Network Working Group A. McMillan
Internet-Draft Morphoss
Intended status: Standards Track C. Daboo
Expires: April 16, 2013 Apple Inc.
October 15, 2012

Aggregated Service Discovery
draft-daboo-aggregated-service-discovery-01

Abstract

This specification describes how clients can discover multiple services to configure themselves with a minimum of user-provided information, as short as possible sequence of queries and with a minimum of overhead for administrators of the services.

Status of This Memo

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

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Table of Contents

1. Introduction

There are currently various systems in place for discovery and configuration of individual protocols, but the process can often require an extensive series of requests using different protocols to discover all of the details needed to set up the various client services which an individual might use to interact with an organisation or service provider.

Consider Jason, a new employee at Example Enterprises. Jason needs to configure his e-mail program to use IMAP [RFC3501] + TLS on port 143 against mail.example.com, he needs to send mail on port 8557 via TLS+SMTP to smtp.example.com, his calendar is on port 8443 at https://caldav.example.com:8443/calendar/, and so forth. Some of these things can be discovered relatively easily, with a combination of DNS queries (including SRV lookups, certificate checking, and http requests). However, each protocol has its own requirements and settings and each has to be done separately. Whilst the client can "hide" the multiple service setup from the user, the actual implementation often requires separate code and processes to manage, making it more complex that it needs to be.

This specification defines a single protocol which will allows for discovery of a variety of services in a single call, allowing developers to simplify the coding and user interface in client software, and in particular in multi-function client software such as a combined e-mail and calendar client.

2. Open Issues

  1. XML vs JSON for the document format
  2. Support custom service attributes without the need for formal registration? If so, would we need a "critical" attribute to indicate ones that must be understood?
  3. Is it OK to embed certificate details for the actual services or a root certificate?
  4. Do we want to support delegation of service information to another service discovery document? That might be useful in cases where different services at the same domain are under the control of different "authorities".
  5. Should we define a local area network discovery mechanism? i.e., client connects to local network and immediately sees a set of services it could configure for the user.
  6. Should we specify whether clients should re-check account information on a regular basis for updates, or should we rely on in-protocol account redirection?

3. 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].

The namespace "urn:ietf:params:xml:ns:servicediscovery" is reserved for the XML elements defined in this specification. XML elements defined by individual implementations MUST NOT use the "urn:ietf:params:xml:ns:caldav" namespace, and instead should use a namespace that they control.

4. Overview

The following outlines the steps a client carries out to setup multiple services for a user:

  1. The client software is expected to capture a user identifier and domain name (possibly entered in the form of an email address) from the user, and possibly a password. e.g., 'cyrus@example.com'.
  2. The client would make an initial DNS SRV [RFC2782] query for '_servicediscovery._tcp.example.com'. The result of the SRV lookup will be a hostname that is then used in place of the user supplied domain name for the next steps. If the SRV lookup is unsuccessful, then the user supplied domain name is used for the next steps.
  3. The client then makes an HTTP GET request [RFC2616] against the server, using TLS [RFC2818], requesting the URL 'https://{domain name}/.well-known/servicediscovery?id={user identifier}', where '{domain name}' is the host name determined from step #2, and '{user identifier}' is the user supplied identifier from step #1. The client will follow any redirects and respond to any authentication challenges. Where the user did not provide an appropriate authentication token in the first step the client software will prompt for it at this point
  4. The client will receive an XML document in step #3 conforming to the format described in Section 5. The client parses this document to extract information about the available services. At that point it can either present a list of services to the user so that they can decide exactly what they want setup, or it can automatically setup services for all those it supports.

5. Aggregated Service Discovery Document Format

The aggregated service discovery document is an XML document. The document contains two groups of information: overall service provider information (e.g., name, icon "badge", contact information), and a list of each service supported. Each service will contain some information common to each type of service, and then information specific to each service.

The XML DTD is defined in Appendix Appendix B. Each element is described below. An example of such an aggregated service discovery document for some common services is shown in Appendix Appendix C.

5.1. SD:servicediscovery

The root element for the document

5.1.1. SD:provider

Contains information describing the service provider, that can be used by clients to "group" individual services together under a common name or section when presenting details to the user.

5.1.1.1. SD:name

The name for the service provider.

5.1.1.2. SD:description

The description of the service provider.

5.1.1.3. SD:image

An image that can be used as an "icon" for the service provider. The image data MUST be base64 encoded. The image SHOULD NOT exceed a size of 128 x 128 pixels.

5.1.1.4. SD:contact

Contact information for the service provider.

5.1.1.4.1. SD:email

An email address that can be used to contact the service provider.

5.1.1.4.2. SD:uri

A URI for a webpage providing information about the service provider.

5.1.1.5. SD:manage

A URI for a webpage where a user can manage details of their account. e.g., a place where users can go to add additional (possibly payment required) services.

5.1.2. SD:entry

Provides detail for a specific service

5.1.2.1. SD:service

The service type. This MUST be an IANA registered service type. The service type will determine what additional information is present within the enclosing SD:entry element.

5.1.2.2. SD:application

Identifies the nature of the service to allow similar services to be grouped together. This is used in conjunction with the SD:priority element to group services of different types so that only one of the services within the group will be configured. For example, the IMAP [RFC3501] and POP3 [RFC1939] protocols will have SD:service values of 'imap' and 'pop3' respectively, and they could be given the same SD:application value of 'mailstore'. If the service provider prefers its users to use IMAP over POP3 (assuming the client supports IMAP), then the SD:priority value for the IMAP service would have a higher value than that for the POP3 service.

5.1.2.3. SD:priority

A positive, non-zero integer value that is used by the service provider to indicate a preference for one particular service over another within the same application group. Multiple services within a group may share the same priority, indicating that the service provider does not wish to express a preference. Services with a higher numbered priority are to be preferred over lower numbered ones.

5.1.2.4. SD:host

The hostname of the server providing the service.

5.1.2.5. SD:port

The network port number of the server providing the service.

5.1.2.6. SD:tls

Provides detail of transport layer security to be used with the service.

5.1.2.6.1. SD:required

If present, indicates that clients MUST use transport layer security when connecting to the server providing the service.

5.1.2.6.2. SD:certificate

TODO: not sure we should have this as opposed to relying on normal certificate verification for each service. If present, indicates details about TLS certificates that the server will present to the client during TLS negotiation. Clients can use these certificate details to "short circuit" certificate verification for the service.

5.1.2.7. SD:auth

Provides detail of authentication to be used with the service.

TODO element details

6. Finding the Aggregated Service Discovery Information

A ".well-known" URI is registered by this specification: "servicediscovery" (see Section 10). This URI points to a resource that the client can use to retrieve the aggregated service discovery document for the site. Clients MUST handle HTTP redirects on the ".well-known" URI. Clients MUST handle HTTP authentication on the ".well-known" URI.

When requesting the document clients MUST include a URI query parameter "id" set to the user identifier entered by the user. When responding to the request, the server MUST tailor the aggregated service discovery document for the user making the request and MUST require HTTP authentication by that user before returning the document.

7. Handling multiple, alternative services

The SD:application and SD:priority elements provide a way for a service provider to distinguish multiple services of the same type, as well as allow the client to select the most appropriate service when several alternatives exist.

For example, consider the case of a service provider supporting two separate email retrieval services, one the "primary" account, and the other for "internal" messaging only. It is expected that clients configure accounts for both services. Each service also offers either IMAP [RFC3501] or POP3 [RFC1939] as an email retrieval protocol. In this case the aggregated service discovery document would contain four SD:entry elements: two describing an IMAP service and two describing a POP3 service. Each entry would contain an SD:application element that groups one IMAP and one POP3 service together for each of the "primary" and "internal" account groups. Each entry would also contain an SD:priority element indicating the service providers preference for clients to use either IMAP or POP3. An example of such an aggregated service discovery document is shown in Appendix Appendix D.

When a client retrieves and processes such a document, it would first group services based on the SD:application value. For each group, it iterates over the list of entries in the group, ordered by SD:priority values, and configures an account for the first one it finds with an SD:service that it supports.

8. Internationalization Considerations

Some elements of the service discovery document can contain human readable text that clients might choose to present to a user. Clients SHOULD use the Accept-Language header behavior described in Section 14.4 of [RFC2616] to ensure the server can return a document suitable for the user's chosen language. Servers SHOULD support variations of the service discovery document based on language, returning the appropriate variation in response to client requests. When doing so the xml:lang attribute SHOULD be included on all XML elements in the document that have been localized.

9. Security Considerations

When using an SRV lookup to discover a server hosting the service discovery document, 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 a server hosting a bogus service discovery document with service data 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.

HTTP requests for the service-discovery document MUST be performed via TLS. Clients MUST use the procedure outlined in Section 4.3 of [RFC6125] to verify the service.

10. IANA Considerations

10.1. Namespace Registration

Registration request for the aggregated service discovery namespace:

10.2. Media Type

This section defines the MIME media type for use with the aggregated service discovery XML data.

Type name:
application
Subtype name:
servicediscovery+xml
Required parameters:
none
Optional parameters:
charset as defined for application/xml in [RFC3023]; per [RFC3023], use of the charset property parameter with the value "utf-8" is "STRONGLY RECOMMENDED"
Encoding considerations:
Same as encoding considerations of application/xml as specified in [RFC3023]
Security considerations:
See Section 9.
Interoperability considerations:
This media type provides a format for aggregated service discovery information based on XML.
Published specification:
This specification.
Applications which use this media type:
Applications that configure services.
Additional information:
Magic number(s):
None
File extension(s):
None
Macintosh file type code(s):
None specified.

Person & email address to contact for further information:
IESG
Intended usage:
COMMON
Restrictions on usage:
There are no restrictions on where this media type can be used.
Author:
See the "Author's Address" section of this document.
Change controller:
IETF

10.3. Well-Known URI Registration

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

10.3.1. servicediscovery Well-Known URI Registration

URI suffix:
servicediscovery
Change controller:
IETF.
Specification document(s):
This RFC.
Related information:
None.

11. Acknowledgments

12. References

12.1. 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.
[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.
[RFC3023] Murata, M., St. Laurent, S. and D. Kohn, "XML Media Types", RFC 3023, January 2001.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known Uniform Resource Identifiers (URIs)", RFC 5785, April 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.

12.2. Informative References

[RFC1939] Myers, J.G. and M.T. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003.

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

Changes in -01:

  1. Renamed various elements for clarity.
  2. Added an SD:manage element.
  3. Added a section on handling of multiple, alternative services, together with a second appendix example.

Appendix B. Aggregated Service Discovery Schema

<!ELEMENT servicediscovery (provider?, entry*) >

  <!ELEMENT provider (name?, description?, image?,
                      contact?, manage?) >

    <!ELEMENT name (#PCDATA) >

    <!ELEMENT description (#PCDATA) >

    <!ELEMENT image (#PCDATA) >

    <!ELEMENT contact (email?, uri?) >

      <!ELEMENT email (#PCDATA) >
      <!ELEMENT uri (#PCDATA) >

    <!ELEMENT manage (#PCDATA) >

  <!ELEMENT entry (service, (application, priority)?,
                      host?, port?, tls?, auth?, ANY) >

    <!ELEMENT service (#PCDATA) >

    <!ELEMENT application (#PCDATA) >

    <!ELEMENT priority (#PCDATA) >

    <!ELEMENT host (#PCDATA) >

    <!ELEMENT port (#PCDATA) >

    <!ELEMENT tls (required?, certificate*) >
      <!ELEMENT required EMPTY >
      <!ELEMENT certificate (data|id|hash) >
        <!ELEMENT data (#PCDATA) >
        <!ELEMENT id (#PCDATA) >
        <!ELEMENT hash (#PCDATA) >

    <!ELEMENT auth (type, details?) >
        <!ELEMENT type (#PCDATA) >
        <!ELEMENT details (userpswd|ANY) >
          <!ELEMENT userpswd (userid?, password?) >
            <!ELEMENT userid (#PCDATA) >
            <!ELEMENT password (#PCDATA) >
        

    <!ELEMENT principal-URL (#PCDATA) >
  

Appendix C. Example of multiple services

GET /.well-known/servicediscovery?id=cyrus@example.com HTTP/1.1
Host:example.com:443
Authorization: basic QmFzZTY0IGlzIGVhc3kgdG8gZGVjb2Rl
Content-Type: application/servicediscovery+xml
Content-Length: xxx

<?xml version="1.0" standalone="no"?>
<servicediscovery xmlns="urn:ietf:params:xml:ns:servicediscovery">
  <provider>
    <name>Super-duper ISP</name>
    <description>
      Super-duper ISP is the home for all your data.
    </description>
    <contact>
      <email>superduper@example.com</email>
      <uri>http://www.example.com</uri>
    </contact>
    <manage>http://www.example.com/myaccount.html</manage>
  </provider>
  
  <entry>
    <service>imap</service>
    <application>mail-access</application>
    <priority>2</priority>
    <host>imap.example.com</host>
    <port>143</port>
    <tls>
      <required />
    </tls>
    <auth>
      <type>CRAM-MD5</type>
    </auth>
  </entry>

  <entry>
    <service>pop3</service>
    <application>mail-access</application>
    <priority>1</priority>
    <host>pop.example.com</host>
    <port>110</port>
    <tls>
      <required />
    </tls>
    <auth>
      <type>CRAM-MD5</type>
    </auth>
  </entry>

  <entry>
    <service>submission</service>
    <host>mail.example.com</host>
    <port>587</port>
    <tls>
      <required />
    </tls>
    <auth>
      <type>CRAM-MD5</type>
    </auth>
  </entry>

  <entry>
    <service>caldav</service>
    <host>calendar.example.com</host>
    <port>443</port>
    <tls>
      <required />
    </tls>
    <auth>
      <type>Digest</type>
    </auth>
    <principal-URL>https://calendar.example.com/principals/
users/cyrus</principal-URL>
  </entry>

  <entry>
    <service>carddav</service>
    <host>contacts.example.com</host>
    <port>443</port>
    <tls>
      <required />
    </tls>
    <auth>
      <type>Digest</type>
    </auth>
    <principal-URL>https://contacts.example.com/principals/
users/cyrus</principal-URL>
  </entry>

</servicediscovery>

Appendix D. Example - multiple, alternative mail retrieval services

GET /.well-known/servicediscovery?id=cyrus@example.com HTTP/1.1
Host:example.com:443
Authorization: basic QmFzZTY0IGlzIGVhc3kgdG8gZGVjb2Rl
Content-Type: application/servicediscovery+xml
Content-Length: xxx

<?xml version="1.0" standalone="no"?>
<servicediscovery xmlns="urn:ietf:params:xml:ns:servicediscovery">
  <provider>
    <name>Mail Agrregator ISP</name>
    <description>
      Primary and internal email services.
    </description>
    <contact>
      <email>emails@example.com</email>
      <uri>http://www.example.com</uri>
    </contact>
    <manage>http://www.example.com/myaccount.html</manage>
  </provider>
  
  <entry>
    <service>imap</service>
    <application>primary email</application>
    <priority>2</priority>
    <host>imap.example.com</host>
    <port>143</port>
    <tls>
      <required />
    </tls>
    <auth>
      <type>CRAM-MD5</type>
    </auth>
  </entry>

  <entry>
    <service>pop3</service>
    <application>primary email</application>
    <priority>1</priority>
    <host>pop.example.com</host>
    <port>110</port>
    <tls>
      <required />
    </tls>
    <auth>
      <type>CRAM-MD5</type>
    </auth>
  </entry>

  <entry>
    <service>imap</service>
    <application>internal email</application>
    <priority>2</priority>
    <host>imap.example.com</host>
    <port>143</port>
    <tls>
      <required />
    </tls>
    <auth>
      <type>CRAM-MD5</type>
    </auth>
  </entry>

  <entry>
    <service>pop3</service>
    <application>internal email</application>
    <priority>1</priority>
    <host>pop.example.com</host>
    <port>110</port>
    <tls>
      <required />
    </tls>
    <auth>
      <type>CRAM-MD5</type>
    </auth>
  </entry>

</servicediscovery>

Authors' Addresses

Andrew McMillan Morphoss Ltd 6 Karoro Place Porirua, 5024 New Zealand EMail: andrew@morphoss.com URI: http://www.morphoss.com/
Cyrus Daboo Apple Inc. 1 Infinite Loop Cupertino, CA 95014 USA EMail: cyrus@daboo.name URI: http://www.apple.com/