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Commit 4dc2665e authored by Ilpo Järvinen's avatar Ilpo Järvinen Committed by David S. Miller
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[TCP]: SACK enhanced FRTO 



Implements the SACK-enhanced FRTO given in RFC4138 using the
variant given in Appendix B.

RFC4138, Appendix B:
  "This means that in order to declare timeout spurious, the TCP
   sender must receive an acknowledgment for non-retransmitted
   segment between SND.UNA and RecoveryPoint in algorithm step 3.
   RecoveryPoint is defined in conservative SACK-recovery
   algorithm [RFC3517]"

The basic version of the FRTO algorithm can still be used also
when SACK is enabled. To enabled SACK-enhanced version, tcp_frto
sysctl is set to 2.

Signed-off-by: default avatarIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 288035f9

net/ipv4/tcp_input.c

+65 −11
Original line number Diff line number Diff line
@@ -100,6 +100,7 @@ int sysctl_tcp_abc __read_mostly; 
#define FLAG_ECE		0x40 /* ECE in this ACK				*/

#define FLAG_DATA_LOST		0x80 /* SACK detected data lossage.		*/

#define FLAG_SLOWPATH		0x100 /* Do not skip RFC checks for window update.*/

#define FLAG_ONLY_ORIG_SACKED	0x200 /* SACKs only non-rexmit sent before RTO */



#define FLAG_ACKED		(FLAG_DATA_ACKED|FLAG_SYN_ACKED)

#define FLAG_NOT_DUP		(FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED)
@@ -110,6 +111,8 @@ int sysctl_tcp_abc __read_mostly; 
#define IsFack(tp) ((tp)->rx_opt.sack_ok & 2)

#define IsDSack(tp) ((tp)->rx_opt.sack_ok & 4)



#define IsSackFrto() (sysctl_tcp_frto == 0x2)



#define TCP_REMNANT (TCP_FLAG_FIN|TCP_FLAG_URG|TCP_FLAG_SYN|TCP_FLAG_PSH)



/* Adapt the MSS value used to make delayed ack decision to the
@@ -1159,6 +1162,18 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_ 
						/* clear lost hint */

						tp->retransmit_skb_hint = NULL;

					}

					/* SACK enhanced F-RTO detection.

					 * Set flag if and only if non-rexmitted

					 * segments below frto_highmark are

					 * SACKed (RFC4138; Appendix B).

					 * Clearing correct due to in-order walk

					 */

					if (after(end_seq, tp->frto_highmark)) {

						flag &= ~FLAG_ONLY_ORIG_SACKED;

					} else {

						if (!(sacked & TCPCB_RETRANS))

							flag |= FLAG_ONLY_ORIG_SACKED;

					}

				}



				TCP_SKB_CB(skb)->sacked |= TCPCB_SACKED_ACKED;
@@ -1240,7 +1255,8 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_ 
/* 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

 *  - TCP has never retransmitted anything other than head

 *  - TCP has never retransmitted anything other than head (SACK enhanced

 *    variant from Appendix B of RFC4138 is more robust here)

 */

int tcp_use_frto(struct sock *sk)

{
@@ -1252,6 +1268,9 @@ int tcp_use_frto(struct sock *sk) 
		      tp->snd_una + tp->snd_wnd))

		return 0;



	if (IsSackFrto())

		return 1;



	/* Avoid expensive walking of rexmit queue if possible */

	if (tp->retrans_out > 1)

		return 0;
@@ -1328,9 +1347,18 @@ void tcp_enter_frto(struct sock *sk) 
	}

	tcp_sync_left_out(tp);



	/* Earlier loss recovery underway (see RFC4138; Appendix B).

	 * The last condition is necessary at least in tp->frto_counter case.

	 */

	if (IsSackFrto() && (tp->frto_counter ||

	    ((1 << icsk->icsk_ca_state) & (TCPF_CA_Recovery|TCPF_CA_Loss))) &&

	    after(tp->high_seq, tp->snd_una)) {

		tp->frto_highmark = tp->high_seq;

	} else {

		tp->frto_highmark = tp->snd_nxt;

	}

	tcp_set_ca_state(sk, TCP_CA_Disorder);

	tp->high_seq = tp->snd_nxt;

	tp->frto_highmark = tp->snd_nxt;

	tp->frto_counter = 1;

}
@@ -2566,6 +2594,10 @@ static void tcp_conservative_spur_to_response(struct tcp_sock *tp) 
 * Rationale: if the RTO was spurious, new ACKs should arrive from the

 * original window even after we transmit two new data segments.

 *

 * SACK version:

 *   on first step, wait until first cumulative ACK arrives, then move to

 *   the second step. In second step, the next ACK decides.

 *

 * 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
@@ -2590,18 +2622,40 @@ static int tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag) 
		return 1;

	}



	/* RFC4138 shortcoming in step 2; should also have case c): ACK isn't

	 * duplicate nor advances window, e.g., opposite dir data, winupdate

	if (!IsSackFrto() || IsReno(tp)) {

		/* RFC4138 shortcoming in step 2; should also have case c):

		 * ACK isn't duplicate nor advances window, e.g., opposite dir

		 * data, winupdate

		 */

		if ((tp->snd_una == prior_snd_una) && (flag&FLAG_NOT_DUP) &&

		    !(flag&FLAG_FORWARD_PROGRESS))

			return 1;



		if (!(flag&FLAG_DATA_ACKED)) {

		tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 0 : 3), flag);

			tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 0 : 3),

					    flag);

			return 1;

		}

	} else {

		if (!(flag&FLAG_DATA_ACKED) && (tp->frto_counter == 1)) {

			/* Prevent sending of new data. */

			tp->snd_cwnd = min(tp->snd_cwnd,

					   tcp_packets_in_flight(tp));

			return 1;

		}



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

		    (!(flag&FLAG_FORWARD_PROGRESS) ||

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

			/* RFC4138 shortcoming (see comment above) */

			if (!(flag&FLAG_FORWARD_PROGRESS) && (flag&FLAG_NOT_DUP))

				return 1;



			tcp_enter_frto_loss(sk, 3, flag);

			return 1;

		}

	}



	if (tp->frto_counter == 1) {

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

		tp->frto_counter = 2;