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Content Distribution Networks

mura avatar
mura
23 diapositives · 345 Views · Oct 12, 2014

This article explores the implementation techniques, hashing schemes, redirection strategies, and design space considerations for Content Distribution Networks (CDNs). Topics include caching, replication, server farms, DDoS protection, and redirection overlay. Various redirection strategies such as Consistent Hashing, Random Weight, and Dynamic Replication are discussed in detail. Simulation experiments are conducted to assess capacity, latency, and scalability of different CDN strategies under normal operation and DDoS attacks.

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Content Distribution Networks
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  1. Content Distribution Networks Outline Implementation Techniques Hashing Schemes Redirection Strategies

  2. Design Space • Caching • explicit • transparent (hijacking connections) • Replication • server farms • geographically dispersed (CDN)

  3. Story for CDNs • Traditional: Performance • move content closer to the clients • avoid server bottlenecks • New: DDoS Protection • dissipate attack over massive resources • multiplicatively raise level of resources needed to attack

  4. client client client attacker Denial of Service Attacks (DoS) server

  5. zombie slave attacker zombie zombie zombie Distributed DoS (DDoS) client attacker client server client

  6. R R R R R R R R Redirection Overlay Geographically distributed server clusters Internet Backbone clients Distributed request-redirectors

  7. Techniques • DNS • one name maps onto many addresses • works for both servers and reverse proxies • HTTP • requires an extra round trip • Router • one address, select a server (reverse proxy) • content-based routing (near client) • URL Rewriting • embedded links

  8. Redirection: Which Replica? • Balance Load • Cache Locality • Network Delay

  9. % Hashing Schemes: Modulo • Easy to compute • Evenly distributed • Good for fixed number of servers • Many mapping changes after a single server change svr0 URL (key) svrN

  10. url-0 svr0 svrN svr1 url-1 svr2 Consistent Hashing (CHash) • Hash server, then URL • Closest match • Only local mapping changes after adding or removing servers • Used by State-of-the-art CDNs Unit circle

  11. weightN weight0 weight2 weight1 sort RH svrN svr svr2 svr1 svr0 svr Highest Random Weight (HRW) high URL • Hash(url, svrAddr) • Deterministic order of access set of servers • Different order for different URLs • Load evenly distributed after server changes weight0 low

  12. Redirection Strategies • Random (Rand) • Requests randomly sent to cooperating servers • Baseline case, no pathological behavior • Replicated Consistent Hashing (R-CHash) • Each URL hashed to a fixed # of server replicas • For each request, randomly select one replica • Replicated Highest Random Weight (R-HRW) • Similar to R-CHash, but use HRW hashing • Less likely two URLs have same set of replicas

  13. Redirection Strategies (cont) • Coarse Dynamic Replication (CDR) • Using HRW hashing to generate ordered server list • Walk through server list to find a lightly loaded one • # of replicas for each URL dynamically adjusted • Coarse grained server load information • Fine Dynamic Replication (FDR) • Bookkeeping min # of replicas of URL (popularity) • Let more popular URL use more replicas • Keep less popular URL from extra replication

  14. Simulation • Identifying bottlenecks • Server overload, network congestion… • End-to-end network simulator prototype • Models network, application, and OS • Built on NS + LARD simulators • 100s of servers, 1000s of clients • >60,000 req/s using full-TCP transport • Measure capacity, latency, and scalability

  15. R R R R R R R R R R R R R R R R R S S S S S S S S S C C C C C C C C C C C C C Network Topology WA MI MA IL PA CA NE DC CO GA SD CA TX S – Server, C – Client, R - Router

  16. Simulation Setup • Workload • Static documents from Web Server trace, available at each cooperative server • Attackers from random places, repeat requesting a subset of random files • Simulation process • Gradually increase offered request load • End when servers very heavily overloaded

  17. Capacity: 64 server case Normal Operation A single server can handle ~600 req/s in simulation

  18. Capacity: 64 server case Under Attack (250 zombies, 10 files, avg 6KB) A single server can handle ~600 req/s in simulation

  19. Latency: 64 Servers Under Attack Random’s Max: 11.2k req/s R-CHash Max: 19.8k req/s

  20. Latency At CDR’s Max: 35.1k req/s

  21. Capacity Scalability Normal Operation Under Attack (250 zombies, 10 files)

  22. Various Attacks (32 servers) 1 victim file, 1 KB 10 victim files, avg 6KB

  23. Deployment Issues • Servers join DDoS protection overlay • Same story as Akamai • Get protection and performance • Clients use DDoS protection service • Same story as proxy caching • Incrementally deployable • Get faster response and help others