Re: [Autoconf] Multicast confusion between MANET Arch and AUTOCONF Problem Statement

[Alexandru Petrescu <[e-mail]>]

Emmanuel Baccelli wrote:
> I agree with Ronald. If there was already an easy definition of what 
> is a "MANET link" then the architecture document would not be needed,
>  would it?

I agree with Ronald too that more special mappings between subnets and
links can be made: two subnets on one link, or two links on one subnet.

But I would like the description of the link we're dealing with to be
more precisely described.  We can't simply say that we deal with all
types of wireless and wired links.  That's way too generic.

Saying a link is asymmetric (X reaches Y, Y reaches Z but Z doesn't
reach X) without saying at _what_ layer is it asymmetric: IP or MAC? I
suppose you mean WiFi MAC adhoc mode being asymmetric (3 laptops with
wifi cards in adhoc mode placed linearly at 50m between each).

That is a problem for the link-layer, which can be solved by using a
802.1d bridge on the Y node.

If we want to solve that problem at IP layer then the first thought is
to make a subnet between X and Y and another one between Y and Z.  I
think this is a good IP solution.

But obviously if one makes an IP datagram repeater on Y we obviously get
a link-layer structuring problem with too many packets in the air and no
multicast structure.

I'm trying to say there are simple solutions to that asymmetric
reachability problem that only involve good simple address planning, not
any new protocol.

But if we don't have good agreed definitions then it all turns round and
round.

Sorry, I can't seem to make myself clear...

Alex

> Emmanuel
> 
> Velt, R. (Ronald) in 't a écrit :
>> Hi Alex,
>>> -----Original Message----- From: Alexandru Petrescu 
>>> [mailto:[e-mail]] Sent: dinsdag 20 november 
>>> 2007 16:20 To: [e-mail] Subject: [Autoconf] Multicast 
>>> confusion between MANET Arch and AUTOCONFProblem Statement
>>> 
>>> I think there may be a confusion with respect to the 
>>> interpretation of multicast in 
>>> draft-ietf-autoconf-manetarch-07.txt vs 
>>> draft-ietf-autoconf-statement-02.txt.
>>> 
>>> It seems as if the manetarch accepts that link-layers have a well
>>>  defined multicast behaviour whereas the problem statement 
>>> doesn't.
>>> 
>>> autoconf-statement:
>>>> Traditional solutions assume that a broadcast directly
>>> reaches every
>>>> router or host on the subnetwork, whereas this generally is not
>>>>  the case in MANETs (see [2]).
>>> So a broadcasted message (a special case of multicast) will not 
>>> reach every host on the MANET subnet.
>> 
>> "link" versus "subnetwork" are distinct notions, aren't they? What 
>> is left of a "subnetwork" in an environment where \128 (\32 for 
>> IPv4) prefixes are configured on MANET interfaces? If your local 
>> interface is the only one in the subnet, then all interfaces in the
>>  subnet are trivially reachable :-) "Link" is something else...
>> 
>>> [2] autoconf-manetarch:
>>>> Link-local Multicast/Broadcast Scope On a MANET interface, a 
>>>> packet sent to a link-local multicast or all-ones broadcast
>>> address reaches
>>>> the MANET interfaces of neighboring MANET routers...
>>> So it actually does.
>>> 
>> 
>> No, not all interfaces of all MANET routers. Just those of the 
>> *neighbo(u)ring* MANET routers. See MANET architecture for a 
>> definition of neighbo(u)r.
>> 
>>> I personally think that the autoconf-manetarch definition is more
>>>  inline with how I see relationships between link-layer multicast
>>>  and IP multicast.
>>> 
>>> Or maybe we could define 'subnet' to be a set of hosts and 
>>> routers linked together by the same link-layer technology where a
>>>  multicast behaviour is well defined.  Or similar?
>>> 
>>> What do you think?
>>> 
>>> Alex
>> 
>> Just my 2 cents, Ronald
>> 
>> 
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