How to disallow target refresh (re-INVITE or UPDATE)?

[juha-h]

I haven't found an API function that I could use to disallow target refresh (re-INVITE or UPDATE) and response, for example, with 403 Forbidden. And when I look the possible values of sdp_neg_state, I didn't find a suitable value for it:

/** SDP Negotiation state */
enum sdp_neg_state {
	SDP_NEG_NONE = 0,
	SDP_NEG_LOCAL_OFFER,		/**< SDP offer sent */
	SDP_NEG_REMOTE_OFFER,		/**< SDP offer received */
	SDP_NEG_PREVIEW_ANSWER,		/**< SDP preview answer sent */
	SDP_NEG_DONE			/**< SDP negotiation done */
};

According to call_sdp_change_allowed function negotiation is allowed if this is true:

(call->state == CALL_STATE_ESTABLISHED
		&& sdp_state == SDP_NEG_DONE)
		|| (sdp_state == SDP_NEG_NONE
		|| sdp_state == SDP_NEG_REMOTE_OFFER);

If denying of negotiation is not currently possible, what would be the best way to implement it?

[juha-h]

I haven't got any comments about this. According to https://www.rfc-editor.org/rfc/rfc6141 it is perfectly OK to reject re-INVITE. See, for example, section 3.1, Figure 1.

Is it so that baresip does not currently support rejection of re-INVITE? If it is, could someone give a hint on how to do it?

[juha-h]

It might be possible to reject re-INVITE when handling BEVENT_CALL_REMOTE_SDP by calling

int sipsess_reject(struct sipsess *sess, uint16_t scode, const char *reason,
		   const char *fmt, ...)

but I have not figured out how to access call->sess using baresip API.

[cspiel1]

Let's look in re/src/sipsess/modify.c first! I'll have a look.

[cspiel1]

This is the wrong direction. modify.c handles sending re-INVITE.
We have to look in re/src/sipsess/listen.c. There are already some checks and SIP replies with 481, 500, 491, ... status codes. Also 488 "Not Acceptable Here" and another 488 in line listen.c:283 in the error handling of the offerh().

[cspiel1]

The offerh is sipsess_offer_handler() in call.c. If the re-INVITE contains an SDP, then update_media() will be called, where the BEVENT_CALL_REMOTE_SDP is called. So you're suggestion to handle this is a good idea. And you are right, that currently there is no good option to return here an error code. Finally the sipsess_offer_handler() should return an error. This needs a change in the core, in update_media().

Maybe also BEVENT_CALL_LOCAL_SDP should have an option to return an error. If there is no SDP, this will be the only event.

[juha-h]

Yes, there is target_refresh_handler calling sipsess_refresh_allowed, but I have not find way via the app to disallow sipsess_refresh.

[cspiel1]

Here is not the right location. See where the sess->offerh() is called!

[juha-h]

Another good option would be if I could reject the re-INVITE by calling sip_treply() in my app's BEVENT_CALL_REMOTE_SDP handler followed by bevent_stop().

[cspiel1]

Not sure if we should use a low level API here. Better is to let the sipsess_offer_handler() return an error. Then it runs into this error path, in listen.c:283

		err = sess->offerh(&desc, msg, sess->arg);
		if (err) {
			(void)sip_reply(sip, msg, 488,
					str_error(err, m, sizeof(m)));
			sess->neg_state = SDP_NEG_DONE;
			return;
		}

[cspiel1]

You could try:

--- a/src/call.c
+++ b/src/call.c
@@ -313,8 +313,10 @@ static int update_media(struct call *call)
 {
        debug("call: update media\n");
 
-       bevent_call_emit(BEVENT_CALL_REMOTE_SDP, call,
-                        call->got_offer ? "offer" : "answer");
+       int err = bevent_call_emit(BEVENT_CALL_REMOTE_SDP, call,
+                                  call->got_offer ? "offer" : "answer");
+       if (err)
+               return err;
 
        return call_update_media(call);
 }

The bevent API has an error setter.

[juha-h]

Do you mean that I should change this

static void event_handler(enum bevent_ev ev, struct bevent *event, void *arg)

to

static int event_handler(enum bevent_ev ev, struct bevent *event, void *arg)

and return some error code to prevent call_update_media(call)?

[cspiel1]

No, only the patch above. Because the emit function has the return value. If you want to know how it works than you could look in the code of bevent_call_emit(). So try

• apply the patch for call.c and
• use function bevent_set_error() in your application

[juha-h]

Thanks for the advice. I called bevent_set_error(event, 488) when ua event (CALL_REMOTE_SDP) offer arrived and my app sent this response to the re-INVITE:

TCP 10.207.11.215:5060 -> 10.207.11.215:46816
SIP/2.0 488 Unknown error 488 [488]
Via: SIP/2.0/TCP 10.207.11.215:46816;received=10.207.11.215;branch=z9hG4bK2a88fa7f0c3d4130;rport=46816
To: "Foo" <sip:foo@test.tutpro.com>;tag=908826950b9b8207
From: <sip:test@test.tutpro.com>;tag=cbbaa2ca44fe47a5
Call-ID: 7926667f09943486
CSeq: 48137 INVITE
Server: baresip v78.0.2 (Android 16/aarch64)
Content-Length: 0

Otherwise OK, but first line of the response could be improved:

SIP/2.0 488 Not Acceptable Here

Do I need to call bevent_stop after bevent_set_error(event, 488)?

[cspiel1]

Okay, not so bad. Currently only linux error codes plus the error codes from re_types.h are supported. The texts come from str_error() from re, which fall back to strerror_r() on linux (and Android?). SIP status codes are not supported.

In the following man page you find some of the error nums + their strings.

man 3 errno

[juha-h]

Fine, I can use one of the man 3 errno codes.

[cspiel1]

Do you want to create a PR for the call.c changes? Or should I?

[juha-h]

Christian Spielberger writes:

Do you want to create a PR for the call.c changes? Or should I?

If you have time, please create the PR.

[cspiel1]

Here is the PR: #3702