[c-nsp] Flow Control and 10GE interfaces

[Brian Turnbow]

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Phil Mayers
Sent: lunedì 23 novembre 2009 17.05
To: Gert Doering
Cc: Matthew Melbourne; cisco-nsp at puck.nether.net; Ross Vandegrift
Subject: Re: [c-nsp] Flow Control and 10GE interfaces

Gert Doering wrote:
> Hi,
> 
> On Mon, Nov 23, 2009 at 08:41:58AM -0500, Ross Vandegrift wrote:
>> The answer is very simple: if someone thinks that ethernet flow
>> control is the answer, the burden of proof is on them to answer
>> difficult questions about what the actual problem is, what flow
>> control is going to solve, and why they think that it won't cause more
>> problems than its worth.  At best it does nothing, realistically it
>> interferes with TCP flow control, and at worst it pauses your storage
>> and breaks every client.
> 
> I tend to disagree with this statement in this broadness.  We've seen
> problems where lack of flow control combined with a switch with too-tiny
> buffers and bursty ingress traffic led to buffer overflow on egress, and
> packet loss.  If the switch would use flow control here to space the
> ingress traffic better (that is: stop and restart the flow for milliseconds
> at a time), packet loss would be avoidable.
> 
> Of course, this can indeed fire backwards - as in: one egress port is
> way overloaded, and flow control spreads the pain from there to all other
> egress ports served by the ingress port in question.
> 
> So indeed, flow control is not a panacea.  I agree with this :-)

>An interesting wrinkle (to some) is that stock flow control is not QoS 
>(i.e. 802.1p codepoint) aware - it's all-or-nothing, meaning your 
>low-bandwidth diffserv/EF flow gets paused as well as your less-then 
>best-effort 999.9mbit/sec FTP transfer :o(

>There's a flow control extension somewhere for per-802.1p flow control, 
>but I can't find the references for this.

The nexus family does PFC (no it's not a card, they reused the acronym)
http://www.cisco.com/en/US/prod/collateral/switches/ps9441/ps9670/white_paper_c11-542809.html
Basically enables sending a pause per class.
They did it for FCOE and it is proprietary , the white paper has the standard mumbo jumbo about 
how it is becoming a standard and everyone is adapting cisco's proposal..


Brian

>QoS seems to have gone out of fashion however, so whether this is 
>relevant is another matter ;o)
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