NETINTRO(4)						   BSD Kernel Interfaces Manual 					       NETINTRO(4)

NAME

     networking -- introduction to networking facilities

SYNOPSIS

     #include <sys/socket.h>
     #include <net/route.h>
     #include <net/if.h>

DESCRIPTION

     This section is a general introduction to the networking facilities available in the system.  Documentation in this part of section 4 is bro-
     ken up into three areas: protocol families (domains), protocols, and network interfaces.

     All network protocols are associated with a specific protocol family.  A protocol family provides basic services to the protocol implementa-
     tion to allow it to function within a specific network environment.  These services may include packet fragmentation and reassembly, routing,
     addressing, and basic transport.  A protocol family may support multiple methods of addressing, though the current protocol implementations
     do not.  A protocol family is normally comprised of a number of protocols, one per socket(2) type.  It is not required that a protocol family
     support all socket types.	A protocol family may contain multiple protocols supporting the same socket abstraction.

     A protocol supports one of the socket abstractions detailed in socket(2).	A specific protocol may be accessed either by creating a socket of
     the appropriate type and protocol family, or by requesting the protocol explicitly when creating a socket.  Protocols normally accept only
     one type of address format, usually determined by the addressing structure inherent in the design of the protocol family/network architec-
     ture.  Certain semantics of the basic socket abstractions are protocol specific.  All protocols are expected to support the basic model for
     their particular socket type, but may, in addition, provide non-standard facilities or extensions to a mechanism.	For example, a protocol
     supporting the SOCK_STREAM abstraction may allow more than one byte of out-of-band data to be transmitted per out-of-band message.

     A network interface is similar to a device interface.  Network interfaces comprise the lowest layer of the networking subsystem, interacting
     with the actual transport hardware.  An interface may support one or more protocol families and/or address formats.  The SYNOPSIS section of
     each network interface entry gives a sample specification of the related drivers for use in providing a system description to the config(8)
     program.  The DIAGNOSTICS section lists messages which may appear on the console and/or in the system error log, /var/log/messages (see
     syslogd(8)), due to errors in device operation.

PROTOCOLS

     The system currently supports the Internet protocols, the Xerox Network Systems(tm) protocols, and some of the ISO OSI protocols.	Raw socket
     interfaces are provided to the IP protocol layer of the Internet, and to the IDP protocol of Xerox NS.  Consult the appropriate manual pages
     in this section for more information regarding the support for each protocol family.

ADDRESSING

     Associated with each protocol family is an address format.  All network address adhere to a general structure, called a sockaddr, described
     below. However, each protocol imposes finer and more specific structure, generally renaming the variant, which is discussed in the protocol
     family manual page alluded to above.

	       struct sockaddr {
		   u_char  sa_len;
		   u_char  sa_family;
		   char    sa_data[14];
	   };

     The field sa_len contains the total length of the structure, which may exceed 16 bytes.  The following address values for sa_family are known
     to the system (and additional formats are defined for possible future implementation):

     #define	AF_UNIX      1	  /* local to host (pipes) */
     #define	AF_INET      2	  /* IPv4: UDP, TCP, etc. */
     #define	AF_INET6     30   /* IPv6: UDP, TCP, etc. */
     #define	AF_NS	     6	  /* Xerox NS protocols */
     #define	AF_CCITT     10   /* CCITT protocols, X.25 etc */
     #define	AF_HYLINK    15   /* NSC Hyperchannel */
     #define	AF_ISO	     18   /* ISO protocols */

ROUTING

     Mac OS X provides some packet routing facilities.	The kernel maintains a routing information database, which is used in selecting the appro-
     priate network interface when transmitting packets.

     A user process (or possibly multiple co-operating processes) maintains this database by sending messages over a special kind of socket.  This
     supplants fixed size ioctl(2) used in earlier releases.

     This facility is described in route(4).

INTERFACES

     Each network interface in a system corresponds to a path through which messages may be sent and received.	A network interface usually has a
     hardware device associated with it, though certain interfaces such as the loopback interface, lo(4), do not.

     The following ioctl calls may be used to manipulate network interfaces.  The ioctl is made on a socket (typically of type SOCK_DGRAM) in the
     desired domain.  Most of the requests supported in earlier releases take an ifreq structure as its parameter.  This structure has the form

     struct  ifreq {
     #define	IFNAMSIZ    16
	 char	 ifr_name[IFNAMSIZ];	     /* if name, e.g. "en0" */
	 union {
	     struct    sockaddr ifru_addr;
	     struct    sockaddr ifru_dstaddr;
	     struct    sockaddr ifru_broadaddr;
	     short     ifru_flags;
	     int       ifru_metric;
	     caddr_t   ifru_data;
	 } ifr_ifru;
     #define ifr_addr	   ifr_ifru.ifru_addr	 /* address */
     #define ifr_dstaddr   ifr_ifru.ifru_dstaddr /* other end of p-to-p link */
     #define ifr_broadaddr ifr_ifru.ifru_broadaddr /* broadcast address */
     #define ifr_flags	   ifr_ifru.ifru_flags	 /* flags */
     #define ifr_metric    ifr_ifru.ifru_metric  /* metric */
     #define ifr_data	   ifr_ifru.ifru_data	 /* for use by interface */
     };

     Calls which are now deprecated are:

     SIOCSIFADDR     Set interface address for protocol family.  Following the address assignment, the ``initialization'' routine for the inter-
		     face is called.

     SIOCSIFDSTADDR  Set point to point address for protocol family and interface.

     SIOCSIFBRDADDR  Set broadcast address for protocol family and interface.

     Ioctl requests to obtain addresses and requests both to set and retrieve other data are still fully supported and use the ifreq structure:

     SIOCGIFADDR     Get interface address for protocol family.

     SIOCGIFDSTADDR  Get point to point address for protocol family and interface.

     SIOCGIFBRDADDR  Get broadcast address for protocol family and interface.

     SIOCSIFFLAGS    Set interface flags field.  If the interface is marked down, any processes currently routing packets through the interface
		     are notified; some interfaces may be reset so that incoming packets are no longer received.  When marked up again, the inter-
		     face is reinitialized.

     SIOCGIFFLAGS    Get interface flags.

     SIOCSIFMETRIC   Set interface routing metric.  The metric is used only by user-level routers.

     SIOCGIFMETRIC   Get interface metric.

     There are two requests that make use of a new structure:

     SIOCAIFADDR     An interface may have more than one address associated with it in some protocols.	This request provides a means to add addi-
		     tional addresses (or modify characteristics of the primary address if the default address for the address family is speci-
		     fied).  Rather than making separate calls to set destination or broadcast addresses, or network masks (now an integral fea-
		     ture of multiple protocols) a separate structure is used to specify all three facets simultaneously (see below).  One would
		     use a slightly tailored version of this struct specific to each family (replacing each sockaddr by one of the family-specific
		     type).  Where the sockaddr itself is larger than the default size, one needs to modify the ioctl identifier itself to include
		     the total size, as described in ioctl.

     SIOCDIFADDR     This requests deletes the specified address from the list associated with an interface.  It also uses the if_aliasreq struc-
		     ture to allow for the possibility of protocols allowing multiple masks or destination addresses, and also adopts the conven-
		     tion that specification of the default address means to delete the first address for the interface belonging to the address
		     family in which the original socket was opened.

     SIOCGIFCONF     Get interface configuration list.	This request takes an ifconf structure (see below) as a value-result parameter.  The
		     ifc_len field should be initially set to the size of the buffer pointed to by ifc_buf.  On return it will contain the length,
		     in bytes, of the configuration list.

     /*
     * Structure used in SIOCAIFADDR request.
     */
     struct ifaliasreq {
	     char    ifra_name[IFNAMSIZ];   /* if name, e.g. "en0" */
	     struct  sockaddr	     ifra_addr;
	     struct  sockaddr	     ifra_broadaddr;
	     struct  sockaddr	     ifra_mask;
     };

     /*
     * Structure used in SIOCGIFCONF request.
     * Used to retrieve interface configuration
     * for machine (useful for programs which
     * must know all networks accessible).
     */
     struct ifconf {
	 int   ifc_len; 	     /* size of associated buffer */
	 union {
	     caddr_t	ifcu_buf;
	     struct	ifreq *ifcu_req;
	 } ifc_ifcu;
     #define ifc_buf ifc_ifcu.ifcu_buf /* buffer address */
     #define ifc_req ifc_ifcu.ifcu_req /* array of structures returned */
     };

SEE ALSO

     ioctl(2), socket(2), intro(4), config(5), routed(8)

HISTORY

     The netintro manual appeared in 4.3BSD-Tahoe.

4.2 Berkeley Distribution					 November 30, 1993					 4.2 Berkeley Distribution