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Versions: 00 01 02 03                                                   
Network Working Group                                             S. Roy
Internet-Draft                                                 A. Durand
Expires: August 13, 2004                                        J. Paugh
                                                  Sun Microsystems, Inc.
                                                       February 13, 2004

               Issues with Dual Stack IPv6 on by Default

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at http://

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on August 13, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   This document discusses problems that can occur when dual stack nodes
   that have IPv6 enabled by default are deployed in IPv4 or mixed IPv4
   and IPv6 environments.  The problems include application connection
   delays, poor connectivity, and network security.  Its purpose is to
   raise awareness of these problems so that they can be fixed or worked
   around. The purpose of this document is not to try to specify whether
   IPv6 should be enabled by default or not, but to raise awareness of
   the potential issues involved.

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Table of Contents

   1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  3
   2.      No IPv6 Router . . . . . . . . . . . . . . . . . . . . . .  3
   2.1     Problems with Default Address Selection for IPv6 . . . . .  3
   2.2     Neighbor Discovery's On-Link Assumption Considered
           Harmful  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.3     Transport Protocol Robustness  . . . . . . . . . . . . . .  6
   2.3.1   TCP Implications . . . . . . . . . . . . . . . . . . . . .  6 TCP Connection Termination . . . . . . . . . . . . . . . .  6 Asynchronous Application Notification  . . . . . . . . . .  7
   2.3.2   UDP Implications . . . . . . . . . . . . . . . . . . . . .  7
   2.3.3   SCTP Implications  . . . . . . . . . . . . . . . . . . . .  8
   3.      Other Problematic Scenarios  . . . . . . . . . . . . . . .  8
   3.1     IPv6 Network of Smaller Scope  . . . . . . . . . . . . . .  8
   3.1.1   Alleviating the Scope Problem  . . . . . . . . . . . . . .  8
   3.2     Poor IPv6 Network Performance  . . . . . . . . . . . . . .  8
   3.2.1   Dealing with Poor IPv6 Network Performance . . . . . . . .  9
   3.3     Security . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.3.1   Mitigating Security Risks  . . . . . . . . . . . . . . . . 10
   4.      Application Robustness . . . . . . . . . . . . . . . . . . 10
   5.      Security Considerations  . . . . . . . . . . . . . . . . . 11
           Normative References . . . . . . . . . . . . . . . . . . . 11
           Informative References . . . . . . . . . . . . . . . . . . 11
           Authors' Addresses . . . . . . . . . . . . . . . . . . . . 12
   A.      Acknowledgments  . . . . . . . . . . . . . . . . . . . . . 12
   B.      Changes from draft-ietf-v6ops-v6onbydefault-00 . . . . . . 12
           Intellectual Property and Copyright Statements . . . . . . 14

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1. Introduction

   This document specifically addresses operating system implementations
   that implement the dual stack IPv6 model, and would ship with IPv6
   enabled by default.  It addresses the case where such a system is
   installed and placed in an IPv4 only or mixed IPv4 and IPv6
   environment, and documents potential problems that users on such
   systems could experience if the IPv6 connectivity is non-existent or
   sub-optimal. The purpose of this document is not to try to specify
   whether IPv6 should be enabled by default or not, but to raise
   awareness of the potential issues involved.

   It begins in Section 2 by examining problems within IPv6
   implementations that defeat the destination address selection
   mechanism defined in [ADDRSEL] and contribute to poor IPv6
   connectivity.  Starting with Section 3 it then examines other issues
   that network software engineers and network and systems
   administrators should be aware of when deploying dual stack systems
   with IPv6 enabled.

2. No IPv6 Router

   Consider a scenario in which a dual stack system has IPv6 enabled and
   placed on a link with no IPv6 routers.  The system is using IPv6
   Stateless Address Autoconfiguration [AUTOCONF], so it only has a
   link-local IPv6 address configured.  It also has a single IPv4
   address that happens to be a private address as defined in

   An application on this system is trying to communicate with a
   destination whose name resolves to public and global IPv4 and IPv6
   addresses.  The application uses an address resolution API that
   implements the destination address selection mechanism described in
   Default Address Selection for IPv6 [ADDRSEL].  The application will
   attempt to connect to each address returned in order until one
   succeeds.  Since the system has no off-link IPv6 routes, the optimal
   scenario would be if the IPv4 addresses returned were ordered before
   the IPv6 addresses.  The following sections describe where things can
   go wrong with this scenario.

2.1 Problems with Default Address Selection for IPv6

   The Default Address Selection for IPv6 [ADDRSEL] destination address
   selection mechanism could save the application a few useless
   connection attempts by placing the IPv4 addresses in front of the
   IPv6 addresses.  This would be desired since all IPv6 destinations in
   this scenario are unreachable (there's no route to them), and the
   system's only IPv6 source address is inadequate to communicate with

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   off-link destinations even if it did have an off-link route.

   Let's examine how the destination address selection mechanism behaves
   in the face of this scenario when given one IPv4 destination and one
   IPv6 destination.

   The first rule, "Avoid unusable destinations", could prefer the IPv4
   destination over the IPv6 destination, but only if the IPv6
   destination is determined to be unreachable.  The unreachability
   determination for a destination as it pertains to this rule is an
   implementation detail.  One implementable method is to do a simple
   forwarding table lookup on the destination, and to deem the
   destination as reachable if the lookup succeeds.  The Neighbor
   Discovery on-link assumption mentioned in Section 2.2 makes this
   method somewhat irrelevant, however, as an implementation of the
   assumption could simply be to insert an IPv6 default on-link route
   into the system's forwarding table when the default router list is
   empty.  The side-effect is that the rule would always determine that
   all IPv6 destinations are reachable.  Therefore, this rule will not
   necessarily prefer one destination over the other.

   The second rule, "Prefer matching scope", could prefer the IPv4
   destination over the IPv6 destination, but only if the IPv4
   destination's scope matches the scope of the system's IPv4 source
   address.  Since [ADDRSEL] considers private addresses (as defined in
   [PRIVADDR]) of site-local scope, then this rule will not prefer
   either destination over the other.  The link-local IPv6 source
   doesn't match the global IPv6 destination, and the site-local IPv4
   source doesn't match the global IPv4 destination. The tie-breaking
   rule in this case is rule 6, "Prefer higher precedence".  Since IPv6
   destinations are of higher precedence than IPv4 destinations in the
   default policy table, the IPv6 destination will be preferred.

   The solution in this case could be to add a new rule after rule 2
   (rule 2.5) that avoids non-link-local IPv6 destinations whose source
   addresses are link-local.  Of course, if the host is manually
   assigned a global IPv6 source address, then rule 2 will automatically
   prefer the IPv6 destination, and there is no fix other than to make
   sure rule 1 considers IPv6 destinations unreachable in this scenario.

   Fixing the destination address selection mechanism by adding such a
   rule is only a mitigating factor if applications use standard name
   resolution API's that implement this mechanism, and these
   applications try addresses in the order returned.  This may not be an
   acceptable assumption in some cases, as there are applications that
   use hard coded addresses and address search orders (DNS resolver is
   one example), and/or literal addresses passed in from the user.  Such
   applications will obviously be subject to whatever connection delays

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   are associated with attempting a connection to an unreachable
   destination.  This is discussed in more detail in the next few

2.2 Neighbor Discovery's On-Link Assumption Considered Harmful

   Let's assume that the application described in Section 2 is
   attempting a connection to an IPv6 address first, either because the
   destination address selection mechanism described in Section 2.1
   returned the addresses in that order, or because the application
   isn't trying the addresses in the order returned.  Regardless, the
   user expects that the application will quickly connect to the
   destination.  It is therefore important that the system quickly
   determine that the IPv6 destination is unreachable so that the
   application can try the IPv4 destination.

   Neighbor Discovery's [ND] conceptual sending algorithm states that
   when sending a packet to a destination, if a host's default router
   list is empty, then the host assumes that the destination is on-link.
   This issue is described in detail in [ONLINK].  In summary, this
   assumption makes the unreachability detection of off-link nodes in
   the absence of a default router a lengthy operation.  This is due to
   the cost of attempting Neighbor Discovery link-layer address
   resolution for each destination, and potential transport layer costs
   associated with connection timeouts.  The transport layer issues are
   discussed later in Section 2.3.

   On a network that has no IPv6 routing and no IPv6 neighbors, making
   the assumption that every IPv6 destination is on-link will be a
   costly and incorrect assumption.  If an application has a list of
   addresses associated with a destination and the first 15 are IPv6
   addresses, then the application won't be able to successfully send a
   packet to the destination until the attempts to resolve each IPv6
   address have failed.  This could take 45 seconds
   compounded by any transport timeouts associated with each connection

   If IPv6 hosts don't assume that destinations are on-link as described
   above, then communication with destinations that are not on-link and
   unreachable should immediately fail.  The IPv6 implementation should
   be able to immediately notify applications or the transport layer
   that it has no route to such IPv6 destinations, and applications
   won't waste time waiting for address resolution to fail.

   If hosts need to communicate with on-link destinations in the absence
   of default routers, then then they need to be explicitly configured
   to have on-link routes for those destinations.

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2.3 Transport Protocol Robustness

   Making the same set of assumptions as Section 2.2, regardless of how
   long the network layer takes to determine that the IPv6 destination
   is unreachable, the delay associated with a connection attempt to an
   unreachable destination can be compounded by the transport layer.
   When the unreachability of a destination is obviated by the reception
   of an ICMPv6 destination unreachable message, the transport layer
   should make it possible for the application to deal with this by
   failing the connection attempt, passing ICMPv6 errors up to the
   application, etc... Such messages would be received in the cases
   mentioned in Section 2 in which a node has no default routers and NUD
   fails for destinations assumed to be on-link, and when firewalls or
   other systems that enforce scope boundaries send such ICMPv6 errors
   as described in Section 3.1 and Section 3.3.

   For cases when packets to a destination are essentially black-holed
   and no ICMPv6 errors are generated, there is very little additional
   remedy other than the existing timer mechanisms inside transport
   layers and applications. The following transport layer implication
   discussions deal with the former case, in which ICMPv6 errors are

2.3.1 TCP Implications

   In the case of a socket application attempting a connection via TCP,
   it would be unreasonable for the application to block even after the
   system has received notification that the destination address is
   unreachable via an ICMPv6 Destination Unreachable message.

   Following are some ways of solving TCP related delays associated with
   destination unreachability when ICMPv6 errors are generated. TCP Connection Termination

   One solution is for TCP to abort connections in SYN-SENT or
   SYN-RECEIVED state when it receives an ICMPv6 Destination Unreachable

   It should be noted that the Requirements for Internet Hosts
   [HOSTREQS] document, in section, states that TCP MUST NOT
   abort connections when receiving ICMP Destination Unreachable
   messages that indicate "soft errors", where soft errors are defined
   as ICMP codes 0 (network unreachable), 1 (host unreachable), and 5
   (source route failed), and SHOULD abort connections upon receiving
   the other codes (which are considered "hard errors").  ICMPv6 didn't
   exist when that document was written, but one could extrapolate the
   concept of soft errors to ICMPv6 Type 1 codes 0 (no route to

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   destination) and 3 (address unreachable), and hard errors to the
   other codes. Thus, it could be argued that a TCP implementation that
   behaves as suggested in this section is in conflict with [HOSTREQS].

   When [HOSTREQS] was written, most applications would mostly only try
   one address when establishing communication with a destination.  Not
   aborting a connection was a sane thing to do if re-trying a single
   address was a better alternative over quitting the application
   altogether. With IPv6, and especially on dual stack systems,
   destinations are often assigned multiple addresses (at least one IPv4
   and one IPv6 address), and applications iterate through destination
   addresses when attempting connections.

   Since soft errors conditions are those that would entice an
   application to continue iterating to another address, TCP shouldn't
   make the distinction between ICMPv6 soft errors and hard errors when
   in SYN-SENT or SYN-RECEIVED state.  It should abort a connection in
   those states when receiving any ICMPv6 Destination Unreachable
   message.  When in any other state, TCP would behave as described in

   Many TCP implementations already behave this way, but others do not.
   This should be noted as a best current practice in this case.

   A tangential method of handling the problem in this way would be for
   applications to somehow notify the TCP layer of their preference in
   the matter.  An application could notify TCP that it should abort a
   connection upon receipt of particular ICMPv6 errors.  Similarly, it
   could notify TCP that it should not abort a connection.  This would
   allow existing TCP implementations to maintain their status quo at
   the expense of increased application complexity. Asynchronous Application Notification

   In section, [HOSTREQS] states that there MUST be a mechanism
   for reporting soft TCP error conditions to the application. Such a
   mechanism (assuming one is implemented) could be used by applications
   to cycle through destination addresses.

2.3.2 UDP Implications

   As noted in [HOSTREQS] section, UDP implementations MUST pass
   to the application layer all ICMP error messages that it receives
   from the IP layer.  As a result, proper handling destination
   unreachability by UDP applications is the responsibility of the
   applications themselves.

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2.3.3 SCTP Implications

   According to [SCTPIMP], SCTP ignores all ICMPv6 destination
   unreachable messages.  The existing SCTP specifications do not
   suggest any action on the part of the implementation on reception of
   such messages.  Investigation needs to be done to determine the

3. Other Problematic Scenarios

   This section describes problems that could arise for a dual stack
   system with IPv6 enabled when placed on a network with IPv6

3.1 IPv6 Network of Smaller Scope

   A network that has a smaller scope of connectivity for IPv6 as it
   does for IPv4 could be a problem in some cases.  If applications have
   access to name to address mapping information that is of greater
   scope than the connectivity to those addresses, there is obvious
   potential for suboptimal network performance.  Hosts will attempt to
   communicate with IPv6 destinations that are outside the scope of the
   IPv6 routing, and depending on how the scope boundaries are enforced,
   applications may not be notified that packets are being dropped at
   the scope boundary.

   If applications aren't immediately notified of the lack of
   reachability to IPv6 destinations, then they aren't able to
   efficiently fall back to IPv4. They then have to rely on transport
   layer timeouts which can be minutes in the case of TCP.

   An example of such a network is an enterprise network that has both
   IPv4 and IPv6 routing within the enterprise and has a firewall
   configured to allow some IPv4 communication, but no IPv6

3.1.1 Alleviating the Scope Problem

   To allow applications to correctly fall back to IPv4 when IPv6
   packets are destined beyond their allowed scope, the devices
   enforcing the scope boundary must send ICMPv6 Destination Unreachable
   messages back to senders of such packets.  The sender's transport
   layer should act on these errors as described in Section 2.3.

3.2 Poor IPv6 Network Performance

   Most applications on dual stack nodes will try IPv6 destinations
   first by default due to the Default Address Selection mechanism

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   described in [ADDRSEL].  If the IPv6 connectivity to those
   destinations is poor while the IPv4 connectivity is better (i.e., the
   IPv6 traffic experiences higher latency, lower throughput, or more
   lost packets than IPv4 traffic), applications will still communicate
   over IPv6 at the expense of network performance.  There is no
   information available to applications in this case to advise them to
   try another destination address.

   An example of such a situation is a node which obtains IPv4
   connectivity natively through an ISP, but whose IPv6 connectivity is
   obtained through a configured tunnel whose other endpoint is
   topologically such that most IPv6 communication is done through
   triangular IPv4 paths.  Operational experience on the 6bone shows
   that IPv6 RTT's are poor in such situations.

3.2.1 Dealing with Poor IPv6 Network Performance

   There are few options from the end node's perspective.  One is to
   configure each node to prefer IPv4 destinations over IPv6.  If hosts
   implement the Default Address Selection for IPv6 [ADDRSEL] policy
   table, IPv4 mapped addresses could be assigned higher precedence,
   resulting in applications trying IPv4 for communication first. This
   has the negative side-effect that these nodes will almost never use
   IPv6 unless the only address published in the DNS for a given name is
   IPv6, presumably because of this phenomenon.

   Disabling IPv6 on the end nodes is another solution. The idea would
   be that enabling IPv6 on dual stack nodes is a manual process that
   would be done when the administrator knows that IPv6 connectivity is

3.3 Security

   Enabling IPv6 on a host implies that the services on the host may be
   open to IPv6 communication.  If the service itself is insecure and
   depends on security policy enforced somewhere else on the network
   (such as in a firewall), then there is potential for new attacks
   against the service.

   A firewall, for example, may not be enforcing the same policy for
   IPv4 as for IPv6 traffic.  One possibility is that the firewall could
   have more relaxed policy for IPv6, perhaps by letting all IPv6
   packets pass through, or by letting all IPv4 protocol 41 packets pass
   through. In this scenario, the dual stack hosts within the protected
   network could be subject to different attacks than for IPv4.

   Even if a firewall has a stricter policy or identical policy for IPv6
   traffic than for IPv4 (the extreme case being that it drops all IPv6

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   traffic), IPv6 packets could go through the network untouched if
   tunneled over a transport layer.  This could open the host to direct
   IPv6 attacks.

   A similar problem could exist for VPN software.  A VPN could protect
   all IPv4 packets but transmit all others onto the local subnet
   unprotected.  At least one widely used VPN behaves this way.  This is
   problematic on a dual stack host that has IPv6 enabled on its local
   network.  It establishes its VPN link and attempts to communicate
   with destinations that resolve to both IPv4 and IPv6 addresses.  The
   destination address selection mechanism prefers the IPv6 destination
   so the application sends packets to an IPv6 address.  The VPN doesn't
   know about IPv6, so instead of protecting the packets and sending
   them to the remote end of the VPN, it passes such packets in the
   clear to the local network.  The reason that packets meant to be
   protected would be sent in the clear on the local network is either
   because of the on-link assumption discussed in Section 2.2, or of
   malicious hijacking of traffic by a rogue "fake" router advertising a

3.3.1 Mitigating Security Risks

   The security policy implemented in firewalls, VPN software, or other
   devices, must take a stance whether it applies equally to both IPv4
   and IPv6 traffic.  It is probably desirable for policy to apply
   equally to both IPv4 and IPv6, but the most important thing is to be
   aware of the potential problem, and to make the policy clear to the
   administrator and user.

   There is still a risk that IPv6 packets could be tunneled over a
   transport layer such as UDP, implicitly bypassing security policy.
   Some more complex mechanism could be implemented to apply the correct
   policy to such packets.  This could be easy to do if tunnel endpoints
   are co-located with a firewall, but more difficult if internal nodes
   do their own IPv6 tunneling.

4. Application Robustness

   Enabling IPv6 on a dual stack node is only useful if applications
   that support IPv6 on that node properly cycle through addresses
   returned from name lookups and fall back to IPv4 when IPv6
   communication fails.  Simply cycling through the list of addresses
   returned from a name lookup when attempting connections works in most
   cases for most applications, but there are still cases where that's
   not enough.  Applications also need to be aware that the fact that a
   dual stack destination's IPv6 address is published in the DNS does
   not necessarily imply that all services on that destination function
   over IPv6. This problem, along with a thorough discussion of IPv6

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   application transition guidelines, is discussed in [APPTRANS].

5. Security Considerations

   This document raises security concerns in Section 3.3.

Normative References

   [ADDRSEL]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

              Hong, Y-G., Hagino, J., Savola, P. and M. Castro,
              "Application Aspects of IPv6 Transition", October 2003.


   [ND]       Narten, T., Nordmark, E. and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461, December

   [ONLINK]   Roy, S., Durand, A. and J. Paugh, "IPv6 Neighbor Discovery
              On-Link Assumption Considered Harmful", October 2003.


Informative References

              Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, December 1998.

              Braden, R., "Requirements for Internet Hosts --
              Communication Layers", STD 3, RFC 1122, October 1989.

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

   [SCTPIMP]  Stewart, R., Arias-Rodriguez, I., Poon, K., Caro, A. and
              M. Tuexen, "", November 2003.


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Authors' Addresses

   Sebastien Roy
   Sun Microsystems, Inc.
   1 Network Drive
   Burlington, MA  01801

   EMail: sebastien.roy@sun.com

   Alain Durand
   Sun Microsystems, Inc.
   17 Network Circle
   Menlo Park, CA  94025

   EMail: alain.durand@sun.com

   James Paugh
   Sun Microsystems, Inc.
   17 Network Circle
   Menlo Park, CA  94025

   EMail: james.paugh@sun.com

Appendix A. Acknowledgments

   The authors gratefully acknowledge the contributions of Jim Bound,
   Tim Hartrick, Mika Liljeberg, Erik Nordmark, Pekka Savola, and Ronald
   van der Pol.

Appendix B. Changes from draft-ietf-v6ops-v6onbydefault-00

   o  Clarified in the abstract and introduction that the document is
      meant to raise awareness, and not to specify whether IPv6 should
      be enabled by default or not.

   o  Shortened section Section 2.2 and made reference to [ONLINK].

   o  Added clarification in section Section 2.3 about packets that are
      lost without ICMPv6 notification.

   o  Section Section 2.3 now has subsections for TCP, UDP, and SCTP.

   o  Removed text in Section suggesting that hosts usually were

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      only assigned one address when [HOSTREQS] was written.

   o  Added text in Section suggesting a method for applications
      to advise TCP of their preference for ICMPv6 handling.

   o  Added section Section

   o  Added section Section 2.3.2.

   o  Added section Section 2.3.3.

   o  Strengthened wording in section Section 3.1.1 to suggest that
      devices enforcing scope boundaries must send ICMPv6 Destination
      Unreachable messages.

   o  Clarified that the VPN problem described in Section 3.3 is due to
      a combination of the VPN software and either the on-link
      assumption and/or a "bad guy".

   o  Shortened section Section 4 and made reference to [APPTRANS].

   o  Miscellaneous editorial changes.

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   Funding for the RFC Editor function is currently provided by the
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