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Network Working Group                                         R. AusteinRequest for Comments: 3364                           Bourgeois DilettantUpdates:2673,2874                                          August 2002Category: InformationalTradeoffs in Domain Name System (DNS) Supportfor Internet Protocol version 6 (IPv6)Status of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2002).  All Rights Reserved.Abstract   The IETF has two different proposals on the table for how to do DNS   support for IPv6, and has thus far failed to reach a clear consensus   on which approach is better.  This note attempts to examine the pros   and cons of each approach, in the hope of clarifying the debate so   that we can reach closure and move on.IntroductionRFC 1886 [RFC1886] specified straightforward mechanisms to support   IPv6 addresses in the DNS.  These mechanisms closely resemble the   mechanisms used to support IPv4, with a minor improvement to the   reverse mapping mechanism based on experience with CIDR.RFC 1886 is   currently listed as a Proposed Standard.RFC 2874 [RFC2874] specified enhanced mechanisms to support IPv6   addresses in the DNS.  These mechanisms provide new features that   make it possible for an IPv6 address stored in the DNS to be broken   up into multiple DNS resource records in ways that can reflect the   network topology underlying the address, thus making it possible for   the data stored in the DNS to reflect certain kinds of network   topology changes or routing architectures that are either impossible   or more difficult to represent without these mechanisms.RFC 2874 is   also currently listed as a Proposed Standard.Austein                      Informational                      [Page 1]

RFC 3364           Tradeoffs in DNS Support for IPv6         August 2002   Both of these Proposed Standards were the output of the IPNG Working   Group.  Both have been implemented, although implementation of   [RFC1886] is more widespread, both because it was specified earlier   and because it's simpler to implement.   There's little question that the mechanisms proposed in [RFC2874] are   more general than the mechanisms proposed in [RFC1886], and that   these enhanced mechanisms might be valuable if IPv6's evolution goes   in certain directions.  The questions are whether we really need the   more general mechanism, what new usage problems might come along with   the enhanced mechanisms, and what effect all this will have on IPv6   deployment.   The one thing on which there does seem to be widespread agreement is   that we should make up our minds about all this Real Soon Now.Main Advantages of Going with A6   While the A6 RR proposed in [RFC2874] is very general and provides a   superset of the functionality provided by the AAAA RR in [RFC1886],   many of the features of A6 can also be implemented with AAAA RRs via   preprocessing during zone file generation.   There is one specific area where A6 RRs provide something that cannot   be provided using AAAA RRs: A6 RRs can represent addresses in which a   prefix portion of the address can change without any action (or   perhaps even knowledge) by the parties controlling the DNS zone   containing the terminal portion (least significant bits) of the   address.  This includes both so-called "rapid renumbering" scenarios   (where an entire network's prefix may change very quickly) and   routing architectures such as the former "GSE" proposal [GSE] (where   the "routing goop" portion of an address may be subject to change   without warning).  A6 RRs do not completely remove the need to update   leaf zones during all renumbering events (for example, changing ISPs   would usually require a change to the upward delegation pointer), but   careful use of A6 RRs could keep the number of RRs that need to   change during such an event to a minimum.   Note that constructing AAAA RRs via preprocessing during zone file   generation requires exactly the sort of information that A6 RRs store   in the DNS.  This begs the question of where the hypothetical   preprocessor obtains that information if it's not getting it from the   DNS.   Note also that the A6 RR, when restricted to its zero-length-prefix   form ("A6 0"), is semantically equivalent to an AAAA RR (with one   "wasted" octet in the wire representation), so anything that can be   done with an AAAA RR can also be done with an A6 RR.Austein                      Informational                      [Page 2]

RFC 3364           Tradeoffs in DNS Support for IPv6         August 2002Main Advantages of Going with AAAA   The AAAA RR proposed in [RFC1886], while providing only a subset of   the functionality provided by the A6 RR proposed in [RFC2874], has   two main points to recommend it:   - AAAA RRs are essentially identical (other than their length) to     IPv4's A RRs, so we have more than 15 years of experience to help     us predict the usage patterns, failure scenarios and so forth     associated with AAAA RRs.   - The AAAA RR is "optimized for read", in the sense that, by storing     a complete address rather than making the resolver fetch the     address in pieces, it minimizes the effort involved in fetching     addresses from the DNS (at the expense of increasing the effort     involved in injecting new data into the DNS).Less Compelling Arguments in Favor of A6   Since the A6 RR allows a zone administrator to write zone files whose   description of addresses maps to the underlying network topology, A6   RRs can be construed as a "better" way of representing addresses than   AAAA.  This may well be a useful capability, but in and of itself   it's more of an argument for better tools for zone administrators to   use when constructing zone files than a justification for changing   the resolution protocol used on the wire.Less Compelling Arguments in Favor of AAAA   Some of the pressure to go with AAAA instead of A6 appears to be   based on the wider deployment of AAAA.  Since it is possible to   construct transition tools (see discussion of AAAA synthesis, later   in this note), this does not appear to be a compelling argument if A6   provides features that we really need.   Another argument in favor of AAAA RRs over A6 RRs appears to be that   the A6 RR's advanced capabilities increase the number of ways in   which a zone administrator could build a non-working configuration.   While operational issues are certainly important, this is more of   argument that we need better tools for zone administrators than it is   a justification for turning away from A6 if A6 provides features that   we really need.Austein                      Informational                      [Page 3]

RFC 3364           Tradeoffs in DNS Support for IPv6         August 2002Potential Problems with A6   The enhanced capabilities of the A6 RR, while interesting, are not in   themselves justification for choosing A6 if we don't really need   those capabilities.  The A6 RR is "optimized for write", in the sense   that, by making it possible to store fragmented IPv6 addresses in the   DNS, it makes it possible to reduce the effort that it takes to   inject new data into the DNS (at the expense of increasing the effort   involved in fetching data from the DNS).  This may be justified if we   expect the effort involved in maintaining AAAA-style DNS entries to   be prohibitive, but in general, we expect the DNS data to be read   more frequently than it is written, so we need to evaluate this   particular tradeoff very carefully.   There are also several potential issues with A6 RRs that stem   directly from the feature that makes them different from AAAA RRs:   the ability to build up address via chaining.   Resolving a chain of A6 RRs involves resolving a series of what are   almost independent queries, but not quite.  Each of these sub-queries   takes some non-zero amount of time, unless the answer happens to be   in the resolver's local cache already.  Assuming that resolving an   AAAA RR takes time T as a baseline, we can guess that, on the   average, it will take something approaching time N*T to resolve an   N-link chain of A6 RRs, although we would expect to see a fairly good   caching factor for the A6 fragments representing the more significant   bits of an address.  This leaves us with two choices, neither of   which is very good:  we can decrease the amount of time that the   resolver is willing to wait for each fragment, or we can increase the   amount of time that a resolver is willing to wait before returning   failure to a client.  What little data we have on this subject   suggests that users are already impatient with the length of time it   takes to resolve A RRs in the IPv4 Internet, which suggests that they   are not likely to be patient with significantly longer delays in the   IPv6 Internet.  At the same time, terminating queries prematurely is   both a waste of resources and another source of user frustration.   Thus, we are forced to conclude that indiscriminate use of long A6   chains is likely to lead to problems.   To make matters worse, the places where A6 RRs are likely to be most   critical for rapid renumbering or GSE-like routing are situations   where the prefix name field in the A6 RR points to a target that is   not only outside the DNS zone containing the A6 RR, but is   administered by a different organization (for example, in the case of   an end user's site, the prefix name will most likely point to a name   belonging to an ISP that provides connectivity for the site).  While   pointers out of zone are not a problem per se, pointers to other   organizations are somewhat more difficult to maintain and lessAustein                      Informational                      [Page 4]

RFC 3364           Tradeoffs in DNS Support for IPv6         August 2002   susceptible to automation than pointers within a single organization   would be.  Experience both with glue RRs and with PTR RRs in the IN-   ADDR.ARPA tree suggests that many zone administrators do not really   understand how to set up and maintain these pointers properly, and we   have no particular reason to believe that these zone administrators   will do a better job with A6 chains than they do today.  To be fair,   however, the alternative case of building AAAA RRs via preprocessing   before loading zones has many of the same problems; at best, one can   claim that using AAAA RRs for this purpose would allow DNS clients to   get the wrong answer somewhat more efficiently than with A6 RRs.   Finally, assuming near total ignorance of how likely a query is to   fail, the probability of failure with an N-link A6 chain would appear   to be roughly proportional to N, since each of the queries involved   in resolving an A6 chain would have the same probability of failure   as a single AAAA query.  Note again that this comment applies to   failures in the the process of resolving a query, not to the data   obtained via that process.  Arguably, in an ideal world, A6 RRs would   increase the probability of the answer a client (finally) gets being   right, assuming that nothing goes wrong in the query process, but we   have no real idea how to quantify that assumption at this point even   to the hand-wavey extent used elsewhere in this note.   One potential problem that has been raised in the past regarding A6   RRs turns out not to be a serious issue.  The A6 design includes the   possibility of there being more than one A6 RR matching the prefix   name portion of a leaf A6 RR.  That is, an A6 chain may not be a   simple linked list, it may in fact be a tree, where each branch   represents a possible prefix.  Some critics of A6 have been concerned   that this will lead to a wild expansion of queries, but this turns   out not to be a problem if a resolver simply follows the "bounded   work per query" rule described inRFC 1034 (page 35).  That rule   applies to all work resulting from attempts to process a query,   regardless of whether it's a simple query, a CNAME chain, an A6 tree,   or an infinite loop.  The client may not get back a useful answer in   cases where the zone has been configured badly, but a proper   implementation should not produce a query explosion as a result of   processing even the most perverse A6 tree, chain, or loop.Interactions with DNSSEC   One of the areas where AAAA and A6 RRs differ is in the precise   details of how they interact with DNSSEC.  The following comments   apply only to non-zero-prefix A6 RRs (A6 0 RRs, once again, are   semantically equivalent to AAAA RRs).Austein                      Informational                      [Page 5]

RFC 3364           Tradeoffs in DNS Support for IPv6         August 2002   Other things being equal, the time it takes to re-sign all of the   addresses in a zone after a renumbering event is longer with AAAA RRs   than with A6 RRs (because each address record has to be re-signed   rather than just signing a common prefix A6 RR and a few A6 0 RRs   associated with the zone's name servers).  Note, however, that in   general this does not present a serious scaling problem, because the   re-signing is performed in the leaf zones.   Other things being equal, there's more work involved in verifying the   signatures received back for A6 RRs, because each address fragment   has a separate associated signature.  Similarly, a DNS message   containing a set of A6 address fragments and their associated   signatures will be larger than the equivalent packet with a single   AAAA (or A6 0) and a single associated signature.   Since AAAA RRs cannot really represent rapid renumbering or GSE-style   routing scenarios very well, it should not be surprising that DNSSEC   signatures of AAAA RRs are also somewhat problematic.  In cases where   the AAAA RRs would have to be changing very quickly to keep up with   prefix changes, the time required to re-sign the AAAA RRs may be   prohibitive.   Empirical testing by Bill Sommerfeld [Sommerfeld] suggests that   333MHz Celeron laptop with 128KB L2 cache and 64MB RAM running the   BIND-9 dnssec-signzone program under NetBSD can generate roughly 40   1024-bit RSA signatures per second.  Extrapolating from this,   assuming one A RR, one AAAA RR, and one NXT RR per host, this   suggests that it would take this laptop a few hours to sign a zone   listing 10**5 hosts, or about a day to sign a zone listing 10**6   hosts using AAAA RRs.   This suggests that the additional effort of re-signing a large zone   full of AAAA RRs during a re-numbering event, while noticeable, is   only likely to be prohibitive in the rapid renumbering case where   AAAA RRs don't work well anyway.Interactions with Dynamic Update   DNS dynamic update appears to work equally well for AAAA or A6 RRs,   with one minor exception: with A6 RRs, the dynamic update client   needs to know the prefix length and prefix name.  At present, no   mechanism exists to inform a dynamic update client of these values,   but presumably such a mechanism could be provided via an extension to   DHCP, or some other equivalent could be devised.Austein                      Informational                      [Page 6]

RFC 3364           Tradeoffs in DNS Support for IPv6         August 2002Transition from AAAA to A6 Via AAAA Synthesis   While AAAA is at present more widely deployed than A6, it is possible   to transition from AAAA-aware DNS software to A6-aware DNS software.   A rough plan for this was presented at IETF-50 in Minneapolis and has   been discussed on the ipng mailing list.  So if the IETF concludes   that A6's enhanced capabilities are necessary, it should be possible   to transition from AAAA to A6.   The details of this transition have been left to a separate document,   but the general idea is that the resolver that is performing   iterative resolution on behalf of a DNS client program could   synthesize AAAA RRs representing the result of performing the   equivalent A6 queries.  Note that in this case it is not possible to   generate an equivalent DNSSEC signature for the AAAA RR, so clients   that care about performing DNSSEC validation for themselves would   have to issue A6 queries directly rather than relying on AAAA   synthesis.Bitlabels   While the differences between AAAA and A6 RRs have generated most of   the discussion to date, there are also two proposed mechanisms for   building the reverse mapping tree (the IPv6 equivalent of IPv4's IN-   ADDR.ARPA tree).   [RFC1886] proposes a mechanism very similar to the IN-ADDR.ARPA   mechanism used for IPv4 addresses: the RR name is the hexadecimal   representation of the IPv6 address, reversed and concatenated with a   well-known suffix, broken up with a dot between each hexadecimal   digit.  The resulting DNS names are somewhat tedious for humans to   type, but are very easy for programs to generate.  Making each   hexadecimal digit a separate label means that delegation on arbitrary   bit boundaries will result in a maximum of 16 NS RRsets per label   level; again, the mechanism is somewhat tedious for humans, but is   very easy to program.  As with IPv4's IN-ADDR.ARPA tree, the one   place where this scheme is weak is in handling delegations in the   least significant label; however, since there appears to be no real   need to delegate the least significant four bits of an IPv6 address,   this does not appear to be a serious restriction.   [RFC2874] proposed a radically different way of naming entries in the   reverse mapping tree: rather than using textual representations of   addresses, it proposes to use a new kind of DNS label (a "bit label")   to represent binary addresses directly in the DNS.  This has the   advantage of being significantly more compact than the textual   representation, and arguably might have been a better solution for   DNS to use for this purpose if it had been designed into the protocolAustein                      Informational                      [Page 7]

RFC 3364           Tradeoffs in DNS Support for IPv6         August 2002   from the outset.  Unfortunately, experience to date suggests that   deploying a new DNS label type is very hard: all of the DNS name   servers that are authoritative for any portion of the name in   question must be upgraded before the new label type can be used, as   must any resolvers involved in the resolution process.  Any name   server that has not been upgraded to understand the new label type   will reject the query as being malformed.   Since the main benefit of the bit label approach appears to be an   ability that we don't really need (delegation in the least   significant four bits of an IPv6 address), and since the upgrade   problem is likely to render bit labels unusable until a significant   portion of the DNS code base has been upgraded, it is difficult to   escape the conclusion that the textual solution is good enough.DNAME RRs   [RFC2874] also proposes using DNAME RRs as a way of providing the   equivalent of A6's fragmented addresses in the reverse mapping tree.   That is, by using DNAME RRs, one can write zone files for the reverse   mapping tree that have the same ability to cope with rapid   renumbering or GSE-style routing that the A6 RR offers in the main   portion of the DNS tree.  Consequently, the need to use DNAME in the   reverse mapping tree appears to be closely tied to the need to use   fragmented A6 in the main tree: if one is necessary, so is the other,   and if one isn't necessary, the other isn't either.   Other uses have also been proposed for the DNAME RR, but since they   are outside the scope of the IPv6 address discussion, they will not   be addressed here.Recommendation   Distilling the above feature comparisons down to their key elements,   the important questions appear to be:   (a) Is IPv6 going to do rapid renumbering or GSE-like routing?   (b) Is the reverse mapping tree for IPv6 going to require delegation       in the least significant four bits of the address?   Question (a) appears to be the key to the debate.  This is really a   decision for the IPv6 community to make, not the DNS community.   Question (b) is also for the IPv6 community to make, but it seems   fairly obvious that the answer is "no".Austein                      Informational                      [Page 8]

RFC 3364           Tradeoffs in DNS Support for IPv6         August 2002   Recommendations based on these questions:   (1) If the IPv6 working groups seriously intend to specify and deploy       rapid renumbering or GSE-like routing, we should transition to       using the A6 RR in the main tree and to using DNAME RRs as       necessary in the reverse tree.   (2) Otherwise, we should keep the simpler AAAA solution in the main       tree and should not use DNAME RRs in the reverse tree.   (3) In either case, the reverse tree should use the textual       representation described in [RFC1886] rather than the bit label       representation described in [RFC2874].   (4) If we do go to using A6 RRs in the main tree and to using DNAME       RRs in the reverse tree, we should write applicability statements       and implementation guidelines designed to discourage excessively       complex uses of these features; in general, any network that can       be described adequately using A6 0 RRs and without using DNAME       RRs should be described that way, and the enhanced features       should be used only when absolutely necessary, at least until we       have much more experience with them and have a better       understanding of their failure modes.Security Considerations   This note compares two mechanisms with similar security   characteristics, but there are a few security implications to the   choice between these two mechanisms:   (1) The two mechanisms have similar but not identical interactions       with DNSSEC.  Please see the section entitled "Interactions with       DNSSEC" (above) for a discussion of these issues.   (2) To the extent that operational complexity is the enemy of       security, the tradeoffs in operational complexity discussed       throughout this note have an impact on security.   (3) To the extent that protocol complexity is the enemy of security,       the additional protocol complexity of [RFC2874] as compared to       [RFC1886] has some impact on security.IANA Considerations   None, since all of these RR types have already been allocated.Austein                      Informational                      [Page 9]

RFC 3364           Tradeoffs in DNS Support for IPv6         August 2002Acknowledgments   This note is based on a number of discussions both public and private   over a period of (at least) eight years, but particular thanks go to   Alain Durand, Bill Sommerfeld, Christian Huitema, Jun-ichiro itojun   Hagino, Mark Andrews, Matt Crawford, Olafur Gudmundsson, Randy Bush,   and Sue Thomson, none of whom are responsible for what the author did   with their ideas.References   [RFC1886]    Thomson, S. and C. Huitema, "DNS Extensions to support                IP version 6",RFC 1886, December 1995.   [RFC2874]    Crawford, M. and C. Huitema, "DNS Extensions to Support                IPv6 Address Aggregation and Renumbering",RFC 2874,                July 2000.   [Sommerfeld] Private message to the author from Bill Sommerfeld dated                21 March 2001, summarizing the result of experiments he                performed on a copy of the MIT.EDU zone.   [GSE]       "GSE" was an evolution of the so-called "8+8" proposal                discussed by the IPng working group in 1996 and 1997.                The GSE proposal itself was written up as an Internet-                Draft, which has long since expired.  Readers interested                in the details and history of GSE should review the IPng                working group's mailing list archives and minutes from                that period.Author's Address   Rob Austein   EMail: sra@hactrn.netAustein                      Informational                     [Page 10]

RFC 3364           Tradeoffs in DNS Support for IPv6         August 2002Full Copyright Statement   Copyright (C) The Internet Society (2002).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Austein                      Informational                     [Page 11]