231 strings / 10667 words

Translation Source string Target string
cnp3-ebook/protocols/ipv6 — French
Before explaining the network layer in detail, it is useful to begin by analyzing the service provided by the `datalink` layer. There are many variants of the datalink layer. Some provide a connection-oriented service while others provide a connectionless service. In this section, we focus on connectionless datalink layer services as they are the most widely used. Using a connection-oriented datalink layer causes some problems that are beyond the scope of this chapter. See :rfc:`3819` for a discussion on this topic.
 
cnp3-ebook/protocols/ipv6 — French
There are three main types of datalink layers. The simplest datalink layer is when there are only two communicating systems that are directly connected through the physical layer. Such a datalink layer is used when there is a point-to-point link between the two communicating systems. The two systems can be hosts or routers. :abbr:`PPP (Point-to-Point Protocol)`, defined in :rfc:`1661`, is an example of such a point-to-point datalink layer. Datalink layers exchange `frames` and a datalink :term:`frame` sent by a datalink layer entity on the left is transmitted through the physical layer, so that it can reach the datalink layer entity on the right. Point-to-point datalink layers can either provide an unreliable service (frames can be corrupted or lost) or a reliable service (in this case, the datalink layer includes retransmission mechanisms similar to the ones used in the transport layer). The unreliable service is frequently used above physical layers (e.g. optical fiber, twisted pairs) having a low bit error ratio while reliability mechanisms are often used in wireless networks to recover locally from transmission errors.
 
cnp3-ebook/protocols/ipv6 — French
The second type of datalink layer is the one used in Local Area Networks (LAN). Conceptually, a LAN is a set of communicating devices such that any two devices can directly exchange frames through the datalink layer. Both hosts and routers can be connected to a LAN. Some LANs only connect a few devices, but there are LANs that can connect hundreds or even thousands of devices.
 
cnp3-ebook/protocols/ipv6 — French
A local area network
 
cnp3-ebook/protocols/ipv6 — French
In the next chapter, we describe the organization and the operation of Local Area Networks. An important difference between the point-to-point datalink layers and the datalink layers used in LANs is that in a LAN, each communicating device is identified by a unique `datalink layer address`. This address is usually embedded in the hardware of the device and different types of LANs use different types of datalink layer addresses. Most LANs use 48-bits long addresses that are usually called `MAC` addresses. A communicating device attached to a LAN can send a datalink frame to any other communicating device that is attached to the same LAN. Most LANs also support special broadcast and multicast datalink layer addresses. A frame sent to the broadcast address of the LAN is delivered to all communicating devices that are attached to the LAN. The multicast addresses are used to identify groups of communicating devices. When a frame is sent towards a multicast datalink layer address, it is delivered by the LAN to all communicating devices that belong to the corresponding group.
 
cnp3-ebook/protocols/ipv6 — French
The third type of datalink layers are used in Non-Broadcast Multi-Access (NBMA) networks. These networks are used to interconnect devices like a LAN. All devices attached to an NBMA network are identified by a unique datalink layer address. However, and this is the main difference between an NBMA network and a traditional LAN, the NBMA service only supports unicast. The datalink layer service provided by an NBMA network supports neither broadcast nor multicast.
 
cnp3-ebook/protocols/ipv6 — French
Unfortunately no datalink layer is able to send frames of unlimited side. Each datalink layer is characterized by a maximum frame size. There are more than a dozen different datalink layers and unfortunately most of them use a different maximum frame size. The network layer must cope with the heterogeneity of the datalink layer.
 
cnp3-ebook/protocols/ipv6 — French
IP version 6
 
cnp3-ebook/protocols/ipv6 — French
In the late 1980s and early 1990s the growth of the Internet was causing several operational problems on routers. Many of these routers had a single CPU and up to 1 MByte of RAM to store their operating system, packet buffers and routing tables. Given the rate of allocation of IPv4 prefixes to companies and universities willing to join the Internet, the routing tables where growing very quickly and some feared that all IPv4 prefixes would quickly be allocated. In 1987, a study cited in :rfc:`1752`, estimated that there would be 100,000 networks in the near future. In August 1990, estimates indicated that the class B space would be exhausted by March 1994. Two types of solution were developed to solve this problem. The first short term solution was the introduction of Classless Inter Domain Routing (:term:`CIDR`). A second short term solution was the Network Address Translation (:term:`NAT`) mechanism, defined in :rfc:`1631`. NAT allowed multiple hosts to share a single public IPv4 address.
 
cnp3-ebook/protocols/ipv6 — French
However, in parallel with these short-term solutions, which have allowed the IPv4 Internet to continue to be usable until now, the Internet Engineering Task Force started working on developing a replacement for IPv4. This work started with an open call for proposals, outlined in :rfc:`1550`. Several groups responded to this call with proposals for a next generation Internet Protocol (IPng) :
 
cnp3-ebook/protocols/ipv6 — French
TUBA proposed in :rfc:`1347` and :rfc:`1561`
 
cnp3-ebook/protocols/ipv6 — French
PIP proposed in :rfc:`1621`
 
cnp3-ebook/protocols/ipv6 — French
SIPP proposed in :rfc:`1710`
 
cnp3-ebook/protocols/ipv6 — French
The IETF decided to pursue the development of IPng based on the SIPP proposal. As IP version `5` was already used by the experimental ST-2 protocol defined in :rfc:`1819`, the successor of IP version 4 is IP version 6. The initial IP version 6 defined in :rfc:`1752` was designed based on the following assumptions :
 
cnp3-ebook/protocols/ipv6 — French
IPv6 addresses are encoded as a 128 bits field
 
cnp3-ebook/protocols/ipv6 — French
The IPv6 header has a simple format that can easily be parsed by hardware devices
 
cnp3-ebook/protocols/ipv6 — French
A host should be able to configure its IPv6 address automatically
 
cnp3-ebook/protocols/ipv6 — French
Security must be part of IPv6
 
cnp3-ebook/protocols/ipv6 — French
The IPng address size
 
cnp3-ebook/protocols/ipv6 — French
When the work on IPng started, it was clear that 32 bits was too small to encode an IPng address and all proposals used longer addresses. However, there were many discussions about the most suitable address length. A first approach, proposed by SIPP in :rfc:`1710`, was to use 64 bit addresses. A 64 bits address space was 4 billion times larger than the IPv4 address space and, furthermore, from an implementation perspective, 64 bit CPUs were being considered and 64 bit addresses would naturally fit inside their registers. Another approach was to use an existing address format. This was the TUBA proposal (:rfc:`1347`) that reuses the ISO CLNP 20 bytes addresses. The 20 bytes addresses provided room for growth, but using ISO CLNP was not favored by the IETF partially due to political reasons, despite the fact that mature CLNP implementations were already available. 128 bits appeared to be a reasonable compromise at that time.
 
cnp3-ebook/protocols/ipv6 — French
IPv6 addressing architecture
 
cnp3-ebook/protocols/ipv6 — French
The experience of IPv4 revealed that the scalability of a network layer protocol heavily depends on its addressing architecture. The designers of IPv6 spent a lot of effort defining its addressing architecture :rfc:`3513`. All IPv6 addresses are 128 bits wide. This implies that there are :math:`340,282,366,920,938,463,463,374,607,431,768,211,456 (3.4 \times 10^{38})` different IPv6 addresses. As the surface of the Earth is about 510,072,000 :math:`km^2`, this implies that there are about :math:`6.67 \times 10^{23}` IPv6 addresses per square meter on Earth. Compared to IPv4, which offers only 8 addresses per square kilometer, this is a significant improvement on paper.
 
cnp3-ebook/protocols/ipv6 — French
Textual representation of IPv6 addresses
 
cnp3-ebook/protocols/ipv6 — French
It is sometimes necessary to write IPv6 addresses in text format, e.g. when manually configuring addresses or for documentation purposes. The preferred format for writing IPv6 addresses is ``x:x:x:x:x:x:x:x``, where the ``x`` 's are hexadecimal digits representing the eight 16-bit parts of the address. Here are a few examples of IPv6 addresses :
 
cnp3-ebook/protocols/ipv6 — French
``abcd:ef01:2345:6789:abcd:ef01:2345:6789``
 
cnp3-ebook/protocols/ipv6 — French
``2001:db8:0:0:8:800:200c:417a``
 
cnp3-ebook/protocols/ipv6 — French
``fe80:0:0:0:219:e3ff:fed7:1204``
 
cnp3-ebook/protocols/ipv6 — French
IPv6 addresses often contain a long sequence of bits set to ``0``. In this case, a compact notation has been defined. With this notation, `::` is used to indicate one or more groups of 16 bits blocks containing only bits set to `0`. For example,
 
cnp3-ebook/protocols/ipv6 — French
``2001:db8:0:0:8:800:200c:417a`` is represented as ``2001:db8::8:800:200c:417a``
 
cnp3-ebook/protocols/ipv6 — French
``ff01:0:0:0:0:0:0:101`` is represented as ``ff01::101``
 
cnp3-ebook/protocols/ipv6 — French
``0:0:0:0:0:0:0:1`` is represented as ``::1``
 
cnp3-ebook/protocols/ipv6 — French
``0:0:0:0:0:0:0:0`` is represented as ``::``
 
cnp3-ebook/protocols/ipv6 — French
An IPv6 prefix can be represented as `address/length`, where `length` is the length of the prefix in bits. For example, the three notations below correspond to the same IPv6 prefix :
 
cnp3-ebook/protocols/ipv6 — French
``2001:0db8:0000:cd30:0000:0000:0000:0000`` / ``60``
 
cnp3-ebook/protocols/ipv6 — French
``2001:0db8::cd30:0:0:0:0`` / ``60``
 
cnp3-ebook/protocols/ipv6 — French
``2001:0db8:0:cd30::`` / ``60``
 
cnp3-ebook/protocols/ipv6 — French
IPv6 supports unicast, multicast and anycast addresses. An IPv6 unicast address is used to identify one datalink-layer interface on a host. If a host has several datalink layer interfaces (e.g. an Ethernet interface and a WiFi interface), then it needs several IPv6 addresses. In general, an IPv6 unicast address is structured as shown in the figure below.
 
cnp3-ebook/protocols/ipv6 — French
An IPv6 unicast address is composed of three parts :
 
cnp3-ebook/protocols/ipv6 — French
A `global routing prefix` that is assigned to the Internet Service Provider that owns this block of addresses
 
cnp3-ebook/protocols/ipv6 — French
A `subnet identifier` that identifies a customer of the ISP
 
cnp3-ebook/protocols/ipv6 — French
An `interface identifier` that identifies a particular interface on a host
 
cnp3-ebook/protocols/ipv6 — French
The subnet identifier plays a key role in the scalability of network layer addressing architecture. An important point to be defined in a network layer protocol is the allocation of the network layer addresses. A naive allocation scheme would be to provide an address to each host when the host is attached to the Internet on a first come first served basis. With this solution, a host in Belgium could have address ``2001:db8::1`` while another host located in Africa would use address ``2001:db8::2``. Unfortunately, this would force all routers on the Internet to maintain one route towards each host. In the network layer, scalability is often a function of the number of routes stored on the router. A network will usually work better if its routers store fewer routes and network administrators usually try to minimize the number of routes that are known by their routers. For this, they often divide their network prefix in smaller blocks. For example, consider a company with three campuses, a large one and two smaller ones. The network administrator would probably divide his block of addresses as follows :
 
cnp3-ebook/protocols/ipv6 — French
the bottom half is used for the large campus
 
cnp3-ebook/protocols/ipv6 — French
the top half is divided in two smaller blocks, one for each small campus
 
cnp3-ebook/protocols/ipv6 — French
Inside each campus, the same division can be done, for example on a per building basis, starting from the buildings that host the largest number of nodes, e.g. the company datacenter. In each building, the same division can be done on a per floor basis, ... The advantage of such a hierarchical allocation of the addresses is that the routers in the large campus only need one route to reach a router in the smaller campus. The routers in the large campus would know more routes about the buildings in their campus, but they do not need to know the details of the organization of each smaller campus.
 
cnp3-ebook/protocols/ipv6 — French
To preserve the scalability of the routing system, it is important to minimize the number of routes that are stored on each router. A router cannot store and maintain one route for each of the almost 1 billion hosts that are connected to today's Internet. Routers should only maintain routes towards blocks of addresses and not towards individual hosts. For this, hosts are grouped in `subnets` based on their location in the network. A typical subnet groups all the hosts that are part of the same enterprise. An enterprise network is usually composed of several LANs interconnected by routers. A small block of addresses from the Enterprise's block is usually assigned to each LAN.
 
cnp3-ebook/protocols/ipv6 — French
In today's deployments, interface identifiers are always 64 bits wide. This implies that while there are :math:`2^{128}` different IPv6 addresses, they must be grouped in :math:`2^{64}` subnets. This could appear as a waste of resources, however using 64 bits for the host identifier allows IPv6 addresses to be auto-configured and also provides some benefits from a security point of view, as explained in section ICMPv6_.
 
cnp3-ebook/protocols/ipv6 — French
In practice, there are several types of IPv6 unicast address. Most of the `IPv6 unicast addresses <http://www.iana.org/assignments/ipv6-address-space/ipv6-address-space.xhtml>`_ are allocated in blocks under the responsibility of IANA_. The current IPv6 allocations are part of the `2000::/3` address block. Regional Internet Registries (RIR) such as RIPE_ in Europe, ARIN_ in North-America or AfriNIC in Africa have each received a `block of IPv6 addresses <http://www.iana.org/assignments/ipv6-unicast-address-assignments/ipv6-unicast-address-assignments.xhtml>`_ that they sub-allocate to Internet Service Providers in their region. The ISPs then sub-allocate addresses to their customers.
 
cnp3-ebook/protocols/ipv6 — French
When considering the allocation of IPv6 addresses, two types of address allocations are often distinguished. The RIRs allocate `provider-independent (PI)` addresses. PI addresses are usually allocated to Internet Service Providers and large companies that are connected to at least two different ISPs [CSP2009]_. Once a PI address block has been allocated to a company, this company can use its address block with the provider of its choice and change its provider at will. Internet Service Providers allocate `provider-aggregatable (PA)` address blocks from their own PI address block to their customers. A company that is connected to only one ISP should only use PA addresses. The drawback of PA addresses is that when a company using a PA address block changes its provider, it needs to change all the addresses that it uses. This can be a nightmare from an operational perspective and many companies are lobbying to obtain `PI` address blocks even if they are small and connected to a single provider. The typical size of the IPv6 address blocks are :
 
cnp3-ebook/protocols/ipv6 — French
``/32`` for an Internet Service Provider
 
cnp3-ebook/protocols/ipv6 — French
``/48`` for a single company
 
cnp3-ebook/protocols/ipv6 — French
``/56`` for small user sites
 
cnp3-ebook/protocols/ipv6 — French
``/64`` for a single user (e.g. a home user connected via ADSL)
 
cnp3-ebook/protocols/ipv6 — French
``/128`` in the rare case when it is known that no more than one host will be attached
 
cnp3-ebook/protocols/ipv6 — French
There is one difficulty with the utilization of these IPv6 prefixes. Consider Belnet, the Belgian research ISP that has been allocated the ``2001:6a8::/32`` prefix. Universities are connected to Belnet. UCLouvain uses prefix ``2001:6a8:3080::/48`` while the University of Liege uses ``2001:6a8:2d80::/48``. A commercial ISP uses prefix ``2a02:2788::/32``. Both Belnet and the commercial ISP are connected to the global Internet.
 
cnp3-ebook/protocols/ipv6 — French
The Belnet network advertises prefix ``2001:6a8::/32`` that includes the prefixes from both UCLouvain and ULg. These two subnetworks can be easily reached from any internet connected host. After a few years, UCLouvain decides to increase the redundancy of its Internet connectivity and buys transit service from ISP1. A direct link between UCLouvain and the commercial ISP appears on the network and UCLouvain expects to receive packets from both Belnet and the commercial ISP.
 
cnp3-ebook/protocols/ipv6 — French
Now, consider how a router inside ``alpha.com`` would reach a host in the ``UCLouvain`` network. This router has two routes towards ``2001:6a8:3080::1``. The first one, for prefix ``2001:6a8:3080::/48`` is via the direct link between the commercial ISP and UCLouvain. The second one, for prefix ``2001:6a8::/32`` is via the Internet and Belnet. Since :rfc:`1519` when a router knows several routes towards the same destination address, it must forward packets along the route having the longest prefix length. In the case of ``2001:6a8:3080::1``, this is the route ``2001:6a8:3080::/48`` that is used to forward the packet. This forwarding rule is called the `longest prefix match` or the `more specific match`. All IP routers implement this forwarding rule.
 
cnp3-ebook/protocols/ipv6 — French
To understand the `longest prefix match` forwarding, consider the IPv6 routing below.
 
cnp3-ebook/protocols/ipv6 — French
With the longest match rule, the route ``::/0`` plays a particular role. As this route has a prefix length of `0` bits, it matches all destination addresses. This route is often called the `default` route.
 
cnp3-ebook/protocols/ipv6 — French
a packet with destination ``2a02:2788:2c4:16f::1`` received by router `R` is destined to a host on interface ``eth0`` .
 
cnp3-ebook/protocols/ipv6 — French
a packet with destination ``2001:6a8:3080::1234`` matches three routes : ``::/0``, ``2001:6a8::/32`` and ``2001:6a8:3080::/48``. The packet is forwarded via gateway ``fe80::bad:cafe``
 
cnp3-ebook/protocols/ipv6 — French
a packet with destination ``2001:1890:123a::1:1e`` matches one route : ``::/0``. The packet is forwarded via ``fe80::dead:beef``
 
cnp3-ebook/protocols/ipv6 — French
a packet with destination ``2001:6a8:3880:40::2`` matches two routes : ``2001:6a8::/32`` and ``::/0``. The packet is forwarded via ``fe80::aaaa:bbbb``
 
cnp3-ebook/protocols/ipv6 — French
The longest prefix match can be implemented by using different data structures One possibility is to use a trie. Details on how to implement efficient packet forwarding algorithms may be found in [Varghese2005]_.
 
cnp3-ebook/protocols/ipv6 — French
For the companies that want to use IPv6 without being connected to the IPv6 Internet, :rfc:`4193` defines the `Unique Local Unicast (ULA)` addresses (``fc00::/7``). These ULA addresses play a similar role as the private IPv4 addresses defined in :rfc:`1918`. However, the size of the ``fc00::/7`` address block allows ULA to be much more flexible than private IPv4 addresses.
 
cnp3-ebook/protocols/ipv6 — French
Furthermore, the IETF has reserved some IPv6 addresses for a special usage. The two most important ones are :
 
cnp3-ebook/protocols/ipv6 — French
``0:0:0:0:0:0:0:1`` (``::1`` in compact form) is the IPv6 loopback address. This is the address of a logical interface that is always up and running on IPv6 enabled hosts.
 
cnp3-ebook/protocols/ipv6 — French
``0:0:0:0:0:0:0:0`` (``::`` in compact form) is the unspecified IPv6 address. This is the IPv6 address that a host can use as source address when trying to acquire an official address.
 
cnp3-ebook/protocols/ipv6 — French
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.
 
cnp3-ebook/protocols/ipv6 — French
All IPv6 hosts have several addresses
 
cnp3-ebook/protocols/ipv6 — French
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.
 
cnp3-ebook/protocols/ipv6 — French
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.
 
cnp3-ebook/protocols/ipv6 — French
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``.
 
cnp3-ebook/protocols/ipv6 — French
Finally, :rfc:`4291` defines the structure of the IPv6 multicast addresses [#fmultiiana]_. This structure is depicted in the figure below.
 
cnp3-ebook/protocols/ipv6 — French
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.
 
cnp3-ebook/protocols/ipv6 — French
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.
 
cnp3-ebook/protocols/ipv6 — French
IPv6 packet format
 
cnp3-ebook/protocols/ipv6 — French
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.
 
cnp3-ebook/protocols/ipv6 — French
The IP version 6 header (:rfc:`2460`)
 
cnp3-ebook/protocols/ipv6 — French
Apart from the source and destination addresses, the IPv6 header contains the following fields :
 
cnp3-ebook/protocols/ipv6 — French
`Version` : a 4 bits field set to `6` and intended to allow IP to evolve in the future if needed
 
cnp3-ebook/protocols/ipv6 — French
`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`
 
cnp3-ebook/protocols/ipv6 — French
`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.
 
cnp3-ebook/protocols/ipv6 — French
`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.
 
cnp3-ebook/protocols/ipv6 — French
`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.
 
cnp3-ebook/protocols/ipv6 — French
`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.
 
cnp3-ebook/protocols/ipv6 — French
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]_.
 
cnp3-ebook/protocols/ipv6 — French
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 :
 
cnp3-ebook/protocols/ipv6 — French
``TCP`` uses `Next Header` number ``6``
 
cnp3-ebook/protocols/ipv6 — French
``UDP`` uses `Next Header` number ``17``
 
cnp3-ebook/protocols/ipv6 — French
``SCTP`` uses `Next Header` number ``132``
 
cnp3-ebook/protocols/ipv6 — French
For example, an IPv6 packet that contains an TCP segment would appear as shown in the figure below.
 
cnp3-ebook/protocols/ipv6 — French
An IPv6 packet containing an TCP segment
 
cnp3-ebook/protocols/ipv6 — French
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.
 
cnp3-ebook/protocols/ipv6 — French
:rfc:`2460` defines several types of IPv6 extension headers that could be added to an IPv6 packet :
 
cnp3-ebook/protocols/ipv6 — French
`Hop-by-Hop Options` header. This option is processed by routers and hosts.
 
cnp3-ebook/protocols/ipv6 — French
`Destination Options` header. This option is processed only by hosts.
 
cnp3-ebook/protocols/ipv6 — French
`Routing` header. This option is processed by some nodes.
 
cnp3-ebook/protocols/ipv6 — French
`Fragment` header. This option is processed only by hosts.
 
cnp3-ebook/protocols/ipv6 — French
`Authentication` header. This option is processed only by hosts.