Re: [tcpm] More motivating scenarios for tcpm-ack-pull

[Bob Briscoe <[e-mail]>]

Jon,

 From the experience of recent years, I've changed my understanding of 
what is the appropriate level of 'simplicity' for each change to TCP.

It takes a huge amount of effort over probably a decade for any change 
to become useful (in terms of both standardization effort, and 
subsequent network traversal effort by all the implementers). So it 
makes little sense to judge simplicity solely on  the simplicity of the 
design. Once you take the whole process into account, it becomes worth 
introducing something perhaps slightly more complex that will be more 
useful to more use-cases. IMHO.


Bob

On 06/11/2019 07:36, Jon Crowcroft wrote:
> thanks v. much - especially for more motivating cases/text
>
> we're strongly inclinded (due to context) to keep our particular approach as simple as possible ...(but
> no simpler...) - that said, sharing common mechanism is also, of course, good if it can be done...
>
>> Carles, Jon, (re-sending, this time without accidentally omitting tcpm,
>> also see couple of addenda tagged [Bob adds:])
>>
>>
>>
>> I'm generally supportive of a mechanism to suppress delayed ACKs from the
>> receiver (in any transport protocol including TCP). So thank you.
>>
>>
>>
>> You might want to borrow at least some of the text for additional
>> motivating scenarios for such a facility from the first two posts on a
>> thread for a similar facility in QuiC:
>>
>> https://github.com/quicwg/base-drafts/issues/1978
>>
>> Also pasted at the end.
>>
>>
>> Some is not relevant to your specific ack-pull scheme, 'cos it's proposing
>> a more general facility in QUIC for the sender to the receiver alters its
>> ACK ratio. (theoretically there are more bits available in QUIC, but this
>> particular request has been put on hold while someone goes and finds them
>> ;)
>>
>>
>>
>>       Another motivating example (rather niche)
>>
>>
>> Coincidentally, Just this morning I published a tech report that had to
>> use
>> the hack of overlapping a byte from the previous segment to force a quick
>> ack: TCP Prague Fall-back on Detection of a Classic ECN AQM <https://
>> arxiv.org/pdf/1911.00710.pdf#page9> (the link takes you to Fig 2, which
>> illustrates the hack) I'd rather not have to do these sorts of hacks - it
>> really messes with the segmentation code.
>>
>>
>>
>>       Middlebox traversal of bit 6
>>
>>
>> I recall that someone did a study of traversal of each of the reserved
>> flags (Marcelo Bagnulo maybe?). I don't think it was that good [Bob adds:
>> I
>> mean traversal - not the study!].
>>
>> A good search engine might find it ;)
>>
>>
>>       More alternative approaches:
>>
>>
>> 1/ At one stage, we tried to include a bit for Delayed Ack control in
>> another protocol called Accurate ECN. We were persuaded to take it out,
>> because it was mission creep (outside the scope of the protocol we were
>> working on):
>>
>> https://tools.ietf.org/html/draft-kuehlewind-tcpm-accurate-ecn-03#appendix-B.4
>>
>>
>> 2/ Have you considered using the Urgent Pointer in a novel way? I.e. a
>> non-zero Urgent Pointer field, even tho the URG flag is zero. [Bob adds:]
>>  From memory, traversal was pretty good - much better than bits 4-6. But I
>> think Windows treated it as a potential attack. I believe Fernando Gont
>> did
>> a study on how 'invalid' TCP header fields are handled.
>>
>>
>>
>> You could assign, say, 3 bits of the urgent pointer for a variable ack_exp
>> that is log base 2 of the ack ratio the sender would like. Then the sender
>> can request the receiver uses ack_ratio` = 2^ack_exp, as in the QUIC
>> proposal below, With ack_exp = 0, you get ack_ratio = 2^0 = 1, which has
>> the same effect as ack-pull. But you also have a more general facility for
>> high speed machines to widen out the ack ratio.
>>
>>
>>
>> If this works, you could open up a registry for the other 13 bits. So
>> instead of using up a precious bit, you generate 13 more ;)
>>
>>
>> See:
>> https://tools.ietf.org/agenda/90/slides/slides-90-tcpm-10.pdf#page=5
>>
>>
>> I think there were problems with some implementations and middleboxes
>> (bound to be). I've checked the minutes of the IETF meeting where that
>> slide was presented <https://www.ietf.org/proceedings/90/minutes/
>> minutes-90-tcpm>, but there's no push-back in there. I suggest you delve
>> back into the archives of the tcpm mailing list around the time of that
>> meeting to find out if there were any show-stoppers.
>>
>>
>> Cheers
>>
>>
>>
>> Bob
>>
>>
>> Paste from QUIC thread:
>>
>> /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/
>> \/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
>>
>>
>>
>>       Problem/goals:
>>
>> 1. Getting up to speed fast without inducing much queue is one of the
>>     main areas where latency reductions are needed (for short and long
>>     flows). The sender needs frequent ACKing during this phase in many
>>     approaches (hystart, etc). A Linux TCP receiver starts with
>>     ack_ratio 1 and uses heuristics to identify the end of the sender's
>>     slow-start (or a re-start after idle). This has ossified a certain
>>     slow-start behaviour into all TCP senders (whether Linux or not). We
>>     are trying to new sender behaviours (paced chirping being an
>>     example), but we need a way for the sender to suppress delayed ACKs.
>> 2. At the other end of the scale, for long-running flows, many current
>>     CC approaches (e.g. BBR) use pacing not ACK-clocking so they can use
>>     very few ACKs per RTT. And fewer means less cache ejections for GRO.
>>     But the receiver doesn't know what CC the sender is using, so it
>>     doesn't know how many or few is too many or too few.
>> 3. Some middleboxes and the majority of link technologies (e.g. DOCSIS,
>>     LTE, Satellite) thin TCP ACKs when they detect the upstream is
>>     filled with TCP ACK stream(s), which are unresponsive. Otherwise the
>>     ACKs constrain downstream throughput (and any upstream data either
>>     in the flow itself, or in others). We don't want these links to
>>     attempt to guess which are the QUIC ACKs and try to thin them. QUIC
>>     can and should do this itself. QUIC has all the machinery for the
>>     sender CC to detect ACK congestion, but not the protocol to tell the
>>     receiver to do the thinning. This protocol addition would provide a
>>     sufficient hook for hosts to unilaterally add this to their CC
>>     behaviour.
>>
>>
>>       Some scenarios where the sender's preferred ack_ratio changes
>>       through the connection:
>>
>> 1.
>>
>>     A sender CC that wants the receiver to turn off DelAcks during
>>     flow-start (e.g. it's using hybrid slow-start as in Cubic and wants
>>     to get delay measurements more frequently) sets ack_exp=0 during
>>     flow-start (ack_ratio=1), then increases ack_exp during congestion
>>     avoidance. If it goes idle, then re-starts, it would set ack_exp=0
>>     again.
>>
>>     Note on heuristics: A Linux TCP receiver currently uses a heuristic
>>     to determine when the sender has exited slow-start. However,
>>     heuristics -> ossification. A Linux receiver's heuristic only works
>>     with the current pattern of slow-start. In TCP, when we tried to
>>     improve the pattern on the sender (paced chirping), the heuristic on
>>     the receiver killed us.
>>
>> 2.
>>
>>     Imagine a paced sender has hardware generic receive offload (GRO),
>>     so for a long-running flow it doesn't want a high rate of QUIC ACKs
>>     that are opaque to GRO. Let's say it would prefer at least 8 ACKs
>>     per RTT. Again, it starts with ack_exp=0, but in congestion
>>     avoidance it would use:
>>
>>     ack_ratio <= cwnd_in_packets/8
>>
>>
>> Upshot: By remote controlling the receiver, the server offloads nearly all
>> the ACKs from large downloads, but still focuses its ACK-receiving
>> resources on getting each client up to speed.
>>
>>
>> Note that calculation of ack_ratio lends itself to fast integer
>> arithmetic.
>>
>>
>>
>>
>>
>>
>>
>> Bob
>>
>>
>> --
>> ________________________________________________________________
>> Bob Briscoehttp://bobbriscoe.net/
>>
>>

-- 
________________________________________________________________
Bob Briscoe                               http://bobbriscoe.net/