Subject: Re: MANET & IP Layering

From: Richard <[email-masked]>
Date: Sun, 08 Apr 2001 16:35:51 -0700

I would like to add some thoughts to this interesting discussion
on MANET & IP layering.  Some of what I say may repeat or agree or 
disagree with some things that Fred, Chip, and Kevin have said, 
but I hope this message will offer some new points. Comments
are welcome.

It seems to me that an IP-based MANET network can be used in either 
of the following ways:

  1. At the internetwork layer, as an automonous system (AS)
  2. At the subnetwork layer, as a subnet in an AS

As a subnet, similar to IP over ATM, or more generally IP over NBMA
[RFC 2491], we have IP over MANET (where a MANET is IP-based).
In this case, the MANET can emulate the service provided to the 
internetwork layer by a conventional broadcast subnet.  
This emulation would utilize an efficient MANET broadcast/multicast
mechanism.  For example, a packet sent by an IP router over an ATM 
(or MANET) subnet *appears* to travel one hop, even though it could 
actually be traveling several hops.

For packets generated *within* a MANET, the existing IP rules can 
be followed, e.g., do not forward packets with the limited
broadcast (255.255.255.255) or all-systems multicast (244.0.0.1)
address.  Packets generated by the internetwork layer with these 
addresses should *appear* to be sent only one hop from the 
internetwork layer's perspective, even though they are actually sent 
multiple hops from the MANET's perspective.  One way to accomplish this 
is to encapsulate the internetwork-layer IP packet by attaching a 
MANET-layer IP header.  For example, this attached header could 
translate the limited broadcast or all-systems multicast address 
to the subnet-directed broadcast address for the MANET, so that it 
is broadcast (using an efficient MANET broadcast mechanism as Kevin 
suggested) to all nodes in the MANET regardless of the number of hops.
The TTL of the the MANET-layer IP header would be decremented at 
each hop, while the TTL of the original IP header stays the same.

A subnet-directed broadcast originating from outside the MANET
would be broadcast to all nodes in the MANET, perhaps using the 
same address. In this case, it may not be necessary to attach a
MANET-layer IP header (but it may be necessary to modify the TTL).

As another example, an OSPF Hello packet generated by an internetwork
router (normally sent a single hop to the multicast address 
AllSPFRouters) can be multicast within the MANET to all gateway routers 
belonging to that group (regardless of the number of hops required).  
(Of course, we would like to avoid the redundancy of sending both OSPF 
Hello packets and MANET Hello packets within the MANET, but that is 
another topic.)

Regarding addressing, I agree that all nodes in a MANET subnet 
should have the same IP network/subnet prefix.  In fact,
this is a requirement of a logical IP subnet (LIS), which is 
commonly used to model an NBMA network (e.g., for IP over ATM). 
It is also possible for two MANETs to overlap, in which case
the nodes common to both MANET's need to have both prefixes.
If a MANET is partitioned, each connected component should be 
reconfigured so that the different components use different
(perhaps longer) network prefixes. These prefixes can perhaps be 
determined by the gateway/border router that is selected as home/foreign 
agent. This problem was addressed (for a different context) by 
SRI's Reconstitution Protocol (RP) developed in the late 80's.

Regarding router/agent advertisements, I assume these would be sent
by internetwork gateway routers to announce their presence to 
MANET routers/hosts. I agree that these advertisements contain
important information, but instead of broadcasting these messages to 
all MANET nodes, an alternative (better or worse?) is to convey this 
information in MANET control messages (e.g., similar to the gateway 
announcement messages proposed in the OLSR extensions draft).

Richard

---

From: Richard <[email-masked]>
Date: Mon, 09 Apr 2001 09:58:25 -0700

I wanted to add one more thought to my previous message.
(I have not yet seen any response to my previous message,
but it usually takes a few hours for me to receive email
sent to the manet list.)

A third way to use a MANET is to emulate an OSPF area.
Thus, a MANET routing protocol would be used within the
area instead of OSPF, but would provide the same information
to Area 0 as would OSPF. This seems doable, but of course this 
only works for OSPF (but maybe it can be generalized).  A virtual 
path can be set up across the MANET/OSPF area if necessary.  
An advantage of this approach is that it is not necessary to 
encapsulate an internetwork-level IP packet into a MANET-level
IP packet (assuming both use IPv4).

I also want to point out that I am just giving general ideas.
Many details need to be worked out, and there could easily
be errors in what I have said.

Richard

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Subject: Re: MANET & IP Layering 
Date: Sun, 08 Apr 2001 16:35:51 -0700
From: Richard <[email-masked]>
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Richard

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