Re: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-13.txt

Asad Sajjad Ahmed <asadsa@ifi.uio.no> Thu, 04 March 2021 23:40 UTC

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Date: Fri, 5 Mar 2021 00:39:41 +0100
From: Asad Sajjad Ahmed <asadsa@ifi.uio.no>
To: Bob Briscoe <ietf@bobbriscoe.net>
Cc: Ingemar Johansson S <ingemar.s.johansson@ericsson.com>, "tsvwg@ietf.org" <tsvwg@ietf.org>
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Subject: Re: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-13.txt
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Bob,

(see [ASA] inline for comments, your original reply got lost for me (not 
in the spam folder, so I had to reconstruct the thread from a later reply.)

> > > -----Original Message-----
> > > From: Bob Briscoe <ietf@bobbriscoe.net>
> > > Sent: den 2 mars 2021 14:54
> > > To: Asad Sajjad Ahmed <asadsa@ifi.uio.no>no>; tsvwg@ietf.org; Ingemar
> > > Johansson S <ingemar.s.johansson@ericsson.com>
> > > Subject: Re: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-13.txt
> > > 
> > > Asad and Ingemar,
> > > 
> > > @Asad. Thank you v much for reminding me of that section in RFC3168. The
> > > text in ecn-l4s-id wasn't trying to allow a higher min window than existing
> > > RFCs. Indeed, it was trying to encourage implementers the other way -
> > > lower. But without forcing anyone, because implementing a lower minimum
> > > than 1  requires a step-change in complexity (as your thesis on this subject
> > > taught me).

[ASA] Sorry, that was what I meant. There is no need, I hope, to reinforce the
same requirement in the L4S experiment, as long as RFC3168
requirement about being responsive to congestion at low cwnd stands.
It's also good that the problem and the requirement of RFC3168 is mentioned
here as with very low queuing delay there is significantly higher likelihood 
for the problem to occur.

> > > 
> > > I don't think the first sentence with the SHOULD contradicts RFC3168, but it
> > > seems the second sentence makes it look that way to some people's eyes.
> > > So let's reword it. Also, I notice that it was only applicable to TCP, not all
> > > transports. How about...
> > > 
> > >         A scalable congestion control SHOULD remain responsive to
> > >         congestion when typical RTTs over the public Internet are
> > >         significantly smaller because they are no longer inflated by
> > >         queuing delay.  It would be preferable for the minimum window of a
> > >         scalable congestion control to be lower than the 1 segment minimum
> > >         for TCP mentioned in S.6.1.2 of [RFC3168] (or an equivalent for other
> > >         transports). However, a lower minimum is not set as a formal
> > > requirement
> > >         for L4S experiments (see Appendix A.1.6 for rationale)."

[ASA] Thanks, this new text is good. But, Appendix A.1.6 needs to reflect the
same view (TCP as is does NOT allow a minimum other the maximum RTO you
are allowed to set [RFC6298], i.e., 60 seconds). For Appendix A.1.6, I would 
also rephrase the paragraph I quoted to be more explicit about, when competing 
with unresponsive flow it's a trade off between being fully responsive and 
going back to behaving same as one Not-ECT flow.

This got me thinking, should an unresponsive ECN flow be allowed to keep using 
the ECT(1) code-point? I would say no because when you decide to go 
unresponsive, you made the decision to harm other traffic (!), you are not 
interested in keeping the queue low (!!), you are gambling that the AQM /drops/ 
your packets with the same probability as for Not-ECT flows (and not mark
them) (!!!). This means that you are, essentially, aiming for no benefits
from L4S so why bother signaling to the network that you are responsive.

This would give a more appropriate trade-off, on one side you keep the
benefits of L4S by being responsive while on the other side you withdraw
your benefits, but at the same time be more "protected from starvation". In 
quotes, because you are now as much protected as the Not-ECT flows you once 
tried to starve.

Asad

> > > 
> > > 
> > > @Ingemar, you said your min in SCReAM is 3 datagrams, but you could
> > > configure it lower. Are SCReAM's datagrams smaller than the MTU? Is the
> > > text above clear enough for a non-TCP transport? Whatever, would you set a
> > > different limit?
> > > 
> > > Bob
> > > 
> > > On 25/02/2021 20:06, Asad Sajjad Ahmed wrote:
> > > > Bob,
> > > > 
> > > > Thanks for updating and for have put in a lot of effort into this draft.
> > > > I have a minor comment regarding the cwnd reduction over ECN when
> > > cwnd
> > > > is already very low:
> > > > 
> > > > Current draft gives a formal requirement:
> > > > 
> > > > §4.3:
> > > >        "A scalable congestion control SHOULD remain responsive to
> > > >         congestion when typical RTTs over the public Internet are
> > > >         significantly smaller because they are no longer inflated by
> > > >         queuing delay.  It would be preferable for the minimum window of a
> > > >         scalable congestion control to be lower than the 2 segment minimum
> > > >         of TCP Reno [RFC5681] but this is not set as a formal requirement
> > > >         for L4S experiments (see Appendix A.1.6 for rationale)."
> > > > 
> > > > §A.1.6:
> > > >       "However, the requirement in Section 4.3 is worded as a "SHOULD"
> > > >        because the existence of a minimum window is not all bad.  When
> > > >        competing with an unresponsive flow, a minimum window naturally
> > > >        protects the flow from starvation by at least keeping some data
> > > >        flowing."
> > > > 
> > > > But, this contradict with RFC3168 (PS):
> > > > 
> > > > §6.1.2:
> > > >       "[...] Therefore, the sending TCP MUST reset the
> > > >        retransmit timer on receiving the ECN-Echo packet when the congestion
> > > >        window is one.  The sending TCP will then be able to send a new
> > > >        packet only when the retransmit timer expires."
> > > > 
> > > > The contradiction here is "SHOULD" vs. "MUST". Also TCP CC, as given
> > > > out by RFC5681, does NOT mandate a minimum transmission rate. TCP's
> > > > loss recovery at low cwnd relies on RTO to recover lost segments and
> > > > TCP using ECN is supposed to mimic this behavior [floyd98]. Otherwise,
> > > > you risk starving the Not-ECT set of flow, driving up the queue and
> > > > increasing the marking probability by going unresponsive at cwnd of
> > > > two segments. This is about harm toward other traffic so it make no
> > > > sense to relax a formal "MUST". Of couse, it is desireable that the
> > > > AQM take action and evict packets from non-responsive flows, but this is
> > > not true for all AQM, one example being CoDel.
> > > > But, the minimum cwnd of 2 does not originate from L4S, but has been
> > > > the current practice for a very long time at least under the Linux TCP stack.
> > > > This, obviously, also needs to be addressed.
> > > > 
> > > > Asad
> > > > 
> > > > [floyd98]
> > > > https://protect2.fireeye.com/v1/url?k=b9da725e-e6414b1d-b9da32c5-
> > > 861fc
> > > > b972bfc-e0a4204086eb23c8&q=1&e=417868d5-deeb-4c4f-9d1e-
> > > debe95d3e33c&u=
> > > > http%3A%2F%2Fwww.icir.org%2Ffloyd%2Fpapers%2Fecnsims.pdf (§8)
> > > > 
> > > > On 21/02/22 14:01:03, 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 Transport Area Working Group WG of the
> > > IETF.
> > > > >           Title           : Identifying Modified Explicit Congestion Notification (ECN)
> > > Semantics for Ultra-Low Queuing Delay (L4S)
> > > > >           Authors         : Koen De Schepper
> > > > >                             Bob Briscoe
> > > > > 	Filename        : draft-ietf-tsvwg-ecn-l4s-id-13.txt
> > > > > 	Pages           : 58
> > > > > 	Date            : 2021-02-22
> > > > > 
> > > > > Abstract:
> > > > >      This specification defines the identifier to be used on IP packets
> > > > >      for a new network service called low latency, low loss and scalable
> > > > >      throughput (L4S).  L4S uses an Explicit Congestion Notification (ECN)
> > > > >      scheme that is similar to the original (or 'Classic') ECN approach.
> > > > >      'Classic' ECN marking was required to be equivalent to a drop, both
> > > > >      when applied in the network and when responded to by a transport.
> > > > >      Unlike 'Classic' ECN marking, for packets carrying the L4S
> > > > >      identifier, the network applies marking more immediately and more
> > > > >      aggressively than drop, and the transport response to each mark is
> > > > >      reduced and smoothed relative to that for drop.  The two changes
> > > > >      counterbalance each other so that the throughput of an L4S flow will
> > > > >      be roughly the same as a non-L4S flow under the same conditions.
> > > > >      Nonetheless, the much more frequent control signals and the finer
> > > > >      responses to them result in much more fine-grained adjustments, so
> > > > >      that ultra-low and consistently low queuing delay (typically sub-
> > > > >      millisecond on average) becomes possible for L4S traffic without
> > > > >      compromising link utilization.  Thus even capacity-seeking (TCP-like)
> > > > >      traffic can have high bandwidth and very low delay at the same time,
> > > > >      even during periods of high traffic load.
> > > > > 
> > > > >      The L4S identifier defined in this document distinguishes L4S from
> > > > >      'Classic' (e.g. TCP-Reno-friendly) traffic.  It gives an incremental
> > > > >      migration path so that suitably modified network bottlenecks can
> > > > >      distinguish and isolate existing traffic that still follows the
> > > > >      Classic behaviour, to prevent it degrading the low queuing delay and
> > > > >      low loss of L4S traffic.  This specification defines the rules that
> > > > >      L4S transports and network elements need to follow to ensure they
> > > > >      neither harm each other's performance nor that of Classic traffic.
> > > > >      Examples of new active queue management (AQM) marking algorithms
> > > and
> > > > >      examples of new transports (whether TCP-like or real-time) are
> > > > >      specified separately.
> > > > > 
> > > > > 
> > > > > The IETF datatracker status page for this draft is:
> > > > > https://datatracker.ietf.org/doc/draft-ietf-tsvwg-ecn-l4s-id/
> > > > > 
> > > > > There are also htmlized versions available at:
> > > > > https://tools.ietf.org/html/draft-ietf-tsvwg-ecn-l4s-id-13
> > > > > https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-ecn-l4s-id-13
> > > > > 
> > > > > A diff from the previous version is available at:
> > > > > https://www.ietf.org/rfcdiff?url2=draft-ietf-tsvwg-ecn-l4s-id-13
> > > > > 
> > > > > 
> > > > > 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.
> > > > > Internet-Drafts are also available by anonymous FTP at:
> > > > > ftp://ftp.ietf.org/internet-drafts/
> > > > > 
> > > > > 
> > > --
> > > __________________________________________________________
> > > ______
> > > Bob Briscoe                               https://protect2.fireeye.com/v1/url?k=2a6f2e59-
> > > 75f4171a-2a6f6ec2-861fcb972bfc-e28ad1342845f12e&q=1&e=417868d5-
> > > deeb-4c4f-9d1e-debe95d3e33c&u=http%3A%2F%2Fbobbriscoe.net%2F
> 
> -- 
> ________________________________________________________________
> Bob Briscoe                               http://bobbriscoe.net/
> 

-- 
Asad