INTRO(4N) — Unix Programmer’s Manual

NAME

networking − introduction to networking facilities

SYNOPSIS

#include <sys/socket.h>
#include <net/route.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”, protocol entries “4P”, and network interfaces “4V” (for VAX specific devices) or “4S” (for Sun specific 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. Raw socket interfaces are provided to IP protocol layer of the DARPA Internet, to the IMP link layer (1822), and to Xerox PUP-1 layer operating on top of 3Mb/s Ethernet interfaces. 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:

#defineAF_UNIX1/∗ local to host (pipes, portals) ∗/
#defineAF_INET2/∗ internetwork: UDP, TCP, etc. ∗/
#defineAF_IMPLINK3/∗ arpanet imp addresses ∗/
#defineAF_PUP4/∗ pup protocols: e.g. BSP ∗/

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) ∗/

Routing table entries come in two flavors, for a specific host or for all hosts on a specific network. 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 (the reference count field is non-zero), when it is deleted, the resources associated with it will not be reclaimed until further references to it are released.

The routing code may return EEXIST if requested to add an already existent entry, ESRCH if requested to delete an entry and it couldn’t be found, or ENOBUFS if requested to add an entry and the system was low on resources.

There currently is no support for reading the routing tables; user processes are expected to read the kernel’s memory through /dev/kmem.

The use field is used by the routing code in providing a simple round-robin scheme of route selection when multiple routes to a destination are present; the heuristic is to choose the least used route.

INTERFACES

Each network interface in a system corresponds to a possible 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.

At boot time each interface which has underlying hardware support makes itself known to the system during the autoconfiguration process. Once the interface has acquired its address it is expected to install a routing table entry so that messages may be routed through it. The manner in which an interface finds out its address varies according to the hardware device involved.

The following ioctl calls may be used to manipulate network interfaces. 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;
shortifru_flags;
} ifr_ifru;
#defineifr_addrifr_ifru.ifru_addr/∗ address ∗/
#defineifr_dstaddrifr_ifru.ifru_dstaddr/∗ other end of p-to-p link ∗/
#defineifr_flagsifr_ifru.ifru_flags/∗ flags ∗/
};

SIOCSIFADDR
Set interface address. Following the address assignment, the “initialization” routine for the interface is called.

SIOCGIFADDR
Get interface address.

SIOCSIFDSTADDR
Set point to point address for interface.

SIOCGIFDSTADDR
Get point to point address for interface.

SIOCSIFFLAGS
Set interface flags field. If the interface is marked down, any processes currently routing packets through the interface are notified.

SIOCGIFFLAGS
Get interface flags.

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).
∗/
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 ∗/
};

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