Proactive Handoff

Overview

New deployment scenarios tend to consider the requirement of session continuity for service provisioning, especially multimedia streaming, to limited heterogeneous portable devices roaming among wireless localities. In particular, multimedia streaming should not experience any interruption while clients roam in wired-wireless integrated networks based on the standard best-effort Internet.

The MUM approach aims to proactively overcome Wi-Fi handoff and maintain multimedia session continuity in the wireless Internet by exploiting mobile proxies running on the wired network. Mobile middleware proxies locally support resource-limited clients, avoid packet losses during handoffs, prefetch local buffers with multimedia contents before handoff occurrence.

Architecture

MUM exploits a proxy-based architecture to hide handoff occurrences and consequent data losses to mobile clients. Proxy-based solutions can also reduce client-to-server signaling during handoffs. Continuous services usually adopt connection-less protocols such as UDP, and manage data re-transmissions directly at the application level to react to high jitter and packet losses. In the fixed Internet, the deriving flow-control signaling is fairly limited and present only during network congestions/failures. On the contrary, Wi-Fi hard handoff causes relevant packet losses and is perceived as a network failure at the application level; in traditional multimedia systems, that may produce non-negligible client-to-server signaling and wrong perceptions of client situations at server side.

Middleware proxies located at wired network edges close to their served wireless clients can split the direct client/server connection and significantly reduce both signaling traffic on the service path and QoS degradation at client. It is crucial to have mobile proxies that can follow client roaming at provision time to maintain co-locality with their supported devices during service sessions.

Moreover, MUM distinguishes three main types of handoff:

• Micro handoff (intra-subnet handoff) relates to clients that roam between two different Wi-Fi cells without changing IP addresses, i.e., before and after handoff clients are attached to the same subnets. Nonetheless, given that Wi-Fi handoff is hard, micro handoff may produce packet losses when clients switch from origin to target APs. The total micro handoff duration may even reach 2s, thus being incompatible with the jitter/packet loss requirements of usual WI continuous services.
• Macro handoff (intra-domain handoff) refers to clients that move between two Wi-Fi cells attached to different IP subnets and includes network-layer handoff, i.e., client IP address change. Macro handoff duration exceeds micro handoff of the time needed to get new IP addresses at target subnets, e.g., DHCP requires some seconds to complete address re-configuration.
• Global handoff (inter-domain handoff), instead, regards mobile clients that roam between two Wi-Fi cells attached to different Internet domains. This requires not only address change, but also some additional time to perform authentication, authorization, and accounting operations, usually necessary when entering a new access domain. Moreover, domain change could also require session re-configuration: for instance, in the new domain there could be a new server, functionally equivalent to the one used in the origin domain, with better QoS and/or lower pricing.

The mobile proxy in charge of handoff management is called Handoff Agent (HA), and follows user movements by moving over the fixed network.

Client-side data buffering is a common solution in streaming over wired networks to smooth possible congestions and packet losses along client-to-server paths. Nonetheless, we claim the unsuitability of traditional buffering, i.e., only pre-fetching chunks of multimedia flows at clients, to support multimedia continuity during Wi-Fi handoffs. Therefore, the HA hosts second level buffers that contribute to smooth handoff packet losses, without imposing too heavy data buffeing at clients.

Work in progress...