Application Layer and Socket Programming

15-441: Computer Networking Lecture 3: Application Layer and Socket Programming

Lecture Overview Application layer Client-server Application requirements Background TCP vs. UDP Byte ordering Socket I/O TCP/UDP server and client I/O multiplexing

Applications and Application-Layer Protocols Application: communicating, distributed processes Running in network hosts in “user space” Exchange messages to implement app e.g., email, file transfer, the Web Application-layer protocols One “piece” of an app Define messages exchanged by apps and actions taken User services provided by lower layer protocols application transport network data link physical application transport network data link physical application transport network data link physical

Client-Server Paradigm Typical network app has two pieces: client and server Client: Initiates contact with server (“speaks first”) Typically requests service from server, For Web, client is implemented in browser; for e-mail, in mail reader Server: Provides requested service to client e.g., Web server sends requested Web page, mail server delivers e-mail application transport network data link physical application transport network data link physical request reply

Ftp: The File Transfer Protocol Transfer file to/from remote host Client/server model Client: side that initiates transfer (either to/from remote) Server: remote host ftp: RFC 959 ftp server: port 21 file transfer remote file system user at host FTP server FTP user interface FTP client local file system

Ftp: Separate Control, Data Connections Ftp client contacts ftp server at port 21, specifying TCP as transport protocol Two parallel TCP connections opened: Control: exchange commands, responses between client, server. “ out of band control” Data: file data to/from server Ftp server maintains “state”: current directory, earlier authentication FTP client FTP server TCP control connection port 21 TCP data connection port 20

Ftp Commands, Responses Sample Commands: sent as ASCII text over control channel USER username PASS password LIST return list of files in current directory RETR filename retrieves (gets) file STOR filename stores (puts) file onto remote host Sample Return Codes status code and phrase 331 Username OK, password required 125 data connection already open; transfer starting 425 Can’t open data connection 452 Error writing file

What Transport Service Does an Application Need? Data loss Some apps (e.g., audio) can tolerate some loss Other apps (e.g., file transfer, telnet) require 100% reliable data transfer Timing Some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” Bandwidth Some apps (e.g., multimedia) require minimum amount of bandwidth to be “effective” Other apps (“elastic apps”) make use of whatever bandwidth they get

Transport Service Requirements of Common Apps no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss elastic elastic elastic audio: 5Kb-1Mb video:10Kb-5Mb same as above few Kbps elastic no no no yes, 100’s msec yes, few secs yes, 100’s msec yes and no file transfer e-mail web documents real-time audio/ video stored audio/video interactive games financial apps Application Data loss Bandwidth Time Sensitive

Server and Client TCP/UDP IP Ethernet Adapter Server TCP/UDP IP Ethernet Adapter Clients Server and Client exchange messages over the network through a common Socket API Socket API hardware kernel space user space ports

User Datagram Protocol(UDP): An Analogy Postal Mail Single mailbox to receive messages Unreliable  Not necessarily in-order delivery Each letter is independent Must address each reply Example UDP applications Multimedia, voice over IP UDP Single socket to receive messages No guarantee of delivery Not necessarily in-order delivery Datagram – independent packets Must address each packet Postal Mail Single mailbox to receive letters Unreliable  Not necessarily in-order delivery Letters sent independently Must address each reply

Transmission Control Protocol (TCP): An Analogy TCP Reliable – guarantee delivery Byte stream – in-order delivery Connection-oriented – single socket per connection Setup connection followed by data transfer Telephone Call Guaranteed delivery In-order delivery Connection-oriented Setup connection followed by conversation Example TCP applications Web, Email, Telnet

Network Addressing Analogy 412-268-8000 ext.123 Central Number Applications/Servers Web Port 80 Mail Port 25 Exchange Area Code 412-268-8000 ext.654 IP Address Network No. Host Number Telephone No 15-441 Students Clients Professors at CMU Network Programming Telephone Call Port No. Extension

Concept of Port Numbers Port numbers are used to identify “entities” on a host Port numbers can be Well-known (port 0-1023) Dynamic or private (port 1024-65535) Servers/daemons usually use well-known ports Any client can identify the server/service HTTP = 80, FTP = 21, Telnet = 23, ... /etc/service defines well-known ports Clients usually use dynamic ports Assigned by the kernel at run time TCP/UDP IP Ethernet Adapter NTP daemon Web server port 123 port 80

Names and Addresses Each attachment point on Internet is given unique address Based on location within network – like phone numbers Humans prefer to deal with names not addresses DNS provides mapping of name to address Name based on administrative ownership of host

Internet Addressing Data Structure sin_family = AF_INET selects Internet address family #include < netinet/in.h > /* Internet address structure */ struct in_addr { u_long s_addr ; /* 32-bit IPv4 address */ }; /* network byte ordered */ /* Socket address, Internet style. */ struct sockaddr_in { u_char sin_family ; /* Address Family */ u_short sin_port ; /* UDP or TCP Port# */ /* network byte ordered */ struct in_addr sin_addr ; /* Internet Address */ char sin_zero[8]; /* unused */ };

Byte Ordering Big Endian Sun Solaris, PowerPC, ... Little Endian i386, alpha, ... Network byte order = Big Endian 128 2 194 95 union { u_int32_t addr; /* 4 bytes address */ char c[4]; } un; /* 128.2.194.95 */ un.addr = 0x8002c25f; /* c[0] = ? */ c[0] c[1] c[2] c[3] 95 194 2 128

Byte Ordering Functions Converts between host byte order and network byte order ‘ h’ = host byte order ‘ n’ = network byte order ‘ l’ = long (4 bytes), converts IP addresses ‘ s’ = short (2 bytes), converts port numbers #include <netinet/in.h> unsigned long int htonl (unsigned long int hostlong); unsigned short int htons (unsigned short int hostshort); unsigned long int ntohl (unsigned long int netlong); unsigned short int ntohs (unsigned short int netshort);

A socket is a file descriptor that lets an application read/write data from/to the network socket returns an integer (socket descriptor) fd < 0 indicates that an error occurred socket descriptors are similar to file descriptors AF_INET: associates a socket with the Internet protocol family SOCK_STREAM: selects the TCP protocol SOCK_DGRAM: selects the UDP protocol What is a Socket? int fd; /* socket descriptor */ if ((fd = socket ( AF_INET , SOCK_STREAM , 0)) < 0) } perror(“socket”); exit(1); }

For example: web server What does a web server need to do so that a web client can connect to it? TCP IP Ethernet Adapter Web Server Port 80 TCP Server

Since web traffic uses TCP, the web server must create a socket of type SOCK_STREAM int fd; /* socket descriptor */ if((fd = socket ( AF_INET , SOCK_STREAM , 0)) < 0) { perror(“socket”); exit(1); } socket returns an integer ( socket descriptor ) fd < 0 indicates that an error occurred AF_INET associates a socket with the Internet protocol family SOCK_STREAM selects the TCP protocol Socket I/O: socket()

A socket can be bound to a port int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by bind() */ /* create the socket */ srv.sin_family = AF_INET ; /* use the Internet addr family */ srv.sin_port = htons ( 80 ); /* bind socket ‘fd’ to port 80*/ /* bind: a client may connect to any of my addresses */ srv.sin_addr.s_addr = htonl ( INADDR_ANY ); if( bind (fd, (struct sockaddr*) &srv, sizeof(srv)) < 0) { perror("bind"); exit(1); } Still not quite ready to communicate with a client... Socket I/O: bind()

Socket I/O: listen() listen indicates that the server will accept a connection int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by bind() */ /* 1) create the socket */ /* 2) bind the socket to a port */ if( listen (fd, 5) < 0) { perror(“listen”); exit(1); } Still not quite ready to communicate with a client...

Socket I/O: accept() accept blocks waiting for a connection int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by bind() */ struct sockaddr_in cli; /* used by accept() */ int newfd; /* returned by accept() */ int cli_len = sizeof(cli); /* used by accept() */ /* 1) create the socket */ /* 2) bind the socket to a port */ /* 3) listen on the socket */ newfd = accept (fd, (struct sockaddr*) &cli, &cli_len); if(newfd < 0) { perror("accept"); exit(1); } accept returns a new socket ( newfd ) with the same properties as the original socket ( fd ) newfd < 0 indicates that an error occurred

Socket I/O: accept() continued... struct sockaddr_in cli; /* used by accept() */ int newfd; /* returned by accept() */ int cli_len = sizeof(cli); /* used by accept() */ newfd = accept (fd, (struct sockaddr*) &cli, &cli_len); if(newfd < 0) { perror("accept"); exit(1); } How does the server know which client it is? cli.sin_addr.s_addr contains the client’s IP address cli.sin_port contains the client’s port number Now the server can exchange data with the client by using read and write on the descriptor newfd . Why does accept need to return a new descriptor?

Socket I/O: read() read can be used with a socket read blocks waiting for data from the client but does not guarantee that sizeof(buf) is read int fd; /* socket descriptor */ char buf[512]; /* used by read() */ int nbytes; /* used by read() */ /* 1) create the socket */ /* 2) bind the socket to a port */ /* 3) listen on the socket */ /* 4) accept the incoming connection */ if((nbytes = read (newfd, buf, sizeof(buf))) < 0) { perror(“read”); exit(1); }

TCP Client For example: web client How does a web client connect to a web server ? TCP IP Ethernet Adapter 2 Web Clients

Dealing with IP Addresses IP Addresses are commonly written as strings (“128.2.35.50”), but programs deal with IP addresses as integers. struct sockaddr_in srv; srv.sin_addr.s_addr = inet_addr(“128.2.35.50”); if(srv.sin_addr.s_addr == (in_addr_t) -1) { fprintf(stderr, "inet_addr failed!\n"); exit(1); } Converting a numerical address to a string: struct sockaddr_in srv; char *t = inet_ntoa(srv.sin_addr); if(t == 0) { fprintf(stderr, “inet_ntoa failed!\n”); exit(1); } Converting strings to numerical address:

Translating Names to Addresses Gethostbyname provides interface to DNS Additional useful calls Gethostbyaddr – returns hostent given sockaddr_in Getservbyname Used to get service description (typically port number) Returns servent based on name #include <netdb.h> struct hostent *hp; /*ptr to host info for remote*/ struct sockaddr_in peeraddr; char *name = “www.cs.cmu.edu”; peeraddr.sin_family = AF_INET; hp = gethostbyname(name) peeraddr.sin_addr.s_addr = ((struct in_addr*)(hp->h_addr))->s_addr;

Socket I/O: connect() connect allows a client to connect to a server... int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by connect() */ /* create the socket */ /* connect: use the Internet address family */ srv.sin_family = AF_INET ; /* connect: socket ‘fd’ to port 80 */ srv.sin_port = htons( 80 ); /* connect: connect to IP Address “128.2.35.50” */ srv.sin_addr.s_addr = inet_addr (“ 128.2.35.50 ”); if( connect (fd, (struct sockaddr*) &srv, sizeof(srv)) < 0) { perror(”connect"); exit(1); }

Socket I/O: write() write can be used with a socket int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by connect() */ char buf[512]; /* used by write() */ int nbytes; /* used by write() */ /* 1) create the socket */ /* 2) connect() to the server */ /* Example: A client could “write” a request to a server */ if((nbytes = write (fd, buf, sizeof(buf))) < 0) { perror(“write”); exit(1); }

Review: TCP Client-Server Interaction socket() bind() listen() accept() write() read() read() TCP Server close() socket() TCP Client connect() write() read() close() connection establishment data request data reply end-of-file notification from UNIX Network Programming Volume 1, figure 4.1

UDP Server Example For example: NTP daemon What does a UDP server need to do so that a UDP client can connect to it? UDP IP Ethernet Adapter NTP daemon Port 123

Socket I/O: socket() The UDP server must create a datagram socket… int fd; /* socket descriptor */ if((fd = socket (AF_INET, SOCK_DGRAM , 0)) < 0) { perror(“socket”); exit(1); } socket returns an integer ( socket descriptor ) fd < 0 indicates that an error occurred AF_INET: associates a socket with the Internet protocol family SOCK_DGRAM: selects the UDP protocol

Socket I/O: bind() A socket can be bound to a port int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by bind() */ /* create the socket */ /* bind: use the Internet address family */ srv.sin_family = AF_INET ; /* bind: socket ‘fd’ to port 80*/ srv.sin_port = htons( 80 ); /* bind: a client may connect to any of my addresses */ srv.sin_addr.s_addr = htonl( INADDR_ANY ); if( bind (fd, (struct sockaddr*) &srv, sizeof(srv)) < 0) { perror("bind"); exit(1); } Now the UDP server is ready to accept packets…

Socket I/O: recvfrom() read does not provide the client’s address to the UDP server int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by bind() */ struct sockaddr_in cli; /* used by recvfrom() */ char buf[512]; /* used by recvfrom() */ int cli_len = sizeof(cli); /* used by recvfrom() */ int nbytes; /* used by recvfrom() */ /* 1) create the socket */ /* 2) bind to the socket */ nbytes = recvfrom (fd, buf, sizeof(buf), 0 /* flags */, (struct sockaddr*) &cli, &cli_len); if(nbytes < 0) { perror(“recvfrom”); exit(1); }

Socket I/O: recvfrom() continued... nbytes = recvfrom (fd, buf, sizeof(buf), 0 /* flags */, (struct sockaddr*) cli, &cli_len); The actions performed by recvfrom returns the number of bytes read ( nbytes ) copies nbytes of data into buf returns the address of the client ( cli ) returns the length of cli ( cli_len ) don’t worry about flags

UDP Client Example How does a UDP client communicate with a UDP server ? TCP IP Ethernet Adapter 2 UDP Clients ports

Socket I/O: sendto() write is not allowed Notice that the UDP client does not bind a port number a port number is dynamically assigned when the first sendto is called int fd; /* socket descriptor */ struct sockaddr_in srv; /* used by sendto() */ /* 1) create the socket */ /* sendto: send data to IP Address “128.2.35.50” port 80 */ srv.sin_family = AF_INET ; srv.sin_port = htons( 80 ); srv.sin_addr.s_addr = inet_addr(“ 128.2.35.50 ”); nbytes = sendto (fd, buf, sizeof(buf), 0 /* flags */, (struct sockaddr*) &srv, sizeof(srv)); if(nbytes < 0) { perror(“sendto”); exit(1); }

Review: UDP Client-Server Interaction socket() bind() recvfrom() sendto() UDP Server socket() UDP Client sendto() recvfrom() close() blocks until datagram received from a client data request data reply from UNIX Network Programming Volume 1, figure 8.1

The UDP Server How can the UDP server service multiple ports simultaneously? UDP IP Ethernet Adapter UDP Server Port 2000 Port 3000

UDP Server: Servicing Two Ports What problems does this code have? int s1; /* socket descriptor 1 */ int s2; /* socket descriptor 2 */ /* 1) create socket s1 */ /* 2) create socket s2 */ /* 3) bind s1 to port 2000 */ /* 4) bind s2 to port 3000 */ while(1) { recvfrom (s1, buf, sizeof(buf), ...); /* process buf */ recvfrom (s2, buf, sizeof(buf), ...); /* process buf */ }

Socket I/O: select() maxfds : number of descriptors to be tested descriptors (0, 1, ... maxfds-1) will be tested readfds : a set of fds we want to check if data is available returns a set of fds ready to read if input argument is NULL , not interested in that condition writefds : returns a set of fds ready to write exceptfds : returns a set of fds with exception conditions int select(int maxfds , fd_set * readfds , fd_set * writefds , fd_set * exceptfds , struct timeval *timeout); FD_CLR(int fd, fd_set *fds); /* clear the bit for fd in fds */ FD_ISSET(int fd, fd_set *fds); /* is the bit for fd in fds ? */ FD_SET(int fd, fd_set *fds); /* turn on the bit for fd in fds */ FD_ZERO(fd_set *fds); /* clear all bits in fds */

Socket I/O: select() timeout if NULL, wait forever and return only when one of the descriptors is ready for I/O otherwise, wait up to a fixed amount of time specified by timeout if we don’t want to wait at all, create a timeout structure with timer value equal to 0 Refer to the man page for more information int select(int maxfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval * timeout ); struct timeval { long tv_sec; /* seconds / long tv_usec; /* microseconds */ }

Socket I/O: select() int s1, s2; /* socket descriptors */ fd_set readfds; /* used by select() */ /* create and bind s1 and s2 */ while(1) { FD_ZERO (&readfds); /* initialize the fd set */ FD_SET (s1, &readfds); /* add s1 to the fd set */ FD_SET(s2, &readfds); /* add s2 to the fd set */ if( select (s2+1, &readfds, 0, 0, 0) < 0) { perror(“select”); exit(1); } if( FD_ISSET (s1, &readfds)) { recvfrom (s1, buf, sizeof(buf), ...); /* process buf */ } /* do the same for s2 */ } select allows synchronous I/O multiplexing

TCP IP Ethernet Adapter Web Server Port 80 How can a a web server manage multiple connections simultaneously? Port 8001 More Details About a Web Server

Socket I/O: select() Now the web server can support multiple connections... int fd, next=0; /* original socket */ int newfd[10]; /* new socket descriptors */ while(1) { fd_set readfds; FD_ZERO (&readfds); FD_SET (fd, &readfds); /* Now use FD_SET to initialize other newfd’s that have already been returned by accept() */ select ( maxfd +1, &readfds, 0, 0, 0); if( FD_ISSET (fd, &readfds)) { newfd[ next++ ] = accept (fd, ...); } /* do the following for each descriptor newfd[ n ] */ if( FD_ISSET (newfd[ n ], &readfds)) { read (newfd[ n ], buf, sizeof(buf)); /* process data */ } }

A Few Programming Notes: Representing Packets 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Length | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type: 4-byte integer Length: 2-byte integer Checksum: 2-byte integer Address: 4-byte IP address

A Few Programming Notes: Building a Packet in a Buffer struct packet { u_int32_t type; u_int16_t length; u_int16_t checksum; u_int32_t address; }; /* ================================================== */ char buf[1024]; struct packet *pkt; pkt = (struct packet*) buf; pkt->type = htonl (1); pkt->length = htons (2); pkt->checksum = htons (3); pkt->address = htonl (4);

Socket Programming References Man page usage: man <function name> Textbook Sections 2.6, 2.7 demo programs written in Java Unix Network Programming : Networking APIs: Sockets and XTI (Volume 1) Section 2, 3, 4, 6, 8 ultimate socket programming bible!

Application Layer and Socket Programming

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Application Layer and Socket Programming