Subject: [Autoconf] MANET-ARCH-05

From: Thomas Narten <[email-masked]>
Date: Fri, 21 Sep 2007 17:21:19 -0400

Ian Chakeres writes:

> I'm happy to report that we've received several excellent comments
> regarding MANET-ARCH during WGLC.
> 
> I'd also like to write about a few of the changes made in the arch-05
> revision. Several of the changes were made based on Fred Templin's
> arch-04 comments. I would like to point out that Fred's suggestion to
> change usage of term interface to link in many places was not made.

I just read the draft and I have great difficulty with the "interface"
terminology.  In my view (from a non-manet background), you are
redefining a term that is widely used elsewhere and I find that very
confusing. I think you should use something else. I too was thinking
something with the word "link" in it (e.g., "manet link") would be
better.

Interfaces are a software abstraction, and are generally used to refer
to, well, interfaces to physical (or virtual) networks (or
links). This document uses the term to describe what in my view are
link properties - characteristics of the MANET technology (e.g.,
non-asymetric reachability). That is a characteristic of the technology.

> The use of interface, link, subnet, and several other terms have been
> highly debated and the current usage has been agreed upon by people
> from different areas/backgrounds.

Not by me.

> That said, the new revision of the document will address the fine
> comments we've received during WGLC.
> 
> If you have any additional comments, please send them ASAP.

Can you say something about what changes are upcoming?

In general, I find the document is still too fuzzy, especially with
respect to when it is talking about "manet technology" in an abstract
sense (like what its characteristics are) and when it is talking about
manet in an IP environment (i.e, when unstated assumptions are made
about what address nodes have, what prefixes mean, etc.)

Some random specifics:

   Full-Broadcast Interface (FBI)
      A broadcast interface known to have both transitive and
      bidirectional communication, i.e. it does not exhibit asymmetric
      reachability.  Nodes which are connected to the same link via
      Full-Broadcast Interfaces can all send and receive IP packets
      directly to each other -- all nodes are thus bi-directional
      neighbors.  Ethernet interfaces connected via a single Ethernet
      segment is an example of a FBI.

Hmm. How does "IP packets" fit into the above definition?  Do all
FBIs have an IP address associated with them? and or an IP prefix?

isn't the bi-directional nature a property that has nothing to do with IP?

   Border Router (BR)
      a router that participates in multiple routing regions, and often
      multiple routing protocols.  A BR defines the border between its
      multiple routing regions.  A BR is responsible for presenting a
      consistent picture of the nodes reachable through itself to each
      routing region.  A BR determines the routing information to
      propagate between different routing regions.

Unclear how this relates to subneting and IP. I.e., this definition is
imprecise. What is it that distinguishes a BR from some other type of
router? Is this something determined dynamically? via static
configuration?

Or, how is a BR any different than a normal IP router?

   MANET Interface
      A MANET interface may demonstrate asymmetric reachability (e.g.,
      SBI) and/or neighboring nodes' addresses may not be a priori
      known.  Note: according to the definition of a classic IP
      interface, such an interface satisfies the characteristics of a
      MANET interface -- with the additional nice properties that it
      does not exhibit asymmetric reachability and a set of neighboring
      nodes' addresses are known a priori.

The last part of this sentence seems incorrect. Most nodes do not know
the set of neighboring node addresses. They may know an address prefix
of destinations that can be assumed to be on-link, but that is not the
same as what is written above.

   MANET Router (MNR)
      a MANET router embodies router functionality and may also include
      host functionality, reachable via its loopback interface(s).

I don't understand this second part. A loopback interface is a
software abstraction and is often implemented to look like an
"interface" (oops, now I'm using the normal definition, not the manet
one -- see why redefining this term confusing?). As such, you are now
(presumably) talking about a particular implementation. As such, the
above is not really an appropriate definition.
      
      A MANET router has one or more MANET interfaces.  A MANET router
      may also have zero or more classic IP interfaces to which itself
      (via loopback), hosts, routers, or networks may connect;

There is that funny loopback  definition again.

      i.e. the router may be responsible for several IP prefixes.

Hmm. now you are talking about IP  addresses. 
      A
      MANET router is expected to participate in routing on behalf of
      one or more of its interfaces.  A MANET router is illustrated in
      Figure 1.

also don't fully agree. and loopback terminology is weird.

   MANET
      a routing region consisting of a set of MANET routers that is
      within one or more MANET router hops.  If a MANET connects to
      other routing regions, its border is defined by Border Routers.

I don't understand what this means. E.g., how do you tell what is in
one region and in another?

   Dependent upon the deployment and management strategy, coalescing and
   fragmentation of MANETs may be a supported feature.  In other words,
   if a communication path between two previously separated MANET
   routers or MANETs becomes available, the two MANETs may merge to form
   a single larger MANET.  Similarly, if a communication path between
   two MANET routers is disappears and no alternative path between the
   routers exists, then the MANET may be partitioned into two separate
   MANETs.

Again, what does this mean? Why/how is this important? Does it impact
the IP characteristics? If so, in what way? What  are the implications
(to IP) of a split or merge? 

   Wireless interfaces exhibit challenging characteristics when compared
   to wired interfaces.  Many protocols (e.g.  IPv6 neighbor discovery
   [RFC2461]) do not operate in wireless networks with asymmetric
   reachability.  Wireless interfaces also exhibit dynamic time varying
   performance (e.g. packet loss, data rate) that can significantly
   impact local communication.

Um, this isn't a problem with "wireless" vs. "wired" interfaces per
se, its a problem with links that display such non-standard
characteristics. We need to be more precise!

And to say "IP doesn't work well" is only true if you make some
unstated assumptions about addresses/prefixes and how they are
assigned.

   The act of forwarding packets out of the same interface as the one
   over which they were received often results in duplicate IP packets
   being received at MANET Routers with more than one neighboring MANET
   Router, while also reaching a new subset of MANET Routers.  Thus,
   duplicate packet detection is often an inherent part of MANET
   protocol designs.

Doesn't this depend on what LL address the packet contains? I'm not
sure this statement is true.

   In wireless systems, nodes within the same small geographic region
   are often densely connected with other nearby nodes.  These nodes
   form a set of extended neighbor relationships that is referred to as
   a neighborhood.  A neighborhood is typically composed of several
   nodes, with each node being densely connected to other nodes.

Is this an MANET Neighborhood? If not, is there a formal definition of
neighborhood?

5.1.  General Address Architecture

   This architectural model considers MANET routers as simply routers
   with addressable nodes attached, as illustrated in Figure 1.  The

Vague. What kind of addresses? IP? and what does "addressable node
attached" mean?

   Link-local Multicast/Broadcast Scope
      On a MANET interface, a packet sent to a link-local multicast or
      broadcast addresses reaches the MANET interfaces of neighboring
      MANET routers, regardless of their configured addresses.  Link-
      local packets are never forwarded and since a MANET may span
      several hops, nodes cannot assume that a packet sent to a link-
      local address will reach all MANET routers within a MANET.

Are we talking IP or not? I don't know what "link local" means outside
of the context of an IP link. But where is MANET defined to be an IP
Link?

Thomas

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