Seamless Inter-Domain Handoffs via Simultaneous ...

Seamless Inter-Domain Handoffs via Simultaneous Bindings Hannes Hartenstein, Karl Jonas, and Ralf Schmitz E-mail: {Hannes.Hartenstein|Karl.Jonas|Ralf.Schmitz}@ccrle.nec.de Computer & Communication Research Laboratory, NEC Europe Ltd., 69115 Heidelberg, Germany

Abstract Fast and seamless handoff procedures for IP-based mobile communication networks have recently been a major field of research. Various methods have been proposed that focus on micro-mobility suppport, i.e., on mobility management within a single administrative domain. However, since in future mobile communication networks a user should be able to conveniently roam between various operators and between fixed and mobile as well as public and private networks independently of the different access technologies used, the provisioning of seamless inter-domain handoffs will also become an important objective. In this paper we report on our current activities of implementing and testing a seamless inter-domain handoff mechanism using Mobile IP[1] and simultaneous bindings. With simultaneous bindings a mobile node is allowed to register more than one care-of address with its home agent: packets destined for the mobile node and intercepted by the home agent are then tunneled to all the care-of-addresses currently registered for the mobile node. Thus, a unicast-multicast-unicast type of handoff can be performed. For our study we assume that a mobile node can send/receive IP packets from only one interface at a time. When a mobile node detects (either via link-level information or network-assisted) that it could be better served by another base station, it request its serving mobility agent to set up a new ‘connection’. This request is sent via the old path, i.e., a backward handoff is performed. When the mobile node then completes the handoff by reconfiguring its interface, packets destined for the mobile node are already delivered to the new point of attachment. With this approach the handoff time period, i.e., the time between handoff decision and completion, is split up into the handoff preparation time during which the mobile node is still connected to the old base station, and the time of actual physical interruption which is very small (about 5 ms in our Linux-based implementation). However, the observed ‘seamlessness’ does also depend on how well the streams (sent to the different care-ofaddresses) are synchronized.

1

Introduction

To provide seamless handoffs is an essential service of cellular systems. A user having a real-time conversation on a mobile terminal should not notice when moving from one base station to another one. In this paper we report on a specific type of handoff procedure for IP-based mobile communication networks. In general, the interest in IP-based mobility management comes from the fact that “a single generic mobility handling mechanism that allows roaming between all types of access networks would allow users to conveniently move between fixed and mobile networks, between public and private networks as well as between PLMN’s [public land mobile networks] with different access technologies. The ongoing work in the IETF Mobile IP working group is targeted towards such a mechanism” [2]. Two options exist in order to provide IP mobility: one either assigns a topologically correct address (care-of address, COA) to the mobile host (in addition to its fixed home address) and re-addresses packets destined for the mobile host using its current COA. Or, as

second option, one keeps the primary address all the time and modifies the routing entries all the way up to the mobile host’s current location (‘host-specific routes’). For Mobile IP [1] the second option was dismissed because of the resulting scalability problems in a ‘global scale deployment’. However, both approaches (re-addresssing-based as well as hostspecific routes-based approaches) have been studied and proposed to extend Mobile IP in order to provide fast ‘local’ handoffs. Thus, the current view of IP-based mobility management is that of a two-level hierarchy: the macro mobility management, i.e., the mobility management for handoffs between different domains (inter-domain), is handled by Mobile IP while the micro-mobility management, i.e., the mobility management within a domain (intra-domain), is handled by one of the various micro-mobility extensions to Mobile IP. While currently the notion of a domain has no precise meaning, a domain can be seen as a part of a network where handoffs are performed only by a single micromobility procedure. Generally, in a large network several domains will exist and, thus, intra- and inter-

domain handoffs will take place. While for fast/seamless intra-domain handoffs a wide variety of procedures has been proposed (see, e.g., [3,4]), the question of how seamless inter-domain handoffs can be achieved has not attracted much attention so far. In this paper we will focus on the latter problem. Where do inter-domain handoffs occur? First of all, the notion of micro-mobility has been defined from the perspective of the network and not of the user: micro-mobility is mobility within a certain part of a network. However, a small movement of a user can very well imply a handoff between various domains or networks: a mobile user coming from UMTS coverage into his/her corporate environment would perform a change of operators and/or access technologies. The following give some specific examples of interdomain handoffs: • UMTS/GPRS from/to wireless LANs. • Wireless LAN from/to wireless LAN (different subnet). • UMTS operator to UMTS operator. • RNC (radio network controller) to/from different RNC (here, a RNC can be regarded as a domain where micro-mobility is handled on link layer). Inter-domain handoff procedures suffer from the fact that, in contrast to intra-domain handoffs, no assumptions can be made about the network that connects the domains involved in the handoff. Therefore, it is difficult (or even impossible) to guarantee fast interdomain handoffs. Nevertheless, seamless inter-domain handoffs can be achieved as follows. When measuring the performance of a handoff procedure, one has to distinguish between handoff time, i.e., the duration between decision and completion of a handoff, and handoff delay, i.e., the time of actual interruption of the end-to-end transmission. After a handoff is decided, a preparation phase prior to the physical handoff may be used to set up packet delivery towards the new base station. The preparation phase might include, for example, acquiring a new IP address, informing the corresponding home agent as well as starting to serve the new base station (this phase may in fact also be used for additional issues, e.g., authorization or some quality of service negotiations). When the mobile node has to break the old connection in order to register at a new point of attachment, the whole prepartion phase contributes to the handoff delay (“break before make”). However, when the mobile node can continue to receive/send data from/to the old base station during the preparation phase, the handoff time might be long (since it includes the preparation phase) but the handoff delay can be quite small and, thus, the handoff can be seamless (“make before

Physically disconnect from old base station

Get new IP address

Physically connect with new base station

Inform home agent

Get new IP address

Transmission to new network

Inform mobility agent

Physically disconnect from old base station

Transmission to new network

Physically connect to new base station

Receive Data

Receive Data

(a)

(b)

Figure 1 (a) Break before make approach, (b) make before break approach. The arrows indicate the time of physical interruption.

break”, see Figure 1). Our goal is to achieve seamless inter-domain handoffs -rather than fast handoff timesby setting up simultaneous bindings in order to have packets destined for the mobile node delivered to both the old and new base station at a time. To the best of our knowledge, the first papers describing an IP-level handoff using multiple bindings has been [5]. Sending data for a mobile node to multiple base stations in order to improve handoff performace (so-called soft handover) is a “standard” feature in WCDMA networks. However, it is important to note that WCDMA soft handover cannot be used for providing multiple bindings on IP level as pointed out in [6].

2

Seamless Inter-Domain Handoffs

2.1

Basic Assumptions

For our study we will assume that • the mobile node works in co-located care-of address mode (the only option in Mobile IP for IPv6), i.e., when moving to a new domain the mobile node has to acquire a new (topologically correct) IP address. • the mobile node has only one wireless interface, i.e., it cannot communicate via two or more interfaces simultaneously. We also assume that the mobile node can communicate on IP level with only one base station at a time. However, the mobile node should be able to simultaneously communicate with several base stations on link layer level.

Before proceeding with details of our approach, let us first discuss why the second assumption has been chosen. In the design of handoff mechanisms one is confronted with the trade-off between smoothness versus ‘costs’. One important factor here is the number of independent transmitter/receiver at the mobile terminal. When the mobile terminal is able to communicate simultaneously (on IP level) with the old as well as the new base station, a seamless handoff is easily achieved with Mobile IP: the mobile node can send a registation request via the new base station while listening/sending to the old base station. When the registration request reaches the home agent (or, more general, a switching mobility agent), the packet flow is redirected almost immediately. An example scenario is given by a mobile terminal that is equipped with a WaveLAN and a GPRS interface. However, in many situations the costs for more than one transmitter/receiver unit might be too high to be justified. Then, in the case of only one receiver/sender, the mobile node will not be able to communicate independently over two interfaces simultaneously. Example scenarios are ‘intra-technology’ handoffs as follows: • UMTS operator to/from UMTS operator. • RNC to/from different RNC. • Wireless LAN 802.11 to/from wireless LAN 802.11 (infrastructure mode, different frequency channels). Another scenario is given by software radio terminals that can switch from one access technology to another 1 one but can only use one scheme at a time.

2.2

the following information: a) an IP subnet prefix of the network the base station is attached to and b) the IP address of the base station.2 In the case of IPv6 this information would suffice to acquire a topologically correct IPv6 address by means of the stateless autoconfiguration mechanism. For IPv4, the above information or some other type of base station identifier has to be used to acquire a valid IPv4 address via the existing connection.

AAS

2

1

4

4 6

3

5 1. 2.

Seamless Handoff Procedure

After having justified our assumptions we will now focus on how to achieve seamless inter-domain handoffs. The principal idea is to set up packet delivery to the new base station via the old ‘connection’. With the use of the ‘simultaneous bindings’ option of Mobile IP, packets will be sent from the home agent to the old and the new base station for some specific time. When the mobile node is informed that the packets can now be received also via the new base station, the mobile node has only to reconfigure its interface and will then immediately receive the packets from the new base station. In more detail the procedure works as follows (see Figure 2). First of all, in order to learn about its handoff ‘options’, a mobile node should be able to monitor control channels or control beacons from various base stations (on link layer level). When a mobile node becomes aware of a base station the mobile node likes to switch to, it has to be provided with

HA

3. 4.

5.

6.

Connection between HA and MN is established via the ‘old’ base station. Address assigment server (AAS) is contacted via the old base station to request an IP address of the new domain (or stateless autoconfiguration in IPv6). The MN sends a registration request with the new COA and the simultaneous binding flag set. Packets destined for the MN and intercepted by the HA are sent to the old as well as to the new coCOA. Receiving the registration reply, the MN reconfigures its interface with the new IP address, changes the routing table and sends a unicast ARP request to the new base station in order to indicate its presence. Connection between HA and MN is now established via the new base station and the old connection can be deregistered.

Figure 2 Basic steps in setting up simultaneous bindings via the old base station.

1

Mechanisms that allow a mobile node to download handoff procedures have been studied under the name of reflective handoffs in [7].

2

Here, a base station is a router equipped with a WaveLAN interface.

For this reason, an address assignment server has to be contacted by the mobile node via the old base station. The duty of the address assignment server is to translate the base station identifier into an IP subnet address and to provide a corresponding co-located care-of address. We like to emphasize that in contrast to a standard DHCP-based interface configuration, the mobile node is already connected and only has to obtain an address for another network. When the mobile node gets the new care-of address, it sends a registration request to its home agent with the S-bit (simultaneous binding flag) set.3 The home agent then will make a new mobility binding for the mobile node’s new care-of address in addition to the old binding which will be maintained. From this point of time packets destined for the mobile node and intercepted by the home agent are sent to the old as well as to the new colocated care-of-address. The packets received at the new base station will be discarded as long as the mobile terminal does not make its presence known to the new base station. When the mobile node gets the registration reply for its new care-of address, it can reconfigure its interface with the new IP address, changes the routing table, and sends a packet to the new base station in order to indicate its presence (alternatively other system-specific ‘attach’ procedures can be performed).

2.3

Implementation

Our simultaneous bindings testbed (see Figure 3) is based on Linux and the Dynamics Mobile IP for IPv4 implementation [8]. In order to simulate two different domains, WaveLAN base stations working on two nonoverlapping channels (30 MHz apart) have been used.4 In the following we will describe the processing of simultaneous bindings separately for home agent, base station, and mobile node. Processing at home agent The interception of packets destined for the mobile node is done using DIVERT sockets [9] at the home agent. This allows user-space processing and encapsulation of the IP packets. The home agent maintains a list of registered care-of-addresses for a mobile node; when a packet destined for the mobile node is intercepted by the home agent, it makes a copy for each of the registered care-of-addresses, encapsulate each copy with an IP header showing the 3

The S-bit option might be removed from future versions of the Mobile IP specification. Alternatives to the S-bit option are discussed in Section 3. 4 The WaveLAN cards are operating in direct sequence spread spectrum mode on two different frequency channels.

HA

MN Figure 3 Our test scenario. corresponding care-of-address as destination address (this is standard IP-in-IP encapsulation), and forwards the encapsulated packet. The use of DIVERT sockets introduces some performance penalty with respect to the overall end-to-end delay (between correspondent node and mobile terminal) but has no effect on the actual handoff times or handoff delays, thus, are suited for our experiments. Processing at base station A base station that receives packets for a mobile node that is not yet ‘attached’ to the base station, i.e., no ARP entry is available for the mobile node, will discard all packets for the mobile node until the mobile node registers with the base station. Thus, the duplication of packets does not introduce additional load on the wireless part of the network. Processing at mobile node The processing at the mobile node works as follows: when the mobile node has acquired an IP address for the new domain, the mobile node sends a registration request via the old base station to the home agent. In the registration request the care-of-address field is set to the acquired COA and the S-bit is set. After receiving the corresponding registration reply, the mobile node might perform the handoff by changing the WaveLAN channel, updating the IP address, and changing the routing table. Then, an attach procedure has to be performed (in our case: sending an ARP request in order to update the ARP cache of the base station). When the mobile node has performed the handoff, the mobile node might send a deregistration request in order to tear down the old mobility binding.

2.4

Experimental Results

The actual physical interruption time, i.e., the time needed to change the WaveLAN channel and to change IP address and routing table entries, takes

about 5 ms in our implementation. Thus, truly seamless handoff can be achieved. The time needed to set up multiple bindings, i.e., the time between sending a registration request and receiving the reply, is about 80 ms. However, this number might be drastically reduced in a more optimized version of the code. Note also that this setup time does not contribute to the actual physical interruption time. In order to see the benefit of the simultaneous bindings approach, we compare it with the standard „break before make“ handoff approach. With break before make, we measure the actual physical interruption time as follows. We ‚break’ the old connection by sending a deregistration message. Then the time beween reception of the deregistration reply (via the old base station) and successful registration via the new base station (as indicated by the reply message that correponds to the new registration request) gives the time of physical interruption. When no new IP address has to be acquired, e.g., in the case of preconfigured addresses, the break before make handoff delay is about 60 ms, thus, it is a factor of 12 slower than with simultaneous bindings. With acquisition of IP addresses through means of DHCP taken into account, the break before make handoff delay is about 160 ms. In contrast, as mentioned above, the IP address acquisition does not contribute to the handoff delay in the simultaneous bindings case. In Tables 1 we present a comparison between the simultaneous bindings implementation and the standard „break before make“ handoff with respect to packet loss that occurs during handoff. For the „break before make“ measurements the times for IP address acquisition have been again taken into account. Of course, other factors like admission control or authorization would make the simultaneous bindings approach look even better. Packet size (byte)/ Sent every x ms 20 / 20 20 / 10 20 / 5 100 / 20 100 / 10 100 / 5 1000 / 20 1000 / 10 1000 / 5

Average packet loss BBM Sim. Bind. 8 0 16 0 32 1 8 0 16 0 32 1 8 0 15 0 32 1

Table 1 Packet loss measurements (number of packets, packet size gives UDP payload) comparing the “break before make” (BBM) approach with the simultaneous bindings approach. The handoff delays are about 160 ms for BBM and about 5 ms for simultaneous bindings.

3

Discussion & Related Work

Simultaneous bindings w/o S-bit In Mobile IP it is assumed that the home agent sets up the appropriate mobility binding while processing a registration request. Thus, the home agent itself could decide whether to keep a previous mobility binding or whether to discard it. The S-bit flag only provides a way for the mobile node to explicitly request multiple bindings. Therefore, when the S-bit option will be removed from future versions of the Mobile IP specification, multiple bindings can still be employed, however, it is then the home agent’s responsibility to set them up. The updated internet-draft of Calhoun and Kempf [10] has outlined this option for the case of a Mobile IP for IPv4 scenario with foreign agents. In general, the S-bit flag can be considered a shorthand notation that is redundant as can be seen as follows. We assume that a home agent always maintains multiple bindings, i.e., the previous mobility bindings are kept by default. A mobile node can explicitly remove a binding by sending a deregistration request (a registration request with lifetime 0). Therefore, Mobile IP simultaneous bindings do not depend on the existence of the S-bit flag in the registration request header. Handoff delay vs packet loss The results given in Section 2.4 show packet loss rates that very well correlate with the time of physical interruption. However, the number of lost packets depends also on how well the two independent streams to the two base stations are synchronized: when the delay between home agent and new base station is larger than between home agent and old base station, the mobile node will receive ‘old’ packets at the new base stations. In the other direction (from slow link to fast link), however, more packets could be lost than implied by the handoff delay. The synchronization of both streams becomes an important aspect. Here, IP multicast might improve synchronization since the streams would be lead over the same routers as long as possible. IP multicast would also reduce bandwidth costs in the wired part of the network. A recent paper [11] gives some results on handoffs using IP multicast. We like to emphasize again that the use of simultaneous bindings does not imply waste of bandwidth on the air interface in any case. Since the mobile node might receive a few duplicate packets when performing a handoff, one has to make sure that packet duplication is appropriately handled by the mobile node: on the one hand the Mobile IP daemons are implemented such that duplicate signalling messages are detected, on the other hand higher protocol layers or the applications themselves have to

handle packet duplication by checking sequence numbers or timestamp value. Related work While we have been preparing the final version of this paper, two internet drafts [12,13] have been releasesd proposing a seamless handoff mechanism similar to the one explored in this paper. Furthermore, in [13] handoffs based on simultaneous bindings for the case of a hierarchical Mobile IPv6 scenario are discussed. However, since we have been interested in interdomain handoffs where no assumption about the infrastructure can be made, we believe that hierarchies that are used for enhancing micro-mobility might not be useful in the scenario we have addressed.

4

Conclusions & Perspectives

The simultaneous bindings approach described in this paper represents an example of a network-assisted handoff procedure as it is typical for many cellular systems – now performed on IP level. It has been developed for seamless inter-domain handoffs but could also be used within a domain provided that cells are sufficiently overlapping. The physical interruption time of 5 ms shows a big advantage over traditional make-before-break approaches. As topics for further investigations we have identified the following. First of all, while this paper describes the mechanism for setting up simultaneous bindings via the old (currently used) base station, the handoff decision model, i.e., the algorithm that decides when to set up multiple bindings as well as when to tear down an existing binding, has not been discussed. Optimizations according to some cost criterion that includes seamlessness as well as network load have to be studied by simulation as well as by experiments. Here, the delicate point that one has to deal with lies in the backward handoff, i.e., the handoff should be initiated when the link quality gets worse but is still good enough to perform the handoff. Secondly, the impact of seamless handoffs to TCP performance has to be analyzed. Another important issue for further research are the AAA (authentication, autorization, accounting) aspects of inter-domain handoffs since the address assignment could/should be tightly coupled with the admission control in general.

5

References

[1] C. Perkins (ed.), IP mobility support, RFC 2002, October 1996.

[2] 3GPP Tech. Report 23.923, Combined GSM and MobileIP Mobility Handling in UMTS IP CN, Oct. 1999. [3] A. Campbell, J. Gomez, C.-Y. Wan, Z. Turanyi, A. Valko, Cellular IP, work in progress, internet draft, Oct. 1999. [4] R. Ramjee, T. La Porta, S. Thuel, K. Varadhan, L. Sagarelli, IP micro-mobility support using HAWAII, work in progress, internet draft, June 1999. [5] H. Balakrishnan, S. Seshan, R. Katz: Improving Reliable Transport and Handoff Performance in Cellular Wireless Networks. ACM Wireless Networks Journal, vol. 1, no. 3, Dec. 1995. [6] J. Kempf, P. McCann, P. Roberts: IP Mobility and the CDMA Radio Access Network – Applicability Statement for Soft Handoff. Work in progress, internet draft, April 2000. [7] M. Kounavis, A. Campbell, G. Ito, G. Bianchi, Supporting Programmable Handoff in Mobile Networks, IEEE MoMuC, San Diego, Nov. 1999. [8] Dynamics HUT Mobile IP Implementation at http://www.cs.hut.fi/Research/Dynamics [9] I. Baldine, Divert sockets – mini howt, see http://www.anr.mcnc.org/~divert/doc/howto/dive rt_socket.howto-4html [10] J. Kempf, P. Calhoun: Foreign Agent Assisted Hand-off. Work in progress, internet draft, June 2000. [11] A. Festag, T. Assmakopoloulos, L. Westhoff, A. Wolisz: Rerouting for Handover in Mobile Networks with Connection-Oriented Backbones. Proc. IEEE Conference on High Performance Switching and Routing, Heidelberg, June 2000. [12] K. El Malki, H. Soliman: Fast Handoffs in Mobile IPv4. Work in progress, internet-draft, July 2000. [13] K. El Malki, H. Soliman: Hierarchical Mobile IPv6 and Fast Handoffs. Work in progress, internet draft, June 2000. Hannes Hartenstein received the diploma in mathematics in 1995 and the PhD degree in computer science in 1998, both from Albert-Ludwigs-Universitaet Freiburg, Germany. He has joined the Mobile Communications Group of NEC Europe Ltd. Heidelberg in 1999. Karl Jonas received his diploma and PhD in computer science in 1991 resp. 1999 from the Technical University of Berlin. From 1994 to 1998 he was with the German National Research Centre for Information Technology (GMD) and since 1998 he is a member of the Mobile Communications Group at NEC Laboratories in Heidelberg. Ralf Schmitz studies communications engineering at the University of Applied Sciences Cologne, and is currently doing his diploma thesis on Seamless handoff in Mobile IP via Simultaneous Bindings at NEC, Heidelberg.