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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.
`Encapsulating Security Payload`. This option is processed only by hosts.
The last two headers are used to add security above IPv6 and implement IPSec. They are described in :rfc:`2402` and :rfc:`2406` and are outside the scope of this document.
The `Hop-by-Hop Options` header was designed to make IPv6 easily extensible. In theory, this option could be used to define new fields that were not foreseen when IPv6 was designed. It is intended to be processed by both routers and hosts. Deploying an extension to a network protocol can be difficult in practice since some nodes already support the extensions while others still use the old version and do not understand the extension. To deal with this issue, the IPv6 designers opted for a Type-Length-Value encoding of these IPv6 options. The `Hop-by-Hop Options` header is encoded as shown below.

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../../protocols/ipv6.rst:343
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locale/pot/protocols/ipv6.pot, string 90