Sharing Files with the Session Initiation Protocol (SIP)
draft-garcia-sipping-file-sharing-framework-01

Status of this Memo

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Abstract

This memo proposes a SIP framework used for advertising and searching for shared files within a given community. The memo defines the signaling that users to announce the availability of files stored in their User Agents (UA). It also provides the signaling for users to perform searches of available files and monitor changes in those files. Additionally, this memo describes the signaling used to access a file. These methods can be used in (but are not limited to) SIP peer-to-peer systems based on centralized, semi-centralized or fully distributed architectures.

Table of Contents

1.  Introduction
2.  Definitions and Document Conventions
3.  Use Cases
    3.1.  File Publication
    3.2.  File Search
    3.3.  File Directory Through Presence Information
    3.4.  File Download
4.  Publication of File Metadata
    4.1.  File Metadata Publication in Support of Search Operations
        4.1.1.  Initial File Metadata Publication
        4.1.2.  Publication of Modified File Metadata
        4.1.3.  Actions Performed by the ESC
    4.2.  File Metadata Publication in Support of Directory Operations
5.  Search Operation
    5.1.  Sending a Search Request
    5.2.  Reporting Search Results
    5.3.  Propagating Searches
        5.3.1.  Searching Based on Flooding
        5.3.2.  Searching Based on Distributed Hash Tables (DHT)
    5.4.  Terminating a Search Request
    5.5.  Example of a Search Filter
6.  Directory Operations Through Presence Information
7.  Downloading a file
8.  Security Considerations
9.  IANA Considerations
10.  References
    10.1.  Normative References
    10.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

The Session Initiation Protocol (SIP) (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261] is a text-based protocol for initiating and managing communication sessions. The protocol is extended by the SIP-events framework (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.) [RFC3265] to provide a mechanism whereby a user can subscribe to state changes of resources and get notifications when the state of the resource changes. SIP also provides a publication mechanism (Niemi, A., “Session Initiation Protocol (SIP) Extension for Event State Publication,” October 2004.) [RFC3903] that allows a user to supply resource metadata related to the state and changes in the state of such resource. A 'file' event package (Garcia-Martin, M. and M. Matuszewski, “A Session Initiation Protocol (SIP) Event Package and Data Format for File Metadata Description,” November 2007.) [I‑D.garcia‑sipping‑file‑event‑package] is used to allow SIP User Agents to publish, subscribe, and get notifications of the availability of files, such as images, video files, audio files, etc. The 'file' event package reuses the XML Data Format for descibing files (Garcia-Martin, M. and M. Matuszewski, “An Extensible Data Format (XML) for Describing Files,” November 2007.) [I‑D.garcia‑app‑area‑file‑data‑format]. All these building blocks can be easily combined to provide a mechanism whereby users can supply the availability of files stored in their user agents. The mechanism can also provide a directory search within a publishing community, so that members of the community can search for the availability of files that have been made available by other members of the same community, and then further download selected files.

Think for example of a user, Alice, who wants to make a set of image files available to members of her family. She sets up a SIP peer-to-peer network with her family, and publishes the file metadata describing her available files. The file metadata is stored in the peer nodes, for example, in each Alice's family user agents. Then, Bob, a member of the same SIP peer-to-peer network, wants to acquire those pictures, tagged with a keyword 'vacation'. He defines the search criteria; his SIP UA creates an appropriate filter and sets up a short subscription by sending it to the SIP peer-to-peer network. Then he receives notifications from the different peer nodes, containing a metadata describing the searched files.

In another scenario, a centralized server can be used to aggregate all the state file metadata. This might be useful in cases where several instances of the same file are available at different SIP user agents. The server will act as a state agent (defined in RFC 3265 (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.) [RFC3265]) and Event State Compositor (ESC) (defined in RFC 3903 (Niemi, A., “Session Initiation Protocol (SIP) Extension for Event State Publication,” October 2004.) [RFC3903]). The aggregation the server performs relieves the endpoints from doing the aggregation itself, so this is an interesting scenario in deployments that involve endpoints with limited processing capability and network bandwidth.

A hybrid scenario is also possible, where, for example, User Agents act as secondary nodes (ordinary peers) in a SIP peer-to-peer network. ESCs are primary nodes (super peers). In this scenario, publication of file metadata and search operations takes place between the secondary and the primary nodes. The primary nodes keep the state consistent among themselves proactively according to a well-defined Distributed Hash Table (DHT) algorithm (e.g. Chord), or alternatively, distribute the search request among themselves reactively when a file is needed.

This memo describes a framework where SIP is used for advertising and searching for shared files. The framework defines the signaling used for users to signal the availability of files stored in their User Agents (UA). It also describes the signaling used for users to perform searches of available files and monitor changes in existing files. Additionally, signaling used to download a file from a remote UA is provided. These methods can be used in (but are not limited to) SIP peer-to-peer systems based on centralized, semi-centralized or fully distributed architectures. While other protocols and mechanisms can be used to achieve similar purposes, it is a beneficial to provide the means to use SIP in order to minimize the protocol implementation support, especially in endpoints with limited resources.

2.  Definitions and Document Conventions

In addition to the definitions of RFC 3265 (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.) [RFC3265], and RFC 3903 (Niemi, A., “Session Initiation Protocol (SIP) Extension for Event State Publication,” October 2004.) [RFC3903], this document introduces the following new terms:

Community:

A collection of loosely coupled SIP user agents that agree to share files among members of the community. A community can be composed of, e.g., an enterprise, a group of friends, family members, or members of a club. The community concept implies that there are only duly authenticated and authorized users who can access files.

File metadata:

A set of properties describing a file. The file metadata can include the hash of the file, its name, creation date, Uniform Resource Name (URN), Uniform Resource Identifier (URI), and other relevant information.

File descriptor:

A subset of the file metadata that uniquely identifies the file.

File directory:

A device storing the file descriptors of a set of files; also called ESC in this document.

Search operation:

Signalling issued by a user to get information of the available files and its associated metadata. Typically search operations are delimited with search filters.

Search filter:

A set of properties used in search operations to set the limits of the search, based on user's input. A search filter can consist of, e.g., a file name, file type, a files description, etc.

File transfer operation:

An operation whereby a UA downloads a file from a remote UA.

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119] and indicate requirement levels for compliant implementations.

3.  Use Cases

This section describes a number of use cases that are addressed later in this document. The use cases are just examples, and do not intend to limit the applicability of the file sharing framework.

3.1.  File Publication

Alice is on holiday in Monaco. While visiting the Casino, she sees a famous painting which she takes a picture of with her camera phone. After taking the photo, she tags it with the following tags: Alice, Holiday, Monaco, Painting, Casino. These tags are there to help her and her friends to locate the picture later.

Alice knows that her friends are also interested in art, so she wants to make this picture available for anyone to download. Alice selects the picture of the painting, along with some other pictures she took later that day, in the picture browser application and selects the publish option to make the picture available for others to see. Once the selection of shared files is done, the SIP UA publishes the availability of those pictures towards an Event State Compositor (ESC). The actual files are not transmitted until someone requests them.

File publication is further discussed in Section 4.1.1 (Initial File Metadata Publication).

3.2.  File Search

While talking with Charlie on the phone, Bob learns that Alice is currently on vacation in Monaco. Bob knows that Alice likes to take photos and share them with her friends, so he opens up his search application and types in a few keywords: 'Alice' and 'Monaco'. Once Bob hits the search button, his SIP UA sends the search message to the ESC. After a while, the ESC sends the search results back to Bob's SIP UA in a series of notifications. Now Bob can see the names of all pictures Alice has taken when she was in Monaco. Bob's application may also download and display thumbnails of the pictures. Bob also finds a couple of pictures taken by Alice's friend, Eve, which have been tagged with the tags: 'Alice' and 'Monaco'.

Dave is a student of art. On the bus he meets his friend Eve. While chatting, Eve tells about the painting she has seen on her recent visit in Monaco. Dave wonders if there are some pictures of it, and enters the keywords 'Monaco' and 'Painting' into the application on his mobile phone. Dave hits the search button, and his SIP UA sends the search message to the ESC. After a while, the ESC sends the search results back to his SIP UA in a series of notifications. The application displays a list of files matching the keywords, including the pictures Alice and other visitors have taken. To his surprise, Dave also finds a video stream presenting the art museums of Monaco.

Search operations are further discussed in Section 5 (Search Operation).

3.3.  File Directory Through Presence Information

Charlie is a good friend of Alice. Therefore he is interested to know about new pictures that Alice publishes. In this case he can just subscribe to Alice's presence information. Attached with conventional the presence information, he receives the information about the files Alice is hosting in her UAs.

Instead of periodical searching for files tagged with Alice's name, Charlie can just subscribe to Alice's presence information, and get notification every time Alice adds new pictures to her shared files.

The same file browsing functionality can be used also in multi-user chat between Alice, Charlie, Eve, and Bob. In the chat application, Bob sees names of every participant in the user list displayed on his screen. When clicking anyone's name, he gets list of files that the selected participant is hosting attached with the conventional presence information of this person.

This document does not specify implementation of the file browsing via presence information. A solution is described in the Internet-Draft 'File Descriptions Extension to the PIDF' (Garcia-Martin, M. and M. Matuszewski, “File Descriptions Extension to the Presence Information Data Format (PIDF),” June 2007.) [I‑D.garcia‑sipping‑file‑desc‑pidf].

File directory through presence information is further discussed in Section 6 (Directory Operations Through Presence Information).

3.4.  File Download

Once a Bob has found an interesting file called 'Alice and Eve at the Casino', e.g., by using the search functionality or by browsing Alice's presence information for files, he wants to display that picture on his device. To initiate the download, Bob selects the picture and hits the download button. Bob's SIP UA sends a download request to Alice's SIP UA. Alice's terminal will automatically approve the request and their UAs will establish a file transfer session. After the file transfer, Bob is presented with a dialog of file transfer completion, and asked if he wants to open the file immediately in the picture viewer.

Section 7 (Downloading a file) provides further discussion on downloading files.

4.  Publication of File Metadata

Publication of File Metadata is based on the PUBLISH method specified in RFC 3903 (Niemi, A., “Session Initiation Protocol (SIP) Extension for Event State Publication,” October 2004.) [RFC3903]. We proposed two variants of publication, depending on whether the publication supports search operations or directory operations. To support the former, publication is done with the 'file' event package (Garcia-Martin, M. and M. Matuszewski, “A Session Initiation Protocol (SIP) Event Package and Data Format for File Metadata Description,” November 2007.) [I‑D.garcia‑sipping‑file‑event‑package] and the 'file-metadata' XML format (Garcia-Martin, M. and M. Matuszewski, “An Extensible Data Format (XML) for Describing Files,” November 2007.) [I‑D.garcia‑app‑area‑file‑data‑format]. To support the later, the Presence Information Data Format (PIDF) (Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and J. Peterson, “Presence Information Data Format (PIDF),” August 2004.) [RFC3863] is extended to provide a description of available files together with the presence information of the presentity.

4.1.  File Metadata Publication in Support of Search Operations

4.1.1.  Initial File Metadata Publication

Initial file metadata publication is perfomed to publish metadata about the availability of one or more files to the ESC. Figure 1 (Signaling flow for publication of file metadata) presents the signaling flow required for an Event Publishing Agent (EPA) to publish the availability of one or more files towards the Event State Compositor (ESC).

EPA                                     ESC
 |                                       |
 |  SIP/2.0 PUBLISH                      |
 |  Event: file                          |
 |  (file-metadata document)             |
 | ------------------------------------> |
 |                                       |
 |  200 OK SIP/2.0                       |
 |  SIP-ETag: x                          |
 | <------------------------------------ |
 |                                       |

The EPA performs the initial file metadata publication by sending a PUBLISH (Niemi, A., “Session Initiation Protocol (SIP) Extension for Event State Publication,” October 2004.) [RFC3903] request to the ESC. The PUBLISH request contains a full 'file-metadata' document that contains metadata about one or more files available at the EPA. The 'file-metadata' document is defined in the XML data format for describing files (Garcia-Martin, M. and M. Matuszewski, “An Extensible Data Format (XML) for Describing Files,” November 2007.) [I‑D.garcia‑app‑area‑file‑data‑format]. Each file is described using a set of invariant file metadata and a set of file metadata specific to each instance of the file, given in the <identity> and <instance> child elements of the <file> element.

The invariant file metadata contains the following attributes: the Uniform Resource Name (URN), the MIME type (e.g., image/jpeg), the size and the SHA-1 hash of the file. For each identical copy of the file, the instance-specific metadata contains any of: the SIP URI of the file, the file name, a short description, a set of keywords describing the file, the file creation date, the file modification date, the file read date, a link to an icon, and other file metadata that is associated to the file. Additionally the instance-specific metadata contains the SIP AOR (e.g. URI) and GRUU of the user's endpoint hosting the file.

The PUBLISH request is routed to the ESC. The ESC sends a 200 OK response that, according to RFC 3903 (Niemi, A., “Session Initiation Protocol (SIP) Extension for Event State Publication,” October 2004.) [RFC3903], includes a SIP-ETag header field that contains the entity-tag allocated to the resource. The EPA stores this entity-tag for future references to the publication.

Note that the actual file is not transmitted at any point to the ESC: only the metadata associated with the file is transmitted.

4.1.2.  Publication of Modified File Metadata

Whenever a file is modified or new files are added or deleted from the endpoint, the EPA refreshes the previous publication by sending a new PUBLISH request, as shown in Figure 2 (Signaling flow for publication of modified file metadata). This publication carries a partial 'file-metadata' document that contains a number of XML patch operations that add, remove, or replace XML elements towards the last published 'file-metadata' document.

A file modification occurs, e.g., when an image file is edited to suppress red eyes, an audio file is edited to suppress silence or apply some noise filter, or when some audio/music stream provided by the UE changes its bitrate. Any kind of modification to the file owned by the UE implies a change in the metadata.

RFC 3903 (Niemi, A., “Session Initiation Protocol (SIP) Extension for Event State Publication,” October 2004.) [RFC3903] contains provisions to allow the ESC to distinguish an initial publication from a refreshment-based one with the aid of the entity tags and the SIP-ETag and SIP-If-Match header fields. The SIP-Etags in conjunction with the 'version' attribute of the root element of the 'file' document provide the means to synchronize versions.

EPA                                     ESC
 |                                       |
 |  SIP/2.0 PUBLISH                      |
 |  SIP-If-Match: x                      |
 |  Event: file                          |
 |  (file-metadata document)             |
 | ------------------------------------> |
 |                                       |
 |  200 OK SIP/2.0                       |
 |  SIP-ETag: y                          |
 | <------------------------------------ |
 |                                       |

If a resouce becomes unavailable at the EPA, e.g., as a result of a file deletion, the file metadata publication contains a partial 'file-metadata' document that describes the file to be removed.

4.1.3.  Actions Performed by the ESC

When the ESC receives initial or updated publications, the ESC typically locally stores the published metadata, but in some cases, depending on the usage scenario, storage of metadata will take place in other nodes, for example, in other primary nodes which are members of a DHT.

The ESC may act as a primary node in an overlay SIP P2P network. Thus, upon reception of a publication from one of its secondary nodes, the primary node may need to publish or update the metadata in the overlay P2PSIP network. This step heavily depends on the chosen peer-to-peer algorithm. For example, if the P2PSIP distribution algorithm is based on flooding, the primary node may not need to contact any other primary node, but just wait for search queries from them. However, if the overlay is based on a Distributed Hash Table (DHT) based algorithm, then the primary node may need to update file metadata and store it in the appropriate node. The actual mechanism to update file metadata is dependent on the specific algorithm and out of scope of this memo.

4.2.  File Metadata Publication in Support of Directory Operations

Publication of file metadata in support of directory operations is done by extending the presence information data format (PIDF). PIDF is commonly used in publications and notifications of presence information. The publisher composes a PIDF (Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and J. Peterson, “Presence Information Data Format (PIDF),” August 2004.) [RFC3863] document according to the Presence Data Model (Rosenberg, J., “A Data Model for Presence,” July 2006.) [RFC4479]. The <device> element of the data model contains the file metadata. This is further described in a the Internet-Draft 'File Descriptions Extension to the PIDF' (Garcia-Martin, M. and M. Matuszewski, “File Descriptions Extension to the Presence Information Data Format (PIDF),” June 2007.) [I‑D.garcia‑sipping‑file‑desc‑pidf].

Publication of presence information contains a number of mechanisms that complement publication operations, for example, partial publication, presence authorization rules, etc., are always at the presentity's disposal.

EPA                                     ESC
 |                                       |
 |  SIP/2.0 PUBLISH                      |
 |  Event: presence                      |
 |  (PIDF + data model +                 |
 |   file-metadata in body)              |
 | ------------------------------------> |
 |                                       |
 |  200 OK SIP/2.0                       |
 |  SIP-ETag: x                          |
 | <------------------------------------ |
 |                                       |

Publication of modified file metadata in the PIDF is done similarly to the publication of modified file metadata (see Section 4.1.2 (Publication of Modified File Metadata)), but the event package is set to presence.

5.  Search Operation

The search of shared files is implemented with the SIP event framework defined in RFC 3265 (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.) [RFC3265] in conjunction with the 'file' event package (Garcia-Martin, M. and M. Matuszewski, “A Session Initiation Protocol (SIP) Event Package and Data Format for File Metadata Description,” November 2007.) [I‑D.garcia‑sipping‑file‑event‑package] and a filter document (Khartabil, H., Leppanen, E., Lonnfors, M., and J. Costa-Requena, “An Extensible Markup Language (XML)-Based Format for Event Notification Filtering,” September 2006.) [RFC4661].

The signaling flow for a search operation is shown in Figure 4 (Signaling flow of a search operation).

Subscriber                           Notifier
 |                                       |
 |  SIP/2.0 SUBSCRIBE                    |
 |  Event: file                          |
 |  (search filter in body)              |
 | ------------------------------------> |
 |                                       |
 |  200 OK SIP/2.0                       |
 | <------------------------------------ |
 |                                       |
 |  SIP/2.0 NOTIFY                       |
 |  Event: file                          |
 | <------------------------------------ |
 |                                       |
 |  200 OK SIP/2.0                       |
 | ------------------------------------> |
 |                                       |
 |  SIP/2.0 NOTIFY                       |
 |  Event: file                          |
 |  (file-metadata document in body)     |
 | <------------------------------------ |
 |                                       |
 |  200 OK SIP/2.0                       |
 | ------------------------------------> |
 |                                       |
 |  SIP/2.0 NOTIFY                       |
 |  Event: file                          |
 |  Subscription-State: terminated       |
 |  (file-metadata document in body)     |
 | <------------------------------------ |
 |                                       |
 |  200 OK SIP/2.0                       |
 | ------------------------------------> |
 |                                       |

5.1.  Sending a Search Request

To search for a particular file, the subscriber first builds a filter containing the data of the searched file. The filter can contain, for example, keywords, file names, types of files, etc. The filter conforms to the XML format for filters (Khartabil, H., Leppanen, E., Lonnfors, M., and J. Costa-Requena, “An Extensible Markup Language (XML)-Based Format for Event Notification Filtering,” September 2006.) [RFC4661]. Then the subscriber attaches the filter to a SUBSCRIBE request for the 'file' event package. The subscription duration will be short, typically on the order of a few minutes. This subscription time provides enough time for a primary node in a SIP peer-to-peer network to propagate the search within the overlay network and get responses before the subscription expires. Eventually, the SUBSCRIBE request is sent to a notifier (either a peer or an ESC) that will provide one or more NOTIFY requests including a 'file-metadata' document according to the filtered content.

5.2.  Reporting Search Results

After receiving the SUBSCRIBE request and acknowledging it with a 200 (OK) response, the notifier sends a NOTIFY request to the subscriber. This request may contain a first collection of file metadata about the searched file, if such information is already available in the ESC, in a full 'file-metadata' document. Information may be available immediately in case there is matching metadata stored in the ESC, due to push operations according to the peer-to-peer algorithm, or due to cached information from previous searches. In many cases, however, this NOTIFY request does not contain a 'file-metadata' document about the searched file, and it is sent just because the protocol (RFC 3265 (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.) [RFC3265]) requires an immediate NOTIFY after each successful SUBSCRIBE request. The NOTIFY request is acknowledged with a 200 (OK) response.

The ESC may, depending on algorithm, invoke a search for additional files, whose metadata is stored in other ESCs (see section 4.3). Due to this propagated search, additional matching file descriptors may become known. New matching file descriptors may also become known as a result of PUBLISH requests received by the ESC within the duration of the subscription.

To report matching files, the ESC sends NOTIFY requests to the subscriber. The body of the initial NOTIFY contains follows the procedures of the 'file' event package (Garcia-Martin, M. and M. Matuszewski, “A Session Initiation Protocol (SIP) Event Package and Data Format for File Metadata Description,” November 2007.) [I‑D.garcia‑sipping‑file‑event‑package] and contains a full 'file-metadata' document that is formatted according to the XML Data Format for describing files (Garcia-Martin, M. and M. Matuszewski, “An Extensible Data Format (XML) for Describing Files,” November 2007.) [I‑D.garcia‑app‑area‑file‑data‑format]. The 'file-metadata' document contains metadata about several files that matched the search criteria, including the file name, size, type, icon, hash, SIP URI and UE (GRUU) of the users (and UAs) where the file is available, etc. In some cases, the file metadata that describes a given file will provide more than one location of the file. This will typically be the case when a popular file is available in several endpoints. Then the 'file-metadata' document supplied with the NOTIFY request contains more than one <instance> child element in a given <file>element. It may also be necessary to divide a NOTIFY request into several smaller due to the user's preferences (rate of notifications, bandwidth consumption, and event throttling). NOTIFY requests are acknowledged with 200 (OK) responses.

The initial NOTIFY request contains a full 'file-metadata' document. Once the notifier acquires more metadata, it sends partial 'file-metadata' documents with additions, replacements, or removals. Upon reception of a new partial 'file-metadata' document, the subscriber composes a full 'file-metadata' document, based on the existing previous version plus the partial notification. Then, the subscriber UA has the new full 'file-metadata' document at his disposal, so it can, e.g., display the metadata sequentially to the user, as soon as new results are received.

5.3.  Propagating Searches

In many cases, such as in P2P systems, the metadata is distributed in several ESCs. We consider two special cases:

1. In a flooding based architecture, several or all ESCs need to be queried in order to find the matching file. A given ESC is only aware of file that have been published into its local database.
2. In a DHT based architecture, such as Chord, a specific ESC is responsible for a specific set of metadata.

In both cases, the ESC/ESCs containing the required file metadata may be another ESC than the one receiving the SUBSCRIBE request.

5.3.1.  Searching Based on Flooding

In a flooding based search, the SUBSCRIBE request is first processed by the local ESC itself, and then distributed to all ESCs in the system. The distribution is, however, limited by the value of the Max-Forwards header field. An ESC receiving the SUBSCRIBE request consults its local database to find matching file descriptors and it replies with a NOTIFY request that may contain a 'file-metadata' document if matching file descriptors are found locally. The ESC also acts as an URI-list server (Roach, A., Campbell, B., and J. Rosenberg, “A Session Initiation Protocol (SIP) Event Notification Extension for Resource Lists,” August 2006.) [RFC4662] where the URI-list is locally stored. It then forwards a SUBSCRIBE request with the same filter document to each of the ESCs stored in its neighbor table, providing that the Max-Forwards header field is still positive and provided that the ESC has not yet processed the same request. The generation and maintenance of the neighbor table is out of scope of this memo.

The ESC will receive NOTIFY requests from other neighbor nodes, each of the requests containing a different 'file-metadata' document. The ESC will aggregates and composes a single 'file-metadata' document, and sends partial notifications to the subscriber, according to the rate of notifications.

The subscriber is getting periodic partial notifications, each one adding new files or new instances of existing files to the list of file descriptors.

5.3.2.  Searching Based on Distributed Hash Tables (DHT)

In a DHT based system, a single node (a notifier) is responsible for the metadata related to a given search key (the file). When searching a file, the hash of the file is the key in the DHT. An ESC receiving a SUBSCRIBE request consults its routing table (finger table in Chord) to locate the notifier whose key is the closest one to the search key, and forwards the SUBSCRIBE request to that ESC. Eventually the SUBSCRIBE request reaches the node responsible for the given search key. The definition of 'closest' is depending on the actual DHT used.

5.4.  Terminating a Search Request

When the last results are made available, or when the search operation expires, the server sends a last NOTIFY request to the user, containing the latest available results in a 'file-metadata' document (if any), and setting the Subscription-State header field to "terminated" to indicate the end of the search operation, as per procedures of RFC 3265 (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.) [RFC3265].

The user can also cancel the search operation by sending a re-SUBSCRIBE request that contains a Expires header field set to zero, according also to the procedures of RFC 3265 (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.) [RFC3265].

5.5.  Example of a Search Filter

Figure 4 (Signaling flow of a search operation) provides the signaling flow for a search operation. The SUBSCRIBE request contains a filter body, formatted according to the filter data format (Khartabil, H., Leppanen, E., Lonnfors, M., and J. Costa-Requena, “An Extensible Markup Language (XML)-Based Format for Event Notification Filtering,” September 2006.) [RFC4661]. Figure 5 (Example of a search filter) shows an example of the SUBSCRIBE request carrying a filter. The filter selects a few XML elements of a file that contains the string "vacation" in a <keyword> element.

SUBSCRIBE sip:bob@example.com SIP/2.0
Via: SIP/2.0/UDP alice.example.net;branch=z9hG4bKnashds7
Max-Forwards: 70
From: <sip:alice@example.net>;tag=31415
To: <sip:bob@example.com>
Call-ID: b89rjhnedlrfjflslj40a222
CSeq: 61 SUBSCRIBE
Event: file
Expires: 180
Accept: application/file+xml;q=0.3
Contact: <sip:alice.example.com>
Content-Type: application/simple-filter+xml
Content-Length: [length]

<?xml version="1.0" encoding="UTF-8"?>
<filter-set xmlns="urn:ietf:params:xml:ns:simple-filter">
 <ns-bindings>
   <ns-binding prefix="rs" urn="urn:ietf:params:xml:ns:file"/>
 </ns-bindings>

 <filter id="ad982" uri="sip:bob@example.com">
  <what>
   <include type="xpath">
     /rs:file-set/rs:file
   </include>
   <include type="xpath">
     /rs:file-set/rs:file/rs:identity/rs:urn
   </include>
   <include type="xpath">
     /rs:file-set/rs:file/rs:identity/rs:mime-type
   </include>
   <include type="xpath">
     /rs:file-set/rs:file/rs:identity/rs:size
   </include>
   <include type="xpath">
     /rs:file-set/rs:file/rs:identity/rs:sha-1
   </include>
   <include type="xpath">
     /rs:file-set/rs:file/rs:instance/rs:uri
   </include>
   <include type="xpath">
     /rs:file-set/rs:file/rs:instance/rs:user-uri
   </include>
   <include type="xpath">
      /rs:file-set/rs:file/rs:instance/rs:user-gruu
   </include>
   <include type="xpath">
      /rs:file-set/rs:file/rs:instance/rs:description
   </include>
   <include type="xpath">
      /rs:file-set/rs:file/rs:instance[rs:keyword="vacation"]
   </include>
  </what>
 </filter>
</filter-set>

6.  Directory Operations Through Presence Information

Directory operations through presence information allows an authorized watcher of presence information to be updated on the list available files stored at a presentity's device. In Figure 6 (Signaling flow of a directory operation through presence), the watcher does a regular subscription to the presentity's presence information, either directly between the two endpoints, or with the support of a Presence Agent (PA). Once the subscription is duly authorized, the subscriber receives updated presence information in NOTIFY requests. The request contains a PIDF document structured according to the presence data model. The 'device' part of the data model contains a list of available files that the presentity provides at the subscriber's disposal.

All the presence mechanisms are available also in directory operations. For example, partial notifications, presence authorization rules, filters, etc., are applicable.

Subscriber                              PA
 |                                       |
 |  SIP/2.0 SUBSCRIBE                    |
 |  Event: presence                      |
 |  (search filter in body)              |
 | ------------------------------------> |
 |                                       |
 |  200 OK SIP/2.0                       |
 | <------------------------------------ |
 |                                       |
 |  SIP/2.0 NOTIFY                       |
 |  Event: presence                      |
 | <------------------------------------ |
 |                                       |
 |  200 OK SIP/2.0                       |
 | ------------------------------------> |
 |                                       |
 |  SIP/2.0 NOTIFY                       |
 |  Event: presence                      |
 |  (PIDF + data model +                 |
 |   file descriptor in body)            |
 | <------------------------------------ |
 |                                       |
 |  200 OK SIP/2.0                       |
 | ------------------------------------> |
 |                                       |

7.  Downloading a file

Once the search operation is complete, the user can select whether to do any further operation on a given file, and if so, on which instance to operate. A file can be downloaded, for example, by setting up an MSRP session towards the user's SIP URI, and providing a file description in the SDP offer. This mechanism is described in [I‑D.ietf‑mmusic‑file‑transfer‑mech] (Garcia, M., Isomaki, M., Camarillo, G., Loreto, S., and P. Kyzivat, “A Session Description Protocol (SDP) Offer/Answer Mechanism to Enable File Transfer,” February 2009.). In this case, the SIP INVITE request is addressed (Request-URI) to the URI contained in a <user-gruu> (preferred option) or <user-uri> elements of the chosen <identity> for that <file>. The file requester creates an SDP description of an MSRP session that contains the SDP file description extensions to describe the file. If the hash of the file is available, it is RECOMMENDED to include it, as it uniquely identifies the file.

In other cases, there can be a URN or URI that describes the file in the <urn> or <uri> elements of that <file>. The mechanism to retrieve or receive service from the file is dependent on the type of URI scheme. For example, an HTTP URI requires an HTTP GET request to retrieve the file. Similarly FTP URIs require the establishment of an FTP session.

8.  Security Considerations

TBD

9.  IANA Considerations

This document contains no actions to IANA.

10.  References

10.1. Normative References

10.2. Informative References

Authors' Addresses

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