Fixed Service-Route update upon registration

[sauwming]

To fix #3790.

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

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.

There are a couple issues with the current implementation:

• We do not clear the service route if there is no Service-Route header found in the response.
• We do not clear the service route upon registration failure.

Thanks to @andreas-wehrmann for the report and investigation.

[sauwming]

Attached is the SIPP xml for testing the issue:
uas-register.xml.txt

Test flow:
Test 1:

1. REGISTER-200 OK with Service Route header
2. Confirm that outgoing call contains that header
3. Re-REGISTER-200 OK without any Service Route header
4. Confirm that outgoing call now contains no Route header

Test 2:

1. REGISTER-200 OK with Service Route header
2. Confirm that outgoing call contains that header
3. Re-REGISTER-failed
4. Confirm that outgoing call now contains no Route header

[andreas-wehrmann]

Though I haven't tested it, I believe there may be another issue with pjsua_acc_modify().
From the looks of it, it seems to assume that the route set consists always of the global outbound proxy list followed by the account specific proxy list (if any).

See here (global outbound proxies first):

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

Line 1171 in 4169142

pjsip_route_hdr *r = acc->route_set.next;

and here (account proxies always last):

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

Line 1191 in 4169142

pjsip_route_hdr *r = acc->route_set.prev;

It doesn't seem to account for any additional routes injected via Service-Route
in which case it may be best to simply discard any headers learned via Service-Route
when modifying an account before attempting to modify the route set?

[sauwming]

I believe we don't need to clear the learnt Service-Route headers during account modification, except in rare cases such as modifying the registrar perhaps?

[andreas-wehrmann]

update_service_route() appends the headers to the route set of an account.
So the route set is made up like this: global proxies + account proxies + service route.

My worry is, that the code here:

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

Line 1191 in 4169142

pjsip_route_hdr *r = acc->route_set.prev;

removes the service route headers (if there are any) when it actually wants to remove the proxies of the account.
Or am I not understanding this code correctly?

[sauwming]

You're right. I understand the problem now.

[andreas-wehrmann]

Ok, so the questions remains what to do when?

• Forget all Service-Routes on account modify unconditionally and let it relearn?
• Forget Service-Routes only when there is a change to the global/account proxy list?
• Forget Service-Routes only when there is change to either proxy list and/or the registrar (what about the Contact header and possibly other fields?)

In my naiveté I tend towards the first since it seems to be the option which is least error prone
but I am not sure...

[sauwming]

I'm currently leaning towards the third option (only change in either proxy list or the registrar).
And I will probably open a new PR since it's a different issue (albeit related to this one).