IEEE802.21 Assisted Network Layer Mobility Support Qazi Bouland Mussabbir *, Wenbing Yao ** and John Cosmas *** *School Of Engineering and Design, Brunel University Uxbridge, London, UB83PH, UK, qazi.mussabbir@brunel.ac.uk ** School Of Engineering and Design, Brunel University, eesrwwy@brunel.ac.uk *** School Of Engineering and Design, Brunel University, eestjpc@brunel.ac.uk ABSTRACT The emerging IEEE802.21 standard defines a Media Independent Handover Function (MIHF) that would assist mobile devices to seamlessly roam across heterogeneous access networks. The aim of this paper is to provide a survey of the IEEE 802.21 Media Independent Handover (MIH) Services and present how these services can be used to assist the MIPv6 based network layer mobility. The three primary functional components of the MIHF which includes Event service (ES), Command Service (CS) and Information service (IS) are extensively reviewed. The paper focuses on presenting a scheme using MIHF services to facilitate the network layer handover between 802.11, 802.16 and GPRS access networks, and assist signalling protocols in network layer mobility support. We will show how to get network neighbourhood information through the 802.21 Information Service to aid intelligent handover decisions, and how the use of the link layer indications, well known as triggers or events, to assist the network layer handover initiation. Keywords: Media Independent Handover Function (MIHF), MIES, MICS, MIIS, Fast Mobile IPv6 (FMIPv6) 1 INTRODUCTION In the vision of the 4G wireless communications, it is requisite to provide seamless mobility support across heterogeneous access networks using wireless technologies such as 802.11 (WiFi), 802.16 (WiMax), CDMA, and wired access technologies like LAN, xdsl. Among many proposed mobility management solutions, Mobile IPv6 (MIPv6) [1] has been widely accepted in the academic world and the industry as the front runner for tackling this challenge. Handover performance is a vital part in the end-to-end delay and packet loss control for the QoS provisioning of real time services in heterogeneous networks. In MIPv6, when a handover process is initiated, the Mobile Node (MN) will acquire a new address, called Care-of-Address (CoA), and use Binding Update messages (BUs) to register the CoA with its Home Agent (HA) and Correspondent Node (CN) which will then communicate with the MN directly through the CoA. Handover delay will occur due to the processes of neighbour network discovery, CoA configuration, mobility binding updates, and sometimes through the network-specific authentication and authorization. Various other extension to the Mobile IP protocol within the IETF have been proposed such as hierarchical mobility (HMIPv6) [2] and fast handovers (FMIPv6) [3] to provide signalling and handover optimizations. In order to quickly detect any Layer3 movement (i.e. loss of attachment with default router and discovery of new router), link-layer indication in the form of event might be beneficial. Link layer information of the current and neighbouring access networks, which may use the same or different access technologies, is extremely useful for reducing the handover latency. The link characteristics of these networks may help to select which neighbour network the MN should handover to, and the information of certain link-layer events at either the MN side or the access network side will assist to decide when and how to initiate the MN handover process. In MIPv6, for instance, the MIHF Event Service could drastically reduce the handover latency by providing generic link layer indications. Moreover, MIHF Information Service would allow intelligent handover decisions through prior neighbouring network knowledge. The IEEE802.21 - Media Independent Handover (MIH) Service WG [6], which was formed in 2003, is developing a draft standard to enable handover and interoperability between heterogeneous networks including both 802 and non 802 networks. Within IETF s MIPSHOP Working Group, a few drafts have proposed usage model and scenarios in which the 802.21 framework could facilitate handover across heterogeneous access networks. The intention of this paper is to provide a survey of the work in the IEEE 802.21 WG and present a scheme which uses the MIH services to assist the network layer mobility support based on existing works. The paper takes into account these scenarios in [4] and [5] and extends them into a detailed discussion on how the 802.21 functional components enhance the overall handover process. The overall scenario given in [4] [5] is elaborated in this paper by describing MIH capability discovery, the Event Service registration process and Information Service discovery mechanisms. The rest of this paper will be organized as follow: In section 2, we outline some of the existing work and research related to IEEE802.21 standard. We will outline the 802.21 MIHF functions in section 3, and discuss the three primary functional components: The Event Service,
The Command Service and the Information service. In section 4, we will present a scheme to use MIH services to facilitate the MIPv6 handover procedure between heterogeneous access networks, and assist signalling protocols in network layer mobility support. We will discuss the conclusion in section 5. 2 RELATED WORKS There are several initiatives to optimize mobility across heterogeneous networks. The MIPSHOP Working Group within the IETF and the IEEE802.21 standard Working Group have been working to develop a framework in which the mobility management protocols would use the 802.21 to enhance the handover process. Reference [10] describes the transport and security requirements for the MIH signalling in order to aid IP handover mechanisms. References [4] and [5] outline few usage models of Event, Command and Information services. They also discuss security considerations for these services. In reference [7], 802.21 assisted SIP based mobility Test-bed across heterogeneous access network was implemented. The IEEE802.21 Working Group is addressing various scenarios in detail and is in the process of standardizing a Media Independent Handover Framework. This paper presents this framework and describes a usage scenario in which IP layer handover is optimized using the functional component of the 802.21 framework. 3 MEDIA INDEPENDENT HANDOVER FUNCTION In the mobility management protocol stack of both mobile node and network element, the Media Independent Handover Function (MIHF) is logically defined as a shim layer between the L2 data link layer and L3 network layer [6]. The upper layers are provided services by the MIH function through a unified interface. The services exposed by the unified interface are independent of access technologies. This unified interface is known as Service Access Point (SAP). The lower layer protocols communicate with the MIHF via media dependent SAP. Figure 1 illustrates the IEEE802.21 MIH Handover Framework MIHF defines three main services that facilitate handovers between heterogeneous networks: Media Independent Event Service (MIES), Media Independent Command Service (MICS) and Media Independent Information Service (MIIS). Detailed discussions of each of the services are given below. 3.1 Media Independent Event Service Figure 1 Media Independent Event Services (MIES) provide event reporting, event filtering and event classification corresponding to the dynamic changes in link characteristics, link quality and link status. The MIES report both local and remote events to the upper layers. The upper layers perform registration to receive events from the MIHF using a request/response primitive. Some of the events that have been specified by IEEE 802.21 are Link Up, Link Down, Link Detect, Link Parameter Reports and Link Going Down. 3.2 Media Independent Command Service Media Independent Command Service (MICS) use the MIHF primitives to send commands from higher layers to lower layers. The MICS command are utilized to determine the status of the connected links and also to execute mobile and connectivity decisions of the higher layers to the lower layers. MIH Commands are identified as either being local or remote. Local MIH commands flows from upper layers to the MIH function, and then to lower layers in the local stack. Remote commands, messages propagate from upper layer to the MIHF in one stack to the MIHF in a peer stack (with usage of the MIH protocol). Messages are further propagated to lower layer. 3.3 Media Independent Information Service Media Independent Information Service (MIIS) provides a framework and mechanism for an MIHF entity to discover available neighbouring network information within a geographical area to facilitate the handover process. The primary idea is for the MIIS to provide a set of information elements, the information structure and its representation and a query/response type mechanism for information transfer. Both static and dynamic information is provided by the MIIS. Examples of Static information would include the names and service providers of the mobile terminal s exiting network neighbourhood. Dynamic information would include link layer parameters such as channel information, MAC addresses, security
Figure 2: Network Selection Procedure information, and other higher layer service information to make intelligent handover decision. The information could be made available through lower layers as well as higher layers. In cases where layer 2 information is not available or sufficient to make efficient handover decisions, then higher layer information services may be required. In order to represent the information across different technologies, the MIIS specifies a common way of representing this information by using a standardized format such as XML or ASN.1. 4 NETWORK CONTROLLED LAYER 3 802.21 ASSISTED HANDOVER In this section we present a survey on Network initiated IP layer handover scheme using the functional components of the IEEE802l.21 framework based on the existing IETF s MIPSHOP s working group drafts [4] [5]. We consider the Fast Mobile IPv6(FMIPv6) scheme here. It is however extensible to other handover signalling schemes such as Hierarchical Mobile IPv6(HMIPv6), HIP, SIP etc The motivation behind choosing a network initiated handover is due to the fact that service providers, including those with multiple access technologies, under any given instance would not like to see that any specific part of their network is operating under heavy loads and prefer to balance the traffic across all the available network paths for optimization of paths. In other words, the network wishes to exercise control over the mobile nodes to make use of a certain network path that would mutually benefit the users and service providers themselves. The handover decision will be made by network using the 802.21 services in two steps: Step1) Network Selection and Step2) Handover Control. 4.1 Network Selection The process of selecting a favourable network for a mobile node to transfer or handover the ongoing services to the selected network is known as Network Selection [4]. The network that is selected maybe a different link access technology from the previous one. It is possible that the mobile node, after handover will not experience the same level of QoS when compared to the current link due to the Network Selection. The selection process in general is meant to provide some user benefit in one way or another, such as, cost savings, higher bandwidth etc [4]. Figure 2 illustrates a Network Selection procedure with the help of the mobile node. In the scenario remote Event Service (ES) and remote Information Service (IS) are shown to play an integral part in the Network Selection procedure. The Mobility Management Entity (MME) is assumed to be a core network element, that is, beyond Layer2. The MME functionality utilizes the MIHF (Media Independent Handover Function). The MME implements network selection handover algorithms and utilizes mobility signalling protocols (Fast Mobile IPv6 in this case) and aid
mobility functions. The following subsections describe the details of how MIES and MIIS of 802.21 framework are used in the Network Selection Procedure for Layer 3 handover optimizations. 4.1.1 Discovery, Registration and Indications of 802.21 MIES In this scenario, the mobile node initially performs a registration or attachment to the network on any link, e.g. 3GPP network in this case. The MME and Mobile node will also need to discover each others MIH capabilities before any service related information could be passed between the two entities. In this case, the MIH function in MME and mobile node could exchange message by a request/response mechanism to determine each others MIH capabilities of the link layers. IEEE802.21 defines the semantics of these service primitives and includes source of the requesting entity, the destination identifier the request/response of local or remote MIH function, and the list of supported events and commands. The MME then registers to remote Link Detect event services from the MIHF in the mobile node. The MIH user (upper layer) of the MME would initially send MIH event request message using the MIH_SAP and associated primitives inside its local stack. The request message would then further propagate from the local MIHF to the peer MIHF in the mobile node. The request message in our case would contain the set of events it would like to receive indications for ( Link Detect in our case) with appropriate filter information. The triggers or indications of the events from the link layer will be checked and scoped based on the filter rules set by the MIH user in the MME. This feature would ensure that the protocol does not result in excessive load on either the network or mobile node processing the event notifications/indications from multiple event generating nodes. Figure 3 shows the MIH registration/deregistration flow model. It is shown in the scenario in figure 2, that 802.16 broadcast is received by the mobile node and a link detect event indication is sent by the 802.16 MAC layer to the MIHF Event Service (ES). The MIHF (ES) translates the indication to an ES Link Detect message and sends it to the MIHF (ES) in the network (collocated in the MME) with all the basic information received from the 802.16 broadcast. 4.1.2 Usage of Information Services After the MME in the figure 2 receives the ES Link Detect, it requests via the MIIS query mechanism to an Information Server (IS) to check the suitability of the detected network (802.16) based on the roaming agreements between the two networks [4]. As with various deployment scenarios, the system would need to provide discovery mechanisms, security association (SA) bootstrap, and transport of information services over IP. For the information services, it is possible the network information may be either centrally stored in a server or distributed in each individual access network. In order to identify or discover a valid information server, a layer 2 or layer 3 mechanisms is required. At the time of writing, DHCP (Dynamic Host Control Protocol)[8] was decided as candidate discovery mechanism within IEEE802.21 MIIS specification. Figure 4 shows the three phases in relation to our MIIS usage scenario: Discovery, SA bootstrap, request/response. Figure 4: Information Service Message Exchange Figure 3: MIH Event Registration and flow The MME in our scenario initially uses DHCP to acquire the location of Information Server in terms of IS server IP address, IS server FDQN (Fully Qualified Domain Name) and URI(Uniform Resource Identifier). Before the MME can exchange any messages with the IS server, a set of Security associations (SA) are established. Authentication and encryption must be provided by each SA for the purpose of mobile device anonymity from eavesdroppers. The SA negotiation mechanism depends on the transport layer used, and security services required [9]. For Instance, TLS will be advisable if upper layer protocols use TCP, while ESP using IPSec/ IKE will work in most
Figure 5: Network Initiated Handover situations without regard of the upper layer protocol, so long as the IS protocol identifiers are handled by IKE [9] After the discovery and SA phase, the MME sends a request message to the IS server to check the suitability of the 802.16 detected network. The IS server responds with a response message containing the Information Elements (IE) requested the by MME.The Handover module in the MME in our scenario decides that this particular 802.16 is not a favourable one and takes no action. This decision could be based on static or dynamic information such as roaming agreements, QoS, channel information and higher mobility management service supported by the network. During a later time, the mobile receives beacon information from a 802.11 Access Point (AP). The MAC layer of the mobile node informs the MIH Event service along with the SSID information. The MIHF (ES) scopes and filters the this link layer information against the rules set by the MIH user (upper layer) of the MME. The MIHF (ES) processes another Link Detect event indication message along with SSID information sends it the peer MIHF of the MME. The MME performs an IS query and upon receiving a response determines that the SSID belongs to a favourable network. Thereby, the network selection is complete 4.2 Handover Control Handover Control procedure follows a Network Selection process described in the previous section. The following scenario portrayed in figure 5 shows a network controlled handover process with Fast Mobile IP signalling mechanism. The MME here uses the MIH Control Service (CS) and generates a Link Switch command. This command is in the form a request message is transported to the MIHF in the mobile node. The included parameters in the message may include a make-before-break mechanism to be performed with target link. In our case, the target link is 802.11 network as shown from the Network Selection process earlier. The MIHF (CS) sends an indication to the Mobile IP function of the awaiting link switch along with new link information. If the Mobile IP function does not have valid Access Router Tuple-Info, for instance, then it sends a Proxy Router Solicitation (PrRtrSol) with necessary link layer information, such as, MAC address of the AP. The Proxy Router Advertisement (PrRtrAdv) provides the relevant layer 3 information for the new link. Upon execution of the MIFH (CS), necessary layer 2 association and authentication procedures by sending an associate request to the target 802.11 MAC. After the Layer 2 association, the MIHF (ES) send a Link Up indication to the Mobile IP function. The Mobile Function performs a Fast Binding Update (FBU) with old foreign agent (FA) over the old link. The mobile node receives packets from the new FA that are tunnelled from the old FA. Later, the Mobile IP function in the mobile node performs update procedure to register the new binding with the HA and reroute the tunnel to the new FA in the corresponding 802.11 network link. As the traffic start to use the link, the MIHF (CS) sends a request to that MAC layer release the old link (3GPP radio link). A MIHF (CS)
Link Switch response is sent back to the MME with the termination of the command. [10]E. Hepworth et al, Media Independent Handovers: Statement, Internet Draft, (works in progress), IETF June 2006 5 CONCLUSION In this paper we have presented the three primary functional components defined by the IEEE802.21 standard. We have shown how these services interact with both the upper and lower layers of the mobility protocol stack through generic SAPs to optimize the handover process. The paper presents a scenario in which Network Controlled IP layer handover process is optimized through the usage of the 802.21 framework. The presented scenario outlines network discovery, network selection, pre-configuration and pre-authentication to facilitate a pro-active handover using the MIES, MICS and MIIS. MIH Capability discovery, Event Registration, Information Server discovery mechanisms have also been extensively discussed. REFERENCES [1] D. Johnson et. al., Mobility Support in IPv6. RFC 3755, IETF June 2004 [2] X. Pérez, A MIPv6, FMIPv6 and HMIPv6 handover latency study: Analytical approach, March 2006 available at: http://www.ist-mobydick.org/publications [3] Koodli et al, Fast Handovers for Mobile IPv6, RFC 4608, IETF, July 2005 [4] S. Sreemanthula et al, A Problem Statement for Event Services and Command Services for Media Independent Handovers, Internet Draft (works in progress), IETF March 2006 [5] S. Faccin et al, Some Requirement for Handover Information Services, Internet Draft (works in progress), IETF March 2006 [6] V.Gupta, IEEE802.21 Standard and Metropolitan Area Networks: Media Independent Handover Services, Draft P802.21/D00.05, January 2006 [7] A.Dutta, Y.Oshba and H. Schulzrinne, Seamless Handover across Heterogeneous Networks- An IEEE802.21 Centric Approach, March 2006, available at: http://www1.cs.columbia.edu/~dutta [8] R.Droms, Dynamic Host Control Protocol RFC 2131, IETF March 1997 [9] S.D Park, DHCPv4 Option for Discovering IEEE 802.21 Information Service Locations, Internet Draft (works in progress), IETF March 2005