


Network Working Group                                           C. Daboo
Internet-Draft                                                Apple Inc.
Updates: 4791,CardDAV-RFC-to-be                       September 16, 2010
(if approved)
Intended status: Standards Track
Expires: March 20, 2011


                  Locating CalDAV and CardDAV services
                       draft-daboo-srv-caldav-10

Abstract

   This specification describes how DNS SRV records, DNS TXT records and
   well-known URIs can be used together or separately to locate
   Calendaring Extensions to WebDAV (CalDAV) or vCard Extensions to
   WebDAV (CardDAV) services.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on March 20, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as



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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  CalDAV SRV Service Labels  . . . . . . . . . . . . . . . . . .  4
   4.  CalDAV and CardDAV Service TXT Records . . . . . . . . . . . .  4
   5.  CalDAV and CardDAV Service Well-Known URI  . . . . . . . . . .  5
     5.1.  Example: well-known URI redirects to actual context
           path . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  Client "Bootstrapping" Procedures  . . . . . . . . . . . . . .  5
   7.  Guidance for Service Providers . . . . . . . . . . . . . . . .  8
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
     9.1.  caldav Well-Known URI Registration . . . . . . . . . . . . 10
     9.2.  carddav Well-Known URI Registration  . . . . . . . . . . . 10
     9.3.  SRV Service Label Registration . . . . . . . . . . . . . . 10
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
     11.2. Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Change History (to be removed prior to
                publication as an RFC)  . . . . . . . . . . . . . . . 13



























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1.  Introduction

   [RFC4791] defines the CalDAV calendar access protocol, based on HTTP
   [RFC2616], for accessing calendar data stored on a server.  CalDAV
   clients need to be able to discover appropriate CalDAV servers within
   their local area network and at other domains, e.g., to minimize the
   need for end users to know specific details such as the fully
   qualified domain name (FQDN) and port number for their servers.

   [I-D.ietf-vcarddav-carddav] defines the CardDAV address book access
   protocol based on HTTP [RFC2616], for accessing contact data stored
   on a server.  As with CalDAV, clients also need to be able to
   discover CardDAV servers.

   [RFC2782] defines a DNS-based service discovery protocol that has
   been widely adopted as a means of locating particular services within
   a local area network and beyond, using DNS SRV Resource Records
   (RRs).  This has been enhanced to provide additional service meta-
   data by use of DNS TXT Resource Records as per
   [I-D.cheshire-dnsext-dns-sd].

   This specification defines new SRV service types for the CalDAV
   protocol, and gives an example of how clients can use this together
   with other protocol features to enable simple client configuration.
   SRV service types for CardDAV are already defined in Section 11 of
   [I-D.ietf-vcarddav-carddav].

   Another issue with CalDAV or CardDAV service discovery is that the
   service might not be located at the "root" URI of the HTTP server
   hosting it.  Thus a client needs to be able to determine the complete
   path component of the Request-URI to use in HTTP requests: the
   "context path".  For example, if CalDAV is implemented as a "servlet"
   in a web server "container", the servlet "context path" might be
   "/caldav/".  So the URI for the CalDAV service would be, e.g.,
   "http://caldav.example.com/caldav/" rather than
   "http://caldav.example.com/".  SRV RRs by themselves only provide a
   FQDN and port number for the service, not a path.  Since the client
   "bootstrapping" process requires initial access to the "context path"
   of the service, there needs to be a simple way for clients to also
   discover what that path is.

   This specification makes use of the "well known URI" feature
   [RFC5785] of HTTP servers to provide a well known URI for CalDAV or
   CardDAV services that clients can make use of.  The well known URI
   will point to a resource on the server that is simply a "stub"
   resource that provides a redirect to the actual "context path"
   resource representing the service endpoint.




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2.  Conventions Used in This Document

   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 [RFC2119].

3.  CalDAV SRV Service Labels

   This specification adds two SRV service labels for use with CalDAV:

   _caldav:  Identifies a CalDAV server that uses HTTP without transport
      layer security ([RFC2818]).

   _caldavs:  Identifies a CalDAV server that uses HTTP with transport
      layer security ([RFC2818]).

   Clients MUST honor "Priority" and "Weight" values in the SRV RRs, as
   described by [RFC2782].

   Example: service record for server without transport layer security

       _caldav._tcp     SRV 0 1 80 calendar.example.com.

   Example: service record for server with transport layer security

       _caldavs._tcp    SRV 0 1 443 calendar.example.com.

4.  CalDAV and CardDAV Service TXT Records

   When SRV RRs are used to advertise CalDAV and CardDAV services, it is
   also convenient to be able to specify a "context path" in the DNS to
   be retrieved at the same time.  To enable that, this specification
   uses a TXT RR that follows the syntax defined in Section 6 of
   [I-D.cheshire-dnsext-dns-sd] and defines a "path" key for use in that
   record.  The value of the key MUST be the actual "context path" to
   the corresponding service on the server.

   A site might provide TXT records in addition to SRV records for each
   service.  When present, clients MUST use the "path" value as the
   "context path" for the service in HTTP requests.  When not present,
   clients use the ".well-known" URI approach described next.

   Example: text record for service with transport layer security

       _caldavs._tcp    TXT path=/caldav






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5.  CalDAV and CardDAV Service Well-Known URI

   Two ".well-known" URIs are registered by this specification for
   CalDAV and CardDAV services, "caldav" and "carddav" respectively (see
   Section 9).  These URIs point to a resource that the client can use
   as the initial "context path" for the service they are trying to
   connect to.  The server MUST redirect HTTP requests for that resource
   to the actual "context path" using one of the available mechanisms
   provided by HTTP (e.g., using a 301, 303, 307 response).  Clients
   MUST handle HTTP redirects on the ".well-known" URI.  Servers MUST
   NOT locate the actual CalDAV or CardDAV service endpoint at the
   ".well-known" URI as per Section 1.1 of [RFC5785].

   Servers SHOULD set an appropriate Cache-Control header value (as per
   Section 14.9 of [RFC2616]) in the redirect response to ensure caching
   occurs or does not occur as needed, or as required by the type of
   response generated.  For example, if it is anticipated that the
   location of the redirect might change over time, then a "no-cache"
   value would be used.

   To facilitate "context path's" that might differ from user to user,
   the server MAY require authentication when a client tries to access
   the ".well-known" URI (i.e., the server would return a 401 status
   response to the unauthenticated request from the client, then return
   the redirect response only after a successful authentication by the
   client).

5.1.  Example: well-known URI redirects to actual context path

   A CalDAV server has a "context path" that is "/servlet/caldav".  The
   client will use "/.well-known/caldav" as the path for its
   "bootstrapping" process after it has first found the FQDN and port
   number via an SRV lookup or via manual entry of information by the
   user which the client can parse suitable information from.  When the
   client makes an HTTP request against "/.well-known/caldav", the
   server would issue an HTTP redirect response with a Location response
   header using the path "/servlet/caldav".  The client would then
   "follow" this redirect to the new resource and continue making HTTP
   requests there to complete its "bootstrapping" process.

6.  Client "Bootstrapping" Procedures

   This section describes a procedure that CalDAV or CardDAV clients
   SHOULD use to do their initial configuration based on minimal user
   input.  The goal is to determine an http: or https: URI that
   describes the full path to the user's principal-URL [RFC3744].





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   1.  Processing user input:

       *  For a CalDAV server:

          +  Minimal input from a user would consist of a calendar user
             address and a password.  A calendar user address is defined
             by iCalendar [RFC5545] to be a URI [RFC3986].  Provided a
             user identifier and a domain name can be extracted from the
             URI, this simple "bootstrap" configuration can be done.

          +  If the calendar user address is a "mailto:" [RFC2368] URI,
             the "mailbox" portion of the URI is examined and the
             "local-part" and "domain" portions extracted.

          +  If the calendar user address is an "http:" [RFC2616] or
             "https:" [RFC2818] URI, the "userinfo" and "host" portion
             of the URI [RFC3986] is extracted.

       *  For a CardDAV server:

          +  Minimal input from a user would consist of their email
             address [RFC5322] for the domain where the CardDAV service
             is hosted, and a password.  The "mailbox" portion of the
             email address is examined and the "local-part" and "domain"
             portions extracted.

   2.  Determination of service FQDN and port number:

       *  An SRV lookup for _caldavs._tcp (for CalDAV) or _carddavs._tcp
          (for CardDAV) is done with the extracted "domain" as the
          service domain.

       *  If no result is found, the client can try _caldav._tcp (for
          CalDAV) or _carddav._tcp (for CardDAV) provided non-SSL
          connections are appropriate.

       *  If an SRV record is returned, the client extracts the target
          FQDN and port number.  In the case of multiple SRV records
          returned, the client MUST use the priority and weight fields
          in the record to determine which one to pick (as per
          [RFC2782]).

       *  If an SRV record is not found, the client will need to prompt
          the user to enter the FQDN and port number information
          directly, or use some other heuristic, for example using the
          extracted "domain" as the FQDN and default HTTPS or HTTP port
          numbers.  In this situation clients MUST first attempt an HTTP
          connection with transport layer security.



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   3.  Determination of initial "context path":

       *  When an SRV lookup is done and a valid SRV record returned,
          the client MUST also query for a corresponding TXT record and
          check for the presence of a "path" key in its response.  If
          present, the value of the "path" key is used for the initial
          "context path".

       *  When an initial "context path" has not been determined from a
          TXT record, the initial "context path" is taken to be "/.well-
          known/caldav" (for CalDAV) or "/.well-known/carddav" (for
          CardDAV).

       *  If the initial "context path" derived from a TXT record
          generates HTTP errors when targeted by requests, the client
          SHOULD repeat its bootstrap procedure using the appropriate
          ".well-known" URI instead.

   4.  Determination of user identifier:

       *  The client will need to make authenticated HTTP requests to
          the service.  Typically a "user identifier" is required for
          some form of user/password authentication.  When a user
          identifier is required, clients MUST first use the "mailbox"
          portion of the calendar user address provided by the user in
          the case of a "mailto:" address, and if that results in an
          authentication failure, SHOULD fall back to using the "local-
          part" extracted from the "mailto:" address.  For an "http:" or
          "https:" calendar user address, the "userinfo" portion is used
          as the user identifier for authentication.  This is in line
          with the guidance outlined in Section 7.  If these user
          identifiers result in authentication failure, the client
          SHOULD prompt the user for a valid identifier.

   5.  Connecting to the service:

       *  Subsequent to configuration, the client will make HTTP
          requests to the service.  When using "_caldavs" or "_carddavs"
          services, a transport layer security negotiation is done
          immediately upon connection.  The client MUST do certificate
          verification using the procedure outlined in Section 4 of
          [I-D.saintandre-tls-server-id-check] in regard to verification
          with an SRV RR as the starting point.

       *  The client does a "PROPFIND" [RFC4918] request with the
          request URI set to the initial "context path".  The body of
          the request SHOULD include the DAV:current-user-principal
          [RFC5397] property as one of the properties to return.  Note



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          that clients MUST properly handle HTTP redirect responses for
          the request.  The server will use the HTTP authentication
          procedure outlined in [RFC2617] or use some other appropriate
          authentication schemes to authenticate the user.

       *  If the server returns a 404 Not Found HTTP status response to
          the request on the initial "context path", clients MAY try
          repeating the request on the "root" URI "/" or prompt the user
          for a suitable path.

       *  If the DAV:current-user-principal property is returned on the
          request, the client uses that value for the principal-URL of
          the authenticated user.  With that, it can execute a
          "PROPFIND" request on the principal-URL and discover
          additional properties for configuration (e.g., calendar or
          address book "home" collections).

       *  If the DAV:current-user-principal property is not returned,
          then the client will need to request the principal-URL path
          from the user in order to continue with configuration.

   Once a successful account discovery step has been done, clients
   SHOULD cache the service details that were successfully used (user
   identity, principal-URL with full scheme/host/port details), and re-
   use those when connecting again at a later time.

   If a subsequent connection attempt fails, or authentication fails
   persistently, clients SHOULD re-try the SRV lookup and account
   discovery to "refresh" the cached data.

7.  Guidance for Service Providers

   Service providers wanting to offer CalDAV or CardDAV services that
   can be configured by clients using SRV records need to follow certain
   procedures to ensure proper operation.

   o  CalDAV or CardDAV servers SHOULD be configured to allow
      authentication with calendar user addresses (just taking the
      "mailbox" portion of any "mailto:" URI) or email addresses
      respectively, or "user identifiers" extracted from them.  In the
      former case, the addresses MUST NOT conflict with other forms of
      permitted user login name.  In the latter case, the extracted
      "user identifiers" need to be unique across the server and MUST
      NOT conflict with any login name on the server.

   o  Servers MUST force authentication for "PROPFIND" requests that
      retrieve the DAV:current-user-principal property to ensure that
      the value of the DAV:current-user-principal property returned



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      corresponds to the principal-URL of the user making the request.

   o  If the service provider uses transport layer security, the service
      provider MUST ensure a certificate is installed that can be
      verified by clients using the procedure outlined in Section 4 of
      [I-D.saintandre-tls-server-id-check] in regard to verification
      with an SRV RR as the starting point.

   o  Install the appropriate SRV records for the offered services.
      Optionally include TXT records.

8.  Security Considerations

   Clients that support transport layer security as defined by [RFC2818]
   SHOULD try the "_caldavs" or "_carddavs" services first before trying
   the "_caldav" or "_carddav" services respectively.  If a user has
   explicitly requested a connection with transport layer security, the
   client MUST NOT use any service information returned for the
   "_caldav" or "_carddav" services.  Clients MUST follow the
   certificate verification process specified in
   [I-D.saintandre-tls-server-id-check].

   A malicious attacker with access to the DNS server data, or able to
   get spoofed answers cached in a recursive resolver, can potentially
   cause clients to connect to any server chosen by the attacker.  In
   the absence of a secure DNS option, clients SHOULD check that the
   target FQDN returned in the SRV record matches the original service
   domain that was queried.  If the target FQDN is not in the queried
   domain, clients SHOULD verify with the user that the SRV target FQDN
   is suitable for use before executing any connections to the host.
   Alternatively, if transport layer security is being used for the
   service, clients MUST use the procedure outlined in Section 4 of
   [I-D.saintandre-tls-server-id-check] to verify the service.

   Implementations of TLS [RFC5246], used as the basis for transport
   layer security ([RFC2818]), typically support multiple versions of
   the protocol as well as the older Secure Sockets Layer (SSL)
   protocol.  Because of known security vulnerabilities, clients and
   servers MUST NOT request, offer, or use SSL 2.0.  See Appendix E.2 of
   [RFC5246] for further details.

9.  IANA Considerations

   This document defines two ".well-known" URIs using the registration
   procedure and template from Section 5.1 of [RFC5785].






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9.1.  caldav Well-Known URI Registration

   URI suffix:  caldav

   Change controller:  IETF.

   Specification document(s):  This RFC.

   Related information:  See also [RFC4791].

9.2.  carddav Well-Known URI Registration

   URI suffix:  carddav

   Change controller:  IETF.

   Specification document(s):  This RFC.

   Related information:  See also [I-D.ietf-vcarddav-carddav].

9.3.  SRV Service Label Registration

   Service labels have been registered according to
   <http://www.dns-sd.org/ServiceTypes.html> [1] and will be
   incorporated into IANA once a new registry is available there.

10.  Acknowledgments

   This specification was suggested by discussion that took place within
   the Calendaring and Scheduling Consortium's CalDAV Technical
   Committee.  The author thanks the following for their contributions:
   Stuart Cheshire, Bernard Desruisseaux, Eran Hammer-Lahav, Helge Hess,
   Arnaud Quillaud, Wilfredo Sanchez, and Joe Touch.

11.  References

11.1.  Normative References

   [I-D.cheshire-dnsext-dns-sd]          Cheshire, S. and M. Krochmal,
                                         "DNS-Based Service Discovery",
                                         draft-cheshire-dnsext-dns-sd-06
                                         (work in progress), March 2010.

   [I-D.ietf-vcarddav-carddav]           Daboo, C., "vCard Extensions to
                                         WebDAV (CardDAV)",
                                         draft-ietf-vcarddav-carddav-10
                                         (work in progress),
                                         November 2009.



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   [I-D.saintandre-tls-server-id-check]  Saint-Andre, P. and J. Hodges,
                                         "Representation and
                                         Verification of Domain-Based
                                         Application Service Identity in
                                         Certificates Used with
                                         Transport Layer Security", draf
                                         t-saintandre-tls-server-id-
                                         check-09 (work in progress),
                                         August 2010.

   [RFC2119]                             Bradner, S., "Key words for use
                                         in RFCs to Indicate Requirement
                                         Levels", BCP 14, RFC 2119,
                                         March 1997.

   [RFC2368]                             Hoffman, P., Masinter, L., and
                                         J. Zawinski, "The mailto URL
                                         scheme", RFC 2368, July 1998.

   [RFC2616]                             Fielding, R., Gettys, J.,
                                         Mogul, J., Frystyk, H.,
                                         Masinter, L., Leach, P., and T.
                                         Berners-Lee, "Hypertext
                                         Transfer Protocol -- HTTP/1.1",
                                         RFC 2616, June 1999.

   [RFC2617]                             Franks, J., Hallam-Baker, P.,
                                         Hostetler, J., Lawrence, S.,
                                         Leach, P., Luotonen, A., and L.
                                         Stewart, "HTTP Authentication:
                                         Basic and Digest Access
                                         Authentication", RFC 2617,
                                         June 1999.

   [RFC2782]                             Gulbrandsen, A., Vixie, P., and
                                         L. Esibov, "A DNS RR for
                                         specifying the location of
                                         services (DNS SRV)", RFC 2782,
                                         February 2000.

   [RFC2818]                             Rescorla, E., "HTTP Over TLS",
                                         RFC 2818, May 2000.

   [RFC3744]                             Clemm, G., Reschke, J., Sedlar,
                                         E., and J. Whitehead, "Web
                                         Distributed Authoring and
                                         Versioning (WebDAV)
                                         Access Control Protocol",



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                                         RFC 3744, May 2004.

   [RFC3986]                             Berners-Lee, T., Fielding, R.,
                                         and L. Masinter, "Uniform
                                         Resource Identifier (URI):
                                         Generic Syntax", STD 66,
                                         RFC 3986, January 2005.

   [RFC4791]                             Daboo, C., Desruisseaux, B.,
                                         and L. Dusseault, "Calendaring
                                         Extensions to WebDAV (CalDAV)",
                                         RFC 4791, March 2007.

   [RFC4918]                             Dusseault, L., "HTTP Extensions
                                         for Web Distributed Authoring
                                         and Versioning (WebDAV)",
                                         RFC 4918, June 2007.

   [RFC5246]                             Dierks, T. and E. Rescorla,
                                         "The Transport Layer Security
                                         (TLS) Protocol Version 1.2",
                                         RFC 5246, August 2008.

   [RFC5322]                             Resnick, P., Ed., "Internet
                                         Message Format", RFC 5322,
                                         October 2008.

   [RFC5397]                             Sanchez, W. and C. Daboo,
                                         "WebDAV Current Principal
                                         Extension", RFC 5397,
                                         December 2008.

   [RFC5785]                             Nottingham, M. and E. Hammer-
                                         Lahav, "Defining Well-Known
                                         Uniform Resource Identifiers
                                         (URIs)", RFC 5785, April 2010.

11.2.  Informative References

   [RFC5545]                             Desruisseaux, B., "Internet
                                         Calendaring and Scheduling Core
                                         Object Specification
                                         (iCalendar)", RFC 5545,
                                         September 2009.

URIs

   [1]  <http://www.dns-sd.org/ServiceTypes.html>



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Appendix A.  Change History (to be removed prior to publication as an
             RFC)

   Changes in -09:

   1.  IESG Review: minor editorial changes.

   2.  GenART Review: minor editorial changes.

   3.  GenART Review: "guideline" -> "procedure".

   4.  GenART Review: "port" -> "port number".

   5.  GenART Review: added definition of "context path".

   6.  GenART Review: clarified OPTIONAL nature of suggested client
       procedure.

   7.  GenART Review: clarified that TXT lookup is an additional query.

   8.  IESG Review: now allow any HTTP redirect response, not just 301.

   9.  IESG Review: added text on cache interaction with redirect.

   Changes in -10:

   1.  AD Review: make client procedure a SHOULD.

   Changes in -08:

   1.  Clarify that email address is a valid input in Section 7 for
       CardDAV.

   2.  Clarified aspects of DAV:current-user-principal handling for
       servers.

   3.  Added additional text to indicate TXT being used in abstract and
       introduction.

   Changes in -07:

   1.  Add password to required minimal user input

   2.  Section 3 -> Section 4 of server-id check draft.

   Changes in -06:





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   1.  Last call comments: Revised title, abstract and text to indicate
       that SRV and .well-known can be done separately.

   2.  Revised IANA section to use dns-sd registry for now.

   3.  Added optional TXT RR with path key for service context path in
       the DNS

   4.  Re-organized client bootstrap to take account of TXT and to call-
       out the different "phases" involved via a numbered list.

   Changes in -05:

   1.  AD Review: Added "Updates" for 4791 and CardDAV.

   2.  AD Review: Changed SHOULD to MUST for honoring priority and
       weight.

   3.  AD Review: Added additional reference to 3986 when talking about
       userinfo/host portions of the URI.

   4.  AD Review: Changed section reference for tls-server-id-check
       draft.

   5.  AD Review: Changed should to SHOULD when describing PROPFIND
       request and made 5397 normative.

   6.  AD Review: Made 3744 and 5322 normative references.

   7.  AD Review: Added IANA SRV registration request.

   Changes in -04:

   1.  Added addition text to client guidelines indicating that clients
       cache the discovery details and can re-do discovery if
       connections later fail.

   2.  Changed principal-URI to principal-URL.

   Changes in -03:

   1.  Updated to RFC 5785 reference.

   2.  Added SSL v2 restriction from srv-email document added after IESG
       review.

   3.  Tweaked client/server guidelines to better match HTTP challenge/
       response authentication mechanism.



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Internet-Draft          SRV for CalDAV & CardDAV          September 2010


   Changes in -02:

   1.  Re-organized introduction.

   2.  Brought terminology into line with srv-email document which has
       been through last call.

   3.  Brought security section into line with srv-email document which
       has been through last call.

   Changes in -01:

   1.  Added discovery of CardDAV service.

   2.  Now makes use of well-known URIs for the service "context path".

   3.  Updated to RFC 5545 reference.

   4.  Added reference to certificate verification spec.

Author's Address

   Cyrus Daboo
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   USA

   EMail: cyrus@daboo.name
   URI:   http://www.apple.com/





















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