsua-lib/pjsua_acc.c

  Line 2057 in ca0c440

  if (uri_cnt == 0)

  
  I believe this check can be removed altogether?

• The other issue is: update_service_route() is only called if the request returns with a 2XX code, which means the Service-Route headers aren't cleared in case of an error response either; see:
  

  pjproject/pjsip/src/pjsua-lib/pjsua_acc.c

  Line 2421 in ca0c440

  } else if (PJSIP_IS_STATUS_IN_CLASS(param->code, 200)) {

Also, thanks for your explanation @TheQue42
In my case the service provider seems to use the header to direct traffic towards certain proxies, but only under certain conditions which are out of my control.

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According to the RFC (https://datatracker.ietf.org/doc/html/rfc3608.html#section-6.1), you are correct:

If
   the UA refreshes the registration, the stored value of the Service-
   Route is updated according to the Service-Route header field of the
   latest 200 class response.  If there is no Service-Route header field
   in the response, the UA clears any service route for that address-
   of-record previously stored by the UA.  If the re-registration
   request is refused or if an existing registration expires and the UA
   chooses not to re-register, the UA SHOULD discard any stored service
   route for that address-of-record.

And we already did more or less the above, with the exception that we don't clear the outbound proxies:
https://github.com/pjsip/pjproject/blob/master/pjsip/src/pjsua-lib/pjsua_acc.c#L1993

• /* Find and parse Service-Route headers */
• /* Update account's route set
• /* First remove all routes which are not the outbound proxies */
• /* Then append the Service-Route URIs */

Not sure if we should remove the outbound proxies here.

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IMHO, since the Service-Route and outbound proxy config are two separate functions, I don't see why you would want to clear the outbound routes. At least since the original purpose of the service-route was to find the S-CSCF in an IMS network, and that shouldn't mean that you should all-of-sudden disregard the configuration on how to get "out of the local network", (since in IMS you must pass through the P-CSCF).

In my mind, if a "roaming" client received a "SIP default route" from a local/roaming network via DHCP, then that is the local network saying "you have to use this to get out of this network", and then having the home network(which shouldnt be able to know to much about a roaming network) override this with a service-route, seems counter-intuitive.

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Thank you @andreas-wehrmann . I have updated the PR based on your findings.

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