1. CS 372 – introduction to computer networks* Wednesday June 30
Announcements:
Assignment 2 is posted.
* Based in part on slides by Bechir Hamdaoui and Paul D. Paulson.
Acknowledgement: slides drawn heavily from Kurose & Ross
Chapter 2, slide:

2. Chapter 2: Application layer
Principles of network applications
app architectures
app requirements
Web and HTTP
File transfer: FTP
P2P file sharing
Socket programming with TCP
Chapter 2, slide:

3. Web and HTTP First some terminologies: Web page consists of objects
Object can be HTML file, JPEG image, Java applet, audio file,…
Web page consists of base HTML-file which includes several referenced objects
Each object is addressable by a URL
Example URL (Uniform Resource Locator):
host name
path name
Chapter 2, slide:

4. HTTP overview: app architecture
HTTP: hypertext transfer protocol
Web’s appl-layer protocol
client/server model
client: browser that requests, receives, “displays” Web objects
server: Web server sends objects in response to requests
HTTP 1.0: RFC 1945
HTTP 1.1: RFC 2068
HTTP request
PC running
Explorer
HTTP response
HTTP request
Server
running
Apache Web
server
HTTP response
Mac running
Navigator
Chapter 2, slide:

5. HTTP overview (continued)
Uses TCP:
client initiates TCP connection (creates socket) to server, port 80
server accepts TCP connection from client
HTTP messages exchanged between browser (HTTP client) and Web server (HTTP server)
TCP connection closed
HTTP is “stateless”
server maintains no information about past client requests
aside
Protocols that maintain “state” are complex!
past history (state) must be maintained
if server/client crashes, their views of “state” may be inconsistent, must be reconciled
Chapter 2, slide:

6. HTTP connections Nonpersistent HTTP
At most one object is sent over a TCP connection.
HTTP/1.0 uses nonpersistent HTTP
Persistent HTTP
Multiple objects can be sent over single TCP connection between client and server.
HTTP/1.1 uses persistent connections in default mode
Chapter 2, slide:

7. Nonpersistent HTTP
(contains text,
references to 10
jpeg images)
Suppose user enters URL
1a. HTTP client initiates TCP connection to HTTP server (process) at on port 80
1b. HTTP server at host waiting for TCP connection at port 80. “accepts” connection, notifying client
2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDepartment/home.index
3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket
time
Chapter 2, slide:

8. Nonpersistent HTTP (cont.)
4. HTTP server closes TCP connection.
5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects
time
6. Steps 1-5 repeated for each of 10 jpeg objects
Chapter 2, slide:

9. Non-Persistent HTTP: Response time
Definition of RTT (round trip time): time to send a small packet to travel from client to server and back.
Response time:
one RTT to initiate TCP connection
one RTT for HTTP request and first few bytes of HTTP response to return
file transmission time
= 2RTT + transmit time
time to
transmit
file
initiate TCP
connection
RTT
request
received
time
Chapter 2, slide:

10. Non-Persistent HTTP: issues
time to
transmit
file
initiate TCP
connection
RTT
request
received
time
Nonpersistent HTTP:
Name some issues??
requires 2 RTTs per object
E.g., a 10 object-page needs ~ 20 RTTs
Open/close TCP connection for each object => OS overhead
Any ideas for improvement?
Chapter 2, slide:

11. Persistent HTTP Persistent HTTP
server leaves connection open after sending response
subsequent HTTP messages between same client/server sent over open connection
reduces response time
Persistent without pipelining:
client issues new request only when previous response has been received
one RTT for each referenced object
Persistent with pipelining:
default in HTTP/1.1
client sends requests as soon as it encounters a referenced object
as little as one RTT for all the referenced objects
Chapter 2, slide:

12. HTTP request message two types of HTTP messages: request, response
ASCII (human-readable format)
request line
(GET, POST,
HEAD commands)
GET /somedir/page.html HTTP/1.1
Host:
User-agent: Mozilla/4.0
Connection: close
Accept-language:fr
(extra carriage return, line feed)
header
lines
Carriage return,
line feed
indicates end
of message
Chapter 2, slide:

13. HTTP response message status line (protocol status code status phrase)
HTTP/ OK
Connection close
Date: Thu, 06 Aug :00:15 GMT
Server: Apache/1.3.0 (Unix)
Last-Modified: Mon, 22 Jun 1998 …...
Content-Length: 6821
Content-Type: text/html
data data data data data ...
header
lines
Entity body:
data, e.g.,
requested
HTML file
Chapter 2, slide:

14. HTTP response status codes
In first line in server -> client response message.
A few sample codes:
200 OK
request succeeded, requested object later in this message
301 Moved Permanently
requested object moved, new location specified later in this message (Location:)
400 Bad Request
request message not understood by server
404 Not Found
requested document not found on this server
505 HTTP Version Not Supported
Chapter 2, slide:

15. Trying out HTTP (client side) for yourself
1. Telnet to your favorite Web server:
telnet cis.poly.edu 80
Opens TCP connection:
to port 80 (default HTTP port)
at cis.poly.edu (~ IP address)
anything typed is sent there
2. Type in a GET HTTP request:
By typing this in (hit carriage
return twice), you send
this minimal (but complete)
GET request to HTTP server
GET /~ross/ HTTP/1.1
Host: cis.poly.edu
3. Look at response message sent by HTTP server!
Try it at home or lab!
Chapter 2, slide:

16. Web and HTTP: Review Question
A HTTP request consists of:
1 basic html object
2 referenced JPEG objects
All objects of size = 106bits
RTT = 1 second
Transmission rate = 1Mbps
Consider transmission delay of
objects only
Question: how long it takes to receive the entire page:
Non-persistent connection
Persistent without pipelining
Persistent with pipelining
time to
transmit
file
initiate TCP
connection
RTT
request
received
time
Chapter 2, slide:

17. Web and HTTP: Review Question
A HTTP request consists of:
1 basic html object
2 referenced JPEG objects
All objects of size = 106bits
RTT = 1 second
Transmission rate = 1Mbps
Consider transmission delay of
objects only
Answer: (transmit time = 1 sec)
a) 3+3+3=9 sec
initiate + request + transmit for each of all 3
b) =7 sec
initiate + (request + transmit for each of all 3)
c) 1+2+3=6 sec initiate + (request + transmit for basic) + (one request for 2 + two transmits, one for each of the 2 objects)
time to
transmit
file
initiate TCP
connection
RTT
request
received
time
Chapter 2, slide:

18. User-server state: cookies
Example:
Susan always access Internet always from PC
visits specific e-commerce site for first time
when initial HTTP requests arrives at site, site creates:
unique ID
entry in backend database for ID
Many major Web sites use cookies
Four components:
1) cookie header line of HTTP response message
2) cookie header line in HTTP request message
3) cookie file kept on user’s host, managed by user’s browser
4) back-end database at Web site

19. Cookies: keeping “state” (cont.)
client
server
ebay 8734
usual http request msg
Amazon server
creates ID
1678 for user
create
entry
cookie file
usual http response
Set-cookie: 1678
ebay 8734
amazon 1678
usual http request msg
cookie: 1678
cookie-
specific
action
access
usual http request msg
cookie: 1678
cookie-
spectific
action
access
usual http response msg
backend
database
one week later:
ebay 8734
amazon 1678
usual http response msg

20. Cookies (continued) Cookies and privacy: What cookies can bring:
aside
Cookies and privacy:
cookies permit sites to learn a lot about you
you may supply name and to sites
What cookies can bring:
authorization
shopping carts
recommendations
user session state (Web )
How to keep “state”:
protocol endpoints: maintain state at sender/receiver over multiple transactions
cookies: http messages carry state

21. Web caches (or proxy server)
Goal: satisfy client request without involving origin server
If page is needed, browser requests it from the Web cache
Q: what if object not in cache??
cache requests object from origin server, then returns object to client
origin
server
HTTP response
Proxy
server
HTTP request
client
HTTP request
HTTP response
client
origin
server
Chapter 2, slide:

22. More about Web caching cache acts as both client and server
typically cache is installed by ISP (university, company, residential ISP)
Why Web caching?
reduce response time for client request
reduce traffic on an institution’s access link.
Chapter 2, slide:

23. Caching example Assumptions Consequences origin servers
avg. object size = 0.1x106bits
avg. request rate from institution to origin servers = 10/sec
Internet delay = 2 sec
Consequences
utilization on LAN = 10%
(LAN: local area network)
utilization on access link = 100%
total delay = Internet delay + access delay + LAN delay
= 2 sec + minutes + milliseconds
public
Internet
1 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache
Chapter 2, slide:

24. Caching example (cont)
origin
servers
possible solution
increase bandwidth of access link to, say, 10 Mbps
consequence
utilization on LAN = 10%
utilization on access link = 10%
Total delay = Internet delay + access delay + LAN delay
= 2 sec + msecs + msecs
often a costly upgrade
public
Internet
10 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache
Chapter 2, slide:

25. Caching example (cont)
origin
servers
possible solution: web cache
suppose hit rate is 0.4
consequence
40% requests will be satisfied almost immediately
60% requests satisfied by origin server
utilization of access link reduced to 60%
total avg delay will also be reduced
public
Internet
1 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache
Chapter 2, slide:

26. Web cache (cont) Solution Advantages are obvious:
Upon receiving a web request, a cache must consult origin server to check whether the requested page is up-to-date
Conditional GET method
What:
Sent by cache to origin server: check page status
When:
For each new request: client checks with cache
Advantages are obvious:
Reduce response time
Reduce internet traffic
Any problems with caches??
Local cache copies of web pages may not be up-to-date??
What do we do then?
Chapter 2, slide:

27. If-modified-since: <date> If-modified-since: <date>
Conditional GET
cache
server
HTTP request msg
If-modified-since: <date>
HTTP response
HTTP/1.0
304 Not Modified
object
not
modified
Goal: don’t send object if cache has up-to-date version
How: cache specifies date of cached copy in HTTP request
If-modified-since: <date>
Server: response contains no object if cached copy is up-to-date:
HTTP/ Not Modified
HTTP request msg
If-modified-since: <date>
HTTP response
HTTP/ OK
<data>
object
modified
Chapter 2, slide:

28. Chapter 2: Application layer
Principles of network applications
Web and HTTP
FTP
P2P file sharing
Socket programming with TCP
Chapter 2, slide:

29. FTP: the file transfer protocol
user
interface
client
file transfer
FTP
server
user
at host
remote file
system
local file
system
transfer file to/from remote host
client/server model
client: that initiates transfer (to or from remote)
server: remote host
ftp: RFC 959
Chapter 2, slide:

30. FTP: separate control, data connections
FTP client contacts FTP server to open TCP control connection
client browses remote directory by sending commands over the TCP control connection.
TCP control connection
port 21
TCP data connection
port 20
FTP
client
FTP
server
when server receives file transfer command, server opens 2nd TCP data connection (for file) to client
after transferring one file, server closes TCP data connection.
Chapter 2, slide:

31. FTP: separate control, data connections
More on FTP
server opens a separate TCP data connection for each file to transfer
TCP control stays on
control connection: called: “out of band”
FTP
client
server
TCP control connection
port 21
TCP data connection
port 20
FTP server maintains “state”: current directory, earlier authent. => limit on concurrent connections
Chapter 2, slide:

32. Chapter 2: Application layer
Principles of network applications
Web and HTTP
FTP
P2P file sharing
Socket programming with TCP
Chapter 2, slide:

33. File sharing approaches
server
peers
Alice
Bob
There are 2 approaches
Centralized:
Client-server architecture
Distributed:
P2P architecture
(e.g., BitTorrent)
obtain list
of peers
trading
chunks
peer
server
Chapter 2, slide:

34. File sharing: P2P vs. client-server architectures
Single point of failure
Not scalable
More secure
Bottleneck
P2P
Fault-tolerant
Scalable
Less secure
Better
Robustness to failure
Scalability
Security
Performance
Chapter 2, slide:

35. Comparing Client-Server, P2P architectures
Question : What is the file distribution time: from one server to N hosts? us u2 d1 d2 u1 uN dN
Server
Network (with
abundant bandwidth)
File, size F
us: server upload bandwidth
ui: client/peer i upload bandwidth
di: client/peer i download bandwidth
Chapter 2, slide:

36. Client-server: file distribution time
server sequentially sends N copies:
NF/us time
client i takes F/di time to download F u2 u1 d1 d2 us
Network (with
abundant bandwidth) dN uN
= dcs > max { NF/us, F/min(di) } i
Time to distribute F
to N clients using
client/server approach
increases linearly in N
(for large N)
lower bound is also
achievable
Chapter 2, slide:

37. P2P: file distribution time
server must send one copy: F/us time
client i takes F/di time to download
Server F u2 u1 d1 d2 us
Network (with
abundant bandwidth) dN
NF bits must be downloaded
- NF bits must be uploaded
- Fastest possible upload rate
(assuming all nodes sending file chunks to same peer) is:
us + Sui uN
i=1,N
dP2P > max { F/us, F/min(di) , NF/(us + Sui) }
i=1,N
Chapter 2, slide:

38. Comparing Client-server, P2P architectures
Chapter 2, slide:

39. Chapter 2: Application layer
Principles of network applications
app architectures
app requirements
Web and HTTP
FTP
P2P file sharing
Socket programming with TCP (assignment 2)
Chapter 2, slide:

40. Assignment 2- programming part
Building a Multi-Threaded Web Server
Read Chapter 2, Section 7 first
Good examples on how to write socket-programs in Java using TCP
Then do your assignment
Socket/Java programming: relatively simple and easy to do
Chapter 2, slide:

41. Chapter 2: Summary our study of network apps now complete!
specific protocols:
HTTP
FTP
P2P
socket programming (assignment 2)
application architectures
client-server
P2P
application service requirements:
reliability, bandwidth, delay
Internet transport service model
connection-oriented, reliable: TCP
unreliable, datagrams: UDP
Chapter 2, slide: