man 2 socket  ;; the 2 is for system calls; 
              ;;  open, close, read, write
              ;; a 3 would be for library function
              ;;  printf, fgets, fopen

Create a socket:
  socket(AF_INET, SOCK_DGRAM, 0);
         ^^^^^^^  "internet" : ip addresses, potentially ipv4 addresses
                  ^^^^^^^^^^ datagram -> UDP (messages that fit in a packet)
                            (as opposed to SOCK_STREAM ->TCP, 
                              text, ordered bytes, reliable)
                              ^ switches between datagram socket 
                                implementations, of which there's 
                                only one.


Fragmentation ; to be covered a bit more later.
  Large IP packet, typically larger than 1500 bytes, would be
   split into fragments to be reassembled at the destination.
  For long-distance links, we use Path MTU discovery.
    MTU == Maximum Transmission Unit ; for Ethernet, 1500 bytes.
  This informs the sender what maximum size packet won't experience
   fragmentation.

  Fragmentation does happen; typically for NFS (network file 
   system) traffic.  Servers tend to be local because they 
   trust IP addresses, you want to send 4096 bytes at a time.

  The maximum size of an IP packet is 65,535 bytes.  (deduct 20 
    for the IP header, 8 for the udp header)

  Jumbo frames (gig ether, -> ~9000 byte MTUs) to be loosely 
    standardized.
  
  Small MTU -> multiplexing (other people get a chance)
               ability to detect bit errors. (2-bit errors, 3-bit errors)
  Large MTU -> efficiency, perhaps fewer headers wasting bandwidth,
               fewer times to lookup where a packet goes.

Ports
  1. Ephemeral - allocated to clients, doesn't matter what they are,
       allow the kernel to tell which conversation a packet belongs to.
  2. Bound - allocated to servers, often well known ( 41710 ), allows
       clients to contact a specific service.  

 Conversation is identified by the 5-tuple:
   Source port, 
   Destination port,
   Source address,
   Destination IP address,
   Protocol (TCP or UDP)

    Implication is that you can send a message to one of our
    servers (at dest port 41710) from a socket (bound to) a
    different port.

  /etc/services lists all the IANA-allocated ports.
    ssh = 22, http = 80, ntp = 123.

 assignment: send/broadcast a message, then wait for messages and print them.

  man 2 bind  ;;  all the include files you need, are listed here.

    bind - will give us a port.   we can ask for one, or let it come.
    bind - the addresses and ports are in network byte order.

#include <sys/socket.h>
  struct sockaddr_in {  /* internet endpoint address, IP address 
                           and port */
     sa_family_t sin_family;   /* AF_INET ; PF_INET */
     in_port_t sin_port;       /* ...sin_port = htons(41710); */
     struct in_addr sin_addr;  /* ...sin_addr.s_addr = "224.0.50.112"; */
                    /* ...sin_addr.s_addr = inet_addr("224.0.50.112");*/
                    /* ...sin_addr.s_addr = htonl(0xc000.....);*/
                    /* ...sin_addr.s_addr = INADDR_ANY;  
                          if you were a server */
                    /* ...sin_addr.s_addr = INADDR_LOCALHOST;   */
                    /* ...sin_addr.s_addr = inet_addr("127.0.0.1"); */
                    /* ...sin_addr.s_addr = htonl(0x7f000001); */ */
  }; 

** that was an address, how to fill in the fields

setsockopt
  allow more than one process to bind the same port.
  --- for most servers, this would be bad.

  int one = 1;
  setsockopt(socket, SOL_SOCKET, SO_REUSEADDR, 
             &one, sizeof(one));

  before the bind.
  
  allow us to configure the multicast membership bits.
  --- 

  


 




man 2 sendto ;; takes the struct sockaddr_in as destination





  (don't let me forget the relevant setsockopt)
   


  





  

























               


packets