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tcp: prefer packet timing to TS-ECR for RTT

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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: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Yuchung Cheng 2013-07-22 16:20:46 -07:00 committed by David S. Miller
parent 375fe02c91
commit 5b08e47caf
2 changed files with 18 additions and 50 deletions
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1
include/net/tcp.h
View File

@ -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)

65
net/ipv4/tcp_input.c
View File

@ -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;