Lightweight Support for Real-Time Communication in the Future Internet (LightComm)
The objective of this project is to design and develop lightweight support for real-time communication in the Internet. The work is embedded into the efforts of the IETF working group on Congestion and Pre-Congestion Notification (PCN). It aims at the standardization of admission control and flow termination on pre-congestion notification.
A link is "pre-congested" with regard to a certain rate threshold if its traffic rate is above that threshold within some tolerance. This can be effectively controlled by virtual queue or token bucket based algorithms. Packets may be marked in case of pre-congestion such that this marking information notifies egress nodes that rate thresholds have been exceeded somewhere in the network.
PCN-based admission control and flow termination associates an admissible rate and a supportable rate with each link in the network. If the admissible rate is exceeded, all packets are marked with "admission-stop" and if the supportable rate is exceeded, the packets exceeding this rate are marked with "excess traffic". The egress node monitors the markings of its received packets. If the majority of the packets of a specific ingress-egress-aggregate is marked with "admission-stop", the corresponding ingress node stops the admission of further flows. If the egress node receives packets with "excess-traffic" marks from a specific ingress-egress-aggregate, the ingress node is notified to terminate flows to reduce the traffic load in the network. This is only the basic idea, but many variants thereof are proposed.
The advantages of that approach are:
- Interior nodes can be unaware of any reservations, they just perform traffic metering and marking.
- If a link or node failure occurs, traffic is rerouted and possibly causes congestion on backup paths. However, this concept allows to set the admissible rates low enough such that the admitted traffic can still be carried by the network after a protected failure. This is called resilient admission control.
- If a major network outage happens and severe overload occurs inspite of careful capacity overprovisioning, the flow termination function guarantees that the network can be brought back to a state without congestion.
The purpose of this project is to work out suitable PCN mechanisms and to conduct performance studies for discussion in IETF to support the standardization process.