Network Programming

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Network Programming

• Eddie Aronovich
• mail eddiea_at_mta.ac.il

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How did it started ?
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How can we write communication ?

• Application Program Interface (API)
• Sockets
• TLI (Transport Layer Interface)
• System calls
• Library functions

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What is it for ?

• Communication systems provides 3 services
• Information resource Sharing
• Distance gapping
• Backup abilities

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Fundamentals

• Server - An entity which gives services
• Client - An entity which requests services
• Transport layer - To transfer the data

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The 7 layers model
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The 7 Layer model in real life
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The message is built in one side...
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And striped in the dest. side
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From Lynx to Netscapeor chat client

• Presentation

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Descriptors

• Everything in Unix is a file
• Descriptor is an index into an array

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Memory Buffers

• Contains Socket Address Structure
• Headers
• Data

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The Basics of communication

• LectureII

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Processing Techniques
Decentralized
Centralized
WEB
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The Client ?? Server Model

• The server roles
• Give service as asked
• Wait the client to appeal to him
• The client roles
• Start the communication process
• Asks the wanted service

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Design considerations

• Serve single or multiple users
• Use reliable or unreliable protocol
• Software updates

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Client and server roles in UDP

• Server roles
• Bind a port
• Wait for a message to come
• Send reply
• Client roles
• Send a message
• Get the reply

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User Datagram Protocol

• Simple protocol
• Connectionless
• Unreliable

• RFC 768

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Socketip_addr, port number

• API, an interface for the program to contact with
  communication.
• Enable usage of regular file commands as read,
  write and so on.
• The sockets are structures passed from kernel to
  process and vice versa.

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What the socket struct contains ?

• Socket type stream, dgram, raw,
• Socket options broadcast, OOB...
• Time to linger wait before close the socket
• Socket state flags
• Protocol Control Block
• Protocol Handle

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The socket address structs

• Generalstruct sockaddr uint8_t sa_len
  /Len of socket struct / sa_family_t sa_fami
  ly /Addr family as AF_INET /
  char sa_data20 /Protocol Address /
• IP V4 address socket
• struct sockaddr_in
• uint8_t sin_len /The socket length /
• sa_family_t sin_family /AF_INET for IP
  addresses /
• unit16_t sin_port /The port id 16-bit port
  num /
• struct in_addr sin_addr /The IP address
  32-bit /
• char sin_zero 8 / FFU - Must be zero /

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How the socket is created

• The system call passes identifiers for address
  family(e.g.AF_UNIX, AF_INET,etc.), socket type
  and protocol.
• Socket data structure is allocated.
• Pointer from the fd table to other i-node struct
  which points to the socket.

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User Datagram Protocol (rfc 768)

• Data transport layer protocol(Fragment packets
  to fit local MTU)
• Used to make available datagram packet switched
  mode
• Connectionless protocol
• Used when RTT is important or no connection needed

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UDP header
Source port number (16 bit) Destination port number (16 bit)
Length (16 bit) UDP Checksum (16 bit)
Data Data
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UDP by network monitor
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Lets do it in UDP - client
C\TEMP\udp-cli-c.htm
C\TEMP\udp-cli-c.htm
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Lets do it in UDP - server
C\TEMP\udp-srv-c.html
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TCP

• LectureIII

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Transmission Control Protocolrfc 793

• Reliability
• Sequenced data transfer
• Flow control
• Full duplex

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TCP Header
Source port number (16 bit) Source port number (16 bit) Source port number (16 bit) Source port number (16 bit) Source port number (16 bit) Source port number (16 bit) Source port number (16 bit) Source port number (16 bit) Dest port number (16 bit)
Sequence number (32 bit) Sequence number (32 bit) Sequence number (32 bit) Sequence number (32 bit) Sequence number (32 bit) Sequence number (32 bit) Sequence number (32 bit) Sequence number (32 bit) Sequence number (32 bit)
Acknowledgement number (32 bit) Acknowledgement number (32 bit) Acknowledgement number (32 bit) Acknowledgement number (32 bit) Acknowledgement number (32 bit) Acknowledgement number (32 bit) Acknowledgement number (32 bit) Acknowledgement number (32 bit) Acknowledgement number (32 bit)
Header length (4 bit) F.F.U (6 bit) URG ACK PSH RST SYN FIN Window size (16 bit)
TCP Checksum (16 bit) TCP Checksum (16 bit) TCP Checksum (16 bit) TCP Checksum (16 bit) TCP Checksum (16 bit) TCP Checksum (16 bit) TCP Checksum (16 bit) TCP Checksum (16 bit) Urgent Pointer (16 bit)
Options Options Options Options Options Options Options Options Options
Data Data Data Data Data Data Data Data Data
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Ack Flow control

• Acknowledgements are sent by byte.
• The acks can be sent for group of packets (but
  should arrive before time out of first packet).
• Window size is used for flow control and is
  controlled by receiver.

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How connection starts ?
connect() listen()
connect returns. listen()
connect returns. accept()
connect returns. accept returns.
accept returns.
SYN j
SYN k, ACK j1
ACK k1
Connection Established
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How we do it in C ?
Server Client
socket() socket()
bind() connect()
listen() connect()
accept()
New file descriptor is given
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Ending Sessions

• Passive close (FIN arrives) Vs. Active close

FIN M
ACK M1
FIN N
ACK N1
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Internet Protocol

• LectureIV

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Network Layer

• Passing packet from source host to destination
  host (cross networks if needed)
• Independent of the datalink layer
• QoS
• Flow control

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Internet Protocol

• Connectionless
• Unreliable
• Best effort

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Address

• IP address consist 32bit and is divided into
  network id.,host id
• Part of host id can be used for subnet mask
• Some special addresses are defined (as net-addr,
  broadcast-addr, multicast-addr)

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Addresses and Classes

• Class A
• Addresses 0-127.X.X.X
• Class B
• Addresses 128-191.0-255.X.X
• Class C
• Addresses 192-223.0-255.0-255.X

01XXXXXX
10XXXXXX
110XXXXX
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Masks

• Mask is used to refine network division.
• 1b in mask symbolize bit belongs to network
  address.
• 0b in mask symbolize bit belongs to host
  address.

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Address conventions

• Network address filled with 0 in host address.
• Broadcast address filled with 1 in host
  address.
• First, last subnet and address arent used

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How routing is done ?

• The router compares destination IP with each
  Network subnet address.
• Unknown destination is routed to default.
• Routers update each other with appropriate
  algorithms.

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VI????? I/O Multiplexing
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I/O Models

• Blocking I/O
• Nonblocking I/O
• I/O Multiplexing
• Signal driven
• Asynchronous I/O

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Non-Blocking I/O

• No blocking is done (Error is returned)
• Reading is done in loop (until data arrives)
• Polling costs CPU time
• Implementation in real time systems only

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Signal Driven I/O

• Signal handler should be started

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