Week 10

Week 10 #

Sockets #

Another way for two processes to communicate

Characteristics of Sockets #

  • Has 2 sides: Server and Client
    • Server: has a publishable address
    • Client: contacts Server by published address
  • Unrelated processes, even on different computers, can contact each other through sockets

Socket Varieties #

By Domain #

  • Unix Domain: Local to the computer, address is a filename
  • IPv4: Over the network, 32-bit address + 16-bit port number
  • IPv6: 128-bit address and over the network

By Abstraction Level #

  • Datagram:
    • Per packet
    • Packet boundary preserved, packet order is not
    • Unnoticed packet loss
  • Stream:
    • Network stack works hard to confirm, timeout, resend
    • Preserves data order
    • No packet boundary

Stream Socket Workflow #

Client #

  1. Call socket, creates the socket fd
  2. Fill in the address struct and use connect to connect to the server at the address
  3. Use the socket fd to communicate with the server
  4. Close the fd when done

Server #

  1. Call socket to create the server socket fd
  2. Fill in the address struct and use bind to bind the sfd to the address
  3. Call listen
  4. Loop
    1. Call accept(sfd) to wait for client to connect, gets back a client fd
    2. Use the cfd to communicate with client, close when done
  5. Close the server socket fd if no longer waiting for clients

Creating Sockets #

int socket(int family, int type, int protocol); // returns socket fd > 0, -1 if error
  • family: AF_UNIX, AF_INET (IPv4), AF_INET6 (IPv6)
  • type: SOCK_DGRAM, SOCK_STREAM
  • protocol: 0

IPv4 Addresses and Port struct #

  • Adresses are 32-bit, and identifies network interfaces (computers)
  • Each byte is seperated by dots. (ex 142.1.96.164)
  • dig can look up IP Addresses from domain names by asking Domain Name Servers (DNS)
  • Port struct:
    struct sockaddr_in {
  sa_family_t     sin_family;  // AF_INET
  in_port_t       sin_port;    // port, need to be in network byte order
  struct in_addr  sin_addr;    // IPv4 address, also in NBO
};

struct in_addr {
  uint32_t        s_addr;
}
  • Special addresses
    • 127.0.0.1: Loopback (You can see this example when using localhost or any web server running on your computer)
    • 0.0.0.0: Request binding to all network interfaces

Endians and Network Byte Order #

  • Big Endian (Network Byte Order): Left to right (ex. 772 in decimal = 03 04)
  • Little Endian: Bytes are swapped (ex. 772 in decimal = 04 03)
  • Use the library functions htonl (32-bit) and htons (16-bit) to convert from Little to Big Endian

bind, accept and connect #

int bind(int fd, const struct sockaddr *addr, socklen_t addrlen);
  • sockaddr: sockaddr_in (IPv4), sockaddr_in6 (IPv6), sockaddr_un (UNIX)
int accept(int fd, struct sockaddr *client_addr, socklen_t addrlen);
  • Returns new socket cfd for talking to client
  • client_addr will recieve the address of client
int connect(int fd, const struct sockaddr *server_addr, socklen_t addrlen);
  • Returns 0 if success, -1 on error
  • fd can now talk to the server

Broken Pipes #

  • If one end of the pipe is closed before the other one, the processes gets SIGPIPE
  • The default action is the processes gets killed
  • Eg.
    $ uniq longFile.txt | head -1 
# 'head' end of pipe closes after the first line
# but 'uniq' is still running, hence the process just ends
  • To override the default, set action to SIG_IGN (ignore)