Subject: Re: (mobile-ip) changes in draft-ietf-mobileip-tunnel-reverse-04.txt

From: Charles Perkins <[email-masked]>
Date: Thu, 28 Aug 1997 13:23:21 -0700 (PDT)

> Dave Johnson wrote:
> > [snip]
> > Exactly -- the topology and number of hops needed to get from one
> > point to another in an ad hoc network may be highly variable over
> > time.  I think ad hoc routing is best done by treating the ad hoc
> > network as a single subnetwork layer, looking like a single hop to IP.
> > [snip]
> 
> I agree with Dave.  In this model, a packet's TTL s not decremented
> as it ``travels'' inside the ad hoc (sub)network.  
> 
> My original question to Charlie Perkins, which started us on 
> this thread, arose from Charlie's assumption that the TTL would be 
> decremented in the ad hoc network.  Charlie, is that what you meant?

Yes, it is what I meant.  Moreover, I think that the reasons for
treating the ad-hoc cloud as a single subnetwork layer need to be
examined very closely.

What does TTL mean anyway?  Conventionally, it means number of
hops.  When subnetwork layers start distorting the meaning of TTL
by tunneling or ATM-style methods, it seems to me that TTL becomes
meaningless as a routing metric, or for that matter an end-to-end
statistic.  I find myself reluctant to relegate TTL to such an
ignominous end.  Worse, if ad-hoc networking involves routing, and
routing involves TTL, by eviscerating TTL one essentially stipulates
that only the most coarse-grained integration with infrastructure
routing is envisioned.  I didn't yet understand why such a heavy-handed
approach was required.

There is still room for a more integrated approach between ad-hoc
networks and existing infrastructure.  To disregard this in order
to solve a Mobile IP problem that has other solutions available
seems premature to me.

The larger question is whether an ad-hoc cloud is a single subnetwork
layer.  I really don't understand the need for this, although I agree
that solutions have been developed that make this simplification.
For instance, what happens if the ad-hoc network contains nodes with
more than one kind of network interface?  Are we modeling subnetworks
as a witches brew of entities whose only common denominator is that nobody
yet thought of a way to do fast enough routing protocols for them?

While I agree that TTL is hardly the world's most elegant routing
metric, I don't think that progress lies along the path of discarding
the only game in town.

> Regards, _Andrea

Regards,
Charlie P.

---

From: Erik Nordmark <[email-masked]>
Date: Thu, 28 Aug 1997 15:26:17 -0700 (PDT)

> ignominous end.

I think the TTL already has such limited semantics in the IP architecture.
It is definitely a routing metric in the sense of the metrics used by
routing protocols.
All it is really used for (which indirectly defines its semantics) is
preventing infinite loops and for IPv4 multicast scope control.

For instance, in the discussions that lead to RFC 1933 (IPv6 transition
mechanisms) tere was an issue whether an IPv6 tunnel over IPv4 (which could
span the whole Internet) should be represented as a single hop in the
IPv6 hop limit or as the same number of hops as there are IPv4 hops inside
the tunnel. The result of this discussion was that an IPv4 tunnel
should be treated as a single hop just like multiple link-layer hops over 
e.g. ATM or X.25.

This is to illustrate that most folks view TTL as just a loop supression
mechanism without any implied meaning of "distance" or "metric".
Presumably the routing protocols will carry a more meaningful metric.

   Erik