Subject: [manet] OLSR questions

From: [email-masked] (Ioannis C Avramopoulos)
Date: Mon, 11 Mar 2002 22:29:44 -0500 (EST)

Dear MANET Researchers,

I have some questions on OLSR and I would greatly appreciate it if any of
you took the time to answer. My questions are the following:

- What are the advantages of having a LOST_LINK type over having no such
type (version 4 for example did not have such link type)?

- Why should a node have multiple interfaces all running the OLSR protocol?

- Does the IEEE 802.11 specification make any provision for the scenario of
a node possessing multiple interfaces?

- Are MPRs elected per interface or per node? In other words is it possible
for a node that possesses multiple interfaces to elect a different MPR-set
for each of its interfaces? If MPRs are elected per interface what is the
point of doing so?

- It is stated in the draft that the MPR-set is constructed such that any
symmetric 2-hop interface is reached. Why is it desirable to reach any
symmetric 2-hop interface and not just any node possessing at least one
symmetric 2-hop interface?

- How does a node determine which interfaces should be used to (re)transmit
network-wide broadcast messages?

- It is stated that HELLO messages are transmitted on all interfaces. Isn't
this a waste of resources?

Thank you in advance.
Ioannis Avramopoulos

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From: [email-masked] (Ioannis C Avramopoulos)
Date: Tue, 12 Mar 2002 15:20:24 -0500 (EST)

Dear Laurent,

thank you for your clarifications. I still have some questions. See below.

Ioannis
 
>> - What are the advantages of having a LOST_LINK type over having no
>>such type (version 4 for example did not have such link type)?

>It allows quicker update of one hop and especially two hop
>neighborhood. A node announcing a neighbor loss allows its neighbors
>to remove the link (without waiting for expiration) from their 2-hop
>neighbor set.

Lets assume that initially nodes A and B have a bidirectional link and at
some time the link becomes unidirectional from A to B (A->B). After the
timeout of A expires and A has not received any HELLO from B, A may instead
of listing B with link type LOST_LINK may not list B at all. We can simply
add a rule in B to consider the link with A unidirectional if B is not
listed in A's HELLO (instead of considering A as a unidirectional link if B
is listed as LOST_LINK) and therefore we don't have to wait for the
timeout.

Similarly if we have another node C that is a neighbor of A and a 2-hop
neighbor of B (C-A-B) when the topology becomes C-A->B if B is not listed
in A's HELLO we may simply add a rule in C that purges B from the 2-hop
neighborhood.

If the aforementioned rule is added I still don't see why a LOST_LINK type
is beneficial.

>several radio interfaces with directional antennas (and possibly same
>radio channel).

>> - Does the IEEE 802.11 specification make any provision for the
>>scenario of a node possessing multiple interfaces?

>No. It is in charge of the routing layer.

If you have several interfaces with directional antennas then it is
possible that they overlap. Packets that are transmitted over all
interfaces simultaneously will collide at nodes that are within the
overlap. Since IEEE 802.11 does not arbitrate access to the broadcast
channel and it is the responsibility of the routing layer then the routing
layer should deal with such cases.

I have one more general remark. I believe that considering multiple
interfaces makes the presentation of neighbor sensing not very
straightforward (in version 4 for example where multiple interfaces are not
considered the presentation is straighforward). Maybe you could consider
presenting neighbor sensing separately for the cases of single and multiple
interfaces and more clearly state the objectives and assumptions of
multiple interfaces. But this is just my opinion...

Thanks again
Ioannis