Re: [alto] Discussion II: Unifying cost-mode and cost-type to a single type

["Scharf, Michael (Michael)" <[e-mail]>]

> I appreciate the observations related to cost and believe 
> some discussion is warranted but some observations:
> 
> - Lower is better is mostly what you get in routers. For 
> example, OSPF has a well defined metric but you can import 
> BGP routes into OSPF (and
> vice-versa) with default or different metrics. You can add 
> static routes, changes preferences, etc.
> 
> An ALTO Server is no different. It needs to take inputs from 
> different sources and come up with a cost metric.

Yes. One can indeed use different input sources...

> If we need
> to define what cost means I suggest we take the definition 
> from routing folks (for example, ref PCE/BGP). Do they have 
> one? For example, if I dump the forwarding table of a router 
> and it shows that for a certain destination the cost is "50" 
> through a certain interface,  what can I infer from it given 
> the mash of different inputs a router can take to calculate that?

... and there may be input metrics other than BGP/OSPF/... routing cost.

> - Different ALTO Servers might be a problem if they are in 
> different domains. ALTO servers within a single domain that 
> need to exchange cost maps need to use the same algorithms.
> 
> - Different ALTO servers in different domains in another 
> issue. This was an issue we worked on:
> 
> https://tools.ietf.org/html/draft-penno-alto-cdn-03#page-17
> 
> I believe implementations just use BGP cost.

While BGP cost is certainly a very relevant input parameter, we've shown an implementation that can also calculate the ALTO cost e.g. from delay (Section IV.C): http://dx.doi.org/10.1109/ICIN.2012.6376038

How to calculate the cost value really depends on the requirements and policies of the ALTO server operator.

Michael