[Olsr-users] Route-Flapping | Out of Control [Deutsch]

[Markus Kittenberger]

On Thu, Dec 23, 2010 at 9:16 PM, Michael Rack <
(spam-protected)> wrote:

>
> > OLSR packets are sent over multicast.  In IEEE 802.11, unicast and
> > multicast packets use different link-layer protocols, and it's fairly
> > usual to see much higher loss rates for multicast than for unicast
> > packets.
> OLSR packets are set via broadcast, not multicast ;)
> multicast is a routed subnet via IGMP in range 224.0.0.0 - 239.255.255.255.
>
routet or not, or which iprange, makes no difference at layers < 3

regarding ethernet layer 2 any mac adress having the last bit of the first
byte set to 1 is a multicast,..
this includes broadcasts FF:FF:FF:FF:FF:FF or various types of multicasts,..

>  Broadcasts should not be dropped anyway.
>
having higher loss rate, does not imply the packets are actively dropped,..

i think what juliusz meant was that multicasts (including broadcasts) are
not acked or retransmitted in most IEEE 802.11 modes (e.g. WDS links are an
exception)
thats why multicasts are usually easier lost than unicasts,..

but osbridges are definetely not "always" IEEE 802.11 conform,..
(i think you mentioned that this link is done with osbridges,..)

(to compensate this expectable higher loss rates) broadcasts are usually
sent with another (usually lower) bitrate:
the multicastrate

which is afair "unconfigureable" on osbridges,..
(but osbridge have a setting called "Multicast Optimization", which imho is
better turned off *G,or at least try if changing it dos have effects on your
problem,..)

but osbridges are definetely not "always" IEEE 802.11 conform,..

e.g. when used in ptp-bridge mode, they do somewhat wdslike things (but not
exactly wds)
(but they will handle broadcasts like unicast in this mode afair,.. (i.e the
ack anr retransmit))

btw. i had once an rb411(bridge mode)-osbridge(ptp-client) link having
(sometimes) unbelieveable high olsr packet loss (about 90%, while the link
itself was doing fine), imho not explainable by lack of retransmits alone.

i temporary fixed this by tunneling the traffic through this link (which
transformed all broadcasts to unicasts)
(and later i replaced the osbridge against a routerboard,..)

another approach would be to configure olsrd to use unicasts instead of
broadcasts *G (i.e. ip4broadcast=neighbor_ip on both sides) (of course this
only makes sense on a ptp link!!)

but when using 2 osbridges i never had such problems (but i don`t use them
since years ago for many other reasons,..)

Markus

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