Communication Networks

1
Communication Networks

• Recitation 1

2
Administrative

• David Raz
• Schreiber M21, (640)6455
• Email davidraz_at_post.tau.ac.il
• website http//www.cs.tau.ac.il/davidraz/course
  s/comnet06/
• Grader Hadas Zur zurhadas_at_post.tau.ac.il

3
TCP/IP Socket Programming
4
What is a socket?

• An interface between application and the network
• The application can send/receive data to/from the
  network -- communicate

Application
Network API
Protocol A
Protocol B
Protocol C
5
A Socket-eye view of the Internet
medellin.cs.columbia.edu (128.59.21.14)
newworld.cs.umass.edu (128.119.245.93)
cluster.cs.columbia.edu (128.59.21.14,
128.59.16.7, 128.59.16.5, 128.59.16.4)

• Each host machine has an IP address

6
Ports

• Each host has 65,536 ports
• Some ports are reserved for specific apps
• 20,21 FTP
• 23 Telnet
• 80 HTTP

Port 0
Port 1
Port 65535
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Address Pair

• An address is an IPport
• A socket provides an interface to an IPport pair

Remote IP 123.45.6.78 Remote Port 3726
Local IP 111.22.3.4 Local Port 2249
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Functions needed

• Specify local and remote communication endpoints
• Initiate a connection
• Send and receive data
• Terminate a connection

9
Socket Creation in C socket()

• int s socket(domain, type, protocol)
• s socket descriptor (an integer, like a
  file-handle)
• domain integer, communication domain
• e.g., PF_INET (IPv4 protocol) typically used
• type communication type
• SOCK_STREAM reliable, 2-way, connection-based
  service
• SOCK_DGRAM unreliable, connectionless,
• protocol specifies protocol (see file
  /etc/protocols for a list of options) - usually
  set to 0

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Socket Descriptor Data Structure
Descriptor Table
Family PF_INET Service SOCK_STREAM Local IP
111.22.3.4 Remote IP 123.45.6.78 Local Port
2249 Remote Port 3726
0
1
2
3
4
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Two essential types of sockets

• SOCK_DGRAM
• a.k.a. UDP
• unreliable delivery
• no order guarantees
• no notion of connection
• can send or receive

• SOCK_STREAM
• a.k.a. TCP
• reliable delivery
• in-order guaranteed
• connection-oriented
• bidirectional

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The bind() function

• associates and (can exclusively) reserves a port
  for use by the socket
• Done by the server
• int status bind(sockid, addrport, size)
• status error status, -1 if bind failed
• sockid integer, socket descriptor
• addrport struct sockaddr, the (IP) address and
  port of the machine. (address usually set to
  INADDR_ANY chooses a local address)
• size the size (in bytes) of the addrport
  structure

13
The struct sockaddr

• The Internet-specific
• struct sockaddr_in
• short sin_family
• u_short sin_port
• struct in_addr sin_addr
• char sin_zero8
• sin_family AF_INET
• sin_port port (0-65535)
• sin_addr IP-address
• sin_zero unused

• The generic
• struct sockaddr
• u_short sa_family
• char sa_data14
• sa_family
• specifies which address family is being used
• determines how the remaining 14 bytes are used

14
Address and port byte-ordering

• Problem
• different machines / OSs use different word
  orderings
• little-endian lower bytes first
• big-endian higher bytes first
• these machines may communicate with one another
  over the network

Big-Endian machine
Little-Endian machine
12.40.119.128
128.119.40.12
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Network Byte Order

• All values stored in a sockaddr_in must be in
  network byte order.
• Whenever the source of the address isnt the
  network, use htonl and htons

Common Mistake Ignoring Network Byte Order
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Example

• int sock
• sock socket(PF_INET, SOCK_STREAM, 0)
• if (socklt0) / ERROR /
• struct sockaddr_in myaddr
• myaddr.sin_family AF_INET
• myaddr.sin_port htons( 80 )
• myaddr.sin_addr htonl( INADDR_ANY )
• bind(sock, myaddr, sizeof(myaddr))

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Connection Setup (SOCK_STREAM)

• A connection occurs between two kinds of
  participants
• passive waits for an active participant to
  request connection
• active initiates connection request to passive
  side
• Once connection is established, passive and
  active participants are similar
• both can send receive data
• either can terminate the connection

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Connection setup contd

• Active participant
• step 2 request establish connection
• step 4 data transfer

• Passive participant
• step 1 listen (for incoming requests)
• step 3 accept (a request)
• step 4 data transfer
• The accepted connection is on a new socket
• The old socket continues to listen

Passive Participant
Active 1
Active 2
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Connection est. listen() accept()

• Called by passive participant
• int status listen(sock, queuelen)
• status 0 if listening, -1 if error
• sock integer, socket descriptor
• queuelen integer, of active participants that
  can wait for a connection
• int s accept(sock, name, namelen)
• s integer, the new socket (used for
  data-transfer)
• sock integer, the orig. socket (being listened
  on)
• name struct sockaddr, address of the active
  participant
• namelen sizeof(name) value/result parameter

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Connection est. connect()

• Called by active participant
• int status connect(sock, name, namelen)
• status 0 if successful connect, -1 otherwise
• sock integer, socket to be used in connection
• name struct sockaddr address of passive
  participant
• namelen integer, sizeof(name)

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Server example
int sock sock socket(PF_INET, SOCK_STREAM,
0) if (socklt0) / ERROR / struct
sockaddr_in myaddr myaddr.sin_family
AF_INET myaddr.sin_port htons( 80
) myaddr.sin_addr htonl( INADDR_ANY
) bind(sock, myaddr, sizeof(myaddr))
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Server example
listen(sock, 5) int new_sock struct sockaddr_in
their_addr sin_size sizeof(struct
sockaddr_in) new_sock accept(sock, (struct
sockaddr )their_addr, sin_size)
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Client example
int sock sock socket(PF_INET, SOCK_STREAM,
0) if (socklt0) / ERROR / struct
sockaddr_in dest_addr dest_addr.sin_family
AF_INET dest_addr.sin_port htons( 80
) dest_addr.sin_addr inet_addr(128.2.5.10)
connect(sock, (struct sockaddr )dest_addr,
sizeof(struct sockaddr))