cnp3-ebook/protocols/ipv6 — English @ Weblate

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177 ../../protocols/ipv6.rst:627

	English Rate limitation of ICMP messages 32/10000 · 32
178 ../../protocols/ipv6.rst:629

	English High-end hardware based routers use special purpose chips on their interfaces to forward IPv6 packets at line rate. These chips are optimized to process `correct` IP packets. They are not able to create ICMP messages at line rate. When such a chip receives an IP packet that triggers an ICMP message, it interrupts the main CPU of the router and the software running on this CPU processes the packet. This CPU is much slower than the hardware acceleration found on the interfaces [Gill2004]_. It would be overloaded if it had to process IP packets at line rate and generate one ICMP message for each received packet. To protect this CPU, high-end routers limit the rate at which the hardware can interrupt the main CPU and thus the rate at which ICMP messages can be generated. This implies that not all erroneous IP packets cause the transmission of an ICMP message. The risk of overloading the main CPU of the router is also the reason why using hop-by-hop IPv6 options, including the router alert option is discouraged [#falert]_. 1033/10000 · 1033
179 ../../protocols/ipv6.rst:633

	English The IPv6 subnet 15/10000 · 15
180 ../../protocols/ipv6.rst:635

	English 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. 571/10000 · 571
181 ../../protocols/ipv6.rst:637

	English 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 : 671/10000 · 671
182 ../../protocols/ipv6.rst:639

	English the source MAC address 22/10000 · 22
183 ../../protocols/ipv6.rst:640

	English the destination MAC address 27/10000 · 27
184 ../../protocols/ipv6.rst:641

	English some multiplexing information to indicate the network layer protocol that is responsible for the payload of the frame 117/10000 · 117
185 ../../protocols/ipv6.rst:643

	English 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. 445/10000 · 445
186 ../../protocols/ipv6.rst:645

	English 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. 1376/10000 · 1376
187 ../../protocols/ipv6.rst:649

	English Interactions between IPv6 and the datalink layer 48/10000 · 48
188 ../../protocols/ipv6.rst:653

	English 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]_. 299/10000 · 299
189 ../../protocols/ipv6.rst:671

	English 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. 737/10000 · 737
190 ../../protocols/ipv6.rst:673

	English Converting MAC addresses in host identifiers 44/10000 · 44
191 ../../protocols/ipv6.rst:675

	English 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. 240/10000 · 240
192 ../../protocols/ipv6.rst:680

	English A MAC address 13/10000 · 13
193 ../../protocols/ipv6.rst:682

	English 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. 325/10000 · 325
194 ../../protocols/ipv6.rst:687

	English A MAC address converted into a 64 bits host identifier 54/10000 · 54
195 ../../protocols/ipv6.rst:689

	English 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. 531/10000 · 531
196 ../../protocols/ipv6.rst:692

	English The next step is to connect the LAN to the Internet. For this, a router is attached to the LAN. 95/10000 · 95

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