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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on April 3, 2012. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must Matsumoto, et al. Expires April 3, 2012 [Page 1] Internet-Draft RFC3484 Bis October 2011 include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. 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 Matsumoto, et al. Expires April 3, 2012 [Page 2] Internet-Draft RFC3484 Bis October 2011 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 Matsumoto, et al. Expires April 3, 2012 [Page 3] Internet-Draft RFC3484 Bis October 2011 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 Matsumoto, et al. Expires April 3, 2012 [Page 4] Internet-Draft RFC3484 Bis October 2011 (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 Matsumoto, et al. Expires April 3, 2012 [Page 5] Internet-Draft RFC3484 Bis October 2011 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 Matsumoto, et al. Expires April 3, 2012 [Page 6] Internet-Draft RFC3484 Bis October 2011 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 Matsumoto, et al. Expires April 3, 2012 [Page 7] Internet-Draft RFC3484 Bis October 2011 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. Matsumoto, et al. Expires April 3, 2012 [Page 8] Internet-Draft RFC3484 Bis October 2011 [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 Matsumoto, et al. Expires April 3, 2012 [Page 9] Internet-Draft RFC3484 Bis October 2011 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. Matsumoto, et al. Expires April 3, 2012 [Page 10] Internet-Draft RFC3484 Bis October 2011 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. Matsumoto, et al. Expires April 3, 2012 [Page 11] Internet-Draft RFC3484 Bis October 2011 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 Matsumoto, et al. Expires April 3, 2012 [Page 12]