Ipv6 Pmtu Discovery - 3Com 4510G Configuration Manual

2) If node B uses this IPv6 address, node B returns an NA message. The NA message contains the
IPv6 address of node B.
3) Node A learns that the IPv6 address is being used by node B after receiving the NA message from
node B. Otherwise, node B is not using the IPv6 address and node A can use it.
Router/prefix discovery and address autoconfiguration
Router/prefix discovery means that a node locates the neighboring routers, and learns the prefix of the
network where the host is located, and other configuration parameters from the received RA message.
Stateless address autoconfiguration means that a node automatically generates an IPv6 address
according to the information obtained through router/prefix discovery.
The router/prefix discovery is implemented through RS and RA messages. The router/prefix discovery
procedure is as follows:
1) After started, a node sends an RS message to request the router for the address prefix and other
configuration information for the purpose of autoconfiguration.
2) The router returns an RA message containing information such as prefix information option. (The
router also regularly sends an RA message.)
3) The node automatically generates an IPv6 address and other information for its interface
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. After receiving a periodic RA message, the node updates
the preferred lifetime and valid lifetime of the address prefix accordingly.
An automatically generated address is applicable within the valid lifetime and is removed when the
valid lifetime times out.
Redirection
When a host is started, its routing table may contain only the default route to the gateway. When
certain conditions are satisfied, the gateway sends an ICMPv6 redirect message to the source host so
that the host can select a better next hop to forward packets (similar to the ICMP redirection function in
IPv4).
The gateway sends an IPv6 ICMP 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 IPv6 ICMP redirect message.
The selected route is not the default route.
The forwarded IPv6 packet does not contain any routing extension header.

IPv6 PMTU Discovery

The links that a packet passes from the source to the destination may have different MTUs. In IPv6,
when the packet size exceeds the path MTU ( the minimum MTU of all links), the packet will be
fragmented at the source end so as to reduce the processing pressure of forwarding devices and
utilize network resources properly.
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