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RFC 3958 - Domain-Based Application Service Location Using SRV R


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Network Working Group                                          L. Daigle
Request for Comments: 3958                                     A. Newton
Category: Standards Track                                 VeriSign, Inc.
                                                            January 2005

    Domain-Based Application Service Location Using SRV RRs and the
              Dynamic Delegation Discovery Service (DDDS)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This memo defines a generalized mechanism for application service
   naming that allows service location without relying on rigid domain
   naming conventions (so-called name hacks).  The proposal defines a
   Dynamic Delegation Discovery System (DDDS) Application to map domain
   name, application service name, and application protocol dynamically
   to target server and port.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Straightforward-NAPTR (S-NAPTR) Specification  . . . . . . . .  3
       2.1.  Key Terms. . . . . . . . . . . . . . . . . . . . . . . .  3
       2.2.  S-NAPTR DDDS Application Usage . . . . . . . . . . . . .  4
             2.2.1.  Ordering and Preference. . . . . . . . . . . . .  4
             2.2.2.  Matching and Non-matching NAPTR Records. . . . .  4
             2.2.3.  Terminal and Non-terminal NAPTR Records. . . . .  5
             2.2.4.  S-NAPTR and Successive Resolution. . . . . . . .  5
             2.2.5.  Clients Supporting Multiple Protocols. . . . . .  6
   3.  Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.  Guidelines for Application Protocol Developers . . . . .  6
             3.1.1.  Registration of Application Service and
                     Protocol Tags. . . . . . . . . . . . . . . . . .  7
             3.1.2.  Definition of Conditions for Retry/Failure . . .  7
             3.1.3.  Server Identification and Handshake  . . . . . .  8
       3.2.  Guidelines for Domain Administrators . . . . . . . . . .  8

       3.3.  Guidelines for Client Software Writers . . . . . . . . .  8
   4.  Illustrations  . . . . . . . . . . . . . . . . . . . . . . . .  9
       4.1.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . .  9
       4.2.  Service Discovery within a Domain  . . . . . . . . . . .  9
       4.3.  Multiple Protocols . . . . . . . . . . . . . . . . . . . 10
       4.4.  Remote Hosting . . . . . . . . . . . . . . . . . . . . . 11
       4.5.  Sets of NAPTR RRs  . . . . . . . . . . . . . . . . . . . 12
       4.6.  Sample Sequence Diagram  . . . . . . . . . . . . . . . . 13
   5.  Motivation and Discussion  . . . . . . . . . . . . . . . . . . 14
       5.1.  So Why Not Just SRV Records? . . . . . . . . . . . . . . 15
       5.2.  So Why Not Just NAPTR Records? . . . . . . . . . . . . . 15
   6.  Formal Definition of <Application Service Location>
       Application of DDDS  . . . . . . . . . . . . . . . . . . . . . 16
       6.1.  Application-Unique String  . . . . . . . . . . . . . . . 16
       6.2.  First Well-Known Rule  . . . . . . . . . . . . . . . . . 16
       6.3.  Expected Output  . . . . . . . . . . . . . . . . . . . . 16
       6.4.  Flags  . . . . . . . . . . . . . . . . . . . . . . . . . 16
       6.5.  Service Parameters . . . . . . . . . . . . . . . . . . . 17
             6.5.1.  Application Services . . . . . . . . . . . . . . 17
             6.5.2.  Application Protocols  . . . . . . . . . . . . . 17
       6.6.  Valid Rules  . . . . . . . . . . . . . . . . . . . . . . 17
       6.7.  Valid Databases  . . . . . . . . . . . . . . . . . . . . 18
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
       7.1.  Application Service Tag IANA Registry  . . . . . . . . . 18
       7.2.  Application Protocol Tag IANA Registry . . . . . . . . . 18
       7.3.  Registration Process . . . . . . . . . . . . . . . . . . 19
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
       10.1. Normative References . . . . . . . . . . . . . . . . . . 21
       10.2. Informative References . . . . . . . . . . . . . . . . . 21
   Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
       A.  Pseudo-pseudocode for S-NAPTR. . . . . . . . . . . . . . . 22
           A.1.  Finding the First (Best) Target. . . . . . . . . . . 22
           A.2.  Finding Subsequent Targets . . . . . . . . . . . . . 23
       B.  Availability of Sample Code. . . . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 25

1.  Introduction

   This memo defines a generalized mechanism for application service
   naming that allows service location without relying on rigid domain
   naming conventions (so-called name hacks).  The proposal defines a
   Dynamic Delegation Discovery System (DDDS -- see [4]) Application to
   map domain name, application service name, and application protocol
   dynamically to target server and port.

   As discussed in section 5, existing approaches to using DNS records
   for dynamically determining the current host for a given application
   service are limited in terms of the use cases supported.  To address
   some of the limitations, this document defines a DDDS Application to
   map service+protocol+domain to specific server addresses by using
   both NAPTR [5] and SRV ([3]) DNS resource records.  This can be
   viewed as a more general version of the use of SRV and/or a very
   restricted application of the use of NAPTR resource records.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119 [1].

2.  Straightforward-NAPTR (S-NAPTR) Specification

   The precise details of the specification of this DDDS application are
   given in Section 6.  This section defines the usage of the DDDS
   application.

2.1.  Key Terms

   "Application service" is a generic term for some type of application,
   independent of the protocol that may be used to offer it.  Each
   application service will be associated with an IANA-registered tag.
   For example, retrieving mail is a type of application service that
   can be implemented by different application-layer protocols (e.g.,
   POP3, IMAP4).  A tag, such as "RetMail", could be registered for it.
   (Note that this has not been done, and there are no plans to do so at
   the time of this writing.)

   An "application protocol" is used to implement the application
   service.  These are also associated with IANA-registered tags.  Using
   the mail example above, "POP3" and "IMAP4" could be registered as
   application protocol tags.  If multiple transports are available for
   the application, separate tags should be defined for each transport.

   The intention is that the combination of application service and
   protocol tags should be specific enough that finding a known pair
   (e.g., "RetMail:POP3" would be sufficient for a client to identify a
   server with which it can communicate.

   Some protocols support multiple application services.  For example,
   LDAP is an application protocol and can be found supporting various
   services (e.g., "whitepages", "directory enabled networking".

2.2.  S-NAPTR DDDS Application Usage

   As defined in section 6, NAPTR records are used to store application
   service+protocol information for a given domain.  Following the DDDS
   standard, these records are looked up, and the rewrite rules
   (contained in the NAPTR records) are used to determine the successive
   DNS lookups until a desirable target is found.

   For the rest of this section, refer to the set of NAPTR resource
   records for example.com, shown in the figure below, where "WP" is the
   imagined application service tag for "white pages" and "EM" is the
   application service tag for an imagined "Extensible Messaging"
   application service.

   example.com.
   ;;       order pref flags
   IN NAPTR 100   10   ""    "WP:whois++"      ( ; service
                             ""                  ; regexp
                             bunyip.example.     ; replacement
                                               )
   IN NAPTR 100   20   "s"   "WP:ldap"         ( ; service
                             ""                  ; regexp
                            _ldap._tcp.myldap.example.com. ; replacement
                                               )
   IN NAPTR 200   10   ""    "EM:protA"        ( ; service
                             ""                  ; regexp
                             someisp.example.    ; replacement
                                               )
   IN NAPTR 200   30   "a"   "EM:protB"          ; service
                             ""                  ; regexp
                             myprotB.example.com.; replacement
                                               )

2.2.1.  Ordering and Preference

   A client retrieves all the NAPTR records associated with the target
   domain name (example.com, above).  These are to be sorted in terms of
   increasing ORDER and increasing PREF within each ORDER.

2.2.2.  Matching and Non-Matching NAPTR Records

   Starting with the first sorted NAPTR record, the client examines the
   SERVICE field to find a match.  In the case of the S-NAPTR DDDS
   application, this means a SERVICE field that includes the tags for
   the desired application service and a supported application protocol.

   If more than one NAPTR record matches, they are processed in
   increasing sort order.

2.2.3.  Terminal and Non-terminal NAPTR Records

   A NAPTR record with an empty FLAG field is "non-terminal" -- that is,
   more NAPTR RR lookups are to be performed.  Thus, to process a NAPTR
   record with an empty FLAG field in S-NAPTR, the REPLACEMENT field is
   used as the target of the next DNS lookup -- for NAPTR RRs.

   In S-NAPTR, the only terminal flags are "S" and "A".  These are
   called "terminal" NAPTR lookups because they denote the end of the
   DDDS/NAPTR processing rules.  In the case of an "S" flag, the
   REPLACEMENT field is used as the target of a DNS query for SRV RRs,
   and normal SRV processing is applied.  In the case of an "A" flag, an
   address record is sought for the REPLACEMENT field target (and the
   default protocol port is assumed).

2.2.4.  S-NAPTR and Successive Resolution

   As shown in the example set above, it is possible to have multiple
   possible targets for a single application service+protocol pair.
   These are to be pursued in order until a server is successfully
   contacted or all possible matching NAPTR records have been
   successively pursued through terminal lookup and server contact.
   That is, a client must backtrack and attempt other resolution paths
   in the case of failure.

   "Failure" is declared, and backtracking must be used, when

   o  the designated remote server (host and port) fails to provide
      appropriate security credentials for the *originating* domain;

   o  connection to the designated remote server otherwise fails -- the
      specifics terms of which are defined when an application protocol
      is registered; or

   o  the S-NAPTR-designated DNS lookup fails to yield expected results
      -- e.g., no A RR for an "A" target, no SRV record for an "S"
      target, or no NAPTR record with appropriate application service
      and protocol for a NAPTR lookup.  Except in the case of the very
      first NAPTR lookup, this last is a configuration error: the fact
      that example.com has a NAPTR record pointing to "bunyip.example"
      for the "WP:Whois++" service and protocol means the administrator
      of example.com believes that service exists.  If bunyip.example
      has no "WP:Whois++" NAPTR record, the application client MUST
      backtrack and try the next available "WP:Whois++" option from
      example.com.  As there is none, the whole resolution fails.

   An application client first queries for the NAPTR RRs for the domain
   of a named application service.  The first DNS query is for the NAPTR
   RRs in the original target domain (example.com, above).

2.2.5.  Clients Supporting Multiple Protocols

   In the case of an application client that supports more than one
   protocol for a given application service, it MUST pursue S-NAPTR
   resolution completely for one protocol, exploring all potential
   terminal lookups in PREF and ORDER ranking, until the application
   connects successfully or there are no more possibilities for that
   protocol.

   That is, the client MUST NOT start looking for one protocol, observe
   that a successive NAPTR RR set supports another of its preferred
   protocols, and continue the S-NAPTR resolution based on that
   protocol.  For example, even if someisp.example offers the "EM"
   service with protocol "ProtB", there is no reason to believe that it
   does so on behalf of example.com (as there is no such pointer in
   example.com's NAPTR RR set).

   It MAY choose which protocol to try first based on its own
   preference, or on the PREF ranking in the first set of NAPTR records
   (i.e., those for the target named domain).  However, the chosen
   protocol MUST be listed in that first NAPTR RR set.

   It MAY choose to run simultaneous DDDS resolutions for more than one
   protocol, in which case the requirements above apply for each
   protocol independently.  That is, do not switch protocols mid-
   resolution.

3.  Guidelines

3.1.  Guidelines for Application Protocol Developers

   The purpose of S-NAPTR is to provide application standards developers
   with a more powerful framework (than SRV RRs alone) for naming
   service targets, without requiring each application protocol (or
   service) standard to define a separate DDDS application.

   Note that this approach is intended specifically for use when it
   makes sense to associate services with particular domain names (e.g.,
   e-mail addresses, SIP addresses, etc).  A non-goal is having all
   manner of label mapped into domain names in order to use this.

   This document does not address how to select the domain for which the
   service+protocol is being sought.  Other conventions will have to
   define how this might be used (e.g., new messaging standards can
   define what domain to use from their URIs or how to step down from
   foobar.example.com to example.com, if applicable).

   Although this document proposes a DDDS application that does not use
   all the features of NAPTR resource records, it is not intended to
   imply that DNS resolvers should fail to implement all aspects of the
   NAPTR RR standard.  A DDDS application is a client use convention.

   The rest of this section outlines the specific elements that protocol
   developers must determine and document to make use of S-NAPTR.

3.1.1.  Registration of Application Service and Protocol Tags

   Application protocol developers who wish to make use of S-NAPTR must
   make provisions for registering any relevant application service and
   application protocol tags, as described in section 7.

3.1.2.  Definition of Conditions for Retry/Failure

   One other important aspect that must be defined is the expected
   behaviour for interacting with the servers that are reached via S-
   NAPTR.  Specifically, under what circumstances should the client
   retry a target that was found via S-NAPTR?  What should it consider a
   failure that causes it to return to the S-NAPTR process to determine
   the next serviceable target, which by definition will have a lower
   preference ranking.

   For example, if the client gets a "connection refused" message from a
   server, should it retry for some (protocol-dependent) period of time?
   Or should it try the next-preferred target in the S-NAPTR chain of
   resolution?  Should it only try the next-preferred target if it
   receives a protocol-specific permanent error message?

   The most important thing is to select one expected behaviour and
   document it as part of the use of S-NAPTR.

   As noted earlier, failure to provide appropriate credentials to
   identify the server as being authoritative for the original target
   domain is always considered a failure condition.

3.1.3.  Server Identification and Handshake

   As noted in section 8, use of the DNS for server location increases
   the importance of using protocol-specific handshakes to determine and
   confirm the identity of the server that is eventually reached.

   Therefore, application protocol developers using S-NAPTR should
   identify the mechanics of the expected identification handshake when
   the client connects to a server found through S-NAPTR.

3.2.  Guidelines for Domain Administrators

   Although S-NAPTR aims to provide a "straightforward" application of
   DDDS and use of NAPTR records, it is still possible to create very
   complex chains and dependencies with the NAPTR and SRV records.

   Therefore, domain administrators are called upon to use S-NAPTR with
   as much restraint as possible while still achieving their service
   design goals.

   The complete set of NAPTR, SRV, and A RRs "reachable" through the S-
   NAPTR process for a particular application service can be thought of
   as a "tree".  Each NAPTR RR that is retrieved points to more NAPTR or
   SRV records; each SRV record points to several A record lookups.
   Even though a particular client can "prune" the tree to use only
   those records referring to application protocols supported by the
   client, the tree could be quite deep, and retracing the tree to retry
   other targets can become expensive if the tree has many branches.

   Therefore,

   o  fewer branches is better: For both NAPTR and SRV records, provide
      different targets with varying preferences where appropriate
      (e.g., to provide backup services) but don't look for reasons to
      provide more; and

   o  shallower is better: Avoid using NAPTR records to "rename"
      services within a zone.  Use NAPTR records to identify services
      hosted elsewhere (i.e., where you cannot reasonably provide the
      SRV records in your own zone).

3.3.  Guidelines for Client Software Writers

   To understand DDDS/NAPTR properly, an implementor must read [4].
   However, the most important aspect to keep in mind is that if the
   application cannot successfully connect to one target, the
   application will be expected to continue through the S-NAPTR tree to
   try the (less preferred) alternatives.

4.  Illustrations

4.1.  Use Cases

   The basic intended use cases for which S-NAPTR has been developed are
   as follows

   o  Service discovery within a domain.  For example, this can be used
      to find the "authoritative" server for some type of service within
      a domain (see the specific example in section 4.2).

   o  Multiple protocols.  This is already common today as new
      application services are defined, and is increasingly a problem.
      It includes the case of extensible messaging (a hypothetical
      service), which can be offered with multiple protocols (see
      section 4.3).

   o  Remote hosting.  Each of the above use cases applies within the
      administration of a single domain.  However, one domain operator
      may elect to engage another organization to provide an application
      service.  See section 4.4 for an example that cannot be served by
      SRV records alone.

4.2.  Service Discovery within a Domain

   There are occasions when it is useful to be able to determine the
   "authoritative" server for a given application service within a
   domain.  This is "discovery", as there is no a priori knowledge as to
   whether or where the service is offered; it is therefore important to
   determine the location and characteristics of the offered service.

   For example, there is growing discussion of having a generic
   mechanism for locating the keys or certificates associated with
   particular application (servers) operated in (or for) a particular
   domain.  The following is a hypothetical case for storing application
   key or certificate data for a given domain: the premise is that a
   credentials registry (CredReg) service has been defined as a leaf
   node service holding the keys/certs for the servers operated by (or
   for) the domain.  It is assumed that more than one protocol is
   available to provide the service for a particular domain.  This
   DDDS-based approach is used to find the CredReg server that holds the
   information.

   Thus, the set of NAPTR records for thinkingcat.example might look
   like this:

thinkingcat.example.
;;       order pref flags
IN NAPTR 100   10   ""    "CREDREG:ldap:iris.beep"   ( ; service
                          ""                           ; regexp
                          theserver.thinkingcat.example. ; replacement

Note that the application service might be offered in another domain
using a different set of application protocols:

anotherdomain.example.
;;       order pref flags
IN NAPTR 100   10   ""    "CREDREG:iris.lwz:iris.beep"  ( ; service
                          ""                              ; regexp
                          foo.anotherdomain.example.      ; replacement
                                                        )

4.3.  Multiple Protocols

   Extensible messaging, a hypothetical application service, will be
   used for illustrative purposes.  (For an example of a real
   application service with multiple protocols, see [9] and [10]).
   Assuming that "EM" was registered as an application service, this
   DDDS application could be used to determine the available services
   for delivery to a target.

   Two particular features of this hypothetical extensible messaging
   should be noted:

   1. Gatewaying is expected to bridge communications across protocols.

   2. Extensible messaging servers are likely to be operated out of a
      different domain than that of the extensible messaging address,
      and servers of different protocols may be offered by independent
      organizations.

   For example, "thinkingcat.example" may support its own servers for
   the "ProtA" extensible messaging protocol but rely on outsourcing
   from "example.com" for "ProtC" and "ProtB" servers.

   Using this DDDS-based approach, thinkingcat.example can indicate a
   preference ranking for the different types of servers for the
   extensible messaging service, yet the out-sourcer can independently
   rank the preference and ordering of servers.  This independence is
   not achievable through the use of SRV records alone.

   Thus, to find the EM services for thinkingcat.example, the NAPTR
   records for thinkingcat.example are retrieved:

thinkingcat.example.
;;   order pref flags
IN NAPTR 100  10   "s"   "EM:ProtA"                  (   ; service
                         ""                              ; regexp
                        _ProtA._tcp.thinkingcat.example. ; replacement
                                                     )
IN NAPTR 100  20   "s"   "EM:ProtB"                  (   ; service
                         ""                              ; regexp
                         _ProtB._tcp.example.com.        ; replacement
                                                     )
IN NAPTR 100  30   "s"   "EM:ProtC"                  (   ; service
                         ""                              ; regexp
                         _ProtC._tcp.example.com.        ; replacement
                                                     )

   Then the administrators at example.com can manage the preference
   rankings of the servers they use to support the ProtB service:

   _ProtB._tcp.example.com.
    ;;    Pref Weight Port  Target
   IN SRV 10    0     10001 bigiron.example.com.
   IN SRV 20    0     10001 backup.em.example.com.
   IN SRV 30    0     10001 nuclearfallout.australia-isp.example.

4.4.  Remote Hosting

   In the Instant Message hosting example in Section 4.3, the service
   owner (thinkingcat.example) had to host pointers to the hosting
   service's SRV records in the thinkingcat.example domain.

   A better approach is to have one NAPTR RR in the thinkingcat.example
   domain point to all the hosted services.  The hosting domain has
   NAPTR records for each service to map them to whatever local hosts it
   chooses (this may change from time to time).

thinkingcat.example.
;;      order pref flags
IN NAPTR 100  10   "s"   "EM:ProtA"                ( ; service
                         ""                          ; regexp
                        _ProtA._tcp.thinkingcat.example. ; replacement
                                                   )
IN NAPTR 100  20   ""    "EM:ProtB:ProtC"          ( ; service
                         ""                          ; regexp
                         thinkingcat.example.com.    ; replacement
                                                   )

   Then the administrators at example.com can break out the individual
   application protocols and manage the preference rankings of the
   servers they use to support the ProtB service (as before):

thinkingcat.example.com.
;;      order pref flags
IN NAPTR 100  10   "s"   "EM:ProtC"                ( ; service
                         ""                          ; regexp
                         _ProtC._tcp.example.com.    ; replacement
                                                   )
IN NAPTR 100  20   "s"   "EM:ProtB"                ( ; service
                         ""                          ; regexp
                         _ProtB._tcp.example.com.    ; replacement
                                                   )

_ProtC._tcp.example.com.
 ;;    Pref Weight Port  Target
IN SRV 10    0     10001 bigiron.example.com.
IN SRV 20    0     10001 backup.em.example.com.
IN SRV 30    0     10001 nuclearfallout.australia-isp.example.

4.5.  Sets of NAPTR RRs

   Note that the above sections assume that there was one service
   available (via S-NAPTR) per domain.  Often, this will not be the
   case.  Assuming that thinkingcat.example had the CredReg service set
   up as described in Section 4.2 and had the extensible messaging
   service set up as described in Section 4.4, then a client querying
   for the NAPTR RR set from thinkingcat.com would get the following
   answer:

thinkingcat.example.
;;       order pref flags
IN NAPTR 100   10   "s"   "EM:ProtA"               ( ; service
                          ""                         ; regexp
                          _ProtA._tcp.thinkingcat.example. ; replacement
                                                   )
IN NAPTR 100   20   ""    "EM:ProtB:ProtC"         ( ; service
                          ""                         ; regexp
                          thinkingcat.example.com.   ; replacement
                                                   )
IN NAPTR 200   10   ""    "CREDREG:ldap:iris-beep" ( ; service
                          ""                         ; regexp
                          bouncer.thinkingcat.example. ; replacement
                                                   )

   Sorting them by increasing "ORDER", the client would look through the
   SERVICE strings to determine whether there was a NAPTR RR that
   matched the application service it was looking for, with an
   application protocol it could use.  The client would use the first
   (lowest PREF) record that matched to continue.

4.6.  Sample sequence diagram

   Consider the example in section 4.3.  Visually, the sequence of steps
   required for the client to reach the final server for a "ProtB"
   service for EM for the thinkingcat.example domain is as follows:

   Client   NS for                NS for
            thinkingcat.example   example.com    backup.em.example.com
                |                     |                  |
     1 -------->|                     |                  |
     2 <--------|                     |                  |
     3 ------------------------------>|                  |
     4 <------------------------------|                  |
     5 ------------------------------>|                  |
     6 <------------------------------|                  |
     7 ------------------------------>|                  |
     8 <------------------------------|                  |
     9 ------------------------------------------------->|
    10 <-------------------------------------------------|
    11 ------------------------------------------------->|
    12 <-------------------------------------------------|
   (...)

   1.  The name server (NS) for thinkingcat.example is reached with a
       request for all NAPTR records.

   2.  The server responds with the NAPTR records shown in section 4.3.

   3.  The second NAPTR record matches the desired criteria; it has an
       "s" flag and a replacement fields of "_ProtB._tcp.example.com".
       So the client looks up SRV records for that target, ultimately
       making the request of the NS for example.com.

   4.  The response includes the SRV records listed in Section 4.3.

   5.  The client attempts to reach the server with the lowest PREF in
       the SRV list -- looking up the A record for the SRV record's
       target (bigiron.example.com).

   6.  The example.com NS responds with an error message -- no such
       machine!

   7.  The client attempts to reach the second server in the SRV list
       and looks up the A record for backup.em.example.com.

   8.  The client gets the A record with the IP address for
       backup.em.example.com from example.com's NS.

   9.  The client connects to that IP address, on port 10001 (from the
       SRV record), by using ProtB over tcp.

   10. The server responds with an "OK" message.

   11. The client uses ProtB to challenge that this server has
       credentials to operate the service for the original domain
       (thinkingcat.example)

   12. The server responds, and the rest is EM.

5.  Motivation and Discussion

   Increasingly, application protocol standards use domain names to
   identify server targets and stipulate that clients should look up SRV
   resource records to determine the host and port providing the server.
   This enables a distinction between naming an application service
   target and actually hosting the server.  It also increases
   flexibility in hosting the target service, as follows:

   o  The server may be operated by a completely different organization
      without having to list the details of that organization's DNS
      setup (SRVs).

   o  Multiple instances can be set up (e.g., for load balancing or
      secondaries).

   o  It can be moved from time to time without disrupting clients'
      access, etc.

   This approach is quite useful, but section 5.1 outlines some of its
   inherent limitations.

   That is, although SRV records can be used to map from a specific
   service name and protocol for a specific domain to a specific server,
   SRV records are limited to one layer of indirection and are focused
   on server administration rather than on application naming.
   Furthermore, although the DDDS specification and use of NAPTR allows
   multiple levels of redirection before the target server machine with
   an SRV record is located, this proposal requires only a subset of
   NAPTR strictly bound to domain names, without making use of the
   REGEXP field of NAPTR.  These restrictions make the client's

   resolution process much more predictable and efficient than it would
   be with some potential uses of NAPTR records.  This is dubbed "S-
   NAPTR" -- a "S"traightforward use of NAPTR records.

5.1.  So Why Not Just SRV Records?

   An expected question at this point is: this is so similar in
   structure to SRV records, why are we doing this with DDDS/NAPTR?

   Limitations of SRV include the following:

   o  SRV provides a single layer of indirection; the outcome of an SRV
      lookup is a new domain name for which the A RR is to be found.

   o  the purpose of SRV is to address individual server administration
      issues, not to provide application naming: As stated in [3], "The
      SRV RR allows administrators to use several servers for a single
      domain, to move services from host to host with little fuss, and
      to designate some hosts as primary servers for a service and
      others as backups".

   o  Target servers by "service" (e.g., "ldap") and "protocol" (e.g.,
      "tcp") in a given domain.  The definition of these terms implies
      specific things (e.g., that protocol should be one of UDP or TCP)
      without being precise.  Restriction to UDP and TCP is insufficient
      for the uses described here.

   The basic answer is that SRV records provide mappings from protocol
   names to host and port.  The use cases described herein require an
   additional layer -- from some service label to servers that may in be
   hosted within different administrative domains.  We could tweak SRV
   to say that the next lookup could be something other than an address
   record, but this is more complex than is necessary for most
   applications of SRV.

5.2.  So Why Not Just NAPTR Records?

   This is a trick question.  NAPTR records cannot appear in the wild;
   see [4].  They must be part of a DDDS application.

   The purpose here is to define a single, common mechanism (the DDDS
   application) to use NAPTR when all that is desired is simple DNS-
   based location of services.  This should be easy for applications to
   use -- a few simple IANA registrations, and it's done.

   Also, NAPTR has very powerful tools for expressing "rewrite" rules.
   This power (==complexity) makes some protocol designers and service
   administrators nervous.  The concern is that these rewrites can
   translate into unintelligible, noodle-like rule sets that are
   difficult to test and administer.

   The proposed DDDS application specifically uses a subset of NAPTR's
   abilities.  Only "replacement" expressions are allowed, not "regular
   expressions".

6.  Formal Definition of <Application Service Location> Application of
    DDDS

   This section formally defines the DDDS application, as described in
   [4].

6.1.  Application-Unique String

   The Application Unique String is domain label for which an
   authoritative server for a particular service is sought.

6.2.  First Well-Known Rule

   The "First Well-Known Rule" is identity -- that is, the output of the
   rule is the Application-Unique String, the domain label for which the
   authoritative server for a particular service is sought.

6.3.  Expected Output

   The expected output of this Application is the information necessary
   for a client to connect to authoritative server(s) (host, port,
   protocol) for a particular application service within a given domain.

6.4.  Flags

   This DDDS Application uses only 2 of the Flags defined for the URI/
   URN Resolution Application ([6]): "S" and "A".  No other Flags are
   valid.

   Both are for terminal lookups.  This means that the Rule is the last
   one and that the flag determines what the next stage should be.  The
   "S" flag means that the output of this Rule is a domain label for
   which one or more SRV [3] records exist.  "A" means that the output
   of the Rule is a domain name and should be used to lookup address
   records for that domain.

   Consistent with the DDDS algorithm, if the Flag string is empty the
   next lookup is for another NAPTR record (for the replacement target).

6.5.  Service Parameters

   Service Parameters for this Application take the form of a string of
   characters that follow this ABNF ([2]):

      service-parms = [ [app-service] *(":" app-protocol)]
      app-service   = experimental-service  / iana-registered-service
      app-protocol  = experimental-protocol / iana-registered-protocol
      experimental-service      = "x-" 1*30ALPHANUMSYM
      experimental-protocol     = "x-" 1*30ALPHANUMSYM
      iana-registered-service   = ALPHA *31ALPHANUMSYM
      iana-registered-protocol  = ALPHA *31ALPHANUM
      ALPHA         =  %x41-5A / %x61-7A   ; A-Z / a-z
      DIGIT         =  %x30-39 ; 0-9
      SYM           =  %x2B / %x2D / %x2E  ; "+" / "-" / "."
      ALPHANUMSYM   =  ALPHA / DIGIT / SYM
      ; The app-service and app-protocol tags are limited to 32
      ; characters and must start with an alphabetic character.
      ; The service-parms are considered case-insensitive.

   Thus, the Service Parameters may consist of an empty string, an app-
   service, or an app-service with one or more app-protocol
   specifications separated by the ":" symbol.

   Note that this is similar to, but not the same as the syntax used in
   the URI DDDS application ([6]).  The DDDS DNS database requires each
   DDDS application to define the syntax of allowable service strings.
   The syntax here is expanded to allow the characters that are valid in
   any URI scheme name (see [8]).  As "+" (the separator used in the
   RFC3404 service parameter string) is an allowed character for URI
   scheme names, ":" is chosen as the separator here.

6.5.1.  Application Services

   The "app-service" must be an IANA-registered service; see Section 7
   for instructions on registering new application service tags.

6.5.2.  Application Protocols

   The protocol identifiers valid for the "app-protocol" production are
   standard, registered protocols; see section 7 for instructions on
   registering new application protocol tags.

6.6.  Valid Rules

   Only substitution Rules are permitted for this application.  That is,
   no regular expressions are allowed.

6.7.  Valid Databases

   At present only one DDDS Database is specified for this Application.
   [5] specifies that a DDDS Database using the NAPTR DNS resource
   record contain the rewrite rules.  The Keys for this database are
   encoded as domain-names.

   The First Well-Known Rule produces a domain name, and this is the Key
   used for the first look up.  The NAPTR records for that domain are
   requested.

   DNS servers MAY interpret Flag values and use that information to
   include appropriate NAPTR, SRV, or A records in the Additional
   Information portion of the DNS packet.  Clients are encouraged to
   check for additional information but are not required to do so.  See
   the Additional Information Processing section of [5] for more
   information on NAPTR records and the Additional Information section
   of a DNS response packet.

7.  IANA Considerations

   This document calls for two IANA registries: one for application
   service tags, and one for application protocol tags.

7.1.  Application Service Tag IANA Registry

   IANA has established and will maintain a registry for S-NAPTR
   Application Service Tags, listing at least the following information
   for each such tag:

   o  Application Service Tag: A string conforming with the IANA-
      registered-service defined in section 6.5.

   o  Defining publication: The RFC used to define the Application
      Service Tag, as defined in the registration process, below.

   An initial Application Service Tag registration is contained in [9].

7.2.  Application Protocol Tag IANA Registry

   IANA has established and will maintain a registry for S-NAPTR
   Application Protocol Tags, listing at least the following information
   for each such tag:

   o  Application Protocol Tag: A string conforming with the iana-
      registered-protocol defined in section 6.5.

   o  Defining publication: The RFC used to define the Application
      Protocol Tag, as defined in the registration process, below.

   An initial Application Protocol Tag registration is defined in [10].

7.3.  Registration Process

   All application service and protocol tags that start with "x-" are
   considered experimental, and no provision is made to prevent
   duplicate use of the same string.  Implementors use them at their own
   risk.

   All other application service and protocol tags are registered based
   on the "specification required" option defined in [7], with the
   further stipulation that the "specification" is an RFC (of any
   category).

   No further restrictions are placed on the tags except that they must
   conform with the syntax defined below (Section 6.5).

   The defining RFC must clearly identify and describe, for each tag
   being registered,

   o  application protocol or service tag,

   o  intended usage,

   o  interoperability considerations,

   o  security considerations (see section 8 of this document for
      further discussion of the types of considerations that are
      applicable), and

   o  any relevant related publications.

8.  Security Considerations

   The security of this approach to application service location is only
   as good as the security of the DNS queries along the way.  If any of
   them is compromised, bogus NAPTR and SRV records could be inserted to
   redirect clients to unintended destinations.  This problem is hardly
   unique to S-NAPTR (or NAPTR in general).  A full discussion of the
   security threats pertaining to DNS can be found in [11].

   To protect against DNS-vectored attacks, secured DNS (DNSSEC) [12]
   can be used to ensure the validity of the DNS records received.

   Whether or not DNSSEC is used, applications should define some form
   of end-to-end authentication to ensure that the correct destination
   has been reached.  Many application protocols such as HTTPS, BEEP,
   and IMAP define the necessary handshake mechanisms to accomplish this
   task.  Newly defined application protocols should take this into
   consideration and incorporate appropriate mechanisms.

   The basic mechanism works as follows:

   1. During some portion of the protocol handshake, the client sends to
      the server the original name of the desired destination (i.e., no
      transformations that may have resulted from NAPTR replacements,
      SRV targets, or CNAME changes).  In certain cases where the
      application protocol does not have such a feature but TLS may be
      used, it is possible to use the "server_name" TLS extension.

   2. The server sends back to the client a credential with the
      appropriate name.  For X.509 certificates, the name would be in
      either the subjectDN or the subjectAltName field.  For Kerberos,
      the name would be a service principle name.

   3. Using the matching semantics defined by the application protocol,
      the client compares the name in the credential with the name sent
      to the server.

   4. If the names match and the credentials have integrity, there is
      reasonable assurance that the correct end point has been reached.

   5. The client and server establish an integrity-protected channel.

   Note that this document does not define either the handshake
   mechanism, the specific credential naming fields, nor the name-
   matching semantics.  Definitions of S-NAPTR for particular
   application protocols MUST define these.

9.  Acknowledgements

   Many thanks to Dave Blacka, Patrik Faltstrom, Sally Floyd, and Ted
   Hardie for discussion and input that have (hopefully!) provoked
   clarifying revisions to this document.

10.  References

10.1.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [3]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
        specifying the location of services (DNS SRV)", RFC 2782,
        February 2000.

   [4]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        One: The Comprehensive DDDS", RFC 3401, October 2002.

   [5]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        Three: The Domain Name System (DNS) Database", RFC 3403, October
        2002.

   [6]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        Four: The Uniform Resource Identifiers (URI)", RFC 3404, October
        2002.

   [7]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

10.2.  Informative References

   [8]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
        Resource Identifiers (URI): Generic Syntax", RFC 2396, August
        1998.

   [9]  Newton, A. and M. Sanz, "IRIS:  A Domain Registry (dreg) Type
        for the Internet Registry Information Service (IRIS)", RFC 3982,
        January 2005.

   [10] Newton, A. and M. Sanz, "Using the Internet Registry Information
        Service (IRIS) over the Blocks Extensible Exchange Protocol
        (BEEP)", RFC 3983, January 2005.

   [11] Atkins, D. and R. Austein, "Threat Analysis Of The Domain Name
        System", Work in Progress, April 2004.

   [12] Arends, R., Larson, M., Austein, R., and D. Massey, "Protocol
        Modifications for the DNS Security Extensions", Work in
        Progress, May 2004.

Appendix A.  Pseudo-Pseudocode for S-NAPTR

A.1.  Finding the First (Best) Target

   Assuming the client supports 1 protocol for a particular application
   service, the following pseudocode outlines the expected process to
   find the first (best) target for the client, using S-NAPTR.

   target = [initial domain]
   naptr-done = false

   while (not naptr-done)
    {
     NAPTR-RRset = [DNSlookup of NAPTR RRs for target]
     [sort NAPTR-RRset by ORDER, and PREF within each ORDER]
     rr-done = false
     cur-rr = [first NAPTR RR]

     while (not rr-done)
        if ([SERVICE field of cur-rr contains desired application
             service and application protocol])
           rr-done = true
           target= [REPLACEMENT target of NAPTR RR]
        else
           cur-rr = [next rr in list]

        if (not empty [FLAG in cur-rr])
           naptr-done = true
    }

   port = -1

   if ([FLAG in cur-rr is "S"])
    {
     SRV-RRset = [DNSlookup of SRV RRs for target]
     [sort SRV-RRset based on PREF]
     target = [target of first RR of SRV-RRset]
     port = [port in first RR of SRV-RRset]
    }

   ; now, whether it was an "S" or an "A" in the NAPTR, we
   ; have the target for an A record lookup

   host = [DNSlookup of target]

   return (host, port)

A.2.  Finding Subsequent Targets

   The pseudocode in Appendix A is crafted to find the first, most
   preferred host-port pair for a particular application service and
   protocol.  If, for any reason, that host-port pair did not work
   (connection refused, application-level error), the client is expected
   to try the next host-port in the S-NAPTR tree.

   The pseudocode above does not permit retries -- once complete, it
   sheds all context of where in the S-NAPTR tree it finished.
   Therefore, client software writers could

   o  entwine the application-specific protocol with the DNS lookup and
      RRset processing described in the pseudocode and continue the S-
      NAPTR processing if the application code fails to connect to a
      located host-port pair;

   o  use callbacks for the S-NAPTR processing; or

   o  use an S-NAPTR resolution routine that finds *all* valid servers
      for the required application service and protocol from the
      originating domain and that provides them in a sorted order for
      the application to try.

Appendix B.  Availability of Sample Code

   Sample Python code for S-NAPTR resolution is available from
   http://www.verisignlabs.com/pysnaptr-0.1.tgz

Authors' Addresses

   Leslie Daigle
   VeriSign, Inc.
   21355 Ridgetop Circle
   Dulles, VA  20166
   US

   EMail: leslie@verisignlabs.com; leslie@thinkingcat.com

   Andrew Newton
   VeriSign, Inc.
   21355 Ridgetop Circle
   Dulles, VA  20166
   US

   EMail: anewton@verisignlabs.com

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