RFC 3057
ISDN Q.921-User Adaptation Layer
Network Working Group K. Morneault Request for Comments: 3057 Cisco Systems Category: Standards Track S. Rengasami M. Kalla Telcordia Technologies G. Sidebottom Nortel Networks February 2001 ISDN Q.921-User Adaptation Layer Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved.Abstract
This document defines a protocol for backhauling of ISDN Q.921 User messages over IP using the Stream Control Transmission Protocol (SCTP). This protocol would be used between a Signaling Gateway (SG) and Media Gateway Controller (MGC). It is assumed that the SG receives ISDN signaling over a standard ISDN interface.
Table of Contents
1. Introduction................................................. 2 1.1 Scope..................................................... 2 1.2 Terminology............................................... 3 1.3 IUA Overview.............................................. 4 1.4 Services Provided by the IUA Layer........................ 9 1.5 Functions Implemented by the IUA Layer.................... 12 1.6 Definition of IUA Boundaries.............................. 14 2. Conventions.................................................. 16 3. Protocol Elements............................................ 17 3.1 Common Message Header..................................... 17 3.2 IUA Message Header........................................ 20 3.3 Description of Messages................................... 22 4. Procedures................................................... 45 4.1 Procedures to Support Service in Section 1.4.1............ 45 4.2 Procedures to Support Service in Section 1.4.2............ 46 4.3 Procedures to Support Service in Section 1.4.3............ 47 5. Examples...................................................... 56 5.1 Establishment of associations between SG and MGC examples.. 56 5.2 ASP Traffic Fail-over Examples............................. 58 5.3 Q.921/Q.931 primitives backhaul Examples................... 59 5.4 Layer Management Communication Examples.................... 61 6. Security..................................................... 61 6.1 Threats.................................................... 61 6.2 Protecting Confidentiality ................................ 62 7. IANA Considerations.......................................... 62 7.1 SCTP Payload Protocol Identifier........................... 62 7.2 IUA Protocol Extensions.................................... 62 8. Acknowledgements............................................. 64 9. References................................................... 64 10. Authors' Addresses........................................... 65 11. Full Copyright Statement..................................... 661. Introduction
In this document, the term Q.921-User refers to an upper layer which 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 [4]. 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 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
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. Q.921-User - Any protocol normally using the services of the ISDN Q.921 (e.g., Q.931, QSIG, etc.). 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. Association - An association refers to a SCTP association. The association will provide the transport for the delivery of Q.921-User protocol data units and IUA adaptation layer peer messages. 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. 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. 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. 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 back-up MGC). Fail- over also applies upon the return to service of a previously unavailable process. 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. Host - The computing platform that the ASP process is running on.1.3 IUA Overview
The architecture that has been defined [4] 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, 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 [3])
IUA - ISDN User Adaptation Layer Protocol
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 NOT be a requirement and TCP can
be used as the underlying common transport protocol.
1.3.2 Support for the management of SCTP associations between the SG
and ASPs
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 state of remote ASP(s) are also 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 Signaling Network Architecture
A Signaling Gateway is used to support the transport of Q.921-User signaling traffic to one or more distributed ASPs (e.g., MGCs). Clearly, the IUA protocol is not designed to meet the performance and reliability requirements for such transport by itself. However, the conjunction of distributed architecture and redundant networks does allow for a sufficiently reliable transport of signaling traffic over IP. The IUA protocol is flexible enough to allow its operation and management in a variety of physical configurations, enabling Network Operators to meet their performance and reliability requirements. To meet the ISDN signaling reliability and performance requirements for carrier grade networks, Network Operators SHOULD ensure that there is no single point of failure provisioned in the end-to-end network architecture between an ISDN node and an IP ASP. Depending of course on the reliability of the SG and ASP functional elements, this can typically be met by the provision of redundant QOS-bounded IP network paths for SCTP Associations between SCTP End Points, and redundant Hosts, and redundant SGs. The distribution of ASPs within the available Hosts is also important. For a particular Application Server, the related ASPs SHOULD be distributed over at least two Hosts. An example logical network architecture relevant to carrier-grade operation in the IP network domain is shown in Figure 1 below:
Host1 ******** ************** * *_________________________________________* ******** * * * _________* * ASP1 * * * SG1 * SCTP Associations | * ******** * * *_______________________ | * * ******** | | ************** | | ******** | | * *_______________________________| * * | * SG2 * SCTP Associations | * *____________ | * * | | Host2 ******** | | ************** | |_________________* ******** * |____________________________* * ASP1 * * * ******** * * * ************** . . . Figure 2 - Logical Model Example For carrier grade networks, the failure or isolation of a particular ASP SHOULD NOT cause stable calls to be dropped. This implies that ASPs need, in some cases, to share the call state or be able to pass the call state between each other. However, this sharing or communication of call state information is outside the scope of this document.1.3.4 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 fail-over model supports an n+k redundancy model, where n ASP(s) are 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. To avoid a single point of failure, it is recommended that a minimum of two ASPs be in the list, resident in separate hosts and therefore available over different SCTP Associations. For example, in the network shown in Figure 2, all messages from a particular D Channel (Interface Identifier) could be sent to ASP1 in Host1 or ASP1 in Host2. The AS list at SG1 might look like the following: Interface Identifier(s) - Application Server #1 ASP1/Host1 - State=Up, Active ASP1/Host2 - State=Up, Inactive In this 1+1 redundancy case, ASP1 in Host1 would be sent any incoming message for the Interface Identifiers registered. ASP1 in Host2 would normally be brought to the active state upon failure of, or loss of connectivity to, ASP1/Host1. In this example, both ASPs are Up, meaning that the related SCTP association and far-end IUA peer is ready. The AS List at SG1 might also be set up in load-share mode as shown below: Interface Identifier(s) - Application Server #1 ASP1/Host1 - State=Up, Active ASP1/Host2 - State=Up, Active In this case, both the ASPs would be sent a portion of the traffic. In the process of fail-over, it is recommended that in the case of ASPs supporting call processing, stable calls do not get released. It is possible that calls in transition MAY fail, although measures of communication between the ASPs involved can be used to mitigate this problem. For example, the two ASPs MAY share call state via shared memory, or MAY use an ASP to ASP protocol to pass call state information. The ASP to ASP protocol is outside the scope of this document.1.3.5 Client/Server Model
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 UDP/TCP) Registered User Port Number Assignment for IUA is 9900.1.4 Services Provided by the IUA Layer
1.4.1 Support for transport of Q.921/Q.931 boundary primitives
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 which 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 which 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 and MGC
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 pointed out in [2], which are shown below: M-TEI STATUS The M-TEI STATUS primitives are used to request, confirm and indicate the status (assigned/unassigned) of a 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).1.4.3 Support for management of active associations between SG and MGC
A set of primitives between the IUA layer and the Layer Management are 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 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 TEIs and 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 failover 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 back-up 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.1.5.3 SCTP Stream Management
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.1.5.4 Seamless Network Management Interworking
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.1.5.5 Congestion Management
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 DATA1.6.2 Definition of IUA/Q.931 boundary
DL-ESTABLISH DL-RELEASE DL-DATA DL-UNIT DATA1.6.3 Definition of SCTP/IUA Boundary
An example of the upper layer primitives provided by SCTP are available in Reference [3] section 10.1.6.4 Definition of IUA/Layer-Management Boundary
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.
2.0 Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
they appear in this document, are to be interpreted as described in
[RFC2119].
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