GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc6764

Internet Engineering Task Force (IETF) C. Daboo Request for Comments: 6764 Apple Inc. Updates: 4791, 6352 February 2013 Category: Standards Track ISSN: 2070-1721

          Locating Services for Calendaring Extensions to
      WebDAV (CalDAV) and vCard Extensions to WebDAV (CardDAV)

Abstract

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

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6764.

Copyright Notice

 Copyright (c) 2013 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
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Daboo Standards Track [Page 1] RFC 6764 SRV for CalDAV & CardDAV February 2013

Table of Contents

 1. Introduction ....................................................2
 2. Conventions Used in This Document ...............................3
 3. CalDAV SRV Service Labels .......................................3
 4. CalDAV and CardDAV Service TXT Records ..........................4
 5. CalDAV and CardDAV Service Well-Known URI .......................4
    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. Well-Known URI Registrations ...............................9
         9.1.1. caldav Well-Known URI Registration .................10
         9.1.2. carddav Well-Known URI Registration ................10
    9.2. Service Name Registrations ................................10
         9.2.1. caldav Service Name Registration ...................10
         9.2.2. caldavs Service Name Registration ..................11
         9.2.3. carddav Service Name Registration ..................11
         9.2.4. carddavs Service Name Registration .................12
 10. Acknowledgments ...............................................12
 11. References ....................................................12
    11.1. Normative References .....................................12
    11.2. Informative References ...................................14

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.
 [RFC6352] 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 RRs as per [RFC6763].

Daboo Standards Track [Page 2] RFC 6764 SRV for CalDAV & CardDAV February 2013

 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
 [RFC6352].
 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 an
 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 use.  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.

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

Daboo Standards Track [Page 3] RFC 6764 SRV for CalDAV & CardDAV February 2013

 Clients MUST honor Priority and Weight values in the SRV RRs, as
 described by [RFC2782].
 Example: service record for server without TLS
     _caldav._tcp     SRV 0 1 80 calendar.example.com.
 Example: service record for server with TLS
     _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
 [RFC6763] 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 TLS
     _caldavs._tcp    TXT path=/caldav

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, or 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

Daboo Standards Track [Page 4] RFC 6764 SRV for CalDAV & CardDAV February 2013

 location of the redirect might change over time, then a "no-cache"
 value would be used.
 To facilitate "context paths" 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, from which the client can parse suitable information.  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].
 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 "bootstrapping" configuration can be done.
        +  If the calendar user address is a "mailto:" [RFC6068] URI,
           the "mailbox" portion of the URI is examined, and the
           "local-part" and "domain" portions are 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.

Daboo Standards Track [Page 5] RFC 6764 SRV for CalDAV & CardDAV February 2013

  • 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 are 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-TLS

        connections are appropriate.
  • If an SRV record is returned, the client extracts the target

FQDN and port number. If multiple SRV records are 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 TLS.
 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 "bootstrapping" procedure using the
        appropriate ".well-known" URI instead.

Daboo Standards Track [Page 6] RFC 6764 SRV for CalDAV & CardDAV February 2013

 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 TLS negotiation is done immediately upon
        connection.  The client MUST do certificate verification using
        the procedure outlined in Section 6 of [RFC6125] 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
        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).

Daboo Standards Track [Page 7] RFC 6764 SRV for CalDAV & CardDAV February 2013

  • 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 reuse
 those when connecting again at a later time.
 If a subsequent connection attempt fails, or authentication fails
 persistently, clients SHOULD retry 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 with "user identifiers" extracted from them.  In
    the former case, the addresses MUST NOT conflict with other forms
    of a 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
    corresponds to the principal-URL of the user making the request.
 o  If the service provider uses TLS, the service provider MUST ensure
    a certificate is installed that can be verified by clients using
    the procedure outlined in Section 6 of [RFC6125] in regard to
    verification with an SRV RR as the starting point.  In particular,
    certificates SHOULD include SRV-ID and DNS-ID identifiers as
    appropriate, as described in Section 8.
 o  Service providers should install the appropriate SRV records for
    the offered services and optionally include TXT records.

Daboo Standards Track [Page 8] RFC 6764 SRV for CalDAV & CardDAV February 2013

8. Security Considerations

 Clients that support TLS 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 TLS, the client MUST NOT use any service
 information returned for the "_caldav" or "_carddav" services.
 Clients MUST follow the certificate-verification process specified in
 [RFC6125].
 A malicious attacker with access to the DNS server data, or that is
 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 TLS is being used for the service,
 clients MUST use the procedure outlined in Section 6 of [RFC6125] to
 verify the service.  When the target FQDN does not match the original
 service domain that was queried, clients MUST check the SRV-ID
 identifier in the server's certificate.  If the FQDN does match,
 clients MUST check any SRV-ID identifiers in the server's certificate
 or, if no SRV-ID identifiers are present, MUST check the DNS-ID
 identifiers in the server's certificate.
 Implementations of TLS [RFC5246], used as the basis for TLS
 ([RFC2818]), typically support multiple versions of the protocol as
 well as the older SSL (Secure Sockets Layer) 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

9.1. Well-Known URI Registrations

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

Daboo Standards Track [Page 9] RFC 6764 SRV for CalDAV & CardDAV February 2013

9.1.1. caldav Well-Known URI Registration

 URI suffix:  caldav
 Change controller:  IETF
 Specification document(s):  This RFC
 Related information:  See also [RFC4791].

9.1.2. carddav Well-Known URI Registration

 URI suffix:  carddav
 Change controller:  IETF
 Specification document(s):  This RFC
 Related information:  See also [RFC6352].

9.2. Service Name Registrations

 This document registers four new service names as per [RFC6335].  Two
 are defined in this document, and two are defined in [RFC6352],
 Section 11.

9.2.1. caldav Service Name Registration

 Service Name:  caldav
 Transport Protocol(s):  TCP
 Assignee:  IESG <iesg@ietf.org>
 Contact:  IETF Chair <chair@ietf.org>
 Description:  Calendaring Extensions to WebDAV (CalDAV) - non-TLS
 Reference:  [RFC6764]
 Assignment Note:  This is an extension of the http service.  Defined
    TXT keys: path=<context path>

Daboo Standards Track [Page 10] RFC 6764 SRV for CalDAV & CardDAV February 2013

9.2.2. caldavs Service Name Registration

 Service Name:  caldavs
 Transport Protocol(s):  TCP
 Assignee:  IESG <iesg@ietf.org>
 Contact:  IETF Chair <chair@ietf.org>
 Description:  Calendaring Extensions to WebDAV (CalDAV) - over TLS
 Reference:  [RFC6764]
 Assignment Note:  This is an extension of the https service.  Defined
    TXT keys: path=<context path>

9.2.3. carddav Service Name Registration

 Service Name:  carddav
 Transport Protocol(s):  TCP
 Assignee:  IESG <iesg@ietf.org>
 Contact:  IETF Chair <chair@ietf.org>
 Description:  vCard Extensions to WebDAV (CardDAV) - non-TLS
 Reference:  [RFC6352]
 Assignment Note:  This is an extension of the http service.  Defined
    TXT keys: path=<context path>

Daboo Standards Track [Page 11] RFC 6764 SRV for CalDAV & CardDAV February 2013

9.2.4. carddavs Service Name Registration

 Service Name:  carddavs
 Transport Protocol(s):  TCP
 Assignee:  IESG <iesg@ietf.org>
 Contact:  IETF Chair <chair@ietf.org>
 Description:  vCard Extensions to WebDAV (CardDAV) - over TLS
 Reference:  [RFC6352]
 Assignment Note:  This is an extension of the https service.  Defined
    TXT keys: path=<context path>

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

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

Daboo Standards Track [Page 12] RFC 6764 SRV for CalDAV & CardDAV February 2013

 [RFC3744]  Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, "Web
            Distributed Authoring and Versioning (WebDAV)
            Access Control Protocol", 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.
 [RFC6068]  Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
            URI Scheme", RFC 6068, October 2010.
 [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
            Verification of Domain-Based Application Service Identity
            within Internet Public Key Infrastructure Using X.509
            (PKIX) Certificates in the Context of Transport Layer
            Security (TLS)", RFC 6125, March 2011.
 [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
            Cheshire, "Internet Assigned Numbers Authority (IANA)
            Procedures for the Management of the Service Name and
            Transport Protocol Port Number Registry", BCP 165,
            RFC 6335, August 2011.
 [RFC6352]  Daboo, C., "CardDAV: vCard Extensions to Web Distributed
            Authoring and Versioning (WebDAV)", RFC 6352, August 2011.
 [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
            Discovery", RFC 6763, February 2013.

Daboo Standards Track [Page 13] RFC 6764 SRV for CalDAV & CardDAV February 2013

11.2. Informative References

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

Author's Address

 Cyrus Daboo
 Apple Inc.
 1 Infinite Loop
 Cupertino, CA  95014
 USA
 EMail: cyrus@daboo.name
 URI:   http://www.apple.com/

Daboo Standards Track [Page 14]

/data/webs/external/dokuwiki/data/pages/rfc/rfc6764.txt · Last modified: 2013/02/20 20:54 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki