.Dd June 3, 2017
.Dt IP 4
.Os
.Sh NAME
.Nm ip
.Nd internet protocol
.Sh SYNOPSIS
.In sys/socket.h
.In netinet/in.h
.Ft int
.Fn socket AF_INET type protocol
.Sh DESCRIPTION
The Internet Protocol version 4 is the original network layer protocol of the
Internet and provides best-effort delivery of datagrams between hosts.
It provides for addressing of hosts, routing over packet-switched networks,
fragmentation and reassembly of datagrams across networks with small maximum
transmission unit sizes; but it does not provide guarantee of delivery,
avoidance of delivering multiple times, ordering, flow control, nor data
integrity.
Its protocol family
.Xr inet 4
can be layered on top of the Internet Protocol to provide the enhanced service
of the transport layer.
For instance, the Transmission Control Protocol
.Xr tcp 4
can be used to provide multiplexed reliable communication across the Internet,
while the User Datagram Protocol
.Xr udp 4
can be used to provide low-overhead multiplexed unreliable communication across
the Internet.
.Pp
Datagrams contain a header followed by a datagram of the above protocol layer.
The header contains the Internet Protocol version (4), the header size, the
desired type of service, the datagram size, information for the reassembly of
fragmented datagrams, the remaining time this datagram has left to live, the
protocol number of the above protocol layer, a checksum of the header, the
address of the source host and the address of the destination host, and an
optional set of options.
.Pp
An incoming datagram on a network interface will be received and passed to the
higher level protocol if the following conditions hold:
.Pp
.Bl -bullet -compact
.It
The checksum is valid.
.It
The protocol is Internet Protocol version 4 and the packet is well-formed.
.It
The source address is neither the broadcast address
.Pq 255.255.255.255
or the subnet's broadcast address.
.It
If the network interface is not the loopback network interface
.Xr lo 4 ,
neither the source nor the destination belong to the loopback subnet
.Pq 127.0.0.0/24
.It
The destination address is either the local address (and the link layer
destination address was not a broadcast address) of the network interface, the
broadcast address of the network interface, or the broadcast address
.Pq 255.255.255.255 .
.El
.Sh ERRORS
Socket operations can fail due to these error conditions, in addition to the
error conditions of link layer and the error conditions of the invoked function.
.Bl -tag -width [EADDRNOTAVAIL]
.It Bq Er EACCES
A datagram was sent to a broadcast address, but
.Dv SO_BROADCAST
is turned off.
.It Bq Er EADDRNOTAVAIL
The socket cannot be bound to the requested address because no network interface
had that address or broadcast address.
.It Bq Er ECONNREFUSED
The destination host of a datagram was not listening on the port.
.It Bq Er EHOSTDOWN
The destination host of a datagram is not up.
.It Bq Er EHOSTUNREACH
The destination host of a datagram was unreachable.
.It Bq Er EMSGSIZE
The datagram was too large to be sent because it exceeded the maximum
transmission unit (MTU) on the path between the local and remote address.
.It Bq Er ENETDOWN
The network interface used to deliver a datagram isn't up.
.It Bq Er ENETUNREACH
The destination network of a datagram was unreachable.
.It Bq Er ENOBUFS
There was not enough memory available for network packets.
.El
.Sh SEE ALSO
.Xr arp 4 ,
.Xr icmp 4 ,
.Xr inet 4 ,
.Xr ping 4 ,
.Xr tcp 4 ,
.Xr udp 4 ,
.Xr kernel 7
.Sh STANDARDS
.Rs
.%A J. Postel (ed.)
.%D September 1981
.%R STD 5
.%R RFC 791
.%T Internet Protocol - DARPA Internet Program Protocol Specification
.%Q USC/Information Sciences Institute
.Re
.Pp
.Rs
.%A Internet Engineering Task Force
.%A R. Braden (ed.)
.%D October 1989
.%R STD 3
.%R RFC 1122
.%T Requirements for Internet Hosts -- Communication Layers
.%Q USC/Information Sciences Institute
.Re
.Sh BUGS
The implementation is incomplete and has known bugs.
.Pp
Fragmented datagrams are not yet supported and are discarded on receipt.
The fragment identification field is always set to 0, preventing the proper
reassembly of multiple datagrams that became fragmented around the same time.
.Pp
Options are not yet supported and are ignored.
.Pp
The 4-byte address space allows only a maximum of 4294967296 addresses and is
being exhausted.
The Internet Protocol version 6 replaces version 4 and provides a 16-byte
address space instead.
.Pp
There is no routing table that can be configured.
Routing happens by searching for the first appropriate network interface that
can transmit the datagram.
If multiple network interfaces have a default route, the packet is sent using
the default route of the network interface with the lowest index number.