Internet Engineering Task Force (IETF)
Request for Comments: 6343
Category: Informational
ISSN: 2070-1721
B. Carpenter
Univ. of Auckland
August 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 (ISPs), 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 minimize both user dissatisfaction and help-desk calls.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are 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

Copyright Notice

Copyright © 2011 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 ( 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. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. Principles of Operation .........................................3
      2.1. Router 6to4 ................................................3
      2.2. Anycast 6to4 ...............................................4
   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 .................14
      4.5. Content Providers and Their ISPs ..........................15
   5. Tunnels Managed by ISPs ........................................17
   6. Security Considerations ........................................17
   7. Acknowledgements ...............................................18
   8. References .....................................................18
      8.1. Normative References ......................................18
      8.2. Informative References ....................................18

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 cause either 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; 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 [RFC3484-REVISE].
  1. Ensuring that application software deals gracefully with connectivity problems [EYEBALLS-IPV6].
  1. Some content providers have chosen to avoid the problem by hiding their IPv6 address except from customers of pre-qualified networks [DNSWHITE].

A companion document [HISTORIC] proposes to reclassify 6to4 as Historic. However, this will not remove the millions of existing hosts and CPEs that implement 6to4. Hence, the advice in this document remains necessary.

2. Principles of Operation

There are two variants of 6to4 that 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 that therefore inherit 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. The relay will decapsulate the packet and forward it 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 2002::/16 in the vicinity of the remote IPv6 host.

Of course, there are 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. Therefore, it 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 three-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 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 [EYEBALLS-IPV6]).

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

  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 3056.

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.

  1. 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 If the relay is subject to ingress filtering, only packets from its native IPv4 address can be transmitted. Therefore, there are only three combinations that can succeed:

  1. No protocol 41 filter, with the relay using its native IPv4 source address.
  1. No protocol 41 filter, with the relay using the anycast IPv4 source address and with no ingress filter.
  1. A stateful protocol 41 firewall, with the relay using the anycast IPv4 source address and with no ingress filter.
  1. 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.

  1. 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.
  1. 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 (PMTUD) 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 Maximum Segment Size (MSS) negotiation mechanism [RFC2923]. 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 also advertise a smaller MTU; in this case, TCP MSS negotiation will definitely fail.

  1. 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.
  1. 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 [CGN] between its customers and the Internet, and is using global public address space as if it were private space to do so.

  1. 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.
  1. 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 fallback to IPv4 connectivity [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 misbehavior by devices acting as local IPv6 routers or connection-sharing devices but issuing Router Advertisement (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 behavior, the resulting rogue RA messages will indeed convey a 2002::/16 prefix.

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. To avoid operational problems and customer dissatisfaction, 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 6to4.
  1. Similarly, host operating systems should not enable Anycast 6to4 by default; it should always be left to the user to switch it on.
  1. 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.
  1. 6to4 implementations should adopt updated IETF recommendations on address selection [RFC3484-REVISE].
  1. 6to4 relay implementations must carefully follow Section 3.2 of [RFC4213] to ensure correct handling of MTU issues.
  1. 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).

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!)
  1. If so, is it functional and stable?
  1. If so, is the ping time reasonably short?
  1. 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 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 [RFC6269].) 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 customers.

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 Improvement (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 packets.

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 addresses.

Such an operator may automatically fall into one of the following two categories (transit provider or content provider), so the guidelines in Sections 4.4 or 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 Improvement (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, 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 Internet exchange point (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 accepted.
  1. 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.
  1. 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.
  1. 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 behavior.)
  1. Protocol 41 must not be filtered in any IPv4 network or firewalls.
  1. 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, 1000 pps or more might be needed. If ICMPv6 "Packet Too Big" error messages are rate limited, users will experience PMTUD failure.
  1. 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.
  1. Of course, the relay must 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 the servers are dual stacked. The following applies to content servers as such, but equally to web hosting servers, servers that form part of a content distribution network, load balancers in front of a server farm, and HTTP caches. 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 customers.

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 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 is to embed the relay function directly in the content server or first hop router. This is straightforward, since it can be achieved by enabling a local 6to4 interface, 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 to 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 unicast reverse path forwarding (uRPF) [RFC3704] 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 Layer 2 Tunneling Protocol version 2 (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 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 [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. 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, Ray Hunter, Geoff Huston, Eric Kline, Victor Kuarsingh, Martin Levy, David Malone, Alexey Melnikov, Martin Millnert, Keith Moore, Gabi Nakibly, Michael Newbery, Phil Pennock, Pekka Savola, Mark Smith, Nathan Ward, James Woodyatt, and others.

8. References

8.1. Normative References

   [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.

8.2. Informative References

   [Aben]            Aben, E., "6to4 - How Bad is it Really?", 2010, <ht
   [Anderson]        Anderson, T., "IPv6 dual-stack client loss in
                     Norway", 2010, <>.
   [CGN]             Perreault, S., Yamagata, I., Miyakawa, S.,
                     Nakagawa, A., and H. Ashida, "Common requirements
                     for Carrier Grade NAT (CGN)", Work in Progress,
                     July 2011.
   [DNSWHITE]        Livingood, J., "IPv6 AAAA DNS Whitelisting
                     Implications", Work in Progress, June 2011.
   [EYEBALLS-IPV6]   Wing, D. and A. Yourtchenko, "Happy Eyeballs:
                     Trending Towards Success with Dual-Stack Hosts",
                     Work in Progress, October 2010.
   [HISTORIC]        Troan, O., "Request to move Connection of IPv6
                     Domains via IPv4 Clouds (6to4) to Historic status",
                     Work in Progress, June 2011.
   [Huston-a]        Huston, G., "Flailing IPv6", 2010, <http://
   [Huston-b]        Huston, G., "Two Simple Hints for Dual Stack
                     Servers", 2010, <
   [RFC1918]         Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot,
                     G., and E. Lear, "Address Allocation for Private
                     Internets", BCP 5, RFC 1918, February 1996.
   [RFC2923]         Lahey, K., "TCP Problems with Path MTU Discovery",
                     RFC 2923, September 2000.
   [RFC3053]         Durand, A., Fasano, P., Guardini, I., and D. Lento,
                     "IPv6 Tunnel Broker", RFC 3053, January 2001.
   [RFC3484-REVISE]  Matsumoto, A., Kato, J., Fujisaki, T., and T.
                     Chown, "Update to RFC 3484 Default Address
                     Selection for IPv6", Work in Progress, July 2011.
   [RFC3704]         Baker, F. and P. Savola, "Ingress Filtering for
                     Multihomed Networks", BCP 84, RFC 3704, March 2004.
   [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.
   [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.
   [RFC6269]         Ford, M., Boucadair, M., Durand, A., Levis, P., and
                     P. Roberts, "Issues with IP Address Sharing",
                     RFC 6269, June 2011.
   [Savola]          Savola, P., "Observations of IPv6 Traffic on a 6to4
                     Relay", ACM SIGCOMM CCR 35 (1) 23-28, 2006.
   [TUNNEL-LOOPS]    Nakibly, G. and F. Templin, "Routing Loop Attack
                     using IPv6 Automatic Tunnels: Problem Statement and
                     Proposed Mitigations", Work in Progress, May 2011.

Author's Address

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