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This document clarifies the existing terminology confusion among references to IPv6/IPv4 encapsulations and IPv4/IPv6 transition mechanisms.
2. Definition of 'in' and 'over'
4. Security Considerations
5. IANA Considerations
7.1. Normative References
7.2. Informative References
§ Author's Address
§ Intellectual Property and Copyright Statements
A number of IETF documents have used in an "interchangeable" way several expressions such as:
Note that this list is not exhaustive and many other similar expressions may be also in use. For example, in some situations other encapsulating layers are used (i.e., 6/UDP/4), and similar expressions are being used as well.
As a consequence, documents from vendors, product manuals, publications, papers, books, tutorials, training material and many others, use also those expressions.
However not all those documents, including the IETF ones, are actually referring to the same IPv6/IPv4 encapsulations or IPv4/IPv6 transition mechanisms.
The result of this terminology confusion is a number of errors among vendors, operators, engineers and end-users when designing and documenting products, or when setting up transition mechanisms, being the cause of unnecessary operational costs, specially for beginners which try to setup IPv6 for their first occasions.
IP in IP Tunneling was defined by  (Simpson, W., “IP in IP Tunneling,” October 1995.). Afterwards  (Gilligan, R. and E. Nordmark, “Transition Mechanisms for IPv6 Hosts and Routers,” April 1996.), obsoleted by  (Gilligan, R. and E. Nordmark, “Transition Mechanisms for IPv6 Hosts and Routers,” August 2000.), defines the basic IPv4/IPv6 transition mechanisms, including the encapsulation of IPv6 packets in IPv4 ones (by means of protocol 41); this document is being updated as "ietf-v6ops-mech-v2".
Similarly,  (Conta, A. and S. Deering, “Generic Packet Tunneling in IPv6 Specification,” December 1998.), defines the encapsulation of IPv4 packets in IPv6 ones.
A first conclusion can be extracted from this documents (even if some of them actually use, some times, "over"): Those encapsulation mechanisms are the ones that should use the "in" terminology. Note that they are just encapsulation mechanisms, which often are used by several transition mechanisms.
Furthermore,  (Carpenter, B. and C. Jung, “Transmission of IPv6 over IPv4 Domains without Explicit Tunnels,” March 1999.) specify a transition mechanism, which uses 6in4 (encapsulation of IPv6 packets in IPv4 ones), creating a virtual IPv6 link over an IPv4 multicast infrastructure. This transition mechanism is named as "6over4". This name was chosen because the mechanism (use of IPv4 multicast for Neighbor Discovery) is exactly analogous to the mechanism for IPv6 over Ethernet (use of Ethernet multicast for Neighbor Discovery).
Clearly, "6in4" and "6over4" are quite different things, actually 6over4 is a transition mechanism which uses 6in4 as the procedure for encapsulating IPv6 packets in IPv4 multicast infrastructures.
The fact that they are different things and specially the requirement for a (IPv4) multicast infrastructure for 6over4, makes necessary to clarify the difference and avoid confusions among both.
Engineers operating networks and their customers (for example when setting up tunnels), often do not read all the IETF documents, so they could easily misunderstand if a tunnel is "6over4" or "6in4" (specially when vendor documents use both terms to actually name "6in4", possibly because the misusage of those terms in  (Gilligan, R. and E. Nordmark, “Transition Mechanisms for IPv6 Hosts and Routers,” April 1996.),  (Gilligan, R. and E. Nordmark, “Transition Mechanisms for IPv6 Hosts and Routers,” August 2000.)), and this creates some extra troubleshooting time and confusion.
In order to avoid this kind of confusion, it should be understood the difference between 6over4 and 6in4 (and any other equivalent terminologies). Future documents should take this in consideration.
It is recommended as well that new documents which describe other encapsulations and protocols, such as IPv4 in IPv6, IPv6 in UDP/IPv4, IPv6 in IPv6, etc., for consistency reasons, use "in" instead of "over".
It is also recommended that existing documents are amended ASAP to avoid the existing confusion.
This document does not have any protocol-related security considerations.
This document does not have any specific IANA considerations.
The author would like to acknowledge the inputs of Brian Carpenter.
|||Simpson, W., “IP in IP Tunneling,” RFC 1853, October 1995.|
|||Gilligan, R. and E. Nordmark, “Transition Mechanisms for IPv6 Hosts and Routers,” RFC 1933, April 1996.|
|||Gilligan, R. and E. Nordmark, “Transition Mechanisms for IPv6 Hosts and Routers,” RFC 2893, August 2000.|
|||Conta, A. and S. Deering, “Generic Packet Tunneling in IPv6 Specification,” RFC 2473, December 1998 (TXT, HTML, XML).|
|||Carpenter, B. and C. Jung, “Transmission of IPv6 over IPv4 Domains without Explicit Tunnels,” RFC 2529, March 1999.|
|Jordi Palet Martinez|
|San Jose Artesano, 1|
|Alcobendas - Madrid|
|E-28108 - Spain|
|Phone:||+34 91 151 81 99|
|Fax:||+34 91 151 81 98|
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