Re: [tcpm] [tsvwg] New Version Notification for draft-grimes-tcpm-tcpsce-00.txt

[Jonathan Morton <[e-mail]>]

> On 21 Jul, 2019, at 9:01 pm, Scharf, Michael <[e-mail]> wrote:
> 
> Please be aware that there is also draft-ietf-tcpm-generalized-ecn-04, which in the current version has a dependency on draft-ietf-tcpm-accurate-ecn for certain cases.
>  
> Just to state the obvious: These documents are work in progress in TCPM and TCPM always welcomes feedback.

I just skimmed through it to remind myself of certain details.  Most of it only applies once AccECN has completed negotiation, so the same arguments apply.

I do however note that SYN is described as the most important packet to protect with ECN, and this is of course sent before AccECN negotiation has completed.  Further, if the SYN-ACK then indicates that AccECN is *not* supported by the remote end - which would be the case for both an SCE endpoint and any conventional one - a conservative IW of 1 segment SHOULD be conservatively selected, with no modification for the increasingly common IW10 case (the default for current versions of Linux).  This incurs a flow completion time delay of approximately 3 RTTs, which could be perceptible to end users.

A less extreme response may be justified here, given that the strongest signal that may have been missed by the lack of ECN feedback is a CE mark, for which the most conservative TCP response is to halve the cwnd.  So for an IW4 sender, the IW should be reduced to 2, and for an IW10 sender, the IW should be reduced to 5.  This would reduce the flow completion penalty to 1 RTT when encountering a non-AccECN endpoint.

 - Jonathan Morton