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Commit d551e454 authored by Ilpo Järvinen's avatar Ilpo Järvinen Committed by David S. Miller
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[TCP] FRTO: RFC4138 allows Nagle override when new data must be sent 



This is a corner case where less than MSS sized new data thingie
is awaiting in the send queue. For F-RTO to work correctly, a
new data segment must be sent at certain point or F-RTO cannot
be used at all. RFC4138 allows overriding of Nagle at that
point.

Implementation uses frto_counter states 2 and 3 to distinguish
when Nagle override is needed.

Signed-off-by: default avatarIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 575ee714

include/net/tcp.h

+5 −0
Original line number Diff line number Diff line
@@ -1199,9 +1199,14 @@ static inline struct sk_buff *tcp_send_head(struct sock *sk) 


static inline void tcp_advance_send_head(struct sock *sk, struct sk_buff *skb)

{

	struct tcp_sock *tp = tcp_sk(sk);



	sk->sk_send_head = skb->next;

	if (sk->sk_send_head == (struct sk_buff *)&sk->sk_write_queue)

		sk->sk_send_head = NULL;

	/* Don't override Nagle indefinately with F-RTO */

	if (tp->frto_counter == 2)

		tp->frto_counter = 3;

}



static inline void tcp_check_send_head(struct sock *sk, struct sk_buff *skb_unlinked)

net/ipv4/tcp_input.c

+8 −5
Original line number Diff line number Diff line
@@ -2637,7 +2637,9 @@ static void tcp_undo_spur_to_response(struct sock *sk, int flag) 
 *                  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.

 * and TCP falls back to conventional RTO recovery. F-RTO allows overriding

 * of Nagle, this is done using frto_counter states 2 and 3, when a new data

 * segment of any size sent during F-RTO, state 2 is upgraded to 3.

 *

 * Rationale: if the RTO was spurious, new ACKs should arrive from the

 * original window even after we transmit two new data segments.
@@ -2666,7 +2668,7 @@ static int tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag) 
		inet_csk(sk)->icsk_retransmits = 0;



	if (!before(tp->snd_una, tp->frto_highmark)) {

		tcp_enter_frto_loss(sk, tp->frto_counter + 1, flag);

		tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3), flag);

		return 1;

	}
@@ -2692,7 +2694,7 @@ static int tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag) 
			return 1;

		}



		if ((tp->frto_counter == 2) &&

		if ((tp->frto_counter >= 2) &&

		    (!(flag&FLAG_FORWARD_PROGRESS) ||

		     ((flag&FLAG_DATA_SACKED) && !(flag&FLAG_ONLY_ORIG_SACKED)))) {

			/* RFC4138 shortcoming (see comment above) */
@@ -2709,14 +2711,15 @@ static int tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag) 
		if (!tcp_send_head(sk) ||

		    after(TCP_SKB_CB(tcp_send_head(sk))->end_seq,

				     tp->snd_una + tp->snd_wnd)) {

			tcp_enter_frto_loss(sk, tp->frto_counter + 1, flag);

			tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3),

					    flag);

			return 1;

		}



		tp->snd_cwnd = tcp_packets_in_flight(tp) + 2;

		tp->frto_counter = 2;

		return 1;

	} else /* frto_counter == 2 */ {

	} else {

		switch (sysctl_tcp_frto_response) {

		case 2:

			tcp_undo_spur_to_response(sk, flag);

net/ipv4/tcp_output.c

+4 −2
Original line number Diff line number Diff line
@@ -1035,8 +1035,10 @@ static inline int tcp_nagle_test(struct tcp_sock *tp, struct sk_buff *skb, 
	if (nonagle & TCP_NAGLE_PUSH)

		return 1;



	/* Don't use the nagle rule for urgent data (or for the final FIN).  */

	if (tp->urg_mode ||

	/* Don't use the nagle rule for urgent data (or for the final FIN).

	 * Nagle can be ignored during F-RTO too (see RFC4138).

	 */

	if (tp->urg_mode || (tp->frto_counter == 2) ||

	    (TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN))

		return 1;