15-9
z
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.
z
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 that 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:
z
The receiving interface is the forwarding interface.
z
The selected route itself is not created or modified by an ICMPv6 Redirect message.
z
The selected route is not the default route.
z
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 MTU of a link, the packet is fragmented at the source end of
the link to reduce the processing pressure on intermediate devices and use network resources
effectively.
The path MTU (PMTU) discovery mechanism is to find the minimum MTU of all links in the path
between a source and a destination.
shows how a source host discovers the PMTU
to a destination host.
Figure 15-5
PMTU discovery process
The PMTU discovery process is:
1) The source host compares its MTU with the packet to be sent, performs necessary
fragmentation, and sends the resulting packet to the destination host.
