Streaming Internet Messaging Attachments
draft-ietf-lemonade-streaming-06

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Abstract

This document describes a method for streaming multimedia attachments received by a resource constrained and/or mobile device from an IMAP server. It allows such clients, which often have limits in storage space and bandwidth, to play video and audio e-mail content.

The document describes a profile for making use of the IMAP URLAUTH extension (RFC 4467), the Network Announcement SIP Media Service (RFC 4240), and the Media Server Control Markup Language (RFC 5022).

Conventions Used 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 RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [KEYWORDS].

In examples, "C:" and "S:" indicate lines sent by the client and server respectively. If a single "C:" or "S:" label applies to multiple lines, then some of the line breaks between those lines are for editorial clarity only and may not be part of the actual protocol exchange.

Table of Contents

1.  Introduction
2.  Mechanism
    2.1.  Overview of Mechanism
    2.2.  Client use of GENURLAUTH Command
    2.3.  Client Determination of Media Server Capabilities
    2.4.  Client Use of the Media Server Announcement Service
    2.5.  Media Negotiation and Transcoding
    2.6.  Client Use of the Media Server MSCML IVR Service
    2.7.  Media Server Use of IMAP Server
    2.8.  Protocol Diagrams
        2.8.1.  Announcement Service Protocol Diagram
        2.8.2.  IVR Service Protocol Diagram
    2.9.  Media Server Discovery
3.  Security Considerations
4.  IANA Considerations
5.  Digital Rights Management (DRM) Issues
6.  Deployment Considerations
7.  Contributors
8.  References
    8.1.  Normative References
    8.2.  Informative References
§  Author's Address
§  Intellectual Property and Copyright Statements

1.  Introduction

Email clients on resource and/or network constrained devices, such as mobile phones, may have difficulties in retrieving and/or storing large attachments received in a message. For example, on a poor network link, the latency required to download the entire attachment may not be acceptable to the user. Conversely, even on a high-speed network, the device may not have enough storage space to secure the attachment once retrieved.

For certain media, such as audio and video, there is a solution: the media can be streamed to the device, using protocols such as RTP (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) [RTP]. Streaming can be initiated and controlled using protocols such as SIP (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [SIP] and particularly the media server profiles as specified in RFC 4240 (Burger, E., Van Dyke, J., and A. Spitzer, “Basic Network Media Services with SIP,” December 2005.) [NETANN] or MSCML (Van Dyke, J., Burger, E., and A. Spitzer, “Media Server Control Markup Language,” Sep 2007.) [MSCML]. Streaming the media to the device addresses both the latency issue, since the client can start playing the media relatively quickly, and the storage issue, since the client does not need to store the media locally. A tradeoff is that the media cannot be viewed/played when the device is offline.

Examples of the types of media that would benefit from the ability to stream such media to the device include:

• Voice or Video mail messages received as an attachment
• Audio clips such as ring tones received as an attachment
• Video clips, such as movie trailers, received as an attachment

The client may wish to present the user with the ability to use simple "VCR"-style controls such as pause, fast-forward and rewind. In consideration of this, the document presents two alternatives for streaming media - a simple mechanism which makes use of the announcement service of RFC 4240, and a more complex mechanism which allows VCR controls, based on MSCML (RFC 5022) (Van Dyke, J., Burger, E., and A. Spitzer, “Media Server Control Markup Language,” Sep 2007.) [MSCML]. The choice of which mechanism to use is up to the client, for example it may be based on limitations of the client or the configured media server. This document presents suggestions for determining which of these streaming services are available.

2.  Mechanism

2.1.  Overview of Mechanism

The proposed mechanism for streaming media to messaging clients is a profile for making use of several existing mechanisms, namely:

1. IMAP URLAUTH Extension (RFC 4467) (Crispin, M., “Internet Message Access Protocol (IMAP) - URLAUTH Extension,” May 2006.) [URLAUTH] - Providing the ability to generate an IMAP URL (RFC 5092) (Newman, C., Melnikov, A., and S. Maes, “IMAP URL Scheme,” Jan 2007.) [IMAPURL] that allows anonymous access from external systems to specific message parts, e.g. an audio clip.
2. URLFETCH Binary Extension [URLFETCH_BINARY] (Cridland, D., “URLAUTH Binary Extension,” Sep 2007.) - Providing the ability to specify BINARY and BODYPARTSTRUCTURE arguments to the URLFETCH command.
3. Media Server Announcement Service (RFC 4240) (Burger, E., Van Dyke, J., and A. Spitzer, “Basic Network Media Services with SIP,” December 2005.) [NETANN] - Providing the ability for a media server to stream media using a reference provided by the media server client in a URL.
4. Media Server Interactive Voice Response (IVR) Service (RFC 5022) (Van Dyke, J., Burger, E., and A. Spitzer, “Media Server Control Markup Language,” Sep 2007.) [MSCML] - Providing the ability to stream media as above, but with VCR-style controls.

The approach is shown in the following figure:

+--------------+
|              |
| Email Client |^
|              | \
+--------------+  \
    ^           ^  \
    |            \  \ (5)
    | (1),        \  \
    | (2)          \  \
    |           (3),\  \
    |           (6)  \  \
    |                 \  \
    v                  v  v
+--------------+       +----------------+
|              |  (4)  |                |
| IMAP Server  |<----->|  Media Server  |
|              |       |                |
+--------------+       +----------------+

The proposed mechanism has the following steps:

1. Client determines from MIME headers of a particular message that a particular message part (attachment) should be streamed to the user. Note that no assumptions are made about how/when/if the client contacts the user of the client about this decision. User input MAY be required in order to initiate the proposed mechanism.
2. Client constructs an IMAP URL referencing the message part, and uses the GENURLAUTH (Crispin, M., “Internet Message Access Protocol (IMAP) - URLAUTH Extension,” May 2006.) [URLAUTH] command to generate a URLAUTH authorized IMAP URL.
3. Client connects to a SIP Media Server using the Announcement Service as specified in RFC 4240 (Burger, E., Van Dyke, J., and A. Spitzer, “Basic Network Media Services with SIP,” December 2005.) [NETANN], or the IVR Service as specified in RFC 5022 (Van Dyke, J., Burger, E., and A. Spitzer, “Media Server Control Markup Language,” Sep 2007.) [MSCML], and passes the URLAUTH authorized URL to the media server.
4. Media Server connects to the IMAP Server specified in the referenced URL, and uses the IMAP URLFETCH (Crispin, M., “Internet Message Access Protocol (IMAP) - URLAUTH Extension,” May 2006.) [URLAUTH] command to retrieve the message part.
5. Media server streams the retrieved message part to the client using RTP (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) [RTP].
6. The media server or the client terminate the media streaming, or the streaming ends naturally. The SIP session is terminated by either client or server.

It should be noted that the proposed mechanism makes several assumptions about the mobile device, as well as available network services, namely:

• Mobile device is provisioned with, or obtains via some dynamic mechanism (see Section 2.9 (Media Server Discovery)), the location of a media server which supports either RFC 4240 (Burger, E., Van Dyke, J., and A. Spitzer, “Basic Network Media Services with SIP,” December 2005.) [NETANN] and/or RFC 5022 (Van Dyke, J., Burger, E., and A. Spitzer, “Media Server Control Markup Language,” Sep 2007.) [MSCML].
• Media Server(s) used by the mobile device support the IMAP URL (Newman, C., Melnikov, A., and S. Maes, “IMAP URL Scheme,” Jan 2007.) [IMAPURL] scheme for the announcement and/or IVR services
• IMAP Server used by the mobile device supports generating anonymous IMAP URLs using the URLAUTH mechanism as well as the IMAP BINARY (Cridland, D., “URLAUTH Binary Extension,” Sep 2007.) [URLFETCH_BINARY] extension

2.2.  Client use of GENURLAUTH Command

The decision to make use of streaming services for a message part will usually be predicated on the content type of the message part. Using the capabilities of the IMAP FETCH command, clients determine the MIME (Freed, N., Borenstein, N., Moore, K., Klensin, J., and J. Postel, “Multipurpose Internet Mail Extensions (MIME),” November 1996.) [MIME] Content-Type of particular message parts and based on local policies or heuristics, decide that streaming for that message part will be attempted.

Once the client has determined that a particular message part requires streaming, the client generates an IMAP URL that refers to the message part according to the method described in RFC 5092 (Newman, C., Melnikov, A., and S. Maes, “IMAP URL Scheme,” Jan 2007.) [IMAPURL]. The client then begins the process of generating an URLAUTH URL, by appending ";EXPIRE=<datetime>" and ";URLAUTH=<access>" to the initial URL.

The ";EXPIRE=<datetime>" parameter is optional, however it SHOULD be used, since the use of anonymous URLAUTH authorized URLs is a security risk, and doing so ensures that at some point in the future, permission to access that URL will cease.

The <access> portion of the URLAUTH parameter SHOULD be 'authuser' if a media server discovery mechanism such as that discussed in Section 2.9 (Media Server Discovery) specifies that the media server is an authorised user of the IMAP server. Without specific prior knowledge of such a configuration (either through a suitable discovery mechanism or by an out of band mechanism), the client MUST use the 'anonymous' access identifier.

The client uses the URL generated as a parameter to the GENAUTHURL command, using the INTERNAL authorization mechanism. The URL returned by a successful response to this command will then be passed to the media server. If no successful response to the GENURLAUTH command is received, then no further action will be possible with respect to streaming media to the client.

Examples:

C: a122 UID FETCH 24356 (BODYSTRUCTURE)
S: * 26 FETCH (BODYSTRUCTURE (("TEXT" "PLAIN"
S: ("CHARSET" "US-ASCII") NIL
S: NIL "7BIT" 1152 23)("VIDEO" "MPEG"
NIL NIL "BASE64" 655350)) UID 24356)
S: a122 OK FETCH completed.
C: a123 GENURLAUTH "imap://joe@example.com/INBOX/;uid=24356/;
section=1.2;expire=2006-12-19T16:39:57-08:00;
urlauth=anonymous" INTERNAL
S: * GENURLAUTH "imap://joe@example.com/INBOX/;uid=24356/;
section=1.2;expire=2006-12-19T16:39:57-08:00;
urlauth=anonymous:
internal:238234982398239898a9898998798b987s87920"
S: a123 OK GENURLAUTH completed

C: a122 UID FETCH 24359 (BODYSTRUCTURE)
S: * 27 FETCH (BODYSTRUCTURE (("TEXT" "PLAIN"
S: ("CHARSET" "US-ASCII") NIL
S: NIL "7BIT" 1152 23)("AUDIO" "G729"
NIL NIL "BASE64" 87256)) UID 24359)
S: a122 OK FETCH completed.
C: a123 GENURLAUTH "imap://joe@example.com/INBOX/;uid=24359/;
section=1.3;expire=2006-12-19T16:39:57-08:00;
urlauth=anonymous" INTERNAL
S: * GENURLAUTH "imap://joe@example.com/INBOX/;uid=24359/;
section=1.3;expire=2006-12-20T18:31:45-08:00;
urlauth=authuser:
internal:098230923409284092384092840293480239482"
S: a123 OK GENURLAUTH completed

2.3.  Client Determination of Media Server Capabilities

Once an authorized IMAP URL has been generated, it is up to the client to pass that URL to a suitable media server that is capable of retrieving the URL via IMAP, and streaming the content to the client using the RTP (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) [RTP] protocol.

This section specifies the behaviour of clients that have not determined, (either statically through configuration, or dynamically through a discovery process as discussed in Section 2.9 (Media Server Discovery)), the capabilities of the media server with respect to the services (i.e. RFC 4240 or 5022) supported by that media server. Clients that have determined those capabilities should use the mechanisms described in Section 2.4 (Client Use of the Media Server Announcement Service) or Section 2.6 (Client Use of the Media Server MSCML IVR Service), as appropriate.

If the client supports the MSCML IVR service, then it SHOULD attempt to contact the media server using the MSCML protocol by sending a SIP INVITE which has the service indicator "ivr". Due to issues described in Section 3 (Security Considerations), the client SHOULD use a suitable end-to-end encryption method, such as S/MIME (Ramsdell, B. and B. Ramsdell, “S/MIME Version 3.1 Message Specification",” July 2004.) [SMIME].

Assuming the media server responds to the INVITE without error, the client can carry on using the MSCML IVR service as specified in Section 2.6 (Client Use of the Media Server MSCML IVR Service). If the media server responds with an error indicating that the "ivr" service is not supported, then if the client supports it, the client SHOULD attempt to contact the media server using the Announcement Service, as described in Section 2.4 (Client Use of the Media Server Announcement Service).

The following example shows an example SIP INVITE using the "ivr" service indicator:

C: INVITE sip:ivr@ms2.example.com SIP/2.0
< SIP Header fields omitted for reasons of brevity >

2.4.  Client Use of the Media Server Announcement Service

Assuming the client or media server does not support use of the MSCML protocol, the media server announcement service is used, as described in RFC 4240 (Burger, E., Van Dyke, J., and A. Spitzer, “Basic Network Media Services with SIP,” December 2005.) [NETANN]. This service allows the client to send a SIP INVITE to a special username ('annc') at the media server (the "announcement" user), supplying the URL obtained as per Section 2.2 (Client use of GENURLAUTH Command).

The SIP INVITE is constructed as shown in the examples below; note that as per RFC 4240, the play parameter is mandatory, and specifies the authorized IMAP URL to be played.

Examples of valid SIP INVITE URIs sent to the media server announcement service:

C: sip:annc@ms2.example.net;
play=imap://joe@example.com/INBOX/;uid=24356/;section=1.2;
expire=2006-12-19T16:39:57-08:00;urlauth=anonymous:
internal:238234982398239898a9898998798b987s87920


C: sip:annc@ms1.example.net;
play=imap://fred@example.com/INBOX/;uid=24359/;section=1.3;
expire=2006-12-20T18:31:45-08:00;urlauth=authuser:
internal:098230923409284092384092840293480239482

If the client receives a 200 (OK) response, the media server has successfully retrieved the content from the IMAP server and the negotiated RTP stream will shortly begin after the ACK.

There are many possible response codes, however a response code of 404 received from the media server indicates that the content could not be found or could not be retrieved for some reason. For example, the media server may not support the use of IMAP URLs. At this point, there are several options to the client, such as using alternate media servers, or giving up in attempting to stream the required message part.

2.5.  Media Negotiation and Transcoding

This document uses standards and protocols from two traditionally separate application areas: Mobile Email (primarily IMAP) and Internet Telephony/Streaming (e.g. SIP/RTP). Since the document primarily addresses enhancing the capabilities of mobile email, it is felt worthwhile to give some examples of simple SIP/SDP exchanges, and discussing capabilities such as media negotiation (using SDP) and media transcoding.

In the below example, the client contacts the media server using the SIP INVITE command to contact the Announcement service (see Section 2.4 (Client Use of the Media Server Announcement Service)), advertising support for a range of audio and video codecs (using SDP (Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” July 2006.) [SDP]), and in response the media server advertises only a set of audio codecs. This process is identical for the IVR service, except that the IVR service does not use the SIP Request-URI to indicate the content to be played; instead this is carried in a subsequent SIP INFO request.

The client and server now know from the SDP advertised by both client and server that communication must be using the subset of audio codecs supported by both client and server (in the example SDP, it is clear that the server does not support any video codecs). The media server may perform transcoding (i.e. converting between codecs) on the media received from the IMAP server in order to satisfy the codecs supported by the client: for example the media server may downgrade the video retrieved from the IMAP server to the audio component only.

For clients using the Announcement service, the media server MUST return an error to the INVITE if it cannot find a common codec between the client, server and media, and it cannot transcode to a suitable codec. Similarly, for clients using the MSCML IVR service, the media server MUST return a suitable error response to the <playcollect> request.

Example SIP INVITE and SDP Media Negotiation

C: INVITE sip:annc@ms2.example.com;
play=imap://joe@example.com/INBOX/;uid=24356/;section=1.2;
expire=2006-12-19T16:39:57-08:00;urlauth=anonymous:
internal:238234982398239898a9898998798b987s87920 SIP/2.0
C: From: UserA <sip:UAA@example.com>
C: To: NetAnn <sip:annc@ms2.example.com>
C: Call-ID: 8204589102@example.com
C: CSeq: 1 INVITE
C: Contact: <sip:UAA@192.0.2.40>
C: Content-Type: application/sdp
C: Content-Length: 481
C:
C: v=0
C: o=UserA 2890844526 2890844526 IN IP4 192.0.2.40
C: s=Session SDP
C: c=IN IP4 192.0.2.40
C: t=3034423619 0
C: m=audio 9224 RTP/AVP 0 8 3 98 101
C: a=alt:1 1 : 01BB7F04 6CBC7A28 192.0.2.40 9224
C: a=fmtp:101 0-15
C: a=rtpmap:98 ilbc/8000
C: a=rtpmap:101 telephone-event/8000
C: a=recvonly
C: m=video 9226 RTP/AVP 105 34 120
C: a=alt:1 1 : 01BCADB3 95DFFD80 192.0.2.40 9226
C: a=fmtp:105 profile=3;level=20
C: a=fmtp:34 CIF=2 QCIF=2 MAXBR=5120
C: a=rtpmap:105 h263-2000/90000
C: a=rtpmap:120 h263/90000
C: a=recvonly


S: SIP/2.0 200 OK
S: From: UserA <sip:UAA@example.com>
S: To: NetAnn <sip:annc@ms2.example.com>
S: Call-ID: 8204589102@example.com
S: CSeq: 1 INVITE
S: Contact: <sip:netann@192.0.2.41>
S: Content-Type: application/sdp
S: Content-Length: 317
S:
S: v=0
S: o=NetAnn 2890844527 2890844527 IN IP4 192.0.2.41
S: s=Session SDP
S: c=IN IP4 192.0.2.41
S: t=3034423619 0
S: m=audio 17684 RTP/AVP 0 8 3 18 98 101
S: a=rtpmap:0 PCMU/8000
S: a=rtpmap:8 PCMA/8000
S: a=rtpmap:3 GSM/8000
S: a=rtpmap:18 G729/8000
S: a=fmtp:18 annexb=no
S: a=rtpmap:98 iLBC/8000
S: a=rtpmap:101 telephone-event/8000
S: a=fmtp:101 0-16


C: ACK sip:netann@192.0.2.41 SIP/2.0
C: From: UserA <sip:UAA@example.com>
C: To: NetAnn <sip:annc@ms2.example.com>
C: Call-ID: 8204589102@example.com
C: CSeq: 1 ACK
C: Content-Length: 0

2.6.  Client Use of the Media Server MSCML IVR Service

Once the client has determined that the media server supports the IVR service, it is up to the client to generate a suitable MSCML request to initiate streaming of the required media.

When using the IVR service, the initial SIP invite is used only to establish that the media server supports the MSCML IVR service, and to negotiate suitable media codecs. Once the initial SIP INVITE and response to that INVITE have been completed successfully, the client must generate a SIP INFO request with MSCML in the body of the request to initiate streaming.

The <playcollect> request is used, as this allows the use of DTMF digits to control playback of the media, such as fast-forward or rewind.

Since the playcollect request is used purely for its VCR capabilities, there is no need for the media server to perform DTMF collection, therefore the playcollect attributes "firstdigittimer", "interdigittimer" and "extradigittimer" SHOULD all be set to "0ms", which will have the effect of causing digit collection to cease immediately the media has finished playing.

The "ffkey" and "rwkey" attributes of <playcollect> are used to control fast forward and rewind behaviour, with the "skipinterval" attribute being used to control the 'speed' of these actions.

The <prompt> tag is used to specify the media to be played, and SHOULD have a single <audio> tag that gives the URL of the media, as per the Section 2.2 (Client use of GENURLAUTH Command). The audio-specific name of the tag is historical, as the tag can be used for video as well as audio content. The "stoponerror" attribute SHOULD be set to "yes", as then meaningful error messages will be returned by the media server in the event of problems such as retrieving the media from the IMAP server.

An example SIP INFO request using the <playcollect> request is shown at the end of this section.

It should be noted that under normal (i.e. non-error) conditions, the response to the <playcollect> request is a SIP 200 (OK) response. The media server then streams the media, and only when the media has finished playing (naturally or due to a user request), does the media server send a <playcollect> response, which includes details of the media played, such as length, and any digits collected.

The client may suspend playback of the media at any time by either sending the DTMF escape key (specified as an attribute to the <playcollect> request) or by sending a <stop> request to the media server in a SIP INFO request. Upon receipt of the request, the media server will acknowledge it, and then cease streaming of the media, followed by a SIP INFO request containing the <playcollect> response.

If the media server cannot play the media for any reason, for example if it cannot retrieve the media from the IMAP server, streaming will not take place, and the <playcollect> response will be sent, usually with meaningful values in the <error_info> element.

The following gives an example dialog between a client and media server, including a rewind request, and termination of the playback by use of the escape key. Some elements of the SIP dialog such as full SIP header fields and SDP are omitted for reasons of brevity. (The protocol diagram in Section 2.8.2 (IVR Service Protocol Diagram) shows the high-level message flow between all the components, including the IMAP server.)

C: INVITE sip:ivr@ms.example.com SIP/2.0
C: From: UserA <sip:UAA@example.com>
C: To: IVR <sip:ivr@ms.example.com>
C: Call-ID: 3298420296@example.com
C: CSeq: 1 INVITE
C: Contact: <sip:UAA@192.0.2.40>
C: Content-Type: application/sdp
C: Content-Length: XXX
C:
C: <SDP Here>


S: SIP/2.0 200 OK
S: From: UserA <sip:UAA@example.com>
S: To: IVR <sip:ivr@ms.example.com>
S: Call-ID: 3298420296@example.com
S: CSeq: 1 INVITE
S: Contact: <sip:ivr@192.0.2.41>
S: Content-Type: application/sdp
S: Content-Length: XXX
S:
S: <SDP Here>


C: ACK sip:ivr@ms.example.com SIP/2.0
C: From: UserA <sip:UAA@example.com>
C: To: IVR <sip:ivr@ms2.example.com>
C: Call-ID: 3298420296@example.com
C: CSeq: 1 ACK
C: Content-Length: 0


C: INFO sip:ivr@192.0.2.41 SIP/2.0
C: From: UserA <sip:UAA@example.com>
C: To: IVR <sip:ivr@ms.example.com>
C: Call-ID: 3298420296@example.com
C: CSeq: 2 INFO
C: Content-Type: application/mediaservercontrol+xml
C: Content-Length: 423
C:
C: <?xml version="1.0"?>
C: <MediaServerControl version="1.0">
C: <request>
C: <playcollect id="332985001"
C: firstdigittimer="0ms" interdigittimer="0ms" extradigittimer="0ms"
C: skipinterval="6s" ffkey="6" rwkey="4" escape="*">
C: <prompt stoponerror="yes"
C: locale="en_US" offset="0" gain="0" rate="0"
C: delay="0" duration="infinite" repeat="0">
C: <audio url="imap://joe@example.com/INBOX/;uid=24356/;section=1.2;
expire=2006-12-19T16:39:57-08:00;urlauth=anonymous:
internal:238234982398239898a9898998798b987s87920">
C: </prompt>
C: </playcollect>
C: </request>
C: </MediaServerControl>

S: SIP/2.0 200 OK
S: From: UserA <sip:UAA@example.com>
S: To: IVR <sip:ivr@ms.example.com>
S: Call-ID: 3298420296@example.com
S: CSeq: 2 INFO
S: Contact: <sip:ivr@192.0.2.41>
S: Content-Length: 0

S: <Media Server retrieves media from IMAP Server and streams to client>

C: <Client streams 6 key>

S: <Media Server fast forwards media by 6 seconds>

C: <Client streams * key>

S: <Media Server stops streaming>

S: INFO sip:UAA@192.0.2.40 SIP/2.0
S: From: IVR <sip:ivr@ms.example.com>
S: To: UserA <sip:UAA@example.com>
S: Call-ID: 3298420296@example.com
S: CSeq: 5 INFO
S: Contact: <sip:ivr@192.0.2.41>
S: Content-Type: application/mediaservercontrol+xml
S: Content-Length: XXX
S:
S: <?xml version="1.0"?>
S: <MediaServerControl version="1.0">
S: <response id="332985001" request="playcollect"
code="200" S: reason="escapekey" playduration="34s"
S: playoffset="34s" digits="" />
S: </MediaServerControl>

C: SIP/2.0 200 OK
C: From: IVR <sip:ivr@ms.example.com>
C: To: UserA <sip:UAA@example.com>
C: Call-ID: 3298420296@example.com
C: CSeq: 5 INFO
C: Content-Length: 0


C: BYE sip:ivr@192.0.2.41 SIP/2.0
C: From: UserA <sip:UAA@example.com>
C: To: IVR <sip:ivr@ms.example.com>
C: Call-ID: 3298420296@example.com
C: CSeq: 6 BYE
C: Content-Length: 0

S: SIP/2.0 200 OK
S: From: UserA <sip:UAA@example.com>
S: To: IVR <sip:ivr@ms.example.com>
S: Call-ID: 3298420296@example.com
S: CSeq: 6 BYE
S: Contact: <sip:ivr@192.0.2.41>
S: Content-Length: 0

2.7.  Media Server Use of IMAP Server

This section describes how the media server converts the IMAP URL received via the announcement or IVR service into suitable IMAP commands for retrieving the content.

The media server first connects to the IMAP server specified in the URL. Once connected, the media server SHOULD use TLS (Dierks, T. and E. Rescorla, “The TLS Protocol,” April 2006.) [TLS] to encrypt the communication path.

If the media server is configured as an authorized user of the IMAP server, it SHOULD authenticate to the IMAP server using the credentials for that user. This document does not go into the details of IMAP authentication, but the authentication SHOULD NOT use the LOGIN command over a non-encrypted communication path.

If the media server is not configured as an authorized user of the IMAP server, then the behaviour specified in IMAP URL bis4 (Newman, C., Melnikov, A., and S. Maes, “IMAP URL Scheme,” Jan 2007.) [IMAPURL] MUST be followed. That is, if the server advertises AUTH=ANONYMOUS IMAP capability, the media server MUST use the AUTHENTICATE command with ANONYMOUS [ANONYMOUS] SASL mechanism. If SASL ANONYMOUS is not available, the user name "anonymous" is used with the "LOGIN" command and the password is supplied as the Internet e-mail address of the administrative contact for the media server.

Once authenticated, the media server issues the URLFETCH command, using the URL supplied in the 'play' parameter of the SIP INVITE (or audio tag of the MSCML). If the IMAP server does not advertise URLAUTH=BINARY in its capability string, then the media server returns a suitable error code to the client.

The additional parameters to the URLFETCH command specified in (URLFETCH BINARY) (Cridland, D., “URLAUTH Binary Extension,” Sep 2007.) [URLFETCH_BINARY] are used by the media server to tell the IMAP server to remove any transfer encoding and return the content type of the media (as content type information is not contained in the IMAP URL).

A successful URLFETCH command will return the message part containing the media to be streamed. If the URLFETCH was unsuccessful, then the media server MUST return an appropriate error code to the client with an appropriate reason code.

Assuming the content is retrieved successfully, the media server returns a 200 (OK) response code to the client. After an ACK is received, an RTP stream is delivered to the client using the parameters negotiated in the SDP.

If appropriate, the media server MAY choose to implement connection caching, in which case connection and disconnection from the IMAP server are handled according to whatever algorithm the media server chooses. For example, the media server may know, a priori, that it will always access the same IMAP server using the same login credentials with an access pattern that would benefit from connection caching, without unduly impacting server resources.

Examples:

C: a001 LOGIN anonymous null
S: a001 OK LOGIN completed.
C: a002 URLFETCH ( "imap://joe@example.com/INBOX/;uid=24356/;section=1.2;
expire=2006-12-19T16:39:57-08:00;urlauth=anonymous:
internal:238234982398239898a9898998798b987s87920" BODYPARTSTRUCTURE BINARY )
S: * URLFETCH "imap://joe@example.com/INBOX/;uid=24356/;
section=1.2;expire=2006-12-19T16:39:57-08:00;urlauth=anonymous:
internal:238234982398239898a9898998798b987s87920"
(BODYPARTSTRUCTURE ("VIDEO" "MPEG" () NIL NIL "BINARY" 655350 )) (BINARY ~{655350}
S: [ ~655350 octets of binary data, containing NUL octets ])
S: a002 OK URLFETCH completed.
C: a003 LOGOUT
S: a003 OK LOGOUT completed.

2.8.  Protocol Diagrams

This section gives examples of using the mechanism described in the document to stream media from a media server to a client, fetching the content from an IMAP server. In all of the examples, the IMAP, SIP and RTP protocols use the line styles "-", "=", and "+", respectively.

2.8.1.  Announcement Service Protocol Diagram

The following diagram shows the protocol interactions between the email client, the IMAP server and the media server when the client uses the Announcement Service.

Client                     IMAP Server                   Media Server
  |   FETCH (BODYSTRUCTURE)     |                              |
  |---------------------------->|                              |
  |           OK                |                              |
  |<----------------------------|                              |
  |   GENURLAUTH                |                              |
  |---------------------------->|                              |
  |           OK                |                              |
  |<----------------------------|                              |
  |                             |                              |
  |                          SIP INVITE                        |
  |===========================================================>|
  |                             |                              |
  |                             |          URLFETCH            |
  |                             |<-----------------------------|
  |                             |             OK               |
  |                             |----------------------------->|
  |                             |                              |
  |                          200 OK                            |
  |<===========================================================|
  |                          ACK                               |
  |===========================================================>|
  |                             |                              |
  |                    Stream Message Part (RTP)               |
  |<+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++|
  |                             |                              |
  |                            BYE                             |
  |<===========================================================|
  |                          200 OK                            |
  |===========================================================>|

2.8.2.  IVR Service Protocol Diagram

The following diagram shows a simplified view of the protocol interactions between the email client, the IMAP server and the media server when the client uses the MSCML IVR Service.

Client                     IMAP Server                   Media Server
  |   FETCH (BODYSTRUCTURE)     |                              |
  |---------------------------->|                              |
  |           OK                |                              |
  |<----------------------------|                              |
  |   GENURLAUTH                |                              |
  |---------------------------->|                              |
  |           OK                |                              |
  |<----------------------------|                              |
  |                             |                              |
  |                          SIP INVITE                        |
  |===========================================================>|
  |                             |                              |
  |                          200 OK                            |
  |<===========================================================|
  |                          ACK                               |
  |===========================================================>|
  |                             |                              |
  |                          SIP INFO (playcollect)            |
  |===========================================================>|
  |                             |                              |
  |                          200 OK                            |
  |<===========================================================|
  |                             |                              |
  |                             |          URLFETCH            |
  |                             |<-----------------------------|
  |                             |             OK               |
  |                             |----------------------------->|
  |                             |                              |
  |                    Stream Message Part (RTP)               |
  |<+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++|
  |                             |                              |
  |                          SIP INFO (e.g., DTMF ff)          |
  |===========================================================>|
  |                          200 OK                            |
  |<===========================================================|
  |                             |                              |
  |                    Continue streaming (RTP)                |
  |<+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++|
  |                             |                              |
  |                (Streaming Ends or is terminated)           |
  |                             |                              |
  |                     SIP INFO (playcollect response)        |
  |<===========================================================|
  |                            BYE                             |
  |===========================================================>|
  |                           200 OK                           |
  |<===========================================================|

2.9.  Media Server Discovery

This section discusses possibilities for the automatic discovery of suitable media servers to perform streaming operations. Defining such a mechanism to perform media server discovery is outside the scope of this document, however it is worthwhile exploring the issues involved.

There are two possibilities for clients with regard to determining the hostname and port number information of a suitable media server:

1. No discovery of media servers is required: clients are configured with suitable media server information in an out-of-band manner.
2. Discovery of media servers is required: clients use the discovery mechanism described in this section to determine a suitable media server to use for streaming multimedia message parts.

There are several scenarios where media server discovery would be a requirement for streaming to be successful:

• Client is not configured with the address of any media servers.
• Client is configured with the address of one or more media servers, but the IMAP server is configured to only accept URLFETCH requests from specific media servers (for security or site policy reasons), and thus streaming would fail due to the media server not being able to retrieve the media from the IMAP server.

There is also a scenario where media server location would improve the security of the streaming mechanism, by avoiding the use of completely anonymous URLs. For example, the client could discover a media server address that was an authorised user of the IMAP server, which would allow the client to generate a URL, which was secure in that it could *only* be accessed by a specific (presumably trusted) media server.

One possible solution could involve using the IMAP METADATA (Daboo, C., “IMAP METADATA Extension,” Apr 2008.) [METADATA] extension, via the use of an entry that provides the contact information for suitable media servers for use with the IMAP server, as well as perhaps the streaming services provided by those media server. Media Server discovery should be optional: clients are free to use pre-configured information about media servers, or to fall-back to such information if no suitable discovery mechanism is available. .

3.  Security Considerations

This document proposes the use of URLAUTH (Crispin, M., “Internet Message Access Protocol (IMAP) - URLAUTH Extension,” May 2006.) [URLAUTH] "pawn tickets", received over IMAP (Crispin, M., “Internet Message Access Protocol - Version 4rev1,” March 2003.) [IMAP], and transmitted over SIP (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [SIP], possibly within the MSCML payload of RFC 5022 (Van Dyke, J., Burger, E., and A. Spitzer, “Media Server Control Markup Language,” Sep 2007.) [MSCML], in order to stream media received in messages. As such, the security considerations in all these documents apply to this specification.

In summary, as the authorized URLs may grant access to data, implementors of this specification need to consider the following:

• Use of an anonymous pawn ticket grants access to any client of the IMAP server without requiring any authentication credentials. Even if one grants an authorized pawn ticket, any client that happens to be an authorized user of the IMAP server will have access granted. The security mechanisms referenced above SHOULD be used to prevent unauthorized access to the pawn ticket.
• If the announcement service is used to set up streaming, then RFC 4240 (Burger, E., Van Dyke, J., and A. Spitzer, “Basic Network Media Services with SIP,” December 2005.) [NETANN] specifies that the pawn ticket is passed in the Request-URI. Thus, if the SIP communication channel is not secured with TLS (e.g. using the sips URI scheme (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [SIP]), untrusted third-parties may be able to intercept the pawn ticket. Using sips in this situation protects the pawn ticket from untrusted third-parties, however, it still allows proxies access to the pawn ticket.
• To fully protect the pawn ticket, the IVR service, RFC 5022 (Van Dyke, J., Burger, E., and A. Spitzer, “Media Server Control Markup Language,” Sep 2007.) [MSCML], which uses MSCML to carry the pawn ticket in the body of the request, is RECOMMENDED, using an end-to-end encryption of the MSCML payload, such as S/MIME (Ramsdell, B. and B. Ramsdell, “S/MIME Version 3.1 Message Specification",” July 2004.) [SMIME].
• If the media server location is retrieved from the IMAP server (see Section 2.9 (Media Server Discovery)), there is an implicit level of trust in the media server used, since the media server has access to both the pawn ticket and the content to be streamed. Trust in media servers configured by out-of-band mechanisms is beyond the scope of this document.

4.  IANA Considerations

This document has no actions for IANA.

5.  Digital Rights Management (DRM) Issues

This document does not specify any Digital Rights Management (DRM) mechanisms for controlling access to and copying of the media to be streamed. This is intentional. A reference to a piece of media content is created using the URLAUTH (Crispin, M., “Internet Message Access Protocol (IMAP) - URLAUTH Extension,” May 2006.) [URLAUTH] command; any DRM required thus should be implemented within the media itself, as implementing checks within URLAUTH could affect any use of the URLAUTH command, such as the BURL (Newman, C., “Message Submission BURL Extension,” May 2006.) [BURL] command for message submission.

The use of URLAUTH in this specification is believed to be pursuant with, and used only for, the execution of those rights to be expected when media is sent via traditional internet messaging, and causes no duplication of media content which is not essentially provided by the action of sending the message; that is, the use of the content for downloading and viewing, which *is* implicitly granted by the sender of the message, in as much as the sender has the right to grant such rights.

The document authors believe that if DRM is a requirement for Internet messaging, then a suitable DRM mechanism should be created. How such a mechanism would work is outside the scope of this document.

6.  Deployment Considerations

This document assumes an Internet deployment where there are no network restrictions between the different components. Specifically, it does not address issues that can occur when network policies restrict the communication between different components, especially between the media server and the IMAP server, and between the client the media server. In particular, RFC 5022 states that "It is unlikely, but not prohibited, for end-user SIP UACs to have a direct signaling relationship with a media server." This caveat makes it likely that firewalls and other network security mechanisms will be configured to block direct end-user access to media servers.

In order for either of the streaming mechanisms described in this document to work, local administrators MUST relax firewalls policies such that appropriate SIP UACs running on mobile devices are permitted to access the media servers directly using the SIP protocol. The detail of how the restrictions are relaxed, (for example, only allowing clients connecting from the network space owned/maintained by the operator of the media server) is a matter of local policy, and so is outside the scope of this document.

7.  Contributors

Eric Burger, eburger@standardstrack.com

Eric Burger provided the initial inspiration for this document, along with advice and support on aspects of the media server IVR and Announcement services, as well as help with the IETF process.

Many people made helpful comments on the document, including Alexey Melnikov, Dave Cridland, Martijn Koster, and a variety of folks in the Lemonade and Sipping WGs.

8.  References

8.1. Normative References

8.2. Informative References

Author's Address

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