Network Working Group Malleswar Kalla INTERNET-DRAFT Selvam Rengasami Telcordia Technologies Ken Morneault Cisco Systems Greg Sidebottom Nortel Networks Expires in six months March 2000 ISDN Q.921-User Adaptation Layer Status of This Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as 'work in progress.' The list of current Internet-Drafts can be accessed at http//www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http//www.ietf.org/shadow.html. To learn the current status of any Internet-Draft, please check the '1id-abstracts.txt' listing contained in the Internet- Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Abstract This Internet Draft defines a protocol for backhauling of ISDN Q.921 User messages over IP using the Simple 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. Kalla, Rengasami, Morneault, & Sidebottom [Page 1] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 TABLE OF CONTENTS 1. Introduction.....................................................3 1.1 Scope.........................................................3 1.2 Terminology...................................................3 1.3 Signaling Transport Architecture..............................4 1.4 Services Provided by the IUA Layer............................8 1.5 Functions Implemented by the IUA Layer.......................10 1.6 Definition of IUA Boundaries.................................11 2. Protocol Elements...............................................12 2.1 Common Message Header........................................12 2.2 IUA Message Header...........................................13 2.3 Description of Messages......................................14 3. Procedures......................................................24 3.1 Procedures to Support Service in Section 1.4.1...............24 3.2 Procedures to Support Service in Section 1.4.2...............24 3.3 Procedures to Support Service in Section 1.4.3...............25 4. Examples.........................................................32 4.1 Establishment of associations between SG and MGC examples.....32 4.2 ASP Traffic Fail-over Examples................................34 4.3 Q.921/Q.931 primitives backhaul Examples......................36 4.4 Layer Management Communication Examples.......................37 5. Security........................................................37 5.1 Threats.......................................................37 5.2 Protecting Confidentiality ...................................38 6. IANA Considerations.............................................38 7. Acknowledgements................................................38 8. References......................................................39 9. Author's Addresses..............................................39 Kalla, Rengasami, Morneault, & Sidebottom [Page 2] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 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. 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). 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 draft supports both ISDN Primary Rate Access (PRA) as well as Basic Rate Access (BRA) including the support for both point-to-point mode and point-to-multipoint modes of communication. QSIG adaptation layer requirements do not differ from Q.931 adaptation layer, hence the procedures described in this draft are also applicable to 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 signalling channel. This signalling 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 signalling 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 other layer to MGC for call processing. For the purposes of this draft 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. Kalla, Rengasami, Morneault, & Sidebottom [Page 3] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 Stream - A stream refers to a SCTP stream. For the purposes of this document, a stream will be mapped to an ISDN signalling channel. 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. Application Server Process Path (ASP Path or just Path) - A Path to a remote Application Server Process instance. A Path maps 1:1 to an SCTP association. Fail-over - The capability to re-route signalling 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. Network Byte Order: Most significant byte first, a.k.a Big Endian. Host - The computing platform that the ASP process is running on. Stream - A stream refers to an SCTP stream. 1.3 Signaling Transport Architecture 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 functions 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 (Q.931). Kalla, Rengasami, Morneault, & Sidebottom [Page 4] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 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 | | | | +----+ +-----+ | | | |UDP | | UDP | | | | +----+ +-----+ | | | | IP + | IP | +-----+ +-----+----+ +-----+ NIF - Nodal Interworking Function EP - ISDN End Point SCTP - Simple Control Transmission Protocol (Refer to [3]) IUA - ISDN User Adaptation Layer Protocol 1.3.2 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 description cannot in itself meet any performance and reliability requirements for such transport. A physical network architecture is required, with data on the availability and transfer performance of the physical nodes involved in any particular exchange of information. However, the IUA protocol must be flexible enough allow its operation and management in a variety of physical configurations that will enable Network Operators to meet their performance and reliability requirements. To meet the stringent ISDN signaling reliability and performance requirements for carrier grade networks, these 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. Kalla, Rengasami, Morneault, & Sidebottom [Page 5] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 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 physical 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 - Physical Model Example For carrier grade networks, Operators should ensure that under failure or isolation of a particular ASP, stable calls are not lost. 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 is outside the scope of this document. 1.3.3 ASP Fail-over Model and Terminology The IUA 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. Kalla, Rengasami, Morneault, & Sidebottom [Page 6] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 The Application Server is in practical terms a list of all ASPs currently registered 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 ASPs 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. 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 1, all messages from a particular D Channel could be sent to ASP1 in Host1 or ASP1 in Host2. The AS list at SG1 might look like this: Interface Identifiers - 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 loadshare mode: Interface Identifier {x) - 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 or fail-back, it is recommended that in the case of ASPs supporting call processing, stable calls do not fail. It is possible that calls in transition may fail, although measures of communication between the ASPs involved can be used to mitigate this. 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. Kalla, Rengasami, Morneault, & Sidebottom [Page 7] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 1.3.4 Client/Server Model The SG takes on the role of server while the ASP is the client. ASPs must 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 SDUs containing Q.931 PDUs 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 SDUs containing Q.931 PDUs which are to be transmitted, by the Q.921 layer using the unacknowledged information transfer service. Kalla, Rengasami, Morneault, & Sidebottom [Page 8] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 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 MIUA-TEI STATUS The MIUA-TEI STATUS primitives are used to request, confirm and indicate the status (in service/out of service) of a TEI. To facilitate reporting of errors that arise because of backhauling Q.931 scenario, the following primitive is defined 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 The IUA layer on the SG keeps the state of various ASPs it is associated with. A set of primitives between IUA layer and the Layer Management are defined below to help the Layer Management manage the association(s) between SG and MGC. The IUA layer can be instructed by the Layer Management to establish SCTP association to a peer IUA node. This can be achieved using the following primitive M-SCTP ESTABLISH The M-SCTP ESTABLISH primitives are used to request, indicate, and confirm the establishment of 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 following primitive M-SCTP STATUS The M-SCTP STATUS primitives are used to request and indicate the status of the underlying SCTP association(s). Kalla, Rengasami, Morneault, & Sidebottom [Page 9] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 The Layer Management may need to inform the IUA layer of a user 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 following primitive. M-ASP STATUS The M-ASP STATUS primitives are used to request and indicate the status of the Application Server Process. 1.5 Functions Implemented by the IUA Layer 1.5.1 Mapping The IUA layer must maintain a map of the Interface ID to a physical interface on the Signaling Gateway. A physical interface would be a T1 line, E1 line, etc. In addition, for a given interface the SG must be able to identify the associated signalling channel. IUA layers on both SG and MGC need to maintain the status of TEIs, SAPIs. 1.5.2 Status of ASPs The IUA layer on the SG must maintain the state of various ASPs it is associated with. The state of an ASP changes because of reception of peer-to-peer messages or reception of indications from the local SCTP association. ASP state transition procedures are described in section 3.3.1. 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, IUA layers are not aware of the stream information from the peer IUA layers. For the purposes of this draft, it is assumed that a separate stream will be 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. Kalla, Rengasami, Morneault, & Sidebottom [Page 10] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 1.5.5 Management Inhibit/Uninhibit Local Management may wish to stop traffic across an SCTP association in order to temporarily remove the association from service or to perform testing and maintenance activity. The function could optionally be used to manage the start of traffic on to a newly available SCTP association. 1.5.6 Active Association Control At an SG, an Application Server list may contain active and inactive ASPs to support ASP loads-haring 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). 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 The upper layer primitives provided by SCTP are available in Reference [3] section 9. 1.6.4 Definition of IUA/Layer-Management Boundary M-ERROR M-NOTIFY M-SCTP ESTABLISH M-SCTP STATUS M-ASP STATUS MIUA-TEI STATUS Kalla, Rengasami, Morneault, & Sidebottom [Page 11] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 2. Protocol Elements This section describes the format of various messages used in this protocol. 2.1 Common Message Header The protocol messages for Q.921 User Adaptation require a message header which contains the adaptation layer version, the message type, and message length. All types of messages contain this header. This message header is common among all SCN signaling protocol adaptation layers. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version | Spare | Message Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1 Common Header Format All fields in an IUA message MUST be transmitted in the network byte order, unless otherwise stated. 2.1.1 Version The version field (vers) contains the version of the IUA adaptation layer. The supported versions are: 0000 0001 Release 1.0 protocol 2.1.2 Message Types The valid message types are defined in Section 2.2.2 and the message contents are described in Section 2.3. Each message can contain parameters. The following list contains the message names for the defined messages. Kalla, Rengasami, Morneault, & Sidebottom [Page 12] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 Q.921/Q.931 Boundary Primitives Transport (QAUP) Messages Data Request Message 0501 Data Indication Message 0502 Unit Data Request Message 0503 Unit Data Indication Message 0504 Establish Request 0505 Establish Confirm 0506 Establish Indication 0507 Release Request 0508 Release Confirm 0509 Release Indication 0510 Application Server Process Maintenance (ASPM) messages ASP Up 0301 ASP Down 0302 ASP Active 0401 ASP Inactive 0402 Management (MGMT) Messages Error Indication 0000 Notify 0001 TEI Status Request 0101 TEI Status Confirm 0102 TEI Status Indication 0103 2.1.3 Message Length The Message length defines the length of the message in octets, not including the common Message header. 2.2 IUA Message Header In addition to the common message header, there will be a specific message header for QAUP and TEI related MGMT messages. The IUA message header will immediately follow the common message header in these messages. This message header will contain the Interface Identifier and Data Link Connection Identifier(DLCI). The Interface Identifier identifies the physical interface terminating the signalling channel at the SG for which the signaling messages are sent/received. The format of the Interface Identifier parameter is an 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. Kalla, Rengasami, Morneault, & Sidebottom [Page 13] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag (0x1) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DLCI | Spare | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2 IUA Message Header The Tag value for Interface Identifier is 0x1. The length is always set to a value of 4. The DLCI format is shown below in Figure 3. 0 1 2 3 4 5 6 7 +-----+-----+-----+-----+-----+-----+-----+-----+ | 0 | SPR | SAPI | +-----------------------------------------------+ | 1 | TEI | +-----------------------------------------------+ Figure 3 DLCI Format SPR Spare, 2nd bit in octet 1 SAPI Service Access Point Identifier, 3rd thru 8th bits in octet 1 TEI Terminal Endpoint Identifier, 2nd thru 8th bits in octet 2 The DLCI field (including the SAPI and TEI) is coded in accordance with Q.921. 2.3 IUA Messages The following section defines the messages and parameter contents. The IUA messages will use the common message header (Figure 2) and the IUA message header (Figure 3). 2.3.1 Q.921-User Backhauled Messages Kalla, Rengasami, Morneault, & Sidebottom [Page 14] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 2.3.1.1 Establish Messages (Request, Confirm, Indication) The Establish Messages are used to establish a data link on the signalling channel or to confirm that a data link on the signalling channel has been established. The Establish messages contain the common message header followed by IUA message header. It does not contain any additional parameters. 2.3.1.2 Release Messages (Request, Indication, Confirmation) The Release Request message is used to release the data link on the signalling channel. The Release Confirm and Indication messages are used to indicate that the data link on the signaling channel has been released. The Release messages contain the common message header followed by IUA message header. The Release confirm message is in response to a Release Request message and it does not contain any additional parameters. The Release Request and Indication messages contain the following parameters REASON 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reason | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The valid values for Reason are shown in the following table. Define Value Description RELEASE_MGMT 0x0 Management layer generated release. RELEASE_PHYS 0x1 Physical layer alarm generated release. RELEASE_DM 0x2 Specific to a request. Indicates Layer 2 should release and deny all requests from far end to establish a data link on the signalling channel (i.e. if SABME is received send a DM) RELEASE_OTHER 0x3 Other reasons Note Only RELEASE_MGMT, RELEASE_DM and RELEASE_OTHER are valid reason codes for a Release Request message. 2.3.1.3 Data Messages (Request, Indication) The Data message contains an ISDN Q.921-User Protocol Data Unit (PDU) corresponding to acknowledged information transfer service. Kalla, Rengasami, Morneault, & Sidebottom [Page 15] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 The Data messages contain the common message header followed by IUA message header. The Data message contains the following parameters PROTOCOL DATA 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The protocol data contains upper layer signalling message e.g. Q.931, QSIG. 2.3.1.4 Unit Data Messages (Request, Indication) The Unit Data message contains an ISDN Q.921-User Protocol Data Unit (PDU) corresponding to unacknowledged information transfer service. The Unit Data messages contain the common message header followed by IUA message header. The Unit Data message contains the following parameters PROTOCOL DATA 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2.3.2 Application Server Process Maintenance (ASPM) Messages The ASPM messages will only use the common message header. 2.3.2.1 ASP UP (ASPUP) The ASP UP (ASPUP) message is used to indicate to a remote IUA peer that the Adaptation layer is ready to receive traffic or maintenance messages. The ASPUP message contains the following parameters: Adaptation Layer Identifier (optional) Info String (optional) Kalla, Rengasami, Morneault, & Sidebottom [Page 16] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 The format for ASPUP Message parameters is as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag (0x2) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Adaptation Layer Identifier* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag (0x4) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | INFO String* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Adaptation Layer Identifier (ALI) is a string that identifies the adaptation layer. This string must be set to "IUA". The ALI would normally only be used in the initial ASP Up message across a new SCTP association to ensure both peers are assuming the same adaptation layer protocol. The optional INFO String parameter can carry any meaningful 8-BIT ASCII character string along with the message. Length of the INFO String parameter is from 0 to 255 characters. No procedures are presently identified for its use but the INFO String may be used for debugging purposes. Note: Strings are padded to 32-bit boundaries. The length field indicates the end of the string. 2.3.2.2 ASP Down (ASPDN) The ASP Down (ASPDN) message is used to indicate to a remote IUA peer that the adaptation layer is not ready to receive traffic or maintenance messages. The ASPDN message contains the following parameters: Reason INFO String (Optional) Kalla, Rengasami, Morneault, & Sidebottom [Page 17] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 The format for the ASPDN message parameters is as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reason | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag (0x4) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | INFO String* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The format and description of the optional Info String parameter is the same as for the ASP Up message (See Section 2.3.2.1.) The Reason parameter indicates the reason that the remote IUA adaptation layer is unavailable. The valid values for Reason are shown in the following table. Value Description 0x1 Management Inhibit 2.3.2.3 ASP Active (ASPAC) The ASPAC message is sent by an ASP to indicate to an SG that it is Active and ready to be used. The ASPAC message contains the following parameters: Type Interface Identifier (Optional) INFO String (Optional) The format for the ASPAC message is as follows: Kalla, Rengasami, Morneault, & Sidebottom [Page 18] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag (0xx) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface Identifiers* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag (0x4) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | INFO String* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Type parameter identifies the traffic mode of operation of the ASP within an AS. The valid values for Type are shown in the following table. Value Description 0x1 Over-ride 0x2 Load-share 0x3 New traffic Within a particular Interface Identifier, only one Type can be used. The Over-ride value indicates that the ASP is operating in Over-ride mode, where the ASP takes over all traffic in an Application Server (i.e., primary/back-up operation), over-riding any currently active ASPs in the AS. In loadshare mode, the ASP will share in the traffic distribution with any other currently active ASPs. In New traffic mode the ASP wishes to take on traffic in the AS but does not expect to receive messages related to calls that are pending completion in another ASP The optional Interface Identifiers parameter contains a list of Interface Identifier integers indexing the Application Server traffic that the sending ASP is configured/registered to receive. There is one-to-one relationship between an Interface Identifier and an AS Name. An SG that receives an ASPAC with an incorrect type for a particular Interface Identifier will respond with an Error Message. The format and description of the optional Info String parameter is the same as for the ASP Up message (See Section 2.3.2.1.) Kalla, Rengasami, Morneault, & Sidebottom [Page 19] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 2.3.2.4 ASP Inactive (ASPIA) The ASPIA message is sent by an ASP to indicate to an SG that it is no longer an active ASP to be used from within a list of ASPs. The SG will respond with an ASPIA message and either discard incoming messages or buffer for a timed period and then discard. The ASPIA message contains the following parameters: Type Interface Identifiers (Optional) INFO String (Optional) The format for the ASPIA message parameters is as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag (0xx) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface Identifiers* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag (0x4) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | INFO String* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Type parameter identifies the traffic mode of operation of the ASP within an AS. The valid values for Type are shown in the following table. Value Description 0x1 Over-ride 0x2 Load-share 0x3 Graceful Withdrawal The format and description of the optional Interface Identifiers and Info String parameters is the same as for the ASP Active message (See Section 2.3.2.3.) 2.3.3 Layer Management (MGMT) Messages Kalla, Rengasami, Morneault, & Sidebottom [Page 20] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 2.3.3.1 Error (ERR) The ERR message is sent when an invalid value is found in an incoming message. The ERR message contains the following parameters: Error Code Diagnostic Information (optional) The format for the ERR message is as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag (0xx) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Diagnostic Information* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Error Code parameter indicates the reason for the Error Message. The Error parameter value can be one of the following values: Invalid Version 0x1 Invalid Interface Identifier 0x2 Invalid Adaptation Layer Identifier 0x3 Invalid Message Type 0x4 Invalid Stream Identifier 0x5 Invalid TEI 0x6 The optional Diagnostic information can be any information germane to the error condition, to assist in identification of the error condition. In the case of an Invalid Version Error Code the Diagnostic information includes the supported Version parameter. In the other cases, the Diagnostic information may be the first 40 bytes of the offending message. 2.3.3.2 Notify (NTFY) The Notify message used to provide autonomous notification of IUA events. Kalla, Rengasami, Morneault, & Sidebottom [Page 21] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 The NTFY message contains the following parameters: Status Type Status Identification Interface Identifiers (Optional) INFO String (Optional) The format for the NTFY message is as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Status Type | Status Identification | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag (0xx) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface Identifiers* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tag (0x4) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | INFO String* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Status Type parameter identifies the type of the Notify message. Following are the valid Status Type values: Value Description 0x1 Application Server state change (AS_State_Change) 0x2 Application Server Process state change (ASP_State_Change) 0x3 Other The Status Information parameter contains more detailed information for the notification, based on the value of the Status Type. If the Status Type is AS_State_Change the following Status Information values are used: Value Description 0x1 Application Server Down (AS_Down) 0x2 Application Server Up (AS_Up) 0x3 Application Server Active (AS_Active) 0x4 Application Server Pending (AS_Pending) Kalla, Rengasami, Morneault, & Sidebottom [Page 22] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 These notifications are sent from an SG to an ASP upon a change in status of a particular Application Server. The value reflects the new state of the Application Server. If the Status type is ASP_State_Change, the Status Information values are: Value Description 0x1 Application Server Process (ASP) Down 0x2 Application Server Process (ASP) Up 0x3 Application Server Process (ASP) Active These notifications are sent from an SG to an ASP upon a change in status of a particular Application Server process within the ASP list of a particular Application Server. The value reflects the new state of the Application Server Process. If the Status Type is Other, then the following Status Information values are defined: Value Description 0x1 Insufficient ASP resources active in AS This notification is not based on the SG reporting the state change of an ASP or AS. For the value defined the SG is indicating to an ASP(s) in the AS that another ASP is required in order to handle the load of the AS. The format and description of the optional Interface Identifiers and Info String parameters is the same as for the ASP Active message (See Section 2.3.2.3.) 2.3.3.3 TEI Status Messages (Request, Confirm and Indication) The TEI Status messages are exchanged between IUA layer peers to request, confirm and indicate the status of a particular TEI. The TEI Status messages contain the common message header followed by IUA message header. The TEI Status Request message does not contain any additional parameters. The TEI Status Indication, and Confirm messages contain the following parameters STATUS Kalla, Rengasami, Morneault, & Sidebottom [Page 23] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Status | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The valid values for Status are shown in the following table. Define Value Description IN_SERVICE 0x0 TEI is in service OUT_OF_SERVICE 0x1 TEI is out of service 3.0 Procedures The IUA layers needs to respond to various primitives it receives from other layers as well as messages it receives from the peer-to-peer messages. This section describes various procedures involved in response to these events. 3.1 Procedures to support service in section 1.4.1 These procedures achieve the IUA layer's "Transport of Q.921/Q.931 boundary" service. 3.1.1 Q.921 or Q.931 primitives procedures On receiving these primitives from the local layer, the IUA layer will send the corresponding QAUP message (Data, Unit Data, Establish, Release) to its peer. While doing so, the IUA layer needs to fill various fields of the common and specific headers correctly. In addition the message needs to be sent on the SCTP stream that corresponds to the D channel. 3.1.2 QAUP message procedures On receiving QAUP messages from a peer IUA layer, the IUA layer on an SG or MGC needs to invoke the corresponding layer primitives (DL-ESTABLISH, DL-DATA, DL-UNIT DATA, DL-RELEASE) to the local Q.921 or Q.931 layer. 3.2 Procedures to support service in section 1.4.2 These procedures achieve the IUA layer's "Support for Communication between Layer Managements" service. Kalla, Rengasami, Morneault, & Sidebottom [Page 24] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 3.2.1 Layer Management primitives procedures On receiving these primitives from the local layer, the IUA layer will send the corresponding MGMT message (TEI Status, Error) to its peer. While doing so, the IUA layer needs to fill various fields of the common and specific headers correctly. 3.2.2 MGMT message procedures On receiving MGMT messages the IUA layer needs to invoke the corresponding Layer Management primitives (MIUA-TEI STATUS, M-ERROR) to the local layer management. 3.3 Procedures to support service in section 1.4.3 These procedures achieve the IUA layer's "Support for management of active associations between SG and MGC" service. 3.3.1 State Maintenance The IUA layer on the SG needs to maintain the states of each ASP as well as the state of the AS. 3.3.1.1 ASP States The state of the each ASP, in each AS that it is configured, is maintained in the IUA layer on the SG. The state of an ASP changes due to events. The events include * Reception of messages from peer IUA layer at that ASP * Reception of some messages from the peer IUA layer at other ASPs in the AS * Reception of indications from SCTP layer * Switch-over Time triggers The ASP state transition diagram is shown in Figure 4. The possible states of an ASP are: ASP-DOWN: Application Server Process is unavailable and/or the SCTP association is down. Initially all ASPs will be in this state. ASP-UP: The remote IUA peer at the ASP is available (and the SCTP association is up) but application traffic is stopped. ASP-ACTIVE: The remote IUA peer at the ASP is available and application traffic is active. Kalla, Rengasami, Morneault, & Sidebottom [Page 25] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 ASP-ACT-OLD: The remote IUA peer at the ASP is available and application traffic is active, but for draining of current calls only (i.e., no new calls) ASP-ACT-NEW: The remote IUA peer at the ASP is available and application traffic is active, but for new calls only (i.e., not for traffic related to completing calls in another ASP). Figure 4: ASP State Transition Diagram +-------------+ |----------------------| | | ASPIA (GW)/ ____| ASP-ACTIVE |<--------\ | Some other / +-------------+ | | ASPAC (NT) / ^ | | Ts | / ASPAC | | ASPIA | | / (LS,OR) | | (LS,OR) | | V | v | | +-------------+ +-------------+ +-------------+ | | | | | | | | | ASP-ACT-OLD |----->| ASP-UP |------>| ASP-ACT-NEW | | +-------------+ Ts / +-------------+ ASPAC +-------------+ | | ASPIA ^ | (NT) | |<---| | | | | | | | ASPDN/ | ASPUP | | ASPDN/ | SCTP CDI | | | SCTP CDI | ASPDN/ | | v | SCTP | +-------------+ | CDI | | | | |------------------>| |<-------------| | ASP-DOWN | +-------------+ SCTP CDI: The local SCTP layer's Communication Down Indication to the Upper Layer Protocol (IUA) on an SG. The local SCTP will send this indication when it detects the loss of connectivity to the ASP's peer SCTP layer. Ts: Switch-over Time Triggers. This timer is configurable by the Operator on a per AS basis. LS : Value of Type parameter in ASPIA and ASPAC messages is equal to Load-share. OR : Value of Type parameter in ASPIA and ASPAC messages is equal to Over-ride. Kalla, Rengasami, Morneault, & Sidebottom [Page 26] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 NT : Value of Type parameter in ASPAC message is equal to New traffic. GW : Value of Type parameter in ASPIA message is equal to Graceful withdrawal. 3.3.1.2 AS States The state of the AS is maintained in the IUA layer on the SG. The state of an AS changes due to events. These events include: * ASP state transitions * Recovery timer triggers The possible states of an AS are: AS-DOWN: The Application Server is unavailable. This state implies that all related ASPs are in the ASP-DOWN state for this AS. Initially the AS will be in this state. AS-UP: The Application Server is available but no application traffic is active (i.e., one or more related ASPs are in the ASP-UP state, but none in the ASP-Active state). AS-ACTIVE: The Application Server is available and application traffic is active. This state implies that one ASP is in the ASP-ACTIVE state. AS-PENDING: An active ASP has transitioned from active to inactive or down and it was the last remaining active ASP in the AS. A recovery timer T(r) will be started and all incoming SCN messages will be queued by the SG. If an ASP becomes active before T(r) expires, the AS will move to AS-ACTIVE state and all the queued messages will be sent to the active ASP. If T(r) expires before an ASP becomes active, the SG stops queuing messages and discards all previously queued messages. The AS will move to AS-UP if at least one ASP is in ASP-UP state, otherwise it will move to AS-DOWN state. Kalla, Rengasami, Morneault, & Sidebottom [Page 27] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 Figure 5: AS State Transition Diagram +----------+ one ASP trans ACTIVE +-------------+ | |------------------------>| | | AS-UP | | AS-ACTIVE | | | | | | |< -| | +----------+ \ / +-------------+ ^ | \ Tr Trigger / ^ | | | \ at least one / | | | | \ ASP in UP / | | | | \ / | | | | \ / | | | | \ /---/ | | one ASP | | \ / one ASP | | Last ACTIVE ASP trans | | all ASP \-/----\ trans to | | trans to UP or to UP | | trans to / \ ACTIVE | | DOWN | | DOWN / \ | | | | / \ | | | | / \ | | | | /all ASP \ | | | v / trans to \ | v +----------+ / DOWN \ +-------------+ | |<--/ -| | | AS-DOWN | | AS-PENDING | | | | (queueing) | | |<------------------------| | +----------+ Tr Trigger no ASP +-------------+ in UP state Tr = Recovery Timer 3.3.2 ASPM procedures for primitives Before the establishment of an SCTP association the ASP state at both the SG and ASP is assumed to be "Down". As the ASP is responsible for initiating the setup of an SCTP association to an SG, the IUA layer at an ASP receives an M-SCTP ESTABLISH request primitive from the Layer Management, the IUA layer will try to establish an SCTP association with the remote IUA peer at an SG. Upon reception of an eventual SCTP-Communication Up confirm primitive from the SCTP, the IUA layer will invoke the primitive M-SCTP ESTABLISH confirm to the Layer Management. At the SG, the IUA layer will receive an SCTP Communication Up indication primitive from the SCTP. The IUA layer will then invoke the primitive M-SCTP ESTABLISH indication to the Layer Management. Kalla, Rengasami, Morneault, & Sidebottom [Page 28] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 Once the SCTP association is established, The IUA layer at an ASP will then find out the state of its local IUA-user from the Layer Management using the primitive M-ASP STATUS. Based on the status of the local IUA-User, the local ASP IUA Application Server Process Maintenance (ASPM) function will initiate the ASPM procedures, using the ASP-Up/-Down/-Active/-Inactive messages to convey the ASP-state to the SG - see Section 3.3.3. If the IUA layer subsequently receives an SCTP-Communication Down indication from the underlying SCTP layer, it will inform the Layer Management by invoking the M-SCTP STATUS indication primitive. The state of the ASP will be moved to "Down" at both the SG and ASP. At an ASP, the Layer Management may try to reestablish the SCTP association using M-SCTP ESTABLISH request primitive. 3.3.3 ASPM procedures for peer-to-peer messages All ASPM messages are sent on a sequenced stream to ensure ordering. SCTP stream 0 is used. 3.3.3.1 ASP-Up After an ASP has successfully established an SCTP association to an SG, the SG waits for the ASP to send an ASP-Up message, indicating that the ASP IUA peer is available. The ASP is always the initiator of the ASP-Up exchange. When an ASP-Up message is received at an SG and internally the ASP is not locked-out for local management reasons, the SG marks the remote ASP as Up. The SG responds with an Notify (ASP-Up) message to the ASP in acknowledgement. The SG sends a Notify (ASP-Up) message in response to a received ASP-Up message from the ASP even if the ASP is already marked as Up at the SG. If for any local reason the SG cannot respond with an ASP-Up, the SG responds to a ASP-Up with a ASP-Down message. At the ASP, the Notify (ASP-Up) message received from the SG is not acknowledged by the ASP. If the ASP does not receive a response from the SG, or an ASP-Down is received, the ASP may resend ASP-Up messages every 2 seconds until it receives a Notify (ASP-Up) message from the SG. The ASP may decide to reduce the frequency (say to every 5 seconds) if a Notify (ASP-Up) is not received after a few tries. The ASP must wait for the Notify (ASP-Up) message from the SG before sending any ASP traffic control messages (ASPAC or ASPIA) or Data messages or it will risk message loss. If the SG receives Data messages before an ASP Up is received, the SG should discard. Kalla, Rengasami, Morneault, & Sidebottom [Page 29] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 3.3.3.1.1 IUA Version Control If a ASP-Up message with an unsupported version is received, the receiving end responds with an Error message, indicating the version the receiving node supports. This is useful when protocol version upgrades are being performed in a network. A node upgraded to a newer version should support the older versions used on other nodes it is communicating with. Because ASPs initiate the ASP-Up procedure it is assumed that the Error message would normally come from the SG. 3.3.3.2 ASP-Down The ASP will send an ASP-Down to an SG when the ASP is to be removed from the list of ASPs in all Application Servers that it is a member. The SG marks the ASP as Down and returns an Notify (ASP-Down) message to the ASP if one of the following events occur: - an ASP-Down message is received from the ASP, - another ASPM message is received from the ASP and the SG has locked out the ASP for management reasons. The SG sends a Notify (ASP-Down) message in response to a received ASP-Down message from the ASP even if the ASP is already marked as Down at the SG. If the ASP does not receive a response from the SG, the ASP may send ASP-Down messages every 2 seconds until it receives a ASP-Down message from the SG or the SCTP association goes down. The ASP may decide to reduce the frequency (say to every 5 seconds) if an ASP-Down is not received after a few tries. 3.3.3.3 ASP-Active Any time after the ASP has received a Notify (ASP-Up) acknowledgement from the SG, the ASP sends an ASP-Active (ASPAC) to the SG indicating that the ASP is ready to start processing traffic. In the case where an ASP is configured/registered to process the traffic for more than one Application Server across an SCTP association, the ASPAC contains one or more Interface Identifiers to indicate for which Application Servers the ASPAC applies. When an ASP Active (ASPAC) message is received, the SG responds to the ASP with a Notify message acknowledging that the ASPAC was received and starts sending traffic for the associated Application Server(s) to that ASP. Kalla, Rengasami, Morneault, & Sidebottom [Page 30] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 There are three modes of Application Server traffic handling in the SG IUA - Over-ride, Load-balancing and New Traffic. The Type parameter in the ASPAC messge indicates the mode used in a particular Application Server. If the SG determines that the mode indicates in an ASPAC is incompatible with the traffic handling mode currently used in the AS, the SG responds with an Error message indicating Invalid Traffic Handling Mode. In the case of an Over-ride mode AS, reception of an ASPAC message at an SG causes the redirection of all traffic for the AS to the ASP which sent the ASPAC. Any previously active ASP in the AS is now considered Inactive and will no longer receive traffic within the AS. The SG responds to the ASPAC with a Notify (ASP-Active) message to the ASP. The SG sends a Notify (ASP-Inactive) to any previously active ASP in the AS. In the case of a Loadshare mode AS, reception of an ASPAC message at an SG causes the direction of traffic to the ASP sending the ASPAC, in addition to all the other ASPs that are currently active in the AS. The algorithm at the SG for loadsharing traffic within an AS to all the active ASPs is application and network dependent. The algorithm could, for example be round-robin or based on information in the Data message, such as Interface ID, depending on the requirements of the application and the call state handling assumptions of the collection of ASPs in the AS. The SG responds to the ASPAC with a Notify (ASP-Active) message to the ASP. In the case of a New Traffic mode AS, reception of an ASPAC message at an SG causes the direction of traffic to the ASP sending the ASPAC. However, traffic related to completing calls in another ASP is not sent to the new ASP (i.e., new calls only). How an SG accomplishes the differentiation of old and new calls and any loadsharing of traffic is application and implementation dependent. The SG responds to the ASPAC with a Notify (ASP-Active_New) message to the ASP. After a configurable time Ts, the ASP is moved to the ASP-Active state and a Notify (ASP-Active) is sent to the ASP. 3.3.3.4 ASP Inactive When an ASP wishes to withdraw from receiving traffic the ASP sends an ASP Inactive (ASPIA) to the SG. In the case where an ASP is configured/registered to process the traffic for more than one Application Server across an SCTP association, the ASPIA contains one or more Interface Ids to indicate for which Application Servers the ASPIA applies. Kalla, Rengasami, Morneault, & Sidebottom [Page 31] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 There are three modes of Application Server traffic handling in the SG IUA when withdrawing an ASP from service - Over-ride, Load-balancing and Graceful Withdrawal. The Type parameter in the ASPIA messge indicates the mode used in a particular Application Server. If the SG determines that the mode indicates in an ASPAC is incompatible with the traffic handling mode currently used in the AS, the SG responds with an Error message indicating Invalid Traffic Handling Mode. In the case of an Over-ride mode AS, where normally another ASP has already taken over the traffic within the AS with an Over-ride ASPAC, the ASP which sent the ASPIA is already considered by the SG to be Inactive. A Notify (ASP_Up) message is resent to the ASP. In the case of a Loadshare mode AS, the SG moves the ASP to the Inactive state and the AS traffic is re-allocated across the remaining active ASPs per the loadsharing algorithm currently used within the AS. A Notify (ASP-Up) message is sent to the ASP after the traffic is halted to the ASP. In the case of Graceful Withdrawal, the SG diverts all traffic related to new calls to other active ASPs and therafter sends only traffic related to incomplete calls to the ASP. A Notify (ASP-Act_Old) is sent to the ASP and the ASP is moved to the Active_Old state. When the outstanding calls are drained, or after a configurable time Ts, the SG moves the ASP to the Up state and sends a Notify (ASP-Up) message to the ASP. If no other ASPs are Active in the Application Server, the SG either discards all incoming messages (except messages related to an Active_Old ASP) for the AS or starts buffering the incoming messages for T(r)seconds after which messages will be discarded. T(r) is configurable by the network operator. If the SG receives an ASPAC from an ASP in the AS before expiry of T(r), the buffered traffic is directed to the ASP and the timer is cancelled. 4. Examples 4.1 Establishment of Association and Traffic between SGs and ASPs 4.1.1 Single ASP in an Application Server (1+0 sparing) This scenario shows the example IUA message flows for the establishment of traffic between an SG and an ASP, where only one ASP is configured within an AS (no backup). It is assumed that the SCTP association is already set-up. Kalla, Rengasami, Morneault, & Sidebottom [Page 32] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 SG ASP1 | |<---------ASP Up----------| |------NTFY (ASP-Up)------>| | - | |<-------ASP Active--------| |----NTFY (ASP_Active)---->| | | 4.1.2 Two ASPs in Application Server (1+1 sparing) This scenario shows the example IUA message flows for the establishment of traffic between an SG and two ASPs in the same Application Server, where ASP1 is configured to be Active and ASP2 a standby in the event of communication failure or the withdrawal from service of ASP1. ASP2 may act as a hot, warm, or cold standby depending on the extent to which ASP1 and ASP2 share call state or can communicate call state under failure/withdrawal events. The example message flow is the same whether the ASP-Active messages are Over-ride or Load-share mode although typically this example would use an Over-ride mode. SG ASP1 ASP2 | | | |<--------ASP Up----------| | |------NTFY (ASP-Up)----->| | | | | |<-----------------------------ASP Up----------------| |----------------------------NTFY (ASP-Up)---------->| | | | | | | |<-------ASP Active-------| | |----NTFY(ASP-Active)---->| | | | | 4.1.3 Two ASPs in an Application Server (1+1 sparing, load-sharing case) This scenario shows a similar case to Section 4.1.2 but where the two ASPs are brought to active and loadshare the traffic load. In this case, one ASP is sufficient to handle the total traffic load. Kalla, Rengasami, Morneault, & Sidebottom [Page 33] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 SG ASP1 ASP2 | | | |<---------ASP Up---------| | |-------NTFY(ASP-Up)----->| | | | | |<------------------------------ASP Up---------------| |-----------------------------NTFY(ASP Up)---------->| | | | | | | |<--ASP Active (Ldshr)----| | |----NTFY(ASP-Active)---->| | | | | |<----------------------------ASP Active (Ldshr)-----| |-----------------------------NTFY(ASP-Active)------>| | | | 4.1.4 Three ASPs in an Application Server (n+k sparing, load-sharing case) This scenario shows the example IUA message flows for the establishment of traffic between an SG and three ASPs in the same Application Server, where two of the ASPs are brought to active and share the load. In this case, a minimum of two ASPs are required to handle the total traffic load (2+1 sparing). SG ASP1 ASP2 ASP3 | | | | |<------ASP Up-------| | | |----NTFY(ASP-Up)--->| | | | | | | |<--------------------------ASP Up-------| | |------------------------NTFY(ASP-Up)--->| | | | | | |<---------------------------------------------ASP Up--------| |--------------------------------------------NTFY(ASP-Up)--->| | | | | | | | | |<-ASP Act. (Ldshr)--| | | |---NTFY(ASP-Act.)-->| | | | | | | |<--------------------ASP Act. (Ldshr)---| | |----------------------NTFY(ASP-Act.)--->| | | | | | 4.2 ASP Traffic Fail-over Examples Kalla, Rengasami, Morneault, & Sidebottom [Page 34] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 4.2.1 (1+1 Sparing, withdrawal of ASP, Back-up Over-ride) Following on from the example in Section 4.1.2, and ASP withdraws from service: SG ASP1 ASP2 | | | |<-----ASP Inactive-------| | |---NTFY(ASP Inactive)--->| | |--------------------NTFY(ASP-Inactive) (Optional)-->| | | | |<------------------------------ ASP Active----------| |-----------------------------NTFY(ASP-Active)------>| | | Note: If the SG detects loss of the IUA peer (IUA heartbeat loss or detection of SCTP failure), the initial SG-ASP1 ASP Inactive message exchange would not occur. 4.2.2 (1+1 Sparing, Back-up Over-ride) Following on from the example in Section 4.1.2, and ASP2 wishes to over-ride ASP1 and take over the traffic: SG ASP1 ASP2 | | | |<------------------------------ ASP Active----------| |-----------------------------NTFY(ASP-Active)------>| |----NTFY(ASP-Inactive)-->| | | | 4.2.3 (n+k Sparing, Load-sharing case, withdrawal of ASP) Following on from the example in Section 4.1.4, and ASP1 withdraws from service: SG ASP1 ASP2 ASP3 | | | | |<----ASP Inact.-----| | | |--NTFY(ASP-Inact.)->| | | | | | | |---------------------------------NTFY(Ins. ASPs)(Optional)->| | | | | |<-----------------------------------------ASP Act. (Ldshr)--| |-------------------------------------------ASP Act. (Ack)-->| | | | | Kalla, Rengasami, Morneault, & Sidebottom [Page 35] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 The Notify message to ASP3 is optional, as well as the ASP-Active from ASP3. The optional Notify can only occur if the SG maintains knowledge of the minimum ASP resources required for example if the SG knows that n+k = 2+1 for a loadshare AS and n currently equals 1. Note: If the SG detects loss of the ASP1 IUA peer (IUA heartbeat loss or detection of SCTP failure), the first SG-ASP1 ASP Inactive message exchange would not occur. 4.3 Q.921/Q.931 primitives backhaul Examples An example of the message flows for establishing a data link on a signalling channel, passing PDUs and releasing a data link on a signalling channel is shown below. An active association between MGC and SG is established (section 4.1) prior to the following message flows. SG MGC <----------- Establish Request Establish Response ----------> <----------- Data Request Data Indication -----------> <----------- Data Request Data Indication -----------> <----------- Data Request <----------- Data Request Data Indication -----------> <----------- Release Request (RELEASE_MGMT) Release Confirm ----------> An example of the message flows for a failed attempt to establish a data link on the signalling channel is shown below. In this case, the gateway has a problem with its physical connection (e.g. Red Alarm), so it cannot establish a data link on the signalling channel. Kalla, Rengasami, Morneault, & Sidebottom [Page 36] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 SG MGC <----------- Establish Request (ESTABLISH_START) Release Indication ----------> (RELEASE_PHYS) 4.4 Layer Management Communication Examples An example of the message flows for communication between Layer Management modules between SG and MGC is shown below. An active association between MGC and SG is established (section 4.1) prior to the following message flows. SG MGC <----------- Data Request Error ----------> (INVALID_TEI) <----------- TEI Status TEI Status ----------> 5.0 Security IUA is designed to carry signaling messages for telephony services. As such, IUA must involve the security needs of several parties: the end users of the services; the network providers and the applications involved. Additional requirements may come from local regulation. While having some overlapping security needs, any security solution should fulfill all of the different parties' needs. 5.1 Threats There is no quick fix, one-size-fits-all solution for security. As a transport protocol, IUA has the following security objectives: * Availability of reliable and timely user data transport. * Integrity of user data transport. * Confidentiality of user data. IUA runs on top of SCTP. SCTP [3] provides certain transport related security features, such as: * Blind Denial of Service Attacks * Flooding * Masquerade * Improper Monopolization of Services Kalla, Rengasami, Morneault, & Sidebottom [Page 37] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 When IUA is running in professionally managed corporate or service provider network, it is reasonable to expect that this network includes an appropriate security policy framework. The "Site Security Handbook" [6] should be consulted for guidance. When the network in which IUA runs in involves more than one party, it may not be reasonable to expect that all parties have implemented security in a sufficient manner. In such a case, it is recommended that IPSEC is used to ensure confidentiality of user payload. Consult [7] for more information on configuring IPSEC services. 5.2 Protecting Confidentiality Particularly for mobile users, the requirement for confidentiality may include the masking of IP addresses and ports. In this case application level encryption is not sufficient; IPSEC ESP should be used instead. Regardless of which level performs the encryption, the IPSEC ISAKMP service should be used for key management. 6.0 IANA Considerations A request will be made to IANA to assign an IUA value for the Payload Protocol Identifier in SCTP Payload Data chunk. The following SCTP Payload Protocol Identifier will be registered: IUA tbd The SCTP Payload Protocol Identifier is included in each SCTP Data chunk, to indicate which protocol the SCTP is carrying. This Payload Protocol Identifier is not directly used by SCTP but may be used by certain network entities to identify the type of information being carried in a Data chunk. The User Adaptation peer may use the Payload Protocol Identifier as a way of determining additional information about the data being presented to it by SCTP. 7.0 Acknowledgements The authors would like to thank Ming-te Chao, Keith Drage, Stephen Lorusso, John Loughney, Neil Olson, Norm Glaude, Michael Tuexen, Nikhil Jain, Dan Brendes and Heinz Prantner for their valuable comments and suggestions. Kalla, Rengasami, Morneault, & Sidebottom [Page 38] Internet Draft ISDN Q.921 User Adaptation Layer Mar 2000 8.0 References [1] ITU-T Recommendation Q.920, 'Digital Subscriber Signalling System No. 1 (DSS1) - ISDN User-Network Interface Data Link Layer - General Aspects' [2] T1S1.7/99-220 Contribution, 'Back-hauling of DSS1 protocol in a Voice over Packet Network' [3] Simple Control Transmission Protocol, draft-ietf-sigtran-sctp-07.txt, March 2000 [4] Media Gateway Control Protocol (MGCP), draft-huitema-megaco-mgcp-v1-03.txt, August 1999 [5] Architectural Framework for Signaling Transport, RFC 2719 , October 1999 [6] Site Security Handbook, RFC 2196, September 1997 [7] Security Architecture for the Internet Protocol, RFC 2401 9.0 Author's Addresses Malleswar Kalla Tel +1-973-829-5212 Telcordia Technologies EMail kalla@research.telcordia.com MCC 1J211R 445 South Street Morristown, NJ 07960 USA Selvam Rengasami Tel +1-732-758-5260 Telcordia Technologies EMail srengasa@telcordia.com NVC-2Z439 331 Newman Springs Rd Red Bank, NJ 07701 USA Ken Morneault Tel +1-703-484-3323 Cisco Systems Inc. EMail kmorneau@cisco.com 13615 Dulles Technology Drive Herndon, VA. 20171 USA Greg Sidebottom Tel +1-613-763-7305 Nortel Networks EMail gregside@nortelnetworks.com 3685 Richmond Rd, Nepean, Ontario Canada K2H5B7 Kalla, Rengasami, Morneault, & Sidebottom [Page 39]