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https://www.7key.jp/rfc/2185/rfc2185_34.html#source
https://www.7key.jp/rfc/2185/rfc2185_34.html#translation
Clearly tunneling is useful only if communication can be achieved in both directions. However, different forms of tunneling may be used in each direction, depending upon the local environment, the form of address of the two hosts which are exchanging IPv6 packets, and the policies in use.
Table 1 summarizes the form of tunneling that will result given each possible combination of host capabilities, and given one possible set of policy decisions. This table is derived directly from the requirements for automatic tunneling discussed above.
The example in table 1 uses a specific set of policy decisions: It is assumed in table 1 that the source host will transmit a native IPv6 where possible in preference over encapsulation. It is also assumed that where tunneling is needed, host to host tunneling will be preferred over host to router tunneling. Other combinations are therefore possible if other policies are used.
Due to a specific policy choice, the default sending rules in [1] may not be followed.
Note that IPv6-capable hosts which do not have any local IPv6 router must be given an IPv4-compatible v6 address in order to make use of their IPv6 capabilities. Thus, there are no entries for IPv6-capable hosts which have an incompatible IPv6 address and which also do not have any connectivity to any local IPv6 router. In fact, such hosts could communicate with other IPv6 hosts on the same local network without the use of a router. However, since this document focuses on routing and router implications of IPv6 transition, direct communication between two hosts on the same local network without any intervening router is outside the scope of this document.
Also, table 1 does not consider manually configured point-to-point tunnels. Such tunnels are treated as if they were normal point-to- point links. Thus any two IPv6-capable devices which have a manually configured tunnel between them may be considered to be directly connected.
-----------------+------------------+-------------------------- Host A | Host B | Result -----------------+------------------+-------------------------- v4-compat. addr. | v4-compat. addr. | host to host tunneling no local v6 rtr. | no local v6 rtr. | in both directions -----------------+------------------+-------------------------- v4-compat. addr. | v4-compat. addr. | A->B: host to host tunnel no local v6 rtr. | local v6 rtr. | B->A: v6 forwarding plus | | rtr->host tunnel -----------------+------------------+-------------------------- v4-compat. addr. | incompat. addr. | A->B: host to rtr tunnel no local v6 rtr. | local v6 rtr. | plus v6 forwarding | | B->A: v6 forwarding plus | | rtr to host tunnel -----------------+------------------+-------------------------- v4-compat. addr. | v4-compat. addr. | end to end native v6 local v6 rtr. | local v6 rtr. | in both directions -----------------+------------------+-------------------------- v4-compat. addr. | incompat. addr. | end to end native v6 local v6 rtr. | local v6 rtr. | in both directions -----------------+------------------+-------------------------- incompat. addr. | incompat. addr. | end to end native v6 local v6 rtr. | local v6 rtr. | in both directions -----------------+------------------+-------------------------- Table 1: Summary of Automatic Tunneling Combinations
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