1. Application layer 1 Principles of network applications 2 Web and HTTP
3 FTP
4 Electronic Mail
5 DNS
2: Application Layer

2. Application architectures
Client-server
Peer-to-peer (P2P)
Hybrid of client-server and P2P
2: Application Layer

3. Client-server architecture
In client-server architecture there is an always-on host called server
Which services requests from many other hosts called clients
Clients do not directly communicate with each other
Server has a fixed well-known address called an IP address
A single server host is incapable of keeping up with all the requests from its clients Ex. News website server can quickly become overload if it has only one server handling all of its requests
For this reason clusters of hosts sometime referred to as a server farm are often used to create a powerful virtual server in client server architecture

4. Client-server architecture
always-on host (called server)
permanent IP address
server farms for scaling
clients:
communicate with server
may be intermittently connected
may have dynamic IP addresses
do not communicate directly with each other
client/server
2: Application Layer

5. Pure P2P architecture no always-on server
arbitrary end systems directly communicate with each other
Pair of hosts called peers
peers are intermittently connected and change IP addresses
Peers communicate without passing through some special server
Highly scalable but difficult to manage
peer-peer
2: Application Layer

6. Hybrid of client-server and P2P
Client server and P2P are two common architectures for network applications
However many applications are organized as hybrids of the client server and P2P architecture
2: Application Layer

7. Processes communicating
Process: program running within a host.
Process can be thought of as a program that is running within end system
within same host, two processes communicate using inter-process communication (defined by OS).
processes in different hosts communicate by exchanging messages
Client process: process that initiates communication
Server process: process that waits to be contacted
Note: applications with P2P architectures have client processes & server processes
2: Application Layer

8. Sockets process sends/receives messages to/from its socket
socket analogous to door
sending process shoves message out door
sending process relies on transport infrastructure on other side of door which brings message to socket at receiving process
Process is analogous to a house and socket is analogous to its door
process
TCP with
buffers
socket
host or
server
process
TCP with
buffers
socket
host or
server
Internet
controlled
by OS
2: Application Layer

9. Addressing processes to receive messages, process must have identifier
host device has unique 32-bit IP address
Q: does IP address of host on which process runs be enough for identifying the process?
A: No, many processes can be running on same host
identifier includes both IP address and port numbers associated with process on host.
Example port numbers:
HTTP server: 80
Mail server: 25
to send HTTP message to web server:
IP address:
Port number: 80
2: Application Layer

10. Application layer 2.1 Principles of network applications
app architectures
app requirements
2.2 Web and HTTP
2.4 Electronic Mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 Socket programming with TCP
2.8 Socket programming with UDP
2: Application Layer

11. Web and HTTP First some jargon 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:
host name
path name
2: Application Layer 11

12. 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 request
PC running
Explorer
HTTP response
HTTP request
Server
running
Apache Web
server
HTTP response
Mac running
Navigator
2: Application Layer

13. HTTP overview (continued)
Uses TCP:
client initiates TCP connection (creates socket) to server
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
2: Application Layer

14. The first line of an HTTP request message is called the request line
The subsequent lines are called the header lines
The request line has three fields
The method field, the URL field, & the HTTP version field
The method field can take on several different values including GET, POST, HEAD
But the majority of HTTP request messages use the GET method (used when browser request an object)

15. HTTP request message two types of HTTP messages: request, response
ASCII (human-readable format)
Cr: carriage return and Lf: Line feed
with each element separated by SP characters. No CR or LF is allowed except in the final CRLF sequence.
request line
(GET, POST,
HEAD commands)
GET /somedir/page.html HTTP/1.1
Host:
User-agent: Mozilla/4.0(browser type)
Connection: close (it doesn’t need persistent connection)
Accept-language:fr (prefers french version otherwise default version)
(extra carriage return, line feed)
header
lines
Carriage return,
line feed
indicates end
of message
2: Application Layer

16. HTTP request message: general format
2: Application Layer

17. 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: side that initiates transfer (either to/from remote)
server: remote host
ftp server: port 21
2: Application Layer

18. FTP: separate control, data connections
FTP uses two parallel TCP connections to transfer a file
Control connection & Data connection
The control connection is used for sending control information between two hosts such as a user identification, password etc
The data connection is used to actually send a file
2: Application Layer 18

19. FTP: separate control, data connections
client
server
TCP control connection
port 21
TCP data connection
port 20
FTP client contacts FTP server at port 21, TCP is transport protocol
client authorized over control connection (ID & Password)
client browses remote directory by sending commands over control connection.
when server receives file transfer command, server opens 2nd TCP connection (for file) to client
after transferring one file, server closes data connection.
server opens another TCP data connection to transfer another file.
FTP server maintains “state”: current directory, earlier authentication
2: Application Layer

20. Electronic Mail: SMTP messages must be in 7-bit ASCII
uses TCP to reliably transfer message from client to server, port 25
direct transfer: sending server to receiving server
three phases of transfer
handshaking (greeting)
transfer of messages
closure
command/response interaction
commands: ASCII text
response: status code and phrase
messages must be in 7-bit ASCII
2: Application Layer

21. Scenario: Alice sends message to Bob
1) Alice uses UA to compose message and “to”
2) Alice’s UA sends message to her mail server; message placed in message queue
3) Client side of SMTP opens TCP connection with Bob’s mail server
4) SMTP client sends Alice’s message over the TCP connection (SMTP handshaking)
5) Bob’s mail server places the message in Bob’s mailbox
6) Bob invokes his user agent to read message at his convenience
mail
server
mail
server 1
user
Agent/Bob'
user
Agent/Alice 2 3 6 4 5
2: Application Layer

22. SMTP: final words Comparison with HTTP:
SMTP uses persistent connections
SMTP requires message (header & body) to be in 7-bit ASCII
SMTP server uses CRLF.CRLF to determine end of message
Comparison with HTTP:
HTTP: pull
SMTP: push
both have ASCII command/response interaction
HTTP: each object encapsulated in its own response msg
SMTP: multiple objects sent in multipart msg
2: Application Layer

23. Both protocols are used to transfer files from one host to another
Comparison with HTTP
Both protocols are used to transfer files from one host to another
HTTP transfer files from a web server to a web client (a browser)
SMTP transfer files (that is messages) from one mail server to another mail server
When transferring the files both persistent HTTP & SMTP use persistent connections
HTTP is mainly a pull protocol-someone loads information on a web server & users use HTTP to pull the information from the server at their convenience
2: Application Layer 23

24. SMTP is primarily a push protocol
Comparison with HTTP
SMTP is primarily a push protocol
The sending mail server pushes the file to the receiving mail server
2: Application Layer 24

25. Mail message format header body
SMTP: protocol for exchanging msgs
standard for text message format:
header lines, e.g.,
To:
From:
Subject:
different from SMTP commands!
body
the “message”, ASCII characters only
header
blank
line
body
2: Application Layer

26. DNS: Domain Name System
People: many identifiers:
Internet hosts, routers:
IP address (32 bit) - used for addressing datagrams
“name”, e.g., ww.yahoo.com - used by humans
2: Application Layer