diff --git a/src/main/p4io/cmd.h b/src/main/p4io/cmd.h
index b5c354a..ece1127 100644
--- a/src/main/p4io/cmd.h
+++ b/src/main/p4io/cmd.h
@@ -3,6 +3,8 @@
 
 #include <stdint.h>
 
+#define P4IO_SOF 0xAA
+
 enum p4ioemu_p4io_command {
     P4IO_CMD_INIT = 0x00,
     P4IO_CMD_GET_DEVICE_INFO = 0x01,
@@ -23,7 +25,7 @@ enum p4ioemu_p4io_command {
 #define P4IO_MAX_PAYLOAD 60
 
 struct p4io_cmd_header {
-    uint8_t AA;
+    uint8_t sof;
     uint8_t cmd;
     uint8_t seq_num;
     uint8_t payload_len;
diff --git a/src/main/p4iodrv/usb.c b/src/main/p4iodrv/usb.c
index 511336a..ca18109 100644
--- a/src/main/p4iodrv/usb.c
+++ b/src/main/p4iodrv/usb.c
@@ -118,7 +118,7 @@ bool p4io_usb_transfer(
         return false;
     }
 
-    cmd_buf.header.AA = 0xAA;
+    cmd_buf.header.sof = P4IO_SOF;
     cmd_buf.header.cmd = cmd;
     cmd_buf.header.seq_num = seq_no;
     cmd_buf.header.payload_len = req_payload_len;
@@ -145,7 +145,7 @@ bool p4io_usb_transfer(
         memcpy(resp_payload, cmd_buf.payload, *resp_payload_len);
         *resp_payload_len = cmd_buf.header.payload_len;
 
-        if(cmd_buf.header.AA != 0xAA) {
+        if(cmd_buf.header.sof != P4IO_SOF) {
             log_warning("Response bad header");
             return false;
         }
diff --git a/src/main/p4ioemu/device.c b/src/main/p4ioemu/device.c
index a3684ed..ee27e3b 100644
--- a/src/main/p4ioemu/device.c
+++ b/src/main/p4ioemu/device.c
@@ -141,7 +141,7 @@ static HRESULT p4ioemu_p4io_bulk_read(void *resp, uint32_t nbytes)
         payload_len = p4ioemu_p4io_cmd_buffer_resp_len;
     }
 
-    package->header.AA = 0xAA;
+    package->header.sof = P4IO_SOF;
     package->header.cmd = p4ioemu_p4io_last_cmd;
     package->header.seq_num = p4ioemu_p4io_last_seq_num;
     package->header.payload_len = payload_len;
@@ -164,7 +164,7 @@ static HRESULT p4ioemu_p4io_bulk_write(const void *req, uint32_t nbytes)
 
     package = (struct p4io_cmd_package *) req;
 
-    if (package->header.AA != 0xAA) {
+    if (package->header.sof != P4IO_SOF) {
         log_warning("Command on bulk endpoint too short: %d", nbytes);
         p4ioemu_p4io_dump_buffer(req, nbytes);