INTRO(4N)

NAME
networking - introduction to networking facilities

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

DESCRIPTION
This section briefly describes the networking facilities available in the system. Documentation in this part of section 4 is broken up into three areas: protocol families (domains), protocols, and network interfaces. Entries describing a protocol family are marked ``4F,'' while entries describing protocol use are marked ``4P.'' Hardware support for network interfaces are found among the standard ``4'' entries.

All network protocols are associated with a specific protocol family. A protocol family provides basic services to the protocol implementation 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 architecture. 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, /usr/adm/messages (see syslogd(8)), due to errors in device operation.

PROTOCOLS
The system currently supports the DARPA Internet protocols and the Xerox Network Systems(tm) protocols. Raw socket interfaces are provided to the IP protocol layer of the DARPA Internet, to the IMP link layer (1822), 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. The following address formats are used by the system (and additional formats are defined for possible future implementation):

#define	AF_UNIX	1	/* local to host (pipes, portals) */
#define	AF_INET	2	/* internetwork: UDP, TCP, etc. */
#define	AF_IMPLINK	3	/* arpanet imp addresses */
#define	AF_PUP	4	/* pup protocols: e.g. BSP */
#define	AF_NS	6	/* Xerox NS protocols */
#define	AF_HYLINK	15	/* NSC Hyperchannel */

ROUTING
The network facilities provided limited packet routing. A simple set of data structures comprise a ``routing table'' used in selecting the appropriate network interface when transmitting packets. This table contains a single entry for each route to a specific network or host. A user process, the routing daemon, maintains this data base with the aid of two socket-specific ioctl(2) commands, SIOCADDRT and SIOCDELRT. The commands allow the addition and deletion of a single routing table entry, respectively. Routing table manipulations may only be carried out by super-user.

A routing table entry has the following form, as defined in <net/route.h>;

struct rtentry {
	u_long	rt_hash;
	struct	sockaddr rt_dst;
	struct	sockaddr rt_gateway;
	short	rt_flags;
	short	rt_refcnt;
	u_long	rt_use;
	struct	ifnet *rt_ifp;
};

with rt_flags defined from,

#define	RTF_UP	0x1		/* route usable */
#define	RTF_GATEWAY	0x2		/* destination is a gateway */
#define	RTF_HOST	0x4		/* host entry (net otherwise) */
#define	RTF_DYNAMIC	0x10		/* created dynamically (by redirect) */

Routing table entries come in three flavors: for a specific host, for all hosts on a specific network, for any destination not matched by entries of the first two types (a wildcard route). When the system is booted and addresses are assigned to the network interfaces, each protocol family installs a routing table entry for each interface when it is ready for traffic. Normally the protocol specifies the route through each interface as a ``direct'' connection to the destination host or network. If the route is direct, the transport layer of a protocol family usually requests the packet be sent to the same host specified in the packet. Otherwise, the interface is requested to address the packet to the gateway listed in the routing entry (i.e. the packet is forwarded).

Routing table entries installed by a user process may not specify the hash, reference count, use, or interface fields; these are filled in by the routing routines. If a route is in use when it is deleted (rt_refcnt is non-zero), the routing entry will be marked down and removed from the routing table, but the resources associated with it will not be reclaimed until all references to it are released. The routing code returns EEXIST if requested to duplicate an existing entry, ESRCH if requested to delete a non-existent entry, or ENOBUFS if insufficient resources were available to install a new route. User processes read the routing tables through the /dev/kmem device. The rt_use field contains the number of packets sent along the route.

When routing a packet, the kernel will first attempt to find a route to the destination host. Failing that, a search is made for a route to the network of the destination. Finally, any route to a default (``wildcard'') gateway is chosen. If multiple routes are present in the table, the first route found will be used. If no entry is found, the destination is declared to be unreachable.

A wildcard routing entry is specified with a zero destination address value. Wildcard routes are used only when the system fails to find a route to the destination host and network. The combination of wildcard routes and routing redirects can provide an economical mechanism for routing traffic.

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. Unless specified otherwise, the request takes an ifrequest structure as its parameter. This structure has the form

struct	ifreq {
	char	ifr_name[16];		/* name of interface (e.g. "ec0") */
	union {
		struct	sockaddr ifru_addr;
		struct	sockaddr ifru_dstaddr;
		struct	sockaddr ifru_broadaddr;
		short	ifru_flags;
		int	ifru_metric;
	} 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	/* routing metric */
};

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

SIOCGIFADDR
    Get interface address for protocol family.

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

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

SIOCSIFBRDADDR
    Set broadcast 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 interface is reinitialized.

SIOCGIFFLAGS
    Get interface flags.

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

SIOCGIFMETRIC
    Get interface metric.

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