--- pptpgre.c.orig
+++ pptpgre.c
@@ -403,15 +403,20 @@
 			stats.rx_truncated++;
 			return 0;
 		}
-		/* check for out-of-order sequence number */
-		if (seq == gre.seq_recv + 1) {
+		/* check for out-of-order sequence number
+		 * N.B.: some client implementations violate RFC 2637
+		 * and start their sequence numbers at 1 instead of 0,
+		 * so we have to introduce a kludge to deal with it. 
+		 * on wrap we may allow an out of order packet to pass
+		*/
+		if (seq == gre.seq_recv + 1 || seq == 1) {
 			if (pptpctrl_debug)
 				syslog(LOG_DEBUG, "GRE: accepting packet #%d", 
 				       seq);
 			stats.rx_accepted++;
 			gre.seq_recv = seq;
 			return cb(cl, buffer + ip_len + headersize, payload_len);
-		} else if (seq == gre.seq_recv) {
+		} else if (!seq_greater(seq, gre.seq_recv)) {
 			if (pptpctrl_debug)
 				syslog(LOG_DEBUG,
 				       "GRE: discarding duplicate or old packet #%d (expecting #%d)",