Subject: Re: [mobile-ip] RE: any ideas for this problem?

From: gabriel montenegro <[email-masked]>
Date: Fri, 09 May 2003 17:35:46 +0200

Ed Remmell wrote:
> To clarify, the problem that Gabriel reports w/ regards to 802.11 using
> different link speeds for broadcast/multicast packets versus for unicast
> packets, as I read it, it only has to do with the access point transmitting
> to the MN: i.e.  since the router uses a faster link speed (i.e. with less
> range) for the unicast packets that it sends to the MN than for multicast
> RSs (i.e. with more range), then there is the potential for the MN to be
> able to receive the RSs from the router, but not the unicast packets from
> the same router - and this means that the fact that the MN is able to
> receive RSs from its router is not a good indication of being reachable from
> its router w/ respect to unicast. However, as I've said above, this is
> really a non-issue - it is all handled correctly by the 802.11 MAC/PHYS.

(Still trying to catch up...) I guess this discussion relates to
issue 297 which is close to being resolved finally.

But, just to clarify: this is a problem that has
been observed in the field and was reported last year in this paper:

H. Lundgren, E. Nordström, C. Tschudin,  "Coping with Communication Gray Zones
in IEEE 802.11b based Ad hoc Networks", WoWMoM 2002.

Reception of an 802.11b broadcast from a node does not guarantee unicast reception from
(or unicast transmission to) that node. However, this was seen in adhoc mode.
Perhaps the AP function in infrastructure mode takes care of this, as you say.

-gabriel

---

From: "Ed Remmell" <[email-masked]>
Date: Fri, 9 May 2003 11:23:12 -0700

Gabriel -

I looked at this paper some.

The information we gleaned from the IEEE 802.11 specification, as well as
the information provided by Intersil (a vendor of 802.11 chipsets) says that
an 802.11-compliant access point will not have this problem, because it
downshifts the link-speed that it uses to send unicast packets to the MN to
compensate (the reception of unicast packets is ACK'ed by the MN at the MAC
layer, so the router can tell if the MN is able to receive its unicast
transmissions).

On the other hand, it is possible that the MN can hear the router's RAs, and
because of the functionality described above can also hear unicast packets
sent to it by the router, however the router can't hear the MN's
transmissions (this is the "reachable from the router" but not
bi-directionally reachable situation). Some kind of probing method is needed
to detect this situation; after removing much requirements text (including
text on probing) from the MIPv6 movement detection section, what we are left
with currently is Neighbor Unreachability Detection probing (which works,
but is slow). I'm OK with the current lack of detail in this section. If you
think we need to add back in more text on probing, please speak up.

Thanks.
- Ed

> -----Original Message-----
> From: gabriel montenegro [mailto:[email-masked]]
> Sent: Friday, May 09, 2003 8:36 AM
> To: Ed Remmell
> Cc: Jari Arkko; Jason Schmidlapp; Mobile IPv6 mailling list
> Subject: Re: [mobile-ip] RE: any ideas for this problem?
>
>
> Ed Remmell wrote:

---

From: "Ed Remmell" <[email-masked]>
Date: Fri, 9 May 2003 11:35:49 -0700

> layer, so the router can tell if the MN is able to receive its unicast

Oops, this should have said:
"so the access point can tell if the MN is able to receive its unicast
transmissions)."

- Ed

---

From: gabriel montenegro <[email-masked]>
Date: Fri, 09 May 2003 20:42:08 +0200

Ed Remmell wrote:
...
> an 802.11-compliant access point will not have this problem, because it

Yes, perhaps 'good' AP's won't have it so in 'infrastructure mode' you may
be mostly fine, but the other 802.11 mode of operation ('adhoc mode') may encounter
this issue more commonly.

...
> If you

I'm on the "less is more" camp on this one, so please don't add more detail.

tnx,

-gabriel