diff --git a/include/net/tcp.h b/include/net/tcp.h
index b8fdc6b..637ee49 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1713,4 +1713,19 @@
 	return dopt;
 }
 
+/* locally generated TCP pure ACKs have skb->truesize == 2
+ * (check tcp_send_ack() in net/ipv4/tcp_output.c )
+ * This is much faster than dissecting the packet to find out.
+ * (Think of GRE encapsulations, IPv4, IPv6, ...)
+ */
+static inline bool skb_is_tcp_pure_ack(const struct sk_buff *skb)
+{
+	return skb->truesize == 2;
+}
+
+static inline void skb_set_tcp_pure_ack(struct sk_buff *skb)
+{
+	skb->truesize = 2;
+}
+
 #endif	/* _TCP_H */