Exit Zen
71
../../protocols/ipv6.rst:241
English
The last type of unicast IPv6 addresses are the `Link Local Unicast` addresses. These addresses are part of the `fe80::/10` address block and are defined in :rfc:`4291`. Each host can compute its own link local address by concatenating the `fe80::/64` prefix with the 64 bits identifier of its interface. Link local addresses can be used when hosts that are attached to the same link (or local area network) need to exchange packets. They are used notably for address discovery and auto-configuration purposes. Their usage is restricted to each link and a router cannot forward a packet whose source or destination address is a link local address. Link local addresses have also been defined for IPv4 :rfc:`3927`. However, the IPv4 link local addresses are only used when a host cannot obtain a regular IPv4 address, e.g. on an isolated LAN.
0/8410 · 841
72
../../protocols/ipv6.rst:259
English
All IPv6 hosts have several addresses
0/370 · 37
73
../../protocols/ipv6.rst:261
English
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.
0/2450 · 245
74
../../protocols/ipv6.rst:265
English
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.
0/10330 · 1033
75
../../protocols/ipv6.rst:289
English
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``.
0/3120 · 312
76
../../protocols/ipv6.rst:291
English
Finally, :rfc:`4291` defines the structure of the IPv6 multicast addresses [#fmultiiana]_. This structure is depicted in the figure below.
0/1380 · 138
77
../../protocols/ipv6.rst:314
English
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.
0/7130 · 713
78
../../protocols/ipv6.rst:316
English
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.
0/2750 · 275
79
../../protocols/ipv6.rst:321
English
IPv6 packet format
0/180 · 18
80
../../protocols/ipv6.rst:323
English
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.
0/2400 · 240
81
../../protocols/ipv6.rst:329
English
The IP version 6 header (:rfc:`2460`)
0/370 · 37
82
../../protocols/ipv6.rst:331
English
Apart from the source and destination addresses, the IPv6 header contains the following fields :
0/960 · 96
83
../../protocols/ipv6.rst:333
English
`Version` : a 4 bits field set to `6` and intended to allow IP to evolve in the future if needed
0/960 · 96
84
../../protocols/ipv6.rst:334
English
`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`
0/1830 · 183
85
../../protocols/ipv6.rst:335
English
`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.
0/2410 · 241
86
../../protocols/ipv6.rst:336
English
`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.
0/1690 · 169
87
../../protocols/ipv6.rst:337
English
`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.
0/1980 · 198
88
../../protocols/ipv6.rst:338
English
`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.
0/1930 · 193
89
../../protocols/ipv6.rst:341
English
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]_.
0/7020 · 702
90
../../protocols/ipv6.rst:343
English
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 :
0/4000 · 400