diff --git a/include/protocol.h b/include/protocol.h
new file mode 100644
index 0000000..461ed55
--- /dev/null
+++ b/include/protocol.h
@@ -0,0 +1,52 @@
+#pragma once
+
+#include <stdint.h>
+#include <arpa/inet.h>
+
+#define OWD_MAGIC 0x4f574454
+#define OWD_VERSION 1
+
+#define OWD_DECL_DATA_END char _data_end[0]
+#define OWD_DATA_LEN(HDR) (offsetof(__typeof__(HDR), _data_end))
+
+enum owd_msg_type {
+  OWD_MSG_ERR,
+  OWD_MSG_SYNC_REQUEST,
+  OWD_MSG_SYNC_RESPONSE,
+};
+
+enum owd_error_code {
+  OWD_ERR_BAD_HEADER,
+  OWD_ERR_BAD_TYPE,
+};
+
+struct owd_timestamp {
+  uint32_t ts_upper;
+  uint32_t ts_lower;
+};
+
+struct owdhdr {
+  uint32_t magic;
+  uint8_t version;
+  uint8_t msg_type;
+  uint16_t length;
+};
+
+struct owd_msg_err {
+  struct owdhdr hdr;
+  uint8_t err;
+  OWD_DECL_DATA_END;
+};
+
+struct owd_msg_sync_response {
+  struct owdhdr hdr;
+  struct owd_timestamp ts;
+  OWD_DECL_DATA_END;
+};
+
+static inline void owd_hdr_populate(struct owdhdr *hdr, uint8_t msg_type, uint16_t length) {
+  hdr->magic = htonl(OWD_MAGIC);
+  hdr->version = OWD_VERSION;
+  hdr->msg_type = msg_type;
+  hdr->length = htons(length);
+}