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

[Ilpo Järvinen <[e-mail]>]

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.
> 
>  
> 
>                                                                             
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>  
> 
> 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
> 
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