RE: [ippm] WGLC on draft-ietf-ippm-bw-capacity-04.txt

["Romascanu, Dan \(Dan\)" <[e-mail]>]

I have several concerns related to this work.

1. I believe that the layering model and the terminology used in this
draft are not accurate enough. The layering model mixed layer 2 and
layer 3 devices and links. The draft uses non-standard terminology - for
example the term 'Ethernet switch' in section 2.1, which has no formal
definition here or some place else. I would suggest that this work
considers using the IEEE 802 terminology, specifically in this example
the one used to define bridges in IEEE 802.1
2. The definition of 'node' in 2.1 is central in defining further the
concepts of 'link' and 'path'. However, the definition is confusing to
me when it says
'We define nodes as hosts, routers, Ethernet switches, or any other
   device where the input and output links have different
   characteristics. '
What these 'different characteristics' are? Is an Ethernet bridge
('switch') where the ingress and egress ports are at the same speed and
which makes no change to the packet format a node according to this
definition? What about a IEEE 802.3 repeater? 
3. Section 2.2 defines the 'physical link capacity' - it is not clear to
me why the role of section is at all - however if it's already there it
looks to me that it intents to refer rather to link layer and not
physical layer. Also note that the majority of the links in Ethernet
have the capacity to change their speed (capacity) as a result of
management actions or auto-negotiation
4. In general I believe that we should look at the current
instrumentation in network devices. Routers are usually instrumented to
measure ifSpeed as pr section 3.1.7 in RFC2863, while Ethernet devices
will measure ifSpeed as per Section 3.2.9 in RFC3635. It would be good
if the IP layer capacity definitions would consider what can be measured
by this instrumentation at the ports that are placed at the extremity of
the link. A discussion of relationship with current ifSpeed definition
in other IETF standards is needed IMO
5. I have a major concern about the way path and path capacity are being
defined. Beyond the mix of layer 2 and layer 3 devices which would make
measurement very difficult at a layer 2 bridge port which typically has
no IP layer awareness, the definition of path capacity in 2.3.2 and
2.3.3 seems to make the assumption that the nodes have no influence in
the path capacity. This is not always the case. Packets can be dropped
by nodes, and the forwarding engine in a bridge for example has a
capacity of its own which is not guaranteed to be always better than the
one of the links. 
6. In the discussion in 3.3 I believe that the packet size has by far
the major contribution in the IP layer capacity, and actually the MTU
which is not mentioned at all. 

Regards,

Dan



 
 

> -----Original Message-----
> From: Henk Uijterwaal [mailto:[e-mail]] 
> Sent: Wednesday, December 06, 2006 10:21 AM
> To: [e-mail]
> Subject: [ippm] WGLC on draft-ietf-ippm-bw-capacity-04.txt
> 
> IPPM group,
> 
> The draft "Defining Network Capacity" 
> (draft-ietf-ippm-bw-capacity-04.txt)
> has been discussed extensively in this group and appears to 
> be stable by now.  We like to start a WGLC on it in order to 
> move it forward.  Please raise any remaining issues by 
> Wednesday, December 20, 2006, 12:00 UTC.
> 
> An URL for the draft is:
> http://www.ietf.org/internet-drafts/draft-ietf-ippm-bw-capacity-04.txt
> 
> Matt & Henk
> 
> --
> --------------------------------------------------------------
> ----------------
> Henk Uijterwaal                           Email: 
> henk.uijterwaal(at)ripe.net
> RIPE Network Coordination Centre          
> http://www.amsterdamned.org/~henk
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