1. Lecture 15 Overview

2. Network API Application Programming Interface
Services that provide the interface between application and protocol software
often by the operating system
Application
Network API
Protocol A
Protocol B
Protocol C
CPE 401/601 Lecture 15 : Socket Programming

3. Network API wish list Generic Programming Interface
Support multiple communication protocol suites (families)
Address (endpoint) representation independence
Provide special services for Client and Server?
Support for message oriented and connection oriented communication
Work with existing I/O services
when this makes sense
Operating System independence
CPE 401/601 Lecture 15 : Socket Programming

4. Socket
A socket is an abstract representation of a communication endpoint
Sockets work with Unix I/O services just like files, pipes & FIFOs
Sockets needs to
establishing a connection
specifying communication endpoint addresses
CPE 401/601 Lecture 15 : Socket Programming

5. Creating a Socket int socket(int family, int type, int proto);
family specifies the protocol family
PF_INET for TCP/IP
type specifies the type of service
SOCK_STREAM, SOCK_DGRAM
protocol specifies the specific protocol
usually 0, which means the default
CPE 401/601 Lecture 15 : Socket Programming

6. socket()
The socket() system call returns a socket descriptor (small integer) or -1 on error
socket() allocates resources needed for a communication endpoint
but it does not deal with endpoint addressing
CPE 401/601 Lecture 15 : Socket Programming

7. Specifying an Endpoint Address
Sockets API is generic
There must be a generic way to specify endpoint addresses
TCP/IP requires an IP address and a port number for each endpoint address
Other protocol suites (families) may use other schemes
CPE 401/601 Lecture 15 : Socket Programming

8. sockaddr sockaddr_in6 sockaddr sockaddr_in sa_len length AF_INET6
sa_family
sa_data
length
AF_INET
port
addr
zero
sockaddr
sockaddr_in
sockaddr_in6
AF_INET6
Flow-label
Scope ID
28 bytes
variable
16 bytes
CPE 401/601 Lecture 15 : Socket Programming

9. struct sockaddr_in (IPv4)
struct sockaddr_in { uint8_t sin_len; sa_family_t sin_family; in_port_t sin_port; struct in_addr sin_addr; char sin_zero[8]; }; struct in_addr { in_addr_t s_addr;
Length of structure (16)
AF_INET
16 bit
Port number
Make structure 16 bytes
32 bit
IPv4 address
CPE 401/601 Lecture 15 : Socket Programming

10. struct sockaddr_in (IPv6)
struct sockaddr_in6 { uint8_t sin6_len; sa_family_t sin6_family; in_port_t sin6_port; uint32_t sin6_flowinfo; struct in6_addr sin6_addr; uint32_t sin6_scope_id; }; struct in6_addr { uint8_t s6_addr[16];
Length of structure (28)
AF_INET6
Port number
Flow label
Scope of address
128 bit
IPv6 address
CPE 401/601 Lecture 15 : Socket Programming

11. Network Byte Order
All values stored in a sockaddr_in must be in network byte order.
sin_port a TCP/IP port number
sin_addr an IP address
uint16_t htons(uint16_t);
uint16_t ntohs(uint_16_t);
uint32_t htonl(uint32_t);
uint32_t ntohl(uint32_t);
CPE 401/601 Lecture 15 : Socket Programming

12. TCP/IP Addresses
We don’t need to deal with sockaddr structures since we will only deal with a real protocol family.
We can use sockaddr_in structures
BUT: The C functions that make up the sockets API expect structures of type sockaddr
CPE 401/601 Lecture 15 : Socket Programming

13. Assigning an address to a socket
The bind() system call is used to assign an address to an existing socket.
int bind( int sockfd,
const struct sockaddr *myaddr,
int addrlen);
You can give bind() a sockaddr_in structure:
int bind( mysock,
(struct sockaddr*) &myaddr,
sizeof(myaddr) );
CPE 401/601 Lecture 15 : Socket Programming

14. bind() Example
int mysock,err; struct sockaddr_in myaddr; mysock = socket(PF_INET,SOCK_STREAM,0); myaddr.sin_family = AF_INET; myaddr.sin_port = htons( portnum ); myaddr.sin_addr = htonl( ipaddress); err=bind(mysock, (sockaddr *) &myaddr, sizeof(myaddr));
CPE 401/601 Lecture 15 : Socket Programming

15. Port schmort - who cares ?
Clients typically don’t care what port they are assigned
When you call bind you can tell it to assign you any available port:
myaddr.port = htons(0);
CPE 401/601 Lecture 15 : Socket Programming

16. What is my IP address ?
How can you find out what your IP address is so you can tell bind() ?
There is no realistic way for you to know the right IP address to give bind()
what if the computer has multiple network interfaces?
specify the IP address as: INADDR_ANY, this tells the OS to take care of things.
CPE 401/601 Lecture 15 : Socket Programming

17. IPv4 Address Conversion
int inet_aton(char *, struct in_addr *);
Convert ASCII dotted-decimal IP address to network byte ordered 32 bit value.
Returns 1 on success, 0 on failure.
char *inet_ntoa(struct in_addr);
Convert network byte ordered value to ASCII dotted-decimal (a string).
CPE 401/601 Lecture 15 : Socket Programming

18. IPv4 & IPv6 Address Conversion
int inet_pton(int, const char*, void*);
(family, string_ptr, address_ptr)
Convert IP address string to network byte ordered 32 or 128 bit value
1 on success, -1 on failure, 0 on invalid input
char *inet_ntop(int, const void*, char*, size_t);
(family, address_ptr, string_ptr, length)
Convert network byte ordered value to IP address string
x:x:x:x:x:x:x:x or x:x:x:x:x:x:a.b.c.d
CPE 401/601 Lecture 15 : Socket Programming

19. Other socket system calls
General Use
read()
write()
close()
Connection-oriented (TCP)
connect()
listen()
accept()
Connectionless (UDP)
send()
recv()
CPE 401/601 Lecture 15 : Socket Programming

20. Lecture 16 TCP and UDP Sockets
CPE 401 / 601
Computer Network Systems
slides are modified from Dave Hollinger
slides are modified from Dave Hollinger

21. TCP Sockets Programming
Creating a passive mode (server) socket
Establishing an application-level connection
send/receive data
Terminating a connection
CPE 401/601 Lecture 16 : TCP Socket Programming

22. TCP state diagram

23. Creating a TCP socket family: AF_INET, AF_INET6, AF_LOCAL, …
int socket(int family,int type,int proto);
family: AF_INET, AF_INET6, AF_LOCAL, …
type: SOCK_STREAM, SOCK_DGRAM, SOCK_SEQPACKET, SOCK_RAW
protocol: IPPROTO_TCP, IPPROTO_UDP, IPPROTO_SCTP
int sock;
sock = socket(PF_INET, SOCK_STREAM, 0);
if (sock<0) { /* ERROR */ }
CPE 401/601 Lecture 16 : TCP Socket Programming

24. Binding to well known address
int bind(int sockfd, const struct sockaddr *myaddr, socklen_t addrlen); int mysock; struct sockaddr_in myaddr; mysock = socket(PF_INET,SOCK_STREAM,0); myaddr.sin_family = AF_INET; myaddr.sin_port = htons( 80 ); myaddr.sin_addr = htonl( INADDR_ANY ); bind(mysock, (sockaddr *) &myaddr, sizeof(myaddr));
CPE 401/601 Lecture 16 : TCP Socket Programming

25. Establishing a passive mode TCP socket
Address already determined
Tell the kernel to accept incoming connection requests directed at the socket address
3-way handshake
Tell the kernel to queue incoming connections for us
CPE 401/601 Lecture 16 : TCP Socket Programming

26. listen() sockfd is the TCP socket
int listen(int sockfd, int backlog);
sockfd is the TCP socket
already bound to an address
backlog is the number of incoming connections the kernel should be able to keep track of (queue for us)
Sum of incomplete and completed queues
CPE 401/601 Lecture 16 : TCP Socket Programming

27. Accepting an incoming connection
Once we call listen(), the O.S. will queue incoming connections
Handles the 3-way handshake
Queues up multiple connections
When our application is ready to handle a new connection
we need to ask the O.S. for the next connection
CPE 401/601 Lecture 16 : TCP Socket Programming

28. accept() sockfd is the passive mode TCP socket
int accept( int sockfd,
struct sockaddr* cliaddr,
socklen_t *addrlen);
sockfd is the passive mode TCP socket
initiated by socket(), bind(), and listen()
cliaddr is a pointer to allocated space
addrlen is a value-result argument
must be set to the size of cliaddr
on return, will be set to be the number of used bytes in cliaddr
CPE 401/601 Lecture 16 : TCP Socket Programming

29. accept() return value
accept() returns a new socket descriptor (small positive integer) or -1 on error
After accept returns a new socket descriptor, I/O can be done using the read() and write() system calls
read() and write() operate a little differently on sockets!
vs. file operation!
CPE 401/601 Lecture 16 : TCP Socket Programming

30. Terminating a TCP connection
int close(int sockfd);
Either end of the connection can call the close() system call
What if there is data being sent?
If the other end has closed the connection, and there is no buffered data, reading from a TCP socket returns 0 to indicate EOF.
CPE 401/601 Lecture 16 : TCP Socket Programming

31. Client Code TCP clients can call connect() which:
takes care of establishing an endpoint address for the client socket
Attempts to establish a connection to the specified server
3-way handshake
no need to call bind first, the O.S. will take care of assigning the local endpoint address
TCP port number, IP address
CPE 401/601 Lecture 16 : TCP Socket Programming

32. connect() sockfd is an already created TCP socket
int connect( int sockfd,
const struct sockaddr *server,
socklen_t addrlen);
sockfd is an already created TCP socket
server contains the address of the server
connect() returns 0 if OK, -1 on error
No response to SYN segment (3 trials)
RST signal
ICMP destination unreachable (3 trials)
CPE 401/601 Lecture 16 : TCP Socket Programming

33. Reading from a TCP socket
int read(int fd, char *buf, int max);
By default read() will block until data is available
reading from a TCP socket may return less than max bytes
whatever is available
You must be prepared to read data 1 byte at a time!
CPE 401/601 Lecture 16 : TCP Socket Programming

34. Writing to a TCP socket
int write(int fd, char *buf, int num);
write might not be able to write all num bytes on a nonblocking socket
readn(), writen() and readline() functions
CPE 401/601 Lecture 16 : TCP Socket Programming

35. Metaphor for Good Relationships
To succeed in relationships*:
you need to establish your own identity.
you need to be open & accepting.
you need to establish contacts.
you need to take things as they come, not as you expect them.
you need to handle problems as they arise.
bind()
accept()
connect()
read might return 1 byte
check for errors
*Copyright Dr. Laura’s Network Programming Corp.
CPE 401/601 Lecture 16 : TCP Socket Programming

36. UDP Sockets

37. UDP Sockets Programming
Creating UDP sockets
Client
Server
Sending data
Receiving data
Connected Mode
CPE 401/601 Lecture 16 : UDP Socket Programming

38. Creating a UDP socket
int socket(int family, int type, int proto); int sock; sock = socket(PF_INET, SOCK_DGRAM, 0); if (sock<0) { /* ERROR */ }
CPE 401/601 Lecture 16 : UDP Socket Programming

39. Binding to well known address
typically done by server only
int mysock;
struct sockaddr_in myaddr;
Mysock=socket(PF_INET,SOCK_DGRAM,0);
myaddr.sin_family = AF_INET;
myaddr.sin_port = htons(1234);
myaddr.sin_addr = htonl(INADDR_ANY);
bind(mysock, &myaddr,
sizeof(myaddr));
CPE 401/601 Lecture 16 : UDP Socket Programming

40. Sending UDP Datagrams sockfd is a UDP socket
ssize_t sendto( int sockfd, void *buff,
size_t nbytes, int flags,
const struct sockaddr* to,
socklen_t addrlen);
sockfd is a UDP socket
buff is the address of the data (nbytes long)
to is the destination address
Return value is the number of bytes sent,
or -1 on error.
CPE 401/601 Lecture 16 : UDP Socket Programming

41. sendto()
The return value of sendto() indicates how much data was accepted by the O.S. for sending as a datagram
not how much data made it to the destination.
There is no error condition that indicates that the destination did not get the data!!!
You can send 0 bytes of data!
CPE 401/601 Lecture 16 : UDP Socket Programming

42. Receiving UDP Datagrams
ssize_t recvfrom( int sockfd, void *buff,
size_t nbytes, int flags,
struct sockaddr* from,
socklen_t *fromaddrlen);
sockfd is a UDP socket
buff is the address of a buffer (nbytes long)
from is the address of a sockaddr
Return value is the number of bytes received and put into buff, or -1 on error
CPE 401/601 Lecture 16 : UDP Socket Programming

43. recvfrom()
If buff is not large enough, any extra data is lost forever...
You can receive 0 bytes of data!
The sockaddr at from is filled in with the address of the sender
CPE 401/601 Lecture 16 : UDP Socket Programming

44. More recvfrom()
recvfrom doesn’t return until there is a datagram available,
unless you do something special
You should set fromaddrlen before calling
If from and fromaddrlen are NULL we don’t find out who sent the data
CPE 401/601 Lecture 16 : UDP Socket Programming

45. Typical UDP client code
Create UDP socket
Create sockaddr with address of server
Call sendto(), sending request to the server
No call to bind() is necessary!
Possibly call recvfrom()
if we need a reply
CPE 401/601 Lecture 16 : UDP Socket Programming

46. Typical UDP Server code
Create UDP socket and bind to well known address
Call recvfrom() to get a request, noting the address of the client
Process request and send reply back with sendto()
CPE 401/601 Lecture 16 : UDP Socket Programming

47. Typical UDP Communication
CPE 401/601 Lecture 16 : UDP Socket Programming

48. UDP Echo Server NEED TO CHECK FOR ERRORS!!!
int mysock; struct sockaddr_in myaddr, cliaddr; char msgbuf[MAXLEN]; socklen_t clilen; int msglen; mysock = socket(PF_INET,SOCK_DGRAM,0); myaddr.sin_family = AF_INET; myaddr.sin_port = htons( S_PORT ); myaddr.sin_addr = htonl( INADDR_ANY ); bind(mysock, &myaddr, sizeof(myaddr)); while (1) { clilen=sizeof(cliaddr); msglen=recvfrom(mysock,msgbuf,MAXLEN,0, cliaddr,&clilen); sendto(mysock,msgbuf,msglen,0,cliaddr,clilen); }
NEED TO CHECK
FOR ERRORS!!!
CPE 401/601 Lecture 16 : UDP Socket Programming

49. Debugging Debugging UDP can be difficult
Write routines to print out sockaddrs
Use trace, strace, ptrace, truss, etc
Include code that can handle unexpected situations
CPE 401/601 Lecture 16 : UDP Socket Programming

50. Timeout when calling recvfrom()
It might be nice to have each call to recvfrom() return after a specified period of time even if there is no incoming datagram
We can do this by using SIGALRM and wrapping each call to recvfrom() with a call to alarm()
CPE 401/601 Lecture 16 : UDP Socket Programming

51. recvfrom()and alarm()
signal(SIGALRM, sig_alrm); alarm(max_time_to_wait); if (recvfrom(…)<0) if (errno==EINTR) /* timed out */ else /* some other error */ /* no error or time out - turn off alarm */ alarm(0);
There are some other (better) ways to do this
CPE 401/601 Lecture 16 : UDP Socket Programming

52. Connected mode A UDP socket can be used in a call to connect()
This simply tells the O.S. the address of the peer
No handshake is made to establish that the peer exists
No data of any kind is sent on the network as a result of calling connect() on a UDP socket
CPE 401/601 Lecture 16 : UDP Socket Programming

53. Connected UDP Once a UDP socket is connected:
can use sendto() with a null dest address
can use write() and send()
can use read() and recv()
only datagrams from the peer will be returned
Asynchronous errors will be returned to the process
OS Specific, some won’t do this!
CPE 401/601 Lecture 16 : UDP Socket Programming

54. Asynchronous Errors
What happens if a client sends data to a server that is not running?
ICMP “port unreachable” error is generated by receiving host and sent to sending host
The ICMP error may reach the sending host after sendto() has already returned!
The next call dealing with the socket could return the error
CPE 401/601 Lecture 16 : UDP Socket Programming

55. Back to UDP connect()
Connect() is typically used with UDP when communication is with a single peer only.
It is possible to disconnect and connect the same socket to a new peer
More efficient to send multiple datagrams to the same user
Many UDP clients use connect()
Some servers (TFTP)
CPE 401/601 Lecture 16 : UDP Socket Programming