Subject: Re: [PATCH 3/3] Revert Backoff [v3]: Calculate TCP's connection close threshold as a time value.

From: Damian Lukowski <[email-masked]>
Date: Mon, 04 Jun 2012 19:50:47 +0200

Hi Jerry,

please verify, I understood you correctly.

You have set TCP_RTO_MIN to a lower value, e.g. 0.002 seconds to improve
your internal low-latency traffic. Because of the improvement, R1
timeouts are triggered too fast for external high-RTT traffic. Is that
correct?
If so, may I suggest to set tcp_retries1 to a higher value? For
TCP_RTO_MIN == 0.002 and tcp_retries1 ==  10, R1 will be calculated to
approximately 4 seconds.

Is that ok?

Best regards
 Damian

Am Freitag, den 01.06.2012, 15:58 -0700 schrieb Jerry Chu:
> > From: Damian Lukowski <[email-masked]>
> > Date: Wed, Aug 26, 2009 at 3:16 AM
> > Subject: [PATCH 3/3] Revert Backoff [v3]: Calculate TCP's connection close
> > threshold as a time value.
> > To: Netdev <[email-masked]>
> >
> >
> > RFC 1122 specifies two threshold values R1 and R2 for connection timeouts,
> > which may represent a number of allowed retransmissions or a timeout value.
> > Currently linux uses sysctl_tcp_retries{1,2} to specify the thresholds
> > in number of allowed retransmissions.
> >
> > For any desired threshold R2 (by means of time) one can specify tcp_retries2
> > (by means of number of retransmissions) such that TCP will not time out
> > earlier than R2. This is the case, because the RTO schedule follows a fixed
> > pattern, namely exponential backoff.
> >
> > However, the RTO behaviour is not predictable any more if RTO backoffs can
> > be
> > reverted, as it is the case in the draft
> > "Make TCP more Robust to Long Connectivity Disruptions"
> > (http://tools.ietf.org/html/draft-zimmermann-tcp-lcd).
> >
> > In the worst case TCP would time out a connection after 3.2 seconds, if the
> > initial RTO equaled MIN_RTO and each backoff has been reverted.
> >
> > This patch introduces a function retransmits_timed_out(N),
> > which calculates the timeout of a TCP connection, assuming an initial
> > RTO of MIN_RTO and N unsuccessful, exponentially backed-off retransmissions.
> >
> > Whenever timeout decisions are made by comparing the retransmission counter
> > to some value N, this function can be used, instead.
> >
> > The meaning of tcp_retries2 will be changed, as many more RTO
> > retransmissions
> > can occur than the value indicates. However, it yields a timeout which is
> > similar to the one of an unpatched, exponentially backing off TCP in the
> > same
> > scenario. As no application could rely on an RTO greater than MIN_RTO, there
> > should be no risk of a regression.
> 
> This looks like a typical "fix one problem, introducing a few more" patch :(.
> What do you mean by "no application could rely on an RTO greater than
> MIN_RTO..."
> above? How can you make the assumption that RTO is not too far off
> from TCP_RTO_MIN?
> 
> While you tried to address a problem where the retransmission count
> was high but the actual
> timeout duration was too short, have you considered the other case
> around, i.e., the timeout
> duration is long but the retransmission count is too short? This is
> exactly what's happening
> to us with your patch. We've much reduced TCP_RTO_MIN for our internal
> traffic, but not
> noticing your change has severely shortened the R1 & R2 recommended by
> RFC1122 for our
> long haul traffic until now. In many cases R1 threshold was met upon
> the first retrans timeout.
> 
> I think retransmits_timed_out() should check against both time
> duration and retrans count
> (icsk_retransmits).
> 
> Thought?
> 
> Jerry
> 
> >
> > Signed-off-by: Damian Lukowski <[email-masked]>
> > ---
> >  include/net/tcp.h    |   18 ++++++++++++++++++
> >  net/ipv4/tcp_timer.c |   11 +++++++----
> >  2 files changed, 25 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/net/tcp.h b/include/net/tcp.h
> > index c35b329..17d1a88 100644
> > --- a/include/net/tcp.h
> > +++ b/include/net/tcp.h
> > @@ -1247,6 +1247,24 @@ static inline struct sk_buff
> > *tcp_write_queue_prev(struct sock *sk, struct sk_bu
> >  #define tcp_for_write_queue_from_safe(skb, tmp, sk)                    \
> >        skb_queue_walk_from_safe(&(sk)->sk_write_queue, skb, tmp)
> >
> > +static inline bool retransmits_timed_out(const struct sock *sk,
> > +                                        unsigned int boundary)
> > +{
> > +       int limit, K;
> > +       if (!inet_csk(sk)->icsk_retransmits)
> > +               return false;
> > +
> > +       K = ilog2(TCP_RTO_MAX/TCP_RTO_MIN);
> > +
> > +       if (boundary <= K)
> > +               limit = ((2 << boundary) - 1) * TCP_RTO_MIN;
> > +       else
> > +               limit = ((2 << K) - 1) * TCP_RTO_MIN +
> > +                       (boundary - K) * TCP_RTO_MAX;
> > +
> > +       return (tcp_time_stamp - tcp_sk(sk)->retrans_stamp) >= limit;
> > +}
> > +
> >  static inline struct sk_buff *tcp_send_head(struct sock *sk)
> >  {
> >        return sk->sk_send_head;
> > diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> > index a3ba494..2972d7b 100644
> > --- a/net/ipv4/tcp_timer.c
> > +++ b/net/ipv4/tcp_timer.c
> > @@ -137,13 +137,14 @@ static int tcp_write_timeout(struct sock *sk)
> >  {
> >        struct inet_connection_sock *icsk = inet_csk(sk);
> >        int retry_until;
> > +       bool do_reset;
> >
> >        if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
> >                if (icsk->icsk_retransmits)
> >                        dst_negative_advice(&sk->sk_dst_cache);
> >                retry_until = icsk->icsk_syn_retries ? :
> > sysctl_tcp_syn_retries;
> >        } else {
> > -               if (icsk->icsk_retransmits >= sysctl_tcp_retries1) {
> > +               if (retransmits_timed_out(sk, sysctl_tcp_retries1)) {
> >                        /* Black hole detection */
> >                        tcp_mtu_probing(icsk, sk);
> >
> > @@ -155,13 +156,15 @@ static int tcp_write_timeout(struct sock *sk)
> >                        const int alive = (icsk->icsk_rto < TCP_RTO_MAX);
> >
> >                        retry_until = tcp_orphan_retries(sk, alive);
> > +                       do_reset = alive ||
> > +                                  !retransmits_timed_out(sk, retry_until);
> >
> > -                       if (tcp_out_of_resources(sk, alive ||
> > icsk->icsk_retransmits < retry_until))
> > +                       if (tcp_out_of_resources(sk, do_reset))
> >                                return 1;
> >                }
> >        }
> >
> > -       if (icsk->icsk_retransmits >= retry_until) {
> > +       if (retransmits_timed_out(sk, retry_until)) {
> >                /* Has it gone just too far? */
> >                tcp_write_err(sk);
> >                return 1;
> > @@ -385,7 +388,7 @@ void tcp_retransmit_timer(struct sock *sk)
> >  out_reset_timer:
> >        icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
> >        inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto,
> > TCP_RTO_MAX);
> > -       if (icsk->icsk_retransmits > sysctl_tcp_retries1)
> > +       if (retransmits_timed_out(sk, sysctl_tcp_retries1 + 1))
> >                __sk_dst_reset(sk);
> >
> >  out:;
> > --
> > 1.6.3.3
> >
> > --
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