On Fri, Oct 11, 2013 at 2:04 PM, Alvaro Retana (aretana)
<aret...@cisco.com>wrote:
> <WG Chair Hat Off>
>
> Hi!
>
> On 10/10/13 5:07 PM, "Alia Atlas" <akat...@gmail.com> wrote:
>
> . . .
>
>
>>> 1.
>>> - Root Selection. No algorithm is provided. There's a reference
>>> to I-D.atlas-ospf-mrt, where a suggestion is made ("..the router with
>>> the
>>> highest GADAG Root Selection Priority is picked to be the GADAG
>>> Root").
>>> IMHO, the algorithm should be specified in this draft, where the
>>> requirement to carry the Priority is defined so that the extensions
>>> draft(s) can show how to implement it in OSPF (or any other
>>> protocol)..not
>>> the other way around.
>>>
>>> [Alia] The Root Selection is described in
>> draft-ietf-rtgwg-mrt-frr-architecture in Sec 7 as
>> '"GADAG Root Selection Priority: Among the routers in the MRT Island and
>> with the highest priority advertised, an implementation MUST pick the
>> router with the highest Router ID to be the GADAG root."
>>
>>
>> This goes back to my comment above about the capabilities and the
>> default profile definition: If this is part of the algorithm it should be
>> specified in the algorithm draft, not somewhere else.
>>
>
> [Alia] Ah - the GADAG root selection is part of the MRT profile but not,
> IMHO, part of the algorithm... The same Lowpoint algorithm can be used
> with different GADAG root selection. For example, for multicast, one might
> do an MRT profile where the multicast S is picked to be the GADAG root -
> and the same Lowpoint algorithm is run.
>
>
> But the performance of the algorithm can be affected by the selection of
> the root. In 6.1 you wrote: "The particular choice of a common GADAG root
> is expected to affect the quality of the MRT alternate paths, with a more
> central common GADAG root resulting in shorter MRT alternate path lengths."
> For the results you chose a centrally located node to be the root. What
> would have happened if the root had been selected randomly? (Or the
> equivalent which would be to assign random Priorities.)
>
[Alia] There is variability, of course, but we found that up to the first
50th percentile of centrality nodes were fairly similar. This is really a
"don't locate it in Timbuktu" if your network is mostly in NA kind of thing.
> I guess my point here is that one of the reasons the Lowpoint algorithm
> performs well is because of how the root is selected. We could go back and
> forth all day about whether the root selection should be part of the
> "default algorithm" or just part of the "default profile"..but I think it
> would be more important to stress (even mandate) that the location of the
> nodes be considered when assigning the Priority.
>
[Alia] Absolutely - the operator should do some basic analysis and pick a
node for GADAG root that is basically well connected and in the spine/core
of the network. I'm sorry if the difference between "default algorithm"
and "default profile" seems like pedantry - I'm trying to set up MRT to
work for multiple purposes (multicast live-live and MRT-FRR) and keep the
right parts separated.
Note that the current text ("Off-line analysis that considers the
> centrality of a router may help determine how good a choice a particular
> router is for the role of GADAG root." -- which is too soft for my taste)
> is in the algorithm draft, not in the architecture draft (which defines the
> profile), so it may not be completely clear..
>
[Alia] Sure - I see your point and would be happy to add in some comments
there. I'm afraid that the analysis giving clue as to importance of GADAG
root selection came while I was updating drafts and may not have gone
through them sufficiently.
Alia
> Thanks!
>
> Alvaro.
>
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