sec-address-allocation.tex

   1 \subsection{Address allocation}
   2 Considering the TCP/IP protocol suite, in order to be able to communicate on the
   3 IP layer, a device needs to configure one of its network interfaces with
   4 an IP address that can be reached from the network that the device wants to connect
   5 to. There are several ways of achieving this:
   6
   7 \begin{itemize}
   8   \item The IP address is pre-selected by a human and stored in the device
   9     configuration. This process is cumbersome when more than one device needs to
  10     be configured, and humans have to remember which addresses are configured on
  11     which device, and which addresses are still free to use on additional
  12     devices.
  13
  14   \item Deploying a central server that assigns network addresses
  15     to the devices in the network, which in order query the server (for example
  16     via broadcast or on a lower layer) for a unique address before they use the
  17     IP layer for the first time. This technique supersedes the dependency of
  18     human memory for mapping IP addresses to devices, and can easily be combined
  19     with additional central configuration management, like the automatic
  20     assignment of a static gateway, or a DNS resolver.\\
  21     The DHCP protocol is one example of central IP address management.
  22
  23   \item A third alternative is the use of a distributed protocol which enables
  24     the devices on the network to choose addresses in accordance with other
  25     devices on the same network, so no IP address is used more than once. In
  26     respect to the Internet of Things, this approach has the advantage that
  27     those devices can easily be used in different scenarios without needing
  28     central deployment at all, and also allowing them to change their addresses
  29     dynamically, therefore easily reacting to changes in the network. \\
  30     There are two major protocols which are used for dynamic configuration of IP
  31     addresses. In the IPv4 world, Link-Local Addressing~\cite{rfc3927} is often
  32     used, and in IPv6 networks, Stateless Address
  33     Autoconfiguration~\cite{rfc4862} is a fundamental feature built into every
  34     device.
  35
  36 \end{itemize}
  37
  38 \paragraph{IPv4 Link-Local Addressing}
  39
  40 \term{Link-Local Addressing}, also known as \term{Automatic Private IP
  41 Addressing (APIPA)} or \term{Zeroconf}, uses the IPv4 subnet
  42 \code{169.254.0.0/16} for addressing. Every device first chooses a random
  43 address from that address space.  Then it checks if the chosen address is used
  44 by any other device on the network by probing the chosen address, which is
  45 usually done using the ARP protocol. If the probing process results that the
  46 address is not used on the network (e.~g.  no device returned an ARP response
  47 during a random time interval), the device claims its chosen address and uses it
  48 for communication on the IPv4 layer. If the chosen address is already used, the
  49   device continues the process, subsequently choosing a new random address and
  50   trying to claim it, until a free address has been found.
  51
  52 \paragraph{IPv6 Stateless Address Autoconfiguration}
  53
  54 Similar to IPv4 Link-Local Addressing, devices configured with \term{IPv6
  55 Stateless Addressing Autoconfiguration} use an IPv6 address from the subnet
  56 \code{fe80::/64}. First, an \term{interface identifier} is generated using the
  57 interface's MAC address. Since MAC addresses must be unique in the network, a
  58 unique IPv6 address is obtained by combining the subnet prefix and the interface
  59 identifier. To ensure that no other device exists with this generated IPv6
  60 address, the device performs \term{Duplicate Address Detection} on the network
  61 through \term{Neighbor Advertisement} messages and listening for \term{Neighbor
  62 Solicitation} messages. If such messages are received from other hosts, the
  63 configured address cannot be used by the device and must be discarded.
  64 Therefore, in order to use IPv6 effectively, it must be guarranteed that MAC
  65 addresses are unique on the network.
  66
  67 In contrast to IPv4 Link-Local Addressing, IPv6 Stateless Address
  68 Autoconfiguration can also be used with a central server. In this case, a
  69 central server broadcasts \term{Router Solicitation} messages on the network
  70 which contain a global network prefix. The hosts on the network can then use
  71 that prefix instead to configure a global IPv6 address.
  72
  73 % vim: set ft=tex et ts=2 sw=2 :