Re: [tcpm] Linux doesn’t implement RFC3465

Yuchung Cheng <ycheng@google.com> Fri, 30 July 2021 18:05 UTC

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From: Yuchung Cheng <ycheng@google.com>
Date: Fri, 30 Jul 2021 11:04:32 -0700
Message-ID: <CAK6E8=ep0wNzLq59GnenSAZSq3STTgERBAr6bTMqn0txg==18A@mail.gmail.com>
To: Vidhi Goel <vidhi_goel=40apple.com@dmarc.ietf.org>
Cc: Neal Cardwell <ncardwell@google.com>, Mark Allman <mallman@icir.org>, Extensions <tcpm@ietf.org>
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Subject: Re: [tcpm] =?utf-8?q?Linux_doesn=E2=80=99t_implement_RFC3465?=
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On Thu, Jul 29, 2021 at 6:03 PM Vidhi Goel <vidhi_goel=
40apple.com@dmarc.ietf.org> wrote:

> Well, perhaps.  L=2 was designed to exactly counteract delayed ACKs.
>>> So, it isn't exactly a new magic number.  We could wave our hands
>>> and say "5 seems OK" or "10 seems OK" or whatever.  And, I am sure
>>> we could come up with something that folks felt was fine.  However,
>>> my feeling is that if we want to worry about bursts then let's worry
>>> about bursts in some generic way.  And, if you have some way to deal
>>> with bursts then L isn't needed.  And, if you don't have a way to
>>> deal with bursts then a conservative L seems fine.  But, perhaps
>>> putting the effort into a generic mechanism instead of cooking yet
>>> another magic number we need to periodically refresh is probably a
>>> better way to spend effort.
>>>
>>
>> Yes, I very much agree that "putting the effort into a generic mechanism
>> instead of cooking yet another magic number we need to periodically refresh
>> is probably a better way to spend effort.”
>>
>>
>> I agree that defining such a number doesn’t fully solve the problem but
>> it gives some recommendation for implementations that don’t do pacing. So,
>> defining a somewhat less restrictive value for L (5 or 10) would be a last
>> resort for implementations that don’t pace.
>>
> How about putting a number 10, and also put all the rationales to follow
> to decide a higher or lower value. It's never one-size for all.
>
>
> That sounds great. Something on the lines of,
>
>  “This document RECOMMENDS using mechanisms like Pacing to control how
> many bytes are sent to the network at a point of time. But if it is not
> possible to implement pacing, an implementation MAY implicitly pace their
> traffic by applying a limit L to the increase in congestion window per ACK
> during slow start. In modern stacks, acknowledgments are aggregated for
> various reason, CPU optimization, reducing network load etc. Hence it is
> common for a sender to receive an aggregated ACK that acknowledges more
> than 2 segments. For example, a stack that implements GRO could aggregate
> packets up to 64Kbytes or ~44 segments before passing on to the TCP layer
> and this would result in a single ACK to be generated by the TCP stack.
> Given that an initial window of 10 packets in current deployments has
> been working fine, the draft makes a recommendation to set L=10 during slow
> start. This would mean that with every ACK, we are probing for a new
> capacity by sending 10 packets in addition to the previously discovered
> capacity. Implementations MAY choose to set a lower limit if they believe
> an increase of 10 is too aggressive."
>
> Does this sound like what we would like to say?
>
Thanks for taking a shot. I would put more description on Pacing to ensure
better implementation. How about:
"Pacing here refers to spread packet transmission following a rate based on
the congestion window and round trip." with a citation of
https://datatracker.ietf.org/doc/html/rfc7661#section-4.4.2



I would also refer to IW RFC 6928 in case it gets increased / updated a few
years later.
Hmm maybe we should also move RFC6928 to the standard track :-)


> -
> Vidhi
>
> On Jul 29, 2021, at 1:47 PM, Yuchung Cheng <
> ycheng=40google.com@dmarc.ietf.org> wrote:
>
>
>
> On Thu, Jul 29, 2021 at 1:19 PM Vidhi Goel <vidhi_goel=
> 40apple.com@dmarc.ietf.org> wrote:
>
>> Well, perhaps.  L=2 was designed to exactly counteract delayed ACKs.
>>> So, it isn't exactly a new magic number.  We could wave our hands
>>> and say "5 seems OK" or "10 seems OK" or whatever.  And, I am sure
>>> we could come up with something that folks felt was fine.  However,
>>> my feeling is that if we want to worry about bursts then let's worry
>>> about bursts in some generic way.  And, if you have some way to deal
>>> with bursts then L isn't needed.  And, if you don't have a way to
>>> deal with bursts then a conservative L seems fine.  But, perhaps
>>> putting the effort into a generic mechanism instead of cooking yet
>>> another magic number we need to periodically refresh is probably a
>>> better way to spend effort.
>>>
>>
>> Yes, I very much agree that "putting the effort into a generic mechanism
>> instead of cooking yet another magic number we need to periodically refresh
>> is probably a better way to spend effort.”
>>
>>
>> I agree that defining such a number doesn’t fully solve the problem but
>> it gives some recommendation for implementations that don’t do pacing. So,
>> defining a somewhat less restrictive value for L (5 or 10) would be a last
>> resort for implementations that don’t pace.
>>
> How about putting a number 10, and also put all the rationales to follow
> to decide a higher or lower value. It's never one-size for all.
>
> Also I believe it's time to move ABC into the standards track, in the era
> of (bigger and bigger) stretch ACKs.
>
>
>> Thanks,
>> Vidhi
>>
>>
>>
>> On Jul 29, 2021, at 8:19 AM, Neal Cardwell <ncardwell@google.com> wrote:
>>
>>
>>
>> On Thu, Jul 29, 2021 at 10:06 AM Mark Allman <mallman@icir.org> wrote:
>>
>>>
>>> >>     (b) If there is no burst mitigation then we have to figure out
>>> >>         if L is still useful for this purpose and whether we want to
>>> >>         retain it.  Seems like perhaps L=2 is sensible here.  L was
>>> >>         never meant to be some general burst mitigator.  However,
>>> >>         ABC clearly *can* aggravate bursting and so perhaps it makes
>>> >>         sense to have it also try to limit the impact of the
>>> >>         aggravation (in the absence of some general mechanism).
>>> >
>>> > Even if recommending a static L value, IMHO L=2 is a bit
>>> > conservative.
>>>
>>> Well, perhaps.  L=2 was designed to exactly counteract delayed ACKs.
>>> So, it isn't exactly a new magic number.  We could wave our hands
>>> and say "5 seems OK" or "10 seems OK" or whatever.  And, I am sure
>>> we could come up with something that folks felt was fine.  However,
>>> my feeling is that if we want to worry about bursts then let's worry
>>> about bursts in some generic way.  And, if you have some way to deal
>>> with bursts then L isn't needed.  And, if you don't have a way to
>>> deal with bursts then a conservative L seems fine.  But, perhaps
>>> putting the effort into a generic mechanism instead of cooking yet
>>> another magic number we need to periodically refresh is probably a
>>> better way to spend effort.
>>>
>>
>> Yes, I very much agree that "putting the effort into a generic mechanism
>> instead of cooking yet another magic number we need to periodically refresh
>> is probably a better way to spend effort."
>>
>>>
>>> >>   - During slow starts that follow RTOs there is a general
>>> >>     problem that just because the window slides by X bytes
>>> >>     doesn't say anything about the *network*, as that sliding can
>>> >>     happen because much of the data was likely queued for the
>>> >>     application on the receiver.  So, e.g., you can RTO and send
>>> >>     one packet and get an ACK back that slides the window 10
>>> >>     packets.  That doesn't mean 10 packets left.  It means one
>>> >>     packet left the network and nine packets are eligible to be
>>> >>     sent to the application.  So, it is not OK to set the cwnd to
>>> >>     1+10 = 11 packets in response to this ACK.  Here L should
>>> >>     exist and be 1.
>>> >
>>> > AFAICT this argument only applies to non-SACK connections. For
>>> > connections with SACK (the vast majority of connections over the
>>> > public Internet and in datacenters), it is quite feasible to
>>> > determine how many packets really left the network (and Linux TCP
>>> > does this; see below).
>>>
>>> If you have an accurate way to figure out how many of the ACKed
>>> bytes left the network and how many were just buffered at the
>>> receiver then I see no problem with increasing based on byte count
>>> as you do in the initial slow start.
>>>
>>> (I don't remember what the paper you cite says, but my guess is it's
>>> often the case that L=1 is a reasonable substitute for something
>>> complicated here.  But, perhaps I am running the simulation in my
>>> head wrong ... it has been a while, admittedly!)
>>>
>>> > Yes, offload mechanisms are so pervasive in practice,
>>>
>>> I am trying to build a mental model here.  How pervasive would you
>>> guess these are?  And, where in the network?  I have assumed that
>>> they are for sure pervasive in data centers and server farms, but
>>> not for the vast majority of Internet-connected devices.
>>>
>>
>> From my impression looking at public Internet traces, aggregation
>> mechanisms that cause TCP ACKs for more than 2 segments are very common. I
>> suspect that's because the majority of public Internet traffic these days
>> has a bottleneck that is either wifi, cellular, or DOCSIS, and all of these
>> have a shared medium with a large latency overhead for L2 MAC control of
>> gets to speak next. So a lot of batching happens, both in big batches of
>> data that arrive at the client in the same L2 medium time slot, and big
>> batches of ACKs that accumulate while the client waits (often several
>> milliseconds, sometimes even tens of milliseconds) for its chance to send a
>> big stretch ACK or batch of ACKs.
>>
>> This brings up a related point: even if there is some ABC-style per-ACK L
>> limit on cwnd increases, the time structure of most public Internet ACK
>> streams is massively bursty because of these aggregation mechanisms
>> inherent in L2 behavior on most public Internet bottlenecks (wifi,
>> cellular, DOCSIS). So even if there is a limit L that limits the per-ACK
>> behavior to be smooth, if there is no pacing of data segments then the data
>> transmit time structure will still be bursty because the ACK arrivals these
>> days are very bursty.
>>
>> best regards,
>> neal
>>
>>
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