On Wed, Jan 13, 2021 at 9:12 PM Enke Chen <enkechen2...@gmail.com> wrote:
>
> From: Enke Chen <enc...@paloaltonetworks.com>
>
> The TCP session does not terminate with TCP_USER_TIMEOUT when data
> remain untransmitted due to zero window.
>
> The number of unanswered zero-window probes (tcp_probes_out) is
> reset to zero with incoming acks irrespective of the window size,
> as described in tcp_probe_timer():
>
> RFC 1122 4.2.2.17 requires the sender to stay open indefinitely
> as long as the receiver continues to respond probes. We support
> this by default and reset icsk_probes_out with incoming ACKs.
>
> This counter, however, is the wrong one to be used in calculating the
> duration that the window remains closed and data remain untransmitted.
> Thanks to Jonathan Maxwell <jmaxwel...@gmail.com> for diagnosing the
> actual issue.
>
> In this patch a separate counter is introduced to track the number of
> zero-window probes that are not answered with any non-zero window ack.
> This new counter is used in determining when to abort the session with
> TCP_USER_TIMEOUT.
>
I think one possible issue would be that local congestion (full qdisc)
would abort early,
because tcp_model_timeout() assumes linear backoff.
Neal or Yuchung can further comment on that, it is late for me in France.
packetdrill test would be :
0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0 bind(3, ..., ...) = 0
+0 listen(3, 1) = 0
+0 < S 0:0(0) win 0 <mss 1460>
+0 > S. 0:0(0) ack 1 <mss 1460>
+.1 < . 1:1(0) ack 1 win 65530
+0 accept(3, ..., ...) = 4
+0 setsockopt(4, SOL_TCP, TCP_USER_TIMEOUT, [3000], 4) = 0
+0 write(4, ..., 24) = 24
+0 > P. 1:25(24) ack 1
+.1 < . 1:1(0) ack 25 win 65530
+0 %{ assert tcpi_probes == 0, tcpi_probes; \
assert tcpi_backoff == 0, tcpi_backoff }%
// install a qdisc dropping all packets
+0 `tc qdisc delete dev tun0 root 2>/dev/null ; tc qdisc add dev
tun0 root pfifo limit 0`
+0 write(4, ..., 24) = 24
// When qdisc is congested we retry every 500ms therefore in theory
// we'd retry 6 times before hitting 3s timeout. However, since we
// estimate the elapsed time based on exp backoff of actual RTO (300ms),
// we'd bail earlier with only 3 probes.
+2.1 write(4, ..., 24) = -1
+0 %{ assert tcpi_probes == 3, tcpi_probes; \
assert tcpi_backoff == 0, tcpi_backoff }%
+0 close(4) = 0
> Cc: sta...@vger.kernel.org
> Fixes: 9721e709fa68 ("tcp: simplify window probe aborting on USER_TIMEOUT")
> Reported-by: William McCall <william.mcc...@gmail.com>
> Signed-off-by: Enke Chen <enc...@paloaltonetworks.com>
> ---
> include/linux/tcp.h | 5 +++++
> net/ipv4/tcp.c | 1 +
> net/ipv4/tcp_input.c | 3 ++-
> net/ipv4/tcp_output.c | 2 ++
> net/ipv4/tcp_timer.c | 5 +++--
> 5 files changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index 2f87377e9af7..c9415b30fa67 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -352,6 +352,11 @@ struct tcp_sock {
>
> int linger2;
>
> + /* While icsk_probes_out is for unanswered 0 window probes, this
> + * counter is for 0-window probes that are not answered with any
> + * non-zero window (nzw) acks.
> + */
> + u8 probes_nzw;
>
> /* Sock_ops bpf program related variables */
> #ifdef CONFIG_BPF
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index ed42d2193c5c..af6a41a5a5ac 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2940,6 +2940,7 @@ int tcp_disconnect(struct sock *sk, int flags)
> icsk->icsk_rto = TCP_TIMEOUT_INIT;
> icsk->icsk_rto_min = TCP_RTO_MIN;
> icsk->icsk_delack_max = TCP_DELACK_MAX;
> + tp->probes_nzw = 0;
> tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
> tp->snd_cwnd = TCP_INIT_CWND;
> tp->snd_cwnd_cnt = 0;
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index c7e16b0ed791..4812a969c18a 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -3377,13 +3377,14 @@ static void tcp_ack_probe(struct sock *sk)
> {
> struct inet_connection_sock *icsk = inet_csk(sk);
> struct sk_buff *head = tcp_send_head(sk);
> - const struct tcp_sock *tp = tcp_sk(sk);
> + struct tcp_sock *tp = tcp_sk(sk);
>
> /* Was it a usable window open? */
> if (!head)
> return;
> if (!after(TCP_SKB_CB(head)->end_seq, tcp_wnd_end(tp))) {
> icsk->icsk_backoff = 0;
> + tp->probes_nzw = 0;
> inet_csk_clear_xmit_timer(sk, ICSK_TIME_PROBE0);
> /* Socket must be waked up by subsequent tcp_data_snd_check().
> * This function is not for random using!
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index f322e798a351..1b64cdabc299 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -4084,10 +4084,12 @@ void tcp_send_probe0(struct sock *sk)
> /* Cancel probe timer, if it is not required. */
> icsk->icsk_probes_out = 0;
> icsk->icsk_backoff = 0;
> + tp->probes_nzw = 0;
> return;
> }
>
> icsk->icsk_probes_out++;
> + tp->probes_nzw++;
> if (err <= 0) {
> if (icsk->icsk_backoff < net->ipv4.sysctl_tcp_retries2)
> icsk->icsk_backoff++;
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index 6c62b9ea1320..87e9f5998b8e 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -349,6 +349,7 @@ static void tcp_probe_timer(struct sock *sk)
>
> if (tp->packets_out || !skb) {
> icsk->icsk_probes_out = 0;
> + tp->probes_nzw = 0;
> return;
> }
>
> @@ -360,8 +361,8 @@ static void tcp_probe_timer(struct sock *sk)
> * corresponding system limit. We also implement similar policy when
> * we use RTO to probe window in tcp_retransmit_timer().
> */
> - if (icsk->icsk_user_timeout) {
> - u32 elapsed = tcp_model_timeout(sk, icsk->icsk_probes_out,
> + if (icsk->icsk_user_timeout && tp->probes_nzw) {
> + u32 elapsed = tcp_model_timeout(sk, tp->probes_nzw,
> tcp_probe0_base(sk));
>
> if (elapsed >= icsk->icsk_user_timeout)
> --
> 2.29.2
>
