1. Mail (smtp), VoIP (sip, rtp)
Network Applications
Two or more end systems (Hosts)
Client server paradigm
Client:
initiates contact with server (“speaks first”)
typically requests service from server,
for Web, client is implemented in browser; for , in mail reader
Server:
provides requested service to client
e.g., Web server sends requested Web page, mail server delivers
Peer to peer paradigm
Peers come and leave at any time
A peer is both a client and a server
Scalability is the advantage
Many hybrid peer-to-peer and client-server in real world
One or more application Protocols
define
type of messages
syntax of messages: fields and their meaning
actions taken
user services provided by lower layer protocols
Mail (smtp), VoIP (sip, rtp)
Application Layer

2. Identifying Applications
Q: how does a process “identify” the other process with which it wants to communicate?
IP address of host running other process
32-bit
Unique ID
“port number” - allows receiving host to determine to which local process the message should be delivered
found in RFC 1700
RFC
The combination of IP address and port number is also called “socket”
When a client application talks to a server, we need
client ip address, client port number
source ip address, source port number
Server: 1. it is always open; 2. it has a known IP address and a known port number
Explain RFC (request for comments) by IETF (Internet Engineering Task Force)
Socket = (ip address, port #)
TCP connection 4-tuple
Application Layer

3. The Web: introduction Web page:
consists of “objects”
addressed by a URL
Most Web pages consist of:
base HTML page, and
several referenced objects.
URL has two components: host name and path name:
User agent for Web is called a browser:
Internet Explorer
Firefox
Server for Web is called Web server:
Apache (public domain)
MS Internet Information Server
Web is the killer application that brought about the fast growth of the current Internet.
Application Layer

4. The Web: the http protocol
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
http1.0: RFC 1945 (before 1997)
http1.1: RFC 2068 (after 1998)
http request
PC running
Explorer
http response
http request
Server
running
NCSA Web
server
http response
http defines how web clients (browsers) request web pages from servers and how servers transfer pages to clients
Mac running
Navigator
Application Layer

5. The http protocol: more
http: TCP transport service:
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
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
http uses TCP as its transport service
Stateless: server does not maintain any information about client connections,
Application Layer

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
Application Layer

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
Application Layer

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
22 roundtrips
Q: how many TCP connections?
Q2: If the browser opens one TCP connection
at one time, how many round trips altogether?
Application Layer

9. Response time modeling
Definition of RTT: 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
Explain why RTT is bigger than transmission time?
Application Layer

10. Persistent HTTP Persistent HTTP
server leaves connection open after sending response
subsequent HTTP messages between same client/server are sent over connection
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
How many round trips do we need? Non-persistent, persistent without pipelining, persistent with pipelining
Application Layer

11. http message format: request
two types of http messages: request, response
http request message:
ASCII (human-readable format)
request line
(GET, POST,
HEAD commands)
GET /somedir/page.html HTTP/1.0
User-agent: Mozilla/4.0
Host:
Accept: text/html, image/gif,image/jpeg
Accept-language:fr
(extra carriage return, line feed)
header
lines
ASCII texts
Request line: method, the URL field, the HTTP version field
Header lines:
Accept-language: the user prefers to receive a French version of the object, if such an object exists; otherwise, the server should send its default version.
Carriage return,
line feed
indicates end
of message
Application Layer

12. http request message: general format
GET
POST
HEAD
asks server to leave requested object out of response
HTTP/1.1
GET, POST, HEAD
PUT
uploads file in entity body to path specified in URL field
DELETE
deletes file specified in the URL field
POST method fills in the entity body with what user enters into the browser forms
Application Layer

13. Uploading form input Post method: Web page often includes form input
Input is uploaded to server in entity body
URL method:
Uses GET method
Input is uploaded in URL field of request line:
Application Layer

14. http message format: response
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
Connection: close, server tells the client that it will close the connection after sending this message
Date: indicates the time and date when the server retrieves the object from its file system and sends the response message
Application Layer

15. 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
Show how to capture packets exchanged on the network
Application Layer

16. 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 server port) at cis.poly.edu.
Anything typed in sent
to port 80 at cis.poly.edu
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/1.0
3. Look at response message sent by http server!
Application Layer

17. User-Server Identification
Server is stateless
How to identify a user
Authentication
Cookies
Caching
Conditional GET
Authentication: restrict access to certain users
Cookies: generate responses based on users
Application Layer

18. User-server interaction: authentication
client
server
Authentication goal: control access to server documents
stateless: client must present authorization in each request
authorization: typically name, password
authorization: header line in request
usual http request msg
401: authorization req.
WWW authenticate:
usual http request msg
+ Authorization:line
usual http response msg
Show the ethereal captured session
usual http request msg
+ Authorization:line
time
usual http response msg
Browser caches name & password so
that user does not have to repeatedly enter it.
Application Layer

19. 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
Application Layer

20. Cookies: keeping “state” (cont.)
client
server
http request
http response +
Set-cookie: 1678
cookie: 1678
http response
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
Application Layer

21. Cookies (continued) What cookies can bring: user accounts
shopping carts
Web portals
Advertising
Secretly collecting users’ browsing habits
What to do?
Application Layer

22. Web Caches (proxy server)
Goal: satisfy client request without involving origin server
user sets browser: Web accesses via web cache
client sends all http requests to web cache
if object at web cache, web cache immediately returns object in http response
else requests object from origin server, then returns http response to client
origin
server
Proxy
server
http request
http request
client
http response
http response
http request
http request
Advantages:
Reduce response time
Reduce server load
Reduce internet traffic
http response
http response
client
origin
server
Application Layer

23. Conditional Get First time cache request Subsequent cache request
First time server response
Cache saves last modification time of the object
Subsequent cache request
Subsequent server response
GET /fruit/kiwi.gif HTTP/1.1
Host:
GET /fruit/kiwi.gif HTTP/1.1
Host:
If-modified-since: Wed, 2 Jul :23:24
HTTP/ OK
Date: Mon, 7 Jul :39:29
Server: Apache/1.3.0 (Unix)
Last-Modified: Wed, 2 Jul :23:24
Content-Type: image/gif
(data data data …)
HTTP/ Not Modified
Date: Mon, 14 Jul :39:29
Server: Apache/1.3.0 (Unix)
(empty entity body …)
Application Layer

24. Conditional GET: client-side caching
server
Goal: don’t send object if client has up-to-date cached version
client: specify 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>
object
not
modified
HTTP response
HTTP/1.0
304 Not Modified
HTTP request msg
If-modified-since: <date>
object
modified
HTTP response
HTTP/ OK
<data>
Application Layer

25. HTTPS Secure version of HTTP Procedure: Client connects,
Systems exchange fresh public keys
Systems agree on a symmetric (fast) protocol (e.g. 3DES)
Server presents certificate
Client accepts or rejects
(Client presents certificate)
Application Layer

26. Review Questions (T/F)
Networked applications interact with each other using interprocess communication.
The routers run all the five layers in the protocol stack.
If an HTTP request message uses the Accept-language: fr header, and the server only has an English version of the object, then the server will return the 404 Document Not Found error message
When authorization is required for a web site, the browser only needs to send one HTTP request with Authorization: header line, including the username and password entered by the user; for subsequent HTTP requests, the browser does not send Authorization: header line.
F, F, F, F
Application Layer

27. Review Questions (T/F)
Networked applications interact with each other using interprocess communication.
The routers run all the five layers in the protocol stack.
If an HTTP request message uses the Accept-language: fr header, and the server only has an English version of the object, then the server will return the 404 Document Not Found error message
When authorization is required for a web site, the browser only needs to send one HTTP request with Authorization: header line, including the username and password entered by the user; for subsequent HTTP requests, the browser does not send Authorization: header line.
F, F, F, F
Application Layer