Mobile-IP Working Group Yong-Geun Hong Internet Draft Myung-Ki Shin draft-hong-mobileip-applicability-00.txt Hyoung-Jun Kim Expires: December 2003 ETRI Woo-Suck Cha Gi-Hwan Cho Chonbuk University Considerations of FMIPv6 in 802.11 networks draft-hong-mobileip-applicability-00.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 to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document describes the applicability of Fast Handovers for Mobile IPv6 in 802.11 networks. Fast Handovers for Mobile IPv6 proposes a set of protocol enhancements to reduce handover latency due to IP protocol operations as small as possible by the help of L2 information. 802.11 networks are considered as a popular wireless infrastructure to meet broadband network service requirement in the future. If Fast Handovers for Mobile IPv6 is applied to wireless network, 802.11 might be a practicable target for deployment. At this point some Fast Handover methods for Mobile IPv6 are thought to be directly applicable to 802.11 networks and some others are not. Hong, et al. Expires: December 2003 [Page 1] Considerations of FMIPv6 in 802.11 networks June 2003 Table of Contents 1. Introduction...................................................2 2. Terminology....................................................3 3. Applicability of FMIPv6 in 802.11 networks.....................4 3.1 Layer 2 Handover in 802.11 networks........................4 3.1.1 Scan procedure.......................................5 3.1.2 Reassociation procedure..............................5 3.2 L2 triggers................................................5 3.2.1 L2 triggers applicable to FMIPv6.....................5 3.2.2 L2 triggers applicable to 802.11 networks............6 3.3 Fast handover mechanisms in FMIPv6.........................6 3.4 Considerations of the fast handovers in 802.11 networks....7 4. Security Considerations........................................8 References........................................................8 Acknowledgments...................................................8 Author's Addresses................................................9 1. Introduction Fast Handovers for Mobile IPv6 (FMIPv6) [1] describes a set of protocol enhancements to reduce handover latency due to IP protocol operations as small as possible by the help of L2 handover information. FMIPv6 allows a Mobile Node (MN) to keep using its Care of Address (CoA) until it establishes itself as a Mobile IPv6 end- point of its New Access Router (NAR) and expeditiously establish a new CoA. 802.11 networks is a popular wireless network nowadays. If the FMIPv6 is applied to wireless network, 802.11 might be a practicable target for deployment. The basic idea of FMIPv6 is to set up a routing path (tunnel) between two access routers (so, PAR and NAR) to enable a MN to send and receive IP packets while the MN establishes itself as a Mobile IPv6 end-point. In FMIPv6, L2 triggers by the MN or the network (i.e., PAR or NAR) could be used to establish this tunnel. The anticipation handover initiation of FMIPv6 is significant only whenever these triggers are invoked before a new wireless link is established. If not, the triggers are used for reducing the L3 movement detection overhead. Triggers which are used for initiating a handover must have some information to assist the fast handover procedure thereafter. The information could include a link-layer identifier, such as a base station id or BSSID in 802.11 networks. In the previous draft of Mobile IPv6 Fast Handover for 802.11 Networks [2], the 802.11 handover operations are divided into 6 steps : scan, join, authentication, association or reassociation, IAPP operations and sending L2 update frame. In the paper of Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process [3], Hong, et al. Expires: December 2003 [Page 2] Considerations of FMIPv6 in 802.11 networks June 2003 the scan (probe) phase is the dominating component of the overall L2 handover delay (the scan phase accounts for more than 90% delay of L2 handover). For the efficiency of FMIPv6, it is clear that triggers must be made before or during the scan phase. In 802.11 network environments, if triggers are invoked after the scan phase, so 90% of L2 handover time passed away, these triggers may be few effective to FMIPv6. Even if the MN can invoke triggers during or before the scan phase with a prospective AP, it might be impossible to guarantee that the MN will move to the AP actually. In 802.11 networks, it therefore can not be guaranteed to go ahead the L3 handover before the completion of L2 handover. In this draft, when FMIPv6 is applied to 802.11 networks, its applicability is dealt with. With considering the deployment of 802.11 networks, some L2 triggers might not be always applicable to FMIPv6 handover. L2-ST and L2-TT might not be applicable, since L2 handover in 802.11 networks must be initiated by the MN. L2-MT can be selectively applicable under the particular conditions. On the other hand, L2-LU and L2-LD might be applicable to FMIPv6 in 802.11 networks. These are meaningful since they eliminate the L3 movement detection time. 2. Terminology L2 handover Layer 2 handover L3 handover Layer 3 handover AP Access Point PAP Previous Access Point NAP New Access Point MN Mobile Node PAR Previous Access Router NAR New Access Router RtSolPr Router Solicitation Proxy PrRtAdv Proxy Router Advertisement F-BU Fast Binding Update CoA Care of Address Hong, et al. Expires: December 2003 [Page 3] Considerations of FMIPv6 in 802.11 networks June 2003 AAR Anchor Access Router HI Handover Initiate HACK Handover Acknowledge HTT Handover To Third 3. Applicability of FMIPv6 in 802.11 networks 3.1 Layer 2 Handover in 802.11 networks L2 handover refers to the mechanism of messages exchanged by AP and MN resulting in a transfer of physical point of network connectivity. Thus L2 handover is a link layer function carried out by at least three participating entries, MN, PAP, and NAP. When a MN moves from its current AP to another AP, there is no standardized method which gets the MN to initiate L2 handover operations. Usually, it is implemented as a result of decaying the signal quality condition on the current AP. That is, when a MN approaches to the border of its current AP's coverage area, the MN repeatedly monitors the link signal quality to the current AP. The signal values are reiteratively compared with a predefined threshold. If the value becomes smaller than the threshold, the MN will initiate L2 handover procedure. Figure 1 depicts the message sequence in 802.11 handover procedure [4, 5]. MN neighbor APs -------------------|----Probe Request ----------->+-----+ ^ |<------------ Probe Reply-----| AP1 | | | +-----+ | Scan |----Probe Request ----------->+-----+ | Procedure |<------------ Probe Reply-----| AP2 | | | +-----+ | | : : v | : : -------------------| (Choose a NAP) ^ |---Authentication Request --->+-----+ | Reassociation |<--- Authentication Reply-----| | | Procedure | | New | | |---Reassociation Request ---->| AP | v |<--- Reassociation Reply------+-----+ -------------------| Figure 1: 802.11 L2 handover operation Hong, et al. Expires: December 2003 [Page 4] Considerations of FMIPv6 in 802.11 networks June 2003 L2 handover in 802.11 can be divided into two distinct procedures in the time consuming point of view : scan procedure and reassociation procedure as described below. 3.1.1 Scan procedure Whenever L2 handover is initiated, the MN needs to find out a potential AP to be associated with. This is accomplished by a scan procedure which is a MAC layer function. The Scan Procedure consists of the following steps. The MN sends "Probe Request" frames to an assigned channel and listens for "Probe Response" frames from all APs on the channel. These are iterated until all range of channels are scanned. The MN creates a list of APs prioritized by the signal strength of received of "Probe Response" frames. The MN selects an AP as a prospective NAP that has the strongest signal strength among them. 3.1.2 Reassociation procedure The MN now attempts to reassociate with the prospective NAP selected during the scan procedure was performed. The reassociation procedure typically involves an authentication and a reassociation with the NAP and requires the following steps to be taken. The MN sends an "Authentication Request" frame to the NAP. If the MN has been correctly authenticated by the NAP, the NAP replies with an "Authentication Response" frame to the MN. Upon successful authentication process, the MN sends a "Reassociation Request" frame to the NAP. If the MN has been correctly reassociated, the NAP replies with a "Reassociation Response" frame to the MN. When the MN receives a "Reassociation Response" frame, L2 handover procedure completes. Based on L2 handover procedure, L2 triggers are classified and identified whether they are applicable to FMIPv6 and to IEEE 802.11 networks respectively. 3.2 L2 triggers 3.2.1 L2 triggers applicable to FMIPv6 FMIPv6 relies on the existence of generic L2 triggers. Fast handover mechanisms in FMIPv6 make use of the following L2 triggers : L2-MT, L2-ST, L2-TT, L2-LU, L2-LD. These triggers can be classified into two types based on the time of their occurrence. L2-MT, L2-ST and L2-TT may be taken place in the link of the current AP before the new link of a NAP is configured. L2-LU and L2-LD occur after the new link of a NAP is connected and represent the completion of L2 handover. These triggers contain the L2 handover information causing Hong, et al. Expires: December 2003 [Page 5] Considerations of FMIPv6 in 802.11 networks June 2003 the trigger to be fired, what entities will receive the trigger and some parameters like as the L2 address of entities. 3.2.2 L2 triggers applicable to 802.11 networks Note that L2 triggers have to contain the information about the next attachment point where the MN will move to. Particularly, this information is very important to L2-MT, L2-ST, and L2-TT which make use of it to initiate L3 handover procedure before the link of a NAP is connected. Again, 802.11 handover is divided into two procedures : scan procedure, reassociation procedure. Since the information such as the NAP's L2 address is determined after the scan procedure is successfully finished. L2-MT, L2-ST, and L2-TT may be initiated after the scan procedure. But, L2-LU and L-LD may be naturally initiated when the reassociation procedure is successfully finished. As a result of an analysis of 802.11 handover procedure, L2-LU and L2-LD can be naturally applicable to 802.11 networks. These triggers are useful since they eliminate the L3 move detection time. However, L2-MT, L2-ST, and L2-TT can not be applicable to 802.11 networks. There are basically two main reasons why they are. - First, according to the resulting of an empirical analysis of the IEEE 802.11 MAC Layer Handoff Process described in [3], the scan procedure delay is more than 90% of the overall L2 handover delay. The NAP's L2 information involved by L2 triggers may be acquired after the scan procedure, L2- MT, L2-ST, and L2-TT containing the NAP's L2 information can be taken place after scan procedure is performed. Since the scan procedure consumes most all L2 handover delay, these triggers might be few effective to FMIPv6. - Seconds, the MN's movement might be influenced by the ping- pong effect and the radio conditions, even the authentication or the reassociation between the MN and the prospective NAP may be failed. Therefore, there is no guarantee that the MN will actually move to the NAP selected during the scan procedure. Nevertheless, some considerations for applicability of L2-MT are discussed to improve the fast handover in 802.11 networks in section 3.4 3.3 Fast handover mechanisms in FMIPv6 In the anticipation handover initiation described in FMIPv6 [1], the MN or the PAR may initiate a fast handover by sending the protocol messages such as RtSolPr or PrRtAdv. For the mobile- initiated anticipated handover, the MN sends a RtSolPr message to the Hong, et al. Expires: December 2003 [Page 6] Considerations of FMIPv6 in 802.11 networks June 2003 PAR, but on the other hand, in the case of network-initiated anticipated handover the PAR sends a PrRtAdv message to the MN. The RtSolPr message sent by the MN requests some information the IP address, L2 address and network prefix of the NAR which the MN will move to. The PrRtAdv message sent by the PAR then supplies the information requested by the RtSolPr message to the MN. Once the MN receives the PrRtAdv message from the PAR, the MN responds to the PAR with a F-BU message which is binding to the new CoA. In optimization using link-layer assisted features, it describes how the tunnel between the PAR and the NAR could be set up by using L2 triggers, without IP signaling messages, so RtSolPr or PrRtAdv. On the other hand, the anticipation handover initiation method is not considering a specific L2 technology. Either the PAR that receives L2-ST or NAR that receives L2-TT initiates a tunnel establishment between PAR and NAR. The tunnel establishment is performed by exchanging HI/HACK messages between them. In three party handover, three party handover method occurs when the MN is on the AAR, and moves to PAR, then moves to NAR before completing MIP registration at PAR. Hence PAR that receives L2-ST (or NAR that receives L2-TT) from Layer 2 must inform NAR to contact AAR about changing the radio directed end of the tunnel. So the PAR and NAR exchange a HTT/HI or HACK/HTT pair. Finally the NAR must perform a new tunnel establishment with AAR. When the tunnel re-establishment is successful, the previous tunnel between AAR and PAR is canceled. Note that even if FMIPv6 was only considering L2-ST and L2-TT triggers in the optimization using the link-layer assisted features and the three party handover, it would be meaningful to make use of L2-MT trigger if it is helpful to the fast handover in 802.11 networks. 3.4 Considerations of the fast handovers in 802.11 networks As mentioned earlier, since L2 handover in 802.11 networks is always initiated by MN, L2-ST and L2-TT are not basically applicable. However, L2-MT might be selectively applicable to improve the fast handover in 802.11 networks, in the particular environments that the MN's movement pattern is known ahead, so anticipated, and/or that the radio condition of 802.11 networks is stable, so determinant. The method to anticipate the MN's move direction is out of scope in this document. To make use of L2-MT trigger, one possible way is to start the fast handover procedure on deciding the prospective NAP, so finishing the scan procedure. Assuming that the authentication or the reassociation after the fast handover is few possible to be failed in the particular environments, L2-MT must improve the fast handover in Hong, et al. Expires: December 2003 [Page 7] Considerations of FMIPv6 in 802.11 networks June 2003 802.11 networks. More aggressively, L2-MT may be triggered on deciding the prospective NAP even during the scan procedure. When the MN meets an AP which has the signal strength over a pre-defined threshold, it assigns the AP as the prospective NAP, and starts the fast handover procedure without concerning the scan procedure thereafter. To do this, the MN must terminate intentionally the scan procedure in 802.11 handover, when it decides the prospective NAP. As a result the anticipation handover initiation in FMIPv6 is now applicable in 802.11 networks. A MN received L2-MT sends a RtSolPr message to the PAR, then goes ahead the fast handover procedure. 4. Security Considerations The security issues are not studied yet. References [1] Koodli, R., "Fast Handovers for Mobile IPv6", draft-ietf-mobileip-fast-mipv6-06 (work in progress), March 2003. [2] P. McCann, "Mobile IPv6 Fast Handovers for 802.11 Networks", draft-mccann-mobileip-80211fh-01.txt (work in progress), October 2002. [3] Mitra, A., Shin, M., and Arbaugh, W., "An empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process", CS-TR-4395, University of Maryland Department of Computer Science, September 2002 [4] "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications", ANSI/IEEE Std 802.11. 1999 Edition [5] Lucent Technologies, "roaming with WaveLAN/IEEE 82.11 WaveLAN" Technical ulletin 021.A, Dec. 1998 Acknowledgments Thanks to Pete McCann for his pioneering Internet Draft "Mobile IPv6 Fast Handovers for 802.11 Networks". Thanks to the authors of "An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process" for providing the detailed analysis of the 802.11 handoff process. Hong, et al. Expires: December 2003 [Page 8] AR-Based MD and CoA Configuration June 2003 Author's Addresses Yong-Guen Hong ETRI PEC 161 Gajeong-Dong, Yuseong-Gu, Daejeon 305-350, Korea Tel : +82 42 860 6447 Fax : +82 42 861 5404 E-mail : yghong@etri.re.kr Myung-Ki Shin ETRI PEC 161 Gajeong-Dong, Yuseong-Gu, Daejeon 305-350, Korea Tel : +82 42 860 4847 Fax : +82 42 861 5404 E-mail : mkshin@etri.re.kr Hyoung-Jun Kim ETRI PEC 161 Gajeong-Dong, Yuseong-Gu, Daejeon 305-350, Korea Tel : +82 42 860 6576 Fax : +82 42 861 5404 E-mail : khj@etri.re.kr Woo-Suck Cha Chonbuk University 664-141 Duckjin-Dong Duckjin-Gu Jeonju Jeonbuk 561-756, Korea Tel : +82 63 270 3437 Fax : +82 63 270 3403 E-mail : wscha@dcs.chonnuk.ac.kr Gi-Hwan Cho Chonbuk University 664-141 Duckjin-Dong Duckjin-Gu Jeonju Jeonbuk 561-756, Korea Tel : +82 63 270 3437 Fax : +82 63 270 3403 E-mail : ghcho@dcs.chonnuk.ac.kr Hong, et al. Expires: December 2003 [Page 9]