Ipv6 Pmtu Discovery - HP A5120 EI Series Configuration Manual

If Host B uses this IPv6 address, Host B returns an NA message. The NA message contains the IPv6
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address of Host B.
Host A learns that the IPv6 address is being used by Host B after receiving the NA message from
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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 in the following steps:
At startup, a node sends an RS message to request from any available router the address prefix
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and other configuration information for autoconfiguration.
A router returns an RA message containing information such as Prefix Information options. (The
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router also periodically sends an RA message.)
The node automatically generates an IPv6 address and other configuration information according
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to the address prefix and other configuration parameters in the RA message.
NOTE:

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 may contain only a default route to the gateway in its routing table. When certain
conditions are satisfied, 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 satisfied.
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.

The IPv6 packet to be forwarded does not contain any routing header.


IPv6 PMTU discovery

The links that a packet passes from a source to a destination may have different MTUs. In IPv6, when the
packet size exceeds the path MTU (PMTU) 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 PMTU discovery mechanism is designed to find the minimum MTU of all links in the path between a
source and a destination. Figure 55 shows how a source host discovers the PMTU to a destination host.
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