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. Morneault, et al. Standards Track [Page 1] RFC 3057 ISDN Q.921-User Adaptation Layer February 2001 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..................................... 66 1. 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 Morneault, et al. Standards Track [Page 2] RFC 3057 ISDN Q.921-User Adaptation Layer February 2001 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 Morneault, et al. Standards Track [Page 3] RFC 3057 ISDN Q.921-User Adaptation Layer February 2001 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: Morneault, et al. Standards Track [Page 4] RFC 3057 ISDN Q.921-User Adaptation Layer February 2001 ****** 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. Morneault, et al. Standards Track [Page 5] RFC 3057 ISDN Q.921-User Adaptation Layer February 2001 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: Morneault, et al. Standards Track [Page 6] RFC 3057 ISDN Q.921-User Adaptation Layer February 2001 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 uniqu