Network Working Group                                             S. Roy
Internet-Draft                                                  J. Paugh
Expires: February 2005                                         A. Durand
                                                  Sun Microsystems, Inc.
                                                               July 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
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   This Internet-Draft will expire on October 30, 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 insecurity.  The purpose of
   this memo is to raise awareness of these problems so that they can be
   fixed or worked around, not to try to specify whether IPv6 should be
   enabled by default or not.

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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
   3.  Other Problematic Scenarios  . . . . . . . . . . . . . . . . .  6
     3.1   IPv6 Network of Smaller Scope  . . . . . . . . . . . . . .  6
       3.1.1   Alleviating the Scope Problem  . . . . . . . . . . . .  7
     3.2   Poor IPv6 Network Performance  . . . . . . . . . . . . . .  7
     3.3   Security . . . . . . . . . . . . . . . . . . . . . . . . .  8
       3.3.1   Mitigating Security Risks  . . . . . . . . . . . . . .  8
   4.  Application Robustness . . . . . . . . . . . . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.1   Normative References . . . . . . . . . . . . . . . . . . . .  9
   6.2   Informative References . . . . . . . . . . . . . . . . . . . 10
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
   A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   B.  Changes from draft-ietf-v6ops-v6onbydefault-02 . . . . . . . . 11
   C.  Changes from draft-ietf-v6ops-v6onbydefault-01 . . . . . . . . 11
   D.  Changes from draft-ietf-v6ops-v6onbydefault-00 . . . . . . . . 11
       Intellectual Property and Copyright Statements . . . . . . . . 13

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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 systems are
   installed and placed in IPv4-only or mixed IPv4 and IPv6
   environments, 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.

   This memo begins in Section 2 by examining problems within IPv6
   implementations that defeat the destination address selection
   mechanism defined in [RFC3484] 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
   is placed on a link with no IPv6 routers.  The system is using IPv6
   Stateless Address Autoconfiguration [RFC2462], 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 [RFC1918].

   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 [RFC3484].  The application will
   attempt to connect to each address, in the order they were returned,
   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
   what things can go wrong with this scenario.

2.1  Problems with Default Address Selection for IPv6

   The Default Address Selection for IPv6 [RFC3484] 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
   off-link destinations even if it did have an off-link route.

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   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", would prefer the IPv4
   destination over the IPv6 destination, but only if the IPv6
   destination were 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 [RFC3484] considers private addresses (as defined in
   [RFC1918]) 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
   selected 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 and/or literal
   addresses passed in from the user.

   For example, one such application is the DNS resolver.  In this case,

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   a configuration file usually contains a list of literal addresses to
   be used as DNS name servers.  The resolver client tries these servers
   in the order that they appear in the file, bypassing address
   selection rules.

   Such applications will obviously be subject to whatever connection
   delays 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 [RFC2461] 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
   [I-D.ietf-v6ops-onlinkassumption].  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 costly
   and incorrect.  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 (MAX_MULTICAST_SOLICIT *
   RETRANS_TIMER * 15).  This could be compounded by any transport
   timeouts associated with each connection attempt, bringing the
   timeouts to even dozens of minutes.

   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

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   be able to immediately notify applications or the transport layer
   that it has no route to such IPv6 destinations, so that 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.

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 or API to deal with this.
   It could fail the connection attempt, pass ICMPv6 errors up to the
   application, or pass them up to an API that is handling this for the
   application, etc.

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.

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   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
   described in [RFC3484].  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.

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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 a 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 may not be enforcing the same policy for IPv4 as for IPv6
   traffic, which could be due to misconfiguration of the firewall.  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
   traffic), IPv6 packets could go through the network untouched if
   tunneled over a transport layer.  This could open the host to direct
   IPv6 attacks.  It should be noted that IPv4 packets can also be
   tunneled, so this is not a new security concern for IPv6.  Firewalls
   must be deliberately and properly configured.

   A similar problem could exist for virtual private network (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.

   This is problematic for a number of reasons.  The first is that if
   the node has a default IPv6 route, the packets will be forwarded
   off-link to an unknown destination.  Another is if no legitimate
   router is on-link and the node makes the on-link assumption discussed
   in Section 2.2, the packets will simply be sent onto the local link
   to be potentially viewed by a node spoofing the destination.  A third
   is if a rogue IPv6 router exists on-link.  In that case the malicious
   node will simply be sent all IPv6 packets in the clear.

3.3.1  Mitigating Security Risks

   The security policy implemented in firewalls, VPN software, or other

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   devices, must take a stance whether it applies equally to both IPv4
   and IPv6 traffic.  It is probably desirable for the 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 the security
   policy.  Some more complex mechanisms 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
   application transition guidelines, is discussed in

5.  Security Considerations

   This document raises security concerns in Section 3.3.  They are
   summarized below:

   o  Firewalls need to be configured properly to have deliberate
      security policies for IPv6 packets, including IPv6 packets
      encapsulated in other layers.

   o  Implementations of virtual private networks need to have a
      deliberate IPv6 security policy that doesn't allow packets to
      accidentally appear in the clear when they were intended to be
      sent securely over the VPN.

6.  References

6.1  Normative References


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              Shin, M., "Application Aspects of IPv6 Transition",
              draft-ietf-v6ops-application-transition-03 (work in
              progress), June 2004.

              Roy, S., Durand, A. and J. Paugh, "IPv6 Neighbor Discovery
              On-Link Assumption Considered Harmful",
              draft-ietf-v6ops-onlinkassumption-02 (work in progress),
              May 2004.

   [RFC2461]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461, December

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

6.2  Informative References

              Stewart, R., "Stream Control Transmission Protocol (SCTP)
              Implementors Guide", draft-ietf-tsvwg-sctpimpguide-10
              (work in progress), December 2003.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

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

   [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, December 1998.

Authors' Addresses

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


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


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


Appendix A.  Acknowledgments

   The authors gratefully acknowledge the contributions of Jim Bound,
   Fernando Gont, Tony Hain, Tim Hartrick, Mika Liljeberg, Erik
   Nordmark, Kacheong Poon, Pekka Savola, Randall Stewart, and Ronald
   van der Pol.

Appendix B.  Changes from draft-ietf-v6ops-v6onbydefault-02

   o  Removed all text suggesting solutions to the problems described
      by this draft.

   o  Removed the all sub-sections of Section 2.3 that offered solutions
      to the problems being presented.

   o  Removed Section 3.2.1, which described a solution to dealing with
      poor IPv6 network performance.

Appendix C.  Changes from draft-ietf-v6ops-v6onbydefault-01

   o  Added specificity to the DNS resolver problem in Section 2.1.

   o  Added a few paragraphs in Section describing potential
      drawbacks to TCP aborting connections ins SYN-SENT or SYN-RECEIVED

   o  Added Section describing how a higher level API could be
      used to manage connections.

   o  Expanded Section 2.3.3 to describe desired SCTP behavior when
      encountering soft errors.

   o  Added a summary of security concerns to Section 5.

   o  Miscellaneous editorial changes.

Appendix D.  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.

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   o  Shortened Section 2.2 and made reference to

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

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

   o  Removed text in Section suggesting that hosts usually were
      only assigned one address when [RFC1122] was written.

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

   o  Added Section

   o  Added Section 2.3.2.

   o  Added Section 2.3.3.

   o  Strengthened wording in 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 4 and made reference to

   o  Miscellaneous editorial changes.

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Intellectual Property Statement

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   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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