MOBILE-IP Working Group Karim El Malki, Ericsson INTERNET-DRAFT Hesham Soliman, Ericsson Expires: January 2001 July 04, 2000 Fast Handoffs in Mobile IPv4 draft-elmalki-mobileip-fast-handoffs-02.txt 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 cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This document is an individual submission to the IETF. Comments should be directed to the authors. Abstract This draft describes a method to achieve Fast Handoffs in Mobile IPv4. Fast Handoffs are required in Mobile IPv4 in order to limit the period of service disruption experienced by a wireless Mobile Node when moving between Foreign Agents. This requirement becomes even more important when supporting real-time services. Fast Handoffs involve anticipating the movement of MNs by sending multiple copies of the traffic to potential Mobile Node movement locations (i.e. FAs). Both a flat and a Hierarchical Mobile IPv4 model are considered. The Hierarchical MIPv4 model in Regional Tunnel Management [1] already offers improvements to Mobile IP handoffs by providing local Home Agent functionality. Some additions are made to the operation of this existing Hierarchical model to achieve Fast Handoffs and limit or avoid triangle routing within the hierarchical domain. Karim El Malki and Hesham Soliman [Page 1] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 TABLE OF CONTENTS 1. Introduction..................................................2 2. Fast Handoffs.................................................4 2.1 Initiating Fast Handoffs through the "previous" FA............5 3. Hierarchical Mobile IPv4 Network..............................6 4. Fast Handoffs in Hierarchical MIPv4...........................8 5. Traffic Routing in Mobile IPv4 Hierarchies...................10 6. Regional Deregistration for Fast Handoffs....................12 7. Smooth Handoffs between Hierarchies (GFAs)...................12 8. IPv6 Considerations..........................................13 9. Security Considerations......................................13 10. Acknowledgements.............................................14 11. References...................................................14 12. Addresses....................................................14 1. Introduction Fast Handoffs anticipate the movement of wireless Mobile Nodes (MNs) by utilizing simultaneous bindings in order to send multiple copies of the traffic to potential Mobile Node movement locations. In this way, Fast Handoffs coupled to layer 2 mobility can help in achieving seamless handoffs between Foreign Agents by eliminating the delay period required to perform a Registration following a Mobile IP handoff (i.e. following a handoff between subnets/FAs). An alternative method to perform improved handoffs, namely Smooth Handoffs, is described in [3]. The method for Fast Handoffs addresses the need to support services having strict delay bounds (i.e. real-time) which in certain cases may be hard to support if traffic has to be forwarded between FAs using Smooth Handoffs. Also, in the non-realtime case it may be possible that the new FA receives buffered traffic from the previous FA (smooth handoff) and traffic from the HA which could cause some out-of-order and delayed packets to be delivered to the MN. In some cases this may affect the performance of higher level protocols (i.e. TCP). This same situation will not arise using Fast Handoffs. Karim El Malki and Hesham Soliman [Page 2] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 This draft considers both the normal Mobile IP model [2] and the hierarchical Mobile IP model [1]. These are shown in Figure 1 where the Access Points (APs) or Radio Access Networks (RANs) are used to provide a MN with wireless L2 access. Simultaneous bindings are described in [2] and may be achieved by setting the "S" bit in the Mobile IP Registration Request message sent by the MN. In this way, the receiving agent (HA, GFA or Regional FA) will add a new binding for the MN without removing any which are existing and have not expired. _________ _________ | | | | | HA |--------| (GFA) |________ |_________| |_________| \ / | \ \ \ ... ... ... \ \ ______/_ _\______ | | | | | | | FA2 | | FA1 | | |________| |________| | ____|___ ____|___ ____|___ | | | | | | |AP/RAN 2| |AP/RAN 1| |AP/RAN 3| |________| |________| |________| | ____|___ | | CN | MN | |________| Figure 1: Flat (HA only) and Hierarchical (HA and GFA) MIPv4 model Fast Handoffs may be applied to normal Mobile IP by performing registrations with the HA using simultaneous bindings. This is described in [2] and the method to anticipate MN movement by interacting with the wireless L2 is described later in this draft. However, having multiple simultaneous bindings for MNs at the HA will cause the HA to send multiple copies of data packets towards mutliple FAs which may be in the same region or domain. In terms of bandwidth usage this would not be efficient unless the HA is close to the FAs in question, but this is not always the case. Also, if the round-trip time between HA and FAs is not negligible this may slow down the MN's new Registration and therefore the Mobile IP handoff. The Hierarchical MIPv4 model addresses these problems. Karim El Malki and Hesham Soliman [Page 3] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 The Hierarchical Mobile IPv4 scheme introduced in Regional Tunnel Management [1] allows a Mobile Node to perform registrations locally with a Gateway Foreign Agent (GFA) in order to reduce the number of signalling messages to the home network. This achieves a reduction in the signalling delay when a Mobile Node moves between Foreign Agents and therefore improves the performance of such handoffs. This draft describes Fast Handoffs in Hierarchical Mobile IPv4 using Regional Registrations [1] and provides a method to avoid or minimise triangle routing within the hierarchical domain. As for Mobile IPv4, hierarchical networks utilizing Regional Tunnel management will suffer from triangle routing. The worst case will involve communication between Mobile Nodes connected to the same Foreign Agent, or to any other Foreign Agent within the same hierarchy, since packets will be routed through the respective home networks. In this draft, triangle routing between nodes within the hierarchical domain is eliminated by direct routing through Regional Foreign Agents (i.e. FA2 and FA1 in Fig. 1) or alternatively reduced by routing through the Gateway Foreign Agent (GFA). This draft is applicable to multi-level Hierarchical Mobile IPv4 (HMIPv4) networks. HMIPv4 networks utilizing Regional Tunnel Management with Fast Handoffs and local routing optimizations offer advantages which are especially important for the support of real-time services. 2. Fast Handoffs Fast Handoffs address the need to achieve seamless Mobile IP Handoffs when the MN moves between FAs. This is done by "bicasting" traffic to the "previous" FA and "new" FA while the MN is moving between them. The anticipation of the MN's movement is achieved by tight coupling with Layer 2 functionality which is dependent on the type of access technology used. "Bicasting" is achieved through simultaneous bindings, where the MN activates the "S" bit in the Registration Request. When a Registration Request has the "S" bit set, the receiving HA, which has an existing binding for the MN, will add the relevant new binding for the MN but will also maintain any other existing bindings it had for the MN. Similarly, in HMIPv4, when a Regional Registration Request has the "S" bit set, the receiving FA/HA or GFA which has an existing binding with the MN will add the relevant new binding for the MN but will also maintain any other existing bindings it had for the MN. When the MN has multiple active bindings with FAs, it may or may not receive multiple copies of the same traffic directed to it. The use of simultaneous bindings does not necessarily mean that the MN is receiving packets contemporarily from multiple sources. This depends on the characteristics of the access (L2) technology. The "bicasting" of packets is used to anticipate the MN's movement and speed up Karim El Malki and Hesham Soliman [Page 4] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 handoffs by sending a copy of the data to the FA which the MN is moving to. Until the MN actually completes the L2 handoff to the new FA, the data "copy" reaching this FA may be discarded. In this way the total handoff delay is limited to the time needed to perform the L2 handoff. Thus, Fast Handoffs coupled to the L2 access potentially result in loss-less IP-layer mobility. As described in 2.1, depending on the L2 characteristics, it is also possible for an MN to initiate a Fast Handoff through the "previous" FA without having direct access to the "new" FA. 2.1 Initiating Fast Handoffs through the "previous" FA In the case in which the wireless L2 technology allows the MN to be data-connected to multiple wireless access points simultaneously, the MN may solicit advertisements from FAs before completing handoffs. In this case "bicasting" may not be necessary. Some existing wireless L2 technologies and their implementations do not allow a MN to be data-connected to multiple wireless access points simultaneously. Thus, in order to perform a Fast Handoff it is necessary for some form of interworking between layers 2 and 3. It should be noted that the method by which an FA determines when a MN has initiated an L2 handoff is outside the scope of this draft and may involve interaction with L2 messaging. Also, the interaction between L2 and L3 should allow the Mobile Node to perform a L2 handoff only after having performed the L3 Fast Handoff described in this draft. That is, the L2 handoff may be performed after the MN's Registration with the "new" FA which produces a simultaneous binding at the GFA/HA. This Registration may be transmitted more than once to reduce the probability that it is lost due to errors on the wireless link. A Fast Handoff in this case requires the MN to receive "new" agent advertisements through the "old" wireless access points, and to perform a registration with the "new" FA through the "old" wireless access point. Two ways of performing this follow. I. Inter-FA Solicitation This solution assumes that the FA with which the MN is currently registered is aware of the IP address of the "new" FA which the MN is moving to. The method by which the current FA is informed of this may depend on interaction with L2 and is outside the scope of this draft. Once the current FA is aware of the address of the FA which the MN will move to, it will send the "new" FA an agent solicitation message. The "new" FA will reply to the current FA by sending it an Karim El Malki and Hesham Soliman [Page 5] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 agent advertisement with appropriate extensions. The current FA will then send the agent advertisement to the MN's address. As a consequence, the MN, being eager to perform new registrations, will send a registration request to the "new" FA through the "old" wireless access point served by the current FA. II. Piggy-backing Advertisements on L2 messaging Let us take Figure 1 as an example, where a MN initiates an L2 handoff from AP/RAN1 to AP/RAN2 (Note that it may not be the MN which takes decisions on handoffs). It is assumed that when an L2 handoff is initiated, AP/RAN1 and AP/RAN2 perform L2 messaging procedures to negotiate the L2 handoff. Since the MN is not attached to AP/RAN2 yet, FA2 is unaware of the IP address of the MN and cannot send an advertisement to it. Therefore it is necessary for the L2 procedures to interwork with Mobile IP. Once a L2 handoff is initiated, such that AP/RAN2 and AP/RAN1 are in communication, it is possible for AP/RAN2 to solicit an advertisement from FA2 and transfer it to AP/RAN1. Once this is received by the MN, the MN can perform a registration directed to FA2 even though the MN has no data-connection to AP/RAN2 yet. The precise definition of such L2 procedures is outside the scope of Mobile IP. 3. Hierarchical Mobile IPV4 Network The Regional Tunnel Management draft [1] describes a two-level Mobile IPv4 hierarchy (i.e. GFA and one level of FAs). In [1] Annex A briefly describes the possibility of having multiple FA levels. In the generic case, there may be multiple levels of FAs and one (or more) "root" GFA within an administrative domain. The procedures described in this draft do not limit the number of hierarchical levels of FAs. In addition, a MN may be attached directly to any FA within the hierarchy and may move between FAs from different levels in the hierarchy. Figure 2 describes the Hierarchical MIPv4 network. In Figure 2, Access Points (APs) or Radio Access Networks (RANs) consisting of multiple access points, are used to provide a MN with L2 access. These may be connected at any level of the hierarchy as shown in the figure. It is important to note that there may be multiple paths between a MN and the GFA (Gateway Foreign Agent). FA1 and FA2 will be referred to as the lowest level regional FAs in the hierarchy. Also, the "common route" regional FA is defined as the first regional FA in common for the route of communication between hosts connected to regional FAs. In Figure 2, FA3 is the Karim El Malki and Hesham Soliman [Page 6] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 "common route" regional FA for communication between a host connected to FA2 (i.e. CN) and a host connected to FA1 (i.e. MN). A regional FA found in a hierarchical level between the GFA and the lowest level FAs will be referred to as an intermediate regional FA. ________ ________ | | | | | AAAL |------| GFA |______________________ |________| |________| \ / | \ \ ... ... ... \ ____|___ \ | | \ | FA3 |_______________ \ |________| | | ______/_ _\______ | | | | | | | | | FA2 | | FA1 | | | |________| |________| | | ____|___ ____|___ ____|___ ____|___ | | | | | | | | |AP/RAN 2| |AP/RAN 1| |AP/RAN 3| |AP/RAN 4| |________| |________| |________| |________| | ____|___ CN | | | MN | |________| Figure 2: A Multi-level Hierarchical MIPv4 domain As described in [1], a regional FA announces itself and its GFA in the Agent Advertisement; in the first and last address in the care-of address field in the Mobility Agent Advertisement extension [2]. If there is a hierarchy of foreign agents between the GFA and the announcing foreign agent, the foreign agent MAY include the corresponding addresses in order between its own address (first) and the GFA address (last). For example, in Figure 2, FA1 MAY advertise in the order: FA1, FA3 .. GFA. This draft supplements [1] with the following for MIPv4: - limitation of triangle routing for communication between hosts within the administrative domain - Fast Handoffs within the administrative domain - Considerations on Regional Deregistration Regional Tunnel Management allows Regional Registrations within an administrative domain in order to avoid always having to perform registrations through the Home Agent, which is often distant from the Karim El Malki and Hesham Soliman [Page 7] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 Mobile Node's current location. However, it is also part of Regional Tunnel Management that the Mobile Node's registration with the Home Agent be renewed before its expiry. Therefore it is assumed that the MN will send one or more Registrations (using the GFA address as care-of address) to the Home Agent before the MN's Registration lifetime with the Home Agent expires. As specified in Regional Tunnel Management, the GFA's address always appears to the Home Agent as the Mobile Node's care-of address. In this way, some of the Home Agent's functionality is performed locally in the GFA. It is assumed in this draft and in [1] that regional FAs and the GFA share a common security association. 4. Fast Handoffs in Hierarchical MIPv4 When there is a hierarchy of foreign agents between the GFA and the announcing foreign agent, the announcing foreign agent MAY include the corresponding addresses in order between its own address (first) and the GFA address (last) in the Mobility Agent Advertisement extension of its Agent Advertisements. If there are only two hierarchical levels, a foreign agent announces itself and a GFA in the Agent Advertisement; in the first and last address in the care-of address field in the Mobility Agent Advertisement extension. There must be at least one care-of address in the Mobility Agent Advertisement extension. If there is only one care-of address it is the address of the GFA, and the MN is connected directly to it. When the MN receives an Agent Advertisement with a Mobility Agent extension and the "I" bit set, as specified in [1], it should perform actions according to the following movement detection mechanisms. In a Hierarchical Mobile IP network such as the one described in this draft, the MN MUST be: - "Eager" to perform new bindings - "Lazy" in releasing existing bindings The above means that the MN will perform Regional Registrations with any "new" FA from which it receives an advertisement (Eager). The method by which the MN determines whether the FA is a "new" FA is described in [2] and may make use of an FA-NAI extension. However the MN should not release existing bindings until it no longer receives advertisement from the relative FA and the lifetime of its existing binding expires (Lazy). It should be noted that the MN may add a Hierarchical FA extension to Registration Requests in order to identify the exact FA path to be followed by the Registration Request. This extension must not be removed by regional FAs. If the MN has at least one existing binding with a FA, additional Karim El Malki and Hesham Soliman [Page 8] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 simultaneous regional registrations will be performed requesting a short lifetime. This is done in order to limit the lifetime of bindings which the MN only needs temporarily and therefore limit bandwidth usage. This is the case when the MN is moving between FAs and uses Fast Handoffs to achieve loss-less IP mobility. The lifetime of additional "auxiliary" bindings needed for Fast Handoffs is thus limited. The remaining issue is the choice of the appropriate HA address in the Regional Registration Request when the MN has at least an existing active regional binding. Two options follow: 1) Mobility Agent extension advertises FA and GFA address only In this case it is assumed that there is always a single path from the MN to the GFA. The MN will always perform Regional Registrations using the GFA address as HA address and the advertising FA as care-of address. As the Regional Registration Request is relayed towards the GFA, each FA receiving it will check whether it has an existing binding with the MN and whether the Regional Registration has the "S" bit set to request for simultaneous bindings. If this is true and the Regional Registration is validated by the GFA, these FAs will activate the simultaneous binding upon receiving the (successful) Regional Registration Reply from the GFA. Therefore it is not necessary to advertise to the MN all of the FA addresses in the hierarchical branch, thus reducing bandwidth usage over wireless. 2) Mobility Agent Advertisement extension advertises complete order of FAs in the branch In specific cases where multiple regional FA levels and multiple paths from the MN to the GFA are present and are advertised, it may be necessary for the MN to identify the "common route" FA using the complete list of FAs in the hierarchical branch. It is assumed that the GFA advertises only one care-of address on all its interfaces towards the MN. The MN must cache the Mobility Agent Advertisement extensions for its active bindings. When it receives an advertisement from a "new" FA which has a different Mobility Agent Adv. extension, it will be eager to perform a new binding. The MN compares the IP addresses in the new Mobility Agent Adv. extension with the ones it has cached for its active binding(s). If there is an IP address in common between these extensions, named "common route" FA or GFA, the MN will use that IP address as HA address and destination address of its Regional Registration Request in which the "S" bit will be set. The care-of address is the advertising FA's address. The MN may add a Hierarchical FA extension to the Regional Registration Request, in Karim El Malki and Hesham Soliman [Page 9] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 order to identify the regional FA path to be followed by the Request up the hierarchy. A Regional FA receiving a Regional Registration Request with it's own address as HA address may return a Regional Registration Reply to the MN. If there is no IP address in common between the extensions, then the MN must have moved into a new hierarchy and the GFA advertised in the new extension must be different from the one in the previously cached extension(s). When the MN moves between administrative domains (i.e. changes GFA) then the MN should use the new GFA's IP address as care-of address in its new Registration Request to the HA and may add the Hierarchical FA extension as described previously. If the MN has at least one existing active binding when it moves to the new GFA, it may perform a smooth handoff as explained in section 6. The MN is able to perform this option to implement Fast Handoffs only if its binding lifetime with the GFA or HA does not expire during the period needed by the MN to complete its handoff. Intermediate regional FAs are able to accept the MN's regional registration (simultaneous binding) only if the intermediate regional FA has an existing active binding for the MN. The resulting simultaneous binding may therefore have a maximum possible lifetime equal to the lifetime remaining in its previously existing active binding. Once the registration lifetime with the GFA or HA is about to expire, the MN must perform a new Mobile IP registration with the HA. 5. Traffic Routing in Mobile IPv4 Hierarchies The GFA and intermediate regional FAs will hold a binding for a registered MN to the "next" FA in the hierarchical branch towards the MN. The Regional Registration Requests (and Regional Registration Replies if the MN includes the Hierarchical Foreign Agent extension in its Regional Registration Request) containing the Hierarchical Foreign Agent extension [1] will allow a hop-by-hop route along this branch to be created within the hierarchy for the MN. This procedure is specified in [1]. The complete order of FAs in the branch MAY be advertised to the MN in the Mobility Agent Advertisement extension. In this case the Hierarchical Foreign Agent extension MAY be present also in the Regional Registration Reply received by the intermediate regional FAs in the branch and the MN, although the order of the addresses is inverted with respect to that in the Regional Registration Request. If the packets for the MN are tunnelled from the HA, then the GFA should change the source and destination IP address of the encapsulating header to the "next" FA towards the MN. The "next" FA may be the lowest level FA. The same procedure will be performed by intermediate regional FAs if any are present. When packets reach the Karim El Malki and Hesham Soliman [Page 10] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 lowest level FA they will be detunnelled and sent to the MN. Otherwise, if the regional FAs or GFA receive packets for MNs which are not tunnelled (i.e. sent by other hosts within the hierarchy), then routing between MNs in the same hierarchy (or between any host in the hierarchy and a MN in the same hierarchy) may be performed: - through the Home Agent - through the GFA - through any regional FA (including the GFA) In order to reduce triangle routing and the associated unnecessary latency and tunnelling overhead for communication between hosts within the same administrative domain, it is preferred to route using the last two options, namely through the GFA or through any regional FA. As an example of routing through any regional FA, in Figure 2 the path of communication between the CN and the MN would go through FA2, FA3 and FA1. The most efficient routing is using the shortest path through any regional FA. However the decision on which option to adopt depends on the particular implementation and deployment. These two methods will be described in the following sections. I. Routing between nodes only through the GFA Routing between MNs (or between a fixed host and a MN) in the same hierarchy may be performed through the GFA. The GFA does this by tunnelling packets for a MN to the appropriate "next" FA using the information it has for the MN in its Regional binding. Therefore, packets generated within the hierarchical domain and directed to the MN's home address reach the GFA and are tunnelled by the GFA to the "next" regional FA. Following this, the same hierarchical routing procedure described previously for traffic coming from the HA applies. II. Routing between the nodes through any regional FA (shortest path) Routing between MNs (or between a fixed host and a MN) in the same hierarchy may be performed through any regional FA in the hierarchy. Any number of levels of regional FAs may be present in the hierarchy. Packets sent between MNs in the same hierarchy will be routed through the shortest path of connected FAs in the hierarchy. This shortest path goes through the closest regional FA that is able to interconnect the nodes: the "common route" regional FA. In Figure 2, this is FA3. It is possible that the closest regional FA able to interconnect the nodes is the GFA. If the regional FAs or GFA receives packets for the MN which are not tunnelled (i.e. sent by other hosts within the hierarchy) then Karim El Malki and Hesham Soliman [Page 11] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 routing will be performed using any existing active regional bindings by tunnelling packets to the "next" regional FA towards the MN. Following this, the same hierarchical routing procedure described previously for traffic coming from the HA applies. 6. Regional Deregistration for Fast Handoffs Regional deregistration is described in [1]. In this draft we apply the deregistration procedure to Fast Handoffs. When the MN performs a regional registration with a "new" regional FA, then a regional deregistration must be performed with the MN's old location, which may include all the FAs in its old regional branch. This is necessary to avoid incorrect routing of packets (see section 3) by the "previous" FA(s) in the old regional branch during the interval in which the MN has moved but the "previous" FA(s)'s regional binding lifetime for the MN has not yet expired. As stated in [1] it is also a problem since, if old locations are not deregistered, it is possible that tunnels are not correctly redirected when a mobile node moves back to a previous regional foreign agent. The regional deregistration is performed by a regional FA upon the first time it receives a valid Regional Registration Request, without the "S" bit set, from a MN which had previously set the "S" bit in its regional registration(s). This regional FA may respond with a Registration reply and may perform the Regional deregistration by sending a Binding Update with zero lifetime to the "next" regional FA in the MN's old regional branch, setting the Binding Update's care-of address to the the previous care-of address it had registered for the MN (i.e. the "previous" lowest level FA). The Binding Update is relayed down towards the previous care-of address, and each regional foreign agent in the hierarchy receiving this notification removes its binding for the MN. In this way, the MN updates all the Regional FAs in the "old" hierarchical branch between the "common route" FA and the "old" lowest level FA. It is assumed that GFA/FAs within the same hierarchical domain share a Security Association which can be used to perform this deregistration. The MN will be able to perform regional deregistrations through intermediate regional FAs if the GFA shares its GFA-MN security association with the regional FAs (further described in Ch.9). Otherwise the regional deregistration will be performed by the GFA. 7. Smooth Handoffs between Hierarchies (GFAs) When the MN moves between domains it receives Mobility Agent extensions containing a new GFA IP address. The MN registers with its HA using the new GFA IP address as care-of address. In order to improve inter-domain handoffs it may use the Previous Foreign Agent Karim El Malki and Hesham Soliman [Page 12] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 extension in the Regional Registration Request [3]. This results in a smooth handoff between the domains. A new flag is required in the Binding Update message to perform a smooth handoff while maintaining the existing binding in the "previous" FA. This is the "S" bit for the simultaneous binding. This simultaneous binding is necessary in the case in which the MN only momentarily moves "forward" to the new domain, then returns back to the "previous" FA (domain) before its "previous" binding expires. In this case the binding for the MN with the "previous" FA must be maintained. Following is the new Binding Update message with the "S" flag added which replaces one bit of the Reserved space. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type |A|I|M|G|S| Rsv | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Mobile Node Home Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Care-of Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Identification + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extensions ... +-+-+-+-+-+-+-+- 8. IPv6 Considerations A Hierarchical model which supports the Fast Handoffs concept for MIPv6 is described in [4]. 9. Security Considerations As in [1], it is assumed that the mobility agents within a hierarchical domain (i.e. GFA and Regional FAs) share a security association. The GFA address is the MN's global care-of-address stored in the HA. The GFA may therefore authenticate the MN by interacting with the AAAL and receives the MN-FA session key information. Since FAs at all levels of the hierarchy should be able to authenticate MN (Regional) Registrations, it should be possible for the GFA to distribute this MN-FA key to other Regional FAs in the path taken by the (Regional) Registration Reply message from GFA to MN. This may be performed using the hierarchical domain's shared SA and a new GFA-FA key extension added to the Regional Registration Karim El Malki and Hesham Soliman [Page 13] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 Reply message sent by the GFA through Regional FAs to the MN. This new extension should be added to the Registration Reply message by the GFA, read by all Regional FAs in the path to the MN and removed by the lowest-level FA before forwarding the Reply to the MN. This new extension should be a subtype of the Generalized Key Reply Extension and is to be specified later. 10. Acknowledgements This draft replaces the previous draft on Fast Handoffs (draft-elmalki-mobileip-fast-handoffs-01). The authors would like to thank the co-authors of that draft: N. A. Fikouras and S. Cvetkovic. 11. References [1] E. Gustafsson, A. Jonsson and C. Perkins, " Mobile IP Regional Tunnel Management ", draft-ietf-mobileip-reg-tunnel-02.txt (work in progress), March 2000. [2] C. Perkins, Editor. "IP Mobility Support", RFC 2002, October 1996. [3] C. Perkins and D. Johnson, "Route Optimization in Mobile IP", draft-ietf-mobileip-optim-09.txt (work in progress), February 2000. [4] H. Soliman and K. El Malki, "Hierarchical Mobile IPv6 and Fast Handoffs", draft-soliman-mobileip-hmipv6-00 (work in progress), June 2000 12. Addresses The working group can be contacted via the current chairs: Basavaraj Patil Phil Roberts Nokia Corporation Motorola M/S M8-540 6000 Connection Drive 1501 West Shure Drive Irving, TX 75039 Arlington Heights, IL 60004 USA USA Phone: +1 972-894-6709 Phone: +1 847-632-3148 EMail: Raj.Patil@nokia.com EMail: QA3445@email.mot.com Fax : +1 972-894-5349 Karim El Malki and Hesham Soliman [Page 14] INTERNET-DRAFT Fast Handoffs in MobileIPv4 July 04, 2000 Questions about this memo can be directed to: Karim El Malki Ericsson Radio Systems AB Access Networks Research SE-164 80 Stockholm SWEDEN Phone: +46 8 7573561 Fax: +46 8 7575720 E-mail: Karim.El-Malki@era.ericsson.se Hesham Soliman Ericsson Australia 61 Rigall St., Broadmeadows Melbourne, Victoria 3047 AUSTRALIA Phone: +61 3 93012049 Fax: +61 3 93014280 E-mail: Hesham.Soliman@ericsson.com.au This Internet-Draft expires in January 2001. Karim El Malki and Hesham Soliman [Page 15]