Network Working Group S. Roy
Internet-Draft A. Durand
Expires: November 30, 2003 J. Paugh
Sun Microsystems, Inc.
June 2003
Dual Stack IPv6 on by Default
draft-ietf-v6ops-v6onbydefault-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). 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 security. Its purpose is to
raise awareness of these problems so that they can be fixed or worked
around.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. No IPv6 Router . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Problems with Default Address Selection for IPv6 . . . . . . 4
2.2 Neighbor Discovery's On-Link Assumption Considered
Harmful . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.1 Other Problems with the On-Link Assumption . . . . . . . . . 6
2.3 Transport Protocol Robustness . . . . . . . . . . . . . . . 7
3. Other Problematic Scenarios . . . . . . . . . . . . . . . . 9
3.1 IPv6 Network of Smaller Scope . . . . . . . . . . . . . . . 9
3.1.1 Alleviating the Scope Problem . . . . . . . . . . . . . . . 9
3.2 Poor IPv6 Network Performance . . . . . . . . . . . . . . . 9
3.2.1 Dealing with Poor IPv6 Network Performance . . . . . . . . . 10
3.3 Security . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3.1 Mitigating Security Risks . . . . . . . . . . . . . . . . . 11
4. Application Robustness . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . 13
Normative References . . . . . . . . . . . . . . . . . . . . 14
Informative References . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . 17
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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.
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.
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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
[PRIVADDR].
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
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.
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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
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 [ND] conceptual sending algorithm dictates 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.
For an IPv6 enabled host deployed on a network that has no IPv6
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routers as is the case in this scenario, the result is that
link-layer address resolution must be performed on all IPv6 addresses
to which the host sends packets. The Application will not receive
acknowledgment of the unreachability of destinations that are not
on-link until at least address resolution has failed, which is no
less than three seconds (MAX_MULTICAST_SOLICIT * RETRANS_TIMER), plus
any transport layer connection timeouts which could be minutes in the
case of TCP. The delay with respect to TCP is 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 (45 seconds), which could be compounded by any
transport timeouts associated with each connection attempt.
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 will immediately fail. The IPv6 implementation should be
able to immediately notify applications 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, then then
they need to be explicitly configured to have on-link routes for
those destinations.
2.2.1 Other Problems with the On-Link Assumption
The on-link assumption is problematic in ways not directly related to
the scenario described, but they should still be briefly mentioned
here.
One problem is that default routes are not special. The on-link
assumption as stated in [ND] would have a host assume that all
destinations are on-link when its default router list is empty.
Clearly it shouldn't make this assumption if it has an off-link route
that covers the destination and that route isn't a default route.
The absence of a default route does not mean that destinations are
not reachable through more specific routes.
Another problem is that the on-link assumption behavior is undefined
on multi-homed hosts. When such a host has no default routers and it
is trying to reach a destination to which it has no route, should it
try NUD on all interfaces at once? Should it simply realize that the
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destination is unreachable? The latter is the simplest solution.
Determining that a destination is unreachable when there is no route
to that destination is the simple solution for all cases, not just
the multi-homed case.
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. In
order for our application to quickly fail from an unreachable address
to a potentially reachable one, the transport layer should notify the
application by failing a connection attempt, or passing ICMPv6 errors
up to the application, etc...
In the case of a socket application attempting a connection via TCP,
it would be unreasonable for the connect() function to block even
after the system has received notification that the address is
unreachable via an ICMPv6 Destination Unreachable message. Therefore,
TCP should abort connections in SYN-SENT or SYN-RECEIVED state when
it receives an ICMPv6 Destination Unreachable message. This helps 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.
It should be noted that the Requirements for Internet Hosts
[HOSTREQS] document, in section 4.2.3.9., 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
destination) and 3 (address unreachable), and hard errors to the
other codes.
When [HOSTREQS] was written, hosts were usually assigned a single
address, and 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
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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. It should make this distinction when a
connection is in any other state.
Many TCP implementations already behave this way, but others do not.
This should be noted as a best current practice in this case.
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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.
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 should 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
By default in dual stack nodes, applications will try IPv6
destinations first. 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.
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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 routes. 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
adequate.
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 from 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
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 drop 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
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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.
3.3.1 Mitigating Security Risks
Establishing a security policy that is the same for IPv4 and IPv6
would help mitigate this risk.
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.
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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.
One example is an application that resolves a destination name to a
list of addresses with the intent to contact multiple services at
that destination (i.e., http and IMAP). The application tries to
contact the first service via the first IPv6 address in the list and
succeeds. The success of the connection results in the application
throwing away the list of addresses it was given by the name lookup
and remembering this single IPv6 address as the usable address for
the destination. The second service it tries, however, is only
implemented for IPv4. The application tries to communicate to the
second service using the same IPv6 address, the connection fails, and
no fall back to IPv4 occurs because the application is outside of the
context of cycling through the list of addresses returned from the
name lookup.
Applications should not assume that because a service is reachable at
an IPv6 address, that other services at that destination are also
reachable via that IPv6 address.
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5. Security Considerations
This document raises security concerns in Section 3.3.
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Normative References
[ADDRSEL] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[ND] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
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Informative References
[AUTOCONF]
Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[HOSTREQS]
Braden, R., "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC 1122, October 1989.
[PRIVADDR]
Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
Authors' Addresses
Sebastien Roy
Sun Microsystems, Inc.
1 Network Drive
UBUR02-212
Burlington, MA 01801
EMail: sebastien.roy@sun.com
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
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Appendix A. Acknowledgments
The authors gratefully acknowledge the contributions of Jim Bound,
Mika Liljeberg, Erik Nordmark, Pekka Savola, and Ronald van der Pol.
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