3com S7906E Manuel De Montage

 

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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: 

z 

The receiving interface is the forwarding interface. 

z 

The selected route itself is not created or modified by an IPv6 ICMP redirect message. 

z 

The selected route is not the default route. 

z 

The forwarded IPv6 packet does not contain any routing extension header. 

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. 

The path MTU (PMTU) discovery mechanism is to find the minimum MTU of all links in the path from the 

source to the destination. 

 shows the working procedure of PMTU discovery. 

Figure 1-5 Working procedure of PMTU discovery 

 

 

The working procedure of the PMTU discovery is as follows: 

1)  The source host uses its MTU to send packets to the destination host. 

2)  If the MTU supported by a forwarding interface is smaller than the packet size, the forwarding 

device will discard the packet and return an ICMPv6 error packet containing the interface MTU to 

the source host. 

3)  After receiving the ICMPv6 error packet, the source host uses the returned MTU to send packets to 

the destination. 

4)  Step 2 to step 3 are repeated until the destination host receives the packet. In this way, the 

minimum MTU of all links in the path from the source host to the destination host is determined. 

Before IPv6 dominates the Internet, high-efficient, seamless IPv6 transition technologies are needed to 

enable communication between IPv4 and IPv6 networks. There are several IPv6 transition technologies, 

which can be used in different environments and periods, such as dual stack (RFC 2893), tunneling 

(RFC 2893), and NAT-PT (RFC 2766).