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Re: [tcpm] draft-ietf-tcpm-accurate-ecn-09: AccECN option: why not EE1B first?

Mirja Kuehlewind <mirja.kuehlewind@ericsson.com> Tue, 12 November 2019 16:09 UTC

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From: Mirja Kuehlewind <mirja.kuehlewind@ericsson.com>
To: Neal Cardwell <ncardwell=40google.com@dmarc.ietf.org>, "tcpm@ietf.org" <tcpm@ietf.org>
CC: Bob Briscoe <research@bobbriscoe.net>
Thread-Topic: [tcpm] draft-ietf-tcpm-accurate-ecn-09: AccECN option: why not EE1B first?
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Date: Tue, 12 Nov 2019 16:09:13 +0000
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Subject: Re: [tcpm] draft-ietf-tcpm-accurate-ecn-09: AccECN option: why not EE1B first?
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Hi Neal,

that's a good point. The order of the field was based on what is expected to be the most likely case. Currently most traffic, if it uses ECN at all, will set the codepoint to ECT(0). With deployment of L4S this could change in future, however, there are also still ways to extend the negotiation in the AccECN TCP handshake to e.g. support this case better in future. 

Mirja



On 11.11.19, 18:25, "tcpm on behalf of Neal Cardwell" <tcpm-bounces@ietf.org on behalf of ncardwell=40google.com@dmarc.ietf.org> wrote:

    A particular question in regards to the AccECN option:
    
      https://tools.ietf.org/html/draft-ietf-tcpm-accurate-ecn-09#section-3.2.6
    
    The AccECN option spec lists the EE0B field first, with the EE1B field
    at the end.
    
    Given that L4S plans on using ECT(1) for data packets, and unchanging
    counter values can be omitted from the end of the AccECN option, why
    not list EE1B first, and EE0B last? With EE1B first and EE0B last it
    seems that in the common case for an L4S connection the (unchanged)
    EE0B could be omitted, allowing 4 extra bytes of payload per packet.
    AFAICT this extra 4 bytes would increase goodput for applications
    using IPv4 or IPv6 with an MTU of 1500 by about 0.3%, by my
    back-of-the-envelope calculations.
    
    best,
    neal
    
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