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Router Advertisements and Hop Limits
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``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
 
This message is sent from the link-local address of the router on the subnet. Its destination is the IPv6 multicast address that targets all IPv6 enabled hosts (i.e. ``ff02::1``). The `Cur Hop Limit` field, if different from zero, allows specifying the default `Hop Limit` that hosts should use when sending IPv6 packets from this subnet. ``64`` is a frequently used value. The `M` and `O` bits are used to indicate that some information can be obtained from DHCPv6. The `Router Lifetime` parameter provides the expected lifetime (in seconds) of the sending router acting as a default router. This lifetime enables planning the replacement of a router by another one in the same subnet. The `Reachable Time` and the `Retrans Timer` parameter are used to configure the utilization of the NDP protocol on the hosts attached to the subnet.
 
Several options can be included in the Router Advertisement message. The simplest one is the MTU option that indicates the MTU to be used within the subnet. Thanks to this option, it is possible to ensure that all devices attached to the same subnet use the same MTU. Otherwise, operational problems could occur. The `Prefix` option is more important. It provides information about the prefix(es) that is (are) advertised by the router on the subnet.
 
The Prefix information option
 
The key information placed in this option are the prefix and its length. This allows the hosts attached to the subnet to automatically configure their own IPv6 address. The `Valid` and `Preferred` `Lifetimes` provide information about the expected lifetime of the prefixes. Associating some time validity to prefixes is a good practice from an operational viewpoint. There are some situations where the prefix assigned to a subnet needs to change without impacting the hosts attached to the subnet. This is often called the IPv6 renumbering problem in the literature :rfc:`7010`. A very simple scenario is the following. An SME subscribes to one ISP. Its router is attached to another router of this ISP and advertises a prefix assigned by the ISP. The SME is composed of a single subnet and all its hosts rely on stateless address configuration. After a few years, the SME decides to change of network provider. It connects its router to the second ISP and receives a different prefix from this ISP. At this point, two prefixes are advertised on the SME's subnet. The old prefix can be advertised with a short lifetime to ensure that hosts will stop using it while the new one is advertised with a longer lifetime. After sometime, the router stops advertising the old prefix and the hosts stop using it. The old prefix can now be returned back to the first ISP. In larger networks, renumbering an IPv6 remains a difficult operational problem [LeB2009]_.
 
Upon reception of this message, the host can derive its global IPv6 address by concatenating its 64 bits identifier with the received prefix. It concludes the SLAAC by sending a Neighbor Solicitation message targeted at its global IPv6 address to ensure that no other host is using the same IPv6 address.
 
Router Advertisements and Hop Limits
 
ICMPv6 Router Advertisements messages are regularly sent by routers. They are destined to all devices attached to the local subnet and no router should ever forward them to another subnet. Still, these messages are sent inside IPv6 packets whose `Hop Limit` is always set to ``255``. Given that the packet should not be forwarded outside of the local subnet, the reader could expect instead a `Hop Limit` set to ``1``. Using a `Hop Limit` set to ``255`` provides one important benefit from a security viewpoint and this hack has been adapted in several Internet protocols. When a host receives a `Router Advertisement` message, it expects that this message has been generated by a router attached to the same subnet. Using a `Hop Limit` of ``255`` provides a simple check for this. If the message was generated by an attacker outside the subnet, it would reach the subnet with a decremented `Hop Limit`. Checking that the `Hop Limit` is set to ``255`` is a simple [#fsend]_ verification that the packet was generated on this particular subnet. :rfc:`5082` provides other examples of protocols that use this hack and discuss its limitations.
 
Routers regularly send Router Advertisement messages. These messages are triggered by a timer that is often set at approximately 30 seconds. Usually, hosts wait for the arrival of a Router Advertisement message to configure their address. This implies that hosts could sometimes need to wait 30 seconds before being able to configure their address. If this delay is too long, a host can also send a `Router Solicitation` message. This message is sent towards the multicast address that corresponds to all IPv6 routers (i.e. ``FF01::2``) and the default router will reply.
 
The last point that needs to be explained about ICMPv6 is the `Redirect` message. This message is used when there is more than one router on a subnet as shown in the figure below.
 
In this network, ``router1`` is the default router for all hosts. The second router, ``router2`` provides connectivity to a specific IPv6 subnet, e.g. ``2001:db8:abcd::/48``. These two routers attached to the same subnet can be used in different ways. First, it is possible to manually configure the routing tables on all hosts to add a route towards ``2001:db8:abcd::/48`` via ``router2``. Unfortunately, forcing such manual configuration boils down all the benefits of using address auto-configuration in IPv6. The second approach is to automatically configure a default route via ``router1`` on all hosts. With such route, when a host needs to send a packet to any address within ``2001:db8:abcd::/48``, it will send it to ``router1``. ``router1`` would consult its routing table and find that the packet needs to be sent again on the subnet to reach ``router2``. This is a waste of time. A better approach would be to enable the hosts to automatically learn the new route. This is possible thanks to the ICMPv6 `Redirect` message. When ``router1`` receives a packet that needs to be forwarded back on the same interface, it replies with a `Redirect` message that indicates that the packet should have been sent via ``router2``. Upon reception of a `Redirect` message, the host updates it forwarding table to include a new transient entry for the destination reported in the message. A timeout is usually associated with this transient entry to automatically delete it after some time.
 
An alternative is the Dynamic Host Configuration Protocol (DHCP) defined in :rfc:`2131` and :rfc:`3315`. DHCP allows a host to automatically retrieve its assigned IPv6 address, but relies on server. A DHCP server is associated to each subnet [#fdhcpserver]_. Each DHCP server manages a pool of IPv6 addresses assigned to the subnet. When a host is first attached to the subnet, it sends a DHCP request message in a UDP segment (the DHCP server listens on port 67). As the host knows neither its IPv6 address nor the IPv6 address of the DHCP server, this UDP segment is sent inside a multicast packet target at the DHCP servers. The DHCP request may contain various options such as the name of the host, its datalink layer address, etc. The server captures the DHCP request and selects an unassigned address in its address pool. It then sends the assigned IPv6 address in a DHCP reply message which contains the datalink layer address of the host and additional information such as the subnet mask, the address of the default router or the address of the DNS resolver. The DHCP reply also specifies the lifetime of the address allocation. This forces the host to renew its address allocation once it expires. Thanks to the limited lease time, IP addresses are automatically returned to the pool of addresses when hosts are powered off.
 
Both SLAAC and DHCPv6 can be extended to provide additional information beyond the IPv6 prefix/address. For example, :rfc:`6106` defines options for the ICMPv6 ND message that can carry the IPv6 address of the recursive DNS resolver and a list of default domain search suffixes. It is also possible to combine SLAAC with DHCPv6. :rfc:`3736` defines a stateless variant of DHCPv6 that can be used to distribute DNS information while SLAAC is used to distribute the prefixes.
 
Footnotes
 
The full list of allocated IPv6 multicast addresses is available at http://www.iana.org/assignments/ipv6-multicast-addresses
 
The IANA_ maintains the list of all allocated Next Header types at http://www.iana.org/assignments/protocol-numbers/
 
When IPv4 was designed, the situation was different. The IPv4 header includes a checksum that only covers the network header. This checksum is computed by the source and updated by all intermediate routers that decrement the TTL, which is the IPv4 equivalent of the `HopLimit` used by IPv6.
 
Until a few years ago, all hosts replied to `Echo request` ICMP messages. However, due to the security problems that have affected TCP/IP implementations, many of these implementations can now be configured to disable answering `Echo request` ICMP messages.
 
For a discussion of the issues with the router alert IP option, see http://tools.ietf.org/html/draft-rahman-rtg-router-alert-dangerous-00 or http://tools.ietf.org/html/draft-rahman-rtg-router-alert-considerations-03
 
For simplicity, you assume that each datalink layer interface is assigned a 64 bits MAC address. As we will see later, today's datalink layer technologies mainly use 48 bits MAC addresses, but the smaller addresses can easily be converted into 64 bits addresses.
 
:rfc:`4291` and :rfc:`4861` explain in more details how the IPv6 multicast address is determined from the target IPv6 unicast address. These details are outside the scope of this book, but may matter if you try to understand a packet trace.
 
The DAD algorithm is also used with `link-local` addresses.
 

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../../protocols/ipv6.rst:791
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locale/fr/LC_MESSAGES/protocols/ipv6.po, string 215