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Date: Sat, 20 Jul 2013 22:39:28 -0700
From: Yuchung Cheng <ycheng@...gle.com>
To: davem@...emloft.net, ncardwell@...gle.com, edumazet@...gle.com
Cc: nanditad@...gle.com, netdev@...r.kernel.org,
Yuchung Cheng <ycheng@...gle.com>
Subject: [PATCH 2/4 net-next] 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: Yuchung Cheng <ycheng@...gle.com>
---
include/net/tcp.h | 1 -
net/ipv4/tcp_input.c | 67 ++++++++++++++--------------------------------------
2 files changed, 18 insertions(+), 50 deletions(-)
diff --git a/include/net/tcp.h b/include/net/tcp.h
index f9777db..c586847 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -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)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 988e816..b6357ec 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -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);
+
+ /* 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).
+ */
+ if (flag & FLAG_RETRANS_DATA_ACKED)
+ seq_rtt = -1;
-/* Read draft-ietf-tcplw-high-performance before mucking
- * with this code. (Supersedes RFC1323)
- */
-static void tcp_ack_saw_tstamp(struct sock *sk, int flag)
-{
/* 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@....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;
--
1.8.3
--
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