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Versions: 00 01                                                         
          INTERNET-DRAFT                                      Martin Calsyn
          Expires: Sept 1998                                 Lisa Dusseault
                                                      Microsoft Corporation
                                                                Gordon Mohr
                                                                  Activerse
       
                        RVP: A Presence Notification Protocol
                               draft-calsyn-rvp-01.txt
       
       1.   Status of this Memo
       
          This document is an Internet-Draft.  Internet-Drafts are working
          documents of the Internet Engineering Task Force (IETF), its
          areas, and its working groups.  Note that other groups may also
          distribute working documents as Internet-Drafts.
       
          Internet-Drafts are draft documents valid for a maximum of six
          months and may be updated, replaced, or obsoleted by other
          documents at any time.  It is inappropriate to use Internet-
          Drafts as reference material or to cite them other than as "work
          in progress."
       
          To view the entire list of current Internet-Drafts, please check
          the "1id-abstracts.txt" listing contained in the Internet-Drafts
          Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
          (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East
          Coast), or ftp.isi.edu (US West Coast).
       
       2.   Abstract
       
          A  new  kind of application is emerging on the Internet, driven
          by the desire of individuals to know instantly whether another
          individual is online,  to be notified when another individual
          arrives online, and to send messages in ‘real time’.  These are
          all special cases of the more general problem of Internet-wide
          notification.
       
          This protocol for facilitating rendezvous between users, known
          herein as RVP, is designed  to enable such notifications and
          messages across a loosely coupled (federated) constellation of
          servers.  RVP is designed to address the need for notification in
          a secure, reliable and scaleable fashion.  RVP encompasses the
          client-server and server-server interactions.
       
          The authors recognize that there is significant overlap with
          between RVP and HTTP, though there are also significant
          differences.  We expect RVP to evolve and either converge or
          diverge with HTTP and/or other existing protocols.  The protocol
          described in this document represents a very viable starting
          point for a discussion of a standardized solution to the problem.
       
          Comments on this draft are solicited and should be sent to the
          authors or the RVP discussion list.  To join the RVP discussion
       
       
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          list, send email with the body "subscribe" to rvp-
          request@iastate.edu.
       
       3.   Contents
       
          1. Status of this Memo.........................................1
          2. Abstract....................................................1
          3. Contents....................................................2
          4. Introduction................................................4
             4.1.  Problem to be solved..................................4
             4.2.  Terminology...........................................4
             4.3.  Definitions...........................................4
             4.4.  Protocol Properties...................................5
                4.4.1. General format....................................5
                4.4.2. Minimal State.....................................5
                4.4.3. Transport-Protocol Neutral........................6
                4.4.4. Text-based........................................6
          5. Overview of RVP Functionality...............................6
                5.1.1. Naming and Location of RVP Objects................6
                5.1.2. RVP Object Schemas................................7
                5.1.3. RVP Clients.......................................7
                5.1.4. Home Servers......................................8
                5.1.5. Proxies...........................................8
                5.1.6. Server-Server Interactions........................8
                5.1.7. Subscriptions.....................................9
                5.1.8. Multiple client issues...........................10
          6. Security...................................................10
             6.1.  Authentication.......................................10
             6.2.  Content Protection...................................10
             6.3.  Server-to-Server Security............................11
             6.4.  Privacy Issues.......................................11
          7. RVP Methods................................................11
             7.1.  GET method (from HTTP)...............................11
             7.2.  PUT method (from HTTP)...............................12
             7.3.  MKCOL (from DAV).....................................12
             7.4.  RMCOL (from DAV).....................................12
             7.5.  ACL (from DAV).......................................12
             7.6.  SUBSCRIBE (introduced here)..........................12
                Reply-To header.........................................12
                7.6.2. Notification-type header.........................12
                7.6.3. Using XML to specify properties..................12
             7.7.  UNSUBSCRIBE..........................................13
             7.8.  NOTIFY...............................................13
          8. Responses..................................................14
             8.1.  GET response.........................................14
             8.2.  SUBSCRIBE response...................................14
                8.2.1. Privacy-level....................................14
             8.3.  Refer................................................15
          9. Transport Considerations for Datagrams and Connections.....15
             9.1.  Choice of Transport..................................15
                9.1.1. Matching Responses To Requests...................16
             9.2.  UDP Retry Policy.....................................16
             9.3.  Mixed-Transport Through Proxies......................17
          10. Protocol Details..........................................17
             10.1. Identity URLs, Location URLs, and default port.......17
       
       
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             10.2. Headers..............................................18
                10.2.1.Accept-charset...................................18
                10.2.2.Accept-encoding..................................18
                10.2.3.Accept-Language..................................18
                10.2.4.Allow............................................19
                10.2.5.Authorization....................................19
                10.2.6.Reply-To.........................................19
                10.2.7.Content-Encoding.................................19
                10.2.8.Content-Language.................................19
                10.2.9.Content-Length...................................19
                10.2.10.Content-Type....................................19
                10.2.11.Date............................................19
                10.2.12.From............................................19
                10.2.13.Host............................................19
                10.2.14.If-Modified-Since...............................20
                10.2.15.If-Unmodified-Since.............................20
                10.2.16.Last-Modified...................................20
                10.2.17.Request-ID......................................20
                10.2.18.Server..........................................20
                10.2.19.Session-ID......................................20
                10.2.20.Subscription-ID.................................20
                10.2.21.Via.............................................20
                10.2.22.XML-body........................................20
             10.3. Responses............................................21
             10.4. Proxying and Referral................................21
             10.5. Peer to peer Messaging...............................22
                10.5.1.Getting direct responses to requests.............23
                10.5.2.Responding directly to notifications.............23
                10.5.3.Receiving instant messages directly..............23
                10.5.4.Receiving Requests Directly......................23
             10.6. Proxy and Referral Conflicts.........................24
          11. Examples..................................................24
             11.1. Subscribing directly to status changes...............24
             11.2. Subscribing through a proxy to a group:..............26
             11.3. Setting ACLs on the status property..................27
             11.4. Sending an instant message...........................27
             11.5. Inviting a user to a session defined by SIP..........28
          12. REFERENCES................................................28
          13. Authors' Addresses........................................29
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
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       4.   Introduction
       
       4.1. Problem to be solved
       
          With the rise of real-time interaction systems, including text,
          audio and video over computer networks, there is a need for a
          user to be able to determine if his or her peers are online.
          Along with this need for status information, users wish to
          exchange brief, perishable, and perhaps time-critical messages.
       
          Users also want to exchange messages with groups of users with
          common interests.  For instance, a given user might want to send
          a message to all users currently logged in who have indicated
          interest in a gaming session.  Like individual users, the online
          status of a group can be published.
          A similar need exists for reliable, secure notifications and
          messages between people and automated processes, for business and
          systems management purposes.
          Existing protocols, such as SIP for session initiation, LDAP for
          user information and SMTP for messaging, overlap somewhat with
          the problem domain defined here.  However, these protocols lack
          key features such as scalability, latency, data replication, and
          integration.  This has led to many independent, centralized,
          monolithic and incompatible vendor solutions.  The popularity of
          these non-interoperable solutions is an indication of the need to
          address the problem.
       
          This draft is an attempt to address the problem space in a
          generalized fashion that allows different server and client
          implementations to interact within a secure internet-wide
          infrastructure.
       
          The PIP Requirements draft [3] discusses in more detail the
          nature of this problem and requirements for solutions.
       4.2. Terminology
          In this document, the key words "MUST", "MUST NOT", "REQUIRED",
          "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",
          "MAY" and "OPTIONAL" are to be interpreted as described in RFC
          2119 [1] and indicate requirement levels for compliant RVP
          implementations.
       
       4.3. Definitions
       
          This specification uses a number of terms to refer to the roles
          played by participants in RVP communications and the kinds of
          messages passed between them.
       
          Client: A software process or portion of a process which connects
          to RVP server(s) but hosts no RVP objects. Client and server
          functionality may exist within a single process.
       
       
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          Content: MIME-encapsulated data transmitted in the body of an RVP
          transaction
          Node:  A named vertex in an RVP tree, which can map to an object.
       
          Property: A named attribute of an RVP object.
       
          Request: A query for information initiated by a server or client
          and targeted at a server.  Requests result in one or more
          responses.
       
          Response: The returned result of a request.  A server may return
          one or more responses in answer to a request.
       
          Server: A software process which can host RVP objects and can
          accept connections from other entities sending messages to or
          requesting data from those objects.  Alternatively, a pure proxy
          server does not host RVP objects but can still accept connections
          on behalf of another process which does host RVP objects.
       
          Subscription: A subscription is a request that will result in
          multiple responses, one for each change in the subscribed
          property or one for each message sent to the subscribed object.
       
          User: A single human; a person who might operate one or more
          instances of the client software; a human represented by a RVP
          ‘user’ object.
       
       4.4. Protocol Properties
       
          RVP is designed to be a useful notification protocol for various
          purposes, but its primary purpose is notifying users of property
          changes on other user objects.  This draft covers both generic
          notification mechanisms and specific people presence issues.
          Most of the generic notification mechanisms are discussed in this
          draft.  Most of the specific people-presence issues are covered
          in the user and group schemas of the schema draft [2].
       
       4.4.1.    General format
       
          The RVP protocol generally fits the HTTP [5] model.  Where we
          have used HTTP protocol elements, we defer to the HTTP definition
          of those elements.  RVP-specific information is general
          transferred in new commands or by specifying the RVP namespace in
          XML-formatted data within the XML body of an HTTP protocol
          element.  Use of XML in this way might allow a RVP server to
          coexist with an HTTP server.  XML is used in order to provide
          flexibility and extensibility.
       
          Parsing of messages should follow HTTP recommendations.  Port 80
          may be used.
       
       4.4.2.    Minimal State
       
       
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          An effort has been made to minimize the amount of state that a
          running server must maintain.  A server is required to maintain a
          list of the subscriptions it currently holds while running.
          Subscriptions for a given client may be discarded if the
          connection to that client is broken, or if the publishing server
          receives a message that the subscriber is offline, but the
          publishing server should inform the subscriber that their
          subscription was dropped.  For most objects, clients are required
          to re-subscribe whenever reconnecting.
       
          Issue: should subscriptions be dropped if the server goes down?
       
       4.4.3.    Transport-Protocol Neutral
       
          RVP is able to utilize both UDP and TCP as transport protocols.
          Servers MUST implement both UDP and TCP transport. Clients MUST
          support TCP and MAY support UDP interactions.
       
          Abbreviated headers should be defined to facilitate UDP
          interactions between servers for cases where the transaction will
          fit wholly within the MTU.  Transactions between servers that do
          not fit within the MTU are sent via TCP link.
       
       4.4.4.    Text-based
          RVP is text-based.  This allows easy implementation in languages
          such as Tcl and Perl, allows easy debugging, and most
          importantly, makes RVP flexible and extensible.
       
       5.   Overview of RVP Functionality
       
       5.1.1.    Naming and Location of RVP Objects
       
          Every RVP object has an authoritative address, which is a URL.
          This URL serves not only as a unique name, but also a pointer to
          the location where the object usually resides.  For more
          information on RVP URLs and finding addresses and servers, see
          [4].
       
          A single RVP server can host a hierarchy of nodes that correspond
          to objects.  Each object must have certain properties, and each
          node may have child nodes. A client or peer server can attempt
          to:
       
           - Get or put properties on objects
           - Add or remove subscriptions on objects
           - Create new child nodes
           - Send notifications/messages to an object
       
          All of the above operations are gated by access control entries
          on each object.
       
       
       
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          A node has a persistent location in this hierarchy, on this
          server, throughout its life.  If a set of content moves away from
          a node and users should still be able to find that content, then
          the server should link, refer or proxy to the new node.
       
       5.1.2.    RVP Object Schemas
       
          In order that clients can query known properties of objects, a
          required minimal schema is defined for each object.  Individual
          implementations and individual instances of servers can extend
          this schema, but must implement the minimal schema.  The minimal
          schema, along with schemas for user and group objects, are
          defined separately in [2].
       
       5.1.3.    RVP Clients
       
          An RVP client is any endpoint process that uses the RVP protocol
          on behalf of a user.  A client can also act as its server by
          hosting rvp objects.  Clients that do not host objects can only
          converse with servers.  Clients that do host objects can converse
          with other clients as well as servers.  Note that Client-to-
          client interaction may enhance performance, but it results in a
          reduction in security by directly exposing client IP addresses.
          In client-to-client interaction, both clients act as their own
          servers.
       
          In the remainder of this document, client requirements are for
          the process or portion of a process that interfaces with the
          user.  Server requirements are for the process or portion of a
          process that hosts objects, subscribes directly to publishing
          servers, or otherwise needs to be contacted directly.  Nothing
          precludes a single process from acting as both client and server.
          The referral and proxy mechanisms discussed later in this
          document allow users who have their content hosted on a server
          while offline to switch to hosting their own content while
          online.
       
          We require that a client be able to receive messages such as
          instant messages and notifications.  This can be done in three
          ways:
          - The client maintains an open TCP connection to their proxy
          server. All queries, subscriptions and notifications from outside
          are sent to the proxy server, which sends messages to the client
          as appropriate. The user's canonical RVP address as listed for
          instant messages and the Reply-To address for subscriptions must
          be on their proxy server which is also their home server, OR the
          home server with the canonical address can refer to the proxy.
          The user's RVP address as listed for queries and subscriptions
          must be on their home server.
       
           - The client listens an open port, accepting connections ONLY
          from their home server.  All queries, subscriptions and
          notifications from outside are sent to the server, and the server
       
       
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          opens up a connection to the client and sends whichever messages
          are appropriate.  RVP address restrictions are as above.
           - The client listens on an open port, accepting connections from
          any source.  The client may now act as its own server.  Messages
          that are sent to the home server may be redirected to the client.
          The user can then be addressed at either their home server or at
          their local client & server machine.  Any client that does this
          must follow all RVP requirements for a server.
       
          A client process representing a user must, as part of its
          initialization, PUT values for properties to that user’s object
          (wherever it is hosted).  These properties indicate the user’s
          status and availability for various types of interactions.  The
          same client process must update the properties as they change.
       
          A client process representing an automated process could get and
          put properties, subscribe to content, or send content out as part
          of it’s operation.  Examples include a fortune-cookie server, a
          system for announcing server status, or a stock transaction
          system.
       
       5.1.4.    Home Servers
       
          A user's home server is the server that most consistently hosts
          their data, particularly while the user is offline.  This is also
          the server pointed to by the user's canonical RVP address.  The
          home server may also act as a proxy.
       
       5.1.5.    Proxies
       
          Sending requests to a proxy can reduce network traffic.  The
          proxy can maintain only one subscription for public data, which
          means that if many of that proxy's users subscribe to the same
          data, the number of messages is greatly reduced.
       
          For controlled data, the proxy must send one subscription request
          for every RVP object that wishes to subscribe, so that the
          publishing server may check ACLs.  However, the number of
          messages is still low: the publishing server need only send one
          response to the proxy, which fans responses out to the individual
          subscribers.
       
          A proxy server may also be the home server for a user.  If the
          proxy server is different from the home server, and the client
          does not accept incoming connections from any source, then the
          client must either maintain a connection to the proxy or listen
          on a port accepting messages from the proxy.
       
       5.1.6.    Server-Server Interactions
       
          An RVP server can be any process that hosts RVP objects, proxies
          RVP messages, or receives RVP messages directly from any sender.
       
       
       
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          Servers interact with other servers using the same protocol as
          client-to-server interactions.
       
          Upon receiving a message, a server may respond by processing that
          message locally, updating its local persistent or dynamic data,
          proxying the message, or responding with an error.
       
          Because this represents a certain amount of dynamic state in the
          server, the client must refresh subscriptions whenever restoring
          a connection to a server and the server may throw away
          subscriptions when a client disconnects.  Some connectionless
          interactions with servers need to be refreshed periodically.
       
          Client-server connections may be via UDP or TCP/IP and server-
          server connections may be via UDP or TCP.  TCP and UDP
          interactions and client-server/server-server interactions are
          defined below in such a way to minimize dynamic server state
          while preserving network bandwidth and giving the clients a
          predictable level of reliability.
       
       5.1.7.    Subscriptions
       
          Subscriptions are usually registered ultimately at the server
          that publishes information.  The exception is the case where a
          proxy that receives a new subscription request already has a
          subscription to that data AND the data is public.  Since the
          proxy will already be getting notifications on that data, the
          proxy can fan out notifications to individual end-points.  Since
          the first subscription always goes to the publishing server, and
          the response indicates whether the data is public or controlled,
          the proxy knows how to handle additional subscriptions.
       
          A subscription request always includes:
           - The source of the subscription (for access control)
           - The call-back address
           - Desired timeout value for subscription
          The response to a subscription always includes:
           - Whether the events being subscribed to are public or
          controlled
           - What the subscription-ID is (so it can be referred to if it
          needs to be cancelled)
           - What timeout was assigned to the subscription
          - Whether the subscription must be cancelled if the subscriber
          goes offline
       
          If a client subscribes to a proxy server which in turn subscribes
          to a publishing server, then the client's RVP address is listed
          as the source, but the proxy's address is listed as the call-back
          address.  The subscription response (OK, failure) is proxied back
          to the originator.
       
       
       
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          The subscription originator stores the timeout value (a date/time                    formatted with RFC 1123 as specified in schema draft), so that
          the originator can refresh the subscription before it times out.
          When the originator wishes to cancel the subscription, it can do
          so by specifying the subscription-ID.  The same mechanism is used
          when the client goes offline, for any subscriptions listed as
          such.
          Issues:  Can one person subscribe another person?  How should we
          handle Reply-Tos that are different from the subscription
          requester?
       
       5.1.8.    Multiple client issues
       
          The issues for multiple clients connected at the same time
          haven't been worked out yet.  Which ones should receive instant
          messages?  If one client is "idle" and one client is "busy" what
          is the user marked as?
       
          Is there any way for another user to query the status of a
          particular client?  E.g. to see if the desktop is idle while the
          laptop is active & vice-versa?  Also, idea of addressing IM's to
          a particular online client?
       
          If multiple clients are connected, and the home server refers to
          one client, that client is primary.  Other clients must send
          their status to the primary client.
       
       6.   Security
       
       6.1. Authentication
       
          Users may log in to their home RVP server, but this is not
          required.  Each request MAY contain user credentials, using HTTP
          syntax, in order to authenticate the user to the server which
          will actually process the request.
       
          Upon receiving a request for the return or modification of
          information, the server processing the request MUST validate the
          authentication and honor any access control list entries which
          might gate the completion of the request.
          Return codes in HTTP style should be provided for indicating
          insufficient access rights.
       
       6.2. Content Protection
          In addition to authentication contained in the headers and
          intended for server authorization, the content (contained in the
          entity) MAY be signed and or sealed according to prevailing MIME
          encoding, signing and encryption standards.
       
       
       
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          This content signing/sealing is the mechanism for ensuring
          against message-stream modification, replay and/or repudiation.
       
       6.3. Server-to-Server Security
       
          Servers may exchange server credentials along with user
          credentials. Servers which are acting on a request will use
          embedded credentials to authenticate and authorize the requesting
          user or process.  Server implementations MAY choose to support
          SSL or other authentication mechanism, but this is generally
          redundant and highly consumptive of CPU resources.
       
          Private messages should be secured from endpoint to endpoint.
       
       6.4. Privacy Issues
       
          Each RVP server MUST support and honor ACLs placed on objects and
          properties in order to preserve the privacy of users.  These ACLs
          MUST allow the user to determine who may subscribe to, get
          properties from, put properties to, make/delete nodes within, and
          send messages to their user object and its child nodes.  The ACLs
          MUST support access-granting and access-denying entries as
          described in [6] and SHOULD also support the ask-first qualifier.
          Ask-first is essentially a deny access control entry which alerts
          the user to the access attempt and allows the user to grant
          access to people who have been denied access by that entry.
       
          RVP servers MUST NOT reveal the network address of a connected
          user to any other server or client, except through a LINK
          property set by the user.
       
       7.   RVP Methods
       
          RVP messages use the generic message format of RFC822. Messages
          consist of a start-line, one or more header fields, an empty line
          (CRLF), and an optional message-body.  The start-line contains the
          method name, a URL, and protocol version information.
          Generic message:
       
           [method] [URL] RVP/1.0
            *message-header
            CRLF
            [message-body]
       
       7.1. GET method (from HTTP)
       
          The GET method conforms to HTTP 1.1.  Supported headers include
          request-ID, Date, Expires, From Via, content-length, content-
          type, authorization, organization, proxy-authorization, user-
          agent.
       
       
       
       
       
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          In RVP, the GET method is used primarily to query properties such
          as the directory entry of a user object, or the number of members
          of a group object.  The property itself is listed in the XML-body
          of the message.  Where the property is unique to RVP, RVP is
          indicated as the namespace.
       
          Responses to the GET method are as defined by HTTP.
       
       7.2. PUT method (from HTTP)
       
          The put method is used as in HTTP/1.1.  It is used in RVP
          primarily to set property values that must be saved by the
          server.  Once again, properties and values are identified via XML
          syntax.  Responses to the PUT method are as defined by HTTP.
       
       7.3. MKCOL (from DAV)
       
          This method results in the creation of a new child node.  The
          method is defined in [7].  Responses to MKCOL as used in RVP
          should follow the DAV recommendations.
       
       7.4. RMCOL (from DAV)
       
          This method results in the removal of a node and all nodes and
          properties within and below it.  Also defined in [7]. Responses
          to RMCOL as used in RVP should follow the DAV recommendations.
       
       7.5. ACL (from DAV)
       
          See DAV ACL draft [6]
       
       7.6. SUBSCRIBE (introduced here)
       
          The Subscribe method is new and will be fully defined in this
          document. This is a new semantic method in the space of methods
          defined for HTTP and its derivatives.  Care has been taken to make
          this method flexible enough to meet the needs of other applications,
          which may require subscription and notification methods.
       
          Responses to the SUBSCRIBE method are defined later in this
          document.
       
       7.6.1.    Reply-To header
       
       7.6.2.    Notification-type header
       
          The notification-type header is used to specify what the user
          wants to subscribe to.  Events could include DEL, CREATE, MOVE,
          ANY, or could be defined in a very granular way with "XML".  A
          value of "XML" in the notification-type header indicates that the
          notification-type is defined in the XML-body of the message.
       7.6.3.    Using XML to specify properties
       
       
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          The body of the Subscribe message is text/XML, stating what
          properties the subscriber wishes to subscribe to.  Using XML
          makes subscriptions very flexible: users can subscribe to a
          particular value of a property, or to a property being refreshed
          as well as changed.  A subset of the full capabilities of XML
          will be used by RVP implementations designed to do presence
          information for people.  This subset is not yet fully defined and
          the authors are aware that they need to work on it.
          XML uses namespaces to avoid confusion in property names.  For
          RVP-specific properties, the namespace is 'rvp:'.  Other possible
          namespaces could be `dav:' or `http://www.iana.org/vcard/' or a
          custom namespace like `http://host.org/namespaces/engineering'.
       
          In the XML-body of subscriptions, the RVP-specific field "event-
          type" is defined. The content of the event-type field could be
          any of the following examples:
            Value="offline"
            Value=[any value of any property]
            Query
            Subscribe
            Change
            Update
            Refresh
            ...
       
          The event-type is used along with the property field to indicate
          what kind of event the subscriber is interested in.  For example,
          if the property is "status" and the event-type is "change", this
          is the way to monitor the online status of a user.  The
          "value=..." is used when the subscriber wishes to be notified of
          a specific value of a property.
       
          A server implementation MUST support the "change" event-type.
          The properties that must be supported are defined in the RVP
          Schema specification [2].  There will be other event-types
          defined which the implementation MAY support.
       
          The examples below show the most common contents of an XML-body
          (i.e. for subscribing to a user's status) and should also provide
          an idea how new kinds of notifications could be defined.
       
       7.7. UNSUBSCRIBE
       
          This new method is used to cancel a subscription before it times
          out.  The Subscription-ID (provided by the publishing server) is
          used, along with the RVP address of the node being subscribed to,
          to specify uniquely which subscription should be cancelled.
       
          It is up to the server implementation to decide who can cancel
          subscriptions.
       
       7.8. NOTIFY
       
       
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          The NOTIFY method is introduced for two reasons: it is how
          subscriptions are responded to and how instant messages are sent.
       
          It has been suggested that a notification protocol use POST to
          send notifications and instant messages, because these types of
          messages fit into the initial definition of POST.  However,
          others argue that overloading an existing method causes problems
          such as difficulties implementing firewall policies.  The issue
          has not been closed yet.
          The NOTIFY method also requires a Reply-To header.  It can use
          XML to specify what are recommended to be taken upon reception of
          the notification.
       
       8.   Responses
       
       8.1. GET response
       
          200 OK
       
          The GET response as used in RVP typically provides the value of
          the property that was queried.  Format is like HTTP 1.1.
       
          The value of the property queried is included in the body of the
          response.  The format of the property is indicated by MIME type
          in the content-type header.
       
       8.2. SUBSCRIBE response
       
          Responses to SUBSCRIBE methods must include a response code
          (typically an error or OK/SUBSCRIBED).
       
          A successful response to a SUBSCRIBE method must include privacy-
          level and Subscription-ID headers.
       
       8.2.1.    Privacy-level
       
          The privacy-level of a subscription indicates whether this
          subscription may be shared.
       
          A privacy level of PUBLIC means that anybody can subscribe to the
          same data, and will receive the same responses.  This can be used
          by proxies to reduce the amount of messages: if a proxy receives
          a second identical subscription to public data, the proxy need
          not subscribe a second time, but can simply forward the incoming
          data to all users subscribed to it.  The proxy is responsible for
          maintaining a subscription list in this case.
          A privacy level of CONTROLLED means that access to this
          subscription is controlled, but all subscribers will receive the
          same information.  A publishing server can use this to cut down
          on the number of notifications sent for the data.  When a proxy
       
       
       
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          receives two identical subscriptions, the proxy must forward both
          subscriptions so that the publishing server can return an error
          if either of those subscriptions is not allowed.  The publishing
          server now maintains the full list of subscribers, even if some
          of those subscribers use the same proxy.  When sending out
          notifications for that subscription, the publishing server can
          send only one notification to each unique Reply-To address.  The
          XML body of the notification must include a fan-out list with the
          list of all subscribers known to the publishing server that had
          that Reply-To address.  The proxy server need not maintain that
          same subscription list, but if it does it should reconcile the
          information with the received fan-out list.
          PRIVATE information is access-controlled and may not be identical
          for each subscriber, so no fan-out or single-instancing of
          subscriptions may be done.
       
       8.3. Refer
       
          Could use HTTP response "302 Moved Temporarily" for refer
       
       9.   Transport Considerations for Datagrams and Connections
       
          RVP message transport differs from HTTP/1.1 in two prominent
          ways:
       
           - Like HTTP/1.1, TCP connections are assumed to be persistent,
          and multiple requests on such connections may be pipelined --
          sent before responses to earlier requests have been received.
          However, RVP persistent TCP connections may carry requests and
          responses in either direction, and the responses may be delivered
          out-of-order relative to the requests that prompted them.
       
           - RVP offers the option of using connectionless, unreliable UDP
          as a transport.
       
          Except as otherwise noted, the policies and error-recovery
          behavior of RVP applications must comply with section 8 of the
          HTTP/1.1 specification. [5]
       
       9.1. Choice of Transport
       
          An RVP client or server may choose to use either TCP or UDP when
          sending a request to a remote server. A likely basis for making
          the choice would be the size of the request (or expected
          response) and the maximal transmission unit (MTU) between source
          and destination: TCP's reliability becomes more useful with each
          multiple of the MTU required to send a message.
       
       
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          Responses must use the same transport as the request which
          generated them. (Loosening this requirement deserves further
          investigation, as in the case where a small request, efficient to
          send over UDP, generates a large response, best sent by TCP.
          However, since this requires that the requestor accept inbound
          TCP, and additionally must be ready for a response to be the
          first message on an inbound connection, additional transport-
          negotiation constructs may be necessary to enable this scenario.)
       
          A mechanism (TBD) may be provided for a servers to specify that a
          particular resource is only available through one or the other
          transport.
       
       9.1.1.    Matching Responses To Requests
       
          RVP's UDP and persistent TCP pipelining allow the possibility
          that responses to outstanding requests will arrive out-of-order.                              Additionally, UDP responses are not associated with a request via
          a persistent connection, and if a request is retried while a
          response is in transit, a requestor may receive duplicate
          responses.
       
          Thus, requests MAY include a "Request-ID" header, which, if
          present, MUST be echoed in the response to a request.
       
       9.2. UDP Retry Policy
       
          When using UDP transport, responses serve as the confirmation
          that the matching request was received. The lack of a timely
          response indicates either that the request should be resent or
          considered undeliverable, in accordance with the retry and
          timeout policies the requestor has adopted.
       
          RVP applications are free to choose their own policies for
          resending unacknowledged requests, within the following
          parameters:
       
           - A single request should be resent no more than 10 times.
           - A single request should be resent no more than once per
          second.
           - A single request should not be resent more than 90 seconds
          after the initial send.
           - If multiple retries fail to generate a received response
          within the desired amount of time, an application should wait at
          least 30 seconds before performing any operation which
          effectively restarts the retry-cycle on a similar transaction
          with the same remote host.
       
       
       
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          For example, an acceptable retry policy within these parameters
          would be: resend a request every 3 seconds, up to 5 times, until
          a response is received; give up if no response is received within
          30 seconds after the initial send, and refrain from sending a
          similar request to the same destination until 60 seconds after
          the initial send.
       
          Beyond these guidelines, applications are free to choose their
          own retry repetition, frequency, and timeout policies. By
          separate, explicit agreement (through mechanisms unspecified),
          communicating RVP programs may adopt retry policies outside these
          parameters.
       
          If responses are lost or delayed in transit, the responder may
          receive duplicates of an already-handled request. Subsequent
          requests from the same host and port, bearing the same "Request-
          ID", MUST be handled in a way that renders them idempotent (as
          defined in Section 9.1.2 of HTTP/1.1 [5]) within a reasonably
          long time window. (The above retry parameters suggest 3-5 minutes
          is a reasonably long window for recognizing duplicates.)
       
          (Open issue: Does DNS provide a better model for UDP retries?)
       
       9.3. Mixed-Transport Through Proxies
       
          An RVP client may intially send a request via reliable TCP to its
          proxy, which may then use unreliable UDP to forward that request
          to its final destination. In all such cases, it is the
          responsibility of the RVP application which chose UDP transport
          to implement a retry and timeout policy. If a timely response is
          not received, the proxy machine must return a 504 ("Gateway
          Timeout") error-response to the request originator owed a
          reliable response.
       
       10.  Protocol Details
       
       10.1.     Identity URLs, Location URLs, and default port
       
          RVP identities and locations are both described as URL addresses,
          as specified in RFC1738. The scheme of a RVP URL is "rvp", and
          the syntax follows the "Common Internet Scheme Syntax" described
          in Section 3.1 of RFC1738. Further information about RVP URLs is
          available in [4].
       
          It is always clear from context whether an RVP URL address is
          being used as an identity or a location. An identity URL may also
       
       
       
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          be interpreted as the "canonical location" to interact with an
          object.
       
          For example, identity URLs are used in the "From" header, and as
          the principals in ACLs. Location URLs are used in the "Reply-To"
          header, and as the destination URLs of "LINK" properties (see
          section 9.X). Location URLs are not typically visible or useful
          for end-users.
       
          If not explicitly specified, the default port value in an RVP URL
          is XXXX. (TBD)
       
       10.2.     Headers
       
          Header fields follow the same generic header format as that given
          in Section 3.1 of RFC 822 [5].  Each header field consists of a
          name followed by a colon (":") and the field value. Field names
          are case insensitive. The field value may be preceded by any
          amount of leading white space (LWS), though a single space (SP)
          is preferred. Header fields can be extended over multiple lines
          by preceding each extra line with at least one SP or horizontal
          tab (HT). Applications SHOULD follow HTTP "common form" when
          generating these constructs, since there might exist some
          implementations that fail to accept anything beyond the common
          forms.
       
          The order in which header fields are received is not significant
          if the header fields have different field names. Multiple header
          fields with the same field-name may be present in a message if
          and only if the entire field-value for that header field is
          defined as a comma-separated list (i.e., #(values)). It MUST be
          possible to combine the multiple header fields into one "field-
          name: field-value" pair, without changing the semantics of the
          message, by appending each subsequent field-value to the first,
          each separated by a comma. The order in which header fields with
          the same field-name are received is therefore significant to the
          interpretation of the combined field value, and thus a proxy MUST
          NOT change the order of these field values when a message is
          forwarded.
       
          Field names are not case sensitive, although their values may be.
       
       
       10.2.1.   Accept-charset
       
       
          As in HTTP/1.1, support for the ISO-8859-1 character set can be
          assumed.
       
       10.2.2.   Accept-encoding
       
          As in HTTP/1.1
       
       10.2.3.   Accept-Language
       
       
       
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          As in HTTP/1.1
       
       10.2.4.   Allow
       
          As in http/1.1: used in 405 (Method Not Allowed) response to
          indicate which methods are allowed by a resource.  In RVP, the
          resource will be either a RVP object or a property.
       
       10.2.5.   Authorization
       
          See section 14.8 of the HTTP/1.1 specification [5]
       
       10.2.6.   Reply-To
       
          Used with Subscribe, GET, notifications, etc. to specify where to
          send a response.
       
       10.2.7.   Content-Encoding
       
          See section 14.12 of the HTTP/1.1 specification [5].
       
       10.2.8.   Content-Language
       
          See section 14.13 of the HTTP/1.1 specification [5].
       
       10.2.9.   Content-Length
       
          See section 14.14 of the HTTP/1.1 specification [5].
       
       10.2.10.  Content-Type
       
          See section 14.18 of the HTTP/1.1 specification [5].
       
       10.2.11.  Date
       
          See section 14.19 of the HTTP/1.1 specification [5].
       
       10.2.12.  From
       
          The RVP object that the message is from.  Can be different from
          Reply-To.
       
          Requests MUST contain this header.  Responses SHOULD contain this
          header.  The 'From' header contains the RVP URL for the object
          that represents the user or process that initiated the request.
       
          See section 14.22 of the HTTP/1.1 specification [5].
       
       10.2.13.  Host
       
          See section 14.23 of the HTTP/1.1 specification [5]: The Host
          request-header field specifies the Internet host and port number
       
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          of the resource being requested, as obtained from the original
          URL given by the user or referring resource.
       
       10.2.14.  If-Modified-Since
       
          See section 14.24 of the HTTP/1.1 specification [5].  Used as a
          conditional in a GET request.
       
       10.2.15.  If-Unmodified-Since
       
          See section 14.28 of the HTTP/1.1 specification [5].  Used as a
          conditional in a GET request.
       
       10.2.16.  Last-Modified
       
          See section 14.29 of the HTTP/1.1 specification [5].  Used in a
          response to a GET request or the first response to a SUBSCRIBE.
       
       10.2.17.  Request-ID
       
       
          A token to uniquely identify a request to the requestor, opaque
          to the destination of a request. When generating a response to a
          request that has a "Request-ID" header, the "Request-ID" header
          must be including verbatim in the response
       
       10.2.18.  Server
       
          See section 14.39 of the HTTP/1.1 specification [5].  Optionally
          used to specify server software version.
       
       10.2.19.  Session-ID
       
          Sent with a NOTIFY message when there is no subscription-ID; used
          to maintain context in replies to that notification.
       
       10.2.20.  Subscription-ID
       
          Sent with a successful response to a subscription message.  Used
          in future notifications which result from that subscription, or
          to cancel subscription.
       
       10.2.21.  Via
       
          As in HTTP/1.1, used when a message is proxied.
       
       10.2.22.  XML-body
       
          Used to specify ACL levels with the ACL method, and sometimes to
          specify notification-type.  Not fully defined yet.  In
          particular, the authors need to define what subset of all
          possible XML semantics must be supported by an implementation.
       
       
       
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          It must be possible for a simple implementation to refuse complex
          XML bodies not defined as required in this draft.
       
       10.3.     Responses
       
          HTTP/1.1 responses are expanded to include these new codes or
          enhanced meaning:
          RVP/1.0 207 SUBSCRIBE
          RVP/1.0 ??? REFER  (or we could use 302 "Moved Temporarily")
       
          Existing HTTP/1.1 responses used in RVP:
       
          HTTP/1.1 400 Bad Request  (bad syntax)
          HTTP/1.1 401 Unauthorized (Need to get authenticated first)
          HTTP/1.1 402 Forbidden (ACLs forbid user to do that)
          HTTP/1.1 404 Not Found (object or property not found)
          HTTP/1.1 501 Not Implemented (method not understood by server)
          HTTP/1.1 503 Service Unavailable (i.e. too busy)
       
       10.4.     Proxying and Referral
       
          The default behavior for a RVP object on a server is for that
          server to respond to queries and subscriptions and receive
          instant messages for that object.  However, there are exceptional
          circumstances.  Both servers and clients can optionally support
          these features.
       
          Each RVP object may have a property named "LINK".  The value of
          the LINK property is [link-method], [RVP-URL].  When the property
          is set using the PUT method, there may be a timeout property
          included with standard timeout syntax (never, date/time, logout).
          If the expiration is reached before the property value is
          refreshed, the server should reset the LINK property to NULL.
       
          The link-method can be PROXY, REFER, REFER-NOTIFICATIONS or LINK.
          The RVP-URL must be a full URL, including DNS address and path.
       
          Proxying a node means that rather than respond for the object,
          the server will forward requests to the address listed in the
          RVP-URL, and will also proxy replies. If the server is ever
          unable to proxy because the host has become unavailable, it
          should immediately timeout the LINK property and go back to
          handling queries itself.   Proxies are intended to be used with
          firewalls: a RVP proxy can sit on a firewall and route messages.
       
          Referring an object means that the server will respond to any
          request, subscription or instant message with a referral to the
          address listed in the RVP-URL, requiring the originating client
          of the request, subscription or message to repeat it to the
          referred address.  It may be desirable for the server to PING the
          referral address periodically to ensure that the referral is
          still valid: if the host is no longer available the value of LINK
          might go back to null.
       
       
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          "REFER-NOTIFICATIONS" indicates that:
           - Unsolicited messages to the user should go to the referral
          address
           - Requests should continue to come to the object's home
           - Notifications in response to a subscription should continue to
          be sent to the Reply-To specified in the subscription.  The
          Reply-To address should always be tried first.
       
          A LINK value of LINK typically indicates a link to the "real"
          home of the content requested: this allows content to be linked
          to from more than one node.  One node is the real home for the
          object and all duplicate nodes link to the real home.
          When an entity decides to start hosting an object by setting the
          LINK field on that object's home node, it may wish to download
          all the object data including the subscription list from the
          server that was previously responsible for it. Ideally the GET
          command which asks for all the node’s data would be bundled with
          the PUT command which switches responsibility using the LINK
          property.  If this is not possible, then the GET command should
          be done before the PUT LINK property command, because once the
          LINK property is in place the server will not respond directly to
          requests for that node and its contents are effectively hidden.
          The only request the server will respond directly to is a request
          to reset the LINK property.
       
          If the server does support the LINK property, then for any
          message to that server, the server must check each node and sub-
          node in the hierarchy starting from the root until it reaches the
          target node, checking each node for a LINK entry.  If a LINK
          entry is encountered at any point, the server needs to follow the
          functionality defined by the link-method.
          If the server does not support the LINK property at all, it
          should respond to a PUT LINK command with an error that means
          "property not supported".  If the server does not allow the
          sender to modify the LINK property, it should respond with an
          error that means "access denied".  If the server supports the
          LINK property but not the PROXY functionality, then if it
          receives a command to set the link_method of the LINK property to
          PROXY, it should respond with an error that means "syntax not
          supported".  Same thing if the REFER or LINK functionality is not
          supported.
       
       10.5.     Peer to peer Messaging
       
          Peer to peer communication can be desirable to lighten the load
          on servers.  This can be done in several ways.
       
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       10.5.1.   Getting direct responses to requests
       
          When the client initiates communication by sending a message (a
          query, subscription or instant message), the client can send the
          message directly to the publishing server rather than to a proxy
          server.  The sender header should include the full RVP Reply-To
          URL (e.g rvp://lisadu.host2.com/lisadu) as well as the full RVP
          identity of the user (e.g. rvp://rvp.host2.com/users/lisadu).
          When the response comes, it comes directly back to the client. In
          sending messages directly to the publishing server, the client
          reveals its DNS address and is therefore more open to attack, but
          the risk is limited by the fact that the IP address only goes to
          targets chosen by the user.  If the initial message goes outside
          a firewall, that firewall must be able to pass incoming RVP
          messages (responses) to the RVP Reply-To URL.
       
       10.5.2.   Responding directly to notifications
       
          When the client receives a notification from a trusted source
          through the client's local proxy and wishes to continue the
          conversation, the client can choose to respond directly to that
          source rather than through the local proxy.  Once again the
          Reply-To header points to the client machine and the From header
          points to the user's RVP object location.
       
       10.5.3.   Receiving instant messages directly
       
          If the client is willing to accept instant messages (unsolicited
          notifications) directly, it should set the LINK property for its
          RVP object on the server to REFER-NOTIFICATIONS plus its full
          client-side RVP URL.  The timeout for this property should be set
          to a reasonable value in case the client goes offline
          unexpectedly.  When the server receives unsolicited notifications
          (notifications which are not in response to a subscription) it
          can respond with a referral message including the REFER-
          NOTIFICATIONS and the client-side RVP URL.
          If the client is willing to accept unsolicited notifications and
          its server does not support the LINK property, then when the
          server proxies instant messages the client can respond with a
          success message and include the REFER-NOTIFICATIONS field,
          timeout, and URL itself.  The proxy will forward this back to the
          original sender, who can now start using the client URL until the
          timeout listed.
          Queries and subscriptions should still be sent to the Reply-To
          address used in the original request message.
       
       10.5.4.   Receiving Requests Directly
       
          If a client wishes to handle queries and subscriptions to its
          user object data as well, it must ask the server to refer all
          messages, using the LINK property as outlined in the section on
          "Proxying, Referring and Linking nodes".
       
       
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       10.6.     Proxy and Referral Conflicts
       
          Note that the referral functionality allows clients to assume
          responsibility for their own user’s nodes, which means that the
          client becomes its own server.  In the case where a user switches
          between clients (presumably on different hardware), this could
          lead to conflict.  If client A sets the LINK property to proxy to
          A, then client B tries to sets the LINK property to proxy to B,
          what should happen?
       
           - If B does not have sufficient access, server responds with an
          error meaning "access denied".
       
           - Or, if the server does not allow a client to take over node
          which is already referred or proxied, the server can reply with
          an error meaning "locked" or something like that
       
           - Or, if the link_method of the LINK property is set to REFER,
          the server responds with a referral to client A.
       
           - Or, if the link_method of the LINK property is set to PROXY,
          the server proxies the PUT command to client A.
       
          When client A receives the PUT command for the LINK property,
          either through proxy or directly, it realizes that another client
          is trying to take control of the node.  The options for client A
          are now:
       
           - Reply with an error "access denied" which is either proxied or
          sent directly to client B
       
           - Reply with a success message which is either proxied or sent
          directly to client B, after setting everything right on the
          server by refreshing all data on the node, including setting the
          LINK property to the new value given by client B.
       
       11.  Examples
       
       11.1.     Subscribing directly to status changes
       
          This example shows a client acting as its own server in
          subscribing directly to a user's RVP status.  In this case the
          client MUST accept incoming connections because the publishing
          server needs to send notifications back to the client.
       
          Client to publishing server:
       
            SUBSCRIBE rvp://host.com/joeuser rvp/1.0
              Notification-type: XML
              Reply-To: rvp://host2.com/lisadu
              From: rvp://host2.com/lisadu
              Content-type: text/XML
              Content-length: xyz
       
       
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              [CRLF]
                <?xml version="1.0"?>
                <?namespace name="rvp:" as="rvp">
                <property><rvp:status/></property>
                <rvp:event-type>change</rvp:event-type>
                <rvp:timeout> 06 Nov 1998 08:49:37 GMT </rvp:timeout>
                <rvp:subscriber-id> rvp://host2.com/lisadu
                  </rvp:subscriber-id>
       
          Publishing server to client (success):
       
            RVP/1.0 200 SUBSCRIBE
              Subscription-ID: 84
              Content-type: text/xml
              From: rvp://host.com/joeuser
              [CRLF]
                <?xml version="1.0"?>
                <?namespace name="rvp:" as="rvp">
                <property><rvp:status>online</rvp:status></property>
                <rvp:event-type>change</rvp:event-type>
                <rvp:timeout> 06 Nov 1998 08:49:37 GMT </rvp:timeout>
                <privacy-level>controlled</privacy-level>
       
          Notice in this example that the publishing server set a lower
          timeout for the subscription than was requested by the
          subscriber.
       
          Notification sent as a result of this subscription:
       
            NOTIFY rvp://host.com/joeuser
              Subscription-ID: 84
              Reply-To: rvp://host.com/joeuser
              From: rvp://host.com/joeuser
              Notification-type: XML
              Content-type: text/XML
              Content-length: xyz
              [CRLF]
                <?xml version="1.0"?>
                <?namespace name="rvp:" as="r">
                <Property><rvp:status>online</rvp:status></property>
                <rvp:event-type>change</rvp:event-type>
          Refresh method for this subscription:
       
            Not yet defined.
       
          Unsubscribe method for this subscription:
       
            UNSUBSCRIBE rvp://host.com/joeuser rvp/1.0
              Call-id: 84
       
       11.2.     Subscribing through a proxy to a group:
       
       
       
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          Message from client to proxy:
       
            SUBSCRIBE rvp://host.com/groups/rvpdisc
              Notification-type: XML
              Reply-To: rvp://client.host2.com/lisadu
              From: rvp://rvp.host2.com/lisadu
              XML-body:
                <?namespace name="rvp:" as="rvp">
                <rvp:event-type>message</rvp:event-type>
                <rvp:subscriber-id> rvp://rvp.host2.com/lisadu
                  </rvp:subscriber-id>
       
          Note that the Reply-To address is a URL to the user's client
          machine, whereas the subscriber-id is a pointer to a user's RVP
          object.  Sometimes these are the same, but in this case the
          object is hosted on a RVP server, whereas the user receives
          messages at their client.  Also note that no timeout is defined,
          because the user wants to use the max or default timeout for that
          subscription list.
       
          Message from proxy to publishing server:
       
            SUBSCRIBE rvp://host.com/groups/rvpdisc
              Notification-type: XML
              Reply-To: rvp://proxy.host2.com/lisadu
              From: rvp://rvp.host2.com/lisadu
              Via: RVP/1.0 rvp://proxy.host2.com/lisadu
              Content-type: text/XML
              Content-length: xyz
              [CRLF]
                <?xml version="1.0"?>
                <?namespace name="rvp:" as="rvp">
                <rvp:event-type>message</rvp:event-type>
                <rvp:subscriber-id>rvp://rvp.host2.com/lisadu
                  </rvp:subscriber-id>
       
          Successful response from publishing server to proxy:
       
            RVP/1.0 200 SUBSCRIBE
              Subscription-ID: 1238
              Content-type: text/xml
              Content-length: xyz
              From: rvp://host.com
              [CRLF]
                <?xml version="1.0"?>
                <?namespace name="rvp:" as="rvp">
                <rvp:event-type>message</rvp:event-type>
                <rvp:timeout> 06 Nov 1998 08:49:37 GMT </rvp:timeout>
                <privacy-level>controlled</privacy-level>
       
          Response forwarded from proxy to client:
       
            RVP/1.0 200 SUBSCRIBE
              Subscription-ID: 1238
       
       
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              Content-type: text/xml
              Body:
                [same body]
       
          Notification from publishing server (list server) to proxy:
       
            NOTIFY rvp://host.com/groups/rvpdisc RVP/1.0
              Subscription-ID: 1238
              Reply-To: rvp://fred.host3.com/fred
              From: rvp://host3.com/users/fred
              Notification-type: XML
              XML-body:
                <?xml version="1.0"?>
                <?namespace name="rvp:" as="r">
                <r:event-type>Message</r:event-type>
                <r:priority>high</r:priority>
                <r:action>display</r:action>
                <r:fanout>rvp://rvp.host2.com/lisadu</r:fanout>
              Content-type: text/plain
              [CRLF]
              Hey everybody, I've got something important to say.
       
          The same message is forwarded from the proxy to the subscriber,
          minus the fan-out list.  The fan-out only shows one user, because
          only one user on this proxy is currently subscribed to the list.
       
          The original sender of the message is the user 'fred' on another
          system.  His client machine address is listed as the Reply-To
          because responses to messages to this list are intended to go to
          the sender.  (On other lists, the Reply-To might be the list
          itself, and the source would still be fred)
       
       11.3.     Setting ACLs on the status property
       
          Client to home server:
       
            ACL rvp://host.com/joeuser RVP/1.0
              Content-type: text/XML
              [CRLF]
                <?xml version="1.0"?>
                <?namespace name="rvp:" as="r">
                <property><r:status></property>
                <begin>
                  grant subscribe rvp://host.com/lisadu
                  deny subscribe rvp://host.com/baduser
                </begin>
       
       11.4.     Sending an instant message
       
          Client to target's home server:
       
            NOTIFY rvp://host.com/joeuser RVP/1.0
              Reply-To: rvp://fred.host3.com/fred
              From: rvp://host3.com/users/fred
       
       
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              Notification-type: XML
              Session-ID: 4321
              XML-body:
                <?xml version="1.0"?>
                <?namespace name="rvp:" as="r">
                <r:event-type>Message</r:event-type>
                <r:priority>high</r:priority>
                <r:action>display</r:action>
              Content-type: text/plain
              Content-length: 7
              [CRLF]
              Hi Joe.
       
          The action taken by the home server depends whether the home
          server refers instant messages or proxies them. If the home
          server proxies, it should send a notification to the client with
          the Reply-To replaced by its own RVP address.  When it receives a
          success message, and forward the success message back to
          rvp://fred.host3.com/fred.
       
          If the home server refers instant messages to Joe's proxy or to
          Joe's machine, it replies back with a referral response:
       
            RVP/1.0 ??? REFER
              Session-ID: 4321
              Referral-type: Notifications
              Referral-address: rvp://proxy.host.com/joeuser RVP/1.0
       
          The referral-type header indicates which kinds of messages should
          go to the referred machine rather than to the home server.  The
          only defined values so far are "notifications" and "all". In this
          case, requests (queries & subscriptions) for information should
          still go to the home server, but notifications (typically
          intended for the user's eyes) should go to the referred address
          IF THERE IS NO REPLY-TO SPECIFIED FOR THAT NOTIFICATION.
       
          The session-ID can also be used for future messages in the
          conversation between Joe and Fred, to provide context.
       
       11.5.     Inviting a user to a session defined by SIP
       
          Example is not done yet.  It will be a NOTIFY message with a
          content-type of application/SIP and a body which is a SIP
          message.
       
       12.  REFERENCES
       
          [1] S. Bradner, "Key words for use in RFCs to indicate
          requirement levels," RFC 2119, Internet Engineering Task Force,
          Mar. 1997.
       
          [2] Dusseault L. and Bone J. "RVP Schemas", INTERNET-DRAFT <draft-
          dusseault-rvp-schema-00.txt>, March 1998.
       
       
       
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          INTERNET-DRAFT                RVP                   March 13, 1997
       
       
          [3] Dusseault L., "Presence Information Protocol Requirements",
          INTERNET-DRAFT <draft-dusseault-pipr-00.txt>, February 1998.
       
          [4] Dusseault L. and Mohr G.  " Addressing and Location for RVP",
          INTERNET-DRAFT <draft-dusseault-rvp-addr-00.txt>, March 1998.
       
          [5] Fielding R., Gettys J., Mogul J., Frystyk H. and Berners-Lee
          T., "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2068, January
          1997.
       
          [6] Leach P. J. and Goland Y. Y., "WebDAV ACL Protocol",
          INTERNET-DRAFT <draft-ietf-webdav-acl-00.txt>, November 1997
       
          [7] Goland Y. Y., Whitehead E. J., Faizi A., Carter S. R. and
          Jensen D., "Extensions for Distributed Authoring on the World
          Wide Web -- WEBDAV", INTERNET-DRAFT <draft-ietf-webdav-protocol-
          07>, January 1998.
       
       13.  Authors' Addresses
       
       
          Martin Calsyn
       
          Microsoft Corporation
          One Microsoft Way
          Redmond, WA 98052
       
          EMail: martinca@microsoft.com
          Fax: (425) 936-7329
       
       
          Lisa Dusseault
       
          Microsoft Corporation
          One Microsoft Way
          Redmond, WA 98052
       
          EMail:  lisadu@microsoft.com
          Fax: (425) 936-7329
       
       
          Gordon Mohr
       
          Activerse, Inc.
          1301 W. 25th St
          Suite 500
          Austin, TX 78705
       
          Email: gojomo@activerse.com
       
       
       
       
       
       
       
       
          Calsyn, Dusseault & Mohr                                  [Page 29]