in_addr_t    /* uint32_t (unsigned 32-bit int): <netinet/in.h> */

in_port_t    /* uint16_t (unsigned 16-bit int): <netinet/in.h> */

sa_family_t  /* <sys/socket.h> */

uint8_t      /* unsigned 8-bit int : <sys/types.h>  */

struct in_addr {

  in_addr_t s_addr;	/* normally: uint32_t */

};

struct sockaddr_in {

  uint8_t           sin_len;	   /* length of structure, 16 */

  sa_family_t	    sin_family;	   /* AF_INET */

  in_port_t	    sin_port;	   /* 16-bit TCP or UDP port
				      network-byte-ordered */

  struct in_addr    sin_addr;	   /* server IP addy;
				      network-byte-ordered */

  char		    sin_zero[8];   /* unused */

};

byteorder (Stevens pg. 77):

 let's say we have a 16-bit value: 01010110 10100111

 then for big-endian, we'll write this:

  A[0] = 01010110  // write most significant byte first

  A[1] = 10100111  // then write 2nd most signif byte

 then for little-endian, we'll write this:

  A[0] = 10100111  // then write 2nd most signif byte 2nd to last

  A[1] = 01010110  // write most significant byte last

 ===============================================
 ==> order of bits within a byte DOES NOT CHANGE
 ===============================================

 ==> only the order of bytes within a multi-byte value

 another e.g., we have: 0x0102

  big endian:    first byte is 0x01, 2nd byte is 0x02

  little endian: first byte is 0x02, 1st byte is 0x01

"big endian" : the big end of the multibyte value (its MSB) is
	       stored at the starting address of the value

"little endian" : the little end of the multibyte value (its LSB)
		  is stored at the starting address of the value

host byte order: whatever byte ordering is used by a given system

<netinet/in.h>

 uint16_t  htons( uint16_t host16bitvalue );

 uint32_t  htonl( uint32_t host32bitvalue );

 uint16_t  ntohs( uint16_t net16bitvalue );

 uint32_t  ntohl( uint32_t net32bitvalue );

---
FTP
---

 PORT h1,h2,h3,h4,p1,p2

 // h1 is highest order byte of IP addy; h2 is second highest order byte of IP addy, ...
 // p1 is highest order byte of port, p2 is ...

 --> bind(), getsockname(), 
 
 --> client sends PORT command; server initiates connection back to client
     in order to xfer data

 --> FTP client opens new (passive) socket

     socket()

     bind()

     listen()
     
 --> client sends PORT command to server on control connection

     -- specifies local addy of new socket (getsockname())

 --> server connects to address specified by PORT

 --> client sends RETR/LIST over control connection

 --> client accept()s server connection

 Type (ASCII/image) for data transfer

  - exe, gz   files use image

  - text files use ASCII?

  - NVT ASCII: <CNTRL-F> should be used to end a line of text, this is "\r\n"

 Look at FTP response codes.

RETRIEVE (RETR) causes server to xfer a copy of the file specified 

PWD causes server to print working directory

LIST causes server to send a directory listing if a directory is specified
else if a filename is sent, server sends that file's attributes; if no
file or directory name is sent, server returns contents of current working dir