Network Working Group N. Greene
Request for Comments: 2805 Nortel Networks
Category: Informational M. Ramalho
Cisco Systems
B. Rosen
Marconi
April 2000
Media Gateway Control Protocol Architecture and Requirements
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 (2000). All Rights Reserved.
Abstract
This document describes protocol requirements for the Media Gateway
Control Protocol between a Media Gateway Controller and a Media
Gateway.
Table of Contents
1. IntrodUCtion .............................................. 3
2. Terminology ............................................... 3
3. Definitions ............................................... 3
4. Specific functions assumed within the MG .................. 5
5. Per-Call Requirements ..................................... 6
5.1. Resource Reservation ................................. 6
5.2. Connection Requirements .............................. 7
5.3. Media Transformations ................................ 8
5.4. Signal/Event Processing and Scripting ................ 9
5.5. QoS/CoS .............................................. 10
5.6. Test Support ......................................... 11
5.7. Accounting ........................................... 11
5.8. Signalling Control ................................... 11
6. Resource Control .......................................... 12
6.1. Resource Status Management ........................... 12
6.2. Resource Assignment .................................. 13
7. Operational/Management Requirements ....................... 13
7.1. Assurance of Control/Connectivity .................... 13
7.2. Error Control ........................................ 14
7.3. MIB Requirements ..................................... 15
8. General Protocol Requirements ............................. 15
8.1. MG-MGC Association Requirements ...................... 16
8.2. Performance Requirements ............................. 17
9. Transport ................................................. 17
9.1. Assumptions made for underlying network .............. 17
9.2. Transport Requirements ............................... 18
10. Security Requirements .................................... 18
11. Requirements specific to particular bearer types ......... 19
11.1. Media-specific Bearer types ......................... 20
11.1.1. Requirements for TDM PSTN (Circuit) ............ 20
11.1.2. Packet Bearer type ............................. 22
11.1.3. Bearer type requirements for ATM ............... 23
11.2. Application-Specific Requirements ................... 26
11.2.1. Trunking Gateway ............................... 26
11.2.2. Access Gateway ................................. 27
11.2.3. Trunking/Access Gateway with fax ports ......... 27
11.2.4. Trunking/Access Gateway with text telephone .... 28
11.2.5. Network Access Server .......................... 29
11.2.6. Restricted Capability Gateway .................. 30
11.2.7. Multimedia Gateway ............................. 31
11.2.8. Audio Resource Function ........................ 32
11.2.9. Multipoint Control Units ........................ 42
12. References ............................................... 43
13. Acknowledgements ......................................... 43
14. Authors" Addresses ....................................... 44
15. Full Copyright Statement ................................. 45
1. Introduction
This document describes requirements to be placed on the Media
Gateway Control Protocol. When the Word protocol is used on its own
in this document it implicitly means the Media Gateway Control
Protocol.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC2119 [1] and
indicate requirement levels for the protocol.
3. Definitions
* Connection
Under the control of a Media Gateway Controller (MGC), the Media
Gateway (MG) realizes connections. In this document, connections are
associations of resources hosted by the MG. They typically involve
two terminations, but may involve more.
* Line or Loop
An analogue or digital access connection from a user terminal which
carries user media content and telephony access signalling (DP, DTMF,
BRI, proprietary business set).
* Media Gateway (MG) function
A Media Gateway (MG) function provides the media mapping and/or
transcoding functions between potentially dissimilar networks, one of
which is presumed to be a packet, frame or cell network. For
example, an MG might terminate switched circuit network (SCN)
facilities (trunks, loops), packetize the media stream, if it is not
already packetized, and deliver packetized traffic to a packet
network. It would perform these functions in the reverse order for
media streams flowing from the packet network to the SCN.
Media Gateways are not limited to SCN <-> packet/frame/cell
functions: A conference bridge with all packet interfaces could be an
MG, as well as an (IVR) interactive voice recognition unit, an audio
resource function, or a voice recognition system with a cell
interface.
* Media Gateway unit (MG-unit)
An MG-unit is a physical entity that contains an MG function and may
also contain other functions, e.g. an SG function.
* Media Gateway Controller (MGC) function
A Media Gateway Controller (MGC) function controls a MG.
* Media Resource
Examples of media resources are codecs, announcements, tones, and
modems, interactive voice response (IVR) units, bridges, etc.
* Signaling Gateway (SG) function
An SG function receives/sends SCN native signalling at the edge of a
data network. For example the SG function may relay, translate or
terminate SS7 signaling in an SS7-Internet Gateway. The SG function
may also be co-resident with the MG function to process SCN
signalling associated with line or trunk terminations controlled by
the MG, such as the "D" channel of an ISDN PRI trunk.
* Termination
A termination is a point of entry and/or exit of media flows relative
to the MG. When an MG is asked to connect two or more terminations,
it understands how the flows entering and leaving each termination
are related to each other.
Terminations are, for instance, DS0"s, ATM VCs and RTP ports. Another
word for this is bearer point.
* Trunk
An analog or digital connection from a circuit switch which carries
user media content and may carry telephony signalling (MF, R2, etc.).
Digital trunks may be transported and may appear at the Media Gateway
as channels within a framed bit stream, or as an ATM cell stream.
Trunks are typically provisioned in groups, each member of which
provides equivalent routing and service.
* Type of Bearer
A Type of Bearer definition provides the detailed requirements for
its particular application/bearer type. A particular class of Media
Gateway, for example, would support a particular set of Bearer types.
4. Specific functions assumed within the MG
This section provides an environment for the definition of the
general Media Gateway Control Protocol requirements.
MGs can be architected in many different ways depending where the
media conversions and transcoding (if required) are performed, the
level of programmability of resources, how conferences are supported,
and how associated signalling is treated. The functions assumed to be
within the MG must not be biased towards a particular architecture.
For instance, announcements in a MG could be provided by media
resources or by the bearer point resource or termination itself.
Further, this difference must not be visible to MGC: The MGC must be
able to issue the identical request to two different implementations
and achieve the identical functionality.
Depending on the application of the MG (e.g., trunking, residential),
some functions listed below will be more prominent than others, and
in some cases, functions may even disappear.
Although media adaptation is the essence of the MG, it is not
necessary for it to be involved every time. An MG may join two
terminations/resources of the same type (i.e., the MG behaves as a
switch). The required media conversion depends on the media type
supported by the resources being joined together.
In addition to media adaptation function, resources have a number of
unique properties, for instance:
* certain types of resources have associated signalling
capabilities (e.g., PRI signalling, DTMF),
* some resources perform maintenance functions (e.g., continuity
tests),
* the MGC needs to know the state changes of resources (e.g., a
trunk group going out of service),
* the MG retains some control over the allocation and control of
some resources (e.g., resource name space: RTP port numbers).
Therefore, an MG realizes point-to-point connections and conferences,
and supports several resource functions. These functions include
media conversion, resource allocation and management, and event
notifications. Handling termination associated signalling is either
done using event notifications, or is handled by the signalling
backhaul part of a MG-unit (i.e. NOT directly handled by the MG).
MGs must also support some level of system related functions, such as
establishing and maintaining some kind of MG-MGC association. This is
essential for MGC redundancy, fail-over and resource sharing.
Therefore, an MG is assumed to contain these functions:
* Reservation and release, of resources
* Ability to provide state of resources
* Maintenance of resources - It must be possible to make
maintenance operations independent of other termination
functions, for instance, some maintenance states should not
affect the resources associated with that resource . Examples of
maintenance functions are loopbacks and continuity tests.
* Connection management, including connection state.
* Media processing, using media resources: these provide services
such as transcoding, conferencing, interactive voice recognition
units, audio resource function units. Media resources may or may
not be directly part of other resources.
* Incoming digit analysis for terminations, interpretation of
scripts for terminations
* Event detection and signal insertion for per-channel signalling
* Ability to configure signalling backhauls (for example, a
Sigtran backhaul)
* Management of the association between the MGC and MG, or between
the MGC and MG resources.
5. Per-Call Requirements
5.1. Resource Reservation
The protocol must:
a. Support reservation of bearer terminations and media resources
for use by a particular call and support their subsequent
release (which may be implicit or eXPlicit).
b. Allow release in a single exchange of messages, of all resources
associated with a particular set of connectivity and/or
associations between a given number terminations.
c. The MG is not required (or allowed) by the protocol to maintain
a sense of future time: a reservation remains in effect until
explicitly released by the MGC.
5.2. Connection Requirements
The protocol must:
a. Support connections involving packet and circuit bearer
terminations in any combination, including "hairpin" connections
(connections between two circuit connections within the same
MG).
b. Support connections involving TDM, Analogue, ATM, IP or FR
transport in any combination.
c. Allow the specification of bearer plane (e.g. Frame Relay, IP,
etc.) on a call by call basis.
d. Support unidirectional, symmetric bi-directional, and asymmetric
bi-directional flows of media.
e. Support multiple media types (e.g. audio, text, video, T.120).
f. Support point-to-point and point-to-multipoint connections.
g. Support creation and modification of more complex flow
topologies e.g. conference bridge capabilities. Be able to add
or delete media streams during a call or session, and be able to
add or suBTract participants to/from a call or session.
h. Support inclusion of media resources into call or session as
required. Depending on the protocol and resource type, media
resources may be implicitly included, class-assigned, or
individually assigned.
i. Provide unambiguous specification of which media flows pass
through a point and which are blocked at a given point in time,
if the protocol permits multiple flows to pass through the same
point.
j. Allow modifications of an existing termination, for example, use
of higher compression to compensate for insufficient bandwidth
or changing transport network connections.
k. Allow the MGC to specify that a given connection has higher
priority than other connections.
l. Allow a reference to a port/termination on the MG to be a
logical identifier,
with a one-to-one mapping between a logical identifier and a
physical port.
m. Allow the MG to report events such as resource reservation and
connection completion.
5.3. Media Transformations
The Protocol must:
a. Support mediation/adaptation of flows between different types of
transport
b. Support invocation of additional processing such as echo
cancellation.
c. Support mediation of flows between different content encoding
(codecs, encryption/decryption)
d. Allow the MGC to specify whether text telephony/FAX/data modem
traffic is to be terminated at the MG, modulated/demodulated,
and converted to packets or forwarded by the MG in the media
flow as voice band traffic.
e. Allow the MGC to specify that Dual-Tone MultiFrequency (DTMF)
digits or other line and trunk signals and general Multi-
Frequency (MF) tones are to be processed in the MG and how these
digits/signals/tones are to be handled. The MGC must be able to
specify any of the following handling of such
digits/signals/tones:
1. The digits/signals/tones are to be encoded normally in the audio
RTP stream (e.g., no analysis of the digits/signals/tones).
2. Analyzed and sent to the MGC.
3. Received from the MGC and inserted in the line-side audio
stream.
4. Analyzed and sent as part of a separate RTP stream (e.g., DTMF
digits sent via a RTP payload separate from the audio RTP
stream).
5. Taken from a separate RTP stream and inserted in the line-side
audio stream.
6. Handled according to a script of instructions. For all but the
first case, an option to mute the digits/signals/tones with
silence, comfort noise, or other means (e.g., notch filtering of
some telephony tones) must be provided. As detection of these
events may take up to tens of milliseconds, the first few
milliseconds of such digit/signal/tone may be encoded and sent
in the audio RTP stream before the digit/signal/tone can be
verified. Therefore muting of such digits/signals/tones in the
audio RTP stream with silence or comfort noise is understood to
occur at the earliest opportunity after the digit/signal/tone is
verified.
f. Allow the MGC to specify signalled flow characteristics on
circuit as well as on packet bearer connections, e.g. u-law/a-
law.
g. Allow for packet/cell transport adaptation only (no media
adaptation) e.g. mid-stream (packet-to-packet)
transpacketization/transcoding, or ATM AAL5 to and from ATM AAL2
adaptation.
h. Allow the transport of audio normalization levels as a setup
parameter, e.g., for conference bridging.
i. Allow conversion to take place between media types e.g., text to
speech and speech to text.
5.4. Signal/Event Processing and Scripting
The Protocol must:
a. Allow the MGC to enable/disable monitoring for specific
supervision events at specific circuit terminations
b. Allow the MGC to enable/disable monitoring for specific events
within specified media streams
c. Allow reporting of detected events on the MG to the MGC. The
protocol should provide the means to minimize the messaging
required to report commonly-occurring event sequences.
d. Allow the MGC to specify other actions (besides reporting) that
the MG should take upon detection of specified events.
e. Allow the MGC to enable and/or mask events.
f. Provide a way for MGC to positively acknowledge event
notification.
g. Allow the MGC to specify signals (e.g., supervision, ringing) to
be applied at circuit terminations.
h. Allow the MGC to specify content of extended duration
(announcements, continuous tones) to be inserted into specified
media flows.
i. Allow the MGC to specify alternative conditions (detection of
specific events, timeouts) under which the insertion of
extended-duration signals should cease.
j. Allow the MGC to download, and specify a script to be invoked on
the occurrence of an event.
k. Specify common events and signals to maximize MG/MGC
interworking.
l. Provide an extension mechanism for implementation defined events
and signals with, for example, IANA registration procedures. It
may be useful to have an Organizational Identifier (i.e. ITU,
ETSI, ANSI, ) as part of the registration mechanism.
m. The protocol shall allow the MGC to request the arming of a
mid-call trigger even after the call has been set up.
5.5. QoS/CoS
The Protocol must:
a. Support the establishment of a bearer channel with a specified
QoS/CoS.
b. Support the ability to specify QoS for the connection between
MGs, and by direction.
c. Support a means to change QoS during a connection, as a whole
and by direction.
d. Allow the MGC to set QOS thresholds and receive notification
when such thresholds cannot be maintained.
e. Allow the jitter buffer parameters on RTP channels to be
specified at connection setup.
5.6. Test Support
The protocol must:
a. Support of the different types of PSTN Continuity Testing (COT)
for both the originating and terminating ends of the circuit
connection (2-wire and 4- wire).
b. Specifically support test line operation (e.g. 103, 105, 108).
5.7. Accounting
The protocol must:
a. Support a common identifier to mark resources related to one
connection.
b. Support collection of specified accounting information from MGs.
c. Provide the mechanism for the MGC to specify that the MG report
accounting information automatically at end of call, in mid-call
upon request, at specific time intervals as specified by the MGC
and at unit usage thresholds as specified by the MGC.
d. Specifically support collection of:
* start and stop time, by media flow,
* volume of content carried (e.g. number of packets/cells
transmitted, number received with and without error, inter-
arrival jitter), by media flow,
* QOS statistics, by media flow.
e. Allow the MGC to have some control over which statistics are
reported, to enable it to manage the amount of information
transferred.
5.8. Signalling Control
Establishment and provisioning of signalling backhaul channels (via
SIGTRAN for example) is out of scope. However, the MG must be
capable of supporting detection of events, and application of signals
associated with basic analogue line, and CAS type signalling. The
protocol must:
a. Support the signalling requirements of analogue lines and
Channel Associated Signaling (CAS).
b. Support national variations of such signalling.
c. Provide mechanisms to support signalling without requiring MG-
MGC timing constraints beyond that specified in this document.
d. Must not create a situation where the MGC and the MG must be
homologated together as a mandatory requirement of using the
protocol;
i.e. it must be possible to optionally conceal signaling type
variation from the MGC.
6. Resource Control
6.1. Resource Status Management
The protocol must:
a. Allow the MG to report changes in status of physical entities
supporting bearer terminations, media resources, and facility-
associated signalling channels, due to failures, recovery, or
administrative action. It must be able to report whether a
termination is in service or out of service.
b. Support administrative blocking and release of TDM circuit
terminations.
Note: as the above point only relates to ISUP-controlled circuits, it
may be unnecessary to require this since the MGC controls their use.
However, it may be meaningful for MF and R2-signalled trunks, where
supervisory states are set to make the trunks unavailable at the far
end.
c. Provide a method for the MGC to request that the MG release all
resources under the control of a particular MGC currently in
use, or reserved, for any or all connections.
d. Provide an MG Resource Discovery mechanism which must allow an
MGC to discover what resources the MG has. Expressing resources
can be an arbitrarily difficult problem and the initial release
of the protocol may have a simplistic view of resource
discovery.
At a minimum, resource discovery must enumerate the names of
available circuit terminations and the allowed values for
parameters supported by terminations.
The protocol should be defined so that simple gateways could
respond with a relatively short, pre-stored response to the
discovery request mechanism. In general, if the protocol defines
a mechanism that allows the MGC to specify a setting or
parameter for a resource or connection in the MG, and MGs are
not required to support all possible values for that setting or
parameter, then the discovery mechanism should provide the MGC
with a method to determine what possible values such settings or
parameters are supported in a particular MG.
e. Provide a mechanism to discover the current available resources
in the MG, where resources are dynamically consumed by
connections and the MGC cannot reasonably or reliably track the
consumption of such resources. It should also be possible to
discover resources currently in use, in order to reconcile
inconsistencies between the MGC and the MG.
f. Not require an MGC to implement an SNMP manager function in
order to discover capabilities of an MG that may be specified
during context establishment.
6.2. Resource Assignment
The protocol must:
a. Provide a way for the MG to indicate that it was unable to
perform a requested action because of resource exhaustion, or
because of temporary resource unavailability.
b. Provide an ability for the MGC to indicate to an MG the resource
to use for a call (e.g. DS0) exactly, or indicate a set of
resources (e.g. pick a DS0 on a T1 line or a list of codec
types) via a "wild card" mechanism from which the MG can select
a specific resource for a call (e.g. the 16th timeslot, or
G.723).
c. Allow the use of DNS names and IP addresses to identify MGs and
MGCs. This shall not preclude using other identifiers for MGs or
MGCs when other non IP transport technologies for the protocol
are used.
7. Operational/Management Requirements
7.1. Assurance of Control/Connectivity
To provide assurance of control and connectivity, the protocol must
provide the means to minimize duration of loss of control due to loss
of contact, or state mismatches.
The protocol must:
a. Support detection and recovery from loss of contact due to
failure/congestion of communication links or due to MG or MGC
failure.
Note that failover arrangements are one of the mechanisms which
could be used to meet this requirement.
b. Support detection and recovery from loss of synchronized view of
resource and connection states between MGCs and MGs. (e.g.
through the use of audits).
c. Provide a means for MGC and MG to provide each other with
booting and reboot indications, and what the MG"s configuration
is.
d. Permit more than one backup MGC and provide an orderly way for
the MG to contact one of its backups.
e. Provide for an orderly switchback to the primary MGC after it
recovers. How MGCs coordinate resources between themselves is
outside the scope of the protocol.
f. Provide a mechanism so that when an MGC fails, connections
already established can be maintained. The protocol does not
have to provide a capability to maintain connections in the
process of being connected, but not actually connected when the
failure occurs.
g. The Protocol must allow the recovery or redistribution of
traffic without call loss.
7.2. Error Control
The protocol must:
a. Allow for the MG to report reasons for abnormal failure of lower
layer connections e.g. TDM circuit failure, ATM VCC failure.
b. Allow for the MG to report Usage Parameter Control (UPC) events.
c. Provide means to ameliorate potential synchronization or focused
overload of supervisory/signaling events that can be detrimental
to either MG or MGC operation. Power restoration or signaling
transport re-establishment are typical sources of potentially
detrimental signaling showers from MG to MGC or vice-versa.
d. Allow the MG to notify the MGC that a termination was terminated
and communicate a reason when a terminations is taken out-of-
service unilaterally by the MG due to abnormal events.
e. Allow the MGC to acknowledge that a termination has been taken
out-of-service.
f. Allow the MG to request the MGC to release a termination and
communicate a reason.
g. Allow the MGC to specify, as a result of such a request its
decision to take termination down, leave it as is or modify it.
7.3. MIB Requirements
The Protocol must define a common MG MIB, which must be extensible,
but must:
a. Provide information on:
* mapping between resources and supporting physical entities.
* statistics on quality of service on the control and signalling
backhaul interfaces.
* statistics required for traffic engineering within the MG.
b. The protocol must allow the MG to provide to the MGC all
information the MGC needs to provide in its MIB.
c. MG MIB must support implementation of H.341 by either the MG,
MGC, or both acting together.
8. General Protocol Requirements
The protocol must:
a. Support multiple operations to be invoked in one message and
treated as a single transaction.
b. Be both modular and extensible. Not all implementations may wish
to support all of the possible extensions for the protocol. This
will permit lightweight implementations for specialized tasks
where processing resources are constrained. This could be
accomplished by defining particular profiles for particular uses
of the protocol.
c. Be flexible in allocation of intelligence between MG and MGC.
For example, an MGC may want to allow the MG to assign
particular MG resources in some implementations, while in
others, the MGC may want to be the one to assign MG resources
for use.
d. Support scalability from very small to very large MGs: The
protocol must support MGs with capacities ranging from one to
millions of terminations.
e. Support scalability from very small to very large MGC span of
control: The protocol should support MGCs that control from one
MG to a few tens of thousands of MGs.
f. Support the needs of a residential gateway that supports one to
a few lines, and the needs of a large PSTN gateway supporting
tens of thousands of lines. Protocol mechanisms favoring one
extreme or the other should be minimized in favor of more
general purpose mechanism applicable to a wide range of MGs.
Where special purpose mechanisms are proposed to optimize a
subset of implementations, such mechanisms should be defined as
optional, and should have minimal impact on the rest of the
protocol.
g. Facilitate MG and MGC version upgrades independently of one
another. The protocol must include a version identifier in the
initial message exchange.
h. Facilitate the discovery of the protocol capabilities of the one
entity to the other.
i. Specify commands as optional (they can be ignored) or mandatory
(the command must be rejected), and within a command, to specify
parameters as optional (they can be ignored) or mandatory (the
command must be rejected).
8.1. MG-MGC Association Requirements
The Protocol must:
a. Support the establishment of a control relationship between an
MGC and an MG.
b. Allow multiple MGCs to send control messages to an MG. Thus, the
protocol must allow control messages from multiple signalling
addresses to a single MG.
c. Provide a method for the MG to tell an MGC that the MG received
a command for a resource that is under the control of a
different MGC.
d. Support a method for the MG to control the rate of requests it
receives from the MGC (e.g. windowing techniques, exponential
back-off).
e. Support a method for the MG to tell an MGC that it cannot handle
any more requests.
8.2. Performance Requirements
The protocol must:
a. Minimize message exchanges between MG and MGC, for example
during boot/reboot, and during continuity tests.
b. Support Continuity test constraints which are a maximum of 200ms
cross-MGC IAM (IAM is the name given to an SS7 connection setup
msg) propagation delay, and a maximum of 200ms from end of
dialing to IAM emission.
c. Make efficient use of the underlying transport mechanism. For
example, protocol PDU sizes vs. transport MTU sizes needs to be
considered in designing the protocol.
d. Not contain inherent architectural or signaling constraints that
would prohibit peak calling rates on the order of 140
calls/second on a moderately loaded network.
e. Allow for default/provisioned settings so that commands need
only contain non-default parameters.
9. Transport
9.1. Assumptions made for underlying network
The protocol must assume that the underlying network:
a. May be over large shared networks: proximity assumptions are not
allowed.
b. Does not assure reliable delivery of messages.
c. Does not guarantee ordering of messages: Sequenced delivery of
messages associated with the same source of events is not
assumed.
d. Does not prevent duplicate transmissions.
9.2. Transport Requirements
The protocol must:
a. Provide the ability to abort delivery of obsolete messages at
the sending end if their transmission has not been successfully
completed. For example, aborting a command that has been
overtaken by events.
b. Support priority messages: The protocol shall allow a command
precedence to allow priority messages to supercede non-priority
messages.
c. Support of large fan-out at the MGC.
d. Provide a way for one entity to correlate commands and responses
with the other entity.
e. Provide a reason for any command failure.
f. Provide that loss of a packet not stall messages not related to
the message(s) contained in the packet lost.
Note that there may be enough protocol reliability requirements here
to warrant a separate reliable transport layer be written apart from
the Media Gateway Control Protocol. Also need to compare Megaco
reliable transport requirements with similar Sigtran requirements.
10. Security Requirements
Security mechanisms may be specified as provided in underlying
transport mechanisms, such as IPSEC. The protocol, or such
mechanisms, must:
a. Allow for mutual authentication at the start of an MGC-MG
association
b. Allow for preservation of the of control messages once the
association has been established.
c. Allow for optional confidentiality protection of control
messages. The mechanism should allow a choice in the algorithm
to be used.
d. Operate across untrusted domains in a secure fashion.
e. Support non-repudiation for a customer-located MG talking to a
network operator"s MGC.
f. Define mechanisms to mitigate denial of service attacks
Note: the protocol document will need to include an extended
discussion of security requirements, offering more precision on each
threat and giving a complete picture of the defense including non-
protocol measures such as configuration.
g. It would be desirable for the protocol to be able to pass
through commonly-used firewalls.
11. Requirements specific to particular bearer types
The bearer types listed in Table 1 can be packaged into different
types of MGs. Examples are listed in the following sections. How
they are packaged is outside the scope of the general Media Gateway
control protocol. The protocol must support all types of bearer types
listed in Table 1.
Table 1: Bearer Types and Applications
Bearer Type Applications Transit Network
================================================================
Trunk+ISUP trunking/access IP, ATM, FR
Voice,Fax,NAS,
Multimedia
Trunk+MF trunking/access IP, ATM, FR
Voice,Fax,NAS,
Multimedia
ISDN trunking/access IP, ATM, FR
Voice,Fax,NAS,
Multimedia
Analogue Voice,Fax, IP, ATM, FR
Text Telephony
Termination in a Restricted Voice,Fax, IP, ATM, FR
Capability Gateway Text Telephony
Application Termination IVR,ARF, Announcement Server,
Voice Recognition Server,...
Multimedia H.323 H.323 Multimedia IP, ATM, FR
Gateway and MCU
Multimedia H.320 H.323 GW and MCU ISDN, IP, ATM, FR
11.1. Media-specific Bearer Types
This section describes requirements for handling terminations
attached to specific types of networks.
11.1.1. Requirements for TDM PSTN (Circuit)
This bearer type is applicable to a Trunking GW, Access GW, ...
The protocol must allow:
a. the MGC to specify the encoding to use on the attached circuit.
b. In general, if something is set by a global signalling protocol
(e.g. ISUP allows mu-Law or A-Law to be signaled using ISUP)
then it must be settable by the protocol.
c. TDM attributes:
* Echo cancellation,
* PCM encoding or other voice compression (e.g. mu-law or A-law),
* encryption,
* rate adaptation (e.g. V.110, or V.120).
d. for incoming calls, identification of a specific TDM circuit
(timeslot and facility).
e. for calls outgoing to the circuit network, identification of a
specific circuit or identification of a circuit group with the
indication that the MG must select and return the identification
of an available member of that group.
f. specification of the default encoding of content passing to and
from a given circuit, possibly on a logical or physical circuit
group basis.
g. specification at any point during the life of a connection of
variable ASPects of the content encoding, particularly including
channel information capacity.
h. specification at any point during the life of a connection of
loss padding to be applied to incoming and outgoing media
streams at the circuit termination.
i. specification at any point during the life of a connection of
the applicability of echo cancellation to the outgoing media
stream.
j. Multi-rate calls to/from the SCN.
k. H-channel (n x 64K) calls to/from the SCN.
l. B channel aggregation protocols for creating high speed channels
for multimedia over the SCN.
m. Modem terminations and negotiations.
The protocol may also allow:
n. specification of sub-channel media streams,
o. specification of multi-channel media streams.
11.1.2. Packet Bearer Type
The protocol must be able to specify:
a. ingress and egress coding (i.e. the way packets coming in and
out are encoded) (including encryption).
b. near and far-end ports and other session parameters for RTP and
RTCP.
The protocol must support reporting of:
c. re-negotiation of codec for cause - for further study
d. on Trunking and Access Gateways, resources capable of more than
one active connection at a time must also be capable of mixing
and packet duplication.
The protocol must allow:
e. specification of parameters for outgoing and incoming packet
flows at separate points in the life of the connection (because
far-end port addresses are typically obtained through a separate
signalling exchange before or after the near-end port addresses
are assigned).
f. the possibility for each Media Gateway to allocate the ports on
which it will receive packet flows (including RTCP as well as
media streams) and report its allocations to the Media Gateway
Controller for signalling to the far end. Note that support of
different IP backbone providers on a per call basis would
require that the ports on which packets flow be selected by the
MGC. (but only if the IP address of the MG is different for each
backbone provider).
g. the specification at any point during the life of a connection
of RTP payload type and RTP session number for each RTP-
encapsulated media flow.
h. the ability to specify whether outgoing flows are to be uni-cast
or multi-cast. Note that on an IP network this information is
implicit in the destination address, but in other networks this
is a connection parameter.
i. invoking of encryption/decryption on media flows and
specification of the associated algorithm and key.
The protocol should also allow:
j. the MGC to configure non-RTP (proprietary or other) encapsulated
packet flows.
11.1.3. Bearer type requirements for ATM
This bearer type is applicable to Trunking GW, Access GW, ....
11.1.3.1. Addressing
a. The protocol must be able to specify the following termination
attributes:
* VC identifier,
* VC identifier plus AAL2 slot, and variant of these allowing the
gateway to choose (part of) the identifier,
* remote termination network address, remote MG name.
b. Allow specification of an ATM termination which is to be
assigned to an MG connection as a VC identifier, a VC identifier
plus AAL2 slot, a wild-carded variant of either of these. A
remote termination network address, or a remote MG name could
also be used when the MG can select the VC and change the VC
during the life of the connection by using ATM signalling.
c. Provide an indication by the MG of the VC identifier and
possibly AAL2 slot of the termination actually assigned to a
connection.
d. Provide a means to refer subsequently to that termination.
e. Refer to an existing VCC as the physical interface + Virtual
Path Identifier (VPI) + Virtual Circuit Identifier (VCI).
f. Where the VCC is locally established (SVCs signalled by the
Gateway through UNI or PNNI signalling or similar), the VCC must
be indirectly referred to in terms which are of significance to
both ends of the VCC. For example, a global name or the ATM
address of the ATM devices at each end of the VCC. However, it
is possible/probable that there may be several VCCs between a
given pair of ATM devices. Therefore the ATM address pair must
be further resolved by a VCC identifier unambiguous within the
context of the ATM address pair.
g. refer to a VCC as the Remote GW ATM End System Address + VCCI.
h. allow the VCCI to be selected by the MG or imposed on the MG.
i. support all ATM addressing variants (e.g. ATM End System Address
(AESA) and E.164).
11.1.3.2. Connection related requirements
The protocol must:
a. Allow for the de-coupling of creation/deletion of the narrow-
band connection from the creation/deletion of the underlying
VCC.
b. Allow for efficient disconnection of all connections associated
with a physical port or VCC. As an example, this could aggregate
disconnections across a broadband circuit which experienced a
physical error.
c. Allow the connection established using this protocol to be
carried over a VCC, which may be a:
* PVC or SPVC,
* an SVC established on demand, either by the MGC itself or by a
broker acting on its behalf or,
* an SVC originated as required by the local MG, or by the remote
end to the local MG through UNI or PNNI signalling.
d. Allow ATM transport parameters and QoS parameters to be passed
to the MG.
e. Allow blocking and unblocking of a physical interface, a VCC or
an AAL1/AAL2 channel.
The protocol should:
f. Where a VCC is required to be established on a per narrow-band
call basis, allow all necessary information to be passed in one
message.
11.1.3.3. Media adaptation
The protocol must:
a. Allow AAL parameters to be passed to the MG.
b. Allow AAL1/AAL2 multiple narrow-band calls to be mapped to a
single VCC. For AAL2, these calls are differentiated within each
VCC by a AAL2 channel identifier. An AAL2 connection may span
more than 1 VCC and transit AAL2 switching devices. ITU
Q.2630.1 [2] defines an end-to-end identifier called the Served
User Generated Reference (SUGR). It carries information from the
originating user of the AAL2 signalling protocol to the
terminating user transparently and unmodified.
c. Allow unambiguous binding of a narrow band call to an AAL2
connection identifier, or AAL1 channel, within the specified
VCC.
d. Allow the AAL2 connection identifier, or AAL1 channel, to be
selected by the MG or imposed on the MG.
e. Allow the use of the AAL2 channel identifier (cid) instead of
the AAL2 connection identifier.
f. Allow the AAL2 voice profile to be imposed or negotiated before
the start of the connection. AAL2 allows for variable length
packets and varying packet rates, with multiple codecs possible
within a given profile. Thus a given call may upgrade or
downgrade the codec within the lifetime of the call. Idle
channels may generate zero bandwidth. Thus an AAL2 VCC may vary
in bandwidth and possibly exceed its contract. Congestion
controls within a gateway may react to congestion by modifying
codec rates/types.
g. Allow the MGC to instruct the MG on how individual narrow-band
calls behave under congestion.
h. Allow for the MGC to specify an AAL5 bearer, with the following
choices:
* Per ATM Forum standard AF-VTOA-0083 [4],
* RTP with IP/UDP,
* RTP without IP/IDP per H.323v2 Annex C [5],
* Compressed RTP per ATM Forum AF-SAA-0124.000 [6].
i. Allow unambiguous binding of a narrow band call to an AAL1
channel within the specified VCC. (In AAL1, multiple narrow-band
calls may be mapped to a single VCC.)
11.1.3.4. Reporting requirements
The protocol should:
a. Allow any end-of-call statistics to show loss/restoration of
underlying VCC within the calls duration, together with duration
of loss.
b. Allow notification, as requested by MGC, of any congestion
avoidance actions taken by the MG.
The protocol must:
c. Allow for ATM VCCs or AAL2 channels to be audited by the MGC.
d. Allow changes in status of ATM VCCs or AAL2 channels to be
notified as requested by the MGC.
e. Allow the MGC to query the resource and endpoint availability.
Resources may include VCCs, and DSPs. VCCs may be up or down.
End-points may be connection-free, connected or unavailable.
11.1.3.5. Functional requirements
The protocol must:
a. Allow an MGC to reserve a bearer, and specify a route for it
through the network.
11.2. Application-Specific Requirements
11.2.1. Trunking Gateway
A Trunking Gateway is an interface between SCN networks and Voice
over IP or Voice over ATM networks. Such gateways typically
interface to SS7 or other NNI signalling on the SCN and manage a
large number of digital circuits.
The protocol must:
a. Provide circuit and packet-side loopback.
b. Provide circuit-side n x 64kbs connections.
c. Provide subrate and multirate connections for further study.
d. Provide the capability to support Reporting/generation of
per-trunk CAS signalling (DP, DTMF, MF, R2, J2, and national
variants).
e. Provide the capability to support reporting of detected DTMF
events either digit-by-digit, as a sequence of detected digits
with a flexible mechanism For the MG to determine the likely end
of dial string, or in a separate RTP stream.
f. Provide the capability to support ANI and DNIS generation and
reception.
11.2.2. Access Gateway
An Access Gateway connects UNI interfaces like ISDN (PRI and BRI) or
traditional analog voice terminal interfaces, to a Voice over IP or
Voice over ATM network, or Voice over Frame Relay network.
The Protocol must:
a. Support detection and generation of analog line signaling
(hook-state, ring generation).
b. Provide the capability to support reporting of detected DTMF
events either digit-by-digit, as a sequence of detected digits
with a flexible mechanism For the MG to determine the likely end
of dial string, or in a separate RTP stream.
c. Not require scripting mechanisms, event buffering, digit map
storage when implementing restricted function (1-2 line)
gateways with very limited capabilities.
d. Provide the capability to support CallerID generation and
reception.
Proxying of the protocol is for further study.
11.2.3. Trunking/Access Gateway with fax ports
a. the protocol must be able to indicate detection of fax media.
b. the protocol must be able to specify T.38 for the transport of
the fax.
c. the protocol must be able to specify G.711 encoding for
transport of fax tones across a packet network.
11.2.4. Trunking/Access Gateway with text telephone access ports
An access gateway with ports capable of text telephone communication,
must provide communication between text telephones in the SCN and
text conversation channels in the packet network.
Text telephone capability of ports is assumed to be possible to
combine with other options for calls as described in section 11.2.6
(e.) on "Adaptable NASes".
The port is assumed to adjust for the differences in the supported
text telephone protocols, so that the text media stream can be
communicated T.140 coded in the packet network without further
transcoding [7].
The protocol must be capable of reporting the type of text telephone
that is connected to the SCN port. The foreseen types are the same as
the ones supported by ITU-T V.18: DTMF, EDT, Baudot-45, Baudot-50,
Bell, V.21, Minitel and V.18. It should be possible to control which
protocols are supported. The SCN port is assumed to contain ITU-T
V.18 functionality [8].
The protocol must be able to control the following functionality
levels of text telephone support:
a. Simple text-only support: The call is set into text mode from
the beginning of the call, in order to conduct a text-only
conversation.
b. Alternating text-voice support: The call may begin in voice mode
or text mode and, at any moment during the call, change mode on
request by the SCN user. On the packet side, the two media
streams for voice and text must be opened, and it must be
possible to control the feeding of each stream by the protocol.
c. Simultaneous text and voice support: The call is performed in a
mode when simultaneous text and voice streams are supported. The
call may start in voice mode and during the call change state to
a text-and-voice call.
A port may implement only level a, or any level combination of a, b
and c, always including level a.
The protocol must support:
d. A text based alternative to the interactive voice response, or
audio resource functionality of the gateway when the port is
used in text telephone mode.
e. Selection of what national translation table to be used between
the Unicode based T.140 and the 5-7 bit based text telephone
protocols.
f. Control of the V.18 probe message to be used on incoming calls.
11.2.5. Network Access Server
A NAS is an access gateway, or Media Gateway (MG), which terminates
modem signals or synchronous HDLC connections from a network (e.g.
SCN or xDSL network) and provides data access to the packet network.
Only those requirements specific to a NAS are described here.
Figure 1 provides a reference architecture for a Network Access
Server (NAS). Signaling comes into the MGC and the MGC controls the
NAS.
+-------+ +-------+
Signaling
-----------+ MGC + AAA

+---+---+ +--+----+

Megaco_______________


+---+---+ ~~~~~
Bearer ( )
-----------+ NAS +-------( IP )
( )
+-------+ ~~~~~~
Figure 1: NAS reference architecture
The Protocol must support:
a. Callback capabilities:
* Callback
b. Modem calls. The protocol must be able to specify the modem
type(s) to be used for the call.
c. Carriage of bearer information. The protocol must be able to
specify the data rate of the TDM connection (e.g., 64 kbit/s, 56
kbit/s, 384 kbit/s), if this is available from the SCN.
d. Rate Adaptation: The protocol must be able to specify the type
of rate adaptation to be used for the call including indicating
the subrate, if this is available from the SCN (e.g. 56K, or
V.110 signaled in Bearer capabilities with subrate connection of
19.2kbit/s).
e. Adaptable NASes: The protocol must be able to support multiple
options for an incoming call to allow the NAS to dynamically
select the proper type of call. For example, an incoming ISDN
call coded for "Speech" Bearer Capability could actually be a
voice, modem, fax, text telephone, or 56 kbit/s synchronous
call. The protocol should allow the NAS to report back to the
MGC the actual type of call once it is detected.
The 4 basic types of bearer for a NAS are:
1. Circuit Mode, 64-kbps, 8-khz structured, Speech
2. Circuit Mode, 64-kbps, 8-khz structured, 3.1-khz, Audio
3. Circuit Mode, 64-kbps, 8-khz structured, Unrestricted Digital
Transmission-Rate Adapted from 56-kbps
4. Circuit Mode, 64-kbps, 8-khz structure, Unrestricted Digital
Transmission
f. Passage of Called and Calling Party Number information to the
NAS from the MGC. Also, passage of Charge Number/Billing Number,
Redirecting Number, and Original Call Number, if known, to the
NAS from the MGC. If there are other Q.931 fields that need to
be passed from the MGC to the MG, then it should be possible to
pass them [9].
g. Ability for the MGC to direct the NAS to connect to a specific
tunnel, for example to an LNS, or to an AAA server.
h. When asked by the MGC, be able to report capability information,
for example, connection types (V.34/V90/Synch ISDN..), AAA
mechanism (RADIUS/DIAMETER/..), access type (PPP/SLIP/..) after
restart or upgrade.
11.2.6. Restricted Capability Gateway
The requirements here may also be applied to small analog gateways,
and to cable/xDSL modems. See also the section on access gateways.
The Protocol must support:
a. The ability to provide a scaled down version of the protocol.
When features of the protocol are not supported, an appropriate
error message must be sent. Appropriate default action must be
defined. Where this is defined may be outside the scope of the
protocol.
b. The ability to provide device capability information to the MGC
with respect to the use of the protocol.
11.2.7. Multimedia Gateway
The protocol must have sufficient capability to support a multimedia
gateway. H.320 and H.324 are characterized by a single data stream
with multiple media streams multiplexed on it.
If the mapping is from H.320 or H.324 on the circuit side, and H.323
on the packet side, it is assumed that the MG knows how to map
respective subchannels from H.320/H.324 side to streams on packet
side. If extra information is required when connecting two
terminations, then it must be supplied so that the connections are
not ambiguous.
The Multimedia Gateway:
1) should support Bonding Bearer channel aggregation,
2) must support 2xB (and possibly higher rates) aggregation via
H.221,
3) must be able to dynamically change the size of audio, video and
data channels within the h.320 multiplex,
4) must react to changes in the H.320 multiplex on 20 msec
boundaries,
5) must support TCS4/IIS BAS commands,
6) must support detection and creation of DTMF tones,
7) should support SNMP MIBS as specified in H.341 [3]
a. If some of the above cannot be handled by the MGC to MG protocol
due to timing constraints, then it is likely that the H.245 to
H.242 processing must take place in the MG. Otherwise, support
for this functionality in the multimedia gateway are protocol
requirements.
b. It must be possible on a call by call basis for the protocol to
specify different applications. Thus, one call might be PSTN to
PSTN under SS7 control, while the next might be ISDN/H.320 under
SS7 control to H.323. This is only one example; the key
requirement is that the protocol not prevent such applications.
11.2.8. Audio Resource Function
An Audio Resource Function (ARF) consists of one or more functional
modules which can be deployed on an stand alone media gateway server
IVR, Intelligent Peripheral, speech/speaker recognition unit, etc. or
a traditional media gateway. Such a media gateway is known as an
Audio Enabled Gateway (AEG) if it performs tasks defined in one or
more of the following ARF functional modules:
Play Audio,
DTMF Collect,
Record Audio,
Speech Recognition,
Speaker Verification/Identification,
Auditory Feature Extraction/Recognition, or
Audio Conferencing.
Additional ARF function modules that support human to machine
communications through the use of telephony tones (e.g., DTMF) or
auditory means (e.g. speech) may be appended to the AEG definition
in future versions of these requirements.
Generic scripting packages for any module must support all the
requirements for that module. Any package extension for a given
module must include, by inheritance or explicit reference, the
requirements for that given module.
The protocol requirements for each of the ARF modules are provided in
the following subsections.
11.2.8.1. Play Audio Module
a. Be able to provide the following basic operation:
- request an ARF MG to play an announcement.
b. Be able to specify these play characteristics:
- Play volume
- Play speed
- Play iterations
- Interval between play iterations
- Play duration
c. Permit the specification of voice variables such as DN, number,
date, time, etc. The protocol must allow specification of both
the value (eg 234-3456), and well as the type (Directory
number).
d. Using the terminology that a segment is a unit of playable
speech, or is an abstraction that is resolvable to a unit of
playable speech, permit specification of the following segment
types:
- A provisioned recording.
- A block of text to be converted to speech.
- A block of text to be displayed on a device.
- A length of silence qualified by duration.
- An algorithmically generated tone.
- A voice variable, specified by type and value. Given a variable
type and value, the IVR/ARF unit would dynamically assemble the
phrases required for its playback.
- An abstraction that represents a sequence of audio segments.
Nesting of these abstractions must also be permitted.
An example of this abstraction is a sequence of audio segments, the
first of which is a recording of the words "The number you have
dialed", followed by a Directory Number variable, followed by a
recording of the words "is no longer in service".
- An abstraction that represents a set of audio segments and which
is resolved to a single segment by a qualifier. Nesting of
these abstractions must be permitted.
For example take a set of audio segments recorded in different
languages all of which express the semantic concept "The number you
have dialed is no longer in service". The set is resolved by a
language qualifier. If the qualifier is "French", the set resolves to
the French version of this announcement.
In the case of a nested abstraction consisting of a set qualified by
language at one level and and a set qualified by gender at another
level, it would be possible to specify that an announcement be
played in French and spoken by a female voice.
e. Provide two different methods of audio specification:
- Direct specification of the audio components to be played by
specifying the sequence of segments in the command itself.
- Indirect specification of the audio components to be played by
reference to a single identifier that resolves to a provisioned
sequence of audio segments.
11.2.8.2. DTMF Collect Module
The DTMF Collect Module must support all of the requirements in the
Play Module in addition to the following requirements:
a. Be able to provide the following basic operation:
- request an AEG to play an announcement, which may optionally
terminated by DTMF, and then collect DTMF
b. Be able to specify these event collection characteristics:
- The number of attempts to give the user to enter a valid DTMF
pattern.
c. With respect to digit timers, allow the specification of:
- Time allowed to enter the first digit.
- Time allowed for user to enter each digit subsequent to the
first digit.
- Time allowed for user to enter a digit once the maximum expected
number of digits has been entered.
d. To be able to allow multiple prompt operations DTMF digit
collection, voice recording (if supported), and/or speech
recognition analysis (if supported) provide the following types
of prompts:
- Initial Prompt
- Reprompt
- Error prompt
- Failure announcement
- Success announcement.
e. To allow digit pattern matching, allow the specification of:
- maximum number of digits to collect.
- minimum number of digits to collect.
- a digit pattern using a regular expression.
f. To allow digit buffer control, allow the specification of:
- Ability to clear digit buffer prior to playing initial prompt
(default is not to clear buffer).
- Default clearing of buffer following playing of un-interruptible
announcement segment.
- Default clearing of buffer before playing a re-prompt in
response to previous invalid input.
g. Provide a method to specify DTMF interruptibility on a per audio
segment basis.
h. Allow the specification of definable key sequences for DTMF
digit collection to:
- Discard collected digits in progress, replay the prompt, and
resume DTMF digit collection.
- Discard collected digits in progress and resume DTMF digit
collection.
- Terminate the current operation and return the terminating key
sequence to the MGC.
i. Provide a way to ask the ARF MG to support the following
definable keys for digit collection and recording. These keys
would then be able to be acted upon by the ARF MG:
- A key to terminate playing of an announcement in progress.
- A set of one or more keys that can be accepted as the first
digit to be collected.
- A key that signals the end of user input. The key may or may
not be returned to the MGC along with the input already
collected.
- Keys to stop playing the current announcement and resume playing
at the beginning of the first segment of the announcement, last
segment of the announcement, previous segment of the
announcement, next segment of the announcement, or the current
announcement segment.
11.2.8.3. Record Audio Module
The Record Module must support all of the requirements in the Play
Module as in addition to the following requirements:
a. Be able to provide the following basic operation:
- request an AEG to play an announcement and then record voice.
b. Be able to specify these event collection characteristics:
- The number of attempts to give the user to make a recording.
c. With respect to recording timers, allow the specification of:
- Time to wait for the user to initially speak.
- The amount of silence necessary following the last speech
segment for the recording to be considered complete.
- The maximum allowable length of the recording (not including
pre- and post- speech silence).
d. To be able to allow multiple prompt operations for DTMF digit
collection (if supported), voice recording (if supported),
speech recognition analysis (if supported) and/or speech
verification/identification (if supported) and then to provide
the following types of prompts:
- Initial Prompt
- Reprompt
- Error prompt
- Failure announcement
- Success announcement.
e. Allow the specification of definable key sequences for digit
recording or speech recognition analysis (if supported) to:
- Discard recording in progress, replay the prompt, and resume
recording.
- Discard recording in progress and resume recording.
- Terminate the current operation and return the terminating key
sequence to the MGC.
f. Provide a way to ask the ARF MG