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draft-ietf-nimrod-mobility



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Nimrod Working Group                                          Ram Ramanathan
Internet Draft                                  BBN Systems and Technologies
February 1995                                           Expires 30 July 1995
draft-ietf-nimrod-mobility-00.txt




    Mobility Support for Nimrod :  Requirements and Solution Approaches



                            Status of this Memo



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                                  Abstract


We discuss the issue of mobility in Nimrod.  We identify the requirements
that Nimrod has of any solution for mobility support.  We also classify and
compare existing approaches for supporting mobility within an internetwork
and discuss their advantages and disadvantages.  Finally, as an example, we
outline the mechanisms to support mobility in Nimrod using the scheme
currently being developed within the IETF - namely, the Mobile-IP protocol.


Internet Draft            Nimrod Mobility Support              February 1995


Contents


1 Introduction                                                             1

2 Mobility :  A Modular Perspective                                        1


3 Effects of Mobility                                                      2

4 Approaches                                                               5


5 A Solution using IETF Mobile-IP                                          8

  5.1 Overview  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  8

  5.2 Protocol Details  . . . . . . . . . . . . . . . . . . . . . . . . .  9

6 Security Considerations                                                 14


7 Summary                                                                 14

8 Acknowledgements                                                        15


9 Author's Address                                                        15
























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1 Introduction


The nature of emerging applications makes the support for mobility essential
for any future routing architecture.  It is the intent of Nimrod to allow
physical devices as well as networks to be mobile.

Nimrod, as a routing and addressing architecture, does not directly concern
itself with mobility.  That is, Nimrod does not propose a solution for the
mobility problem.  There are two chief reasons for this.  First, mobility is
a non-trivial problem whose implications and requirements are still not well
understood and will perhaps be understood only when a mobile internetwork is
deployed on a large scale.  Second, a number of groups (for instance the
Mobile-IP working group of the IETF) are studying the problem by itself and
it is not our intention to duplicate those efforts.

This attitude towards mobility is consistent with Nimrod's general
philosophy of flexibility, adaptability and incremental change.

While a mobility solution is not part of the ``core'' Nimrod architecture,
Nimrod does require that the solution have certain characteristics.  It is
the purpose of this document to discuss some of these requirements and
evaluate approaches towards meeting them.

We begin by identifying the precise nature of the functionality needed to
accommodate mobile entities (section 2).  Following that, we discuss the
effects of mobility on Nimrod (section 3).  Next, we classify current and
possible approaches to a solution for mobility (section 4) and finally (in
section 5) we describe how mobility can be implemented using the IETF's
Mobile-IP protocol.

This document uses many terms and concepts from the Nimrod Architecture
document [CCS94] and some terms and concepts (in section 5) from the Nimrod
Functionality document [Ste94].  Much of the discussion assumes that you
have read at least the Nimrod Architecture document [CCS94].


2 Mobility :  A Modular Perspective


Nimrod has a basic feature that helps accommodate mobility in a graceful and
natural manner, namely, the separation of the endpoint naming space from the
locator space.  The Nimrod architecture [CCS94] associates an endpoint with
a globally unique endpoint identifier (EID) and an endpoint label (EL). The
location of the endpoint within the Internetwork topology is given by its
locator.  When an endpoint moves, its EID and EL remain the same, but its
locator might change.  Nimrod can route a packet to the endpoint after the
move, provided it is able to obtain its new locator.

Thus, providing a solution to mobility in the context of Nimrod may be
perceived as one of maintaining a dynamic association between the endpoints

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and the locators.  Extending this viewpoint further, one can think of
mobility-capable Nimrod as essentially consisting of two ``modules'' :  the
Nimrod routing module and the dynamic association module (DAM). The DAM is
an abstraction, embodying the functionality pertinent to maintaining the
dynamic association.  This is a valuable paradigm because it facilitates the
comparison of various mobility schemes from a common viewpoint.  Our
discussion will be structured based on the DAM abstraction and will be in
two parts, the themes of which are :


  o What constitutes mobility for the DAM and Nimrod?  Is the realization
    of mobility as a mobility ``module'' that interacts with Nimrod viable?
    What then are the interactions between Nimrod and such a module?  These
    points will be discussed in section 3.

  o What are some of the approaches one can take in engineering the DAM
    functionality?  We classify some approaches and compare them in section
    4.


A word of caution:  the DAM should not be thought of as something equivalent
to the current day Domain Name Service (DNS) - the DAM is a more general
concept than that.  For instance, consider a mobility solution for Nimrod
similar to the scheme described in [Sim94].  Very roughly, this approach is
as follows:  Every endpoint is associated with a ``home'' locator.  If the
endpoint moves, it tells a ``home representative'' about its new locator.
Packets destined for the endpoint sent to the old locator are picked up by
the home representative and sent to the new locator.  In this scheme, the
DAM embodies the functionalities implemented by all of the home
representatives in regard to tracking the mobile hosts.  The point is that
the association maintenance, while required in some form or other, may not
be an explicitly distinct part, but implicit in the way mobility is handled.

Thus, the DAM is merely an abstraction useful to our discussion and should
not be construed as dictating a design.

In summary, we view the Nimrod architecture as carrying a functional
``stub'' for mobility, the details of the stub being deferred for later.
The stub will be elaborated when a solution that meets the requirements of
Nimrod becomes available (for instance from the IETF Mobile-IP research).
We do not, however, preclude the modification of any such solutions to meet
the Nimrod requirements or preclude the development of an independent
solution within Nimrod.


3 Effects of Mobility


One consequence of mobility is the change in the locator of an endpoint.
However, not all instances of mobility result in a locator change (for
instance, there is no locator change if a host moves within a LAN) and a

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change in the locator is not the only possible effect of mobility.  Mobility
might also cause a change in the topology map.  This typically happens when
entire networks move (e.g., an organization relocates, a wireless network in
a train or plane moves between cells, etc.).  If the network is a Nimrod
network, we might have a change in the connectivity of the node representing
the network and hence a change in the map.

In this section, we consider the effects of mobility on the two ``modules''
identified above:  Nimrod, which provides routing to a locator, and a
hypothetical instantiation of the DAM, which provides a dynamic
endpoint-locator association, for use by Nimrod.  We consider four scenarios
based on whether or not the topology and an endpoint's locator changes and
comment on the effect of the scenarios on Nimrod and the DAM.


Scenario 1 .  Neither the locator nor the topology changes.  This is the
    trivial case and affects neither the DAM nor Nimrod.  An example of
    this scenario is when a workstation is moved to a new interface on the
    same local area network(1) or when mobility is handled transparently
    (by lower layers).

Scenario 2 .  The locator changes but the topology remains the same.  This
    is the case when an endpoint moves from one node to another, thereby
    changing its locator.  The DAM is affected in this case, since it has
    to note the new endpoint-locator association and indicate this to
    Nimrod if necessary.  The effect on Nimrod is related to obtaining this
    change from the DAM. For instance, Nimrod may be informed of this
    change or ask for the association if and when it finds out that the
    mobile host cannot be reached.

Scenario 3 .  The locator does not change but the topology changes.  One
    way this could happen is if a network node moves and changes its
    neighbors (topology change) but remains within the same enclosing node.
    The DAM is not affected because the endpoint-locator association has
    not changed.  Nimrod is affected in the sense that the topology map
    would now have to be updated.

Scenario 4 .  Both the locator and the topology change.  If a network node
    moves out of its enclosing node, we have a change both in the map and
    in the locators of the devices in the network.  In this case, both
    Nimrod and the DAM are affected.


In scenarios 3 and 4, it may not be sufficient to simply let Nimrod handle
the topological change using the update mechanisms described in [Ste94].
These mechanisms are likely to be optimized for relatively slow changes.

------------------------------
 1. This is not true  for all LANs, only those  in which all interfaces  are
part of the same Nimrod node.


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Mobile wireless networks (in trains and cars for instance) are likely to
produce more frequent changes in topology.  Therefore, it might be necessary
that topological updates caused by mobility be handled using additional
mechanisms.  For instance, one might send specific updates to appropriate
node representatives, so that packets entering that node can be routed using
the new topology.  We observe that accommodating mobility of networks,
especially the fast moving ones, might require a closer interaction between
Nimrod and the DAM than required for endpoint mobility.  It is beyond the
scope of this document to specify the nature of this interaction; however,
we note that a solution to mobility should handle the case when a network as
a whole moves.  Current trends [WJ92] indicate that such situations are
likely to be common in future when wireless networks will be present in
trains, airplanes, cars, ships, etc.

In summary, if we discount the movement of networks, i.e., assume no
topology changes, it appears that the mobility solution can be kept fairly
independent of Nimrod and in fact can be accommodated by an implementation
of the DAM. However, to accommodate network mobility (scenarios 3 and 4), it
might be necessary for Nimrod routing/routers to get involved with mobility.

Beyond the constraints imposed by the interaction with Nimrod, it is
desirable that the mobility solution have some general features.  By
general, we mean that these are not Nimrod specific.  However, their
paramount importance in future applications makes them worth mentioning in
this document.  The desirable features are :


  o Support of both off-line and on-line mobility.  Off-line mobility (or
    portability) refers to the situation in which a session is torn down
    during the move, while on-line mobility refers to the situation in
    which the session stays up during the move.  While currently much of
    the mobility is off-line, trends indicate that a large part of mobility
    in the future is likely to be on-line.  A solution that only supports
    off-line mobility would probably have limited applications in future.

  o Scalability.  One of the primary goals of Nimrod is scalability, and it
    would be contrary to our design goals if the mobility solution does not
    scale.  The Internet is rapidly growing and with the advent of Personal
    Communication Systems (PCS) [WJ92], the number and rapidity of mobile
    components in the Internet is also likely to increase.  Thus, there are
    three directions in which scalability is important :  size of the
    network, number of mobile entities and the frequency of movement of the
    mobile entities.

    Note that for any given system with minimum response time (to a move)
    of o seconds, if the mobile entity changes attachment points faster
    than 1=o changes per second, the system will fail to track the entity.
    Augmenting traditional location tracking mechanisms with special
    techniques such as predictive routing might be necessary in this case.
    Hooks in the mobility solution for such augmentation is a desirable
    feature.

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  o Security.  It is likely that in future, there will be increased demand
    for secure communications.  Apart from the non-mobility specific
    security mechanisms, the solution should address the following :


     --  Authentication.  The information sent by a mobile host about its
         location should be authenticated to prevent impersonation.
         Additionally, there should be mechanisms to decide if a mobile
         user who wishes to join a network has the privileges to do so or
         not.

     --  Denial of service.  The schemes employed for handling mobility in
         general could be a drain on the resources if not controlled
         carefully.  Specifically, the resource intensive portions of the
         protocol should be guarded so that inappropriate use of them does
         not cause excessive load on the network.


4 Approaches


As discussed in section 2, the problem of mobility in the context of Nimrod
may be viewed as one of maintaining a dynamic association (DAM) and
communicating this association and changes therein to Nimrod.  Approaches to
mobility may be classified based on how different aspects of the DAM are
addressed.

Our classification identifies two aspects to the mobility solution :


 1. How and where to maintain the dynamic association between endpoints and
    locators?  This may be perceived as a problem of database maintenance.
    The database may be maintained in a centralized fashion, wherein a
    single entity maintains the association and updates are sent to it by
    the mobile host or in a distributed fashion, wherein there are a number
    of entities that store the associations.

    A (distributed) database that stores the endpoint-locator mapping is
    required by Nimrod even in the absence of mobility.  If this service
    can accommodate dynamic update and retrieval requests at the rate
    produced by mobility, this service is a candidate for a solution.
    However, we note that the availability of such a system should not be a
    requirement for the mobility solution.

 2. Where to do the remapping between the endpoint and locator, in case of
    a change in association?  By remapping, we mean associate a new locator
    with the endpoint.  Some candidates are :  the source, the ``home''
    location of the host that has moved and any router (say, between the
    source and the destination) in the network.



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                         (Re-mapping location)
                                   _
                                   v
          -----------------------------------------
          _            _Source _  Home  _ Routers _
          -----------------------------------------
 (Assoc.  _Centralized _  A1   _   X    _    X    _
  maint)-> ----------------------------------------
          _Distributed _  X    _   A2   _   A3    _
          ----------------------------------------


     Table 1 : Classification of approaches based on how the association
               is maintained and where the remapping is done.



Many of the existing approaches and perhaps some new approaches to the
problem of mobile internetworking may be seen to be instantiations of a
combination of a dynamic association method and a remapping method.  We
consider some combinations as illustrated in Table 1.  We discuss three
combinations (marked A1 - A3 in the table) and examine their advantages and
disadvantages in the context of our requirements.  The other combinations
(marked X in the table) are possible, but do not represent a substantially
different class of solutions from the ones discussed and hence are not
considered here.

Note that this is but one classsification of mobility schemes and that the
remapping and endpoint-locator maintenance strategies mentioned in the table
are not exhaustive.  The main intention is to help understand better the
kinds of approaches that would be most suitable for Nimrod.

In the following, we use the term source to refer to the endpoint that is
attempting to communicate with or sending packets to a mobile endpoint.  The
source could be static or mobile.  We use the term mobile destination to
refer to the endpoint that is the intended destination of the source's
packets.



A1 .  In this approach, all endpoint-locator mappings are maintained at a
    centralized location.  The source queries the database to get the
    locator of the mobile destination.  Alternatively, the database can
    send updates to the source when the mobile destination moves.

    The main advantage of this scheme is its simplicity.  Also, no
    modification to routers is required, and the route from the source to a
    mobile destination is direct.

    The main disadvantage of this scheme is vulnerability - if the

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    centralized location goes down, all information is lost.  While this
    scheme may be sufficient for small networks with low mobility, it does
    not scale adequately to be a long term solution for Nimrod.

A2 .  This approach uses distributed association maintenance with remapping
    done at the home.  This is the approach that is being used by the
    Mobile-IP working group of the IETF for the draft proposal and by the
    Cellular Digital Packet Data (CDPD) consortium.  In this approach,
    every mobile endpoint is associated with a ``home'' and a ``home
    representative'' keeps track of the location of every mobile endpoint
    associated with it.  A protocol between a mobile endpoint and the home
    representative is used to keep the information up-to-date.  The source
    sends the packet using the home locator of the mobile destination, and
    the home representative forwards the packet to the mobile destination.

    The advantage of this scheme is that it is fairly simple and does not
    involve either the source or the routers in the network.  Furthermore,
    the mobile destination can keep its location secret (known only to the
    home representative) - this is likely to be a desirable feature for
    mobile hosts in some applications.  Finally, most of the control
    information is confined to the node containing the home representative
    and the mobile host and this is a plus for scalability.

    The main disadvantage is a problem often referred to as triangular
    routing.  That is, the packets have to go from the source to the home
    representative first before going to the mobile destination.  This is
    especially inefficient if, for instance, both the source and mobile
    destination are in, say, England and the home representative is in,
    say, Australia.  Also, there is still some vulnerability, since if the
    home representative becomes unreachable, the location of all of the
    mobile hosts it tracks is lost and communication from most sources to
    the mobile host is cut-off.  It is also not clear how well this scheme
    will scale to mobile internetworks of the future.

    Nevertheless, we feel that this approach or a modification thereof
    might be a viable first-cut mobility solution for Nimrod.

A3 .  In each of the previous cases, the routers in the network were not
    involved in tracking the location of the mobile host.  In this
    approach, state is maintained in the routers.  An example is the
    approach proposed in [TYT91] wherein the packets sent by a mobile host
    are snooped and state is created.  The packets contain the mobile
    host's home location and its new location.  This mapping is maintained
    at some routers in the network.  When a packet intended for the mobile
    host addressed to its home location enters such a router, a translation
    is made and the packet is redirected to the new location.

    An alternate mechanism is to maintain the mapping in all of the border
    routers (e.g., border routers) in the node within which the movement
    took place.  A packet from outside the node intended for a destination
    within the node would typically enter the node through one of the

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    border routers.  Using the mapping, the border router could figure out
    the most recent locator of the mobile destination and send the packet
    directly to that locator.  If most of the movements are within low
    level nodes, this would scale to large numbers of movements.
    Furthermore, the packet takes an optimal path (or as optimal as one can
    get with a hierarchical network) to the new location within the time it
    takes for the node representative to get the new information, which is
    typically quite small for low-level nodes.

    The main disadvantage of this scheme is that routers have to be
    involved.  However, future requirements in regard to scalability and
    response time might necessitate such an approach.  Furthermore, this
    solution has closer ties with Nimrod routing and is better suited to
    handling scenarios 3 and 4 where the topology changes as a result of
    mobility.


All of these approaches seem potentially capable of handling scenarios 1 and
2 of the previous section.  Scenarios 3 and 4 are best handled by an
approach similar to A3.  However, approaches like A3 are more complex and
involve more Nimrod entities (e.g., routers) than may be desirable.

We have tried to bring out the various issues governing mobility in Nimrod.
In the final analysis, the tradeoffs between the various options will have
to be examined vis-a-vis our particular requirements (for instance, the need
to support network mobility) in adopting a solution.  It is likely that
general requirements such as scalability and security will also influence
the direction of the approach to mobility in Nimrod.


5 A Solution using IETF Mobile-IP


The Mobile-IP Working Group of the IETF is in the process of standardizing a
protocol that allows an IPv4 capable network to support mobile hosts.  In
this section, we outline how mobility can be implemented within Nimrod using
the same mechanism and indeed, the same protocol headers defined in [Sim94].
Not all functionality described in [Sim94] are covered - only those that
form the ``core'' of mobility support.

In order to follow this section, the reader is required to have some
familiarity with the IETF Mobile-IP protocol (see [Sim94]).


5.1 Overview


The general scheme employed by the IETF Mobile-IP protocol is as follows.  A
Mobile Host (MH) has a predefined Home Agent (HA) that is responsible for
the MH's whereabouts.  Typically, the MH spends most of its time in the
network containing the HA. Let us assume that the MH wanders to a new

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network.  The MH then contacts a Foreign Agent (FA) at the new network that
will act on its behalf and sends a registration request to the HA via the
FA. This serves the purpose of informing the HA of the MH's new whereabouts
and also is a means of verification of the MH's authenticity.  It also
contains the address of the FA as the new Care-of-Address.  A correspondent
host (CH) wishing to send a message to the MH uses the MH's Home IP address.
This message is captured by the HA and tunnelled using encapsulation to the
FA whereupon the FA decapsulates and sends the original message to the MH.

If the MH can get itself a new transient address then there is no need for a
Foreign Agent.  The transient address will be sent as the Care-of-Address.
The packets will be tunnelled directly to this address by the Home Agent.
Note, however, that some networks may require that a mobile host go through
a Foreign Agent.

A fundamental difference between IP and Nimrod is that in the latter an
endpoint has both a (topologically sensitive) locator and a (topologically
insensitive) endpoint-id (EID). In IP, the IP address serves as both the EID
and the locator.  Thus, it should be possible to use the Mobile-IP protocol
for providing mobility support in Nimrod by simply using the EID of the MH
wherever its Home IP Address was being used and by appropriately using the
EID and locator of the FA and HA in place of their IP addresses.  We give
below the details of the protocol fields and the actions taken by the MH, FA
and HA to show that this is possible and that it is quite simple.


5.2 Protocol Details


There are two kinds of protocol headers relevant to our discussion - the
Mobile-IP Protocol (MIPP) headers and the headers for data packets
transported by Nimrod.  It is our intent that Nimrod use, as much as
possible, the next generation IP (IPng) header.  However, as of date, no
IPng protocol has been standardized and therefore, for the purposes of the
following description, we use the concept of a Nimrod Protocol (NP) packet
header.  The NP header contains as a subset fields that would eventually be
present in the IPng header.

In the scheme given below, the MIPP header is enclosed within the NP packet
(i.e., MIPP operates over NP). The details of the fields constituting the NP
header are beyond the scope of this document.  However, without venturing
into bit lengths, etc., we identify below a few fields that are relevant to
our discussion:


  o Source EID (S-EID) : The endpoint ID of the source entity originating
    the packet.

  o Destination EID (D-EID) : The endpoint ID of the destination.

  o Source locator (S-LOC) : Locator of the entity originating the packet.

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  o Destination locator (D-LOC) : Locator of the destination.


The MIPP header fields are described in [Sim94].

In what follows, we describe the values that must be assigned to the
relevant NP and MIPP fields in order for Nimrod to work with Mobile-IP.
There are three phases we must consider :  agent discovery, registration and
forwarding [Sim94].  A pictorial summary of the control and data packets is
given in Figure 1.


Agent Discovery In this phase, the MH discovers the foreign agent, if any,
that will act on its behalf.  In MIPP, this is done using the ICMP Router
Discovery messages.

When an MH attaches to a Nimrod network (node), foreign agent discovery is
done as follows.  We assume that a link-level connection is established
between the MH and a node N belonging to the network.  For instance, this
node could be a wireless equipped base station that establishes a signalling
channel for communication with the MH.

If the MH is itself a node then N and the MH execute an arc formation
procedure between themselves as described in [Ste94].  This results in a
locator being assigned to the MH and to the arcs between N and MH.

If the MH is not a node but only an endpoint, then MH initiates locator
acquisition procedure as described in [Ste94].  This results in a locator
being assigned to the MH.

The MH then sends a Foreign Agent Request message to N. This message
contains, amongst other information, the EID and locator of the MH. If N is
not itself the foreign agent, then we assume that it knows of and has the
ability to reach a foreign agent.

The foreign agent (FA) notes the EID of the MH in its Visitor List and sends
a Foreign Agent Reply to the MH. This contains the EID and the locator of
the FA and will be used as the ``Care-of-Address'' (COA) of the MH for a
prespecified period.


Registration In the registration phase, infomation is exchanged between the
MH and the Home Agent (HA). The HA could, for instance, be the endpoint
representative of the endpoint in its home location.  The registration
procedure is used to create a mobility binding which the HA uses to forward
data packets intended for the MH. Another purpose of registration is to
verify the authenticity of the MH.

There are four parts to the registration.  We describe the values assigned
to the relevant fields.  Recall that there are two headers we must create -
the Nimrod Protocol (NP) header and the Mobile-IP Protocol (MIPP) header.

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The NP fields are as described in section 5.2 and the MIPP fields are as
in [Sim94].  The fields mh-eid(mh-loc), fa-eid(fa-loc), ha-eid(ha-loc) are
used to refer to the EID (locator) of the mobile host, foreign agent and
home agent respectively.


 1. The MH sends a Registration Request to the prospective Foreign Agent to
    begin the registration process.


       o NP fields :  S-EID = mh-eid; D-EID = fa-eid; S-LOC = mh-loc ;
         D-LOC = mh-loc.

       o MIPP fields :  Home Agent = ha-eid; Home Address = mh-eid;
         Care-of-Address = fa-eid.


    Note that the mh-loc is known to the MH by virtue of the locator
    acquisition (see paragraph on ``Agent Discovery'') and that the fa-eid
    is known to the MH from the Foreign Agent Reply.  The FA caches the
    mh-eid for future reference.

 2. The Foreign Agent relays the request by sending a Registration Request
    to the Home Agent, to ask the Home Agent to provide the requested
    service.


       o NP fields :  S-EID = fa-eid; D-EID = ha-eid; S-LOC = fa-loc; D-LOC
         = ha-loc.

       o MIPP fields :  Same as in (copied from) (1) above.


    The HA caches the (Home Address, Care-of-Address) as a mobility
    binding.  Optionally, for efficiency, it may also cache fa-loc.

 3. The Home Agent sends a Registration Reply to the Foreign Agent to grant
    or deny service.


       o NP fields :  S-EID = ha-eid; D-EID = fa-eid; S-LOC = ha-loc; D-LOC
         = fa-loc.

       o MIPP fields :  Home Address = mh-eid; code = as in [Sim94].


    The S-EID and D-EID fields are taken from the Request and swapped, as
    are the S-LOC and D-LOC fields.  The Home Address in the MIPP is the
    same as the Home Address in the Request.  The code indicates whether or
    not permission was granted by the Home Agent.


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    +--------+
    _  CH    _
    +--------+
        V
        V
    #--------------#
    _mh-eid _ data _ = P(orig)
    #--------------#
        V
    +--------+  *----------------*   +--------+ *--------------* +------+
    _        _  _fa-eid _ mh-eid _   _        _ _ ha-eid_mh-eid_ _      _
    _        _  *----------------*   _        _ *--------------* _      _
    _  HA    _------<-REG REQ-<------_  FA    _----<-REG REQ-<---_ MH   _
    _        _   2                   _        _  1               _      _
    _ mh-eid _                    3  _ mh-eid _                4 _      _
    _   _    _------>-REG REPL->-----_   _    _---->-REG REPL->--_      _
    _   v    _  *----------------*   _   v    _ *--------------* _      _
    _ fa-eid _  _mh-eid _ yes/no _   _ mh-loc _ _mh-eid_yes/no _ _      _
    _        _  *----------------*   _        _ *--------------* _      _
    _        _  #------------------# _        _ #---------#      _      _
    _        _>>_       #--------# _>_        _>_ P (orig)_>>>>> _      _
    +--------+5 _fa-eid _ P(orig)_ _ +--------+ #---------#  6   +------+
                _       #--------# _
                #------------------#

     Figure 1 : The control and data packets for mobility handling using
                the Mobile-IP protocol. The packets bordered as # denote
                data packets and those bordered * denote control packets.
                Only the crucial information conveyed in each message is
                shown (i.e., locators and EIDs in packet headers are not
                shown. The associations maintained at HA and FA are shown.




 4. The Foreign Agent sends a copy of the Registration Reply to the MH to
    inform it of the disposition of its request.


       o NP fields :  S-EID = fa-eid; D-EID = mh-eid; S-LOC = fa-loc; D-LOC
         = mh-loc.

       o MIPP fields :  Same as (copied from) (3) above.


    At this point the MH is registered with the HA (provided the
    registration request is approved by the HA) and packets can be
    forwarded to the MH.



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Forwarding Data We describe the manner in which a packet from the
correspondent host (CH) intended for the MH is encapsulated and forwarded by
the HA.


  o At HA : Suppose that a packet P intended for MH arrives at HA. For
    instance, P first comes to the router for the local network and the
    router finds that MH is unreachable.  The router then forwards P to the
    HA for possible redirection.

    The HA extracts the destination EID from the NP header for P. If no
    match is found in its mobility binding, then the MH is deemed as
    unreachable.  If a match is found, the corresponding fa-eid is
    extracted.  A new header is prepended to P. For this header, S-EID =
    ha-eid, D-EID = fa-eid, S-LOC = ha-loc and D-LOC = fa-loc.  The fa-loc
    may be obtained from the Association Database [CCS94].  Alternatively,
    if it was cached in (2) above, it could be obtained from the cache.

  o At FA : By a special bit(s) in the Nimrod Protocol packet header(TBD),
    the FA knows that the packet is an encapsulated one.  It removes the
    wrapping and looks at the EID in P. If that EID is found in the Visitor
    List then the FA knows the locator of the MH and can deliver the packet
    to the MH. Otherwise, the packet is discarded.


Other Issues


We have not addressed a number of issues such as deregistration,
authentication, etc.  The mobility specific portion of authentication can be
adapted from the specification in [Sim94]; deregistration can be done in a
manner similar to registration.

The protocol in [Sim94] describes a registration scheme without the
involvement of the Foreign Agent.  This is done when the MH obtains a
transient IP address using some link-level protocol (e.g.  PPP). A similar
scheme can be given in the context of Nimrod.  In this case, the MH obtains
its locator (typically inherited from the node to which it attaches) and
sends this locator as its Care-of-Address in the Registration Request.  The
HA, while forwarding, uses this as the locator in the outer NP header and
thus the encapsulated packet is delivered directly to the MH which then
decapsulates it.  No Foreign Agent Discovery is needed.  Apart from this,
the fields used are as described for the scheme with the FA.

We note however that many networks may require that the registration be
through a Foreign Agent, for purposes of security, billing etc.






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6 Security Considerations


The registration protocol between a mobile host and the network (for
instance, in the mobile-ip protocol, the MH and the HA) contains security
mechanisms to validate access, prevent impersonation etc.

This document is not a protocol specification and therefore does not contain
a description of security mechanisms for Nimrod.


7 Summary


  o Nimrod permits physical devices to be mobile, but does not specify a
    particular solution for routing in the face of mobility.

  o The fact that the endpoint naming (EID) space and the locator space are
    separated in Nimrod helps in accommodating mobility in a graceful and
    natural manner.  Mobility may be percieved, essentially, as dynamism in
    the endpoint - locator association.

  o Nimrod allows two kinds of mobility:


     --  Endpoint mobility.  For example, when a host in a network moves.
         This might cause a change in the locator associated with the host,
         but does not cause a change in the topology map for Nimrod.

     --  Network mobility.  For example, when a router or an entire network
         moves.  This might cause a change in the topology in addition to
         the locator.


  o Endpoint mobility may be handled by maintaining a dynamic association
    between endpoints and locators.  However, network mobility requires
    addressing the topology change problem as well.

  o Apart from the ability to handle network mobility, it is desirable that
    the mobility solution be scalable to large networks and large numbers
    of mobile devices and provide security mechanisms.

  o There are a number of existing and emerging solutions to mobility.  In
    particular, adaptation of solutions developed by the IETF is a first
    cut possibility for Nimrod.  As the description given in section 5
    shows, it is relatively easy to implement the scheme being designed by
    the Mobile-IP working group in the context of Nimrod.





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8 Acknowledgements


We thank Isidro Castineyra (BBN), Charles Lynn (BBN), Martha Steenstrup
(BBN) and other members of the Nimrod Working Group for their comments and
suggestions on this draft.


9 Author's Address


Ram Ramanathan
BBN Systems and Technologies
10 Moulton Street
Cambridge, MA 02138
Phone :  (617) 873-2736
Email :  ramanath@bbn.com


References


[CCS94] I. Castineyra, J. N. Chiappa, and M. Steenstrup. The nimrod
        architecture. Working Draft, July 1994.
        (draft-castineyra-routing-arch-00.ps (.txt)).

[Sim94] W. A. Simpson. Ip mobility support. Working Draft, May 1994.
        (draft-ietf-mobileip-protocol-00.txt).

[Ste94] M. Steenstrup. A perspective on nimrod functionality. Working
        Draft, July 1994. (draft-steenstrup-fun-perspective-00.ps (.txt)).

[TYT91] F. Teraoka, Y. Yokote, and M. Tokoro. A network architecture
        providing host migration transparency. In Proceedings of ACM
        SIGCOMM, 1991.

[WJ92]  K. A. Wimmer and J. B. Jones. Global development of pcs. IEEE
        Communications Magazine, pages 22--27, Jun 1992.














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