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