CNT5505

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CNT5505 DATA/COMUTER COMMUNICATIONS Spring 2012
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About myself

• Zhenghao Zhang
• Office Love 164 Phone [phone]
• Email zzhang_at_cs.fsu.edu
• URL http//www.cs.fsu.edu/zzhang
• Research area
• Wireless networks
• Network security
• Peer to peer networks
• Optical networks

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Office Hours

• Love 164
• 330pM 530PM, Thursday, or by appointments
  through email

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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

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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

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Course Materials

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

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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

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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

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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.

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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

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Course Project

• There will be several projects.
• Not coding intensive

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Questions and Concerns?
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Computer Communication A motivation example

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

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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

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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)

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Step 2 Getting IP address (Contd)

• 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

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Step 2 Getting IP address (Contd)

• 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?

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Step 2 Getting IP address (Contd)

• 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.

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Step 2 Getting IP address (Contd)

• 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?

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Step 2 Getting IP address (Contd)

• 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.

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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

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Step 3 establishing HTTP connection (Contd)

• 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.

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Step 3 establishing HTTP connection (Contd)

• There are many options (paths) to reach the
  destination.
• How to choose the best path?
• How to maintain the path information?

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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

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Step 4 Web page request and retrieval (Contd)

• 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?

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Step 5 web page rendering

• Browser displays the content of the web page

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Socket Programming
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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

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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.

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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

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Sockets Conceptual View
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Connection-Oriented Application

• Server gets ready to service clients
• Creates a socket
• Binds an address (IP interface, port number) to
  the socket
• Servers 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

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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

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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

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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_zero8 / not used /
• struct in_addr
• u_long s_addr / 32 bit host address /

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Bind Example
int sdstruct sockaddr_in ma sd
socket(AF_INET, SOCK_STREAM, 0) ma.sin_family
AF_INETma.sin_port htons(5100)ma.sin_addr.s_
addr htonl(INADDR_ANY)if (bind(sd, (struct
sockaddr ) ma, sizeof(ma)) ! -1)
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Connecting to Server

• int connect(int sd, struct sockaddr addr, int
  len)
• sd socket descriptor returned by socket()
• addr pointer to sockaddr structure containing
  servers address (IP address and port)
• len length of address structure
• Returns 0 if success, -1 otherwise

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Connect Example
int sdstruct sockaddr_in sa sd
socket(AF_INET, SOCK_STREAM, 0) sa.sin_family
AF_INETsa.sin_port htons(5100)sa.sin_addr.s_
addr inet_addr(128.101.34.78)if
(connect(sd, (struct sockaddr ) sa, sizeof(sa))
! -1)
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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 clients address
• len length of address structure
• Blocks until connection requested or error
• returns a new socket descriptor on success

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Connection-oriented Server
int sd, cd, calenstruct sockaddr_in ma,
ca sd socket(AF_INET, SOCK_STREAM,
0)ma.sin_family AF_INETma.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
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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

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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()

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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

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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

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Timeout in Select

• Wait indefinitely till there is an event
• Pass NULL to the timeout argument
• Dont 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

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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)

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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

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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 /
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Servers and Services

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

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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.