Download 3rd Edition: Chapter 2 - Universidad de Sevilla

Departamento de
Tecnología Electrónica
Computer Networking
Chapter 2
The Application layer
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
2009.
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
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
Creating a network app
write programs that
application
transport
network
data link
physical
 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
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
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
Client-server architecture
server:
 always-on host
 permanent IP address
 server farms for scaling
clients:
client/server
 communicate with server
 may be intermittently
connected
 may have dynamic IP
addresses
 do not communicate directly
with each other
Application 2-7
Pure P2P architecture



no always-on server
arbitrary end systems
directly communicate
peer-peer
peers are intermittently
connected and change IP
addresses
highly scalable but difficult to
manage
Application 2-8
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
How is app layer implemented?
Web browsers, e.g.: Mozilla firefox, Chrome, Internet Explore, Safari,…
User interface
User interface
Application
Application Protocol
Application
A_PDU
SAP
SAP
Transport
Transport
Internet
Application 2-10
App-layer protocol defines

types of exchanged
messages,
 e.g., request, response

message syntax:
 what fields are in messages
& how fields are
distinguished

public-domain protocols:
 defined in RFCs
 allows for interoperability
 e.g., HTTP, SMTP
proprietary protocols:
 e.g., Skype
message semantics
 meaning of information in
fields

rules for when and how
processes send & respond
to messages
Application 2-11
Process communication
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.
client process: process that
initiates communication
server process: process that
waits to be contacted
 Note: applications with P2P
architectures have both
client processes & server
processes
Application 2-12
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
server
client
process
controlled by
app developer
process
socket
socket
TCP with
buffers,
variables
Internet
TCP with
buffers,
variables
controlled
by OS
 API: (1) choice of transport protocol; (2) ability to fix a few
parameters (more on this later)
Application 2-13
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)
Note
For every IP address you have an associated name
(hostname) which are used to identify the network
devices by humans and applications.
For example, www.dte.us.es = 150.214.141.196
More on Names and IPs on next section…


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, email client, Skype…
Application 2-14
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
http://www.iana.org/assignments/port-numbers

Socket is identified by the pair:

IP address

Port number
Application 2-15
Example
DTE web server
Interfaz Usuario
Interfaz Usuario
Aplicación
Aplicación
150.214.141.196, 80
Client IP address, port
Transport
Transport
Internet
Application 2-16
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
process1
process2
socket1
socket2
OS Internet
Communication Service
Localhost is usually asociated to IP address
127.0.0.1, but other IPs may be used….
More on Chapter 4…
Q: How can the communication
service on the host able to
distinguish each process?
Application 2-17
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
Timing
 some apps (e.g., Internet
telephony, interactive
games) require low delay
to be “effective”
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, …
Application 2-18
Transport service requirements of common apps
Data loss
Throughput
Time Sensitive
file transfer
e-mail
Web documents
real-time audio/video
no loss
no loss
no loss
loss-tolerant
no
no
no
yes, 100’s msec
stored audio/video
interactive games
instant messaging
loss-tolerant
loss-tolerant
no loss
elastic
elastic
elastic
audio: 5kbps-1Mbps
video:10kbps-5Mbps
same as above
few kbps up
elastic
Application
yes, few secs
yes, 100’s msec
yes and no
Application 2-19
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
Internet apps: application, transport protocols
Application
e-mail
remote terminal access
Web
file transfer
streaming multimedia
Internet telephony
Application
layer protocol
Underlying
transport 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)
TCP
TCP
TCP
TCP
TCP or UDP
typically UDP
Application 2-21
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
DNS: Domain Name System
people: many identifiers:
 SSN, name, passport #
Internet hosts, routers:
 IP address (32 bit) - used for
addressing datagrams
 “name”, e.g.,
www.yahoo.com - 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 applicationlayer protocol
Application 2-23
http://www.dte.us.es/index.html
HTTP client
DNS server
DNS client
DNS adds an additional delay
HTTP server
Internet
Application 2-24
DNS
DNS services
 hostname to IP address
translation (direct)
 IP address to hostname
translation (inverse)
 host aliasing
 Canonical, alias names

mail server aliasing
Which transport service does it
use?
load distribution
 Not reliable
 replicated Web servers: set of IP
 UDP, port 53
addresses for one canonical


Why not centralize DNS?
 single point of failure
 traffic volume
 distant centralized database
 maintenance
… doesn’t scale!
@domain
name
Application 2-25
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
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
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
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:
www.informatica.us.es/DTE/pic.gif
host name
path name
Application 2-29
HTML language format
It allows formatting web pages; created in 1991. 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.
Aside
You can view the HTML code in any webpage with
your favourite navigator, right-click -> view source code.
Application 2-30
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
PC running
IExplorer
Server
running
Apache Web
server
Linux running
Firefox
Application 2-31
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
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
Non-persistent HTTP
suppose user enters URL:
(contains text & references to 13 objects)
http://www.dte.us.es/personal/smartin/lab3/referencias.html
1a. HTTP client initiates TCP connection
to HTTP server (process) at
www.dte.us.es on port 80
1b. HTTP server at host
www.dte.us.es 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
Nonpersistent HTTP (cont.)
4. HTTP server closes TCP connection.
5. HTTP client receives response
time
message containing html file,
displays html. Parsing html file,
finds 13 referenced objects
(images, scripts, …)
6. Steps 1-5 repeated for each of 13
objects with different unique URLs
Application 2-35
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
initiate TCP
connection
RTT
request
file
RTT
file
received
time
time to
transmit
file
time
Application 2-36
Persistent HTTP
non-persistent HTTP issues:
 requires 2 RTTs per object
 OS overhead for each TCP
connection
Note that:
Parallel HTTP connections:
 browsers often open parallel TCP
connections to fetch referenced
objects faster. They are requested
sequentally per each connection
(persistent or not).
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
Application 2-37
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
HTTP request message
Note:
<CR>: Carriage-Return : \r
<LF>: Line-Feed: \n
request line
(GET, POST,
HEAD commands)
header
lines
\r & \n at start
of line indicates
end of header lines
UD
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-8859-1,utf-8;q=0.7\r\n
Keep-Alive: 115\r\n
Connection: keep-alive\r\n
\r\n
Application 2-39
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
HTTP request message: general format
request
line
header
lines
PCI
PDU
body
UD
Application 2-41
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: www.somesearchengine.com\r\n
…
Application 2-42
Method types
HTTP/1.0 (RFC-1945)
 GET
 POST
 HEAD
 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
HTTP response message
status line
(protocol
status code
status phrase)
header
lines
data, e.g.,
requested
HTML file
HTTP/1.1 200 OK\r\n
Date: Sun, 26 Sep 2010 20:09:20 GMT\r\n
Server: Apache/2.0.52 (CentOS)\r\n
Last-Modified: Tue, 30 Oct 2007 17:00:02
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-88591\r\n
\r\n
data data data data data ...
Application 2-44
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
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
Trying out HTTP (client side) for yourself
1. Telnet to your favourite Web server:
telnet www.dte.us.es 80 opens TCP connection to port 80
(default HTTP server port) at www.dte.us.es
anything typed in sent
to port 80 at www.dte.us.es
2. type in a GET HTTP request:
GET /docencia/ HTTP/1.1
Host: www.dte.us.es
by typing this, you send
this minimal (but complete)
GET request to HTTP server
3. look at response message sent by HTTP server!
(or use Wireshark!)
Q: What happens if you send “hello”?
Application 2-47
User-server state: cookies
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
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
Application 2-48
Cookies: Example
Client visits AMAZON
ebay 8734
cookie file
AMAZON server
usual http request msg
usual http response
Set-cookie:1678
ebay 8734
amazon 1678
usual http request msg
cookie:1678
one week later:
usual http response msg
Amazon server
creates ID
1678 for user create
entry
cookiespecific
action
access
access
ebay 8734
amazon 1678
usual http request msg
cookie:1678
usual http response msg
backend
database
cookiespecific
action
Application 2-49
Cookies (continued)
Note
what cookies can bring:
 authorization
 shopping carts
 recommendations
 user session state (Web email)
cookies and privacy:
 cookies allow sites to
learn a lot about you
 you may supply personal
information to sites
(name, e-mail,…)
Application 2-50
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
client
Proxy
server
client
origin
server
Application 2-51
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
Conditional GET


Goal: don’t send object if
cache-proxy
cache has up-to-date
cached version
HTTP request msg
If-modified-since: <date>
Cache-proxy: specify date
of cached copy in HTTP
request
HTTP response
If-modified-since: <date>

server: response contains
no object if cached copy
is up-to-date:
Last-modified:<date>
object
modified
before
<date>
HTTP/1.0
304 Not Modified
HTTP request msg
If-modified-since: <date>
HTTP/1.0 304 Not Modified
Or modified object is
sended with the lastmodified date:
server
HTTP response
HTTP/1.0 200 OK
Last-modified: <date>
object
modified
after
<date>
<data>
Application 2-53
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
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 streamoriented
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
Application protocol
Server
Client
User interface
User interface
Application
Application
Server IP address, port
Client IP address, port
Transport
Transport
Internet
Application 2-56
Socket-programming using TCP
Socket: a door between application process and end-endtransport protocol (UCP or TCP)
TCP service: reliable transfer of bytes from one process to
another
controlled by
application
developer
controlled by
operating
system
process
process
socket
TCP with
buffers,
variables
socket
TCP with
buffers,
variables
client or
server
internet
controlled by
application
developer
controlled by
operating
system
client or
server
Application 2-57
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
Client/server socket interaction: TCP
Server (running on hostid)
Wait for primitive:
Connection.indication
Send primitive:
Connection.response
Client
create socket,
port=x, for
incoming request:
welcomeSocket =
ServerSocket()
TCP
connection
setup
wait for incoming
connection request
connectionSocket =
welcomeSocket.accept()
read request from
connectionSocket
write reply to
connectionSocket
close
connectionSocket
hostid = IP or hostname
create socket,
connect to hostid, port=x
clientSocket =
Socket()
send request using
clientSocket
Send primitive:
Connection.request
Wait for primitive:
Connection.confirm
read reply from
clientSocket
close
clientSocket
Application 2-59
Java Stream


Client
Process
inFromUser
inFromUser
input
input
stream
stream
monitor
monitor
process
output
output
stream
stream
inFromServer
inFromServer

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
outToServer
outToServer

keyboard
keyboard
client
TCP
clientSocket
socket
to network
To
transport
layer
input
input
stream
stream
TCP
socket
from network
From
transport
layer
Application 2-60
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
Example: Java client (TCP)
import java.io.*;
import java.net.*;
class TCPClient {
create
input stream
create
clientSocket object
of type Socket,
connect to server
create
output stream
attached to socket
This package defines Socket()
and ServerSocket() classes
public static void main(String argv[]) throws Exception
{
server name,
String sentence;
e.g., www.dte.us.es
String modifiedSentence;
T_ICI
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname", 6789);
T_ICI
DataOutputStream outToServer =
new DataOutputStream(clientSocket.getOutputStream());
server port #
Application 2-62
Example: Java client (TCP), cont.
BufferedReader inFromServer =
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
create
input stream
attached to socket
Send primitive:
Data.request
sentence = inFromUser.readLine();
send line
to server
PDU
outToServer.writeBytes(sentence + '\n');
read line
from server
Wait for primitive:
Data.indication
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER: " + modifiedSentence);
close socket
clientSocket.close();
(clean up behind yourself!)
}
}
Application 2-63
Example: Java server (TCP)
import java.io.*;
import java.net.*;
class TCPServer {
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
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()));
Application 2-64
Example: Java server (TCP), cont
create output
stream, attached
to socket
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
PDU
Wait for primitive:
clientSentence = inFromClient.readLine();
Data.indication
read in line
from socket
capitalizedSentence = clientSentence.toUpperCase() + '\n';
write out line
to socket
outToClient.writeBytes(capitalizedSentence);
}
}
}
Send primitive:
Data.request
PDU
end of while loop,
loop back and wait for
another client connection
Application 2-65
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
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
application viewpoint:
UDP provides unreliable transfer
of groups of bytes (“datagrams”)
between client and server
UDP: transmitted data may be
received out of order, or some
may be lost
Application 2-67
Client/server socket interaction: UDP
Server (running on hostid)
create socket,
port= x.
serverSocket =
DatagramSocket()
read datagram from
serverSocket
write reply to
serverSocket
specifying
client address,
port number
hostid = IP or hostname
Client
create socket,
clientSocket =
DatagramSocket()
Wait primitive:
Data.indication
Create datagram with server IP and
port=x; send datagram via
clientSocket
Send primitive:
Data.request
read datagram from
clientSocket
close
clientSocket
Application 2-68
Example: client (UDP datagrams)
input
input
stream
stream
Client
Process
monitor
monitor
inFromUser
inFromUser
keyboard
keyboard
process
Input: receives packet
receivePacket
inFromServer
UDP
output
stream
packet
outToServer
(recall that TCP sent
“byte stream”)
sendPacket
Output: sends packet
client
UDP
clientSocket
socket
to network
To
transport
layer
(recall that TCP
received “byte
stream”)
UDP
input
stream
packet
TCP
UDP
socket
from network
From
transport
layer
socket
Application 2-69
Example: Java client (UDP)
import java.io.*;
import java.net.*;
class UDPClient {
public static void main(String args[]) throws Exception
{
create
input stream
create
client socket
translate
hostname to IP
address using DNS
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();
Application 2-70
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);
Send primitive:
T_IDU
Data.request
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
send T_IDU
create new T_IDU
clientSocket.receive(receivePacket);
receive T_IDU
Wait for primitive:
Data.indication
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close();
}
}
Application 2-71
Example: Java server (UDP)
import java.io.*;
import java.net.*;
Create UDP socket
at port 9876
(known by client)
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)
{
create new T_IDU
receive
T_IDU
Wait for primitive:
Data.indication
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
Application 2-72
Example: Java server (UDP) (cont.)
String sentence = new String(receivePacket.getData());
get sender IP addr
and port # from
T_ICI
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
create T_IDU
with A_PDU to
send, length, IP
addr and port #
sendData = capitalizedSentence.getBytes();
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress,
port);
serverSocket.send(sendPacket);
Send T_IDU
}
}
}
end of while loop,
loop back and wait for
another datagram
Application 2-73
Departamento de
Tecnología Electrónica
Computer Networking – Chapter 2: Application layer
PROBLEMS AND EXERCISES
Application 2-74
Pr1: True or false?
a)
With non-persistent connections between browser and origin
server, it is possible for a single TCP segment to carry two different
HTTP request messages.
b) 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.
c) These two different Web pages www.mit.edu/index.html and
www.dte.us.es/index.html can be sent over the same persistent
connection.
d) The “Date:” header in the HTTP response message indicates when
the object in the response was last modified.
e) HTTP response messages never have an empty message body.
Application 2-75
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
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/20040804 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: enus,enjq=0.5<cr><lf>AcceptEncoding:zip,deflate<cr><lf>AcceptCharset: ISO-8859-1,utf-8;q=0.7,*jq=0.7<cr><lf>Keep-Alive:
300<cr><If>Connection:keep-alive<cr><lf><cr><lf>
a) What is the URL of the document requested by the browser?
b)
What version of HTTP is the browser running?
c) Does the browser request a non-persistent or a persistent connection?
d) What is the IP address of the host on which the browser is running?
e) What type of browser initiates this message? Why is the browser type needed in an HTTP request
message?
Application 2-77
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/1.1 200 OK<cr><lf>Date: Tue, 07 Mar 2008
12:39:45GMT<cr><lf>Server: Apache/2.0.52 (Fedora) <cr><lf>LastModified: Sat, 10 Dec2005 18:27:46 GMT<cr><lf>ETag: "526c3-f22a88a4c80"<cr><lf>AcceptRanges: bytes<cr><lf>Content-Length:
3874<cr><lf> Keep-Alive: timeout=max=100<cr><lf>Connection: KeepAlive<cr><lf>Content-Type: text/html; charset= ISO-88591<cr><lf><cr><lf><!doctype html public "// w3c//dtd html 4.0
transitional//en"><lf><html><lf> <head><lf> <meta httpequiv="Content-Type“ content="text/htmlj charset=iso-8859-1"><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)
a) Was the server able to successfully find the document or not? What time was the document
reply provided?
b) When was the document last modified?
c) How many bytes are there in the returned document?
d) What are the first 5 bytes of the returned document?
e) Did the server agree to a persistent connection?
Application 2-78
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
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:
a)Non-persistent HTTP with no parallel TCP connections?
b)Non-persistent HTTP with the browser configured for 5 parallel
connections?
c)Persistent HTTP?
Application 2-80
Pr7: Proxy (I)
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:
a) utilization on LAN = 15%
b) utilization on access link = 100%
c) As access link uses is 100%, aL/R ~ 1, queues can
grow indefinitely and with them the associated
delay. Total delay: increses indefinitely.
origin
servers
public
Internet
1.5 Mbps
access link
10 Mbps LAN
Institutional network
Application 2-81
Pr7: Proxy (II)
One possible solution:
increase bandwidth of access link
to, say, 10 Mbps
consequences
a) utilization on LAN ?
b) utilization on access link ?
c) average total delay ?
d) Comments ?

public
Internet
10 Mbps
access link
10 Mbps LAN
Institutional network
Application 2-82
Pr7: Proxy (III)
Origin
servers
Another possible solution:
 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