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Re: [8023-CMSG] Purpose



Title:
Hugh,
 
Re: cut through market penetration: Bring this back up in 2-3 years and we will compare notes. 
 
Re: preemption backward compatibility: Of course 802.3 would create mechanisms to ensure that a switch implementing pre-emption would plug and play with one that didn't. You simply make the default mode equal to the existing mode. I know that you know this. 802.3 did for Link Ag. It did for OAM. It did for.... Come on Hugh, this isn't that hard.
 
jonathan
-----Original Message-----
From: owner-stds-802-3-cm@LISTSERV.IEEE.ORG [mailto:owner-stds-802-3-cm@LISTSERV.IEEE.ORG]On Behalf Of Hugh Barrass
Sent: Monday, May 03, 2004 9:33 AM
To: STDS-802-3-CM@LISTSERV.IEEE.ORG
Subject: Re: [8023-CMSG] Purpose

Jonathan,

In line:

Jonathan Thatcher wrote:
Hugh,
 
Well, I certainly can't get on board with the idea of 40 or 100 Gb/s being cheap or simple. At least not in the next couple of years.
 
I never thought of myself as small-frame-phobic. I always thought of myself as a lover of improved cost-performance.
 
You are correct that geometry matters if you want low latency.
 
Regarding your comment of this being a niche of a niche, to some of us, being a part of a 1 B$ a year and rapidly growing niche within a 50B $ a year niche is worthy of consideration. It doesn't especially bother me that this might be embarrassingly small and not worthy of consideration for the largest vendors.
You (and others) would be wise not to make assumptions about what large vendors consider worthy of attention (or not). The reason why I classify it as a niche of a niche is that I expect most of this large and interesting market will be satisfied by products based on standard technologies (adapted from LAN and WAN applications). I also expect that that there will be a significant niche demanding higher performance (in the range discussed below) that will require more exotic architecture. To satisfy this niche, end-station vendors will need hardware acceleration; switch vendors may use cut-through and, as a result, hardware will be significantly more expensive. Then there is a niche-of-a-niche that will need faster layer 2 operation than Ethernet can provide. I expect that such a market could use existing supercomputer-defined interfaces or may be small enough to tolerate custom or proprietary solutions. I do not see that the niche-of-a-niche warrants the making of a new standard for the whole of Ethernet.

If there is any demand for preemption , then I would expect that cut-through switching would have a significant segment of the current market as it is a tried-and-true technology that is fully compliant with current standards. What is the current penetration of cut-through switches in new switch sales?
 
Of course a switch implementing pre-emption would interoperate with a switch that didn't. Really Hugh, that kind of FUD is beneath you.
Of course it won't! You would have to define some mechanism for backward compatibility that involves discovery and negotiation before pre-emption is used. If, for any reason, a switch were to use preemption on an interface connected to a switch which doesn't understand preemption then the receiving switch would see a jumbled frames. At best this would lead to packet loss, at worst it would cause a very high false packet acceptance rate. I would expect that such a mix of PCS capabilities introduced into the market would generate a far worse number of user issues than simply adding (or changing) a protocol frame.

Hugh.
 
jonathan
-----Original Message-----
From: owner-stds-802-3-cm@LISTSERV.IEEE.ORG [mailto:owner-stds-802-3-cm@LISTSERV.IEEE.ORG]On Behalf Of Hugh Barrass
Sent: Friday, April 30, 2004 11:22 AM
To: STDS-802-3-CM@LISTSERV.IEEE.ORG
Subject: Re: [8023-CMSG] Purpose

Jonathan,

I don't know why you're so scared of smaller frames - anyone who wants smaller latency should prefer smaller frames. If you reduce the MTU to 500 bytes (not to 48) the equation swings in favor of existing standards:

6 x 500 x 8/10k + 0.5 = 2.9 vs 0.8 - still a 3x improvement using preemption, but getting closer. Bear in mind that this is an extreme worst case comparison. Averages will be almost identical because the preempting packet can arrive at any time during the preempted frame; the preempted frame might not be of maximum size; the link may be idle when the preempting packet arrives; plus of course the packet in progress may be a high priority packet also.

Of course if we start adding in more delays the difference gets yet smaller (both delays increase similarly). e.g.

Your example allows only 15m per link, you will start to run into geometrical problems if you want to aggregate very large numbers of nodes with only 15m per link. If there are fewer nodes then you need to re-architect you interconnect matrix because 6 hops should be able to accommodate many thousands of end stations.

Your example must be assuming very aggressive cut-through switch architecture (cut through has lost popularity in recent years, shame). If you want to conform to the requirements of bridging then you should wait for both the source & dest MAC address to be received before you transmit (unless you are a repeater!). Since you are advocating preemption, I would also assume that you must wait for the COS/TOS tag. That extra 10 bytes will be difficult to avoid. Of course, if you decide that the error propagation of cut-through makes the technique unfavorable then you have a full 64 bytes of latency to wait for the CRC of the incoming frame.

Regarding jumbo frames and complexity of end station devices, I would expect that any device capable of filling  a 10Gbps pipe will require some hardware acceleration. For hardware implementations there really is no significant difference between encapsulating 1500 byte frames vs 500 byte (or even smaller) frames. Hardware which performs this high speed operation has the advantage that it is seamlessly compatible with any other equipment that might be connected to it. On the other hand, if a switch started using a preemption mechanism when connected to any existing hardware then it could be anybody's guess what would result. My assertion is that a small reduction in MTU for the local network will yield results which are close enough to your extreme examples to make the applicable space where a new standard is demanded very small indeed. As I said, it's a niche of a niche.

Better to spend our effort on cheap and simple 40Gig (or even 100Gig) and make this whole argument moot (yes, at 100G the max length frame can be stored in 25m of wire).

Hugh.