Ability to set the 802.11s TTL for outgoing packets

Javier Cardona javier at cozybit.com
Tue Oct 10 15:12:49 EDT 2006

Hi Lennart,

> So OLPC isn't really implementing that protocol that shall become
> 802.11s?

The intent is to follow 802.11s, yes.  Right now we have implemented a
subset of it such as the route discovery and path selection protocols,
which follow the HWMP protocol defined in Sec. 11A.  But some
things, like the new Type and Subtypes defined for Mesh Data (Sect, or the Mesh Forwarding Control field (Sect. are
not supported by the existing hardware.  The priority is to have
working firmware rather than full compatibility with a standard that's
still being developed.

Also, it's important to keep in mind that we are working from existing
code that we want to leverage as much as possible.  For instance, the
firmware is capable of acting simultaneously as a mesh node as well as
an infrastructure client.

> (...)
> > The TTL in data frames would determine how many hops a data packet
> > would travel.  This is not implemented yet.
> Do you have any idea when this will be available?

We are working on this right now.  I would say that there will be an
official release in two months or so.

> All what I need is a way to reach just the hosts "nearby". Because the
> CPU of nodes nearby might not be powered on or running Avahi I need
> the functionality to broadcast with a TTL > 1.
> It's not really important whether this is actually implemented with
> multicasting or broadcasting. I assume that most of the OLPC machines
> *will* run Avahi, hence in effect there is not much of a difference if
> I send my packets with a broadcast or with a multicast.
> Please note that I will never use large TTLs, i.e. those > 10. Hence,
> a dirty solution could be to define some magic MAC addresses like
> FF:FF:FF:FF:FF:FE which behave like a broadcast with TTL=1 and so
> on. If you did this there wouldn't be a need to change the frame
> format.

We will probably implement this as a sockopt, as you suggested earlier.

> I am currently doing lots of simulations for my new protocol. For that
> I generate random mesh networks. However, I am not quite sure what
> parameters to choose for these networks. My job is to get Avahi to
> scale for up to 10.000 hosts, so that's one parameter. I read
> somehwere that the diameter of the mesh graph is expected to be low,
> in the range of 4-5. Is this true? What about the degree of the the
> vertices (in the sense of graph theory, i.e. the number of connected
> other vertices)? What is the maximum degree of vertices that is
> expected in real-world networks? What are the parameters the Marvell
> hardware has been designed for?

The number's I've heard are around 2000 nodes, and I have no info
about the degree of the vertices.

> One last thing: is cross-posting to the libertas and olpc-networking
> mailing lists a good idea? I started this because I didn't know which
> one to use, but perhaps we should move this thread to only one of
> them. (probably libertas-dev?)

I don't know either ;)  So I'll just follow your lead for now...


Javier Cardona
cozybit Inc.
e javier_AT_cozybit.com

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