Internet DRAFT - draft-cook-lemonade-streaming
draft-cook-lemonade-streaming
Lemonade N. Cook
Internet-Draft Openwave
Expires: November 20, 2006 May 19, 2006
Streaming Multimedia Messaging Attachments
draft-cook-lemonade-streaming-00
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document, part of the Lemonade Working Group, describes the Best
Current Practice for streaming multimedia attachments received by a
resource constrained and/or mobile device from a IMAP server. It
allows such clients, which are limited in storage space and
bandwidth, to play video and audio content which is attached to email
messages. The document describes a profile for making use of the
existing IMAP URLAUTH extension [2] and the "Netann" SIP Media
Service as described in RFC 4240 [1].
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Conventions Used in this Document
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in RFC 2119 [7].
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 the line breaks between those lines are for
editorial clarity only and are not part of the actual protocol
exchange.
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1. Introduction
Email clients on resource and/or network constrained devices, such a
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 SIP [5]
and particularly the media server profile as specified in RFC 4240
[1]. Streaming the media to the device addresses both the latency
issue, since the client can start playing the media immediately, 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 ringtones received as an attachment
+ Video clips such as movie trailers received as an attachment
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2. Mechanism
2.1. Overview of Mechanism
The proposed mechanism for streaming media to messaging clients is a
profile for making use of two existing mechanisms, namely:
1. IMAP URLAUTH Extension [2] - Providing the ability to generate an
IMAP URL [4] which allows anonymous access from external systems
to specific message parts; for example, an audio clip.
2. Media Server Announcement Service RFC 4240 [1] - Providing the
ability for a media server to stream media using a reference
provided by the media server client in a URL.
However, it should be noted that this document proposes a extension
to the SIP Parameter Registry [8], in order to accomodate the passing
of a content transfer encoding parameter.
The approach is shown in the following figure:
+--------------+
| |
| Email Client |^
| | \
+--------------+ \
| \ \
| \ \ (5)
| (1), \ \
| (2) \ \
| (3),\ \
| (6) \ \
| \ \
v v \
+--------------+ +----------------+
| | (4) | |
| IMAP Server |<------| Media Server |
| | | |
+--------------+ +----------------+
Figure 1
The proposed mechanism has the following steps:
1. Client determines from headers of a particular message that a
particular message part (attachment) should be streamed to the
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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 [2] 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 [1], 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 [2] command to
retrieve the message part.
5. Media server streams the retrieved message part to the client
using RTP [6].
6. Media server terminates the SIP session.
It should be noted that the proposed mechanism makes several
assumptions about the mobile device, as well as the network services,
namely:
+ Mobile device is provisioned with address or addresses of a
media server which supports the announcement service defined in
RFC 4240 [1]
+ Media Server(s) used by the mobile device support the IMAP URL
[4] scheme for the announcement service
+ IMAP Server used by the mobile device supports generating
anonymous IMAP URLs using the URLAUTH mechanism
This document assumes that there are no network restrictions between
the different components. Specifically it does not address the
issues that could occur when the media server and the IMAP server are
provided by different entities, and network security protocols
restrict the communication between the different components,
especially between the media server and the IMAP server.
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 can
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determine the MIME [9] 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 2192 [4].
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 'anonymous'.
This document makes the assumption that Media Servers are unlikely to
be configured as authorized users to IMAP servers. In this case,
without specific prior knowledge of such a configuration, the client
MUST use the 'anonymous' access identifier. In the event that the
client does have prior knowledge of a media server which is
configured for authorized access to the media server, the 'authuser'
access identifier SHOULD be used.
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 BODY[1.2.MIME]
S: * 26 FETCH (BODY[1.2.MIME] {127}
S: Content-Transfer-Encoding: base64
S: Content-Type: video/mpeg;
S: UID 24356 FLAGS (\Seen))
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
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C: a122 UID FETCH 24359 BODY[1.2.MIME]
S: * 26 FETCH (BODY[1.3.MIME] {127}
S: Content-Transfer-Encoding: base64
S: Content-Type: audio/G729;
S: UID 24359 FLAGS (\Seen))
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 Use of the Media Server Announcement Service
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 [6]protocol.
The media server announcement service is used, as described in RFC
4240 [1]. 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 the previous section.
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.
The content-type parameter is optional in RFC 4240, however it MUST
be supplied here, using the Content-Type header returned by the IMAP
server for the message part. The reason for supplying the content-
type parameter is that when the media server issues a URLFETCH
command to retrieve the message part, the message part will be
returned without any content type information. Since the media
server is not likely to have authorized access to other sections in
that message, for example the MIME section, then it may fail to
stream the content if the content type is not supplied as a parameter
to the SIP INVITE URI.
Similarly, the message well be encoded with a content transfer
encoding such as base 64. However, RFC 4240 does not include a
method for communicating content transfer encoding to the media
server as part of the announcement service, nor does the URLFETCH
command include a mechanism for retrieving message parts without
encoding (c.f. the [BINARY] extension to IMAP). Therefore, an
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extension parameter is required, namely a 'content-transfer-encoding'
parameter, using the value of the Content-Transfer-Encoding MIME
header returned by the IMAP server for the message part. The
content-transfer-encoding parameter MUST be supplied if a Content-
Transfer-Encoding header for the message part existed in the original
message.
Examples of valid SIP INVITE URIs sent to the media server
announcement service:
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; \
content-type=video/mpeg; \
content-transfer-encoding=base64
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; \
content-type=audio/G729; \
content-transfer-encoding=base64
If the SIP INVITE is successful, as indicated by a 200 OK response,
the client can assume that the media server has successfully
retrieved the content from the IMAP server, and that the negotiated
RTP stream will shortly begin.
An unsuccessful 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.
This document does not go into the details of the SIP [5], SDP [11]
or RTP [6] protocols; these are well described elsewhere.
2.4. Media Server Use of IMAP Server
A media server receiving a SIP INVITE to the annc user must first
attempt to retrieve the content indicated in the play parameter,
before returning a response to the client. This section describes
how the media server converts the received IMAP URL into suitable
IMAP commands for retrieving the content.
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The media server first connects to the IMAP server specified in the
URL. Once connected, the media server MAY choose to use TLS [10] 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, it MUST authenticate to the IMAP server using the LOGIN
command, with the username of "anonymous". However, in this case, if
the URL supplied in the 'play' parameter of the SIP invite specifies
an authorization of 'authuser', then the media server SHOULD NOT
attempt to contact the IMAP server, but SHOULD instead immediately
return a response code of 404 with a reason phrase of 'Not authorized
to access resource'. reason.
Once authenticated, the media server issues the URLFETCH command,
using the URL supplied in the 'play' parameter of the SIP invite. A
successful URLFETCH command will return the message part, which must
be decoded by the media server, according to the content-transfer-
encoding header provided by the client (if any). If the URLFETCH was
unsuccessful, then the media server MUST return a response code of
404 with an appropriate reason code.
Assuming the content is retrieved and decoded successfully, the media
server returns a 200 OK response code, and after an ACK is received,
an RTP stream is delivered to the client using the parameters
negotiated in the SDP.
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.
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"
S: * URLFETCH "imap://joe@example.com/INBOX/;uid=24356/; \
section=1.2;expire=2006-12-19T16:39:\57-08:00;urlauth=anonymous: \
internal:238234982398239898a9898998798b987s87920" {36}
S: U2kgdmlzIHBhY2VtLCBwYXJhIGJlbGx1bS4K
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S: a002 OK URLFETCH completed.
C: a003 LOGOUT
S: a003 OK LOGOUT completed.
2.5. Protocol Diagram
The following diagram shows a simplified view of the protocol
interactions between the email client, the IMAP server and the media
server. The IMAP, SIP and RTP protocols are distinguished with
different line styles.
Client IMAP Server Media Server
| FETCH BODY[MIME] | |
|---------------------------->| |
| GENURLAUTH | |
|---------------------------->| |
| | |
| SIP INVITE |
|===========================================================>|
| | |
| | URLFETCH |
| |<-----------------------------|
| | |
| 200 OK |
|<===========================================================|
| ACK |
|===========================================================>|
| | |
| Stream Message Part (RTP) |
|<+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++|
| | |
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3. Security Considerations
This document proposes the use of URLAUTH "pawn-tickets" to grant
access to message parts that are required to be streamed by the media
server. If an anonymous pawn-ticket is used, that grants access to
any client of the IMAP server, without requiring any authentication
credentials. Even if an authorized pawn-ticket is granted, access is
granted to any client that happens to be an authorized user of the
IMAP server.
Clearly, any third parties which gain access to the pawn-tickets can
potentially access private data. To minimise this, implementors
should consider the following:
IMAP clients SHOULD use encryption such as TLS [10] to secure the
communication path to the IMAP server
The media server SHOULD use encryption such as TLS secure the
communication path to the IMAP server
However, the communication path between the client and the media
server uses SIP, which currently has no mechanism to secure the data
contained in the SIP INVITE URI. Thus, even if an encrypted
communication path is used to the IMAP server, the potential for
third parties to gain access to the pawn-ticket still exists.
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4. Contributors
5. References
[1] Burger, E., "Basic Network Media Services with SIP", RFC 4240,
December 2005.
[2] Crispin, M., "Internet Message Access Protocol (IMAP) - URLAUTH
Extension", draft-ietf-lemonade-urlauth-08.txt (work in
progress), October 2005.
[3] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003.
[4] Newman, C., "IMAP URL Scheme", RFC 2192, September 1997.
[5] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[6] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, July 2003.
[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
[8] Camarillo, G., "The Internet Assigned Number Authority (IANA)
Uniform Resource Identifier (URI) Parameter Registry for the
Session Initiation Protocol (SIP)", RFC 3969, BCP 99,
December 2004.
[9] Freed, N., Borenstein, N., Moore, K., Klensin, J., and J.
Postel, "Multipurpose Internet Mail Extensions (MIME)",
RFC 2045, RFC 2046, RFC 2047, RFC 2048, RFC 2049,
November 1996.
[10] Dierks, T. and C. Allen, "The TLS Protocol", RFC 2246,
January 1999.
[11] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
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Author's Address
Neil L Cook
Openwave Systems
e-mail: neil.cook@openwave.com
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