Network Working Group A. Matsumoto
Internet-Draft J. Kato
Intended status: Standards Track T. Fujisaki
Expires: April 3, 2012 NTT
T. Chown
University of Southampton
October 2011
Update to RFC 3484 Default Address Selection for IPv6
draft-ietf-6man-rfc3484-revise-05.txt
Abstract
RFC 3484 describes algorithms for source address selection and for
destination address selection. The algorithms specify default
behavior for all Internet Protocol version 6 (IPv6) implementations.
This document specifies a set of updates that modify the algorithms
and fix the known defects.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 3, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Specification . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Changes related to the default policy table . . . . . . . 3
2.1.1. ULA in the policy table . . . . . . . . . . . . . . . 4
2.1.2. Teredo in the policy table . . . . . . . . . . . . . . 4
2.1.3. 6to4, Teredo, and IPv4 prioritization . . . . . . . . 4
2.1.4. Deprecated addresses in the policy table . . . . . . . 5
2.1.5. Renewed default policy table . . . . . . . . . . . . . 5
2.2. The longest matching rule . . . . . . . . . . . . . . . . 5
2.3. Utilize next-hop for source address selection . . . . . . 6
2.4. Private IPv4 address scope . . . . . . . . . . . . . . . . 6
2.5. Deprecation of site-local unicast address . . . . . . . . 7
2.6. Anycast addresses for candidate source addresses . . . . . 7
3. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Normative References . . . . . . . . . . . . . . . . . . . 8
5.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9
Appendix B. Past Discussion . . . . . . . . . . . . . . . . . . . 9
B.1. The longest match rule . . . . . . . . . . . . . . . . . . 9
B.2. NAT64 prefix issue . . . . . . . . . . . . . . . . . . . . 10
B.3. ISATAP issue . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix C. Revision History . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
The IPv6 addressing architecture [RFC4291] allows multiple unicast
addresses to be assigned to interfaces. Because of this IPv6
implementations need to handle multiple possible source and
destination addresses when initiating communication. RFC 3484
[RFC3484] specifies the default algorithms, common across all
implementations, for selecting source and destination addresses so
that it is easier to predict the address selection behavior.
Since RFC 3484 was specified, some issues have been identified with
the algorithms specified there. The issues include the longest match
algorithm used in Rule 9 of destination address selection breaking
DNS round-robin techniques, and prioritization of poor IPv6
connectivity using transition mechanisms over native IPv4
connectivity.
There have also been some significant changes to the IPv6 addressing
architecture that require changes in the RFC 3484 policy table. Such
changes include the deprecation of site-local unicast addresses
[RFC3879] and of IPv4-compatible IPv6 addresses, and the introduction
of Unique Local Addresses [RFC4193].
This document specifies a set of updates that modify the algorithms
and fix the known defects.
2. Specification
2.1. Changes related to the default policy table
The default policy table is defined in RFC 3484 Section 2.1 as
follows:
Prefix Precedence Label
::1/128 50 0
::/0 40 1
2002::/16 30 2
::/96 20 3
::ffff:0:0/96 10 4
The changes that should be included into the default policy table are
those rules that are universally useful and do no harm in every
reasonable network environment. The changes we should consider for
the default policy table are listed in this sub-section.
The policy table is defined to be configurable. The changes that are
useful locally but not universally can be put into the policy table
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manually or by using the policy distribution mechanism proposed as a
DHCP option [I-D.ietf-6man-addr-select-opt].
2.1.1. ULA in the policy table
RFC 5220 [RFC5220] sections 2.1.4, 2.2.2, and 2.2.3 describe address
selection problems related to ULAs [RFC4193]. These problems can be
solved by either changing the scope of ULAs to site-local, or by
adding an entry for the default policy table that has its own label
for ULAs.
Centrally assigned ULAs [I-D.ietf-ipv6-ula-central] have been
proposed, and are assigned fc00::/8. Using the different labels for
fc00::/8 and fd00::/8 makes sense if we assume the same kind of
address block is assigned in the same or adjacent network. However,
we cannot expect that the type of ULA address block and network
adjacency commonly have any relationships.
Regarding the scope of ULAs, ULAs have been specified with a global
scope because the reachability of ULAs was intended to be restricted
by the routing system. Since the ULAs will not be exposed outside of
their reachability domain, if a ULA is available as a candidate
destination address, it can be expected to be reachable.
If we change the scope of ULAs to be smaller than global, we can
prioritize ULA to ULA communication over GUA to GUA communication.
At the same time, however, finer-grained configuration of ULA address
selection will be impossible. For example, even if you want to
prioritize communication related to the only /48 ULA prefix used in
your site, and do not want to prioritize communication to any other
ULA prefix, such a policy cannot be implemented in the policy table.
So, this draft proposes the use of the policy table to differentiate
ULAs from GUAs.
2.1.2. Teredo in the policy table
Teredo [RFC4380] is defined and has been assigned 2001::/32. This
address block should be assigned its own label in the policy table.
Teredo's priority should be less than or equal to 6to4, considering
its characteristic of being a transitional tunnel mechanism.
2.1.3. 6to4, Teredo, and IPv4 prioritization
Regarding the prioritization between IPv4 and these transitional
mechanisms, their connectivity is known to usually be worse than
IPv4. These mechanisms are said to be the last resort access method
to IPv6 resources. 6to4 should have higher precedence than Teredo,
given that 6to4 host to 6to4 host communication can be over IPv4
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(which can result in a more optimal path) and that 6to4 should not
used behind a NAT device.
2.1.4. Deprecated addresses in the policy table
IPv4-compatible IPv6 addresses (::/96) are deprecated [RFC4291].
IPv6 site-local unicast addresses (fec0::/10) are deprecated
[RFC3879]. 6bone testing addresses [RFC3701] has also been phased
out.
These addresses were removed from the current specification.
Considering the inappropriate use of these address blocks, especially
in outdated implementations and bad effects brought by them, they
should be labeled differently from the legitimate address blocks as
long as the address block is reserved by IANA.
2.1.5. Renewed default policy table
After applying these updates, the default policy table will be:
Prefix Precedence Label
::1/128 60 0
fc00::/7 50 1
::/0 40 2
::ffff:0:0/96 30 3
2002::/16 20 4
2001::/32 10 5
::/96 1 10
fec0::/10 1 11
3ffe::/16 1 12
2.2. The longest matching rule
This issue is related to the longest matching rule, which was found
by Dave Thaler. It causes a malfunction of the DNS round robin
technique, as described below. It is common for both IPv4 and IPv6.
When a destination address DA, DB, and the source address of DA
Source(DA) are on the same subnet and Source(DA) == Source(DB), DNS
round robin load-balancing cannot function. By considering prefix
lengths that are longer than the subnet prefix, this rule establishes
preference between addresses that have no substantive differences
between them. The rule functions as an arbitrary tie-breaker between
the hosts in a round robin, causing a given host to always prefer a
given member of the round robin.
By limiting the calculation of common prefixes to a maximum length
equal to the length of the subnet prefix of the source address, rule
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9 can continue to favor hosts that are nearby in the network
hierarchy without arbitrarily sorting addresses within a given
network. This modification could be written as follows:
Rule 9: Use longest matching prefix.
When DA and DB belong to the same address family (both are IPv6 or
both are IPv4): If CommonPrefixLen(DA & Netmask(Source(DA)),
Source(DA)) > CommonPrefixLen(DB & Netmask(Source(DB)), Source(DB)),
then prefer DA. Similarly, if CommonPrefixLen(DA &
Netmask(Source(DA)), Source(DA)) < CommonPrefixLen(DB &
Netmask(Source(DB)), Source(DB)), then prefer DB.
2.3. Utilize next-hop for source address selection
RFC 3484 source address selection rule 5 says that the address that
is attached to the outgoing interface should be preferred as the
source address. This rule is reasonable considering the prevalence
of ingress filtering described in BCP 38 [RFC2827]. This is because
an upstream network provider usually assumes it receives packets from
their customer that only have the delegated addresses as the source
addresses.
This rule, however, is not effective in an environment such as that
described in RFC 5220 Section 2.1.1, where a host has multiple
upstream routers on the same link and has addresses delegated from
each upstream router on a single interface.
Also, DHCPv6 assigned addresses are not associated like SLAAC
assigned addresses to a next-hop gateway, so implementations usually
can't apply this heuristic in a DHCPv6 network.
So, a new rule 5.1 that utilizes next-hop information for source
address selection is inserted just after rule 5.
Rule 5.1: Use an address assigned by the selected next-hop.
If SA is assigned by the selected next-hop that will be used to send
to D and SB is assigned by a different next-hop, then prefer SA.
Similarly, if SB is assigned by the next-hop that will be used to
send to D and SA is assigned by a different next-hop, then prefer SB.
2.4. Private IPv4 address scope
When a packet goes through a NAT, its source or destination address
can get replaced with another address with a different scope. It
follows that the result of the source address selection algorithm may
be different when the original address is replaced with the NATed
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address.
The algorithm currently specified in RFC 3484 is based on the
assumption that a source address with a small scope cannot reach a
destination address with a larger scope. This assumption does not
hold if private IPv4 addresses and a NAT are used to reach public
IPv4 addresses.
Due to this assumption, in the presence of both a NATed private IPv4
address and a transitional address (like 6to4 or Teredo), the host
will choose the transitional IPv6 address to access dual-stack peers
[I-D.denis-v6ops-nat-addrsel]. Choosing transitional IPv6
connectivity over native IPv4 connectivity, particularly where the
transitional connectivity is unmanaged, is not considered to be
generally desirable.
This issue can be fixed by changing the address scope of private IPv4
addresses to global.
2.5. Deprecation of site-local unicast address
RFC 3484 contains a few "site-local unicast" and "fec0::"
descriptions. It's better to remove examples related to site-local
unicast addresses, or change the examples to use ULAs. Possible
points to be re-written are listed below.
- 2nd paragraph in RFC 3484 Section 3.1 describes the scope
comparison mechanism.
- RFC 3484 Section 10 contains examples for site-local addresses.
2.6. Anycast addresses for candidate source addresses
RFC 3484 Section 4 states that anycast addresses, as well as
multicast addresses and the unspecified address, MUST NOT be included
in a candidate set of source address. Now that RFC 4291 Section 2.6
[RFC4291] removed the restrictions on using IPv6 anycast addresses as
the source address of an IPv6 packet, this restriction of RFC 3484
should also be removed.
3. Security Considerations
No security risk is found that degrades RFC 3484.
4. IANA Considerations
Address type number for the policy table may have to be assigned by
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IANA.
5. References
5.1. Normative References
[RFC1794] Brisco, T., "DNS Support for Load Balancing", RFC 1794,
April 1995.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC3701] Fink, R. and R. Hinden, "6bone (IPv6 Testing Address
Allocation) Phaseout", RFC 3701, March 2004.
[RFC3879] Huitema, C. and B. Carpenter, "Deprecating Site Local
Addresses", RFC 3879, September 2004.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
[RFC5220] Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
"Problem Statement for Default Address Selection in Multi-
Prefix Environments: Operational Issues of RFC 3484
Default Rules", RFC 5220, July 2008.
5.2. Informative References
[I-D.chown-addr-select-considerations]
Chown, T., "Considerations for IPv6 Address Selection
Policy Changes", draft-chown-addr-select-considerations-03
(work in progress), July 2009.
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[I-D.denis-v6ops-nat-addrsel]
Denis-Courmont, R., "Problems with IPv6 source address
selection and IPv4 NATs", draft-denis-v6ops-nat-addrsel-00
(work in progress), February 2009.
[I-D.ietf-6man-addr-select-opt]
Matsumoto, A., Fujisaki, T., Kato, J., and T. Chown,
"Distributing Address Selection Policy using DHCPv6",
draft-ietf-6man-addr-select-opt-01 (work in progress),
June 2011.
[I-D.ietf-ipv6-ula-central]
Hinden, R., "Centrally Assigned Unique Local IPv6 Unicast
Addresses", draft-ietf-ipv6-ula-central-02 (work in
progress), June 2007.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
October 2010.
Appendix A. Acknowledgements
Authors would like to thank to Dave Thaler, Pekka Savola, Remi Denis-
Courmont, Francois-Xavier Le Bail, and the members of 6man's address
selection design team for their invaluable contributions to this
document.
Appendix B. Past Discussion
This section summarizes discussions we had before related to address
selection mechanisms.
B.1. The longest match rule
RFC 3484 defines that destination address selection rule 9 should be
applied to both IPv4 and IPv6, which spoils the DNS-based load
balancing technique that is widely used in the IPv4 Internet today.
When two or more destination addresses are acquired from one FQDN,
rule 9 states that the longest matching destination and source
address pair should be chosen. As in RFC 1794, the DNS-based load
balancing technique is achieved by not re-ordering the destination
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addresses returned from the DNS server. Rule 9 defines a
deterministic rule for re-ordering hosts, hence the technique
described in RFC 1794 is not available anymore.
Regarding this problem, there was discussion in IETF and other places
like below.
Discussion: The possible changes to RFC 3484 are as follows:
1. To delete Rule 9 completely.
2. To apply Rule 9 only for IPv6 and not for IPv4. In IPv6,
hierarchical address assignment generally used at present, hence
the longest matching rule is beneficial in many cases. In IPv4,
as stated above, the DNS based load balancing technique is widely
used.
3. To apply Rule 9 for IPv6 conditionally and not for IPv4. When
the length of matching bits of the destination address and the
source address is longer than N, rule 9 is applied. Otherwise,
the order of the destination addresses do not change. The value
of N should be configurable and it should be 32 by default. This
is simply because the two sites whose matching bit length is
longer than 32 are probably adjacent.
Now that IPv6 PI addresses are being introduced by RIRs, hierarchical
address assignment is not always maintained anymore. It seems that
the longest matching algorithm may not worth the adverse effect of
disabling the DNS-based load balance technique.
B.2. NAT64 prefix issue
The NAT64 WKP has recently been defined[RFC6052]. It depends site by
site whether NAT64 should be preferred over IPv4, in other words
NAT44, or NAT44 over NAT64. So, the issue of local site policy
should be solved by manual policy table changes locally, or by use of
the proposed DHCP-based policy distribution mechanism.
B.3. ISATAP issue
Where a site is using ISATAP [RFC5214], there is generally no way to
differentiate an ISATAP address from a native address without
interface information. However, a site will assign a prefix for its
ISATAP overlay, and can choose to add an entry for that prefix to the
policy table if it wishes to change the default preference for that
prefix.
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Appendix C. Revision History
05:
6bone testing addresses were back in the default policy table.
Section 2.6 for allowing anycast source address were added.
04:
Added comment about ISATAP.
03:
ULA address selection issue was expanded.
6to4, Teredo and IPv4 prioritization issue was elaborated.
Deprecated address blocks in policy table section was elaborated.
In appendix, NAT64 prefix issue was added.
02:
Suresh Krishnan's suggestions for better english sentences were
incorporated.
A new source address selection rule that utilizes the next-hop
information is included in Section 2.3.
Site local address prefix was corrected.
01:
Re-structured to contain only the actual changes to RFC 3484.
00:
Published as a 6man working group item.
03:
Added acknowledgements.
Added longest matching algorithm malfunction regarding local DNS
round robin.
The proposed changes section was re-structured.
The issue of 6to4/Teredo and IPv4 prioritization was included.
The issue of deprecated addresses was added.
The renewed default policy table was changed accordingly.
02:
Added the reference to address selection design team's proposal.
01:
The issue of private IPv4 address scope was added.
The issue of ULA address scope was added.
Discussion of longest matching rule was expanded.
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Authors' Addresses
Arifumi Matsumoto
NTT SI Lab
Midori-Cho 3-9-11
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 3334
Email: arifumi@nttv6.net
Jun-ya Kato
NTT SI Lab
Midori-Cho 3-9-11
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 2939
Email: kato@syce.net
Tomohiro Fujisaki
NTT PF Lab
Midori-Cho 3-9-11
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 7351
Email: fujisaki@syce.net
Tim Chown
University of Southampt on
Southampton, Hampshire SO17 1BJ
United Kingdom
Email: tjc@ecs.soton.ac.uk
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