Ability to set the 802.11s TTL for outgoing packets

Lennart Poettering mzbycp at 0pointer.de
Wed Oct 4 20:36:20 EDT 2006


I am currently doing some scalability work for the Avahi mDNS/DNS-SD
responder (see avahi.org) for usage in OLPC. For that I'd like to be
able to set the 802.11s TTL for outgoing packets. The background is
that to minimize the necessary traffic I'd like to take the mesh
topology into account when distributing the Avahi service information
over the network. Right now I can read the mesh forwarding table from
userspace. However, it's not yet possible to multicast packets to just
a subset of the hosts in the mesh, depending on the
proximity. However, such a feature would become very handy in my work,
and probably even for other projects. (Think of an RTP video cast to
only the hosts nearby)

After reading through the 80211s draft I came the conclusion that the
"Mesh TTL field" ( in the packet header could be used for
what I am looking for. Setting it to 1 should allow me to multicast
packets to just my direct neighbors and nobody else. (or did I miss
anything? the 802.11s draft is not that verbose in its current

The marvell hardware interface right now doesn't seem to allow setting
the TTL for each outgoing packet as I understood the driver. (Actually
it doesn't even allow setting the TTL at all, not even globally)

Fixing up the driver to pass this value along in the TxPD structure
doesn't seem too difficult to me. However I wonder how feasible it is
to get the firmware improved to accept this field in each TxPD
message. Does anyone at OLPC have direct access to the firmware
sources? Or is Marvell open to requests like this one?

What I have in mind in the end is that I can use a setsockopt() or a
sendmsg() field for setting the mesh TTL for outgoing packets, much
like it is done with IP ttls with the IPPROTO_IP::IP_TTL sockopt.

Any thoughts or ideas?


Lennart Poettering; lennart [at] poettering [dot] net
ICQ# 11060553; GPG 0x1A015CC4; http://0pointer.net/lennart/
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