1. Departamento de Tecnología Electrónica
Computer Networking
Some of these slides are given as copyrighted material from:
Computer Networking: A Top Down Approach , 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April
Chapter 2 The Application layer

2. Chapter 2: The Application Layer
Our goals:
conceptual, implementation aspects of network application protocols
transport-layer service models
client-server paradigm
peer-to-peer paradigm
learn about protocols by examining popular application-layer protocols
HTTP
DNS
programming network applications
socket API
Application 2-2

3. Some network apps e-mail web instant messaging remote login
P2P file sharing
multi-user network games
streaming stored video (e.g. YouTube)
voice over IP (VoIP)
real-time video conferencing
cloud computing …
Application 2-3

4. Creating a network app write programs that
application
transport
network
data link
physical
write programs that
run on (different) end systems
communicate over network
e.g., web server software communicates with browser software
No need to write software for network-core devices
network-core devices do not run user applications
applications on end systems allows for rapid app development, propagation
application
transport
network
data link
physical
application
transport
network
data link
physical
Application 2-4

5. Chapter 2: Application layer
2.1 Principles of network applications
2.2 DNS
2.3 Web and HTTP
2.4 Socket programming with TCP
2.5 Socket programming with UDP
Application 2-5

6. Application architectures
client-server
peer-to-peer (P2P)
hybrid of client-server and P2P
Note:
IP address: it uniquely identifies the devices (end systems, routers, etc...) connected to a TCP/IP network. The ISP assigns a static (fixed) or dynamic (variable) IP addr. More soon ...
Application 2-6

7. Client-server architecture
always-on host
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
Application 2-7

8. Pure P2P architecture no always-on server
arbitrary end systems directly communicate
peers are intermittently connected and change IP addresses
highly scalable but difficult to manage
peer-peer
Application 2-8

9. Hybrid of client-server and P2P
Skype
voice-over-IP P2P application
centralized server: finding address of remote party
client-client connection: direct (not through server)
Instant messaging
chatting between two users is P2P
centralized service: client presence detection/location
user registers its IP address with central server when it comes online
user contacts central server to find IP addresses of buddies
Application 2-9

10. How is app layer implemented?
Web browsers, e.g.: Mozilla firefox, Chrome, Internet Explore, Safari,…
User interface
User interface
Application Protocol
Application
Application
A_PDU
SAP
SAP
Transport
Transport
Internet
Application 2-10 10

11. App-layer protocol defines
types of exchanged messages,
e.g., request, response
message syntax:
what fields are in messages & how fields are distinguished
message semantics
meaning of information in fields
rules for when and how processes send & respond to messages
public-domain protocols:
defined in RFCs
allows for interoperability
e.g., HTTP, SMTP
proprietary protocols:
e.g., Skype
Application 2-11

12. Process communication
client process: process that initiates communication
server process: process that waits to be contacted
process: program running within a host.
within the same host, communication is held using inter-process communication (defined by OS).
In different hosts, communication is done by exchanging messages (PDUs). Communication services (generally defined by OS) are used.
Note: applications with P2P architectures have both client processes & server processes
Application 2-12

13. Sockets (SAP) process sends/receives messages to/from its socket
socket analogous to door
sending process sends message out door
sending process relies on transport infrastructure on the other side of the door, bringing message to socket at receiving process
process
TCP with
buffers,
variables
socket
client
process
TCP with
buffers,
variables
socket
server
controlled by
app developer
Internet
controlled
by OS
API: (1) choice of transport protocol; (2) ability to fix a few parameters (more on this later)
Application 2-13

14. How is a socket identified?
If you want to send a friend a mail you need to know his address to reach his home mailbox
Each end system has unique 32-bit IP address (IPv4)
Q: Is your friend’s address enough to make the letters reach him?
A: No. Several people may be living at the same home.
Several app protocols can be running on one end system
Web browser, client, Skype…
For every IP address you have an associated name (hostname) which are used to identify the network devices by humans and applications.
For example, =
More on Names and IPs on next section…
Note
Application 2-14

15. How is a socket identified?
Each app protocol is identified by a port number
Client port number and server port number may not be the same
example port numbers:
HTTP server: 80
DNS server: 53
Mail server: 25
ICANN (Internet Corporation for Assigned Names and Numbers) is responsible for public app protocols port registering
Socket is identified by the pair:
IP address
Port number
Application 2-15

16. Example DTE web server Internet Aplicación Aplicación Transport
Interfaz Usuario
Interfaz Usuario
Aplicación
Aplicación
, 80
Client IP address, port
Transport
Transport
Internet
Application 2-16 16

17. OS Internet Communication Service
Localhost: Connecting 2 process on the same host
localhost: is a “reserved name” related to a particular IP address which always identify our own end system.
It’s useful to connect network applications on a single host (without any other physical network connection).
In general, it allows interprocess comunications in the end system using the same Internet protocol stack.
Note
Localhost is usually asociated to IP address , but other IPs may be used…. More on Chapter 4…
OS Internet Communication Service
socket1
process1
socket2
process2
Q: How can the communication service on the host able to distinguish each process?
Application 2-17 17 17

18. What transport service does an app need?
Data loss
some apps (e.g., audio) can tolerate some loss
other apps (e.g., file transfer, telnet) require 100% reliable data transfer
Throughput
some apps (e.g., multimedia) require minimum amount of throughput to be “effective”
other apps (“elastic apps”) make use of whatever throughput they get
Security
encryption, data integrity, …
Timing
some apps (e.g., Internet telephony, interactive games) require low delay to be “effective”
Application 2-18

19. Transport service requirements of common apps
Application
file transfer
Web documents
real-time audio/video
stored audio/video
interactive games
instant messaging
Data loss
no loss
loss-tolerant
Throughput
elastic
audio: 5kbps-1Mbps
video:10kbps-5Mbps
same as above
few kbps up
Time Sensitive no
yes, 100’s msec
yes, few secs
yes and no
Application 2-19

20. Internet transport protocols services
TCP service:
connection-oriented: setup required between client and server processes
reliable transport between sending and receiving process
flow control: sender won’t overwhelm receiver
congestion control: throttle sender when network overloaded
does not provide: timing, minimum throughput guarantees, security
UDP service:
unreliable data transfer between sending and receiving process
does not provide: connection setup, reliability, flow control, congestion control, timing, throughput guarantee, or security
Q: Why UDP?
Application 2-20

21. Internet apps: application, transport protocols
layer protocol
SMTP [RFC 2821]
Telnet [RFC 854]
HTTP [RFC 2616]
FTP [RFC 959]
HTTP (e.g., YouTube), RTP [RFC 1889]
SIP, RTP, propietary
(e.g., Skype)
Underlying
transport protocol
TCP
TCP or UDP
typically UDP
Application
remote terminal access
Web
file transfer
streaming multimedia
Internet telephony
Application 2-21

22. Chapter 2: Application layer
2.1 Principles of network applications
2.2 DNS
2.3 Web and HTTP
2.4 Socket programming with TCP
2.5 Socket programming with UDP
Application 2-22

23. DNS: Domain Name System
people: many identifiers:
SSN, name, passport #
Internet hosts, routers:
IP address (32 bit) - used for addressing datagrams
“name”, e.g., - used by humans
Q: map between IP address and name, and viceversa?
Domain Name System:
distributed database implemented in hierarchy of many name servers
application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation)
note: core Internet function, implemented as application- layer protocol
Application 2-23

24. HTTP client HTTP server Internet DNS server DNS client
DNS server
DNS client
DNS adds an additional delay
Internet
HTTP server
Application 2-24

25. DNS DNS services Why not centralize DNS?
hostname to IP address translation (direct)
IP address to hostname translation (inverse)
host aliasing
Canonical, alias names
mail server aliasing
@domain
load distribution
replicated Web servers: set of IP addresses for one canonical name
Why not centralize DNS?
single point of failure
traffic volume
distant centralized database
maintenance
… doesn’t scale!
Which transport service does it use?
Not reliable
UDP, port 53
Application 2-25

26. DNS: caching and updating records
once (any) name server learns mapping, it caches mapping
cache entries timeout (disappear) after some time
TLD servers typically cached in local name servers
Thus root name servers not often visited
update/notify mechanisms among DNS servers proposed IETF standard
RFC 2136
Application 2-26

27. DNS: how does it work? (simplified!)
If an application have a hostname and not the associated IP address it requests the resolution to the DNS service in the same host (reverse resolution is also available). El DNS service check if there’s an entry in the DNS cache for the hostname or IP address and then…
… if there’s an entry in the cache:
It returns the requested resolution to the application.
… if there’s no entry in the cache:
The DNS service sends a DNS REQUEST (DNS_PDU) to the DNS Server IP address configured on the host (at least one is usually configured) and wait for the DNS REPLY (another type of DNS_PDU) from the DNS Server it was requested. The reply is then added to the cache to accelerate future requests and it’s also returned to the application.
If it’s not possible to resolve the hostname or IP address, it notifies the application.
Note
When a DNS Server receives a DNS REQUEST, its behaviour is the same as a host, it first tries its own cache and if it’s not available it generates a Request to another DNS Server in a higher hierarchy, and the Reply is then forwarded back to the client.
Application 2-27 27 27

28. Chapter 2: Application layer
2.1 Principles of network applications
2.2 DNS
2.3 Web and HTTP
2.4 Socket programming with TCP
2.5 Socket programming with UDP
Application 2-28

29. Web and HTTP First, a review… 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 (Uniform Resource Locator)
example URL:
host name
path name
Application 2-29

30. HTML language format Aside
It allows formatting web pages; created in It includes elements with TAGS in <>. Each element usually have 4 fields: a starting tag (like “<html>”) & an ending tag (like “</html>”), some attributes (in starting tag) and some contents (between the tags).
<!DOCTYPE html…> remarks document start (optional)
<html>document</html> start/finish of document
<head>headers</head> header content (not visible to the end user, like title, styles, metainformation, etc…)
<body>webpage</body> it contains the body of the webpage, with:
from <h1> to <h6> headings
<table>table</table> creates a table with fils/cols
<a href=“URL”>link</a> hiperlink: when the user clicks on the link, the URL object is requested.
<img src=“URL”> object reference, navigator requests the URL object inmediately to be shown.
You can view the HTML code in any webpage with
your favourite navigator, right-click -> view source code.
Aside
Application 2-30 30 30

31. 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
IExplorer
HTTP response
HTTP request
Server
running
Apache Web
server
HTTP response
Linux running
Firefox
Application 2-31

32. HTTP overview (continued)
Uses TCP:
client initiates TCP connection (creates socket) to server, port 80 in server
server accepts TCP connection from client
HTTP messages (A_PDU’s: 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
note
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 synchronized
Application 2-32

33. HTTP connections non-persistent HTTP
one object sent over TCP connection.
persistent HTTP
multiple objects can be sent over single TCP connection between client, server.
Application 2-33

34. (contains text & references to 13 objects)
Non-persistent HTTP
(contains text & references to 13 objects)
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 with pathname /personal/smartin/lab3/r eferencias.html
3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket
time
Application 2-34

35. Nonpersistent HTTP (cont.)
4. HTTP server closes TCP connection.
5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 13 referenced objects (images, scripts, …)
time
6. Steps 1-5 repeated for each of 13 objects with different unique URLs
Application 2-35

36. Non-Persistent HTTP: Response time
RTT (Round-Trip Time): time for a PDU with a few bytes 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
Application 2-36

37. Persistent HTTP non-persistent HTTP issues: persistent HTTP
requires 2 RTTs per object
OS overhead for each TCP connection
persistent HTTP
server leaves connection open after sending response
subsequent HTTP messages between same client/server sent over open connection
client sends requests as soon as it finds a referenced object
Each referenced object takes only 1 RTT
Parallel HTTP connections:
browsers often open parallel TCP connections to fetch referenced objects faster. They are requested sequentally per each connection (persistent or not).
Note that:
Application 2-37

38. Mensajes HTTP (HTTP_PDU)
2 types of messages:
HTTP Request:
Sent by the client
Takes neccessary information Transporta información (HTTP_PCI) to get an object from the server (HTTP_UD)
ASCII chracters (intelligible text)
HTTP Response:
Sent by the server
Takes the object (HTTP_UD) requested by the client and some control information (HTTP_PCI)
Aplicación 2-38

39. HTTP request message Note: UD request line (GET, POST, HEAD commands)
<CR>: Carriage-Return : \r
<LF>: Line-Feed: \n UD
request line
(GET, POST,
HEAD commands)
GET /index.html HTTP/1.1\r\n
Host: www-net.cs.umass.edu\r\n
User-Agent: Firefox/3.6.10\r\n
Accept: text/html,application/xhtml+xml\r\n
Accept-Language: en-us,en;q=0.5\r\n
Accept-Encoding: gzip,deflate\r\n
Accept-Charset: ISO ,utf-8;q=0.7\r\n
Keep-Alive: 115\r\n
Connection: keep-alive\r\n
\r\n
header
lines
\r & \n at start
of line indicates
end of header lines
Application 2-39

40. Some HTTP headers CLIENT can send
Host: hostname (name of the web site)
User-Agent: web_navigator_version
Accept-xxx: preferences_list_for_xxx_feature
Connection: keep-alive
Client if asking the server to stablish persistant connection
Keep-Alive: nnn
Client if asking the server to keep the persistant connection alive for nnn seconds.
Aplicación 2-40 40 40

41. HTTP request message: general format
line
header
lines
PCI
PDU
body UD
Application 2-41

42. Uploading form input
Web page often includes form input (with some parameters that are sended to server). There are two methods to send:
POST method:
input is uploaded to server in entity body
GET method:
Inputs are uploaded in URL field of request line (separated by “?” and “&”):
GET /animalsearch?monkeys&banana HTTP/1.1\r\n
Host: …
Application 2-42

43. Method types HTTP/1.0 (RFC-1945) GET POST HEAD HTTP/1.1 (RFC-2616)
Identical to GET but object is not included in response (only corresponding headers)
HTTP/1.1 (RFC-2616)
GET, POST, HEAD
PUT
uploads file in entity body to path specified in URL field
DELETE
deletes file specified in the URL field
Application 2-43

44. HTTP response message status line (protocol status code status phrase)
HTTP/ OK\r\n
Date: Sun, 26 Sep :09:20 GMT\r\n
Server: Apache/ (CentOS)\r\n
Last-Modified: Tue, 30 Oct :00:02 GMT\r\n
ETag: "17dc6-a5c-bf716880"\r\n
Accept-Ranges: bytes\r\n
Content-Length: 2652\r\n
Keep-Alive: timeout=10, max=100\r\n
Connection: Keep-Alive\r\n
Content-Type: text/html; charset=ISO \r\n
\r\n
data data data data data ...
header
lines
data, e.g.,
requested
HTML file
Application 2-44

45. Some HTTP headers the SERVER can send
Date: date
Last-Modified: (last modification of the requested object)
Server: web_server_version
Content-Type: object_type (HTML, image, text …)
Content-Length: body_length (in bytes)
Connection: keep-alive
Server announces the connection will be persistant.
Keep-Alive: timeout=ttt, max=nnn
Server will close the connection after ttt seconds of inactivity or otherwise after nnn seconds.
Application 2-45 45 45

46. HTTP response status codes
status code appears in 1st line in serverclient response message.
some sample codes:
200 OK
request succeeded, requested object later in this msg
301 Moved Permanently
requested object moved, new location specified later in this msg (Location:)
400 Bad Request
request msg not understood by server
404 Not Found
requested document not found on this server
505 HTTP Version Not Supported
Application 2-46

47. Trying out HTTP (client side) for yourself
1. Telnet to your favourite Web server:
telnet 80
opens TCP connection to port 80
(default HTTP server port) at
anything typed in sent
to port 80 at
2. type in a GET HTTP request:
by typing this, you send
this minimal (but complete)
GET request to HTTP server
GET /docencia/ HTTP/1.1
Host:
3. look at response message sent by HTTP server! (or use Wireshark!)
Q: What happens if you send “hello”?
Application 2-47

48. User-server state: cookies
example:
Susan always accesses Internet from her PC
visits specific e-commerce site for first time (e.g. Amazon)
when initial HTTP requests arrives at site, site creates:
unique ID
entry in back-end database for ID
many 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
Application 2-48

49. Cookies: Example Client visits AMAZON AMAZON server Amazon 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-
specific
action
access
usual http response msg
backend
database
one week later:
ebay 8734
amazon 1678
usual http response msg
Application 2-49

50. Cookies (continued) cookies and privacy: what cookies can bring:
Note
cookies and privacy:
cookies allow sites to learn a lot about you
you may supply personal information to sites (name, ,…)
what cookies can bring:
authorization
shopping carts
recommendations
user session state (Web e- mail)
Application 2-50

51. Web caches (proxy server)
Goal: satisfy client request without involving origin server
user sets browser: Web accesses via cache-proxy
browser sends all HTTP requests to cache-proxy
object in cache: cache returns object
Else, cache requests object from origin server, then returns object to client
origin
server
HTTP request
HTTP request
client
HTTP response
HTTP response
HTTP request
Proxy
server
HTTP response
client
origin
server
Application 2-51

52. More about Web caching - Proxy
cache acts as both client (of the original server) and server (of the user client)
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.
enables “poor” content providers to effectively deliver content (but so does P2P file sharing)
Application 2-52

53. Conditional GET
Goal: don’t send object if cache has up-to-date cached version
Cache-proxy: 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
Or modified object is sended with the last- modified date:
Last-modified:<date>
cache-proxy
server
HTTP request msg
If-modified-since: <date>
object
modified
before
<date>
HTTP response
HTTP/1.0
304 Not Modified
HTTP request msg
If-modified-since: <date>
object
modified
after
<date>
HTTP response
HTTP/ OK
Last-modified: <date>
<data>
Application 2-53

54. Chapter 2: Application layer
2.1 Principles of network applications
2.2 DNS
2.3 Web and HTTP
2.4 Socket programming with TCP
2.5 Socket programming with UDP
Application 2-54

55. Socket programming
Goal: learn how to build client/server application that communicate using sockets
Socket API
introduced in BSD4.1 UNIX, 1981
explicitly created, used, released by apps
client/server paradigm
two types of transport service via socket API:
unreliable datagram
reliable, byte stream- oriented
socket
A local-host,
application-created,
OS-controlled interface (a “door”) where
application processes can both send and
receive messages to/from another application processes
Application 2-55

56. Application protocol Server Client Internet Application Application
User interface
User interface
Application
Application
Server IP address, port
Client IP address, port
Transport
Transport
Internet
Application 2-56 56

57. Socket-programming using TCP
Socket: a door between application process and end-end- transport protocol (UCP or TCP)
TCP service: reliable transfer of bytes from one process to another
controlled by
application
developer
controlled by
application
developer
process
TCP with
buffers,
variables
socket
process
TCP with
buffers,
variables
socket
controlled by
operating
system
controlled by
operating
system
internet
client or
server
client or
server
Application 2-57

58. Socket programming with TCP
Client must contact server
server process must first be running
server must open a socket (door) that welcomes client’s contact
Client contacts server by:
creating client TCP socket
specifying IP address, port number of server process
when client creates socket: client TCP establishes connection to server TCP
when contacted by client, server TCP creates new socket for server process to communicate with client
allows server to talk with multiple clients
source port numbers used to distinguish clients (more in Chap 3)
application viewpoint
TCP provides reliable, in-order
transfer of bytes (“pipe”)
between client and server
Application 2-58

59. Server (running on hostid)
Client/server socket interaction: TCP
Server (running on hostid)
Client
create socket,
port=x, for
incoming request:
welcomeSocket =
ServerSocket()
Wait for primitive: Connection.indication
close
connectionSocket
read reply from
clientSocket
TCP
connection
setup
create socket,
connect to hostid, port=x
clientSocket =
Socket()
Send primitive: Connection.response
wait for incoming
connection request
connectionSocket =
welcomeSocket.accept()
Send primitive: Connection.request
Wait for primitive: Connection.confirm
send request using
clientSocket
read request from
connectionSocket
write reply to
hostid = IP or hostname
Application 2-59

60. Java Stream
keyboard
monitor
stream is a sequence of characters that flow into or out of a process.
input stream is attached to some input source for the process, e.g., keyboard or socket.
output stream is attached to an output source, e.g., monitor or socket.
Ex.: we are going to use 3 streams and 1 socket in the client
input stream
inFromUser
Client
process
output stream
input stream
outToServer
inFromServer
client TCP socket
To transport layer
From transport layer
Application 2-60

61. Socket programming with TCP
Example client-server app:
1) client reads line from standard input (inFromUser stream) , sends to server via socket (outToServer stream)
2) server reads line from socket
3) server converts line to uppercase, sends back to client
4) client reads, prints modified line from socket (inFromServer stream)
Application 2-61

62. Example: Java client (TCP)
import java.io.*;
import java.net.*;
class TCPClient {
public static void main(String argv[]) throws Exception {
String sentence;
String modifiedSentence;
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname", 6789);
DataOutputStream outToServer =
new DataOutputStream(clientSocket.getOutputStream());
This package defines Socket()
and ServerSocket() classes
server name,
e.g.,
T_ICI
create
input stream
create
clientSocket object
of type Socket,
connect to server
server port #
T_ICI
create
output stream
attached to socket
Application 2-62

63. Example: Java client (TCP), cont.
BufferedReader inFromServer =
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
sentence = inFromUser.readLine();
outToServer.writeBytes(sentence + '\n');
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER: " + modifiedSentence);
clientSocket.close(); }
create
input stream
attached to socket
Send primitive: Data.request
PDU
send line
to server
Wait for primitive: Data.indication
read line
from server
close socket
(clean up behind yourself!)
Application 2-63

64. Example: Java server (TCP)
import java.io.*;
import java.net.*;
class TCPServer {
public static void main(String argv[]) throws Exception {
String clientSentence;
String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789);
while(true) {
Socket connectionSocket = welcomeSocket.accept();
BufferedReader inFromClient =
new BufferedReader(new
InputStreamReader(connectionSocket.getInputStream()));
create
welcoming socket
at port 6789
wait, on welcoming
socket accept() method
for client contact create,
new socket on return
create input
stream, attached
to socket
Application 2-64

65. Example: Java server (TCP), cont
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
outToClient.writeBytes(capitalizedSentence); }
create output
stream, attached
to socket
PDU
read in line
from socket
Wait for primitive: Data.indication
write out line
to socket
PDU
Send primitive: Data.request
end of while loop,
loop back and wait for
another client connection
Application 2-65

66. Chapter 2: Application layer
2.1 Principles of network applications
2.2 DNS
2.3 Web and HTTP
2.4 Socket programming with TCP
2.5 Socket programming with UDP
Application 2-66

67. Socket programming with UDP
UDP: no “connection” between client and server
no handshaking
sender explicitly attaches IP address and port of destination to each message (PDU) that wants to send
server must extract IP address, port of sender from received message
UDP: transmitted data may be received out of order, or some may be lost
application viewpoint:
UDP provides unreliable transfer
of groups of bytes (“datagrams”)
between client and server
Application 2-67

68. Server (running on hostid)
Client/server socket interaction: UDP
Server (running on hostid)
create socket,
clientSocket =
DatagramSocket()
Client
Create datagram with server IP and
port=x; send datagram via clientSocket
create socket,
port= x.
serverSocket =
DatagramSocket()
Wait primitive: Data.indication
read datagram from
serverSocket
Send primitive: Data.request
close
clientSocket
read datagram from
write reply to
serverSocket
specifying client address,
port number
hostid = IP or hostname
Application 2-68

69. Example: client (UDP datagrams)
keyboard
monitor
input stream
inFromUser
Client
process
Input: receives packet (recall that TCP received “byte stream”)
Output: sends packet (recall that TCP sent “byte stream”)
UDP packet
UDP packet
sendPacket
receivePacket
client UDP socket
UDP socket
To transport layer
From transport layer
Application 2-69

70. Example: Java client (UDP)
import java.io.*;
import java.net.*;
class UDPClient {
public static void main(String args[]) throws Exception {
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
DatagramSocket clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("hostname");
byte[] sendData = new byte[1024];
byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
create
input stream
create
client socket
translate
hostname to IP
address using DNS
Application 2-70

71. Example: Java client (UDP) (cont.)
T_ICI
create T_IDU with A_PDU,
length, IP addr, port
A_PDU
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
clientSocket.send(sendPacket);
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
clientSocket.receive(receivePacket);
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close(); }
T_IDU
send T_IDU
Send primitive: Data.request
create new T_IDU
receive T_IDU
Wait for primitive: Data.indication
Application 2-71

72. Example: Java server (UDP)
import java.io.*;
import java.net.*;
class UDPServer {
public static void main(String args[]) throws Exception {
DatagramSocket serverSocket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
Create UDP socket
at port 9876 (known by client)
Wait for primitive: Data.indication
create new T_IDU
receive
T_IDU
Application 2-72

73. Example: Java server (UDP) (cont.)
String sentence = new String(receivePacket.getData());
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress,
port);
serverSocket.send(sendPacket); }
get sender IP addr and port # from T_ICI
create T_IDU with A_PDU to send, length, IP addr and port #
Send T_IDU
end of while loop,
loop back and wait for
another datagram
Application 2-73

74. PROBLEMs AND EXERCISES
Departamento de Tecnología Electrónica
Computer Networking – Chapter 2: Application layer
PROBLEMs AND EXERCISES
Application 2-74

75. Pr1: True or false?
With non-persistent connections between browser and origin server, it is possible for a single TCP segment to carry two different HTTP request messages.
A user requests a Web page that consists of some text and two images. For this page, the client sends one request message and receives three response messages.
These two different Web pages and can be sent over the same persistent connection.
The “Date:” header in the HTTP response message indicates when the object in the response was last modified.
HTTP response messages never have an empty message body.
Application 2-75 75

76. Pr2: Application-Transport
Consider an HTTP client that wants to retrieve a Web document from a given URL. HTTP server’s IP address is initially unknown.
What transport and application-layer protocols besides HTTP are needed in this scenario?
Application 2-76 76

77. Pr3: HTTP client headers
Consider the following string of ASCll characters that were captured by Wireshark when the browser sent an HTTP GET message (i .e., this is the actual content of an HTTPGET message). The characters <cr><l/> are carriage return and line-feed characters (that is, the character string <cr> in the text below represents the single carriage-return character that was contained at that point in the HTTP header). Answer the following questions, indicating where in the HTTP GET message below you found the answer.
 GET /cs453/index.html HTTP/l.l<cr><lf>Host: gaia.cs.umass.edu<cr><lf>User-Agent: Mozilla/5.0 (WindowsiUi Windows NT 5.1i en-USi rv:1.7.2) Gecko/ Netscape/7.2 (ax) <cr><lf>Accept:ext/xml, application/xml, application/xhtml+xml, text/htmliq=0.9, text/plain iq=0.8,image/png,*/*;q=0.5<cr><lf>Accept-Language: en- us,enjq=0.5<cr><lf>AcceptEncoding:zip,deflate<cr><lf>Accept- Charset: ISO ,utf-8;q=0.7,*jq=0.7<cr><lf>Keep-Alive: 300<cr><If>Connection:keep-alive<cr><lf><cr><lf>
What is the URL of the document requested by the browser?
 What version of HTTP is the browser running?
Does the browser request a non-persistent or a persistent connection?
What is the IP address of the host on which the browser is running?
What type of browser initiates this message? Why is the browser type needed in an HTTP request message?
Application 2-77 77

78. Pr4: HTTP server headers
The text below shows the reply sent from the server in response to the HTTP GET message in the question above. Answer the following questions, pointing out where in the message below you found the answer.
HTTP/ OK<cr><lf>Date: Tue, 07 Mar :39:45GMT<cr><lf>Server: Apache/ (Fedora) <cr><lf>Last- Modified: Sat, 10 Dec :27:46 GMT<cr><lf>ETag: "526c3-f22- a88a4c80"<cr><lf>AcceptRanges: bytes<cr><lf>Content-Length: 3874<cr><lf> Keep-Alive: timeout=max=100<cr><lf>Connection: Keep- Alive<cr><lf>Content-Type: text/html; charset= ISO <cr><lf><cr><lf><!doctype html public "// w3c//dtd html 4.0 transitional//en"><lf><html><lf> <head><lf> <meta http- equiv="Content-Type“ content="text/htmlj charset=iso "><lf> <meta name="GENERATOR" content="Mozilla/4.79 [en] (Windows NT 5.0j U) Netscape]"><lf> <title>CMPSCI 453 / 591 / NTU-ST550A Spring 2005 homepage</title><lf></head><lf> <much more document text following here (not shown)
Was the server able to successfully find the document or not? What time was the document reply provided?
When was the document last modified?
How many bytes are there in the returned document?
What are the first 5 bytes of the returned document?
Did the server agree to a persistent connection?
Application 2-78 78

79. Pr5: RTT with DNS (I)
Suppose you click on a link within your Web browser to obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS request is necessary to obtain the IP address. Suppose that RTT for a DNS request is RTTDNS.
Besides, suppose that the Web page associated with the link contains exactly one object, consisting of a small amount of HTML text (assume object transmission time = 0). Let RTT0 be the RTT between the local host and the Web server.
How long does it take since the client clicks on the link until the client receives the object?
Application 2-79 79

80. Pr6: RTT with DNS (II)
Referring to Problem Pr5, suppose the HTML file references eight very small objects on the same server. Despite of transmission times, how long does it take to get the web page using:
Non-persistent HTTP with no parallel TCP connections?
Non-persistent HTTP with the browser configured for 5 parallel connections?
Persistent HTTP?
Application 2-80 80

81. Institutional network
Pr7: Proxy (I)
origin
servers
This figure shows two networks (institutional and public). The institutional network is a high speed LAN. A router in the institutional network and a router in the Internet are connected by 1.5 Mbps. The origin servers are attached to the Internet but are located all over the globe. Suppose that:
The average object size = 100 Kbits
The average request rate from institution’s browsers to origin servers = 15 requests/sec
HTTP request message are negligibly small
Delay from institutional router to any origin server and back to router = 2 sec
We can calculate the following:
utilization on LAN = 15%
utilization on access link = 100%
As access link uses is 100%, aL/R ~ 1, queues can grow indefinitely and with them the associated delay. Total delay: increses indefinitely.
public
Internet
1.5 Mbps
access link
10 Mbps LAN
Institutional network
Application 2-81

82. Institutional network
Pr7: Proxy (II)
One possible solution:
increase bandwidth of access link to, say, 10 Mbps
consequences
utilization on LAN ?
utilization on access link ?
average total delay ?
Comments ?
public
Internet
10 Mbps
access link
10 Mbps LAN
Institutional network
Application 2-82

83. Pr7: Proxy (III) Another possible solution: Origin servers
Install proxy-cache server
Success rate of 0.4:
40% requests replied by the proxy inmediately
60% requests forwared to the origin web servers
Consequences:
utilization on LAN ?
utilization on access link ?
average total delay ?
Comments ?
Public
Internet
1.5 Mbps
Access link
Institutional
network
10 Mbps LAN
Institutional
proxy
Aplicación 2-83 83 83