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INTERNET-DRAFT                                                A. O'Neill
Internet Engineering Task Force                          BT Laboratories
draft-oneill-ema-00.txt                                   S. Corson
                                                  University of Maryland
                                                         Ansible Systems
                                                         22 October 1999

                       Edge Mobility Architecture

                          Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   This draft concurs with the authors of Cellular IP and HAWAII
   regarding the need for domain-based routing support in handling edge
   mobility.  It suggests that a general routing solution might be
   advantageous and presents the loose framework of a possible routing
   architecture. It advocates the creation of a specific IETF working
   group to address an overall architecture for edge mobility routing,
   specific extensions to existing routing protocols to accomplish that
   architecture, and extensions to existing Internet technologies to
   support this architecture.

1. Introduction

   Internet routing protocols have been traditionally designed from an
   assumption that the location of an IP interface in the topology is
   static. In addition, they assume that address allocation within the

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   topology will aim to provide multiple levels of IP address
   aggregation such that routing protocols can deal with address
   prefixes, rather than large numbers of host routes. Within this
   framework, traditional intra-domain protocols, such as OSPF, need
   only react to infrequent changes to the network due to link or router
   failures or permanent modifications to the addressing scheme or the

   Mobile Ad hoc NETwork (MANET) routing protocols have been developed
   to address what could be considered to be an extreme scenario,
   whereby the mobile nodes have permanent IP addresses which can
   rapidly roam through an ad hoc topology, leading to the need for
   alternative routing technology and the general loss of aggregation

   A third family of routing protocols is now under investigation for
   the case in which the core topology is essentially fixed but the end
   systems are mobile. This is the classical edge mobility scenario that
   is today supported by cellular networks, primarily as part of the
   cellular technology elements (GSM, GPRS etc). Migrating this routing
   function to the layer 3 IP routing protocol, to release all the end-
   to-end internetworking benefits which has aided the deployment of the
   Internet, would tend to suggest a fusion of the MANET and traditional
   routing protocol architectures. The primary aim is to move the IP
   interface location in the routing topology as the mobile changes base
   stations so that active IP sessions are maintained. This is clearly
   only one of the possible models and this draft does not make any
   judgments as to the pro's and con's of this particular mobility
   model, but is simply using it as a precursor for the discussion of
   the related hand-over architecture.

   These edge mobility networks can be considered to have a single IP
   routing protocol which runs between routers in the domain, with some
   of those routers being the edge base stations equipped with a
   (potential) diversity of radio technologies such as CDMA, TDMA and
   Radio LANs etc. The radio layers are assumed to provide the well
   known layer 2 hand-over models and other capabilities including
   break-before-make, make-before-break, power measurement, mobile
   assisted hand-over, paging and security features. To facilitate
   internetworking, inter-base station coordination is assumed to use
   IP-based communication using messages which are abstractions of the
   messages which are today carried in cellular technology-specific
   messages, often via central processing elements.

   This brief draft proposes a general approach to the support of this
   edge mobility function, with the aim of being generic enough to
   support a range of different routing protocols, as well as enabling
   hand-over between diverse types of cellular technology through

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   capability exchange between radio-equipped base stations. It does not
   deal with the details of these mechanisms as they are specific to the
   type of routing protocol selected. This draft does also not discuss
   the effects of the architecture on DNS, DHCP and infrastructure
   services, nor does it contribute to the debate on the appropriate
   layer and model for mobile terminal paging.

2. Mobile Routing Architecture

   The architecture proposed assumes that modifications to either MANET
   or traditional routing protocols are possible which will enable these
   protocols to comply with this architecture and hence facilitate a
   message set and control model which has a degree of protocol
   independence. The details of these modifications are left to later
   drafts, both from the authors of this draft, and likely contributions
   from other researchers in this area. Clearly, the actual detailed
   mechanisms, message content/timing and performance are going to be
   dependent on the type of intra-domain routing protocol that forms the
   basis for the mobility extensions.

   The architecture has six main components, the first being the use of
   mobile IP across provider boundaries to facilitate the temporary
   movement of an IP address (on a mobile terminal interface) away from
   its home domain whilst maintaining active sessions. The mobile IP
   tunnels are initiated by a base station (HA) at the inter-domain
   boundary to the base station (FA) in the foreign domain, and are
   extended to further base stations in the foreign domain using normal
   Mobile IP foreign agent mechanisms. This bounds subsequent
   discussions to intra-domain routing issues. The other five components

      a) the provision of a modified intra-domain routing protocol which
      provides prefix-based routing within a domain, with each prefix
      representing a block of IP addresses allocated to each base
      station in the domain, as well as host routes to support mobile
      terminal migration away from the allocating (IP address) base

      b) the provision and use of a virtual link for routing exchange
      and messaging between adjacent base stations to exchange
      capabilities, and to effectively and locally manage the hand-over
      of the responsibility for, and routing of, the mobile terminal and
      it's associated IP address,

      c) the ability to inject a host route for the mobile,

      d) the ability to poison the existing route to the mobile via the
      old base station

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      e) the provision and use of a temporary tunnel to redirect packets
      in flight between the old base station and the new base station
      whilst routing converges, and

      f) a method to return the allocated IP address to the allocating
      base station on mobile session termination at a different base
      station in the same domain.

   The reasons for each of these components will be explained in the
   following sections which will give examples for CDMA (make-before-
   break) and TDMA (break-before-make) handover.

2.1 Mobile Session Start-Up

   The mobile connects to the nearest / best base station and is brought
   into the IP routing domain by requesting and being allocated an IP
   address out of the block of addresses managed by that base station.
   The base station will be advertising the IP address prefix associated
   with that address block into the intra-domain routing protocol such
   that 'at home' mobiles have a proactively and permanently advertised
   route, and are immediately reachable to all hosts in the internet.
   As the mobile moves base stations, this IP address moves with them so
   that higher-layer sessions are unaffected. This is accomplished by
   modifying the intra-domain routing using hosts routes to overrule
   (longest match) or overwrite the underlying proactive base station
   prefix routing. Placing an appropriate set of messages over IP
   ensures that a wide range of radio technology specific hand-over
   models can be accommodated within a single IP model to allow for
   internetworking of IP over those diverse technologies.

2.2 Break before Make

   TDMA technology such as GSM only allows the mobile to be connected to
   a single base station at a time, with a data path dead-time incurred
   during hand-over. The 'inject' route feature is therefore invoked
   when the old and new base stations have been identified, and have
   agreed to the hand-over by creating the dynamic redirect tunnel
   between themselves. Note that for efficiency purposes a single
   redirect tunnel could be pre-configured between adjacent base
   stations to support all inter-base station hand-overs, and dynamic
   mobile-specific redirect state temporarily installed against that
   aggregate tunnel. Either way, the base stations collaborate to
   locally inject the new route into the routing domain. When the mobile
   disconnects at the radio layer from the old base station, the new
   base station, through the inter-base station virtual link or tunnel,
   is immediately known to be the next best hop, and packets will be
   immediately redirected down the tunnel to the new base station. Some
   time later the mobile will attach to the new base station and will

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   immediately receive in-flight and locally cached packets. The base
   stations then again collaborate to poison the old route that points
   to the old base station, and to propagate the new route from the new
   base station. When routing has converged, the old base station will
   detect this and eliminate the redirect state associated with the
   temporary tunnel. Packets to the mobile will then head towards the
   new base station route. The reception of the mobile is then confirmed
   through acknowledgement messages to the old base station which is
   used to confirm hand-over of responsibility for the mobile and it's
   IP address in the system.

2.3 Make Before Break

   CDMA technology enables a mobile terminal to be connected to two base
   stations at the same time and to undertake measurements to establish
   the preferred channel and hand-over time. In this system it is
   therefore important to support concurrent routing paths from all
   potential senders to the mobile via the two concurrent base stations.
   This requires the inject route feature from the base stations to be
   invoked before the mobile leaves the old station, and for the poison
   route feature to only be invoked when the hand-over to the new base
   station is triggered, rather than simply on connection to the new
   base station. Finally, it is necessary to ensure that some data is
   simultaneously received over the concurrent paths so that the mobile
   is able to make comparative path quality measurements.

2.4 Hybrid Model

   When a hybrid node is able to support both TDMA and CDMA (or other
   combinations of technology) then a consistent set of base station and
   mobile IP messages makes hand-over of the concurrent sessions between
   TDMA and CDMA base stations possible. This is achieved by the base
   stations understanding each others capabilities, and holding up and
   synchronising the inject and poison stages as appropriate.

2.5 Mobile Switch Off

   It is clear that the migration of IP addresses away from the
   allocating base station can lead to address exhaustion and a gradual
   degradation over time of the usefulness of the proactively advertised
   base station address block prefix. It is therefore critical that at
   the moment that the mobile finishes active sessions, at a distant
   base station, that the IP address is returned to the home base
   station. This can be modelled as a virtual mobile moving from the
   distant base station back to the home base station and then locally
   returning the IP address. This can be accomplished using similar
   mechanisms which are used to support real inter-base station hand-
   overs, with the base stations acting as  proxies for the virtual

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   mobile. Their aim is to co-ordinate the removal of all host specific
   routing entries in the domain as a result of previous mobility away
   from the home base station.

3. Comparison to Alternatives

3.1 Mobile IP

   Mobile IP [MobileIP] is a well known technique for supporting edge
   mobility through the use of stateful intelligence and the permanent
   use of tunnels. Mobile IP is used in our proposal as the only
   credible means to support hand-over between Autonomous Systems whilst
   trying to preserve the current IP interface address and dependent IP
   sessions. Mobile IP effectively takes the position that IP routing
   should not inherently try to support IP interface movement due to the
   implications on the scaling of the intra-domain routing. It is
   therefore deemed better to add functionality to hide the interface
   movement from the routing protocol(s). The authors of this draft
   fully concur with this position for traditional routing protocols
   such as OSPF, where the consequence of mobility in any reasonable
   domain is clearly disastrous for routing state and message overhead.
   However, we believe other routing approaches can achieve sufficient
   scaling to be commercially useful, and the case for Mobile IP against
   a routing solution becomes a more detailed comparison of interactions
   with other IP / cellular protocol features, system reliability,
   system overhead, time to market and ultimately cost etc. We believe
   that with suitable routing technology, the Mobile IP solution will in
   many cases be inappropriate, and we would encourage others to work on
   such technology, in competition to our detailed proposals that will
   be published when finished.

3.2 Cellular IP

   Cellular IP [CellularIP] is an existing proposal which to some extent
   fits aspects of this architecture, in particular in that it uses
   Mobile IP inter-domain and advocates use of a constant in-session
   address. It provides for handover-based redirection and soft state-
   based maintenance of host-specific routing and paging entries.  These
   entries point to a central domain router, and the redirections modify
   a set of default routes collectively forming a tree.


   HAWAII [HAWAII] is another proposal similar to Cellular IP.  It also
   advocates the use of a constant in-session address and Mobile IP
   inter-domain.  It also provides for handover-based redirection of
   host-specific routing paths rooted at a central core router, although
   its handover and paging mechanisms are more complex than those of

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   Cellular IP.

   Cellular IP and HAWAII differ from the proposed architecture in that
   here host routes modify a traditional, prefix-routed mesh topology
   and form route sets other than trees leading to reduced
   configuration, greater resilience, shorter data path lengths and
   topological design freedom.  In addition, these approaches appear not
   to make provision for the temporary tunnelling of packets in flight
   whilst redirect routing converges, which can lead to data packet loss
   depending on topology, convergence time, link speeds, control packet
   loss recovery times and traffic load.

4. IETF Working Group Activity

   British Telecommunications (BT) would welcome the creation of a
   specific IETF working group to address an overall architecture for
   edge mobility routing, specific extensions to existing routing
   protocols to accomplish that architecture, and extensions to existing
   Internet technology (DNS, DHCP, AAA etc) to support this
   architecture. We would be happy for Cellular IP and HAWAII to be
   covered by such a working group assuming the respective authors

5. Security

   This draft is too general to explicitly address security issues.
   Certainly host and router authentication must be handled in the
   architecture, and various mechanisms already exist for these
   purposes.  For example, host authentication during dynamic address
   assignment is possible via [RADIUS].  Router authentication could be
   handled via shared key exchange as is common in many routing
   algorithms.  Additionally, mechanisms would be required for
   authenticating Mobile IP messages and the discussion of possible
   approaches in [HAWAII] applies here as well.  Additionally, source
   address checking would be necessary.

6. References

   [CellularIP] A. Valko, ``Cellular IP: A New Approach to Internet Host
   Mobility," ACM Computer Communication Review, Vol. 29, No. 1, January

   [HAWAII] R. Ramjee, T. La Porta, S. Thuel, K. Varadhan and L.
   Salgarelli, ``IP micro-mobility support using HAWAII," Internet
   Draft, Work in Progress, June 1999.

   [MobileIP] C.E. Perkins, ``IP Mobility Support," Internet RFC 2002,
   Oct 1996.

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   [RADIUS] C. Rigney, A. Rubens, W. Simpson and S. Willens, ``Remote
   Authentication Dial in User Service (RADIUS),'' Request for Comments
   2138, Apr 1997.

Appendix A -- IPR Statement

   British Telecommunications plc has patent applications relevant to
   the architecture mentioned in this draft and to modifications of
   routing protocols. The modifications seek to achieve the scaling and
   performance objectives required for commercial cellular IP systems.

   British Telecommunications plc is currently considering giving an
   undertaking to the IETF to grant licences under the patents resulting
   from the patent applications on fair and reasonable terms so that
   vendors can develop systems based on IETF specifications for
   commercial deployment in a timely and cost-effective manner.

   British Telecommunications plc would also encourage the IETF to seek
   similar undertakings from others re licensing of their patents which
   could otherwise hamper the development and deployment of the IETF
   specifications related to this technology.

Author's Addresses

   Alan O'Neill
   BT Laboratories
   (+44) 1473-646459

   M. Scott Corson
   University of Maryland
   Ansible Systems
   (+1) 301-405-6630

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