V6OPS                                                       B. Carpenter
Internet-Draft                                         Univ. of Auckland
Intended status: Informational                            April 26, 2011
Expires: October 28, 2011

                Advisory Guidelines for 6to4 Deployment


   This document provides advice to network operators about deployment
   of the 6to4 technique for automatic tunneling of IPv6 over IPv4.  It
   is principally addressed to Internet Service Providers, including
   those that do not yet support IPv6, and to Content Providers.  Some
   advice to implementers is also included.  The intention of the advice
   is to minimise both user dissatisfaction and help desk calls.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 28, 2011.

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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Principles of Operation  . . . . . . . . . . . . . . . . . . .  4
     2.1.  Router 6to4  . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Anycast 6to4 . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Problems Observed  . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Advisory Guidelines  . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Vendor Issues  . . . . . . . . . . . . . . . . . . . . . . 10
     4.2.  Consumer ISPs, and enterprise networks, that do not
           support IPv6 in any way  . . . . . . . . . . . . . . . . . 11
       4.2.1.  Anycast address availability . . . . . . . . . . . . . 11
       4.2.2.  Protocol 41  . . . . . . . . . . . . . . . . . . . . . 11
       4.2.3.  IPv4 prefix issues . . . . . . . . . . . . . . . . . . 12
       4.2.4.  DNS issues . . . . . . . . . . . . . . . . . . . . . . 12
       4.2.5.  Rogue Router Advertisements  . . . . . . . . . . . . . 12
       4.2.6.  Planning for IPv6 deployment . . . . . . . . . . . . . 13
     4.3.  Consumer ISPs, and enterprise networks, that do
           support IPv6 . . . . . . . . . . . . . . . . . . . . . . . 13
     4.4.  Transit ISPs and Internet Exchange Points  . . . . . . . . 13
     4.5.  Content providers and their ISPs . . . . . . . . . . . . . 15
   5.  Tunnels Managed by ISPs  . . . . . . . . . . . . . . . . . . . 16
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   9.  Change log [RFC Editor: please remove] . . . . . . . . . . . . 18
   10. Informative References . . . . . . . . . . . . . . . . . . . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20

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1.  Introduction

   A technique for automatic tunneling of IPv6 over IPv4, intended for
   situations where a user may wish to access IPv6-based services via a
   network that does not support IPv6, was defined a number of years
   ago.  It is known as 6to4 [RFC3056] [RFC3068] and is quite widely
   deployed in end systems, especially desktop and laptop computers.
   Also, 6to4 is supported in a number of popular models of CPE routers,
   some of which have it enabled by default, leading to quite widespread
   unintentional deployment by end users.

   Unfortunately, experience shows that the method has some problems in
   current deployments that can lead to connectivity failures.  These
   failures either cause long retry delays or complete failures for
   users trying to connect to services.  In many cases, the user may be
   quite unaware that 6to4 is in use, and when the user contacts a help
   desk, in all probability the help desk is unable to correctly
   diagnose the problem.  Anecdotally, many help desks simply advise
   users to disable IPv6, thus defeating the whole purpose of the
   mechanism, which was to encourage early adoption of IPv6.

   The main goal of the present document is to offer advice to network
   operators on how to deal with this situation more constructively than
   by disabling 6to4.  It briefly describes the principle of operation,
   then describes the problems observed, and finally offers specific
   advice on the available methods of avoiding the problems.  Note that
   some of this advice applies to ISPs that do not yet support IPv6,
   since their customers and help desks are significantly affected in
   any case.

   Other advice applies to content providers and implementers, but this
   document does not discuss aspects that are mainly outside the scope
   of network operators:
   1.  Operating system preferences between IPv4 and IPv6 when both
       appear to be available [I-D.ietf-6man-rfc3484-revise].
   2.  Ensuring that application software deals gracefully with
       connectivity problems [I-D.wing-v6ops-happy-eyeballs-ipv6].
   3.  Some content providers have chosen to avoid the problem by hiding
       their IPv6 address except from customers of pre-qualified
       networks [I-D.ietf-v6ops-v6-aaaa-whitelisting-implications].

   A companion document [I-D.ietf-v6ops-6to4-to-historic] reclassifies
   6to4 as Historic.  However, this will not remove the millions of
   existing hosts and customer premises equipments that implement 6to4.
   Hence, the advice in this document remains necessary.

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2.  Principles of Operation

   There are two variants of 6to4 which are referred to here as "Router
   6to4" and "Anycast 6to4".  To understand Anycast 6to4, it is
   necessary first to understand Router 6to4.

2.1.  Router 6to4

   Router 6to4 is the original version, documented in [RFC3056].  The
   model assumes that a user site operates native IPv6, but that its ISP
   provides no IPv6 service.  The site border router acts as a 6to4
   router.  If its external global 32-bit IPv4 address is V4ADDR, the
   site automatically inherits the IPv6 prefix 2002:V4ADDR::/48.  (The
   explanation in RFC 3056 is somewhat confusing, as it refers to the
   obsolete "Top Level Aggregator" terminology.)  The prefix 2002:
   V4ADDR::/48 will be used and delegated for IPv6 service within the
   user site.

   Consider two such site border routers, with global IPv4 addresses and, and therefore inheriting the IPv6
   prefixes 2002:c000:2aa::/48 and 2002:c000:2bb::/48 respectively.  The
   routers can exchange IPv6 packets by encapsulating them in IPv4 using
   protocol number 41, and sending them to each other at their
   respective IPv4 addresses.  In fact, any number of 6to4 routers
   connected to the IPv4 network can directly exchange IPv6 packets in
   this way.

   Some 6to4 routers are also configured as "Relay routers."  They
   behave as just described, but in addition they obtain native IPv6
   connectivity with a normal IPv6 prefix.  They announce an IPv6 route
   to 2002::/16.  For example, assume that the 6to4 router at is a relay router, whose address on the 6to4 side is
   2002:c000:2bb::1.  Suppose that a host with the 6to4 address 2002:
   c000:2aa::123 sends an IPv6 packet to a native IPv6 destination such
   as 2001:db8:123:456::321.  Assume that the 6to4 router at
   has its IPv6 default route set to 2002:c000:2bb::1, i.e. the relay.
   The packet will be delivered to the relay, encapsulated in IPv4.
   After decapsulation, the relay will forward the packet into native
   IPv6 for delivery.  When the remote host replies, the packet (source
   2001:db8:123:456::321, destination 2002:c000:2aa::123) will find a
   route to 2002::/16 and hence be delivered to a 6to4 relay.  The
   process will be reversed and the packet will be encapsulated and
   forwarded to the 6to4 router at for final delivery.

   Note that this process does not require the same relay to be used in
   both directions.  The outbound packet will go to whichever relay is
   configured as the default IPv6 router at the source router, and the
   return packet will go to whichever relay is announcing a route to

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   2002::/16 in the vicinity of the remote IPv6 host.

   There are of course many further details in RFC 3056, most of which
   are irrelevant to current operational problems.

2.2.  Anycast 6to4

   Router 6to4 assumes that 6to4 routers and relays will be managed and
   configured cooperatively.  In particular, 6to4 sites need to
   configure a relay router willing to carry their outbound traffic,
   which becomes their default IPv6 router (except for 2002::/16).  The
   objective of the anycast variant, defined in [RFC3068], is to avoid
   any need for such configuration.  The intention was to make the
   solution available for small or domestic users, even those with a
   single host or simple home gateway rather than a border router.  This
   is achieved quite simply, by defining as the default IPv4
   address for a 6to4 relay, and therefore 2002:c058:6301:: as the
   default IPv6 router address for a 6to4 site.

   Since Anycast 6to4 implies a default configuration for the user site,
   it does not require any particular user action.  It does require an
   IPv4 anycast route to be in place to a relay at  As with
   Router 6to4, there is no requirement that the return path goes
   through the same relay.

3.  Problems Observed

   It should be noted that Router 6to4 was not designed to be an
   unmanaged solution.  Quite the contrary: RFC 3056 contains a number
   of operational recommendations intended to avoid routing issues.  In
   practice, there are few if any deployments of Router 6to4 following
   these recommendations.  Mostly, Anycast 6to4 has been deployed.  In
   this case, the user site (either a single host or a small broadband
   gateway) discovers that it doesn't have native IPv6 connectivity, but
   that it does have a global IPv4 address and can resolve AAAA queries,
   and therefore assumes that it can send 6to4 packets to

   Empirically, 6to4 appears to suffer from a significant level of
   connection failure; see [Aben] and [Huston-a].  In experiments
   conducted on a number of dual stack web servers, the TCP connection
   failure rate has been measured.  In these experiments, the client's
   connection attempt to a server was considered to have failed when the
   server received a TCP SYN packet and sent a SYN/ACK packet in
   response, but received no ACK packet to complete the initial TCP
   3-way handshake.  The experiment conducted by Aben recorded a failure
   rate of between 9% and 20% of all 6to4 connection attempts.  The
   experiment conducted by Huston has recorded a failure rate of between

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   9% and 19% of all 6to4 clients.  In this latter experiment it was
   further noted that between 65% to 80% of all 6to4 clients who failed
   to connect using 6to4 were able to make a successful connection using
   IPv4, while the remainder did not make any form of IPv4 connection
   attempt, successful or otherwise, using the mapped IPv4 address as a
   source address.  No connection attempts using embedded RFC 1918 IPv4
   addresses were recorded by the server.

   There have been several possible reasons offered for this form of
   6to4 connection failure.  One is the use of private IPv4 addresses
   embedded in the 6to4 address, making the return path for the 6to4
   tunnel infeasible, and the second is the use of local filters and
   firewalls that drop incoming IP packets that use IP protocol 41.  If
   the former case were prevalent it would be reasonable to expect that
   a significant proportion of failed 6to4 connections would use
   embedded IPv4 addresses that are either drawn from the private use
   (RFC 1918) address ranges, contrary to RFC 3056, or from addresses
   that are not announced in the Internet's IPv4 inter-domain routing
   table.  Neither case was observed to any significant volume in the
   experiments conducted by Huston.  Furthermore, the experimental
   conditions were varied to use a return 6to4 tunnel with either the
   native IPv4 source address of the dual stack server or an IPv4 source
   address of  No change in the 6to4 connection failure
   rate was observed between these two configurations; however, other
   operators have reported significant problems when replying from the
   native address, caused by stateful firewalls at the user site.  Given
   that the server used its own 6to4 relay for the return path, the only
   difference in the IP packet itself between the successful IPv4
   connections and the failed 6to4 connections was the IP protocol
   number, which was 6 (TCP) for the successful IPv4 connections and 41
   (IPv6 payload) for the failed 6to4 connections.  The inference from
   these experiments is that one likely reason for the high connection
   failure rate for 6to4 connections is the use of local filters close
   to the end-user that block incoming packets with protocol 41, in some
   cases made worse by stateful firewalls if the source address is not

   In a dual stack context this connection failure rate was effectively
   masked by the ability of the client system to recover from the
   failure and make a successful connection using IPv4.  In this case
   the only effect on the client system was a delay in making the
   connection of between 7 and 20 seconds as the client's system timed
   out on the 6to4 connection attempts (see

   This experience and further analysis shows that specific operational
   problems with Anycast 6to4 include:

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   1.  Outbound Black Hole: does not generate 'destination
       unreachable' but in fact packets sent to that address are
       dropped.  This can happen due to routing or firewall
       configuration, or even because the relay that the packets happen
       to reach contains an ACL such that they are discarded.

       This class of problem arises because the user's ISP is accepting
       a route to despite the fact that it doesn't go
       anywhere useful.  Either the user site or its ISP is dropping
       outbound Protocol 41 traffic, or the upstream operator is
       unwilling to accept incoming 6to4 packets from the user's ISP.
       The latter is superficially compatible with the design of Router
       6to4 (referred to as "unwilling to relay" in RFC 3056).  However,
       the simple fact of announcing a route to in IPv4,
       coupled with the behavior described in RFC 3068, amounts to
       announcing a default route for IPv6 to all 6to4 sites that
       receive the IPv4 route.  This violates the assumptions of RFC

       The effect of this problem on users is that their IPv6 stack
       believes that it has 6to4 connectivity, but in fact all outgoing
       IPv6 packets are black-holed.  The prevalence of this problem is
       hard to measure, since the resulting IPv6 packets can never be
       observed from the outside.
   2.  Inbound Black Hole: In this case, 6to4 packets sent to are correctly delivered to a 6to4 relay, and reply
       packets are returned, but they are dropped by an inbound Protocol
       41 filter.  As far as the user is concerned, the effect is the
       same as the previous case: IPv6 is a black hole.  Many enterprise
       networks are believed to be set up in this way.  Connection
       attempts due to this case can be observed by IPv6 server
       operators, in the form of SYN packets from addresses in 2002::/16
       followed by no response to the resulting SYN/ACK.  From the
       experiments cited above, this appears to be a significant problem
       in practice.

       This problem is complicated by three variables: the firewall
       applying the Protocol 41 filter may be stateless or stateful; the
       relay may source its packets from its native IPv4 address or from; packets from the relay may be subject to IPv4
       ingress filtering.  If the Protocol 41 filter is stateless, 6to4
       will never succeed.  If it is stateful, the firewall will drop
       inbound packets from addresses that have not been seen in
       outbound traffic on the same port.  In this case, 6to4 will only
       succeed if the packets are sourced from  But if the
       relay is subject to ingress filtering, only packets from its
       native IPv4 address can be transmitted.  There are therefore only
       three combinations that can succeed:

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       1.  No Protocol 41 filter, with the relay using its native IPv4
           source address.
       2.  No Protocol 41 filter, with the relay using the anycast IPv4
           source address and with no ingress filter.
       3.  A stateful Protocol 41 firewall, with the relay using the
           anycast IPv4 source address and with no ingress filter.
   3.  No Return Relay: If the Outbound Black Hole problem does not
       occur, i.e. the outgoing packet does reach the intended native
       IPv6 destination, the target system will send a reply packet, to
       2002:c000:2aa::123 in our example above.  Then 2002::/16 may or
       may not be successfully routed.  If it is not routed, the packet
       will be dropped (hopefully with 'destination unreachable').
       According to RFC 3056, an unwilling relay "MUST NOT advertise any
       2002:: routing prefix into the native IPv6 domain"; therefore,
       conversely, if this prefix is advertised the relay must relay
       packets regardless of source and destination.  However, in
       practice the problem arises that some relays reject packets that
       they should relay, based on their IPv6 source address.

       Whether the native IPv6 destination has no route to 2002::/16, or
       it turns out to have a route to an unwilling relay, the effect is
       the same: all return IPv6 packets are black-holed.  While there
       is no direct evidence of the prevalence of this problem, it
       certainly exists in practice.
   4.  Large RTT: In the event that none of the above three problems
       applies, and a two-way path does in fact exist between a 6to4
       host and a native host, the round trip time may be quite large
       and variable since the paths to the two relays are unmanaged and
       may be complex.  Overloaded relays might also cause highly
       variable RTT.
   5.  PMTUD Failure: A common link MTU size observed on the Internet
       today is 1500 bytes.  However, when using 6to4 the path MTU is
       less than this due to the encapsulation header.  Thus a 6to4
       client will normally see a link MTU that is less than 1500, but a
       native IPv6 server will see 1500.  It has been observed that Path
       MTU Discovery does not always work, and this can lead to
       connectivity failures.  Even if a TCP SYN/ACK exchange works, TCP
       packets with full size payloads may simply be lost.  This problem
       is apparently exacerbated in some cases by failure of the TCP MSS
       negotiation mechanism.  These failures are disconcerting even to
       an informed user, since a standard 'ping' from the client to the
       server will succeed, because it generates small packets, and the
       successful SYN/ACK exchange can be traced.  Also, the failure may
       occur on some paths but not others, so a user may be able to
       fetch web pages from one site, but only ping another.

       Additionally, there is a problem if 6to4 is enabled on a router
       and it advertises the resulting prefix on a LAN, but does not

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       also advertise a smaller MTU; in this case TCP MSS negotiation
       will definitely fail.
   6.  Reverse DNS Failure: Typically a 6to4-addressed host will not
       have a reverse DNS delegation.  If reverse DNS is used as a
       pseudo-security check, it will fail.
   7.  Bogus Address Failure: By design, 6to4 does not work and will not
       activate itself if the available V4ADDR is a private address
       [RFC1918].  However it will also not work if the available V4ADDR
       is a "bogon", i.e. a global address that is being used by the
       operator as a private address.  A common case of this is a legacy
       wireless network using as if it was a private address.
       In this case, 6to4 will assume it is connected to the global
       Internet, but there is certainly no working return path.

       This failure mode will also occur if an ISP is operating a
       Carrier Grade NAT between its customers and the Internet, and is
       using global public address space as if it were private space to
       do so.
   8.  Faulty 6to4 Implementations: It has been reported that some 6to4
       implementations attempt to activate themselves even when the
       available IPv4 address is an RFC 1918 address.  This is in direct
       contradiction to RFC 3056, and will produce exactly the same
       failure mode as Bogus Address Failure.  It is of course outside
       the ISP's control.
   9.  Difficult Fault Diagnosis: The existence of all the above failure
       modes creates a problem of its own: very difficult fault
       diagnosis, especially if the only symptom reported by a user is
       slow access to web pages, caused by a long timeout before
       fallback to IPv4.  Tracking down anycast routing problems and
       PMTUD failures is particularly hard.

   The practical impact of the above problems, which are by no means
   universal as there is considerable successful use of Anycast 6to4,
   has been measured at a fraction of 1% loss of attempted connections
   to dual-stack content servers [Anderson].  This is because a small
   fraction of client hosts attempt to connect using 6to4, and up to 20%
   of these experience one of the above failure modes.  While this seems
   low, it amounts to a significant financial impact for content
   providers.  Also, end users frustrated by the poor response times
   caused by fall-back to IPv4 connectivity
   [I-D.wing-v6ops-happy-eyeballs-ipv6] are considered likely to
   generate help desk calls with their attendant costs.

   A rather different operational problem caused incidentally by 6to4 is
   that, according to observations made at the University of Southampton
   by Tim Chown and James Morse, and at other sites, rogue Router
   Advertisements [RFC6104] often convey a 2002::/16 prefix.  This
   appears to be due to misbehaviour by devices acting as local IPv6

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   routers or connection-sharing devices but issuing RA messages on the
   wrong interface.  Such a device, if it obtains IPv6 connectivity via
   an upstream link to the Internet, should only issue the corresponding
   RA messages on its downstream link to the nodes intended to share its
   Internet connection.  Issuing RA messages on the upstream link will
   perturb any other IPv6 hosts on that link.  If 6to4 routing is
   enabled by default on a device that exhibits this faulty behaviour,
   the resulting rogue RA messages will indeed convey a 2002::/16

4.  Advisory Guidelines

   There are several types of operator involved, willingly or
   unwillingly, in the Anycast 6to4 scenario and they will all suffer if
   things work badly.  There is a clear incentive for each of them to
   take appropriate action, as described below.

   This document avoids formal normative language, because it is highly
   unlikely that the guidelines apply universally.  Each operator will
   make its own decisions about which of the following guidelines are
   useful in its specific scenario.

4.1.  Vendor Issues

   Although this document is aimed principally at operators, there are
   some steps that implementers and vendors of 6to4 should take.
   1.  Some vendors of routers, including customer premises equipment,
       have not only included support for 6to4 in their products, but
       have enabled it by default.  This is bad practice - it should
       always be a conscious decision by a user to enable 6to4.  Many of
       the above problems only occur due to unintentional deployment of
   2.  Similarly, host operating systems should not enable Anycast 6to4
       by default; it should always be left to the user to switch it on.
   3.  Any 6to4 implementation that attempts to activate itself when the
       available IPv4 address is an RFC 1918 address is faulty and needs
       to be updated.
   4.  6to4 implementations should adopt updated IETF recommendations on
       address selection [I-D.ietf-6man-rfc3484-revise].
   5.  6to4 relay implementations must carefully follow section 3.2 of
       [RFC4213] to ensure correct handling of MTU issues.
   6.  6to4 router or connection-sharing implementations must avoid
       issuing rogue RAs [RFC6104].  Additionally, where 6to4 is being
       enabled by a node for Internet connection sharing purposes, and
       the node supports [RFC4191], then it should set the Router
       Advertisement router preference bits to 11 (low preference).

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4.2.  Consumer ISPs, and enterprise networks, that do not support IPv6
      in any way

4.2.1.  Anycast address availability

   To reduce the negative impact of Anycast 6to4 deployed (probably
   unknowingly) by users, and consequent user dissatisfaction and help
   desk calls, such ISPs should check in sequence:
   1.  Does the ISP have a route to  (This means an
       explicit route, or knowledge that the default upstream provider
       has an explicit route.  A default route doesn't count!)
   2.  If so, is it functional and stable?
   3.  If so, is the ping time reasonably short?
   4.  If so, does the relay willingly accept 6to4 traffic from the
       ISP's IPv4 prefixes?  (Note that this is an administrative as
       well as a technical question - is the relay's operator willing to
       accept the traffic?)

   Unless the answer to all these questions is 'yes', the operator
   should consider blocking the route to and generating an
   IPv4 'destination unreachable' message.  This may cause some 6to4
   implementations to fall back to IPv4 more quickly.  There is little
   operational experience with this, however.

   Some implementations also perform some form of 6to4 relay
   qualification.  For example, one host implementation (Windows) tests
   the Protocol 41 reachability by sending an ICMPv6 echo request with
   Hop Limit=1 to the relay, expecting a response or Hop Limit exceeded
   error back.  Lack of any response indicates that the 6to4 relay does
   not work so 6to4 is turned off [Savola].

   A more constructive approach for such an ISP is to seek out a transit
   provider who is indeed willing to offer outbound 6to4 relay service,
   so that the answer to each of the questions above is positive.

4.2.2.  Protocol 41

   ISPs in this class should always allow protocol 41 through their
   network and firewalls.  Not only is this a necessary condition for
   6to4 to work, but it also allows users who want to use a configured
   IPv6 tunnel service to do so.

   Some operators, particularly enterprise networks, silently block
   Protocol 41 on security grounds.  Doing this on its own is bad
   practice, since it contributes to the problem and harms any users who
   are knowingly or unknowingly attempting to run 6to4.  The strategic
   solution is to deploy native IPv6, making Protocol 41 redundant.  In
   the short term, experimentation could be encouraged by allowing

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   Protocol 41 for certain users, while returning appropriate ICMP
   responses as mentioned above.  Unfortunately, if this is not done,
   the 6to4 problem cannot be solved.

4.2.3.  IPv4 prefix issues

   Operators should never use "bogon" address space such as the example
   of for customers, since IPv4 exhaustion means that all
   such addresses are likely to be in real use in the near future.
   (Also see [I-D.ietf-intarea-shared-addressing-issues].)  An operator
   that is unable to immediately drop this practice should ensure that generates IPv4 'destination unreachable'.  It has been
   suggested that they could also run a dummy 6to4 relay at that address
   which always returns ICMPv6 'destination unreachable' as a 6to4
   packet.  However, these techniques are not very effective, since most
   current end-user 6to4 implementations will ignore them.

   If an operator is providing legitimate global addresses to customers
   (neither RFC 1918 nor bogon addresses), and also running Carrier
   Grade NAT (Large Scale NAT) between this address space and the global
   address space of the Internet, then 6to4 cannot work properly.  Such
   an operator should also take care to return 'destination unreachable'
   for 6to4 traffic.  Alternatively, they could offer untranslated
   address space to the customers concerned.

4.2.4.  DNS issues

   A customer who is intentionally using 6to4 may also need to create
   AAAA records, and the operator should be able to support this, even
   if the DNS service itself runs exclusively over IPv4.  However,
   customers should be advised to consider carefully whether their 6to4
   service is sufficiently reliable for this.

   Operators could, in principle, offer reverse DNS support for 6to4
   users [RFC5158], although this is not straightforward for domestic

4.2.5.  Rogue Router Advertisements

   Paradoxically, operators in this category should consider whether
   they need to defend themselves against rogue IPv6 RA messages
   [RFC6105], since such messages may appear from devices seeking to
   operate as 6to4 routers, and confuse any user devices with IPv6
   enabled by default.  Eventually, the measures being designed by the
   IETF Source Address Validation Improvements (SAVI) working group will
   assist with this problem.  In the short term, IPv4-only operators may
   choose to filter out packets with the IPv6 Ethertype (0x86DD) in
   their access equipment; this will definitively remove rogue RA

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4.2.6.  Planning for IPv6 deployment

   Enterprise operators who have complete administrative control of all
   end-systems may choose to disable 6to4 in those systems as an
   integral part of their plan to deploy IPv6.

   Some IPv4 operators have chosen to install a 6to4 relay, connected
   via an IPv6-in-IPv4 tunnel to an IPv6 operator, as a first step
   before native IPv6 deployment.  The routing guidelines in Section 4.4
   would apply.  However, offering genuine IPv6 service to interested
   customers, even if tunneled, would generally be a better first step.

4.3.  Consumer ISPs, and enterprise networks, that do support IPv6

   Once an operator does support IPv6 service, whether experimentally or
   in production, it is almost certain that users will get better
   results using this service than by continuing to use 6to4.
   Therefore, these operators are encouraged to advise their users to
   disable 6to4 and they should not create DNS records for any 6to4

   Such an operator may automatically fall into one of the following two
   categories (transit provider or content provider), so the guidelines
   in Section 4.4 or in Section 4.5 will apply instead.

   Operators in this category should make sure that no routers are
   unintentionally or by default set up as active 6to4 relays.
   Unmanaged 6to4 relays will be a source of problems.

   Operators in this category should consider whether they need to
   defend themselves against rogue RA messages with an RA Guard solution
   [RFC6105].  If RA Guard is not available, it may help in some cases
   if at least one legitimate IPv6 router per LAN supports [RFC4191] and
   sets the Router Advertisement router preference bits to 01 (high
   preference).  Eventually, the measures being designed by the IETF
   Source Address Validation Improvements (SAVI) working group will
   assist with this problem.

4.4.  Transit ISPs and Internet Exchange Points

   We assume that transit ISPs have IPv6 connectivity.  To reduce the
   negative impact of Anycast 6to4 on all their client networks, it is
   strongly recommended that they each run an Anycast 6to4 relay
   service.  This will have the additional advantage that they will
   terminate the 6to4 IPv4 packets, and can then forward the
   decapsulated IPv6 traffic according to their own policy.  Otherwise,

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   they will blindly forward all the encapsulated IPv6 traffic to a
   competitor who does run a relay.

   Although most modern Internet Exchange Points do not offer IP layer
   services, an IXP could choose to operate an Anycast 6to4 relay
   service for the benefit of its customers.  If so, it should follow
   the recommendations in this section.

   It is of critical importance that routing to this service is
   carefully managed:
   1.  The IPv4 prefix must be announced only towards
       client IPv4 networks whose outbound 6to4 packets will be
   2.  The IPv6 prefix 2002::/16 must be announced towards native IPv6.
       The relay must accept all traffic towards 2002::/16 that reaches
       it, so the scope reached by this announcement should be carefully
       planned.  It must reach all client IPv6 networks of the transit
       ISP.  If it reaches a wider scope, the relay will be offering a
       free ride to non-clients.
   3.  As discussed in item 2 of Section 3, the choice of IPv4 source
       address used when the relay sends 6to4 packets back towards a
       6to4 user is important.  The best choice is likely to be, not the relay's unicast IPv4 address, unless ingress
       filtering is an issue.  This is to avoid failure if the user is
       behind a stateful firewall.
   4.  The relay should be capable of responding correctly to ICMPv6
       echo requests encapsulated in IPv4 protocol 41, typically with
       outer destination address and inner destination
       address 2002:c058:6301::.  (As noted previously, some 6to4 hosts
       are known to send echo requests with Hop Limit = 1, which allows
       them to rapidly detect the presence or absence of a relay in any
       case, but operators cannot rely on this behaviour.)
   5.  Protocol 41 must not be filtered in any IPv4 network or
   6.  As a matter of general practice, which is essential for 6to4 to
       work well, IPv6 PMTUD must be possible, which means that ICMPv6
       must not be blocked anywhere [RFC4890].  This also requires that
       the relay has a sufficiently high ICMP error generation
       threshold.  For a busy relay, a typical default rate limit of 100
       packets per second is too slow.  On a busy relay, 1000pps or more
       might be needed.  If ICMPv6 "Packet Too Big" error messages are
       rate-limited, users will experience PMTUD failure.
   7.  The relay must have adequate performance, and since load
       prediction is extremely hard, it must be possible to scale it up
       or, perhaps better, to replicate it as needed.  Since the relay
       process is stateless, any reasonable method of load sharing
       between multiple relays will do.

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   8.  The relay must of course be connected directly to global IPv4
       space, with no NAT.

   Operators in this category should make sure that no routers are
   unintentionally or by default set up as active 6to4 relays.
   Unmanaged 6to4 relays will be a source of problems.

4.5.  Content providers and their ISPs

   We assume that content providers and their ISPs have IPv6
   connectivity, and that content servers are dual stacked.  There is a
   need to avoid the situation where a client host, configured with
   Anycast 6to4, succeeds in sending an IPv6 packet to the server, but
   the 6to4 return path fails as described above.  To avoid this, there
   must be a locally positioned 6to4 relay.  Large content providers are
   advised to operate their own relays, and ISPs should do so in any
   case.  There must be a 2002::/16 route from the content server to the
   relay.  As noted in the previous section, the corresponding route
   advertisement must be carefully scoped, since any traffic that
   arrives for 2002::/16 must be relayed.

   Such a relay may be dedicated entirely to return traffic, in which
   case it need not respond to the 6to4 anycast address.

   Nevertheless, it seems wisest to ensure that when the relay sends
   6to4 packets back towards a 6to4 user, they should have
   as their IPv4 source address (not the relay's unicast IPv4 address).
   As noted above, this is to avoid problems if the user is behind a
   stateful firewall that drops UDP packets from addresses that have not
   been seen in outbound traffic.  However, it is also necessary that is not blocked by upstream ingress filtering - this needs
   to be tested.

   Without careful engineering, there is nothing to make the return path
   as short as possible.  It is highly desirable to arrange the scope of
   advertisements for 2002::/16 such that content providers have a short
   path to the relay, and the relay should have a short path to the ISP
   border.  Care should be taken about shooting off advertisements for
   2002::/16 into BGP4; they will become traffic magnets.  If every ISP
   with content provider customers operates a relay, there will be no
   need for any of them to be advertised beyond each ISP's own

   Protocol 41 must not be filtered in the ISP's IPv4 network or
   firewalls.  If the relays are placed outside the content provider's
   firewall, the latter may filter protocol 41 if desired.

   The relay must have adequate performance, and since load prediction

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   is extremely hard, it must be possible to scale it up or, perhaps
   better, to replicate it as needed.  Since the relay process is
   stateless, any reasonable method of load sharing between multiple
   relays will do.

   The relay must of course be connected directly to global IPv4 space,
   with no NAT.

   An option for content servers is to embed the relay function directly
   in the content server.  This is in fact trivial, since it can be
   achieved by enabling a local 6to4 interface on the server, and using
   it to route 2002::/16 for outbound packets.  (This might not allow
   use of as the source address.)  Further details are to be
   found at [Huston-b].  However, in this case Protocol 41 must be
   allowed by the firewalls.

   Content providers who do embed the relay function in this way could,
   in theory, accept inbound 6to4 traffic as well.  This is highly
   unadvisable since, according the the rules of 6to4, they would then
   have to relay traffic for other IPv6 destinations too.  So they
   should not be reachable via  Also, they should certainly
   not create an AAAA record for their 6to4 address - their inbound IPv6
   access should be native, and advertising a 6to4 address might well
   lead to uRPF ingress filtering problems.

   To avoid the path MTU problem described above, content servers should
   also set their IPv6 MTU to a safe value.  From experience, 1280 bytes
   (the minimum allowed for IPv6) is recommended; again see [Huston-b].
   Of course, ICMPv6 "Packet Too Big" must not be blocked or rate-
   limited anywhere [RFC4890].

   Reverse DNS delegations are highly unlikely to exist for 6to4
   clients, and are by no means universal for other IPv6 clients.
   Content providers (and in fact all service providers) should not rely
   on them as a pseudo-security check for IPv6 clients.

   Operators and content providers should make sure that no routers are
   unintentionally or by default set up as active 6to4 relays.
   Unmanaged 6to4 relays will be a source of problems.

5.  Tunnels Managed by ISPs

   There are various ways, such as tunnel brokers [RFC3053], 6rd
   [RFC5969], and L2TPv2 hub-and-spoke [RFC5571], by which Internet
   Service Providers can provide tunneled IPv6 service to subscribers in
   a managed way, in which the subscriber will acquire an IPv6 prefix
   under a normal provider-based global IPv6 prefix.  Most of the issues

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   described for 6to4 do not arise in these scenarios.  However, for
   IPv6-in-IPv4 tunnels used by clients behind a firewall, it is
   essential that IPv4 Protocol 41 is not blocked.

   As a matter of general practice, IPv6 PMTUD must be possible, which
   means that ICMPv6 "Packet Too Big" must not be blocked or rate-
   limited anywhere [RFC4890].

6.  Security Considerations

   There is a general discussion of security issues for IPv6-in-IPv4
   tunnels in [RFC6169] and [I-D.ietf-v6ops-tunnel-loops] discusses
   possible malicious loops.  [RFC3964] specifically discusses 6to4
   security.  In summary, tunnels create a challenge for many common
   security mechanisms, simply because a potentially suspect packet is
   encapsulated inside a harmless outer packet.  All these
   considerations apply to the automatic mechanisms discussed in this
   document.  However, it should be noted that if an operator provides
   well managed servers and relays for 6to4, non-encapsulated IPv6
   packets will pass through well defined points (the native IPv6
   interfaces of those servers and relays) at which security mechanisms
   may be applied.

   A blanket recommendation to block Protocol 41 is not compatible with
   mitigating the 6to4 problems described in this document.

7.  IANA Considerations

   This document makes no request of the IANA.

8.  Acknowledgements

   Useful comments and contributions were made by Emile Aben, Mikael
   Abrahamsson, Tore Anderson, Hermin Anggawijaya, Jack Bates, Cameron
   Byrne, Tim Chown, Remi Despres, Jason Fesler, Wes George, Philip
   Homburg, Geoff Huston, Eric Kline, Victor Kuarsingh, Martin Levy,
   David Malone, Martin Millnert, Keith Moore, Gabi Nakibly, Michael
   Newbery, Pekka Savola, Mark Smith, Nathan Ward, James Woodyatt, and

   This document was produced using the xml2rfc tool [RFC2629].

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9.  Change log [RFC Editor: please remove]

   draft-ietf-v6ops-6to4-advisory-01: Updated with WG Last Call
   comments, 2011-04-26

   draft-ietf-v6ops-6to4-advisory-00: WG draft, updated with WG
   comments, 2011-03-31

   draft-carpenter-v6ops-6to4-teredo-advisory-03: updated with
   additional security reference and additional comments, 2011-03-12

   draft-carpenter-v6ops-6to4-teredo-advisory-02: updated after further
   comments, removed references to Teredo, 2011-02-24

   draft-carpenter-v6ops-6to4-teredo-advisory-01: updated after WG
   discussion, 2011-02-10

   draft-carpenter-v6ops-6to4-teredo-advisory-00: original version,

10.  Informative References

   [Aben]     Aben, E., "6to4 - How Bad is it Really?", 2010, <https://

              Anderson, T., "IPv6 dual-stack client loss in Norway",
              2010, <http://www.fud.no/ipv6/>.

              Huston, G., "Flailing IPv6", 2010,

              Huston, G., "Two Simple Hints for Dual Stack Servers",

              Matsumoto, A., Kato, J., and T. Fujisaki, "Update to RFC
              3484 Default Address Selection for IPv6",
              draft-ietf-6man-rfc3484-revise-02 (work in progress),
              March 2011.

              Ford, M., Boucadair, M., Durand, A., Levis, P., and P.

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              Roberts, "Issues with IP Address Sharing",
              draft-ietf-intarea-shared-addressing-issues-05 (work in
              progress), March 2011.

              Troan, O., "Request to move Connection of IPv6 Domains via
              IPv4 Clouds (6to4) to Historic status",
              draft-ietf-v6ops-6to4-to-historic-00 (work in progress),
              April 2011.

              Nakibly, G. and F. Templin, "Routing Loop Attack using
              IPv6 Automatic Tunnels: Problem Statement and Proposed
              Mitigations", draft-ietf-v6ops-tunnel-loops-06 (work in
              progress), March 2011.

              Livingood, J., "IPv6 AAAA DNS Whitelisting Implications",
              (work in progress), February 2011.

              Wing, D. and A. Yourtchenko, "Happy Eyeballs: Trending
              Towards Success with Dual-Stack Hosts",
              draft-wing-v6ops-happy-eyeballs-ipv6-01 (work in
              progress), October 2010.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [RFC3053]  Durand, A., Fasano, P., Guardini, I., and D. Lento, "IPv6
              Tunnel Broker", RFC 3053, January 2001.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3068]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
              RFC 3068, June 2001.

   [RFC3964]  Savola, P. and C. Patel, "Security Considerations for
              6to4", RFC 3964, December 2004.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

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   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

   [RFC4890]  Davies, E. and J. Mohacsi, "Recommendations for Filtering
              ICMPv6 Messages in Firewalls", RFC 4890, May 2007.

   [RFC5158]  Huston, G., "6to4 Reverse DNS Delegation Specification",
              RFC 5158, March 2008.

   [RFC5571]  Storer, B., Pignataro, C., Dos Santos, M., Stevant, B.,
              Toutain, L., and J. Tremblay, "Softwire Hub and Spoke
              Deployment Framework with Layer Two Tunneling Protocol
              Version 2 (L2TPv2)", RFC 5571, June 2009.

   [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd) -- Protocol Specification",
              RFC 5969, August 2010.

   [RFC6104]  Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
              Problem Statement", RFC 6104, February 2011.

   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              February 2011.

   [RFC6169]  Krishnan, S., Thaler, D., and J. Hoagland, "Security
              Concerns with IP Tunneling", RFC 6169, April 2011.

   [Savola]   Savola, P., "Observations of IPv6 Traffic on a 6to4
              Relay", ACM SIGCOMM CCR 35 (1) 23-28, 2006.

Author's Address

   Brian Carpenter
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland,   1142
   New Zealand

   Email: brian.e.carpenter@gmail.com

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