.\" Copyright (c) 1983, 1990 The Regents of the University of California.
.\" %sccs.include.redist.man%
.\" @(#)netintro.4 6.9 (Berkeley) %G%
networking \- introduction to networking facilities
.\" 2.94 went to 2.6, 3.64 to 3.30
.if n .ta .84i 2.6i 3.30i
This section is a general introduction to the networking facilities
Documentation in this part of section
4 is broken up into three areas:
.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.
All network protocols are associated with a specific
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
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
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
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
The DIAGNOSTICS section lists messages which may appear on the console
and/or in the system error log,
due to errors in device operation.
The system currently supports the DARPA 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 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.
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
The field sa_len contains the total length of the of the structure,
which may exceed 16 bytes.
The following address values for
(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_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 */
provides some packet routing facilities.
The kernel maintains a routing information database, which
is used in selecting the appropriate network interface when
A user process (or possibly multiple co-operating processes)
maintains this database by sending messages over a special kind
This supplants fixed size
used in earlier releases.
This facility is described in
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,
calls may be used to manipulate network interfaces.
is made on a socket (typically of type SOCK_DGRAM)
Most of the requests supported in earlier releases
structure as its parameter. This structure has the form
char ifr_name[IFNAMSIZE]; /* if name, e.g. "en0" */
struct sockaddr ifru_addr;
struct sockaddr ifru_dstaddr;
struct sockaddr ifru_broadaddr;
#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:
Set interface address for protocol family. Following the address
assignment, the ``initialization'' routine for
Set point to point address for protocol family and interface.
Set broadcast address for protocol family and interface.
requests to obtain addresses and requests both to set and
retreive other data are still fully supported
Get interface address for protocol family.
Get point to point address for protocol family and interface.
Get broadcast address for protocol family and interface.
Set interface flags field. If the interface is marked down,
any processes currently routing packets through the interface
some interfaces may be reset so that incoming packets are no longer received.
When marked up again, the interface is reinitialized.
Set interface routing metric.
The metric is used only by user-level routers.
There are two requests that make use of a new structure:
An interface may have more than one address associated with it
in some protocols. This request provides a means to
add additional addresses (or modify characteristics of the
primary address if the default address for the address family
is specified). Rather than making separate calls to
set destination or broadcast addresses, or network masks
(now an integral feature of multiple protocols)
a separate structure is used to specify all three facets simultaneously:
.ta \w'struct 'u +\w'struct 'u +\w'sockaddr 'u +\w'ifra_broaddadr 'u
char ifra_name[IFNAMSIZ]; /* if name, e.g. "en0" */
struct sockaddr ifra_addr;
struct sockaddr ifra_broadaddr;
struct sockaddr ifra_mask;
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
identifier itself to include the total size, as described in
This requests deletes the specified address from the list
associated with an interface. It also uses the if_aliasreq
structure to allow for the possibility of protocols allowing
multiple masks or destination addresses, and also adopts the
convention 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.
Get interface configuration list. This request takes an
structure (see below) as a value-result parameter. The
field should be initially set to the size of the buffer
On return it will contain the length, in bytes, of the
* Structure used in SIOCGIFCONF request.
* Used to retrieve interface configuration
* for machine (useful for programs which
* must know all networks accessible).
int ifc_len; /* size of associated buffer */
#define ifc_buf ifc_ifcu.ifcu_buf /* buffer address */
#define ifc_req ifc_ifcu.ifcu_req /* array of structures returned */
socket(2), ioctl(2), intro(4), config(8), routed(8C)
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