[TCP] FRTO: Comment cleanup & improvement

Moved comments out from the body of process_frto() to the head
(preferred way; see Documentation/CodingStyle). Bonus: it's much
easier to read in this compacted form.

FRTO algorithm and implementation is described in greater detail.
For interested reader, more information is available in RFC4138.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1236,22 +1236,22 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 	return flag;
 }
 
+/* F-RTO can only be used if these conditions are satisfied:
+ *  - there must be some unsent new data
+ *  - the advertised window should allow sending it
+ */
 int tcp_use_frto(const struct sock *sk)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 
-	/* F-RTO must be activated in sysctl and there must be some
-	 * unsent new data, and the advertised window should allow
-	 * sending it.
-	 */
 	return (sysctl_tcp_frto && sk->sk_send_head &&
 		!after(TCP_SKB_CB(sk->sk_send_head)->end_seq,
 		       tp->snd_una + tp->snd_wnd));
 }
 
-/* RTO occurred, but do not yet enter loss state. Instead, transmit two new
- * segments to see from the next ACKs whether any data was really missing.
- * If the RTO was spurious, new ACKs should arrive.
+/* RTO occurred, but do not yet enter Loss state. Instead, defer RTO
+ * recovery a bit and use heuristics in tcp_process_frto() to detect if
+ * the RTO was spurious.
  */
 void tcp_enter_frto(struct sock *sk)
 {
@@ -2489,6 +2489,30 @@ static void tcp_conservative_spur_to_response(struct tcp_sock *tp)
 	tcp_moderate_cwnd(tp);
 }
 
+/* F-RTO spurious RTO detection algorithm (RFC4138)
+ *
+ * F-RTO affects during two new ACKs following RTO. State (ACK number) is kept
+ * in frto_counter. When ACK advances window (but not to or beyond highest
+ * sequence sent before RTO):
+ *   On First ACK,  send two new segments out.
+ *   On Second ACK, RTO was likely spurious. Do spurious response (response
+ *                  algorithm is not part of the F-RTO detection algorithm
+ *                  given in RFC4138 but can be selected separately).
+ * Otherwise (basically on duplicate ACK), RTO was (likely) caused by a loss
+ * and TCP falls back to conventional RTO recovery.
+ *
+ * Rationale: if the RTO was spurious, new ACKs should arrive from the
+ * original window even after we transmit two new data segments.
+ *
+ * F-RTO is implemented (mainly) in four functions:
+ *   - tcp_use_frto() is used to determine if TCP is can use F-RTO
+ *   - tcp_enter_frto() prepares TCP state on RTO if F-RTO is used, it is
+ *     called when tcp_use_frto() showed green light
+ *   - tcp_process_frto() handles incoming ACKs during F-RTO algorithm
+ *   - tcp_enter_frto_loss() is called if there is not enough evidence
+ *     to prove that the RTO is indeed spurious. It transfers the control
+ *     from F-RTO to the conventional RTO recovery
+ */
 static void tcp_process_frto(struct sock *sk, u32 prior_snd_una)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
@@ -2497,25 +2521,16 @@ static void tcp_process_frto(struct sock *sk, u32 prior_snd_una)
 
 	if (tp->snd_una == prior_snd_una ||
 	    !before(tp->snd_una, tp->frto_highmark)) {
-		/* RTO was caused by loss, start retransmitting in
-		 * go-back-N slow start
-		 */
 		tcp_enter_frto_loss(sk);
 		return;
 	}
 
 	if (tp->frto_counter == 1) {
-		/* First ACK after RTO advances the window: allow two new
-		 * segments out.
-		 */
 		tp->snd_cwnd = tcp_packets_in_flight(tp) + 2;
-	} else {
+	} else /* frto_counter == 2 */ {
 		tcp_conservative_spur_to_response(tp);
 	}
 
-	/* F-RTO affects on two new ACKs following RTO.
-	 * At latest on third ACK the TCP behavior is back to normal.
-	 */
 	tp->frto_counter = (tp->frto_counter + 1) % 3;
 }