IP Micro-Mobility Protocols Andrew T. Campbell Javier Gomez-Castellanos campbell @ comet.cohtmbia.edu javierg@ comet.columbia.edu COMET Group, Center for Telecommunications Research Columbia University, New York, NY, USA The IETF Mobile IP Working Group is discussing a number of enhancements to the base protocol to reduce the latency, packet loss and signaling overhead experienced during hand- off. bz this article, we discuss a number of "micro-mobility protocols" that extend Mobile IP with fast handoff and paging capabilities. The aim of this article is not to provide an ex- haustive survey of these protocols. Rather, we discuss the motivation behind micfv-mobility, present common characteristics that a number of proposals share and briefly describe some of the key contributions discussed by the working group. In the longer tetwt there is a need to understand the differences between many of the micro-mobility proposals discussed in this article in terms of complexity of the design choice, and performance di~emnces. As part of that process we have recently made available the Columbia Micro-mobility Suite (CMS). The CMS software is freely available from the web (comet.columbia.edu/micromobility) and includes ns source code extensions for Celhdar IP, Hawaii and Hierarchical Mobile IP. I. Introduction Wireless access to the Internet may outstrip all other forms of access in the near future. It is likely that mo- bile users will expect similar levels of service quality as wireline users. Such a vision presents a number of technical challenges for Mobile IP in terms of perfor- mance and scalability. Recently, a number of micro-mobility protocols" have been discussed in the IETF Mobile IP Working Group that address some of these performance and scalability issues. Micro-mobility protocols are de- signed for environments where mobile hosts change their point of attachment to the network so frequently that the basic Mobile IP protocol [1 ] tunneling mech- anism introduces network overhead in terms of in- creased delay, packet loss and signaling. For exam- ple, many real-time wireless applications (e.g., voice- over-IP) would experience noticeable degradation of service with frequent handoff. Establishment of new tunnels can introduce additional delays in the hand- off process causing packet loss and delayed delivery of data to applications. This delay is inherent in the round-trip incurred by Mobile IP as the registration re- quest is sent to the home agent and the response sent back to the foreign agent. Micro-mobility protocols aim to handle local movement (e.g., within a domain) of mobile hosts without interaction with the Mobile IP enabled Internet. This has the benefit of reducing delay and packet loss during handoff and eliminating registration between mobile hosts and possibly distant home agents when mobile hosts remain inside their local coverage areas. Eliminating registration in this manner reduces the signaling load experienced by the core network in support of mobility. As the numbers of wireless users grow so will the signaling overhead associated with mobility man- agement. In cellular networks registration and pag- ing techniques are used to minimize the signaling overhead and optimize mobility management perfor- mance. Currently, Mobile IP supports registration but not paging. An important characteristic of micro- mobility protocols is their ability to reduce the sig- naling overhead related to frequent mobile migrations taking into account a mobile host's operational mode (i.e., active or idle). When wireless access to Internet becomes the norm then Mobile IP will have to pro- vide efficient and scalable location tracking in sup- port of idle users, and paging in support of active communications. Support for "passive connectivity" to the wireless Internet balances a number of impor- tant design considerations. For example, only keeping the approximate location information of idle users re- quires significantly less signaling and thus reduces the load over the air interface and in the core network. Re- ducing signaling over the air interfaces in this manner also has the benefit of preserving the power reserves of mobile hosts. In this article we discuss a number of micro- mobility protocols. Our aim is not to provide an ex- haustive survey of the field. Rather, we discuss the motivation behind the work, present some common Mobile Corrr~Y~Z~ing and Communications Review, Volume 4, Number 4 45characteristics that a number of proposals share and briefly describe some of the key contributions dis- cussed in the Mobile IP working group over the past several years. II. Motivation Micro-mobility protocols aim to support fast hand- off control with minimum or zero packet loss, and to minimize signaling through the introduction of pag- ing techniques thereby reducing registration to a min- imum. These enhancements are necessary for the In- ternet to scale to support very large volumes of wire- less subscribers. In this section we discuss a number of issues that motivate the design of micro-mobility protocols. Fast Handoff. Support for fast handoff, which reduces delay and packet loss during handoff, is an important attributed of micro-mobility protocols. A number of design choices influence handoff perfor- mance including handoff control, buffering and for- warding techniques, radio behavior, movement detec- tion and prediction, and coupling and synchronization between the IP and radio layers. Tighter coupling be- tween layers, for example, may minimize handoff la- tency but may impact the general applicability of the solution. The working group has considered a num- ber of contributions that cover a wide set of design choices. Many proposals discuss seamless handoff (i.e., zero or near zero loss) where data is forwarded between the old and new access points during hand- off. Many of these approaches support fairly com- plex signaling, buffering and synchronization proce- dures. Layer three movement detection (e.g., eager cell switching) plays an important roll in handoff per- formance. The delay associated with recognizing and registering at a new access point can have a significant impact on mobility and data delivery.