Basic socket programming

Network Programming with Sockets Reading: Stevens 3rd ed., Ch. 3-6, or 2 nd ed. Beej's Guide to Network Programming http://beej.us/guide/bgnet/

Outline Basic socket programming Concurrent communication Libnet and libcap library

Sockets process sends/receives messages to/from its socket socket analogous to door sending process shoves message out door sending process relies on transport infrastructure on other side of door which brings message to socket at receiving process Internet controlled by app developer process TCP with buffers, variables socket host or server process TCP with buffers, variables socket host or server

Client-Server Model Asymmetric Communication Client sends requests Server sends replies Server/Daemon Well-known name (e.g., IP address + port) Waits for contact Processes requests, sends replies Client Initiates contact Waits for response Client Server Client Client Client

Client-Server Communication Model Service Model Concurrent: Server processes multiple clients’ requests simultaneously Sequential: Server processes only one client’s requests at a time Hybrid: Server maintains multiple connections, but processes responses sequentially Client and server categories are not disjoint A server can be a client of another server A server can be a client at the same time Example?

TCP Service Reliable Data Transfer Guarantees delivery of all data Exactly once if no catastrophic failures Ordered Data Transfer Guarantees in-order delivery of data If A sends M1 followed by M2 to B, B never receives M2 before M1 Regulated Data Flow Monitors network and adjusts transmission appropriately Prevents senders from wasting bandwidth Reduces global congestion problems Data Transmission Full-Duplex byte stream Connection setup and teardown

UDP Services User Datagram Protocol Service Provides a thin layer over IP 16-bit port space (distinct from TCP ports) allows multiple recipients on a single host

UDP Services Unit of Transfer Datagram (variable length packet) Unreliable No guaranteed delivery Drops packets silently Unordered No guarantee of maintained order of delivery Unlimited Transmission No flow control

Byte Ordering Big Endian vs. Little Endian Little Endian (Intel, DEC): Least significant byte of word is stored in the lowest memory address Big Endian (Sun, SGI, HP): Most significant byte of word is stored in the lowest memory address Network Byte Order = Big Endian Allows both sides to communicate Must be used for some data (i.e. IP Addresses) Good form for all binary data Most significant Byte 0xAA Least significant Byte 0xBB Least significant Byte 0xBB Most significant Byte 0xAA 0x1000 0x1001 Big-Endian (Network-Byte-Order) Little-Endian Memory address

Byte Ordering Functions 16- and 32-bit conversion functions (for platform independence) Examples: int m, n; short int s,t; m = ntohl (n) net-to-host long (32-bit) translation s = ntohs (t) net-to-host short (16-bit) translation n = htonl (m) host-to-net long (32-bit) translation t = htons (s) host-to-net short (16-bit) translation

BSD Sockets BSD Sockets PF_INET sockets PF_PACKET Socket PF_NETLINK Socket TCP UDP IP Network device SOCK_ STREAM SOCK_ DGRAM SOCK_ RAW

BSD Socket Structure include/linux/net.h struct socket { socket_state state;/* SS_CONNECTED .. */ unsigned long flags; struct proto_ops *ops;/*protocols do most everything*/ struct fasync_struct *fasync_list; struct file *file; struct sock *sk; /*hold protocol specific info*/ wait_queue_head_t wait; short type;/*SOCKET_STREAM */ unsigned char passcred; }; struct sock { … struct sk_buff_head sk_receive_queue; struct sk_buff_head sk_write_queue; }

struct proto_ops { int family; struct module *owner; int (*release) (struct socket *sock); int (*bind) (struct socket *sock, struct sockaddr *myaddr, int sockaddr_len); int (*connect) (struct socket *sock, struct sockaddr *vaddr, int sockaddr_len, int flags); int (*socketpair)(struct socket *sock1, struct socket *sock2); int (*accept) (struct socket *sock, struct socket *newsock, int flags); int (*getname) (struct socket *sock, struct sockaddr *addr, int *sockaddr_len, int peer); unsigned int (*poll) (struct file *file, struct socket *sock, struct poll_table_struct *wait); int (*ioctl) (struct socket *sock, unsigned int cmd, unsigned long arg); int (*listen) (struct socket *sock, int len); int (*shutdown) (struct socket *sock, int flags); int (*setsockopt)(struct socket *sock, int level, int optname, char __user *optval, int optlen); int (*getsockopt)(struct socket *sock, int level, int optname, char __user *optval, int __user *optlen); int (*sendmsg) (struct kiocb *iocb, struct socket *sock, struct msghdr *m, size_t total_len); int (*recvmsg) (struct kiocb *iocb, struct socket *sock, struct msghdr *m, size_t total_len, int flags); … };

Socket Address Structure Socket address struct sockaddr { short sa_family; char sa_data[14]; }; Internet address: struct sockaddr_in { short sin_family; /* e.g., AF_INET */ ushort sin_port; /* TCP/UDP port */ struct in_addr sin_addr; /* IP address */ unsigned char sin_zero[8]; /* Same size as struct sockaddr */ }; IP address: struct in_addr { in_addr_t s_addr; /* 32-bit IP address */ }; all but sin_family in network byte order

Address Access/Conversion Functions All binary values are network byte ordered struct hostent* gethostbyname (const char* hostname); Translate English host name to IP address (uses DNS) struct hostent* gethostbyaddr (const char* addr, size_t len, int family); Translate IP address to English host name (not secure) Better used in combination of gethostbyname() to validate the results char* inet_ntoa (struct in_addr inaddr); Translate IP address to ASCII dotted-decimal notation (e.g., “128.32.36.37”) int gethostname (char* name, size_t namelen); Read host’s name (use with gethostbyname to find local IP) (/etc/hosts)

Structure: hostent The hostent data structure (from /usr/include/netdb. h ) canonical domain name and aliases list of addresses associated with machine also address type and length information struct hostent { char* h_name; /* official name of host */ char** h_aliases; /* NULL-terminated alias list */ int h_addrtype /* address type (AF_INET) */ int h_length; /* length of addresses (4B) */ char** h_addr_list; /* NULL-terminated address list */ #define h_addr h_addr_list[0];/* backward-compatibility */ };

Address Access/Conversion Functions in_addr_t inet_addr (const char* strptr); Translate dotted-decimal notation to IP address (Network Byte Order); returns -1 on failure, thus cannot handle broadcast value “255.255.255.255” struct sockaddr_in ina; ina.sin_addr.s_addr = inet_addr("10.12.110.57"); int inet_aton (const char *strptr, struct in_addr *inaddr); Translate dotted-decimal notation to IP address; returns 1 on success, 0 on failure struct sockaddr_in my_addr; my_addr.sin_family = AF_INET; // host byte order my_addr.sin_port = htons(MYPORT); // short, network byte order inet_aton("10.12.110.57",&(my_addr.sin_addr)); memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct

Sockets API Creation and Setup Establishing a Connection (TCP) Sending and Receiving Data Tearing Down a Connection (TCP)

Socket Functions blocks until connection from client TCP Client TCP Server socket() listen() accept() socket() bind() Well-known port connect() write() TCP three-way handshaking data (request) read() process request

Socket Functions blocks until connection from client TCP Server TCP Client read() data (reply) close() read() close() socket() connect() write() TCP three-way handshaking data (request) read() process request write()

Socket Creation and Setup Include file <sys/socket.h> Create a socket int socket (int domain, int type, int protocol); Returns file descriptor or -1. Bind a socket to a local IP address and port number int bind (int sockfd, struct sockaddr* myaddr, int addrlen); Put socket into passive state (wait for connections rather than initiate a connection). int listen (int sockfd, int backlog); Accept connections int accept (int sockfd, struct sockaddr* cliaddr, int* addrlen); Returns file descriptor or -1.

Functions: socket int socket (int domain, int type, int protocol); Create a socket. Returns file descriptor or -1. Also sets errno on failure. domain : protocol family (same as address family) PF_INET for IPv4 other possibilities: PF_INET6 (IPv6), PF_UNIX or PF_LOCAL (Unix socket), PF_ROUTE (routing) type : style of communication SOCK_STREAM for TCP (with PF_INET ) SOCK_DGRAM for UDP (with PF_INET ) protocol : protocol within family typically 0 getprotobyname(), /etc/protocols for list of protocols

Function: bind int bind (int sockfd, struct sockaddr* myaddr, int addrlen); Bind a socket to a local IP address and port number. Returns 0 on success, -1 and sets errno on failure. sockfd : socket file descriptor (returned from socket ) myaddr : includes IP address and port number IP address: set by kernel if value passed is INADDR_ANY , else set by caller port number: set by kernel if value passed is 0, else set by caller addrlen : length of address structure = sizeof (struct sockaddr_in) socket.socket_state = TCP_CLOSE;

TCP and UDP Ports Allocated and assigned by the Internet Assigned Numbers Authority see RFC 1700 or ftp://ftp.isi.edu/in-notes/iana/assignments/port-numbers private/ephemeral ports 49152-65535 registered services/ephemeral ports 1024-49151 registered and controlled, also used for identity verification super-user only 513-1023 standard services (see /etc/services ) super-user only 1-512

Functions: listen int listen (int sockfd, int backlog); Put socket into passive state (wait for connections rather than initiate a connection). Returns 0 on success, -1 and sets errno on failure. sockfd : socket file descriptor (returned from socket ) backlog : bound on length of unaccepted connection queue (connection backlog); kernel will cap, thus better to set high socket.socket_state = TCP_LISTEN;

Functions: accept int accept (int sockfd, struct sockaddr* cliaddr, int* addrlen); Accept a new connection. Returns file descriptor or -1. Also sets errno on failure. sockfd : socket file descriptor (returned from socket ) cliaddr : IP address and port number of client (returned from call) addrlen : length of address structure = pointer to int set to sizeof (struct sockaddr_in) addrlen is a value-result argument: the caller passes the size of the address structure, the kernel returns the size of the client’s address (the number of bytes written)

Accept (cont'd) A new socket was cloned from the listening socket If there are no incoming connection to accept Non-Blocking—accept operation failed and throw away the new socket Blocking—accept operation was added to the wait queue (default)

TCP Connection Setup client server socket socket connect bind listen accept connect completes connection added to incomplete queue connection moved to complete queue Synchronize (SYN) J SYN K, acknowledge (ACK) J+1 ACK K+1

server #include <stdio.h> #include <stdlib.h> #include <errno.h> #include <string.h> #include <sys/types.h> #include <netinet/in.h> #include <sys/socket.h> #include <sys/wait.h> #define PORT 3490 #define BACKLOG 10 /* how many pending connections queue will hold */

server main() { int sockfd, new_fd; /* listen on sock_fd, new connection on new_fd */ struct sockaddr_in my_addr; /* my address */ struct sockaddr_in their_addr; /* connector addr */ int sin_size; if ((sockfd = socket(PF_INET, SOCK_STREAM, 0))==-1){ perror("socket"); exit(1); }

server my_addr.sin_family = AF_INET; /* host byte order */ my_addr.sin_port = htons(MYPORT); /* short, network byte order */ my_addr.sin_addr.s_addr = htonl(INADDR_ANY); /* INADDR_ANY allows clients to connect to any one of the host’s IP address */ bzero(&(my_addr.sin_zero), 8); /* zero the struct */ if (bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr)) == -1) { perror("bind"); exit(1); }

server if (listen(sockfd, BACKLOG) == -1) { perror("listen"); exit(1); } while(1) { /* main accept() loop */ sin_size = sizeof(struct sockaddr_in); if ((new_fd = accept(sockfd, (struct sockaddr*) &their_addr,&sin_size)) == -1) { perror("accept"); continue; } printf("server: got connection from %s\n", inet_ntoa(their_addr.sin_addr));

Establishing a Connection Include file <sys/socket.h> int connect (int sockfd, struct sockaddr* servaddr, int addrlen); Connect to another socket. Returns 0 on success, -1 and sets errno on failure. sockfd : socket file descriptor (returned from socket ) servaddr : IP address and port number of server addrlen : length of address structure = sizeof (struct sockaddr_in)

Connect Before connecting, socket.socket_state = SS_UNCONNECTED; Add the sock to tcp_listening_hash waiting for server’s response

client if ((sockfd = socket (PF_INET, SOCK_STREAM, 0)) == -1) { perror (“socket”); exit (1); } their_addr.sin_family = AF_INET; /* interp’d by host */ their_addr.sin_port = htons (PORT); their_addr.sin_addr = *((struct in_addr*)he->h_addr); bzero (&(their_addr.sin_zero), 8); /* zero rest of struct */ if (connect (sockfd, (struct sockaddr*)&their_addr, sizeof (struct sockaddr)) == -1) { perror (“connect”); exit (1); }

Sending and Receiving Data Write/Read data to/from a stream (TCP) or “connected” datagram (UDP) socket. int write (int sockfd, char* buf, size_t nbytes); int read (int sockfd, char* buf, size_t nbytes); Write/Read a datagram to/from a UDP socket. int sendto (int sockfd, char* buf, size_t nbytes, int flags, struct sockaddr* destaddr , int addrlen); int recvfrom (int sockfd, char* buf, size_t nbytes, int flags, struct sockaddr* srcaddr , int* addrlen);

Functions: write int write (int sockfd, char* buf, size_t nbytes); Write data to a stream (TCP) or “connected” datagram (UDP) socket. Returns number of bytes written or -1. Also sets errno on failure. sockfd : socket file descriptor (returned from socket ) buf : data buffer nbytes : number of bytes to try to write Some reasons for failure or partial writes: process received interrupt or signal kernel resources unavailable (e.g ., buffers)

Functions: read int read (int sockfd, char* buf, size_t nbytes); Read data from a stream (TCP) or “connected” datagram (UDP) socket. Returns number of bytes read or -1. Also sets errno on failure. Returns 0 if socket closed. sockfd : socket file descriptor (returned from socket ) buf : data buffer nbytes : number of bytes to try to read

Tearing Down a Connection int close (int sockfd); Close a socket. Returns 0 on success, -1 and sets errno on failure. int shutdown (int sockfd, int howto); Force termination of communication across a socket in one or both directions. Returns 0 on success, -1 and sets errno on failure.

Functions: close int close (int sockfd); Close a socket. Returns 0 on success, -1 and sets errno on failure. sockfd : socket file descriptor (returned from socket ) Closes communication on socket in both directions. All data sent before close are delivered to other side (although this aspect can be overridden). After close , sockfd is not valid for reading or writing.

Functions: shutdown int shutdown (int sockfd, int howto); Force termination of communication across a socket in one or both directions. Returns 0 on success, -1 and sets errno on failure. sockfd : socket file descriptor (returned from socket ) howto : SHUT_RD to stop reading SHUT_WR to stop writing SHUT_RDWR to stop both

UDP Connection Example client server socket socket sendto bind recvfrom sendto recvfrom close

Functions: sendto int sendto (int sockfd, char* buf, size_t nbytes, int flags, struct sockaddr* destaddr, int addrlen); Send a datagram to another UDP socket. Returns number of bytes written or -1. Also sets errno on failure. sockfd : socket file descriptor (returned from socke t) buf : data buffer nbytes : number of bytes to try to read flags : see man page for details; typically use 0 destaddr : IP address and port number of destination socket addrlen : length of address structure = sizeof (struct sockaddr_in)

Functions: recvfrom int recvfrom (int sockfd, char* buf, size_t nbytes, int flags, struct sockaddr* srcaddr, int* addrlen); Read a datagram from a UDP socket. Returns number of bytes read (0 is valid) or -1. Also sets errno on failure. sockfd : socket file descriptor (returned from socket ) buf : data buffer nbytes : number of bytes to try to read flags : see man page for details; typically use 0 srcaddr : IP address and port number of sending socket (returned from call) addrlen : length of address structure = pointer to int set to sizeof (struct sockaddr_in)

Basic socket programming

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