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- Drafts. 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 http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract 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 O'Neill, Corson Expires 22 April 2000 [Page 1]
INTERNET-DRAFT Edge Mobility Architecture 22 October 1999 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 topology. 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 opportunities. 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 O'Neill, Corson Expires 22 April 2000 [Page 2]
INTERNET-DRAFT Edge Mobility Architecture 22 October 1999 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 are: 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 station, 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 O'Neill, Corson Expires 22 April 2000 [Page 3]
INTERNET-DRAFT Edge Mobility Architecture 22 October 1999 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 O'Neill, Corson Expires 22 April 2000 [Page 4]
INTERNET-DRAFT Edge Mobility Architecture 22 October 1999 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 O'Neill, Corson Expires 22 April 2000 [Page 5]
INTERNET-DRAFT Edge Mobility Architecture 22 October 1999 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. 3.3 HAWAII 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 O'Neill, Corson Expires 22 April 2000 [Page 6]
INTERNET-DRAFT Edge Mobility Architecture 22 October 1999 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 concur. 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 1999. [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. O'Neill, Corson Expires 22 April 2000 [Page 7]
INTERNET-DRAFT Edge Mobility Architecture 22 October 1999 [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 alan.w.oneill@bt.com M. Scott Corson University of Maryland Ansible Systems (+1) 301-405-6630 corson@isr.umd.edu O'Neill, Corson Expires 22 April 2000 [Page 8]