CNT5505 DATA/COMUTER COMMUNICATIONS Spring 2012

1. CNT5505 DATA/COMUTER COMMUNICATIONS Spring 2012 Computer Science, FSU

2. About myself • Zhenghao Zhang • Office: Love 164; Phone: 644-1685 • Email: zzhang@cs.fsu.edu • URL: http://www.cs.fsu.edu/~zzhang • Research area: • Wireless networks • Network security • Peer to peer networks • Optical networks Computer Science, FSU

3. Office Hours • Love 164 • 3:30pM – 5:30PM, Thursday, or by appointments through email Computer Science, FSU

4. What is CNT5505 about? • General purpose computer networks • Not specialized networks (e.g., telephone or cable) • Fundamental principles • Not survey of existing protocol standards • Focus on network software architecture • Only discuss some relevant network hardware • Designing and building network systems Computer Science, FSU

5. Course Prerequisites • A rudimentary understanding of computer architecture, and operating systems would be helpful • Basic understanding of algorithm analysis • C/C++ or Java programming is required • Socket programming • Unix programming • Event multiplexing, timer • Provided executable code and template of project in C/C++ • You can develop the project in Java • Basic probability theory may be needed to understand some performance analysis Computer Science, FSU

6. Course Materials • Required textbook • “Computer Networks,'' by Andrew S. Tanenbaum, Prentice Hall, 5th edition • Class notes, other assigned readings • Materials on the Internet Computer Science, FSU

7. Class Information • Class website • http://www.cs.fsu.edu/~zzhang/CNT5505_Spring_2012.htm • Go to my website and click teaching • Check the Announcements and your email account regularly Computer Science, FSU

8. Course Requirements • Do assigned readings • Be prepared; read textbook/lectures before class • Attend and participate in class activities • Please ask and answer questions in (and out of) class • Attendance will be considered in the final letter grade • Workload • Homework assignments. • Projects. • One midterm • One final Computer Science, FSU

9. Policies and Guidelines • Homework and Assignments usually have two due times. • Please work early and make the first due time. • 10% penalty for submission by the second due time. • Zero if later than the second due time. • No make-up exam, no incomplete • unless proof of emergency • Scholastic behaviors • Follow the Academic Honor Code. • Acknowledge reference/credits if receive help. • You may end up “F” for dishonesty. It is not worth it. Computer Science, FSU

10. Important dates • Check the course website • For homework assignments • Hand in hard-copy in class on due dates • Preferably typed instead of handwritten • For course project • Submit by email • Midnight on the due dates • Demo time will be announced later Computer Science, FSU

11. Course Project • There will be several projects. • Not coding intensive Computer Science, FSU

12. Questions and Concerns? Computer Science, FSU

13. Computer Communication: A motivation example • What happens behind the scene when I type http://www.google.com in the address bar of my iPhone? Computer Science, FSU

14. Step 1: on local host • Browser figures out what to do with the URL: http://www.google.com/ • Three components in URL • Where: www.google.com • What: (retrieving file index.html) • How: through HTTP protocol • Talk to http daemon on www.google.com to get file index.html through HTTP protocol Computer Science, FSU

15. Step 2: translating domain name to IP address • Each machine on the Internet identified by one or more IP address • Browser translating domain name (www.google.com) to corresponding IP address using domain name service (DNS) Computer Science, FSU

16. Step 2: Getting IP address (Cont’d) • Browser calls UDP protocol entity to send a message to 128.186.120.179/53 (assuming that is our DNS server) • The UDP message to the DNS server is “What is the IP address of www.google.com?” • The DNS server sends a message back: 64.233.161.99 • Actually situation is complicated than this • www.google.com is associated with multiple IP addresses Computer Science, FSU

17. Step 2: Getting IP address (Cont’d) • How can my iPhone talk to the DNS server? • It will have to first talk to the Access Point (AP) by sending frames. • What is a frame? Computer Science, FSU

18. Step 2: Getting IP address (Cont’d) • The frame is the wireless signal sent by the Wi-Fi interface of my iPhone that carries bits. • The wireless signal is the oscillating wave. • The frame encapsulates the IP packet. Computer Science, FSU

19. Step 2: Getting IP address (Cont’d) • What if another iPhone also wants to talk to the same AP? • What if someone just walks by and blocks my line-of-sight path to the AP? Computer Science, FSU

20. Step 2: Getting IP address (Cont’d) • The AP relays the IP packet from my iPhone to the DNS server, assuming they are in the same Ethernet LAN. • Note that the AP is not relaying the Wi-Fi frame. • The AP may have to use the ARP protocol to find the Ethernet address of the DNS server, then encapsulates the IP packet in an Ethernet frame. Computer Science, FSU

21. Step 3: establishing HTTP connection • Calls TCP entity to set up a connection to 64.233.161.99 /80 • TCP protocol calls IP to send a datagram to 64.233.161.99 Computer Science, FSU

22. Step 3: establishing HTTP connection (Cont’d) • Turns out that www.google.com is far away. • Need to forward to the first-hop router (128.186.120.1) • find the Ethernet address of first-hop router using arp • forward packet to first-hop router • (second router, third router) …... • www.google.com receives a packet. Computer Science, FSU

23. Step 3: establishing HTTP connection (Cont’d) • There are many options (paths) to reach the destination. • How to choose the best path? • How to maintain the path information? Computer Science, FSU

24. Step 4: Web page request and retrieval • Use TCP to send requests • TCP entity calls IP to send a datagram • ….. • www.google.com responses with the content of index.html Computer Science, FSU

25. Step 4: Web page request and retrieval (Cont’d) • The webpage may contain lots of data (images, videos). • We will receive a bunch of IP packets. • What if some of the IP packets are dropped by the routers? • We will be missing some parts in a picture • How can the google server figure out how fast to send the data? Computer Science, FSU

26. Step 5: web page rendering • Browser displays the content of the web page Computer Science, FSU

27. Socket Programming

28. Interprocess Communication • Within a single system • Pipes, FIFOs • Message Queues • Semaphores, Shared Memory • Across different systems • BSD Sockets • Transport Layer Interface (TLI) • Reference • Unix Network Programming by Richard Stevens Computer Science, FSU

29. BSD Socket API • Introduced in 1981 BSD 4.1 UNIX • Sockets is just a convenient interface for the processes on different hosts to communicate. • Just like if you want to write to the hard disk, you can operate the hard disk directly, but it is much more convenient to open a file and write to the file using the interface provided by the system. Computer Science, FSU

30. Socket • A 5-tuple associated with a socket • {protocol, local IP address, local port, remote IP address, remote port} • Complete socket is like a file descriptor • Both send() and recv() through same socket

31. Sockets: Conceptual View Computer Science, FSU

32. Connection-Oriented Application • Server gets ready to service clients • Creates a socket • Binds an address (IP interface, port number) to the socket • Server’s address should be made known to clients • Client contacts the server • Creates a socket • Connects to the server • Client has to supply the address of the server when using connect() • Accepts connection requests from clients • Further communication is specific to application Computer Science, FSU

33. Creating a socket int socket(int family, int service, int protocol) • family: symbolic name for protocol family • AF_INET, AF_UNIX • type: symbolic name for type of service • SOCK_STREAM, SOCK_DGRAM, SOCK_RAW • protocol: further info in case of raw sockets • typically set to 0 Returns socket descriptor Computer Science, FSU

34. Binding Socket with an Address int bind(int sd, struct sockaddr *addr, int len) • sd: socket descriptor returned by socket() • addr: pointer to sockaddr structure containing address to be bound to socket • len: length of address structure Returns 0 if success, -1 otherwise Computer Science, FSU

35. Specifying Socket Address struct sockaddr_in { short sin_family; /* set to AF_INET */ u_short sin_port; /* 16 bit port number */ struct in_addr sin_addr; /* 32 bit host address */ char sin_zero[8]; /* not used */ }; struct in_addr { u_long s_addr; /* 32 bit host address */ }; Computer Science, FSU

36. Bind Example int sd;struct sockaddr_in ma; sd = socket(AF_INET, SOCK_STREAM, 0); ma.sin_family = AF_INET;ma.sin_port = htons(5100);ma.sin_addr.s_addr = htonl(INADDR_ANY);if (bind(sd, (struct sockaddr *) &ma, sizeof(ma)) != -1) … Computer Science, FSU

37. Connecting to Server int connect(int sd, struct sockaddr *addr, int len) • sd: socket descriptor returned by socket() • addr: pointer to sockaddr structure containing server’s address (IP address and port) • len: length of address structure Returns 0 if success, -1 otherwise Computer Science, FSU

38. Connect Example int sd;struct sockaddr_in sa; sd = socket(AF_INET, SOCK_STREAM, 0); sa.sin_family = AF_INET;sa.sin_port = htons(5100);sa.sin_addr.s_addr = inet_addr(“128.101.34.78”);if (connect(sd, (struct sockaddr *) &sa, sizeof(sa)) != -1) … Computer Science, FSU

39. Connection Acceptance by Server int accept(int sd, struct sockaddr *from, int *len) • sd: socket descriptor returned by socket() • from: pointer to sockaddr structure which gets filled with client’s address • len: length of address structure Blocks until connection requested or error • returns a new socket descriptor on success Computer Science, FSU

40. Connection-oriented Server int sd, cd, calen;struct sockaddr_in ma, ca; sd = socket(AF_INET, SOCK_STREAM, 0);ma.sin_family = AF_INET;ma.sin_port = htons(5100);ma.sin_addr.s_addr = htonl(INADDR_ANY);bind(sd, (struct sockaddr *) &ma, sizeof(ma)); listen(sd, 5); calen = sizeof(ca); cd = accept(sd, (struct sockaddr *) &ca, &calen); …read and write to client treating cd as file descriptor… Computer Science, FSU

41. More on Socket Descriptor • socket() fills the protocol component • local IP address/port can be filled by bind() • remote IP address/port by accept() in case of server • in case of client both local and remote by connect() • accept() returns a new complete socket • Original one can be used to accept more connections Computer Science, FSU

42. Streams and Datagrams • Connection-oriented reliable byte stream • SOCK_STREAM based on TCP • No message boundaries • Multiple write() may be consumed by one read() • Connectionless unreliable datagram • SOCK_DGRAM based on UDP • Message boundaries are preserved • Each sendto() corresponds to one recvfrom() Computer Science, FSU

43. Input/Output Multiplexing • Polling • Nonblocking option using fcntl()/ioctl() • Waste of computer resources • Asynchronous I/O • Generates a signal on an input/output event • Expensive to catch signals • Wait for multiple events simultaneously • Using select() system call • Process sleeps till an event happens Computer Science, FSU

44. Select System Call int select(int maxfdp1, fd_set *readfds,fd_set *writefds, fd_set *exceptfds,struct timeval *timeout) • maxfdp1: largest numbered file descriptor + 1 • readfds: check if ready for reading • writefds: check if ready for writing • exceptfds: check for exceptional conditions • timeout: specifies how long to wait for events Computer Science, FSU

45. Timeout in Select • Wait indefinitely till there is an event • Pass NULL to the timeout argument • Don’t wait beyond a fixed amount of time • Pass pointer to a timeval structure specifying the number of seconds and microseconds. • Just poll without blocking • Pass pointer to a timeval structure specifying the number of seconds and microseconds as 0 Computer Science, FSU

46. Working with File Descriptor Set • Set is represented by a bit mask • Keep a descriptor in/out the set, turn on/off corresponding bit • Using FD_ZERO, FD_SET and FD_CLR • Use FD_ISSET to check for membership • Example: • Make descriptors 1 and 4 members of the readset fd_set readset; FD_ZERO(&readset); FD_SET(1, &readset); FD_SET(4, &readset); • Check if 4 is a member of readset • FD_ISSET(4, &readset); Computer Science, FSU

47. Return Values from Select • Arguments readfds etc are value-result • Pass set ofdescriptors you are interested in • Select modifies the descriptor set • Keeps the bit on if an event on the descriptor • Turns the bit off if no event on the descriptor • On return, test the descriptor set • Using FD_ISSET Computer Science, FSU

48. Select Example fd_set readset; FD_ZERO(&readset); FD_SET(0, &readset); FD_SET(4, &readset); select(5, &readset, NULL, NULL, NULL); if (FD_ISSET(0, &readset) { /* something to be read from 0 */} if (FD_ISSET(4, &readset) { /* something to be read from 4 */} Computer Science, FSU

49. Servers and Services • Mapping between names and addresses (DNS) • Host name to address: gethostbyname() • Host address to name: gethostbyaddr() Computer Science, FSU

50. send() and recv() • Once the sockets have been set up, the bytes can be sent and receive using send() and recv(). • send(): • ssize_t send(int socket, const void *buffer, size_t length, int flags); • Returns the number of bytes sent. -1 if error. • To send, fill the buffer to length, and call send(). • recv(): • ssize_t recv(int socket, void *buffer, size_t length, int flags); • Returns the number of bytes received. If returns 0, means the connection is down. -1 if error. • When the socket fd is set, call recv(), and check the return value and buffer.