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Network applications: some jargon Process: program running user agent: interfaces within a host. with user “above” and network “below”. within same host, two processes communicate implements user using interprocess interface & communication (defined application-level by OS). protocol Web: browser processes running in E-mail: mail reader different hosts streaming audio/video: communicate with an media player application-layer protocol 2: Application Layer 1 Applications and application-layer protocols Application: communicating, distributed processes e.g., e-mail, Web, P2P file sharing, instant messaging running in end systems (hosts) exchange messages to implement application application transport network data link physical Application-layer protocols one “piece” of an app define messages exchanged by apps and actions taken use communication services provided by lower layer protocols (TCP, UDP) application transport network data link physical application transport network data link physical 2: Application Layer 2 App-layer protocol defines Types of messages exchanged, eg, request & response messages Syntax of message types: what fields in messages & how fields are delineated Semantics of the fields, ie, meaning of information in fields Rules for when and how processes send & respond to messages Public-domain protocols: defined in RFCs allows for interoperability eg, HTTP, SMTP Proprietary protocols: eg, KaZaA 2: Application Layer 3 Client-server paradigm Typical network app has two pieces: client and server Client: application transport network data link physical initiates contact with server (“speaks first”) typically requests service from server, Web: client implemented in browser; e-mail: in mail reader Server: provides requested service to client request reply application transport network data link physical e.g., Web server sends requested Web page, mail server delivers e-mail 2: Application Layer 4 Processes communicating across network process sends/receives messages to/from its socket socket analogous to door sending process shoves message out door sending process asssumes transport infrastructure on other side of door which brings message to socket at receiving process host or server host or server 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 2: Application Layer 5 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 host-local, application-created, OS-controlled interface (a “door”) into which application process can both send and receive messages to/from another application process 2: Application Layer 6 Socket-programming using TCP Socket: a door between application process and endend-transport 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 host or server internet socket TCP with buffers, variables controlled by application developer controlled by operating system host or server 2: Application Layer 7 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 application viewpoint TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server 2: Application Layer 8 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. 2: Application Layer 9 Socket programming with TCP Client Process process input stream output stream inFromServer 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) outToServer Example client-server app: monitor inFromUser keyboard input stream client TCP clientSocket socket to network TCP socket from network 2: Application Layer 10 Client/server socket interaction: TCP Server (running on hostid) Client create socket, port=x, for incoming request: welcomeSocket = ServerSocket() TCP wait for incoming connection request connection connectionSocket = welcomeSocket.accept() read request from connectionSocket write reply to connectionSocket close connectionSocket setup create socket, connect to hostid, port=x clientSocket = Socket() send request using clientSocket read reply from clientSocket close clientSocket 2: Application Layer 11 Example: Java client (TCP) import java.io.*; import java.net.*; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence; Create input stream Create client socket, connect to server Create output stream attached to socket BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); Socket clientSocket = new Socket("hostname", 6789); DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream()); 2: Application Layer 12 Example: Java client (TCP), cont. Create input stream attached to socket BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); sentence = inFromUser.readLine(); Send line to server outToServer.writeBytes(sentence + '\n'); Read line from server modifiedSentence = inFromServer.readLine(); System.out.println("FROM SERVER: " + modifiedSentence); clientSocket.close(); } } 2: Application Layer 13 Example: Java server (TCP) import java.io.*; import java.net.*; class TCPServer { Create welcoming socket at port 6789 Wait, on welcoming socket for contact by client 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())); 2: Application Layer 14 Example: Java server (TCP), cont Create output stream, attached to socket DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream()); Read in line from socket clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; Write out line to socket outToClient.writeBytes(capitalizedSentence); } } } End of while loop, loop back and wait for another client connection 2: Application Layer 15 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 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 lost 2: Application Layer 16 Client/server socket interaction: UDP Server (running on hostid) create socket, port=x, for incoming request: serverSocket = DatagramSocket() read request from serverSocket write reply to serverSocket specifying client host address, port number Client create socket, clientSocket = DatagramSocket() Create, address (hostid, port=x, send datagram request using clientSocket read reply from clientSocket close clientSocket 2: Application Layer 17 Example: Java client (UDP) input stream Client process monitor inFromUser keyboard Process Input: receives packet (TCP received “byte stream”) UDP packet receivePacket packet (TCP sent “byte stream”) sendPacket Output: sends client UDP clientSocket socket to network UDP packet UDP socket from network 2: Application Layer 18 Example: Java client (UDP) import java.io.*; import java.net.*; Create input stream Create client socket Translate hostname to IP address using DNS 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(); 2: Application Layer 19 Example: Java client (UDP), cont. Create datagram with data-to-send, length, IP addr, port DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); Send datagram to server clientSocket.send(sendPacket); Read datagram from server clientSocket.receive(receivePacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); } } 2: Application Layer 20 Example: Java server (UDP) import java.io.*; import java.net.*; Create datagram socket at port 9876 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 space for received datagram Receive datagram DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); serverSocket.receive(receivePacket); 2: Application Layer 21 Example: Java server (UDP), cont String sentence = new String(receivePacket.getData()); Get IP addr port #, of sender InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase(); sendData = capitalizedSentence.getBytes(); Create datagram to send to client DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); Write out datagram to socket serverSocket.send(sendPacket); } } } End of while loop, loop back and wait for another datagram 2: Application Layer 22 Processes communicating across network process sends/receives messages to/from its socket socket analogous to door sending process shoves message out door sending process asssumes transport infrastructure on other side of door which brings message to socket at receiving process host or server host or server 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 2: Application Layer 23 Addressing processes: For a process to receive messages, it must have an identifier Every host has a unique 32-bit IP address Q: does the IP address of the host on which the process runs suffice for identifying the process? Answer: No, many processes can be running on same host Identifier includes both the IP address and port numbers associated with the process on the host. Example port numbers: HTTP server: 80 Mail server: 25 2: Application Layer 24 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 Q: map between IP addresses and name ? 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” 2: Application Layer 25 DNS name servers Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance doesn’t scale! 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 2: Application Layer 26 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 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 k RIPE London i NORDUnet Stockholm m WIDE Tokyo e NASA Mt View, CA f Internet Software C. Palo Alto, CA b USC-ISI Marina del Rey, CA l ICANN Marina del Rey, CA 13 root name servers worldwide 2: Application Layer 27 Simple DNS example host surf.eurecom.fr wants IP address of gaia.cs.umass.edu root name server 2 4 5 1. contacts its local DNS server, dns.eurecom.fr 2. dns.eurecom.fr contacts local name server dns.eurecom.fr root name server, if necessary 1 6 3. root name server contacts authoritative name server, dns.umass.edu, if requesting host necessary surf.eurecom.fr 3 authorititive name server dns.umass.edu gaia.cs.umass.edu 2: Application Layer 28 DNS example root name server Root name server: may not know authoritative name server may know intermediate name server: who to contact to find authoritative name server 6 2 7 local name server dns.eurecom.fr 1 8 requesting host 3 intermediate name server dns.umass.edu 4 5 authoritative name server dns.cs.umass.edu surf.eurecom.fr gaia.cs.umass.edu 2: Application Layer 29 DNS: iterated queries recursive query: iterated query: contacted server replies with name of server to contact “I don’t know this name, but ask this server” iterated query 2 puts burden of name resolution on contacted name server heavy load? root name server 3 4 7 local name server dns.eurecom.fr 1 8 requesting host intermediate name server dns.umass.edu 5 6 authoritative name server dns.cs.umass.edu surf.eurecom.fr gaia.cs.umass.edu 2: Application Layer 30 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/dnsind-charter.html 2: Application Layer 31 DNS protocol, messages DNS 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 2: Application Layer 32 DNS records DNS: distributed db storing resource records (RR) RR format: (name, Type=A name is hostname value is IP address value, type,ttl) Type=CNAME name is alias name for some “cannonical” (the real) name www.ibm.com is really Type=NS servereast.backup2.ibm.com name is domain (e.g. value is cannonical name foo.com) value is IP address of Type=MX authoritative name value is name of mailserver server for this domain associated with name 2: Application Layer 33 DNS protocol, messages Name, type fields for a query RRs in reponse to query records for authoritative servers additional “helpful” info that may be used 2: Application Layer 34