ChorusOS man pages section 7P: Protocols

INET(7P)

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 on top of the Internet Protocol (IP) transport layer, using 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 i386 based machines, 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 use the following addressing structure,

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

Sockets can be created with a INADDR_ANY local alddress to enable "wildcard" matching on incoming messages. The address in a connect(2POSIX) or sendto(2POSIX) call may be given as INADDR_ANY to mean ``this host.'' The distinguished address INADDR_BROADCAST is allowed as 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 transport protocol, Internet Control Message Protocol (ICMP), 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 the 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 must absolutely break addresses into their component parts, the following ioctl(2POSIX) commands on a datagram socket in the Internet domain.

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, 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:

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").
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.
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 3/4, 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 (for example, 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(3POSIX) MIB. In addition to the variables supported by the transport protocols (for which see the respective manual pages), the following general variables are defined:

IPCTL_FORWARDING

(ip.forwarding)

Boolean: enable/disable forwarding of IP packets (default false).

IPCTL_SENDREDIRECTS

(ip.redirect)

Boolean: enable/disable sending of ICMP redirects in response to unforwardable IP packets (default true).

IPCTL_DEFTTL

(ip.ttl)

Integer: default time-to-live ("TTL") to use for outgoing IP packets (default 64).

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).

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Evolving

WARNING

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

ChorusOS 4.0 Last Revised December 1999