Subject: [Autoconf] MANET subnet model

From: "Yi, Seung" <[email-masked]>
Date: Tue, 24 Oct 2006 17:35:25 -0700

Hi all,

Some of us at Boeing have been discussing about the MANET subnet model
issue brought up in the list and came up with some idea we'd like to
share. Following is a short description of our view. Please direct your
comments and questions to the list so that we can continue the
discussion.

Thanks,

- Seung

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IPv6 Subnet Model for MANETs

In this text, we outline candidate models for assigning IPv6 addresses
in a MANET. We organize our discussion by separately discussing the
assignment of i) link-local addresses to a MANET interface, ii)
non-link-local addresses to a MANET interface, and iii) non-link-local
addresses to a MANET router's non-MANET interfaces.  Definitions and
terminology are borrowed from the MANET architecture I-D
(draft-chakeres-manet-arch-01.txt). 

Any two routers that can communicate via the underlying layer-2
technology without decrementing the Hop Count are said to be on the same
link; under this definition, the links and on-link neighbors may be
time-varying on a MANET.

1.  Link-local addresses on MANET interfaces
We assume that each MANET Router (MR) configures a link-local unicast
address on its MANET interface.  In standard IPv6 operation, a
link-local address is generated without relying on external support per
RFC2462 with interface identifiers configured from EUI-64 identifiers
(RFC4291), Cryptographically Generated Addresses (CGAs- RFC3972), etc.

A key challenge with link-local addresses is providing duplicate address
detection (DAD).  To satisfy the uniqueness on the link, the link-local
address for a MR's MANET interface must be unique among all MANET
interfaces on the link.  However, assuming mobility the neighbor
relationship of an MR may change over time.  Since RFC 2462 (DAD) is
performed only when the link-local address is configured, there is a
chance that two nodes that were originally not on-link with respect to
one another might configure the same link-local address.  If this occurs
and the two nodes then move into each other's transmission range, this
situation would violate the on-link address uniqueness condition. This
calls for extending the DAD functionality in three potential directions:

a. Operational guarantees - Guarantee address uniqueness using
out-of-band mechanisms. This may include providing a large-enough
address space for a small number of nodes so that random address
selection can be virtually collision-free, managed configuration of
interface identifiers, etc.

b. Pre-service DAD using Neighbor Discovery Proxies (RFC 4389) - Use ND
proxies to propagate DAD messages across links with colliding addresses
in the MANET. This technique is known to have conflicts with Secure
Neighbor Discovery (SEND), but can be mitigated by "partial proxying"
that covers only the DAD function.   
  
c. In-service DAD - Constantly monitor addresses of other nodes to
detect any potential address conflict. When detected, the conflict must
be resolved (mechanism to be specified).

2.  Non-link-local addresses and prefixes
MRs can configure non-link-local addresses and prefixes such as Unique
Local Addresses (ULA), MANET Local Addresses (MLA) or globally routable
IP addresses.  Two concepts of operations include prefix assignment and
prefix delegation.    How to assign or acquire addresses/prefixes is out
of scope of this discussion.

2.1  Assignment from prefix to a MANET interface
With prefix assignment, multiple MRs each configures one or more
addresses (and the addresses' associated prefix lengths) to their MANET
interface. This case is distinct from prefix delegation in that in
prefix assignment, the router does not own the whole prefix.

One possible configuration is that each MR can assign addresses from the
same prefix to a MANET interface so that interfaces on multiple MRs
belong to the same subnet. This method is feasible only when all MANET
nodes that configure addresses from the same prefix are operationally
assured to be reachable in one IP hop on the same link.  If all nodes
within this prefix are not reachable in one IP hop on the same link,
then a multi-link subnet exists
[draft-iab-multilink-subnet-issues-00.txt].

Another possible configuration is for each MR to assign prefixes to a
MANET interface so that no two MRs belong to a same subnet.  MANET
routing protocol behavior need not change since it will still inject
appropriate routes to provide connectivity.  The simplest case would be
configuring MANET interfaces with an address with a /128 prefix length.
However, an application might make an incorrect assumption; e.g., a host
application might assume that the MANET interface is configured with a
/64 instead of a /128. If a host makes this assumption, it could fail to
connect with another node in the same subnet due to an incorrect on-link
assumption. But, the IPv6 scenario should be no different than for IPv4
since IPv4 applications must honor CIDR prefix assignments on interfaces
in order to avoid incorrect on-link assumptions.

2.2   Delegation from prefix to a non-MANET interface
With prefix delegation, an MR acquires/chooses one or more prefixes for
assignment to non-MANET interfaces and other downstream hosts on those
links.  For applications on the router itself that require a
non-link-local address, a technique of delegating a small prefix to the
router's loopback interface and having the router's applications bind to
an address in that prefix has proven to be operationally useful.  To an
attached node, this set-up appears as a normal link where link-local
scope addressing and on-link determination works as usual. Again, MANET
routing protocol behavior does not change and the MR injects appropriate
routes over its MANET interface to provide connectivity.

2.3 Combined model
Prefix assignment and prefix delegation are not disjoint.  An MR can
both assign addresses from shared prefixes to its MANET interface and
receive prefix delegations for assignment to non-MANET interfaces, but
it should keep the prefixes for each operation disjoint.  An MR should
not use a same prefix for both assignment and delegation on different
interfaces.

3.  Prefix Aggregation
Section 2 presents certain implications about prefix aggregation in
MANETs.  MANET border routers may be able to present the MANET as a
whole to external entities by enforcing a common network prefix for the
all the nodes inside the MANET. For instance, all of the addresses in
the prefixes delegated to each MR may be aggregated.  However, there is
an operational risk to doing so if the MANET becomes partitioned.  

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From: "Charles E. Perkins" <[email-masked]>
Date: Wed, 25 Oct 2006 23:38:31 -0700

Hello,

I have a few comments on your text.

> link; under this definition, the links and on-link neighbors may be
I think it would be better to make sure that any time a packet goes
from one node to another distinct node the hop count is decremented.
A router decrements the hop count before forwarding to a node
on the same link.
> address on its MANET interface.

I don't think this is necessary.  It should be allowed, but not mandated.

> of scope of this discussion.
What exactly does it mean to say that acquiring addresses is out of scope?

Regards,
Charlie P.

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Please address return e-mail to [email-masked]

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From: "Yi, Seung" <[email-masked]>
Date: Mon, 30 Oct 2006 11:01:45 -0800

Charlie,

I believe Fred answered question 1. For question 2, I tried to convey
that a subnet model is not tied to any particular address management
mechanism. We may use one or some of existing mechanisms such as DHCP or
we can invent a completely new mechanism/protocol for MANET.

Regards,

- Seung

-----Original Message-----
From: Charles E. Perkins [mailto:[email-masked]] 
Sent: Wednesday, October 25, 2006 11:39 PM
To: Yi, Seung
Cc: [email-masked]
Subject: Re: [Autoconf] MANET subnet model

> 2.  Non-link-local addresses and prefixes MRs can configure 
> non-link-local addresses and prefixes such as Unique Local Addresses 
> (ULA), MANET Local Addresses (MLA) or globally routable IP addresses.

> Two concepts of operations include prefix assignment and
What exactly does it mean to say that acquiring addresses is out of
scope?

Regards,
Charlie P.