[OLPC Networking] Re: Ability to set the 802.11s TTL for outgoing packets

[Javier Cardona]

Hi Lennart,

The data frames in the current implementation of the libertas mesh
routing protocol do not include the newer Mesh Forwarding Control
field (where the TTL counter lives).  Instead, they are in the
standard 4-address WDS format, which is the format supported by the on
chip MAC controller.

However, a TTL counter is used in mesh management frames.  Management
frames are used to discover the routes used to forward unicast frames.
 The TTL counter in mgmt frames limits the number of hops that a route
request will travel.  Routing loops are possible in roots manually
created by users (and this would cause infinite retransmissions!), but
not in routes discovered by the mesh routing protocol.   This TTL is
now a compile time variable, but could be made available to the driver
if needed.

We are currently working with Marvell in implementing support for
in-mesh broadcast, for which we'll have to find a way to implement TTL
in data frames.  But until that is complete, broadcast requests are
not forwarded, and that could be just what you need:  in the current
released firmware, a broadcast frame will only reach its direct
neighbors.

In summary:

The TTL in data frames would determine how many hops a data packet
would travel.  This is not implemented yet.

The TTL in management frames determines how far a node will look for
nodes it has traffic for.  This is a compile time variable that could
easily be exported to userland.

In the current release, broadcast frames only reach a node's direct neighbors.

Please let me know what would be the requirements to support your mDNS
responder.

Javier

-- 
Javier Cardona
cozybit Inc.
e javier_AT_cozybit.com