Content-type: text/html Man page of netintro

netintro

Section: Environments, Tables, and Troff Macros (7)
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NAME

netintro, networking - Introduction to socket networking facilities  

SYNOPSIS

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

DESCRIPTION

This section is a general introduction to the networking facilities available in the system. Documentation in this part of Section 7 is broken up into three areas: protocol families (domains), protocols, and network interfaces.

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 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 the reference page for the socket() function. 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 and 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 nonstandard 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, address formats, or both. 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 program. The ERRORS section lists messages which may appear on the console and/or in the system error log, /var/log/messages (see the syslogd function), due to errors in device operation.

The system currently supports the DARPA Internet protocols. Raw socket interfaces are provided to the IP layer of the DARPA Internet. Consult the appropriate manual pages in this section for more information regarding this support.  

Addressing

Associated with each protocol family is an address format. All network address adhere to a general structure, called a sockaddr. However, each protocol imposes finer and more specific structure, generally renaming the variant.

Both the 4.3BSD and 4.4BSD sockaddr structures are supported by Tru64 UNIX. The default sockaddr structure is the 4.3BSD structure, which is as follows:

struct sockaddr {        u_short sa_family;
       char    sa_data[14];
};

If the compile-time option _SOCKADDR_LEN is defined before the sys/socket.h header file is included, however, the 4.4BSD sockaddr structure is defined, which is as follows: struct sockaddr {        u_char  sa_len;
       u_char  sa_family;
       char    sa_data[14];
};

The 4.4BSD sockaddr structure provides for a sa_len field, which contains the total length of the structure. Unlike the 4.3BSD sockaddr structure, this length may exceed 16 bytes.

The following address values for sa_family are known to 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. */
 

Routing

The UNIX operating system provides packet routing facilities. The kernel maintains a routing information database, which is used in selecting the appropriate network interface when transmitting packets.

A user process (or possibly multiple cooperating processes) maintains this database by sending messages over a special kind of socket. This supplants fixed size ioctl's used in earlier releases.

This facility is described in the files reference page for the route function.  

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, 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. Most of the requests supported in earlier releases take an ifreq structure as its parameter. This structure has the following 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 */
};

Calls which are now deprecated are: Set interface address for protocol family. Following the address assignment, the ``initialization'' routine for the interface is called. Set point to point address for protocol family and interface. Set broadcast address for protocol family and interface.

All ioctl requests to obtain addresses and requests both to set and retrieve other data are still fully supported and use the ifreq structure: Get interface address for protocol family. Get point to point address for protocol family and interface. Get broadcast address for protocol family and interface. 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. Get interface flags. Set interface routing metric. The metric is used only by user-level routers. Get interface metric.

There are three requests that make use of a new structure: An interface may have more than one address associated with it in some protocols. This request provides a means to add additional addresses (or modify characteristics of the primary address if the default address for the address family is specified). Rather than making separate calls to set destination addresses, broadcast addresses, or network masks (now an integral feature of multiple protocols) a separate structure is used to specify all three facets simultaneously: struct
 ifaliasreq {        char    ifra_name[IFNAMSIZ];            /* if name, e.g. "en0" */
       struct  sockaddr        ifra_addr;
       struct  sockaddr        ifra_broadaddr;
       struct  sockaddr        ifra_mask;
};

One would use a slightly tailored version of this struct are specific to each family (replacing each sockaddr by one of the family-specific type). Where the sockaddr itself is larger than the default size, one needs to modify the ioctl identifier itself to include the total size. This request deletes the specified address from the list associated with an interface. It uses the if_aliasreq structure to permit protocols to allow multiple masks or destination addresses, and it adopts the convention that specification of the default address means to delete the first address for the interface belonging to the address family in which the original socket was opened. 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 contains 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 */
};  

RELATED INFORMATION

Functions: socket(2), ioctl(2).

Files: config(8), routed(8).

Network Programmer's Guide delim off


 

Index

NAME
SYNOPSIS
DESCRIPTION
Addressing
Routing
Interfaces
RELATED INFORMATION

This document was created by man2html, using the manual pages.
Time: 02:40:20 GMT, October 02, 2010