1. Department of Computer Science University of Calgary
CPSC 441 Tutorial-3
Department of Computer Science
University of Calgary

2. Web Caching Benefits Case Study
Reduce response time of a client request
Reduce traffic in the access link
Case Study

3. 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:
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
problem!
institutional
network
10 Mbps LAN

4. Case Study: Fatter access link
Assumptions:
Access link rate: 1.54 Mbps
Everything else remains the same as ‘Default Scenario’
consequences:
LAN utilization: 15%
Access link utilization = 97%
Total delay = Internet delay + access delay + LAN delay
= 2 sec + minutes + usecs
154 Mbps
origin
servers
public
Internet
0.97%
1.54 Mbps
access link
154 Mbps
institutional
network
usecs
10 Mbps LAN
Cost: increased access link speed (not cheap!)

5. Case Study: Install local cache
assumptions:
Install local web cache
Everything else remains the same as ‘Default Scenario’
consequences:
LAN utilization: 15%
access link utilization =
total delay = Internet delay + access delay + LAN delay =
origin
servers
public
Internet ?
1.54 Mbps
access link ?
institutional
network
How to compute link
utilization, delay?
10 Mbps LAN
local web
cache
Cost: web cache (cheap!)

6. Case Study: Install local cache
Calculating access link utilization, delay with cache:
suppose cache hit rate is 0.4
40% requests satisfied at cache, 60% requests satisfied at origin
origin
servers
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
Access link utilization = 0.9/1.54 * 100 = 58%
1.54 Mbps
access link
institutional
network
Total delay
= 0.6 * (delay from origin servers) +0.4 * (delay when satisfied at cache)
= 0.6 (2 + ~msecs for access link and LAN) (~usecs for LAN)
= ~ 1.2 secs
less than with 154 Mbps link (and cheaper too!)
10 Mbps LAN
local web
cache