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8.3 Common Tunneling Mechanisms > 8.3.4 Handling IPv4 ICMP errors - Pg. 330

Chapter 8 Encapsulating nodes that have a large number of tunnels might not be able to store the IPv4 Path MTU for all tunnels. Such nodes can, at the expense of additional fragmentation in the network, avoid using the IPv4 Path MTU algorithm across the tunnel and instead use the MTU of the link layer (under IPv4) in the above algorithm instead of the IPv4 path MTU. In this case the Don't Fragment bit must not be set in the encapsu- lating IPv4 header. 8.3.3 Hop Limit IPv6-over-IPv4 tunnels are modeled as "single-hop." That is, the IPv6 hop limit is decremented by 1 when an IPv6 packet traverses the tunnel. The single-hop model serves to hide the existence of a tunnel. The tun- nel is opaque to users of the network, and is not detectable by network diagnostic tools such as traceroute. The single-hop model is implemented by having the encapsulating and decapsulating nodes process the IPv6 hop limit field as they would if they were forwarding a packet on to any other datalink. That is, they decre- ment the hop limit by 1 when forwarding an IPv6 packet. (The originating node and final destination do not decrement the hop limit.) The TTL of the encapsulating IPv4 header is selected in an implementation dependent manner. The current suggested value is published in the "Assigned Numbers RFC. Implementations may provide a mechanism to allow the administrator to configure the IPv4 TTL such as the one specified in the IP Tunnel MIB. 8.3.4 Handling IPv4 ICMP errors In response to encapsulated packets it has sent into the tunnel, the encapsulating node might receive IPv4 ICMP error messages from IPv4 routers inside the tunnel. These packets are addressed to the encapsulating node because it is the IPv4 source of the encapsulated packet. The ICMP "packet too big" error messages are handled according to IPv4 Path MTU Discovery and the resulting path MTU is recorded in the IPv4 layer. The recorded path MTU is used by IPv6 to determine if an IPv6 ICMP "packet too big" error has to be generated as described in section 8.3.2. The handling of other types of ICMP error messages depends on how much information is included in the "packet in error" field, which holds the encapsulated packet that caused the error. Many older IPv4 routers return only 8 bytes of data beyond the IPv4 header of the packet in error, which is not enough to include the address fields of the IPv6 header. More modern IPv4 routers are likely to return enough data beyond the IPv4 header to include the entire IPv6 header and possibly even the data beyond that. If the offending packet includes enough data, the encapsulating node may extract the encapsulated IPv6 packet and use it to generate an IPv6 ICMP message directed back to the originating IPv6 node, as shown in Figure 8.2. 330