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diff --git a/doc/rfc/rfc3364.txt b/doc/rfc/rfc3364.txt new file mode 100644 index 00000000..189c0d2a --- /dev/null +++ b/doc/rfc/rfc3364.txt @@ -0,0 +1,619 @@ + + + + + + +Network Working Group R. Austein +Request for Comments: 3364 Bourgeois Dilettant +Updates: 2673, 2874 August 2002 +Category: Informational + + + Tradeoffs in Domain Name System (DNS) Support + for 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. + +Introduction + + RFC 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 2002 + + +Main 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 2002 + + +Potential 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 less + + + +Austein 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 in RFC 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 2002 + + +Transition 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 protocol + + + +Austein 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 2002 + + +Acknowledgments + + 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.net + + + + + + + + + + + + + + + + +Austein Informational [Page 10] + +RFC 3364 Tradeoffs in DNS Support for IPv6 August 2002 + + +Full 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] + |