1. Introduction
In this document, the term Q.921-User refers to an upper layer that
uses the services of Q.921, not the user side of ISDN interface [1].
Examples of the upper layer would be Q.931 and QSIG.
This section describes the need for ISDN Q.921-User Adaptation (IUA)
layer protocol as well as how this protocol shall be implemented.
1.1. Scope
There is a need for Switched Circuit Network (SCN) signaling protocol
delivery from an ISDN Signaling Gateway (SG) to a Media Gateway
Controller (MGC) as described in the Framework Architecture for
Signaling Transport [5]. The delivery mechanism SHOULD meet the
following criteria:
* Support for transport of the Q.921/Q.931 boundary primitives
* Support for communication between Layer Management modules on SG
and MGC
* Support for management of SCTP active associations between SG
and MGC
This document supports both ISDN Primary Rate Access (PRA) as well as
Basic Rate Access (BRA) including the support for both point-to-point
and point-to-multipoint modes of communication. This support
includes Facility Associated Signaling (FAS), Non-Facility Associated
Signaling (NFAS), and NFAS with backup D channel. QSIG adaptation
layer requirements do not differ from Q.931 adaptation layer; hence,
the procedures described in this document are also applicable for a
QSIG adaptation layer. For simplicity, only Q.931 will be mentioned
in the rest of this document.
1.2. Terminology
Application Server (AS) - A logical entity serving a specific
application instance. An example of an Application Server is a MGC
handling the Q.931 and call processing for D channels terminated by
the Signaling Gateways. Practically speaking, an AS is modeled at
the SG as an ordered list of one or more related Application Server
Processes (e.g., primary, secondary, tertiary).
Application Server Process (ASP) - A process instance of an
Application Server. Examples of Application Server Processes are
primary or backup MGC instances.
Association - An association refers to an SCTP association. The
association will provide the transport for the delivery of Q.921-User
protocol data units and IUA adaptation layer peer messages.
Backhaul - A SG terminates the lower layers of an SCN protocol and
backhauls the upper layer(s) to MGC for call processing. For the
purposes of this document, the SG terminates Q.921 and backhauls
Q.931 to MGC.
Fail-over - The capability to re-route signaling traffic as required
between related ASPs in the event of failure or unavailability of the
currently used ASP (e.g., from primary MGC to backup MGC). Fail-over
also applies upon the return to service of a previously unavailable
process.
Host - The computing platform that the ASP process is running on.
Interface - For the purposes of this document, an interface supports
the relevant ISDN signaling channel. This signaling channel MAY be a
16-kbps D channel for an ISDN BRA as well as 64-kbps primary or
backup D channel for an ISDN PRA. For QSIG, the signaling channel is
a Qc channel.
Interface Identifier - The Interface Identifier identifies the
physical interface at the SG for which the signaling messages are
sent/received. The format of the Interface Identifier parameter can
be text or integer, the values of which are assigned according to
network operator policy. The values used are of local significance
only, coordinated between the SG and ASP. Significance is not
implied across SGs served by an AS.
Layer Management - Layer Management is a nodal function that handles
the inputs and outputs between the IUA layer and a local management
entity.
Network Byte Order - Most significant byte first, a.k.a big endian.
Stream - A stream refers to an SCTP stream: a uni-directional logical
channel established from one SCTP endpoint to another associated SCTP
endpoint, within which all user messages are delivered in sequence
except for those submitted to the un-ordered delivery service.
Q.921-User - Any protocol normally using the services of the ISDN
Q.921 (e.g., Q.931, QSIG, etc.).
1.3. IUA Overview
The architecture that has been defined [5] for SCN signaling
transport over IP uses multiple components, including an IP transport
protocol, a signaling common transport protocol, and an adaptation
module to support the services expected by a particular SCN signaling
protocol from its underlying protocol layer.
This document defines an adaptation module that is suitable for the
transport of ISDN Q.921-User (e.g., Q.931) messages.
1.3.1. Example: SG to MGC
In a Signaling Gateway (SG), it is expected that the ISDN signaling
is received over a standard ISDN network termination. The SG then
provides interworking of transport functions with IP Signaling
Transport, in order to transport the Q.931 signaling messages to the
MGC where the peer Q.931 protocol layer exists, as shown below:
****** ISDN ****** IP *******
* EP *---------------* SG *--------------* MGC *
****** ****** *******
+-----+ +-----+
|Q.931| (NIF) |Q.931|
+-----+ +----------+ +-----+
| | | | IUA| | IUA |
| | | +----+ +-----+
|Q.921| |Q.921|SCTP| |SCTP |
| | | +----+ +-----+
| | | | IP | | IP |
+-----+ +-----+----+ +-----+
NIF - Nodal Interworking Function
EP - ISDN End Point
SCTP - Stream Control Transmission Protocol (Refer to [4,8])
IUA - ISDN User Adaptation Layer Protocol
Figure 1. IUA in the SG to MGC Application
It is recommended that the IUA use the services of the Stream Control
Transmission Protocol (SCTP) as the underlying reliable common
signaling transport protocol. The use of SCTP provides the following
features:
- explicit packet-oriented delivery (not stream-oriented)
- sequenced delivery of user messages within multiple streams,
with an option for order-of-arrival delivery of individual user
messages,
- optional multiplexing of user messages into SCTP datagrams,
- network-level fault tolerance through support of multi-homing
at either or both ends of an association,
- resistance to flooding and masquerade attacks, and
- data segmentation to conform to discovered path MTU size.
There are scenarios without redundancy requirements and scenarios in
which redundancy is supported below the transport layer. In these
cases, the SCTP functions above MAY be determined to not be required
and TCP MAY be used as the underlying common transport protocol.
The IUA layer at the SG maintains the availability state of all
dynamically registered remote ASPs, in order to manage the SCTP
associations and the traffic between the SG and ASPs. As well, the
active/inactive states of remote ASP(s) are maintained. Active ASPs
are those currently receiving traffic from the SG.
The IUA layer MAY be instructed by local management to establish an
SCTP association to a peer IUA node. This can be achieved using the
M-SCTP ESTABLISH primitive to request, indicate, and confirm the
establishment of an SCTP association with a peer IUA node.
The IUA layer MAY also need to inform local management of the status
of the underlying SCTP associations using the M-SCTP STATUS request
and indication primitive. For example, the IUA MAY inform local
management of the reason for the release of an SCTP association,
determined either locally within the IUA layer or by a primitive from
the SCTP.
1.3.3. ASP Fail-over Model and Terminology
The IUA layer supports ASP fail-over functions in order to support a
high availability of call processing capability. All Q.921-User
messages incoming to an SG are assigned to a unique Application
Server, based on the Interface Identifier of the message.
The Application Server is, in practical terms, a list of all ASPs
configured to process Q.921-User messages from certain Interface
Identifiers. One or more ASPs in the list are normally active (i.e.,
handling traffic) while any others MAY be unavailable or inactive, to
be possibly used in the event of failure or unavailability of the
active ASP(s).
The IUA layer supports an n+k redundancy model (active-standby, load
sharing, broadcast) where n is the minimum number of redundant ASPs
required to handle traffic and k ASPs are available to take over for
a failed or unavailable ASP. Note that 1+1 active/standby redundancy
is a subset of this model. A simplex 1+0 model is also supported as
a subset, with no ASP redundancy.
It is recommended that the SG and ASP be able to support both client
and server operation. The peer endpoints using IUA SHOULD be
configured so that one always takes on the role of client and the
other the role of server for initiating SCTP associations. The
default orientation would be for the SG to take on the role of server
while the ASP is the client. In this case, ASPs SHOULD initiate the
SCTP association to the SG.
The SCTP and TCP Registered User Port Number Assignment for IUA is
9900.
In the backhaul scenario, the Q.921/Q.931 boundary primitives are
exposed. IUA layer needs to support all of the primitives of this
boundary to successfully backhaul Q.931.
This includes the following primitives [1]:
DL-ESTABLISH
The DL-ESTABLISH primitives are used to request, indicate, and
confirm the outcome of the procedures for establishing multiple frame
operation.
DL-RELEASE
DL-RELEASE primitives are used to request, indicate, and confirm the
outcome of the procedures for terminating a previously established
multiple frame operation, or for reporting an unsuccessful
establishment attempt.
DL-DATA
The DL-DATA primitives are used to request and indicate layer 3
(Q.931) messages that are to be transmitted, or have been received,
by the Q.921 layer using the acknowledged information transfer
service.
DL-UNIT DATA
The DL-UNIT DATA primitives are used to request and indicate layer 3
(Q.931) messages that are to be transmitted, by the Q.921 layer using
the unacknowledged information transfer service.
1.4.2. Support for Communication between Layer Management Modules on SG
It is envisioned that the IUA layer needs to provide some services
that will facilitate communication between Layer Management modules
on the SG and MGC. These primitives are shown below:
M-TEI STATUS
The M-TEI STATUS primitives are used to request, confirm, and
indicate the status (assigned/unassigned) of an ISDN Terminal
Endpoint Identifier (TEI).
M-ERROR
The M-ERROR primitive is used to indicate an error with a received
IUA message (e.g., interface identifier value is not known to the
SG).
A set of primitives between the IUA layer and the Layer Management is
defined below to help the Layer Management manage the SCTP
association(s) between the SG and MGC. The IUA layer can be
instructed by the Layer Management to establish an SCTP association
to a peer IUA node. This procedure can be achieved using the M-SCTP
ESTABLISH primitive.
M-SCTP ESTABLISH
The M-SCTP ESTABLISH primitives are used to request, indicate, and
confirm the establishment of an SCTP association to a peer IUA node.
M-SCTP RELEASE
The M-SCTP RELEASE primitives are used to request, indicate, and
confirm the release of an SCTP association to a peer IUA node.
The IUA layer MAY also need to inform the status of the SCTP
associations to the Layer Management. This can be achieved using the
M-SCTP STATUS primitive.
M-SCTP STATUS
The M-SCTP STATUS primitives are used to request and indicate the
status of the underlying SCTP association(s).
The Layer Management MAY need to inform the IUA layer of an AS/ASP
status (i.e., failure, active, etc.), so that messages can be
exchanged between IUA layer peers to stop traffic to the local IUA
user. This can be achieved using the M-ASP STATUS primitive.
M-ASP STATUS
The ASP status is stored inside IUA layer on both the SG and MGC
sides. The M-ASP STATUS primitive can be used by Layer Management to
request the status of the Application Server Process from the IUA
layer. This primitive can also be used to indicate the status of the
Application Server Process.
M-ASP-UP
The M-ASP-UP primitive can be used by Layer Management to send a ASP
Up message for the Application Server Process. It can also be used
to generate an ASP Up Acknowledgement.
M-ASP-DOWN
The M-ASP-DOWN primitive can be used by Layer Management to send a
ASP Down message for the Application Server Process. It can also be
used to generate an ASP Down Acknowledgement.
M-ASP-ACTIVE
The M-ASP-UP primitive can be used by Layer Management to send a ASP
Active message for the Application Server Process. It can also be
used to generate an ASP Active Acknowledgement.
M-ASP-INACTIVE
The M-ASP-UP primitive can be used by Layer Management to send a ASP
Inactive message for the Application Server Process. It can also be
used to generate an ASP Inactive Acknowledgement.
M-AS STATUS
The M-AS STATUS primitive can be used by Layer Management to request
the status of the Application Server. This primitive can also be
used to indicate the status of the Application Server.
1.5. Functions Implemented by the IUA Layer
1.5.1. Mapping
The IUA layer MUST maintain a map of the Interface Identifier to a
physical interface on the Signaling Gateway. A physical interface
would be a T1 line, E1 line, etc., and could include the Time-
Division Multiplexing (TDM) timeslot. In addition, for a given
interface the SG MUST be able to identify the associated signaling
channel. IUA layers on both SG and MGC MAY maintain the status of
ISDN Terminal Endpoint Identifiers (TEIs) and Service Access Point
Identifiers (SAPIs).
The SG maps an Interface Identifier to an SCTP association/stream
only when an ASP sends an ASP Active message for a particular
Interface Identifier. It MUST be noted, however, that this mapping
is dynamic and could change at any time due to a change of ASP state.
This mapping could even temporarily be invalid, for example, during
fail-over of one ASP to another. Therefore, the SG MUST maintain the
states of AS/ASP and reference them during the routing of an messages
to an AS/ASP.
One example of the logical view of relationship between D channel,
Interface Identifier, AS, and ASP in the SG is shown below:
/---------------------------------------------------+
/ /------------------------------------------------|--+
/ / v |
/ / +----+ act+-----+ +-------+ -+--+-|+--+-
D chan1-------->|IID |-+ +-->| ASP |--->| Assoc | v
/ +----+ | +----+ | +-----+ +-------+ -+--+--+--+-
/ +->| AS |--+ Streams
/ +----+ | +----+ stb+-----+
D chan2-------->|IID |-+ | ASP |
+----+ +-----+
where IID = Interface Identifier
Note that an ASP can be in more than one AS.
1.5.2. Status of ASPs
The IUA layer on the SG MUST maintain the state of the ASPs it is
supporting. The state of an ASP changes because of reception of
peer-to-peer messages (ASPM messages as described in Section 3.3.2)
or reception of indications from the local SCTP association. ASP
state transition procedures are described in Section 4.3.1.
At a SG, an Application Server list MAY contain active and inactive
ASPs to support ASP load-sharing and fail-over procedures. When, for
example, both a primary and a backup ASP are available, IUA peer
protocol is required to control which ASP is currently active. The
ordered list of ASPs within a logical Application Server is kept
updated in the SG to reflect the active Application Server
Process(es).
Also the IUA layer MAY need to inform the local management of the
change in status of an ASP or AS. This can be achieved using the
M-ASP STATUS or M-AS STATUS primitives.
SCTP allows a user-specified number of streams to be opened during
the initialization. It is the responsibility of the IUA layer to
ensure proper management of these streams. Because of the
unidirectional nature of streams, an IUA layer is not aware of the
stream number to Interface Identifier mapping of its peer IUA layer.
Instead, the Interface Identifier is in the IUA message header.
The use of SCTP streams within IUA is recommended in order to
minimize transmission and buffering delay, therefore improving the
overall performance and reliability of the signaling elements. It is
recommended that a separate SCTP stream is used for each D channel.
The IUA layer on the SG SHOULD pass an indication of unavailability
of the IUA-User (Q.931) to the local Layer Management, if the
currently active ASP moves from the ACTIVE state. The Layer
Management could instruct Q.921 to take some action, if it deems
appropriate.
Likewise, if an SCTP association fails, the IUA layer on both the SG
and ASP sides MAY generate Release primitives to take the data links
out-of-service.
If the IUA layer becomes congested (implementation dependent), it MAY
stop reading from the SCTP association to flow control from the peer
IUA.
1.6. Definition of IUA Boundaries
1.6.1. Definition of IUA/Q.921 Boundary
DL-ESTABLISH
DL-RELEASE
DL-DATA
DL-UNIT DATA
1.6.2. Definition of IUA/Q.931 Boundary
DL-ESTABLISH
DL-RELEASE
DL-DATA
DL-UNIT DATA
1.6.3. Definition of SCTP/IUA Boundary
An example of the upper layer primitives provided by SCTP are
available in Section 10 of RFC 2960prop [4].
M-SCTP ESTABLISH request
Direction: LM -> IUA
Purpose: LM requests ASP to establish an SCTP association with an SG.
M-STCP ESTABLISH confirm
Direction: IUA -> LM
Purpose: ASP confirms to LM that it has established an SCTP
association with an SG.
M-SCTP ESTABLISH indication
Direction: IUA -> LM
Purpose: SG informs LM that an ASP has established an SCTP
association.
M-SCTP RELEASE request
Direction: LM -> IUA
Purpose: LM requests ASP to release an SCTP association with SG.
M-SCTP RELEASE confirm
Direction: IUA -> LM
Purpose: ASP confirms to LM that it has released SCTP association
with SG.
M-SCTP RELEASE indication
Direction: IUA -> LM
Purpose: SG informs LM that ASP has released an SCTP association.
M-SCTP STATUS request
Direction: LM -> IUA
Purpose: LM requests IUA to report status of SCTP association.
M-SCTP STATUS indication
Direction: IUA -> LM
Purpose: IUA reports status of SCTP association.
M-ASP STATUS request
Direction: LM -> IUA
Purpose: LM requests SG to report status of remote ASP.
M-ASP STATUS indication
Direction: IUA -> LM
Purpose: SG reports status of remote ASP.
M-AS-STATUS request
Direction: LM -> IUA
Purpose: LM requests SG to report status of AS.
M-AS-STATUS indication
Direction: IUA -> LM
Purpose: SG reports status of AS.
M-NOTIFY indication
Direction: IUA -> LM
Purpose: ASP reports that it has received a NOTIFY message
from its peer.
M-ERROR indication
Direction: IUA -> LM
Purpose: ASP or SG reports that it has received an ERROR
message from its peer.
M-ASP-UP request
Direction: LM -> IUA
Purpose: LM requests ASP to start its operation and send an ASP UP
message to the SG.
M-ASP-UP confirm
Direction: IUA -> LM
Purpose: ASP reports that is has received an ASP UP Acknowledgement
message from the SG.
M-ASP-DOWN request
Direction: LM -> IUA
Purpose: LM requests ASP to stop its operation and send an ASP DOWN
message to the SG.
M-ASP-DOWN confirm
Direction: IUA -> LM
Purpose: ASP reports that is has received an ASP DOWN
Acknowledgement message from the SG.
M-ASP-ACTIVE request
Direction: LM -> IUA
Purpose: LM requests ASP to send an ASP ACTIVE message to the SG.
M-ASP-ACTIVE confirm
Direction: IUA -> LM
Purpose: ASP reports that is has received an ASP ACTIVE
Acknowledgement message from the SG.
M-ASP-INACTIVE request
Direction: LM -> IUA
Purpose: LM requests ASP to send an ASP INACTIVE message to the SG.
M-ASP-INACTIVE confirm
Direction: IUA -> LM
Purpose: ASP reports that is has received an ASP INACTIVE
Acknowledgement message from the SG.
M-TEI STATUS request
Direction: LM -> IUA
Purpose: LM requests ASP to send a TEI status request to the SG.
M-TEI STATUS indication
Direction: IUA -> LM
Purpose: ASP reports that is has received a TEI status indication
from the SG.
M-TEI STATUS confirm
Direction: IUA -> LM
Purpose: ASP reports that is has received a TEI status confirm from
the SG.
