[OLPC Networking] Path discovery sanity

[Javier Cardona]

Scott,

On Tue, May 13, 2008 at 10:23 AM, C. Scott Ananian <cscott at cscott.net> wrote:
>  It may also be worth comparing to a pure proactive approach
>  such as OLSR, which is also permitted under 802.11s and may scale
>  further than HWMP.

The draft defines an "extensibility framework" for path selection
protocols, which allows the definition of vendor proprietary
protocols.  But the only path selection protocol proposed for
standarization is HWMP.
OLSR was included in the past as an optional alternative to the
mandatory HWMP, but it was removed last year.
The group concluded that there were no usage scenarios in which OLSR
would provide clear benefits over (the proactive part of) HWMP.

> In contrast, the author of the HWMP protocol only expects it to scale to 50 nodes (see attached).

The original HWMP specification (
https://mentor.ieee.org/802.11/file/06/11-06-1778-01-000s-hwmp-specification.doc
) does not specify a limit in the number of nodes.  Bahr takes the 32
node
limit from the 11s PAR
(http://standards.ieee.org/board/nes/projects/802-11s.pdf) which is
actually
a design guideline and not a theoretical limit.  In fact, that limit
probably comes from the medium
access mechanism and not from the path selection protocol.

>  A reactive approach is much more sensitive to the loss of a single
>  packet during route-finding;

That's just because the proactive approach transmits more redundant
information.
The reactive approach can be made as "insensitive" by increasing redundancy.
The main trade-off between proactive and reactive methods is latency vs.
management overhead.

>  Hybrid approaches (such as the one specified in
>  802.11s but not actually implemented by OLPC) may combine the
>  strengths of both.

Yes.  Or the weaknesses.  It all depends how you tune it ;)

Cheers,

Javier

-- 
Javier Cardona
cozybit Inc.