[Cake] lower bounds for latency

Dave Taht dave.taht at gmail.com
Fri Jun 5 21:58:56 EDT 2015

On Fri, Jun 5, 2015 at 6:02 PM, David Lang <david at lang.hm> wrote:
> On Fri, 5 Jun 2015, Dave Taht wrote:
>> bob's been up to good stuff lately..
>> http://bobbriscoe.net/projects/latency/sub-mss-w.pdf
> one thing that looks wrong to me. He talks about how TCP implementations
> cannot operate at less than two packets per RTT. It's not clear what he
> means. Does he mean two packets in flight per RTT? or two packets worth of
> buffering per RTT?


I also disagree with this statement:

"It is always wrong to send smaller packets more
often, because the constraint may be packet pro-
cessing, not bits."

because I believe (but would have to go look at some data to make
sure) that we're good with packet sizes down into the 300 byte range
on most hardware, and thus could (and SHOULD) also reduce the mss in
these cases to keep the "signal strength" up at a sustainable levels.

I do recall that bittorrent used to reduce the mss and were asked to
stop (a decade ago), when they got as far down as 600 bytes or so.

but the rest I quite liked.

> Two packets in flight per RTT would make sense as a minimum, but two packets
> worth of buffering on N devices in the path doesn't.
> using the example of a 6ms RTT. Depending on the equipment involved, this
> could have from one to several devices handling the packets between the
> source and the destination. Saying that each device in the path must have
> two packets worth of buffering doesn't make sense. At a given line speed and
> data rate, you will have X packets in flight. the number of devices between
> the source and the destination will not change X.


> If the requirement is that there are always at least two packets in flight
> in a RTT, it doesn't then follow that both packets are going to be in the
> buffer of the same device at the same time. I spoke with a vendor promising
> 7ms Los Angeles to Los Vegas. For the vast majority of that 7ms the packets
> are not in the buffers of the routers, but exist only as light in the fiber
> (I guess you could view the fiber acting as a buffer in such conditions)
> where is the disconnect between my understanding and what Bob is talking
> about?

Flight, not buffering. Redefining the goal of an aqm to keep packets
in flight rather than achieve a fixed queuing delay is what this is
about, and improving tcps to also keep packets in flight with
subpacket windows is part of his answer.

I like getting away from a target constant for delay (why 5ms when 5us
is doable) and this is an interesting way to think about it from both

And I was nattering about how I didn't like delayed acks just a few hours ago.

> David Lang
>> It was weird, only last night I was thinking upon the real lower
>> bounds on what was needed to keep a flow going in tcp at X,Y,Z rtts
>> (in the context of being dissatisified with the stanford result, and
>> not "quite" in the context of "buffering"), and he nails that in the
>> first paragraph.
>> Have to work through his prescription though....
>> --
>> Dave Täht
>> What will it take to vastly improve wifi for everyone?
>> https://plus.google.com/u/0/explore/makewififast
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Dave Täht
What will it take to vastly improve wifi for everyone?

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