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IETF RFC 6732
Last modified on Sunday, September 30th, 2012
 Permanent link to RFC 6732
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Independent Submission V. Kuarsingh, Ed.
Request for Comments: 6732 Rogers Communications
Category: Informational Y. Lee
ISSN: 2070-1721 Comcast
O. Vautrin
Juniper Networks
September 2012
6to4 Provider Managed Tunnels
 Abstract
6to4 Provider Managed Tunnels (6to4-PMT) provide a framework that can
help manage 6to4 tunnels operating in an anycast configuration. The
6to4-PMT framework is intended to serve as an option for operators to
help improve the experience of 6to4 operation when conditions of the
network may provide sub-optimal performance or break normal 6to4
operation. 6to4-PMT supplies a stable provider prefix and forwarding
environment by utilizing existing 6to4 relays with an added function
of IPv6 Prefix Translation. This operation may be particularly
important in NAT444 infrastructures where a customer endpoint may be
assigned a non-RFC 1918 address, thus breaking the return path for
anycast-based 6to4 operation. 6to4-PMT has been successfully used in
a production network, implemented as open source code, and
implemented by a major routing vendor.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6732.
Kuarsingh, et al. Informational PAGE 1 
RFC 6732 6to4 Provider Managed Tunnels September 2012
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction ....................................................3
2. Motivation ......................................................3
3. 6to4 Provider Managed Tunnels ...................................5
3.1. 6to4 Provider Managed Tunnel Model .........................5
3.2. Traffic Flow ..............................................5
3.3. Prefix Translation ........................................6
3.4. Translation State .........................................7
4. Deployment Considerations and Requirements ......................7
4.1. Customer Opt-Out ...........................................7
4.2. Shared CGN Space Considerations ............................8
4.3. End-to-End Transparency ....................................8
4.4. Path MTU Discovery Considerations ..........................9
4.5. Checksum Management ........................................9
4.6. Application Layer Gateways .................................9
4.7. Routing Requirements .......................................9
4.8. Relay Deployments .........................................10
5. Security Considerations ........................................10
6. Acknowledgements ...............................................10
7. References .....................................................11
7.1. Normative References ......................................11
7.2. Informative References ....................................11
Kuarsingh, et al. Informational PAGE 2
RFC 6732 6to4 Provider Managed Tunnels September 2012
1. Introduction
6to4 [RFC 3056] tunneling, along with the anycast operation described
in [RFC 3068], is widely deployed in modern Operating Systems and
off-the-shelf gateways sold throughout retail and Original Equipment
Manufacturer (OEM) channels. Anycast-based 6to4 [RFC 3068] allows for
tunneled IPv6 connectivity through IPv4 clouds without explicit
configuration of a relay address. Since the overall system utilizes
anycast forwarding in both directions, flow paths are difficult to
determine, tend to follow separate paths in either direction, and
often change based on network conditions. The return path is
normally uncontrolled by the local operator and can contribute to
poor performance for IPv6 and can also act as a breakage point. Many
of the challenges with 6to4 are described in [RFC 6343]. A specific
critical use case for problematic anycast 6to4 operation is related
to conditions in which the consumer endpoints are downstream from a
northbound Carrier-Grade NAT (CGN) [RFC 6264] function when assigned
non-RFC 1918 IPv4 addresses, which are not routed on interdomain
links.
Operators that are actively deploying IPv6 networks and operate
legacy IPv4 access environments may want to utilize the existing 6to4
behavior in customer site resident hardware and software as an
interim option to reach the IPv6 Internet in advance of being able to
offer full native IPv6. Operators may also need to address the
brokenness related to 6to4 operation originating from behind a
provider NAT function. 6to4-PMT offers an operator the opportunity to
utilize IPv6 Prefix Translation to enable deterministic traffic flow
and an unbroken path to and from the Internet for IPv6-based traffic
sourced originally from these 6to4 customer endpoints.
6to4-PMT translates the prefix portion of the IPv6 address from the
6to4-generated prefix to a provider-assigned prefix that is used to
represent the source. This translation will then provide a stable
forward and return path for the 6to4 traffic by allowing the existing
IPv6 routing and policy environment to control the traffic. 6to4-PMT
is primarily intended to be used in a stateless manner to maintain
many of the elements inherent in normal 6to4 operation.
Alternatively, 6to4-PMT can be used in a stateful translation mode
should the operator choose this option.
2. Motivation
Many operators endeavor to deploy IPv6 as soon as possible so as to
ensure uninterrupted connectivity to all Internet applications and
content through the IPv4 to IPv6 transition process. The IPv6
preparations within these organizations are often faced with both
financial challenges and timing issues related to deploying IPv6 to
Kuarsingh, et al. Informational PAGE 3
RFC 6732 6to4 Provider Managed Tunnels September 2012
the network edge and related transition technologies. Many of the
new technologies available for IPv4 to IPv6 transition will require
the replacement of the organization's Customer Premises Equipment
(CPE) to support technologies like IPv6 Rapid Deployment (6RD)
[RFC 5969], Dual-Stack Lite [RFC 6333], and native dual-stack.
Operators face a number of challenges related to home equipment
replacement. Operator-initiated replacement of this equipment will
take time due to the nature of mass equipment refresh programs or may
require the consumer to replace their own gear. Replacing consumer
owned and operated equipment, compounded by the fact that there is
also a general unawareness of what IPv6 is, also adds to the
challenges faced by operators. It is also important to note that
6to4 is present in much of the equipment found in networks today that
do not as of yet, or will not, support 6RD and/or native IPv6.
Operators may still be motivated to provide a form of IPv6
connectivity to customers and would want to mitigate potential issues
related to IPv6-only deployments elsewhere on the Internet.
Operators also need to mitigate issues related to the fact that 6to4
operation is often on by default, and may be subject to erroneous
behavior. The undesired behavior may be related to the use of
non-RFC 1918 addresses on CPE equipment that operate behind large
operator NATs or other conditions as described in a general advisory
as laid out in [RFC 6343].
6to4-PMT allows an operator to help mitigate such challenges by
leveraging the existing 6to4 deployment base, while maintaining
operator control of access to the IPv6 Internet. It is intended for
use when better options, such as 6RD or native IPv6, are not yet
viable. One of the key objectives of 6to4-PMT is to also help
reverse the negative impacts of 6to4 in CGN environments. The
6to4-PMT operation can also be used immediately with the default
parameters that are often enough to allow it to operate in a 6to4-PMT
environment. Once native IPv6 is available to the endpoint, the
6to4-PMT operation is no longer needed and will cease to be used
based on correct address selection behaviors in end hosts [RFC 6724].
6to4-PMT thus helps operators remove the impact of 6to4 in CGN
environments, deals with the fact that 6to4 is often on by default,
and allows access to IPv6-only endpoints from IPv4-only addressed
equipment. Additionally, it provides relief from many challenges
related to mis-configurations in other networks that control return
flows via foreign relays. Due to the simple nature of 6to4-PMT, it
can also be implemented in a cost-effective and simple manner,
allowing operators to concentrate their energy on deploying native
IPv6.
Kuarsingh, et al. Informational PAGE 4
RFC 6732 6to4 Provider Managed Tunnels September 2012
3. 6to4 Provider Managed Tunnels
3.1. 6to4 Provider Managed Tunnel Model
The 6to4 managed tunnel model behaves like a standard 6to4 service
between the customer IPv6 host or gateway, and the 6to4-PMT Relay
(within the provider domain). The 6to4-PMT Relay shares properties
with 6RD [RFC 5969] by decapsulating and forwarding encapsulated IPv6
flows within an IPv4 packet to the IPv6 Internet. The model provides
an additional function that translates the source 6to4 prefix to a
provider-assigned prefix that is not found in 6RD [RFC 5969] or
traditional 6to4 operation.
The 6to4-PMT Relay is intended to provide a stateless (or stateful)
mapping of the 6to4 prefix to a provider supplied prefix.
| 6to4-PMT Operation |
+-----+ 6to4 Tunnel +--------+ +------+ IPv6 +----+
| CPE |-------------|6to4 BR |--| PT66 |--------- |Host|
+-----+ IPv4 +--------+ +------+ Provider +----+
Network Prefix
Unified or Separate
Functions/Platforms
Figure 1: 6to4-PMT Functional Model
This mode of operation is seen as beneficial when compared to broken
6to4 paths and/or environments where 6to4 operation may be functional
but highly degraded.
3.2. Traffic Flow
Traffic in the 6to4-PMT model is intended to be controlled by the
operator's IPv6 peering operations. Egress traffic is managed
through outgoing routing policy, and incoming traffic is influenced
by the operator-assigned prefix advertisements using normal
interdormain routing functions.
The routing model is as predictable as native IPv6 traffic and legacy
IPv4-based traffic. Figure 2 provides a view of the routing topology
needed to support this relay environment. The diagram references
PrefixA as 2002::/16 and PrefixB as the example 2001:db8::/32.
Kuarsingh, et al. Informational PAGE 5
RFC 6732 6to4 Provider Managed Tunnels September 2012
| 6to4 IPv4 Path | Native IPv6 Path |
----------- ----------- -------------
/ IPv4 Net \ / IPv6 Net \ / IPv6 Internet \
+------+ +--------+ +-------+ +---------+
| CPE | PrefixA |6to4-PMT| PrefixB |Peering| |IPv6 HOST|
+------+ +--------+ +-------+ +---------+
\ / \ / \ /
---------- ------------ --------------
IPv4 6to4 IPv6 Provider IPv6 Prefix
Anycast Prefix Propagation
Figure 2: 6to4-PMT Flow Model
Traffic between two 6to4-enabled devices would use the IPv4 path for
communication according to [RFC 3056] unless the local host still
prefers traffic via a relay. 6to4-PMT is intended to be deployed in
conjunction with the 6to4 relay function in an attempt to help
simplify its deployment. The model can also provide the ability for
an operator to forward both 6to4-PMT (translated) and normal 6to4
flows (untranslated) simultaneously based on configured policy.
3.3. Prefix Translation
IPv6 Prefix Translation is a key part of the system as a whole. The
6to4-PMT framework is a combination of two concepts: 6to4 [RFC 3056]
and IPv6 Prefix Translation. IPv6 Prefix Translation, as used in
6to4-PMT, has some similarities to concepts discussed in [RFC 6296].
6to4-PMT would provide prefix translation based on specific rules
configured on the translator that maps the 6to4 2002::/16 prefix to
an appropriate provider assigned prefix. In most cases, a ::/32
prefix would work best in 6to4-PMT that matches common Regional
Internet Registry (RIR) prefix assignments to operators.
The provider can use any prefix mapping strategy they so choose, but
the simpler the better. Simple direct bitmapping can be used, or
more advanced forms of translation should the operator want to
achieve higher address compression. More advanced forms of
translation may require the use of stateful translation.
Figure 3 shows a 6to4 Prefix with a Subnet-ID of "0000" mapped to a
provider-assigned, globally unique prefix (2001:db8::/32). With this
simple form of translation, there is support for only one Subnet-ID
per provider-assigned prefix. In characterization of deployed OSs
and gateways, a Subnet-ID of "0000" is the most common default case
followed by Subnet-ID "0001". Use of the Subnet-ID can be referenced
in [RFC 4291]. It should be noted that in normal 6to4 operation, the
endpoint (network) has access to 65,536 (16-bits) Subnet IDs. In the
Kuarsingh, et al. Informational PAGE 6
RFC 6732 6to4 Provider Managed Tunnels September 2012
6to4-PMT case as described above using the mapping in Figure 3, all
but the one Subnet-ID used for 6to4-PMT would still operate under
normal 6to4 operation.
Pre-Relayed Packet [Provider Access Network Side]
0 16 32 48 64 80 96 112 128 Bits
| ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- |
2002 : 0C98 : 2C01 : 0000 : xxxx : xxxx : xxxx : xxxx
| ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- |
| | | | | |
---- ---- | | | |
| | | | | |
| ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- |
2001 : 0db8 : 0c98 : 2c01 : xxxx : xxxx : xxxx : xxxx
| ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- |
Post-Relayed Packet [Internet Side]
Figure 3: 6to4-PMT Prefix Mapping
3.4. Translation State
It is preferred that the overall system use deterministic prefix
translation mappings such that stateless operation can be
implemented. This allows the provider to place N number of relays
within the network without the need to manage translation state.
Deterministic translation also allows a customer to employ inward
services using the translated (provider prefix) address.
If stateful operation is chosen, the operator would need to validate
state and routing requirements particular to that type of deployment.
The full body of considerations for this type of deployment is not
within this scope of this document.
4. Deployment Considerations and Requirements
4.1. Customer Opt-Out
A provider enabling this function should offer a method to allow
customers to opt-out of such a service should the customer choose to
maintain normal 6to4 operation irrespective of degraded performance.
In cases where the customer is behind a CGN device, the customer
would not be advised to opt-out and can be assisted in turning off
6to4.
Kuarsingh, et al. Informational PAGE 7
RFC 6732 6to4 Provider Managed Tunnels September 2012
Since the 6to4-PMT system is targeted at customers who are relatively
unaware of IPv6 and IPv4, and normally run network equipment with a
default configuration, an opt-out strategy is recommended. This
method provides 6to4-PMT operation for non-IPv6 savvy customers whose
equipment may turn on 6to4 automatically and allows savvy customers
to easily configure their way around the 6to4-PMT function.
Capable customers can also disable anycast-based 6to4 entirely and
use traditional 6to4 or other tunneling mechanisms if they are so
inclined. This is not considered the normal case, and most endpoints
with auto-6to4 functions will be subject to 6to4-PMT operation since
most users are unaware of its existence. 6to4-PMT is targeted as an
option for stable IPv6 connectivity for average consumers.
4.2. Shared CGN Space Considerations
6to4-PMT operation can also be used to mitigate a known problem with
6to4 occurring when shared address space [RFC 6598] or Global Unicast
Addresses (GUA) are used behind a CGN and not routed on the Internet.
Non-RFC 1918, yet unrouted (on interdomain links) address space would
cause many deployed OSs and network equipment to potentially
auto-enable 6to4 operation even without a valid return path (such as
behind a CGN function). The operator's desire to use non-RFC 1918
addresses, such as shared address space [RFC 6598], is considered
highly likely based on real world deployments.
Such hosts, in normal cases, would send 6to4 traffic to the IPv6
Internet via the anycast relay, which would in fact provide broken
IPv6 connectivity, since the return path flow is built using an IPv4
address that is not routed or assigned to the source network. The
use of 6to4-PMT would help reverse these effects by translating the
6to4 prefix to a provider-assigned prefix, masking this automatic and
undesired behavior.
4.3. End-to-End Transparency
The 6to4-PMT mode of operation removes the traditional end-to-end
transparency of 6to4. Remote hosts would connect to a 6to4-PMT-
serviced host using a translated IPv6 address versus the original
6to4 address based on the 2002::/16 well-known prefix. This can be
seen as a disadvantage of the 6to4-PMT system. This lack of
transparency should also be contrasted with the normal operating
state of 6to4 that provides connectivity that is uncontrolled and
often prone to high latency. The lack of transparency is, however, a
better form of operation when extreme poor performance, broken IPv6
connectivity, or no IPv6 connectivity is considered as the
alternative.
Kuarsingh, et al. Informational PAGE 8
RFC 6732 6to4 Provider Managed Tunnels September 2012
4.4. Path MTU Discovery Considerations
The MTU will be subject to a reduced value due to standard 6to4
tunneling operation. Under normal 6to4 operation, the 6to4 service
agent would send an ICMP Packet Too Big Message as part of Path MTU
discovery as described in [RFC 4443] and [RFC 1981], respectively. In
6to4-PMT operation, the PMT Service agent should be aware of the
reduced 6to4 MTU and send ICMP messages using the translated address
accordingly.
It is also possible to pre-constrain the MTU at the upstream router
from the 6to4-PMT service agents that would then have the upstream
router send the appropriate ICMP Packet Too Big Messages.
4.5. Checksum Management
Checksum management for 6to4-PMT can be implemented in one of two
ways. The first deployment model is based on the stateless 6to4-PMT
operational mode. In this case, checksum modifications are made
using the method described in [RFC 3022], Section 4.2. The checksum
is modified to match the parameters of the translated address of the
source 6to4-PMT host. In the second deployment model in which
stateful 6to4-PMT translation is used, the vendor can implement
checksum-neutral mappings as defined in [RFC 6296].
4.6. Application Layer Gateways
Vendors can choose to deploy Application Layer Gateways (ALGs) on
their platforms that perform 6to4-PMT if they so choose. No ALGs
were deployed as part of the open source and vendor product
deployments of 6to4-PMT. In the vendor deployment case, the same
rules were used as with their NPTv6 [RFC 6296] base code.
4.7. Routing Requirements
The provider would need to advertise the well-known IP address range
used for normal anycast 6to4 [RFC 3068] operation within the local
IPv4 routing environment. This advertisement would attract the 6to4
upstream traffic to a local relay. To control this environment and
make sure all northbound traffic lands on a provider-controlled
relay, the operator may filter the anycast range from being
advertised from customer endpoints toward the local network (upstream
propagation).
The provider would not be able to control route advertisements inside
the customer domain, but that use case is not in scope for this
document. In that case, it is likely that the end network/customer
understands 6to4 and is maintaining their own relay environment and
Kuarsingh, et al. Informational PAGE 9
RFC 6732 6to4 Provider Managed Tunnels September 2012
therefore would not be subject to the operators 6to4 and/or PMT
operation.
Within their own network, the provider would also likely want to
advertise the 2002::/16 range to help bridge traditional 6to4 traffic
within the network (native IPv6 to 6to4-PMT-based endpoint). It
would also be advised that the local 6to4-PMT operator not leak the
well-known 6to4 anycast IPv4 prefix to neighboring Autonomous Systems
to prevent PMT operation for neighboring networks. Policy
configuration on the local 6to4-PMT Relay can also be used to
disallow PMT operation should the local provider service downstream
customer networks.
4.8. Relay Deployments
The 6to4-PMT function can be deployed onto existing 6to4 relays (if
desired) to help minimize network complexity and cost. 6to4-PMT has
already been developed on Linux-based platforms that are package
add-ons to the traditional 6to4 code. The only additional
considerations beyond normal 6to4 relay operation would include the
need to route specific IPv6 provider prefix ranges used for 6to4-PMT
operation towards peers and transit providers.
5. Security Considerations
6to4-PMT operation would be subject to the same security concerns as
normal 6to4 operation as described in [RFC 6169]. 6to4-PMT is also
not plainly perceptible by external hosts, and local entities appear
as native IPv6 hosts to the external hosts.
6. Acknowledgements
Thanks to the following people for their textual contributions and/or
guidance on 6to4 deployment considerations: Dan Wing, Wes George,
Scott Beuker, JF Tremblay, John Brzozowski, Chris Metz, and Chris
Donley.
Additional thanks to the following for assisting with the coding and
testing of 6to4-PMT: Marc Blanchet, John Cianfarani, Tom Jefferd, Nik
Lavorato, Robert Hutcheon, and Ida Leung.
Kuarsingh, et al. Informational PAGE 10
RFC 6732 6to4 Provider Managed Tunnels September 2012
7. References
7.1. Normative References
[RFC 3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC 3068] Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
RFC 3068, June 2001.
7.2. Informative References
[RFC 1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC 3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
[RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC 4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 4443, March
2006.
[RFC 5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) -- Protocol Specification", RFC
5969, August 2010.
[RFC 6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security
Concerns with IP Tunneling", RFC 6169, April 2011.
[RFC 6264] Jiang, S., Guo, D., and B. Carpenter, "An Incremental
Carrier-Grade NAT (CGN) for IPv6 Transition", RFC 6264,
June 2011.
[RFC 6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
Translation", RFC 6296, June 2011.
[RFC 6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee,
"Dual-Stack Lite Broadband Deployments Following IPv4
Exhaustion", RFC 6333, August 2011.
[RFC 6343] Carpenter, B., "Advisory Guidelines for 6to4 Deployment",
RFC 6343, August 2011.
Kuarsingh, et al. Informational PAGE 11
RFC 6732 6to4 Provider Managed Tunnels September 2012
[RFC 6598] Weil, J., Kuarsingh, V., Donley, C., Liljenstolpe, C., and
M. Azinger, "IANA-Reserved IPv4 Prefix for Shared Address
Space", BCP 153, RFC 6598, April 2012.
[RFC 6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, September 2012.
Authors' Addresses
Victor Kuarsingh (editor)
Rogers Communications
8200 Dixie Road
Brampton, Ontario L6T 0C1
Canada
EMail: victor.kuarsingh@gmail.com
URI: http://www.rogers.com
Yiu L. Lee
Comcast
One Comcast Center
Philadelphia, PA 19103
U.S.A.
EMail: yiu_lee@cable.comcast.com
URI: http://www.comcast.com
Olivier Vautrin
Juniper Networks
1194 N Mathilda Avenue
Sunnyvale, CA 94089
U.S.A.
EMail: olivier@juniper.net
URI: http://www.juniper.net
Kuarsingh, et al. Informational PAGE 12
RFC TOTAL SIZE: 28156 bytes
PUBLICATION DATE: Sunday, September 30th, 2012
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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