The Socket Interface

Chapter 22

Introduction

• This chapter reviews one example of an Application Program Interface (API) which is the interface between applications and TCP/IP protocols

• Why did we wait so long to cover this?– The interface architecture is not standardized

– It is not appropriate to tie the protocols to a particular API because no single interface architecture works well on all systems

• Interface details depend on the operating system

• This interface has become a de facto standard

• It forms the basis for Microsoft’s Windows Socket interface

UNIX I/O and Network I/O

• UNIX was designed in the late 1960’s as a timesharing system for single process computers– It is a process-oriented operating system in which each

application program executes as a user level process

– An application program interacts with the operating system by making system calls

• System calls act like procedure calls in that they take arguments and return values

UNIX I/O and Network I/O

• The UNIX I/O primitives follow the pattern:

open-read-write-close– Open is called to specify and get permission from the

file or device to be used• A small integer file descriptor is returned for future reference

– Read transfers data to the user process

– Write transfers data form the user process to the file or device

– Close informs the operating system that operations are complete

Adding Network I/O to UNIX

• More complex network protocols were added to BSD UNIX

• The protocol interface allows programmers to:– create server code that awaits connections passively

– create client code that forms connections actively

– allow sending application programs to specify the destination address with each datagram instead of binding destinations at the time they call open

– use other protocol suites besides just TCP/IP• i.e. a 32 bit address must be identified as an IP address

The Socket Abstraction

• A socket is a generalization of the UNIX file access mechanism that provides an endpoint for communication– Applications request the operating system to create a

socket when one is needed

– The system returns an integer that is used by the application to reference the socket

• Sockets can be created without binding to a specific destination address

The Socket Abstraction

• The application can choose to:– Supply a destination address each time it uses the

socket• when sending datagrams

– Bind the destination address to the socket• and avoid specifying the destination repeatedly as in a TCP

connection

• When it makes sense, sockets perform like UNIX files or devices– Data is transferred using read and write

Creating a Socket

• The socket function uses three arguments and returns an integer result

result = socket (pf, type, protocol)

– pf specifies the protocol family• TCP/IP, Xerox, Apple, and UNIX

– type specifies type of communication• reliable stream, connectionless datagram, raw type (allows access to

low-level protocols or network interfaces)

– protocol specifies one protocol if a family has multiple protocols that support one type of service

• i.e. a single family might have 2 connectionless deliveries

Creating a Socket

• The UNIX pipe mechanism– An interprocess communication mechanism that uses a

reliable stream delivery service

– The calling process creates both endpoints for communication simultaneously

– Function socketpair creates two sockets simultaneously

socketpair (pf, type, protocol, sarray)• the two socket descriptors are place in sarray which is a two-

element integer array

Socket Inheritance and Termination• UNIX uses the fork and exec system calls to start new

application programs– First, fork creates a copy of the currently executing application program

• This copy inherits access to all open sockets and it inherits access to all open files

– Then exec is called so that the new copy can execute

• The operating system keeps a reference count associated with each socket, so it knows how many applications have access to

it • When a process finishes using a socket, it calls close

– The reference count is decremented

Specifying a Local Address

• In many cases, sockets are created without care for which local address is assigned

• However, server processes at well-known ports must be able to specify that port– After a socket has been created, a server binds it to a local

address

bind (socket, localaddr, addrlen)• socket is socket descriptor (integer) of the socket to be bound

• localaddr is a structure shown in Figure 22.1that specifies the local address to which the socket should be bound

• addrlen specifies length of the address in bytes

Specifying a Local Address

– The structure sockaddr (in Figure 22.1) has:• a 16-bit ADDRESS FAMILY that identifies the protocol suite

to which the address belongs and determines what follows

• an address of up to 14 octets

– The format of a socket address structure for a TCP/IP address is shown in Figure 22.2

Connecting Sockets to Destination Addresses• Initially a socket is not associated with any foreign

address– The function connect binds a permanent destination to a

socket, placing it in a connected state

connect (socket, destaddr, addrlen)• destaddr is a socket address structure that specifies the

destination address to which the socket should be bound

– A connect must be made for reliable transport• It is not necessary for connectionless datagram service, but may

be used; the destination will not have to be specified with each datagram

Sending Data Through a Socket

• Once an application has established a socket, it uses the socket to transmit data– write - requires buffer and length

– writev - requires a vector of pointers to blocks of data

– send - has flags that could indicate out-of-band data

– sendto and sendmsg - require a destination; sendmsg is used with a message structure

• send, write and writev only work with connected sockets, as

they do not specify a destination address

Receiving Data Through a Socket• An application may receive data through a socket

– read - when a socket is connected• read (descriptor, buffer, length)

– readv - allows reading from noncontiguous locations

– recv - uses flags similar to send

– recvfrom - allows input from an unconnected socket• recvfrom (socket, buffer, length, flags, fromaddr,len)

– recvmsg - analogous to sendmsg with structure

Obtaining Local and Remote Socket Addresses• Newly created processes inherit the set of open

sockets from the process that created them– Sometimes the newly created process needs to

determine the destination address to which a socket connects

• getpeername - determines the address of the peer (the remote end) to which a socket connects

– getpeername ( socket, destaddr, addrlen)

• getsockname - returns the local address associated with a socket

– getsockname ( socket, localaddr, addrlen)

Obtaining and Setting Socket Options• In addition to binding a socket to a local address or

connecting it to a destination address, we need a way for the application to control the socket– get or set timeout parameters– control allocation of buffer space– control processing of out-of-band data

• Instead of providing different commands for each option, two operations are given:– getsockopt - application requests information of socket

– setsockopt - sets an option based on values from getsockopt

Specifying a Queue Length for a Server• Typically, a server

– creates a socket

– binds it to a well-known port

– listens for requests

• A new request may arrive before the server has finished with an old request– The server uses listen to put the socket in a passive

mode ready to accept simultaneous requests at the socket (only for sockets that have reliable stream)

listen (socket, qlength)

How a Server Accepts Connections• A server uses socket, bind and listen commands

– to create a socket, bind it to a well-known port, and specify queue length for listening

• At this point the socket is not connected to a specific

foreign destination – the server waits for a connection, using the accept function

newsock = accept (socket, addr, addrlen)

– when a connection request arrives, the system fills in the addr (a pointer to a structure of type sockaddr)

– the system creates a new socket that has its destination connected to the requesting client

How a Server Accepts Connections• The server can handle connection requests

– iteratively - server handles the request itself, closes the new socket and calls accept to get the next connection request

– concurrently - server creates a slave which inherits a copy of the new socket, the slave services the request and closes the socket

• Multiple processes use the same local port, but they must connect to different destinations

– When a TCP segment arrives, it is sent to the socket connected to the segment’s source

• In a concurrent server there is one process per client and one process that accepts connections

– The socket used by the master server has a wildcard for the foreign destination

Other System Calls

• select - allows a single process to wait for connections on mulitple sockets

• gethostname - allows user processes to access the host name

• sethostname - allows privileged processes to set the host name

• setdomainname - notifies the O.S. on each host as to which domain it is in

• getdomainname - allows processes to retrieve the domain name

Other System Calls

• gethostbyname - takes a domain name and returns a pointer to a structure of information for that host

• gethostbyaddr - same as above, except it accepts a host address as an argument

• getnetbyname - obtains and formats database entry given the domain name of a network

• getnetbyaddr - to get information about a network given its address

• getservbyname - maps a named service onto a port number• getservbyport - allows the caller to obtain an entry from the

services database given the port number assigned to it

Socket Library Calls

• System calls pass control to the O.S.• Library routines are bound into an application

program as shown in Figure 22.5

Network Byte Order Conversion Routines• Conversion routines for converting between the

local machine byte order and the network standard byte order– ntohs - network order to local host order (short 2-bytes)

– ntohl - network order to local host order (long 4-bytes)

– htons - local host order to network order (short 2-bytes)

– htonl - local host order to network order (long 4-bytes)

An Example Client

• Implementation of whois client– Allows a client on one machine to get information about a user

on a remote system

– The client is an application program that a user invokes with two arguments

• the name of a remote machine

• the name of a user on that machine

– The client calls • gethostbyname to map the remote machine name into an IP address

• getservbyname to find the well-known port for the whois service

– The client creates a socket and binds it to the whois protocol port (43) on the destination machine

An Example Server

• Listens on the whois port• Returns requested information in response to a

request from any client• Information is taken from the UNIX password file

on the server’s machine

Summary

• The interface between an application program and TCP/IP protocols depends on the O.S.

• To use TCP, a program creates a socket, binds addresses to it, accepts incoming connections, communicates with reads and writes, and closes the socket

• System calls and library routines help programmers

For Next Time

• Read Chapter 23• Class presentations start with Chapter 24