Change packet_index functions to more closely match RFC

This became necessary because of the way Asterisk handles Sequence
numbers when changing SSRC.  They continue to increment a single
sequence number even though the SSRC is different, on switching back
this causes the packet_index function to interpret this as many lost
packets.  The previous function had dead-spots that would not adjust
the packet_index at all if the difference fell in these ranges.  These
gaps always resulted in behavior contra what would happen in webrtc
clients.

daemon/rtp.c

@@ -137,8 +137,6 @@ error:
 
 static u_int64_t packet_index(struct crypto_context *c, struct rtp_header *rtp) {
 	u_int16_t seq;
-	u_int64_t index;
-	long long int diff;
 
 	seq = ntohs(rtp->seq_num);
 	/* rfc 3711 section 3.3.1 */
@@ -146,24 +144,24 @@ static u_int64_t packet_index(struct crypto_context *c, struct rtp_header *rtp)
 		c->last_index = seq;
 
 	/* rfc 3711 appendix A, modified, and sections 3.3 and 3.3.1 */
-	index = (c->last_index & 0xffffffff0000ULL) | seq;
-	diff = index - c->last_index;
-	if (diff >= 0) {
-		if (diff < 0x8000)
-			c->last_index = index;
-		else if (index >= 0x10000)
-			index -= 0x10000;
-	}
-	else {
-		if (diff >= -0x8000)
-			;
-		else {
-			index += 0x10000;
-			c->last_index = index;
-		}
+	u_int16_t s_l = (c->last_index & 0x00000000ffffULL);
+	u_int32_t roc = (c->last_index & 0xffffffff0000ULL) >> 16;
+	u_int32_t v = 0;
+
+	if (s_l < 0x8000) {
+		if (((seq - s_l) > 0x8000) && roc > 0)
+			v = (roc - 1) % 0x10000;
+		else
+			v = roc;
+	} else {
+		if ((s_l - 0x8000) > seq)
+			v = (roc + 1) % 0x10000;
+		else
+			v = roc;
 	}
 
-	return index;
+	c->last_index = (u_int64_t)(((v << 16) | seq) & 0xffffffffffffULL);
+	return c->last_index;
 }
 
 void rtp_append_mki(str *s, struct crypto_context *c) {

kernel-module/xt_RTPENGINE.c

@@ -1889,8 +1889,10 @@ static u_int64_t packet_index(struct re_crypto_context *c,
 {
 	u_int16_t seq;
 	u_int64_t index;
-	long long int diff;
 	unsigned long flags;
+	u_int16_t s_l;
+	u_int32_t roc;
+	u_int32_t v;
 
 	// Keep the crypt context SSRC in sync.
 	if (unlikely(!c->ssrc))
@@ -1906,25 +1908,27 @@ static u_int64_t packet_index(struct re_crypto_context *c,
 	if (unlikely(!s->last_index))
 		s->last_index = seq;
 
-	/* rfc 3711 appendix A, modified, and sections 3.3 and 3.3.1 */
-	index = ((u_int64_t) c->roc << 16) | seq;
-	diff = index - s->last_index;
-	if (diff >= 0) {
-		if (diff < 0x8000)
-			s->last_index = index;
-		else if (index >= 0x10000)
-			index -= 0x10000;
-	}
-	else {
-		if (diff >= -0x8000)
-			;
-		else {
-			index += 0x10000;
-			c->roc++;
-			s->last_index = index;
-		}
+  /* rfc 3711 appendix A, modified, and sections 3.3 and 3.3.1 */
+	s_l = (s->last_index & 0x00000000ffffULL);
+	roc = (s->last_index & 0xffffffff0000ULL) >> 16;
+	v = 0;
+
+	if (s_l < 0x8000) {
+		if (((seq - s_l) > 0x8000) && roc > 0)
+			v = (roc - 1) % 0x10000;
+		else
+			v = roc;
+	} else {
+		if ((s_l - 0x8000) > seq)
+			v = (roc + 1) % 0x10000;
+		else
+			v = roc;
 	}
 
+	index = (v << 16) | seq;
+	s->last_index = index;
+	c->roc = v;
+
 	spin_unlock_irqrestore(&c->lock, flags);
 
 	return index;