[tcpm] Comments on draft-ietf-tcpm-alternativebackoff-ecn-03
"Black, David" <David.Black@dell.com> Wed, 15 November 2017 14:46 UTC
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From: "Black, David" <David.Black@dell.com>
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To: "tcpm@ietf.org" <tcpm@ietf.org>
Thread-Topic: Comments on draft-ietf-tcpm-alternativebackoff-ecn-03
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Date: Wed, 15 Nov 2017 14:45:40 +0000
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Subject: [tcpm] Comments on draft-ietf-tcpm-alternativebackoff-ecn-03
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I volunteered to review this draft in Prague, so in classic "just before the deadline" IETF style, here are some comments. The draft is applicable to use of AQM in general, but seems to limit its focus/analysis to modern AQM mechanisms such as PIE and CoDel. Some broader discussion of older AQM mechanisms would be a good idea, although it may not be necessary to go all the way back to RED. As part of that, this text at the end of Section 4.1: ([RFC7567<https://tools.ietf.org/html/rfc7567>] notes the current status of RED as an AQM method.) should be strengthened to indicate that current usage of RED is limited. --Section 3: This specification describes an update to the congestion control algorithm of an ECN-capable TCP transport protocol. It allows a TCP stack to update the TCP sender response when it receives feedback indicating reception of a CE-marked packet. It RECOMMENDS that a TCP sender multiplies the slow start threshold (ssthresh) by 0.8 times of the FlightSize (with its minimum value set to 2 * SMSS) and reduces the cwnd in congestion avoidance following reception of a TCP segment that sets the ECN-Echo flag (defined in [RFC3168<https://tools.ietf.org/html/rfc3168>]). This text has several problems including: - it's not clear what the update is - "allows" is too weak a verb - "RECOMMENDS" is not an RFC 2119 keyword Attempted rewrite, including additional editorial changes: This specification updates the congestion control algorithm of an ECN-capable TCP transport protocol by changing the the TCP sender response to feedback from the TCP receiver that indicates reception of a CE-marked packet, i.e., receipt of a packet with the ECN-Echo flag (defined in [RFC3168<https://tools.ietf.org/html/rfc3168>]) set. The updated TCP sender response specification is that the slow start threshold (ssthresh) SHOULD be multiplied by 0.8 times the FlightSize with the result increased to the minimum ssthresh value of 2 * SMSS if necessary. The TCP sender also reduces the the cwnd value to that new ssthresh value. In Section 4.3, the text is not clear that the same cwnd reduction applies to both ECN and packet loss. Section 5: OLD The currently published ECN specification requires that the congestion control response to a CE-marked packet is the same as the response to a dropped packet [RFC3168<https://tools.ietf.org/html/rfc3168>]. The specification is currently being updated to allow for specifications that do not follow this rule [I-D.ECN-exp<https://tools.ietf.org/html/draft-ietf-tcpm-alternativebackoff-ecn-03#ref-I-D.ECN-exp>]. The present specification defines such an experiment and has thus been assigned an Experimental status before being proposed as a Standards-Track update. NEW The original ECN specification, RFC 3168 [RFC3168], required that the congestion control response to a CE-marked packet be the same as the response to a dropped packet [RFC3168<https://tools.ietf.org/html/rfc3168>]. That requirement has been relaxed by RFC YYYY [I-D.ECN-exp<https://tools.ietf.org/html/draft-ietf-tcpm-alternativebackoff-ecn-03#ref-I-D.ECN-exp>] to enable experimentation with different congestion control responses. The present specification is one such experiment; if the experiment succeeds, the changed congestion control response will be published in a standards track RFC to encourage deployment. In: To evaluate the benefit, this experiment therefore requires support in AQM routers (except to enable an ECN-marking mechanism [RFC3168<https://tools.ietf.org/html/rfc3168>] [RFC7567<https://tools.ietf.org/html/rfc7567>]) for ECN-marking of packets carrying the ECN Capable Transport, ECT(0), codepoint [RFC3168<https://tools.ietf.org/html/rfc3168>]. I don't understand the parenthetical "(except to ...)" text - can that just be deleted? Thanks, --David ---------------------------------------------------------------- David L. Black, Distinguished Engineer Dell EMC, 176 South St., Hopkinton, MA 01748 +1 (508) 293-7953 Mobile: +1 (978) 394-7754 David.Black@dell.com<mailto:David.Black@dell.com> ----------------------------------------------------------------
- [tcpm] Comments on draft-ietf-tcpm-alternativebac… Black, David
- Re: [tcpm] Comments on draft-ietf-tcpm-alternativ… Black, David