SIP WG A. B. Roach
Internet-Draft Tekelec
Expires: July 12, 2009 January 8, 2009
A SIP Event Package for Subscribing to Changes to an HTTP Resource
draft-roach-sip-http-subscribe-01
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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,
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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 events
framework, for doing so.
This document further proposes that the HTTP work necessary to make
such a mechanism work be extensible to support protocols other than
SIP for monitoring HTTP resources.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Associating Monitoring SIP URIs with HTTP URLs . . . . . . . . 3
2.1. Monitoring a Single HTTP Resource . . . . . . . . . . . . 4
2.2. Monitoring Multiple HTTP Resources . . . . . . . . . . . . 4
2.3. Rationale: Other Approaches Considered . . . . . . . . . . 6
3. HTTP Change Event Package . . . . . . . . . . . . . . . . . . 6
3.1. Event Package Name . . . . . . . . . . . . . . . . . . . . 6
3.2. Event Package Parameters . . . . . . . . . . . . . . . . . 6
3.3. SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 6
3.4. Subscription Duration . . . . . . . . . . . . . . . . . . 7
3.5. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 7
3.5.1. Use of message/http in HTTP Monitor Event Package . . 7
3.6. Notifier processing of SUBSCRIBE requests . . . . . . . . 8
3.7. Notifier generation of NOTIFY requests . . . . . . . . . . 9
3.8. Subscriber processing of NOTIFY requests . . . . . . . . . 9
3.9. Handling of forked requests . . . . . . . . . . . . . . . 9
3.10. Rate of notifications . . . . . . . . . . . . . . . . . . 9
3.11. State Agents . . . . . . . . . . . . . . . . . . . . . . . 9
4. Example Message Flow . . . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5.1. New Link Relation: monitor . . . . . . . . . . . . . . . . 10
5.2. New Link Relation: monitor-group . . . . . . . . . . . . . 10
5.3. New SIP Event Package: http-monitor . . . . . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
The Session Initiation Protocol (SIP) [2] is increasingly being used
in systems that are tightly coupled with Hypertext Transport Protocol
(HTTP) [1] 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, such as is
described in [11] and [12]. 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
[3], for subscribing to changes in the resource referenced by an HTTP
server. Such subscriptions do not carry the content associated with
the resource -- 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 SIPS URI to be used for such
subscriptions can be determined from the HTTP server.
2. 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 to return
that URI within an HTTP transaction.
In particular, HTTP servers use link relations -- such as the HTTP
Link: header [7], the the HTML <link/> element [8], and the Atom
<atom:link/> element [4] -- to convey the URI or URIs that can be
used to discover changes to the resource. This document defines
behavior for SIP and SIPS URIs in such link relations. 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. Those clients that support processing
of HTML bodies 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 types of link relations.
Because a single resource may have the ability to be monitored via
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multiple protocols, it is perfectly legal for an HTTP response to
contain multiple link relationships with relations that allow for
monitoring of changes. Implementors are cautioned to process all
link relations to locate a one that corresponds with their preferred
change monitoring protocol.
2.1. Monitoring a Single HTTP Resource
If an HTTP server wishes to offer the ability to subscribe to a
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 change state 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 Section 3.
2.2. 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 [5] and/or RFC 5367 [6]. 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 ablity 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 [6]. This
RLS service is not, however, required to accept URI lists that
include monitoring URIs that are not associated with resources served
by its related HTTP server. This allows RLS functionality to be
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implemented without requiring back-end subscriptions. If a server
wishes to reject such requests, the "403 Forbidden" response code is
appropriate.
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-list 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.
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2.3. Rationale: Other Approaches Considered
[This section will be removed before publication as an RFC]
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 ([10], 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 [13] 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.
3. HTTP Change Event Package
3.1. Event Package Name
The name of this event package is "http-monitor".
3.2. Event Package Parameters
This event package defines no parameters. [TODO: should we define a
simple filter that allows subscribers to request the body be sent in
notifications? Something like "body=true"?]
3.3. SUBSCRIBE Bodies
This event package defines no bodies to be used in the SUBSCRIBE
message. Future extensions may define filter criteria to be sent in
the SUBSCRIBE bodies.
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3.4. 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 other 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 type of subscriptions, expirations
as long as 604800 (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 the absence of an expires value in a subscription, the notifier
can assume a default expiration period according to local policy.
This local policy may 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 (one day)
when the client fails to provide one.
3.5. NOTIFY Bodies
By default, the bodies of NOTIFY messages for the http-monitor event
package will be of content-type "message/http," as defined in RFC
2616 [1].
3.5.1. Use of message/http in HTTP Monitor Event Package
The message/http NOTIFY bodies used in the HTTP monitor event package
reflect the response that would be returned if the client performed
an HTTP HEAD operation on the HTTP resource.
An example of a message/http body as used in this event package is
shown below.
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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, unless returning a null state as described in
Section 3.7. Inclusion of a Last-Modified header is also
RECOMMENDED. Additionally, the message/http 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 [1] for details.
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 in the form of a filter. Filters for this event package are
out of scope for this specification.
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 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 a 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").
3.6. 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.
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3.7. Notifier generation of NOTIFY requests
NOTIFY messages should be 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 may
return a body that is zero bytes long.
3.8. Subscriber processing of NOTIFY requests
Upon receipt of a NOTIFY message, subscriber should use 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.
3.9. 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 [3].
3.10. 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.
3.11. 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 [9].
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4. Example Message Flow
Subscriber 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 | |
|-------------------------------------->|
| |(7) SIP PUBLISH |
| |------------------>|
| |(8) SIP 200 OK |
| |<------------------|
|(9) SIP NOTIFY | |
|<--------------------------------------|
|(10) SIP 200 | |
|-------------------------------------->|
| | |
| | |
[TBD: include full example messages]
5. IANA Considerations
[TBD: these sections need some prose to describe which registry we're
putting the values in to]
5.1. 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 XXXX [[Note to RFC Editor: replace with the RFC
number for this specification]]
5.2. New Link Relation: monitor-group
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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 XXXX [[Note to RFC Editor: replace with the RFC
number for this specification]]
5.3. New SIP Event Package: http-monitor
Package Name: http-monitor
Type: package
Contact: Adam Roach, adam.roach@tekelec.com
Reference: RFC XXXX [[Note to RFC Editor: replace with the RFC
number for this specification]]
6. 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.
7. References
7.1. Normative References
[1] 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.
[2] 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.
[3] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[4] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom Syndication
Format", RFC 4287, December 2005.
[5] Roach, A., Campbell, B., and J. Rosenberg, "A Session
Initiation Protocol (SIP) Event Notification Extension for
Resource Lists", RFC 4662, August 2006.
[6] Camarillo, G., Roach, A., and O. Levin, "Subscriptions to
Request-Contained Resource Lists in the Session Initiation
Protocol (SIP)", RFC 5367, October 2008.
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[7] Nottingham, M., "Link Relations and HTTP Header Linking",
draft-nottingham-http-link-header-03 (work in progress),
November 2008.
[8] 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>.
7.2. Informative References
[9] Niemi, A., "Session Initiation Protocol (SIP) Extension for
Event State Publication", RFC 3903, October 2004.
[10] Dusseault, L., "HTTP Extensions for Web Distributed Authoring
and Versioning (WebDAV)", RFC 4918, June 2007.
[11] Griffin, K. and J. Rosenberg, "Representational State Transfer
(REST) for Feature Configuration in Session Initiation
Protocol (SIP)", draft-griffin-bliss-rest-00 (work in
progress), October 2008.
[12] Zourzouvillys, T., "Automatic Call Handling (ACH) Configuration
Requirements",
draft-zourzouvillys-bliss-ach-config-requirements-00 (work in
progress), October 2008.
[13] 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>.
Author's Address
Adam Roach
Tekelec
17210 Campbell Rd.
Suite 250
Dallas, TX 75252
US
Email: adam@nostrum.com
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