Homework

2: Application Layer 1

Homework Collect Homework 1 Assign Homework 2

Problems Ch 2#1,4,6,7 (two graded)

Due Wednesday, 10 September


Chapter 2 outline

2.1 Principles of app layer protocols

2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS

2.6 Socket programming with TCP

2.7 Socket programming with UDP

2.8 Building a Web server

2.9 Content distribution Network Web caching Content distribution

networks P2P file sharing


DNS: Domain Name System

People: many identifiers: SSN, name, passport

#

Internet hosts, routers: IP address (32 bit) -

used for addressing datagrams

“name”, e.g., gaia.cs.umass.edu - used by humans

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

complexity at network’s “edge”


DNS name servers

no server has all name-to-IP address mappings

local name servers: each ISP, company has

local (default) name server host DNS query first goes

to local name server

authoritative name server: for a host: stores that

host’s IP address, name can perform name/address

translation for that host’s name

Why not centralize DNS? single point of failure traffic volume distant centralized

database maintenance

doesn’t scale!


DNS: Root name servers contacted by local name server that can not resolve name root name server:

contacts authoritative name server if name mapping not known

gets mapping returns mapping to local name server

b USC-ISI Marina del Rey, CAl ICANN Marina del Rey, CA

e NASA Mt View, CAf Internet Software C. Palo Alto, CA

i NORDUnet Stockholm

k RIPE London

m WIDE Tokyo

a NSI Herndon, VAc PSInet Herndon, VAd U Maryland College Park, MDg DISA Vienna, VAh ARL Aberdeen, MDj NSI (TBD) Herndon, VA

13 root name servers worldwide


Simple DNS example

host surf.eurecom.fr wants IP address of gaia.cs.umass.edu

1. contacts its local DNS server, dns.eurecom.fr

2. dns.eurecom.fr contacts root name server, if necessary

3. root name server contacts authoritative name server, dns.umass.edu, if necessary

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

authorititive name serverdns.umass.edu

local name serverdns.eurecom.fr

1

23

4

5

6


DNS example

Root name server: may not know

authoritative name server

may know intermediate name server: who to contact to find authoritative name server


gaia.cs.umass.edu

root name server


1

23

4 5

6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8


DNS: iterated queries

recursive query: puts burden of

name resolution on contacted name server

heavy load?

iterated query: contacted server

replies with name of server to contact

“I don’t know this name, but ask this server”


gaia.cs.umass.edu

root name server


1

23

4

5 6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

iterated query


DNS: caching and updating records once (any) name server learns mapping, it

caches mapping cache entries timeout (disappear) after

some time update/notify mechanisms under design by

IETF RFC 2136 http://www.ietf.org/html.charters/dnsext-charter.html


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

RR format: (name, value, type, ttl)

Type=A name is hostname value is IP address

Type=CNAME name is alias name for some

“canonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com value is canonical name

Type=MX value is name of mailserver

associated with name


DNS protocol, messagesDNS protocol : query and reply messages, both with same message format

msg header identification: 16 bit #

for query, reply to query uses same #

flags: query or reply recursion desired recursion available reply is authoritative


DNS protocol, messages

Name, type fields for a query

RRs in responseto query

records forauthoritative servers

additional “helpful”info that may be used


DNS Tools

nslookup: can find IP given name can find name given IP can show other RR

whois: can show information about domain and owner can show information about “owner” of an IP

address or network online whois servers

http://www.arin.net/whois/index.html http://network-tools.com/

dig and host: Show more detail Not on Suns

common on Linux Online:

http://www.dnsstuff.com/ http://www.ip-plus.net/tools/ dns_config.en.html


Email SPAM

Problem: I get over 100 messages per day. Perhaps 30% are SPAM. How can I stop it?

NEVER respond to SPAM:• From address is often bogus.• When valid, it just verifies that your address works.

SPAM is a violation of most ISP’s AUPs.• Complain to the ISP, and get the spammer

disconnected. Use tools and knowledge

• Examine mail headers• Use traceroute• Use whois


Sample SPAMReturn-Path: <[email protected]>Received: from mx02.mrf.mail.rcn.net (mx02.mrf.mail.rcn.net [207.172.4.51]) by briansbooks.com (8.9.3/8.8.7) with ESMTP id PAA22178 for <[email protected]>; Tue, 22 Jan 2002 15:22:59 -0500From: [email protected]: from [211.251.72.130] (helo=spray.no) by mx02.mrf.mail.rcn.net with smtp (Exim 3.34 #5) id 16T7U6-0004iN-00; Tue, 22 Jan 2002 15:25:35 -0500Reply-To: <[email protected]>Message-ID: <006b42c45cbe$5652b6c4$0ee32ab0@uihupb>To: [email protected]: Support Follow up. (2141@4)MiME-Version: 1.0Content-Type: text/html; charset="iso-8859-1"X-Mailer: MIME-tools 5.503 (Entity 5.501)Importance: NormalDate: Tue, 22 Jan 2002 15:25:35 -0500

<META HTTP-EQUIV="Content-Type" CONTENT="text/html;charset=iso-8859-1"><HTML>[…]


Chapter 2 outline

2.1 Principles of app layer protocols


SMTP, POP3, IMAP

2.5 DNS







Socket programming

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

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


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


Socket programming with TCPClient 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 (more in Chap 3)

TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server

application viewpoint


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, eg, keyboard or socket.

An output stream is attached to an output source, eg, monitor or socket.


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 socket3) 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


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


Example: Java echo 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


Example: Java echo client (TCP), cont.

BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

sentence = inFromUser.readLine();

while (sentence != null) { outToServer.writeBytes(sentence + '\n');

modifiedSentence = inFromServer.readLine();

System.out.println("FROM SERVER: " + modifiedSentence);

sentence = inFromUser.readLine(); } clientSocket.close(); } }

Createinput stream

attached to socket

Send lineto server

Read linefrom server


Example: Java echo 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


Example: Java echo server (TCP), cont

DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine();

while(clientSentence != null) { capitalizedSentence = clientSentence.toUpperCase() + '\n';

outToClient.writeBytes(capitalizedSentence); clientSentence = inFromClient.readLine(); } connectionSocket.close(); } }

}

Read in linefrom socket

Create outputstream,

attached to socket

Write out lineto socket

End of while loop,loop back and wait foranother client connection


Example: C echo client (TCP)

#include <sys/types.h> /* basic system data types */#include <sys/socket.h> /* basic socket definitions */#include <netinet/in.h>#include <stdio.h>#include <unistd.h>

int main(int argc, char **argv){ int sockfd; struct sockaddr_in servaddr;

if (argc != 2) { printf("usage: tcpcli <IPaddress>\n"); exit(-1); }

sockfd = socket(AF_INET, SOCK_STREAM, 0);Create

TCP socket

Define socket address structure

Helpfulincludes


Example: C echo client (TCP), cont.

bzero(&servaddr, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_port = htons(6789); servaddr.sin_addr.s_addr = inet_addr(argv[1]);

connect(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr));

str_cli(stdin, sockfd); /* do it all */

exit(0);}

Fill in socketstructure with

server information

Establish connectionwith server

Work with the established socket


Example: C echo client (TCP), cont.void str_cli(FILE *fp, int sockfd){ char sendline[MAXLINE], recvline[MAXLINE];

while (fgets(sendline, MAXLINE, fp) != NULL) {

write(sockfd, sendline, strlen(sendline));

if (readline(sockfd, recvline, MAXLINE) == 0) { printf("str_cli: server terminated prematurely\n"); exit(-1); }

fputs(recvline, stdout); }}

Get line of text from stdin

Send line to server

Get line from server

Write the line to stdout

Readline is also user defined.


Example: C echo server (TCP)#include <sys/types.h> /* basic system data types */#include <sys/socket.h> /* basic socket definitions */#include <netinet/in.h>#include <stdio.h>#include <unistd.h>

#define MAXLINE 1024#define LISTENQ 16 /* max size of queue */

int main(int argc, char **argv){ int listenfd, connfd; pid_t childpid; int clilen; struct sockaddr_in cliaddr, servaddr;

listenfd = socket(AF_INET, SOCK_STREAM, 0);

bzero(&servaddr, sizeof(servaddr));

CreateTCP socket

Define socket address structure

Helpfulincludes


Example: C echo server (TCP), cont.

servaddr.sin_family = AF_INET; servaddr.sin_addr.s_addr = htonl(INADDR_ANY); servaddr.sin_port = htons(6789);

bind(listenfd, (struct sockaddr *) &servaddr, sizeof(servaddr));

listen(listenfd, LISTENQ);

for ( ; ; ) { clilen = sizeof(cliaddr); connfd = accept(listenfd, (struct sockaddr *) &cliaddr, &clilen);

str_echo(connfd); /* process the request */ close(connfd); /* close connected socket */ }} /* end main */

Fill in structure to accept conns from any local interface

Convert socket toa listening socket

Wait until new conn. is established

Assign structure to the socket

Work with the established conn.


Example: C echo server (TCP), cont.

void str_echo(int sockfd){ ssize_t n; char line[MAXLINE];

for ( ; ; ) { if ( (n = readline(sockfd, line, MAXLINE)) == 0) return; /* connection closed by other end */

write(sockfd, line, n); }} /* end str_echo */

Get line from client

Write line back to client

Readline is also user defined.


Chapter 2 outline

2.1 Principles of app layer protocols clients and servers app requirements


SMTP, POP3, IMAP

2.5 DNS







Socket programming with UDP

UDP: 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


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


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


Example: Java echo 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]; String sentence = inFromUser.readLine(); while (sentence != null) { byte[] receiveData = new byte[1024];

sendData = sentence.getBytes();

Createinput stream

Create client socket

Translate hostname to IP

address using DNS

Get line from STDIN

Convert to byte array


Example: Java echo client (UDP), cont.

DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 6789); 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); String sentence = inFromUser.readLine(); } clientSocket.close(); } }

Create datagram with data-to-send,

length, IP addr, port

Send datagramto server

Read datagramfrom server

Convert to String

Send to STDOUTLook for more input


Example: Java echo server (UDP)

import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(6789); byte[] sendData = new byte[1024]; while(true) { byte[] receiveData = new byte[1024]; DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);

serverSocket.receive(receivePacket);

Createdatagram socket

at port 6789

Create space forreceived datagram

Receivedatagra

m


Example: Java echo 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

Convert to caps,convert to bytes


Example: C echo client (UDP)

[…headers…]

int main(int argc, char **argv) { int sockfd; struct sockaddr_in servaddr;

if (argc != 2) { printf("usage: udpcli <IPaddress>"); exit(-1); } bzero(&servaddr, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_port = htons(6789); servaddr.sin_addr.s_addr = inet_addr(argv[1]);

sockfd = socket(AF_INET, SOCK_DGRAM, 0);

dg_cli(stdin, sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr));

exit(0);}

CreateUDP socket


Set up socketparameters


Example: C echo client (UDP) cont.

voiddg_cli(FILE *fp, int sockfd, const struct sockaddr *pservaddr, int servlen){ int n; char sendline[MAXLINE], recvline[MAXLINE + 1];

while (fgets(sendline, MAXLINE, fp) != NULL) {

sendto(sockfd, sendline, strlen(sendline), 0, pservaddr, servlen);

n = recvfrom(sockfd, recvline, MAXLINE, 0, NULL, NULL);

recvline[n] = 0; /* null terminate */ fputs(recvline, stdout); }}

Write out datagramto socket

Get line from STDIN

Send line to STDOUT

Receive datagramFrom socket


Example: C echo server (UDP)#include <sys/types.h> /* basic system data types */#include <sys/socket.h> /* basic socket definitions */#include <netinet/in.h>#include <stdio.h>#include <unistd.h>#define MAXLINE 1024

int main(int argc, char **argv){ int sockfd; struct sockaddr_in servaddr, cliaddr;

sockfd = socket(AF_INET, SOCK_DGRAM, 0);

bzero(&servaddr, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_addr.s_addr = htonl(INADDR_ANY); servaddr.sin_port = htons(6789);

bind(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr)); dg_echo(sockfd, (struct sockaddr *) &cliaddr, sizeof(cliaddr));}

CreateUDP socket


Bind socket tospecified address


Example: C echo server (UDP) cont.

voiddg_echo(int sockfd, struct sockaddr *pcliaddr, int clilen){ int n; int len; char mesg[MAXLINE];

for ( ; ; ) { len = clilen; n = recvfrom(sockfd, mesg, MAXLINE, 0, pcliaddr, &len);

sendto(sockfd, mesg, n, 0, pcliaddr, len); }}

Send datagramback to client

Read nextdatagram

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