Network Working Group Tom George INTERNET-DRAFT Alcatel Ram Dantu Cisco Systems Malleswar Kalla Telcordia Hanns Juergen Schwarzbauer Siemens Greg Sidebottom
Nortel NetworksKen Morneault Cisco Systems Expires September 2001 March 2,January 2002 July 20, 2001 SS7 MTP2-User Peer-to-Peer Adaptation Layer <draft-ietf-sigtran-m2pa-02.txt><draft-ietf-sigtran-m2pa-03.txt> 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- DraftsInternet-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 supporting the transport of SS7 MTPSignaling System Number 7 (SS7) Message Transfer Part (MTP) Layer 3 signaling messages over IPInternet Protocol (IP) using the services of the Stream Control Transmission Protocol (SCTP). This protocol would be used between SS7 Signaling Points employing the MTP Level 3 protocol. The SS7 Signaling Points may also employ standard SS7 links using the SS7 Message Transfer Part (MTP)MTP Layer 2 to provide transport of MTP Layer 3 signaling messages. TABLE OF CONTENTS 1. Introduction............................................. 4 1.1 Scope................................................. 4 1.2 Terminology........................................... 4 1.3 Abbreviations......................................... 5 1.4 Conventions........................................... 6 1.5 Signaling Transport Architecture...................... 6 1.6 Services Provided by M2PA............................. 78 1.7 Functions Provided by M2PA............................ 89 1.8 Definition of the M2PA Boundaries..................... 9Boundaries.....................10 1.9 Differences Between M2PA and M2UA.....................11M2UA.....................12 2. Protocol Elements........................................13Elements........................................14 2.1 Common Message Header.................................13Header.................................14 2.2 M2PA Messages.........................................14Messages.........................................16 3. M2PA Link States.........................................16State Control..................................19 4. Procedures...............................................19Procedures...............................................22 4.1 Procedures to Support MTP2 Features...................19Features...................22 4.2 Procedures to Support the MTP3/MTP2 Interface.........26Interface.........31 5. Examples of M2PA Procedures..............................31Procedures..............................36 5.1 Link Initialization (Alignment).......................31(Alignment).......................36 5.2 Message Transmission and Reception....................33Reception....................39 5.3 Link Status Indication................................33Indication................................39 5.4 Link Status Message (Processor Outage)................34Outage)................40 5.5 Level 2 Flow Control..................................35Control..................................41 5.6 MTP3 Signaling Link Congestion........................37Congestion........................43 5.7 Link Deactivation.....................................38Deactivation.....................................44 5.8 Link Changeover.......................................39Changeover.......................................45 6. Security.................................................41Security.................................................47 6.1 Threats...............................................47 6.2 Protecting Confidentiality............................47 7. IANA Considerations......................................42Considerations......................................48 7.1 SCTP Payload Protocol Identifier......................48 7.2 M2PA Protocol Extensions..............................48 8. Acknowledgements.........................................42Acknowledgements.........................................49 9. References...............................................43References...............................................50 10. Author's Addresses.......................................44Authors' Addresses.......................................51 1. Introduction 1.1 Scope There is a need for Switched Circuit Network (SCN) signaling protocol delivery over an IP network. This includes delivery frommessage transfer between the following: - a SignallingSignaling Gateway (SG) toand a Media Gateway Controller (MGC) or - a SG and an IP Signaling Point, as described in the Framework Architecture for Signalling Transport .Point (IPSP) - an IPSP and an IPSP This could allow for convergence of some signaling and data networks. SCN signaling nodes would have access to databases and other devices in the IP network domain that do not employ SS7 signaling links. Likewise, IP telephony applications would have access to SS7 services. There may also be operational cost and performance advantages when traditional signaling links are replaced by IP network "connections". The delivery mechanism described in this document allows for full MTP3 message handling and network management capabilities between any two SS7 nodes, communicating over an IP network. An SS7 node equipped with an IP network connection is called an IP Signaling Point (IPSP). The IPSPs function as traditional SS7 nodes using the IP network instead of SS7 links. The delivery mechanism should - Support seamless operation of MTP3 protocol peers over an IP network connection. - Support the MTP Level 2 / MTP Level 3 interface boundary. - Support management of SCTP transport associations and traffic instead of MTP2 Links. - Support asynchronous reporting of status changes to management. 1.2 Terminology MTP - The Message Transfer Part of the SS7 protocol . . MTP2 - MTP Level 2, the MTP signaling link layer. MTP3 - MTP Level 3, the MTP signaling network layer. MTP2-User - A protocol that normally uses the services of MTP Level 2. The only MTP2 user is MTP3. The MTP2 user is equivalent to the M2PA user. Signaling End Point (SEP) - A node in an SS7 network that originates or terminates signaling messages. One example is a central office switch. IP Signaling Point (IPSP) - An SS7 Signaling Point with an IP network connection used for SS7 over IP. Signaling Gateway (SG) - A signaling agent that receives/sends SCN native signaling at the edge of the IP network . In this context, an SG is an SS7 Signaling Point that has both an IP network connection used for SS7 over IP, and a traditional (non-IP) link to an SS7 network. Signaling Transfer Point (STP) - A node in an SS7 network that routes signaling messages based on their destination point code in the SS7 network. Association - An association refers to a SCTP association . The association provides the transport for MTP3 protocol data units and M2PA adaptation layer peer messages. Network Byte Order - Most significant byte first, also known as "Big Endian". See , Appendix B Data Transmission Order. Stream - A stream refers to a SCTP stream . 1.3 Abbreviations BSNT - Backward Sequence Number to be Transmitted FSNC - Forward Sequence Number of last message accepted by remote level 2 LI - Length Indicator MSU - Message Signal Unit SCCP - Signaling Connection Control Part SCN - Switched Circuit Network SCTP - Stream Control Transmission Protocol SIF - Signaling Information Field SIO - Service Information Octet SLC - Signaling Link Code SS7 - Signaling System Number 7 SSN - Stream Sequence Number STP - Signal Transfer Point 1.4 Conventions The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in . 1.5 Signaling Transport Architecture The architecture that has been defined  for Switched Circuit Network (SCN) signaling transport over IP uses multiple components, including an IP transport protocol, the Stream Control Transmission Protocol (SCTP), and an adaptation module to support the services expected by a particular SCN signaling protocol from its underlying protocol layer. Within this framework architecture, this document defines an SCN adaptation module that is suitable for the transport of SS7 MTP3 messages. Figure 1 shows the seamless interworking at the MTP3 layer. MTP3 is adapted to the SCTP layer using the MTP2 User Peer-to-peer Adaptation Layer (M2PA). All the primitives between MTP3 and MTP2 are supported by M2PA. The SCTP association acts as one SS7 link between the IPSPs. An IPSP may have the Signaling Connection Control Part (SCCP) and other SS7 layers above MTP3. ******** IP ******** * IPSP *--------* IPSP * ******** ******** +------+ +------+ | TCAP | | TCAP | +------+ +------+ | SCCP | | SCCP | +------+ +------+ | MTP3 | | MTP3 | +------+ +------+ | M2PA | | M2PA | +------+ +------+ | SCTP | | SCTP | +------+ +------+ | IP | | IP | +------+ +------+ IP - Internet Protocol IPSP - IP Signaling Point SCTP - Stream Control Transmission Protocol (see Reference ) Figure 1: M2PA Symmetrical Peer-to-Peer Architecture An IPSP may have SCCP and other SS7 layers above MTP3.Figure 2 shows an example. Theexample of M2PA used in a Signaling Gateway (SG). The SG is an IPSP equipped with both traditional SS7 and IP network connections. In effect, the Signaling Gateway acts as an STP.a Signal Transfer Point (STP). Any of the nodes in the diagram could have SCCP or other SS7 layers. STPs may or may not be present in the SS7 path between the SEP and the SG. ******** SS7 *************** IP ******** * SEP *--------* SG *--------* IPSP * ******** *************** ******** +------+ +------+ | TCAP | | TCAP | +------+ +------+ | SCCP | | SCCP | +------+ +-------------+ +------+ | MTP3 | | MTP3 | | MTP3 | +------+ +------+------+ +------+ | MTP2 | | MTP2 | M2PA | | M2PA | +------+ +------+------+ +------+ | MTP1 | | MTP1 | SCTP | | SCTP | | | | +------+ +------+ | | | | IP | | IP | +------+ +------+------+ +------+ SEP - SS7 Signaling Endpoint Figure 2: M2PA in IP Signaling Gateway Figure 2 is only an example. Other configurations are possible. For example, IPSPs without traditional SS7 links could use the protocol layers MTP3/M2PA/SCTP/IP to route SS7 messages in a network with all IP links. Another example, related to Figure 2, is that two SGs could be connected over an IP network to form an SG mated pair similar to the way STPs are provisioned in traditional SS7 networks. 1.5.1 Point Code Representation The MTP specification requires that each node with an MTP3 layer is representedidentified by an SS7 point code. In particular, each IPSP must have its own SS7 point code. 1.6 Services Provided by M2PA The SS7 MTP3/MTP2 (MTP2-User) interface is retained in the IPSP. The M2PA protocol layer is required to provide the equivalent set of services to its user as provided by MTP Level 2 to MTP Level 3. These services are described in the following subsections. 1.6.1 Support for MTP Level 2 / MTP Level 3 interface boundary This interface is the same as the MTP2/MTP3 interface described in ,  and , with the addition of support for larger sequence numbers in  and . Because SCTP uses larger sequence numbers than MTP, the MTP3 Changeover procedure must use the Extended Changeover Order and Extended Changeover Acknowledgment messages described in . and . This will allow for use of the SCTP stream sequence numbers in the changeover messages. Also, the following MTP3/MTP2 primitives must use the larger sequence numbers: - BSNT IndicationConfirmation - Retrieval Request and FSNC 1.6.2 Support for peer-to-peer communication In SS7, MTP Level 2 sends three types of messages, known as signal units: Message Signal Units (MSUs), Link Status Signal Units (LSSUs), and Fill-In Signal Units (FISUs). MSUs originate at a higher level than MTP2, and are destined for a peer at another node. Likewise, M2PA passes these messages from MTP3 to SCTP as data for transport across a link. These are called User Data messages in M2PA. LSSUs allow peer MTP2 layers to exchange status information. Analogous messages are needed for M2PA. The Link Status message serves this purpose. FISUs are sent when no other signal units are waiting to be sent. This purpose is served by the heartbeat messages in SCTP. FISUs also carry acknowledgment of messages. This function is performed by SCTP. Therefore, it is unnecessary for M2PA to provide a protocol data unit like the FISU. 1.7 Functions Provided by M2PA 1.7.1 Support of MTP3/MTP2 Primitives M2PA receives the primitives sent from MTP3 to its lower layer. M2PA processes these primitives or maps them to appropriate primitives at the M2PA/SCTP interface. Likewise, M2PA sends primitives to MTP3 like those used in the MTP3/MTP2 interface. 1.7.2 MTP2 Functionality M2PA provides MTP2 functionality that is not provided by SCTP. This includes - Data retrieval to support the MTP3 changeover procedure - Reporting of link status changes to MTP3 - Processor outage procedure - Link alignment procedure 1.7.3 Mapping of SS7 and IP Entities For each IP link, theThe M2PA layer must maintain a map of theeach of its SS7 linklinks to its SCTP association and itsthe corresponding IP destination.SCTP association. 1.7.4 SCTP Stream Management SCTP allows a user-specified number of streams to be opened during the initialization. It is the responsibility of the M2PA layer to ensure proper management of the streams allowed within each association. M2PA uses two streams in each direction infor each association. Stream 0 in each direction is designated for Link Status messages. Stream 1 is designated for User Data and Proving Data messages. Separating the Link Status and User Data messages onto separate stream allows M2PA to prioritize the messages in a manner similar to MTP2. 1.7.5 Retention of MTP3 in the SS7 Network M2PA allows MTP3 to perform all of its Message Handling and Network Management functions with IPSPs as with other SS7 nodes. 1.8 Definition of the M2PA Boundaries 1.8.1 Definition of the M2PA / MTP Level 3 boundary The upper layer primitives provided by M2PA are the same as those provided by MTP2 to MTP3. These primitives are described in , , and . Following is a list of the primitives. Primitives sent from MTP3 to M2PA: Data Request - Used to send a Data Message for transmission. Start Request - Used to establishactivate a link. Stop Request - Used to releasedeactivate a link. Retrieve BSNT Request - Request the BSNT for the changeover procedure. Retrieval Request and FSNC - Request retrieval of unacknowledged and unsent messages. This request includes the FSNC received from the remote end. Local Processor Outage Request - Informs M2PA of a local processor outage condition. Local Processor Outage Recovered Request - Informs M2PA that a local processor outage condition has ceased. Flush Buffers Request - Requests that all transmit and receive buffers be emptied. Continue Request - Requests that processing resume after a processor outage. Emergency Request - This is ignored by M2PA.Requests that M2PA use the emergency alignment procedure. Emergency Ceases Request - This is ignored by M2PA.Requests that M2PA use the normal alignment procedure. Primitives sent from M2PA to MTP3: Data Indication - Used to deliver received Data Message to MTP3. Congestion Indication - Indicates change in congestion level.status. The indication includes the congestion level,status, if the protocol is using the optional congestion levels. The indication also includes the discard level.status. In Service Indication - Indicates that the link is in service. Out of Service Indication - Indicates that the link is out of service. Retrieved Messages Indication - Indicates delivery of unacknowledged and unsent messages. Retrieval Complete Indication - Indicates that delivery of unacknowledged and unsent messages is complete. BSNT Confirm - Replies to the BSNT Request. The confirmation includes the BSNT. BSNT Not Retrievable Confirm - Replies to the BSNT Request when the BSNT cannot be determined. Remote Processor Outage Indication - Indicates processor outage at remote end. Remote Processor Recovered Indication - Indicates recovery from processor outage at remote end. 1.8.2 Definition of the Lower Layer Boundary between M2PA and SCTP The upper layer primitives provided by SCTP are described in Reference  Section 10 "Interface with Upper Layer". 1.9 Differences Between M2PA and M2UA The MTP2 User Adaptation Layer (M2UA)  also adapts the MTP3 layer to the SCTP/IP stack. It does so through a backhauling architecture . This section intends to clarify some of the differences between the M2PA and M2UA approaches. A possible M2PA architecture is shown in Figure 3. Here the IPSP's MTP3 uses its underlying M2PA as a replacement for MTP2. CommmunicationCommunication between the two layers MTP3/M2PA is defined by the same primitives as in SS7 MTP3/MTP2. M2PA performs functions similar to MTP2. A comparable architecture for M2UA is shown in Figure 4. In M2UA, the MGC's MTP3 uses the SG's MTP2 as its lower SS7 layer. InLikewise, the SG's MTP2 uses the MGC's MTP3 as its upper SS7 layer. In SS7, commmunicationcommunication between the MTP3 and MTP2 layers is defined by primitives. In M2UA, the MTP3/MTP2 communication is defined as M2UA messages and sent over the IP connection. ******** SS7 *************** IP ******** * SEP *--------* SG *--------* IPSP * ******** *************** ******** +------+ +-------------+ +------+ | SCCP | | SCCP | | SCCP | +------+ +-------------+ +------+ | MTP3 | | MTP3 | | MTP3 | +------+ +------+------+ +------+ | MTP2 | | MTP2 | M2PA | | M2PA | +------+ +------+------+ +------+ | MTP1 | | MTP1 | SCTP | | SCTP | | | | +------+ +------+ | | | | IP | | IP | +------+ +------+------+ +------+ Figure 3: M2PA in IP Signaling Gateway ******** SS7 *************** IP ******** * SEP *--------* SG *--------* MGC * ******** *************** ******** +------+ +------+ | SCCP | | SCCP | +------+ +------+ | MTP3 | (NIF) | MTP3 | +------+ +------+------+ +------+ | MTP2 | | MTP2 | M2UA | | M2UA | +------+ +------+------+ +------+ | MTP1 | | MTP1 | SCTP | | SCTP | | | | +------+ +------+ | | | | IP | | IP | +------+ +------+------+ +------+ NIF - Nodal Interworking Function Figure 4: M2UA in IP Signaling Gateway M2PA and M2UA are similar in that: a. Both transport MTP3 data messages. b. Both present an MTP2 upper interface to MTP3. Differences between M2PA and M2UA include: a. M2PA: IPSP processes MTP3-to-MTP2MTP3/MTP2 primitives. M2UA: MGC transports MTP3-to-MTP2MTP3/MTP2 primitives tobetween the SG's MTP2 and the MGC's MTP3 (via the NIF) for processing. b. M2PA: SG-IPSP connection is an SS7 link. M2UA: SG-MGC connection is not an SS7 link. It is an extension of MTP to a remote entity. c. M2PA: SG is an SS7 node with a point code. M2UA: SG is not an SS7 node and has no point code. d. M2PA: SG can have upper SS7 layers, e.g., SCCP. M2UA: SG does not have upper SS7 layers since it has no MTP3. e. M2PA: relies on MTP3 for management procedures. M2UA: uses M2UA management procedures. Potential users of M2PA and M2UA should be aware of these differences when deciding how to use them for SS7 signaling transport over IP networks. 2. Protocol Elements This section describes the format of various messages used in this protocol. All fields in an M2PA message must be transmitted in the network byte order, i.e., most significant byte first, unless otherwise stated. 2.1 Common Message Header The protocol messages for M2PA require a message header structure which contains a version, message type and message length. This message header is common among all SCN adaptation layers.The header structure is shown in Figure 5. 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 Class | Message Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |Figure 5: Common Message Header 2.1.1 Version The version field contains the version of the M2PA adapation layer.M2PA. The supported versions are: 01Value Version ----- ------- 1 Release 1.0 of M2PA protocol 2.1.2 Message TypeSpare The valid message types are defined belowSpare field SHOULD be set to all zeroes (0's) by the sender and ignored by the message contentsreceiver. The Spare field SHOULD NOT be used for proprietary information. 2.1.3 Message Class The following List contains the valid Message Classes: Value (decimal) Message Class --------- ------------- 11 M2PA Messages Other values are described in Section 2.2. Each message can contain parameters.invalid for M2PA. 2.1.4 Message Type The following list contains the message types for the defined messages. MTP2 User Adaptatation Messages TypeValue (Hex)Message Type ----- ------------ 1 User Data 06012 Link Status 0602 2.1.33 Proving Data Other values are invalid. 2.1.4 Message Length The Message lengthLength defines the length of the message in octets, notincluding the header.Common Header. 2.2 M2PA Messages The following section defines the messages and parameter contents. An M2PA message consists of a Common Message Header followed by the data appropriate to the message. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Common Message Header | ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Message Data | ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2.2.1 User Data The User Data is the data sent from the MTP3 in the form of the contiguous LI, SIO, and SIF octets of the MSU ( Q.703, section 2.2 Signal Unit Format). The LI octet includes the two undefined bits between the SIO and LI fields.MTP3. The format for the User Data 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Data | ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NoThe Data field contains the following fields of the MTP Message Signal Unit (MSU): - Length Indicator (LI), including the two undefined bits between the SIO and LI fields. - Service Information Octet (SIO) - Signaling Information Field (SIF) The MTP MSU described in  Q.703, section 2.2 Signal Unit Format, and  T1.111.3 section 2.2 Signal Unit Format. M2PA does not add padding is addedto the MTP3 message. Note that the Data field contains only the LI, SIF, and SIO octets. TheSHALL NOT contain other components of the MTP2MTP MSU format (the Flag, BSN, BIB, FSN, FIB, CK) are not included in M2PA. It is not necessary to put the message length in theformat: - Flag - Backward Sequence Number (BSN) - Backward Indicator Bit (BIB) - Forward Sequence Number (FSN) - Forward Indicator Bit (FIB) - Check bits (CK) The Data field SHALL be transmitted in the byte order as defined by MTP3. It is not necessary to put the message length in the LI octet as in MTP2. The LI octet is included because the two spare bits in the LI octet are used by MTP3 in aat least one national version of SS7 to carry MTP3 information. For example, the Japan TTC standard uses these spare bits as aan MTP3 Message Priority field. See , section 14 "Common Characteristics of message signal unit formats", section 14.2 (A) Priority Indicator (PRI). For versions of MTP that do not use these two bits, the entire octet is spare. Therefore in M2PA the format of the LI octet is: 0 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | spare |PRI| (followed by SIO, SIF) +-+-+-+-+-+-+-+-+ PRI - Priority used only in national MTP defined in . Spare for other MTP versions. Since the LI octet is not used for a message length, there is no need to support the expanded LI field in , Q.703 Annex A. Therefore the LI field in M2PA is always one octet. Note: In the SS7 Recommendations, the format of the messages and fields within the messages are based on bit transmission order. In these recommendations the Least Significant Bit (LSB) of each field is positioned to the right. The received SS7 fields are populated octet by octet as received into the 4-octet word as shown below. As an example, in the ANSI MTP protocol, the Data field format is shown below: |MSB---------------------------------------------------------LSB| 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | spare |PRI| SIO | SIF octet | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ : \ / : / \ : \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | ... | ... | SIF octet | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Within each octet the Least Significant Bit (LSB) per the SS7 Recommendations is to the right (e.g., bit 15 of SIO is the LSB). 2.2.2 Link Status The MTP2 Link Status message can be sent between M2PA peers to indicate link status. This message performs a function similar to the the Link Status Signal Unit in MTP2. Except as modified later in this section, the format for the Link Status 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | State | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The valid values for State are shown in the following table. Value (decimal) Description --------- ----------- 1 In ServiceAlignment 2 Proving Normal 3 Proving Emergency 4 Ready 5 Processor Outage 36 Processor Outage Ended 47 Busy 58 Busy Ended 3. M2PA Link States2.2.3 Proving Data The M2PA link moves from one state to another in response to various events.Proving Data message is used during the proving period. The format for the 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Data | ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ It is recommended that the length of the Data field be similar to the size of the User Data messages that will be carried on the link. It is recommended that the Data field contain a number pattern which varies among the Proving Data messages, and that will allow the SCTP checksum to be used to verify the accuracy of transmission. 3. M2PA Link State Control The M2PA link moves from one state to another in response to various events. The events that may result in a change of state include: - MTP3 primitive requests - SCTP notifications - Receipt of Status messages from the peer M2PA - Expiration of certain timers Figure 6 illustrates state changes together with the causing events. Note that some of the error conditions are not shown in the state diagram. Following is a list of the M2PA Link States and a description of each. IDLE - State of the link during power-up initialization. OOS - Out Of Service. Power-up initialization is complete. AIP - Alignment In Progress. M2PA is attempting to establish SCTP association. INS-LOCALexchange Alignment messages with its peer. PROVING - In Service Local. AssociationM2PA is established.sending Proving Data messages to its peer. ALIGNED READY - Proving is complete. M2PA is waiting foruntil peer to verify that it is In Service.completes proving. INS - In Service. Link is ready for traffic. RETRIEVAL - Link no longer carries traffic. M2PA is waiting for request for message retrieval from MTP3. +-----------+ | IDLE | +-----------+ | | Power On | | +--------------------------+ | | (Associate) | V V | +-----------+ +------->|| +------>| OOS |<---------------------+ ^|<--+ | | +-----------+ | Link Configured | | | | | Server AND(Associate) | | | +-----+ | | Client AND| | MTP3 Start| | MTP3 Start | | MTP3 Stop | | | | VV | | +-----------+ | | |+<------| AIP |--------------------->+|----------------------->+ | +-----------+ SCTP Comm Error | | | OR SCTP Comm Lost | | | OR T1 Expiry | | | | | | Receive LS Alignment | | | OR LS Proving | | MTP3 Stop | | | ORV | | +-----------+ | +<------| PROVING |----------------------->+ | +-----------+ SCTP Comm Error | | | OR SCTP Comm Lost | | | | | | SCTP Comm UpT2 Expiry | | MTP3 Stop | | | V | | +-----------+ | +<------| ALIGNED | | INS-LOCAL |--------------------->+| READY |----------------------->+ +-----------+ MTP3 Stop| | ORSCTP Comm Error | | OR SCTP Comm Lost | | OR T1T3 Expiry | | | | Link StatusReceive LS Proving Complete | | In Service receivedOR Receive User Data | | | V | +-----------+ | | INS | | +-----------+ | | Retrieval| | | | | | | | | | MTP3 Stop Complete| | OR SCTP Comm Error OR| | OR SCTP Comm Lost MTP3 Start| | OR T6 Expiry | | | V | +-----------+ +<-------| RETRIEVAL| | RETRIEVAL |----------------------->+ +-----------+ Retrieval Complete OR MTP3 Start Figure 6: M2PA Link State Transition Diagram.Diagram Figure 7 illustrates state changes in the M2PA management of the SCTP association together with the causing events. Note that some of the error conditions are not shown in the state diagram. Following is a list of the M2PA Association States and a description of each. IDLE - State of the association during power-up initialization. ASSOCIATE - M2PA is attempting to establish an SCTP association. ESTABLISHED - SCTP association is established. +-----------+ +------------------->| IDLE | | +-----------+ | | | (Issue SCTP | Associate | Abort) | (Issue SCTP associate) | | | | +----------------------+ | | | (Issue SCTP | | V V associate) | | Abort +-----------+ | +<-------------------| ASSOCIATE |------------------->+ | +-----------+ SCTP Comm Error | | | | | | | | | SCTP Comm Up | | | | | V | | Abort +-------------+ | +<-------------------| ESTABLISHED |----------------->+ +-------------+ SCTP Comm Error OR SCTP Comm Lost Figure 7: M2PA Association State Transition Diagram 4. Procedures 4.1 Procedures to Support MTP2 Features 4.1.1 Signal Unit Format, Delimitation, Acceptance Messages for transmission across the network must follow the format described in section 2. SCTP provides reliable, in-sequence delivery. Therefore the related functionality of MTP2 is not needed. SCTP does not provide functions related to Link State Control in MTP2. These functions must be provided by M2PA. 4.1.2 Link Alignment Link alignment includes the establishment of an SCTP associationMTP and a handshaking procedure between theSCTP Entities This section describes how M2PA peers to verify that the association is ready to be used as a link.relates MTP and SCTP entities. To prevent duplicate associations from being established, it must be decided in advance which endpoint initiates the establishment of the association. In a pair of endpoints, the endpoint that initiates the establishment of the association is called the client. The other endpoint is the server. An endpoint may be a client in its relationship with one endpoint, and a server in its relationship with another endpoint. The designations of client and server are needed only to decide which endpoint initiates the establishment of the association. After that, the endpoints function as peers. The client initiates the association using the server's IP address and the M2PA well-known port number as the destination endpoint. In orderIf only one association is to allow forbe established between these two IP addresses, then the client should use its own IP address and the M2PA well-known port number as the source endpoint. If it is desirable to create multiple linksassociations (for multiple links) between the two endpoints,IP addresses, the client uses a different local port number for each link. It must be decided in advance which local ports are used by the client. Each of theseassociation. The client ports must be knownM2PA should establish the association for a link when the link is configured for operation by MTP signaling management. Whenever the association is terminated, the client M2PA should establish the association as soon as the termination procedure is complete. The client M2PA establishes an association by sending the SCTP ASSOCIATE primitive to SCTP. The client should attempt to establish the server.association periodically until it is successful. Once the association is established and MTP3 has issued its Start Request, M2PA begins the alignment procedure. The M2PA at either end may initiate the alignment procedure first. There is no client/server distinction once the SCTP association is established. Each combination of client IP address/port and server IP address/port (i.e., each association) must be mapped to the same Signaling Link Code (SLC) in the client and server, so that each endpoint knows which link is being created at the time the SCTP association is established. However, M2PA does not do any processing based on the SLC. Following are examples of the relationships between associations and links. Note that a link is an SCTP association identified by two endpoints, in this case a client and server. Each endpoint is identified by an IP address and port number. Each association is mapped to an SLC. Figure 78 shows a case with two IPSPs, each with two IP addresses. Two associations are the links that connect the two IPSPs. Since these links are in the same link set, they must have different SLCs. Table 1 shows the relationships in tabular form. Table 1 is only conceptual. The actual method for mapping the SCTP associations to the SLCs is implementation dependent. IPSP X IPSP Y +-------------+ +-------------+ | | SCTP | | | IPA | association 1 | IPB | | port = PW +---------------+ port = PW | | SLC = a | | SLC = a | | Client | | Server | | | | | | | SCTP | | | IPC | association 2 | IPD | | port = PW +---------------+ port = PW | | SLC = b | | SLC = b | | Client | | Server | | | | | +-------------+ +-------------+ IPx = IP address PW = Well-known port number for M2PA Figure 7:8: Associations and Links - Two IPSPs with Two EndpointsIP Addresses Each +-------------+---------------------------------------+-----+ | Association | Client | Server | SLC | | +------------+------+------------+------+ | | | IP address | Port | IP address | Port | | +=============+============+======+============+======+=====+ | 1 | IPA | PW | IPB | PW | a | +-------------+------------+------+------------+------+-----+ | 2 | IPC | PW | IPD | PW | b | +-------------+------------+------+------------+------+-----+ Table 1: Associations and SLCs - Two IPSPs with Two EndpointsIP Addresses Each Figure 89 and Table 2 show an example with three IPSPs. Note that in this example, the two links are in different link sets. Therefore, it is possible that the values a and b may be equal. IPSP X IPSP Y +-------------+ +-------------+ | | SCTP | | | IPA | association 1 | IPB | | port = PW +---------------+ port = PW | | SLC = a | | SLC = a | | Client | | Server | | | | | | | SCTP | | | IPC | association 2 | | | port = PW +-------+ | | | SLC = b | | | | | Client | | | | | | | | | +-------------+ | +-------------+ | | | IPSP Z | | +-------------+ | | | | | IPD | +-------+ port = PW | | SLC = b | | Server | | | | | | | | | | | | | | | +-------------+ IPx = IP address PW = Well-known port number for M2PA Figure 8:9: Associations and Links - One IPSP Connected to Two IPSPs +-------------+---------------------------------------+-----+ | Association | Client | Server | SLC | | +------------+------+------------+------+ | | | IP address | Port | IP address | Port | | +=============+============+======+============+======+=====+ | 1 | IPA | PW | IPB | PW | a | +-------------+------------+------+------------+------+-----+ | 2 | IPC | PW | IPD | PW | b | +-------------+------------+------+------------+------+-----+ Table 2: Associations and SLCs - One IPSP Connected to Two IPSPs Figure 910 and Table 3 show two associations between the same endpoints. This is accomplished by using different port numbers for each association at the client. IPSP X IPSP Y +-------------+ +-------------+ | | SCTP | | | IPA | association 1 | IPB | | port = P1 +---------------+ port = PW | | SLC = a | | SLC = a | | Client | | Server | | | | | | | SCTP | | | IPA | association 2 | IPB | | port = PW +---------------+ port = PW | | SLC = b | | SLC = b | | Client | | Server | | | | | +-------------+ +-------------+ IPx = IP address P1, P2P1 = Pre-selected port numbersnumber for Client PW = Well-known port number for M2PA Figure 9:10: Associations and SLCs - Multiple Associations Between EndpointsTwo IP Addresses +-------------+---------------------------------------+-----+ | Association | Client | Server | SLC | | +------------+------+------------+------+ | | | IP address | Port | IP address | Port | | +=============+============+======+============+======+=====+ | 1 | IPA | P1 | IPB | PW | a | +-------------+------------+------+------------+------+-----+ | 2 | IPA | P2PW | IPB | PW | b | +-------------+------------+------+------------+------+-----+ Table 3: Associations and SLCs - Multiple Associations Between EndpointsTwo IP Addresses The association shall contain two streams in each direction. Stream 0 is designated for Link Status messages. Stream 1 is designated for User Data and Proving Data messages. If4.1.3 Link Alignment The purposes of the alignment procedure are: 1. To provide a handshaking procedure so that both endpoints are prepared to send SS7 traffic, and to prevent traffic from being sent before the other end is ready. 2. Verify that the SCTP association is not established, the client M2PA sends the ASSOCIATE primitive to SCTP. The client should attemptsuitable for use as an SS7 link. 3. Optionally, to establishovercome the SCTP slow start period. Link alignment takes place after the association periodically until itis successful.established. If SCTP fails to establish the association, and M2PA hadhas received a Start Request from its MTP3, then M2PA shall report to MTP3 that the link is out of service. If M2PA has an SCTP association ID for that association, it should ABORT the association. The association ID is a number provided by the SCTP used to identify an association.Once the association is established,established and M2PA has received a Start Request from MTP3, M2PA invokessends the GETSRTTREPORT primitiveLink Status Alignment message to determine the Smooth Round Trip Time (SRTT) from SCTP.its peer. If M2PA has not already received the SRTT exceedsLink Status Alignment message from its maximum allowed value (which is implementation dependent),peer, then M2PA should use the ABORT primitive to endstarts timer T1. (Note that if the association. Ifremote M2PA hadhas not received a Start Request from its MTP3, it will not send the Link Status Alignment message to the local M2PA. Eventually timer T1 in the local M2PA will expire.) M2PA stops timer T1 when it has received the Link Status Alignment message from its peer. If timer T1 expires, then M2PA shall reportreports to MTP3 that the link is out of service. OnceM2PA has received a Start Request from MTP3,should leave the association is established, the SRTT is determined to be satisfactory, and ifestablished. M2PA waits for MTP3 has not deactivatedto initiate the link, then: (a) If there is no local processor outage condition,alignment procedure again. When M2PA sends ahas both sent and received the Link Status of In ServiceAlignment message, it has completed alignment and moves to its peer. (b) If there is a local processor outage condition,the proving state. M2PA starts the proving period timer T2. During the proving period, M2PA sends Link Status Processor OutageProving messages to its peer. When MTP3 sends Local Processor Recovered, thenpeer at an interval defined by the protocol parameter Status_Interval. M2PA sends Link Status Processor Outage Endedeither the Proving Normal or Proving Emergency message, according to its peer, followed by Link Status In Service. Ifthe Emergency and Emergency Ceases commands from MTP3. M2PA has not receiveduses the value of T2 corresponding to the Normal or Emergency state. However, if M2PA receives a Link Status In ServiceProving Emergency message from its peerpeer, then M2PA shall use the Emergency value for T2. Also while T2 is running, M2PA shall send Proving Data messages on the User Data stream. These messages are sent at a rate equal to the time it sendsprotocol parameter Proving_Data_Rate. When the Link Status In Service, M2PA startsproving period timer T1. Timer T1T2 expires, M2PA shall determine the association's performance as described in section 4.1.6 Error Monitoring. If the association's performance is stopped wheninadequate, M2PA receivesshall report to MTP3 that the link is out of service. M2PA should leave the association established. M2PA waits for MTP3 to initiate the alignment procedure again. If the association's performance is satisfactory, M2PA shall start the timer T3 and send Link Status In Service fromReady messages to its peer. Ifpeer at interval Status_Interval. These messages are used to verify that both ends have completed proving. M2PA does not receiveshall stop timer T3 when it receives a Link Status In ServiceProving Complete or User Data message from its peer before T1its peer. If timer T3 expires, then M2PA reports to MTP3 that the link is out of service. ThenM2PA usesshould leave the ABORT primitiveassociation established. M2PA waits for MTP3 to endinitiate the association. Recommended value of T1 is 5-50 seconds.alignment procedure again. Note that if the serverM2PA has notalready received a Start RequestLink Status Ready message from its MTP3,peer when it will not sendfinishes checking the association's performance, there is no need to start timer T3. M2PA can just send Link Status In Service message to the client. Eventually the client will ABORT the association. The client will then attemptReady to establishthe association.peer and continue along. When all of the association is established,following are true: (a) M2PA has received a Start Request from MTP3. (b) M2PA's proving period T2 has expired. (c) M2PA has sent a Link Status In ServiceReady to its peer, andpeer. (d) M2PA has received a Link Status Ready OR User Data message from its peer. then M2PA shall send Link In Service fromto its peer, andMTP3. If there is noa local processor outage condition, thenM2PA sends Link In ServiceStatus Processor Outage to its MTP3.peer. When the local processor outage condition ends, then M2PA shall send Link Status Processor Outage Ended to its peer. M2PA shall attempt to complete the alignment procedure during the local processor outage condition. If M2PA receives a Link Status ofProcessor Outage during alignment, and M2PA had received a Start Request from its MTP3, M2PA shall report Remote Processor Outage to MTP3. M2PA shall ignore the Emergency andRecommended values: T1 Alignment - Range: 1-60 seconds Default: 10 seconds T2 Proving - Normal - Range: 1-60 seconds Default: 10 seconds Emergency Ceases commands from MTP3. 4.1.3- Range: 400-600 milliseconds Default: 500 milliseconds T3 Ready - Range: 1-60 seconds Default: 10 seconds Status_Interval - implementation dependent. Proving_Data_Rate - implementation dependent. 4.1.4 Processor Outage A processor outage occurs when M2PA cannot transfer messages because of a condition at a higher layer than M2PA. When M2PA detects a local processor outage, it sends a Link Status message to its peer with status Processor Outage. M2PA shall discard any User Data messages received. M2PA shallalso cease sending User Data messages to SCTP for transmission. M2PA should periodically send a Link Status Processor Outage message as long as there is a local processor outage. The peer M2PA, upon receiving the Link Status Processor Outage message, shall report Remote Processor Outage to its MTP3. The peer M2PA ceases sending User Data messages. M2PA stops the Remote Congestion timer T6 if it is running. MTP3 sends a Flush Buffers or Continue command to M2PA. When the processor outage ceases, MTP3 sends a Local Processor Recovered indication to M2PA. The local M2PA notifies its peer by sending a Link Status message with status Processor Outage Ended. The peer notifiesuses the Remote Processor Recovered Indication to notify its MTP3 that the remote processor outage condition has ceased. 188.8.131.52.5 Level 2 Flow Control Notification of receive congestion from SCTP to M2PA is implementation dependent. This section assumes that there isM2PA has some formmeans of determining when SCTP is in receive congestion, such as a receive congestion notification from SCTP to M2PA. Since SCTP has its own congestion control, the purpose of the M2PA level 2 flow control is to monitor the association and decide if it should be aborted. If M2PA receives notification from its lower layer SCTPdetermines that SCTP is in receive congestion for an association, M2PA shall send a Link Status Busy message to its peer on that association. M2PA should periodically send a Link Status Busy message as long as its SCTP is in receive congestion. When the peer M2PA receives the Link Status Busy message, it shall start the Remote Congestion timer T6. If timer T6 expires, M2PA shall use the ABORT primitive to end the association and take the link out of service. The peer M2PA shall continue transmitting messages to SCTP while its T6 timer is running, i.e., while the other end is Busy. If M2PA receives notification from its lower layer SCTPdetermines that SCTP is no longer in receive congestion for the association, M2PA shall send a Link Status Busy Ended message to its peer on that association. When the peer M2PA receives the Link Status Busy Ended message, it shall stop timer T6. Recommended value of T6 is 1-6 seconds. 184.108.40.206.6 Error Monitoring If M2PA loses the SCTP association for a link, M2PA shall report to MTP3 that the link is out of service. As long as the SCTP association is up, M2PA shall regularly invokemonitor the SCTP GETSRTTREPORT primitiveassociation performance. It is recommended that M2PA use the following data to determine the performance of the association: - Smooth Round Trip Time (SRTT)(SRTT). This can be obtained from SCTP. If the the SRTT exceedsSCTP by invoking the maximum acceptable value,SCTP GETSRTTREPORT primitive. - Frequency of SCTP retransmissions. - Frequency of SCTP Gap Acknowledgements received. If these values are not acceptable, the link is considered failed and taken out of service. The acceptable values of these data are implementation dependent. The interval between successive SRTT requests andchecks of the maximum acceptable SRTTperformance data should be a configurable parameter. Its value are determined by the parameters: SRTT_measurement_interval Range: 1 - 1000 seconds. Default: 5 seconds. SRTT_max Range: 0-65,535 milliseconds. Default: 1000 milliseconds. 4.1.6is implementation dependent. 4.1.7 Transmission and Reception Priorities In MTP, Link Status messages have priority over User Data messages ( Q.703, section 11.2). To achieve this in M2PA, M2PA shall send Link Status and User Data messages shall be sent via SCTPon separate streams.streams in its SCTP association. All messages are sent using the ordered delivery option. M2PA SHOULD give higher priority to reading the Link Status stream over the User Data stream. M2PA SHOULD give higher priority to receiving notificationsSHOULD give higher priority to receiving notifications from SCTP over reading either the Link Status stream or the User Data stream. Implementation Note: If the SCTP implementation allows streams to have different priorities for sending messages, then M2PA SHOULD set the Link Status stream to a higher priority than the User Data stream. See  for a possible extension to SCTP to allow for stream priorities. 4.1.8 M2PA Version Control A node upgraded to a newer version of M2PA SHOULD support the older versions used on other nodes with which it is communicating. If that is the case, then alignment can proceed normally. In particular, it is recommended that for future modifications to this protocol: - Any newer version should be able to process the messages from a lower version. - A newer version of M2PA should refrain from SCTP over reading either the Link Status stream orsending messages to an older version of M2PA messages that the User Data stream. Implementation Note:older version cannot process. - If an older version of M2PA receives a message that it cannot process, it should discard the SCTP implementation allows streams to havemessage. - In cases where different prioritiesprocessing is done in two versions for sending messages,the same format of a message, then M2PA SHOULD setthe Link Status streamnewer version should contain procedures to recognize this and handle it appropriately. In case a higher priority thannewer version of M2PA is incompatible with an older version, the User Data stream. See  fornewer version should recognize this and prevent the alignment of the link. If a possible extension to SCTP to allow for stream priorities.Link Status Alignment message with an unsupported version is received by the newer version, the receiving end's M2PA shall not complete the alignment procedure. 4.2 Procedures to Support the MTP3/MTP2 Interface 4.2.1 Sending/receiving messages When MTP3 sends a message for transmission to M2PA, M2PA passes the corresponding M2PA message to SCTP using the SEND primitive. M2PA Link Status messages are passed to SCTP using the SEND primitive. Link Status and User Data messages shall be sent via SCTP on separate streams. When M2PA receives a User Data message from SCTP, M2PA passes the message to MTP3. If M2PA receives a message from SCTP with an invalid Message Class or unsupported Message Type in the Common Message Header, M2PA shall discard the message. 4.2.2 Link activation and restoration When MTP3 requests that M2PA activate or restore a link by a Start Request, M2PA shall follow the alignment procedure in section 220.127.116.11.1.3. 4.2.3 Link deactivation When MTP3 requests that M2PA deactivate a link by a Stop command, M2PA shall send an ABORT primitive to SCTP. 4.2.4 Flush Buffers, Continue The Flush Buffers and Continue commands allow M2PA to resume normal operations (i.e., transmission of messages to SCTP and receiving messages from SCTP) after a processor outage (local and/or remote) ceases. If M2PA receives a Flush Buffers command from MTP3, M2PA: (a) shall not transmit any messages to SCTP that are currently waiting to be transmitted to SCTP. These messages shall be discarded. (b) shall discard all messages currently waiting to be passed to MTP3. If M2PA receives either a Flush Buffers or Continue command from MTP3, and the processor outage condition ceases, M2PA shall resume receiving and transmitting messages. 4.2.5 MTP3 Signaling Link Congestion Notification of transmit congestion from SCTP to its upper layer (M2PA) is implementation dependent. Nevertheless, M2PA should receive notification from SCTP adequate to allow MTP3 to meet its requirements for signaling link transmit congestion in  Q.704, section 3.8. M2PA shall use the Congestion Indication primitive to notify its upper layer MTP3 when transmit buffer occupancy crossesof changes in the signaling link congestion onset, discard,status and abatement thresholds.the signaling link discard status. For national networks with multiple congestion threshold levels, M2PA shall notify MTP3 when transmit buffer occupancy crosses each levelof the congestion onset, discard,and abatement thresholds.discard status levels. Note: M2PA does not discard messages because of transmit congestion. Discarding of messages due to transmit congestion is performed by MTP3. 4.2.6 Changeover The objective of the changeover is to ensure that signaling traffic carried by the unavailable signaling link is diverted to the alternative signaling link(s) as quickly as possible while avoiding message loss, duplication, or mis-sequencing. For this purpose, the changeover procedure includes data retrieval, which is performed before opening the alternative signaling links to the diverted traffic. Data retrieval consists of these steps: (1) buffer updating, i.e., identifying all those User Data messages in the retransmission buffer of the unavailable signaling link which have not been received by the far end SCTP, as well as untransmitted messages, and (2) transferring those messages to the transmission buffers of the alternate links. Note that only User Data messages are retrieved and transmitted over the alternate links. Link Status messages shall not be retrieved and transmitted over the alternate links. References to stream sequence numbers in this section refer only to the User Data stream's stream sequence numbers. In order to support changeover in M2PA, the SCTP Stream Sequence Numbers must be used in place of the Forward and Backward Sequence Numbers (FSN/BSN) of SS7. Stream Sequence Numbers used by SCTP are 16 bits long. MTP2's Forward and Backward Sequence Numbers are only seven bits long. Hence it is necessary for MTP3 to accomodateaccommodate the larger SSNs. This is done through the use of the Extended Changeover Order (XCO) and Extended Changeover Acknowledgement (XCA) messages instead of the Changeover Order (COO) and Changeover Acknowledgement (COA) messages. The XCO and XCA messages are specified in Reference  section 18.104.22.168.8.1 and Reference  T1.111.4, section 15.4. Only the XCO and XCA messages from  or  are required. These messages have a 24-bit field for the sequence number.The upper 8 bits of the 24 bit field should be set to 0, and theSSN is placed in the lower 16 bits.XCO/XCA message as explained in  and . (Note that the Stream Sequence Numbers are used instead of the Transmission Sequence Numbers. The Transmission Sequence Numbers are 32 bits long, and therefore would not fit in the XCO and XCA messages.)messages. Furthermore, TSNs do not number User Data messages consecutively. TSNs also number Link Status and SCTP-originated messages, which should not be retrieved during the changeover procedure.) Also, the following MTP3/MTP2 primitives must use the larger sequence numbers: - BSNT IndicationConfirmation - Retrieval Request and FSNC For data retrieval, MTP3 requests the Backward Sequence Number to be Transmitted (BSNT) from M2PA through the Retrieve BSNT request. Normally, SCTP receives messages in order, in which case the BSNT is the last message received by SCTP. However, because of congestion or a failure condition, the sequence numbers of the acknowledged messages may have gaps. In particular, the SCTP SACK (selective acknowledgement message) message can have several of these gaps. In this case, it is necessary to scan through these gaps and find the sequence number before the first gap. This is the number considered as the BSNT and communicated to MTP3. M2PA sends the BSNT value to MTP3 in the BSNT indication.confirmation. In the same way, the remote end also detects its BSNT. The MTP3 layers exchange BSNT values through the XCO/XCAXCO and XCA messages. The BSNT received from the other end is called the FSNC. When MTP3 receives the FSNC from the other end, MTP3 retrieves all the unsent and unacknowledged messages starting with sequence number (FSNC + 1). This is accomplished through a Retrieval Request and FSNC request. After all the messages are sent from M2PA to MTP3, M2PA sends a Retrieval Complete indication to MTP3. As an example of how the BSNT is determined, suppose the following SSNs had been received by SCTP on the Data Stream when it is time to do the changeover procedure: 1-10, 13, 14, 16. Then M2PA tells its upper layer that the last message it received (the BSNT) was 10. SCTP has not delivered 13, 14, and 16 to M2PA because to do so would violate ordered delivery within the stream. The value of 10 is transmitted to the remote end by MTP3 in the XCO/XCA message. So the remote end will retransmit 11-16 on an alternate link. If there are any messages on the SCTP receive queue, M2PA SHOULD receive these messages and deliver them to MTP3. Note that SCTP does not deliver incoming messages after the first gap (if any) in the SSNs, since this would violate ordered delivery within the stream. In the example above, this would mean that messages 1-10 SHOULD be received. Otherwise, these unreceived messages might be lost, since SCTP might have acknowledged them. Note that the sequence numbers and messages requested by MTP3 may be obtained by M2PA from SCTP via the Communication Lost primitive . Retrieval of messages is an optional feature in SCTP.SCTP that is required by M2PA. To perform data retrieval, it is necessary that this option be implemented, and thatSCTP identify the SSNs of the messages are identified.that M2PA retrieves. SCTP must retain the messages for retrieval by MTP3/M2PA whenever an association is aborted. SCTP must be able to return messages to M2PA so that M2PA can perform retrieval for MTP3. There are various ways that this can be implemented, such as: (1) SCTP provides a way for M2PA to request retrieval of messages for a specified stream and SSN(s). (2) SCTP retrieves all messages and identifies the stream and SSN of each message. M2PA then must select the appropriate messages to pass up to MTP3. M2PA must be able to respond to the BSNT request from MTP3. There are various ways of implementing this, such as having SCTP provide the BSNT. It is helpful for M2PA to have access to the first and last SSN in SCTP's transmission queue. This information could be used to determine if the FSNC received from the remote end is a valid value. If M2PA receives a Retrieve BSNT request from MTP3, M2PA shall respond with the BSNT indication.confirmation. The BSNT value is the SCTP stream sequence number of the last message received by SCTP User Data stream before any gaps in the stream sequence numbers. (Note that any messages received with a stream sequence number greater than this BSNT value have been acknowledged by the receiving SCTP, but have not been passed up to M2PA. These messages are discarded by the receiving SCTP and are not delivered to the upper layer M2PA. Therefore these messages should be retransmitted by the far end on the alternate link.) If M2PA receives a Retrieval Request and FSNC request from MTP3, M2PA shall retrieve from SCTP in order and deliver to MTP3: (a) any transmitted User Data messages beginning with the first unacknowledged message with stream sequence number greater than FSNC. (b) any untransmitted User Data messages in SCTP. (c) any untransmitted User Data messages M2PA has not delivered to SCTP for transmission. Then M2PA shall send the Retrieval Complete indication to MTP3. For emergency changover,changeover, MTP3 retrieves only the unsent messages for transmission on the alternate link(s). If M2PA receives a Retrieval Request and FSNC request with no FSNC value, or with an invalid FSNC, then M2PA shall retrieve from SCTP in order and deliver to MTP3: (a) any untransmitted User Data messages in SCTP. (b) any untransmitted User Data messages M2PA has not delivered to SCTP for transmission. Then M2PA shall send the Retrieval Complete indication to MTP3. Note:22.214.171.124 Multiple User Data Streams and Changeover The changeover procedure makes it impossibleproblematic for M2PA to have multiple User Data streams in aone direction for onea link. Buffer updating would have to be done for each User Data stream separately to avoid duplication or loss of messages. But MTP3 provides for only one XCOXCO/XCA message for sending the last-received SSN. M2PA links with multiple User Data streams would be possible if a multiple-BSNT XCO/XCA message is defined in MTP3, or MTP3 allows multiple XCO/XCA messages (one for each User Data stream) to be sent during a changeover. This is beyond the scope of this document. Another attempt to solve this problem and allow for multiple User Data streams, without changes to MTP3 messages and procedures, is to introduce sequence numbering of User Data messages at the M2PA layer. The M2PA sequence numbers would be used instead of SCTP SSNs in the XCO/XCA messages. However, since the M2PA messages would be delivered over multiple streams, there could be a gap in the M2PA sequence numbers at the receiving end when the changeover procedure begins. There would be a possibility of losing the messages in the gap, or duplicating messages after the gap. 5. Examples of M2PA Procedures In general, messages passed between MTP3 and M2PA are the same as those passed between MTP3 and MTP2. M2PA interprets messages from MTP3 and sends the appropriate message to SCTP. Likewise, messages from SCTP are used to generate a meaningful message to MTP3. Note that throughout this section, the primitives between MTP3 and M2PA are named using the MTP terminology . Communications between M2PA and SCTP are named using SCTP terminology. 5.1 Link Initialization (Alignment) An example of the message flow to bring an SS7 link in service is shown below. Alignment is done by both ends of the link. To simplify the diagram, alignment is shown on one end only. It is assumed in this example that SCTP has been initialized. MTP3 M2PA SCTP SCTP M2PA MTP3 ---- ---- ---- ---- ---- ---- Out of Service <------------Emergency OR Emergency Ceases ------------> Start ------------> Associate ------------> (SCTP Association procedure) Communication Up Communication Up <------------ ------------> Even though the SCTP association is established, it is important that M2PA not send MTP3 data at this point. It must be confirmed that both ends of the link are ready for traffic. Otherwise, messages could be lost. The endpoints must exchange In Service messages.MTP3 M2PA SCTP SCTP M2PA MTP3 ---- ---- ---- ---- ---- ---- Link Status Alignment ------------------------------------> Start timer T1 Link Status Alignment <------------------------------------ Stop timer T1 Start timer T2 Proving period begins. (Messages from remote end not shown.) Link Status Proving Proving Data ------------------------------------> ------------------------------------> ------------------------------------> ------------------------------------> ------------------------------------> ------------------------------------> Timer T2 expires Proving period ends. Check association performance. Get SRTT Report ------------> Send Link Status In ServiceReady until the remote end completes its proving period. Start timer T3 Link Status Ready ------------------------------------> ------------------------------------> ------------------------------------> ------------------------------------> Link Status In ServiceReady <------------------------------------ Stop timer T3 In Service In Service <------------ ------------> At this point, MTP3 may begin sending data messages. 5.2 Message Transmission and Reception Messages are transmitted using the Data Request primitive from MTP3 to M2PA. The diagram shows the case where the Link is In Service. The message is passed from MTP3 of the source to MTP3 of the destination. MTP3 M2PA SCTP SCTP M2PA MTP3 ---- ---- ---- ---- ---- ---- Message for transmission ------------> Send (Data Message) ------------> (SCTP sends message) Receive ------------> Received message ------------> 5.3 Link Status Indication If SCTP sends a Communication Lost primitive to M2PA, M2PA notifies MTP3 that the link is out of service. MTP3 responds in its usual way. MTP3 M2PA SCTP SCTP M2PA MTP3 ---- ---- ---- ---- ---- ---- Communication Lost <------------ Out of Service <------------ 5.4 Link Status Message (Processor Outage) This example shows how M2PA responds to a local processor outage. M2PA sends a Link Status message to its peer. The peer M2PA notifies MTP3 of the outage. MTP3 can then follow the processor outage procedures in . MTP3 M2PA SCTP SCTP M2PA MTP3 ---- ---- ---- ---- ---- ---- M2PA detects Local Processor Outage Link Status Processor Outage ------------> (SCTP sends message) Receive ------------> Remote Processor Outage ------------> Link Status Processor Outage Ended ------------> (SCTP sends message) Receive ------------> Remote Processor Outage Ceases ------------> 5.5 Level 2 Flow Control This illustrates the Level 2 Flow Control procedure. In the first diagram, congestion ceases before timer T6 expires. The second diagram shows the case where T6 expires. MTP3 M2PA SCTP SCTP M2PA MTP3 ---- ---- ---- ---- ---- ---- Implementation dependent indication of receive congestion onset <------------ Link Status Busy ------------------------------------> Start Timer T6 Implementation dependent indication of receive congestion abatement <------------ Link Status Busy Ended ------------------------------------> Stop Timer T6 MTP3 M2PA SCTP SCTP M2PA MTP3 ---- ---- ---- ---- ---- ---- Implementation dependent indication of receive congestion onset <------------ Link Status Busy ------------------------------------> Start Timer T6 : : Timer T6 Expires Abort <------------ Comm Lost ------------> Out of Service ------------> 5.6 MTP3 Signaling Link Congestion In this example, it is assumed that SCTP notifies M2PA of congestion onset and abatement. The notification includes the congestion level, if there are levels of congestion defined. MTP3 M2PA SCTP SCTP M2PA MTP3 ---- ---- ---- ---- ---- ---- Implementation dependent indication of transmit congestion onset (level) <------------ Congestion Indication (level) <------------ Implementation dependent indication of transmit congestion abatement (level) <------------ Congestion Indication (level) <------------ 5.7 Link Deactivation The MTP3 can request that a SS7-IP link be taken out-of-service. M2PA uses the Abort message as shown below. MTP3 M2PA SCTP SCTP M2PA MTP3 ---- ---- ---- ---- ---- ---- Stop ------------> Abort ------------> (SCTP performs its termination procedure) Communication Lost <------------ Out of Service <------------ 5.8 Link Changeover In this example, MTP3 performs a changeover because the link went out of service. MTP3 selects a different link for retransmittingto retransmit the unacknowledged and unsent messages. Note that in this example, the sequence numbers and messages requested by MTP3 are sent from SCTP to M2PA in the Communication Lost primitive. In general, the retrieval of sequence numbers and messages is implementation dependent. MTP3 M2PA SCTP SCTP M2PA MTP3 ---- ---- ---- ---- ---- ---- Communication Lost <------------ Out of Service <------------ Retrieve BSNT ------------> (M2PA locates first gap in received messages) BSNT IndicationConfirmation <------------ XCO (BSNT) on another link ------------------------------------------------------------> Retrieve BSNT <------------ BSNT IndicationConfirmation ------------> XCA (BSNT) <------------------------------------------------------------ Retrieval Request and FSNC ------------> (M2PA locates first gap in acknowledgements) Retrieved Message <------------ Retrieved Message <------------ Retrieval Complete <------------ Send messages on another link. 6. Security M2PA is designed to carry signaling messages for telephony services. As such, M2PA 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. 6.1 Threats There is no quick-fix, one-size-fits-all solution for security. As a transport protocol, M2PA has the following security objectives: - Availability of reliable and timely user data transport. - Integrity of user data transport. - Confidentiality of user data. M2PA runs on top of SCTP. SCTP  provides certain transport related security features, such as: - Blind Denial of Service Attacks - Flooding - Masquerade - Improper Monopolization of Services When M2PA 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"  SHOULD be consulted for guidance. When the network in which M2PA runs involves more than one party,party (e.g., a non-private network), 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 be used to ensure confidentiality of user payload. Consult  for more information on configuring IPSEC services. 6.2 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. 7. IANA Considerations 7.1 SCTP Payload Protocol Identifier The SCTP (and UDP/TCP) Registered User Port Number Assignment for M2PA is TBD. The value assigned by IANA for the Payload Protocol Identifier in the SCTP Payload Data chunk is M2PA 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. 7.2 M2PA Protocol Extensions This protocol may be extended through IANA in three ways: - through definition of additional message classes, - through definition of additional message types, and - through definition of additional message parameters. The definition and use of new message classes, types, and parameters is an integral part of SIGTRAN adaptation layers. Thus, these extensions are assigned by IANA through an IETF Consensus action as defined in . The proposed extension must in no way adversely affect the general working of the protocol. 7.2.1 IETF Defined Message Classes The documentation for a new message class MUST include the following information: (a) A long and short name for the message class. (b) A detailed description of the purpose of the message class. 7.2.2 IETF Defined Message Types Documentation of the message type MUST contain the following information: (a) A long and short name for the new message type. (b) A detailed description of the structure of the message. (c) A detailed definition and description of the intended use of each field within the message. (d) A detailed procedural description of the use of the new message type within the operation of the protocol. (e) A detailed description of error conditions when receiving this message type. When an implementation receives a message type which it does not support, it MUST discard the message. 7.2.3 IETF-defined Parameter Extension Documentation of the message parameter MUST contain the following information: (a) Name of the parameter type. (b) Detailed description of the structure of the parameter field. (c) Detailed definition of each component of the parameter value. (d) Detailed description of the intended use of this parameter type, and an indication of whether and under what circumstances multiple instances of this parameter type may be found within the same message type. 8. Acknowledgements The authors would like to thank the following for their valuable comments and suggestions: Brian Tatum, Jeff Copley, Monique Bernard, Wayne Davis, Cliff Thomas, Brian Bidulock, Ian Rytina, Al Varney. 9. References  ITU-T Recommendation Q.700, 'Introduction To ITU-T Signalling System No. 7 (SS7)'(SS7)'.  ITU-T Recommendation Q.701-Q.705, 'Signalling System No. 7 (SS7) - Message Transfer Part (MTP)'(MTP)'.  ANSI T1.111-2000, American National Standard for Telecommunications - Signaling System Number 7 (SS7) - Message Transfer Part (MTP), 20002000.  RFC 2719, Framework Architecture for Signaling Transport, October 19991999.  RFC 2960, Stream Control Transmission Protocol, October 20002000.  SS7 MTP2-User Adaptation Layer, draft-ietf-sigtran-m2ua-07.txt, February 2001draft-ietf-sigtran-m2ua-09.txt, July 2001.  ITU-T Recommendation Q.2210, 'Message transfer part level 3 functions and messages using the services of ITU-T Recommendation Q.2140'Q.2140'.  Bradner, S. "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Telecommunication Technology Committee (TTC) Standard JT-Q704, 'Message Transfer Part Signaling Network Functions', April 28, 1992.  ITU-T Recommendation Q.2140, 'B-ISDN ATM Adaptation Layer', February 19951995.  RFC 2196, Site Security Handbook, September 19971997.  RFC 2401, Security Architecture for the Internet Protocol, November 19981998.  SCTP Stream BasedExtensions for Dynamic Reconfiguration of IP Addresses and Enforcement of Flow Limiting Methods, draft-ietf-sigtran-srwnd-sctp-00.txt, January 31, 2001and Message Limits, draft-ietf-tsvwg-addip-sctp-02.txt, June 29, 2001.  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.  RFC 791, Internet Protocol, September 1981. 10. Author'sAuthors' Addresses Tom George Tel: +1-972-519-3168 Alcatel USAUSA, Inc. EMail: email@example.comTom.George@usa.alcatel.com 1000 Coit Road Plano, TX 75075 USA Ram Dantu, Ph.D. Tel: +1-469-255-0716 Cisco Systems Inc. EMail: firstname.lastname@example.org 17919 Waterview Parkway Dallas, TX 75252 USA Malleswar Kalla Tel: +1-973-829-5212 Telcordia Technologies EMail: email@example.com MCC 1J211R 445 South Street Morristown, NJ 07960 USA Hanns Juergen Schwarzbauer Tel: +49-89-722-24236 SIEMENS AG HannsJuergen.Schwarzbauer@icn.siemens.de Hofmannstr. 51 81359 Munich Germany Greg Sidebottom Tel: +1-613-763-7305 Nortel Networks EMail: firstname.lastname@example.org 3685 Richmond Rd, Nepean,Kanata, Ontario EMail: email@example.com Canada K2H5B7Ken Morneault Tel: +1-703-484-3323 Cisco Systems Inc. EMail: firstname.lastname@example.org 13615 Dulles Technology Drive Herndon, VA.VA 20171 USA This Internet Draft expires September 2001.January 2002.