| This paper presents an IP QoS solution that supports per flow based Call Admission Control (CAC) with Multilevel Priority and Preemption (MLPP). The solution can be used for both wired networks and wireless ad hoc networks and is fully distributed. The principles behind the design of the QoS solution, the implementation in Linux and the results from lab tests are presented.
The flow based CAC is based on passive monitoring of real-time traffic load and individual real-time traffic flows. Passive monitoring is performed on each IP-network device supporting the QoS solution. A flow is given by the 4-tuple: [DST_IP, PROTO, DST_PORT, TOS].
The CAC initially admits all new flows into the network. When the total real-time load exceeds a given threshold, the CAC triggers the MLPP function. The MLPP function preempts the lowest priority and/or latest arrived flows until the real time-load is below the theshold again. Each time a new real-time flow is discovered, the CAC waits for a very short number of packets (e.g. 10) or milliseconds before the bandwidth of the total real-time traffic is recalculated. Thus, although false admissions will occur, the banwidth threshold will typically only be exceeded for a short period before the MLPP function takes effect.
The flow priorites are given by the IP ToS field, and the routers should support multiple strict priority real time classes accordingly (similar to multiple EF classes). Simple signalling is used to report preemption of flows back to the source, such that the source and the nodes along the reverse path (reverse path flow state may be maintained to support such signalling) is able to stop preempted flows. However, the signalling traffic and signalling state is kept to a minimum. The QoS solution therefore works well in conjunction with IPsec devices.
The QoS solution has been implemented on Linux using PCAP packet capture library for monitoring real-time flows and traffic load. IPTABLES is used to block flows and ICMP is used for signalling back to the source and ICMP snooping is used to enable reverse path preemptions. Linux TC (Traffic Control) is used to support Priority Queuing with four strict priority queues for the real time traffic. Other lower priority queues is used for the data traffic. The CAC ansures that resources for data traffic is available.
The solution works with 802.11 WLAN based ad hoc networks, by using promiscuous mode packet capturing. Currently a simple packet length compensation function is used to estimate the physical WLAN traffic load. By mapping the IP level ToS priorites to WLAN, the solution also seamlessly integrates with the 802.11e Enhanced Distributed Coordination Function (EDCF). |