Re: [Bloat] dslreports bufferbloat tests

[Jonathan Morton <chromatix99@gmail.com>]

> On 20 Apr, 2016, at 09:06, jb <justin@dslr.net> wrote:
> 
> So this particular snap from the first 200 packets of a download there
> are poor RTTs but it also picked up the IP ecn_capable flag (but not
> the IP ecn congestion flag) and it picked up the TCP ece and cwr flags
> on.

The TCP ECE and CWR flags are always set during ECN negotiation.  If there are no CE marks subsequently, they will not appear again once the data phase begins.  You can therefore distinguish between ECN capability of the host and the network.

For a “download” test, your sender will see ECE flags inbound and generate CWR flags outbound *during the data phase* only if there is an ECN aware AQM on the path at the bottleneck.  It will not see the CE marks themselves, as those are only present between the bottleneck and the receiver.

For an “upload" test, your receiver will see the inbound CE marks from an AQM active on the bottleneck, respond to them with ECE flags, and receive CWR flags in reply.  Again, only during the data phase.

 - Jonathan Morton