NAME
intro - introduction to networking facilities

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

DESCRIPTION
This section briefly describes the transport layer and below
networking facilities available in the system.

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
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 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_UP0x1/* route usable */
#define RTF_GATEWAY0x2/* destination is a gateway */
#define RTF_HOST0x4/* host entry (net otherwise) */
#define RTF_DYNAMIC0x10/* 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 {
#define IFNAMSIZ 16
char ifr_name[IFNAMSIZE]; /* 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 */
};

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 */
};