2: Application Layer1 Homework r Collect Homework 1 r Assign Homework 2 m Problems Ch 2#1,4,6,7 (two graded) m Due Wednesday, 10 September

2: Application Layer2 Chapter 2 outline r 2.1 Principles of app layer protocols r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail m SMTP, POP3, IMAP r 2.5 DNS r 2.6 Socket programming with TCP r 2.7 Socket programming with UDP r 2.8 Building a Web server r 2.9 Content distribution m Network Web caching m Content distribution networks m P2P file sharing

2: Application Layer3 DNS: Domain Name System People: many identifiers: m SSN, name, passport # Internet hosts, routers: m IP address (32 bit) - used for addressing datagrams m “name”, e.g., gaia.cs.umass.edu - used by humans Domain Name System: r distributed database implemented in hierarchy of many name servers r application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) m note: core Internet function, implemented as application-layer protocol m complexity at network’s “edge”

2: Application Layer4 DNS name servers r no server has all name- to-IP address mappings local name servers: m each ISP, company has local (default) name server m host DNS query first goes to local name server authoritative name server: m for a host: stores that host’s IP address, name m can perform name/address translation for that host’s name Why not centralize DNS? r single point of failure r traffic volume r distant centralized database r maintenance doesn’t scale!

2: Application Layer5 DNS: Root name servers r contacted by local name server that can not resolve name r root name server: m contacts authoritative name server if name mapping not known m gets mapping m returns mapping to local name server b USC-ISI Marina del Rey, CA l ICANN Marina del Rey, CA e NASA Mt View, CA f Internet Software C. Palo Alto, CA i NORDUnet Stockholm k RIPE London m WIDE Tokyo a NSI Herndon, VA c PSInet Herndon, VA d U Maryland College Park, MD g DISA Vienna, VA h ARL Aberdeen, MD j NSI (TBD) Herndon, VA 13 root name servers worldwide

2: Application Layer6 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 host surf.eurecom.fr gaia.cs.umass.edu root name server authorititive name server dns.umass.edu local name server dns.eurecom.fr

2: Application Layer7 DNS example Root name server: r may not know authoritative name server r may know intermediate name server: who to contact to find authoritative name server requesting host surf.eurecom.fr gaia.cs.umass.edu root name server local name server dns.eurecom.fr authoritative name server dns.cs.umass.edu intermediate name server dns.umass.edu 7 8

2: Application Layer8 DNS: iterated queries recursive query: r puts burden of name resolution on contacted name server r heavy load? iterated query: r contacted server replies with name of server to contact r “I don’t know this name, but ask this server” requesting host surf.eurecom.fr gaia.cs.umass.edu root name server local name server dns.eurecom.fr authoritative name server dns.cs.umass.edu intermediate name server dns.umass.edu 7 8 iterated query

2: Application Layer9 DNS: caching and updating records r once (any) name server learns mapping, it caches mapping m cache entries timeout (disappear) after some time r update/notify mechanisms under design by IETF m RFC 2136 m

2: Application Layer10 DNS records DNS: distributed db storing resource records (RR) r 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) r Type=A  name is hostname  value is IP address r Type=CNAME  name is alias name for some “canonical” (the real) name is really servereast.backup2.ibm.com  value is canonical name r Type=MX  value is name of mailserver associated with name

2: Application Layer11 DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header r identification: 16 bit # for query, reply to query uses same # r flags: m query or reply m recursion desired m recursion available m reply is authoritative

2: Application Layer12 DNS protocol, messages Name, type fields for a query RRs in response to query records for authoritative servers additional “helpful” info that may be used

2: Application Layer13 DNS Tools nslookup: r can find IP given name r can find name given IP r can show other RR whois: r can show information about domain and owner r can show information about “owner” of an IP address or network r online whois servers m m dig and host:  Show more detail  Not on Suns  common on Linux  Online:   dns_config.en.html

2: Application Layer14 SPAM Problem: r I get over 100 messages per day. r Perhaps 30% are SPAM. r How can I stop it? m NEVER respond to SPAM: From address is often bogus. When valid, it just verifies that your address works. m SPAM is a violation of most ISP’s AUPs. Complain to the ISP, and get the spammer disconnected. m Use tools and knowledge Examine mail headers Use traceroute Use whois

2: Application Layer15 Sample SPAM Return-Path: Received: from mx02.mrf.mail.rcn.net (mx02.mrf.mail.rcn.net [ ]) by briansbooks.com (8.9.3/8.8.7) with ESMTP id PAA22178 for ; Tue, 22 Jan :22: From: Received: from [ ] (helo=spray.no) by mx02.mrf.mail.rcn.net with smtp (Exim 3.34 #5) id 16T7U6-0004iN-00; Tue, 22 Jan :25: Reply-To: Message-ID: To: Subject: Support Follow up. MiME-Version: 1.0 Content-Type: text/html; charset="iso " X-Mailer: MIME-tools (Entity 5.501) Importance: Normal Date: Tue, 22 Jan :25: […]

2: Application Layer16 Chapter 2 outline r 2.1 Principles of app layer protocols r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail m SMTP, POP3, IMAP r 2.5 DNS r 2.6 Socket programming with TCP r 2.7 Socket programming with UDP r 2.8 Building a Web server r 2.9 Content distribution m Network Web caching m Content distribution networks m P2P file sharing

2: Application Layer17 Socket programming Socket API r introduced in BSD4.1 UNIX, 1981 r explicitly created, used, released by apps r client/server paradigm r two types of transport service via socket API: m unreliable datagram m 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

2: Application Layer18 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 with buffers, variables socket controlled by application developer controlled by operating system host or server process TCP with buffers, variables socket controlled by application developer controlled by operating system host or server internet

2: Application Layer19 Socket programming with TCP Client must contact server r server process must first be running r server must have created socket (door) that welcomes client’s contact Client contacts server by: r creating client-local TCP socket r specifying IP address, port number of server process r When client creates socket: client TCP establishes connection to server TCP r When contacted by client, server TCP creates new socket for server process to communicate with client m allows server to talk with multiple clients m 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

2: Application Layer20 Stream jargon r A stream is a sequence of characters that flow into or out of a process. r An input stream is attached to some input source for the process, eg, keyboard or socket. r An output stream is attached to an output source, eg, monitor or socket.

2: Application Layer21 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) Client process client TCP socket

2: Application Layer22 Client/server socket interaction: TCP wait for incoming connection request connectionSocket = welcomeSocket.accept() create socket, port= x, for incoming request: welcomeSocket = ServerSocket() create socket, connect to hostid, port= x clientSocket = Socket() close connectionSocket read reply from clientSocket close clientSocket Server (running on hostid ) Client send request using clientSocket read request from connectionSocket write reply to connectionSocket TCP connection setup

2: Application Layer23 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()); Create input stream Create client socket, connect to server Create output stream attached to socket

2: Application Layer24 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(); } Create input stream attached to socket Send line to server Read line from server

2: Application Layer25 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())); Create welcoming socket at port 6789 Wait, on welcoming socket for contact by client Create input stream, attached to socket

2: Application Layer26 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 line from socket Create output stream, attached to socket Write out line to socket End of while loop, loop back and wait for another client connection

2: Application Layer27 Example: C echo client (TCP) #include /* basic system data types */ #include /* basic socket definitions */ #include int main(int argc, char **argv) { int sockfd; struct sockaddr_in servaddr; if (argc != 2) { printf("usage: tcpcli \n"); exit(-1); } sockfd = socket(AF_INET, SOCK_STREAM, 0); Create TCP socket Define socket address structure Helpful includes

2: Application Layer28 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 socket structure with server information Establish connection with server Work with the established socket

2: Application Layer29 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.

2: Application Layer30 Example: C echo server (TCP) #include /* basic system data types */ #include /* basic socket definitions */ #include #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)); Create TCP socket Define socket address structure Helpful includes

2: Application Layer31 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 to a listening socket Wait until new conn. is established Assign structure to the socket Work with the established conn.

2: Application Layer32 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.

2: Application Layer33 Chapter 2 outline r 2.1 Principles of app layer protocols m clients and servers m app requirements r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail m SMTP, POP3, IMAP r 2.5 DNS r 2.6 Socket programming with TCP r 2.7 Socket programming with UDP r 2.8 Building a Web server r 2.9 Content distribution m Network Web caching m Content distribution networks m P2P file sharing

2: Application Layer34 Socket programming with UDP UDP: no “connection” between client and server r no handshaking r sender explicitly attaches IP address and port of destination to each packet r 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

2: Application Layer35 Client/server socket interaction: UDP close clientSocket Server (running on hostid ) read reply from clientSocket create socket, clientSocket = DatagramSocket() Client Create, address ( hostid, port=x, send datagram request using clientSocket create socket, port= x, for incoming request: serverSocket = DatagramSocket() read request from serverSocket write reply to serverSocket specifying client host address, port number

2: Application Layer36 Example: Java client (UDP) Output: sends packet (TCP sent “byte stream”) Input: receives packet (TCP received “byte stream”) Client process client UDP socket

2: Application Layer37 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(); Create input stream Create client socket Translate hostname to IP address using DNS Get line from STDIN Convert to byte array

2: Application Layer38 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 datagram to server Read datagram from server Convert to String Send to STDOUT Look for more input

2: Application Layer39 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); Create datagram socket at port 6789 Create space for received datagram Receive datagram

2: Application Layer40 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 addr port #, of sender Write out datagram to socket End of while loop, loop back and wait for another datagram Create datagram to send to client Convert to caps, convert to bytes

2: Application Layer41 Example: C echo client (UDP) […headers…] int main(int argc, char **argv) { int sockfd; struct sockaddr_in servaddr; if (argc != 2) { printf("usage: udpcli "); 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); } Create UDP socket Work with the established socket Set up socket parameters

2: Application Layer42 Example: C echo client (UDP) cont. void dg_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 datagram to socket Get line from STDIN Send line to STDOUT Receive datagram From socket

2: Application Layer43 Example: C echo server (UDP) #include /* basic system data types */ #include /* basic socket definitions */ #include #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)); } Create UDP socket Work with the established socket Bind socket to specified address

2: Application Layer44 Example: C echo server (UDP) cont. void dg_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 datagram back to client Read next datagram