4.1cBSD/usr/man/man4/intro.4n

.TH INTRO 4N "19 March 1982"
.UC 4
.SH NAME
net \- introduction to networking facilities
.SH SYNOPSIS
.B #include <sys/socket.h>
.br
.B #include <net/route.h>
.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 on 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'',
protocol entries ``4P'', and network interfaces ``4V'' (for VAX
specific devices) or ``4S'' (for Sun specific 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
outputing 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 useable */
#define	RTF_GATEWAY	0x2		/* destination is a gateway */
#define	RTF_HOST	0x4		/* host entry (net otherwise) */
.fi
.PP
Routing table entries come in two flavors, for a specific
host or for all hosts on a specific network.  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 receipient (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 (the reference count field is non-zero),
when it is deleted, the resources associated with it will not
be reclaimed until further references to it are released. 
.PP
The routing code may return EEXIST if
requested to add an already existant entry, ESRCH if
requested to delete an entry and it couldn't be found,
or ENOBUFS if requested to add an entry and the system was low
on resources.
.PP
There currently is no support for reading the routing tables;
user processes are expected to read the kernel's memory through
.IR /dev/kmem .
.PP
The use field is used by the routing code in providing a simple
round-robin scheme of route selection when multiple routes to
a destination are present; the heuristic is to choose the least
used route.
.SH SEE ALSO
config(8), socket(2)