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The support of IP-based real-time services in the next-generation mobile systems requires the coupling of mobility with quality of service. The mobile node can experience disruptions or even intermittent disconnections of an ongoing real-time session due to handovers. The duration of such interruptions is called disruption time or handover delay and can heavily affect user satisfaction. Therefore, this delay needs to be minimized to provide good quality VoIP services. In this paper, we focus on the network-layer mobility, specifically on mobile Internet protocols (MIP) since they are natural candidates for providing network layer mobility. Using analytical models, we evaluate MIPv4 and MIPv6 and compare their performances in terms of handover delay for VoIP services. To optimize the handover delay, we propose to use the adaptive retransmission timer described in this paper. The results obtained using the adaptive timer technique show that for 3% frame error rate and a 9.6 Kbps channel, the handoff delay is about 0.67 s for MIPv6 and 0.32 s for MIPv4. MIPv4 gives an appropriate support to VoIP services in terms of handover delay but the end-to-end delay may be affected by the triangular routing avoided in MIPv6.