Network Working Group Ken Morneault
INTERNET-DRAFT Cisco Systems
Mallesh Kalla
Telcordia
Greg Sidebottom
Nortel Networks
Ram Dantu
IPMobile
Tom George
Alcatel
Expires in six months March 2000
SS7 MTP2-User Adaptation Layer
<draft-ietf-sigtran-m2ua-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
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Abstract
This Internet Draft defines a protocol for backhauling of SS7 MTP2
User signaling 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 SS7 signaling over a standard SS7 interface using the
SS7 Message Transfer Part (MTP) to provide transport. The Signaling
Gateway would act as a Signaling Link Terminal.
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TABLE OF CONTENTS
1. Introduction..............................................2
1.1 Scope..................................................2
1.2 Terminology............................................3
1.3 Signaling Transport Architecture.......................3
1.4 Services Provide by the M2UA Adaptation Layer..........6
1.5 Function Provided by the M2UA Layer....................8
1.6 Definition of the M2UA Boundaries......................9
2. Protocol Elements.........................................9
2.1 Common Message Header.................................10
2.2 M2UA Message Header...................................11
2.3 M2UA Messages.........................................11
3. Procedures...............................................20
3.1 Procedures to Support Service in Section 1.4.1........20
3.2 Procedures to Support Service in Section 1.4.2........21
3.3 Procedures to Support Service in Section 1.4.3........21
4. Examples of MTP2 User Adaptation (M2UA) Procedures.......26
4.1 Establishment of associations between SG and MGC......26
examples
4.2 MTP Level 2 / MTP Level 3 Boundary Examples...........28
4.3 Layer Management Communication Examples...............29
5. Security.................................................30
6. IANA Considerations......................................31
7. Acknowledgements.........................................31
8. References...............................................32
9. Author's Addresses.......................................33
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1. Introduction
1.1 Scope
There is a need for SCN signaling protocol delivery from an Signaling
Gateway (SG) to a Media Gateway Controller (MGC) or IP Signaling Point
(IPSP). The delivery mechanism should meet the following criteria:
* Support for MTP Level 2 / MTP Level 3 interface boundary
* Support for communication between Layer Management modules on SG and
MGC
* Support for management of active associations between the SG and MGC
In other words, the Signaling Gateway will transport MTP Level 3
messages to a Media Gateway Controller (MGC) or IP Signaling Point
(IPSP). In the case of delivery from an SG to an IPSP, both the SG and
IPSP function as traditional SS7 nodes using the IP network as a new
type of SS7 link. This allows for full MTP Level 3 message handling
and network management capabilities.
1.2 Terminology
MTP2-User - A protocol that normally uses the services of MTP Level 2
(i.e. MTP3).
Interface - For the purposes of this document, an interface is a SS7
signaling link.
Association - An association refers to a SCTP association. The
association will provide the transport for the delivery of protocol
data units for one or more interfaces.
Backhaul - Refers to the transport of signaling from the point of
interface for the associated data stream (i.e., SG function in the MGU)
back to the point of call processing (i.e., the MGCU), if this is not
local [4].
Application Server (AS) - A logical entity serving a specific application
instance. An example of an Application Server is a MGC handling the
MTP Level 3 and call processing for SS7 links 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 signaling traffic as required
to an alternate Application Server Process, or group of ASPs, within
an Application Server in the event of failure or unavailability of a
currently used Application Server Process. Fail-back may apply upon
the return to service of a previously unavailable Application Server
Process.
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Signaling Link Terminal (SLT) - Refers to the means of performing all
of the functions defined at MTP level 2 regardless of their
implementation [2].
Network Byte Order: Most significant byte first, a.k.a Big Endian.
Layer Management - Layer Management is a nodal function in an SG or
ASP that handles the inputs and outputs between the M2UA layer and a
local management entity.
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 framework architecture that has been defined for SCN signaling
transport over IP [5] uses multiple components, including a signaling
common transport protocol and an adaptation module to support the
services expected by a particular SCN signaling protocol from its
underlying protocol layer.
Within this framework architecture, this document defines a SCN
adaptation module that is suitable for the transport of SS7 MTP2 User
messages. The only SS7 MTP2 User is MTP3. The M2UA uses the services
of the Simple Common Transport protocol [6] as the underlying reliable
signaling common transport protocol.
In a Signaling Gateway, it is expected that the SS7 MTP2-User signaling
is transmitted and received from the PSTN over a standard SS7 network
interface, using the SS7 Message Transfer Part Level 1 and Level 2 [3,4]
to provide reliable transport of the MTP3-User signaling messages to and
from an SS7 Signaling End Point (SEP) or Signaling Transfer Point (STP).
The SG then provides a functional inter-working of transport functions
with the IP transport, in order to transfer the MTP2-User signaling
messages to and from an Application Server Process where the peer MTP2-
User protocol layer exists.
1.3.1 Example - SG to MGC
In a Signaling Gateway, it is expected that the SS7 signaling is
received over a standard SS7 network termination, using the SS7 Message
Transfer Part (MTP) to provide transport of SS7 signaling messages to
and from an SS7 Signaling End Point (SEP) or SS7 Signaling Transfer
Point (STP). In other words, the SG acts as a Signaling Link Terminal
(SLT) [2]. The SG then provides interworking of transport functions
with IP Signaling Transport, in order to transport the MTP3 signaling
messages to the MGC where the peer MTP3 protocol layer exists, as shown
below:
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****** SS7 ****** IP *******
*SEP *-----------* SG *-------------* MGC *
****** ****** *******
+----+ +----+
|S7UP| |S7UP|
+----+ +----+
|MTP + |MTP |
| L3 | (NIF) |L3 |
+----+ +----+----+ +----+
|MTP | |MTP |M2UA| |M2UA|
| | | +----+ +----+
|L2 | |L2 |SCTP| |SCTP|
|L1 | |L1 +----+ +----+
| | | |IP | |IP |
+----+ +---------+ +----+
NIF - Nodal Interworking Function
SEP - SS7 Signaling Endpoint
IP - Internet Protocol
SCTP - Simple Control Transmission Protocol
(Refer to Reference [5])
Figure 1 M2UA in the SG to MGC Application
Note: STPs may be present in the SS7 path between the SEP and the SG.
1.3.2 Signaling Network Architecture
A Signaling Gateway will support the transport of MTP2-User signaling
traffic received from the SS7 network to one or more distributed ASPs
(e.g., MGCs). Clearly, the M2UA 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 M2UA 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 SS7 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 SS7 node and an IP ASP. Depending
of course on the reliability of the SG and ASP functional elements, this
can typically be met by the spreading links in a linkset across SGs, the
provision of redundant QOS-bounded IP network paths for SCTP Associations
between SCTP End Points, and redundant Hosts. 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 2 below:
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******** **************
* *_________________________________________* ******** * 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 or transactions are not
lost. This implies that ASPs need, in some cases, to share the call/-
transaction state or be able to pass the call/transaction state between
each other. Also, in the case of ASPs performing call processing,
coordination may be required with the related Media Gateway to transfer
the MGC control for a particular trunk termination. However, this
sharing or communication is outside the scope of this document.
1.3.4 ASP Fail-over Model and Terminology
The M2UA supports ASP fail-over functions in order to support a high
availability of call and transaction processing capability. All MTP2-
User messages incoming to an SG from the SS7 network are assigned to a
unique Application Server, based on the Interface Identifier of the
message.
The Application Server is in practical terms a list of all ASPs currently
registered to process MTP2-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 ôn+kö redundancy model, where ônö ASPs
is the minimum number of redundant ASPs required to handle traffic and
ôkö ASPs are available to take over for a failed or unavailable ASP.
Note that ô1+1ö active/standby redundancy is a subset of this model.
A simplex ô1+0ö model is also supported as a subset, with no ASP
redundancy.
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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 to DPC x could be sent to ASP1
in Host1 or ASP1 in Host2. The AS list at SG1 might look like this:
Interface Identiers - Application Server #1
ASP1/Host1 - State=Up, Active
ASP1/Host2 - State=Up, Inactive
In this ô1+1ö 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 M2UA peer is ready.
The AS List at SG1 might also be set up in loadshare mode:
Interface Identifiers - 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 ôtransitionö 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.
1.3.5 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 M2UA
is 2904.
1.4 Services Provided by the M2UA Adaptation Layer
The SS7 MTP3/MTP2(MTP2-User) interface is retained at the termination
point in the IP network, so that the M2UA protocol layer is required to
provide the equivalent set of services to its users as provided by the
MTP Level 2 to MTP Level 3.
This includes the following services:
1.4.1 Support for MTP Level 2 / MTP Level 3 interface boundary
Also provision is made for protocol elements that enable a seamless, or
as seamless as possible, operation of the MTP2-User peers in the SS7 and
IP domains. This includes
Data
Provides the ability to transport MTP2 User information (in this case,
MTP Level 3 PDUs).
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Link Establish
Provides the ability to request MTP Level 2 to bring SS7 links
in-service.
Link Release
Provides the ability to request MTP Level 2 to take SS7 links out-of-
service. Also, provides mechanism for MTP2 to autonomously indicate
that SS7 link(s) have gone out-of-service.
Link State
Provides the ability to request state change or information on a
per link basis. Some examples would be the forcing of Local Processor
Outage or flushing buffers.
Link Status
Provides a means for asynchronous notification of link state changes to
be reported to the upper layer (MTP Level 3). An example would be the
reporting of remote processor outage event.
Data Retrieval
Provides a mechanism to perform SS7 link changeover procedure in
the case of a SS7 link failure.
1.4.2 Support for communication between Layer Management modules
on SG and MGC
It is envisioned that the M2UA layer needs to provide some messages that
will facilitate communication between Layer Management modules on the SG
and MGC.
To facilitate reporting of errors that arise because of backhauling MTP
Level 3 scenario, the following primitive is defined:
M-ERROR
The M-ERROR message is used to indicate an error with a received
M2UA 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 M2UA layer on the SG keeps the state of various ASPs it is associated
with. A set of primitives between M2UA layer and the Layer Management
are defined below to help the Layer Management manage the association(s)
between the SG and the MGC.
M-SCTP ESTABLISH
The M-SCTP ESTABLISH primitive is used to request, indicate and confirm
the establishment of SCTP association to a peer M2UA node.
The M2UA layer may also need to inform the status of the SCTP
association(s) to the Layer Management. This can be achieved using
the following primitive.
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M-SCTP STATUS
The M-SCTP STATUS primitive is used to request and indicate the status
of underlying SCTP association(s).
The Layer Management may need to inform the M2UA layer of a user status
(i.e., failure, active, etc.), so that messages can be exchanged between
M2UA layer peers to stop traffic to the local M2UA user. This can be
achieved using the following primitive.
M-ASP STATUS
The M-ASP STATUS primitive is used by the Layer Management to indicate
the status of the local M2UA user to the M2UA layer.
1.5 Functions Provided by the M2UA Layer
1.5.1 Mapping
The M2UA layer must maintain a map of a Interface ID to a physical
interface on the Signaling Gateway. A physical interface would be a
V.35 line, T1 line/timeslot, E1 line/timeslot, etc. The M2UA layer
must also maintain a map of Interface Identifier to SCTP association
and to the related stream within the association.
1.5.2 Flow Control / Congestion
It is possible for the M2UA layer to be informed of IP network congestion
by means of an implementation-dependent function (i.e. an indication
from the SCTP). If the M2UA layer receives this indication, the action(s)
taken are implementation dependent.
1.5.3 SCTP Stream Management
SCTP allows user specified number of streams to be opened during the
initialization. It is the responsibility of the M2UA layer to ensure
proper management of these streams. SCTP streams provide a means for
avoiding head of line blocking. For that reason, a stream should be used
per SS7 signaling link terminated by the Signaling Gateway. The SS7
signaling link can be identified by the optional Interface Identifier
in the M2UA specific message header (refer to Section 2.2).
1.5.4 Seamless SS7 Network Management Interworking
If the SG loses the SCTP association to the MGC, it should follow
MTP 2 processor outage procedures [2].
1.5.5 Management Inhibit/Uninhibit
Local Management at an ASP or SG 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 control the start of traffic on to a newly-
available SCTP association.
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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, M2UA 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 the M2UA Boundaries
1.6.1 Definition of the M2UA / MTP Level 3 boundary
DATA
ESTABLISH
RELEASE
STATE
STATUS
RETRIEVAL
DATA RETRIEVAL
DATA RETRIEVAL COMPLETE
1.6.2 Definition of the M2UA / MTP Level 2 boundary
DATA
ESTABLISH
RELEASE
STATE
STATUS
RETRIEVAL
DATA RETRIEVAL
DATA RETRIEVAL COMPLETE
1.6.3 Definition of the Lower Layer Boundary between M2UA and SCTP
The upper layer and layer management primitives provided by SCTP are
provided in Reference [6] Section 9.
1.6.4 Definition of Layer Management / M2UA Boundary
M-ERROR
M-SCTP ESTABLISH
M-SCTP STATUS
M-ASP STATUS
2.0 Protocol Elements
This section describes the format of various messages used in this
protocol.
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2.1 Common Message Header
The protocol messages for MTP2-User Adaptation require a message
structure which contains a version, message type, message length, and
message contents. This message header is common among all 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 3 Common Message Header
All fields in an M2UA 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 M2UA adapation
layer. The supported versions are:
0000 0001 Release 1.0 protocol
2.1.2 Message Type
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 types for the defined messages.
MTP2 User Adaptatation (MAUP) Messages
Data 0601
Establish Request 0602
Establish Confirm 0603
Release Request 0604
Release Confirm 0605
Release Indication 0606
State Request 0607
State Confirm 0608
State Indication 0609
Data Retrieval Request 060a
Data Retrieval Confirm 060b
Data Retrieval Indication 060c
Data Retrieval Complete Indication 060d
Application Server Process Maintenance (ASPM) Messages
ASP Up (ASPUP) 0301
ASP Down (ASPDN) 0302
ASP Active (ASPAC) 0401
ASP Inactive (ASPIA) 0402
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Management (MGMT) Messages
Error 0000
2.1.3 Message Length
The Message length defines the length of the message in octets, not
including the header.
2.2 M2UA Message Header
In addition to the common message header, there will be a M2UA specific
message header. The M2UA specific message header will immediately
follow the common message header, but will only be used with MAUP and
MGMT messages.
This message header will contain the Interface Identifier. The
Interface Identifier identifies the physical interface at the SG for
which the signaling messages are sent/received. The format of the
Interface Identifier parameter 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.
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=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 M2UA Message Header
The Tag value for Interface Identifier is 0x1. The length is always
set to a value of 4.
2.3 M2UA Messages
The following section defines the messages and parameter contents. The
M2UA messages will use the common message header (Figure 3) and the
M2UA message header (Figure 4).
2.3.1 MTP2 User Adaptation Messages
2.3.1.1 Data
The Data message contains an SS7 MTP2-User Protocol Data Unit (PDU). The
Data message contains the protocol data.
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The format for the Data 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Protocol Data field contains the MTP2-User application message in
network byte order.
2.3.1.2 Establish (Request, Confirmation)
The Establish Request message is used to establish the link or to
indicate that the channel has been established. Note that the gateway
may already have the SS7 link established at its layer. If so, upon
receipt of an Establish Request, the gateway takes no action except to
send an Establish Confirm.
The mode (Normal of Emergency) for bringing the link in service is
defaulted to Normal. The State Request (described in Section 2.3.1.4
below) can be used to change the mode to Emergency.
2.3.1.3 Release (Request, Indication, Confirmation)
This Release Request message is used to release the channel. The
Release Confirm and Indication messages are used to indicate that the
channel has been released.
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_SIOS 0x2 Receipt of SIOS
RELEASE_T6 0x3 Release due to expiration of Timer T6
RELEASE_T7 0x4 Release due to expiration of Timer T7
RELEASE_BSN 0x5 Release due to invalid BSN (2 of 3)
RELEASE_FIB 0x6 Release due to invalid FIB (2 of 3)
RELEASE_SUERM 0x7 Release due to failure reported by SUERM
RELEASE_IAC 0x8 Release due to initial alignment failed
RELEASE_OTHER 0x9 Other reason SS7 link out-of-service
2.3.1.4 State (Request, Confirm)
The State Request message can be sent from a MGC to cause an action
on a particular SS7 link supported by the Signaling Gateway. The
gateway sends a State Confirm to the MGC if the action has been success-
fully completed. The State Confirm reflects that state value received
in the State Request message.
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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.
Define Value Description
STATUS_LPO_SET 0x0 Request local processor outage
STATUS_LPO_CLEAR 0x1 Request local processor outage
recovered
STATUS_EMER_SET 0x2 Request emergency alignment
procedure
STATUS_EMER_CLEAR 0x3 Request normal alignment (cancel
emergency) procedure
STATUS_FLUSH_BUFFERS 0x4 Flush transmit and retransmit
buffers
STATUS_CONTINUE 0x5 Continue
2.3.1.5 State Indication
The MTP2 State Indication message can be sent from a gateway to an
ASP to indicate a condition on a link.
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.
Define Value Description
EVENT_ENTER_LPO 0x0 Entered local processor outage
EVENT_EXIT_LPO 0x1 Exited local processor outage
EVENT_ENTER_CONG 0x2 Entered a congested state
EVENT_EXIT_CONG 0x3 Exited a congested state
EVENT_PHYS_UP 0x4 Physical interface up
EVENT_PHYS_DOWN 0x5 Physical interface down
EVENT_PROTOCOL_ERR 0x6 Protocol error occurred
EVENT_REM_ENTER_CONG 0xc Remote entered congestion
EVENT_REM_EXIT_CONG 0xd Remote exited congestion
EVENT_REM_ENTER_PO 0xe Remote entered processor outage
EVENT_REM_EXIT_PO 0xf Remote exited processor outage
2.3.1.6 Retrieval (Request, Confirm)
The MTP2 Retrieval Request message is used during the MTP Level 3
changeover procedure to request the BSN, to retrieve PDUs from the
retransmit queue or to flush PDUs from the retransmit queue.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Action |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fsn_bsn |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The valid values for Action are shown in the following table.
Define Value Description
ACTION_RTRV_BSN 0x1 Retrieve the backward sequence number
ACTION_RTRV_MSGS 0x2 Retrieve the PDUs from the retransmit
queue
ACTION_DROP_MSGS 0x3 Drop the PDUs in the retransmit queue
In the Retrieval Request message, the fsn_bsn field contains the FSN of
the far end if the action is ACTION_RTRV_MSGS.
When the Signaling Gateway sends a Retrieval Confirm to this request,
it echos the action and puts the BSN in the fsn_bsn field if the action
was ACTION_RTRV_BSN. If there is a failure in retrieving the BSN, the
fsn_bsn should contain a -1 (0xffffffff).
2.3.1.7 Retrieval Indication
The Retrieval Indication message is sent by the Signaling Gateway
with a PDU from the retransmit queue. The Retrieval Indication
message does not contain the Action or fsn_bsn fields, just a PDU from
the retransmit queue.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PDU from retransmit queue |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.3.1.8 Retrieval Complete Indication
The MTP2 Retrieval Complete Indication message is exactly the same as
the MTP2 Retrieval Indication message except that it also indicates that
it contains the last PDU from the retransmit queue.
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 M2UA peer
that the Adaptation layer is ready to receive traffic or maintenance
messages.
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
The ASPUP message contains the following parameters:
Adaptation Layer Identifer (optional)
Protocol Identifier (optional)
INFO String (optional)
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 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.
The optional Adaptation Layer Identifier (ALI) is a string that
identifies the adaptation layer. This string must be set to "M2UA"
which results in a length of 4. 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.
Note: Strings are padded to 32-bit boundaries. The length field
indicates the end of the string.
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
2.3.2.2 ASP Down (ASPDN)
The ASP Down (ASPDN) message is used to indicate to a remote M2UA 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)
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 M2UA
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:
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
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
Within a particular Routing Context, 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/transactions that are pending completion in another ASP.
An SG that receives an ASPAC with an incorrect type for a particular
Interface Identifier will respond with an Error Message.
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.
If no Interface Identifiers are present, then the message is intended
for all Interface Identifiers supported by the SG.
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.)
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
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
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.) If no Interface Identifiers are present, then the
message is intended for all Interface Identifiers supported by the SG.
2.3.3 Layer Management (MGMT) Messages
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 Traffic Handling Mode 0x5
Invalid Stream Identifier 0x5
The optional Diagnostic information can be any information germain 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.
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
2.3.3.2 Notify (NTFY)
The Notify message used to provide autonomous notification of M2UA events.
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)
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
0x4 Application Server Process (ASP) Active_Old
0x5 Application Server Process (ASP) Active_New
Morneault, et al [Page 20]
Internet Draft SS7 MTP2 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 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.)
3.0 Procedures
The M2UA 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 M2UA layer's "Transport of MTP Level 2 /
MTP Level 3 boundary" service.
3.1.1 MTP Level 2 / MTP Level 3 Boundary Procedures
On receiving a primitive from the local upper layer, the M2UA layer will
send the corresponding MAUP message (see Section 2) to its peer. The
M2UA layer must fill in 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 SS7 link.
3.1.2 MAUP Message Procedures
On receiving MAUP messages from a peer M2UA layer, the M2UA layer on an
SG or MGC needs to invoke the corresponding layer primitives to the
local MTP Level 2 or MTP Level 3 layer.
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
3.2 Procedures to Support Service in Section 1.4.2
These procedures achieve the M2UA layer's "Support for Communication
between Layer Managements" service.
3.2.1 Layer Management Primitives Procedure
On receiving these primitives from the local layer, the M2UA layer will
send the corresponding MGMT message (Error) to its peer. The M2UA layer
must fill in the various fields of the common and specific headers
correctly.
3.2.2 MGMT message procedures
Upon receipt of MGMT messages the M2UA layer must invoke the corresponding
Layer Management primitives (M-ERROR) to the local layer management.
3.3 Procedures to Support Service in Section 1.4.3
These procedures achieve the M2UA layer's "Support for management of
active associations between SG and MGC" service.
3.3.1 State Maintenance
The M2UA layer on the SG maintains the state of each AS, in each
Appliction Server that it is configured to receive traffic.
3.3.1.1 ASP States
The state of each ASP, in each AS that it is configured, is maintained
in the M2UA layer in the SG. The state of a particular ASP in a
particular AS changes due to events. The events include:
* Reception of messages from the peer M2UA layer at the ASP
* Reception of some messages from the peer M2UA layer at other ASPs
in the AS
* Reception of indications from the 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: The remote M2UA peer at the ASP is unavailable and/or the SCTP
association is down. Initially all ASPs will be in this state.
ASP-UP: The remote M2UA peer at the ASP is available (and the SCTP
association is up) but application traffic is stopped.
ASP-ACTIVE: The remote M2UA peer at the ASP is available and application
traffic is active (for a particular Routing Context or set of Routing
Contexts).
ASP-ACT-OLD: The remote M2UA peer at the ASP is available and application
traffic is active (for a particular Routing Context or set of Routing
Contexts), but for draining of current call/transactions only (i.e., no
new calls/transactions)
ASP-ACT-NEW: The remote M2UA peer at the ASP is available and application
traffic is active (for a particular Routing Context or set of Routing
Contexts), but for new calls/transactions only (i.e., not for traffic
related to completing calls/transactions in another ASP).
+-------------+
+----------------------| |
| Some other /| ASP-ACTIVE |<--------\
| ASP / +-------------+ |
| Takeover / ^ | | Ts
| / ASP | | ASP |
| / Active | | Inactive |
| v | v |
| +-------------+ +-------------+ +-------------+
| | | | | | |
| | ASP-ACT-OLD |----->| ASP-UP |------>| ASP-ACT-NEW |
| +-------------+ Ts / +-------------+ ASP +-------------+
| | ASP Inactive ^ | Takeover |
|<---| | | |
| | | |
ASP Down/ | ASP | | ASP Down / | ASP
SCTP CDI | Up | | SCTP CDI | Down/
| | v | SCTP
| +-------------+ | CDI
| | | |
+------------------>| |<-------------+
| ASP-DOWN |
+-------------+
SCTP CDI: The local SCTP layer's Communication Down Indication to the
Upper Layer Protocol (M2UA) 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.
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
3.3.1.2 AS States
The state of the AS is maintained in the M2UA 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.
+----------+ one ASP trans ACTIVE +-------------+
| |------------------------>| |
| AS-UP | | AS-ACTIVE |
| | | |
| |< -| |
+----------+ \ / +-------------+
^ | \ Tr Trigger / ^ |
| | \ at least one / | |
| | \ ASP in UP / | |
| | \ / | |
| | \ / | |
| | \ /---/ | |
one ASP | | \ / one ASP | | ACTIVE ASP
trans | | all ASP \-/----\ trans | | trans to UP or
to UP | | trans to / \ ACTIVE | | ACTIVE ASP
| | DOWN / \ | | SCTP CLN
| | / \ | |
| | / \ | |
| | /all ASP \ | |
| v / trans to \ | v
+----------+ / DOWN \ +-------------+
| |<--/ -| |
| AS-DOWN | | AS-PENDING |
| | | (queueing) |
| |<------------------------| |
+----------+ Tr Trigger no ASP +-------------+
in UP state
Tr = Recovery Timer
Figure 5: AS State Transition Diagram
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
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 M2UA layer at an ASP receives an M-SCTP
ESTABLISH request primitive from the Layer Management, the M2UA layer
will try to establish an SCTP association with the remote M2UA peer at
an SG. Upon reception of an eventual SCTP-Communication Up confirm
primitive from the SCTP, the M2UA layer will invoke the primitive M-SCTP
ESTABLISH confirm to the Layer Management.
At the SG, the M2UA layer will receive an SCTP Communication Up
indication primitive from the SCTP. The M2UA layer will then invoke the
primitive M-SCTP ESTABLISH indication to the Layer Management.
Once the SCTP association is established, The M2UA layer at an ASP will
then find out the state of its local M2UA-user from the Layer Management
using the primitive M-ASP STATUS. Based on the status of the local
M2UA-User, the local ASP M2UA 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 M2UA 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 æ0Æ is used.
3.3.3.2 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 M2UA 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
æUpÆ. 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 ôUpö 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.
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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.
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 ôDownö 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 ôDownö 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 M2UA 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.
3.3.3.4 ASP-Active
Anytime 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.
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
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.
There are two modes of Application Server traffic handling in the SG
M2UA - 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 ôInvalid Traffic
Handling Modeö.
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 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/transactions in another ASP
is not sent to the new ASP (i.e., new calls/transactions only). How an
SG accomplishes the differentiation of old and new transactions 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. Most likely, the New Traffic
mode would not be used in M2UA.
3.3.3.5 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 Routing
Contexts to indicate for which Application Servers the ASPIA applies.
There are two modes of Application Server traffic handling in the SG
M2UA 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 ôInvalid Traffic Handling Modeö.
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
ôInactiveö. 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
ôInactiveö state and the AS traffic is re-allocated across the remaining
ôactiveö ASPs per the laoadsharing 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/transactions to other "active" ASPs and therafter sends only
traffic related to incomplete transactons 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/transactions 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. Most likely, Graceful Withdrawal
will not be used with M2UA.
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
If no other ASPs are ôActiveö in the Application Server, the SG either
discards all incoming messages (except messages related to an ôActive_Oldö
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.
4.0 Examples of MTP2 User Adaptation (M2UA) Procedures
4.1 Establishment of associations between SG and MGC examples
4.1.1 Single ASP in an Application Server (ô1+0ö sparing)
This scenario shows the example M2UA 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.
SG ASP1
|
|<---------ASP Up----------|
|------NTFY (ASP-Up)------>|
| |
|<-------ASP Active--------|
|----NTFY (ASP_Active)---->|
| |
4.1.2 Two ASPs in Application Server (ô1+1ö sparing)
This scenario shows the example M2UA message flows for the establishment
of traffic between an SG and two ASPs in the same Application Server,
where ASP1 is configured to be ôactiveö and ASP2 a ôstandbyö 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/transaction 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----------| |
|-------TFY (ASP-Up)----->| |
| | |
|<-----------------------------ASP Up----------------|
|----------------------------NTFY (ASP-Up)---------->|
| | |
| | |
|<-------ASP Active-------| |
|----NTFY(ASP-Active)---->| |
| | |
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4.1.3 Two ASPs in an Application Server (ô1+1ö sparing, load-sharing case)
This scenario shows a similar case to Section 4.1.2 but where the two
ASPs are brought to ôactiveö and loadshare the traffic load. In this
case, one ASP is sufficient to handle the total traffic load.
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 (ôn+kö sparing, load-sharing case)
This scenario shows the example M2UA 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 ôactiveö 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.)--->| |
| | | |
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4.2 ASP Traffic Fail-over Examples
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 M2UA peer (M2UA 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)-->|
| | | |
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
ôn+kö = ô2+1ö for a loadshare AS and ônö currently equals ô1ö.
Note: If the SG detects loss of the ASP1 M2UA peer (M2UA heartbeat loss
or detection of SCTP failure), the first SG-ASP1 ASP Inactive message
exchange would not occur.
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
4.3 SG to MGC, MTP Level 2 to MTP Level 3 Boundary Procedures
4.3.1 SS7 Link Alignment
The MGC can request that a SS7 link be brought into alignment using the
normal or emergency procedure. An example of the message flow to bring
a SS7 link in-service using the normal alignment procedure is shown
below.
SG ASP
| |
|<---------Establish Req--------------|
| |
|----------Establish Cfm------------->|
| |
An example of the message flow to bring a SS7 link in-service using the
emergency alignment procedure.
SG ASP
| |
|<----State Req (STATUS_EMER_SET)-----|
| |
|-----State Cfm (STATUS_EMER_SET)---->|
| |
|<---------Establish Req--------------|
| |
|----------Establish Cfm------------->|
| |
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4.3.2 SS7 Link Release
The MGC can request that a SS7 link be taken out-of-service. It uses
the Release Request message as shown below.
SG ASP
| |
|<-----Release Req (RELEASE_MGMT)-----|
| |
|------------Release Cfm------------->|
| |
The SG can autonomously indicate that a SS7 link has gone out-of-service
as shown below.
SG ASP
| |
|------Release Ind (RELEASE_PHYS)---->|
| |
4.3.3 Set and Clear Local Processor Outage
The MGC can set a Local Processor Outage condition. It uses the
State Request message as shown below.
SG ASP
| |
|<-----State Req (STATUS_LPO_SET)-----|
| |
|------State Cfm (STATUS_LPO_SET)---->|
| |
The MGC can clear a Local Processor Outage condition. It uses the
State Request message as shown below.
SG ASP
| |
|<----State Req (STATUS_LPO_CLEAR)----|
| |
|-----State Cfm (STATUS_LPO_CLEAR)--->|
| |
4.3.4 Notification of Processor Outage (local or remote)
The SG can indicate a Local or Remote Processor Outage condition. It
uses the State Indication message as shown below.
SG ASP
| |
|-----State Ind (EVENT_ENTER_LPO)---->|
| |
|-----State Ind (EVENT_EXIT_LPO)----->|
| |
SG ASP
| |
|-----State Ind (EVENT_ENTER_RPO)---->|
| |
|-----State Ind (EVENT_EXIT_RPO)----->|
| |
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4.3.5 SS7 Link Changeover
An example of the message flow for a changeover is shown below. In this
example, there were three messages in the retransmission queue that
needed to be retrieved.
SG ASP
| |
|----Retrieval Req (MTP2_RTRV_BSN)--->|
| |
|------Retrieval Cfm (with BSN)------>|
| |
|---Retrieval Req (MTP2_RTRV_MSGS)--->|
| with FSN |
| |
|-----------Retrieval Cfm------------>|
| |
|-----------Retrieval Ind------------>|
|-----------Retrieval Ind------------>|
|-------Retrieval Complete Ind------->|
| |
Note: The number of Retrieval Indication is dependent on the number of
messages in the retransmit queue that have been requested. Only one
Retrieval Complete Indication should be sent.
5.0 Security
5.1 Introduction
M2UA is designed to carry signaling messages for telephony services. As such,
M2UA 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.
5.2 Threats
There is no quick fix, one-size-fits-all solution for security. As a
transport protocol, M2UA has the following security objectives:
* Availability of reliable and timely user data transport.
* Integrity of user data transport.
* Confidentiality of user data.
M2UA runs on top of SCTP. SCTP [6] provides certain transport related
security features, such as:
* Blind Denial of Service Attacks
* Flooding
* Masquerade
* Improper Monopolization of Services
When M2UA 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" [9]
should be consulted for guidance.
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
When the network in which M2UA 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 [10] for more
information on configuring IPSEC services.
5.3 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 M2UA value for the Payload
Protocol Identifier in SCTP Payload Data chunk. The following SCTP Payload
Protocol Identifier will be registered:
M2UA 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 Ian Rytina, Hanns Juergen Schwarzbauer
and ZhangYi for their valuable comments and suggestions.
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
8.0 References
[1] ITU-T Recommendation Q.700, 'Introduction To ITU-T Signalling
System No. 7 (SS7)'
[2] ITU-T Recommendation Q.701-Q.705, 'Signalling System No. 7 (SS7) -
Message Transfer Part (MTP)'
[3] ANSI T1.111 'Signalling System Number 7 - Message Transfer Part'
[4] Bellcore GR-246-CORE 'Bell Communications Research Specification
of Signaling System Number 7', Volume 1, December 1995
[5] Framework Architecture for Signaling Transport, draft-ietf-sigtran-
framework-arch-03.txt, June 1999
[6] Simple Control Transmission Protocol, draft-ietf-sigtran-sctp-07.txt,
March 2000
[7] Media Gateway Control Protocol (MGCP), draft-huitema-megaco-mgcp-
v1-03.txt, August 1999
[8] ITU-T Recommendation Q.2210, 'Message transfer part level 3
functions and messages using the services of ITU-T
Recommendation Q.2140'
[9] RFC 2196, "Site Security Handbook", B. Fraser Ed., September 1997
[10] RFC 2401, "Security Architecture for the Internet Protocol", S.
Kent, R. Atkinson, November 1998.
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Internet Draft SS7 MTP2 User Adaptation Layer Mar 2000
9.0 Author's Addresses
Ken Morneault Tel: +1-703-484-3323
Cisco Systems Inc. EMail: kmorneau@cisco.com
13615 Dulles Technology Drive
Herndon, VA. 20171
USA
Malleswar Kalla Tel: +1-973-829-5212
Telcordia Technologies EMail: kalla@research.telcordia.com
MCC 1J211R
445 South Street
Morristown, NJ 07960
USA
Greg Sidebottom Tel: +1-613-763-7305
Nortel Networks EMail: gregside@nortelnetworks.com
3685 Richmond Rd,
Nepean, Ontario
Canada K2H5B7
Ram Dantu, Ph.D. Tel +1-972-234-6070 extension 211
IPmobile EMail rdantu@ipmobile.com
1651 North Glenville, Suite 216
Richardson, TX 75081
USA
Tom George Tel: +1-972-519-3168
Alcatel USA EMail: tom.george@usa.alcatel.com
1000 Coit Road
Plano, TX 74075
USA
This Internet Draft expires September 2000.
