[tcpm] Summary of updates in draft-ietf-tcpm-accurate-ecn-17

Bob Briscoe <ietf@bobbriscoe.net> Mon, 07 March 2022 23:25 UTC

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Subject: [tcpm] Summary of updates in draft-ietf-tcpm-accurate-ecn-17
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The AccECN draft has just been updated to roll up all the changes over 
the last month since draft-16 (3 Feb 22).
All the changes have already been discussed on this list, but they are 
enumerated below for convenience. The last normative change below about 
ACK filtering has been discussed before, but the precise wording hasn't 
been posted to the list until now.

As always, the full diff can be accessed from the datatracker (via the 
above link).

Normative (Changes intended to make a technical difference are *in bold*)

  * §3.1.1
    Negotiation during the TCP handshake
    Reordered the requirements for feeding back the incoming IP-ECN
    field on the SYN-ACK to make more sense {no technical change, just
  * §3.2
    AccECN Feedback
    made "MUST increment byte counters" conditional on support for
    sending of the AccECN TCP Option
  * §
    Testing for Mangling of the IP/ECN Field
    If continuous CE-marking is detected, Data Sende:
      o SHOULD NOT send ECN-capable packets and *then SHOULD NOT respond
        to any ECN feedback* {added second part of this requirement}
      o *Throughout, it MUST remain in the AccECN feedback mode*
        {additional requirement}
      o and it MUST continue to feed back any ECN markings on arriving
        packets (in its role as Data Receiver) {just reworded a bit}
  * §
    Data Receiver Safety Procedures
    Increment-Triggered ACKs:
      o Clarified that the condition about 'new data' means 'newly
        delivered data' {no difference in meaning, just clearer}
      o Changed to 'In either case, *'n' MUST be no greater than 7*.'
        (was 6)
  * §
    Usage of the AccECN TCP Option
    Condition for 'SHOULD include an AccECN TCP Option on every
    scheduled ACK':
      o 'that acknowledges new data' -> 'if any byte counter has
        incremented since the last ACK' {*includes new markings on
  * §3.3.3
    Requirements for TCP ACK Filtering
      o 'SHOULD preserve the correct operation of AccECN feedback'
        {*previously it described how this SHOULD be done, now it just
        gives the high level requirement*}


  * §3.2.1
    Initialization of Feedback Counters
    Made initialization of ECT(1) feedback counter r.e1b the same as
    r.e0b, now that we have the two different TCP Option orders.


  * Made other parts of the spec consistent with the above changes
  * Called out the updates to other RFCs in the abstract
  * Increased depth of table of contents to 4 (was 3)
  * Added titles to all tables (xml2rfc now adds a 'Table xx' caption to
    the HTML rendering whether or not there is a caption title, and even
    if suppress-title=true i enabled).
  * Other minor edits


On 07/03/2022 23:17, internet-drafts@ietf.org wrote:
> A New Internet-Draft is available from the on-line Internet-Drafts directories.
> This draft is a work item of the TCP Maintenance and Minor Extensions WG of the IETF.
>          Title           : More Accurate ECN Feedback in TCP
>          Authors         : Bob Briscoe
>                            Mirja Kühlewind
>                            Richard Scheffenegger
> 	Filename        : draft-ietf-tcpm-accurate-ecn-17.txt
> 	Pages           : 60
> 	Date            : 2022-03-07
> Abstract:
>     Explicit Congestion Notification (ECN) is a mechanism where network
>     nodes can mark IP packets instead of dropping them to indicate
>     incipient congestion to the end-points.  Receivers with an ECN-
>     capable transport protocol feed back this information to the sender.
>     ECN was originally specified for TCP in such a way that only one
>     feedback signal can be transmitted per Round-Trip Time (RTT).  Recent
>     new TCP mechanisms like Congestion Exposure (ConEx), Data Center TCP
>     (DCTCP) or Low Latency Low Loss Scalable Throughput (L4S) need more
>     accurate ECN feedback information whenever more than one marking is
>     received in one RTT.  This document updates the original ECN
>     specification to specify a scheme to provide more than one feedback
>     signal per RTT in the TCP header.  Given TCP header space is scarce,
>     it allocates a reserved header bit previously assigned to the ECN-
>     Nonce.  It also overloads the two existing ECN flags in the TCP
>     header.  The resulting extra space is exploited to feed back the IP-
>     ECN field received during the 3-way handshake as well.  Supplementary
>     feedback information can optionally be provided in a new TCP option,
>     which is never used on the TCP SYN.  The document also specifies the
>     treatment of this updated TCP wire protocol by middleboxes, updating
>     BCP 69 with respect to ACK filtering.
> The IETF datatracker status page for this draft is:
> https://datatracker.ietf.org/doc/draft-ietf-tcpm-accurate-ecn/
> There is also an HTML version available at:
> https://www.ietf.org/archive/id/draft-ietf-tcpm-accurate-ecn-17.html
> A diff from the previous version is available at:
> https://www.ietf.org/rfcdiff?url2=draft-ietf-tcpm-accurate-ecn-17
> Internet-Drafts are also available by rsync at rsync.ietf.org::internet-drafts
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Bob Briscoehttp://bobbriscoe.net/