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Re: [8023-CMSG] Proposed Upper Layer Compatibility Objective



Title: Message
Hi Gary,
 
I agree we can't ignore "How does what we do at L2 impact UPL". Yikes - there are many ULP's, not just DS.
 
I agree that "We leave the discard decisions up to the UPL".
 
I can't tell you exactly how CM will impact DS until I know what CM does? I am consulting with one of the original DS authors, Steve Blake, he and I use to work together a few companies ago when we were both at IBM, this was when DS was standardized. We discussed this point this morning and the general feeling was that a FDX point-to-point (PTP) L2 Ethernet link should not be making implicit or explicit selective packet drop decisions, that this would in a negative way impact the operation of DS. 
 
If what CM does is to channelize traffic across a PTP link (high or low BW, short or long distance, inside a chassis across an Ethernet backplane or externally across a physical copper or optical link) into 'N' number of Classes (CoS transmission buffers) then when one Class is "some how" determined to be using too much BW and that Class is "Turned OFF" for some period of time there is a high likelihood that packets in that Class will be dropped. When this happens you've just impacted DS ability to do its drop probability calculations properly........... more to come on this as we better understand what CM does.  
 
NOTE: 802.1p has already defined a L2 "marker" that is used as a form of L2 rate control. 802.1p is used by intermediate L2 switches (between router hops; DS Hops) to provide intermediate L2 class of service prioritization. Most switch/router products that are worth purchasing use this feature along with DSCP (L3) to prioritize traffic across their available ingress & egress ports.
 
 
Regards,
 
   - Jeff
----- Original Message -----
Sent: Thursday, May 13, 2004 11:48 AM
Subject: Re: [8023-CMSG] Proposed Upper Layer Compatibility Objective

Jeff,
 
I am perfectly fine with NOT having such an objective if you think we can get away with it. I'm not sure we can ignore how, what we propose to do at L2, will affect the operation of the upper layers.
 
On the subject of DiffServ, I would simply propose that, within the limited scope of the high speed, short range, L2 interconnects we are trying to enhance, we leave the discard decisions up to the upper layers while maintaining the desired latency and traffic differentiation qualities within layer 2. We have shown through simulations that layer 2 rate control mechanisms can eliminate (or significantly reduce frame) discards within layer 2 (effectively pushing the discard decision up to L3). We have also shown that rate control combined with prioritization in layer 2 can maintain excellent latency and traffic differentiation qualities.
 
Maybe you can explain to us how this is likely to affect the operation of DiffServ. I haven't dug deep enough into DiffServ to know if it is counting on L2 devices to discard. My intuition is telling me it is likely to improve the operation of DiffServ, as well as other upper layer protocols of interest.
 
Gary
 
 
 
 
 -----Original Message-----
From: owner-stds-802-3-cm@LISTSERV.IEEE.ORG [mailto:owner-stds-802-3-cm@LISTSERV.IEEE.ORG] On Behalf Of Jeff Warren
Sent: Wednesday, May 12, 2004 7:58 PM
To: STDS-802-3-CM@LISTSERV.IEEE.ORG
Subject: Re: [8023-CMSG] Proposed Upper Layer Compatibility Objective

Gary,
 
You mentioned improved operation of DiffServ as a goal for CM. DS is a collection of a number of RFC's, here's the basic set of DS RFCs. 
  • RFC2474 "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers"
  • RFC2475 "An Architecture for Differentiated Services"
  • RFC2597 "Assured Forwarding PHB"
  • RFC2598 "An Expedited Forwarding PHB" 

What did you have in mind for supporting ULP's such as DS? For example this L3 protocol's purpose in life is to decide drop probabilities for individual packets on a hop-by-hop basis. For example assured forwarding has three levels of drop precedence (red, yellow, green). Then there's expedited forwarding in the highest priority queue, plus best effort, etc......

 

I've been wondering how an "Ethernet" standard is going to assist a L3 protocol such as DS by classification MAC traffic? Would you propose duplication of or cooperation with DS protocols? Maybe you let DS do its thing and present a CM enabled Ethernet egress port with remarked packets that are fortunate enough to have passed across the devices fabric w/o being dropped. This CM enabled Ethernet port would then align the offered MAC load to the queues it has available, it does this for example by inspecting DSCP values and comparing these DSCP values to a predefined buffer table. Bla bla bla......  

 

The lights haven't gone off for me yet, I don't see the value in CM supporting DS because switch manufactures have already figured this one out and implemented this concept of multiple egress queues working at line rate. Plus these implementations require global knowledge of networking policies to configure them properly, and more importantly to standardize them were talking about linking behavior of ingress and egress ports, knowledge of system wide (e.g. switch or router) buffer management capabilities, all very much vendor specific capabilities that would be very difficult to get everyone to agree to.  

 

Regards,

 

   - Jeff Warren     

 

 

 

----- Original Message -----
Sent: Wednesday, May 12, 2004 5:44 PM
Subject: [8023-CMSG] Proposed Upper Layer Compatibility Objective

My A.R. from last meeting (thank you Ben).

Here's a first shot at a Upper Layer Compatibility objective:

The objective is:
"To define 802.3 congestion control support that, at a minimum, will do nothing to degrade the operation of existing upper layer protocols and flow/congestion control mechanisms, but has the explicit goal of
facilitating the improved operation of some existing and emerging protocols, over 802.3 full-duplex link technology."


If we can narrow the scope and still make it meaningful, I'm all for it.

I have attached RFC3168 (on ECN) as a reference. It contains a very good overview of congestion control at the TCP and IP layers. I would also consider this and DiffServ as examples of existing ULPs we would want to do our part to improve the operation of. IMO, what we can do at the 802.3 level to better support these will also provide the support we need for improved operation of some emerging protocols such as iSCSI and RDMA.

Gary <<rfc3168.txt>>