Kevin,
My understanding is that 802.1au, "lossless Ethernet", was designed
primarily to allow Fibre Channel to be carried over 10 GbE so that SAN and
LAN can share a common infrastructure in datacenters. I don't believe
anyone
intends for it to be enabled for traffic classes carrying TCP.
Well, QCN requires a L2 MAC sender, network and receiver cooperation (thus
you need fancy "CNA" converged network adapters, to start using it - these
would be reaction/reflection points; plus the congestion points - switches -
would need HW support too; nothing one can buy today; higher-grade
(carrier?) switches may have the reaction/reflection points built into them,
and could use legacy 802.3x signalling outside the 802.1Qau cloud).
The following may be too simplistic
Once the hardware has a reaction point support, it classifies traffic, and
calculates the per flow congestion of the path (with flow really being the
classification rules by the sender), the intermediates / receiver sample the
flow and return the congestion back to the sender - and within the sender, a
token bucket-like rate limiter will adjust the sending rate of the
appropriate flow(s) to adjust to the observed network conditions.
http://www.stanford.edu/~balaji/presentations/au-prabhakar-qcn-description.pdf
http://www.ieee802.org/1/files/public/docs2007/au-pan-qcn-details-053007.pdf
The congestion control loop has a lot of similarities to TCP CC as you will
note...
Also, I haven't found out how fine-grained the classification is supposed to
be (per L2 address pair? Group of flows? Which hashing then to use for
mapping L2 flows into those groups between reaction/congestion/reflection
points...).
Anyway, for the here and now, this is pretty much esoteric stuff not
relevant in this context :)
Best regards,
Richard
----- Original Message -----
From: "Kevin Gross" <[email protected]>
To: <[email protected]>
Sent: Monday, May 16, 2011 3:15 PM
Subject: Re: [Bloat] Jumbo frames and LAN buffers
All the stand-alone switches I've looked at recently either do not support
802.3x or support it in the (desireable) manner described in the last
paragraph of the linked blog post. I don't believe Ethernet flow control
is
a factor in current LANs. I'd be interested to know the specifics if
anyone
sees it differently.
My understanding is that 802.1au, "lossless Ethernet", was designed
primarily to allow Fibre Channel to be carried over 10 GbE so that SAN and
LAN can share a common infrastructure in datacenters. I don't believe
anyone
intends for it to be enabled for traffic classes carrying TCP.
Kevin Gross
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Jim Gettys
Sent: Monday, May 16, 2011 5:24 AM
To: [email protected]
Subject: Re: [Bloat] Jumbo frames and LAN buffers
Not necessarily out of knowledge or desire (since it isn't usually
controllable in the small switches you buy for home). It can cause
trouble even in small environments as your house.
http://virtualthreads.blogspot.com/2006/02/beware-ethernet-flow-control.html
I know I'm at least three consumer switches deep, and it's not by choice.
- Jim
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