Design Choices When Expanding DNS
draft-iab-dns-choices-05

Status of this Memo

By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

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.”

The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.

This Internet-Draft will expire on August 21, 2008.

Abstract

This note discusses how to extend the DNS with new data for a new application. DNS extension discussions too often focus on reuse of the TXT Resource Record Type. This document lists different mechanisms to extend the DNS, and concludes that the use of a new DNS Resource Record Type is the best solution.

Table of Contents

1.  Introduction
2.  Background
3.  Extension mechanisms
    3.1.  Place selectors inside the RDATA of existing Resource Record Types
    3.2.  Add a prefix to the owner name
    3.3.  Add a suffix to the owner name
    3.4.  Add a new Class
    3.5.  Add a new Resource Record Type
4.  Zone boundaries are invisible to applications
5.  Why adding a new Resource Record Type is the preferred solution
6.  Conclusion and Recommendation
7.  Creating A New Resource Record Type
8.  IANA Considerations
9.  Security Considerations
10.  Acknowledgements
11.  References
    11.1.  Normative References
    11.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

The DNS stores multiple categories of data. The two most commonly used categories are infrastructure data for the DNS system itself (NS and SOA Resource Records) and data which have to do with mappings between domain names and IP addresses (A, AAAA and PTR Resource Records). There are other categories as well, some of which are tied to specific applications like email (MX Resource Records), while others are generic Resource Record Types used to convey information for multiple protocols (SRV and NAPTR Resource Records).

When storing data in the DNS for a new application, the data are usually tied to a "normal" domain name, so that the application can query for the data it wants, while minimizing the impact on existing applications.

Historically, extending DNS to store application data tied to a domain name has been done in different ways at different times. MX Resource Records were created as a new Resource Record Type specifically designed to support electronic mail. SRV records are a generic type which use a prefixing scheme in combination with a base domain name. Records associated with ENUM use a suffixing scheme. NAPTR records add selection data inside the RDATA. It is clear that the methods used to add new data types to the DNS have been inconsistent, and the purpose of this document is to attempt to clarify the implications of each of these methods, both for the applications that use them and for the rest of the DNS.

This document talks extensively about use of DNS wildcards. Many people might think use of wildcards is not something that happens today. In reality though, wildcards are in use, especially for certain application-specific data such as MX Resource Records. Because of this, the choice has to be made with existence of wildcards in mind.

Another overall issue that must be taken into account is what the new data in the DNS are to describe. In some cases they might be completely new data. In other cases they might be metadata tied to data that already exist in the DNS. An example of new data is key information for SSH and data used for fighting spam (metadata tied to MX Resource Records). If the new data are tied to data that already exist in the DNS, an analysis should be made as to whether having (for example) address records and SSH key information in different DNS zones is a problem, and if it is, whether the specification must require all of the related data to be in the same zone.

This document does not talk about what one should store in the DNS. It also doesn't discuss whether DNS should be used for service discovery, or whether DNS should be used for storage of data specific for the service. In general, DNS is a protocol that, apart from holding metadata that makes the DNS itself function (NS, SOA, DNSSEC Resource Record Types, etc), only holds references to service locations (SRV, NAPTR, A, AAAA Resource Record Types), but there are exceptions (such as MX Resource Records).

2.  Background

See [RFC2929] (Eastlake 3rd, D., Brunner-Williams, E., and B. Manning, “Domain Name System (DNS) IANA Considerations,” September 2000.) for a brief summary of DNS query structure. Readers interested in the full story should start with the base DNS specification in [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.), and continue with the various documents that update, clarify, and extend the base specification.

When composing a DNS query, the parameters used by the protocol are a triple: a DNS name, a DNS class, and a DNS record Type. Every Resource Record matching a particular name, type and class is said to belong to the same "RRSet", and the whole RRSet is always returned to the client that queries for it. Splitting an RRSet is a protocol violation, because it results in coherency problems with the DNS caching mechanism.

Some discussions around extensions to the DNS include arguments around MTU size. Note that most discussions about DNS and MTU size are about the size of the whole DNS packet, not about the size of a single RRSet.

Almost all DNS query traffic is carried over UDP, where a DNS message must fit within a single UDP packet. DNS response messages are almost always larger than DNS query messages, so message size issues are almost always about responses, not queries. The base DNS specification limits DNS messages over UDP to 512 octets; EDNS0 [RFC2671] (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.) specifies a mechanism by which a client can signal its willingness to receive larger responses, but deployment of EDNS0 is not universal, in part because of firewalls that block fragmented UDP packets or EDNS0. If a response message won't fit in a single packet, the name server returns a truncated response, at which point the client may retry using TCP. DNS queries over TCP are not subject to this length limitation, but TCP imposes significantly higher per-query overhead on name servers than UDP. It is also the case that the policies in deployed firewalls far too often is such that it blocks DNS over TCP, so using TCP might not in reality be an option. There are also risks (although possibly small) that a change of routing while a TCP flow is open create problems when the DNS servers are deployed in an anycast environment.

3.  Extension mechanisms

The DNS protocol is intended to be extensible to support new kinds of data. This section examines the various ways in which this sort of extension can be accomplished.

3.1.  Place selectors inside the RDATA of existing Resource Record Types

For a given query name, one might choose to have a single RRSet (all Resource Records sharing the same name, type, and class) shared by multiple applications, and have the different applications use selectors within the Resource Record data (RDATA) to determine which records are intended for which applications. This sort of selector mechanism is usually referred to "subtyping", because it is in effect creating an additional type subsystem within a single DNS Resource Record Type.

Examples of subtyping include NAPTR Resource Records (see [RFC3761] (Faltstrom, P. and M. Mealling, “The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM),” April 2004.)) and the original DNSSEC KEY Resource Record Type ([RFC2535] (Eastlake 3rd, D., “Domain Name System Security Extensions,” March 1999.)) (before it was updated by [RFC3445] (Massey, D. and S. Rose, “Limiting the Scope of the KEY Resource Record (RR),” December 2002.)).

All DNS subtyping schemes share a common weakness: With subtyping schemes it is impossible for a client to query for just the data it wants. Instead, the client must fetch the entire RRSet, then select the Resource Records in which it is interested. Furthermore, since DNSSEC signatures operate on complete RRSets, the entire RRSet must be re-signed if any Resource Record in it changes. As a result, each application that uses a subtyped Resource Record incurs higher overhead than any of the applications would have incurred had they not been using a subtyping scheme. The fact the RRSet is always passed around as an indivisible unit increases the risk the RRSet will not fit in a UDP packet, which in turn increases the risk that the client will have to retry the query with TCP, which substantially increases the load on the name server. More precisely: having one query fail over to TCP is not a big deal, but since the typical ratio of clients to servers in today's deployed DNS is very high, having a substantial number of DNS messages fail over to TCP may cause the queried name servers to be overloaded by TCP overhead.

Because of the size limitations, using a subtyping scheme to list a large number of services for a single domain name risks triggering truncation and fallback to TCP, which may in turn force the zone administrator to announce only a subset of available services.

3.2.  Add a prefix to the owner name

By adding an application-specific prefix to a domain name, we get a different name/class/type triple, and therefore a different RRSet. One problem with adding prefixes has to do with wildcards, especially if one has records like

*.example.com. IN MX 1 mail.example.com.

and one wants records tied to those names. Suppose one creates the prefix "_mail". One would then have to say something like

_mail.*.example.com. IN X-FOO A B C D

but DNS wildcards only work with the "*" as the leftmost token in the domain name (see also [RFC4592] (Lewis, E., “The Role of Wildcards in the Domain Name System,” July 2006.)).

Even when a specific prefix is chosen, the data will still have to be stored in some Resource Record Type. This Resource Record Type can either be a record Type that has an appropriate format to store the data or a new Resource Record Type. This implies that some other selection mechanism has to be applied as well, such as ability to distinguish between the records in an RRSet given they have the same Resource Record Type. Because of this, one needs to both register a unique prefix and define what Resource Record Type is to be used for this specific service.

If the record has some relationship with another record in the zone, the fact that the two records can be in different zones might have implications on the trust the application has in the records. For example:

example.com.      IN MX    10 mail.example.com.
_foo.example.com. IN X-BAR "metadata for the mail service"

In this example, the two records might be in two different zones, and because of this might be signed by two different organizations when using DNSSEC.

3.3.  Add a suffix to the owner name

Adding a suffix to a domain name changes the name/class/type triple, and therefore the RRSet. In this case, since the query name can be set to exactly the data one wants the size of the RRSet is minimized. The problem with adding a suffix is that it creates a parallel tree within the IN class. Further, there is no technical mechanism to ensure that the delegation for "example.com" and "example.com._bar" are made to the same organization. Furthermore, data associated with a single entity will now be stored in two different zones, such as "example.com" and "example.com._bar", which, depending on who controls "_bar", can create new synchronization and update authorization issues.

One way of solving the administrative issues is by using the DNAME Resource Record Type specified in [RFC2672] (Crawford, M., “Non-Terminal DNS Name Redirection,” August 1999.).

Even when using a different name, the data will still have to be stored in some Resource Record Type. This Resource Record Type can either be a "kitchen-sink record" or a new Resource Record Type. This implies that some other mechanism has to be applied as well, with implications detailed in other parts of this note.

In [RFC2163] (Allocchio, C., “Using the Internet DNS to Distribute MIXER Conformant Global Address Mapping (MCGAM),” January 1998.) an infix token is inserted directly below the TLD, but the result is equivalent to adding a suffix to the owner name (instead of creating a TLD one is creating a second level domain).

3.4.  Add a new Class

DNS zones are class-specific in the sense that all the records in that zone share the same class as the zone's SOA record and the existence of a zone in one class does not guarantee the existence of the zone in any other class. In practice, only the IN class has ever seen widespread deployment, and the administrative overhead of deploying an additional class would almost certainly be prohibitive.

Nevertheless, one could in theory use the DNS class mechanism to distinguish between different kinds of data. However, since the DNS delegation tree (represented by NS Resource Records) is itself tied to a specific class, attempting to resolve a query by crossing a class boundary may produce unexpected results because there is no guarantee that the name servers for the zone in the new class will be the same as the name servers in the IN class. The MIT Hesiod system used a scheme like this for storing data in the HS class, but only on a very small scale (within a single institution), and with an administrative fiat requiring that the delegation trees for the IN and HS trees be identical.

Even when using a different class, the data will still have to be stored in some Resource Record Type or another. This Resource Record Type can either be a "kitchen-sink record" or a new Resource Record Type. This implies that some other mechanism has to be applied as well, with implications detailed in other parts of this note.

3.5.  Add a new Resource Record Type

When adding a new Resource Record Type to the system, entities in four different roles have to be able to handle the new Type:

1. There must be a way to insert the new Resource Records into the zone of the Primary Master name server. For some server implementations, the user interface only accepts record Types which it understands (perhaps so that the implementation can attempt to validate the data). Other implementations allow the zone administrator to enter an integer for the Resource Record Type code and the RDATA in Base64 or hexadecimal encoding (or even as raw data). [RFC3597] (Gustafsson, A., “Handling of Unknown DNS Resource Record (RR) Types,” September 2003.) specifies a standard generic encoding for this purpose.
2. A slave authoritative name server must be able to do a zone transfer, receive the data from some other authoritative name server, and serve data from the zone even though the zone includes records of unknown Types. Historically, some implementations have had problems parsing stored copies of the zone file after restarting, but those problems have not been seen for a few years.
3. A caching resolver (most commonly a recursive name server) will cache the records which are responses to queries. As mentioned in [RFC3597] (Gustafsson, A., “Handling of Unknown DNS Resource Record (RR) Types,” September 2003.),there are various pitfalls where a recursive name server might end up having problems.
4. The application must be able to get the RRSet with a new Resource Record Type. The application itself may understand the RDATA, but the resolver library might not. Support for a generic interface for retrieving arbitrary DNS Resource Record Types has been a requirement since 1989 (see [RFC1123] (Braden, R., “Requirements for Internet Hosts - Application and Support,” October 1989.) Section 6.1.4.2). Some stub resolver library implementations neglect to provide this functionality and cannot handle unknown Resource Record Types, but implementation of a new stub resolver library is not particularly difficult, and open source libraries that already provide this functionality are available.

4.  Zone boundaries are invisible to applications

Regardless of the possible choices above we have seen a number of cases where the application made assumptions about the structure of the namespace and the location where specific information resides. We take a small sidestep to argue against such approaches.

The DNS namespace is a hierarchy, technically speaking. However, this only refers to the way names are built from multiple labels. DNS hierarchy neither follows nor implies administrative hierarchy. That said, it cannot be assumed that data attached to a node in the DNS tree is valid for the whole subtree. Technically, there are zone boundaries partitioning the namespace and administrative boundaries (or policy boundaries) may even exist elsewhere.

The false assumption has lead to an approach called "tree climbing", where a query that does not receive a positive response (either the requested RRSet was missing or the name did not exist) is retried by repeatedly stripping off the leftmost label (climbing towards the root) until the root domain is reached. Sometimes these proposals try to avoid the query for the root or the TLD level, but still this approach has severe drawbacks:

• Technically, the DNS was built as a query - response tool without any search capability [RFC3467] (Klensin, J., “Role of the Domain Name System (DNS),” February 2003.). Adding the search mechanism imposes additional burden on the technical infrastructure, in the worst case on TLD and root name servers.
• For reasons similar to those outlined in RFC 1535, querying for information in a domain outside the control of the intended entity may lead to incorrect results and may also put security at risk. Finding the exact policy boundary is impossible without an explicit marker which does not exist at present. At best, software can detect zone boundaries (e.g., by looking for SOA Resource Records), but some TLD registries register names starting at the second level (e.g., CO.UK), and there are various other "registry" types at second, third or other level domains that cannot be identified as such without policy knowledge external to the DNS.

To restate, the zone boundary is purely a boundary that exists in the DNS for administrative purposes, and applications should be careful not to draw unwarranted conclusions from zone boundaries. A different way of stating this is that the DNS does not support inheritance, e.g. a wildcard MX RRSet for a TLD will not be valid for any subdomain of that particular TLD.

5.  Why adding a new Resource Record Type is the preferred solution

By now, the astute reader might be be wondering what conclusions to draw from all the issues presented so far. We will now attempt to clear up the reader's confusion by following the thought processes of a typical application designer who wishes to store data in the DNS, showing how such a designer almost inevitably hits upon the idea of just using TXT Resource Record, why this is a bad thing, and why a new Resource Record Type should be allocated instead.

The overall problem with most solutions has to do with two main issues:

• No semantics to prevent collision with other use
• Space considerations in the DNS message

A typical application designer is not interested in the DNS for its own sake, but rather as a distributed database in which application data can be stored. As a result, the designer of a new application is usually looking for the easiest way to add whatever new data the application needs to the DNS in a way that naturally associates the data with a DNS name.

As explained in Section 3.4 (Add a new Class), using the DNS class system as an extension mechanism is not really an option, and in fact most users of the system don't even realize that the mechanism exists. As a practical matter, therefore any extension is likely to be within the IN class.

Adding a new Resource Record Type is the technically correct answer from the DNS protocol standpoint (more on this below), but doing so requires some DNS expertise, due to the issues listed in Section 3.5 (Add a new Resource Record Type). As a result, this option is usually not considered. Note that according to [RFC2929] (Eastlake 3rd, D., Brunner-Williams, E., and B. Manning, “Domain Name System (DNS) IANA Considerations,” September 2000.), some Types require IETF Consensus, while others only require a specification.

The application designer is thus left with the prospect of reusing some existing DNS Type within the IN class, but when the designer looks at the existing Types, almost all of them have well-defined semantics, none of which quite match the needs of the new application. This has not completely prevented proposals from reusing existing Resource Record Types in ways incompatible with their defined semantics, but it does tend to steer application designers away from this approach.

For example, Resource Record Type 40 was registered for the SINK record Type. This Resource Record Type was discussed in the DNSIND working group of the IETF, and it was decided at the 46th IETF to not move the I-D forward to become an RFC because of the risk of encouraging application designers to use the SINK Resource Record Type instead of registering a new Resource Record Type, which would result in infeasibly large SINK RRsets.

Eliminating all of the above leaves the TXT Resource Record Type in the IN class. The TXT RDATA format is free form text, and there are no existing semantics to get in the way. Furthermore, the TXT Resource Record can obviously just be used as a bucket in which to carry around data to be used by some higher level parser, perhaps in some human readable programming or markup language. Thus, for many applications, TXT Resource Records are the "obvious" choice. Unfortunately, this conclusion, while understandable, is also wrong, for several reasons.

The first reason why TXT Resource Records are not well suited to such use is precisely the lack of defined semantics that make them so attractive. Arguably, the TXT Resource Record is misnamed, and should have been called the Local Container record, because the lack of defined semantics means that a TXT Resource Record means precisely what the data producer says it means. This is fine, so long as TXT Resource Records are being used by human beings or by private agreement between data producer and data consumer. However, it becomes a problem once one starts using them for standardized protocols in which there is no prior relationship between data producer and data consumer. The reason for this is that there is nothing to prevent collisions with some other incompatible use of TXT Resource Records. This is even worse than the general subtyping problem described in Section 3.1 (Place selectors inside the RDATA of existing Resource Record Types), because TXT Resource Records don't even have a standardized selector field in which to store the subtype. [RFC1464] (Rosenbaum, R., “Using the Domain Name System To Store Arbitrary String Attributes,” May 1993.) tried, but it was not a success. At best a definition of a subtype is reduced to hoping that whatever scheme one has come up with will not accidently conflict with somebody else's subtyping scheme, and that it will not be possible to mis-parse one application's use of TXT Resource Records as data intended for a different application. Any attempt to impose a standardized format within the TXT Resource Record format would be at least fifteen years too late even if it were put into effect immediately; at best, one can restrict the syntax that a particular application uses within a TXT Resource Record and accept the risk that unrelated TXT Resource Record uses will collide with it.

Using one of the naming modifications discussed in Section 3.2 (Add a prefix to the owner name) and Section 3.3 (Add a suffix to the owner name) would address the subtyping problem, but each of these approaches brings in new problems of its own. The prefix approach (that for example SRV Resource Records use) does not work well with wildcards, which is a particular problem for mail-related applications, since MX Resource Records are probably the most common use of DNS wildcards. The suffix approach doesn't have wildcard issues, but, as noted previously, it does have synchronization and update authorization issues, since it works by creating a second subtree in a different part of the global DNS name space.

The next reason why TXT Resource Records are not well suited to protocol use has to do with the limited data space available in a DNS message. As alluded to briefly in Section 3.1 (Place selectors inside the RDATA of existing Resource Record Types), typical DNS query traffic patterns involve a very large number of DNS clients sending queries to a relatively small number of DNS servers. Normal path MTU discovery schemes do little good here because, from the server's perspective, there isn't enough repeat traffic from any one client for it to be worth retaining state. UDP-based DNS is an idempotent query, whereas TCP-based DNS requires the server to keep state (in the form of TCP connection state, usually in the server's kernel) and roughly triples the traffic load. Thus, there's a strong incentive to keep DNS messages short enough to fit in a UDP datagram, preferably a UDP datagram short enough not to require IP fragmentation.

Subtyping schemes are therefore again problematic because they produce larger Resource RRSets than necessary, but verbose text encodings of data are also wasteful, since the data they hold can usually be represented more compactly in a Resource Record designed specifically to support the application's particular data needs. If the data that need to be carried are so large that there is no way to make them fit comfortably into the DNS regardless of encoding, it is probably better to move the data somewhere else, and just use the DNS as a pointer to the data, as with NAPTR.

6.  Conclusion and Recommendation

Given the problems detailed in Section 5 (Why adding a new Resource Record Type is the preferred solution), it is worth reexamining the oft-jumped-to conclusion that specifying a new Resource Record Type is hard. Historically, this was indeed the case, but recent surveys suggest that support for unknown Resource Record Types [RFC3597] (Gustafsson, A., “Handling of Unknown DNS Resource Record (RR) Types,” September 2003.) is now widespread, and that lack of support for unknown Types is mostly an issue for relatively old software that would probably need to be upgraded in any case as part of supporting a new application. One should also remember that deployed DNS software today should support DNSSEC, and software recent enough to do so will likely support both unknown Resource Record Types [RFC3597] (Gustafsson, A., “Handling of Unknown DNS Resource Record (RR) Types,” September 2003.) and EDNS0 [RFC2671] (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.).

Of all the issues detailed in Section 3.5 (Add a new Resource Record Type), provisioning the data is in some respects the most difficult. The problem here is less difficult for the authoritative name servers themselves than the front-end systems used to enter (and perhaps validate) the data. Hand editing does not work well for maintenance of large zones, so some sort of tool is necessary, and the tool may not be tightly coupled to the name server implementation itself. Note, however, that this provisioning problem exists to some degree with any new form of data to be stored in the DNS, regardless of data format, Resource Record type, or naming scheme. Including the TXT Resource Record Type. Adapting front-end systems to support a new Resource Record type may be a bit more difficult than reusing an existing type, but this appears to be a minor difference in degree rather than a difference in kind.

Given the various issues described in this note, we believe that:

• there is no magic solution which allows a completely painless addition of new data to the DNS, but
• on the whole, the best solution is still to use the DNS Resource Record Type mechanism designed for precisely this purpose, and
• of all the alternate solutions, the "obvious" approach of using TXT Resource Records is almost certainly the worst.

This especially for the two reasons outlined above (lack of semantics and its implications, and size leading to the need to use TCP).

7.  Creating A New Resource Record Type

The process for creating a new Resource Record Type is specified in [I‑D.ietf‑dnsext‑2929bis] (3rd, D., “Domain Name System (DNS) IANA Considerations,” August 2007.).

8.  IANA Considerations

This document does not require any IANA actions.

9.  Security Considerations

DNS RRSets can be signed using DNSSEC. DNSSEC is almost certainly necessary for any application mechanism that stores authorization data in the DNS. DNSSEC signatures significantly increase the size of the messages transported, and because of this, the DNS message size issues discussed in Section 3.1 (Place selectors inside the RDATA of existing Resource Record Types) and Section 5 (Why adding a new Resource Record Type is the preferred solution) are more serious than they might at first appear.

Adding new Resource Record Types (as discussed in Section 3.5 (Add a new Resource Record Type)) might conceivably trigger bugs and other bad behavior in software that is not compliant with [RFC3597] (Gustafsson, A., “Handling of Unknown DNS Resource Record (RR) Types,” September 2003.), but most such software is old enough and insecure enough that it should be updated for other reasons in any case. Basic API support for retrieving arbitrary Resource Record Types has been a requirement since 1989 (see [RFC1123] (Braden, R., “Requirements for Internet Hosts - Application and Support,” October 1989.)).

Any new protocol that proposes to use the DNS to store data used to make authorization decisions would be well advised not only to use DNSSEC but also to encourage upgrades to DNS server software recent enough not to be riddled with well-known exploitable bugs. Because of this, support for new Resource Record Types will not be as hard as people might think at first.

10.  Acknowledgements

This document has been created during a number of years, with input from many people. The question on how to expand and use the DNS is sensitive, and a document like this can not please everyone. The goal is instead to describe the architecture and tradeoffs, and make some recommendations about best practices.

People that have helped include: Dean Andersson, Loa Andersson, Mark Andrews, John Angelmo, Roy Badami, Dan Bernstein, Alex Bligh, Nathaniel Borenstein, Stephane Bortzmeyer, Brian Carpenter, Leslie Daigle, Elwyn Davies, Mark Delany, Richard Draves, Martin Duerst, Donald Eastlake, Robert Elz, Jim Fenton, Tony Finch, Jim Gilroy, Olafur Gudmundsson, Eric Hall, Philip Hallam-Baker, Ted Hardie, Bob Hinden, Paul Hoffman, Geoff Houston, Christian Huitema, Johan Ihren, John Klensin, Olaf Kolkman, Ben Laurie, William Leibzon, John Levine, Edward Lewis, David MacQuigg, Allison Manking, Bill Manning, Danny McPherson, David Meyer, Pekka Nikander, Masataka Ohta, Douglas Otis, Michael Patton, Jonathan Rosenberg, Anders Rundgren, Miriam Sapiro, Pekka Savola, Chip Sharp, James Snell, Dave Thaler, Michael Thomas, Paul Vixie, Sam Weiler, Florian Weimer, Bert Wijnen, and Dan Wing.

Members of the IAB when this document was made available were: Loa Andersson, Leslie Daigle, Elwyn Davies, Kevin Fall, Russ Housley, Olaf Kolkman, Barry Leiba, Kurtis Lindqvist, Danny McPherson, David Oran, Eric Rescorla, Dave Thaler, and Lixia Zhang.

11.  References

11.1. Normative References

11.2. Informative References

Authors' Addresses

Full Copyright Statement

Copyright © The IETF Trust (2008).

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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.

Intellectual Property

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.