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../../protocols/ipv6.rst:839
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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.
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../../protocols/ipv6.rst:841
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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.
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../../protocols/ipv6.rst:843
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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
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../../protocols/ipv6.rst:846
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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.
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../../protocols/ipv6.rst:848
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: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.
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../../protocols/ipv6.rst:850
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The DAD algorithm is also used with `link-local` addresses.
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../../protocols/ipv6.rst:852
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Using a datalink layer address to derive a 64 bits identifier for each host raises privacy concerns as the host will always use the same identifier. Attackers could use this to track hosts on the Internet. An extension to the Stateless Address Configuration mechanism that does not raise privacy concerns is defined in :rfc:`4941`. These privacy extensions allow a host to generate its 64 bits identifier randomly every time it attaches to a subnet. It then becomes impossible for an attacker to use the 64-bits identifier to track a host.
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../../protocols/ipv6.rst:857
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Using a `Hop Limit` of ``255`` prevents one family of attacks against ICMPv6, but other attacks still remain possible. A detailed discussion of the security issues with IPv6 is outside the scope of this book. It is possible to secure NDP by using the `Cryptographically Generated IPv6 Addresses` (CGA) defined in :rfc:`3972`. The Secure Neighbor Discovery Protocol is defined in :rfc:`3971`. A detailed discussion of the security of IPv6 may be found in [HV2008]_.
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../../protocols/ipv6.rst:859
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In practice, there is usually one DHCP server per group of subnets and the routers capture on each subnet the DHCP messages and forward them to the DHCP server.
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