Re: [tcpm] New Version Notification fordraft-ietf-tcpm-accurate-ecn-11.txt

[Bob Briscoe <[e-mail]>]

Ilpo,

Can you please describe the idea more fully? I cannot understand it from 
the description, although I thought I understood it when you explained 
it to me (off list).

Here's my understanding of the problem, for the benefit of the list 
(Ilpo and I have been discussing this off list, and I don't think the 
list will understand without this):

* This is all in the context of a Receiver that is sending AccECN TCP 
Options. It doesn't have to send these options at all but, if it does, 
they complement and help disambiguate the 3-bit ACE counter in the main 
TCP header (which is always used).
* Further context: Even if a receiver is sending the AccECN Option, it 
can choose to sometimes send ACKs without an AccECN Option, and the 
sender can then assume that all the bytes ACK'd had the same ECN marking 
(perhaps wrongly if there have been ACK losses).
* The proposed rules for when to use the AccECN Option are here:
https://tools.ietf.org/html/draft-ietf-tcpm-accurate-ecn-11#section-3.2.3.3

The first rule is:

    Change-Triggered ACKs:  If an arriving packet increments a different
       byte counter to that incremented by the previous packet, the Data
       Receiver SHOULD immediately send an ACK with an AccECN Option,
       without waiting for the next delayed ACK [...].

       Even though this bullet is stated as a "SHOULD", it is important
       for a transition to immediately trigger an ACK if at all possible,
       ...

Originally Ilpo thought this made the sender implementation too 
difficult, even in very common delayed ACK cases, e.g.:

     ECT1
ACK#1 (ECT1 counter)
     ECT1
     CE
ACK#2 (ECT1 & CE counters)
     ECT1
ACK#3 (ECT1 counter)
     ECT1
     ECT1
ACK#4 (no AccECN Option)
     ECT1
     ECT1
ACK#5 (no AccECN Option)
...

In ACK#2, both ECT1 and CE counters have incremented, so how does the 
sender know which counter triggered this ACK? it needs to know this to 
guess which counter to increment if it gets an ACK without an AccECN Option.
My answer: the sender can infer that CE triggered the second ACK shown, 
because in the previous ACK, only ECT1 was incrementing.

Ilpo was concerned that, in the case where ACK#3 is lost or thinned, the 
sender could assume from the subsequent lack of any AccECN Options that 
more and more CE was arriving, when actually more and more ECT1 was 
arriving.

I agreed, but my argument was that there are more "SHOULD" rules for the 
receiver, such as:

       *  It SHOULD always include an AccECN Option if the r.ceb counter
          is incrementing and it MAY include an AccECN Option if r.ec0b
          or r.ec1b is incrementing

So, certainly it is possible that there will be ambiguous occasions when 
there is no AccECN Option. However, the idea is not to omit an AccECN 
option at all unless there is no space (e.g. because some other option 
takes priority). So these cases where assumptions have to be made are 
not intended to be routine - the rules are only written as SHOULDs in 
order to give the receiver flexibility if it needs the space to send 
other options. it is not expected to break all the SHOULD rules all the 
time.

Nonetheless, it did make sense to add this rule, at Ilpo's suggestion:

       *  It SHOULD include a counter that has continued to increment on
          the next scheduled ACK following a change-triggered ACK;

For example, ACks #3 and #5 below are due to this rule:

     ECT1
ACK#1 (ECT1 counter)
     ECT1
     CE
ACK#2 (ECT1 & CE counters)
     CE
     CE
ACK#3 (CE counter)
     ECT1
ACK#4 (ECT1 counter)
     ECT1
     ECT1
ACK#5 (ECT1 counter)
     ECT1
     ECT1
ACK#6 (no AccECN Option)
...


But I pointed out that, of course, this doesn't fully solve the problem, 
because this second AccECN Option could be lost, just as easily as the 
first (obviously a lower probability that they are both lost, but this 
could be common with heavy ACK thinning).

It was at this point that Ilpo suggested that a 2-bit field in a 1-byte 
option could specify which was the last counter to have changed.

However, now I re-read both Ilpo's offlist and the onlist emails, I'm 
not sure I understand the rules of the scheme, whereas I thought I did.


Ilpo, can you write a really precise and full spec of your proposal, 
stating all assumptions?
Are ACKs without any AccECN Option allowed? Or are you saying this 1 
byte option MUST always be included?



Bob

On 10/03/2020 06:38, Scharf, Michael wrote:
>
> Yep, if one byte was added, there would probably be remaining bits left.
>
> RFC 5218 Section 2.2.1: „Protocols that are extensible are more likely 
> to be wildly successful in terms of being used for purposes outside 
> their original design.“
>
> Michael
>
> *Von: *Ilpo Järvinen <mailto:[e-mail]>
> *Gesendet: *Montag, 9. März 2020 23:32
> *An: *Scharf, Michael <mailto:[e-mail]>
> *Cc: *Bob Briscoe <mailto:[e-mail]>; tcpm IETF list 
> <mailto:[e-mail]>; Richard Scheffenegger 
> <mailto:[e-mail]>; Mirja Kuehlewind 
> <mailto:[e-mail]>
> *Betreff: *Re: [tcpm] New Version Notification 
> fordraft-ietf-tcpm-accurate-ecn-11.txt
>
> Hi Michael,
>
> I agree with out  that it's somewhat painful to deal with the marker bit
>
> at the beginning for the option field order.
>
> Another challenge I've come across is to correctly reproduce the
>
> Accurate ECN state at the sender which would benefit from having
>
> a few extra bits available.
>
> It currently requires guessing at every option arrival what the receiver
>
> is doing next (what counter it will be increasing next) which is not very
>
> robust to ACK losses. On change-triggered ACK, two byte counters are
>
> increasing rather than one. ACK losses will easily confuse approach that
>
> just assumes back-and-forth transitions. Thus, I suggested sending one
>
> extra option after the change-triggered ACK to allow sender to see
>
> unambiguous counter delta between the change-triggered ACK and that
>
> next ACK (it is now in -11).
>
> To allow the sender to immediately sync with the receiver on any
>
> AccECN option arrival, it would make sense to encode that information
>
> into the option itself. It would need 2 extra bits into the option
>
> so adding that one byte there would help. ...It might be possible to
>
> combine it to field order and define that the first encoded field is
>
> the currently increasing counter (the order of the remaining two fields
>
> still need to be addressed somehow).
>
> -- 
>
>  i.
>
> On Mon, 9 Mar 2020, Michael Scharf wrote:
>
> >
>
> > With chair hat off, I really wonder if the solution to encode the two
>
> > different orders in the TCP Option is an example for good and robust
>
> > protocol engineering.
>
> >
>
> >
>
> >
>
> > For instance, the current design makes it hard to decode the field in a
>
> > monitoring tool (such as Wireshark). Also, as far as I understand, 
> it does
>
> > not allow to switch the encoding during a connection, which limits
>
> > flexibility. We almost certainly do not understand *now* all future use
>
> > cases of this Standard.
>
> >
>
> >
>
> >
>
> > Unless I miss something, there would be several other solutions:
>
> >
>
> >
>
> >
>
> > First, IMHO, we have enough TCP option codepoints left to spend two
>
> > codepoints if there is a good reason for doing so. As compared to the
>
> > current design proposal in -10/-11, spending two different option kinds
>
> > would look to me like *much* better protocol engineering.
>
> >
>
> >
>
> >
>
> > Second, if the TCPM community insists in only one option kind 
> codepoint for
>
> > whatever reason, IMHO one could add one „sub-type“ byte to the 
> option. The
>
> > TCP Option field has to be multiples of 4 byte, i.e., if a segment only
>
> > contains a 11 byte AccECN TCP option, an additional NOP TCP option 
> is needed
>
> > for padding, no? So, what downside have 12 bytes as compared to 11 
> bytes?
>
> > For the shorter variants, the overhead of a „sub-type“ field 
> increases, but
>
> > it may still be within reasonable limits. What do I miss?
>
> >
>
> >
>
> >
>
> > Third, one could use different lengths for the different orders, e.g.,
>
> > lenths 5/8/11 for type 0 and 6/9/12 for type 12. Is this not possible?
>
> >
>
> >
>
> >
>
> > In all these cases, the resulting protocol looks simpler and more 
> robust to
>
> > me. What prevents us from using the KISS principle?
>
> >
>
> >
>
> >
>
> > Michael
>
> >
>
> >
>
> >
>
> >
>
> >
>
> >
>
> >
>
> > Von: Bob Briscoe
>
> > Gesendet: Freitag, 6. März 2020 04:34
>
> > An: tcpm IETF list
>
> > Cc: Richard Scheffenegger; Mirja Kuehlewind
>
> > Betreff: Re: [tcpm] New Version Notification for
>
> > draft-ietf-tcpm-accurate-ecn-11.txt
>
> >
>
> >
>
> >
>
> > tcpm,
>
> >
>
> > You will have seen draft-10 then draft-11 in quick succession, as 
> already
>
> > explained.
>
> > The diffs from draft-09 to -10 were those that had built up since 
> Jul'19.
>
> > The diffs from draft-10 to -11 were solely those for the change from EXP
>
> > track to STD track.
>
> > Draft-10 doesn't seem to display in the list of links to each 
> version, but
>
> > you can manually write the URL.
>
> >
>
> > The main technical changes in draft-10 were numerous - many will be
>
> > recognized from list discussion since Jul'19.
>
> > Particular thanks to Ilpo Järvinen who identified many niggles (and 
> their
>
> > solutions) while writing and testing a full Linux implementation 
> (based on
>
> > Olivier Tilmans's, in turn based on Mirja's).
>
> >
>
> >  *  Allowed 2 different orders of the fields in the AccECN Option
>
> >  *  Reflect IP-ECN field of SYN/ACK only on ACK of SYN/ACK, not also on
>
> >     first data packet
>
> >      +  greatly simplifies implementation, esp with TFO.
>
> >      +  repeating on first data packet was for reliable delivery, 
> which is
>
> >         now achieved with ACE counter (see next bullet)
>
> >  *  Increment the ACE counter if CE on SYN/ACK (but still not if CE 
> on SYN)
>
> >      +  Reliable delivery of feedback of CE on SYN/ACK
>
> >  *  Redefine 'first packet' as first to arrive, not first in 
> sequence in 2
>
> >     cases:
>
> >      +  Handshake reflection on the ACK of the SYN/ACK
>
> >      +  In the test for zeroing of ACE
>
> >      +  Reason: greatly simplifies implementation
>
> >  *  if ACE could have wrapped more than once, SHOULD assume “safest 
> likely
>
> >     case”
>
> >     not "conservatively assume" it did cycle
>
> >      +  Reason: avoid unnecessary hit on performance
>
> >  *  More robustness (with flexibility) in rules for when to include an
>
> >     AccECN Option
>
> >      +  Change-triggered AccECN Option as SHOULD, not MUST
>
> >      +  SHOULD follow change-triggered AccECN Option with another 
> (removes
>
> >         ambiguity if ACK thinning or loss)
>
> >      +  when same counter continues to increment, SHOULD consistently
>
> >         include it every n ACKs
>
> >      +  Made rule about precedence of SACK conditional (max 2 SACK 
> blocks)
>
> >      +  MAY exclude counters that have not changed for the whole 
> connection
>
> >  *  Allowed an AccECN server not to implement RFC3168 ECN (all 
> clients still
>
> >     have to)
>
> >  *  Precluded mixed capability negotiation from either end
>
> >      +  reduces freedom to choose SYN & SYN/ACK fall-back strategies
>
> >      +  to prevent cases where each end's outcome after handshake 
> could be
>
> >         inconsistent (in reordering corner-cases)
>
> >  *  Reserved the codepoint combination used by the historic nonce case
>
> >  *  Merged in a number of points from RFC3168 that we hadn't covered
>
> >      +  (a whole new subsection about obligations to do with ECN)
>
> >  *  Explicit about checking "acceptable packets"
>
> >      +  before counting their ECN markings or before counting the ECN
>
> >         feedback they carry
>
> >  *  Required retransmitted Fallback SYN to use same ISN
>
> >      +  allows servers to detect ECN downgrade SYN attacks
>
> >  *  Handled corner cases like In-window SYN during TIME-WAIT
>
> >
>
> >
>
> >
>
> > Bob
>
> >
>
> > On 06/03/2020 02:24, [e-mail] wrote:
>
> >
>
> >
>
> >
>
> > A new version of I-D, draft-ietf-tcpm-accurate-ecn-11.txt
>
> >
>
> > has been successfully submitted by Bob Briscoe and posted to the
>
> >
>
> > IETF repository.
>
> >
>
> >
>
> >
>
> > Name: draft-ietf-tcpm-accurate-ecn
>
> >
>
> > Revision: 11
>
> >
>
> > Title:           More Accurate ECN Feedback in TCP
>
> >
>
> > Document date:   2020-03-05
>
> >
>
> > Group:           tcpm
>
> >
>
> > Pages:           58
>
> >
>
> > URL: https://www.ietf.org/internet-drafts/draft-ietf-tcpm-accurat
>
> > e-ecn-11.txt
>
> >
>
> > Status: https://datatracker.ietf.org/doc/draft-ietf-tcpm-accurate-ec
>
> > n/
>
> >
>
> > Htmlized: https://tools.ietf.org/html/draft-ietf-tcpm-accurate-ecn-11
>
> >
>
> > Htmlized: https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-accura
>
> > te-ecn
>
> >
>
> > Diff: https://www.ietf.org/rfcdiff?url2=draft-ietf-tcpm-accurate-e
>
> > cn-11
>
> >
>
> >
>
> >
>
> > 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 is 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 specifies 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, that was
>
> >
>
> >    previously used for the ECN-Nonce which has now been declared
>
> >
>
> >    historic.  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.
>
> >
>
> >
>
> >
>
> >
>
> >
>
> >
>
> >
>
> >
>
> >
>
> >
>
> > Please note that it may take a couple of minutes from the time of 
> submission
>
> >
>
> > until the htmlized version and diff are available at tools.ietf.org.
>
> >
>
> >
>
> >
>
> > The IETF Secretariat
>
> >
>
> >
>
> >
>
> >
>
> >
>
> >
>
> >
>
> >
>

-- 
________________________________________________________________
Bob Briscoe                               http://bobbriscoe.net/