NAME

IP − Internet Protocol

SYNOPSIS

#include <sys/socket.h>
#include <netinet/in.h>

s = socket(AF_INET, SOCK_RAW, proto);

t = t_open ("/dev/rawip", O_RDWR);

d = open ("/dev/ip", O_RDWR);

DESCRIPTION

IP is the internetwork datagram delivery protocol that is central to the Internet protocol family.  Programs may use IP through higher-level protocols such as the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP), or may interface directly to IP.  See tcp(4P) and udp(4P).  Direct access may be via the socket interface (using a “raw socket”) or the Transport Level Interface (TLI).  The protocol options defined in the IP specification may be set in outgoing datagrams. 

The STREAMS driver /dev/rawip is the TLI transport provider that provides raw access to IP.  The device /dev/ip is the multiplexing STREAMS driver that implements the protocol processing of IP.  Raw IP sockets are connectionless and are normally used with the sendto() and recvfrom() calls, [(see send(2) and recv(2)] although the connect(2) call may also be used to fix the destination for future datagrams [in which case the read(2) or recv(2) and write(2) or send(2) calls may be used].  If proto is zero, the default protocol, IPPROTO_RAW, is used.  If proto is non-zero, that protocol number will be set in outgoing datagrams and will be used to filter incoming datagrams.  An IP header will be generated and prepended to each outgoing datagram; received datagrams are returned with the IP header and options intact. 

A single socket option, IP_OPTIONS, is supported at the IP level.  This socket option may be used to set IP options to be included in each outgoing datagram.  IP options to be sent are set with setsockopt() [see getsockopt(2)].  The getsockopt(2) call returns the IP options set in the last setsockopt() call.  IP options on received datagrams are visible to user programs only using raw IP sockets.  The format of IP options given in setsockopt() matches those defined in the IP specification with one exception: the list of addresses for the source routing options must include the first-hop gateway at the beginning of the list of gateways.  The first-hop gateway address will be extracted from the option list and the size adjusted accordingly before use.  IP options may be used with any socket type in the Internet family. 

At the socket level, the socket option SO_DONTROUTE may be applied.  This option forces datagrams being sent to bypass the routing step in output.  Normally, IP selects a network interface to send the datagram, and possibly an intermediate gateway, based on an entry in the routing table.  See routing(4).  When SO_DONTROUTE is set, the datagram will be sent using the interface whose network number or full IP address matches the destination address.  If no interface matches, the error ENETUNRCH will be returned. 

Raw IP datagrams can also be sent and received using the TLI connectionless primitives. 

Datagrams flow through the IP layer in two directions: from the network up to user processes and from user processes down to the network.  Using this orientation, IP is layered above the network interface drivers and below the transport protocols such as UDP and TCP.  The Internet Control Message Protocol (ICMP) is logically a part of IP. See icmp(4P). 

IP provides for a checksum of the header part, but not the data part of the datagram.  The checksum value is computed and set in the process of sending datagrams and checked when receiving datagrams.  IP header checksumming may be disabled for debugging purposes by patching the kernel variable ipcksum to have the value zero. 

IP options in received datagrams are processed in the IP layer according to the protocol specification.  Currently recognized IP options include: security, loose source and record route (LSRR), strict source and record route (SSRR), record route, stream identifier, and internet timestamp. 

The IP layer will normally forward received datagrams that are not addressed to it.  Forwarding is under the control of the kernel variable ipforwarding: if ipforwarding is zero, IP datagrams will not be forwarded;  if ipforwarding is one, IP datagrams will be forwarded.  ipforwarding is usually set to one only in machines with more than one network interface (internetwork routers).  This kernel variable can be patched to enable or disable forwarding. 

The IP layer will send an ICMP message back to the source host in many cases when it receives a datagram that can not be handled.  A “time exceeded” ICMP message will be sent if the “time to live” field in the IP header drops to zero in the process of forwarding a datagram.  A “destination unreachable” message will be sent if a datagram can not be forwarded because there is no route to the final destination, or if it can not be fragmented.  If the datagram is addressed to the local host but is destined for a protocol that is not supported or a port that is not in use, a destination unreachable message will also be sent.  The IP layer may send an ICMP “source quench” message if it is receiving datagrams too quickly.  ICMP messages are only sent for the first fragment of a fragmented datagram and are never returned in response to errors in other ICMP messages. 

The IP layer supports fragmentation and reassembly.  Datagrams are fragmented on output if the datagram is larger than the maximum transmission unit (MTU) of the network interface.  Fragments of received datagrams are dropped from the reassembly queues if the complete datagram is not reconstructed within a short time period. 

Errors in sending discovered at the network interface driver layer are passed by IP back up to the user process. 

SEE ALSO

connect(2), getsockopt(2), read(2), recv(2), send(2), write(2), inet(4F) icmp(4P), tcp(4P), udp(4P).  CX/UX Networking Reference Manual

DIAGNOSTICS

A socket operation may fail with one of the following errors returned:

EACCESS A IP broadcast destination address was specified and the caller was not the privileged user. 

EISCONN An attempt was made to establish a connection on a socket which already had one, or to send a datagram with the destination address specified and the socket was already connected. 

EMSGSIZE An attempt was made to send a datagram that was too large for an interface, but was not allowed to be fragmented (such as broadcasts). 

ENETUNREACH An attempt was made to establish a connection or send a datagram, where there was no matching entry in the routing table, or if an ICMP “destination unreachable” message was received. 

ENOTCONN A datagrem was sent, but no destination address was specified, and the socket had not been connected. 

ENOBUFS The system ran out of memory for fragmentation buffers or other internal data structure. 

EADDRNOTAVAIL
An attempt was made to create a socket with a local address that did not match any network interface, or an IP broadcast destination address was specified and the network interface does not support broadcast. 

The following errors may occur when setting or getting IP options:

EINVAL An unknown socket option name was given. 

EINVAL The IP option field was improperly formed; an option field was shorter than the minimum value or longer than the option buffer provided. 

NOTES

Raw sockets should receive ICMP error packets relating to the protocol; currently such packets are simply discarded. 

Users of higher-level protocols such as TCP and UDP should be able to see received IP options. 

CX/UX Networking