+/**
+ * Check if duplicate ack as per RFC5681 Sec. 2
+ */
+always_inline u8
+tcp_ack_is_dupack (tcp_connection_t * tc, vlib_buffer_t * b, u32 prev_snd_wnd,
+ u32 prev_snd_una)
+{
+ return ((vnet_buffer (b)->tcp.ack_number == prev_snd_una)
+ && seq_gt (tc->snd_nxt, tc->snd_una)
+ && (vnet_buffer (b)->tcp.seq_end == vnet_buffer (b)->tcp.seq_number)
+ && (prev_snd_wnd == tc->snd_wnd));
+}
+
+/**
+ * Checks if ack is a congestion control event.
+ */
+static u8
+tcp_ack_is_cc_event (tcp_connection_t * tc, vlib_buffer_t * b,
+ u32 prev_snd_wnd, u32 prev_snd_una, u8 * is_dack)
+{
+ /* Check if ack is duplicate. Per RFC 6675, ACKs that SACK new data are
+ * defined to be 'duplicate' as well */
+ *is_dack = tc->sack_sb.last_sacked_bytes
+ || tcp_ack_is_dupack (tc, b, prev_snd_wnd, prev_snd_una);
+
+ /* If reneging, wait for timer based retransmits */
+ if (PREDICT_FALSE (tcp_is_lost_fin (tc) || tc->sack_sb.is_reneging))
+ return 0;
+
+ return (*is_dack || tcp_in_cong_recovery (tc));