[PATCH] Add a new TCP_IGNOREIDLE socket option

[Kevin Oberman]

On Sat, Feb 9, 2013 at 6:41 AM, Alfred Perlstein <bright at mu.org> wrote:
> On 2/7/13 12:04 PM, George Neville-Neil wrote:
>>
>> On Feb 6, 2013, at 12:28 , Alfred Perlstein <bright at mu.org> wrote:
>>
>>> On 2/6/13 4:46 AM, John Baldwin wrote:
>>>>
>>>> On Wednesday, February 06, 2013 6:27:04 am Randall Stewart wrote:
>>>>>
>>>>> John:
>>>>>
>>>>> A burst at line rate will *often* cause drops. This is because
>>>>> router queues are at a finite size. Also such a burst (especially
>>>>> on a long delay bandwidth network) cause your RTT to increase even
>>>>> if there is no drop which is going to hurt you as well.
>>>>>
>>>>> A SHOULD in an RFC says you really really really really need to do it
>>>>> unless there is some thing that makes you willing to override it. It is
>>>>> slight wiggle room.
>>>>>
>>>>> In this I agree with Andre, we should not be *not* doing it. Otherwise
>>>>> folks will be turning this on and it is plain wrong. It may be fine
>>>>> for your network but I would not want to see it in FreeBSD.
>>>>>
>>>>> In my testing here at home I have put back into our stack max-burst.
>>>>> This
>>>>> uses Mark Allman's version (not Kacheong Poon's) where you clamp the
>>>>> cwnd at
>>>>> no more than 4 packets larger than your flight. All of my testing
>>>>> high-bw-delay or lan has shown this to improve TCP performance. This
>>>>> is because it helps you avoid bursting out so many packets that you
>>>>> overflow
>>>>> a queue.
>>>>>
>>>>> In your long-delay bw link if you do burst out too many (and you never
>>>>> know how many that is since you can not predict how full all those
>>>>> MPLS queues are or how big they are) you will really hurt yourself even
>>>>> worse.
>>>>> Note that generally in Cisco routers the default queue size is
>>>>> somewhere between
>>>>> 100-300 packets depending on the router.
>>>>
>>>> Due to the way our application works this never happens, but I am fine
>>>> with
>>>> just keeping this patch private.  If there are other shops that need
>>>> this they
>>>> can always dig the patch up from the archives.
>>>>
>>> This is yet another time when I'm sad about how things happen in FreeBSD.
>>>
>>> A developer come forward with a non-default option that's very useful for
>>> some specific workloads, specifically one that contributes much time and $$$
>>> to the project and the community rejects the patches even though it's been
>>> successful in other OSes.
>>>
>>> It makes zero sense.
>>>
>>> John, can you repost the patch?  Maybe there is a way to refactor this
>>> somehow so it's like accept filters where we can plug in a hook for TCP?
>>>
>>> I am very disappointed, but not surprised.
>>>
>> I take away the complete opposite feeling.  This is how we work through
>> these issues.
>> It's clear from the discussion that this need not be a default in the
>> system,
>> and is a special case.  We had a reasoned discussion of what would be best
>> to do
>> and at least two experts in TCP weighed in on the effect this change might
>> have.
>>
>> Not everything proposed by a developer need go into the tree, in
>> particular since these
>> discussions are archived we can always revisit this later.
>>
>> This is exactly how collaborative development should look, whether or not
>> the patch
>> is integrated now, next week, next year, or ever.
>
>
> I agree that discussion is great, we have all learned quite a bit from it,
> about TCP and the dangers of adjusting buffering without considerable
> thought.  I would not be involved in FreeBSD had this type of discussion and
> information not be discussed on the lists so readily.
>
> However, the end result must be far different than what has occurred so far.
>
> If the code was deemed unacceptable for general inclusion, then we must find
> a way to provide a light framework to accomplish the needs of the community
> member.
>
> Take for instance someone who is starting a company that needs this
> facility.  Which OS will they choose?  One who has integrated a useful
> feature?  Or one who has rejected it and left that code in the mailing list
> archives?
>
> As much as expert opinion is valuable, it must include understanding and
> need of handling special cases and the ability to facilitate those special
> cases for our users and developers.

This is a subject rather near to my heart, having fought battles with
congestion back in the dark days of Windows when it essentially
defaulted to TCPIGNOREIDLE. It was a huge pain, but it was the only
way Windows did TCP in the early days. It simply did not implement
slow-start. This was really evil, but in the days when lots of links
were 56K and T-1 was mostly used for network core links, the Internet,
small as it was back then, did not melt, though it glowed a
frightening shade of red fairly often. Today too many systems running
like this would melt thins very quickly.

OTOH, I can certainly see cases, like John's,  where it would be very
beneficial. And, yes, Linux has it. (I don't see this a relevant in
any way except as proof tat not enough people have turned it on to
cause serious problems... yet!) It seems a shame to make everyone who
really has a need develop their own patches or dig though old mail to
find John's.

What I would like to see is a way to have it available, but make it
unlikely to be enabled except in a way that would put up flashing red
warnings and sound sirens to warn people that it is very dangerous and
can be a way to blow off a few of one's own toes.

One idea that popped into my head (and may be completely ridiculous,
is to make its availability dependent on a kernel option and have
warning in NOTES about it contravening normal and accepted practice
and that it can cause serious problems both for yourself and for
others using the network.

I might also note that almost all higher performance (1G and faster)
networks already have a form of this...TSO. In case you hadn't
noticed, TSO will take a large buffer and transmit it as multiple
segments which are transmitted back to back with NO delay or awareness
of congestion. I can confirm that even this limited case can and does
sometimes result in packet loss when router queues are inadequate to
handle the load.
-- 
R. Kevin Oberman, Network Engineer
E-mail: kob6558 at gmail.com