Web Caching Web Caching and wireless networks Next generation - PowerPoint PPT Presentation

Web Caching Web Caching and wireless networks Next generation Wireless Networks Helsinki university of technology César Fernández Gago Jean-Baptiste Escoyez 1

Index ■ Web caching concepts ■ Web caching issues  How does it works?  Content delivery  Cache consistency ➔ Wireless issues  Cache cooperation ➔ Wireless issues 2

Introduction: Concepts ■ Web caching  Different scheme: multiple readers, one writer  Browser Cache & Proxy: ➔ Improve response time ➔ Save bandwidth ➔ Real-life usage: hierarchy, reverse ■ Other proxy services ➔ Security - Ensure security policies . Virus . Authentication & Billing (Wireless) 3

Http1.1: How does it work? ¿? Ø Cache miss Slow Hit conditional Fast Hit GET / HTTP/1.1 Host: www.google.fi Proxy-Connection: keep-alive Accept-Encoding: gzip,deflate 4

Http1.1: Optimizing a website ■ Expires ■ Cache-control header:  max-age=[seconds] ➔ s-maxage=[seconds] HTTP/1.1 200 OK  public Date: Fri, 30 Oct 2004 13:19:41 GMT Server: Apache/1.3.5 (Unix)  no-cache Cache-Control: max-age=3600, must-revalidate Expires: Fri, 30 Oct 1998 14:19:41 GMT  no-store Last-Modified: Mon, 29 Jun 2004 02:28:12 GMT ETag: "3e86-410-3596fbbd"  must-revalidate Content-Length: 1040 Content-Type: text/html ➔ proxy-revalidate ■ Validators: Etag, last-modified.. 5

Content delivery ■ On demand request-response (Scales bad) ■ Prefetching  Proxy caching = 26%, prefetching = 60%  Different techniques: ➔ Dependency graph (learning patterns) ➔ Prediction scheme ➔ Server Volumes ■ Multicast push  Asynchronous multicast push (AMP)  Continuous multicast push (CMP)  Geographic push caching ■ Optimizations: send only “differences” 6

Cache Consistency : Problem ■ Multiple copies of objects created  How and when renewing the copies? ■ Goals  Avoid stale copies  Keep non useful traffic as low as possible Sub-domain Internet Client Proxy Server 7

Cache Consistency : Polling ■ Solution 1: Polling every time request poll doc1.html (vx) doc1.html doc1.html (v*) copy returned retrieve if stale Sub-domain Internet Client Proxy Server ■ Benefits : Strong cache consistency ■ Drawbacks : Cache hits are always slow 8

Cache Consistency : Polling ■ Solution 2 : (Widely employed)  Assign a TTL to each document poll if TTL expired request doc1.html doc1.html (vx) doc1.html (v*) copy returned retrieve and reset TTL if stale increase TTL if up-to-date Sub-domain Internet Client Proxy Server ■ Benefits : More fast hits ■ Drawbacks : Weak cache consistency (5% stale)  Can lead to problems (e.g. Weather forecast) 9

Cache Consistency : Invalidation ■ Solution 3 :  The server helps the proxy in maintaining consistency  Invalidation protocols ➔ When the proxy makes a request, - Piggyback cache validation (PCV) : the proxy provides some other potentially stale copies for server validating - Piggyback cache invalidation (PCI) : the server provides some copies which have been updates since last access . Use of volumes - Volume lease : . The client receive a lease from the server . During the lease validity the client can retreive copies from proxy . When the lease expire the client has to renew it 0 1

Cache Cooperation ■ Idea  Makes the proxies cooperating ➔ To improve the access latency ➔ To reduce the traffic load ■ Different techniques  Hierarchical caching ➔ Manually configured  Intercache communication ➔ Protocols to share data ➔ Use of volumes  Transparent caching ➔ Router redirect traffic to the proxy 1 1

The wireless problem ■ Wireless networks:  Save bandwidth, Low power, Client disconnections  Location dependent information services  Movements ➔ Temporal dependent updates ➔ Location dependent updates 2 1

The wireless problem: Cache invalidation ■ Update propagation ■ Update invalidation ■ Stateful-based protocols  Keeping track of each client ■ Stateless-based protocols  asynchronous ➔ If disconnected ?  synchronous (periodic) ➔ Waiting for a long time 3 1

The wireless problem : Cache invalidation ■ Cache invalidation  Bit vector with compression (BVC)  Grouped bit vector with compression (GBVC)  Implicit scope information (ISI) ■ Caching routes  On demand routing protocol (Dynamic Source route protocol) 4 1

The Wireless Problem: Cache cooperation ■ Classical cooperation scheme no valid anymore  Hierarchical cache : too different  Distributed cache : too complexe, heavy ■ Cooperative caching scheme  Cache line: ID, Tag, State bytes, Link fields, Client list and Origine 5 1

Conclusion ■ Cache technologies for wireless networks are not mature yet ■ The possible improvements are not very significant ■ Not a lot of common data to share  Caching issues only have sense in location dependency problems  The wireless schemes are very different of the classical schemes ■ Caching on fixed hosts links is not influenced by wireless networks problems 6 1

Questions? Questions? 7 1

References ➔ Performance Analysis of Location-Dependent Cache Invalidation Schemes for Mobile Environments Jianliang Xu, Member, IEEE, Xueyan Tang, and Dik Lun Lee, March 03 ➔ Caching Strategies in On-Demand Routing Protocols for Wireless Ad Hoc Networks, Yih-Chun Hu, David B. Johnson, Carnegie Mellon University ➔ Cache invalidation scheme for mobile computing systems with real-time data, CM SIGMOD Record archive, Volume 29 , Issue 4 (December 2000) ➔ Network Cache Model for Wireless Proxy Caching James Z., Wang Zhidian, Du Pradip, K Srimani, Department of Computer Science, Clemson University 8 1