HP VPN Firewall Appliances Network Management Configuration Guide

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3. Host A learns that the IPv6 address is being used by Host B after receiving the NA message from
Host B. If receiving no NA message, Host A decides that the IPv6 address is not in use and uses this
address.
Router/prefix discovery and address autoconfiguration
Router/prefix discovery enables a node to locate the neighboring routers and to learn from the received
RA message configuration parameters such as the prefix of the network where the node is located.
Stateless address autoconfiguration enables a node to generate an IPv6 address automatically
according to the information obtained through router/prefix discovery.
Router/prefix discovery is implemented through RS and RA messages as follows:
1. At startup, a node sends an RS message to request the address prefix and other configuration
information for autoconfiguration.
2. A router returns an RA message containing information such as Prefix Information options. (The
router also periodically sends an RA message.)
3. The node automatically generates an IPv6 address and other configuration information according
to the address prefix and other configuration parameters in the RA message.
In addition to an address prefix, the Prefix Information option also contains the preferred lifetime and
valid lifetime of the address prefix. Nodes update the preferred lifetime and valid lifetime accordingly
through periodic RA messages.
An automatically generated address is applicable within the valid lifetime and is removed when the valid
lifetime expires.
Redirection
A newly started host might have only a default route to the gateway in its routing table. When certain
conditions are met, the gateway sends an ICMPv6 Redirect message to the source host, so the host can
select a better next hop to forward packets (similar to the ICMP redirection function in IPv4).
The gateway sends an ICMPv6 Redirect message when the following conditions are met:
• The receiving interface is the forwarding interface.
• The selected route itself is not created or modified by an ICMPv6 Redirect message.
• The selected route is not the default route.
IPv6 path MTU discovery
The links that a packet passes from a source to a destination might have different MTUs. In IPv6, when the
packet size exceeds the path MTU of a link, the packet is fragmented at the source end of the link to
reduce the processing pressure on intermediate devices and to use network resources effectively.
The path MTU discovery mechanism is designed to find the minimum MTU of all links in the path
between a source and a destination. Figure 361 sho
ws how a source host dis
covers the path MTU to a
destination host.