SOCKETS Lecture #3

The Socket Interface Funded by ARPA (Advanced Research Projects Agency) in Developed at UC Berkeley Objective: to transport TCP/IP software to UNIX The socket interface has become a de facto standard.

Approach Define functions that support network communications in general, and use parameters to make TCP/IP communication a special case. Socket calls refer to all TCP/IP protocols as a single protocol family.

TCP/IP provides peer-to-peer communication. Is a protocol that provides basic mechanisms to transfer data provides connectionless and connection- oriented servers.

Concurrency Main() { int I; fork(); for(I=0; I<5; I++) { printf(“I = %d\n”, I); fflush(stdout); } exit(0); }

Fork (Cont’d) main() { int pid; pid = fork(); if ( pid == 0 ) printf(“The child process\n”); else printf(“The parent process\n”); exit(0); }

Basic I/O Functions in UNIX Open close read write lseek ioctl

Using I/O in UNIX int desc;... desc = open(“file”, O_RDWR, 0); read(desc, buffer, 128); … close(desc);

Using UNIX I/O with TCP/IP They extended the conventional UNIX I/O facilities It became possible to use file descriptors for network communication Extended the read and write system calls so they work with the new network descriptors.

Descriptor Table Internal data structure for file 0

Internal data structure for file 0 Family: PF_INET... Service: SOCK_STREAM Local IP: Remote IP: Local Port: Remote Port:

Passive/Active Socket A passive socket is used by a server to wait for an incoming connection. An active socket is used by a client to initiate a connection.

Sockets When a socket is created it does not contain information about how it will be used. TCP/IP protocols define a communication endpoint to consist of an IP address and a protocol port number.

Socket Functions socket: create a descriptor for use in network communication. connect: connect to a remote peer (used by client) write: send outgoing data across a connection. read: acquire incoming data from a connection.

Socket Functions (Cont’d) close: terminate communication and deallocate a descriptor. bind: bind a local IP address and protocol number to a socket. listen: place the socket in passive mode and set the number of incoming TCP connections the system will enqueue. accept: accept the next incoming connection (by server).

Data Conversion Functions The functions htons, ntohs, htonl, and ntohl are used to convert binary integers between the host’s native byte order and the network standard byte order. This makes the source code portable to any machine, independent of the the native byte order

socket() bind() listen() accept() read() write() procees request get a blocked client Server Process TCPUDP socket() connect() write() read() Client Process socket() bind() sendto() recvfrom() Client Process socket() bind() recvfrom() sendto() Server Process get a blocked client process request

TCP vs. UDP TCP (Transmission Control Protocol –Connection-oriented –Reliability in delivery of messages –Splitting messages into datagrams –keep track of order (or sequence) –Use checksums for detecting errors

TCP vs. UDP (Cont’d) UDP (User Datagram Protocols) –Connectionless –No attempt to fragment messages –No reassembly and synchronization –In case of error, message is retransmitted –No acknowledgment

Selecting UDP Remote procedures are idempotent Server and client messages fit completely within a packet. The server handles multiple clients (UDP is stateless)

Selecting TCP Procedures are not idempotent Reliability is a must Messages exceed UDP packet size

OSI Layers vs. TCP/IP Network Hardware Interface IP TCP UDP User Application 5-7. Session 4. Transport 3. Network 1-2. Data Link/ Physical

Client Architecture Simpler than servers Most clients do not explicitly handle concurrent interactions with multiple servers. Most client software executes as a conventional program. Clients, unlike servers, do not require special privileged ports. Most clients rely on OS for security.

Socket Address Structure struct sockaddr_in { shortsin_family; u_short sin_port; struct in_addr sin_addr; charsin_zero[8]; };

Domain Name Structure struct hostent { char *h_name;/* official name of host */ char **h_aliases;/* host’s alias names */ int h_addrtype;/* address type */ char **h_addr_list; /*list of addresses from name server */ };

Example: Char *host = “eve.kean.edu”; struct hostent *hp; … hp = gethostbyname(host); “inet_addr()” takes an ASCII string that contains a dotted decimal address and returns the equivalent IP address in binary.

Service Structure struct servent { char *s_name;/* service name */ char **s_aliases;/* alias list */ int s_port;/* port number */ char *s_proto;/* protocol to use */ };

Example: struct servent *sp;.. sp = getservbyname(“smtp”, “”tcp”); sp->s_port has the port number.

The Protocol Structure struct protoent { char *p_name;/* official protocol name */ char **p_aliases;/* allowed aliases */ int p_proto;/* official protocol number */ }; Example: struct protent *pp; pp = getprotobyname(“udp”);

ACCEPT accept(s, address, len) Used by servers to accept the next incoming connection. Returns the socket descriptor of the new socket. Used only with TCP

s, from, to: socket descriptor address: pointer to the struct sockaddr len, fromlen, tolen: size of sockaddr name: character string protocol: character string Qlen: integer buffer: character array flags: integer

BIND Bind( s, address, len ) binds a local IP and protocol number to a socket. Used by servers primarily Returns 0 if successful, or -1 in case of error.

CLOSE close(s) Terminates communication and deallocates the socket. Returns 0 or -1

CONNECT connect( s, address, len ) Used to specify the remote end point address Used with TCP and UDP Returns 0 or -1

GETHOSTBYNAME gethostbyname( name ) translates host name to an IP address. Returns a pointer to a hostent structure, if successful; otherwise it returns 0

GETPROTOBYNAME getprotobyname( name ) Translates protocol’s name to its official integer value. Returns a pointer to the protoent structure; otherwise it returns 0.

LISTEN listen( s, Qlen) It puts the socket in a receiving mode to accept incoming requests. It also sets a limit on the queue size for incoming connection requests. Returns 0 or -1.

GETSERVBYNAME getservbyname( name, protocol ) Used to map a service name to a protocol port number. Returns a pointer to a servent structure, if successful; otherwise it returns 0.

READ read( s, buffer, len) Used to get input from a socket. Returns 0 in case of error, or the number of bytes read in case of success.

RECV recv( s, buffer, len, flags ) gets the next incoming message from a socket. Flags Control bits that specify whether to receive out-of-band data and whether to look ahead for messages. Returns the number of bytes in the message, or -1 in case of error.

RECVFROM recvfrom(s, buffer, len, flags, from,fromlen) Gets the next message that arrives at a socket and records the sender’s address. Returns the number of bytes in the message, or -1 in case of error.

SELECT select(numfds, refds, wrfds, exfds, time); Provides asynchronous I/O by permitting a single process to wait for the first of any file descriptors in a specified set to become ready. The caller can also specify a maximum timeout for the wait.

SELECT (Cont’d) numfds: number of file descriptors in set refds: address of file descriptors for input wrfds: address of file descriptors for output exfds: address of file descriptors for exceptions time: maximum time to wait (or zero) Returns the number of ready file descriptors, 0 if time limit reached, or -1 for error.

SEND send( s, msg, len, flags) To transfer a message to another machine. Returns the number of characters sent, or -1 in case of error.

SENDTO sendto( s, msg, len, flags, to, tolen) To send a message using the destination structure to. Returns the number of bytes sent, or -1 in case of error.

SOCKET socket(family, type, protocol ) To create a socket. Returns the integer descriptor for the socket, or -1 in case of error.

WRITE write( s, buffer, len) write(Allows an application to transfer data to a remote machine. Returns the number of bytes transferred or - 1 in case of error.

References “Internetworking with TCP/IP, VOL III, Client-Server Programming and Applications”, D. Comer and D. Stevens, Prentice Hall