Subject: Re: Dynamic DNS and TTL value 0, part II

From: Mikael Latvala <[email-masked]>
Date: Wed, 15 Dec 1999 15:44:19 +0200

[email-masked] wrote:
> 
> This isn't any different from any other situation.  You haven't raised any
> issues about mobility, or issues about service provider roaming.  

Yes I did, although rather superficially in my earlier message. Mobility
(including roaming between service providers) is naturally one of the
important issues. I just thought that I should limit the conversation to
the topics relevant to this WG. However, because this lack of mobility
aspect in my messages have been brought up in other responses I am going
to digress and try to address this issue briefly.

The TTL approaching 0 does not solve the mobility issues. For example,
to make sure that systems can perform handovers from one access network
to another (e.g. from 802.11 to GPRS), or between service providers
(=cellular operators) there must be some sort of tunneling mechanism so
that on-going connections are not interrupted when a handover is
performed (handover always implies that there is a on-going connection
when the system decides that the access point should be changed).
MobileIP is probably the best know tunneling mechanism today although
one can do similar things with L2TP or GTP which is GPRS's proprietary
tunneling mechanism.

Roaming is a bit trickier in a sense that at least on the paper one
could envison it to be done by updating a DNS entry, which is pretty
much how it is done today in cellular systems (visited PLMN telling
mobile phone's Home Location Register (HLR) where the mobile phone is
currently).

In the new brave world it is very possible that we are going to see 3
different mobility management mechanism in cellular systems. MobileIP is
going to be used to provide mobility on a macro level (e.g. roaming
between cellular operators), some IP-based micro mobility mechanism
inside a core network of a cellular operator (e.g. HAWAII or CellularIP,
see MIP WG's home page), and level 2 mobility functionality inside the
radio access network (e.g. BSS in GSM, or RNS in UMTS).

The obvious question is why we want to mess with TTL if cellular systems
must implement MobileIP anyways. The answer to that is that with
MobileIP there is this problem called triagle-routing which is not only
bad for the core network because of the extra load it puts on it but
because it increases the end-to-end delay in many cases above acceptable
levels for voice (200ms). There is a solution to fix that
(draft-ietf-mobileip-optim-08.txt) but in IPv4 world that means that any
host wanting to benefit from that solution must be modified. Not good.

Therefore MobileIP should be used only when necessary. In other times
wired/wireless hosts should be notified ASAP of the new location (i.e.
new IP address) of a mobile host so that there is no need to establish
tunnels between old and new access points.

> Naming is the least of your problems. 

True, not only that it is realtively minor compared to others but that
it is also a very futuristic problem. However, it is still a problem and
that's why I started this thread on this list.

> The issue of DHCP lease times is something that the service provider can decide
> based on its service model. How mobile is mobile?  Does the user stay in > a single service area for a while, or are they racing down the highway, and
> probably change providers every 90 seconds. Both cases might exist, but
> probably have different service providers, or at least different DHCP pools. 
> TTL is something that the mobile device software needs to set based on the DHCP
> lease time.
> 

Good question. I have tried to get some mobility pattern numbers from
cellular operators but they seem to be rather reluctant to give those
out. One thing for sure, although we might see in the future more
competitors on cellular sector, the existing ones are unlikely going
disappear. Which means that you are very unlikely going to see scenarios
where you enter a new coverage area operated by a different cellular
operator every 90 sec. For example, here in Finland my cellular operator
has covered 99,9% of the country which means that I have to get a new
service provider only when I visit another country. From a personal
experinece I know that that is also the case  in many places in the
State, i.e. you lose you digital coverage when you drive far away from
the metropolitan area. Of course, if one were happy with an analog
service s/he could move freely almost anywhere in the States.

> Today, you aren't going to add a route per device.  It might happen in the 
> future, by the time such mobile devices are widely used. Though frankly, many
> people in large ISP core engineering groups are of the opinion that such
> services will never be offered, and they don't need equipment with capabilities
> to do QOS, or bandwidth reservations through the core, or handle route tables
> with millions of routes. There are a few small voices for such services. And
> possibly some vendors who can build the routers to do the job. But whatever you
> do, there is no free lunch.

Hmmm ... I wonder what those folks would say about
draft-ietf-mobileip-optim-08.txt or draft-valko-cellularip-01.txt where
routing table entries are updated when ever a mobile host changes its
access point.

/Mikael

---

From: Harald Tveit Alvestrand <[email-masked]>
Date: Fri, 17 Dec 1999 14:48:26 +0100

>radio access network (e.g. BSS in GSM, or RNS in UMTS).

I would see tethered-IP (what's currently called mobile-IP) as a tool for 
keeping a connection alive while the handset is moving between providers - 
one can live with the extra delay introduced by roaming from my 802.11 
office to the GSM cell outside, I would think.

OTOH, I can't see it working when I fly to the States and get a call from 
the guy at the other end of the bus; linking through my home office is 
suboptimal. (I live in Norway)

So I would see four levels:

- DNS at phone-turn-on time, and when needed and no calls are open
- Mobile-IP for inter-technology roaming inside a city, while calls are open
- Micro-mobility within an operator (I know nothing about this)
- Basestation mobility within a technology (GSM basestation switching)

Ain't we got fun?

>service s/he could move freely almost anywhere in the States.

OTOH, if I go to Germany, where my GSM provider has roaming deals with all 
major networks, my cellular *does* change providers every 5 minutes.
I've seen it change providers 3 times between the plane and the exit when 
landing at Heathrow.
The world is a strange place.

                       Harald

--
Harald Tveit Alvestrand, EDB Maxware, Norway
[email-masked]