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Re: [pilc] IESG Review of LINK

Some comments on Allison's original context:

Allison Mankin wrote:

From Alex Zinin:
I didn't know how to formulate my concerns at first, because it
appeared that in addition to being not quite correct in some places,
the discussion in text actually goes in a wrong direction, and
touches on some seemingly irrelevant points without discussing those
that should have been.

So, I decided to start with comments on the actual text and then
move to the meta question...

> 17 Routing
> > Many subnetworks provide their own internal routing mechanisms.

The text should probably say why some do and some don't. E.g., if a
subnetwork technology can be used to construct a multi-node network,
where internal nodes do not run IP, and where the network is
self-contained and can be used for service independent from IP
routing, then subnetwork-level routing function is usually provided
(FR, ATM, Ethernet bridging and STP). If the technology is primarily
used for single-hop packet framing among IP nodes, subnetwork-level
routing is not needed.
The issue isn't whether it's a multi-node network; the issue is whether the media is switched or not.

Possible text: (?)
--------------------------------
Multipoint subnetworks may be single-hop or multi-hop. In either case, IP considers the subnetwork as a single shared link, i.e., IP assumes there is one or more subnetwork addresses that can broadcast to all nodes on the subnetwork with one subnetwork packet. IP also assumes that multi-hop multipoint subnetworks provide their own routing, such that IP can inject the subnetwork packet with the appropriate destination subnetwork address, with no further path information.
--------------------------------

 > Since routing is the major function of the Internet layer, the
 > question naturally arises as to the proper division of function
 > between routing at the Internet layer and routing in the subnet.

 and the text does not address this question, nor does it talk
 about the interactions among the routing function within the
 subnetwork and at the IP level.
I believe the above addresses the interaction - i.e., that in general IP assumes there is NO interaction, that the subnetwork can be considered a single shared link and be done.

> In general, routing in a subnetwork and at IP is more > complementary than competitive. Routing algorithms often have > difficulty scaling to very large networks, and a division of labor > between IP and a large subnetwork can often make the routing > problem more tractable for both.

In fact, experience shows that this is exactly the opposite.

When a large number of IP routers is connected via a subnetwork that
hides internal routing details from IP, this usually results in O(N^2)
adjacencies, flooding storms, and unforeseen convergence interactions.
In most cases, the physical topology of a subnetwork is orders of
magnitude simpler than the logical IP topology on top of it. This is
because the topology graph within the subnetwork is determined by
physical links and nodes, while the topology graph at the IP level is
essentially driven by the connectivity matrix.
But the link connectivity matrix is insufficient to express the reachability of IP addresses, which is what IP needs to compute.

 With the same number of nodes, routing scales better if it operates
 over the physical topology rather than when it's layered up.
Link routing doesn't scale when viewed as a transit; in that case, the link subnet looks more like an AS, and IP routing convergence faulters when interior routing protocols (e.g., ones that assume broadcast or multicast) are used over a multihop subnetwork.

These are good reasons not to use large-scale subnetworks as transits in dynamically routed IP networks, or to better manage the route advertisements in a BGP-like fashion.

...
 OK, meta issues now.

What I would expect this section to talk about is:

1. Difference between the _function_ of IP routing and the routing
      function found in some subnetworks and analysis of why
      certain subnetwork technologies have the routing function.

(see contributed paragraph above)


2. Give a recommendation that unless there is a very compelling
      reason, routing should be left to IP.

(see contributed paragraph above)


3. If the routing function is still implemented in the subnetwork
      technology, explain what considerations should be kept in mind:

   a) Connectivity graph and its affects on IP routing, such as:
           - number of adjacencies/sessions per router to maintain
           - broadcast & connectivity (if routing treats the subnet as
               broadcast, any-to-any connectivity needs to be ensured)
           - flooding implications
           - ...
Perhaps a paragraph warning about how transit subnetworks end up looking like AS's ;-)??

b) Rerouting, convergence and timing with multiple layers:
- effects of topology change and rerouting within subnetwork
on IP routing (e.g., if the subnetwork has slow rerouting,
IP will start reconverging)
- effects of reconvergence at the IP level on the subnet
(e.g. dynamic connection establishment)

- combination of the two
This hits a different concept. IP doesn't really like having links appear and disappear; it expects the subnetwork to be largely static. That point might be useful to raise (?).

c) Connection model for connection-oriented subnetworks
If the model assumes temporary connectivity between
nodes (e.g., PSTN or ISDN) how will this affect IP routing
(adjacencies going up/down, applicability of e.g. demand circuit
extension in OSPF)
Such systems can be 'faked' by having routing proxies on both sides of the idle links that lie about reachability, and connect only when there is traffic in place. I.e., when the links are made on-demand, it can be useful to let IP consider the links as always there, and kill the proxies only when connection establishment fails.

d) Optimality and traffic engineering
it's addressed in the text to some extent, but more thought
should be given to such aspects as how traffic engineering
within the network (pure IP and MPLS) would be done, i.e.,
is it just "enough BW", or the subnetwork provides TE
functionality as in ATM, for example...
Shifting sands again. ;-)

Joe

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