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The IPv6 subnet
Until now, we have focused our discussion on the utilization of IPv6 on point-to-point links. Although there are point-to-point links in the Internet, mainly between routers and sometimes hosts, most of the hosts are attached to datalink layer networks such as Ethernet LANs or WiFi networks. These datalink layer networks play an important role in today's Internet and have heavily influenced the design of the operation of IPv6. To understand IPv6 and ICMPv6 completely, we first need to correctly understand the key principles behind these datalink layer technologies.
As explained earlier, devices attached to a Local Area Network can directly exchange frames among themselves. For this, each datalink layer interface on a device (host, router, ...) attached to such a network is identified by a MAC address. Each datalink layer interface includes a unique hardwired MAC address. MAC addresses are allocated to manufacturers in blocks and interface is numbered with a unique address. Thanks to the global unicity of the MAC addresses, the datalink layer service can assume that two hosts attached to a LAN have different addresses. Most LANs provide an unreliable connectionless service and a datalink layer frame has a header containing :
the source MAC address
the destination MAC address
some multiplexing information to indicate the network layer protocol that is responsible for the payload of the frame
LANs also provide a broadcast and a multicast service. The broadcast service enables a device to send a single frame to all the devices attached to the same LAN. This is done by reserving a special broadcast MAC address (typically all bits of the address are set to one). To broadcast a frame, a device simply needs to send a frame whose destination is the broadcast address. All devices attached to the datalink network will receive the frame.
The broadcast service allows easily reaching all devices attached to a datalink layer network. It has been widely used to support IP version 4. A drawback of using the broadcast service to support a network layer protocol is that a broadcast frame that contains a network layer packet is always delivered to all devices attached to the datalink network, even if some of these devices do not support the network layer protocol. The multicast service is a useful alternative to the broadcast service. To understand its operation, it is important to understand how a datalink layer interface operates. In shared media LANs, all devices are attached to the same physical medium and all frames are delivered to all devices. When such a frame is received by a datalink layer interface, it compares the destination address with the MAC address of the device. If the two addresses match, or the destination address is the broadcast address, the frame is destined to the device and its payload is delivered to the network layer protocol. The multicast service exploits this principle. A multicast address is a logical address. To receive frames destined to a multicast address in a shared media LAN, a device captures all frames having this multicast address as their destination. All IPv6 nodes are capable of capturing datalink layer frames destined to different multicast addresses.
Interactions between IPv6 and the datalink layer
IPv6 hosts and routers frequently interact with the datalink layer service. To understand the main interactions, it is useful to analyze all the packets that are exchanged when a simple network containing a few hosts and routers is built. Let us first start with a LAN containing two hosts [#fMAC]_.
Hosts ``A`` and ``B`` are attached to the same datalink layer network. They can thus exchange frames by using the MAC addresses shown in the figure above. To be able to use IPv6 to exchange packets, they need to have an IPv6 address. One possibility would be to manually configure an IPv6 address on each host. However, IPv6 provides a better solution thanks to the `link-local` IPv6 addresses. A `link-local` IPv6 address is an address that is composed by concatenating the ``fe80:://64`` prefix with the MAC address of the device. In the example above, host A would use IPv6 `link-local` address ``fe80::0223:45FF:FE67:89ab`` and host B ``fe80::0234:56FF:FE78:9abc``. With these two IPv6 addresses, the hosts can exchange IPv6 packets.
Converting MAC addresses in host identifiers
Appendix A of :rfc:`4291` provides the algorithm used to convert a 48 bits MAC address into a 64 bits host identifier. This algorithm builds upon the structure of the MAC addresses. A MAC address is represented as shown in the figure below.
A MAC address
MAC addresses are allocated in blocks of :math:`2^{20}`. When a company registers for a block of MAC addresses, it receives an identifier. company identifier is then used to populated the `c` bits of the MAC addresses. The company can allocate all addresses in starting with this prefix and manages the `m` bits as it wishes.
A MAC address converted into a 64 bits host identifier
Inside a MAC address, the two bits indicated as `0` and `g` in the figure above play a special role. The first bit indicates whether the address is universal or local. The `g` bit indicates whether this is a multicast address or a unicast address. The MAC address can be converted into a 64 bits host identifier by flipping the value of the `0` bit and inserting ``FFFE``, i.e. ``1111111111111110`` in binary, in the middle of the address as shown in the figure below. The `c`, `m` and `g` bits of the MAC address are not modified.
The next step is to connect the LAN to the Internet. For this, a router is attached to the LAN.
Assume that the LAN containing the two hosts and the router is assigned prefix ``2001:db8:1234:5678/64``. A first solution to configure the IPv6 addresses in this network is to assign them manually. A possible assignment is :
``2001:db8:1234:5678::1`` is assigned to ``router``
``2001:db8:1234:5678::AA`` is assigned to ``hostA``
``2001:db8:1234:5678::BB`` is assigned to ``hostB``
To be able to exchange IPv6 packets with ``hostB``, ``hostA`` needs to know the MAC address of the interface of ``hostB`` on the LAN. This is the `address resolution` problem. In IPv6, this problem is solved by using the Neighbor Discovery Protocol (NDP). NDP is specified in :rfc:`4861`. This protocol is part of ICMPv6 and uses the multicast datalink layer service.
NDP allows a host to discover the MAC address used by any other host attached to the same LAN. NDP operates in two steps. First, the querier sends a multicast ICMPv6 Neighbor Solicitation message that contains as parameter the queried IPv6 address. This multicast ICMPv6 NS is placed inside a multicast frame [#fndpmulti]_. The queried node receives the frame, parses it and replies with a unicast ICMPv6 Neighbor Advertisement that provides its own IPv6 and MAC addresses. Upon reception of the Neighbor Advertisement message, the querier stores the mapping between the IPv6 and the MAC address inside its NDP table. This table is a data structure that maintains a cache of the recently received Neighbor Advertisement. Thanks to this cache, a host only needs to send a Neighbor Solicitation message for the first packet that it sends to a given host. After this initial packet, the NDP table can provide the mapping between the destination IPv6 address and the corresponding MAC address.
The NS message can also be used to verify the reachability of a host in the local subnet. For this usage, NS messages can be sent in unicast since other nodes on the subnet do not need to process the message.
When an entry in the NDP table times out on a host, it may either be deleted or the host may try to validate it by sending the NS message again.
This is not the only usage of the Neighbor Solicitation and Neighbor Advertisement messages. They are also used to detect the utilization of duplicate addresses. In the network above, consider what happens when a new host is connected to the LAN. If this host is configured by mistake with the same address as ``hostA`` (i.e. ``2001:db8:1234:5678::AA``), problems could occur. Indeed, if two hosts have the same IPv6 address on the LAN, but different MAC addresses, it will be difficult to correctly reach them. IPv6 anticipated this problem and includes a `Duplicate Address Detection` Algorithm (DAD). When an IPv6 address [#flinklocal]_ is configured on a host, by any means, the host must verify the uniqueness of this address on the LAN. For this, it multicasts an ICMPv6 Neighbor Solicitation that queries the network for its newly configured address. The IPv6 source address of this NS is set to ``::`` (i.e. the reserved unassigned address) if the host does not already have an IPv6 address on this subnet). If the NS does not receive any answer, the new address is considered to be unique and can safely be used. Otherwise, the new address is refused and an error message should be returned to the system administrator or a new IPv6 address should be generated. The `Duplicate Address Detection` Algorithm can prevent various operational problems that are often difficult to debug.
Few users manually configure the IPv6 addresses on their hosts. They prefer to rely on protocols that can automatically configure their IPv6 addresses. IPv6 supports two such protocols : DHCPv6 and the Stateless Address Autoconfiguration (SLAAC).
The Stateless Address Autoconfiguration (SLAAC) mechanism defined in :rfc:`4862` enables hosts to automatically configure their addresses without maintaining any state. When a host boots, it derives its identifier from its datalink layer address [#fprivacy]_ as explained earlier and concatenates this 64 bits identifier to the `FE80::/64` prefix to obtain its link-local IPv6 address. It then multicasts a Neighbor Solicitation with its link-local address as a target to verify whether another host is using the same link-local address on this subnet. If it receives a Neighbor Advertisement indicating that the link-local address is used by another host, it generates another 64 bits identifier and sends again a Neighbor Solicitation. If there is no answer, the host considers its link-local address to be valid. This address will be used as the source address for all NDP messages sent on the subnet.
To automatically configure its global IPv6 address, the host must know the globally routable IPv6 prefix that is used on the local subnet. IPv6 routers regularly multicast ICMPv6 Router Advertisement messages that indicate the IPv6 prefix assigned to the subnet. The Router Advertisement message contains several interesting fields.
Format of the ICMPv6 Router Advertisement message

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Source string location
../../protocols/ipv6.rst:687
String age
2 years ago
Source string age
2 years ago
Translation file
locale/pot/protocols/ipv6.pot, string 194