lecture 10 tcp/ip application layer (2)
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Lecture 10 TCP/IP Application Layer (2). Outline (Application Layer). Principles of network applications Web and HTTP FTP Electronic Mail SMTP, POP3, IMAP DNS Socket programming with TCP Socket programming with UDP Building a Web server. Internet hosts, routers: - PowerPoint PPT PresentationTRANSCRIPT
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Lecture 10
TCP/IP Application Layer (2)
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Outline (Application Layer)• Principles of network applications
• Web and HTTP
• FTP
• Electronic Mail– SMTP, POP3, IMAP
• DNS
• Socket programming with TCP
• Socket programming with UDP
• Building a Web server
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
DNS: Domain Name SystemInternet hosts, routers:
– IP address (32 bit) - used for addressing datagrams– “name”, e.g., ww.yahoo.com - used by humans
• Hostname to IP address translation
• Host aliasing (canonical hostname)– A host with a complicated hostname can have one or more alias names, E.g.
relay1.west-coast.enterprise.com could have, say, two aliases such as enterprise.com and www.enterprise.com. Alias hostnames, are typically more mnemonic than a canonical hostname. DNS can be invoked by an application to obtain the canonical hostname for a supplied alias hostname as well as the IP address of the host.
• Mail server aliasing– Similar to host aliasing, e.g. the canonical hostname for hotmail might be
something like relay1.west-coast.hotmail.com• Load distribution
– Replicated Web servers: set of IP addresses for one canonical name. DNS server responds with the entire set of IP addresses, but rotates the ordering of the addresses within each reply.
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
DNS: Domain Name System• DNS uses a large number of name servers, organized in a
hierarchical fashion and distributed around the world.• No one name server has all of the mappings for all of the
hosts in the Internet. Instead, the mappings are distributed across the name servers.
• There are three types of name servers: – local name servers, – root name servers, and – authoritative name servers.
Why not centralize DNS?• single point of failure• traffic volume• distant centralized database• maintenance
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Local Name Server
• Each ISP (residential ISP, company, university) has one.– Also called “default name server”
• When a host makes a DNS query, query is sent to its local DNS server– The IP address of the local name server is typically
configured by hand in a host.– If a host requests a translation for another host that is
part of the same local ISP, then the local name server will be able to immediately provide the requested IP address.
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Root Name Servers• contacted by local name server that can not resolve name• root name server:
– If the root name server has a record for the hostname, it sends a DNS reply message to the local name server
– contacts authoritative name server if name mapping not known, gets mapping and returns mapping to local name server
b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA
e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 17 other locations)
i Autonomica, Stockholm (plus 3 other locations)
k RIPE London (also Amsterdam, Frankfurt)
m WIDE Tokyo
a Verisign, Dulles, VAc Cogent, Herndon, VA (also Los Angeles)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 11 locations)
13 root name servers worldwide
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Authoritative Servers• The authoritative name server for a host is a name
server in the host's local ISP
• A name server is authoritative for a host if it always has a DNS record that translates the host's hostname to that host's IP address.
DNS: Caching and Updating Records• once (any) name server learns mapping, it caches
mapping– cache entries timeout (disappear) after some time
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Example1. the host surf.eurecom.fr first sends a
DNS query message to its local name server, dns.eurecom.fr. The hostname to be translated, is gaia.cs.umass.edu.
2. The local name server forwards the query message to a root name server
3. the root name server forwards a query for a host with hostname ending with umass.edu to the name server dns.umass.edu.
4. This name server forwards all queries with hostnames ending with .cs.umass.edu to the name server dns.cs.umass.edu, which is authoritative for all hostnames ending with .cs.umass.edu.
5. The authoritative name server sends the desired mapping to the intermediate name server, dns.umass.edu
6. And so on….
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
DNS Query Types
recursive query:• puts burden of name
resolution on contacted name server
iterated query:• contacted server
replies with name of server to contact
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
DNS Records
DNS: distributed db storing resource records (RR)
• Type=NS– name is domain (e.g.
foo.com)
– value is IP address of authoritative name server for this domain
– e.g. (foo.com, dns.foo.com, NS)
RR format: (name, value, type, ttl)
• Type=A– name is hostname
– value is IP address
– e.g. (relay1.bar.foo.com, 145.37.93.126,A)
• Type=CNAME– name is alias name for some
“cannonical” (the real) name
www.ibm.com is really
servereast.backup2.ibm.com– value is cannonical name
– e.g. (foo.com, relay1.bar.foo.com, CNAME)
• Type=MX– value is name of mail server
associated with name– e.g. (foo.com. mail.bar.foo.com, MX)
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
DNS Protocol, Messages
DNS protocol : query and reply messages, both with same message format
message header• identification: 16 bit #
for query, reply to query uses same #
• flags:– query or reply (0/1)– recursion desired – recursion available– reply is authoritative
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
DNS Protocol, Messages
Name, type fields for a query
RRs in responseto query
records forauthoritative servers
additional “helpful”info that may be used
Programs for Exploring DNS• There are various client programs available for exploring the contents of name servers in the Internet, e.g. nslookup : allows to enter a hostname and they return an IP address. Visit
http://namespace.pgmedia.net/nslookup/ http://www.infobear.com/nslookup-form.cgi
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Outline (Application Layer)• Principles of network applications
• Web and HTTP
• FTP
• Electronic Mail– SMTP, POP3, IMAP
• DNS
• Socket programming with TCP
• Socket programming with UDP
• Building a Web server
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Socket Programming
Socket API• introduced in BSD4.1 UNIX,
1981• explicitly created, used,
released by applications • client/server paradigm • two types of transport service
via socket API: – unreliable datagram – reliable, byte stream-oriented
a host-local, application-created,
OS-controlled interface (a “door”) into which
application process can both send and
receive messages to/from another
application process
socket
Goal: learn how to build client/server application that communicate using sockets
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
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
process
TCP withbuffers,
variables
socket
controlled byapplicationdeveloper
controlled byoperating
system
host orserver
process
TCP withbuffers,
variables
socket
controlled byapplicationdeveloper
controlled byoperatingsystem
host orserver
internet
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Socket Programming with TCP
Client must contact server• server process must first be
running• server must have created socket
(door) that welcomes client’s contact
Client contacts server by:• creating client-local 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
TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server
application viewpoint
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Stream Jargon• A stream is a sequence of characters that flow into or out
of a process.
• An input stream is attached to some input source for the process, e.g., keyboard or socket.
• An output stream is attached to an output source, e.g., monitor or socket.
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Socket Programming with TCPExample client-server
application: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)
outT
oSer
ver
to network from network
inFr
omS
erve
r
inFr
omU
ser
keyboard monitor
Process
clientSocket
inputstream
inputstream
outputstream
TCPsocket
Clientprocess
client TCP socket
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Client/Server Socket Interaction: TCP
wait for incomingconnection requestconnectionSocket =welcomeSocket.accept()
create socket,port=x, forincoming request:welcomeSocket =
ServerSocket()
create socket,connect to hostid, port=xclientSocket =
Socket()
closeconnectionSocket
read reply fromclientSocket
closeclientSocket
Server (running on hostid) Client
send request usingclientSocketread request from
connectionSocket
write reply toconnectionSocket
TCP connection setup
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
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());
Createinput stream
Create client socket,
connect to server
Createoutput stream
attached to socket
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
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(); } }
Createinput stream
attached to socket
Send lineto server
Read linefrom server
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
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()));
Createwelcoming socket
at port 6789
Wait, on welcomingsocket for contact
by client
Create inputstream, attached
to socket
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Example: Java Server (TCP) (cont..)
DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
outToClient.writeBytes(capitalizedSentence); } } }
Read in linefrom socket
Create outputstream,
attached to socket
Write out lineto socket
End of while loop,loop back and wait foranother client connection
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Outline (Application Layer)• Principles of network applications
• Web and HTTP
• FTP
• Electronic Mail– SMTP, POP3, IMAP
• DNS
• Socket programming with TCP
• Socket programming with UDP
• Building a Web server
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Socket Programming with UDPUDP: no “connection” between client and server• no handshaking• sender explicitly attaches IP address and port of
destination to each packet• server must extract IP address, port of sender from
received packet
UDP: transmitted data may be received out of order, or lost
application viewpoint
UDP provides unreliable transfer of groups of bytes (“datagrams”)
between client and server
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Client/Server Socket Interaction: UDP
closeclientSocket
Server (running on hostid)
read reply fromclientSocket
create socket,clientSocket = DatagramSocket()
Client
Create, address (hostid, port=x,send datagram request using clientSocket
create socket,port=x, forincoming request:serverSocket = DatagramSocket()
read request fromserverSocket
write reply toserverSocketspecifying clienthost address,port number
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Example: Java Client (UDP)
sendP
ack
et
to network from network
rece
iveP
ack
et
inF
rom
Use
r
keyboard monitor
Process
clientSocket
UDPpacket
inputstream
UDPpacket
UDPsocket
Output: sends packet (TCP sent “byte stream”)
Input: receives packet (TCP received “byte stream”)
Clientprocess
client UDP socket
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
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();
Createinput stream
Create client socket
Translate hostname to IP
address using DNS
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Example: Java Client (UDP), cont.
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(); }
}
Create datagram with data-to-send,
length, IP addr, port
Send datagramto server
Read datagramfrom server
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
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);
Createdatagram socket
at port 9876
Create space forreceived datagram
Receivedatagra
m
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
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 IP addrport #, of
sender
Write out datagramto socket
End of while loop,loop back and wait foranother datagram
Create datagramto send to client
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Outline (Application Layer)• Principles of network applications
• Web and HTTP
• FTP
• Electronic Mail– SMTP, POP3, IMAP
• DNS
• Socket programming with TCP
• Socket programming with UDP
• Building a Web server
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Building a Simple Web Server
• handles one HTTP request• accepts the request• parses header• obtains requested file from server’s file system• creates HTTP response message:
– header lines + file
• sends response to client• after creating server, you can request file using a
browser (e.g. IE explorer)
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Example: Web Serverimport java.io.*;import java.net.*;import java.util.*;class WebServer{public static void main(String argv[]) throws Exception {
String requestMessageLine;String fileName;ServerSocket listenSocket = new ServerSocket(6789);Socket connectionSocket = listenSocket.accept();BufferedReader inFromClient = new BufferedReader(new
InputStreamReader(connectionSocket.getInputStream()));DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());requestMessageLine = inFromClient.readLine();StringTokenizer tokenizedLine = new StringTokenizer(requestMessageLine);if (tokenizedLine.nextToken().equals("GET")){
fileName = tokenizedLine.nextToken();if (fileName.startsWith("/") == true )
fileName = fileName.substring(1);
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Example: Web Server File file = new File(fileName); int numOfBytes = (int) file.length(); FileInputStream inFile = new FileInputStream (fileName); byte[] fileInBytes = new byte[numOfBytes]; inFile.read(fileInBytes); outToClient.writeBytes("HTTP/1.0 200 Document Follows\r\n");
if (fileName.endsWith(".jpg")) outToClient.writeBytes("Content-Type: image/jpeg\r\n");
if (fileName.endsWith(".gif")) outToClient.writeBytes("Content-Type: image/gif\r\n");
outToClient.writeBytes("Content-Length: " + numOfBytes + "\r\n"); outToClient.writeBytes("\r\n"); outToClient.write(fileInBytes, 0, numOfBytes);
connectionSocket.close(); } else System.out.println("Bad Request Message"); }}
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Reading Material• Chapter 2 – text3 (Kurose)
• Chapter 27, 28 – text1 (Stevens)