diff --git a/include/net/tcp.h b/include/net/tcp.h
index c586847..18fc999 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -284,6 +284,7 @@
 extern int sysctl_tcp_early_retrans;
 extern int sysctl_tcp_limit_output_bytes;
 extern int sysctl_tcp_challenge_ack_limit;
+extern unsigned int sysctl_tcp_notsent_lowat;
 
 extern atomic_long_t tcp_memory_allocated;
 extern struct percpu_counter tcp_sockets_allocated;
@@ -1539,6 +1540,19 @@
 extern void __tcp_v4_send_check(struct sk_buff *skb, __be32 saddr,
 				__be32 daddr);
 
+static inline u32 tcp_notsent_lowat(const struct tcp_sock *tp)
+{
+	return tp->notsent_lowat ?: sysctl_tcp_notsent_lowat;
+}
+
+static inline bool tcp_stream_memory_free(const struct sock *sk)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+	u32 notsent_bytes = tp->write_seq - tp->snd_nxt;
+
+	return notsent_bytes < tcp_notsent_lowat(tp);
+}
+
 #ifdef CONFIG_PROC_FS
 extern int tcp4_proc_init(void);
 extern void tcp4_proc_exit(void);