Internet Engineering Task Force J. Manner Internet-Draft M. Kojo Expires: July, 2001 University of Helsinki T. Suihko VTT Information Technology P. Eardley D. Wisely BT R. Hancock Siemens/Roke Manor Research N. Georganopoulos King's College London January 12, 2001 Mobility Related Terminology 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. This Internet-Draft will expire in July, 2001. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved. Abstract There is a need for common definitions of terminology in the work to be done around IP mobility. This memo defines terms for mobility related terminology. It is intended as a living document for use by the Seamoby working group, and especially for use in Seamoby drafts and in WG discussions. Manner et al Expires July 2001 [Page 1] Internet-Draft Mobility Related Terminology January 2001 Table of Contents 1 Introduction ................................................. 2 2 Definitions .................................................. 3 2.1 Network Components ......................................... 3 2.2 Handover Terminology ....................................... 4 2.2.1 Scope of Handover ........................................ 5 2.2.2 Technologies and Network Interfaces ...................... 5 2.2.3 Handover Control ......................................... 6 2.2.4 Simultaneous connectivity to Access Routers .............. 7 2.2.5 Performance and Functional Requirements .................. 7 2.3 Micro diversity, Macro diversity, and IP diversity ......... 7 2.4 Mobile Host States and Modes ............................... 8 2.5 User, Personal and Host Mobility ........................... 9 2.6 Macro and Micro Mobility ................................... 9 3 Acknowledgement .............................................. 10 4 References ................................................... 11 5 Author's Addresses ........................................... 11 6 Appendix A - Examples ........................................ 14 1. Introduction This document presents a terminology to be used for documents and discussions within the Seamoby Working Group. Other working groups may also take advantage of this terminology in order to create a common terminology for the area of mobility. Some terms and their definitions that are not directly related to the IP world are included for the purpose of harmonizing the terminology, for example, 'Access Point' and 'base station' refer to the same component but 'Access Router' has a very different meaning. The presented terminology may not be adequate to cover mobile ad-hoc networks. The proposed terminology is not meant to 'push' new terminology. Rather the authors would welcome discussion on more exact definitions, better, missing and unnecessary terms. This work is a collaborative enterprise between people from many different engineering backgrounds and so already presents a first step in harmonizing the terminology. Manner et al Expires July 2001 [Page 2] Internet-Draft Mobility Related Terminology January 2001 2. Definitions 2.1. Network Components Note: The fundamental new concept to be introduced is that of the Access Network (AN) which supports enhanced mobility. It is a working assumption that to support routing and QoS for mobile devices, we need specialized routing functions (i.e. not OSPF or other standard IGPs) which are used to maintain forwarding information for these devices as they move physically, and these functions are implemented in IP routers with this additional capability. We can distinguish three types of these: Access Routers (AR) which handle the last hop to the mobile; Access Network Gateways (ANG) which form the boundary on the fixed network side and shield the fixed network from the specialized routing protocols; and (optionally) internal Access Network Routers which may also be needed in some cases to support the protocols. The Access Network consists of the equipment needed to support this specialized routing, i.e. A/ANG/ANR. Mobile Node (MN) An IP node capable of changing its point of attachment to the network. The Mobile Node may have routing functionality. Mobile Host (MH) An IP node capable of changing its point of attachment to the network. The Mobile Host only refers to an end-host without routing support. Access Link (AL) A facility or medium over which a Mobile Host and an Access Point can communicate at the link layer, i.e., the layer immediately below IP. Access Point (AP) An Access Point is a layer 2 device which is connected to an Access Router and offers the wireless link connection to the Mobile Host. Access Points are sometimes called 'base stations' or 'access point transceivers'. An Access Point may be a separate entity or co- located with an Access Router. Access Network Router (ANR) An IP router in the Access Network. An Access Network Router may include Access Network specific functionalities, for example, on mobility and/or QoS. This is to distinguish between ordinary routers and routers that have Access Network-related special functionality. Manner et al Expires July 2001 [Page 3] Internet-Draft Mobility Related Terminology January 2001 Access Router (AR) An Access Network Router residing on the edge of an Access Network and connected to one or more access points. An Access Router offers IP connectivity to MHs. The Access Router may include intelligence beyond a simple forwarding service offered by ordinary IP routers. Access Network Gateway (ANG) An Access Network Gateway that separates an Access Network from other IP-networks. An Access Router and an Access Network Gateway may be the same physical node. The Access Network Gateway looks to the fixed network like a standard IP router. Access Network (AN) An IP network which includes one or more Access Network Routers. The terms Access Network and (administrative) domain are often used interchangeably (e.g., "intra-AN" is "intra-domain") since often an Access Network has its own administration. Serving Access Router (SAR) The Access Router currently offering the connectivity to the Mobile Host. This is usually the point of departure for the Mobile Host as it makes its way towards a new Access Router (then Serving Access Router takes the role of the Old Access Router). There may be several Serving Access Routers serving the Mobile Host at the same time. Old Access Router (OAR) An Access Router that offered connectivity to the Mobile Host prior to a handover. This is the Serving Access Router that will cease or has ceased to offer connectivity to the Mobile Host. New Access Router (NAR) The Access Router that offers connectivity to the Mobile Host after a handover. Candidate Access Router (CAR) An Access Router to which the Mobile Host may move next. A handover scheme may support several Candidate Access Routers. 2.2. Handover Terminology These terms refer to different approaches to supporting different aspects of mobility. - Roaming refers to a particular aspect of user mobility. Roaming is an operator-based term involving formal agreements between operators Manner et al Expires July 2001 [Page 4] Internet-Draft Mobility Related Terminology January 2001 that allows a mobile to get connectivity from a foreign network. Roaming includes, for example, the functionality by which users can communicate their identity to the local AN so that inter-AN agreements can be activated and service and applications in the MH's home network can be made available to the user locally. - Handover (also known as handoff) is the process involved when an active MH (in the Active State, see section 2.4) changes its point of attachment to the network, or when such a change is attempted. The access network may provide particular capabilities to minimize the interruption to sessions in progress. There are different types of handover classified according to different aspects involved in the handover. Some of this terminology follows the description of [5]. 2.2.1. Scope of Handover - Layer 2 Handover: When a MH changes APs (or some other aspect of the radio channel) connected to the same AR's interface then a layer 2 handover occurs. This type of handover is transparent to the routing at the IP layer (or it appears simply as a link layer reconfiguration without any mobility implications). - Intra-AR Handover: This is a handover which changes the AR's IP- layer's interface to the mobile. This causes routing changes internal to the AR. The IP-address by which the MH is reachable does not change. - Intra-AN Handover: When the MH changes ARs inside the same AN then this handover occurs. Such a handover is not necessarily visible outside the AN. In case the ANG serving the MH changes, this handover is seen outside the AN due to a change in the routing paths. The IP- address by which the MH is reachable does not change. Note that the ANG may change for only some of the MH's data flows. - Inter-AN Handover: When the MH moves to a new AN then this handover occurs. This requires some sort of host mobility across ANs, which has to be provided by the external IP core. Note that this would have to involve the assignment of a new IP address to the MH. 2.2.2. Technologies and Network Interfaces - Intra-technology Handover: A handover between equipment of the same technology. Layer 2 handovers are necessarily intra-technology. - Inter-technology Handover: A handover between equipment of different technologies. - Horizontal Handover: from the IP point of view a horizontal handover happens if the MH communicates with the OAR and NAR via the same network interface. Horizontal handover is typically also an Manner et al Expires July 2001 [Page 5] Internet-Draft Mobility Related Terminology January 2001 intra-technology handover but it can be an inter-technology handover if the MH can do a handover between two different technologies without changing the network interface seen by the IP layer. - Vertical Handover: in a vertical handover the MH's network interface to the Access Network changes. Vertical handover is typically an inter-technology handover but it may also be an intra- technology handover if the MH has several interfaces of the same type. The different handover types defined in this section and in section 2.2.1 have no direct relationship. In particular, a MH can do an intra-AN handover of any of types defined above. 2.2.3. Handover Control A handover must be one of the following two types (a): - Mobile-initiated Handover: the MH is the one that makes the initial decision to initiate the handover. - Network-initiated Handover: the network makes the initial decision to initiate the handover. A handover is also one of the following two types (b): - Mobile-controlled Handover (MCHO): the MH has the primary control over the handover process. - Network-controlled Handover (NCHO): the network has the primary control over the handover process. A handover may also be either of these two types (c): - Mobile-assisted handover: information and measurement from the MH are used to decide on the execution of a handover. - Network-assisted handover: a handover where the AN collects information that can be used in a handover decision. A handover is also one of the following two types (d): - Backward handover: a handover either initiated by the OAR, or where the MH initiates a handover via the OAR. - Forward handover: a handover either initiated by the NAR, or where the MH initiates a handover via the NAR. The handover is also either proactive or reactive (e): - Planned handover: a proactive (expected) handover where some signalling can be done in advance of the MH getting connected to the new AR, e.g. building a temporary tunnel from the old AR to the new Manner et al Expires July 2001 [Page 6] Internet-Draft Mobility Related Terminology January 2001 AR. Generally this is a result of a backward handover. - Unplanned handover: a reactive (unexpected) handover, where no signalling is done in advance of the MH's move of the OAR to the new AR. Generally this results from a forward handover. The five handover types (a-e) are orthogonal. Type 'c' may be present in a handover, the other types are always present. 2.2.4. Simultaneous connectivity to Access Routers - Make-before-break handover (MBB): During a MBB handover the MH can communicate simultaneously with the old and new AR. This should not be confused with "soft handover" which relies on macro diversity. - Break-before-make handover (BBM): During a BBM handover the MH does not communicate simultaneously with the old and the new AR. 2.2.5. Performance and Functional Requirements - Handover Latency: Handover latency is the time difference between when a MH is last able to send and/or receive an IP packet by way of the OAR, until when the MH is able to send and/or receive an IP packet through the NAR. Adapted from [5] - Smooth handover: A handover that aims primarily to minimize packet loss, with no explicit concern for additional delays in packet forwarding. - Fast handover: A handover that aims primarily to minimize delay, with no explicit interest in packet loss. - Seamless handover: A handover that is both smooth and fast, thus provides fast lossless handover between two ARs. - Context-aware Handover: A handover that is governed by a certain specific requirement to be fulfilled while handing the connection between two ARs. 2.3. Micro diversity, Macro diversity, and IP diversity Certain air interfaces (e.g. UTRAN FDD mode) require or at least support the concepts of macro diversity combining. Essentially, this refers to the fact that a single MH is able to send and receive over two independent radio channels ('diversity branches') at the same time; the information received over different branches is compared and that from the better branch passed to the upper layers. This can be used both to improve overall performance, and to provide a seamless type of handover at layer 2, since a new branch can be added before the old is deleted. See also [4]. Manner et al Expires July 2001 [Page 7] Internet-Draft Mobility Related Terminology January 2001 It is necessary to differentiate between combining/diversity that occurs layer 1/2 (physical and radio link layers) where the relevant unit of data is the radio frame, and that which occurs at layer 3, the network layer, where what is considered is the IP packet itself. In the following definitions micro- and macro diversity refer to L1/L2 and IP diversity refers to L3. - Micro diversity is the term used for the case where, for example, two antennas on the same transmitter send the same signal to a receiver over a slightly different path to overcome fading. - Macro diversity takes place when the duplicating / combining actions take place over multiple APs, possibly attached to different ARs. This may require support from the network layer to move the radio frames between the base stations and a central combining point. - IP diversity means the splitting and combining of packets at the IP level. 2.4. Mobile Host States and Modes Mobile systems may employ the use of MH states in order to operate more efficiently without degrading the performance of the system. The term 'mode' is also common and means the same as 'state'. A MH is always in one of the following three states: - Active State is when the AN knows the MH's SAR and the MH can send and receive IP packets. The AL may not be active, but the radio layer is able to establish one without assistance from the network layer. The MH has an IP address assigned. - Idle State is when the AN knows the MH's Paging Area, but the MH has no SAR and so packets cannot be delivered to the MH without the AN initiating paging. - Detached State is when the MH is in neither the Active nor Idle State. The MH does not have an IP address from the AN. - Paging is a procedure initiated by the Access Network to move an Idle MH into the Active State. As a result of paging, the MH establishes a SAR and the IP routes are set up. - Location updating is a procedure initiated by the MH, by which it informs the AN that it has moved into a new paging area. - A Paging Area is a part of the Access Network, typically containing a number of ARs/APs, which corresponds to some geographical area. The AN keeps and updates a list of all the Idle MHs present in the area. If the MH is within the radio coverage of the area it will be able to receive paging messages sent within that Paging Area. Manner et al Expires July 2001 [Page 8] Internet-Draft Mobility Related Terminology January 2001 Note: in fact, as well as the MH being in one of these three states, the AN also stores which state it believes the MH is in. Normally these are consistent; the definitions above assume so. 2.5. User, Personal and Host Mobility Different sorts of mobility management may be required of a mobile system. We can differentiate between user, personal and host mobility. - User mobility: refers to the ability of a user to access services from different physical hosts. This usually means, the user has an account on these different hosts or that a host does not restrict users from using the host to access services. - Personal mobility: complements user mobility with the ability to track the user's location and provide the users current location to allow sessions to be initiated by and towards the user by anyone on any other network. Personal mobility is also concerned with enabling associated security, billing and service subscription authorization made between administrative domains. - Host mobility: refers to the function of allowing a mobile host to change its point of attachment to the network, without interrupting IP packet delivery to/from that host. There may be different sub- functions depending on what the current level of service is being provided; in particular, support for host mobility usually implies active and idle modes of operation, depending on whether the host has any current sessions or not. Access Network procedures are required to keep track of the current point of attachment of all the MHs or establish it at will. Accurate location and routing procedures are required in order to maintain the integrity of the communication. Host mobility is often called 'terminal mobility'. 2.6. Macro and Micro Mobility Macro and micro mobility refer to host mobility in wide and local geographical area. Correspondingly, macro- and micro-mobility management refer to the scope of protocol operations in mobility management. - Macro mobility refers literally to 'mobility over a large area'. This includes mobility support and associated address registration procedures that are needed when a mobile host moves between IP domains. Inter-AN handovers typically involve macro-mobility protocols. Mobile-IP can be seen as a means to provide macro mobility. - Micro mobility refers to 'mobility over a small area'. Usually this means mobility within an IP domain with an emphasis on support for active mode using handover, although it may include idle mode procedures also. Micro-mobility protocols exploit the locality of Manner et al Expires July 2001 [Page 9] Internet-Draft Mobility Related Terminology January 2001 movement by confining movement related changes and signalling to the access network. 3. Acknowledgement This work has been performed in the framework of the IST project IST-1999-10050 BRAIN, which is partly funded by the European Union. The authors would like to acknowledge the contributions of their colleagues from Siemens AG, British Telecommunications PLC, Agora Systems S.A., Ericsson Radio Systems AB, France T‰l‰com R&D, INRIA, King's College London, Nokia Corporation, NTT DoCoMo, Sony International (Europe) GmbH, and T-Nova Deutsche Telekom Innovations- gesellschaft GmbH. Some definitions of terminology have been adapted from [1], [2], [3], [5], [7] and [8]. Manner et al Expires July 2001 [Page 10] Internet-Draft Mobility Related Terminology January 2001 4. References [1] Blair, D., Tweedly, A., Thomas, M., Trostle, J., Ramalho, M., "Realtime Mobile IPv6 Framework". Internet Draft (work in progress), November 2000 (draft-blair-rt-mobileipv6-seamoby-00.txt). [2] Deering, S., Hinden, R., "Internet Protocol, Version 6 (IPv6) Specification". Internet Engineering Task Force, Request for Comments (RFC) 2460, December 1998. [3] Gustafsson, E., Jonsson, A., Perkins, C., "Mobile IP Regional Registration". Internet Draft (work in progress), July 2000 (draft-ietf-mobileip-reg-tunnel-03.txt). [4] Kempf, J., McCann, P., Roberts, P., "IP Mobility and the CDMA Radio Access Network: Applicability Statement for Soft Handoff", Internet Draft (work in progress), July 2000 (draft-kempf-cdma-appl-00.txt). [5] MIPv6 Handover Design Team, "Fast Handovers for Mobile IPv6". Internet Draft (work in progress), November 2000 (draft-perkins-mobileip-handover-00.txt). [6] Pandya, R., "Emerging mobile and personal communication systems," IEEE Communications Magazine , vol. 33, pp. 44--52, June 1995. [7] Perkins, C., "IP Mobility Support". Internet Engineering Task Force, Request for Comments (RFC) 2002, October 1996. [8] Ramjee, R., La Porta, T., Thuel, S., Varadhan, K., Salgarelli, L., "IP micro-mobility support using HAWAII". Internet Draft (work in progress), July 2000 (draft-ietf-mobileip-hawaii-01.txt). 5. Author's Addresses Questions about this document may be directed to: Jukka Manner Department of Computer Science University of Helsinki P.O. Box 26 (Teollisuuskatu 23) FIN-00014 HELSINKI Finland Voice: +358-9-191-44210 Fax: +358-9-191-44441 E-Mail: jmanner@cs.helsinki.fi Manner et al Expires July 2001 [Page 11] Internet-Draft Mobility Related Terminology January 2001 Markku Kojo Department of Computer Science University of Helsinki P.O. Box 26 (Teollisuuskatu 23) FIN-00014 HELSINKI Finland Voice: +358-9-191-44179 Fax: +358-9-191-44441 E-Mail: kojo@cs.helsinki.fi Tapio Suihko VTT Information Technology P.O. Box 1203 FIN-02044 VTT Finland Voice: +358-9-456-6078 Fax: +358-9-456-7028 E-Mail: tapio.suihko@vtt.fi Phil Eardley BTexaCT Adastral Park Martlesham Ipswich IP5 3RE United Kingdom Voice: +44-1473-645938 Fax: +44-1473-646885 E-Mail: philip.eardley@bt.com Dave Wisely BTexaCT Adastral Park Martlesham Ipswich IP5 3RE United Kingdom Voice: +44-1473-643848 Fax: +44-1473-646885 E-Mail: dave.wisely@bt.com Robert Hancock Roke Manor Research Ltd Romsey, Hants, SO51 0ZN United Kingdom Voice: +44-1794-833601 Fax: +44-1794-833434 E-Mail: robert.hancock@roke.co.uk Manner et al Expires July 2001 [Page 12] Internet-Draft Mobility Related Terminology January 2001 Nikos Georganopoulos King's College London Strand London WC2R 2LS United Kingdom Voice: +44-20-78482889 Fax: +44-20-78482664 E-Mail: nikolaos.georganopoulos@kcl.ac.uk) Manner et al Expires July 2001 [Page 13] Internet-Draft Mobility Related Terminology January 2001 6. Appendix A - Examples This appendix provides examples for the terminology presented. A.1 Mobility Host mobility is logically independent of user mobility, although in real networks, at least the address management functions are often required to attach the host to the network in the first place. In addition, if the network wishes to determine whether access is authorized (and if so, who to charge for it), then this may be tied to the identity of the user of the terminal. An example of user mobility would be a campus network, where a student can log into the campus network from several workstations and still get his/her files, emails, etc. services automatically. Personal mobility support typically amounts to the maintenance and update of some sort of address mapping database, such as a SIP server or DNS server; it is also possible for the personal mobility support function to take a part in forwarding control messages between end user and correspondent rather than simply acting as a database. SIP is a protocol for session initiation in IP networks. It includes registration procedures which partially support personal mobility (namely, the ability for the network to route a session towards a user at a local IP address). Personal mobility has been defined in [6] as "the ability of end users to originate and receive calls and access subscribed telecommunication services on any terminal in any location, and the ability of the network to identify end users as they move. Personal mobility is based on the use of a unique personal identity (i.e., personal number)." Roaming, in its original (GSM) sense, is the ability of a user to connect to the networks owned by operators other than the one he has a direct formal relationship with. More recently (e.g. in data networks and UMTS) it also refers to the fact that the 'foreign' network may still be able to provide user-customized services, e.g. QoS profiles for specific applications. HAWAII, Cellular IP, Regional Registration and EMA are examples of micro mobility schemes, with the assumption that Mobile IP is used for macro mobility. WLAN technologies such as IEEE 802.11 typically support aspects of user and host mobility in a minimal way. User mobility procedures (for access control and so on) are defined only over the air interface (and the way these are handled within the network is not further defined). PLMNs (GSM/UMTS) typically have extensive support for both user and host mobility. Complete sets of protocols (both over the air and on Manner et al Expires July 2001 [Page 14] Internet-Draft Mobility Related Terminology January 2001 the network side) are provided for user mobility, including customized service provision. Handover for host mobility is also supported, both within access networks, and also within the GSM/UMTS core network for mobility between access networks of the same operator. A.2 Handovers A hard handover is required where a MH is not able to receive or send traffic to two APs simultaneously. In order to move the traffic channel from the old to the new access point the MH abruptly changes the frequency/timeslot/code on which it is transmitting and listening to new values associated with a new access point. A good example of hard handover is GSM where the mobile listens for new base stations, reports back to the network the signal strength and identity of the new base station(s) heard. When the old base station decides that a handover is required it instructs the new base station to set up resources and, when confirmed, instructs the mobile to switch to a new frequency and time slot. This sort of hand over is called hard, mobile assisted, network initiated and backward (meaning that the old base station is responsible for handling the change-over). In a TDMA system, such as GSM, the hard hand over is delayed until the mobile has moved well within the coverage of the new base station. If the handover threshold was set to the point where the new base station signal exceeded the old then there would be a very large number of handovers as the mobile moved through the region between the cells and radio signals fluctuated, this would create a large signalling traffic. To avoid this a large hysteresis is set, i.e. the new base station must be (say) 10dB stronger for handover to occur. If the same was done in W-CDMA then the mobile would be transmitting a powerful signal to the old base station and creating interference for other users, since in CDMA everyone else's transmissions are seen as noise, thus reducing capacity. To avoid this soft handover is used, giving an estimated doubling in capacity. Support for soft handover (in a single mode terminal) is characteristic of radio interfaces which also require macro diversity (bi-casting) for interference limitation but the two concepts are logically independent. A good example of soft handover is the UTRAN FDD mode. W-CDMA is particularly suited to soft handover because of the design of the receivers and transmitters: typically a rake receiver will be used to overcome the multi-path fading of the wide-band channel. Rake receivers have a number of so-called fingers, each effectively separate detectors, that are tuned to the same signal (e.g. spreading code) but delayed by different times. When the delay times are correctly adjusted and the various components properly combined (this is micro diversity combining) the effect of multi-path fading is removed. The rake receiver can also be used to detect signals from Manner et al Expires July 2001 [Page 15] Internet-Draft Mobility Related Terminology January 2001 different transmitters by tuning the fingers to different spreading codes. Soft handover is used in UTRAN FDD mode to also increase capacity. Every handover can be seen as Context-aware Handovers. In PLMNs the context to be fulfilled is that the new AP can accommodate the new mobile, for example, the new GSM cell can serve the incoming phone. Lately, the notion of Context-aware Handovers has been enlarged by, for example, QoS-aware handovers, meaning that the handover is governed by the need to support the QoS-context of the moving mobile in order to keep the service level assured to the user of the MH. A.3 Diversity combining In the case of UMTS it is radio frames that are duplicated at some point in the network (the serving RNC) and sent to a number of Node Bs and, possibly via other (drift) RNCs. The combining that takes place at the serving RNC in the uplink direction is typically based on some simple quality comparison of the various received frames, which implies that the various copies of these frames must contain identical upper layer information. The serving RNC also has to do buffering to take account of the differing time of flight from each Node B to the RNC. A.4 Miscellaneous In a GPRS/UMTS system the Access Network Gateway node would be the GGSN component. The ANG can provide support for mobility of hosts, admission control, policy enforcement, and Foreign Agent functionality. When presenting a mobile network topology, APs and ARs are usually pictured as separate components. This is the case with GSM/GPRS/UMTS presentations, for example. From the IP point of view APs are not directly visible. An AP should only be seen from the MH's or AR's IP- layer as a link (interface) connecting MHs to the AR. When the mobile moves through the network, depending on the mobility mechanism, the OAR will forward packets destined to the old MHs address to the SAR which currently serves the MH. At the same time the handover mechanism may be studying CARs to find the best NAR where the MH will be handed next. Note that when a network includes IP-over-IP tunnels, we need to be very careful about which IP routing and IP address we are discussing. Manner et al Expires July 2001 [Page 16] Internet-Draft Mobility Related Terminology January 2001 Full Copyright Statement Copyright (C) The Internet Society (2000). All Rights Reserved. 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