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Commit 30935cf4 authored by Ilpo Järvinen's avatar Ilpo Järvinen Committed by David S. Miller
Browse files

[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: default avatarIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent bdaae17d

net/ipv4/tcp_input.c

+32 −17
Original line number Diff line number Diff line
@@ -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;

}