[Nml-wg] Metrics for pathfinding

[Paul Boven]

Hi Freek, everyone,

On 06/06/2013 05:45 PM, Freek Dijkstra wrote:
> The problem with capacity is that it is highly non-trivial for the main
> technology in use today, Ethernet. Two notes:
> * Maximum achievable bandwidth of a channel (VLAN) depends on frame size
> and interframe gap, and header size. (Most technologies define bandwidth
> of the payload; to avoid these issues, Ethernet defines the bandwidth of
> the underlying medium)
> * Ethernet does not work with a achievable bandwidth, but instead
> defines Committed Information Rate (CIR), Excess Information Rate (EIR),
> Committed Burst Size (CBS), and Excess Burst Size (EBS).

While I understand that this is a complex issue, we do need a single 
definition of bandwidth for NSI to properly function end-to-end. 
Otherwise, we end up with end-users having to 'fudge' their requests to 
make sure that it fits with any possible underlying technology. So in 
this context, payload capacity is the only metric that makes sense. I 
will bring up this issue in the NSI-WG as well, the current draft of the 
NSI v2.0 spec does not give any precise definition of 'bandwidth'.

> Maybe we have to pick, and make some "experimental" parameter:
> * Do you want a parameter that describes the capacity of the data flow
> including or excluding the header?
> * For Ethernet, do you want a parameter that defines the CIR or PIR
> (=CIR+EIR), or do you prefer two (or four) parameters?

As my goal here is to do reliable pathfinding, I think we should have a 
single, technology agnostic metric that will tell me whether a link can 
theoretically support my request, or not. NSI has no notion of 
CIR/EIR/PIR, so presumably their use of 'bandwidth' matches closes to 
the CIR parameter for Ethernet.

Regards, Paul Boven.
-- 
Paul Boven <boven at jive.nl> +31 (0)521-596547
Unix/Linux/Networking specialist
Joint Institute for VLBI in Europe - www.jive.nl
VLBI - It's a fringe science