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

2. Web Caching
Telnet
CPSC Application Layer

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
Internet delay: 2 sec
Access link rate: 1.54 Mbps
Consider delay for Access link to be negligible when utilization is less than 80%
consequences:
Download traffic: ?
LAN utilization: ?
Access link utilization : ?
Total delay = Internet delay + access delay + LAN delay
= 2 sec + minutes + negligible (usecs to ms)
origin
servers
public
Internet
1.54 Mbps
access link
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: ?
Access link utilization = ?
Total delay = Internet delay + access delay + LAN delay
=2 sec + minutes + negligible (usecs to ms)
154 Mbps
origin
servers
public
Internet
1.54 Mbps
access link
154 Mbps
institutional
network
negligible (usecs to ms)
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: ?
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
Download rate over the access link =?
Access link utilization = ?
1.54 Mbps
access link
institutional
network
Total delay: ?
less than with 154 Mbps link (and cheaper too!)
10 Mbps LAN
local web
cache

7. Key takeaways Benefits of web caching
Reduce response time of a client request
Reduce traffic in the access link
In your assignment-1, you will be implementing a simple web proxy

8. How to use Telnet?
Establishes TCP connection with any internet service (or server) that runs on TCP
No need for implementation of application layer
It is up to the user what kind of application message needs to be send to the server
You can use telnet to test assignment-1 (apart from using a web browser)
CPSC Application Layer

9. # telnet pages.cpsc.ucalgary.ca 80 // tcp connection established
Open a terminal
# telnet pages.cpsc.ucalgary.ca 80
// tcp connection established
GET HTTP/1.0 <Press Enter>
Host: pages.cpsc.ucalgary.ca <Press Enter>
Connection: close <Press Enter>
<Press Enter>
// HTTP request sent
// HTTP response and data get displayed on the terminal
CPSC Application Layer