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Abstract : |
IP multicast is an efficient point-to-multipoint distribution mechanism. However, there are a number of scenarios in which a reverse, multipoint-to-point aggregation mechanism is highly desirable. We introduce a programmable mechanism, called gathercast, to support the aggregation of packets without altering any of the routing or forwarding mechanisms of the Internet. Gathercast is based on active services framework and can be deployed incrementally. It works well within the current IP multicast model. We have implemented it in our own network. One of the aggregation mechanisms that we study in this paper is the combination of small packets using gathercast. Small packets constitute a large fraction of packets in today's Internet. Every packet requires a routing table lookup at every router and incurs the same performance cost irrespective of the size of the packet. Gathercast allows for car pooling, in which multiple small packets to the same destination are combined into a larger packet, thereby reducing the number of packets in the network. Web servers, in particular, are a leading cause of small packets. All the TCP ACKs generated by the clients have the same destination (the web server). These ACKs are just 40 bytes in size. Another example of small packet traffic is ICP (Internet Cache Protocol) queries generated among web proxies. ICP queries are about 60 bytes in size. Gathercast combines such small packets into a larger packet (and regenerates the original packets at the destination), thereby reducing the number of packets seen by routers in the network. Simulation studies show that gathercast-enhanced ICP scales with the number of proxies, and reduces the the number of generated UDP messages by a factor of 16. Simulation studies also show 11 % to 68 % reduction in packet loss and 2 % to 30 % improvement in performance of busy web servers when used in conjunction with gathercast. 1, |
