[c-nsp] Flow Control and 10GE interfaces

[Phil Mayers]

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.

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