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All IPv6 hosts have several addresses
An important consequence of the IPv6 unicast addressing architecture and the utilization of link-local addresses is that each IPv6 host has several IPv6 addresses. This implies that all IPv6 stacks must be able to handle multiple IPv6 addresses.
The addresses described above are unicast addresses. These addresses are used to identify (interfaces on) hosts and routers. They can appear as source and destination addresses in the IPv6 packets. When a host sends a packet towards a unicast address, this packet is delivered by the network to its final destination. There are situations, such as when delivering video or television signal to a large number of receivers, where it is useful to have a network that can efficiently deliver the same packet to a large number of receivers. This is the `multicast` service. A multicast service can be provided in a LAN. In this case, a multicast address identifies a set of receivers and each frame sent towards this address is delivered to all receivers in the group. Multicast can also be used in a network containing routers and hosts. In this case, a multicast address identifies also a group of receivers and the network delivers efficiently each multicast packet to all members of the group. Consider for example the network below.
Assume that ``B`` and ``D`` are part of a multicast group. If ``A`` sends a multicast packet towards this group, then ``R1`` will replicate the packet to forward it to ``R2`` and ``R3``. ``R2`` would forward the packet towards ``B``. ``R3`` would forward the packet towards ``R4`` that would deliver it to ``D``.
Finally, :rfc:`4291` defines the structure of the IPv6 multicast addresses [#fmultiiana]_. This structure is depicted in the figure below.
The low order 112 bits of an IPv6 multicast address are the group's identifier. The high order bits are used as a marker to distinguish multicast addresses from unicast addresses. Notably, the 4-bit `Flags` field indicates whether the address is temporary or permanent. Finally, the `Scope` field indicates the boundaries of the forwarding of packets destined to a particular address. A link-local scope indicates that a router should not forward a packet destined to such a multicast address. An organization local-scope indicates that a packet sent to such a multicast destination address should not leave the organization. Finally the global scope is intended for multicast groups spanning the global Internet.
Among these addresses, some are well known. For example, all hosts automatically belong to the ``ff02::1`` multicast group while all routers automatically belong to the ``ff02::2`` multicast group. A detailed discussion of IPv6 multicast is outside the scope of this chapter.
IPv6 packet format
The IPv6 packet format was heavily inspired by the packet format proposed for the SIPP protocol in :rfc:`1710`. The standard IPv6 header defined in :rfc:`2460` occupies 40 bytes and contains 8 different fields, as shown in the figure below.
The IP version 6 header (:rfc:`2460`)
Apart from the source and destination addresses, the IPv6 header contains the following fields :
`Version` : a 4 bits field set to `6` and intended to allow IP to evolve in the future if needed
`Traffic class` : this 8 bits field indicates the type of service expected by this packet and contains the ``CE`` and ``ECT`` flags that are used by `Explicit Congestion Notification`
`Flow Label` : this field was initially intended to be used to tag packets belonging to the same `flow`. A recent document, :rfc:`6437` describes some possible usages of this field, but it is too early to tell whether it will be really used.
`Payload Length` : this is the size of the packet payload in bytes. As the length is encoded as a 16 bits field, an IPv6 packet can contain up to 65535 bytes of payload.
`Next Header` : this 8-bit field indicates the type [#fianaprotocol]_ of header that follows the IPv6 header. It can be a transport layer header (e.g. `6` for TCP or `17` for UDP) or an IPv6 option.
`Hop Limit` : this 8-bit field indicates the number of routers that can forward the packet. It is decremented by one by each router and prevents packets from looping forever inside the network.
It is interesting to note that there is no checksum inside the IPv6 header. This is mainly because all datalink layers and transport protocols include a checksum or a CRC to protect their frames/segments against transmission errors. Adding a checksum in the IPv6 header would have forced each router to recompute the checksum of all packets, with limited benefit in detecting errors. In practice, an IP checksum allows for catching errors that occur inside routers (e.g. due to memory corruption) before the packet reaches its destination. However, this benefit was found to be too small given the reliability of current memories and the cost of computing the checksum on each router [#fipv4checksum]_.
When a host receives an IPv6 packet, it needs to determine which transport protocol (UDP, TCP, SCTP, ...) needs to handle the payload of the packet. This is the first role of the `Next header` field. The IANA_ which manages the allocation of Internet resources and protocol parameters, maintains an official list of transport protocols [#fianaprotocol]_. The following protocol numbers are reserved :
``TCP`` uses `Next Header` number ``6``
``UDP`` uses `Next Header` number ``17``
``SCTP`` uses `Next Header` number ``132``
For example, an IPv6 packet that contains an TCP segment would appear as shown in the figure below.
An IPv6 packet containing an TCP segment
However, the `Next header` has broader usages than simply indicating the transport protocol which is responsible for the packet payload. An IPv6 packet can contain a chain of headers and the last one indicates the transport protocol that is responsible for the packet payload. Supporting a chain of headers is a clever design from an extensibility viewpoint. As we will see, this chain of headers has several usages.
:rfc:`2460` defines several types of IPv6 extension headers that could be added to an IPv6 packet :
`Hop-by-Hop Options` header. This option is processed by routers and hosts.
`Destination Options` header. This option is processed only by hosts.
`Routing` header. This option is processed by some nodes.
`Fragment` header. This option is processed only by hosts.
`Authentication` header. This option is processed only by hosts.

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../../protocols/ipv6.rst:337
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locale/pot/protocols/ipv6.pot, string 87