Re: [Bloat] Random idea in reaction to all the discussion of TCP flavours - timestamps?

[Justin McCann <jneilm@gmail.com>]

On Thu, Mar 17, 2011 at 6:56 PM, Jonathan Morton <chromatix99@gmail.com> wrote:
>
> On 18 Mar, 2011, at 12:20 am, Rick Jones wrote:
>
>>>> So initialRTO is specced currently to be 3 seconds, with a small but
>>>> non-trivial effort under way to reduce that, but once established
>>>> connections have a minimum RTO of less than or equal to a second don't
>>>> they?
>>>
>>> If the RTT they measure is low enough, then yes.  If the queues
>>> lengthen, the measured RTT goes up and so does the RTO, once the
>>> connection is established.
>>
>> Right.  I should have been more explicit about "You know it won't
>> retransmit any sooner than N." (for some, changing value of N :)
>
> I think there is a minimum value, on the order of 100ms - I don't know precisely.

The current TCP_MIN_RTO is 200ms on Linux
(http://lxr.linux.no/linux+v2.6.38/include/net/tcp.h#L124). Apparently
it's 30ms on FreeBSD, if this old thread still holds true
(http://www.postel.org/pipermail/end2end-interest/2004-November/004402.html).
From the DCTCP paper, it looks like Windows has a 300 ms RTO.

There were several papers the last couple of years about the minimum
RTO causing low throughput in data center with very small nominal
RTTs.  The DCTCP paper has been mentioned here several times already;
it has a lot of relevant discussion about buffer sizing, latency, AQM,
and the difficulties of tuning RED.

     Justin