Network Working Group N. Cook
Request for Comments: 5616 Cloudmark
Category: Informational August 2009
Streaming Internet Messaging Attachments
Abstract
This document describes a method for streaming multimedia attachments
received by a resource- and/or network-constrained device from an
IMAP server. It allows such clients, which often have limits in
storage space and bandwidth, to play video and audio email content.
The document describes a profile for making use of the URLAUTH-
authorized IMAP URLs (RFC 5092), the Network Announcement SIP Media
Service (RFC 4240), and the Media Server Control Markup Language (RFC
5022).
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Overview of Mechanism . . . . . . . . . . . . . . . . . . 3
3.2. Media Server Discovery . . . . . . . . . . . . . . . . . . 5
3.3. Client Use of GENURLAUTH Command . . . . . . . . . . . . . 7
3.4. Client Determination of Media Server Capabilities . . . . 9
3.5. Client Use of the Media Server Announcement Service . . . 10
3.6. Media Negotiation and Transcoding . . . . . . . . . . . . 11
3.7. Client Use of the Media Server MSCML IVR Service . . . . . 13
3.8. Media Server Use of IMAP Server . . . . . . . . . . . . . 17
3.9. Protocol Diagrams . . . . . . . . . . . . . . . . . . . . 18
3.9.1. Announcement Service Protocol Diagram . . . . . . . . 18
3.9.2. IVR Service Protocol Diagram . . . . . . . . . . . . . 19
4. Security Considerations . . . . . . . . . . . . . . . . . . . 21
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
6. Digital Rights Management (DRM) Issues . . . . . . . . . . . . 24
7. Deployment Considerations . . . . . . . . . . . . . . . . . . 24
8. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 25
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 26
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
10.1. Normative References . . . . . . . . . . . . . . . . . . . 26
10.2. Informative References . . . . . . . . . . . . . . . . . . 28
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
before displaying any of it 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
[RTP]. Streaming can be initiated and controlled using protocols
such as SIP [SIP] and particularly the media server profiles as
specified in RFC 4240 [NETANN] or MSCML [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.
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Examples of the types of media that would benefit from the ability to
stream to the device include:
o Voice or video mail messages received as an attachment
o Audio clips such as ring tones received as an attachment
o 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 that makes use of the
announcement service of RFC 4240, and a more complex mechanism which
allows VCR controls, based on MSCML (RFC 5022) [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. 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 [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.
3. Mechanism
3.1. Overview of Mechanism
The proposed mechanism for streaming media to messaging clients is a
profile for making use of several existing mechanisms, namely:
o IMAP URLAUTH Extension [URLAUTH] - Providing the ability to
generate an IMAP URL that allows access by external entities to
specific message parts, e.g., an audio clip.
o URLFETCH Binary Extension [URLFETCH_BINARY] - Providing the
ability to specify BINARY and BODYPARTSTRUCTURE arguments to the
URLFETCH command.
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o Media Server Announcement Service (RFC 4240) [NETANN] - Providing
the ability for a media server to stream media using a reference
provided by the media server client in a URL.
o Media Server Interactive Voice Response (IVR) Service (RFC 5022)
[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 |
| | | |
+--------------+ +----------------+
Figure 1: Proposed Mechanism
The proposed mechanism has the following steps:
(1) The 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) The client constructs an IMAP URL referencing the message part,
and uses the GENURLAUTH [URLAUTH] command to generate a URLAUTH-
authorized IMAP URL.
(3) The client connects to a SIP Media Server using the announcement
service as specified in RFC 4240 [NETANN], or the IVR service as
specified in RFC 5022 [MSCML], and passes the URLAUTH-authorized
URL to the media server.
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(4) The media server connects to the IMAP server specified in the
referenced URL, and uses the IMAP URLFETCH [URLAUTH] command to
retrieve the message part.
(5) The media server streams the retrieved message part to the
client using RTP [RTP].
(6) The media server or the client terminates 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:
o The mobile device is provisioned with, or obtains via some dynamic
mechanism (see Section 3.2), the location of a media server which
supports either RFC 4240 [NETANN] and/or RFC 5022 [MSCML].
o The media server(s) used by the mobile device support the IMAP URL
[IMAPURL] scheme for the announcement and/or IVR services.
o The IMAP server used by the mobile device supports generating
anonymous IMAP URLs using the URLAUTH mechanism as well as the
IMAP URLFETCH BINARY [URLFETCH_BINARY] extension.
3.2. Media Server Discovery
This section discusses possibilities for the automatic discovery of
suitable media servers to perform streaming operations, and provides
for such a mechanism using the IMAP METADATA [METADATA] extension.
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 a discovery
mechanism to determine a suitable media server that will be used
for streaming multimedia message parts.
There are several scenarios where media server discovery would be a
requirement for streaming to be successful:
o Client is not configured with the address of any media servers.
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o 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 discovery 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 authorized user of the IMAP server
for streaming purposes, which would allow the client to generate a
URL, which was secure in that it could *only* be accessed by an
entity that is trusted by the IMAP server to retrieve content. The
issue of trust in media servers is discussed more fully in Section 4.
This document describes using the IMAP METADATA [METADATA] extension,
via the use of a server entry that provides the contact information
for suitable media servers for use with the IMAP server. Media
Server discovery is optional: clients are free to use pre-configured
information about media servers, or to fall back to pre-configured
information if they encounter IMAP servers that do not support either
the METADATA extension or the proposed entry, or that do not provide
a value for the entry.
A METADATA entry with the name of "/shared/mediaServers" is used to
store the locations of suitable media servers known to the IMAP
server. The entry is formatted according to the formalSyntax
specified in Section 8. This consists of a tuple of a URI and
optional "stream" string, where the URI is surrounded by <> symbols,
the URI and "stream" are separated using a colon ":", and tuples are
separated using a ";".
The "stream" string (c.f. the "stream" access identifier from
[ACCESSID]) is used to identify media servers capable of connecting
to the IMAP server as users authorized to retrieve URLs constructed
using the "stream" access identifier. It indicates that the client
MUST create the content URI using the "stream" access identifier.
See Section 3.3 for a description of how the client should make use
of the access identifier when generating IMAP URLs.)
Example values of the /shared/mediaServers METADATA entry (N.B. Any
line-wrapping below is for the purpose of clarity):
"<sip:ivr@ms.example.net:5060>:stream;<sip:annc@
ms1.example.net:5060>;<sips:ivr@ms2.example.net:5061>"
"<sip:ivr@192.0.2.40:5060>;<sip:192.0.2.41:5060>;<sips:annc@
192.0.2.42:5060>:stream"
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It should be noted that the URI specified in the ABNF (in Section 8)
is generic, i.e., not restricted to SIP URIs; however, this document
only specifies how to make use of SIP URIs. Additionally, the
"userinfo" (known as the "service indicator" in RFC 4240 and RFC
4722) component of the URI is optional; if specified, it gives the
client additional information about the media server capabilities.
For example, a "userinfo" component of "annc" indicates that the
media server supports RFC 4240, and "ivr" indicates support for RFC
4722. Section 3.4 further describes how clients should behave if the
"userinfo" component is not present.
Clients SHOULD parse the value of the /shared/mediaServers entry, and
contact a media server using one of the returned URIs. The servers
are returned in order of preference as suggested by the server;
however, it is left to the client to decide if a different order is
more appropriate when selecting the media server(s) to contact, as
well as the selection of alternates under failure conditions.
Administrators configuring the values of the /shared/mediaServers
entry, who do not know the capabilities of the media servers being
configured, SHOULD NOT include a "userinfo" component as part of the
URI. In that case, the client will determine which service to use as
specified in Section 3.4. Note that if a media server supports
multiple services, a URI with the appropriate userinfo component
SHOULD be configured for each service.
Note that even though the media server address can be discovered
dynamically, it is assumed that the necessary security arrangements
between the client and the media server already exist. For example,
the media server could use SIP digest authentication to provide
access only to authenticated clients; in this case, it is assumed the
username and password have already been set up. Likewise, if the
client wants to authenticate the media server using, e.g., TLS and
certificates, it is assumed the necessary arrangements (trust anchors
and so on) already exist. In some deployments, the clients and media
servers may even be willing to rely on the security of the underlying
network, and omit authentication between the client and the media
server entirely. See Section 4 for more details.
3.3. 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 [MIME] Content-Type of particular message parts, and based
on local policies or heuristics, they decide whether streaming for
that message part will be attempted.
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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
[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 (see Section 4), and it ensures that at some point in
the future, permission to access that URL will cease. IMAP server
implementors may choose to reject anonymous URLs that are considered
insecure (for example, with an EXPIRE date too far in the future), as
a matter of local security policy. To prevent this from causing
interoperability problems, IMAP servers that implement this profile
MUST NOT reject GENURLAUTH commands for anonymous URLs on the basis
of the EXPIRE time, if that time is equal to, or less than, 1 hour in
the future.
The <access> portion of the URLAUTH URL MUST be 'stream' (see
[ACCESSID]) if an out-of-band mechanism or the media server discovery
mechanism discussed in Section 3.2 specifies that the media server is
an authorized user of the IMAP server for the purposes of retrieving
content via URLFETCH. Without specific prior knowledge of such a
configuration (either through the discovery mechanism described in
this document, or by an out-of-band mechanism), the client SHOULD use
the 'stream' access identifier, which will cause streaming to fail if
the media server is not an authorized user of the IMAP server for the
purposes of streaming.
However, if the client wishes to take the risk associated with
generating a URL that can be used by any media server (see
Section 4), it MAY use 'anonymous' as the <access> portion of the
URLAUTH URL passed to the GENURLAUTH command. For example, the
client may have been pre-configured with the address of media servers
in the local administrative domain (thus implying a level of trust in
those media servers), without knowing whether those media servers
have a pre-existing trust relationship with the IMAP server to be
used (which may well be in a different administrative domain). See
Section 4 for a full discussion of the security issues.
The client uses the URL generated as a parameter to the GENURLAUTH
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.
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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=stream" INTERNAL
S: * GENURLAUTH "imap://joe@example.com/INBOX/;uid=24359/;
section=1.3;expire=2006-12-20T18:31:45-08:00;
urlauth=stream:
internal:098230923409284092384092840293480239482"
S: a123 OK GENURLAUTH completed
3.4. 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 [RTP] protocol.
This section specifies the behavior of clients that have not
determined (either statically through configuration, or dynamically
through a discovery process as discussed in Section 3.2), 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
Sections 3.5 or 3.7, as appropriate.
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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 that has the service indicator "ivr".
Assuming the media server responds to the INVITE without error, the
client can carry on using the MSCML IVR service as specified in
Section 3.7. 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 3.5.
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 >
3.5. 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 [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 3.3.
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:%2F%2Fjoe@example.com%2FINBOX%2F%3Buid%3D24356%2F%3Bsection
%3D1.2%3Bexpire%3D2006-12-19T16:39:57-08:00%3Burlauth%3Danonymous:
internal:238234982398239898a9898998798b987s87920
C: sip:annc@ms1.example.net;
play=imap:%2F%2Ffred@
example.com%2FINBOX%2F%3Buid%3D24359%2F%3Bsection
%3D1.3%3Bexpire%3D2006-12-20T18:31:45-08:00%3Burlauth%3Dstream:
internal:098230923409284092384092840293480239482
Notice that many of the characters that are used as parameters of the
IMAP URI are escaped, as otherwise they would change the meaning of
the enclosing SIP URI, by being regarded as SIP URI parameters
instead of IMAP URL parameters.
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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.
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.
3.6. 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
to discuss 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 3.5), advertising support for a range of audio and video
codecs (using SDP [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 session description
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 session description below, 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, or 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.
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Example SIP INVITE and SDP Media Negotiation
C: INVITE sip:annc@ms2.example.com;
play=imap:%2F%2Fjoe@example.com%2FINBOX%2F%3Buid%3D24356%2F%3B
section%3D1.2%3Bexpire%3D2006-12-19T16:39:57-08:00%3Burlauth%3D
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
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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
3.7. 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 dual
tone multi-frequency (DTMF) digits to control playback of the media,
such as fast-forward or rewind.
Since the <playcollect> request is used purely for its VCR-like
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 after the media has finished playing.
The "ffkey" and "rwkey" attributes of <playcollect> are used to
control fast-forward and rewind behavior, 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 3.3. The audio-specific name of the tag is
historical, as the tag can be used for video as well as audio
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content. The "stoponerror" attribute SHOULD be set to "yes", so that
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 3.9.2 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
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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
Cook Informational [Page 15]
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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
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3.8. 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 [TLS] to
encrypt the communication path.
If the media server has a user identity on the IMAP server, the media
server SHOULD authenticate itself to the IMAP server using the media
server's user identity.
If the media server is not configured as an authorized user of the
IMAP server, then the behavior specified in IMAP URL [IMAPURL] MUST
be followed. That is, if the server advertises AUTH=ANONYMOUS IMAP
capability, the media server MUST use the AUTHENTICATE command with
the ANONYMOUS [ANONYMOUS] SASL mechanism. If SASL ANONYMOUS is not
available, the username "anonymous" is used with the "LOGIN" command
and the password is supplied as the Internet email 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 post-authentication 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) [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 response to the client.
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
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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.
3.9. 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.
3.9.1. 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.
Cook Informational [Page 18]RFC 5616 Streaming Internet Messaging Attachments August 2009
Client IMAP Server Media Server
| FETCH (BODYSTRUCTURE) | |
|---------------------------->| |
| OK | |
|<----------------------------| |
| GENURLAUTH | |
|---------------------------->| |
| OK | |
|<----------------------------| |
| | |
| SIP INVITE |
|===========================================================>|
| | |
| | URLFETCH |
| |<-----------------------------|
| | OK |
| |----------------------------->|
| | |
| 200 OK |
|<===========================================================|
| ACK |
|===========================================================>|
| | |
| Stream Message Part (RTP) |
|<+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++|
| | |
| BYE |
|<===========================================================|
| 200 OK |
|===========================================================>|
3.9.2. 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.
Cook Informational [Page 19]RFC 5616 Streaming Internet Messaging Attachments August 2009
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 |
|<===========================================================|
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4. Security Considerations
This document proposes the use of URLAUTH [URLAUTH] "pawn tickets",
received over IMAP [IMAP], and transmitted over SIP [SIP], possibly
within the MSCML payload of RFC 5022 [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
with respect to the security implications of using IMAP URLs:
o Use of an anonymous pawn ticket grants access to any client of the
IMAP server without requiring any authentication credentials. The
security mechanisms referenced above (with the caveats specified
below) SHOULD be used to prevent unauthorized access to the pawn
ticket.
o Use of pawn tickets that contain the "stream" access identifier
restricts access to the content to those entities that are
authorized users of the IMAP server for the purposes of streaming
retrieved content. Use of such pawn tickets is thus desirable and
so implementors should consult Section 3.3, which describes when
such pawn tickets should be used.
o If the announcement service is used to set up streaming, then RFC
4240 [NETANN] specifies that the pawn ticket is passed in the
Request-URI, and so untrusted third parties may be able to
intercept the pawn ticket. The SIP communication channel MAY be
secured by using SIPS URIs [SIP], which would provide hop-by-hop
TLS encryption.
o If the IVR service (RFC 5022 [MSCML]) is used to set up and
control streaming, then MSCML is used to carry the pawn ticket in
the body of the request, and so untrusted third parties may be
able to intercept the pawn ticket. This MAY be secured by using
SIPS URIs [SIP], which would provide hop-by-hop TLS encryption.
o Using SIPS URIs in the above situations protects the pawn ticket
from third parties; however, it still allows proxies access to the
pawn ticket, which could result in misuse by malicious proxies;
see note below.
This document describes a mechanism that makes use of two separate
servers to achieve the goal of streaming the content desired by the
client. A major security implication of this is that the media
server and IMAP server may well be located in separate administrative
domains. This leads us to consider the security implications of a
Cook Informational [Page 21]RFC 5616 Streaming Internet Messaging Attachments August 2009
three-way protocol exchange, and the potential trust model implicit
in that tripartite relationship. The security implications of the
individual protocols have already been referenced; therefore, this
section describes the security considerations specific to the three-
way data exchange, as follows:
o The client grants the media server full access to the potentially
private media content specified by the IMAP URL. As a result, the
client is responsible for verifying the authenticity of the media
server to a degree it finds acceptable for the content (we can
refer to this process as determining the "trust" that the client
has in a particular media server). The security mechanisms
provided by SIP [SIP] and RTP [RTP] may be used for this purpose,
as well as out of band mechanisms such as pre-configuration.
o However, since the media server will retrieve content from an IMAP
server on the user's behalf, the issue of security between the
IMAP server and the media server also needs to be considered. A
client has no way of determining (programatically at least) the
security of the exchanges between the media server and the IMAP
server. However, it can determine, using the "stream" token that
is part of the media server discovery mechanism described in
Section 3.2, that the media server has a pre-existing
authentication relationship with the IMAP server for the purposes
of retrieving content using IMAP URLs. The IMAP server
administrator may put prerequisites on media server administrator
before this relationship can be established, for example, to
guarantee the security of the communication between the media
server and the IMAP server.
o The above two security considerations will influence the decision
the client makes with regards to generation of the pawn ticket
that is subsequently passed to the media server. This document
mandates the use of URLs protected with the "stream" access
identifier where the client knows in advance that the "stream"
authentication relationship between media server and IMAP server
exists. However, it does allow the use of anonymous pawn tickets
where the possibility exists that use of "stream" would cause
streaming to fail.
o There exists the possibility of several types of attack by a
malicious media server, SIP proxy, or other network elements even
against pawn tickets protected with the "stream" access
identifier. All of these attacks allow access to the RTP stream,
if not the original content. These attacks include:
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* The client contacts a malicious media server, MS1, that then
proxies the streaming request to a second media server, MS2,
that it has determined or guessed to have "stream"
authorization credentials with the IMAP server specified in the
pawn ticket. The media server can then redirect the streamed
RTP traffic elsewhere.
* Any proxy on the path between the client and the media server
has access to the client's message in cleartext. In this case,
a malicious proxy could perform a man-in-the-middle attack and
could change the message to redirect RTP traffic elsewhere.
* Any network element that is able to "see" the traffic between
the client and the media server (or between any two proxies)
can capture the pawn ticket, and then reissue a request using
that pawn ticket to the same media server. Again the streamed
traffic can be redirected to any desired location.
Media servers handling streaming requests will be making use of pawn-
ticket URLs for the period of time required to process the streaming
request, after which the URL will be forgotten. However, media
servers may log the URLs received from clients, in which case, the
private data contained in the IMAP server could be accessed by a
malicious or curious media server administrator. Even URLs protected
with EXPIRE may be accessed within the period of expiry. Therefore,
media servers SHOULD remove or anonymize the internal portion of the
IMAP URL when logging that URL.
Additionally, many of the security considerations in the Message
Submission BURL Extension apply to this document, particularly around
the use of pawn tickets and prearranged trust relationships such as
those described above.
Message parts that are encrypted using mechanisms such as S/MIME
[SMIME] are designed to prevent third parties from accessing the
data, thus media servers will not be able to fulfill streaming
requests for messages parts that are encrypted.
5. IANA Considerations
IANA has registered the following [METADATA] server entry to be used
for media server discovery, using the [METADATA] registry.
Cook Informational [Page 23]RFC 5616 Streaming Internet Messaging Attachments August 2009
To: iana@iana.org
Subject: IMAP METADATA Entry Registration
Type: Server
Name: /shared/mediaServers
Description: Defines a set of URIs containing the locations of
suitable media servers for streaming multimedia content
Content-type: text/plain; charset=utf-8
Contact: neil.cook@noware.co.uk
6. 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 [URLAUTH] command; thus, any DRM
required should be implemented within the media itself, as
implementing checks within URLAUTH could affect any use of the
URLAUTH command, such as the BURL [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 that is not essentially provided by the
action of sending the message. In other words, the use of the
content for downloading and viewing *is* implicitly granted by the
sender of the message, in as much as the sender has the right to
grant such rights.
The document author believes 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.
7. 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
Cook Informational [Page 24]RFC 5616 Streaming Internet Messaging Attachments August 2009
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 firewall policies
such that appropriate SIP UACs (user agent clients) 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.
8. Formal Syntax
The following syntax specification for the mediaServers METADATA
entry value uses the Augmented Backus-Naur Form (ABNF) notation as
specified in RFC 5234 [ABNF] and the "absolute-URI" definition from
RFC 3986 [RFC3986].
Except as noted otherwise, all alphabetic characters are case-
insensitive. The use of upper or lower case characters to define
token strings is for editorial clarity only. Implementations MUST
accept these strings in a case-insensitive fashion.
Copyright (c) 2009 IETF Trust and the persons identified as authors
of the code. All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
- Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the
distribution.
- Neither the name of Internet Society, IETF or IETF Trust, nor the
names of specific contributors, may be used to endorse or promote
products derived from this software without specific prior written
permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
Cook Informational [Page 25]RFC 5616 Streaming Internet Messaging Attachments August 2009
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
media-servers = ms-tuple *(";" ms-tuple)
ms-tuple = "<" absolute-URI ">" [":" "stream"]
9. Contributors
Eric Burger (eburger@standardstrack.com) 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.
10. References
10.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
January 2008.
[ACCESSID] Cook, N., "Internet Message Access Protocol (IMAP) - URL
Access Identifier Extension", RFC 5593, June 2009.
[ANONYMOUS] Zeilenga, K., "Anonymous Simple Authentication and
Security Layer (SASL) Mechanism", RFC 4505, June 2006.
[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[IMAPURL] Melnikov, A., Ed. and C. Newman, "IMAP URL Scheme",
RFC 5092, October 2007.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[METADATA] Daboo, C., "The IMAP METADATA Extension", RFC 5464,
February 2009.
Cook Informational [Page 26]RFC 5616 Streaming Internet Messaging Attachments August 2009
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME)", RFC 2045, November 1996.
Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII
Text", RFC 2047, November 1996.
Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December
2005.
Freed, N. and J. Klensin, "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures",
BCP 13, RFC 4289, December 2005.
Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996.
[MSCML] Van Dyke, J., Burger, E., Ed., and A. Spitzer, "Media
Server Control Markup Language (MSCML) and Protocol",
RFC 5022, September 2007.
[NETANN] Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network
Media Services with SIP", RFC 4240, December 2005.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RTP] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[SDP] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[SIP] 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.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
Cook Informational [Page 27]RFC 5616 Streaming Internet Messaging Attachments August 2009
[URLAUTH] Crispin, M., "Internet Message Access Protocol (IMAP) -
URLAUTH Extension", RFC 4467, May 2006.
[URLFETCH_BINARY]
Cridland, D., "Extended URLFETCH for Binary and Converted
Parts", RFC 5524, May 2009.
10.2. Informative References
[BURL] Newman, C., "Message Submission BURL Extension",
RFC 4468, May 2006.
[SMIME] Ramsdell, B., Ed., ""Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification"",
RFC 3851, July 2004.
Author's Address
Neil L Cook
Cloudmark
EMail: neil.cook@noware.co.uk
Cook Informational [Page 28]
