Network Working Group P. Blatherwick (Editor)
Request for Comments: 3054 Nortel Networks
Category: Informational R. Bell
Cisco Systems
P. Holland
Circa Communications
(Chair TIA TR-41.3.4)
January 2001
Megaco IP Phone Media Gateway Application Profile
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.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document specifies a particular application of the Megaco/H.248
Protocol for control of Internet telephones and similar appliances:
the Megaco IP Phone Media Gateway. The telephone itself is a Media
Gateway (MG), controlled by the Megaco/H.248 Protocol, with
application control intelligence located in the Media Gateway
Controller (MGC). To achieve a high degree of interoperability and
design efficiency in sUCh end-user devices, a consistent
architectural approach, a particular organization of Terminations and
Packages, and a Protocol Profile are described. The approach makes
use of existing Protocol features and user interface related
Packages, and is thus a straight-forward application of the
Megaco/H.248 Protocol.
1. Introduction
This document represents the current view from the TIA working group
on VoIP (Voice over IP) telephone specification [1], TIA TR-41.3.4,
with the intent of using this as part of its "whole device"
specification as an optional method of device control.
Industry feedback has made it clear that interoperability and
acoustic performance of Internet telephones is key to the rapid and
extensive commercialization of these products. To facilitate this,
the TIA has established working group TR-41.3.4 to develop a standard
for VoIP telephones. The TR-41.3.4 working group has included the
"whole device" within the scope of the standard, so a full range of
requirements including acoustic performance, protocols, methods for
powering and safety are provided. Where possible, the requirements
are based on existing standards, which are included by reference.
The TIA TR-41.3.4 working group has also recognized that its proposed
standard must enable creative application of the equipment, encourage
the development of new capabilities and allow for high levels of
product customization. To achieve this, peer to peer architectures
that are based on protocols such as H.323 or SIP and master/slave
architectures such as Megaco/H.248 Protocol are both necessary and
complementary.
In support of the Megaco/H.248 Protocol development effort, the TR-
41.3.4 working group has considered product enabling issues and
requirements, and has developed an approach to use the Megaco/H.248
Protocol for Internet telephone device control. This document
represents the working group's current view.
This document covers the general requirements of the Megaco IP Phone
application (section 3), architectural approach and MG organization
(section 4), details of specific Termination types used and Packages
supported by each (section 5), and the Megaco IP Phone Protocol
Profile (section 6).
2. Conventions
The keyWords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC2119 [5].
3. General Requirements
The following general requirements were identified to drive the
Megaco IP Phone design [1]:
1. The Megaco IP Phone must meet the basic needs of the business user
from day one;
2. Provide a path for rapid eXPansion to support sophisticated
business telephony features;
3. Flexibility to allow for a very wide range of telephones and
similar devices to be defined, from very simple to very feature
rich;
4. Simple, minimal design;
5. Allow device cost to be appropriate to capabilities provided;
6. Packages and Termination types must have characteristics that
enable reliability;
7. The IP Phone MG shall meet the appropriate Megaco/H.248 Protocol
requirements as provided in the Megaco Requirements document [2]
and be a straight-forward application of the Megaco/H.248 Protocol
[3].
4. Architecture Description
The following subsections describe the general design approach and
organization of the Megaco IP Phone MG.
4.1. Design Approach
Design intent of the Megaco IP Phone is to keep it determinedly
simple while providing required support for fully featured business
telephones and the flexibility to allow for a very wide range of
telephone configurations and similar appliances.
The approach to achieve this goal is to provide a very simple and
direct master/slave control model in which very little feature
intelligence is required in the end device. This design intent
matches the Megaco/H.248 Protocol approach well.
It is important to note that additional functionality, built-in
feature capability or system-specific optimization can easily be
provided, at the option of the implementer, by defining additional
Termination types, Event/Signal Packages, or providing built-in
application capability. This document defines the minimal design
only.
4.2. General Structure
As shown in Figure 1 below, the Megaco IP Phone is organized as a
Media Gateway (MG) that consists of a User Interface Termination and
a set of Audio Transducer Terminations.
Several - potentially thousands - of Megaco IP Phone MGs may be
controlled by a single Media Gateway Controller (MGC). This is
distinguished from the organization between traditional analog or PBX
telephones behind an IP network, where the MGC would control an MG
which in turn controls the collection of telephone devices in
question. In the case of a Megaco IP Phone MG, the MG directly
implements the media terminations like handset, handsfree and
headset, as well as the user interface. In this case, the Megaco IP
Phone itself is the MG.
+---------------+
MGC
+---------------+
^ \ \
v
+---------------------------------------------+
Megaco IP Phone MG
================== Audio Transducer
Terminations:
Audio context(s): + - - - - - - - +
+---------------------+ +-----------+
Context A Handset
+-----------+
RTP +-----+ +-----+ +-----------+
<--------+-+-> Tr Ta2 <-+----- Handsfree
audio +-----+ +-----+ +-----------+
stream +-----------+
+---------------------+ Headset
+-----------+
ETC.
+ - - - - - - - +
+----------------------------------------+
User Interface Termination
+--------------+ +--------------+
Text Display Keypad
+--------------+ +--------------+
+--------------+ +--------------+
Softkeys Indicators
+--------------+ +--------------+
+--------------+
Function Keys ETC.
+--------------+
+----------------------------------------+
+---------------------------------------------+
Figure 1: Megaco IP Phone Termination / Package Model
4.3. Termination / Package Organization
As shown in Figure 1, each Audio Transducer Termination represents an
individually controllable audio input/output element of the telephone
device, such as Handset, Handsfree, Headset, etc. By separating each
audio element as a distinct Termination, more flexible applications
can be easily implemented, such as paging, group listening, and so
on. Since this is actually only the logical view of the device,
represented by protocol, it is also quite possible to simplify
representation of the device by hiding all available audio
input/outputs behind a single Audio Transducer Termination, for
example the Handset, and implement control of multiple real
input/outputs locally inside the device.
All non-audio user interface elements are associated with the User
Interface Termination. This special Termination supports Packages to
implement all user interaction with the telephone user interface,
including Function Keys, Indicators, the Dialpad, etc, as appropriate
for the specific device capabilities (within constraints given in the
section on User Interface Termination). The User Interface
Termination cannot be placed in any Context. This grouping of user
interface elements behind a well-know Termination greatly simplifies
audits to determine actual device configuration, and reduces the
number of Terminations involved in representing user interface.
In addition, TerminationID naming conventions are provided to
identify specific Terminations within the Megaco IP Phone MG and
group them into related sets. These conventions use a set of well
known identifier names to specify the individual Terminations, for
example the User Interface Termination ("ui"), the Handset Audio
Transducer ("at/hs"), or the Handsfree Audio Transducer ("at/hf").
This specific naming is important in this application, especially for
the Audio Transducer Terminations, since the real input/output
elements to which they map on the physical device have very different
functional significance to the end-user, yet they may be represented
in the protocol using exactly the same sets of Packages. Naming
conventions allow the controlling MGC to distinguish this end-user
meaning without specific advance knowledge of physical device
configuration and without the requirement to provide different
Packages for each audio input/output type.
Using these same TerminationID naming conventions in combination with
wildcards, the MGC application can target commands to groups of
related Terminations, for example the collection of all Audio
Transducer Terminations ("at/*"). This is especially useful during
the discover phase, for example to efficiently Audit all available
Audio Transducer Terminations, and to efficiently send commands to a
set of related Terminations in a single command, for example to
simultaneously SuBTract all Audio Transducer Terminations from a
particular Context. Further information on TerminationID naming
conventions and their use can be found under the sections on Control
Interaction and Capability Discovery (next two subsections) and under
Termination Types.
4.4. Control Interaction
To provide control of audio paths, Audio Transducer Terminations are
manipulated using Contexts in the normal way, by sending Add, Move,
Subtract and Modify commands addressed to the specific Terminations
being manipulated. For example creating a Context (Context A)
containing an RTP Termination (Tr) and a Handset Audio Transducer
Termination (Ta1) creates a voice connection to/from the handset.
Moving a Handsfree Audio Transducer Termination (Ta2) into the
Context, and removing the Handset, sets up a handsfree conversation.
This situation is shown in Figure 1. See the section on Audio
Transducer Termination Types for further details on specific Package
support requirements.
User input elements, such as Keypad or Function Keys, generate Events
through Notify commands sent from the User Interface Termination of
the Megaco IP Phone MG to the controlling MGC for handling. These
Events are according to the specific set of Packages supported by the
User Interface Termination of the device. See the section on User
Interface Termination Type for further details on specific Package
support requirements.
User output elements such as the Text Display or Indicators are
controlled by Signals sent by the MGC, addressed to the User
Interface Termination of the Megaco IP Phone MG, generally as part of
a Modify command, using syntax defined in the corresponding Packages.
Since the User Interface Termination cannot be part of any context,
Add, Move and Subtract commands sent to it are not valid. See the
section on User Interface Termination Type for further details on
specific Package support requirements.
Some elements, for example Softkeys, have both user input and output
ASPects, so both react to Signals and generate Events as above.
The TerminationID naming conventions may be used to target commands
to specific Terminations by well known name, for example to Add the
Handsfree Audio Transducer Termination ("at/hf") to a Context. The
naming conventions in combination with wildcards may be used to
efficiently send commands to groups of related Terminations, for
example to simultaneously Subtract all Audio Transducer Terminations
("at/*") from a particular Context.
4.5. Capability Discovery
At startup or service change, the Megaco IP Phone MG identifies
itself to its controlling MGC as being a Megaco IP Phone class of
device by use of the IPPhone Protocol Profile. This is the first and
most important stage of capability discovery, and implicitly provides
a great deal of the necessary information in a single step.
Thereafter, the MGC can make a large number of assumptions regarding
organization and behavior of the MG. See the section on IPPhone
Protocol Profile for further details of ServiceChange operation.
Device capabilities, including the list of all Terminations and
supported Packages for each, are queried through the AuditValue
command. Wildcarded AuditValue commands targeted at the whole MG
(i.e., addressed to ContextID=Null, TerminationID=ALL) return the
list of all Terminations, including the User Interface Termination
and all supported Audio Transducer Terminations. Since the returned
TerminationIDs use well known identifier names, the MGC can derive
the specific audio input/output elements available on the physical
device, and their intended purpose. Further AuditValues commands on
individual named Terminations provide further details of each, for
example for the MGC to query user interface support Packages
available on the User Interface Termination ("ui"). TerminationID
naming conventions in combination with wildcards can be used with
AuditValues commands to query specific Package support for the
collection of all Audio Transducer Terminations ("at/*").
Since the structure of the Megaco IP Phone MG is well known in
advance, by virtue of the IPPhone Protocol Profile, audits can be
efficiently directed at discovering only what additional information
is required by the MGC. Thus the MGC is able to efficiently and
unambiguously discover both the specific user interface capabilities
and the supported audio input/outputs of the Megaco IP Phone MG,
without specific advance knowledge of physical device configuration.
It is not necessary for the MGC to attempt to infer function from
supported Packages within a random collection of Terminations, and a
great deal of behavior common to all Megaco IP Phone MGs can simply
be assumed. This pre-determined organization and behavior therefore
greatly reduces design complexity of both MG and MGC, and greatly
improves interoperability.
5. Termination Types
The Termination types defined for use in the Megaco IP Phone MG are:
* User Interface (implements user interface);
* Audio Transducer (implements audio input/output to the user, and
potentially appears as several individual Terminations
corresponding to individual audio input/outputs on the physical
device);
* RTP (transport of audio streams over IP).
These Termination types represent minimal capabilities to support
fully featured business telephones. Additional Termination types can
be defined to extend these capabilities.
The following subsections describe requirements and constraints on
each type in further detail.
5.1. User Interface Termination Type
The User Interface Termination represents the Megaco IP Phone MG user
interface elements. Megaco IP Phone MGs MUST support exactly one
User Interface Termination.
TerminationID of the User Interface Termination MUST be "ui", used
for both command addressing and command response return. ABNF text
encoding for this MUST be as described in Megaco/H.248 Protocol
Appendix B.1 [3].
Note: If ASN.1 binary encoding is used (OPTIONAL in this
specification), TerminationID for the User Interface Termination MUST
be encoded as described in Megaco/H.248 Protocol Appendix A.1 [3],
with alphabetic characters of the identifier given above mapping to
the equivalent octet string in the ASN.1 encoding.
The User Interface Termination cannot be part of any context, hence
Add, Move and Subtract commands are invalid for this Termination.
The User Interface Termination MAY support the following Packages,
defined in Megaco/H.248 Protocol H.248 Annex G: "User Interface
Elements and Actions Packages" [4].
__________________________________________________________
Package Name Support in User Interface
Termination
__________________________ _____________________________
Text Display dis OPTIONAL
Keypad kp OPTIONAL
Function Key kf OPTIONAL
Indicator ind OPTIONAL
Softkey ks OPTIONAL
Ancillary Input anci OPTIONAL
________________________________________________________
Additional Packages not listed above MAY also be provided where these
are defined to extend to additional user interface elements.
Note: The reasoning to make all Packages optional in the User
Interface Termination is to allow maximum flexibility to create a
very broad range of Internet telephones and similar devices. For
example, anything from a simple hotel lobby phone (handset and
hookswitch only), to conferencing units (handsfree unit and one or
two buttons) to fully featured business telephones (display, rich set
of keys and indicators, both handset and handsfree, etc) could be
designed.
5.2. Audio Transducer Termination Types
The Audio Transducer Terminations are used to control audio
input/output to/from the end user of the device. Megaco IP Phone MGs
MUST support at least one Audio Transducer Termination, which MAY be
chosen from the following well known types (with identifier name):
* Handset ("hs") -- input/output,
* Handsfree ("hf") -- input/output,
* Headset ("ht") -- input/output,
* Microphone ("mi") -- input only,
* Speaker ("sp") -- output only.
TerminationIDs of the Audio Transducer Terminations MUST be of the
form "at/<name>", where <name> is the 2 character identifier listed
above, used for both command addressing and command response return.
If more than one Audio Transducer Termination of a particular type is
implemented, the TerminationIDs of each MUST be of the form
"at/<name>/<num>", where <num> is a 2 digit index number in
hexadecimal format beginning at 01. Examples of valid TerminationIDs
include: "at/hs" (handset), "at/mi/02" (microphone 2), "at/*" (all
audio input/outputs). ABNF text encoding for this MUST be as
described in Megaco/H.248 Protocol Appendix B.1 [3].
Note: If ASN.1 binary encoding is used (OPTIONAL in this
specification), TerminationIDs and wildcards MUST be encoded as
described in Megaco/H.248 Protocol Appendix A.1 [3], with alphabetic
characters of the identifiers given above mapping to octet sub-
strings in the ASN.1 encoding and the '/' character not used.
Additional Audio Transducer Termination types MAY also be defined by
the implementer, however well know identifier names for these are
outside the scope of this specification.
All Audio Transducer type Terminations MUST support the following
Packages, defined in Megaco/H.248 Protocol Annex E [3].
____________________________________________________________
Package Name Support in Audio Transducer
Terminations
____________________________ _____________________________
Basic DTMF Generator dg REQUIRED
Call Progress Tones cg REQUIRED
Generator
__________________________________________________________
Additional Packages not listed above MAY also be provided where
applicable to audio input/output functions.
5.3. RTP Termination Type
Megaco IP Phone MGs MUST support at least one RTP Termination in
order to support audio streams to/from the device, as defined in
Megaco/H.248 Protocol Annex E.12 [3].
No special TerminationID naming convention is defined for RTP
Terminations as part of this specification.
6. IPPhone Protocol Profile
The following subsections provide details of the IPPhone Protocol
Profile, used between Megaco IP Phone MGs and their controlling MGCs.
This includes implicit application-level agreements between the
Megaco IP Phone MG and its controlling MGC on organization and
behavior of the MG, types of Terminations used and the specific
minimum Package support for each, and specific minimum selections on
the transport and encoding methods used.
Use of a this Profile greatly simplifies assumptions necessary by the
MGC regarding MG organization, thereby reducing complexity and cost
of both MG and MGC, and improves interoperability for the specific
Megaco IP Phone application. Since the Profile is specific to the
Megaco IP Phone MG, no other applications of Megaco/H.248 Protocol
are affected.
It is important to note that the IPPhone Profile specifies minimum
functionality only, for interoperability purposes. Additional
Termination types, Package support, transport or encoding methods, or
other capabilities MAY be added at the discretion of the implementer
within the general structure described.
6.1. Profile Descriptor and Usage
Profile name: "IPPhone"
Version: 1
The Megaco/H.248 Protocol [3] describes startup and service change
procedures in detail, including use of Profiles.
In brief, the above Profile name and version are supplied by the
Megaco IP Phone MG on startup or at service change, in the
ServiceChangeDescriptor parameter of the ServiceChange command,
issued to the controlling MGC as part of the registration procedure.
In response, the MGC may 1) accept control by acknowledging the
Service Change, 2) pass control to a different MGC by replying with a
new MGC to try, or 3) refuse control entirely by rejecting the
Service Change. If MGC control is refused, the Megaco IP Phone MG
may attempt registration with other MGCs in its list of MGCs to try.
Once a controlling MGC accepts the IPPhone Profile, both it and the
Megaco IP Phone MG become bound by the Profile rules and constraints
described in subsequent subsections as well as Megaco IP Phone
Termination/Package organization and behavior rules described in
previous sections of this document. Thereafter, any protocol use
outside these rules is considered an error.
6.2 Termination Organization and Package Support
Termination organization, required Termination types, and the
specific Packages supported by each MUST be as described in sections
4 - 5 of this document.
Note that additional Termination types and Package support MAY also
be provided within the general structure described.
6.3. Transport
Megaco IP Phone MGs MUST support Application Layer Framing (ALF) over
UDP transport, as specified in the Megaco/H.248 Protocol Appendix D.1
[3].
Note that this does not imply that the Megaco IP Phone MG cannot
support other transport methods as well. TCP transport is OPTIONAL,
but if used MUST conform to Megaco/H.248 Protocol Appendix D.2 [3].
6.4. Message Encoding
Megaco IP Phone MGs MUST support ABNF text encoding of the protocol,
as specified in the Megaco/H.248 Protocol Appendix B [3].
Note that this does not imply that the Megaco IP Phone MG cannot
support ASN.1 binary encoding as well. ASN.1 binary encoding is
OPTIONAL, but if used MUST conform to Megaco/H.248 Protocol Appendix
A [3].
7. Security Considerations
The Megaco IP Phone Media Gateway Application Profile adds no new
security issues beyond those endemic to all applications of
Megaco/H.248 Protocol [3].
8. References
[1] TIA/EIA, IS-811, Performance and Interoperability Requirements
for Voice-over-IP (VoIP) Feature Telephones,
http://www.tiaonline.org/standards/ip/voip/tia-eia-is-811-
final.pdf
[2] Greene, N., Ramalho, M. and B. Rosen, "Media Gateway Control
Protocol Architecture and Requirements", RFC2805, April 2000.
[3] Cuervo, F., Greene, N., Rayhan, A., Huitema, C., Rosen, B. and J.
Segers, "Megaco Protocol Version 1.0", RFC3015, November 2000.
[4] ITU-T SG16, H.248 Annex G: User Interface Elements and Actions
Packages, Brown, M. & P. Blatherwick, November 2000.
http://www.itu.int/itudoc/itu-t/rec/h/h248anxg.Html
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
9. Authors' Addresses
Peter Blatherwick (Editor)
Nortel Networks
P.O. Box 3511, Stn C
Ottawa, Ontario,
Canada K1Y 4H7
Phone: (613) 763-7539
(613) 724-4726
EMail: blather@nortelnetworks.com
peter.blatherwick@home.com
Bob Bell
Cisco Systems Inc.
576 S. Brentwood Ln.
Bountiful, UT 84010
USA
Phone: (801) 294-3034
EMail: rtbell@cisco.com
Phil Holland
Circa Communications Ltd.
1000 West 14th Street
North Vancouver, British Columbia,
Canada V7P 3P3
Phone: (604) 924-1742
EMail: phil.holland@circa.ca
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