netdev - Re: [PATCH v1 net] TCP_USER_TIMEOUT and tcp_keepalive should conform to RFC5482

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Date:   Wed, 9 Aug 2017 21:59:14 -0700
From:   Jerry Chu <hkchu@...gle.com>
To:     Rao Shoaib <rao.shoaib@...cle.com>
Cc:     David Miller <davem@...emloft.net>, codesoldier1@...il.com,
        Yuchung Cheng <ycheng@...gle.com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v1 net] TCP_USER_TIMEOUT and tcp_keepalive should conform
 to RFC5482

On Wed, Aug 9, 2017 at 8:32 PM, Jerry Chu <hkchu@...gle.com> wrote:
> On Wed, Aug 9, 2017 at 5:47 PM, Rao Shoaib <rao.shoaib@...cle.com> wrote:
>>
>>
>> On 08/09/2017 05:30 PM, David Miller wrote:
>>>
>>> From: Joe Smith <codesoldier1@...il.com>
>>> Date: Wed, 9 Aug 2017 17:20:32 -0700
>>>
>>>> Making Linux conform to standards and behavior that is logical seems
>>>> like a good enough reason.
>>>
>>> That's an awesome attitude to have when we're implementing something
>>> new and don't have the facility already.
>>>
>>> But when we have something already the only important consideration is
>>> not breaking existing apps which rely on that behavior.
>>>
>>> That is much, much, more important than standards compliance.
>>>
>>> If users are confused, just fix the documentation.
>>
>> David,
>>
>> If it was just confusion than sure fixing the documentation is fine. What if
>> the logic is incorrect, does not conform to the standard that is says it is
>
> Not sure what part of logic is "incorrect" when it was a homegrown Linux API
> with no need to conform with any "standard"? Note that the new API was invented
> 7 years ago not out of need for RFC5482. In fact I initially call the option
> TCP_FAILFAST and did not even know the existence of RFC5482 until someone
> around the same time proposed a UTO option specifically for RFC5482 and I
> thought the two can be combined. (This is roughly the memory I can
> recollect so far.)
>
> So you see my focus back then was to devise a "failfast" option whereas RFC5482
> was meant for a "failslow" case. I think that explains why I let the
> option override
> keepalive so a TCP connection can "fail fast" while RFC5482 4.2 tries to prevent
> keepalive failure ahead of UTO, favoring "fail slow".
>
> If we start from a clean slate then perhaps one can argue the semantic
> either way
> but we do not have a clean slate. For that I still slightly favor not
> changing the code
> because the risk of breakage is definitely non-zero and the issue you're having
> seem to be only related to documentation.

One more thing - the proposed patch compares TCP_KEEPIDLE against
TCP_USER_TIMEOUT. But I don't think TCP_KEEPIDLE is what the
"keep-alive
timer" referred to in RFC5482. Linux keepalive implementation seems to use #
of retries (TCP_KEEPCNT) rather than time duration (keep-alive time) to
determine when to quit. If that is the case then your proposed change is not
fully "compliant" either and the best is probably just don't change.

>
> Jerry
>
>> implementing and easy to fix with little or no risk of breakage.
>>
>> The proposed patch changes a feature that no one uses. It also imposes the
>> relation ship between keepalive and timeout values that is required by the
>> RFC and make sense.
>>
>> You are the final authority, if you say we should just fix the documentation
>> than that is fine.
>>
>> Shoaib
>>