Tribblix: manual page: inet6.4p

INET6(4P) Protocols INET6(4P)

NAME

inet6 - Internet protocol family for Internet Protocol version 6

SYNOPSIS

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

DESCRIPTION

The inet6 protocol family implements a collection of protocols that
are centered around the Internet Protocol version 6 (IPv6) and share
a common address format. The inet6 protocol family can be accessed
using the socket interface, where it supports the SOCK_STREAM,
SOCK_DGRAM, and SOCK_RAW socket types, or the Transport Level
Interface (TLI), where it supports the connectionless (T_CLTS) and
connection oriented (T_COTS_ORD) service types.

PROTOCOLS

The Internet protocol family for IPv6 included the Internet Protocol
Version 6 (IPv6), the Neighbor Discovery Protocol (NDP), the Internet
Control Message Protocol (ICMPv6), the Transmission Control Protocol
(TCP), and the User Datagram Protocol (UDP).

TCP supports the socket interface's SOCK_STREAM abstraction and TLI's
T_COTS_ORD service type. UDP supports the SOCK_DGRAM socket
abstraction and the TLI T_CLTS service type. See tcp(4P) and udp(4P).
A direct interface to IPv6 is available using the socket interface.
See ip6(4P). ICMPv6 is used by the kernel to handle and report errors
in protocol processing. It is also accessible to user programs. See
icmp6(4P). NDP is used to translate 128-bit IPv6 addresses into
48-bit Ethernet addresses.

IPv6 addresses come in three types: unicast, anycast, and multicast.
A unicast address is an identifier for a single network interface. An
anycast address is an identifier for a set of interfaces; a packet
sent to an anycast address is delivered to the "nearest" interface
identified by that address, pursuant to the routing protocol's
measure of distance. A multicast address is an identifier for a set
of interfaces; a packet sent to a multicast address is delivered to
all interfaces identified by that address. There are no broadcast
addresses as such in IPv6; their functionality is superseded by
multicast addresses.

For IPv6 addresses, there are three scopes within which unicast
addresses are guaranteed to be unique. The scope is indicated by the
address prefix. The three varieties are link-local (the address is
unique on that physical link), site-local (the address is unique
within that site), and global (the address is globally unique).

The three highest order bits for global unicast addresses are set to
001. The ten highest order bits for site-local addresses are set to
1111 1110 11. The ten highest order bits for link-local addresses are
set to 1111 1110 11. For multicast addresses, the eight highest order
bits are set to 1111 1111. Anycast addresses have the same format as
unicast addresses.

IPv6 addresses do not follow the concept of "address class" seen in
IP.

A global unicast address is divided into the following segments:

o The first three bits are the Format Prefix identifying a
unicast address.

o The next 13 bits are the Top-Level Aggregation (TLA)
identifier. For example, the identifier could specify the
ISP.

o The next eight bits are reserved for future use.

o The next 24 bits are the Next-Level Aggregation (NLA)
identifier.

o The next 16 bits are the Site-Level Aggregation (SLA)
identifier.

o The last 64 bits are the interface ID. This will most
often be the hardware address of the link in IEEE EUI-64
format.

Link-local unicast addresses are divided in this manner:

o The first ten bits are the Format Prefix identifying a
link-local address.

o The next 54 bits are zero.

o The last 64 bits are the interface ID. This will most
often be the hardware address of the link in IEEE EUI-64
format.

Site-local unicast addresses are divided in this manner:

o The first ten bits are the Format Prefix identifying a
site-local address.

o The next 38 bits are zero.

o The next 16 bits are the subnet ID.

o The last 64 bits are the interface ID. This will most
often be the hardware address of the link in IEEE EUI-64
format.

ADDRESSING

IPv6 addresses are sixteen byte quantities, stored in network byte
order. The socket API uses the sockaddr_in6 structure when passing
IPv6 addresses between an application and the kernel. The
sockaddr_in6 structure has the following members:

sa_family_t sin6_family;
in_port_t sin6_port;
uint32_t sin6_flowinfo;
struct in6_addr sin6_addr;
uint32_t sin6_scope_id;
uint32_t __sin6_src_id;

Library routines are provided to manipulate structures of this
form. See inet(3C).

The sin6_addr field of the sockaddr_in6 structure specifies a local
or remote IPv6 address. Each network interface has one or more IPv6
addresses configured, that is, a link-local address, a site-local
address, and one or more global unicast IPv6 addresses. The special
value of all zeros may be used on this field to test for "wildcard"
matching. Given in a bind(3SOCKET) call, this value leaves the local
IPv6 address of the socket unspecified, so that the socket will
receive connections or messages directed at any of the valid IPv6
addresses of the system. This can prove useful when a process neither
knows nor cares what the local IPv6 address is, or when a process
wishes to receive requests using all of its network interfaces.

The sockaddr_in6 structure given in the bind() call must specify an
in6_addr value of either all zeros or one of the system's valid IPv6
addresses. Requests to bind any other address will elicit the error
EADDRNOTAVAI. When a connect(3SOCKET) call is made for a socket that
has a wildcard local address, the system sets the sin6_addr field of
the socket to the IPv6 address of the network interface through which
the packets for that connection are routed.

The sin6_port field of the sockaddr_in6 structure specifies a port
number used by TCP or UDP. The local port address specified in a
bind() call is restricted to be greater than IPPORT_RESERVED (defined
in <netinet/in.h>) unless the creating process is running as the
super-user, providing a space of protected port numbers. In
addition, the local port address cannot be in use by any socket of
the same address family and type. Requests to bind sockets to port
numbers being used by other sockets return the error EADDRINUSE. If
the local port address is specified as 0, the system picks a unique
port address greater than IPPORT_RESERVED. A unique local port
address is also selected when a socket which is not bound is used in
a connect(3SOCKET) or sendto() call. See send(3SOCKET). This allows
programs that do not care which local port number is used to set up
TCP connections by simply calling socket(3SOCKET) and then
connect(3SOCKET), and then sending UDP datagrams with a socket() call
followed by a sendto() call.

Although this implementation restricts sockets to unique local port
numbers, TCP allows multiple simultaneous connections involving the
same local port number so long as the remote IPv6 addresses or port
numbers are different for each connection. Programs may explicitly
override the socket restriction by setting the SO_REUSEADDR socket
option with setsockopt(). See getsockopt(3SOCKET).

In addition, the same port may be bound by two separate sockets if
one is an IP socket and the other an IPv6 socket.

TLI applies somewhat different semantics to the binding of local port
numbers. These semantics apply when Internet family protocols are
used using the TLI.

SOURCE ADDRESS SELECTION

IPv6 source address selection is done on a per destination basis, and
utilizes a list of rules from which the best source address is
selected from candidate addresses. The candidate set comprises a set
of local addresses assigned on the system which are up and not
anycast. If just one candidate exists in the candidate set, it is
selected.

Conceptually, each selection rule prefers one address over another,
or determines their equivalence. If a rule produces a tie, a
subsequent rule is used to break the tie.

The sense of some rules may be reversed on a per-socket basis using
the IPV6_SRC_PREFERENCES socket option (see ip6(4P)). The flag values
for this option are defined in <netinet/in.h> and are referenced in
the description of the appropriate rules below.

As the selection rules indicate, the candidate addresses are SA and
SB and the destination is D.

Prefer the same address
If SA == D, prefer SA. If SB == D,
prefer SB.

Prefer appropriate scope
Here, Scope(X) is the scope of X
according to the IPv6 Addressing
Architecture.

If Scope(SA) < Scope(SB): If Scope(SA)
< Scope(D), then prefer SB and
otherwise prefer SA.

If Scope(SB) < Scope(SA): If Scope(SB)
< Scope(D), then prefer SA and
otherwise prefer SB.

Avoid deprecated addresses
If one of the addresses is deprecated
(IFF_DEPRECATED) and the other is not,
prefer the one that isn't deprecated.

Prefer preferred addresses
If one of the addresses is preferred
(IFF_PREFERRED) and the other is not,
prefer the one that is preferred.

Prefer outgoing interface
If one of the addresses is assigned to
the interface that will be used to send
packets to D and the other is not, then
prefer the former.

Prefer matching label
This rule uses labels which are
obtained through the IPv6 default
address selection policy table. See
ipaddrsel(8) for a description of the
default contents of the table and how
the table is configured.

If Label(SA) == Label(D) and Label(SB)
!= Label(D), then prefer SA.

If Label(SB) == Label(D) and Label(SA)
!= Label(D), then prefer SB.

Prefer public addresses
This rule prefers public addresses over
temporary addresses, as defined in RFC
3041. Temporary addresses are disabled
by default and may be enabled by
appropriate settings in ndpd.conf(5).

The sense of this rule may be set on a
per-socket basis using the
IPV6_SRC_PREFERENCES socket option.
Passing the flag IPV6_PREFER_SRC_TMP or
IPV6_PREFER_SRC_PUBLIC will cause
temporary or public addresses to be
preferred, respectively, for that
particular socket. See ip6(4P) for
more information about IPv6 socket
options.

Use longest matching prefix.

This rule prefers the source address that has the longer matching
prefix with the destination. Because this is the last rule and
because both source addresses could have equal matching prefixes,
this rule does an xor of each source address with the
destination, then selects the source address with the smaller xor
value in order to break any potential tie.

If SA ^ D < SB ^ D, then prefer SA.

If SB ^ D < SA ^ D, then prefer SB.

Applications can override this algorithm by calling bind(3SOCKET)
and specifying an address.

NOTES

The IPv6 support is subject to change as the Internet protocols
develop. Users should not depend on details of the current
implementation, but rather the services exported.

March 30, 2022 INET6(4P)