--- channels/chan_sip.c	2013-03-27 19:51:29.000000000 +0000
+++ ../../asterisk-11.4.0-rc1/channels/chan_sip.c	2013-04-14 19:50:51.968832287 +0000
@@ -12531,6 +12531,11 @@
 	struct ao2_container *candidates;
 	struct ao2_iterator i;
 	struct ast_rtp_engine_ice_candidate *candidate;
+	int srflx_count = 0;
+	char port_buf[6];
+	char *rtp_port;
+	char *rtcp_port;
+	int i_port;
 
 	/* If no ICE support is present we can't very well add the attributes */
 	if (!ice || !(candidates = ice->get_local_candidates(instance))) {
@@ -12549,8 +12554,37 @@
 	while ((candidate = ao2_iterator_next(&i))) {
 		ast_str_append(a_buf, 0, "a=candidate:%s %d %s %d ", candidate->foundation, candidate->id, candidate->transport, candidate->priority);
 		ast_str_append(a_buf, 0, "%s ", ast_sockaddr_stringify_host(&candidate->address));
-		ast_str_append(a_buf, 0, "%s typ ", ast_sockaddr_stringify_port(&candidate->address));
+		/*
+		 * ASTERISK-21383 fix - as per RFC 5245 section 3:
+		 *
+		 * Default Destination/Candidate:  The default destination for a
+      		 *	component of a media stream is the transport address that would be
+      		 *	used by an agent that is not ICE aware.  For the RTP component,
+      		 *	the default IP address is in the c line of the SDP, and the port
+      		 *	is in the m line.  For the RTCP component, it is in the rtcp
+      		 *	attribute when present, and when not present, the IP address is in
+      		 *	the c line and 1 plus the port is in the m line.  A default
+      		 *	candidate for a component is one whose transport address matches
+      		 *	the default destination for that component.
+		 */
+		if(candidate->type == AST_RTP_ICE_CANDIDATE_TYPE_SRFLX) {
+			// don't increment the port +1 for the first srflx candidate, only the second one
+			if(srflx_count == 0) {
+				srflx_count++;
+				ast_str_append(a_buf, 0, "%s typ ", ast_sockaddr_stringify_port(&candidate->address));
 
+			} else {
+				ast_debug(3, "About to increment the port for the second srflx candidate by 1, as we assume rtcp attribute not present in SDP...\n");
+				rtp_port = ast_sockaddr_stringify_port(&candidate->address);
+				i_port = atoi(rtp_port);
+				sprintf(port_buf, "%d", ++i_port);
+				rtcp_port = port_buf;
+				ast_debug(1, "Incremented candidate port for the second srflx component-id (rtcp) by 1 and set to %s\n", rtcp_port);
+				ast_str_append(a_buf, 0, "%s typ ", rtcp_port);
+			}
+		} else {
+			ast_str_append(a_buf, 0, "%s typ ", ast_sockaddr_stringify_port(&candidate->address));
+		}
 		if (candidate->type == AST_RTP_ICE_CANDIDATE_TYPE_HOST) {
 			ast_str_append(a_buf, 0, "host");
 		} else if (candidate->type == AST_RTP_ICE_CANDIDATE_TYPE_SRFLX) {