Issues in Mobile Node Controlled Handovers

Issues in 802.21 Mobile Node Controlled Handovers Rehan Qureshi, Arek Dadej and Qiang Fu Institute for Telecommunications Research University of South Australia Mawson Lakes, SA 5095, Australia Email: first.last@postgrads.unisa.edu.au first.last@unisa.edu.au Abstract Mobile nodes moving across heterogenous access networks need to perform Mobile IP handovers to maintain continuity of Internet access. The IEEE 802.21 framework was defined to support such handovers by providing link layer intelligence and Media Independent Handover control facilities to the user entities. This framework, however, does not provide all controls necessary to perform seamless handovers in heterogeneous network environment. Additions, clarifications and simplifications of some commands may be required to promote uniform approach to implementing seamless handovers in heterogeneous access environment and ensure inter-operability across the range of handover control procedures implemented independently by various manufacturers and providers of network access solutions. Although the current 802.21 draft incorporates recent proposals to solve some of these issues, it falls short of proposing solutions suitable for the case of mobile controlled handovers. This paper proposes definitions of commands for inclusion in the IEEE 802.21 standard, to address the current deficiencies of the standard. I. INTRODUCTION Multiple operators offering network access services, and the increasing number of access technologies available, make the wireless network access environment increasingly heterogeneous. With the availability of a variety of wireless technologies, a mobile node has a greater chance of remaining connected to the internet while on the move e.g. in a building a mobile node can connect via a WiFi network, and on moving outside it can connect via 3GPP or WiMAX. When moving from one access network to another, a mobile node has to perform seamless handover in order to remain connected to the Internet. Due to the heterogenous nature of the networks, these handovers include both the link layer handovers and the network layer handovers. To facilitate this kind of handovers, the IEEE Media Independent Handover (MIH) framework (IEEE 802.21 standard) [1] was defined. The 802.21 MIH framework provides link layer intelligence and other related network information to upper layers to facilitate optimisation of handovers. The network information is obtained from the mobile node s current or prospective point of attachments (PoA) using the MIH protocol. Such information can only be obtained if the PoAs involved are IEEE 802.21 enabled. This, however, is not always the case in the heterogeneous access environment with multiple network providers. A mobile node might need to be capable of performing handovers without network assistance, i.e. on the basis of information available locally from the link layer interfaces. This paper discusses mobile node controlled handovers carried out without any assistance from the network. While attempting to specify handover control procedures for a practical case involving mobile controlled handovers, we came to a realisation that the recent drafts of IEEE 802.21 standard [1], [2] do not provide to the upper layers all the facilities necessary to completely and optimally control handovers. Important details, such as the principles of translation of MIH level commands into link level commands are left to be decided as part of implementation, potentially leading to a variety of MIH support implementations not inter-operable with each other despite full compliance with the 802.21 standard. We also observed that the system of MIH Commands as currently defined may suffer from some internal inconsistencies, and in some cases lack functionality necessary to provide coverage of a wide range of handover scenarios in a simple and intuitively natural manner. Some of these problems are currently being addressed in numerous proposals (including changes of names and structure of MIH Commands) discussed within the 802.21 working group [3] and incorporated into the most recent draft [1], however none of them provide simple solution to mobile controlled handovers in the absence of PoA supplied network information (i.e. when the PoAs are not 802.21 enabled). The section II of the paper presents a brief overview of the IEEE 802.21 framework and the commands defined to support handovers. Section III provides a detailed description of the problems observed when performing make-before-break handovers and when a simple connection to a new link is required. Section IV proposes two new commands for the MIH Users to connect and disconnect a link. Finally, section V concludes the paper. II. OVERVIEW In this paper, we consider a mobile node with multiple network interfaces. Such a node can also be called a multihomed mobile node [4]. The mobile node roams in a heterogenous access environment. The node is IPv6 [5] capable and uses Mobile IPv6 [6] to manage network layer mobility. In order to cope with the heterogenous nature of the access networks and to easily transfer communications from one network to another, the mobile node uses IEEE 802.21 Media Independent Handover mechanisms. The IEEE 802.21 standard provides a platform to support handovers across heterogenous access networks. It specifies Media Independent Handover interfaces to support handovers

Upper Layers MIH User (Mobility Protocol, Handover Policy, etc.) Event Event Command MIH Function Command Information Information Lower Layers (802.3, 802.11, 802.16, 3GPP, 3GPP2) Fig. 1. MIH Function location and services featuring 3GPP, 3GPP2 and both wired and wireless IEEE 802 family of media. The information used to facilitate handovers is exchanged as part of the following IEEE 802.21 services. 1) The Media Independent Event service that detects events and delivers triggers from both local and remote network interfaces. 2) The Media independent Command service that provides a set of commands for the MIH Users to control link states relevant to handover. 3) The Media Independent Information service that provides the information model and the information repository (database) for effective support of handover decisions across heterogeneous access networks. The above services are supported by the Media Independent Handover Function (MIHF) which acts as a facilitator in mobility management and handover processes. The MIH Function attempts to present link layer information from different access technologies to the upper layers in a unified, independent of specific link technology, manner. It translates commands received from a MIH User into one or more Link Commands. It also transfers events received from the link layer to the upper layers (MIH Users). Figure 1 shows the positioning of the MIH Function in a layered protocol model, and the services offered by the MIH. For the case of a mobile node considered in this paper, the MIH User is the Handover Controller entity in the mobile node. The Handover Controller decides when and where to hand over the ongoing communication based on local policies, requirements of the mobile node, and link condition indications received from the MIH Function. In the case considered here, the mobile node is expected to roam into networks that might not be able to support the IEEE 802.21 protocols, thus the Handover Controller makes the handover decisions by itself, without network assistance. The principles of operation and the implementation of Handover Controller are out of the scope of this paper. For performing handovers, the IEEE 802.21 framework provides a MIH User the MIH Switch Command and the MIH HO (Candidate Query / Query Resources / Candidate Commit / Complete) Command suite. These MIH commands are translated by the MIH Function into Link Commands such as Link Connect and Link Disconnect. The use of Link Commands is relatively straightforward, as the commands have well defined meaning in the context of specific link technology. However, the use of MIH Commands needs careful consideration, as the 802.21 standard does not provide clear guidelines for translation of MIH Commands into sequences of Link Commands, i.e. for the design and implementation of the MIH Function. A. MIH Switch Command This command is generated by upper layer entities (e.g. a Handover Controller entity) to switch active session from one link to another. The MIH Function may execute this command using a combination of Link Commands (e.g. Link Connect, Link Disconnect etc.) depending on the parameters defined in the MIH Switch command. The parameters of the MIH Switch command defined in the current 802.21 draft are as follows: MIH Switch.request { DestinationIdentifier, HandoverMode, NewLinkIdentifier, OldLinkIdentifier, OldLinkActions } The HandoverMode parameter in the command specifies either Make-before-Break or Break-before-Make mode of the link switch, while the OldLinkActions parameter specifies the action that should be taken in respect to the old link. The actions on the old link can be Link Disconnect, Link Sleep, Link Power Down, Link No Action etc. It needs to be noted that in the most recent works of the 802.21 Working Group (as of late July 2007), there seems to be an agreement that the Switch command could be abandoned, as its intended effect can be adequately achieved with the use of HO Commit command discussed in the section below. However, as no official document is available as yet to a wider audience that could be used to confirm this, we include in this paper the discussion of issues associated with the use of Switch command in handover control. Moreover, as explained in Section III, the issues that prompted our efforts towards proposing new commands to allow more precise control of links during handovers, remain valid regardless of the specific command (Switch or Commit) used to execute link switch during handovers. B. MIH HO Command Suite The MIH HO (Candidate Query / Query Resources / Candidate Commit / Complete) suite of commands is intended for use by peer MIH Functions to communicate remotely with each other, e.g. in a handover scenario involving interaction between Mobile Node and Network or Network and Network. When all handover control is local to the Mobile Node, the

use of the MIH HO (Candidate Query / Query Resources / Candidate Commit / Complete) suite of commands will also have to be local. As the MIH HO commands support operation in both remote and local mode, it is possible for the Handover Controller (MIH User) to use these commands with the MIH Function collocated within the Mobile Node. The MIH HO Candidate Query commands communicate intention to perform handover and suggest new (candidate) links, the handover mode and optionally suggest new PoAs. The MIH Net HO Candidate Query is sent from network to mobile node and the MIH MN HO Candidate Query is sent from a mobile node to network. The MIH N2N HO Query Resources prompts the candidate network entity to prepare resources for handover. It is intended to be used between the MIHFs of the serving network entity and the candidate network entity. The MIH HO Candidate Commit commands request handover to a specific link and inform what action should be taken on the current link. The MIH Net HO Candidate Commit is sent from network to mobile node and the MIH MN HO Candidate Commit is sent from a mobile node to network. Finally, the MIH HO Complete commands communicate completion of MIH level handover aiding process. The MIH MN HO Complete is sent from a mobile node to network and the MIH N2N HO Complete is used between two network entities. III. LIMITATIONS OF IEEE 802.21 This section will discuss limitations of the current IEEE 802.21 draft in providing adequate support for make-beforebreak handovers and when a simple connection via a specific link needs to be activated. A. Make-before-break Handovers Handover procedure involves connecting to a new link and disconnecting from the old link. The timing of these two events depends on the nature of handover. In a make-before-break handover, first a connection is established to a new link and then the old link is disconnected (make-before-break). In a hard handover, the current link is disconnected first and then the connection to a new link is established (break-beforemake). The case of hard handover is quite simple, however a make-before-break handover brings specific requirements for the functionality of the handover commands. A true make-before-break Mobile IP handover can only be achieved when the old link is disconnected after both layer 2 and layer 3 connectivity is established on the new link. MIPv6 [6] allows a mobile node to use more than one Careof-Addresses (CoA) at a time. This enables a mobile node to perform make-before-break handover by configuring a new CoA on the new link while maintaining its previous CoA (and the old link connectivity) as long as necessary. This way the packets forwarded by the mobile node s home agent (HA) before receiving the new Binding Update can reach the Mobile Node, and the Mobile Node experiences no or little disruption of service. The intuitively natural choice for link switch without network assistance (locally by the Mobile Node) is the MIH Switch command. To achieve Make-before-Break handover with the MIH Switch command, the parameters of the command would have to be set as follows: HandoverMode to Make-before-Break, and the OldLinkActions to either Link Disconnect or Link Sleep. However, the MIH Switch command applied as above has a major drawback and may result in a far from optimal handover. The MIH User has no control over the time when the OldLinkActions will be performed on the old link (i.e. it does not have any means to control the period of time when both links stay connected). Also, the IP layer handover requires more time than the layer 2 handover [7]. The IP layer handover delay includes time to configure new CoA and its HA binding. The time interval between connecting to a new link and disconnecting from the previous one should allow for both these delays. Without explicit control by the MIH User, this could only be achieved by making the delay between the activation of the new link and the deactivation of the old one sufficiently long, resulting in far from optimal handover delay. It would be desirable to allow the MIH User to receive a confirmation that the new link has been connected, and then to issue a command to disconnect the old link after the IP layer connectivity has been established on the new link. This, however, is not available as part of the MIH Switch command functionality. The MIH Switch command s list of possible actions on the old link also contains Link No Action. Thus, the MIH Switch command can also be used with HandoverMode set to Make-before-Break, and the OldLinkActions set to Link No Action. However, this will not be sufficient for implementation of make-before-break handovers. During makebefore-break handovers, the MIH Switch command used as above will enable connection to a new link without disconnecting the previous link, allowing time for the network layer handover. There are no commands, however, defined in the IEEE 802.21, that the upper layers could use to disconnect a particular link at a later time. This leaves the old link connected when it is no longer required. MIP Configuration Fig. 2. Make-before-break handover with MIH Switch command

The other option to perform link switching is to use locally the MIH HO command suite. The IEEE 802.21 draft provides some information on the component MIH HO functions and the intended effects of these commands when used with peer MIHFs. However, the local use of these commands lacks appropriate guidelines. Thus a key challenge in designing local (MN-controlled) handover control procedures is to determine how these commands should be used. The lack of firm guidelines on the MIH Function functionality can potentially lead to inter-operability problems between different implementations, despite their full compliance with the 802.21 standards. The MIH HO Candidate Query command only indicates the intention of handover and suggests options for handover. As the local handover controller issuing this command would itself be the decision maker and does not need to make suggestions to another functional entity, this command has no application in our case of local handover control within the Mobile Node. The MIH HO N2N Query Resources command also has no utility when no network assistance is used. The MIH HO MN Candidate Commit command can be used to begin a locally (Mobile Node) controlled handover procedure. This command has a OldLinkAction parameter that specifies action on the old link once handover procedure has been executed. To prevent old link connection from being disconnected before the MIP configuration (new CoA binding) is completed on the new link, the OldLinkAction can be set to Link No Action. This, however, requires another command that MIH User can use to inform the MIH Function that the MIP configuration is completed and that it should now disconnect the old link. As there are no explicit commands available for a MIH User to issue a disconnect request to the MIH Function, the only other commands left to consider are the MIH HO Complete commands. These commands are intended to be used between Mobile Node and network or between entities at the old and the new networks, to indicate the completion of a handover so that the resources reserved in the old network can be released. This use is meaningless when all handover procedures are dealt with inside the mobile node (locally), as is the case considered in this paper. This command also does not provide sufficient means to request a disconnection of the old link, as its current definition does not include a parameter by which an identifier of the link to be disconnected can be supplied by the MIH User to the MIH Function. B. Link Connection A mobile node may elect to shift its network layer communication to another link and also keep the previous link connected. This can typically happen when a mobile node needs to connect to a link with higher bandwidth to satisfy its increasing needs and wants to maintain previous link for load balancing, added reliability etc. This process will involve scanning and selecting an appropriate link, connecting to it and possibly configuring Primary CoA on the new link. The IEEE 802.21 draft provides MIH Scan command that can be issued to discover neighbouring PoAs. The MIH User can select an appropriate PoA based on its local policy and attempt to connect to it. However, there is no command available for MIH User by which it can request connection to a particular link. The MIH Switch command with HandoverMode set to Make-before-Break, and the OldLinkActions set to Link No Action can be used to simply connect to a new link. However, the MIH Switch command is for switching the links and operates in either Break-before-Make or Makebefore-Break, requiring the old link to be broken during or at the end of handover procedure. Using this command to only connect to a link is, although feasible, counterintuitive. The other option is to use MIH HO command suite solely for connecting to a new link rather than performing a full handover. The MIH MN HO Candidate Commit can be used to connect to a specific link with OldLinkAction parameter set to Link No Action. However, it is not clear in the IEEE 802.21 draft whether the MIH Function always expects a MIH X HO Complete command to follow the MIH MN HO Candidate Commit command, or these commands can be used separately. Also, if both of these commands are required to complete the action, it is not clear what will the MIH Function do on receiving MIH X HO Complete command in this scenario. IV. SOLUTION: PROPOSED NEW COMMANDS One way of dealing with the issue of make-beforebreak handovers could be to add a delay parameter to the MIH Switch primitive that defines the time interval between connecting to a new link and disconnecting the old one. This would provide some time for the MIP to configure and bind a new CoA. However, the time required to complete the binding update procedure depends on the location of HA and the type (quality) of link, and may be difficult to determine precisely. The ideal set of MIH Commands available to the Handover Controller would be one that separates the link connection and link disconnection functions. The IEEE 802.21 draft suggests Handover Controller MIHF Interface 1 Interface 2 MIP Configuration Fig. 3. MIH_Connect. request MIH_Connect. confirm MIH_Disconnect.request MIH_Disconnect.confirm Link_Connect Link_Disconnect Handover with MIH Connect and MIH Disconnect commands

that the MIH Function can execute the MIH Switch command by using a combination of Link Connect and Link Disconnect Commands. If upper layers could generate individual commands to connect and disconnect a link i.e. MIH Connect and MIH Disconnect, far more freedom in performing optimal handover procedures would be available. These commands would facilitate make-before-break handovers and also help in cases where only a specific link needs to be connected. During a make-before-break handover, the MIH User could direct the MIH Function to connect to a specific link using MIH Connect command. Once the link layer connection is established, the MIP configuration could start. As soon as MIP configuration is completed on the new link, the MIH User could direct the MIH Function to disconnect the old link using MIH Disconnect command. In another case, when only a new link needs to be connected, the MIH user could scan the interface using MIH Scan command, and choose the appropriate point of attachment from the PoAs returned in scan results. Once the choice of the appropriate PoA is made, the MIH User could connect to the selected link by issuing a MIH Connect command. The following sections describe the proposed MIH Commands. A. MIH Connect The parameters of MIH Connect.request and MIH Connect.confirm are defined in table I and table II respectively. The Destination and Source Identifier parameters have the same definition and application as defined in the IEEE 802.21 draft for the MIH Commands [1]. The Link Identifier parameter identifies the link or interface for which the command was sent. The MAC New PoA Identifier parameter can hold the MAC address of the PoA the MIH User is requesting to connect to. The Result Code parameter informs the MIH User about the result of connection attempt. B. MIH Disconnect The parameters of MIH Disconnect.request and MIH Disconnect.confirm are defined in table III and table IV respectively. The Destination and Source Identifier TABLE I MIH CONNECT.REQUEST PARAMETERS Destination Identifier Identifier Valid MIHF identifier LinkIdentifier May be one of N/A the 802.x or Cellular networks MACNewPoA MAC Address N/A TABLE II MIH CONNECT.CONFIRM PARAMETERS Source Identifier Identifier Valid MIHF identifier ResultCode Enumerate Success; Failure; Rejected TABLE III MIH DISCONNECT.REQUEST PARAMETERS Destination Identifier Identifier Valid MIHF identifier TABLE IV MIH DISCONNECT.CONFIRM PARAMETERS Source Identifier Identifier Valid MIHF identifier ResultCode Enumerate Success; Failure; Rejected parameters have the same definition and application as defined in IEEE 802.21 standards draft for various MIH Commands [1]. The Link Identifier parameter identifies the link or interface for which the command was sent. The Result Code parameter informs the MIH User about the result of connection attempt. V. CONCLUSION In this paper, we have identified and discussed limitations of current version of IEEE 802.21 standards draft in facilitating make-before-break handovers and providing adequate control to MIH User. Solutions were proposed to facilitate adequate control of handover procedures by the MIH User. These include definitions of commands that allow the MIH User to separately control the activation of the new link, and deactivation of the old one, in the process of make-beforebreak handover. We also observe that the lack of implementation guidelines for the MIH Function in the current 802.21 draft seems to be a major drawback. As the translation between the MIH Commands and the Link commands determines the handover procedures, the appropriate guidelines are necessary to promote a unified approach to handover control. The lack of standardised MIH Function procedures as above will inevitably lead to multiple proprietary implementations and incompatibility between network elements (i.e. inability to cooperatively participate in handover procedures) despite full compliance with the 802.21 standard. REFERENCES [1] Media Independant Handover s, IEEE Standards Draft 802.21, Rev. 4.0, 2007. [2] Media Independant Handover s, IEEE Standards Draft 802.21, Rev. 1.0, 2006. [3] (2007, March) The IEEE 802.21 Working Group website. [Online]. Available: http://www.ieee802.org/21/ [4] C. Ng, E. Paik, T. Ernst, and M. Bagnulo, Analysis of multihoming in network mobility support, IETF, Internet Draft, February 2006, draftietf-nemo-multihoming-issues-05. [5] S. Deering and R. Hinden, Internet Protocol version 6 (IPv6) specification, IETF, RFC 2460, December 1998. [6] D. Johnson, C. Perkins, and J. Arkko, Mobility support in IPv6, IETF, RFC 3775, June 2004. [7] M. Siksik, H. Alnuweiri, and S. Zahir, A detailed characterization of the handover process using mobile IPv6 in 802.11 networks, in IEEE Pacific Rim Conference on Communications, Computers and signal Processing, August 2005, pp. 312 315.