Subject: Re: scope of problem

From: [email-masked]
Date: Mon, 25 Mar 1996 14:35:27 -0500

Friends:

I  have been struggling with some mobile-mesh issues for a  while
and have arrived at the following conclusions:
1.   We  allow  both hosts and routers to be mobile.  Agents  are
     not mobile.
2.   We  have  a  solution for mobile hosts, but it  requires  at
     least  two  network accesses (one to the host  agent  and  a
     second  to the foreign agent) to deliver the information  to
     the  intended destination.  This may be a problem we  cannot
     solve.
3.   We  need  a  delivery service that minimizes the  number  of
     network accesses required to deliver information to multiple
     destinations.   I currently see this service  as  routing  a
     single  datagram  addressed to multiple  destinations  to  a
     single   (host)   agent.   The  agent  then   forwards   the
     information to the individual destinations.  The  agent  can
     take   advantage   of   the  wireless  network's   broadcast
     characteristics and use only one network access  to  forward
     to multiple destinations if they are all at the same foreign
     agent.   My  problem is that it is more likely they  are  at
     different  foreign agents so multiple network  accesses  are
     needed.
4.   I really want a way to associate hosts with foreign agents.

It  may be that I've actually jumped to these conclusions,  so  I
offer some of my thought processes below.

The   wireless   nets  we're  talking  about  are   radio   nets,
specifically  UHF radio nets.  UHF radio networks offer  severely
limited  bandwidth  for a number of reasons.  First,  the  radios
themselves  provide a clock rate of only 16 kbps or so.   Second,
the  environment  is  noisy (up to 10%  BER),  so  forward  error
correction  and encoding techniques are added at the link  layer;
this  reduces the available bandwidth to less than half the clock
rate.   Third, the media itself is shared, half-duplex  and  that
introduces delays in providing media access opportunities  and/or
collisions.  Fourth, these networks support both voice  and  data
communications on the same medium.  So, one of our objectives  is
to  minimize  a  station's  use of the bandwidth.   Luckily,  the
networks  employ broadcast "party line" type media  so  a  single
transmission can be used to deliver to multiple destinations.

We  need  a  reliable, multi-destination transmission feature  to
reduce  the number of network accesses.  The problem we  need  to
overcome  is  that IP permits only one destination  address.   We
cannot  depend  upon  IGMP because the originator  cannot  select
individual destinations of a transmission.  About a year ago, RFC
1770  introduced a "Selective Directive Broadcast"  technique  of
passing  individual addresses through an IP option  to  the  link
layer.   The  technique has some limitations in that  it  doesn't
scale  and  that  there  is no mechanism for  the  originator  to
request  an  acknowledgment  from the  destinations.   Perhaps  a
larger  problem  is  that the technique appears  to  require  the
originator  to have some knowledge of the network's  topology  in
order  to determine which destination addresses should be grouped
into  a  single  option  field.  Can an  originator  assume  that
198.92.10.1  and 198.92.10.28 are both served by the same  router
(agent)?

This  leads me to question what we mean by "mobile".  If  we  say
that  hosts are mobile, then I think we mean that a host can move
from  one  router  to another without affecting  its  ability  to
transmit  or  receive.  This then implies that the  host  can  be
attached to router A or router B and still retain its IP address.
And  that leads me to think that an originator cannot assume that
198.92.10.1 and 198.92.10.28 are both served by the same  router.
UNLESS  he  is  relying  upon  the fact  that  the  destination's
original  router  (host  agent) will forward transmissions  to  a
different  router  (foreign agent) now serving the  mobile  host.
But this means additional transmissions and network accesses.

In  the Army's tactical internet, I believe that hosts take their
IP  address from an attached router.  In this case, hosts are not
mobile.   The user is mobile in the sense that his User Reference
Name  can  be associated with different IP addresses.  This  then
suggests  that a user must register an association  with  a  name
service  each  time  he  moves.  I think that  means  that  other
stations  now need to check with this name service  to  find  out
where   the   user  is.   I'm  not  sure  this  is  a  reasonable
alternative.  (Or am I missing something here?)

Any comments?

:Mike McMahon

---

From: "M. Scott Corson" <[email-masked]>
Date: Thu, 28 Mar 1996 09:44:19 -0500 (EST)

> offer some of my thought processes below.

Mike,

It seems that you're mixing a lot of "mobile IP-ish" terminology
in here, but the problem we're initially focusing on is rather 
separate from the mobile IP problem. 

In the proposed architecture, as I understand it, all network nodes
are "routers", and the network topology is changing.  
The problem is one of routing between the mobile routers in this 
mobile network.  This problem has nothing to do with mobile IP as its
commonly known.

Where notions similar to mobile IP start to filter in is when one
considers whether there is one or multiple hosts per router, 
and more importantly, whether the hosts may "move" between routers.
If there is only one host per router (i.e. the host is a router), 
or if there are multiple hosts per router but the hosts may not 
move between routers, then there is still no need for mobile IP 
functionality within the mobile network.  One may simply associate 
a host with its permanently attached, mobile router.  

However, if hosts may move between mobile routers, then one needs
a mechanism to determine the router at which the host currently 
resides.  Here, mobile IP or something providing similar 
functionality is necessary.  However, this is not the primary 
focus of this group, at least not initially.  Thus, there has not
yet been any need to define anything similar to a mobile IP "agent".

> transmission can be used to deliver to multiple destinations.

A lot of what you say here is true, in parts, for various link layer
technologies.  We have to be very careful and not assume very much 
regarding the link layer implementation.  I think characteristics
such as limited bandwidth and a dynamic topology may generally 
be assumed, but one cannot even say that every node will employ
omni-directional antennas.  Some may employ directional or steerable 
antennas.

> We  need  a  reliable, multi-destination transmission feature  to
                         ^^^^^^^^^^^^^^^^^
                         multicast?

> 198.92.10.1  and 198.92.10.28 are both served by the same  router

I think the IP multicast paradigm can and should be applied to this
mobile network.  Issues regarding IGMP need to be addressed.

> But this means additional transmissions and network accesses.

Previous comment applies.

> alternative.  (Or am I missing something here?)

There are several possible approaches I can conceive of to handle
inter-router host mobility within a mobile network, but the intent
to properly address the issue of "routing" first, so that it's 
possible to handle this second issue.  Perhaps this approach is 
too conservative, but we're already proposing to deal with a lot 
of complexity.

> :Mike McMahon

That's my three cents.

-Scott

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M. Scott Corson                                 Institute for Systems Research
[email-masked]                              A.V. Williams Bldg. (115)
Phone: 301-405-6630 (Rm. 2219 AVW)              University of Maryland     
Fax:   301-314-8586                             College Park, MD  20742

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