diff --git a/proto/protocol.xml b/proto/protocol.xml
index bfbb583..0d7a02b 100644
--- a/proto/protocol.xml
+++ b/proto/protocol.xml
@@ -31,13 +31,12 @@
 
   <!--
 
-    Protocol wire format: [sender-id, length | opcode, ...]
+    Protocol wire format: [sender-id, length, opcode, ...]
     Where:
     - sender-id is one 64-bit integer
-    - length and opcode share one 32-bit integer, and
-      length are the high 16 bits and opcode are the low 16 bits.
+    - length and opcode are 32-bit integers
     - all integers are in the EIS implementation's native byte order.
-    - length is the length of the message in bytes, including the 8 header bytes
+    - length is the length of the message in bytes, including the 16 header bytes
     - sender-id is the id of the object sending the request/event. The sender-id
       0 is reserved for the special "ei_handshake" object.
     - opcode is the event or request-specific opcode, starting at 0.
diff --git a/src/brei-shared.c b/src/brei-shared.c
index 059e11d..62d8b69 100644
--- a/src/brei-shared.c
+++ b/src/brei-shared.c
@@ -42,6 +42,13 @@ struct brei_context {
 	void *log_context;
 };
 
+struct brei_header {
+	object_id_t sender_id;
+	uint32_t msglen;	/* length of message in bytes including this header */
+	uint32_t opcode;
+} _packed_;
+static_assert(sizeof(struct brei_header) == 16, "Unexpected size for brei_header struct");
+
 struct brei_string {
 	uint32_t len;
 	const char str[];
@@ -323,15 +330,14 @@ brei_marshal_message(struct brei_context *brei,
 		     size_t nargs,
 		     va_list args)
 {
-
 	_cleanup_iobuf_ struct iobuf *buf = iobuf_new(128);
 
 	_unref_(brei_result) *result = brei_marshal(brei, buf, signature, nargs, args);
 	if (result)
 		return steal(&result);
 
-	size_t message_len = iobuf_len(buf) + 12;
-	uint32_t header[3] = {0, 0, message_len << 16 | opcode};
+	size_t message_len = iobuf_len(buf) + sizeof(struct brei_header);
+	uint32_t header[4] = {0, 0, message_len, opcode};
 	memcpy(header, &id, sizeof(id));
 	iobuf_prepend(buf, (const char *)header, sizeof(header));
 
@@ -361,20 +367,23 @@ brei_dispatch(struct brei_context *brei,
 		uint32_t *data = (uint32_t*)iobuf_data(buf);
 		size_t len = iobuf_len(buf);
 
-		const size_t headersize = 12; /* object-id, length | opcode */
+		const size_t headersize = sizeof(struct brei_header);
 
 		if (len < headersize)
 			break;
 
-		object_id_t object_id = *(uint64_t*)data; /* 8 bytes for object id */
-		size_t msglen = data[2] >> 16; /* 2 bytes message length */
-		uint32_t opcode = data[2] & 0xFFFF; /* 2 bytes for opcode */
+		const struct brei_header *header = (const struct brei_header*)data;
+		uint32_t msglen = header->msglen;
+
 		if (len < msglen)
 			break;
 
+		object_id_t object_id = header->sender_id;
+		uint32_t opcode = header->opcode;;
+
 		/* Find the object, it is stored in the ei/eis context  */
 		struct brei_object *object = NULL;
-		rc = lookup_object(object_id, &object, user_data);
+		rc = lookup_object(header->sender_id, &object, user_data);
 		if (rc == -ENOENT) {
 			iobuf_pop(buf, msglen);
 			continue;
@@ -395,7 +404,7 @@ brei_dispatch(struct brei_context *brei,
 		if (opcode >= interface->nincoming) {
 			result = brei_result_new(BREI_CONNECTION_DISCONNECT_REASON_PROTOCOL,
 						 "opcode %u exceeds interface %s method count %u",
-						 opcode, interface->name, interface->nincoming);
+						 header->opcode, interface->name, interface->nincoming);
 			goto error;
 		}
 
@@ -564,7 +573,7 @@ MUNIT_TEST(test_brei_send_message)
 	int sock_read = sv[0];
 	int sock_write = sv[1];
 
-	const int header_size = 12;
+	const int header_size = 16;
 
 	{
 		const int msglen = header_size + 8; /* 2 * 4 bytes */
@@ -575,13 +584,15 @@ MUNIT_TEST(test_brei_send_message)
 		munit_assert_int(rc, ==, msglen);
 
 		uint32_t buf[64];
-		uint64_t *buf_id = (uint64_t*)buf;
 		int len = read(sock_read, buf, sizeof(buf));
 		munit_assert_int(len, ==, msglen);
-		munit_assert_int(*buf_id, ==, id);
-		munit_assert_int(buf[2], ==, msglen << 16 | opcode);
-		munit_assert_int(buf[3], ==, 0xff);
-		munit_assert_int(buf[4], ==, 0xdddd);
+
+		const struct brei_header *header = (const struct brei_header*)buf;
+		munit_assert_int(header->sender_id, ==, id);
+		munit_assert_int(header->msglen, ==, msglen);
+		munit_assert_int(header->opcode, ==, opcode);
+		munit_assert_int(buf[4], ==, 0xff);
+		munit_assert_int(buf[5], ==, 0xdddd);
 	}
 	{
 		const int msglen = header_size + 8; /* 2 * 4 bytes */
@@ -592,18 +603,21 @@ MUNIT_TEST(test_brei_send_message)
 		munit_assert_int(rc, ==, msglen);
 
 		uint32_t buf[64];
-		uint64_t *buf_id = (uint64_t*)buf;
 		int len = read(sock_read, buf, sizeof(buf));
 		union {
 			uint32_t bytes;
 			float f;
 		} ufloat;
 		munit_assert_int(len, ==, msglen);
-		munit_assert_int(*buf_id, ==, id);
-		munit_assert_int(buf[2], ==, msglen << 16 | opcode);
-		ufloat.bytes = buf[3];
+
+		const struct brei_header *header = (const struct brei_header*)buf;
+		munit_assert_int(header->sender_id, ==, id);
+		munit_assert_int(header->msglen, ==, msglen);
+		munit_assert_int(header->opcode, ==, opcode);
+
+		ufloat.bytes = buf[4];
 		munit_assert_double_equal(ufloat.f, 1.234, 4/* precision */);
-		munit_assert_int(buf[4], ==, -12);
+		munit_assert_int(buf[5], ==, -12);
 	}
 
 	{
@@ -620,20 +634,23 @@ MUNIT_TEST(test_brei_send_message)
 		munit_assert_int(rc, ==, msglen);
 
 		uint32_t buf[64];
-		uint64_t *buf_id = (uint64_t*)buf;
 		int len = read(sock_read, buf, sizeof(buf));
 		munit_assert_int(len, ==, msglen);
-		munit_assert_int(*buf_id, ==, id);
-		munit_assert_int(buf[2], ==, msglen << 16 | opcode);
-		munit_assert_int(buf[3], ==, -42);
 
-		const struct brei_string *s = (const struct brei_string *)&buf[4];
+		const struct brei_header *header = (const struct brei_header*)buf;
+		munit_assert_int(header->sender_id, ==, id);
+		munit_assert_int(header->msglen, ==, msglen);
+		munit_assert_int(header->opcode, ==, opcode);
+
+		munit_assert_int(buf[4], ==, -42);
+
+		const struct brei_string *s = (const struct brei_string *)&buf[5];
 		munit_assert_int(s->len, ==, strlen0(string));
 		munit_assert_string_equal(s->str, string);
 		munit_assert_int(memcmp(s->str, string, brei_string_proto_length(s->len) - 4), ==, 0);
 
-		munit_assert_int(buf[5 + slen/4], ==, 0xab);
-		munit_assert_int(buf[7 + slen/4], ==, 0xcdef);
+		munit_assert_int(buf[6 + slen/4], ==, 0xab);
+		munit_assert_int(buf[8 + slen/4], ==, 0xcdef);
 	}
 
 	{
@@ -648,18 +665,20 @@ MUNIT_TEST(test_brei_send_message)
 		munit_assert_int(rc, ==, msglen);
 
 		uint32_t buf[64];
-		uint64_t *buf_id = (uint64_t*)buf;
 		int len = read(sock_read, buf, sizeof(buf));
 		munit_assert_int(len, ==, msglen);
-		munit_assert_int(*buf_id, ==, id);
-		munit_assert_int(buf[2], ==, msglen << 16 | opcode);
 
-		const struct brei_string *s1 = (const struct brei_string *)&buf[3];
+		const struct brei_header *header = (const struct brei_header*)buf;
+		munit_assert_int(header->sender_id, ==, id);
+		munit_assert_int(header->msglen, ==, msglen);
+		munit_assert_int(header->opcode, ==, opcode);
+
+		const struct brei_string *s1 = (const struct brei_string*)&buf[4];
 		munit_assert_int(s1->len, ==, strlen0(string1));
 		munit_assert_string_equal(s1->str, string1);
 		munit_assert_int(memcmp(s1->str, string1, brei_string_proto_length(s1->len) - 4), ==, 0);
 
-		const struct brei_string *s2 = (const struct brei_string *)&buf[7];
+		const struct brei_string *s2 = (const struct brei_string *)&buf[8];
 		munit_assert_int(s2->len, ==, strlen0(string2));
 		munit_assert_string_equal(s2->str, string2);
 		munit_assert_int(memcmp(s2->str, string2, brei_string_proto_length(s2->len) - 4), ==, 0);
@@ -691,13 +710,15 @@ MUNIT_TEST(test_brei_send_message)
 		int len = iobuf_recv_from_fd(recv, sock_read);
 
 		uint32_t *buf = (uint32_t*)iobuf_data(recv);
-		uint64_t *buf_id = (uint64_t*)buf;
 		munit_assert_int(len, ==, msglen);
-		munit_assert_int(*buf_id, ==, id);
-		munit_assert_int(buf[2], ==, msglen << 16 | opcode);
-		munit_assert_int(buf[3], ==, 0xab);
+		const struct brei_header *header = (const struct brei_header*)buf;
+		munit_assert_int(header->sender_id, ==, id);
+		munit_assert_int(header->msglen, ==, msglen);
+		munit_assert_int(header->opcode, ==, opcode);
+
+		munit_assert_int(buf[4], ==, 0xab);
 		/* fd is not in data */
-		munit_assert_int(buf[4], ==, 0xcd);
+		munit_assert_int(buf[5], ==, 0xcd);
 
 		_cleanup_close_ int fd = iobuf_take_fd(recv);
 		munit_assert_int(fd, !=, -1);
diff --git a/src/util-macros.h b/src/util-macros.h
index 7fb131e..62243db 100644
--- a/src/util-macros.h
+++ b/src/util-macros.h
@@ -54,6 +54,7 @@
 #define _printf_(_a, _b) __attribute__((format (printf, _a, _b)))
 #define _fallthrough_ __attribute__((fallthrough))
 #define _unused_ __attribute__((unused))
+#define _packed_ __attribute__((packed))
 
 #define run_only_once \
 	static int _once_per_##__func__ = 0; \
diff --git a/test/eiproto.py.tmpl b/test/eiproto.py.tmpl
index 788e5ed..a4c120d 100644
--- a/test/eiproto.py.tmpl
+++ b/test/eiproto.py.tmpl
@@ -63,18 +63,16 @@ class MessageHeader:
 
     @classmethod
     def size(cls) -> int:
-        return 12
+        return 16
 
     @classmethod
     def from_data(cls, data: bytes) -> "MessageHeader":
-        object_id, opcode = struct.unpack("=QI", data[:cls.size()])
-        msglen = opcode >> 16
-        opcode = opcode & 0xFFFF
+        object_id, msglen, opcode = struct.unpack("=QII", data[:cls.size()])
         return cls(object_id, msglen, opcode)
 
     @property
-    def as_tuple(self) -> Tuple[int, int]:
-        return self.object_id, self.msglen << 16 | self.opcode
+    def as_tuple(self) -> Tuple[int, int, int]:
+        return self.object_id, self.msglen, self.opcode
 
 
 @attr.s
@@ -150,7 +148,7 @@ class Interface:
     outgoing: dict[int, str] = attr.ib(default=attr.Factory(list), repr=False)
 
     def format(self, *args, opcode: int, signature: str) -> bytes:
-        encoding = ["=QI"]
+        encoding = ["=QII"]
         arguments = []
         for sig, arg in zip(signature, args):
             if sig in ["u"]:
@@ -181,7 +179,7 @@ class Interface:
         return struct.pack(format, *header.as_tuple, *arguments)
 
     def unpack(self, data, signature: str, names: list[str]) -> Tuple[int, dict[str, Any]]:
-        encoding = ["=QI"]  # the header
+        encoding = ["=QII"]  # the header
         for sig in signature:
             if sig in ["u"]:
                 encoding.append("I")
@@ -212,7 +210,7 @@ class Interface:
         # logger.debug(f"unpacked {format} to {values}")
 
         results = []
-        values = values[2:]  # drop id, length | opcode
+        values = values[3:]  # drop id, length, opcode
 
         # we had to insert the string length into the format, filter the
         # value for that out again.