1. Parallel Access For Mirror Sites in the Internet Yu Cai

2. Paper Parallel Access For Mirror Sites in the Internet Pablo Rodriguez, et al. Infocom 1999

3. What is it about? Parallel download from multiple mirror sites to speed up performance

4. Background Popular sites are mirrored or cached on geographically dispersed multiple sites. HTTP 1.1 byte-range header protocol enable clients to download a specific portion of an object. –For example, a document is 500 k bytes, and mirrored in 5 sites. –We can download the range of byte 0 to 100k from site 1, and 100 to 200k from site 2, and so on so forth. –This mechanism enable the parallel access for multiple mirror sites.

5. Questions Answered question: - practical –What is the downloading unit? –What is the strategy to pick mirror sites Unanswered question: - graphic theory –What is the maximum download speed? –How many mirror sites to pick? –What are those mirror sites? –What is the impact of bottleneck?

6. Contribution of the paper Develop parallel access schemes to download from multiple mirror servers –History based scheme –Dynamic scheme Extensive experimental result

7. History based scheme The download size is proportional to the server/path download rate. Example: –a 600 k byte documents, 3 mirror sites, –download rate for each site is 1, 2 and 3 byte/second –So we can download 0-100k (100k) byte from site 1, 100-300k (200k) byte from site 2, and 300-600k (300k) byte from site 3 –All downloads finish at the same time, no additional waiting time for reassemble.

8. History based scheme History based scheme get the server rate from previous data It works well when the network/server condition don’t change too much and too often. In dynamic environment, it may perform bad due to the inaccurate estimation. –Everyone needs to wait for the slowest download

9. History based scheme

10. Dynamic scheme Partition the document into small units. Request different unit from each server. If a server finish the downloading, the client request another un-downloaded unit from it. For the last unit, all the free servers should start the downloading. Until all units are downloaded.

11. Dynamic scheme Find an appropriate downloading unit. –The unit size should be large enough to reduce the transmission overhead –The unit size should be small enough to provide fine granularity. –It is determined by experiment. –Usually the document should be divided into 4 – 40 units.

12. Dynamic scheme

13. Work left ? Graphic theory analysis –What is the maximum download speed? –How many mirror sites to pick? –What are those mirror sites? –What is the impact of bottleneck?