The Socket Interface

1. The Socket Interface Chapter 21

2. Application Program Interface (API) • Interface used between application programs and TCP/IP protocols • Will look at one example that is a de facto standard • Things to keep in mind • Application/protocol interaction not in the standards • Distinguish between interface and protocols • In practice, details of interface depend on OS • Example is from BSD UNIX operating system • Is a widely accepted, de facto standard • Operations listed are NOT part of the TCP/IP standards

3. UNIX I/O Paradigm • Unix is a process oriented OS • Applications execute as a user level process • Application interacts with OS via system calls • Act just like procedure calls • I/O is open-read-write-close • Call open to get file descriptor for file or device • Call read or write to do I/O • Call close to indicate done with object

4. Adding Network I/O to UNIX • Originally, all I/O was open-read-write-close • Needed to add network protocols to UNIX • Interactions more complex • Passive server code as well as active client code • Specify datagram address when send vs when open • Abandoned the above paradigm • Additions made I/O interface more complex • Needed general mechanism for any protocol interface

5. Socket Abstraction • Socket is basis for network I/O • Mechanism that provides an endpoint for communication • Create when needed; get integer to reference • Application can choose address binding method • TCP connection • Both endpoints must be specified • Use for UDP • Remote endpoint can be left unspecified • Pass as argument each time message is sent

6. Creating a Socket • Use socket function to create a socket result = socket(pf, type, protocol) • pf specifies protocol family • type specifies communication type • Reliable stream; connectionless datagram; raw • protocol specifies a specific protocol in the family • Need more than just family and type • Ex: pipe in UNIX family cannot do packet delivery • Programmer must know protocol family well

7. Socket Inheritance & Termination • Two system calls used to start new application programs • fork • Creates separate copy of currently executing process • New copy inherits all open file descriptors & sockets • exec • Desired application program loaded into the process • Still retains access to inherited sockets (and FDs)

12. Both old and new processes share the existing sockets • OS keeps a count associated with each socket • Knows how many applications are using it • Programmer must ensure it is done in a meaningful way • To close a socket: close(socket) • Or, open sockets closed upon process termination • close actually decrements count; destroy socket when it reaches zero

13. Specifying a Local Address • Upon creation, sockets have no association to local or destination addresses • TCP/IP • No local protocol port number has been assigned • No destination port or IP address has been specified • Client may not care what local address is • Server process at well-know port will care • Uses bind function to establish specific local addrs

14. Connecting Sockets to Destination Addresses • Initially, sockets are unconnected • Not associated with any remote destination • Function connect binds a permanent destination • Form: connect(socket, destaddr, addrlen) • Semantics depend upon underlying protocol • Reliable stream service: build TCP connection • Connectionless service: stores dest address locally

15. Sending Data Through a Socket • Sockets are used to transmit data • Five possible functions • write • write(socket, buffer, length) • buffer contains the address of the data to be sent • Works with connected sockets only • writev • writev(socket, iovector, vectorlen) • iovector contains a sequence of pointers to blocks of bytes • Works with connected sockets only

16. send • send(socket, message, length, flags) • message gives the address of the data • flags controls the transmission • Works with connected sockets only • sendto • sendto(socket, message, length, flags, destaddr, addrlen) • destaddr specifies the socket address structure • sendmsg • sendmsg(socket, messagestruct, flags) • messagestruct is a structure with the required information • Used when sendto makes the program inefficient or hard to read

17. Receiving Data Through a Socket • Analogous five input functions • read(descriptor, buffer, length) • readv(descriptor, iovector, vectorlen) • recv(socket, buffer, length, flags) • recfrom(socket, buffer, length, flags, fromaddr, addrlen) • recmsg(socket, messagestruct, flags)

18. Text gives examples of many socket functions and library routines • Can read over at your convenience

19. How a Server Accepts Connections • Once a socket is established, the server waits for a connection • Uses function accept to do so • newsock = accept(socket, addr, addrlen) • When a request arrives: • addr and addrlen filled in • New socket created that has destination connected to client • Original socket still remains open • Call to accept returns

20. Server can handle connections one of two ways • Iteratively • Server handles request, closes new socket, calls accept • Concurrently • Master creates slave to handle request at new socket • Closes its copy of the new socket • Calls accept • Slave closes socket when it finishes, and then terminates • Multiple processes will be using same local protocol port • Ok because pair of endpoints defines a connection • Master server has wildcard in foreign destination • All other processes have a specific foreign dest

21. Socket Library Calls • Socket API also offers set of library routines • Perform useful functions related to networking • System calls pass control to computer’s OS • Library routines are like other program procedures Figure 21.5

22. Many socket library routines provide database services • Determine names of machines & network services • Determine protocol port numbers • Find out other related information • Sections 21.19 - 21.25 examine groups of library routines

23. Example Client & Server • Text gives example C program using the socket API to access TCP/IP protocols • Implements simple whois client & server • Client is an application that a user invokes • Two arguments: name of remote machine and user • Creates socket, uses TCP, binds socket to whois port • Server only slightly more complex • Listens on well-know whois port • Returns requested info from UNIX password file

24. Summary • API for TCP/IP depends on details of OS • Because TCP/IP protocol SW resides inside OS • Interface not specified by protocol standard • Examined socket API • Originally designed for BSD UNIX • Widely used by many vendors (like Microsoft) • Uses UNIX open-read-write-close paradigm

25. To use TCP, application program must: • Create socket • Bind addresses to it • Accept incoming connections • Communicate using read or write primitives • Close when finished • Many library routines available, also • Socket interface is popular and widely supported • If not have socket facilities in OS, often provide socket library • Underlying OS will use different set of system calls