On Sat, Jul 27, 2019 at 4:23 AM Josh Hunt <joh...@akamai.com> wrote:
>
> The current implementation of TCP MTU probing can considerably
> underestimate the MTU on lossy connections allowing the MSS to get down to
> 48. We have found that in almost all of these cases on our networks these
> paths can handle much larger MTUs meaning the connections are being
> artificially limited. Even though TCP MTU probing can raise the MSS back up
> we have seen this not to be the case causing connections to be "stuck" with
> an MSS of 48 when heavy loss is present.
>
> Prior to pushing out this change we could not keep TCP MTU probing enabled
> b/c of the above reasons. Now with a reasonble floor set we've had it
> enabled for the past 6 months.
And what reasonable value have you used ???
>
> The new sysctl will still default to TCP_MIN_SND_MSS (48), but gives
> administrators the ability to control the floor of MSS probing.
>
> Signed-off-by: Josh Hunt <joh...@akamai.com>
> ---
> Documentation/networking/ip-sysctl.txt | 6 ++++++
> include/net/netns/ipv4.h | 1 +
> net/ipv4/sysctl_net_ipv4.c | 9 +++++++++
> net/ipv4/tcp_ipv4.c | 1 +
> net/ipv4/tcp_timer.c | 2 +-
> 5 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/networking/ip-sysctl.txt
> b/Documentation/networking/ip-sysctl.txt
> index df33674799b5..49e95f438ed7 100644
> --- a/Documentation/networking/ip-sysctl.txt
> +++ b/Documentation/networking/ip-sysctl.txt
> @@ -256,6 +256,12 @@ tcp_base_mss - INTEGER
> Path MTU discovery (MTU probing). If MTU probing is enabled,
> this is the initial MSS used by the connection.
>
> +tcp_mtu_probe_floor - INTEGER
> + If MTU probing is enabled this caps the minimum MSS used for
> search_low
> + for the connection.
> +
> + Default : 48
> +
> tcp_min_snd_mss - INTEGER
> TCP SYN and SYNACK messages usually advertise an ADVMSS option,
> as described in RFC 1122 and RFC 6691.
> diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
> index bc24a8ec1ce5..c0c0791b1912 100644
> --- a/include/net/netns/ipv4.h
> +++ b/include/net/netns/ipv4.h
> @@ -116,6 +116,7 @@ struct netns_ipv4 {
> int sysctl_tcp_l3mdev_accept;
> #endif
> int sysctl_tcp_mtu_probing;
> + int sysctl_tcp_mtu_probe_floor;
> int sysctl_tcp_base_mss;
> int sysctl_tcp_min_snd_mss;
> int sysctl_tcp_probe_threshold;
> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index 0b980e841927..59ded25acd04 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -820,6 +820,15 @@ static struct ctl_table ipv4_net_table[] = {
> .extra2 = &tcp_min_snd_mss_max,
> },
> {
> + .procname = "tcp_mtu_probe_floor",
> + .data = &init_net.ipv4.sysctl_tcp_mtu_probe_floor,
> + .maxlen = sizeof(int),
> + .mode = 0644,
> + .proc_handler = proc_dointvec_minmax,
> + .extra1 = &tcp_min_snd_mss_min,
> + .extra2 = &tcp_min_snd_mss_max,
> + },
> + {
> .procname = "tcp_probe_threshold",
> .data = &init_net.ipv4.sysctl_tcp_probe_threshold,
> .maxlen = sizeof(int),
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index d57641cb3477..e0a372676329 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -2637,6 +2637,7 @@ static int __net_init tcp_sk_init(struct net *net)
> net->ipv4.sysctl_tcp_min_snd_mss = TCP_MIN_SND_MSS;
> net->ipv4.sysctl_tcp_probe_threshold = TCP_PROBE_THRESHOLD;
> net->ipv4.sysctl_tcp_probe_interval = TCP_PROBE_INTERVAL;
> + net->ipv4.sysctl_tcp_mtu_probe_floor = TCP_MIN_SND_MSS;
>
> net->ipv4.sysctl_tcp_keepalive_time = TCP_KEEPALIVE_TIME;
> net->ipv4.sysctl_tcp_keepalive_probes = TCP_KEEPALIVE_PROBES;
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index c801cd37cc2a..dbd9d2d0ee63 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -154,7 +154,7 @@ static void tcp_mtu_probing(struct inet_connection_sock
> *icsk, struct sock *sk)
> } else {
> mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low) >> 1;
> mss = min(net->ipv4.sysctl_tcp_base_mss, mss);
> - mss = max(mss, 68 - tcp_sk(sk)->tcp_header_len);
> + mss = max(mss, net->ipv4.sysctl_tcp_mtu_probe_floor);
> mss = max(mss, net->ipv4.sysctl_tcp_min_snd_mss);
> icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
> }
Existing sysctl should be enough ?
tcp_min_snd_mss documentation could be slightly updated.
And maybe its default value could be raised a bit.
