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, 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 in the system error log /usr/adm/messages due to errors in device operation. 

PROTOCOLS

The system currently supports only the DARPA Internet protocols fully.  A raw socket interface is provided to IP protocol layer of the DARPA Internet. 

ADDRESSING

Associated with a protocol family is an address format.  The following address formats are used by the system:

#defineAF_UNIX1/∗ local to host (pipes, portals) ∗/
#defineAF_INET2/∗ internetwork: UDP, TCP, etc. ∗/

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_longrt_hash;
structsockaddr rt_dst;
structsockaddr rt_gateway;
shortrt_flags;
shortrt_refcnt;
u_longrt_use;
structifnet ∗rt_ifp;
};

with rt_flags defined from,

#defineRTF_UP0x1/∗ route usable ∗/
#defineRTF_GATEWAY0x2/∗ destination is a gateway ∗/
#defineRTF_HOST0x4/∗ host entry (net otherwise) ∗/
#defineRTF_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, 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). 

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 resources associated with it will not be reclaimed until further references to it are released. 

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. 

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

structifreq {
charifr_name[16];/∗ name of interface (e.g. "ec0") ∗/
union {
structsockaddr ifru_addr;
structsockaddr ifru_dstaddr;
structsockaddr ifru_broadaddr;
shortifru_flags;
intifru_metric;
shortifru_mtu;
} ifr_ifru;
#defineifr_addrifr_ifru.ifru_addr/∗ address ∗/
#defineifr_dstaddrifr_ifru.ifru_dstaddr/∗ other end of p-to-p link ∗/
#defineifr_broadaddrifr_ifru.ifru_broadaddr/∗ broadcast address ∗/
#defineifr_flagsifr_ifru.ifru_flags/∗ flags ∗/
#defineifr_metricifr_ifru.ifru_metric/∗ routing metric ∗/
#defineifr_mtuifr_ifru.ifru_mtu/∗ mtu ∗/
};

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

SIOCSIFMTU
set the maximum transfer unit (mtu) of an interface

SIOCGIFMTU
get the maximum transfer unit (mtu) of an interface 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).
 ∗/
structifconf {
intifc_len;/∗ size of associated buffer ∗/
union {
caddr_tifcu_buf;
structifreq ∗ifcu_req;
} ifc_ifcu;
#defineifc_bufifc_ifcu.ifcu_buf/∗ buffer address ∗/
#defineifc_reqifc_ifcu.ifcu_req/∗ array of structures returned ∗/
};

SEE ALSO

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

4BSD