[tcpm] [Technical Errata Reported] RFC8257 (6697)

RFC Errata System <rfc-editor@rfc-editor.org> Tue, 28 September 2021 07:18 UTC

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Subject: [tcpm] [Technical Errata Reported] RFC8257 (6697)
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The following errata report has been submitted for RFC8257,
"Data Center TCP (DCTCP): TCP Congestion Control for Data Centers".

You may review the report below and at:

Type: Technical
Reported by: Vidhi Goel <vidhi_goel@apple.com>

Section: 3.3

Original Text
The below pseudocode follows after DCTCP.Alpha is updated on ACK processing. This is wrong as cwnd should only be reduced using DCTCP.Alpha when ECE is received. 

9. Rather than always halving the congestion window as described in
       [RFC3168], the sender SHOULD update cwnd as follows:

          cwnd = cwnd * (1 - DCTCP.Alpha / 2)

Corrected Text
Instead, a new paragraph for Congestion Response to ECN feedback would be much clearer. First start with RFC 3168's response to ECE and then provide DCTCP's response to ECE.

I am thinking splitting section 3.3 into two sub-sections - 
3.3.1 Computation of DCTCP.Alpha
3.3.2 Congestion Response to ECE at sender

Although RFC 8257 refers to RFC 3168 congestion window halving at step 9, but it is confusing to put it right after step 8.

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RFC8257 (draft-ietf-tcpm-dctcp-10)
Title               : Data Center TCP (DCTCP): TCP Congestion Control for Data Centers
Publication Date    : October 2017
Author(s)           : S. Bensley, D. Thaler, P. Balasubramanian, L. Eggert, G. Judd
Category            : INFORMATIONAL
Source              : TCP Maintenance and Minor Extensions
Area                : Transport
Stream              : IETF
Verifying Party     : IESG