[Bloat] Jumbo frames and LAN buffers

[Richard Scheffenegger]

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" <kevin.gross at avanw.com>
To: <bloat at lists.bufferbloat.net>
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: bloat-bounces at lists.bufferbloat.net
> [mailto:bloat-bounces at lists.bufferbloat.net] On Behalf Of Jim Gettys
> Sent: Monday, May 16, 2011 5:24 AM
> To: bloat at lists.bufferbloat.net
> 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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