Vijay,
On 30 Jan 2012, at 23:06, Vijay K. Gurbani wrote:
> As individual.
>
> Ben: Please see inline.
>
> On 01/30/2012 11:03 AM, Ben Niven-Jenkins wrote:
>> Colleagues,
>>
>> In the current specification of ALTO, are costs always End to End?
>>
>> What I mean by that is when looking at ALTO cost maps is it possible
>> to safely assume that of there is a cost between PIDX& PID Y and a cost
>> between PIDY& PIDZ then the cost between PIDX& PIDZ can be calculated as
>> cost(PIDX,PIDY)+cost(PIDY,PIDZ)? [If this assumption does hold, it is
>> obviously not applicable to ordinal cost types].
>>
>> I suspect the answer is no, but I wanted to check what the
>> definitiveanswer is.
>
> It will be interesting to hear from the authors, but here is my
> two cents.
>
> If I take the cost map and create a graph from it, and if the graph is
> connected and I simply run Dijkstra's shortest path algorithm on it,
> then I suspect the answer is yes.
I guess my question is really whether an ALTO cost map is a graph or a mesh? I
always assumed it was a mesh but then I got wondering...
> More below.
>
>> For example if a cost map contains:
>>
>> "map" : {
>> "PID1": { "PID2": 1 },
>> "PID2": { "PID1": 1, "PID3": 2 },
>> "PID3": { "PID2": 2 }
>
> If one were to construct a graph from the above cost map --- omitting
> edges emanating and terminating at the same vertex and assuming
> bi-directional links --- one would get:
>
> PID-2
> /\
> 1/ \2
> / \
> PID-1 PID-3
>
>> Can one assume that the cost between PID1& PID3 is 3 (PID1->PID2 +
>> PID2->PID3)?
>
> Clearly, PID-3 is reachable from PID-1 via PID-2 at a cost of 3.
I started considering that case but then wondered how to tell the difference
between PID3 being reachable from PID1 via PID2 and PID2, for example, being a
dual homed host/site that isn't capable/configured of routing between its WAN
interfaces?
This might be a case where we'd need a PID property to distinguish "transit"
PIDs from "End Host" PIDs.
Ben
>
> Now, for the second example I get a graph of the form:
>
>> How about if the cost map contains:
>>
>> "map" : {
>> "PID1": { "PID2": 1, "PID3": 3 },
>> "PID2": { "PID1": 1, "PID3": 2 },
>> "PID3": { "PID2": 2, "PID1": 3 }
>
> PID-2
> /\
> 1/ \2
> / \
> PID-1------PID-3
> 3
>
>> Can one assume PID2 is on a path between PID1& PID3?
>
> PID-2 may be on a path, but it may not be the *shortest* path.
> Although in the above example, it really does not matter since the cost
> to reach PID-3 from PID-1 is 3 regardless of whether we go through
> PID-2 or directly from PID-1 to PID-3. Now, if the shortest path
> computation algorithm was optimizing other metrics besides cost of the
> edges (say, the hop count), then the best path from PID-1 to PID-3 would
> be the direct path.
>
> Thanks,
>
> - vijay
> --
> Vijay K. Gurbani, Bell Laboratories, Alcatel-Lucent
> 1960 Lucent Lane, Rm. 9C-533, Naperville, Illinois 60566 (USA)
> Email: vkg@{bell-labs.com,acm.org} / [email protected]
> Web: http://ect.bell-labs.com/who/vkg/
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