linux-kernel - Re: Get amount of fast retransmissions from TCP info

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Message-ID: <CAF0XkCD_US9yVxsU8GDpEqoH3y3LLOGQF0K8fu5cNbjTewXuNw@mail.gmail.com>
Date:   Wed, 3 May 2017 21:47:55 +0200
From:   Lars Erik Storbukås <storbukas.dev@...il.com>
To:     Andreas Petlund <apetlund@...ula.no>
Cc:     Neal Cardwell <ncardwell@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: Get amount of fast retransmissions from TCP info

2017-04-25 0:20 GMT+02:00 Andreas Petlund <apetlund@...ula.no>:
>
>> On 24 Apr 2017, at 23:31, Lars Erik Storbukås <storbukas.dev@...il.com> wrote:
>>
>> 2017-04-24 23:00 GMT+02:00 Neal Cardwell <ncardwell@...gle.com>:
>>> On Mon, Apr 24, 2017 at 4:20 PM, Lars Erik Storbukås
>>> <storbukas.dev@...il.com> wrote:
>>>> 2017-04-24 21:42 GMT+02:00 Neal Cardwell <ncardwell@...gle.com>:
>>>>> On Mon, Apr 24, 2017 at 3:11 PM, Lars Erik Storbukås
>>>>> <storbukas.dev@...il.com> wrote:
>>>>>> I'm trying to get amount of congestion events in TCP caused by
>>>>>> DUPACK's (fast retransmissions), and can't seem to find any variable
>>>>>> in the TCP info struct which hold that value. There are three
>>>>>> variables in the TCP info struct that seem to hold similar congestion
>>>>>> values: __u8 tcpi_retransmits;__u32 tcpi_retrans; __u32
>>>>>> tcpi_total_retrans;
>>>>>>
>>>>>> Does anyone have any pointers on how to find this value in the TCP code?
>>>>>>
>>>>>> Please CC me personally if answering this question. Any help is
>>>>>> greatly appreciated.
>>>>>
>>>>> [I'm cc-ing the netdev list.]
>>>>>
>>>>> Do you need this per-socket? On a per-socket basis, I do not think
>>>>> there are separate totals for fast retransmits and timeout
>>>>> retransmits.
>>>>>
>>>>> If a global number is good enough, then you can get that number from
>>>>> the global network statistics. In "nstat" output they look like:
>>>>>
>>>>>  TcpExtTCPFastRetrans = packets sent in fast retransmit / fast recovery
>>>>>
>>>>>  TcpExtTCPSlowStartRetrans = packets sent in timeout recovery
>>>>>
>>>>> It sounds like TcpExtTCPFastRetrans is what you are after.
>>>>>
>>>>> Hope that helps,
>>>>> neal
>>>>
>>>> Thanks for your answer Neal.
>>>>
>>>> Yes, I need this information per-socket. What would be the most
>>>> appropriate place to update this value?
>>>
>>> Is this for a custom kernel you are building? Or are you proposing
>>> this for upstream?
>>
>> This is currently for a custom kernel.
>>
>>> IMHO the best place to add this for your custom kernel would be in
>>> _tcp_retransmit_skb() around the spot with the comment "Update global
>>> and local TCP statistics". Something like:
>>>
>>>  /* Update global and local TCP statistics. */
>>> ...
>>>  tp->total_retrans += segs;
>>>  if (icsk->icsk_ca_state == TCP_CA_Loss)
>>>    tp->slow_retrans += segs;
>>>  else
>>>    tp->fast_retrans += segs;
>>>
>>
>> Excellent. That seems like a logical place.
>>
>>>> If none of the variables (mentioned above) contain any value in
>>>> regards to fast retransmits, what does the different values represent?
>>>
>>> tcpi_retransmits: consecutive retransmits of lowest-sequence outstanding packet
>>>
>>> tcpi_retrans: retransmitted packets estimated to be in-flight in the network now
>>>
>>> tcpi_total_retrans: total number of retransmitted packets over the
>>> life of the connection
>>>
>>> Can you sketch out why you need to have separate counts for fast
>>> retransmits and timeout/slow-start retransmits?
>>>
>>> neal
>>
>> I'm working on the implementation of a Deadline Aware, Less than Best
>> Effort framework proposed by David A. Hayes, David Ros, Andreas
>> Petlund. A framework for adding both LBE behaviour and awareness of
>> “soft” delivery deadlines to any congestion control (CC) algorithm,
>> whether loss-based, delay- based or explicit signaling-based. This
>> effectively allows it to turn an arbitrary CC protocol into a
>> scavenger protocol that dynamically adapts its sending rate to network
>> conditions and remaining time before the deadline, to balance
>> timeliness and transmission aggressiveness.
>>
>
> Just for the record, the paper is not publicly available yet, so it’s a bit hard to find:)
> It will be published in IFIP Networking in June.
> We will make it available as  soon as the conference regulations allows.
> You can find the abstract here:
> https://www.simula.no/publications/framework-less-best-effort-congestion-control-soft-deadlines
>
> Cheers,
> Andreas Petlund

I also want to count the amount of ECN signals received. Do anyone
have any input on where to place an ECN signal count?

Is any of these locations a logical place to increase the ECN counter
(which I've created in tcp_sock)? Both locations are in the
tcp_input.c.

/* In tcp_fastretrans_alert() */
if (flag & FLAG_ECE) {
    tp->prior_ssthresh = 0;
    tp->ecn_count += 1; // ECN counter
}

or

/* In tcp_enter_recovery() */
if (!tcp_in_cwnd_reduction(sk)) {
    if (!ece_ack)
        tp->prior_ssthresh = tcp_current_ssthresh(sk);
    else
       tp->ecn_count += 1; // ECN counter
    tcp_init_cwnd_reduction(sk);
}
tcp_set_ca_state(sk, TCP_CA_Recovery);

/Lars Erik