Re: [aqm] TCP ACK Suppression

Simon Barber <simon@superduper.net> Sat, 10 October 2015 20:46 UTC

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Subject: Re: [aqm] TCP ACK Suppression
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I'm talking specifically about duplicate ACKs - same ACK, same contents 
- signalling that the receiver has seen a hole. These should be 
communicated as quickly as possible to reduce head of line blocking and 
application latency - hence the TCP spec that you should stop delaying 
your ACKs in this state.

Simon

On 10/8/2015 9:08 PM, David Lang wrote:
> Well, the only situation we are talking about is when an AQM function 
> finds that there are multiple acks for one TCP session in it's queue. 
> The dispute is if these acks are can be considered 'duplicates' (since 
> the later ones will ack data for the earlier ones) and so can be 
> 'deduped' or if there is a significant amount of value in sending 
> every ack through anyway.
>
> David Lang
>
> On Thu, 8 Oct 2015, Simon Barber wrote:
>
>> I like this suggestion - but it's only needed in the case where there 
>> are dup acks.
>>
>> Simon
>>
>> On 10/8/2015 7:09 PM, David Collier-Brown wrote:
>>> One short interjection: send at least two acks, separated by a time 
>>> based on available packets and/or average noise-burst durations on 
>>> links with noise problems...
>>>
>>>
>>> On 08/10/15 08:04 PM, David Lang wrote:
>>>> On Thu, 8 Oct 2015, Joe Touch wrote:
>>>>
>>>>> On 10/8/2015 3:29 PM, David Lang wrote:
>>>>>> On Thu, 8 Oct 2015, Joe Touch wrote:
>>>>>>
>>>>>>> On 10/8/2015 2:31 PM, David Lang wrote:
>>>>>>>> On Thu, 8 Oct 2015, Joe Touch wrote:
>>>>>>>>
>>>>>>>>> On 10/7/2015 12:42 AM, LAUTENSCHLAEGER, Wolfram (Wolfram) wrote:
>>>>>>>>> ...
>>>>>>>>>> Is this topic addressed in some RFC already?
>>>>>>>>>
>>>>>>>>> It's a direct violation of RFC793, which expects one ACK for 
>>>>>>>>> every two
>>>>>>>>> segments:
>>>>>>>>>
>>>>>>>>> 4.2 Generating Acknowledgments
>>>>>>>>>
>>>>>>>>>   The delayed ACK algorithm specified in [Bra89] SHOULD be 
>>>>>>>>> used by a
>>>>>>>>>   TCP receiver.  When used, a TCP receiver MUST NOT 
>>>>>>>>> excessively delay
>>>>>>>>>   acknowledgments.  Specifically, an ACK SHOULD be generated 
>>>>>>>>> for at
>>>>>>>>>   least every second full-sized segment, and MUST be generated 
>>>>>>>>> within
>>>>>>>>>   500 ms of the arrival of the first unacknowledged packet.
>>>>>>>>
>>>>>>>> actually, this is only a violation of the SHOULD section, not 
>>>>>>>> the MUST
>>>>>>>> section.
>>>>>>>
>>>>>>> When you violate a SHOULD, you need to have a good reason that 
>>>>>>> applies
>>>>>>> in a limited subset of cases.
>>>>>>>
>>>>>>> "it benefits me" isn't one of them, otherwise the SHOULD would 
>>>>>>> *always*
>>>>>>> apply.
>>>>>>>
>>>>>>>> And if the Ack packets are going to arrive at wire-speed anyway 
>>>>>>>> (due to
>>>>>>>> other causes), is there really an advantage to having 32 ack 
>>>>>>>> packets
>>>>>>>> arriving one after the other instead of making it so that the 
>>>>>>>> first ack
>>>>>>>> packet (which arrives at the same time) can ack everything?
>>>>>>>
>>>>>>> If the first ACK confirms everything, you're giving the endpoint 
>>>>>>> a false
>>>>>>> sense of how fast the data was received. This is valid only if the
>>>>>>> *last* ACK is the only one you retain, but then you'll increase 
>>>>>>> delay.
>>>>>>
>>>>>> why does it give the server a false sense of how fast the data was
>>>>>> received? the packets don't have timestamps that the server can 
>>>>>> trust,
>>>>>> they are just packets arriving.
>>>>>
>>>>> Well, the only reason we can no longer trust them is that an
>>>>> intermediate device has tampered with them.
>>>>
>>>> no, you could not trust any timestamps in the packets even if 
>>>> nothing changes the packets between endpoints.
>>>>
>>>>> See, this is the problem - the DOCSIS modem wants to do what *it* 
>>>>> wants,
>>>>> assuming everyone else plays by the rules, but it doesn't care 
>>>>> whether
>>>>> it violates the assumptions other parties are making.
>>>>>
>>>>> That's an example of "tragedy of the commons".
>>>>>
>>>>>> And if the server concludes something
>>>>>> different from 32 packets arriving, each acking 2 packet, but all
>>>>>> arriving one after the other at it's wire speed (let's say it's a 
>>>>>> slow
>>>>>> network, only Gig-E) compared to a single packet arriving that 
>>>>>> acks 64
>>>>>> packets of data at once, it's doing something very strange and 
>>>>>> making
>>>>>> assumptions about how the network works that are invalid.
>>>>>
>>>>> Says who? The RFCs say that this assumption SHOULD be reasonable.
>>>>>
>>>>>>> Unless you know that the endpoint supports ABC and pacing, yes, 
>>>>>>> there's
>>>>>>> a very distinct advantage to getting 32 ACKs rather than 1. It also
>>>>>>> helps with better accuracy on the RTT calculation, which is 
>>>>>>> based on
>>>>>>> sampling (and you've killed 97% of the samples).
>>>>>>
>>>>>> the 97% of the samples that I've killed would be producing 
>>>>>> invalid data
>>>>>> for your calculation because they were delayed in returning.
>>>>>
>>>>> Why do you think that is invalid data? That's an accurate measure 
>>>>> of the
>>>>> return path of the ACK stream.
>>>>
>>>> so how do you sanely conclude anything from 32 ack packets arriving 
>>>> at wire speed back-to-back?
>>>>
>>>>> ...
>>>>>>>> And if there is such an advantage, does it outweight the 
>>>>>>>> disadvantages
>>>>>>>> that the extra ack packets cause by causing highly asymmetric 
>>>>>>>> links to
>>>>>>>> be overloaded and drop packets?
>>>>>>>
>>>>>>> Why is it so bad to drop packets?
>>>>>>
>>>>>> because forcing packets for other services to be dropped to make 
>>>>>> room
>>>>>> for acks degrades those other services.
>>>>>
>>>>> Sure, but remember that we're not here to support the cable company's
>>>>> business model. They deployed networks that had severely
>>>>> underprovisioned backchannels so they could use shared channels 
>>>>> rather
>>>>> than routers one step lower in the hierarchy. Now they pull this 
>>>>> stunt
>>>>> so they can fix what's broken with their provisioning model.
>>>>>
>>>>> The trouble is that it has effects for others in the network, not 
>>>>> just
>>>>> the cable company.
>>>>
>>>> It's not just cable companies. the same sort of thing will happen 
>>>> with half-duplex wifi links where acks will accumulate while data 
>>>> packets are flowing in the other direction.
>>>>
>>>> stop trying to say that this is the fault of one subset of industry 
>>>> and recognize that there are lots of legitimate reasons for this.
>>>>
>>>> Highly asymmetric links are not just 'cable companies 
>>>> underprovisioning their networks'. DSL lines are highly asymmetric 
>>>> due to the difference in the cost of the transmitters on each end 
>>>> of the link. As are Satellite IP systems, etc.
>>>>
>>>>>>> TCP isn't supposed to be the most efficient in EVERY corner 
>>>>>>> case. It's
>>>>>>> supposed to *always work* in EVERY corner case.
>>>>>>
>>>>>> I don't see how it fails to work in this case. As people have 
>>>>>> pointed
>>>>>> out, some cable routers have been doing this for 15 years and the
>>>>>> Internet has not imploded from it yet, so the drawbacks of dropping
>>>>>> these already-delayed and redundant ack packets cannot be the
>>>>>> end-of-the-internet that you are painting it to be
>>>>>
>>>>> Oh, right. That argument. We haven't seen it break anything, so it
>>>>> *must* be safe.
>>>>>
>>>>> What would you see if it were broken? Maybe hosts that burst into the
>>>>> net and caused router buffers to overload? Hmmm.
>>>>
>>>> that happens without this, so you can't blame it on the missing acks.
>>>>
>>>>>> We are talking about only doing this in one specific case, the case
>>>>>> where other things have already caused some of the acks to be 
>>>>>> delayed to
>>>>>> the point where later acks have 'caught up' with them on the 
>>>>>> network and
>>>>>> both early and late acks are sitting in the same queue on the same
>>>>>> device waiting to be sent at the same time.
>>>>>
>>>>> They're in a queue. That means the early ones go out before the late
>>>>> ones. You have two choices if you coalesce their information:
>>>>>
>>>>> a) delete the early ACKs
>>>>>
>>>>>     Oh, but you wouldn't do *that* because it would hit *your*
>>>>>     customers with a higher delay.
>>>>
>>>> but by not having to transmit the early acks, the later ack goes 
>>>> out faster, so the customers get less of a delay in getting the 
>>>> data they have received acked.
>>>>
>>>> If the only thing in the queue is acks, then the last ack in the 
>>>> queue goes out as fast as the first ack would. By doing this you 
>>>> transmit less, which can speed up the network overall as the next 
>>>> station can transmit it's data faster (thinking of wifi as an example)
>>>>
>>>> If there are other packets in the queue, you still can delete all 
>>>> the acks except the last one that will fit into the burst with no 
>>>> degredation in how fast data is acknoleged (and you increase the 
>>>> amount of usable data that is sent in that timeslot instead of 
>>>> wasting it on redundant ack packets)
>>>>
>>>>> b) delete the late ACKs and alter the early ones
>>>>>
>>>>>     Giving your customers a false sense of how fast their
>>>>>     data was getting there. Roadrunner pulled stunts like this
>>>>>     in the early 90's too. It's not exactly news.
>>>>
>>>> unless there are other packets in the flow that the ack is jumping, 
>>>> I see no problem with this. You aren't sending out an ack before 
>>>> the data is arrived, you just aren't delaying the last ack 
>>>> unneccessarily.
>>>>
>>>>>> At this point there are three possiblilities
>>>>>>
>>>>>> 1. all the acks get sent back-to-back, wasting bandwith with their
>>>>>> redundancy
>>>>>
>>>>> That's not a waste; that's information.
>>>>
>>>> very low value information at best.
>>>>
>>>>>> 2. send only the newest ack, trashing all the ones that would be 
>>>>>> redundant
>>>>>
>>>>> If you wait to send it last, maybe... but then you're still 
>>>>> encouraging
>>>>> the receiver to burst its next transmissions. We already know that 
>>>>> sort
>>>>> of bursting causes problems (even if *we* don't see them, someone 
>>>>> does).
>>>>
>>>> how would the burst be any different if the server gets 32 acks 
>>>> back to back vs 1 ack that covers everything. The additional acks 
>>>> aren't going to get there any faster than the single one would.
>>>>
>>>>>> 3. the total of the acks that are queued exceeds the next transmit
>>>>>> window, so only some of the acks get sent, the newest one doesn't 
>>>>>> and
>>>>>> gets delayed further.
>>>>>>
>>>>>>
>>>>>> we know that #2 doesn't break the Internet,
>>>>>
>>>>> No, you really don't. What you know is that #2 is cheap and benefits
>>>>> you. Everyone continually doing that *will* break the Internet.
>>>>
>>>> you keep stating that, but you are short on details about why a 
>>>> string of acks at wire speed is better than a single ack covering 
>>>> the same data 'because I say so' doesn't cut it.
>>>>
>>>>>> it's within the range of
>>>>>> responses permitted by the RFC SHOULD.
>>>>>
>>>>> SHOULD means that breaking it needs to be done for a reason. I've 
>>>>> long
>>>>> argued that the SHOULD should never be there in the first place 
>>>>> without
>>>>> explaining why it isn't a MUST or a MAY and the conditions under 
>>>>> which
>>>>> it might be appropriate to violate it. RFC793 doesn't have that 
>>>>> context,
>>>>> unfortunately, but it doesn't mean that any - and every - SHOULD is
>>>>> intended to be willfully ignored at all times.
>>>>>
>>>>>> It decreases load on congested links.
>>>>>                       ^^^^^^^^^
>>>>> severely underprovisioned
>>>>
>>>> no, merely congested for some reason. Any shared media will have 
>>>> the same situation, when another station is transmitting, a queue 
>>>> builds.
>>>>
>>>>>> But you keep insisting that it's a horrible thing to consider doing.
>>>>>
>>>>> The tragedy of the commons is a horrible thing. Just because 
>>>>> something
>>>>> doesn't hurt you or you can't see how it hurts others doesn't mean 
>>>>> there
>>>>> isn't a problem.
>>>>>
>>>>> I've outlined the reasons why this is bad - basically it works only
>>>>> under the assumption that DOCSIS modems get to play by their own 
>>>>> rules
>>>>> and every one else plays fair. If that's not a bad idea, I don't know
>>>>> what is.
>>>>
>>>> you say that it breaks timing assumptions and calculations, but 
>>>> then don't explain how the train of acks arriving at wire speed 
>>>> would let your calculations be any more accurate.
>>>>
>>>> David Lang
>>>>
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>>>
>>>
>>
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