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The network layer
The main objective of the network layer is to allow hosts, connected to different networks, to exchange information through intermediate systems called :term:`router`. The unit of information in the network layer is called a :term:`packet`.
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.
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.
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.
A local area network
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.
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.
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.
IP version 6
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.
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) :
TUBA proposed in :rfc:`1347` and :rfc:`1561`
PIP proposed in :rfc:`1621`
SIPP proposed in :rfc:`1710`
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 :
IPv6 addresses are encoded as a 128 bits field
The IPv6 header has a simple format that can easily be parsed by hardware devices
A host should be able to configure its IPv6 address automatically
Security must be part of IPv6
The IPng address size
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.
IPv6 addressing architecture
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.
Textual representation of IPv6 addresses
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 :
``abcd:ef01:2345:6789:abcd:ef01:2345:6789``
``2001:db8:0:0:8:800:200c:417a``
``fe80:0:0:0:219:e3ff:fed7:1204``
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,
``2001:db8:0:0:8:800:200c:417a`` is represented as ``2001:db8::8:800:200c:417a``
``ff01:0:0:0:0:0:0:101`` is represented as ``ff01::101``
``0:0:0:0:0:0:0:1`` is represented as ``::1``
``0:0:0:0:0:0:0:0`` is represented as ``::``
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 :
``2001:0db8:0000:cd30:0000:0000:0000:0000`` / ``60``
``2001:0db8::cd30:0:0:0:0`` / ``60``
``2001:0db8:0:cd30::`` / ``60``
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.
An IPv6 unicast address is composed of three parts :
A `global routing prefix` that is assigned to the Internet Service Provider that owns this block of addresses
A `subnet identifier` that identifies a customer of the ISP
An `interface identifier` that identifies a particular interface on a host
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 :
the bottom half is used for the large campus
the top half is divided in two smaller blocks, one for each small campus
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.

Ellipsis

The string uses three dots (...) instead of an ellipsis character (…)

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.
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_.
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.