Thursday, September 18, 2008

Supporting Real-Time App in a Integrated Services Packet Network: Architecture and Mechanism (MIT)

The paper introduces the idea of Integrated Services Packet Network (ISPN), which allows real-time (RT) and non-RT traffic to share bandwidth. It classifies three types of services: guaranteed, predicted, and datagram. Guaranteed is real-time service that has a hard deadline of delivery. Clients need to declare a usage rate before being admitted into the network. Predicted is more like "soft" real-time, where the delay bound of delivery is assumed to be adaptive and adjusted by previous experienced delay. Datagram is relatively time-insensitive but is reserved with some minimal amount (10%, chosen in the paper) of BW to ensure progress.

Jitter is a main problem in RT services. Many reasons lead to jitters. The transport layer, in particular, induces jitter from multiplexing packets from different sources. Packets get delayed because of delivery of packets of other unrelated sources. This is the commonly known side-effect of sharing. Interestingly, the author suggests that jitter can be minimized if sharing is prioritized.

Isolation is inevitable for hard real-time services, but isolation is expensive. Providing a integrated service can be seen as tradeoff between cost and performance. At the same time, users can easily switch between RT and non-RT servicing. This may allow future applications to have a more dynamic and finer-grained timing behavior.

For soft RT and non-RT servicing, the author proposes FIFO+, which incoporate the concept of priority into FIFO, to mimize the max delay (jitter). It uses the expected instead of actual arrival time. The expected arrival time is driven by the packet's deadline.

Priority can be set at different granularity. The common way is to associated a priority number with each connection link. In multihop environment, the paper suggests that priority of packets can change at each hop. This leads to a more dynamic and flexible scheduling, since packets can now "accelerate" to catch up with their deadline.

The paper is nice in that it shows a new dimension, timing, in network service. It also shows how to incorporate this new "stricter" service with the traditional services.

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