INET(4)

NAME
inet - Internet protocol family

SYNOPSIS
#include <sys/types.h>
#include <netinet/in.h>

DESCRIPTION
The Internet protocol family is a collection of protocols layered atop the Internet Protocol (IP) transport layer, and utilizing the Internet address format. The Internet family provides protocol support for the SOCK_STREAM, SOCK_DGRAM, and SOCK_RAW socket types; the SOCK_RAW interface provides access to the IP protocol.

ADDRESSING
Internet addresses are four byte quantities, stored in network standard format (on the VAX these are word and byte reversed). The include file <netinet/in.h> defines this address as a discriminated union.

Sockets bound to the Internet protocol family utilize the following addressing structure,

    struct sockaddr_in {
    	u_char		sin_len;
    	sa_family_t	sin_family;
    	u_short		sin_port;
    	struct in_addr	sin_addr;
    	char		sin_zero[8];
    };

Sockets may be created with the local address INADDR_ANY to affect “wildcard” matching on incoming messages. The address in a connect(2) or sendto(2) call may be given as INADDR_ANY to mean “this host”. The distinguished address INADDR_BROADCAST is allowed as a shorthand for the broadcast address on the primary network if the first network configured supports broadcast.

PROTOCOLS
The Internet protocol family is comprised of the IP network protocol, Internet Control Message Protocol (ICMP), Internet Group Management Protocol (IGMP), Transmission Control Protocol (TCP), and User Datagram Protocol (UDP). TCP is used to support the SOCK_STREAM abstraction while UDP is used to support the SOCK_DGRAM abstraction. A raw interface to IP is available by creating an Internet socket of type SOCK_RAW. The ICMP message protocol is accessible from a raw socket.

The 32-bit Internet address contains both network and host parts. However, direct examination of addresses is discouraged. For those programs which absolutely need to break addresses into their component parts, the following ioctl(2) commands are provided for a datagram socket in the Internet domain; they have the same form as the SIOCSIFADDR and SIOCGIFADDR commands (see intro(4)).

SIOCSIFNETMASK
    Set interface network mask. The network mask defines the network part of the address; if it contains more of the address than the address type would indicate, then subnets are in use.

SIOCGIFNETMASK
    Get interface network mask.

ROUTING
The current implementation of Internet protocols includes some routing-table adaptations to provide enhanced caching of certain end-to-end information necessary for Transaction TCP and Path MTU Discovery. The following changes are the most significant:

1.  All IP routes, except those with the RTF_CLONING flag and those to multicast destinations, have the RTF_PRCLONING flag forcibly enabled (they are thus said to be “protocol cloning”).

2.  When the last reference to an IP route is dropped, the route is examined to determine if it was created by cloning such a route. If this is the case, the RTF_PROTO3 flag is turned on, and the expiration timer is initialized to go off in net.inet.ip.rtexpire seconds. If such a route is re-referenced, the flag and expiration timer are reset.

3.  A kernel timeout runs once every ten minutes, or sooner if there are soon-to-expire routes in the kernel routing table, and deletes the expired routes.

A dynamic process is in place to modify the value of net.inet.ip.rtexpire if the number of cached routes grows too large. If after an expiration run there are still more than net.inet.ip.rtmaxcache unreferenced routes remaining, the rtexpire value is multiplied by ¾, and any routes which have longer expiration times have those times adjusted. This process is damped somewhat by specification of a minimum rtexpire value (net.inet.ip.rtminexpire), and by restricting the reduction to once in a ten-minute period.

If some external process deletes the original route from which a protocol-cloned route was generated, the ``child route'' is deleted. (This is actually a generic mechanism in the routing code support for protocol-requested cloning.)

No attempt is made to manage routes which were not created by protocol cloning; these are assumed to be static, under the management of an external routing process, or under the management of a link layer (e.g., ARP for Ethernets).

Only certain types of network activity will result in the cloning of a route using this mechanism. Specifically, those protocols (such as TCP and UDP) which themselves cache a long-lasting reference to route for a destination will trigger the mechanism; whereas raw IP packets, whether locally-generated or forwarded, will not.

  MIB Variables
A number of variables are implemented in the net.inet branch of the sysctl(3) MIB. In addition to the variables supported by the transport protocols (for which the respective manual pages may be consulted), the following general variables are defined:

IPCTL_FORWARDING
    (ip.forwarding) Boolean: enable/disable forwarding of IP packets. Defaults to off.

IPCTL_FASTFORWARDING
    (ip.fastforwarding) Boolean: enable/disable the use of fast IP forwarding code. Defaults to off. When fast forwarding is enabled, IP packets are forwarded directly to the appropriate network interface with a minimal validity checking, which greatly improves the throughput. On the other hand, they bypass the standard procedures, such as IP option processing and ipfirewall(4) checking. It is not guaranteed that every packet will be fast-forwarded.

IPCTL_SENDREDIRECTS
    (ip.redirect) Boolean: enable/disable sending of ICMP redirects in response to unforwardable IP packets. Defaults to on.

IPCTL_DEFTTL
    (ip.ttl) Integer: default time-to-live (“TTL”) to use for outgoing IP packets.

IPCTL_SOURCEROUTE
    (ip.sourceroute) Boolean: enable/disable forwarding of source-routed IP packets (default false).

IPCTL_RTEXPIRE
    (ip.rtexpire) Integer: lifetime in seconds of protocol-cloned IP routes after the last reference drops (default one hour). This value varies dynamically as described above.

IPCTL_RTMINEXPIRE
    (ip.rtminexpire) Integer: minimum value of ip.rtexpire (default ten seconds). This value has no effect on user modifications, but restricts the dynamic adaptation described above.

IPCTL_RTMAXCACHE
    (ip.rtmaxcache) Integer: trigger level of cached, unreferenced, protocol-cloned routes which initiates dynamic adaptation (default 128).

SEE ALSO
ioctl(2), socket(2), sysctl(3), icmp(4), intro(4), ip(4), ipfirewall(4), tcp(4), udp(4) An Introductory 4.3 BSD Interprocess Communication Tutorial, PS1, 7. An Advanced 4.3 BSD Interprocess Communication Tutorial, PS1, 8.

CAVEATS
The Internet protocol support is subject to change as the Internet protocols develop. Users should not depend on details of the current implementation, but rather the services exported.

HISTORY
The inet protocol interface appeared in 4.2BSD. The “protocol cloning” code appeared in FreeBSD 2.1.