C. Tappenden Internet Draft Alcatel Document: draft-tappenden-atm-aal2-package-00 Category: Informational Nov. 2001 Expires: April 2002 H.248 control of AAL2 CID Allocation Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. Abstract: This contribution proposes specific procedures that media gateways shall follow for H.248 Allocation of AAL2 CIDs. This proposal covers gateways using non-AAL2 signalling (no Q.2630.x) with a backward SPVC call setup mode. This contribution attempts to build upon existing RFCs and expand upon the expected behavior of Media Gateways when receiving H.248 messaging designed to set-up and tear down AAL2 connections. 1.0 Introduction: RFC3108 describes parameters that may be used within H.248 to transport AAL2 information between the various media gateways and media gateway controllers. However, it does not outline the expected behavior of the MG when it receives this information. Specifically, RFC3018 does not outline how the H.248 messaging should be tied with SPVC/SVC messaging for AAL2. Not doing so results in many different interpretations and solutions for handling H.248 control of AAL2 and the dynamic allocation/deallocation of multiple CIDs within an AAL2 connection by H.248. The wide variety of interpretations makes it highly unlikely that different vendor's solutions will interoperate. This contribution attempts to resolve some H.248 control of AAL2 CID Allocation Nov. 2001 of those interpretations by proposing procedures that the MG must follow in order to ensure interoperability between different vendor's implementations. This contribution makes no changes to RFC3108 or RFC3015 and does not require any modifications to existing SPVC standards. It merely attempts to clarify how the two sets of standards fit together to ensure a common reference point for vendor interoperability. 1.1 Scope This document proposes specific procedures that media gateways shall follow for H.248 Allocation of AAL2 CIDs. This proposal covers gateways using non-AAL2 signalling (no Q.2630.x) with a backward SPVC call setup mode. This contribution makes no attempt to cover gateways that use AAL2 signalling (Q.2630.x), SVCs or a forward call setup model. It is assumed that gateways using such schemes will not be compatible with gateways using a backward SPVC call setup model. 1.2 Architecture Overview This contribution assumes the same architecture as used in RFC3015 and RFC3108. It assumes that the MGC will not be aware of the underlying ATM network topology and will expect the MGs to negotiate such items as which CID or AAL2 connection to use. In addition, both MGs must support a backward SPVC call setup model. 2.0 Solution Overview With this solution, at the start of a voice call, the Originating Gateway shall select a VPI/VCI for any new SPVC connection and relay to the Terminating MG via H.248 message. The Terminating MG will use that information in setting up any new SPVC. The Terminating MG will select a CID for the chosen SPVC and relay the necessary information back to the Originating MG via H.248 messaging. At the Originating MG, information must be stored linking the SPVC connection and voice call with the incoming H.248 messaging. This shall be done by matching the Terminating MG's ATM address and VPI/VCI obtained through the H.248 Modify message's Remote Descriptor to the SPVC. Refer to the following call flow for more detail. 2.1 Call Flow This call flow details the expected Media Gateway behavior. In this example no connection exists or additional connections are required between the Originating and Terminating Gateways. The Originating Gateway identifies a unique AAL2 connection by using the Terminating MG's NSAP address with the terminating MG's VPI/VCI. 1) Add from MGC to Originating MG Tappenden Informational Page 2 H.248 control of AAL2 CID Allocation Nov. 2001 Add Local { C= ATM - - M = audio $/$/$/$ ... } 2) Add Reply from Originating MG to MGC Add Reply Local { C = ATM NSAP 0123456789... M = audio 1/1/2/1 AAL2/ITU 1 AAL2/custom 100 AAL2/ITU 2 } 3) Add from MGC to Terminating MG Add Local { C = ATM - - M = audio $/$/$/$ ... } Remote { C=ATM NSAP 0123456789... M=audio 1/1/2/1 AAL2/ITU 1 AAL2/custom 100 AAL2/ITU 2 } 4) Set-Up Message launched from Terminating MG to Originating MG Set-Up Message contains the Originating MG's VPI/VCI in the SPVC IE and the Terminating MG's NSAP Address in the Calling Party Number IE 5) Add Reply from Terminating MG to MGC Add Reply Local { C = ATM NSAP 987654321... M = audio 1/3/4/23 AAL2/ITU 1 } Remote { C=ATM NSAP 0123456789... M=audio 1/1/2/1 AAL2/ITU 1 AAL2/custom 100 AAL2/ITU 2 } 6) Modify from MGC to Originating MGC Local { C=ATM NSAP 0123456789... M=audio 1/1/2/1 AAL2/ITU 1 AAL2/custom 100 AAL2/ITU 2 } Remote { C = ATM NSAP 987654321... M = audio 1/3/4/23 AAL2/ITU 1 } 2.1.1 Description of call flow steps: 1) The MGC initiates a call setup using an ADD. 2) The Originating MG replies by sending connection information in the H.248 reply message including the VPI/VCI and NSAP address for any new Tappenden Informational Page 3 H.248 control of AAL2 CID Allocation Nov. 2001 SPVC to connect to. The VPI/VCI identifies the VPI/VCI on the Originating MG's side of the connection. 3) The MGC sends an ADD to the Terminating MG containing the information that the Originating MG sent. 4) The Terminating MG takes the information in the remote descriptor of the H.248 message and uses that information to a) determine that there is no available connection between the two gateways and b) set up a new SPVC connection to the Originating MG. In this case the Terminating MG will take the VPI/VCI sent by the Originating MG and place it into the Set-Up message's SPVC IE, the Terminating MG's NSAP address will be placed in the Set-Up message's Calling Party Number IE so that the Originating MG will know where the SPVC is coming from. This VPI/VCI information will be used to connect to the Originating MG on the Originating MG's side of the SPVC. The Terminating Gateway will route the call based on the ATM address specified by the Originating Gateway. The Terminating MG will also be expected to select the CID in the AAL2 connection to be used. 5) The Terminating Gateway will reply to the ADD message with parameters in the Local descriptor including the VPI/VCI information on the terminating MG's side of the SPVC and the NSAP address for the terminating MG, along with the CID being used 6) The Originating Gateway connects the voice call by identifying the SPVC being used by using the information in the Terminating Gateway's H.248 message. Specifically the Originating MG will identify the SPVC by matching the Terminating MG's NSAP to the Terminating MG's VPI/VCI. The Originating MG will obtain the Terminating MG's VPI/VCI through the Modify message's Remote descriptor. 3.0 Media Gateway Procedures To work successfully, both the Originating and Terminating Media Gateways must follow certain procedures outlined below. 3.1 Media Gateway Controller: The Media Gateway Controller shall use the following Virtual Connection ID format in the media descriptor when setting up and tearing down AAL2 calls using H.248. /// 3.2 Originating Media Gateway: The Originating MG will transfer information in the SDP parameters via H.248 signalling for use by the Terminating MG in setting up the SPVC. At the start of a voice call, the Originating Gateway will not be aware whether a new SPVC has to be set up for this voice call. Thus the Originating MG will select a VPI/VCI for any new SPVC connection and place that information in the media descriptor of the H.248 message. The CID variable will not be relevant at this point of the call. If the call requires a new SPVC to be set-up, the Originating MG will Tappenden Informational Page 4 H.248 control of AAL2 CID Allocation Nov. 2001 accept the incoming SPVC as per standard SPVC processing. The Originating MG will identify the uniqueness of the SPVC through the following two identifiers: 1) The VPI/VCI contained in the Set-Up Message's SPVC IE. This identifies the Originating MG's side of the ATM connection. 2) The ATM Address of the Originating MG contained in the Set-Up message's Called Party Number IE. These elements will allow the Originating MG to uniquely identify the SPVC and later map that SPVC to a voice call when the Originating MG receives an H.248 Modify message later in the voice call setup. Upon receiving an H.248 Modify message once the Terminating MG has set- up the SPVC, the originating MG will connect the voice call according to the information received in the H.248 Modify message. The originating MG will map the following elements of the Modify message to identify a particular SPVC. 1) The VPI/VCI information in the Local descriptor's Virtual Connection ID identifies the Originating MG's side of the ATM connection and maps to the VPI/VCI in the Set-Up message's SPVC IE. If no new SPVC connection was set-up then this information will not be needed. 2) The VPI/VCI information in the Remote descriptor's Virtual Connection ID identifies the Terminating MG's side of the ATM connection. This information is not sent in the SPVC Set-Up message but will be needed to identify additional voice calls that will be added to an existing AAL2 connection. 3) The ATM Address in the Remote Descriptor's Connection Information Line identifies the Terminating MG's ATM address and maps to the Set-Up message's Calling Party Number IE. The Originating MG will need to know the ATM Address of the Terminating MG as well as the VPI/VCI on the Terminating MG's side of the SPVC connection. Thus to tie the SPVC with an incoming H.248 message, the Originating MG will need to match the Originating VPI/VCI with the Terminating MG's ATM address and VPI/VCI once a SPVC is initially established. After that point, the Originating MG only needs to use the Terminating MG's ATM address and VPI/VCI, which is obtained from the H.248 Modify message's Remote descriptor. The Modify message will also identify the CID of the channel the voice call is using. This CID shall be contained within the Virtual Connection ID of the Remote Descriptor. The Originating MG will connect the voice call to the identified SPVC using this CID. 3.3 Terminating Media Gateway: The Terminating MG will be expected to recognize whether a SPVC has been set up between the Terminating and Originating MGs. In addition, the Terminating MG will be expected to know whether any connections between the Originating and Terminating MG have additional capacity for another channel. The Terminating Gateway may be in one of two states: Connected, Not- Connected. Tappenden Informational Page 5 H.248 control of AAL2 CID Allocation Nov. 2001 3.3.1 Connected State: In this state a connection exists between the Terminating and Originating Gateway. This existing connection also has the capability to have an additional voice call added to it. In this case the Terminating Gateway will designate an available CID to be used in that connection for the voice call. The Terminating Gateway will signal that CID to the Originating Gateway in the Add Reply via the following use of H.248 messaging: 1) The VPI/VCI of the Terminating MG's side of the ATM connection will be placed in the Local Descriptor's Media Descriptor. 2) The CID identified for use by the Terminating MG will be placed in the Local Descriptor's Virtual Connection ID. 3) The Terminating MG's ATM Address will be placed in the Local Descriptor's Connection Information Line. In this case the Terminating Gateway shall ignore the connection information sent by the Originating Gateway. The Terminating MG will also always send its ATM address and VPI/VCI in any H.248 message so that the Originating MG will be able to identify the SPVC for the voice call. 3.3.2 Not-Connected State: In this state the Terminating Gateway either does not have a connection to the Originating Gateway or if it does then that connection is not available for use (e.g. due to bandwidth constraints) and thus a new SPVC must be set up between the two gateways. If there is no available connection between the Terminating and Originating Gateway, the Terminating Gateway shall set-up an SPVC connection with the originating Gateway based on the information sent by the Originating Gateway. The following shall occur when the SPVC is set-up: 1) The VPI/VCI contained in the Remote descriptor's Virtual Connection ID shall be placed in the Set-Up message's SPVC IE. This identifies the Originating MG's side of the ATM connection and will be used in establishing the SPVC at the Originating MG. 2) The ATM Address of the Terminating MG shall be placed in the Set-Up message's Calling Party Number IE. The Terminating Gateway will designate an available CID to be used in that connection for the voice call. The Terminating Gateway shall signal that CID to the Originating Gateway in the Add Reply via the following use of H.248 messaging: 1) The VPI/VCI of the Terminating MG's side of the ATM connection will be placed in the Local Descriptor's Media Descriptor. 2) The CID identified for use by the Terminating MG will be placed in the Local Descriptor's Virtual Connection ID. 3) The Terminating MG's ATM Address (from the Set-Up message's Calling Party Number IE) will be placed in the Local Descriptor's Connection Information Line. Disconnecting calls shall follow normal procedures. 4.0 Security Considerations Not sure what statement should be made here. Tappenden Informational Page 6 H.248 control of AAL2 CID Allocation Nov. 2001 5.0 References: 1) Kumar, R. and Mostafa, M., " Conventions for the use of the Session Description Protocol (SDP) for ATM Bearer Connections", RFC 3108, May 2001. 2) Cuervo, F., Greene, N., Rayhan, A., Huitema, C., Rosen, B. and J. Segers, "Megaco Protocol Version 1.0", RFC 3015, November 2000. 6.0 Author's Address: Chris Tappenden Alcatel 600 March Road Ottawa, Ontario Phone: +1 613 784 1104 Canada K2K 2E6 Email: chris.tappenden@alcatel.com Full Copyright Statment "Copyright (C) The Internet Society (date). All Rights Reserved. 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