diff --git a/include/net/tcp.h b/include/net/tcp.h
index b71a446..56b7602 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -793,6 +793,13 @@
 	return tp->packets_out - tcp_left_out(tp) + tp->retrans_out;
 }
 
+#define TCP_INFINITE_SSTHRESH	0x7fffffff
+
+static inline bool tcp_in_initial_slowstart(const struct tcp_sock *tp)
+{
+	return tp->snd_ssthresh >= TCP_INFINITE_SSTHRESH;
+}
+
 /* If cwnd > ssthresh, we may raise ssthresh to be half-way to cwnd.
  * The exception is rate halving phase, when cwnd is decreasing towards
  * ssthresh.