diff --git a/waterutil/tun_ipv4.go b/waterutil/tun_ipv4.go
index 6e249e5..c0aba4c 100644
--- a/waterutil/tun_ipv4.go
+++ b/waterutil/tun_ipv4.go
@@ -1,6 +1,7 @@
 package waterutil
 
 import (
+	"encoding/binary"
 	"net"
 )
 
@@ -12,6 +13,10 @@ func IPv4ECN(packet []byte) byte {
 	return packet[1] & 0x03
 }
 
+func IPv4TotalLength(packet []byte) uint16 {
+	return binary.BigEndian.Uint16(packet[2:4])
+}
+
 func IPv4Identification(packet []byte) [2]byte {
 	return [2]byte{packet[4], packet[5]}
 }