cnt5505 data/comuter communications spring 2012

Computer Science, FSU 1

CNT5505DATA/COMUTER COMMUNICATIONS

Spring 2012


About myselfZhenghao ZhangOffice: Love 164; Phone: 644-1685Email: [email protected]: http://www.cs.fsu.edu/~zzhang

Research area: • Wireless networks • Network security• Peer to peer networks• Optical networks


Office Hours

• Love 164

• 3:30pM – 5:30PM, Thursday, or by appointments through email


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


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


Course Materials• Required textbook

– “Computer Networks,'' by Andrew S. Tanenbaum, Prentice Hall, 5th edition

• Class notes, other assigned readings • Materials on the Internet


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

http://www.cs.fsu.edu/~zzhang/CNT5505_Spring_2012.htm


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


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.


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

Course Project

• There will be several projects.• Not coding intensive



Questions and Concerns?


Computer Communication: A motivation example

• What happens behind the scene when I type http://www.google.com in the address bar of my iPhone?

http://www.google.com/


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


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)


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


• 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?



• 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.



• 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?



• 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.



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

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 arpforward packet to first-hop router

• (second router, third router) …... • www.google.com receives a packet.


http://www.google.com/

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?



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

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?



Step 5: web page rendering

• Browser displays the content of the web page

Socket Programming


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


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.

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


Sockets: Conceptual View


Connection-Oriented Application

1. 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

2. Client contacts the server– Creates a socket– Connects to the server

• Client has to supply the address of the server when using connect()

3. Accepts connection requests from clients4. Further communication is specific to

application


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


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


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 */

};


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)

…


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


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)

…


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


Connection-oriented Serverint 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…


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


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()


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


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


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


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);


Return Values from Select

• Arguments readfds etc are value-result• Pass set of descriptors 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


Select Examplefd_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 */}


Servers and Services

• Mapping between names and addresses (DNS)– Host name to address: gethostbyname()– Host address to name: gethostbyaddr()

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.

cnt5505 data/comuter communications spring 2012

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