Socket API

1
Socket API

• Lecture 3

2
Socket API History

• Starting in 1980, ARPA funded research for TCP/IP
  development
• Adopted in 1983 in BSD v4.2 UNIX
• 1989 Berkeley released networking library freely
  to other operating systems
• deFacto standard for TCP/IP network sockets
• Many other types of APIs for networks, but the
  Berkeley API has most popularity

3
Internet Socket

• Communication endpoint
• either listening or talking
• Identified by the tuple (IP address, port,
  protocol)?
• Addressing
• IPv4 4 octets
• Octet is 0-255 (2 to the 8th power)?
• IPv6 32 octets
• Not all octets are used for endpoint
  identification
• 2 very special addresses
• 0.0.0.0 listening for anyone
• 255.255.255.255 broadcasting to everyone

4
IP Addressing

• IPv4 addressing
• 4 octets
• Classes of addresses

5
IP Addressing CIDR (classless inter-domain
routing)?
6
Original Internet Map
7
Port Numbers

• TCP 0 65535
• UDP 0 65535
• http//www.iana.org/assignments/port-numbers
• Well Known are 0-1023
• Usually have to run as 'root' for servers on
  Well Known
• Many applications listen on tcp and udp for a
  given port number....
• whether they actually use both or not!

8
Networking Layers
9
Networking - Routing
10
TCP state model
11
TCP vs UDP Header
TCP Header
UDP Header
12
Socket Library (API)?

• Original intention of Berkeley Sockets was not to
  define network, only a communication method
• Supports many different types of networks
• Main difference between is not in socket library
  calls, only in address definition (rendezvous)
• Abstraction was/is the key
• first build most atomic functions that do not
  discriminate against any networks/protocols
• second start abstracting out more functions that
  accept arguments to define function in more
  specifics

13
Socket and File Descriptors

• File descriptors are integer values gt 0 that
  identify an open file to a program
• These values are held in a file descriptor table
• Table holds integer values as keys, and the value
  of the key is a pointer to the data structure for
  the open file
• Holds things like v-node, offset/etc..
• Simply, socket descriptors also fall in that
  table, but they point at a different kind of
  structure
• Point to a socket data structure

14
File and Socket descriptor data structure
differences

• File Descriptor Data Structure
• current file offset
• v-node pointer
• status
• flags
• ....etc...
• Socket Descriptor Data Structure
• protocol family
• service
• local and remote IP address
• local and remote port numbers
• etc.....

15
Active or Passive Socket Descriptors

• Active Socket Descriptor
• Client
• Initiating the connection
• Passive Socket Descriptor
• Server
• Listening for incoming active sockets
• Both created in the same way, just how they are
  used!

16
Endpoint Addressing

• Must understand that the socket library is an
  ABSTRACTION, so when we talk about addressing, we
  are describing TCP/IP addressing
• 2 types based on families
• AF_INET Address Family
• PF_INET Protocol Family
• NOTE in the C language, if a VALUE is in all
  caps, it usually is a defined constant with an
  integer value!!
• e.g. AF_INET is really the integer 2, and PF_INET
  is also the same integer (usually quite
  confusing)?

17
Address Structure

• Again, since Berkeley Sockets are an ABSTRACTION,
  there are generic and specific structures to hold
  address types
• Generic struct sockaddr
• Specific struct sockaddr_in (in for Internet)?
• NOTE USE THE SPECIFIC STRUCTURE!
• struct sockaddr_in
• u_char sin_len
• u_short sin_family
• u_short sin_port
• struct in_addr sin_addr
• char sin_zero8

18
Socket Function Calls - socket()?

• include ltsys/types.hgt
• include ltsys/socket.hgt
• int socket(int domain, int type, int
  protocol)
• Returns socket descriptor if OK, -1 on error
• DOMAIN PARAMETER TYPE
• PF_UNIX, PF_LOCAL Local communication
  unix(7) SOCK_STREAM TCP
• PF_INET IPv4 Internet
  protocols ip(7) SOCK_DGRAM UDP
• PF_INET6 IPv6 Internet
  protocols
• PF_IPX IPX - Novell
  protocols
• PF_NETLINK Kernel user interface
  device netlink(7)?
• PF_X25 ITU-T X.25 /
  ISO-8208 protocol x25(7)?
• PF_AX25 Amateur radio AX.25
  protocol
• PF_ATMPVC Access to raw ATM
  PVCs
• PF_APPLETALK Appletalk
  ddp(7)?
• PF_PACKET Low level packet
  interface packet(7)?

19
Socket Function Calls - connect()?

• include ltsys/types.hgt
• include ltsys/socket.hgt
• int connect(int sockfd, const struct
  sockaddr serv_addr, socklen_t addrlen)
• Returns 0 if ok, -1 on error

20
Socket Function Calls - send()?

• include ltsys/types.hgt
• include ltsys/socket.hgt
• ssize_t send(int s, const void buf, size_t
  len, int flags)
• ssize_t sendto(int s, const void buf,
  size_t len, int flags, const
• struct sockaddr to, socklen_t tolen)
• ssize_t sendmsg(int s, const struct msghdr
  msg, int flags)
• Returns number of characters sent, or -1 on
  error

21
Socket Function Calls Recv()

• include ltsys/types.hgt
• include ltsys/socket.hgt
• ssize_t recv(int s, void buf, size_t len, int
  flags)
• ssize_t recvfrom(int s, void buf, size_t len,
  int flags, struct sock- addr from,
  socklen_t fromlen)
• ssize_t recvmsg(int s, struct msghdr msg, int
  flags)
• Returns number of bytes read if ok, 0 on
  shutdown, -1 on error

22
Socket Function Calls - close()?

• include ltunistd.hgt
• int close(int fd)
• Returns 0 if ok, -1 on error

23
Socket Function Calls - bind()?

• include ltsys/types.hgt
• include ltsys/socket.hgt
• int bind(int sockfd, struct sockaddr my_addr,
  socklen_t addrlen)
• Returns 0 if ok, -1 on error

24
Socket Function Calls - listen()?

• include ltsys/socket.hgt
• int listen(int s, int backlog)
• Returns 0 if ok, -1 on error

25
Socket Function Calls - accept()?

• include ltsys/types.hgt
• include ltsys/socket.hgt
• int accept(int s, struct sockaddr addr,
  socklen_t addrlen)
• Returns socket descriptor if ok, -1 on error

26
Socket Functions Client Sequence

• socket
• connect
• send
• recv
• close

27
Socket Functions Server Sequence

• socket
• bind
• listen
• accept
• recv
• send
• close