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Commit 5b08e47c authored by Yuchung Cheng's avatar Yuchung Cheng Committed by David S. Miller
Browse files

tcp: prefer packet timing to TS-ECR for RTT 



Prefer packet timings to TS-ecr for RTT measurements when both
sources are available. That's because broken middle-boxes and remote
peer can return packets with corrupted TS ECR fields. Similarly most
congestion controls that require RTT signals favor timing-based
sources as well. Also check for bad TS ECR values to avoid RTT
blow-ups. It has happened on production Web servers.

Signed-off-by: default avatarYuchung Cheng <ycheng@google.com>
Acked-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 375fe02c

include/net/tcp.h

+0 −1
Original line number Diff line number Diff line
@@ -591,7 +591,6 @@ extern void tcp_initialize_rcv_mss(struct sock *sk); 
extern int tcp_mtu_to_mss(struct sock *sk, int pmtu);

extern int tcp_mss_to_mtu(struct sock *sk, int mss);

extern void tcp_mtup_init(struct sock *sk);

extern void tcp_valid_rtt_meas(struct sock *sk, u32 seq_rtt);

extern void tcp_init_buffer_space(struct sock *sk);



static inline void tcp_bound_rto(const struct sock *sk)

net/ipv4/tcp_input.c

+18 −49
Original line number Diff line number Diff line
@@ -2792,65 +2792,36 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked, 
	tcp_xmit_retransmit_queue(sk);

}



void tcp_valid_rtt_meas(struct sock *sk, u32 seq_rtt)

static inline void tcp_ack_update_rtt(struct sock *sk, const int flag,

				      s32 seq_rtt)

{

	tcp_rtt_estimator(sk, seq_rtt);

	tcp_set_rto(sk);

	inet_csk(sk)->icsk_backoff = 0;

}

EXPORT_SYMBOL(tcp_valid_rtt_meas);

	const struct tcp_sock *tp = tcp_sk(sk);



/* Read draft-ietf-tcplw-high-performance before mucking

 * with this code. (Supersedes RFC1323)

	/* Prefer RTT measured from ACK's timing to TS-ECR. This is because

	 * broken middle-boxes or peers may corrupt TS-ECR fields. But

	 * Karn's algorithm forbids taking RTT if some retransmitted data

	 * is acked (RFC6298).

	 */

static void tcp_ack_saw_tstamp(struct sock *sk, int flag)

{

	if (flag & FLAG_RETRANS_DATA_ACKED)

		seq_rtt = -1;



	/* RTTM Rule: A TSecr value received in a segment is used to

	 * update the averaged RTT measurement only if the segment

	 * acknowledges some new data, i.e., only if it advances the

	 * left edge of the send window.

	 *

	 * See draft-ietf-tcplw-high-performance-00, section 3.3.

	 * 1998/04/10 Andrey V. Savochkin <saw@msu.ru>

	 *

	 * Changed: reset backoff as soon as we see the first valid sample.

	 * If we do not, we get strongly overestimated rto. With timestamps

	 * samples are accepted even from very old segments: f.e., when rtt=1

	 * increases to 8, we retransmit 5 times and after 8 seconds delayed

	 * answer arrives rto becomes 120 seconds! If at least one of segments

	 * in window is lost... Voila.	 			--ANK (010210)

	 */

	struct tcp_sock *tp = tcp_sk(sk);

	if (seq_rtt < 0 && tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)

		seq_rtt = tcp_time_stamp - tp->rx_opt.rcv_tsecr;



	tcp_valid_rtt_meas(sk, tcp_time_stamp - tp->rx_opt.rcv_tsecr);

}



static void tcp_ack_no_tstamp(struct sock *sk, u32 seq_rtt, int flag)

{

	/* We don't have a timestamp. Can only use

	 * packets that are not retransmitted to determine

	 * rtt estimates. Also, we must not reset the

	 * backoff for rto until we get a non-retransmitted

	 * packet. This allows us to deal with a situation

	 * where the network delay has increased suddenly.

	 * I.e. Karn's algorithm. (SIGCOMM '87, p5.)

	 */



	if (flag & FLAG_RETRANS_DATA_ACKED)

	if (seq_rtt < 0)

		return;



	tcp_valid_rtt_meas(sk, seq_rtt);

}

	tcp_rtt_estimator(sk, seq_rtt);

	tcp_set_rto(sk);



static inline void tcp_ack_update_rtt(struct sock *sk, const int flag,

				      const s32 seq_rtt)

{

	const struct tcp_sock *tp = tcp_sk(sk);

	/* Note that peer MAY send zero echo. In this case it is ignored. (rfc1323) */

	if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)

		tcp_ack_saw_tstamp(sk, flag);

	else if (seq_rtt >= 0)

		tcp_ack_no_tstamp(sk, seq_rtt, flag);

	/* RFC6298: only reset backoff on valid RTT measurement. */

	inet_csk(sk)->icsk_backoff = 0;

}



/* Compute time elapsed between (last) SYNACK and the ACK completing 3WHS. */
@@ -2989,8 +2960,6 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets, 
			if (sacked & TCPCB_SACKED_RETRANS)

				tp->retrans_out -= acked_pcount;

			flag |= FLAG_RETRANS_DATA_ACKED;

			ca_seq_rtt = -1;

			seq_rtt = -1;

		} else {

			ca_seq_rtt = now - scb->when;

			last_ackt = skb->tstamp;