Internet Engineering Task Force (IETF)                        A.B. Roach
Request for Comments: 5989                                       Tekelec
Category: Standards Track                                   October 2010
ISSN: 2070-1721

A SIP Event Package for Subscribing to Changes to an HTTP Resource

Abstract

   The Session Initiation Protocol (SIP) is increasingly being used in
   systems that are tightly coupled with Hypertext Transport Protocol
   (HTTP) servers for a variety of reasons.  In many of these cases,
   applications can benefit from being able to discover, in near real-
   time, when a specific HTTP resource is created, changed, or deleted.
   This document proposes a mechanism, based on the SIP Event Framework,
   for doing so.

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/rfc5989.

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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   (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.

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. Associating Monitoring SIP URIs with HTTP URLs ..................3
      3.1. Monitoring a Single HTTP Resource ..........................4
      3.2. Monitoring Multiple HTTP Resources .........................5
   4. HTTP Change Event Package .......................................6
      4.1. Event Package Name .........................................6
      4.2. Event Package Parameters ...................................6
      4.3. SUBSCRIBE Bodies ...........................................7
      4.4. Subscription Duration ......................................7
      4.5. NOTIFY Bodies ..............................................8
           4.5.1. Use of message/http in HTTP Monitor Event Package ...8
      4.6. Notifier Processing of SUBSCRIBE Requests ..................9
      4.7. Notifier Generation of NOTIFY Requests .....................9
      4.8. Subscriber Processing of NOTIFY Requests ...................9
      4.9. Handling of Forked Requests ...............................10
      4.10. Rate of Notifications ....................................10
      4.11. State Agents .............................................10
   5. Example Message Flow ...........................................10
   6. Security Considerations ........................................14
   7. IANA Considerations ............................................15
      7.1. New Link Relations ........................................15
           7.1.1. New Link Relation: monitor .........................15
           7.1.2. New Link Relation: monitor-group ...................16
      7.2. New SIP Event Package: http-monitor .......................16
      7.3. New Event Header Field Parameter: body ....................16
   8. Acknowledgements ...............................................16
   9. References .....................................................17
      9.1. Normative References ......................................17
      9.2. Informative References ....................................18
   Appendix A.  Rationale: Other Approaches Considered ...............19

1. Introduction

   The Session Initiation Protocol (SIP) [3] is increasingly being used
   in systems that are tightly coupled with Hypertext Transport Protocol
   (HTTP) [2] servers for a variety of reasons.  In many of these cases,
   applications can benefit from learning of changes to specified HTTP
   resources in near real-time.  For example, user agent terminals may
   elect to store service-related data in an HTTP tree.  When such
   configuration information is stored and retrieved using HTTP, clients
   may need to be informed when information changes, so as to make
   appropriate changes to their local behavior and user interface.

   This document defines a mechanism, based on the SIP Event Framework
   [4], for subscribing to changes in the resource referenced by an HTTP
   server.  Such subscriptions do not necessarily carry the content
   associated with the resource.  In the cases that the content is not
   conveyed, the HTTP protocol is still used to transfer the contents of
   HTTP resources.  This document further defines a mechanism by which
   the proper SIP and/or Session Initiation Protocol Secure (SIPS) URI
   to be used for such subscriptions can be determined from the HTTP
   server.

2. Terminology

   The capitalized terms "MUST", "SHOULD", "MAY", "SHOULD NOT", and
   "MUST NOT" in this document are to be interpreted as described in RFC
   2119 [1].

   Note that this document discusses both SIP messages and HTTP
   messages.  Because SIP's syntax was heavily based on HTTP's, the
   components of these messages have similar or identical names.  When
   referring to message payloads, HTTP documents have historically
   preferred the hyphenated form "message-body", while SIP documents
   favor the unhyphenated form "message body".  This document conforms
   to both conventions, using the hyphenated form for HTTP, and the
   unhyphenated form for SIP.

3. Associating Monitoring SIP URIs with HTTP URLs

   One of the key challenges in subscribing to the changes of a resource
   indicated by an HTTP URL is determining which SIP URI corresponds to
   a specific HTTP URL.  This specification takes the approach of having
   the HTTP server responsible for the URL in question select an
   appropriate SIP URI for the corresponding resource and return that
   URI within an HTTP transaction.

   In particular, HTTP servers use link relations -- such as the HTTP
   Link header field [10], the HTML <link/> element [11], and the Atom
   <atom:link/> element [5] -- to convey the URI or URIs that can be
   used to discover changes to the resource.  This document defines two
   new link relation types ("monitor" and "monitor-group") for this
   purpose, and specifies behavior for SIP and SIPS URIs in link
   relations of these types.  Handling for other URI schemes is out of
   scope for the current document, although we expect future
   specifications to define procedures for monitoring via other
   protocols.

   Clients making use of the mechanism described in this document MUST
   support the HTTP Link header field.  Those clients that support
   processing of HTML documents SHOULD support the HTML <link/> element;
   those that support processing of Atom documents SHOULD support Atom
   <atom:link/> elements.  These requirements are not intended to
   preclude the use of any other means of conveying link relations.

   The service that provides HTTP access to a resource might provide
   monitoring of that resource using multiple protocols, so it is
   perfectly legal for an HTTP response to contain multiple link
   relationships with relations that allow for monitoring of changes
   (see [10]).  Implementors are cautioned to process all link relations
   to locate one that corresponds with their preferred change monitoring
   protocol.

   These link relations are scoped to a single HTTP entity.  When an
   HTTP resource is associated with multiple entities (for example, to
   facilitate content negotiation), the "monitor" and "monitor-group"
   link relations will generally be different for each entity.

3.1. Monitoring a Single HTTP Resource

   If an HTTP server wishes to offer the ability to subscribe to changes
   in a resource's value using this event package, it returns a link
   relation containing a SIP or SIPS URI with a relation type of
   "monitor" in a successful response to a GET or HEAD request on that
   resource.  If the server supports both SIP and SIPS access, it MAY
   return link relations for both kinds of access.

   A client wishing to subscribe to the state change of an HTTP resource
   obtains a SIP or SIPS URI by sending a GET or HEAD request to the
   HTTP URL it wishes to monitor.  This SIP or SIPS URI is then used in
   a SUBSCRIBE request, according to the event package defined in
   Section 4.

3.2. Monitoring Multiple HTTP Resources

   If a client wishes to subscribe to the state of multiple HTTP
   resources, it is free to make use of the mechanisms defined in RFC
   4662 [6] and/or RFC 5367 [9].  This requires no special support by
   the server that provides resource state information.  These
   approaches, however, require the addition of a Resource List Server
   (RLS) as defined in RFC 4662, which will typically subscribe to the
   state of resources on behalf of the monitoring user.  In many cases,
   this is not a particularly efficient means of monitoring several
   resources, particularly when such resources reside on the same HTTP
   server.

   As a more efficient alternative, if an HTTP server wishes to offer
   the ability to subscribe to the state of several HTTP resources in a
   single SUBSCRIBE request, it returns a link relation containing a SIP
   or SIPS URI with a relation type of "monitor-group" in a successful
   response to a GET or HEAD request on any monitorable resource.  In
   general, this monitor-group URI will be the same for all resources on
   the same HTTP server.

   The monitor-group URI corresponds to an RLS service associated with
   the HTTP server.  This RLS service MUST support subscriptions to
   request-contained resource lists, as defined in RFC 5367 [9].  This
   RLS service MAY, but is not required to, accept URI lists that
   include monitoring URIs that are not associated with resources served
   by its related HTTP server.  Not requiring such functionality allows
   the RLS to be implemented without requiring back-end subscriptions.
   If a server wishes to reject such requests, the "403" (Forbidden)
   response code is appropriate.  Any "403" responses generated for this
   reason SHOULD contain a message body of type "application/
   resource-lists+xml"; this message body lists the offending URI or
   URIs.  See RFC 4826 [7] for the definition of the "application/
   resource-lists+xml" MIME type.

   The HTTP server MUST also return a SIP and/or SIPS link relation with
   a relation type of "monitor" whenever it returns a SIP and/or SIPS
   link relation with a relation type of "monitor-group".  The monitor-
   group URI corresponds only to an RLS, and never an HTTP resource or
   fixed set of HTTP resources.

   If a client wishes to subscribe to the state of multiple HTTP
   resources, and has received monitor-group URIs for each of them, it
   may use the monitor-group URIs to subscribe to multiple resources in
   the same subscription.  To do so, it starts with the set of HTTP
   resources it wishes to monitor.  It then groups these resources by
   their respective monitor-group URIs.  Finally, for each such group,

   it initiates a subscription to the group's monitor-group URI; this
   subscription includes a URI list, as described in RFC 5367.  The URI
   list contains all of the URIs in the group.

      For example: consider the case in which a client wishes to monitor
      the resources http://www.example.com/goat,
      http://www.example.com/sheep, http://www.example.org/llama, and
      http://www.example.org/alpaca.  It would use HTTP to perform HEAD
      and/or GET operations on these resources.  The responses to these
      operations will contain link relations for both monitor and
      monitor-type for each of the four resources.  Assume the monitor
      link for http://www.example.com/goat is sip:a94aa000@example.com;
      for http://www.example.com/sheep, sip:23ec24c5@example.com; for
      http://www.example.org/llama,
      sip:yxbO-UHYxyizU2H3dnEerQ@example.org; and for
      http://www.example.org/alpaca,
      sip:-J0piC0ihB9hfNaJc7GCBg@example.org.  Further, assume the
      monitor-group link for http://www.example.com/goat and
      http://www.example.com/sheep are both sip:httpmon@rls.example.com,
      while the monitor-group link for http://www.example.org/llama and
      http://www.example.org/alpaca are both sip:rls@example.org.

      Because they share a common monitor-group link, the client would
      group together http://www.example.com/goat and
      http://www.example.com/sheep in a single subscription.  It sends
      this subscription to the monitor-group URI
      (sip:httpmon@rls.example.com), with a resource-list containing the
      relevant monitor URIs (sip:a94aa000@example.com and
      sip:23ec24c5@example.com).  It then repeats this process for the
      remaining two HTTP resources, using their monitor-group and
      monitor URIs in the same way.

4. HTTP Change Event Package

4.1. Event Package Name

The name of this event package is "http-monitor".

4.2. Event Package Parameters

   This event package defines a single parameter to be used with the
   Event header field.  The syntax for this parameter is shown below,
   using the ABNF format defined in RFC 5234 [8].  The use of the
   construction "EQUAL" is as defined by RFC 3261 [3].

     body-event-param = "body" EQUAL ( "true" / "false" )

   If present and set to "true" in a SUBSCRIBE request, this parameter
   indicates to the server that the client wishes to receive a message-
   body component in the message/http message bodies sent in NOTIFY
   messages.

   If a server receives a SUBSCRIBE message with an Event header field
   "body" parameter set to "true", it MAY choose to include a message-
   body component in the message/http message bodies that it sends in
   NOTIFY messages.  Alternatively, it MAY decline to send such message-
   bodies, even when this parameter is present, based on local policy.
   In particular, it would be quite reasonable for servers to have a
   policy of not including HTTP message-bodies larger than a relatively
   small number of bytes.

   When absent, the value of this parameter is assumed to be "false".

      Note that this parameter refers to the message-body component of
      the HTTP message, not the message body component of the SIP
      message.

4.3. SUBSCRIBE Bodies

This event package defines no message bodies to be used in the
SUBSCRIBE message.

4.4. Subscription Duration

   Reasonable values for the duration of subscriptions to the http-
   monitor event package vary widely with the nature of the HTTP
   resource being monitored.  Some HTTP resources change infrequently
   (if ever), while others can change comparatively rapidly.  For
   rapidly changing documents, the ability to recover more rapidly from
   a subscription failure is relatively important, so implementations
   will be well served by selecting smaller durations for their
   subscriptions, on the order of 1800 to 3600 seconds (30 minutes to an
   hour).

   Subscriptions to slower-changing resources lack this property, and
   the need to periodically refresh subscriptions render short
   subscriptions wasteful.  For these types of subscriptions,
   expirations as long as 604800 seconds (one week) or even longer may
   well make sense.

   The subscriber is responsible for selecting an expiration time that
   is appropriate for its purposes, taking the foregoing considerations
   into account.  Keep in mind that the goal behind selecting
   subscription durations is to balance server load against time to

   recover in the case of a failure.  In particular, short subscription
   expiration times guard against the loss of subscription server state,
   albeit at the expense of additional load on the server.

   In the absence of an expires value in a subscription, the notifier
   can assume a default expiration period according to local policy.
   This local policy might choose to take various aspects of the
   monitored resource into account, such as its age and presumed period
   of validity.  Absent any other information, it would not be
   unreasonable for a server to assume a default expiration value of
   86400 seconds (one day) when the client fails to provide one.

4.5. NOTIFY Bodies

   By default, the message bodies of NOTIFY messages for the http-
   monitor event package will be of content-type "message/http," as
   defined in RFC 2616 [2].

4.5.1. Use of message/http in HTTP Monitor Event Package

   The message/http NOTIFY message bodies used in the HTTP monitor event
   package reflect a subset of the response that would be returned if
   the client performed an HTTP HEAD operation on the HTTP resource.

   An example of a message/http message body as used in this event
   package is shown below.

     HTTP/1.1 200 OK
     Date: Sat, 13 Nov 2010 17:18:52 GMT
     ETag: 38fe6-58b-1840e7d0
     Content-MD5: 4e3b50421829c7c379a5c6154e560449
     Last-Modified: Sat, 13 Nov 2010 03:29:00 GMT
     Accept-Ranges: bytes
     Content-Location: http://www.example.com/pet-profiles/alpacas/
     Content-Length: 12511
     Content-Type: text/html

   When used in the HTTP monitor event package defined in this document,
   the message/http SHOULD contain at least one of an ETag or Content-
   MD5 header field, unless returning a null state as described in
   Section 4.7.  Inclusion of a Last-Modified header field is also
   RECOMMENDED.  Additionally, the message/http message body MUST
   contain a Content-Location field that identifies the resource being
   monitored.  Note that this is not necessarily the same URL from which
   the link association was originally obtained; see RFC 2616 [2] for
   details.

   Except for the foregoing normative requirements, the decision
   regarding which HTTP header fields to include is at the discretion of
   the notifier.

   When used in the HTTP monitor event package, the message/http MUST
   NOT contain a message-body component, unless the corresponding
   subscription has explicitly indicated the desire to receive such
   bodies as described in Section 4.2.

   If the change to the resource being communicated represents a
   renaming of the HTTP resource, the message/http start line will
   contain the same 3xx-class HTTP response that would be returned if a
   user agent attempted to access the relocated HTTP resource with a
   HEAD request (e.g., "301 Moved Permanently").  The message/http also
   SHOULD contain a Location header field that communicates the new name
   of the resource.

   If the change to the resource being communicated represents a
   deletion of the HTTP resource, the start line will contain the same
   4xx-class HTTP response that would be returned if a user agent
   attempted to access the missing HTTP resource with a HEAD request
   (e.g., "404 Not Found" or "410 Gone").

4.6. Notifier Processing of SUBSCRIBE Requests

   Upon receipt of a SUBSCRIBE request, the notifier applies
   authorization according to local policy.  Typically, this policy will
   be aligned with the HTTP server authorization policies regarding
   access to the resource whose change state is being requested.

4.7. Notifier Generation of NOTIFY Requests

   NOTIFY messages are generated whenever the underlying resource
   indicated by the corresponding HTTP URL has been modified.

   In the case that the notifier has insufficient information to return
   any useful information about the underlying HTTP resource, it MUST
   return a message body that is zero bytes long (subject to any
   mechanisms that would suppress sending of a NOTIFY message).

4.8. Subscriber Processing of NOTIFY Requests

   Upon receipt of a NOTIFY message, the subscriber applies any
   information in the message/http to update its view of the underlying
   HTTP resource.  In most cases, this results in an invalidation of its
   view of the HTTP resource.  It is up to the subscriber implementation
   to decide whether it is appropriate to fetch a new copy of the HTTP
   resource as a reaction to a NOTIFY message.

4.9. Handling of Forked Requests

   Multiple notifiers for a single HTTP resource is semantically
   nonsensical.  In the aberrant circumstance that a SUBSCRIBE request
   is forked, the subscriber SHOULD terminate all but one subscription,
   as described in Section 4.4.9 of RFC 3265 [4].

4.10. Rate of Notifications

   Because the data stored in HTTP for the purpose of SIP services may
   change rapidly due to user input, and because it may potentially be
   rendered to users and/or used to impact call routing, a high degree
   of responsiveness is appropriate.  However, for the protection of the
   network, notifiers for the http-monitor event package SHOULD NOT send
   notifications more frequently than once every second.

4.11. State Agents

   Decomposition of the authority for the HTTP resource into an HTTP
   server and a SIP Events server is likely to be useful, due to the
   potentially different scaling properties associated with serving HTTP
   resources and managing subscriptions.  In the case of such
   decomposition, implementors are encouraged to familiarize themselves
   with the PUBLISH mechanism described in RFC 3903 [14].

5. Example Message Flow

   The following is a simple example message flow, to aid in
   understanding how this event package can be used.  It is included for
   illustrative purposes only, and does not form any portion of the
   specification of the mechanisms defined in this document.

          Client            HTTP Server      SIP Events Server
             |                   |                   |
             |                   |                   |
             |(1) HTTP GET       |                   |
             |------------------>|                   |
             |(2) HTTP 200 OK    |                   |
             |<------------------|                   |
             |(3) SIP SUBSCRIBE  |                   |
             |-------------------------------------->|
             |(4) SIP 200 OK     |                   |
             |<--------------------------------------|
             |(5) SIP NOTIFY     |                   |
             |<--------------------------------------|
             |(6) SIP 200 OK     |                   |
             |-------------------------------------->|
             |                   |                   |
             |                   |                   |
             |        [HTTP document changes]        |
             |                   |                   |
             |                   |                   |
             |                   |(7) SIP PUBLISH    |
             |                   |------------------>|
             |                   |(8) SIP 200 OK     |
             |                   |<------------------|
             |(9) SIP NOTIFY     |                   |
             |<--------------------------------------|
             |(10) SIP 200       |                   |
             |-------------------------------------->|
             |                   |                   |
             |                   |                   |

   The following messages illustrate only the portions of the messages
   that are relevant to the example.  They intentionally elide fields
   that, while typical or mandatory, are not key to understanding the
   foregoing message flow.

   1. The client issues a GET request to retrieve the document
      identified by the URL
      "http://www.example.com/pet-profiles/alpacas/".

     GET /pet-profiles/alpacas/ HTTP/1.1
     Host: www.example.com

   2. The HTTP server responds with the document, and several relevant
      pieces of meta-data.  Of key interest for this example is the Link
      header field with a "rel" parameter of "monitor".  This is the SIP
      URL that the client will use to monitor changes to the state of
      the HTTP resource.  Note that, since the message-body

      is an HTML document, the "monitor" link relation could alternately
      be indicated in the HTML document itself, through the use of a
      <link/> element.

      Note also the presence of the ETag, Content-MD5, and Last-
      Modified header fields.  These can be used by the client to
      identify the version of the entity returned by the HTTP server.

     HTTP/1.1 200 OK
     ETag: 38fe6-58b-1840e7d0
     Content-MD5: 4e3b50421829c7c379a5c6154e560449
     Last-Modified: Sat, 13 Nov 2010 03:29:00 GMT
     Content-Location: http://www.example.com/pet-profiles/alpacas/
     Link: <sip:23ec24c5@example.com>; rel="monitor"
     Link: <sip:httpmon@rls.example.com>; rel="monitor-group"
     Content-Length: 12511
     Content-Type: text/html

     [HTML message-body]

   3. The client sends a SUBSCRIBE request to the SIP URI indicated in
      the "monitor" link relation, indicating an event type of "http-
      monitor".

     SUBSCRIBE sip:23ec24c5@example.com SIP/2.0
     To: <sip:23ec24c5@example.com>
     From: <sip:adam@example.org>;tag=57dac993-0b5b-4f04
     Event: http-monitor
     Contact: <sip:adam@198.51.100.17:2487>

   4. The SIP Events server acknowledges receipt of the subscription
      request, and establishes a dialog for the resulting subscription.

     SIP/2.0 200 OK
     To: <sip:23ec24c5@example.com>;tag=907A953576E6
     From: <sip:adam@example.org>;tag=57dac993-0b5b-4f04
     Contact: <sip:23ec24c5@203.0.113.72>

   5. The SIP Events server sends a NOTIFY message containing the
      current state of the HTTP resource.  The client can compare the
      contents of the ETag, Content-MD5, or Last-Modified header fields
      against those received in the HTTP "200" response to verify that
      it has the most recent version of the entity.

     NOTIFY sip:adam@198.51.100.17:2487 SIP/2.0
     To: <sip:adam@example.org>;tag=57dac993-0b5b-4f04
     From: <sip:23ec24c5@example.com>;tag=907A953576E6
     Contact: <sip:23ec24c5@203.0.113.72>
     Event: http-monitor
     Subscription-State: active
     Content-Type: message/http

     HTTP/1.1 200 OK
     ETag: 38fe6-58b-1840e7d0
     Content-MD5: 4e3b50421829c7c379a5c6154e560449
     Last-Modified: Sat, 13 Nov 2010 03:29:00 GMT
     Content-Location: http://www.example.com/pet-profiles/alpacas/
     Content-Length: 12511
     Content-Type: text/html

6. The client acknowledges receipt of the NOTIFY message.

     SIP/2.0 200 OK
     To: <sip:adam@example.org>;tag=57dac993-0b5b-4f04
     From: <sip:23ec24c5@example.com>;tag=907A953576E6
     Contact: <sip:adam@198.51.100.17:2487>

   7. At some point after the subscription has been established, the
      entity hosted by the HTTP server changes.  It can convey this
      information to a SIP Events server using a SIP PUBLISH request.
      The PUBLISH message body contains information regarding the state
      of the entity.

      Note that SIP PUBLISH is one of many ways such information could
      be conveyed -- any other means of communicating this information
      would also be valid.

     PUBLISH sip:23ec24c5@example.com SIP/2.0
     To: <sip:23ec24c5@example.com>
     From: <sip:webserver@example.com>;tag=03-5gbK652_jNMr-b8-11Z_G-NsLR
     Contact: <sip:webserver@203.0.113.99>
     Event: http-monitor
     Content-Type: message/http

     HTTP/1.1 200 OK
     ETag: 3238e-1a3-b83be580
     Content-MD5: 10a1ef5b223577059fafba867829abf8
     Last-Modified: Sat, 17 Nov 2010 08:17:39 GMT
     Content-Location: http://www.example.com/pet-profiles/alpacas/
     Content-Length: 17481
     Content-Type: text/html

   8. The SIP Events server acknowledges the changed entity state.  Note
      that the value of the SIP-ETag header field is not related to the
      ETag header field associated with the HTTP entity.

     SIP/2.0 200 OK
     To: <sip:23ec24c5@example.com>
     From: <sip:webserver@example.com>;tag=03-5gbK652_jNMr-b8-11Z_G-NsLR
     SIP-ETag: 3psbqi1o5633

9. The SIP events server informs the client of the change in state
for the subscribed resource using a NOTIFY message.

     NOTIFY sip:adam@198.51.100.17:2487 SIP/2.0
     To: <sip:adam@example.org>;tag=57dac993-0b5b-4f04
     From: <sip:23ec24c5@example.com>;tag=907A953576E6
     Contact: <sip:23ec24c5@203.0.113.72>
     Event: http-monitor
     Subscription-State: active
     Content-Type: message/http

     HTTP/1.1 200 OK
     ETag: 3238e-1a3-b83be580
     Content-MD5: 10a1ef5b223577059fafba867829abf8
     Last-Modified: Sat, 17 Nov 2010 08:17:39 GMT
     Content-Location: http://www.example.com/pet-profiles/alpacas/
     Content-Length: 17481
     Content-Type: text/html

  10. The client acknowledges receipt of the changed state.  At this
      point, the client may choose to retrieve a fresh copy of the
      document so that it can act on the new content.  Alternately, it
      may simply mark the previously retrieved document as out of date
      or discard it, choosing to retrieve a new copy at a later point in
      time.

     SIP/2.0 200 OK
     To: <sip:adam@example.org>;tag=57dac993-0b5b-4f04
     From: <sip:23ec24c5@example.com>;tag=907A953576E6
     Contact: <sip:adam@198.51.100.17:2487>

6.  Security Considerations

   Unless secured using Transport Layer Security (TLS), IPsec, or a
   similar technology, the content of the Link header field is not
   secure, private, or integrity-protected.

   Because an unencrypted Link header field can be intercepted, server
   implementations are cautioned not to use the value sent in the Link
   header field as a security token that authenticates a subscriber, or
   that demonstrates authorization to subscribe to a particular
   resource.

   Because an unsecured Link header field can be tampered with -- or
   inserted -- in transit, client implementations need to consider the
   interaction between their application and a forged set of
   notifications.  This issue becomes particularly problematic when the
   change notifications include entity state (using "body=true").

   This mechanism introduces the means to learn information about the
   state of an HTTP resource using an alternate protocol, and
   potentially a different server.  If the HTTP resource is restricted
   using some form of access control, special care MUST be taken to
   ensure that the SIP means of subscribing to the resource state is
   also restricted in the same way.  Otherwise, unauthorized users may
   learn information that was intended to be confidential (including the
   actual resource value, in some cases).

   Similarly, if the HTTP resource is encrypted or integrity protected
   in transit -- for example, by using HTTP over TLS [12] -- then the
   SIP means of subscribing to the HTTP resource MUST also have
   appropriate encryption or integrity protection applied.  Examples of
   mechanisms for providing such protection include the use of the SIPS
   URI scheme [17], and the use of S/MIME bodies [13].

7. IANA Considerations

7.1. New Link Relations

The following entries have been added to the "Link Relation Types"
registry, as created by the "Web Linking" specification [10].

7.1.1. New Link Relation: monitor

   o  Relation Name: monitor

   o  Description: Refers to a resource that can be used to monitor
      changes in an HTTP resource.

   o  Reference: RFC 5989

7.1.2. New Link Relation: monitor-group

   o  Relation Name: monitor-group

   o  Description: Refers to a resource that can be used to monitor
      changes in a specified group of HTTP resources.

   o  Reference: RFC 5989

7.2. New SIP Event Package: http-monitor

   The following entry is to be added to the "SIP Events" registry, as
   created by the SIP Event Framework [4].

   Package Name:  http-monitor

   Type:  package

   Contact:  Adam Roach, adam@nostrum.com

   Reference:  RFC 5989

7.3. New Event Header Field Parameter: body

   The following entry is to be added to the SIP "Header Field
   Parameters and Parameter Values" registry, as created by the SIP
   Change Framework [15].

   Header Field:  Event

   Parameter Name:  body

   Predefined Values:  yes

   Reference:  RFC 5989

8. Acknowledgements

   Thanks to Lisa Dusseault and Mark Nottingham for significant input on
   the mechanisms to bind an HTTP URL to a SIP URI.  Thanks also to Mark
   Nottingham and Theo Zourzouvillys for thorough feedback on early
   versions of this document.  Thanks to Martin Thompson, Shida
   Schubert, John Elwell, and Scott Lawrence for their careful reviews
   and feedback.

9. References

9.1. Normative References

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

   [2]   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.

   [3]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [4]   Roach, A., "Session Initiation Protocol (SIP)-Specific Event
         Notification", RFC 3265, June 2002.

   [5]   Nottingham, M., Ed. and R. Sayre, Ed., "The Atom Syndication
         Format", RFC 4287, December 2005.

   [6]   Roach, A., Campbell, B., and J. Rosenberg, "A Session
         Initiation Protocol (SIP) Event Notification Extension for
         Resource Lists", RFC 4662, August 2006.

   [7]   Rosenberg, J., "Extensible Markup Language (XML) Formats for
         Representing Resource Lists", RFC 4826, May 2007.

   [8]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [9]   Camarillo, G., Roach, A., and O. Levin, "Subscriptions to
         Request-Contained Resource Lists in the Session Initiation
         Protocol (SIP)", RFC 5367, October 2008.

   [10]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

   [11]  Jacobs, I., Hors, A., and D. Raggett, "HTML 4.01
         Specification", World Wide Web Consortium Recommendation REC-
         html401-19991224, December 1999,
         <http://www.w3.org/TR/1999/REC-html401-19991224>.

9.2. Informative References

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

   [13]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail
         Extensions (S/MIME) Version 3.2 Message Specification",
         RFC 5751, January 2010.

   [14]  Niemi, A., "Session Initiation Protocol (SIP) Extension for
         Event State Publication", RFC 3903, October 2004.

   [15]  Camarillo, G., "The Internet Assigned Number Authority (IANA)
         Header Field Parameter Registry for the Session Initiation
         Protocol (SIP)", BCP 98, RFC 3968, December 2004.

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

   [17]  Audet, F., "The Use of the SIPS URI Scheme in the Session
         Initiation Protocol (SIP)", RFC 5630, October 2009.

   [18]  Wachob, G., Reed, D., Chasen, L., Tan, W., and S. Churchill,
         "Extensible Resource Identifier (XRI) Resolution V2.0",
         February 2008, <http://docs.oasis-open.org/xri/2.0/specs/
         xri-resolution-V2.0.html>.

Appendix A. Rationale: Other Approaches Considered

   Several potential mechanisms for retrieving the SIP URI from the HTTP
   server were evaluated.  Of them, link relations were determined to
   have the most favorable set of properties.  Two key candidates that
   were considered but rejected in favor of link relations are discussed
   below.

   The HTTP PROPFIND method ([16], Section 9.1) can be used to retrieve
   the value of a specific property associated with an HTTP URL.
   However, this cannot be done in conjunction with retrieval of the
   document itself, which is usually desirable.  If a PROPFIND approach
   is employed, clients will typically perform both a GET and a PROPFIND
   on resources of interest.  Additionally, the use of PROPFIND requires
   support of the PROPFIND method in HTTP user agents -- which, although
   fairly well implemented, still lacks the penetration of GET
   implementations.

   Similar to PROPFIND, XRDS (Extensible Resource Descriptor Sequence)
   [18] can be used to retrieve properties associated with an HTTP URL.
   It has the advantage of using GET instead of PROPFIND; however, it
   suffers from both the two-round-trip issue discussed above, as well
   as an unfortunately large number of options in specifying how to
   retrieve the properties.

Author's Address

   Adam Roach
   Tekelec
   17210 Campbell Rd.
   Suite 250
   Dallas, TX  75252
   US

   EMail: adam@nostrum.com