[Cake] lower bounds for latency

David Lang david at lang.hm
Fri Jun 5 21:02:55 EDT 2015

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?

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?

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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