1. Block 5: An application layer protocol: HTTP
Jin Sa
5-May-18
Client-Server Programming

2. Client-Server Programming
Outline of block 4
An overview of HTTP
HTTP connections
HTTP Message Format
Web Caching
Cookies
Most of this block is based on chapter 2 of “Computer Networking: A Top Down Approach” by Kurose and Ross, and some lecture slides by Anirban Mahanti
5-May-18
Client-Server Programming

3. Client-Server Programming
HTTP overview
HTTP: hypertext transfer protocol
Web’s application 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 2616
HTTP request
PC running
Explorer
HTTP response
HTTP request
Server
running
Apache Web
server
HTTP response
Mac running
Navigator
5-May-18
Client-Server Programming

4. Client-Server Programming
HTTP Overview
Uses TCP:
client initiates TCP connection (creates socket) to server, port 80
server accepts TCP connection from client
HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)
TCP connection closed
5-May-18
Client-Server Programming

5. Client-Server Programming
HTTP connections
Non-persistent HTTP
At most one object is sent over a TCP connection.
HTTP/1.0 uses non-persistent HTTP
Persistent HTTP
Multiple objects can be sent over single TCP connection between client and server.
HTTP/1.1 allows persistent connections
5-May-18
Client-Server Programming

6. Response time modeling
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
total = 2RTT+transmit time
time to
transmit
file
initiate TCP
connection
RTT
request
received
time
5-May-18
Client-Server Programming

7. Client-Server Programming
Persistent HTTP
Nonpersistent HTTP issues:
requires 2 RTTs per object
OS must work and allocate host resources for each TCP connection
(but browsers often open parallel TCP connections to fetch referenced objects)
Persistent HTTP
server leaves connection open after sending response
subsequent HTTP messages between same client/server are sent over connection
5-May-18
Client-Server Programming

8. HTTP request message format
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
5-May-18
Client-Server Programming

9. Commonly used HTTP Methods
GET: retrieve a file (95% of requests)
HEAD: just get meta-data
POST: submitting a form to a server
5-May-18
Client-Server Programming

10. HTTP response message format
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
data, e.g.,
requested
HTML file
5-May-18
Client-Server Programming

11. Some common HTTP Response Status Codes
200 OK: request succeeded, information returned in response.
301 Moved Permanently:
400 Bad Request: generic error code, request not understood by server
404 Not Found: requested object not on server
505 HTTP Version Not Supported
5-May-18
Client-Server Programming

12. Trying out HTTP (client side) for yourself
1. Telnet to your favorite Web server:
telnet 80
Opens TCP connection to port 80
(default HTTP server port) at
2. Type in a GET HTTP request:
GET /~jsa/index.html HTTP/1.0
By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server
3. Look at response message sent by HTTP server!
5-May-18
Client-Server Programming

13. Client-Server Programming
Web Proxy Caching
origin
server
user sets browser: Web accesses via cache
browser sends all HTTP requests to cache
If object in cache => cache hit: returns object
else cache requests object from origin server, then returns object to client
Proxy
server
HTTP request
HTTP request
client
HTTP response
HTTP response
HTTP request
HTTP request
HTTP response
HTTP response
client
origin
server
Cache acts as both client and server
5-May-18
Client-Server Programming

14. Client-Server Programming
Why Cache?
Reduce response time for client request.
Reduce traffic on an institution’s access link (from router to the Internet)
Reduce Web traffic in the Internet as a whole, therefore improving performance for all applications.
5-May-18
Client-Server Programming

15. Problem with Web caching
Copy of object in the cache may be out of date, i.e. the object on the original server is modified after the copy is cached.
To solve this problem, HTTP has a mechanism called Conditional GET
5-May-18
Client-Server Programming

16. Client-Server Programming
Conditional GET
An HTTP request is a conditional GET if
The request message uses a GET method
The request message contains the following header line:
If-Modified-Since: ….
5-May-18
Client-Server Programming

17. How does Conditional GET work?-1
On behalf of a requesting browse, a proxy cache sends a request message to a Web server:
GET /fruit/kiwi.gif HTTP/1.1
Host:
The Web server sends response to the cache:
HTTP/ OK
Date: Thu, 7 Jul :39:29
Server: Apache/1.3.0(unix)
Last-Modified: Mon, 4 Jul :23:25
Content-Type: image/gif
(data data ….)
5-May-18
Client-Server Programming

18. How does Conditional GET work?-1a
origin server:
Proxy
server
HTTP request
HTTP request
client
HTTP response
HTTP response
5-May-18
Client-Server Programming

19. How does Conditional GET work?-2
Later, another object requests the same object via the cache.
Object still in cache, but it may be changed on the original Web server, so, cache performs an up-to-date check by issuing a conditional GET:
GET /fruit/kiwi.gif HTTP/1.1
Host:
If-modified-since: Mon, 4 Jul :23:25
Note the value of If-modified-since: is the same as the Last-Modified.
5-May-18
Client-Server Programming

20. How does Conditional GET work?-2a
origin server:
Proxy
server
HTTP request
HTTP request
client
HTTP response
HTTP response
HTTP request
HTTP response
client
5-May-18
Client-Server Programming

21. How does Conditional GET work?-3a
Cache
Original server
HTTP request msg
If-modified-since: <date>
object
not
modified
HTTP response
HTTP/1.1
304 Not Modified
5-May-18
Client-Server Programming

22. How does Conditional GET work?-3b
Cache
Original server
HTTP request msg
If-modified-since: <date>
object
modified
HTTP response
HTTP/ OK
<data data ..>
5-May-18
Client-Server Programming

23. Cookies: keeping “state”
Many major Web sites use cookies
Four components:
1) cookie header line in the HTTP response message
2) cookie header line in HTTP request message
3) cookie file kept on user’s host and managed by user’s browser
4) back-end database at Web site
Example:
Susan access Internet always from same PC
She visits a specific e-commerce site for first time
When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID
5-May-18
Client-Server Programming

24. Cookies: keeping “state” (cont.)
client
server
usual http request msg
usual http response +
Set-cookie: 1678
cookie: 1678
usual http response msg
cookie-
specific
action
spectific
Cookie file
ebay: 8734
server
creates ID
1678 for user
entry in backend
database
Cookie file
amazon: 1678
ebay: 8734
access
one week later:
access
Cookie file
amazon: 1678
ebay: 8734
5-May-18
Client-Server Programming

25. Client-Server Programming
Cookies (continued)
Cookies can simplify the Internet shopping experience, e.g. shopping cart
But, They can also be considered as an invasion to privacy, a web site can learn a lot about a user
5-May-18
Client-Server Programming

26. Client-Server Programming
Web & HTTP
The major application on the Internet
A large fraction of traffic is HTTP
Client/server model:
Clients make requests, servers respond to them
Done mostly in ASCII text (helps debugging!)
Various headers and commands
Web Caching & Performance
5-May-18
Client-Server Programming