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Versions: 00 01 02 03 04 draft-daigle-snaptr

Network Working Group                                          L. Daigle
Internet-Draft                                                 A. Newton
Expires: August 15, 2004                                  VeriSign, Inc.
                                                       February 15, 2004

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

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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-

   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

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   This Internet-Draft will expire on August 15, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   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 to target
   server and port, dynamically.

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.    Straightforward-NAPTR (S-NAPTR) Specification  . . . . . . .  4
   2.1   Key Terms  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.2   S-NAPTR DDDS Application Usage . . . . . . . . . . . . . . .  5
   2.2.1 Ordering and Preference  . . . . . . . . . . . . . . . . . .  5
   2.2.2 Matching and non-Matching NAPTR Records  . . . . . . . . . .  5
   2.2.3 Terminal and Non-Terminal NAPTR Records  . . . . . . . . . .  5
   2.2.4 S-NAPTR and Successive Resolution  . . . . . . . . . . . . .  6
   2.2.5 Clients Supporting Multiple Protocols  . . . . . . . . . . .  6
   3.    Guidelines . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.1   Guidelines for Application Protocol Developers . . . . . . .  7
   3.1.1 Registration of application service and protocol tags  . . .  7
   3.1.2 Definition of conditions for retry/failure . . . . . . . . .  8
   3.1.3 Server identification and handshake  . . . . . . . . . . . .  8
   3.2   Guidelines for Domain Administrators . . . . . . . . . . . .  8
   3.3   Guidelines for Client Software Writers . . . . . . . . . . .  9
   4.    Illustrations  . . . . . . . . . . . . . . . . . . . . . . .  9
   4.1   Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.2   Service Discovery within a Domain  . . . . . . . . . . . . . 10
   4.3   Multiple Protocols . . . . . . . . . . . . . . . . . . . . . 10
   4.4   Remote Hosting . . . . . . . . . . . . . . . . . . . . . . . 11
   4.5   Sets of NAPTR RRs  . . . . . . . . . . . . . . . . . . . . . 12
   4.6   Sample sequence diagram  . . . . . . . . . . . . . . . . . . 12
   5.    Motivation and Discussion  . . . . . . . . . . . . . . . . . 14
   5.1   So, why not just SRV records?  . . . . . . . . . . . . . . . 15
   5.2   So, why not just NAPTR records?  . . . . . . . . . . . . . . 15
   6.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 16
   7.    Security Considerations  . . . . . . . . . . . . . . . . . . 16
   8.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
         References . . . . . . . . . . . . . . . . . . . . . . . . . 17
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 18
   A.    Application Service Location Application of DDDS . . . . . . 18
   A.1   Application Unique String  . . . . . . . . . . . . . . . . . 18
   A.2   First Well Known Rule  . . . . . . . . . . . . . . . . . . . 18
   A.3   Expected Output  . . . . . . . . . . . . . . . . . . . . . . 18
   A.4   Flags  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   A.5   Service Parameters . . . . . . . . . . . . . . . . . . . . . 19
   A.5.1 Application Services . . . . . . . . . . . . . . . . . . . . 19
   A.5.2 Application Protocols  . . . . . . . . . . . . . . . . . . . 20
   A.6   Valid Rules  . . . . . . . . . . . . . . . . . . . . . . . . 20
   A.7   Valid Databases  . . . . . . . . . . . . . . . . . . . . . . 20
   B.    Pseudo pseudocode for S-NAPTR  . . . . . . . . . . . . . . . 20
   B.1   Finding the first (best) target  . . . . . . . . . . . . . . 20
   B.2   Finding subsequent targets . . . . . . . . . . . . . . . . . 21
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 23

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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 [6]) Application to
   map domain name, application service name, and application protocol
   to target server and port, dynamically.

   As discussed in Section 5, existing approaches to using DNS records
   to 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 using both
   NAPTR [7] and SRV ([5]) 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",
   document are to be interpreted as described in RFC2119 ([2]).

2. Straightforward-NAPTR (S-NAPTR) Specification

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

2.1 Key Terms

   An "application service" is a generic term for some type of
   application, indpendent of the protocol that may be used to offer it.
   Each application service will be associated with an IANA-registered
   tag.  For example, instant messaging is a type of application
   service, which can be implemented by many different application-layer
   protocols, and the tag "IM" (used as an illustration here) could be
   registered for it.

   An "application protocol" is used to implement the application
   service.  These are also associated with IANA-registered tags.  In
   the case where 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., "IM:ProtC") is sufficient for a client to identify a server
   with which it can communicate.

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   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", etc).

2.2 S-NAPTR DDDS Application Usage

   As outlined in Appendix A, 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.

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

2.2.1 Ordering and Preference

   A client retrieves all of 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, that 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

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   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 NAPTR RR 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 to terminal lookups and servers contacted.
   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) fail 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

   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 application client MUST select
   one protocol to choose The PREF field of the NAPTR RRs may be used by
   the domain administrator to 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 before trying another.j It MAY

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   choose which protocol to try first based on its own preference, or
   from 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.

   That is, what the client MUST NOT do is 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 "IM"
   service with protocol "ProtB", there is no reason to believe it does
   so on behalf of example.com (since there is no such pointer in
   example.com's NAPTR RR set).

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.

   Specifically not addressed in this document is how to select the
   domain for which the service+protocol is being sought.  It is up to
   other conventions to define how that might be used (e.g., instant
   messaging standards can define what domain to use from IM URIs, how
   to step down from foobar.example.com to example.com, and so on, if
   that is applicable).

   Although this document proposes a DDDS application that does not use
   all the features of NAPTR resource records, it does not mean 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 in order to make use of S-

3.1.1 Registration of application service and protocol tags

   Application protocol developers that wish to make use of S-NAPTR must
   make provision to register any relevant application service and
   application protocol tags, as described in Section 6.

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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 (a less preferred target)?

   For example, if the client gets a "connection refused" 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 taret
   domain is always considered a failure condition.

3.1.3 Server identification and handshake

   As noted in Section 7, 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 that are "reachable" through
   the S-NAPTR process for a particular application service can be
   thought of as a "tree".  Each NAPTR RR 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

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   client, the tree could be quite deep, and retracing the tree to retry
   other targets can become expensive if the tree has many branches.


   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, etc), but don't look for
      reasons to provide more.

   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 properly understand DDDS/NAPTR, an implementor must read [6].
   However, the most important aspect to keep in mind is that, if one
   target fails to work for the application, it is expected that the
   application will 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

   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 increasingly common as new
      application services are defined.  This includes the case of
      instant messaging (a 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.

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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", because 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.  Here's a hypothetical case for storing application key or
   certificate data for a given domain.  The premise is that some
   credentials registry (CredReg) service has been defined to be a leaf
   node service holding the keys/certs for the servers operated by (or
   for) the domain.  Furthermore, 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:

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

   Note that another domain, offering the same application service,
   might offer it using a different set of application protocols:

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

4.3 Multiple Protocols

   As it stands, there are several different protocols proposed for
   offering "instant message" services.  Assuming that "IM" was
   registered as an application service, this DDDS application could be
   used to determine the available services for delivering to a target.

   Two particular features of instant messaging should be noted:

   1.  gatewaying is expected to bridge communications across protocols

   2.  instant messaging servers are likely to be operated out of a

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       different domain than the instant messaging address, and servers
       of different protocols may be offered by independent

   For example, "thinkingcat.example" may support its own servers for
   the "ProtA" instant 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 instant
   messaging service, and 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 IM services for thinkingcat.example, the NAPTR
   records for thinkingcat.example are retrieved:

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

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

    ;;    Pref Weight Port  Target
   IN SRV 10    0     10001 bigiron.example.com
   IN SRV 20    0     10001 backup.im.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 way to approach this is to have one NAPTR RR in the
   thinkingcat.example domain pointing to all the hosted services, and
   the hosting domain has NAPTR records for each service to map them to
   whatever local hosts it chooses (and may change from time to time).

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

   and 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):

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

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

4.5 Sets of NAPTR RRs

   Note that the above sections assumed that there was one service
   available (via S-NAPTR) per domain.  Often, that will not be the
   case.  Assuming thinkingcat.example had the CredReg service set up as
   described in Section 4.2 and the instant 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:

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

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

4.6 Sample sequence diagram

   Consider the example in Section 4.3.  Visually, the sequence of steps

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   required for the client to reach the final server for a "ProtB"
   service for IM for the thinkingcat.example domain is as follows:

   Client   NS for                NS for
            thinkingcat.example   example.com    backup.im.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; that 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

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

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

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   9.  the client connects to that IP address, on port 10001 (from the
        SRV record), 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

   12.  the server responds, and the rest is IM.

5. Motivation and Discussion

   Increasingly, application protocol standards are using domain names
   to identify server targets, and stipulating 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:

   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

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

   This is quite useful, but Section 5.1 outlines some of the
   limitations inherent in the approach.

   That is, while 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.  And, while
   the DDDS specification and use of NAPTR allows multiple levels of
   redirection before locating the target server machine with an SRV
   record, 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 with some potential uses of NAPTR
   records.  This is dubbed "S-NAPTR" -- a "S"traightforward use of
   NAPTR records.

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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:

   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 focused on individual server administration,
      not application naming: as stated in [5] "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
   fact 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 that is more complex than is necessary for
   most applications of SRV.

5.2 So, why not just NAPTR records?

   That's a trick question.  NAPTR records cannot appear in the wild --
   see [6].  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 -- some simple IANA registrations and it's done.

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

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

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6. IANA Considerations

   This document calls for 2 IANA registries:  one for application
   service tags, and one for application protocol tags.

   Application service and protocol tags should be defined in an RFC
   (unless the "x-" experimental form is used, in which case they are
   unregistered).  There are no restrictions placed on the tags other
   than that they must conform with the syntax defined below (Appendix
   A.5).  The IANA registries should list the tags and the RFC that
   defines their use.

7. Security Considerations

   The security of this approach to application service location is only
   as good as the security of the DNS servers 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).

   To protect against DNS-vectored attacks, 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, IMAP, etc...  define the necessary handshake mechansims
   to accomplish this task.

   The basic mechanism works in the following way:

   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

   2.  The server sends back to the client a credential with the
       appropriate name.  For X.509 certificates, the name would either
       be in the subjectDN or subjectAltName fields.  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, there is reasonable assurance that the

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       correct end point has been reached.

   It is important to note that this document does not define either the
   handshake mechanism, the specific credenential naming fields, nor the
   name matching semantics.  Definitions of S-NAPTR for particular
   application protocols MUST define these.

8. Acknowledgements

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


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

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

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

   [4]  Eastlake, D., "Domain Name System Security Extensions", RFC
        2535, March 1999.

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

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

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

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

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Authors' Addresses

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

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

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

   EMail: anewton@verisignlabs.com

Appendix A. Application Service Location Application of DDDS

   This section defines the DDDS application, as described in [6].

A.1 Application Unique String

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

A.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.

A.3 Expected Output

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

A.4 Flags

   This DDDS Application uses only 2 of the Flags defined for the
   URI/URN Resolution Application ([8]): "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

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   "S" flag means that the output of this Rule is a domain label for
   which one or more SRV [5] 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).

A.5 Service Parameters

   Service Parameters for this Application take the form of a string of
   characters that follow this ABNF ([3]):

      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  ; "+" / "-" / "."
      ; 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, just 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 ([8]).  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 [1]).  Since "+" (the separator used in the
   RFC3404 service parameter string) is an allowed character for URI
   scheme names, ":" is chosen as the separator here.

A.5.1 Application Services

   The "app-service" must be a registered service [this will be an IANA
   registry; this is not the IANA port registry, because we want to
   define services for which there is no single protocol, and we don't
   want to use up port space for nothing].

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A.5.2 Application Protocols

   The protocol identifiers that are valid for the "app-protocol"
   production are any standard, registered protocols [IANA registry
   again -- is this the list of well known/registered ports?].

A.6 Valid Rules

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

A.7 Valid Databases

   At present only one DDDS Database is specified for this Application.
   [7] specifies a DDDS Database that uses the NAPTR DNS resource record
   to 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
   that is used for the first lookup -- 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 [7] for more
   information on NAPTR records and the Additional Information section
   of a DNS response packet.

Appendix B. Pseudo pseudocode for S-NAPTR

B.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]

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      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]
            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)

B.2 Finding subsequent targets

   The pseudocode in Appendix B is crafted to find the first, most
   preferred, host-port pair for a particular application service an
   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;

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   o  use callbacks for the S-NAPTR processing;

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

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