INTERNET DRAFT James Kempf, Editor Category: Informational Sun Microsystems Title: draft-seamoby-paging-problem-statement-01.txt Date: Feburary 2001 Paging Problem Statement Status of this Memo This document is a working group contribution for the Seamoby Working Group. 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. Copyright (C) The Internet Society 2001. All Rights Reserved. Abstract The IESG has requested that the Seamoby Working Group develop a problem statement about the need for additional protocol work to support paging for seamless IP mobility. The paging design team interpreted this as direction to examine whether location of a mobile node in power saving mode can be supported by the existing Mobile IPv4 and Mobile IPv6 protocols given existing radio link protocols. This draft describes paging, assesses the need for IP paging, and presents a list of recommendations for Seamoby charter items regarding work on paging. The results are specifically directed James Kempf, Editor expires August 2001 [Page 1] INTERNET DRAFT Feburary 2001 toward the task undertaken by the design team, and are not meant to be the definitive word on paging for all time, nor to be binding on Seamoby or other working groups, should the situation with regard to IP mobility protocols or radio link support undergo a major change. 1.0 Introduction Many existing radio link protocols and mobile systems support location of and radio link establishment with mobile nodes that are in power saving mode and hence are not actively listening for delivery of IP packets all the time or are not listening on the radio channels normally associated with delivering IP traffic to mobile nodes. This functionality allows mobile nodes to reduce power consumption and decreases signaling load on the network for tracking mobiles that are not actively participating in IP packet generation or reception. When a mobile is in low power consumption mode, special steps need to be taken to locate the mobile. These steps differ depending on the radio link, but the generic name for this process is paging. In this document, after some initial definitions and material related to more clearly explaining what paging is, we assess the need for paging in existing IP mobility protocols (namely Mobile IP [1] [2]). We then develop a list of work items for the Seamoby working group related to this need. Note that the discussion in this document and the conclusions regarding work items are directed toward existing IP mobility protocols and existing radio link protocols. Should a major change occur in radio link support or the available IP mobility protocols, such as the introduction of a micromobility protocol for IP, the issues examined in this document may need to be revisited. 2.0 Definitions The following definitions are relevent with respect to clarifying the paging functionality: Dormant Mode - A state in which the mobile restricts its ability to receive normal IP traffic by reducing monitoring of radio channels. This allows the mobile to save power and reduces signaling load on the network. Time-slotted Dormant Mode - A dormant mode implementation in which the mobile alternates between periods of not listening for any radio traffic and listening for traffic. Time-slotted dormant mode implementations are typically synchronized with the network so the network can deliver traffic to the mobile during listening James Kempf, Editor expires August 2001 [Page 2] INTERNET DRAFT Feburary 2001 periods. Additionally, the mobile may be restricted to listening on specific signaling channels that, according to current practice, are not typically used to carry IP traffic. Paging - As a consequence of a mobile-bound packet destined for a mobile currently in dormant mode, signaling by the network through radio access points directed to locating the mobile and establishing a last hop connection. This messaging is in addition to simply delivering the packet to the mobile, i.e. last hop routing of packets is NOT considered to be paging. Paging Area - Collection of radio access points that are signaled to locate a dormant mode mobile node. A paging area does not necessarily correspond to an IP subnet. A dormant mode mobile node may be required to signal to the network when it crosses a paging area boundary, in order that the network can maintain a rough idea of where the mobile is located. Paging Channel - A radio channel dedicated to signaling dormant mode mobiles for paging purposes. By current practice, the protocol used on a paging channel is usually dictated by the radio link protocol, although some paging protocols have provision for carrying arbitrary traffic (and thus could potentially be used to carry IP). Traffic Channel - The radio channel on which IP traffic to an active mobile is typically sent. This channel is used by a mobile that is actively sending and receiving IP traffic, and is not continuously active in a dormant mode mobile. For some radio link protocols, this may be the only channel available. Regional Registrations - Signaling from a dormant mode mobile node to the network when the mobile node crosses a paging area boundary to establish the mobile node's presence in the new paging area. 3.0 Discussion of Paging Dormant mode is advantageous to a mobile node and the network for the following reasons: - Power savings. By reducing the amount of time the mobile is required to listen to the radio interface, the drain on the mobile node's battery is reduced. - Reduced signaling for location tracking. By requiring the mobile to only signal when it crosses a paging area boundary rather than when it switches between radio access points, the amount of signaling for tracking the mobile is reduced because paging areas James Kempf, Editor expires August 2001 [Page 3] INTERNET DRAFT Feburary 2001 typically contain many radio access points. - Reduced router state. By removing the need for routers to keep the mobile node's binding in their binding caches, the amount of state in routers is reduced, because the number of mobile nodes in dormant mode may be considerably more than those that are active. In existing radio link protocols, there is a clear distinction between those protocols that support dormant mode only and those that support dormant mode with paging. Radio link protocols that do not support paging have no paging areas, no dedicated paging channel, and no radio link protocol specifically directed towards locating a dormant mode mobile, while radio link protocols that do support paging have these features. Although generalizations always run the risk of being contradicted by specific exceptions, the following comparison of existing radio link protocol support for these two cases may be instructive. 3.1 Dormant Mode Support Only In radio link protocols that only support dormant mode, a dormant mode mobile node typically operates in time slotted mode and there is only one radio channel available, namely the traffic channel. The mobile node periodically wakes up, and, synchronously, the radio access point in the network with which the mobile node is associated delivers any IP packets that have arrived while the mobile node was asleep. Radio access points are required to buffer incoming packets for dormant mode mobiles; exactly how many packets and how long they are buffered are implementation dependent. If the mobile node happens to move out of range of the access point with which it was associated while it is in dormant mode, it discovers this when it awakens and reassociates with a new access point. The new access point then contacts the old access point over the wired backbone, the old access point sends any buffered packets, and the new access point delivers them to the mobile. Radio link protocols with dormant mode support only are typically wireless LAN protocols in unlicensed spectrum in which the mobile node is not charged for using a traffic channel, and hence there is no need for conserving spectrum usage. 3.2 Dormant Mode with Paging Support In radio link protocols with support for paging, the radio link typically supports more than one channel. A dormant mode mobile node may operate in time slotted mode, periodically waking up to listen to the paging channel, or it may simply listen to the paging channel James Kempf, Editor expires August 2001 [Page 4] INTERNET DRAFT Feburary 2001 continuously. The important point is that the mobile does not listen to nor transmit on a traffic channel while in dormant mode. The radio access points are grouped into paging areas, and the radio link protocol supports periodic signaling between the mobile and the network only when the mobile crosses a paging area boundary, for the purpose of giving the network a rough idea of the mobile's location (regional registrations). Some deployments of paging do not even use regional registrations. They use heuristics to determine where the mobile is located when a packet arrives, in which case, no signaling is required while the mobile is in dormant mode. An incoming packet is directed to the paging area where the mobile last reported, or the paging area is determined by heuristics. The network performs a radio link page by sending out a signal on the paging channel. The signal may be repeated until the mobile answers or a timeout occurs. In the former case, the packet is delivered, in the latter, the mobile is assumed to be unreachable. Radio link protocols with paging support tend to be in licensed spectrum where the network operator has an interest in reducing the amount of signaling over traffic channels. Such reduction frees traffic channel spectrum for revenue-producing use, and avoids charging the customer for signalling overhead. 4.0 Is IP Paging Necessary? In this section, we consider whether IP paging support is necessary. We first consider radio link protocols that have no support for paging. We then examine radio link protocols that have paging support. As discussed in the introduction, the focus is on whether the existing IETF mobility protocol, namely Mobile IP, requires enhancement. We also briefly discuss the relationship between paging and a potential future micromobility protocol. 4.1 IP Paging for Dormant Mode Only Radio Links One possible justification for IP paging is for radio links that do not support paging. The reasoning is that an IP paging protocol could allow location of a dormant mode mobile in radio networks that do not support paging in the radio protocol. An important point to keep in mind when considering this possibility is that, for radio links that do support paging, paging is typically used to locate mobiles for which the network has a rough idea of where the mobile is located. More specifically, in order to conserve signaling between the network and the mobile and to reduce power drain on the mobile, the mobile only updates the network about its James Kempf, Editor expires August 2001 [Page 5] INTERNET DRAFT Feburary 2001 location when it crosses a paging area boundary (if even then), which is far less frequent than when it crosses a radio access point boundary. If IP paging is to be of any use to radio link protocols that do not support paging, it must also be the case that it allows the network to maintain a rough idea of where the mobile is, otherwise, the amount of signaling involved in tracking the mobile and power drain on the mobile is not reduced. However, as the description in the previous section indicates, for radio links without paging support, the network always has an *exact* idea of where the mobile is located. When the mobile moves into range of a new radio access point, it re-registers with the access point in that cell allowing the new access point to contact the old and deliver any buffered traffic. Additionally, the new access point at that time may choose to deliver a foreign agent advertisement (for Mobile IPv4) or router advertisement (for Mobile IPv6) to the mobile if the mobile node has changed subnets, so that the mobile can perform Mobile IP re-registration in order to make sure its IP routing is current. There is absolutely no ambiguity in the mobile's location as far as the network is concerned, and so the network can continue to route packets to the mobile node while the mobile is in dormant mode with assurance (modulo buffer overflows and timeouts at the radio access point) that the packets will be delivered to the mobile the next time it wakes up from dormant mode. As a consequence, IP paging provides no advantages for radio link protocols in which the radio link does not have support for paging. 4.2 IP Paging for Radio Links with Paging Support In radio links that do support paging, there are two cases to consider: networks of radio links having a homogeneous radio technology and networks of radio links having heterogeneous radio technologies. We consider whether Mobile IP can support dormant mode location for both these cases. 4.2.1 Homogeneous Technology Networks For homogeneous technology networks, the primary issue is whether signaling involved in Mobile IP is enough to provide support for locating dormant mode mobile nodes. Subnets constitute the unit of signaling for presence in IP. When a mobile node moves from one subnet to another, Mobile IP signaling is required to change the mobile's care-of address. This signaling establishes the mobile's presence in the new subnet. Paging areas constitute the unit of signaling for dormant mode mobile presence at the radio level. Regional registrations or heuristics are used to establish a dormant mode mobile's presence in a particular paging area. James Kempf, Editor expires August 2001 [Page 6] INTERNET DRAFT Feburary 2001 Given these two units of presence at radio and IP layers, the most important point with regard to whether or not paging is needed in homogeneous networks is how subnets are mapped into paging areas. To determine whether additional IP paging protocol work is required, we first need to determine whether we can support location of and radio link establishment with a mobile in dormant mode given some mapping between subnets and paging areas using Mobile IP. Standard Mobile IP requires the mobile node to actively listen for or solicit foreign agent or router advertisements and participate in registering, so it is of little use if the mobile is in dormant mode. However, network assisted handoff techniques used for fast handoff [3] [4] allow the network to track the mobile without requiring any signaling from the mobile. The radio signaling that occurs when a dormant mobile crosses paging area boundaries (provided the operator has configured regional registrations) can be used as the "L2 trigger" for network assisted handoff techniques to allow the network to track the mobile in dormant mode, with perhaps some enhancements to optimize the ability of the network to update the home agent and/or any hierarchical agents about the mobile's location. We need to examine paging area to subnet mappings in order to determine when we can utilize the radio paging area update trigger to trigger network assisted handoff in dormant mode. In general, the mapping between paging areas and subnets can be arbitrary, but we consider here a smooth subset relationship, in which paging areas are subsets of subnets or vice versa. Network topologies in which one subnet is split between two or more paging areas are therefore eliminated. The restriction is arbitrary, but by starting here, we can discover whether additional work is needed. If it turns out that work is needed, then more complex topologies can be investigated in the course of actually doing the work. There are three cases: 1) The topological boundaries of the paging area and subnet are identical. 2) Multiple paging areas are part of the same subnet. 3) Multiple subnets are part of the same paging area. In the case where radio paging areas map one to one onto IP subnets (and hence Mobile IPv4 foreign agents or IPv6 access routers), it is possible to use radio link paging together with Mobile IP network assisted handoff techniques for the network to track the mobile's location. Enhancements to network-assisted handoff techniques can James Kempf, Editor expires August 2001 [Page 7] INTERNET DRAFT Feburary 2001 allow the network to track the mobile as it moves from paging area (== subnet) to paging area. New Mobile IP signaling for the mobile node to inform the network when it is going into dormant mode could possibly optimize this. With network tracking, if a packet for the dormant mobile comes in, it is routed by the hierarchical agent or home agent to the last hop foreign agent or access router at the mobile's last known position, where the radio page is performed. Once the L2/L3 address mapping is known, the packet is delivered. The case where multiple radio paging areas map to a single IP subnet is the same as above, with the exception that the last hop Mobile IPv4 foreign agent or IPv6 access router for the subnet performs paging in multiple paging areas to locate the mobile. In the case where a single radio paging area maps onto multiple IP subnets, it is not possible to directly use enhanced network assisted handoff techniques between last hop access routers or foreign agents to track the mobile's location as it moves, because the mobile does not signal its location when it changes subnets. Within the set of subnets that span the paging area, the mobile's movement is invisible to the L2 paging system, so a packet delivered to the mobile's last known location may result in a page that is answered in a different subnet. In this case, some support is required at the IP level for locating a dormant mode mobile that has moved into a different subnet. 4.2.2 Heterogeneous Technology Networks In a network composed of links with multiple technologies, there may be commonalities in the corresponding radio paging protocols that would allow a mapping to be established between the radio protocols and an abstract IP paging protocol. For example, assume we have a common paging area identifier defined at the IP layer that is mapped to each radio paging protocol by the access points. An IP paging message containing the identifier is sent to multiple access points, where the appropriate radio paging message is sent based on the particular technology implemented by the access points. The results are then returned by the radio paging responses, mapped back into IP by the access points, and delivered back to the origin of the page. Note, however, that the same analysis as applied to homogeneous networks regarding the mapping between paging areas and subnets applies here as well. For example, if there is a one-to-one mapping between radio paging areas and subnets, then there can be only a single technology per subnet, and movement across paging area boundaries by a dual technology mobile in dormant mode on both interfaces can be tracked through mobile IP network assisted handoff. Similarly, if more than one paging area exists per subnet, even if James Kempf, Editor expires August 2001 [Page 8] INTERNET DRAFT Feburary 2001 the areas are from multiple technologies, the foreign agent or access router can page in the multiple areas using the radio paging protocol and a dual mode mobile can be tracked as it crosses paging area boundaries. But if multiple subnets are contained within a paging area or for more complex topologies, the lack of clear identification between subnet boundaries at the IP layer and paging area boundaries at radio layer requires some means of locating the mobile at the IP layer. An additional case to consider is when a single subnet consists of multiple access tchnologies. A wireless access point usually provides L2 bridge behavior to the wired link with which it is connected. If two access points with incompatible technologies and non-overlapping cells are connected to the same subnet, a mobile node with interfaces to both technologies would need paging from both technologies. If reachability can be established simply by ARP or neighbor discovery, no IP paging is needed. However, note that ARP or neighbor discovery requires that a functional traffic channel be available to the mobile, since these protocols are typically implemented for wired networks in which a single channel exists on which all IP traffic is delivered. If the mobile is currently in the sleep phase of a time-slotted dormant mode, or if it is listening to a paging channel it will fail to respond to these requests. In this case, some means of triggering a radio page from IP is necessary to find the mobile. Modifying ARP or neighbor discovery to utilize a paging channel if available is a possible, if somewhat messy, alternative, but a dedicated location protocol may be somewhat cleaner. 4.3 Paging and Micromobility If the Seamoby Working Group decides that an IP micromobility protocol is necessary for intra-subnet mobility, then the above analysis is no longer complete. A micromobility protocol may require some type of paging support. The design team does not want to include any further discussion of paging and micromobility at this point, because it is not clear whether micromobility will be pursued by Seamoby and hence such discussion would be premature. 5.0 What Exactly is the Problem? While the above analysis has identified situations in which location of a mobile in dormant mode may require some action at the IP layer, it is important keep in mind what the problem is. The problem to be solved is the location of a mobile node because it has moved while in dormant mode. IP paging is one solution to the problem, there may be others. James Kempf, Editor expires August 2001 [Page 9] INTERNET DRAFT Feburary 2001 6.0 Recommendations The design group recommends the following charter items for Seamboy: 1) Since the design group has identified at least one network deployment scenerio where existing Mobile IP technology cannot find a mobile in dormant mode, protocol work is necessary to define a way for the network to find a mobile that is currently in dormant mode. 2) The work defined above should be pursued in a way that is maximally consistent with Mobile IP and other existing IETF protocols. The work should also generate recommendations about how to achieve the best match between existing radio paging protocols and IP. 3) If the Seamoby working group decides to pursue a micromobility protocol that requires paging, the Seamoby group should undertake the design of a new paging protocol within the context of that work. 4) There is some evidence that cellular operators' deployments of paging are highly variable, and may, in fact, be suboptimal in many cases with respect to supporting IP. The Seamoby working group should write a BCP which explains how to perform IP subnet to paging area mapping and which techniques to use when, so network designers in wireless networks have a guide when they are setting up their networks. 7.0 Acknowledgements The editor would like to thank the Seamoby paging design team for helping formulate the first draft of the document. Jari Malinen contributed text to Section 4.2. Hesham Soliman and Behcet Sarikaya contributed critical commentary on the first draft, which was important in sharpening the reasoning about what can and can't be expected in the absence of radio layer paging support. 8.0 References [1] C. Perkins, editor. "IP Mobility Support", RFC 2002, October, 1966. [2] Johnson, D., and C. Perkins, "Mobility Support in IPv6", draft- ietf-mobileip-ipv6-13.txt, a work in progress. James Kempf, Editor expires August 2001 [Page 10] INTERNET DRAFT Feburary 2001 [3] Calhoun, P., et. al., "Foreign Agent Assisted Hand-off", draft- calhoun-mobileip-proactive-fa-03.txt, a work in progress. [4] Tsirtsis, G., Editor, "Fast Handovers for Mobile IPv6", draft- designteam-fast-mipv6-01.txt, a work in progress. 9.0 Editor's Address Questions about this memo can be directed to: James Kempf Sun Labs California Sun Microsystems, Inc. 901 San Antonio Rd., UMPK15-214 Palo Alto, CA, 94303 USA Phone: +1 650 786 5890 Fax: +1 650 786 6445 E-Mail: james.kempf@sun.com 10.0 Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this docu- ment itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Inter- net organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permis- sions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WAR- RANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." James Kempf, Editor expires August 2001 [Page 11]