netdev - Re: [PATCHv3 net-next 2/4] ipv4: Use binary search to choose tcp PMTU probe

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Message-ID: <CABrhC0=ZPJwPOJFkuuJ+pJgn4CoVhZ2tJcrbce6kytVVtsy=9g@mail.gmail.com>
Date:	Sat, 28 Feb 2015 18:20:41 -0500
From:	John Heffner <johnwheffner@...il.com>
To:	Fan Du <fan.du@...el.com>
Cc:	edumazet@...gle.com, David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Fan Du <fengyuleidian0615@...il.com>
Subject: Re: [PATCHv3 net-next 2/4] ipv4: Use binary search to choose tcp PMTU probe_size

> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index a2a796c..c418829 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -1837,11 +1837,13 @@ static int tcp_mtu_probe(struct sock *sk)
>         struct tcp_sock *tp = tcp_sk(sk);
>         struct inet_connection_sock *icsk = inet_csk(sk);
>         struct sk_buff *skb, *nskb, *next;
> +       struct net *net = sock_net(sk);
>         int len;
>         int probe_size;
>         int size_needed;
>         int copy;
>         int mss_now;
> +       int interval;
>
>         /* Not currently probing/verifying,
>          * not in recovery,
> @@ -1854,11 +1856,17 @@ static int tcp_mtu_probe(struct sock *sk)
>             tp->rx_opt.num_sacks || tp->rx_opt.dsack)
>                 return -1;
>
> -       /* Very simple search strategy: just double the MSS. */
> +       /* Use binary search for probe_size bewteen tcp_mss_base,
> +        * and current mss_clamp. if (search_high - search_low)
> +        * smaller than a threshold, backoff from probing.
> +        */
>         mss_now = tcp_current_mss(sk);
> -       probe_size = 2 * tp->mss_cache;
> +       probe_size = (icsk->icsk_mtup.search_high +
> +                     icsk->icsk_mtup.search_low) >> 1;
>         size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
> -       if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high)) {
> +       interval = icsk->icsk_mtup.search_high - icsk->icsk_mtup.search_low;
> +       if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high) ||
> +           interval < net->ipv4.sysctl_tcp_probe_threshold) {
>                 /* TODO: set timer for probe_converge_event */
>                 return -1;
>         }

A couple things: the local variable probe_size here is TCP segment
size, while search_low and search_high are IP datagram sizes.  Use
tcp_mtu_to_mss to subtract headers.  Also, I think if you set
sysctl_tcp_probe_threshold <= 0, this will keep probing indefinitely
at search_low (not useful).  You probably want to test interval <
max(1, sysctl_tcp_probe_threshold).

  -John
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