With the introduction of IPv6 came different ways for the same things to get done. For example address resolution can now be done with NDP which stands for Neighbor Discovery Protocol. NDP not only can perform address resolution but also perform DAD (Duplicate Address Detection) and router discovery. These functions are carried out by ICMPv6 packets which are NS (Neighbor Solicitation), NA (Neighbor Advertisement), RS (Router Solicitation), and RA (Router Advertisement).
Neighbor Solicitation- The first message I am going to talk about is NS (Neighbor Solicitation.) These messages can be sent to initiate address resolution or DAD. The source of a NS is usually the IPv6 address of the sender or unspecified (::) when sending a DAD message. The destination for a NS is the solicited-node multicast address of the target IPv6 address for both address resolution and DAD. The NS for address resolution is basically the sending node asking a device on the local link “Who owns this IP address?” The NS for DAD essentially asks “does anyone on this local link already have this address?”
Neighbor Advertisement- The message that is sent in response to a NS is the NA (Neighbor Advertisement.) When it comes to address resolution, this is the IPv4 equivalent of an ARP reply. When a NA is sent in response to an address resolution request via NS, the NA replies via unicast including its MAC address (link-layer address). The only time a NA will be sent for DAD, is when a host on the local link actually has the tentative IPv6 address. A NA can also be sent unsolicited, either to inform the hosts on the local link of a change in MAC address or to inform of a new IPv6 to MAC binding that they need to be aware of.
Router Solicitation- IPv6 has added a new way to explicitly discover routers and also receive network configuration details from said router. The message that initiates this process is a RS (Router Solicitation.) This is a message sent to a multicast address of FF02::2 which is a multicast address for “All Routers.” When a RS is sent to this address, every router on the local link will receive this and is usually sent so a node on the local network can get their network configurations promptly instead of waiting for periodic RA packets. RS messages are usually sent immediately after an interface comes up to prevent delay in configuration.
Router Advertisement- RA (Router Advertisements) packets serve multiple purposes because not only do they help devices learn of the router but the devices that receive an RA will be able to receive information needed to auto configure itself via SLAAC. RA provides network configuration details such as the prefix and prefix length, the router’s IPv6 link-local address, MTU, and hop limit. With this information not only does the receiving device receive the default gateway, but they also get important information to auto-configure via SLAAC if needed to. On top of this, RA packets can also include configuration flags. These configuration flags will let the receiver know whether it needs to obtain DNS server information from another protocol such as DHCPv6. The M flag means use DHCPv6 for an IPv6 address and the O flag means use DHCPv6 for other information such as a DNS server address. RA packets can be sent periodically or sent in response to an RS. Periodic RA packets are sent to FF02::1 which is a “All nodes” multicast address, and RA packets in response to RS packets are typically unicast messages.
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