netdev - Re: [PATCH net-next 2/3] ipv4: Use binary search to choose tcp PMTU probe

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Date:	Mon, 16 Feb 2015 18:59:02 -0500
From:	John Heffner <johnwheffner@...il.com>
To:	Fan Du <fengyuleidian0615@...il.com>
Cc:	Fan Du <fan.du@...el.com>, David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 2/3] ipv4: Use binary search to choose tcp PMTU probe_size

On Mon, Feb 16, 2015 at 12:27 AM, Fan Du <fengyuleidian0615@...il.com> wrote:
> 于 2015年02月14日 01:52, John Heffner 写道:
>
>> On Fri, Feb 13, 2015 at 3:16 AM, Fan Du <fan.du@...el.com> wrote:
>>> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
>>> index 0732b78..9d1cfe0 100644
>>> --- a/net/ipv4/tcp_timer.c
>>> +++ b/net/ipv4/tcp_timer.c
>>> @@ -113,7 +113,7 @@ static void tcp_mtu_probing(struct
>>> inet_connection_sock *icsk, struct sock *sk)
>>>                          struct tcp_sock *tp = tcp_sk(sk);
>>>                          int mss;
>>>
>>> -                       mss = tcp_mtu_to_mss(sk,
>>> icsk->icsk_mtup.search_low) >> 1;
>>> +                       mss = tcp_mtu_to_mss(sk,
>>> icsk->icsk_mtup.search_low);
>>>                          mss = min(net->ipv4.sysctl_tcp_base_mss, mss);
>>>                          mss = max(mss, 68 - tp->tcp_header_len);
>>>                          icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk,
>>> mss);
>>
>>
>> Why did you change this?  I think this breaks black hole detection.
>
> hmm, I misunderstood this part.
> In case of pure black hole detection, lowering the current tcp mss instead
> of search_low,
> will make more sense, as current tcp mss still got lost.
>
> -                       mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low)
>>> 1;
> +                       /* try mss smaller than current mss */
> +                       mss = tcp_current_mss(sk) >> 1;
>
> Black hole seems more like a misconfiguration in administrative level on
> intermediate node,
> rather than a stack issue, why keep shrinking mss to get packet through with
> poor performance?

The idea here is to make it robust to route changes, where the new
path has a lower MTU.  This also protects us against paths that have a
lower MTU than the base mss (which you're also trying to raise in this
patch series).

Thanks,
  -John
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