MOBILE-IP Working Group Youngjune L. Gwon (Editor) INTERNET-DRAFT Atsushi Takeshita Expires: August 23, 2001 DoCoMo USA Labs February 23, 2001 Network Layer Triggered Mobile IPv4 Predictive Handoff Status of this memo This document is an individual contribution for consideration by the Mobile IP Working Group of the Internet Engineering Task Force. Comments should be submitted to the mobile- ip@standards.nortelnetworks.com mailing list. Distribution of this memo is unlimited. 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 draft describes a network layer (layer 3) handoff-triggering mechanism and associated Mobile IPv4 predictive handoff scheme that is free of the link layer (layer 2) triggering events. By utilizing layer 3 information only, the network layer mobility prediction (L3MP) mechanism first predicts the future per packet latency. Then, the L3MP selects the future access point given the predicted values in advance. The future access point suggestion by the L3MP triggers the Mobile IP predictive handoff. The predictive handoff scheme described in this draft encompasses three significant steps before switching the actual access point: L3MP handoff triggering, pre-register, and route pre-optimization. Table of Contents 1. Introduction 2. Terminology Gwon, et al. Expires August 23, 2001 [Page 1] INTERNET-DRAFT Layer 3 Triggered MIPv4 Predictive Handoff Feb 2001 3. Scope of the Solution 4. Link Layer Independency 5. Access Network Considerations 5.1 Layer 3 Signaling from Different Nearby Base Stations 5.2 Signal Strength vs. Per Packet Latency 6. Network Layer Mobility Prediction (L3MP) 6.1 L3MP Latency Prediction (L3MP-LP) 6.2 L3MP Access Point Selection (L3MP-APS) 6.3 Compact Layer 3 Beacon Format 7. Specifying Handoff Triggering Events 8. Mobile IPv4 Predictive Handoff 8.1 Layer 3 Trigger 8.2 Pre-register 8.3 Route Pre-optimization 9. Security Considerations 10. Acknowledgement 11. References 12. Author's Addresses 1. Introduction A number of Mobile IP fast handoff schemes currently proposed in the IETF Mobile-IP Working Group require a handoff trigger specified by the link layer (layer 2) events. The layer 2 triggering events are interpreted to initiate the Mobile IP handoff at the network layer (layer 3). Although a handoff solution should not depend on the mobile wireless access technologies, it is an obscure task to universally specify the layer 2 triggering events from different access technologies. Furthermore, one cannot expect the layer 2 triggers from every wireless device. To avoid this air-interface (physical layer or layer 1) and layer 2 diversity issues, a generalized layer 3-contained methodology or layer 3 triggering mechanism that is independent of the access technologies (layers 1 and 2) must be developed. This draft considers both the layer 3 mobility prediction mechanism (L3MP) and the L3MP triggered predictive Mobile IPv4 handoff. By utilizing the layer 3 information only, the L3MP is an association of two sub-functions: Latency Prediction (L3MP-LP) and Access Point Selection (L3MP-APS). The L3MP-LP predicts the future per packet latency based on the present and the previous latency samples obtained at the layer 3. Thus, the L3MP-LP reflects the future layer 3 communication quality. The L3MP-APS selects the future access point given multiple L3MP-LP values for different access points (e.g. mobility agents and access routers) in prior to an occurrence of the actual handoff. The Mobile IPv4 predictive handoff scheme presented in this draft encompasses three steps performed in advance: predictive handoff triggering, pre-register, and route pre-optimization. Gwon, et al. Expires August 23, 2001 [Page 2] INTERNET-DRAFT Layer 3 Triggered MIPv4 Predictive Handoff Feb 2001 2. Terminology This draft uses terms defined in Mobile IPv4 and other related documents. In addition, this document uses the following terms: Layer 3 Triggering Events Layer 3 contained process that initiates the Mobile IP predictive handoff. L3MP An acronym for layer 3 mobility prediction or network layer mobility prediction (See network layer mobility prediction). L3MP-LP An acronym for L3MP-latency prediction is a process of predicting the future value of per packet latency using the present and previous latency samples. Latency samples are obtained by using ICMP timestamp option on layer 3 beacon. Timestamp is marked by the mobility agents and the mobile node retrieves the packet traveling time upon the reception of the layer 3 beacon. Thus, L3MP-LP is a prediction based layer 3 link evaluation capability. L3MP-APS An acronym for L3MP-access point selection is a process of choosing the access point (access routers or mobility agents) given the L3MP-LP prediction values. Network Layer Mobility Prediction A mechanism that chooses the future access point for a mobile node by encompassing two separate processes: latency prediction (L3MP-LP) and access point selection (L3MP-APS). Pre-register Mobile IPv4 registration before switching the actual access point. Pre-register can only be successful if the handoff of a mobile node is correctly anticipated in timely manner. Otherwise, it will only increase the layer 3 control signaling due to unnecessary pre-register. Route Pre-optimization Route optimization [2] before switching the actual access point, i.e. establishing direct routes between the mobile and correspondent nodes before the actual handoff. 3. Scope of the Solution This draft specifies both the layer 3 handoff-triggering mechanism, namely the L3MP, and the Mobile IPv4 predictive handoff scheme that is triggered by the L3MP. This draft specifies the layer 3 triggering events and the methodology of obtaining such events. This draft also speficies the predictive handoff extentions to the base Gwon, et al. Expires August 23, 2001 [Page 3] INTERNET-DRAFT Layer 3 Triggered MIPv4 Predictive Handoff Feb 2001 Mobile IPv4 specification [1] and the route optimization extention [2]. 4. Link Layer Independency This draft describes yet another approach that utilizes the information available at the layer 3 only for the Mobile IP handoff trigger. It is necessary to develop the layer 3-contained handoff triggering events for the following reasons: a) It is difficult to specify layer 2 triggering events if the access technologies (layers 1 and 2) differ. b) If changes in access technology should not affect layer 3, faster deployment of new access technology is possible. c) Broader concept of handoff such as inter-access technology handoff can be implemented easily, e.g. WLAN to cellular handoff. d) One cannot expect and specify all possible layer 2 triggers from every type of wireless devices. 5. Access Network Considerations This draft generally considers fully IP-based network technologies and therefore possibly beyond the third generation networks. However, the scope of this draft should not be restricted to the fully IP-based or beyond the third generation network technologies. 5.1 Layer 3 Signaling from Different Nearby Base Stations A mobile node is in charge of detecting the layer 3 beacons that are used for layer 3 prediction. This draft considers that the mobile node may simultaneously listen to multiple layer 3 signaling beacons transmitted by different nearby base stations. For the current or the third generation systems, layer 3 beacon may have to be transmitted over either the signaling channel or common shared channels to resolve the situation. 5.2 Signal Strength vs. Per Packet Latency Obtaining per packet latency is a layer 3 analogous process that is similar to the periodic beacon pilot strength measurement occurred at lower layers (layers 1 and 2). Beacon pilot strength measurement is reflected in terms of SIR (CDMA), SNR (TDMA), or raw signal power depending on wireless access technologies. To accomplish the layer 2 like measurement at layer 3, a compact layer 3 beacon packet should be introduced. Furthermore, per packet latency should be predicted. A successful prediction creates a time gap that can be used for preparing the Gwon, et al. Expires August 23, 2001 [Page 4] INTERNET-DRAFT Layer 3 Triggered MIPv4 Predictive Handoff Feb 2001 Mobile IP handoff in advance. 6. Network Layer Mobility Prediction (L3MP) The L3MP is a mechanism that resides within a mobile node. The mobile node is responsible to carry out the L3MP operation by capturing the layer 3 beacons periodically transmitted by the nearby base stations. The compact layer 3 beacon format is described in 6.3. The L3MP consists of two parts: latency prediction (L3MP-LP) and access point selection (L3MP-APS). L3MP-LP is the layer 3 link evaluation capability by predicting the future value of per packet latencies with respect to the different foreign agents or access routers. Then, the foreign agent that is predicted for the best future performance, i.e. giving the smallest L3MP-LP value, is selected by the L3MP-APS. Figure 1 depicts an overall diagram of the L3MP operation in the access network. ____ _____ L3 Beacon ____ L3 Beacon _____ ____ |AR1 |-|BTS1 | ==========> | MN | <========== |BTS2 |-|AR2 | ---- ----- ---- ----- ---- Figure 1: Mobile Node Operating L3MP in the Access Network 6.1 L3MP Latency Prediction (L3MP-LP) There are a number of ways to predict the future value given the present and past per packet latency samples that are obtained by ICMP timestamp on layer 3 beacons. There are some important observations: a) The variation of per packet latency over time, T(t), is a stochastic process. b) The obtained present and past per packet latency samples are samples of T(t). c) More recent latency samples are more correlated to the future outcome of the latency. d) It may be necessary that distorted samples should be sampled out for the prediction accuracy. Estimation theory may be applied to compute the future expected value of the per packet latency given the present and past samples. However, unless the first and the second order distribution of the latency samples are known or properly modeled, estimating the future expected value of a stochastic process involves the complex computation of auto-and-cross correlation matrices. Although relying on estimation theory is a solution, its applicability remains Gwon, et al. Expires August 23, 2001 [Page 5] INTERNET-DRAFT Layer 3 Triggered MIPv4 Predictive Handoff Feb 2001 questionable in reality. On the other hand, adaptive approach is a practical solution that involves less computational complexity. Adaptive L3MP-LP is depicted in Figure 2. The present and previous latency samples and error are inputs for the adaptive predictor. Adaptive prediction is iterative process that the prediction error is also fed back for computation. ^ / / ------------------ T_present ====> | | | | T_previous_1 ====> | | | Adaptive |==========> T_predicted T_previous_2 ====> | Predictor | | | | | .... | | | ------------------ | / | - / T_future + =====> error / | /_____________________| Figure 2: Adaptive L3MP-LP The adaptive prediction accuracy is related with the number of previous samples as its inputs. It can also be modeled in terms of the differences of consecutive samples. A popular algorithm such as least mean square (LMS) method [3] can be applied to efficiently compute adaptive prediction. Gauss-Markov approximation of the user mobility may further reduce the computational burden by using a couple of the previous samples. 6.2 L3MP Access Point Selection (L3MP-APS) Mapping from the predicted future latency value to the future access point selection is performed by a simple method. Among all communicable mobility agents or access points, one that gives the smallest latency prediction value is chosen, e.g. there may be 6 possible nearby base stations connected to different mobility agents in optimal hexagonal cellular network configuration. 6.3 Compact Layer 3 Beacon Format Although there is no definite requirement of layer 3 beaconing period, the layer 3 beacon format must be as compact as possible. Access point entities are required to send this layer 3 beacon so Gwon, et al. Expires August 23, 2001 [Page 6] INTERNET-DRAFT Layer 3 Triggered MIPv4 Predictive Handoff Feb 2001 that any mobile node capable of listening can measure the per packet latency for L3MP. Figure 3 shows a compact layer 3 beacon format for this purpose. ICMP timestamp is included for layer 3 detection and measurement purposes. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + IP Header with Agent's IP address (20 Bytes) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Authentication (Optional) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + ICMP Timestamp (20 Bytes) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Compact Layer 3 Beacon Format 7. Specifying Handoff Triggering Events Layer 3 handoff triggering events are specified by the output of the L3MP, i.e. in the form of mobility agent's or access router's IP addresses, so that a mobile node can actively contact them to carry out the predictive handoff in timely manner. It is noteworthy that the L3MP operates on the basis of optimizing the future latency performance of the mobile node. If multiple agent addresses are available, the preference for each agent also has to be specified. Once the addresses of the mobility agents or access routers are available, the mobile node may start contacting the candidate agent by sending pre-register request. 8. Mobile IPv4 Predictive Handoff The Mobile IPv4 predictive handoff is really a temporary simultaneous binding of a mobile node to two or more care-of-addresses. 8.1 Layer 3 Trigger As specified in Section 7, a layer 3 triggering event causes the predictive handoff to be initiated. The layer 3 trigger is created by the L3MP to provide the candidate mobility agent's IP addresses used for pre-register and route pre-optimization. 8.2 Pre-register Then, the mobile node sends pre-register request to the nFA via oFA Gwon, et al. Expires August 23, 2001 [Page 7] INTERNET-DRAFT Layer 3 Triggered MIPv4 Predictive Handoff Feb 2001 while maintaining the connection with the oFA. This is illustrated in Figure 3. If other secure (not via oFA) way to pre-register is possible, then the mobile node may do it so. It is also necessary if regional registration is not supported, the mobile node should pre-register with its home agent. Pre-register request via oFA ------ ------- ------ | MN |=======>| oFA |=======>| nFA | | |<=======| |<=======| | ------ ------- ------ Pre-register reply via oFA Figure 3: Pre-register to nFA via oFA It should be noted that pre-register has relatively shorter lifetime compared to the registration. The mobile node is responsible for renewing the pre-register to the (regular) Mobile IP registration after actually switching the access point. Pre-regsiter request and reply packet formats are identical to the registration request and reply formats except for the different type numbers (TBD) and the shorter lifetimes. 8.3 Route Pre-optimization Route pre-optimization sets up the direct and optimized routes between the mobile node and its correspondent nodes in prior to the actual handoff. This draft adds two new messages to the route optimization draft [2]: Pre-binding update and route pre-optimization completion messages. These two new messages are very similar to the existing binding update and binding acknowledge messages except for the different type numbers (TBD) and the shorter lifetime. 9. Security Considerations Any mobile node with proper wireless transceiver is capable of capturing the layer 3 beacon in the access network. Therefore, the L3MP operation can create possible security attacks in the access network. It is generally recommended that authentication header should be included in the layer 3 beacon format. Since the nature of the L3MP is to provide the predictive handoff trigger at the layer 3 by predicting the future access point in timely manner, unnecessary compuation regarding the authentication that may cause an extra L3MP operational overhead should be avoided. Security considerations regarding the predictive handoff extention described in this draft conforms with those of the base Mobile IPv4 specification [1]. 10. Acknowledgements Gwon, et al. Expires August 23, 2001 [Page 8] INTERNET-DRAFT Layer 3 Triggered MIPv4 Predictive Handoff Feb 2001 The authors of this draft appreciates advisors from the DoCoMo Communications Laboratories USA's Advisory Board for their valuable comments and inputs. The authors are also in gratitude to our colleagues at Multimedia and Wireless Laboratories of NTT DoCoMo. 11. References [1] Charles Perkins, "IP Mobility Support," RFC 2002, October 1996 [2] C. Perkins and D. Johnson, "Route Optimization in Mobile IP," draft-ietf-mobileip-optim-10.txt (work in progress), November 2000 [3] B. Widrow and S. Stearns, "Adaptive Signal Processing," 1985 by Prentice Hall 12. Authors' Addresses Questions about this memo can be directed to: Youngjune Gwon DoCoMo Communications Laboratories USA, Inc. 181 Metro Drive, Suite 300 San Jose, CA 95110 USA Phone: +1 408 451 4734 Email: gyj@dcl.docomo-usa.com Fax: +1 408 573 1090 Atsushi Takeshita DoCoMo Communications Laboratories USA, Inc. 181 Metro Drive, Suite 300 San Jose, CA 95110 USA Phone: +1 408 451 4705 Email: takeshita@dcl.docomo-usa.com Fax: +1 408 573 1090 Gwon, et al. Expires August 23, 2001 [Page 9]