Network Working Group                                      M. Nottingham
Internet-Draft                                         February 21, 2002
Expires: August 22, 2002

                     Web Active Resource Monitoring

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   This Internet-Draft will expire on August 22, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   WARM is a straw-man proposal for a solution to the RUP requirements
   of the WEBI WG which reuses the Web Architecture (and HTTP).  In
   particular, it provides a mechanism for distributing cache
   invalidations from HTTP servers to clients.

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1   Requirements . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2   WARM Resources . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2.1 Channel Resources  . . . . . . . . . . . . . . . . . . . . .  4
   1.2.2 Subscription Resources . . . . . . . . . . . . . . . . . . .  5
   2.    Overview of Operation  . . . . . . . . . . . . . . . . . . .  5
   2.1   Discovery  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.1.1 Active Discovery . . . . . . . . . . . . . . . . . . . . . .  5
   2.1.2 Passive Discovery  . . . . . . . . . . . . . . . . . . . . .  6
   2.2   Monitoring . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.2.1 Subscription-Based Monitoring  . . . . . . . . . . . . . . .  7
   2.2.2 Polling-Based Monitoring . . . . . . . . . . . . . . . . . .  8
   3.    Relationships to Network Nodes . . . . . . . . . . . . . . .  9
   4.    Using WARM for Cache Invalidation  . . . . . . . . . . . . .  9
   5.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 10
   5.1   The WATCH HTTP request method  . . . . . . . . . . . . . . . 10
   5.2   The Subscription HTTP request header . . . . . . . . . . . . 11
   5.3   The Channel HTTP response header . . . . . . . . . . . . . . 11
   6.    Security Considerations  . . . . . . . . . . . . . . . . . . 11
         References . . . . . . . . . . . . . . . . . . . . . . . . . 11
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 12
   A.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
   B.    Issues/TODO  . . . . . . . . . . . . . . . . . . . . . . . . 12
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 13

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1. Introduction

   WEBI's Resource Update Protocol requirements are broad enough that a
   wide variety of architectural styles could satisfy them.  WARM is a
   straw-man proposal for RUP that concentrates on reusing the Web
   architecture.  This approach has several advantages;

   o  Generality - The Web is most correctly defined as an information
      space, rather than the use of any particular protocol or format.
      A notification protocol that uses the same foundations as the Web
      (namely, resources identified by URIs and HTTP) will be able to
      make statements about any resource on the Web, not just a subset
      of them.

   o  Extensibility/Evolvability - WARM leverages the Web's properties
      of extensibility and evolvability, in turn providing them to
      applications that use it for notifications.

   o  Simplicity - A HTTP-based system is easier for Web publishers and
      HTTP implementors to understand.

   o  Ease of Implementation - Because WARM uses the HTTP, the cost of
      implementing on HTTP devices it is extremely low.  Additionally,
      WARM will be able to use well-understood HTTP mechanisms like
      authentication, SSL/TLS, ETag validation, content negotiation,
      redirection, etc.

   This document defines the WARM architecture and a cache invalidation
   payload for it.  Please direct comments to the WEBI WG mailing list,

1.1 Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [2].

   An implementation is not compliant if it fails to satisfy one or more
   of the MUST or REQUIRED level requirements.  An implementation that
   satisfies all the MUST or REQUIRED level and all the SHOULD level
   requirements is said to be "unconditionally compliant"; one that
   satisfies all the MUST level requirements but not all the SHOULD
   level requirements is said to be "conditionally compliant".

1.2 WARM Resources

   WARM provides for propogation of events concerning Web resources by
   defining two new types of resources; Channel Resources and

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   Subscription Resources.

   Note that these classifications are conveniences; they are not
   fundamentally different from other kinds of resources on the Web.

   WARM effects monitoring by transferring representations of the state
   of Channel Resources and caching it for a short period of time.  If
   subscription-based monitoring is in use, clients expose a
   Subscription Resource, into which representations of the Channel
   Resource's state are copied.  Clients using Polling-based monitoring
   will directly fetch a representation of the Channel Resource's state
   and cache it locally.

   Representations of Channel Resources (whether polled or subscribed)
   have a freshness associated with them, which is functionally similar
   to HTTP cache freshness.  When they become stale, any assertions made
   by the representations SHOULD be considered invalid.

   On their own, these mechanisms allow the transfer of state
   representations in the channel and subscriptions to it; they do not
   describe what the representations of that state are.  This
   specification defines one representation format that can be used to
   maintain coherence in an HTTP cache; other payloads may be defined in
   the future.

1.2.1 Channel Resources

   Channel Resources characterize the state of an arbitrary grouping of
   Web resources.  They are are identified by URIs, are accessed using
   the HTTP, and can be monitored either by polling or through

   Traditional Web resources may be monitored using the same mechanisms;
   they behave as Channel Resources that characterize the state of only
   one Web resource.

   For example, the Channel Resource;A

   embodies a channel that contains the state of an arbitrary number of
   resources.  Those resources may be under control of the same
   authority as the Channel Resource, or they may be elsewhere.

   Informally, the semantics of common HTTP methods on Channel Resources

   o  GET - retrieve a representation of the state of the channel.

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   o  WATCH - subscribe to the channel.

1.2.2 Subscription Resources

   When a Channel Resource is subscribed to using the WATCH method, a
   reference to a Subscription Resource is provided in the Subscription
   HTTP request header.  This allows the Channel Resource to maintain
   state regarding who is subscribed, and to locate them to perform
   operations related to the subscription.

   For example, a Subscription Resource;1

   contains the state associated with a particular subscription to the
   Channel Resource.  Subscription resources might be accessed through
   any number of protocols; this document only defines how they are
   accessed in HTTP.

   Informally, the semantics of common HTTP methods on Subscription
   Resources are:

   o  GET - retrieve a representation of the state of the subscription.

   o  PUT - replace the state of the subscription.

   o  POST - update the freshness of the subscription.

   o  DELETE - terminate the subscription.

2. Overview of Operation

   WARM's operation is composed of two different modes; discovery and

2.1 Discovery

   The appropriate Channel Resource for a given Web resource can be
   discovered either passively or actively.  Discovery is OPTIONAL; some
   deployments may require out-of-band discovery, which is out of scope
   for this document.

2.1.1 Active Discovery

   Active discovery is accomplished by performing a WATCH on the Web

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   For example:


     302 Found

   Here, the location of the appropriate Channel Resource is found by
   examining the target of the redirect.  The semantics of HTTP status
   codes in responses to active discovery requests should be honored as
   they are defined in the HTTP.

   The Resource SHOULD be considered associated with the actively
   discovered Channel Resource until a subsequent WATCH changes the
   association, or the semantics of the Channel Resource's
   representation explicitly change the association.

2.1.2 Passive Discovery

   Clients can passively discover Channel Resources by looking for the
   Channel HTTP response header;


     200 OK

   The Resource SHOULD be considered associated with the passively
   discovered Channel Resource until subsequent representation has a
   different or missing Channel response header, or the semantics of a
   representation of the Channel Resource explicitly change the

   [[[ what about changes to the Channel Resource's state? ]]]

2.2 Monitoring

   Once the appropriate Channel Resource is discovered, its state can be
   monitored through the use of one of two techniques; subscription-
   based monitoring and polling-based monitoring.

   Subscription-based monitoring allows notifications to be sent as
   quickly as network and other resource limitations allow them to be,

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   in combination with a heartbeat mechanism to assure that the channel
   remains available.

   Polling-based monitoring can be used in situations where the Channel
   Resource is unwilling to maintain state about subscriptions, or where
   network conditions (e.g., a firewall) make it impractical to expose a
   Subscription Resource.

2.2.1 Subscription-Based Monitoring

   Clients who wish to use subscription-based monitoring advertise this
   through use of the Subscription HTTP request header, in combination
   with the WATCH method.  For example;

     Accept: text/xml, */*;q=0.0

     200 OK

   A Channel Resourse SHOULD NOT return a successful status code to the
   WATCH method until it has initiated the Subscription Resource (with a
   PUT).  If it cannot do so, it SHOULD return an appropriate client or
   server failure status code.  In disconnected deployments, it MAY
   return 202 Accepted.

   In subscription-based monitoring, the Channel Resource must first
   initialise the state of the Subscription Resource by PUTing a
   representation of the channel into it.  Thereafter, the Channel
   Resource may update the state of the Subscription Resource with
   subsequent PUTs, and forceably delete the subscription with DELETE.
   Integrity of channel connectivity is assured by polling the
   Subscription Resource with the POST method and an empty entity body.

   For example;

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     (initialise/update request)
     Cache-Control: max-age=600
     Content-Type: text/xml
     [entity body]

     (initialise response)
     201 Created

     (update response)
     200 OK

     (heartbeat request)
     Cache-Control: max-age=600
     Content-Length: 0

     (heartbeat response)
     204 No Content

     (delete request)

     (delete response)
     200 OK

   PUT and POST requests to Subscription Resources SHOULD include a
   Cache-Control: max-age header.  Its value is used to determine when
   the next heartbeat should arrive by; if a heartbeat is not received
   by its expiry, the Subscription Resource SHOULD be considered

   [[[ is this a good use of Cache-Control, or would it be more correct
   to define a new header? ]]]

   The semantics of HTTP status codes in responses MUST be honored.  In
   particular, if any request to a Subscription Resource returns 410
   Gone, the Channel Resource SHOULD consider the subscription canceled,
   and cease monitoring.

2.2.2 Polling-Based Monitoring

   Clients using polling-based monitoring make periodic HTTP requests to
   the Channel Resource; the response represents its current state.

   To ensure correctness and efficiency in polling-based monitoring,

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   Channel Resources MUST support ETag validation.  Channel Resources
   SHOULD use the Cache-Control response header for GETs to declare how
   long that representation of the channel should be considered fresh
   (and therefore, how long before the client should poll again).

   For example;

     If-None-Match: "abcde"
     Accept: text/xml, */*;q=0.0

     304 Not Modified
     Cache-Control: max-age=600
     ETag: "abcde"

3. Relationships to Network Nodes

   Because they are located by URIs, Channel and Subscription Resources
   may be located on any addressable network node.  However, it may be
   helpful to illustrate a typical implementation;

      +--------+                                                 +--------+
      |        | ------ request(s) to Channel Resource(s) -----> |        |
      |  HTTP  |                                                 |  HTTP  |
      | Client |                                                 | Server |
      |    +   | <--- request(s) to Subscription Resource(s) --- |  +  +  |
      +--------+                                                 +--------+
           ^                                                        ^  ^
           \ Subscription Resource                 Channel Resource /  |
                                                       Web Resource   /

   This illustration should not be construed to limit the location of a
   Channel Resource to the network node on which the resource(s) it
   characterizes reside, or to prohibit the location of a Subscription
   Resource on a node other than the HTTP client.  Indeed, there are
   many scenarios where it is beneficial to do so, for purposes of
   scalability, managability, assertion of administrative control, or
   for disconnected operation.

4. Using WARM for Cache Invalidation

   WARM may be used to maintain coherence of cached representations.  In
   this application, the payload of notifications is a simple XML
   document identified by the application/xml media type, using a single
   element, 'cache', in the WARM cache namespace;

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     <?xml version="1.0"?>
     <cache xmlns="">

   The content of the element is a nonce generated by the Channel
   Resource to identify its current revision level; it MUST be
   guaranteed by the Channel Resource to be unique in its scope.  When
   any Web resource associated with the Channel Resource becomes stale,
   the Channel Resource state SHOULD change.

   This implies that the cache will track the mapping between Web
   resources and Channel Resources and/or Subscription Resources, so
   that when notifications are received, the appropriate representations
   can be marked stale.  Web resources that are associated with such
   WARM Channel Resources SHOULD be considered fresh until such a
   notification is received, the channel is deleted, or connectivity is

   WARM can also be used with other cache-related payloads; their
   semantics, interactions with cache behaviour, and additional
   association mechanisms are format-dependent.

   For purposes of content negotiation, the media type of this format is
   "[[[ TBD ]]]".

5. IANA Considerations

5.1 The WATCH HTTP request method

   The WATCH method is used to associate a Subscription Resource with a
   Channel Resource, or to locate an appropriate Channel Resource.  If a
   Subscription Resource is associated, regular requests containing
   heartbeat and/or update messages (as described above) will be made to

   watch = "WATCH"

   WATCHing a resource may instigate one or more requests to
   subscription resources, if subscription-based monitoring is in use
   (as evidenced by a Subscription request header).  If content
   negotiation is used to determine the representation sent in response
   to the WATCH, the same representation SHOULD be sent in subsequent
   PUTs to the Subscription Resource.

   Implementations SHOULD interpret HTTP status codes resulting from
   WATCH as defined in RFC2616.  Web resources that are not Channel
   Resources MAY return 303 See Other to direct clients to the

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   appropriate Channel Resource, which may then be monitored by polling
   or subscription.

   WATCH requests MAY contain an entity-body; however, this document
   does not specify a format for them.

5.2 The Subscription HTTP request header

   The Subscription request header is used to indicate the URI of the
   Subscription Resource that the client wishes to associate with a
   Channel Resource.

   subscription = "Subscription" ":" URI

5.3 The Channel HTTP response header

   The Channel response header is used to indicate the URI of the
   Channel Resource associated with the Web resource.

   channel = "Channel" ":" URI

6. Security Considerations

      WARM uses the same confidentiality, integrity, authorization and
      authentication as HTTP does.  Therefore, the use of SSL/TLS, the
      Content-MD5 header and HTTP authentication mechanisms are
      encouraged, and support for them in implementations is
      RECOMMENDED.  Such issues are relevent to both Channel Resources
      and Subscription Resources.

      WARM allows Channel Resources to make statements about Web
      resources in other administrative domains.  Client implementations
      SHOULD be aware of the impliations of this, and be conservative in
      their trust of such statements.

      Certain modes in WARM imply non-trivial resource use by either the
      client or the server; implementations SHOULD limit their use
      through techniques such as increasing the lifetime of
      representations (through the Cache-Control header), limiting the
      number of clients accepted, etc.


   [1]  Fielding, R., "Architectural Styles and the Design of Network-
        based Software Architectures", 2000, <

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement

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        Levels", RFC 2119, March 1997.

Author's Address

   Mark Nottingham


Appendix A. Acknowledgements

   The author would like to thank Paul Prescod for the spark that led to
   WARM, Roy Fielding for his description of the REST architectural
   style [1] that it is built upon, Mark Baker for his insight and
   patience in explaining and applying REST, and Rohit Khare and Adam
   Rifkin for their overview of Internet-Scale Event Notification

   Any error, misconception or bad design in this document is the
   responsibility of the author, not them.

Appendix B. Issues/TODO

   o  Discuss WARM Intermediaries, to scale to large deployments.

   o  Formalize the cache and state models.

   o  how does a client specify authentication credentials for the
      Subscrition Resource?

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