[rmcat] Following up on my ECN feedback question at the meeting today

[Gorry Fairhurst <[e-mail]>]

I have some follow-up comments on things relating to the transport use 
of ECNthat may relate to draft-dt-rmcat-feedback-message-01.

(1) My meeting comment on jabber was (I may have missed the point 
earlier in the talk) that I wanted to check if a new method could 
potentially hide the presence of a CE-Mark (specifically receiving a 
CE-marked packet and not reporting this seems like something we may wish 
to worry about and I could not work out is the proposal ever omitted ECN 
information).

(2) Now looking at the design team draft, this statement is of course OK 
with me:

"If ECN
[RFC3168] is used, it is necessary to report on the 2-bit ECN mark
in received packets, indicating for each packet whether it is
marked not-ECT, ECT(0), ECT(1), or ECN-CE."

And following this it seems to me that there are two very basic pieces 
of information that I’d love to be able to know as a sender when using 
an ECN-capable network:

(2a) It *can* be important to know that an ECT(?) codepoint is being 
received at the IP layer. A sender may well know that it is sending 
either ECT(0) or ECT(1), but doesn’t definitely know these are being 
received unchanged (i.e., the path has not remarked these).
- Reporting all ECN marks would clearly satisfy this - read no further 
if you plan to always do this.
- A count of how many ECT marks were received is really helpful to know 
that the current ECN forwarding path is broken, it could even allow a 
sender to check that any chosen ECT(x) encoding is working, by 
independently setting x=0 or x=1, if it wishes.
- Knowing which specific ECT(?) is received is potentially useful 
information. This could perhaps be used in a method to determine which 
flavour of ECN-marking is supported by a path, and more info about 
whether ECN is being handled correctly by a network (knowing middleboxes 
don’t do unwanted things can always utilise extra information). (As new 
info, the TSVWG is currently considering work proposing to eliminate the 
use of NONCE encoding where an apps sends ECT(0) and ECT(1) randomly. 
This could simplify the need to encode which ECT(?) codepoints were 
received - see draft-black-tsvwg-ecn-experimentation.)

(2b) A sender must *at least* know if there were any CE marks in a 
reporting interval.
- If you report each CE received than this is sufficient. But I have a 
word of caution. *If* an app looses a report with a CE-mark report it 
may never know, and does not impact user experience in terms of loss, 
but the CC still needs to respond to behave correctly for AQM, and may 
impact delay :-).
- It can be very valuable for the sender to know if ANY CE-marked 
packets were received, even if sending not-ECN capable packets at this 
moment (where it is an error that indicates ECT(?) is unreliable).
- Knowing how many CE-marks (or CE-marked bytes) per interval is 
probably really valuable for a CC. I’d think a sender could easily 
utilise more CE per packet information, and may need to do (e.g., to 
utilise L4S should that be wanted in future).
- Knowing each CE-mark is something that any CC can utilise and could be 
needed for L4S. It could also help to know about whether bursts are 
being marked, rather than average rate.
- Some AQMs may use shallow marking and that is something that COULD be 
nice to detect in future from per-packet CE-mark reports. (At the 
moment, I really think we don’t completely know yet how to do this).


So to me, this suggests at least some counters per reporting interval 
are needed - and per-packet info will be much better.

(3) In relation to "Feedback Frequency and Overhead", section 4. I have 
one less useful comment:

Amortising overhead by reporting less often could perhaps result in a 
performance tradeoff. There may well be quite stringent timing 
implications if an RMCAT app wants to use "low latency" ECN - that’s 
something to be watched as the transport as the L4S work in TSVWG 
progresses, but it would be nice to know that the report interval does 
not constrain our options. I’m arguing that the WG may need to note this 
(or consider this), but I for one, don’t yet know how this will evolve 
in future.

In case anyone needs it, draft-ietf-aqm-ecn-benefits-08 may have some 
more info.

Best wishes,

Gorry