"HAWAII: A Domain-based Approach for Supporting Mobility in Wide-Area Wireless Networks"

R. Ramjee, T. La Porta, S. Thuel, K. Varadhan Bell Labs, Lucent Technologies

S. Y. Wang Department of EECS, Harvard

Summary by Andy Reitz.
April 25^th, 2000

Mobile-IP is the current standard for macro-mobility, but it has several deficiencies when it comes to micro-mobility. HAWAII (Handoff-Aware Wireless Access Internet Infrastructure) is an extension to Mobile-IP that corrects the following deficiencies:

• In Mobile-IP, user traffic is disrupted when the mobile host changes domains.
• In Mobile-IP, costly updates to the home agent must be made for every domain change.
• When mobile hosts change domains, they acquire new network paths to the home agent. In turn, this requires the old QoS guarantee to be torn down, and a new one (along the new route) to be established. Because these QoS setups are costly, they should be minimized.
• The requirements for home and foreign agents incur a robustness penalty. This penalty should be reduced.

Essentially, HAWAII attempts to do this by making the assumption that most of the mobility is within a single logical administrative domain (which may be composed of many physical sub-domains), and then taking steps in order to optimize the system for that type of environment. The HAWAII protocol is driven by five design goals: limit the disruption to user traffic, enable the efficient use of access network resources, a drive to enhance scalability, to provide intrinsic support for QoS, and to enhance reliability.

Protocol Overview:

In HAWAII, a domain is a logical aggregation of networks, that sit behind a common router. When a mobile host is no longer in its home domain, then standard Mobile-IP routing occurs from the home agent to the domain router. The HAWAII protocol operates inside the domain, between the domain router and the mobile host. It is important to note that so long as the mobile host stays within this local domain, its care-of address will not change. The protocol operates over the following types of messages:

• Path setup power-up message: This message constructs a route between the mobile host and the domain router. This state information is only known to the domain router, and all routers on the path.
• Path setup update message: The mobile host uses this type of message to update the necessary routers that it has moved within the domain. The routers that receive this message is determined by the path setup scheme, to be discussed later.
• Path refresh message: The routing state in HAWAII is soft, so the mobile host must periodically send these messages so that the routers know that it is still alive, and that its state should be kept.

HAWAII Path Setup Schemes:

Now, the paper launches into a lengthy discussion of different path setup schemes, which form the bulk of this protocol (this is where they gain their primary performance and reliability improvements). The cross-over router is a router that serves as a common parent to the two base stations that are involved in the transition. During the course of the transition, this router will stop sending to the old base station, and start sending to the new one. Furthermore, no routing path information "above" this router will change.

• Multiple Stream Forwarding (MSF): This scheme uses the standard IP routing infrastructure in order to divert packets from the old base station to the new base station. This method can transmit multiple out-of-order streams to the mobile host, and can be subject to transient routing loops.
• Single Stream Forwarding (SSF): This scheme uses interface-based forwarding, a modification to the IP routers that allows packets sent to the old base station to be diverted to the new base station in a single stream. While SSF performs slightly better than MSF, this slight gain comes at the expense of complex implementation.
• Unicast Non-Forwarding (UNF): Optimized for networks in which the mobile host can listen to multiple base stations simultaneously. When the cross-over router detects that the mobile host has moved (gets new path setup from new base station), it automatically routes packets destined for old base station to the new one.
• Multicast Non-Forwarding (MNF): Uses custom designed "dual-casting" scheme, which is suited for networks that force the mobile host to communicate with one sole base station. This scheme is similar to UNF, except that the cross-over router will multicast packets to both base stations for a short time.

Other Points:

HAWAII has a number of other differences that distinguish it from other micro-mobility solutions. After simulation, the authors found that the localized HAWAII schemes caused less disruption to UDP-based audio and video streams than did Mobile-IP schemes. HAWAII's ability to quickly change routes without notifying the home agent allows it to drop less packets, resulting in less disruption. The authors also argue that HAWAII is scalable. Due to how HAWAII distributes its overhead, it results in much less overhead at the most heavily loaded router as compared to Mobile-IP. Furthermore, HAWAII is better at supporting QoS than competing protocols. HAWAII inter-operates well with existing wire-line QoS protocols because it minimizes changes to the care-of address, in effect, making the mobile host appear more permanent that it really is. Finally, the authors argue that HAWAII's use of soft-state refreshes, and the elimination of the HA when mobile in the home domain, allow it to increase reliability over straight Mobile-IP.

Strengths Of This Design:

• Limits change of mobile host's IP address.
• Eliminates need for HA when roaming in home domain.
• Can adapt to changed base stations within the current domain more quickly than Mobile-IP.

Weaknesses Of This Design:

• Requires new routers and/or daemons.
• They assume that base stations have IP routing functionality.
• Requires state in routers, which always breeds scalability concerns.
• HAWAII incurs a higher processing overhead on limited-power mobile host than Mobile-IP.