4.2BSD/usr/man/man4/intro.4n

.TH INTRO 4N "7 July 1983"
.UC 4
.SH NAME
networking \- introduction to networking facilities
.SH SYNOPSIS
.nf
.ft B
#include <sys/socket.h>
#include <net/route.h>
#include <net/if.h>
.fi R
.fi
.SH DESCRIPTION
.de _d
.if t .ta .6i 2.1i 2.6i
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.de _f
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This section briefly describes the networking facilities
available in the system.
Documentation in this part of section
4 is broken up into three areas:
.IR protocol-families ,
.IR protocols ,
and
.IR "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.
.PP
All network protocols are associated with a specific
.IR 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
.IR 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. 
.PP
A protocol supports one of the socket abstractions detailed
in
.IR 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.
.PP
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
.IR config (8)
program.
The DIAGNOSTICS section lists messages which may appear on the console
and in the system error log
.I /usr/adm/messages
due to errors in device operation.
.SH PROTOCOLS
The system currently supports only the DARPA Internet
protocols fully.  Raw socket interfaces are provided to IP protocol
layer of the DARPA Internet, to the IMP link layer (1822), and to
Xerox PUP-1 layer operating on top of 3Mb/s Ethernet interfaces.
Consult the appropriate manual pages in this section for more
information regarding the support for each protocol family.
.SH ADDRESSING
Associated with each protocol family is an address
format.  The following address formats are used by the system:
.sp 1
.nf
._d
#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 */
.fi
.SH 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 
.IR 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.
.PP
A routing table entry has the following form, as defined
in
.RI < net/route.h >;
.sp 1
._f
.nf
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;
};
.sp 1
.fi
with
.I rt_flags
defined from,
.sp 1
.nf
._d
#define	RTF_UP	0x1		/* route usable */
#define	RTF_GATEWAY	0x2		/* destination is a gateway */
#define	RTF_HOST	0x4		/* host entry (net otherwise) */
.fi
.PP
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, each network interface autoconfigured 
installs a routing table entry when it wishes to have packets
sent through it.  Normally the interface specifies the route
through it is 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
may be requested to address the packet to an entity different
from the eventual recipient (i.e. the packet is forwarded).
.PP
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
.RI ( rt_refcnt
is non-zero),
the resources associated with it will not
be reclaimed until further references to it are released. 
.PP
The routing code returns EEXIST if
requested to duplicate an existing entry, ESRCH if
requested to delete a non-existant entry,
or ENOBUFS if insufficient resources were available
to install a new route.
.PP
User processes read the routing tables through the
.I /dev/kmem 
device.
.PP
The
.I rt_use
field contains the number of packets sent along the route.
This value is used to select among multiple
routes to the same destination.  When multiple routes to
the same destination exist, the least used route is selected.
.PP
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.
.SH 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,
.IR lo (4),
do not.
.PP
At boot time each interface which has underlying hardware support
makes itself known to the system during the autoconfiguration
process.  Once the interface has acquired its address it is
expected to install a routing table entry so that messages may
be routed through it.  Most interfaces require some part of
their address specified with an SIOCSIFADDR ioctl before they
will allow traffic to flow through them.  On interfaces where
the network-link layer address mapping is static, only the
network number is taken from the ioctl; the remainder is found
in a hardware specific manner.  On interfaces which provide
dynamic network-link layer address mapping facilities (e.g.
10Mb/s Ethernets), the entire address specified in the ioctl
is used.
.PP
The following 
.I ioctl
calls may be used to manipulate network interfaces.  Unless
specified otherwise, the request takes an
.I ifrequest
structure as its parameter.  This structure has the form
.PP
.nf
.DT
struct	ifreq {
	char	ifr_name[16];		/* name of interface (e.g. "ec0") */
	union {
		struct	sockaddr ifru_addr;
		struct	sockaddr ifru_dstaddr;
		short	ifru_flags;
	} 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_flags	ifr_ifru.ifru_flags	/* flags */
};
.fi
.TP
SIOCSIFADDR
Set interface address.  Following the address
assignment, the ``initialization'' routine for
the interface is called.
.TP
SIOCGIFADDR
Get interface address.
.TP
SIOCSIFDSTADDR
Set point to point address for interface.
.TP
SIOCGIFDSTADDR
Get point to point address for interface.
.TP
SIOCSIFFLAGS
Set interface flags field.  If the interface is marked down,
any processes currently routing packets through the interface
are notified.
.TP
SIOCGIFFLAGS
Get interface flags.
.TP
SIOCGIFCONF
Get interface configuration list.  This request takes an
.I ifconf
structure (see below) as a value-result parameter.  The 
.I ifc_len
field should be initially set to the size of the buffer
pointed to by 
.IR ifc_buf .
On return it will contain the length, in bytes, of the
configuration list.
.PP
.nf
.DT
/*
 * 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 */
};
.fi
.SH SEE ALSO
socket(2),
ioctl(2),
intro(4),
config(8),
routed(8C)