Subject: Re: [PWE3] Transport Area Review: Congestion Control Framework

From: Bob Briscoe <[email-masked]>
Date: Mon, 17 Nov 2008 14:51:04 +0000

David & PWEs

Further to David's points (2) & (3) from back in late Oct...

I had intended to write up my review of this draft, but I've run out 
of time (it's has all been in pen on the draft for some time, but 
converting it to key-strokes has not happened yet, I'm afraid).

1/ My main point would have been that this draft really wants to be 
talking about congestion /policing/, not congestion /control/.

The distinction is that you want the end points to be doing 
congestion control while congestion policing merely detects it 
they're not (or not doing it any thing like enough). Only then would 
a congestion policer intervene. Otherwise a policer should appear transparent.

Congestion policing addresses the issue of safety against congestion 
collapse, while it would also control fairness, but differently from 
how it's been described by David and in the draft.

Even if the policer does kick in (because the PWE3 hasn't been 
behaving), a congestion policer would allow any congestion responsive 
flows in the PWE3 to compensate for any unresponsive flows. This is 
described in the paper below...

As I've run out of time, I'll type up further comments properly 
later. But in the interim, here's very recently accepted paper on 
congestion policing that says what I want to say. The idea is, 
essentially, to limit the rate at which the PWE3 can cause its 
packets to be dropped by bottlenecks - ie not limiting the bit rate 
of all traffic, just the rate at which bits get dropped by 
bottlenecks. This limits the PWE's bit rate only in as much as that 
bit-rate is at the expense of others. Importantly, it makes no 
assumptions about the congestion control that needs to be used by the 
endpoints, whether TCP, TFRC, admission control or nothing. It just 
sets an outer envelope of congestion that the PWE has to keep within.
"Policing Freedom to Use the Internet Resource Pool"
<http://www.cs.ucl.ac.uk/staff/B.Briscoe/pubs.html#polfree>
It's only 6pp.

The paper says loss-detection would need to be added to the network 
for congestion policing to work with loss (rather than just ECN), but 
in PWE3 you're talking about adding loss-detection to the network - 
in this draft.

See you in a few mins in PWE3.

Bob

At 14:46 27/10/2008, [email-masked] wrote:
>                                 Transport Area Review
>                 Pseudowire Congestion Control Framework
>                 draft-ietf-pwe3-congestion-frmwk-01.txt
>
>                                 David L. Black, EMC
>                                 October 27, 2008
>                 ---------------------------------------------
>
>Ordinarily, a review like this would focus on requirements.  Due to the
>unusual circumstances surrounding congestion control for pseudowires,
>this review's primary purpose is to state a number of non-requirements,
>three in particular:
>
>(1) The Fibre Channel pseudo wire is special.  The congestion control
>framework for the Fibre Channel pseudowire (cf.
>draft-ietf-pwe3-fc-encap-08.txt) is specific to that encapsulation and
>is well beyond what appears to be needed and/or appropriate for
>pseudowires in general.
>
>Fibre Channel (FC) implementations are effectively lossless in practice;
>FC frames are dropped under only the most extreme circumstances.  For
>the most common usage of Fibre Channel, carrying SCSI-based I/O via the
>FCP protocol, a dropped FC frame will typically result in the entire I/O
>being retried after a timeout (30 seconds and 60 seconds are common
>values for this timeout).  Typical SCSI I/O latencies are a small
>fraction of a second, and hence this sort of frame loss requiring an
>expensive and delayed I/O retry needs to be a very rare event.  For this
>reason, the FC pseudowire incorporates an reliable delivery protocol
>that includes
>retransmission, and the FC pseudowire's congestion control measures are
>based on that protocol.  In essence, this is an FC-specific adaptation
>to make the FC pseudowire tolerant of (and hence usable over) a broader
>range of network conditions.
>
>In contrast, most pseudowires do not have requirements to avoid losses
>beyond those of the underlying network, and in fact pass through such
>losses to the pseudowire service provided, avoiding retransmission
>entirely at the pseudowire level.  In the absence of retransmission, the
>congestion impact of such a pseudowire can be estimated from its packet
>or frame size, transmission rate, losses experienced at that
>transmission rate and the latency between the endpoints.  The simplified
>TCP throughput equation in Section 3.1 of RFC 5348 indicates the rate at
>which TCP would transmit under such conditions, so the difference
>between the pseudo-wire's actual transmission rate and the corresponding
>TCP transmission rate is an initial indication of the pseudowire's
>congestion impact.  The word "initial" needs to be emphasized - there
>are numerous other considerations, starting with non-requirements (2)
>and (3).
>
>(2) There is no single always-right answer for TCP equivalence of a
>pseudowire, i.e., the number of TCP-equivalent flows represented by a
>pseudowire's traffic.
>
>In the absence of other information, it is reasonable to assume that a
>pseudowire is equivalent to one TCP flow.  On the other hand, other
>information is often available about a pseudo-wire, often indicating
>that it carries traffic equivalent to multiple flows.  Examples include,
>the nature and composition of traffic on a network pseudowire, number of
>hosted or hostable voice connections for a TDM pseudowire, etc.
>Considering possible application use of a pseudowire at one extreme and
>the common service provider use of pseudowires for network provisioning
>and traffic engineering at the other extreme, it does not appear
>possible to span this range of applicability with a single specification
>of the number of TCP-equivalent flows that an individual pseudowire
>should be considered to represent.
>
>(3) There isn't even a single goal for pseudo-wire congestion control.
>The purposes of congestion control appear to vary from fairness wrt
>other traffic to basic safety protection of the network, depending on
>the deployment.
>
>This range of goals follows from a similar premise as the range of
>TCP-equivalent flows in the previous item.  If an application's
>pseudowire is mixed with TCP traffic on the same links, fairness to that
>other TCP traffic is very important.  By contrast, in a
>well-controlled-and-managed service provider network that is provisioned
>entirely with pseudowires, inter-pseudo-wire fairness is a network
>management and traffic engineering issue for OAM systems and the like,
>and the appropriate goal of congestion control is safety (network
>protection), as the network design and traffic engineering will be
>tolerant of some failure scenarios, but unanticipated failures may cause
>situations in which congested traffic has to be removed in order to
>preserve the remainder of the network.
>
>-----------------------
>
>Overall, the pseudowire congestion framework draft is a good starting
>point, but it appears (in 20/20 hindsight) that the TCP flow equivalence
>issue has been raised prematurely.
>
>Based on discussion of the congestion framework draft, there does not
>appear to be a single congestion control problem or solution for
>pseudowires that can span the spectrum from possible application use of
>pseudowires to core network provisioning based entirely on pseudowires.
>Rather, there are likely to be multiple problems and hence multiple
>solutions involved, making determination of applicability important.
>
>The next step suggested by this review is to focus on the problem
>structure. In particular, it has been strongly asserted that the use of
>pseudowires to provision and traffic engineer service provider networks
>has fundamentally different congestion control characteristics, goals
>and requirements from the Internet at large.  The author of this review
>and the Transport Area Directors are inclined to agree, but want to see
>the applicability details.
>
>We also want to put the pwe3 WG on notice that we do not believe
>assertions to the effect that all service provider networks that use
>pseudowires are completely immune from the disruptive effects of:
>         - congestion
>         - lightning bolts
>         - backhoes (e.g., JCB)
>         - ship anchor damage to submarine cables
>         - etc.
>The words "all" and "completely" are crucial in the above sentence.
>
>With the support of the Transport ADs, the transport area advisor to the
>pwe3 WG (David L. Black) is interested in working with the pwe3 WG on a
>draft that describes the nature of a
>well-controlled-and-managed-service-provider-network (shorter name
>clearly needed) for which pseudowire congestion control mechanisms
>should be focused on network protection from unanticipated drastic
>failures.  This could be productively viewed as an applicability
>statement for a basic set of congestion control requirements that may
>cover the vast majority of current pseudowire deployments.  Outside this
>scope of applicability, a more demanding set of congestion requirements
>may be appropriate for unconstrained use of pseudowires.
>
>
>Thanks,
>--David (IETF Transport Directorate Member)
>----------------------------------------------------
>David L. Black, Distinguished Engineer
>EMC Corporation, 176 South St., Hopkinton, MA  01748
>+1 (508) 293-7953             FAX: +1 (508) 293-7786
>[email-masked]        Mobile: +1 (978) 394-7754
>----------------------------------------------------
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____________________________________________________________________________
Bob Briscoe, <[email-masked]>      Networks Research Centre, BT Research
B54/77 Adastral Park,Martlesham Heath,Ipswich,IP5 3RE,UK.    +44 1473 645196 

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