IETF Seamoby Working Group Dirk Trossen INTERNET-DRAFT Govind Krishnamurthi draft-ietf-seamoby-cardiscovery-issues-00.txt Hemant Chaskar Expires: January 2002 Nokia James Kempf Sun Microsystems, Inc. July 2001 Issues in candidate access router discovery for seamless IP handoffs 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 made obsolete 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. Copyright Notice Copyright (c) The Internet Society (2001). All rights reserved. ABSTRACT Handoff in IP mobility protocols involves moving a mobile node's Layer 3 routing reachability information from one access router to another, before or after the mobile node has established a Layer 2 connection with the radio access point that is covered by the new access router. In addition, other context information about the mobile node's packet session may be transferred from the old access router to the new one, in order to minimize the service disruption during the handoff process. While the exact details of how this is accomplished vary depending on the IP mobility and seamless handoff protocols, one common thread required for seamless handoffs is identifying the candidate access routers for the mobile node's handoff. When a collection of candidates has been identified, an algorithm is run to determine the target access router, and this router becomes the target of handoff at Layer 3. This document presents a problem statement describing issues involved in designing a candidate access router discovery protocol. The document does not discuss the algorithm by which the actual target router is identified, nor how the handoff to the target is achieved. Trossen, Krishnamurthi, Chaskar, Kempf [Page i] INTERNET-DRAFT Candidate Access Router Discovery Issues July, 2001 TABLE OF CONTENTS 1. INTRODUCTION 1 2. TERMINOLOGY 1 3. CANDIDATE ACCESS ROUTER DISCOVERY PROBLEM 2 3.1 Anticipated CAR Discovery 2 3.2 Dynamic CAR Discovery 3 3.3 Hybrid CAR Discovery 3 4. SPECIFIC ISSUES IN CAR DISCOVERY 3 4.1 Identifying PAARs 3 4.2 Identifying capabilities of PAARs 4 4.3 Security considerations 5 5. REFERENCES 5 6. AUTHORS' ADDRESS 6 ACKNOWLEDGMENTS Special thanks are due to John Loughney (Nokia) for his input during the preparation of this document. Trossen, Krishnamurthi, Chaskar, Kempf [Page ii] INTERNET-DRAFT Candidate Access Router Discovery Issues July, 2001 1. INTRODUCTION IP mobility protocols enable mobile nodes (MNs) to change the access routers (ARs) by which they access the Internet. An AR providing Internet connectivity to the MN changes when movement of the MN causes a change in the radio access point through which the MN communicates with the wired network, such that the new radio access point is in a new subnet. There are existing solutions [1, 2] that enable MNs to execute handoffs between the ARs. Additionally, work is underway [3, 4, 5, 6, 7], to define protocols that would allow seamless, meaning low latency and low packet loss, handoffs of MNs between the ARs. These handoff solutions assume that the identity of the new AR is known to the old AR. They do not provide a solution to the problem of discovering the target AR for the MN's handoff. What is required is a protocol that would allow an AR or an MN to discover the neighboring ARs for handoff, along with their capabilities. This information can be used for selecting the actual target AR for the MN's handoff. This draft discusses the issues involved in the problem of candidate AR discovery. The draft does not discuss the problem of actually selecting the target AR to which handoff is performed, nor the actual handoff process itself. 2. TERMINOLOGY Access Point (AP) A radio transceiver by which an MN obtains Layer 2 connectivity with the wired network. Access Router (AR) An IP router residing in an access network and connected to one or more APs. An AR offers IP connectivity to MN. Physically Adjacent AR (PAAR) An AR whose coverage area is such that an MN may move from the coverage area of the AR currently serving the MN into the coverage area of this AR. Candidate AR (CAR) This is an AR that is a candidate for MN's handoff. CAR is necessarily a PAAR of the AR currently serving the MN, and also has the capability set required to serve the MN. Target AR (TAR) This is an AR with which the procedures for MN's handoff are initiated. Trossen, Krishnamurthi, Chaskar, Kempf [Page 1] INTERNET-DRAFT Candidate Access Router Discovery Issues July, 2001 TAR Selection Algorithm The algorithm that determines a unique TAR for MN's handoff from the set of CARs. The exact nature and definition of this algorithm is outside the scope of this document. 3. CANDIDATE ACCESS ROUTER DISCOVERY PROBLEM There are two basic approaches to CAR discovery, namely, Anticipated CAR Discovery and Dynamic CAR Discovery. 3.1 Anticipated CAR Discovery In this approach, an AR currently serving the MN identifies all CARs for the MN's handoff, at some point prior to handoff. This information is then immediately available at the time of handoff as input to the TAR Selection Algorithm. Another input into the TAR Selection Algorithm may be the preferences expressed by the MN prior to the handoff, or preferences enforced by the administrative control. At the time of handoff, it may only be required to provide input to TAR Selection Algorithm about the reachability of neighboring ARs from the MN. The advantage of this approach is that the handoff can be executed quickly because the AR has already collected much of the information needed by the TAR Selection Algorithm. Also, this approach does not generate much radio traffic for performing CAR discovery. The Anticipated CAR Discovery problem encompasses the areas outlined in the box in Figure 1. ----------------------------------------------------------------- | All routers | | | | | | Physical adjacency discovery | | v | | Local map of PAARs at current AR | | | | | | Capability identification | | v | | CARs for MN's handoff identified at current AR | | | | --------------------------|-------------------------------------- | | Predefined preferences of the MN AR Reachability | negotiated at admission time or for the MN | administrative preferences | | | v v v TAR Selection Algorithm executed at the time of handoff to determine unique TAR for MN's handoff Figure 1: The Anticipated CAR Discovery Problem Trossen, Krishnamurthi, Chaskar, Kempf [Page 2] INTERNET-DRAFT Candidate Access Router Discovery Issues July, 2001 3.2 Dynamic CAR Discovery In this approach, an MN dynamically obtains information about the available ARs for handoff, along with their capabilities. When the MN detects an AR that has capabilities which match its preferences, the MN may notify the currently serving AR to perform a handoff to this AR using one or more of the seamless handoff protocols. The advantage of this technique is that the MN has fine-grained control over the TAR selection. However, this approach generates more radio traffic during the CAR discovery step. Also, it is required that the MN can listen to or, in some cases, even talk to the PAARs for collecting capability information and negotiating preferences, while still being connected to the current AR. The Dynamic CAR Discovery problem encompasses the areas outlined in the box in Figure 2. -------------------------------------------------------------- | | | PAARs identified by MN | | | | | | Capability identification by MN | | | | | v | | CARs for MN's handoff identified at MN | | | | ---------------------------|---------------------------------- | | TAR selection by MN at the | time of handoff v Unique TAR identity sent to current AR Figure 2: Dynamic CAR Discovery Problem 3.3 Hybrid CAR Discovery In practice, a hybrid of the above techniques for CAR discovery may be used. In the hybrid approach, the partitioning of capability information between the two techniques as well as the partitioning of TAR selection process between the MN and the current AR is possible. 4. SPECIFIC ISSUES IN CAR DISCOVERY The following sections describe the specific issues in the CAR discovery, may it be done in anticipated, dynamic or hybrid way. 4.1 Identifying PAARs A basic requirement for a new AR to be considered as a CAR for MN's handoff is that the coverage area of the new AR be "physically adjacent" to that of the AR currently serving the MN. In other words, Trossen, Krishnamurthi, Chaskar, Kempf [Page 3] INTERNET-DRAFT Candidate Access Router Discovery Issues July, 2001 a new AR must be a PAAR. Note, the physical adjacency of the coverage areas of two ARs is not necessarily implied by their "logical" adjacency. Logical adjacency of two ARs means that there is just one IP hop between the two ARs, while physical adjacency of two ARs implies that the MN can actually move from the coverage area of one AR to another. Conversely, PAARs need not be logically adjacent, and may even have IP addresses in different administrative domains. One approach is to manually configure this physical neighborhood at each AR. However, such an approach has disadvantages and in many cases, may not be feasible. For instance, two ARs that are physically adjacent may be under different administrative control, and thus, are not informed about each other's presence. Even within the same administrative domain, the manual configuration approach demands much network planning in terms of the coverage areas of different ARs. Finally, the manual configuration approach is not suitable if the routers can be physically relocated or their coverage area changed, thus altering the physical adjacency of their coverage areas. Such a scenario is very common when ARs are temporarily introduced in hot spots. Clearly, a more automatic and dynamic mechanism is needed that would allow an AR to discover physically adjacent ARs around itself without much administrative intervention. For the Anticipated CAR Discovery, the process of identifying PAARs requires a protocol that allows ARs to exchange physical adjacency information in advance of handoff. In Dynamic CAR Discovery, the MN automatically serves as the judge of whether an AR is a PAAR. In the latter case, if the MN can hear a Router Advertisement of the new AR, then the new AR is PAAR. 4.2 Identifying capabilities of PAARs: Although not common now, the future generation mobile networks may consist of ARs that are physically adjacent, but are heterogeneous in administrative control, function, capability, link layer technology and resources. An MN that is attached to a given AR may have specific requirements as regards these parameters. For example, the MN may need some hardware or software support from the access router to run certain IP-based applications. Support for QoS, security, multicast, header compression etc. are some other aspects that need to be considered when choosing the CAR for MN's handoff. It may also be required to assure some security association or policy agreement between the current AR and its physical neighbor in order to allow MN's handoff between them. The various capabilities that need to be identified in PAAR for smooth handoff are outside the scope of this draft. However, as an illustration, the capabilities to be identified may include administrative parameters such as ISP, organization and policy information; cost of access parameters; information about radio interfaces; information about availability of any application logic such as multicast support, header compression capability, playout buffer hosting for streaming applications, TCP PEPs and media transcoding functionality; Internet connectivity parameters such as Trossen, Krishnamurthi, Chaskar, Kempf [Page 4] INTERNET-DRAFT Candidate Access Router Discovery Issues July, 2001 need for NAT traversal; resource parameters such as load and so on. In the Anticipated CAR Discovery, capability exchange between ARs occurs over the Internet. Thus, a protocol is needed that would allow ARs to exchange capability information. In the Dynamic CAR Discovery approach, the MN is soliciting or receiving the capabilities of PAARs directly. Some protocol is required to allow the MN to solicit, or the ARs to advertise their capabilities. 4.3 Security considerations AR discovery may allow other nodes to learn the following pieces of information about an AR: - Physical location - Capabilities - State of capabilities. Malicious nodes may use this kind of information to launch attacks of the DoS-style and/or service hijacking. Therefore, the following topics should be covered in any protocol developed for AR discovery: - Authentication of nodes - Security associations between nodes - Message/payload encryption. 5. REFERENCES [1] IP Mobility Support, C. Perkins (Editor), RFC 2002, October 1996. [2] Mobility Support in IPv6, D. Johnson and C. Perkins, draft-ietf-mobileip-ipv6-13.txt, work in progress, November 2000. [3] Low Latency Handoffs in Mobile IPv4, MIPv4 Handoffs Design Team, draft-ietf-mobileip-lowlatency-handoffs-v4-00.txt, work in progress, February 2001. [4] Fast handoffs for Mobile IPv6, MIPv6 handoff Design Team, draft-ietf-mobileip-fast-mipv6-01.txt, work in progress, April 2001. [5] Problem Description: Reasons For Performing Context Transfers Between Nodes in an IP Access Network, O. H. Levkowetz et. al., draft-ietf-seamoby-context-transfer-problem-stat-01.doc, work in progress, May 2001. Trossen, Krishnamurthi, Chaskar, Kempf [Page 5] INTERNET-DRAFT Candidate Access Router Discovery Issues July, 2001 [6] General requirements for a context transfer framework, H. Sayed et. al., draft-ietf-seamoby-ct-reqs-00.txt, work in progress, May 2001. [7] Buffer Management for Smooth handoffs in Mobile IPv6, G. Krishnamurthi, R. Chalmers, and C. Perkins, draft-krishnamurthi-mobileip-buffer6-01.txt, work in progress, March 2001. 6. AUTHORS' ADDRESS Dirk Trossen Communication Systems Laboratory, Nokia Research Center 5 Wayside Road Burlington, MA 01803, USA Phone: +1 781 993 3605 Fax: +1 781 993 1907 E-mail: dirk.trossen@nokia.com Govind Krishnamurthi Communication Systems Laboratory, Nokia Research Center 5 Wayside Road Burlington, MA 01803, USA Phone: +1 781 993 3627 Fax: +1 781 993 1907 E-mail: govind.krishnamurthi@nokia.com Hemant Chaskar Communication Systems Laboratory, Nokia Research Center 5 Wayside Road Burlington, MA 01803, USA Phone: +1 781 993 3785 Fax: +1 781 993 1907 E-mail: hemant.chaskar@nokia.com James Kempf Network and Security Research Center, Sun Labs Sun Microsystems, Inc. 15 Network Circle Menlo Park, CA 94025, USA Phone: +1 650 786 5890 Fax: +1 650 786 6445 E-mail: james.kempf@eng.sun.com Trossen, Krishnamurthi, Chaskar, Kempf [Page 6]