Internet DRAFT - draft-burger-mscl-thoughts
draft-burger-mscl-thoughts
Network Working Group E. Burger
Internet-Draft Cantata Technology, Inc.
Expires: December 7, 2006 June 5, 2006
Media Server Control Language and Protocol Thoughts
draft-burger-mscl-thoughts-01
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Copyright (C) The Internet Society (2006).
Abstract
IP mutli-function Media Server control is a problem that has slowly
bubbled up in importance over the past four years. A driver in the
IETF is the requirements generated by the XCON framework. Many
approaches have been proposed. Some of these proposals are device-
controlled-oriented, such as H.248. Others are server-oriented,
using SIP and application-oriented markup. Before rushing headlong
into a framework for a solution, it is time to step back and try to
understand just what the scope of the problem is. Once consensus is
reached, we can then move forward with a framework for a solution.
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This document describes a number of existing approaches and proposals
to solve the Application Server - Media Server protocol problem,
their characteristics and benefits and drawbacks.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Media Resource Model . . . . . . . . . . . . . . . . . . . 4
2.2. Number of Protocol Messages for a Given Operation . . . . 5
2.3. Network Topology . . . . . . . . . . . . . . . . . . . . . 5
2.4. Protocol Layer Integrity . . . . . . . . . . . . . . . . . 6
2.5. Computer Science Issues . . . . . . . . . . . . . . . . . 7
2.6. Deployment Scale . . . . . . . . . . . . . . . . . . . . . 9
2.7. Compatibility with SIP Model . . . . . . . . . . . . . . . 10
2.8. Security Issues . . . . . . . . . . . . . . . . . . . . . 10
3. Transport Protocols . . . . . . . . . . . . . . . . . . . . . 11
3.1. Pure Device Control . . . . . . . . . . . . . . . . . . . 11
3.2. Pure SIP . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3. SIP With TCP Side Channel . . . . . . . . . . . . . . . . 12
3.4. SIP With INFO . . . . . . . . . . . . . . . . . . . . . . 13
3.5. SIP With SUBSCRIBE/NOTIFY . . . . . . . . . . . . . . . . 14
3.6. SIP With MEDIA . . . . . . . . . . . . . . . . . . . . . . 14
4. Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. H.248 . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2. MSCML . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3. MOML/MSML . . . . . . . . . . . . . . . . . . . . . . . . 18
5. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 20
6. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
8. Informative References . . . . . . . . . . . . . . . . . . . . 22
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 24
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . . . . 26
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1. Introduction
An IP multi-function Media Server is a network server that provides
media processing services to the network.
There are two models for media resource servers. One models the
media resource server as a box of low-level resources, such as RTP
mixers, transcoders, audio play and record resources, video play and
record resources, tone detection and generation resources, and
resources to connect, or "plumb" the resources together. The other
model is that of a server that offers announcement services,
interactive voice response (IVR) services (including speech
recognition and speech synthesis modalities), interactive video
response (IVVR) services, basic mixing services, and enhanced mixing
services.
In general, when we say "multi-function Media Server", we are
referring to the server model.
As the IP Media Server evolved from a box of low-level resources into
a first-class server in the Internet, the protocol interfaces to
control the IP Media Server evolved, as well. When people thought of
the media server as a box of low-level resources, device control
protocols like H.248 [1] seemed appropriate. At the time, the
primary model for control of a media server was from a "SoftSwitch",
or Media Gateway Controller. The principal application was for
playing announcements and collecting a small number of digits. The
Media Gateway Controller already implemented a device control state
machine to control the Media Gateways. Moreover, the Media Gateway
Controller implemented some form of Gateway Control protocol to
control the Media Gateways. Thus it was logical to assume that a
device control protocol, more specifically H.248 from the IETF
perspective, would be appropriate for media resource control.
Although the "SoftSwitch" (traditional telephony) model (and market)
was an early driver for the need for media resources, within two
years it was clear that the primary consumer of media resources would
be Internet-oriented applications. Developers create and deploy
these applications on Internet Application Servers, using Internet
and Web tools and protocols. These Application Servers have no need
to control Media Gateways, and thus do not generally have
implementations of device control protocols such as H.248. Moreover,
Application Servers were much more likely to have HTTP [2] and SIP
[3] and use stimulus-markup, client-server application architectures.
RFC3087 [4] introduced the concept of addressing services as if they
were users in SIP. This meant that it was possible to address
specific resources from an application simply by sending the session
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to a "user" at a media server. However, RFC3087 did not provide any
mechanism to achieve Internet-wide interoperability. What was needed
was some sort of naming convention to address the various services
available at the media server. The netann [5] specification provides
such a naming convention.
Recalling the functions of a multi-function IP Media Server, the
netann specification is directly sufficient for announcements and
simple conferencing.
For Interactive Voice Response (IVR), VoiceXML [6] provides a
standard method for defining voice (and now video) dialogs. However,
there is a need to inform the IP multifunction media server that the
request is for the VoiceXML service and the URI of the initial
document. The netann specification provides this definition.
What is missing is a method for enhanced conference control.
By enhanced conference control, we mean facilities for creating sub-
mixes, recording the mix or a leg, playing media into a mix or leg,
altering the gain on a leg or the mix as a whole, defining which
media is eligible for the mix, and so on.
To date, there have been several proposals, experimental protocols,
and de facto standards to address the enhanced conference control
problem. Factors influencing these protocols include the
application's media resource model (raw resources versus service
server), the desire to leverage existing protocol infrastructure
(such as using SIP Registrars for resource discovery, SIP Proxies for
resource location, scale, and availability), and the expectations of
Internet-scale deployment sizing. The following sections examine
these factors and then look at the various proposals to address them.
As a side note, two XML-based, SIP-transported media server control
markup languages command approximately 100% of the market: MSCML [16]
and MOML [17].
2. Factors
2.1. Media Resource Model
As the Introduction indicated, many new applications use the Internet
model for media resources. That is, applications request media
services from an Internet-oriented, IP multi-function Media Server.
However, some legacy applications, as well as application developers
more comfortable with a telco-oriented approach, would like to model
the media processing function as a set of low-level resources.
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There is no question that with a low-level model, one has the full
flexibility to address any possible requirement. For example,
creating a sidebar conference is simply the manipulation of some
mixer resources and plumbing the selected RTP streams (possibly
through transcoders) to the mixer resources. Likewise, one can
accomplish playing a prompt to a leg by disconnecting the leg from
the mixer, allocating a media player, plumbing the media player to
the RTP port that represents the leg, directing the media player to
play the prompt, then deallocate the media player, and finally re-
plumbing the RTP stream to the mixer.
Conversely, with an Internet server model, applications request media
manipulation using protocols appropriate for applications. For
example, media streams are addressed using application constructs,
such as SIP dialog identifiers. Rather than specifying a sidebar by
manipulating RTP streams directly, the application specifies which
legs the Media Server is to place into a sidebar. In fact, as we
will show below, one can specify complex topologies, such as Agent/
Supervisor/Mark, with fewer messages than using a device control
protocol.
2.2. Number of Protocol Messages for a Given Operation
The number of protocol messages required for a given set of
operations is a factor that can potentially affect the scale of the
deployment.
Too many messages can result in bandwidth problems at the media
server control interface, packet handling problems at either the
media server or application server, and stack processing problems at
either the media server or application server.
Conversely, optimizing on number of messages can result in complex
protocols with a very large number of verbs. This is often in
conflict with engineering principles such as offering a simple
protocol with a small number of verbs.
2.3. Network Topology
In determining the control mechanism, we need to examine the control
topology. Namely, will there be a one-to-one mapping of Application
Servers to Media Servers? Will there be a one-to-many mapping of
Applications Servers to Media Servers? Will there be a many-to-one
mapping of Applications Servers to Media Servers? Or, can there be a
many-to-many mapping of Application Servers to Media Servers.
Answers to this question helps determine the question as to whether
there should be a single control channel per Media Server, single
control channel per Application Server, single control channel per
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session, or single control channel per leg.
Since control channels consume operating system resources, fewer
control channels use fewer operating system resources. Of course,
overall system resource utilization is more complex than simply how
many channels there are at a given node. For example, on most
operating systems, message routing is done in kernel space with
pointer manipulation. However, once in application space, message
routing is often done with buffer copying.
Another aspect influencing the cardinality of control channels is
protocol layer integrity. We will examine this point in the next
section.
2.4. Protocol Layer Integrity
There are many fundamental principles driving the IETF model of
layered protocols. For example, a single TCP socket uses less system
resources that ten thousand TCP sockets. Given that, why do we have
FTP, TELNET, SMTP, NNTP, MGCP, etc.? It would appear to be much more
efficient to establish a single TCP socket between the hosts and
multiplex the different protocols over that socket. One of the
reasons we do not do this is that while we would save on memory and
kernel processing on the TCP socket, we end up spending memory and
kernel processing resources on demultiplexing the TCP stream to
direct the stream to the appropriate application process in user
space.
Likewise, one could multiplex a given protocol over a single channel.
In this case, the decision comes down to programming model. For
example, in the FTP case, it is easier to manage the media and
control separately over separate channels. Many implementations of
FTP has the server FTP daemon spawning separate FTP server processes
to handle requests. In this way the FTP server process can be quite
simple and straightforward.
Another approach has multiple requests physically multiplexed to a
single port, but establish separate logical sessions. One protocol
that uses this model is SIP. All requests go to a single port
(usually 5060), yet in the protocol data unit (PDU), we have a dialog
identifier that identifies which dialog the message belongs to.
The control channel per session model maintains protocol layer
integrity by allowing the kernel to do appropriate routing of
requests to the application.
Multiplexing the control channel requires special considerations.
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If there is a limit of a single control channel at the Media Server,
then, by definition, there can be only a single Application Server
controlling it. This works in a device control model, such as H.248
[1], where a Media Gateway Controller controls an entire Media
Gateway. In order to allow multiple clients to control the server,
one must "virtualize" the server. That is, the server presents what
looks to the client as an entire, self-contained server, while in
fact those self-contained servers are actually logical partitions of
the physical server.
Depending on the server function, such partitioning may be easy or
extremely complex. Let us consider the case of a SIP Application
Server. A SIP Application Server, or Back-to-Back User Agent
(B2BUA), looks to the world like a whole bunch of SIP User Agent
Servers. This is not too difficult to manage, as the SIP User Agent
Servers all generally look alike. On the other hand, consider a SIP
Media Server. The SIP Media Server often has a fixed number of
different types of resources, such as announcement players,
conference bridges, recorders, and so on. Partitioning these
resources can be exceedingly complex.
Some applications benefit from a single control channel model. For
example, the classic SoftSwitch model and the current IMS model
assume that all media processing requests go through a single network
element that, in the words of TRON, is a "Master Control Program."
While many from the telco world are comfortable with having a large,
centralized system, many in the IETF have found time and time again
that a single central server rarely meets the requirements for
Internet scale. Other methods, such as server farms and alternate
return contact addresses, enable theoretically infinite scale.
2.5. Computer Science Issues
Two issues to consider when using a device control protocol are how
long it takes to create an application and the quality of the work
product. Two factors influencing these issues are the program length
and cyclomatic complexity.
There is an interesting result through 30 years of programmer
productivity studies. It turns out that with the exception of the
introduction of compilers, visual editors, and visual debuggers,
programmer productivity has been relatively constant, at 10 to 50
lines of code delivered per day. Thus, reducing the number of lines
of code required for a given function is an important tactic to
achieve the goal of improving either the time-to-market or robustness
of an application. This is one of the reasons why we code in Java,
C++, VB, etc., instead of assembly language.
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Cyclomatic complexity measures the number of branches and function
calls in a given application. Again, 15 years of research have shown
a strong correlation between cyclomaitc complexity and the difficulty
of test and liklihood of bugs in fielded code. This is an intuitive
result: more branches means more test cases, or the collary, that
more branches means more code that testing will miss. However, the
emperical results are more impressive: the higher the cyclomatic
complexity, the more errors found in the field.
Here is a concrete example of how this plays out in practice. iSCSI
[7] defines how one can, over IP, read and write blocks on a disk.
One could then ask, "Why do we access data bases using data base-
oriented protocols, like TDS [8]?" After all, one can do all the
manipulation one needs for a data base application at the disk block
level. Moreover, one can virtualize the target disk, so the
application does not have to have direct control over physical disk
blocks.
We would offer the answer is obvious. Data base application
developers think and operate at the table access level. They don't
care about disk blocks, B-Trees, indices, and so on.
One could argue that supplying a client library that hides the data
base-centric operations from an application would hide the low-level
nature of a disk access protocol from the application. That is, it
would present an application-layer interface to the application. We
offer here that protocol layer integrity comes to play here, as well.
In particular, embedding data base code in the client means that one
cannot have any data base innovation at the server. Everything
occurs at, and is bound to, the client.
Clearly there is a need for a low-level disk access protocol. That
is what drove the iSCSI effort. However, application developers need
a file access protocol like NFS [10]; data base application
developers need a high-level data base access protocol; mail
application developers need a mail transfer protocol like SMTP [11];
and so on.
A similar situation exists in the media processing milieu. The IETF,
with the ITU-T has created a media gateway control protocol, H.248
[1]. Although designed for the media gateway control problem, H.248
has capabilities for controlling arbitrary media functions, albeit at
a very low level. H.248, and, THE MODEL IT REPRESENTS, assumes a
master/slave, low-level device control programming model. This is
analogous to direct disk block manipulation for data access, as
represented by iSCSI. Features accessible via H.248 or protocols in
the style of H.248 include audio players, audio recorders, RTP
termination and origination, mixers, tone detectors and generators,
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and plumbing primitives.
High-level media processing protocols have been proposed, modeling a
media resource server as just that, a server that offers multimedia
processing functions. Services offered by media servers include IVR,
conference mixing, announcements, interactive video, and so on.
Consider the choice of terms: a H.248 device offers "features" while
a media server offers "services". Section 3 examines the different
protocol proposals in detail.
2.6. Deployment Scale
Just how many sessions do we need at any given Media Server? First,
let us consider a Media Server that would handle ALL calls on the
globe.
Take a population of seven billion people. Let us assume that every
person calls one other person, on average, once every week. That
means we are looking at 1 billion calls per day. Calculating the
maximum number of simultaneous calls, let us assume that in any given
populated time zone, up to 1/12th of the population of the world is
actively making calls. The assumption here is that the time zones
dividing the Pacific and Atlantic Oceans are essentially unpopulated
(sorry Greenland and Alaska), while the time zones covering Europe
have a relatively high teledensity. We make this assumption as we
assume that busy hour will rotate around the Earth for a given
application.
With these assumptions, there are about 83 million calls per day in a
given time zone. Since, for most applications, 15% of calls occur
during the busy hour, we are looking at 12.5 million simultaneous
calls.
Now it is time for a reality check. Just how many simultaneous
sessions will any given Application Server or Media Server really
need to handle? In the above example, we found an upper limit of
12.5 million simultaneous sessions ASSUMING ALL CALLS IN THE WORLD GO
THROUGH THE APPLICATION. That is a pretty hefty assumption.
What if we worked it backward? Let us assume that a single
Application Server and Media Server provided voice messaging to the
entire world. Again, let us start with a population of seven billion
people. With a ratio of 200 subscribers per session, we get 35
million sessions. Taking time zones into account, we would be
looking at about 2 million simultaneous sessions.
What is the point of these calculations? It is that the argument
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that one must have a single control channel to effectively scale
services is a bit disingenuous. Namely, if an Application Server
will be handling, say, 100 million users, only a small percentage
will be using the service at any given time. Moreover, if one
architected the Application Server to be a single node, it will have
to handle hundreds of thousands of inbound connections anyway. If
you can handle a few hundreds of thousands of simultaneous
connections, you can probably handle a few two- or three- hundreds of
thousands of connections. To put this into perspective, 100,000
inbound connections represents well over 2 entire IP port address
spaces.
2.7. Compatibility with SIP Model
Various proposals offer to use SIP in some way. The question is,
will one use SIP within the acceptable use of SIP, or will one use it
"because it is there."
For example, does a given protocol proposal leverage the SIP routing
infrastructure, or is it intended for a point-to-point deployment?
Does the server offer SIP-level services, or is it simply using SIP
to transport, or tunnel, device control commands? Does the protocol
preserve layer integrity, by using references in the SIP domain, or
does it require references to the SDP [9] or IP domain?
One measure of compatibility with the SIP model a given proposal
offers is to see what its compatibility with SIP Proxies, as defined
by RFC3261 [3], is. For example, does the proposal require SDP
manipulation? If so, how deep does the manipulation need to be?
Clearly, any SDP manipulation makes the protocol incompatible with
SIP Proxies - SDP modification requires the use of a back-to-back
User Agent (B2BUA). Is the B2BUA simply inserting an m-line in the
SDP to plumb a control channel? Is the B2BUA parsing the SDP to
determine RTP addresses and media types?
The best would be pure proxies, as this will have the highest chance
of avoiding compatibility issues in the future.
2.8. Security Issues
One issue is who is allowed to manipulate what at the Media Server.
For services like announcements, IVR, and IVVR, a straightforward
security model is to have commands come on the same SIP dialog as
what established the media connection. Clearly, if you can create
the connection, you have some kind of relationship with the end
point, if you are not the requesting end point itself.
Other relationships get more complicated. For example, if we have a
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single control pipe from the Application Server, everything is OK if
there is only one Application Server. This is the model for H.248.
However, if we have more than one Application Server, then we have to
ensure a separation of the resources from one Application Server from
another.
One solution for this problem is to partition the Media Server into
multiple virtual Media Servers, each one dedicated to a given
Application Server. This is a suggested model in H.248. However, as
mentioned above in Section 2.4, this may be difficult for server-
centric Media Servers.
3. Transport Protocols
3.1. Pure Device Control
H.248 [1] is the IETF/ITU-T media gateway control protocol. H.248
provides generic session establishment machinery and gateway internal
resource interconnection. H.248 packages define various resources,
including tone detectors, tone generators, audio recorders, and
fixed-function audio prompt resources.
H.248 uses SDP for session negotiation, but it is considerably
different than SIP's SDP offer/answer [12] protocol.
H.248 assumes a single media gateway controller per media gateway.
H.248 uses a single TCP, UDP, or SCTP pipe between the controller and
gateway.
Most H.248 implementations use text encoding over the wire. For
those that are enamored with XML PDU's, H.248 does have an ASN.1 [13]
encoding. This means one can use XER [14] to have an XML wire
protocol.
3.2. Pure SIP
Using the netann [5] convention, one can perform basic media
services, such as announcements and basic mixing. However, SIP does
not provide the necessary controls for enhanced conferencing, such as
gain control, identification of preferred speakers (if they speak,
they have priority in the mix, even if they are not the loudest),
creating sidebar and other topologies (such as Coach/Agent/Mark), and
so on.
Note that Pure SIP uses a single TCP or SCTP socket. However, there
is a separate SIP session per leg.
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3.3. SIP With TCP Side Channel
MRCPv2 [15] is an example of a media processing protocol that uses a
TCP side channel. In MRCPv2, the client uses SIP to route to a
speech server, uses SIP's SDP offer/answer [12] protocol to negotiate
the media codecs, and specifies the protocol machinery for
establishing a side channel transfer protocol, such as TCP or TLS,
for the actual MRCPv2 PDU's.
The MRCPv2 server hands back a unique session identifier to the
client. All subsequent messages relating to a given MRCPv2 session
include the session identifier. This means one can share the side
channel between multiple client instances on the requesting node.
MRCPv2 allows the client to request channel reuse or to request a new
channel at session establishment time. Correspondingly, the MRCPv2
server can insist on a side channel per session, rather than sharing
the side channel amongst sessions.
The MRCPv2 model has the benefit of using the SIP protocol machinery
for session establishment. This includes using the SIP security
mechanisms to authorize the association of the side channel with the
media channel.
MRCPv2 itself has the drawbacks of having a totally different state
machine. The MRCPv2 state machine is optimized for speech services
like speech recognition and speech synthesis. Moreover, the methods
are incompatible with the needs for conference control.
In addition, the MRCPv2 approach rules out the use of the protocol by
SIP Proxies, as the B2BUA must modify the SDP to insert the SDP
m-line for the control channel.
One might ask, "If all we are doing is establishing a TCP connection
to control the media server, what do we need SIP for?" This is a
reasonable question. The key is to be using SIP for media session
establishment. If we are using SIP for media session establishment,
then we need to ensure the URI used for session establishment
resolves to the same node as the node for session control. Using the
SIP routing mechanism, and having the server initiate the TCP
connection back, ensures this works. For example, the URI sip:
myserver.example.com may resolve to sip:
server21.farm12.northeast.example.net, whereas the URI
http://myserver.example.com may resolve to
http://server41.httpfarm.central.example.net. That is, the host part
is NOT NECESSARILY unambiguous.
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3.4. SIP With INFO
Two proposals have been put forward that use the SIP dialog for the
side channel. Both use the INFO method. They are MSCML [16] and
MSML [18].
MSCML uses the SIP Requires and Content-Type headers to ensure
interoperability and preservation of SIP semantics. MSCML correlates
the commands received on the dialog with the dialog's media streams.
In the case of enhanced conferences, where there are global commands
such as conference size, playing to the entire conference, or
recording the entire conference, MSCML has the concept of a
Conference Control Leg. The Conference Control Leg is not associated
with any media dialog. However, it is a SIP dialog in the normal
sense.
MSML relies on a private (non-Internet) agreement between the
Application Server and Media Server to know the context of the INFO
messages. MSML tunnels SDP-layer information over the established
dialog; in the case of media processing, it uses a secondary markup,
MOML [18]. MOML is a device control protocol, with primitives
similar to H.248.
Deployed versions of MOML/MSML do not use SIP, such as for
referencing entities with SIP dialog properties, using SIP semantics
for control, or transparently correlating SIP dialogs with RTP
streams. However, the current version of the MSML specification does
suggest using the SIP Dialog identifier to identify media sessions.
We will touch upon the content of what goes over the side channel in
Section 4.
Using the SIP dialog for the side channel has the benefit of using
the SIP routing network for getting the messages to locate and follow
(in the mobility case) the UAS and UAC. In particular, proxies that
are important for routing can Record-Route, while proxies that are
not needed other than for session establishment can chose to not
Record-Route. Thus the transport of side channel commands places
only a small burden on the SIP routing network.
Note that there are a few problems resulting from the use of INFO.
First, there are no throttling mechanisms, other than that provided
by the underlying transport mechanism (TCP or Connection-Mode SCTP).
If you are using UDP, you are out of luck. Second, even in the case
of MSCML, which is well behaved in that it is guaranteed by the SIP
protocol machinery that both the UAS and UAC will interoperate and
understand the semantics of the MSCML INFO messages, the stacks can
still get other, ill-behaved INFO messages that it may not
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understand. Third, even though this has never happened in the real
world, there is a theoretical problem that INFO message handling may
overwhelm a proxy. In practice, one sizes ones proxies to the total
traffic they need to handle. Moreover, only active element proxies,
such as Edge Proxies, need Record-Route. That said, this might be a
problem in the future.
The following sections explore alternatives that use the SIP Dialog.
3.5. SIP With SUBSCRIBE/NOTIFY
As outlined in the expired draft, INFO Considered Harmful [19], the
events framework (SUBSCRIBE/NOTIFY) addresses all of the problems
with INFO. Namely, event packages must offer throttling mechanisms,
all event packages identify themselves and thus globally
interoperate, and even stupid proxies that Record-Route everything
often decide not to Record-Route SUBSCRIBE and NOTIFY messages.
Of course, SUBSCRIBE/NOTIFY really, really, really should not
(actually, most of us, including me, say "MUST NOT") reuse the SIP
dialog directly associated with the media session. This means we
lose the auto-correlation feature that we have by using the INFO
method.
There is a subtler, yet arguably more important problem with using
SUBSCRIBE/NOTIFY. Namely, the semantics of SUBSCIBE are, "tell me
(monitor) what is going on at the device." Typical uses for
SUBSCRIBE are for presence [20] (what is the state of the user?), MWI
[21] (what is the state of the message store?), and KPML [22] (what
is the state of the key press buffer?). No package changes the state
of the UAS. Using SUBSCRIBE, for example, to play a prompt or to
change the configuration of a mixer, most definitely changes the
state of the UAS.
3.6. SIP With MEDIA
Another approach outlined in INFO Considered Harmful [19] is to
introduce a new method. This was the route taken by PUBLISH [23], as
it was not quite NOTIFY.
Properly defined, a new method can safely share the SIP dialog.
Moreover, it would satisfy the auto-correlation properties used by,
for example, MSCML. Lastly, the semantics would be well defined,
addressing the issues raised by INFO Considered Harmful.
4. Models
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4.1. H.248
H.248 [1] provides:
1. A single control channel between Application Server and Media
Server.
2. The possibility for an XML transport encoding.
3. Total control of media resources, at the assembly language level.
The first item is of use to those whom would want a single control
channel and socket per Application Server. The second item is of use
to those whom love XML. The third item ensures a measure of
capabilities possibility. That is, since the Application explicitly
defines the application-level semantics of media processing at the
media layer, future Applications can define future, unanticipated
topologies.
The drawbacks of H.248 are:
1. Layer violation et al.
2. Market adoption
The first item touches upon virtually every issue raised in
Section 2. By definition, H.248 is a low-level device control
protocol. That means more lines of code for a given function, higher
complexity for a given function, no compatibility with the SIP model
(everything becomes a MGC), and the Application Server must dive deep
into SDP and they media layer to do basic operations.
The second item, while not in itself a determining factor in the
IETF, is important to note as a leading indicator. For many of the
reasons noted above, neither Application Server developers nor Media
Server developers desire H.248 as an Application Server - Media
Server protocol. Moreover, none of the major media server
manufacturers have or plan to offer H.248-based media servers. In a
sense, the market has spoken about this option, even in light of the
1999 declaration (well before there were any enhanced media services)
by 3GPP that H.248 would be the media server (MRFP) interface.
4.2. MSCML
MSCML [16] provides:
1. Automatic correlation, including security associations, between
the control channel and the media session.
2. Preservation of SIP semantics, including being SIP Proxy
friendly.
3. Operations and all semantics are at the SIP dialog layer.
4. Application Servers can be relatively simple, as addressing of
media processing commands is straightforward: send the command
down the associated SIP media dialog.
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5. Establishing a media session is straightforward: INVITE the Media
Server to a session.
6. Strict adherence to the philosophy espoused by, among other
places, the Application Interaction Framework [24].
The drawbacks of MSCML include:
1. Even though MSCML properly uses INFO, using INFO in itself has
theoretical problems with non-interoperating devices.
2. By relying on SIP dialogs, the Application Server uses multiple
SIP dialogs to control, for example, an enhanced conference on
the Media Server.
3. By taking the application layer approach, MSMCL requires one to
two more protocol messages than a device control approach.
The first issue is a result of using INFO.
The second issue is more interesting. For example, the enhanced
conference case, that is, where one needs to play or record into the
entire conference, one has to setup an additional SIP dialog, the
Conference Control Dialog, per conference. In the extreme case of
two-party conferences, this increases the number of SIP dialogs by
50%. Of course, few two-party scenarios require the enhanced
conferencing features, and thus would not increase the number of
dialogs. However, if one did need those features, then the dialog
expansion would occur.
The third issue refers to the situation where the Application Server
wants to place the caller into a conference, but the application
needs to interact with the caller before the application knows which
conference to place them into. In the MSCML model, the application
has to INVITE the caller into a dialog (VoiceXML) or IVR session with
the caller, determine the address of the conference, and then re-
INVITE or REFER the caller into the conference.
Of course, if one uses a low-level device control markup rather than
an application-level markup like VoiceXML, then the number of
protocol messages to implement a voice dialog will swamp the extra
redirect message.
Interestingly, MSML and MSCML exchange the same number of messages to
do the same task.
The re-INVITE model offers total flexibility, in that the application
never has to change if the modality of the IVR step changes. For
example, the IVR step could be to a low-cost audio media resource,
which then places the caller into a high-cost, 30fps, continuous
presence video bridge.
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Application Server Media Server
| |
|INVITE sip:dialog@ms.example.net |
|;voicexml=http://as.example.net/get-id |
|----------------------------------------->|
| |
|200 OK |
|<-----------------------------------------|
| |
|ACK |
|----------------------------------------->|
| |
|GET http://as.example.net/cgi-bin/get-id |
|<-----------------------------------------|
| |
|(VoiceXML script) |
|..........................................|
| |
|POST (result) |
|<-----------------------------------------|
| |
|REFER sip:conf=12345@ms.example.net |
|----------------------------------------->|
| |
|202 ACCEPTED |
|<-----------------------------------------|
| |
|NOTIFY |
|<-----------------------------------------|
| |
|200 OK (NOTIFY) |
|----------------------------------------->|
| |
| |
The downside of the re-INVITE model is that it involves the endpoint
in the SDP renegotiation. This puts an additional burden on the
Application Server and caller device to relay and act upon the
messages.
The REFER model does not involve the calling endpoint. However, it
does have one additional protocol message.
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Application Server Media Server
| |
|INVITE sip:dialog@ms.example.net |
|;voicexml=http://as.example.net/get-id |
|----------------------------------------->|
| |
|200 OK |
|<-----------------------------------------|
| |
|ACK |
|----------------------------------------->|
| |
|GET http://as.example.net/cgi-bin/get-id |
|<-----------------------------------------|
| |
|(VoiceXML script) |
|..........................................|
| |
|POST (result) |
|<-----------------------------------------|
| |
|REFER sip:conf=12345@ms.example.net |
|----------------------------------------->|
| |
|202 ACCEPTED |
|<-----------------------------------------|
| |
|NOTIFY |
|<-----------------------------------------|
| |
|200 OK (NOTIFY) |
|----------------------------------------->|
| |
| |
4.3. MOML/MSML
MSML [18] provides:
1. As of the -04 draft, a SIP Dialog addressing scheme.
2. Arbitrarily complex mixing topologies, on a par with H.248.
3. With MOML [17], the audio prompt, record, DTMF detection, and
other functions of H.248, with the addition of access to speech
resources.
4. Switching between IVR and conferencing can be done without a re-
INVITE or REFER.
The drawbacks of MSML include:
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1. The application has to be aware of and manipulate the media
resource plumbing.
2. With most operations on a par with H.248, why not use H.248?
3. The MSML model assumes everything resides in a single server,
especially with respect to the audio/video example given above.
Application Server Media Server
| |
| |
|INVITE sip:dialog@ms.example.net |
|;moml=cid:foobratz12@ms.example.net * |
|----------------------------------------->|
| |
|200 OK |
|<-----------------------------------------|
| |
|ACK |
|----------------------------------------->|
| |
|GET http://as.example.net/cgi-bin/get-id |
|<-----------------------------------------|
| |
|(VoiceXML script) |
|..........................................|
| |
|POST (result) |
|<-----------------------------------------|
| |
|INFO (MSML <result>) |
|<-----------------------------------------|
| |
|200 OK |
|----------------------------------------->|
| |
|INFO (MSML <join>) |
|----------------------------------------->|
| |
|200 OK |
|<-----------------------------------------|
| |
| |
* The MSML specification does not state how to start a session. We
assume that one starts a MOML session and then send a <msml>
document. The URI of the VoiceXML script, and the programming logic
necessary to start that script, is embedded in the MSML document sent
to the Media Server.
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5. Recommendations
This section is in the spirit of getting a conversation started.
Everything here is opinion. Feel free to argue.
First of all, it is clear there is interest in a standard for the
Application Server - Media Server protocol in the Internet community.
The adoption of MOML/MSML in the developer community and MSCML in the
developer and vendor community is an existence proof of the utility
of, and need for, such a protocol.
The official impetus for this work is the XCON Media Server
Requirements [26]. However, in spite of the fact we have VoiceXML
for application level IVR specification and H.248 for low-level IVR
specification, people keep asking for IVR with conferencing, as
evidenced by the XCON requirements. The problem is this IVR
functionality bleeds out, and thus we need to ensure it is well
thought out before just tossing something in there.
There is a desire to leverage the SIP protocol machinery for media
session establishment, namely the SIP Offer/Answer protocol.
Application developers want to see the Media Server as a server that
offers application-level media processing. That is, modeling the
Media Server as a server that offers IVR, conference mixing, and
other, application-level media processing services.
If application developers want low-level, DSP-level media
manipulation, they already have an IETF protocol, H.248.
If application developers want a single control channel (total,
including session establishment) from the Application Server to the
Media Server, they already have an IETF protocol, H.248.
If application developers want an XML transport encoding for a low-
level protocol or a single control channel, they already have an IETF
protocol, H.248.
Assuming developers do not want H.248, what are the options?
INFO probably isn't it.
That leaves to directions to go. The first is to stick with the SIP
Dialog model of MSCML and the other is to stick with the side channel
model of MRCPv2.
The former would indicate a new method, such as MEDIA. The latter
would indicate a new establishment procedure, such as described in
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the other MSRP [25].
What does all this mean?
WHAT GOES DOWN THE PIPE IS AS IMPORTANT AS THE PIPE ITSELF.
It is easy to identify protocol abuse in the determination of the
control channel. However, even if we have a decent control channel
establishment mechanism, sending the wrong kind of messages down that
channel can render the protocol less than useful.
For example, it is great to use SIP to route messages to a media
server. However, if those messages emulate H.248, but encoded in
XML, it would be much more efficient, cleaner, and avoid the layer
violation by simply using H.248. You can even get H.248 in XML!
Just please, please, please, don't transport it in SIP or a SIP side
channel.
NOTE: This is one of the reasons I pulled out of [25] at the last
minute. What goes in to the pipe is as important as the pipe
itself.
6. Security Considerations
One issue is who is allowed to manipulate what at the Media Server.
For services like announcements, IVR, and IVVR, a straightforward
security model is to have commands come on the same SIP dialog as
what established the media connection. Clearly, if you can create
the connection, you have some kind of relationship with the end
point, if you are not the requesting end point itself.
Other relationships get more complicated. For example, if we have a
single control pipe from the Application Server, everything is OK if
there is only one Application Server. This is the model for H.248.
However, if we have more than one Application Server, then we have to
ensure a separation of the resources from one Application Server from
another.
One solution for this problem is to partition the Media Server into
multiple virtual Media Servers, each one dedicated to a given
Application Server. This is a suggested model in H.248. However, as
mentioned above in Section 2.4, this may be difficult for server-
centric Media Servers.
7. IANA Considerations
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As this is an Informative exploration, there are no IANA
Considerations.
8. Informative References
[1] Groves, C., Pantaleo, M., Anderson, T., and T. Taylor, "Gateway
Control Protocol Version 1", RFC 3525, June 2003.
[2] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[3] 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.
[4] Campbell, B. and R. Sparks, "Control of Service Context using
SIP Request-URI", RFC 3087, April 2001.
[5] Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network Media
Services with SIP", RFC 4240, December 2005.
[6] Burnett, D., Hunt, A., McGlashan, S., Porter, B., Lucas, B.,
Ferrans, J., Rehor, K., Carter, J., Danielsen, P., and S.
Tryphonas, "Voice Extensible Markup Language (VoiceXML) Version
2.0", W3C REC REC-voicexml20-20040316, March 2004.
[7] Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka, M., and E.
Zeidner, "Internet Small Computer Systems Interface (iSCSI)",
RFC 3720, April 2004.
[8] Sybase, Inc., "TDS 5.0 Functional Specification Version 3.4",
URL http://www.sybase.com/content/1013412/tds34.pdf,
August 1999.
[9] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[10] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., Beame,
C., Eisler, M., and D. Noveck, "Network File System (NFS)
version 4 Protocol", RFC 3530, April 2003.
[11] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[12] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
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[13] Telecommunication Standardization Sector of International
Telecommunication Union, "Abstract Syntax Notation One (ASN.1):
Specification of basic notation", ITU-T Recommendation X.680,
July 2002.
[14] Telecommunication Standardization Sector of International
Telecommunication Union, "ASN.1 encoding rules: XML Encoding
Rules (XER)", ITU-T Recommendation X.693, December 2001.
[15] Burnett, D. and S. Shanmugham, "Media Resource Control Protocol
Version 2 (MRCPv2)", draft-ietf-speechsc-mrcpv2-09 (work in
progress), December 2005.
[16] Dyke, J., "Media Server Control Markup Language (MSCML) and
Protocol", draft-vandyke-mscml-08 (work in progress), May 2006.
[17] Saleem, A. and G. Sharratt, "Media Objects Markup Language
(MOML)", draft-melanchuk-sipping-moml-06 (work in progress),
October 2005.
[18] Melanchuk, T. and G. Sharratt, "Media Sessions Markup Language
(MSML)", draft-melanchuk-sipping-msml-05 (work in progress),
March 2006.
[19] Rosenberg, J., "The Session Initiation Protocol (SIP) INFO
Method Considered Harmful", draft-rosenberg-sip-info-harmful-00
(work in progress), January 2003.
[20] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
[21] Mahy, R., "A Message Summary and Message Waiting Indication
Event Package for the Session Initiation Protocol (SIP)",
RFC 3842, August 2004.
[22] Burger, E., "A Session Initiation Protocol (SIP) Event Package
for Key Press Stimulus (KPML)", draft-ietf-sipping-kpml-07
(work in progress), December 2004.
[23] Niemi, A., "Session Initiation Protocol (SIP) Extension for
Event State Publication", RFC 3903, October 2004.
[24] Rosenberg, J., "A Framework for Application Interaction in the
Session Initiation Protocol (SIP)",
draft-ietf-sipping-app-interaction-framework-05 (work in
progress), July 2005.
[25] Boulton, C. and T. Melanchuk, "Media Server Request Protocol",
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draft-boulton-media-server-control-00 (work in progress),
June 2005.
[26] Even, R., "Requirements for a media server control protocol",
draft-even-media-server-req-00 (work in progress),
January 2005.
Appendix A. Contributors
I cannot share blame with anyone on this one.
Appendix B. Acknowledgements
Brooks Gelfand in 1985 made the quote, "If you cannot do it in
assembly language, you cannot do it at all," during an argument I was
having with another engineer about the relative merrits of C versus
Lisp.
The catalyst for this document was the very hard and dedicated work
of Chris Boulton, Tim Melanchuk, and I to bang out the and argue over
the other MSRP draft, starting in April of 2005 and lasting through
the very end of June.
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Author's Address
Eric Burger
Cantata Technology, Inc.
18 Keewaydin Dr.
Salem, NH 03079-2839
USA
Phone: +1 603 890 7587
Fax: +1 603 457 5944
Email: eburger@cantata.com
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