IPv6 Overview 507
■ The selected route is not the default route.
■ The forwarded IPv6 packet does not contain any routing 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 link MTU, the packet
will be fragmented at the source end so as to reduce the processing pressure of
the forwarding device and utilize network resources rationally.
The path MTU (PMTU) discovery mechanism is to find the minimum MTU of all
links in the path from the source to the destination. Figure 154 shows the working
procedure of the PMTU discovery.
Figure 154 Working procedure of the PMTU discovery
The working procedure of the PMTU discovery is as follows:
1 The source host uses its MTU to fragment packets and then sends them to the
destination host.
2 If the MTU supported by the forwarding interface is less 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
fragment the packet again and then sends it.
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.
Introduction to IPv6 DNS In the IPv6 network, a Domain Name System (DNS) supporting IPv6 converts
domain names into IPv6 addresses, instead of IPv4 addresses.
However, just like an IPv4 DNS, an IPv6 DNS also covers static domain name
resolution and dynamic domain name resolution. The function and
implementation of these two types of domain name resolution are the same as
those of an IPv4 DNS. For details, refer to “DNS Overview” on page 971.
Usually, the DNS server connecting IPv4 and IPv6 networks not only contain A
records (IPv4 addresses), but also AAAA records (IPv6 addresses). The DNS server
can convert domain names into IPv4 addresses or IPv6 addresses. In this way, the
DNS server implements the functions of both IPv6 DNS and IPv4 DNS.
Source
MTU = 1500 MTU = 1500 MTU = 1350 MTU = 1400
Packet with MTU = 1500
ICMP error: packet too big;
use MTU = 1350
Packet with MTU = 1350
Packet received
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