diff --git a/ntp_io.c b/ntp_io.c
index ec2fd7b..fc80aca 100644
--- a/ntp_io.c
+++ b/ntp_io.c
@@ -513,7 +513,9 @@ NIO_UnwrapMessage(SCK_Message *message, int sock_fd, double *net_correction)
 
   msg = message->data;
 
-  if (msg->header.type != PTP_TYPE_DELAY_REQ || msg->header.version != PTP_VERSION ||
+  if ((msg->header.type != PTP_TYPE_DELAY_REQ && msg->header.type != PTP_TYPE_SYNC) ||
+      (msg->header.version != PTP_VERSION_2 &&
+       (msg->header.version != PTP_VERSION_2_1 || msg->header.min_sdoid != 0)) ||
       ntohs(msg->header.length) != message->length ||
       msg->header.domain != PTP_DOMAIN_NTP ||
       ntohs(msg->header.flags) != PTP_FLAG_UNICAST ||
@@ -561,7 +563,7 @@ wrap_message(SCK_Message *message, int sock_fd)
 
   memset(ptp_message, 0, PTP_NTP_PREFIX_LENGTH);
   ptp_message->header.type = PTP_TYPE_DELAY_REQ;
-  ptp_message->header.version = PTP_VERSION;
+  ptp_message->header.version = PTP_VERSION_2;
   ptp_message->header.length = htons(PTP_NTP_PREFIX_LENGTH + message->length);
   ptp_message->header.domain = PTP_DOMAIN_NTP;
   ptp_message->header.flags = htons(PTP_FLAG_UNICAST);
diff --git a/ptp.h b/ptp.h
index 8034a2c..4330f4b 100644
--- a/ptp.h
+++ b/ptp.h
@@ -31,7 +31,9 @@
 
 #include "ntp.h"
 
-#define PTP_VERSION 2
+#define PTP_VERSION_2 2
+#define PTP_VERSION_2_1 (2 | 1 << 4)
+#define PTP_TYPE_SYNC 0
 #define PTP_TYPE_DELAY_REQ 1
 #define PTP_DOMAIN_NTP 123
 #define PTP_FLAG_UNICAST (1 << (2 + 8))