Importing rev #8537 from 1.2, never send response to ACK (issue #6308)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@8538 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -11040,7 +11040,9 @@ static int handle_request(struct sip_pvt *p, struct sip_request *req, struct soc
 		ast_log(LOG_DEBUG, "**** Received %s (%d) - Command in SIP %s\n", sip_methods[p->method].text, sip_methods[p->method].id, cmd); 
 
 	if (p->icseq && (p->icseq > seqno)) {
+		if (option_debug)
 			ast_log(LOG_DEBUG, "Ignoring too old SIP packet packet %d (expecting >= %d)\n", seqno, p->icseq);
+		if (req->method != SIP_ACK)
 			transmit_response(p, "503 Server error", req);	/* We must respond according to RFC 3261 sec 12.2 */
 		return -1;
 	} else if (p->icseq && (p->icseq == seqno) && req->method != SIP_ACK &&(p->method != SIP_CANCEL|| ast_test_flag(p, SIP_ALREADYGONE))) {