COMP 3331/9331: Computer Networks and Applications

Transcription

1 COMP 3331/9331: Computer Networks and Applications Week 3 Application Layer: DNS, P2P and Socket Programming Reading Guide: Chapter 2 - Sections

2 Announcements Labs started this Week (Week 3) Labs for Week 4 Socket programming exercise Solutions available at the end of the week on webpage Problem Set Did anyone try to solve the first problem set? Please do so. Solution set soon. Remember mid-semester exam (see announcements) 2

3 Application Layer Recap Principles of Application Layer What are the different types of architectures you would consider while designing a new application? What transport services does an application need? Examples: Web, Interactive gaming Which transport protocols does the Internet provide? What are the differences? Which transport protocol is used by HTTP, FTP, Real-time audio? 3

4 Application layer: Roadmap 2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS 2.6: P2P Networks 2.7 Socket programming 4

5 DNS: Domain Name System People: many identifiers: student #, name, passport # Internet hosts: IP address (32 bit) - used for addressing datagrams e.g: name, e.g., - used by humans Internet core: Uses IP addresses only Q: map between IP addresses and name? Domain Name System: distributed database implemented in hierarchy of many name servers application-layer protocol hosts and name servers communicate to resolve names (address/name translation) note: core Internet function, implemented as application-layer protocol complexity at network s edge 5

6 DNS Example You type in the URL window of your web browser Your browser must establish a TCP connection with the CSE web server To do this, your browser needs to know the IP address of the CSE web server How does it obtain the IP address?? The browser passes on the hostname to the client side of the DNS application running on your machine (gethostbyname() function in UNIX) The DNS client sends out a query for mapping the hostname to an IP address into the DNS hierarchy black box over UDP (destination port number: 53) The DNS client receives a reply with the IP address for The browser can now initiate a TCP connection with the HTTP server process located at that IP address 6

7 DNS DNS services Hostname to IP address translation Host aliasing Canonical and alias names CN: relay1.sydney.wifi.com.au Alias: wifi.com.au Mail server aliasing MS: relay1.sydney.hotmail.com Alias: hotmail.com Load distribution Replicated Web servers: set of IP addresses for one canonical name Rotation policy Content Distribution Networks: use IP address of requesting host to find best suitable server Example: closest, least-loaded, etc Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance doesn t scale 7

8 Distributed, Hierarchical Database Root DNS Servers com DNS servers org DNS servers edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers poly.edu umass.edu DNS servers DNS servers 1 st approx: Client wants IP for Client queries a root server to find com DNS server Client queries com DNS server to get yahoo.com DNS server Client queries yahoo.com authoritative DNS server to get IP address for 8

9 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 Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 11 locations) k RIPE London (also Amsterdam, i Autonomica, Frankfurt) Stockholm (plus 3 other locations) e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA m WIDE Tokyo 13 root name servers worldwide 9

10 TLD and Authoritative Servers Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains au, uk, fr, ca, jp. Network Solutions maintains servers for com TLD Educause for edu TLD Authoritative DNS servers: organization s DNS servers, providing authoritative hostname to IP mappings for organization s servers (e.g., Web and mail). Can be maintained by organization or service provider 10

11 Local Name Server Does not strictly belong to hierarchy 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 Acts as a proxy, forwards query into hierarchy 11

12 Host at cse.unsw.edu.au wants IP address for gaia.cs.umass.edu iterated query: contacted server replies with name of server to contact I don t know this name, but ask this server Example local DNS server dns.cse.unsw.edu.au requesting host cse.unsw.edu.au root DNS server TLD DNS server 6 authoritative DNS server dns.umass.edu gaia.cs.umass.edu 12

13 Recursive queries root DNS server recursive query: puts burden of name resolution on contacted name server heavy load Typically recursive to local DNS and then iterative local DNS server dns.cse.unsw.edu.au requesting host cse.unsw.edu.au TLD DNS server 4 authoritative DNS server dns.umass.edu gaia.cs.umass.edu 13

14 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 TTL field in DNS record indicates how long to cache When passing DNS record to another name server, TTL is updated = (original TTL - time record cached for) TTL often set to 2 days 14

15 DNS records DNS: distributed db storing resource records (RR) RR format: (name, value, type, ttl) Type=A name is hostname value is IP address Type=NS name is domain (e.g. foo.com) value is hostname of authoritative name server for this domain (e.g. dns.foo.com) Type=CNAME name is alias name for some cannonical (the real) name is really servereast.backup2.ibm.com value is cannonical name Type=MX value is name of mailserver associated with name 15

16 Reflections: Which type of DNS records? Q: I type in in my browser. Which type of DNS record will the DNS client on my computer request for? A: Type A Q: I send to cs3331@cse.unsw.edu.au using Outlook. Which type of DNS record will the DNS client on my computer request for? A: Type MX Q: Give an example of a case where a name server contains a type NS record for a hostname? A: The TLD name server for.edu contains a type NS record for the domain mit.edu (e:g: mit.edu, dns.mit.edu, NS) 16

17 bash-3.2$ dig ;; QUESTION SECTION: ; IN A ;; ANSWER SECTION: IN CNAME albeniz.orchestra.cse.unsw.edu.au. albeniz.orchestra.cse.unsw.edu.au IN A ;; AUTHORITY SECTION: orchestra.cse.unsw.edu.au IN NS maestro.orchestra.cse.unsw.edu.au. orchestra.cse.unsw.edu.au IN NS beethoven.orchestra.cse.unsw.edu.au. ;; ADDITIONAL SECTION: maestro.orchestra.cse.unsw.edu.au IN A beethoven.orchestra.cse.unsw.edu.au IN A beethoven.orchestra.cse.unsw.edu.au IN A beethoven.orchestra.cse.unsw.edu.au IN A

18 Inserting records into DNS Example: just created startup Register name muckingaround.com at a registrar (e.g., Network Solutions) Need to provide registrar with names and IP addresses of your authoritative name server (primary and secondary) Registrar inserts two RRs into the com TLD server: (muckingaround.com, dns1.muckingaround.com, NS) (dns1.muckingaround.com, , A) Where do you put in the Type A record for Answer: in the DNS server (dns1.muckingaround.com) If you want to provide accounts such as abc@muckingaround.com, what do you do? Answer: Type MX record for muckingaround.com is added to the DNS server How do people get the IP address of your Web site? 18

19 Wibbly Wobbly Web (self study) DNS poisoning: redirect users to dodgy URLs I setup a server ( and add the following entires into my authoritative DNS server NS ns1.yahoo.com NS ns3.europe.yahoo.com ns1.yahoo.com A ns3.europe.yahoo.com A Once other DNS servers query for this mapping they will cache the mappings (including the yahoo name server mappings) into their caches. Hence the DNS cache is poisoned with false entries for the yahoo name server Solution: Do not allow DNS servers to cache IP address mappings unless they are from authoritative name servers In above example, none of the DNS servers will cache the type A records for Yahoo name servers received from nasty.com s name server DNSSEC: DNS Security Extensions,

20 Reverse DNS lookup IP address è domain name, location Sample application: A website has replicated its content in 4 servers located at Sydney, Oslo, Tokyo and New York When a DNS query reaches the website s DNS server, it performs a reverse DNS lookup to find out your location and provides you with the IP address of the server closest to you Example: If the request comes from a device from Australia, it will provide the IP address of the server in Sydney, rather than the other 3 locations. 20

21 Quiz - DNS What would happen if the DNS query sent out by a local DNS server is lost, given that DNS uses UDP? How is it is possible for CSE to have both a web server and mail server with an alias of cse.unsw.edu.au (i.e. a web server with hostname and addresses such as xyz@cse.unsw.edu.au) 21

22 News break: Internationalised domain names Currently, domain names can only contain a to z, 0 to 9 and (Note that. has a special meaning.) In the near future, domain names can contain alphabets, characters and scripts from all world languages Greek alphabets αβγδ, Swedish ä ö å Chinese, Korean and Japanese characters; Arabic script etc Non-Latin domain name will be coded in unicode DNS works on ASCII For backward compatibility, need a method to encode unicode as ASCII To probe more: ipj_11-1/111_idns.html 22

23 Roadmap: Application layer 2.1 Principles of network applications 2.2 Web and HTTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS 2.6: P2P Applications 2.7 Socket programming with TCP (self study) 2.8 Socket programming with UDP (self study) 23

24 Pure P2P architecture no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses peer-peer Two topics: File distribution Searching for information 24

25 File Distribution: Client-Server vs P2P Question : How much time to distribute file from one server to N peers? File, size F Server u 1 d 1 u 2 u d 2 s u s : server upload bandwidth u i : peer i upload bandwidth d i : peer i download bandwidth u N d N Network (with abundant bandwidth) 25

26 File distribution time: server-client server sequentially sends N copies: NF/u s time client i takes F/d i time to download F Server u N d N u u 2 1 d 1 u d s 2 Network (with abundant bandwidth) Time to distribute F to N clients using client/server approach = d cs >= max { NF/u s, F/min(d i ) } i increases linearly in N (for large N) 26

27 File distribution time: P2P Server server must send one copy: F/u s time client i takes F/d i time to download F u N u u 2 1 d 1 u d s 2 Network (with abundant bandwidth) d N fastest possible upload rate: u s + Σu i d P2P >= max { F/u s, F/min(d i ), NF/(u s + Σu i ) } i 27

28 Server-client vs. P2P: example Client upload rate = u, F/u = 1 hour, u s = 10u, d min u s Minimum Distribution Time P2P Client-Server N

29 File distribution: BitTorrent P2P file distribution tracker: tracks peers participating in torrent torrent: group of peers exchanging chunks of a file obtain list of peers trading chunks peer 29

30 BitTorrent (1) file divided into 256KB chunks. peer joining torrent: has no chunks, but will accumulate them over time registers with tracker to get list of peers, connects to subset of peers ( neighbors ) using TCP while downloading, peer uploads chunks to other peers. peers may come and go once peer has entire file, it may (selfishly) leave or (altruistically) remain 30

31 BitTorrent (2) Pulling Chunks at any given time, different peers have different subsets of file chunks periodically, a peer (Alice) asks each neighbor for list of chunks that they have. Alice sends requests for her missing chunks rarest first Sending Chunks: tit-for-tat Alice sends chunks to four neighbors currently sending her chunks at the highest rate re-evaluate top 4 every 10 secs every 30 secs: randomly select another peer, starts sending chunks newly chosen peer may join top 4 optimistically unchoke 31

32 BitTorrent: Tit-for-tat (1) Alice optimistically unchokes Bob (2) Alice becomes one of Bob s top-four providers; Bob reciprocates (3) Bob becomes one of Alice s top-four providers Devil in details - mini-chunks - random first selection - anti-snubbing Read linked paper on website (NOT FOR EXAM) With higher upload rate, can find better trading partners & get file faster! 32

33 Philosophy behind P2P There is a spirit of sharing and cooperation behind P2P applications Sharing of files There are many other examples of sharing and cooperation in the Internet Linux is produced by many cooperating programmers Wikipedia is created by a team of cooperating authors What else can people share? Current research 33

34 Sharing of information Current research You are driving and you know the traffic condition at where you are If everyone shares his/her local traffic information, you get a traffic map You are driving past a petrol station You know the price at that petrol station If people share this information, you know the price of petrol at many petrol stations 34

35 Current research Automatic collection of petrol prices from mobile phone camera 35

36 Distributed Hash Tables (DHT): History In , academic researchers jumped on to the P2P bandwagon Motivation: frustrated by popularity of all these half-baked P2P apps. We can do better! (so they said) guaranteed lookup success for data in system provable bounds on search time provable scalability to millions of node Hot topic in networking ever since 36

37 Hash table - review (key,value) pairs Centralised hash table all (key,value) pairs on 1 node Distributed hash tables each node has a section of (key,value) pairs Figure src: 37

38 DHT Applications File sharing and backup [CFS, Ivy, OceanStore, PAST, Pastiche ] Web cache and replica [Squirrel, Croquet Media Player] Censor-resistant stores [Eternity] DB query and indexing [PIER, Place Lab, VPN Index] Event notification [Scribe] Naming systems [ChordDNS, Twine, INS, HIP] Distrubted BitTorrent tracker [Kademlia, Vuze] Communication primitives [I3, ] Host mobility [DTN Tetherless Architecture] 38

39 Distributed Hash Table (DHT) DHT = distributed P2P database Database has (key, value) pairs; key: tax file number; value: human name key: content type; value: IP address Peers query DB with key DB returns values that match the key Peers can also insert (key, value) peers 39

40 DHT: Basic Idea 40

41 DHT: Basic Idea (2) 41

42 DHT: Basic Idea (3) 42

43 DHT: Basic Idea (4) 43

44 DHT: Basic Idea (5) Two questions: - how are the nodes connected? - how the key space is partitioned? 44

45 DHT Identifiers Assign integer identifier to each peer in range [0,2 n -1] Each identifier can be represented by n bits Require each key to be an integer in same range To get integer keys, hash original key (Led Ze..) eg, key = h( Led Zeppelin IV ) Common hash function such a SHA-1 This is why they call it a distributed hash table 45

46 How to assign keys to peers? Central issue: Assigning (key, value) pairs to peers Rule: assign key to the peer that has the closest ID greater than or equal to the key Convention in lecture: closest is the immediate successor of the key n=4, range[0-15]; 8 peers: 1,3,4,5,8,10,12,14; key = 13, then successor peer = 14 key = 15, then successor peer = 1 46

47 Circular DHT (1) Peers organised in the form of a ring Overlay network Popular Instantiation: CHORD (paper linked to lecture notes) 8 47

48 Ciruclar DHT: How to search? (1) One extreme: Every node only knows its successor 1 Who s resp I am 3 for key 14? N/2 messages 15 on avg to resolve query, when there are N peers Define closest as closest successor

49 Ciruclar DHT: How to search? (2) Other extreme: Every node knows of every other node 1 Who s resp You are 3 for key 14? 1 message 15 to resolve query BUT large routing tables (N entries)

50 Circular DHT with Shortcuts Who s resp for key 1110? 10 8 Each peer keeps track of IP addresses of predecessor, successor, and a few short cuts Reduced from 6 to 2 messages Possible to design shortcuts so O(log N) neighbors, O(log N) messages in query Active research area

51 Peer Churn To handle peer churn, require each peer to know the IP address of its two successors. Each peer periodically pings its two successors to see if they are still alive Peer 5 abruptly leaves Peer 4 detects; makes 8 its immediate successor; asks 8 who its immediate successor is; makes 8 s immediate successor its second successor What if peer 13 wants to join? 51

52 1 Peer Join Joining peer gets in touch with one of the peers (e.g.: designated contact peer 1 in this example) Node 13 sends a join request This gets forwarded until reaches Node 12 detects that node 13 will be its new successor Node12 informs node 13 -> your predecessor (12) and successor (15) Node 13 joins and informs 12 to change its successor (from 15 to 13) Node 12 s key space is partitioned and appropriate keys transferred to 12 52

53 P2P Example: Skype Skype clients (SC) inherently P2P: pairs of users communicate proprietary application-layer protocol (inferred via reverse engineering) hierarchical overlay with SNs Index maps usernames to IP addresses; distributed over SNs Skype login server Supernode (SN) 53

54 Application: Summary Our study of network apps now complete! Application architectures client-server P2P hybrid application service requirements: reliability, bandwidth, delay Internet transport service model connection-oriented, reliable: TCP unreliable, datagrams: UDP specific protocols: HTTP FTP SMTP, POP, IMAP DNS P2P Networks socket programming 54

55 Application layer: Roadmap 2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS 2.6 P2P Networks 2.7 Socket programming (self study) Note: 6 th Edition uses Phython, older editions used Java 55

56 Application: Summary Most importantly: learned about protocols typical request/reply message exchange: client requests info or service server responds with data, status code message formats: headers: fields giving info about data data: info being communicated control vs. data msgs in-band, out-of-band centralized vs. decentralized stateless vs. stateful reliable vs. unreliable msg transfer complexity at network edge 56

57 Socket programming Goal: learn how to build client/server application that communicate using sockets socket 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 57

58 Socket Programming (self study) You should review this section in your own time (foils from this point to the end has been left as self study) You can learn programming sockets by programming hands on. Run sample codes, do lab-4 and then apply this to your assignment 58

59 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 controlled by application developer controlled by operating system process socket TCP with buffers, variables internet process socket TCP with buffers, variables controlled by application developer controlled by operating system host or server host or server 59

60 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 (more in Chap 3) application viewpoint TCP provides reliable, in-order transfer of bytes ( pipe ) between client and server 60

61 TCP Sockets 61

62 Client/server socket interaction (in Java) Server (running on hostid) create socket, port=x, for incoming request: welcomesocket = ServerSocket() wait for incoming connection request connectionsocket = welcomesocket.accept() TCP connection setup Client create socket, connect to hostid, port=x clientsocket = Socket() read request from connectionsocket send request using clientsocket write reply to connectionsocket close connectionsocket read reply from clientsocket close clientsocket 62

63 Client/Server Interaction (in C) server welcomesocket=socket(): create socket client clientsocket=socket(): create socket bind(welcomesocket, ): specify socket address/port bind(clientsocket): specify socket address optional listen(welcomesocket, ): specify that socket welcomesocket is a listening socket TCP connection setup connectionsocket=accept(welcomesocket, ): get a connected connection from the queue for socket welcomesocket; create a new socket identified by connectionsocket connect(clientsocket, serveraddr, serverport): initialize TCP handshake to server; return until TCP handshake is done read()/write(): do IO on clientsocket read()/write(): do IO on socket connectionsocket close(clientsocket): done close(connectionsocket): done 63

64 ServerSocket ServerSocket() creates an unbound server socket. ServerSocket(int port) creates a server socket, bound to the specified port. ServerSocket(int port, int backlog) creates a server socket and binds it to the specified local port number, with the specified backlog. ServerSocket(int port, int backlog, InetAddress bindaddr) create a server with the specified port, listen backlog, and local IP address to bind to. bind(socketaddress endpoint) binds the ServerSocket to a specific address (IP address and port number). bind(socketaddress endpoint, int backlog) binds the ServerSocket to a specific address (IP address and port number). Socket accept() listens for a connection to be made to this socket and accepts it. close() closes this socket. 64

65 Socket Socket(InetAddress address, int port) creates a stream socket and connects it to the specified port number at the specified IP address. Socket(InetAddress address, int port, InetAddress localaddr, int localport) creates a socket and connects it to the specified remote address on the specified remote port. Socket(String host, int port) creates a stream socket and connects it to the specified port number on the named host. bind(socketaddress bindpoint) binds the socket to a local address. connect(socketaddress endpoint) connects this socket to the server. connect(socketaddress endpoint, int timeout) connects this socket to the server with a specified timeout value. InputStreamget InputStream() returns an input stream for this socket. OutputStreamgetOutputStream() returns an output stream for this socket. close() closes this socket. 65

66 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. 66

67 Socket programming with TCP keyboard monitor 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 process input stream output stream infromuser outtoserver to network infromserver client TCP clientsocket socket input stream from network TCP socket 67

68 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()); 68

69 Example: Java client (TCP), cont. Create input stream attached to socket Send line to server Read line from server 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(); } } 69

70 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())); 70

71 Example: Java server (TCP), cont Create output stream, attached to socket Read in line from socket DataOutputStream outtoclient = new DataOutputStream(connectionSocket.getOutputStream()); clientsentence = infromclient.readline(); Write out line to socket } } } capitalizedsentence = clientsentence.touppercase() + '\n'; outtoclient.writebytes(capitalizedsentence); End of while loop, loop back and wait for another client connection 71

72 Application layer: Roadmap 2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS 2.6 P2P Networks 2.7 Socket programming 72

73 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 73

74 Client/server socket interaction: UDP Server (running on hostid) Client create socket, port=x, for incoming request: serversocket = DatagramSocket() read request from serversocket create socket, clientsocket = DatagramSocket() Create, address (hostid, port=x, send datagram request using clientsocket write reply to serversocket specifying client host address, port number read reply from clientsocket close clientsocket 74

75 Example: Java client (UDP) keyboard monitor Process Client process Output: sends packet (TCP sent byte stream ) input stream UDP packet infromuser sendpacket receivepacket UDP packet Input: receives packet (TCP received byte stream ) client clientsocket UDP socket UDP socket to network from network 75

76 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(); 76

77 Example: Java client (UDP), cont. Create datagram with data-to-send, length, IP addr, port Send datagram to server Read datagram from server } 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(); } 77

78 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]; Create space for received datagram Receive datagram while(true) { DatagramPacket receivepacket = new DatagramPacket(receiveData, receivedata.length); serversocket.receive(receivepacket); 78

79 Example: Java server (UDP), cont Get IP addr port #, of sender String sentence = new String(receivePacket.getData()); InetAddress IPAddress = receivepacket.getaddress(); int port = receivepacket.getport(); String capitalizedsentence = sentence.touppercase(); senddata = capitalizedsentence.getbytes(); Create datagram to send to client Write out datagram to socket } } DatagramPacket sendpacket = new DatagramPacket(sendData, senddata.length, IPAddress, port); serversocket.send(sendpacket); } End of while loop, loop back and wait for another datagram 79

80 Differences with C C usually requires a little more work. Java does a lot of things implicitly Sequence of commands for TCP client: socket(), connect(), write(), read(), write(), read(),, close() Before call to connect() you have to explicitly get the IP address of the server using gethostbyname() Sequence of commands for the TCP server: socket(), bind() to well-known port, listen(), accept(), read(), write(), read(), write(),., close() socket() creates an active socket (client socket that will issue connect() ) listen() converts an unconnected socket into a passive socket, indicates to the OS that incoming connection requests should be directed to the socket accept() is used to welcome the next connection request that arrives 80

81 Programming References Lots of links on the assignment webpage (at the bottom) TCP/IP Sockets in Java: Practical Guide for Programmers, by Kenneth Calvert, Michael Donahoo Unix Network Programming Volume1: Networking APIs, by W. Richard Stevens Any other basic network programming book is sufficient Any basic C, Java book Note: The programming assignment does not require extensive knowledge of network programming, just basics 81

82 Sample Code Sections 2.7 and 2.8 of the text have sample working programs Sample Echo Client/Server Program on website (under Assignment link) Echo service: Client sends text to server and server echoes it back to the client Read and understand the code before executing it Practice Programming Lab: Week 4 82