مدل سازی و تجزیه و تحلیل عملکرد برای ترافیک IPv6 با کلاس های QoS چندگانه

This paper focuses on the modeling and performance analysis for IPv6 traffic with multi-class QoS in virtual private networks (VPN). The multi-class QoS is implemented on differentiated service basis using priority scheme of 4 bits defined in the packet header of IPv6. A VPN-enabled IP router is modeled as a tandem queuing system in which each output link consists of two parallel priority output queues. The high-priority queue is used to carry the delay sensitive traffic while the low-priority queue is used to carry the delay insensitive traffic. On the other hand, multiple thresholds are implemented in each queue, respectively, for packet loss priority control. The performance analysis is done using fluid flow techniques. The numerical results obtained from the analysis show that the differentiated service based on the priority schemes defined in IPv6 is able to effectively satisfy the multi-class QoS requirement for supporting multimedia services in VPN. The performance trade-off between the delay sensitive traffic and delay insensitive traffic in terms of traffic throughput, packet loss probability and end-to-end delay in VPN networks is presented.

Currently, there is a significant interest in the development of virtual private network (VPN) over IP backbone. VPN is an enterprise network based on shared public network infrastructure but employing the same security, management and throughput policies as applied in a private network [13]. Comparing with the existing private networks, VPN is a more cost-effective mean of building and deploying private communication networks for multi-site communication, especially when IPv6 over broadband Internet is implemented. VPN is also able to support multimedia services such as voice, video, data and image transfer applications. For an IP-based VPN [16], the service provider connects multiple IP addresses located at geographically dispersed sites as appearing to be within a private network. As shown in Fig. 1, VPN can be implemented using VPN-enabled router [13] which plays the network layer functions in the TCP/IP protocol suite to support network security, network routing connectivity and QoS parameters. Full-size image (11 K) Fig. 1. VPN over IP backbone. Figure options One of the typical problems with the implementation of VPN over Internet is the difficulty of QoS guarantee. The Internet Engineering Task Force (IETF) has recommended a differentiated service mechanism for the traffic with different QoS requirement on priority basis [17] and [18]. IPv6 [2] is ideally designed for supporting such differentiated services [17]. However, in current IP-based networks, when the data are encrypted, it may be difficult for the network to determine the class-of-service based on packet content in the network layer, especially when a multiple class of QoS is involved. By contrast, this can be done in IP-based VPN which has the advantage that the class of service can be stated outside the VPN envelop of the IP packets [15]. Current QoS issues involved in VPN mainly focus on call admission control level [20] and [21] to determine the network ability for assigning network resources to mission-critical or delay-sensitive services while limiting resources committed to low-priority traffic as an essential component of VPN solution. According to service level agreements (SLA), IP traffic with multi-class of QoS from different users are classified and stored in separate buffers before they are transmitted in the network. The disadvantages of such mechanism are that (1) it complicates the implementation using separate buffers for different QoS classes, (2) the buffer utilization is inferior and (3) the re-sequencing is required at the destination which is highly undesirable for the traffic with different QoS classes but from the same user source. This paper focuses on the issues to improve QoS at IP packet level. The multi-class QoS is implemented on differentiated service basis using priority scheme of 4 bits defined in the packet header of IPv6. A VPN-enabled IP router is modeled as a tandem queuing system in which each output link consists of two space-priority output queues. The high priority queue is used to carry the delay sensitive traffic while the low priority queue for the delay insensitive traffic. On the other hand, multiple thresholds are implemented in each queue, respectively, for packet loss priority control [7]. The performance analysis is done using fluid flow technique [10] and [11]. The evaluation focuses on the performance trade-off between the delay sensitive traffic and delay insensitive traffic in terms of traffic throughput, packet loss probability and end-to-end delay in VPN networks are also presented.

The introduction of multi-class priority defined in IPv6 traffic on priority basis makes the QoS control in VPN more flexible. The numerical results show that incorporating priority scheme defined by IPv6 in VPN is able to improve the performance for the high priority traffic classes. However, the priority schemes do not reduce the total packet loss but do protect the high priority traffic from packet loss while allowing the performance of the low priority traffic to degrade as little as possible, especially when the traffic loading and the threshold value are properly controlled. The behavior of multi-class priority scheme is studied with a variety of traffic conditions. The obtained results show that the high priority traffic improve vastly with the use of multi-class priority scheme under the condition that the proportion of high priority traffic (for both the high priority offered load and the ratio of high priority sources) must be kept to a small percentage. On the other hand, the burstiness of the traffic must also be carefully controlled.