[tcpm] AccECN: summary text about omitting unchanged fields

Re:
  https://www.ietf.org/archive/id/draft-ietf-tcpm-accurate-ecn-20.html

and this passage:

[EXISTING:]
"Whenever a Data Receiver sends an AccECN Option, the rules in Section
3.2.3.3 allow it to omit unchanged fields from the tail of the option, to
help cope with option space limitations, as long as it preserves the order
of the remaining fields and includes any field that has changed."

I'm a little concerned that some readers might see this passage, and not
actually follow the hyperlinked reference and read Section 3.2.3.3, and
might use the summary in this sentence to conclude that it's OK "to omit
unchanged fields from the tail of the option, to help cope with option
space limitations, as long as it preserves the order of the remaining
fields and includes any field that has changed." But if a data receiver
receives only 1 CE-marked data packet, and then the single ACK carrying an
increment of r.ceb is lost, then following that policy could cause the data
receiver to never again send the r.ceb field (because r.ceb was never again
incremented), and thus could cause the data sender to not receive
notification of a CE congestion event. This would defeat the whole purpose
of Accurate ECN.

To reduce the risk of this mis-reading, I'd suggest shortening this forward
reference to Section 3.2.3.3 to something like:

[SUGGESTED:]
"When a Data Receiver sends an AccECN Option it is allowed in certain
circumstances to omit fields from the tail of the option, but when doing so
MUST follow the rules in Section 3.2.3.3.  When omitting fields the Data
Receiver MUST preserve the order of the remaining fields. Omitting fields
helps cope with option space limitations."

Hopefully consolidating into  Section 3.2.3.3 the rules about when omitting
fields is allowed would help avoid misunderstandings that might compromise
the accuracy of Accurate ECN.

best regards,
neal

On Mon, Jul 25, 2022 at 10:47 AM Bob Briscoe <ietf@bobbriscoe.net> wrote:

> tcpm,
>
> Minor diffs from 19 to 20; Summary:
> 1, Section 3.2.3. The AccECN Option
>
> https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-accurate-ecn-20#section-3.2.3
>      As promised, we've added a stronger motivation for the
> RECOMMENDations for partial implementations.
> 2. Section 7 IANA Considerations:
>
> https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-accurate-ecn-20#section-7
>      Added the ExIDs of the two options used by experimental
> implementations.
>
>
> Bob
>
>
> On 25/07/2022 15:21, 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-20.txt
> >    Pages           : 61
> >    Date            : 2022-07-25
> >
> > 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.
> >
> >
> > 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-20.html
> >
> > A diff from the previous version is available at:
> > https://www.ietf.org/rfcdiff?url2=draft-ietf-tcpm-accurate-ecn-20
> >
> >
> > Internet-Drafts are also available by rsync at rsync.ietf.org:
> :internet-drafts
> >
> >
> > _______________________________________________
> > tcpm mailing list
> > tcpm@ietf.org
> > https://www.ietf.org/mailman/listinfo/tcpm
>
> --
> ________________________________________________________________
> Bob Briscoe                               http://bobbriscoe.net/
>
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