TT#90101 don't blindly accept a protocol switch in an answer

An endpoint switching protocols is normally passed through to the peer,
but in an answer that is usually not desirable. Change the default
behaviour to stick to the original protocol that was offered even if the
answerer changes protocols.

Change-Id: Ib288549f4b1c9ab57a6333c6b7dd511537af96f7
(cherry picked from commit 12ac8846de)
(cherry picked from commit 35d9a52c1b)
(cherry picked from commit 4d9856bf90)

README.md

@@ -829,6 +829,14 @@ Optionally included keys are:
 
 	Valid values are: `RTP/AVP`, `RTP/AVPF`, `RTP/SAVP`, `RTP/SAVPF`.
 
+	Additionally the string `accept` can be given in `answer` messages to allow a special case: By
+	default (when no `transport-protocol` override is given) in answer messages, *rtpengine* will
+	use the transport protocol that was originally offered. However, an answering client may answer
+	with a different protocol than what was offered (e.g. offer was for `RTP/AVP` and answer comes
+	with `RTP/AVPF`). The default behaviour for *rtpengine* is to ignore this protocol change and
+	still proceed with the protocol that was originally offered. Using the `accept` option here
+	tells *rtpengine* to go along with this protocol change and pass it to the original offerer.
+
 * `media address`
 
 	This can be used to override both the addresses present in the SDP body

daemon/call.c

@@ -1878,14 +1878,30 @@ static void __update_media_protocol(struct call_media *media, struct call_media
 	/* deduct protocol from stream parameters received */
 	if (other_media->protocol != sp->protocol) {
 		other_media->protocol = sp->protocol;
-		/* if the endpoint changes the protocol, we reset the other side's
+		/* If the endpoint changes the protocol, we reset the other side's
 		 * protocol as well. this lets us remember our previous overrides,
 		 * but also lets endpoints re-negotiate.
-		 * Exception: OSRTP answer/accept. */
-		if (flags && flags->opmode == OP_ANSWER && other_media->protocol && other_media->protocol->rtp
-				&& !other_media->protocol->srtp
-				&& media->protocol && media->protocol->osrtp && flags->osrtp_accept)
-			;
+		 * Answers are a special case: handle OSRTP answer/accept, but otherwise
+		 * answer with the same protocol that was offered, unless we're instructed
+		 * not to. */
+		if (flags && flags->opmode == OP_ANSWER) {
+			// OSRTP?
+			if (other_media->protocol && other_media->protocol->rtp
+					&& !other_media->protocol->srtp
+					&& media->protocol && media->protocol->osrtp)
+			{
+				// accept it?
+				if (flags->osrtp_accept)
+					;
+				else
+					media->protocol = NULL; // reject
+			}
+			// pass through any other protocol change?
+			else if (!flags->protocol_accept)
+				;
+			else
+				media->protocol = NULL;
+		}
 		else
 			media->protocol = NULL;
 	}

daemon/call_interfaces.c

@@ -931,8 +931,13 @@ static void call_ng_process_flags(struct sdp_ng_flags *out, bencode_item_t *inpu
 	call_ng_flags_list(out, input, "T.38", ng_t38_option, NULL);
 #endif
 
-	bencode_get_alt(input, "transport-protocol", "transport protocol", &out->transport_protocol_str);
-	out->transport_protocol = transport_protocol(&out->transport_protocol_str);
+	str transport_protocol_str;
+	bencode_get_alt(input, "transport-protocol", "transport protocol", &transport_protocol_str);
+	if (!str_cmp(&transport_protocol_str, "accept"))
+		out->protocol_accept = 1;
+	else
+		out->transport_protocol = transport_protocol(&transport_protocol_str);
+
 	bencode_get_alt(input, "media-address", "media address", &out->media_address);
 	if (bencode_get_alt(input, "address-family", "address family", &out->address_family_str))
 		out->address_family = get_socket_family_rfc(&out->address_family_str);

include/call_interfaces.h

@@ -25,7 +25,6 @@ struct sdp_ng_flags {
 	str received_from_family;
 	str received_from_address;
 	str media_address;
-	str transport_protocol_str;
 	str address_family_str;
 	const struct transport_protocol *transport_protocol;
 	sockaddr_t parsed_received_from;
@@ -47,6 +46,7 @@ struct sdp_ng_flags {
 	    rev_ptime;
 	GHashTable *sdes_no;
 	int asymmetric:1,
+	    protocol_accept:1,
 	    no_redis_update:1,
 	    unidirectional:1,
 	    trust_address:1,