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

[Jonathan Morton <chromatix99@gmail.com>]

On 17 Mar, 2011, at 8:22 pm, Rick Jones wrote:

>> For the benefit of the 3G folks, here are some helpful axioms to discuss:
>> 
>> 1) Buffering more than a couple of seconds of data (without employing
>> AQM) is unhelpful, and will actually increase network load without
>> increasing goodput.  Unless there is a compelling reason, you should
>> try to buffer less than a second.
>> 
>> This is because congestion and packet-loss information takes longer to
>> influence existing flows, and new flows are more difficult to start. 
>> After about 3 seconds of no information, most TCPs will start
>> retransmission - regardless of whether the packets were physically
>> lost, or are simply languishing in a multi-megabyte buffer somewhere.
> 
> 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.

But the *initial* RTO is the important one for unmanaged queue sizing, because that determines whether a new connection can be started without retransmissions, all else functioning correctly of course.  There is no way to auto-tune that.

Note also that with AQM that can re-order packets, the length of the bulk queue starts to matter much less, because the SYN/ACK packets can bypass most of the traffic.  In that case the RTT measured by the existing bulk flows will be higher than the latency seen by new and interactive flows.

 - Jonathan