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
draft-ietf-v6ops-v6onbydefault-03.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
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
sections.
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
connectivity.
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
communication.
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
[I-D.ietf-v6ops-application-transition].
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
[I-D.ietf-v6ops-application-transition]
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Shin, M., "Application Aspects of IPv6 Transition",
draft-ietf-v6ops-application-transition-03 (work in
progress), June 2004.
[I-D.ietf-v6ops-onlinkassumption]
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
1998.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
6.2 Informative References
[I-D.ietf-tsvwg-sctpimpguide]
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
UBUR02-212
Burlington, MA 01801
EMail: sebastien.roy@sun.com
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Alain Durand
Sun Microsystems, Inc.
17 Network Circle
UMPK17-202
Menlo Park, CA 94025
EMail: alain.durand@sun.com
James Paugh
Sun Microsystems, Inc.
17 Network Circle
UMPK17-202
Menlo Park, CA 94025
EMail: james.paugh@sun.com
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 2.3.1.1 describing potential
drawbacks to TCP aborting connections ins SYN-SENT or SYN-RECEIVED
state.
o Added Section 2.3.1.3 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
[I-D.ietf-v6ops-onlinkassumption].
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 2.3.1.1 suggesting that hosts usually were
only assigned one address when [RFC1122] was written.
o Added text in Section 2.3.1.1 suggesting a method for applications
to advise TCP of their preference for ICMPv6 handling.
o Added Section 2.3.1.2.
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
[I-D.ietf-v6ops-application-transition].
o Miscellaneous editorial changes.
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