Re: [8023-CMSG] {Spam?} Re: [8023-CMSG] Server/NIC analogy
Siamack:
Yes, Pause is widely implemented because it part of the
IEEE 803.3 Standard being added by the 802.3x effort.
However it is NOT widely used, actually it is seldom used.
Be advised that if used at all it must be deployed with great
care to prevent congestion spreading. Because of this and
related issues I would assert that pause has little to NO value.
Friends don't let friends use pause.
Thomas Dineen
Siamack Ayandeh wrote:
> Ben,
>
> It may help to start from a more limited scope with clear value before
> we venture far in to more complex territory.
>
> Clearly there must have been a perceived value in the existing Pause
> mechanism which is part of the standard and widely deployed. This
> mechanism, or a yet to be defined mechanism, can be improved in the
> following sense:
>
> 1) Scope: Can remain as is i.e. a single link. Given that Ethernet is a
> technology that is being used in a wide range of applications from
> inter-chip communication to the local loop for Metro Ethernet services,
> a single link would cover a wide range of applications.
>
> 2) Granularity: Needs to be improved. The granularity can be defined by
> introduction of a grain_ID (I am threading carefully hear & don't use
> flow-ID). How this is mapped to Class of Service, VLAN tags, etc.
> becomes a local matter over a single link and need not be part of a
> standard. It is application dependent. Sure there are problems to be
> solved here but that's why we need a study group.
>
> The need is to create multiple control loops rather than one. How these
> get mapped is a local decision over a single link.
>
> 3) Flow control algorithm: Currently ON/OFF control is in place. This
> is a simple and effective mechanism. Whether it can be improved using
> the so called "rate based" algorithms or some thing else is to be seen
> and is the subject of study for the working group.
>
> In this limited context the study group can add value and produce a
> useful extension to the existing Pause flow control mechanism.
>
> Whether more can be done e.g. to extend the scope to multiple hops will
> no doubt arise and be debated in the course of the study. However the
> ambiguity that currently is floating around this subject should not
> prevent concrete progress in the more limited context.
>
> Regards, Siamack
>
>
>
> Benjamin Brown wrote:
>
>>
>> Gary,
>>
>> You say you're seeing promising results from simulations
>> but you're not ready to share the data. I certainly hope
>> that will change before the presentation deadline for the
>> July meeting in 4 weeks.
>>
>> I don't mean to pick on you but you seem to be the only
>> one that is taking up the flag AND at least suggesting that
>> there is simulation data to back up your claims.
>>
>> As chair of this group, I'm trying to stir up discussion in
>> order to get all the arguments on the table. If there are
>> flaws in these arguments (the "gospels" as you call them)
>> and the exploitation of these flaws has broad market
>> potential and is both technically and economically
>> feasible, then we need to get this information disseminated
>> as soon as possible.
>>
>> I don't think we can try to go through the July meeting
>> without this material and expect to get a continuation of
>> this study group.
>>
>> Regards,
>> Ben
>>
>> McAlpine, Gary L wrote:
>>
>>> Norm,
>>>
>>> I agree with you on many of your points below. A higher granularity of
>>> "flow" than 8 priorities is needed to get any significant improvement
>>> across multiple stages of switching. I know I'm being vague about
>>> exactly what granularity of "flow" on which I want to exert targeted
>>> influence (rate control/backpressure). It's not because I don't know,
>>> it's because any discussions on the subject without data to back the
>>> proposals will "simply" turn into a big rathole. I am busy developing
>>> the data.
>>>
>>> I understand all your arguments below. I've been listening to the same
>>> ones for the last 15 years and, until a few years ago, treating them as
>>> the gospel. It wasn't until I set out to thoroughly understand the
>>> gorey
>>> details through simulations that I realized there were some interesting
>>> flaws in the "old" assumptions that can be very effectively
>>> exploited in
>>> confined networks such as multi-stage cluster interconnects.
>>>
>>> I guess I don't see such a clear boundary of responsibility between
>>> 802.1 and 802.3 as you. I think it's an IEEE problem. And since the
>>> target link technology is Ethernet, then the focus should be on the
>>> 802.3 support required to enable acceptable Ethernet based solutions. I
>>> think 802.1 needs to be part of a complete solution, but only to the
>>> extent of including support for the 802.3 mechanisms.
>>>
>>> Gary
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: owner-stds-802-3-cm@LISTSERV.IEEE.ORG
>>> [mailto:owner-stds-802-3-cm@LISTSERV.IEEE.ORG] On Behalf Of Norman Finn
>>> Sent: Wednesday, June 09, 2004 2:33 AM
>>> To: STDS-802-3-CM@LISTSERV.IEEE.ORG
>>> Subject: Re: [8023-CMSG] Server/NIC analogy
>>>
>>>
>>> Gary,
>>>
>>> McAlpine, Gary L wrote:
>>> >
>>> > I think this discussion is off on a tangent.
>>>
>>> One can reasonably claim that you're the one who's off on a tangent.
>>> One man's tangent is another man's heart of the argument. You keep
>>> saying, "we're just ..." and "we're only ..." and "we're simply ..."
>>> and
>>> failing to acknowledge our "but you're ..." arguments. Specifically:
>>>
>>> You want back pressure on some level finer than whole links. The heart
>>> of the argument, that you are not addressing in your last message, is,
>>> "On exactly what granularity do you want to exert back pressure?"
>>>
>>> The answer to that question is, inevitably, "flows". (I have no
>>> problem
>>> that "flows" are relatively undefined; we dealt with that in Link
>>> Aggregation.) Per-flow back pressure is the "but you're ..." argument.
>>> Hugh Barrass's comments boil down to exactly this point. You want to
>>> have per-flow back pressure.
>>>
>>> The "per-something Pause" suggestions have mentioned VLANs and priority
>>> levels as the granularity. The use of only 8 priority levels, and thus
>>> only 8 flows, is demonstrably insufficient in any system with more than
>>> 9 ports. For whatever granularity you name, you require at least one
>>> queue in each endstation transmitter for each flow in which that
>>> transmitter participates. Unfortunately, this o(n) problem in the
>>> endstations is an o(n**2) problem in the switch. A simple-minded
>>> switch
>>> architecture requires one queue per flow on each inter-switch trunk
>>> port, which means o(n**2) queues per trunk port. The construction of
>>> switches to handle back-pressured flows without requiring o(n**2)
>>> queues
>>> per inter-switch port has been quite thoroughly explored by ATM and
>>> Fibre Channel, to name two. It is
>>> *not* an easy problem to solve.
>>>
>>> At the scale of one switch, one flow per port, and only a few ports, as
>>> Ben suggests, it is easy and quite convenient to ignore the o(n**2)
>>> factor, and assume that the per-link back pressure protocol is the
>>> whole
>>> problem. Unfortunately, as you imply in your e-mail below, the trivial
>>> case of a one-switch "network" is insufficient. As soon as you scale
>>> the system up to even "a few hops", as you suggest, the number of ports
>>> has grown large enough to stress even a 12-bit tag per link.
>>> Furthermore, to assume that a given pair of physical ports will never
>>> want to have multiple flows, e.g. between different processes in the
>>> CPUs, is to deny the obvious.
>>>
>>> In other words, implementing per-flow back pressure, even in networks
>>> with a very small number of switches, very quickly requires very
>>> sophisticated switch architectures.
>>>
>>> For a historical example, just look at Fibre Channel. It started with
>>> very similar goals, and very similar scaling expectations, to what
>>> you're talking about, here. (The physical size was different
>>> because of
>>> the technology of the day, but the number of ports and flows was quite
>>> similar.) Fibre Channel switches are now quite sophisticated, because
>>> the problem they are solving becomes extraordinarily difficult even for
>>> relatively small networks.
>>>
>>> Summary:
>>>
>>> This project, as described by its proponents, is per-flow switching. It
>>> is not the job of 802.3 to work on switching based even on MAC address,
>>> much less per-flow switching. It is essential that anyone who desires
>>> to work on per-flow switching in 802 or any forum become familiar with
>>> what the real problems are, and what solutions exist.
>>>
>>> -- Norm
>>>
>>> > ... There are assumptions being
>>>
>>>
>>>> made here that are off-base. We need to focus our attention on what it
>>>>
>>>>
>>>
>>>
>>>
>>>> is we are trying to enable with new standards. (My numbered items are
>>>> responses to Hugh's numbered items.)
>>>>
>>>> 1. If what we are trying to enable are single stage interconnects for
>>>> backplanes, then wrt the IEEE standards, we're done. We just need to
>>>> get good implementations of NIC's and switches using 802.3x (rate
>>>> control, not XON/XOFF) to meet the requirements (e.g. good enough
>>>> throughput, low latency, low latency variation, no loss due to
>>>> congestion). But ... single stage interconnects are not very
>>>> interesting to people who want to construct larger interconnects to
>>>> tie multiple racks with multiple shelves of blades together into a
>>>> single system.
>>>>
>>>> 2. (Putting on my server hat) We're NOT asking for IEEE to provide
>>>> end-to-end congestion management mechanisms. If IEEE can simply
>>>> standardize some tweaks to the current 802.3 (& 802.1) standards to
>>>> support better congestion visibility at layer 2 and better methods of
>>>> reacting to congestion at layer 2 (more selective rate control and no
>>>> frame drops), then the rest can be left up to the upper layers. There
>>>> are methods that can be implemented in layer 2 that don't prohibit
>>>> scalability. Scalability may be limited to a few hops, but that is all
>>>>
>>>>
>>>
>>>
>>>
>>>> that is needed.
>>>>
>>>> 3. The assumption in item 3 is not entirely true. There are
>>>> relationships (that can be automatically discovered or configured)
>>>> that can be expoited for significantly improved layer 2 congestion
>>>> control.
>>>>
>>>> 4. For backpressure to work, it neither requires congestion to be
>>>> pushed all the way back to the source nor does it require the
>>>> backpressuring device to accurately predict the future. From the layer
>>>>
>>>>
>>>
>>>
>>>
>>>> 2 perspective, the source may be a router. So back pressure only needs
>>>>
>>>>
>>>
>>>
>>>
>>>> to be pushed up to the upper layers (which could be a source endpoint
>>>> or a router). Also, the backpressuring device simply needs to know its
>>>>
>>>>
>>>
>>>
>>>
>>>> own state of congestion and be able to convey clues to that state to
>>>> the surrounding devices. We don't need virtual circuits to supported
>>>> at layer 2 to get "good enough" congestion control.
>>>>
>>>> 5. From an implementation perspective, I believe the queues can go
>>>> either in the MAC or the bridge, depending on the switch
>>>> implementation. (Am I wrong? I haven't seen anything in the interface
>>>> between the bridge and the MAC that would force the queues to be in
>>>> the bridge.) IMO, where they go should NOT be dictated by either 802.1
>>>>
>>>>
>>>
>>>
>>>
>>>> or 802.3. The interface between the bridge and MAC should be defined
>>>> to enable the queues to be place where most appropriate for the switch
>>>>
>>>>
>>>
>>>
>>>
>>>> architecture. In fact, a switch could be implemented such that frame
>>>> payloads bypass the bridge and the bridge only deal with the task of
>>>> routing frame handles from MAC receivers to one or more MAC
>>>> transmitters (Do the 802.1 standards prevent such a design?).
>>>>
>>>> As far as the IETF standards go, they don't seem to rely on layer 2 to
>>>>
>>>>
>>>
>>>
>>>
>>>> drop frames (although we don't yet have a clear answer on this). If a
>>>> router gets overwhelmed, it will drop packets. But if it supports ECN,
>>>>
>>>>
>>>
>>>
>>>
>>>> it can start forwarding ECN notices before becoming overwhelmed. I
>>>> think the jury is still out on whether the upper layers (in a confined
>>>> network) would work better with layer 2 backpressure or layer 2 drops.
>>>>
>>>>> From a datacenter server perspective, there is no doubt in my mind
>>>>
>>>>
>>>>
>>>>> that
>>>>>
>>>>>
>>>> backpressure would be preferrable to drops.
>>>>
>>>> Gary
>>>>
>>>>
>>>
>>>
>>>
>>
>> --
>> -----------------------------------------
>> Benjamin Brown
>> 178 Bear Hill Road
>> Chichester, NH 03258
>> 603-491-0296 - Cell
>> 603-798-4115 - Office
>> benjamin-dot-brown-at-ieee-dot-org
>> (Will this cut down on my spam???)
>> -----------------------------------------
>>
>