1. DotSlash – or how to deal with 15 minutes of fame Weibin Zhao Henning Schulzrinne Columbia University CATT/WICAT Annual Research Review November 14, 2003

2. 11/14/2003 DotSlash2 Web Hotspots A sudden, dramatic surge of request rate Hard to predict – e.g., external reference to web site Build up quickly Large magnitude Short-term overload Long delay or no service One-time event “15 minutes of fame” Small web sites, e.g., on DSL or cable modem Thus, traditional solutions (Akamai, etc.) not economical

3. 11/14/2003 DotSlash3 Existing Mechanisms Capacity planning  high-end servers, LAN clusters, WAN mirrors Fixed capacity, cost CDNs Advance set-up, cost Only effective for large servers Caching Not fully controlled by the server Admission control No service for some clients

4. 11/14/2003 DotSlash4 DotSlash Approach Enable a site to utilize spare capacity at other sites via dynamic collaboration  Rescue Extending capacity dynamically as needed Usage models: mainly for mutual aid, could be commercial Focusing on small sites Advantages Client transparent: no browser changes Low cost: based on existing systems, no additional hardware cost

5. 11/14/2003 DotSlash5 Rescue Example-1: origin.com rescue.com Client1 www.origin.com 1.2.3.4 www.rescue.com 5.6.7.8 (1) www.origin.com (2) 1.2.3.4 (3)(4) HTTP redirect alias1.rescue.com (8) Reverse proxy (5) alias1.rescue.com (6) 5.6.7.8 (9) (10)(7) Dynamic DNS DNS RR origin.com DNS rescue.com DNS cache HTTP redirect & Cache miss

6. 11/14/2003 DotSlash6 Rescue Example-2: origin.com rescue.com www.origin.com 1.2.3.4 www.rescue.com 5.6.7.8 Dynamic DNS DNS RR origin.com DNS rescue.com DNS cache Client2 (2) 5.6.7.8 (1) www.origin.com (4) (3) DNS RR & Cache hit

7. 11/14/2003 DotSlash7 DotSlash Functions Workload monitoring Rescue server discovery Enhanced SLP: mSLP + Select/Sort extensions Request distribution and redirect DNS RR + HTTP redirect Dynamic replication of content Reverse caching proxy Rescue relationship management Allocate/release rescue servers

8. 11/14/2003 DotSlash8 Basic Architecture Distributed system + dynamic virtual hosting Distributed: multiple servers for one site Virtual hosting: one server for multiple sites Components Apache module (Mod_dots), DotSlash daemon (Dotsd), dynamic DNS (BIND), service discovery (enhanced SLP) BINDmSLP HTTP SHM SLP DNS TCP Other Dotsd Client Apache Dotsd Mod_dots

9. 11/14/2003 DotSlash9 Server Status Normal Rescue SOS Allocate rescue serversRelease rescue servers As a rescue serverNot as a rescue server Origin server Ask help from others Rescue server Provide help to others Regular server Otherwise

10. 11/14/2003 DotSlash10 Workload Monitoring Bottlenecks Network bandwidth CPU Memory Metrics Output data rate (HTTP) KBytes/second CPU load average Num concurrent conns MaxClients CPUNetMem Overload Normal OR Gate AND Gate

11. 11/14/2003 DotSlash11 Origin Server Workload monitoring HTTP redirect Use rescue server aliases Don’t redirect requests from rescue servers Redirect Policies WRR based on rescue server capacity (load) URL Hashing (URL locality) for large files DNS round-robin Add/remove rescue server IP addresses

12. 11/14/2003 DotSlash12 Rescue Server Workload monitoring Reverse caching proxy Memory caching: concurrency control among threads Check Host: header: A regular request A reverse proxy request to origin server (active rescue) An HTTP redirect to origin server (expired rescue) Dynamic DNS update Register new aliases

13. 11/14/2003 DotSlash13 Rescue Server Management Allocation Initial allocation: based on origin server load Additional allocation: based on rescue server load Policy: double current capacity Release When load is consistently low Release one rescue server at a time Partial release: the min-capacity rescue server Final release: the last rescue server

14. 11/14/2003 DotSlash14 Other Components Testing tool Use httperf from HP Labs, and extend it to handle HTTP redirects automatically Dynamic DNS Use BIND 9.2.2, set up a testing domain: dotslash.cs.columbia.edu, and only allow updates from specific hosts Service discovery Use SLP with mesh-enhancement (mSLP), and Select, Sort and Attribute List extensions

15. 11/14/2003 DotSlash15 Testing Environment Apache 2.0.47 With proxy and caching modules Enable: caching and mod_dots OS Redhat 9, Linux kernel 2.4.20-20.9, hosts from Clic Lab Redhat 7.3, Linux kernel 2.4.19- 6_planetlab, nodes from PlanetLab

16. 11/14/2003 DotSlash16 Conclusion Preliminary version implemented Basic measurement and control flow Working on service location On-going work Performance measurement Apply to dynamic contents LAMP: Linux+ Apache + MySQL + PHP