Internet Engineering Task Force (IETF) A.B. Roach

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
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.

Roach Standards Track [Page 1]

RFC 5989 SIP HTTP Subscriptions October 2010

Table of Contents

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

Roach Standards Track [Page 2]

RFC 5989 SIP HTTP Subscriptions October 2010

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.

Roach Standards Track [Page 3]

RFC 5989 SIP HTTP Subscriptions October 2010

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.

Roach Standards Track [Page 4]

RFC 5989 SIP HTTP Subscriptions October 2010

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,

Roach Standards Track [Page 5]

RFC 5989 SIP HTTP Subscriptions October 2010

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:[email protected];
for http://www.example.com/sheep, sip:[email protected]; for
http://www.example.org/llama,
sip:[email protected]; and for
http://www.example.org/alpaca,
sip:[email protected]. Further, assume the
monitor-group link for http://www.example.com/goat and
http://www.example.com/sheep are both sip:[email protected],
while the monitor-group link for http://www.example.org/llama and
http://www.example.org/alpaca are both sip:[email protected].

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:[email protected]), with a resource-list containing the
relevant monitor URIs (sip:[email protected] and
sip:[email protected]). 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" )

Roach Standards Track [Page 6]

RFC 5989 SIP HTTP Subscriptions October 2010

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

Roach Standards Track [Page 7]

RFC 5989 SIP HTTP Subscriptions October 2010

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.

Roach Standards Track [Page 8]

RFC 5989 SIP HTTP Subscriptions October 2010

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.

Roach Standards Track [Page 9]

RFC 5989 SIP HTTP Subscriptions October 2010

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.

Roach Standards Track [Page 10]

RFC 5989 SIP HTTP Subscriptions October 2010

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

Roach Standards Track [Page 11]

RFC 5989 SIP HTTP Subscriptions October 2010

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:[email protected]>; rel="monitor"
Link: <sip:[email protected]>; 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:[email protected] SIP/2.0
To: <sip:[email protected]>
From: <sip:[email protected]>;tag=57dac993-0b5b-4f04
Event: http-monitor
Contact: <sip:[email protected]: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:[email protected]>;tag=907A953576E6
From: <sip:[email protected]>;tag=57dac993-0b5b-4f04
Contact: <sip:[email protected]>

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.

Roach Standards Track [Page 12]

RFC 5989 SIP HTTP Subscriptions October 2010

NOTIFY sip:[email protected]:2487 SIP/2.0
To: <sip:[email protected]>;tag=57dac993-0b5b-4f04
From: <sip:[email protected]>;tag=907A953576E6
Contact: <sip:[email protected]>
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:[email protected]>;tag=57dac993-0b5b-4f04
From: <sip:[email protected]>;tag=907A953576E6
Contact: <sip:[email protected]: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:[email protected] SIP/2.0
To: <sip:[email protected]>
From: <sip:[email protected]>;tag=03-5gbK652_jNMr-b8-11Z_G-NsLR
Contact: <sip:[email protected]>
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

Roach Standards Track [Page 13]

RFC 5989 SIP HTTP Subscriptions October 2010

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:[email protected]>
From: <sip:[email protected]>;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:[email protected]:2487 SIP/2.0
To: <sip:[email protected]>;tag=57dac993-0b5b-4f04
From: <sip:[email protected]>;tag=907A953576E6
Contact: <sip:[email protected]>
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:[email protected]>;tag=57dac993-0b5b-4f04
From: <sip:[email protected]>;tag=907A953576E6
Contact: <sip:[email protected]: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.

Roach Standards Track [Page 14]

RFC 5989 SIP HTTP Subscriptions October 2010

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

Roach Standards Track [Page 15]

RFC 5989 SIP HTTP Subscriptions October 2010

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, [email protected]

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.

Roach Standards Track [Page 16]

RFC 5989 SIP HTTP Subscriptions October 2010

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>.

Roach Standards Track [Page 17]

RFC 5989 SIP HTTP Subscriptions October 2010

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>.

Roach Standards Track [Page 18]

RFC 5989 SIP HTTP Subscriptions October 2010

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: [email protected]

Roach Standards Track [Page 19]