CPSC 441
Tutorial-3
Department of Computer Science
University of Calgary
Web Caching

Benefits
 Reduce response time of a client request
 Reduce traffic in the access link

Case Study
Case Study: Default Scenario
Assumptions:






Avg object size: 100K bits
Avg request rate from browsers to
origin servers:15/sec
HTTP request messages are small
Avg data rate to browsers: 1.50 Mbps
Internet delay: 2 sec
Access link rate: 1.54 Mbps
consequences:
problem!
LAN utilization: 15%
 Access link utilization = 97%
 Total delay = Internet delay + access
delay + LAN delay
= 2 sec + minutes + negligible (usecs
to ms)
Note: Utilization of 97% is same as Traffic
Intensity of .97

origin
servers
public
Internet
1.54 Mbps
access link
institutional
network
10 Mbps LAN
Case Study: Fatter access link
Assumptions:


Access link rate: 1.54 Mbps 154 Mbps
Everything else remains the same as
‘Default Scenario’
origin
servers
public
Internet
consequences:



LAN utilization: 15%
Access link utilization = 97% 0.97%
Total delay = Internet delay + access
delay + LAN delay
= 2 sec + minutes + usecs
usecs
1.54 Mbps
154 Mbps
access link
institutional
network
10 Mbps LAN
Cost: increased access link speed (not cheap!)
Case Study: Install local cache
assumptions:


Install local web cache
Everything else remains the same as
‘Default Scenario’
origin
servers
public
Internet
consequences:



LAN utilization: 15%
access link utilization = ?
total delay = Internet delay + access
delay + LAN delay = ?
How to compute link
utilization, delay?
Cost: web cache (cheap!)
1.54 Mbps
access link
institutional
network
10 Mbps LAN
local web
cache
Case Study: Install local cache
Calculating access link
utilization, delay with cache:
 suppose
origin
servers
cache hit rate is 0.4
 40% requests satisfied at cache,
60% requests satisfied at origin
public
Internet
 i.e 60% of requests use access link

Data rate to browsers over access link
= 0.6*1.50 Mbps = 0.9 Mbps
1.54 Mbps
access link
 Access link utilization = 0.9/1.54 * 100
= 58%
 Total
delay
 = 0.6 * (delay from origin servers) +0.4
* (delay when satisfied at cache)
 = 0.6 (2 + ~msecs for access link and
LAN) + 0.4 (~usecs for LAN)
 = ~ 1.2 secs
 less than with 154 Mbps link (and
cheaper too!)
institutional
network
10 Mbps LAN
local web
cache