> I just find it strange that you have hit the multicast problem for routing
> protocols but not for IPv6.
Ah, I now understand that we were speaking about different things.
The issue that Babel used to have with 802.11 multicast was not packet
loss -- Babel is designed to be extremely resilient to packet loss, it
handles 80% loss before breakfast.
The issue was about collisions. 802.11 uses CSMA/CA, which is not as good
at avoiding collisions as the familiar CSMA/CD (which cannot be used on
wireless). Since multicast packets are sent at a glacially slow rate,
multiple nodes multicasting simultaneously can cause the network to
collapse.
In earlier days, engineers from a company that wishes to remain anonymous
were testing Babel by putting 1500 mesh routers, tuned to a single
frequency, in a room, and booting them all simultaneously. They found
that when doing that Babel would cause massive amounts of collisions for
no less than 20 minutes; the network worked fine once the bootstrap had
finished.
The issue was fixed by doing two things: rate-limiting replies to wildcard
requests and adding massive amounts of jitter to "urgent" TLVs (those that
the spec says "MUST be sent in a timely manner"). Of course, this
increases convergence time on wired networks, but something had to give.
There are two lessons to be drawn from that experience:
1. don't put 1500 wifi routers in a single room;
2. making a routing daemon that works well in a variety of conditions is
hard work, and requires large amounts of careful testing.
But please don't take my word for it -- it would be way more helpful if
you could do your own testing in order to find out how well IS-IS works
over wireless, and publish your experimental results. As far as I am
aware, nobody has done that, so this would be a seminal paper in its
(admittedly narrow) niche.
-- Juliusz
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