Request for Comments: 3332 Signatus Technologies
Category: Standards Track K. Morneault
Cisco
J. Pastor-Balbas
Ericsson
Editors
September 2002
Signaling System 7 (SS7) Message Transfer Part 3 (MTP3) -
User Adaptation Layer (M3UA)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This memo defines a protocol for supporting the transport of any SS7
MTP3-User signalling (e.g., ISUP and SCCP messages) over IP using the
services of the Stream Control Transmission Protocol. Also,
provision is made for protocol elements that enable a seamless
operation of the MTP3-User peers in the SS7 and IP domains. This
protocol would be used between a Signalling Gateway (SG) and a Media
Gateway Controller (MGC) or IP-resident Database, or between two
IP-based applications. It is assumed that the SG receives SS7
signalling over a standard SS7 interface using the SS7 Message
Transfer Part (MTP) to provide transport.
Table of Contents
1. Introduction..................................................3
1.1 Scope.........................................................3
1.2 Terminology...................................................4
1.3 M3UA Overview.................................................6
1.4 Functional Areas.............................................10
1.5 Sample Configurations........................................18
1.6 Definition of M3UA Boundaries................................21
2. Conventions..................................................25
3. M3UA Protocol Elements.......................................25
3.1 Common Message Header........................................26
3.2 Variable Length Parameter....................................29
3.3 Transfer Messages............................................31
3.4 SS7 Signalling Network Management (SSNM) Messages............35
3.5 ASP State Maintenance (ASPSM) Messages.......................45
3.6 Routing Key Management (RKM) Messages........................48
3.7 ASP Traffic Maintenance (ASPTM) Messages.....................59
3.8 Management (MGMT) Messages...................................63
4. Procedures...................................................69
4.1 Procedures to Support the M3UA-User .........................69
4.2 Procedures to Support the Management of SCTP Associations ...70
4.3 AS and ASP State Maintenance.................................72
4.4 Routing Key Management Procedures............................87
4.5 Procedures to Support the Availability or Congestion Status
of SS7 Destination...........................................89
4.6 MTP3 Restart.................................................92
5. Examples of M3UA Procedures..................................93
5.1 Establishment of Association and Traffic
Between SGs and ASPs.........................................93
5.2 ASP traffic Failover Examples................................99
5.3 Normal Withdrawal of an ASP from an Application Server
and Teardown of an Association..............................100
5.4 M3UA/MTP3-User Boundary Examples............................101
5.5 Examples of IPSP communication..............................105
6. Security Considerations.....................................108
6.1 Introduction................................................108
6.2 Threats.....................................................108
6.3 Protecting Confidentiality..................................108
7. IANA Considerations.........................................109
7.1 SCTP Payload Protocol Identifier............................109
7.2 M3UA Port Number............................................109
7.3 M3UA Protocol Extensions....................................109
8. References...................................................111
8.1 Normative References........................................111
8.2 Informative References......................................111
9. Acknowledgements.............................................113
10. Document Contributors.......................................113
Appendix A......................................................114
A.1 Signalling Network Architecture.............................114
A.2 Redundancy Models...........................................117
Editors' Addresses..............................................119
Full Copyright Statement........................................120
1. Introduction
This memo defines a protocol for supporting the transport of any SS7
MTP3-User signalling (e.g., ISUP and SCCP messages) over IP using the
services of the Stream Control Transmission Protocol [17]. Also,
provision is made for protocol elements that enable a seamless
operation of the MTP3-User peers in the SS7 and IP domains. This
protocol would be used between a Signalling Gateway (SG) and a Media
Gaway Controller (MGC) or IP-resident Database [11], or between two
IP-based applications.
1.1 Scope
There is a need for Switched Circuit Network (SCN) signalling
protocol delivery from an SS7 Signalling Gateway (SG) to a Media
Gateway Controller (MGC) or IP-resident Database as described in the
Framework Architecture for Signalling Transport [11]. The delivery
mechanism should meet the following criteria:
* Support for the transfer of all SS7 MTP3-User Part messages (e.g.,
ISUP [1,2,3], SCCP [4,5,6], TUP [12], etc.)
* Support for the seamless operation of MTP3-User protocol peers
* Support for the management of SCTP transport associations and
traffic between an SG and one or more MGCs or IP-resident
Databases
* Support for MGC or IP-resident Database process failover and load
sharing
* Support for the asynchronous reporting of status changes to
management
In simplistic transport terms, the SG will terminate SS7 MTP2 and
MTP3 protocol layers [7,8,9] and deliver ISUP, SCCP and/or any other
MTP3-User protocol messages, as well as certain MTP network
management events, over SCTP transport associations to MTP3-User
peers in MGCs or IP-resident Databases.
1.2 Terminology
Application Server (AS) - A logical entity serving a specific Routing
Key. An example of an Application Server is a virtual switch element
handling all call processing for a unique range of PSTN trunks,
identified by an SS7 SIO/DPC/OPC/CIC_range. Another example is a
virtual database element, handling all HLR transactions for a
particular SS7 DPC/OPC/SCCP_SSN combination. The AS contains a set
of one or more unique Application Server Processes, of which one or
more is normally actively processing traffic. Note that there is a
1:1 relationship between an AS and a Routing Key.
Application Server Process (ASP) - A process instance of an
Application Server. An Application Server Process serves as an active
or backup process of an Application Server (e.g., part of a
distributed virtual switch or database). Examples of ASPs are
processes (or process instances) of MGCs, IP SCPs or IP HLRs. An ASP
contains an SCTP endpoint and may be configured to process signalling
traffic within more than one Application Server.
Association - An association refers to an SCTP association. The
association provides the transport for the delivery of MTP3-User
protocol data units and M3UA adaptation layer peer messages.
IP Server Process (IPSP) - A process instance of an IP-based
application. An IPSP is essentially the same as an ASP, except that
it uses M3UA in a point-to-point fashion. Conceptually, an IPSP does
not use the services of a Signalling Gateway node.
Failover - The capability to reroute signalling 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. Failover also applies
upon the return to service of a previously unavailable Application
Server Process.
Host - The computing platform that the process (SGP, ASP or IPSP) is
running on.
Layer Management - Layer Management is a nodal function that handles
the inputs and outputs between the M3UA layer and a local management
entity.
Linkset - A number of signalling links that directly interconnect two
signalling points, which are used as a module.
MTP - The Message Transfer Part of the SS7 protocol.
MTP3 - MTP Level 3, the signalling network layer of SS7
MTP3-User - Any protocol normally using the services of the SS7 MTP3
(e.g., ISUP, SCCP, TUP, etc.).
Network Appearance - The Network Appearance is a M3UA local reference
shared by SG and AS (typically an integer) that together with an
Signaling Point Code uniquely identifies an SS7 node by indicating
the specific SS7 network it belongs to. It can be used to distinguish
between signalling traffic associated with different networks being
sent between the SG and the ASP over a common SCTP association. An
example scenario is where an SG appears as an element in multiple
separate national SS7 networks and the same Signaling Point Code
value may be reused in different networks.
Network Byte Order: Most significant byte first, a.k.a Big Endian.
Routing Key: A Routing Key describes a set of SS7 parameters and
parameter values that uniquely define the range of signalling traffic
to be handled by a particular Application Server. Parameters within
the Routing Key cannot extend across more than a single Signalling
Point Management Cluster.
Routing Context - A value that uniquely identifies a Routing Key.
Routing Context values are either configured using a configuration
management interface, or by using the routing key management
procedures defined in this document.
Signalling Gateway Process (SGP) - A process instance of a Signalling
Gateway. It serves as an active, backup, load-sharing or broadcast
process of a Signalling Gateway.
Signalling Gateway - An SG is a signaling agent that receives/sends
SCN native signaling at the edge of the IP network [11]. An SG
appears to the SS7 network as an SS7 Signalling Point. An SG
contains a set of one or more unique Signalling Gateway Processes, of
which one or more is normally actively processing traffic. Where an
SG contains more than one SGP, the SG is a logical entity and the
contained SGPs are assumed to be coordinated into a single management
view to the SS7 network and to the supported Application Servers.
Signalling Process - A process instance that uses M3UA to communicate
with other signalling processes. An ASP, an SGP and an IPSP are all
signalling processes.
Signalling Point Management Cluster (SPMC) - The complete set of
Application Servers represented to the SS7 network under a single MTP
entity (Signalling Point) in one specific Network Appearance. SPMCs
are used to aggregate the availability, congestion, and user part
status of an MTP entity (Signalling Point) that is distributed in the
IP domain, for the purpose of supporting MTP3 management procedures
towards the SS7 network. In some cases, the SG itself may also be a
member of the SPMC. In this case, the SG availability /congestion
/User_Part status should also be taken into account when considering
any supporting MTP3 management actions.
Stream - A stream refers to an SCTP stream; a unidirectional logical
channel established from one SCTP endpoint to another associated SCTP
endpoint, within which all user messages are delivered in-sequence
except for those submitted to the unordered delivery service.
1.3 M3UA Overview
1.3.1 Protocol Architecture
The framework architecture that has been defined for SCN signalling
transport over IP [11] uses multiple components, including a common
signalling transport protocol and an adaptation module to support the
services expected by a particular SCN signalling protocol from its
underlying protocol layer.
Within the framework architecture, this document defines an MTP3-User
adaptation module suitable for supporting the transfer of messages of
any protocol layer that is identified to the MTP Level 3 as an MTP
User. The list of these protocol layers includes, but is not limited
to, ISDN User Part (ISUP) [1,2,3], Signalling Connection Control Part
(SCCP) [4,5,6] and Telephone User Part (TUP) [12]. TCAP [13,14,15]
or RANAP [16] messages are transferred transparently by the M3UA
protocol as SCCP payload, as they are SCCP-User protocols.
It is recommended that M3UA use the services of the Stream Control
Transmission Protocol (SCTP) [17] as the underlying reliable common
signalling transport protocol. This is to take advantage of various
SCTP features such as:
- Explicit packet-oriented delivery (not stream-oriented),
- Sequenced delivery of user messages within multiple streams,
with an option for order-of-arrival delivery of individual
user messages,
- Optional multiplexing of user messages into SCTP datagrams,
- Network-level fault tolerance through support of multi-homing
at either or both ends of an association,
- Resistance to flooding and masquerade attacks, and
- Data segmentation to conform to discovered path MTU size.
Under certain scenarios, such as back-to-back connections without
redundancy requirements, the SCTP functions above might not be a
requirement and TCP MAY be used as the underlying common transport
protocol.
1.3.2 Services Provided by the M3UA Layer
The M3UA Layer at an ASP or IPSP provides the equivalent set of
primitives at its upper layer to the MTP3-Users as provided by the
MTP Level 3 to its local MTP3-Users at an SS7 SEP. In this way, the
ISUP and/or SCCP layer at an ASP or IPSP is unaware that the expected
MTP3 services are offered remotely from an MTP3 Layer at an SGP, and
not by a local MTP3 layer. The MTP3 layer at an SGP may also be
unaware that its local users are actually remote user parts over
M3UA. In effect, the M3UA extends access to the MTP3 layer services
to a remote IP-based application. The M3UA layer does not itself
provide the MTP3 services. However, in the case where an ASP is
connected to more than one SG, the M3UA layer at an ASP should
maintain the status of configured SS7 destinations and route messages
according to the availability and congestion status of the routes to
these destinations via each SG.
The M3UA layer may also be used for point-to-point signalling between
two IP Server Processes (IPSPs). In this case, the M3UA layer
provides the same set of primitives and services at its upper layer
as the MTP3. However, in this case the expected MTP3 services are not
offered remotely from an SGP. The MTP3 services are provided but the
procedures to support these services are a subset of the MTP3
procedures due to the simplified point-to-point nature of the IPSP to
IPSP relationship.
1.3.2.1 Support for the Transport of MTP3-User Messages
The M3UA layer provides the transport of MTP-TRANSFER primitives
across an established SCTP association between an SGP and an ASP or
between IPSPs.
At an ASP, in the case where a destination is reachable via multiple
SGPs, the M3UA layer must also choose via which SGP the message is to
be routed or support load balancing across the SGPs, minimizing
missequencing.
The M3UA layer does not impose a 272-octet signalling information
field (SIF) length limit as specified by the SS7 MTP Level 2 protocol
[7,8,9]. Larger information blocks can be accommodated directly by
M3UA/SCTP, without the need for an upper layer segmentation/re-
assembly procedure as specified in recent SCCP or ISUP versions.
However, in the context of an SG, the maximum 272-octet block size
must be followed when interworking to a SS7 network that does not
support the transfer of larger information blocks to the final
destination. This avoids potential ISUP or SCCP fragmentation
requirements at the SGPs. The provisioning and configuration of the
SS7 network determines the restriction placed on the maximum block
size. Some configurations (e.g., Broadband MTP [21]) may permit
larger block sizes.
1.3.2.2 Native Management Functions
The M3UA layer provides the capability to indicate errors associated
with received M3UA messages and to notify, as appropriate, local
management and/or the peer M3UA.
1.3.2.3 Interworking with MTP3 Network Management Functions
At the SGP, the M3UA layer provides interworking with MTP3 management
functions to support seamless operation of the user SCN signalling
applications in the SS7 and IP domains. This includes:
- Providing an indication to MTP3-Users at an ASP that a destination
in the SS7 network is not reachable.
- Providing an indication to MTP3-Users at an ASP that a destination
in the SS7 network is now reachable.
- Providing an indication to MTP3-Users at an ASP that messages to a
destination in the SS7 network are experiencing SS7 congestion.
- Providing an indication to the M3UA layer at an ASP that the routes
to a destination in the SS7 network are restricted.
- Providing an indication to MTP3-Users at an ASP that a MTP3-User
peer is unavailable.
The M3UA layer at an ASP keeps the state of the routes to remote SS7
destinations and may initiate an audit of the availability, the
restricted or the congested state of remote SS7 destinations. This
information is requested from the M3UA layer at the SGP.
The M3UA layer at an ASP may also indicate to the SG that the M3UA
layer itself or the ASP or the ASP's Host is congested.
1.3.2.4 Support for the Management of SCTP Associations between the SGP
and ASPs.
The M3UA layer at the SGP maintains the availability state of all
configured remote ASPs, to manage the SCTP Associations and the
traffic between the M3UA peers. As well, the active/inactive and
congestion state of remote ASPs is maintained.
The M3UA layer MAY be instructed by local management to establish an
SCTP association to a peer M3UA node. This can be achieved using the
M-SCTP_ESTABLISH primitives (See Section 1.6.3 for a description of
management primitives.) to request, indicate and confirm the
establishment of an SCTP association with a peer M3UA node. In order
to avoid redundant SCTP associations between two M3UA peers, one side
(client) SHOULD be designated to establish the SCTP association, or
M3UA configuration information maintained to detect redundant
associations (e.g., via knowledge of the expected local and remote
SCTP endpoint addresses).
Local management MAY request from the M3UA layer the status of the
underlying SCTP associations using the M-SCTP_STATUS request and
confirm primitives. Also, the M3UA MAY autonomously inform local
management of the reason for the release of an SCTP association,
determined either locally within the M3UA layer or by a primitive
from the SCTP.
Also the M3UA layer MAY inform the local management of the change in
status of an ASP or AS. This MAY be achieved using the M-ASP_STATUS
request or M-AS_STATUS request primitives.
1.3.2.5 Support for the Management of Connections to Multiple SGPs
As shown in Figure 1 an ASP may be connected to multiple SGPs. In
such a case a particular SS7 destination may be reachable via more
than one SGP and/or SG, i.e., via more than one route. As MTP3 users
only maintain status on a destination and not on a route basis, the
M3UA layer must maintain the status (availability, restriction,
and/or congestion of route to destination) of the individual routes,
derive the overall availability or congestion status of the
destination from the status of the individual routes, and inform the
MTP3 users of this derived status whenever it changes.
1.4 Functional Areas
1.4.1 Signalling Point Code Representation
For example, within an SS7 network, a Signalling Gateway might be
charged with representing a set of nodes in the IP domain into the
SS7 network for routing purposes. The SG itself, as a signalling
point in the SS7 network, might also be addressable with an SS7 Point
Code for MTP3 Management purposes. The SG Point Code might also be
used for addressing any local MTP3-Users at the SG such as a local
SCCP layer.
An SG may be logically partitioned to operate in multiple SS7 network
appearances. In such a case, the SG could be addressable with a
Point Code in each network appearance, and represents a set of nodes
in the IP domain into each SS7 network. Alias Point Codes [8] may
also be used within an SG network appearance.
Where an SG contains more than one SGP, the MTP3 routeset, SPMC and
remote AS/ASP states of each SGP SHOULD be coordinated across all the
SGPs. Rerouting of traffic between the SGPs MAY also be supported.
Application Servers can be represented under the same Point Code of
the SG, their own individual Point Codes or grouped with other
Application Servers for Point Code preservation purposes. A single
Point Code may be used to represent the SG and all the Application
Servers together, if desired.
If an ASP or group of ASPs is available to the SS7 network via more
than one SG, each with its own Point Code, the ASP(s) will typically
be represented by a Point Code that is separate from any SG Point
Code. This allows, for example, these SGs to be viewed from the SS7
network as "STPs", each having an ongoing "route" to the same ASP(s).
Under failure conditions where the ASP(s) become(s) unavailable from
one of the SGs, this approach enables MTP3 route management messaging
between the SG and SS7 network, allowing simple SS7 rerouting through
an alternate SG without changing the Destination Point Code Address
of SS7 traffic to the ASP(s).
Where a particular AS can be reached via more than one SGP, the
corresponding Routing Keys in the SGPs should be identical. (Note:
It is possible for the SGP Routing Key configuration data to be
temporarily out-of-sync during configuration updates).
+--------+
| |
+------------+ SG 1 +--------------+
+-------+ | SS7 links | "STP" | IP network | ----
| SEP +---+ +--------+ +---/ \
| or | |* | ASPs |
| STP +---+ +--------+ +---\ /
+-------+ | | | | ----
+------------+ SG 2 +--------------+
| "STP" |
+--------+
Figure 1 Example with mated SGs
* Note:. SG-to-SG communication (i.e., "C-links") is recommended for
carrier grade networks, using an MTP3 linkset or an equivalent, to
allow rerouting between the SGs in the event of route failures. Where
SGPs are used, inter-SGP communication might be used. Inter-SGP
protocol is outside of the scope of this document.
The following example shows a signalling gateway partitioned into two
network appearances.
SG
+-------+ +---------------+
| SEP +--------------| SS7 Ntwk |M3UA| ----
+-------+ SS7 links | "A" | | / \
|__________| +-----------+ ASPs |
| | | \ /
+-------+ | SS7 Ntwk | | ----
| SEP +--------------+ "B" | |
+-------+ +---------------+
Figure 2 Example with multiple Network
1.4.2 Routing Contexts and Routing Keys
1.4.2.1 Overview
The distribution of SS7 messages between the SGP and the Application
Servers is determined by the Routing Keys and their associated
Routing Contexts. A Routing Key is essentially a set of SS7
parameters used to filter SS7 messages, whereas the Routing Context
parameter is a 4-byte value (integer) that is associated to that
Routing Key in a 1:1 relationship. The Routing Context therefore can
be viewed as an index into a sending node's Message Distribution
Table containing the Routing Key entries.
Possible SS7 address/routing information that comprise a Routing Key
entry includes, for example, the OPC, DPC, SIO found in the MTP3
routing label, or MTP3-User specific fields (such as the ISUP CIC,
SCCP subsystem number). Some example Routing Keys are: the DPC
alone, the DPC/OPC combination, the DPC/OPC/CIC combination, or the
DPC/SSN combination. The particular information used to define an
M3UA Routing Key is application and network dependent, and none of
the above examples are mandated.
An Application Server Process may be configured to process signalling
traffic related to more than one Application Server, over a single
SCTP Association. In ASP Active and ASP Inactive management
messages, the signalling traffic to be started or stopped is
discriminated by the Routing Context parameter. At an ASP, the
Routing Context parameter uniquely identifies the range of signalling
traffic associated with each Application Server that the ASP is
configured to receive.
1.4.2.2 Routing Key Limitations
Routing Keys SHOULD be unique in the sense that each received SS7
signalling message SHOULD have a full or partial match to a single
routing result. It is not necessary for the parameter range values
within a particular Routing Key to be contiguous. For example, an AS
could be configured to support call processing for multiple ranges of
PSTN trunks that are not represented by contiguous CIC values.
1.4.2.3 Managing Routing Contexts and Routing Keys
There are two ways to provision a Routing Key at an SGP. A Routing
Key may be configured statically using an implementation dependent
management interface, or dynamically using the M3UA Routing Key
registration procedure.
When using a management interface to configure Routing Keys, the
message distribution function within the SGP is not limited to the
set of parameters defined in this document. Other implementation
dependent distribution algorithms may be used.
1.4.2.4 Message Distribution at the SGP
To direct messages received from the SS7 MTP3 network to the
appropriate IP destination, the SGP must perform a message
distribution function using information from the received MTP3-User
message.
To support this message distribution, the SGP might, for example,
maintain the equivalent of a network address translation table,
mapping incoming SS7 message information to an Application Server for
a particular application and range of traffic. This could be
accomplished by comparing elements of the incoming SS7 message to
currently defined Routing Keys in the SGP.
These Routing Keys could in turn map directly to an Application
Server that is enabled by one or more ASPs. These ASPs provide
dynamic status information regarding their availability, traffic
handling capability and congestion to the SGP using various
management messages defined in the M3UA protocol.
The list of ASPs in an AS is assumed to be dynamic, taking into
account the availability, traffic handling capability and congestion
status of the individual ASPs in the list, as well as configuration
changes and possible failover mechanisms.
Normally, one or more ASPs are active (i.e., currently processing
traffic) in the AS but in certain failure and transition cases it is
possible that there may be no active ASP available. Broadcast,
loadsharing and backup scenarios are supported.
When there is no matching Routing Key entry for an incoming SS7
message, a default treatment MAY be specified. Possible solutions
are to provide a default Application Server at the SGP that directs
all unallocated traffic to a (set of) default ASP(s), or to drop the
message and provide a notification to layer management. The
treatment of unallocated traffic is implementation dependent.
1.4.2.5 Message Distribution at the ASP
The ASP must choose an SGP to direct a message to the SS7 network.
This is accomplished by observing the Destination Point Code (and
possibly other elements of the outgoing message such as the SLS
value). The ASP must also take into account whether the related
Routing Context is active or not (See Section 4.3.4.3).
Implementation Note: Where more than one route (or SGP) is possible
for routing to the SS7 network, the ASP could, for example, maintain
a dynamic table of available SGP routes for the SS7 destinations,
taking into account the SS7 destination
availability/restricted/congestion status received from the SGP(s),
the availability status of the individual SGPs and configuration
changes and failover mechanisms. There is, however, no M3UA messaging
to manage the status of an SGP (e.g., SGP-Up/Down/Active/Inactive
messaging).
Whenever an SCTP association to an SGP exists, the SGP is assumed to
be ready for the purposes of responding to M3UA ASPSM messages (Refer
to Section 3).
1.4.3 SS7 and M3UA Interworking
In the case of SS7 and M3UA interworking, the M3UA adaptation layer
is designed to provide an extension of the MTP3 defined user
primitives.
1.4.3.1 Signalling Gateway SS7 Layers
The SG is responsible for terminating MTP Level 3 of the SS7
protocol, and offering an IP-based extension to its users.
From an SS7 perspective, it is expected that the Signalling Gateway
transmits and receives SS7 Message Signalling Units (MSUs) to and
from the PSTN over a standard SS7 network interface, using the SS7
Message Transfer Part (MTP) [7,8,9] to provide reliable transport of
the messages.
As a standard SS7 network interface, the use of MTP Level 2
signalling links is not the only possibility. ATM-based High Speed
Links can also be used with the services of the Signalling ATM
Adaptation Layer (SAAL) [18,19].
Note: It is also possible for IP-based interfaces to be present,
using the services of the MTP2-User Adaptation Layer (M2UA) [27] or
M2PA [28].
These could be terminated at a Signalling Transfer Point (STP) or
Signalling End Point (SEP). Using the services of MTP3, the SG could
be capable of communicating with remote SS7 SEPs in a quasi-
associated fashion, where STPs may be present in the SS7 path between
the SEP and the SG.
1.4.3.2 SS7 and M3UA Interworking at the SG
The SGP provides a functional interworking of transport functions
between the SS7 network and the IP network by also supporting the
M3UA adaptation layer. It allows the transfer of MTP3-User
signalling messages to and from an IP-based Application Server
Process where the peer MTP3-User protocol layer exists.
For SS7 user part management, it is required that the MTP3-User
protocols at ASPs receive indications of SS7 signalling point
availability, SS7 network congestion, and remote User Part
unavailability as would be expected in an SS7 SEP node. To
accomplish this, the MTP-PAUSE, MTP-RESUME and MTP-STATUS indication
primitives received at the MTP3 upper layer interface at the SG need
to be propagated to the remote MTP3-User lower layer interface at the
ASP.
MTP3 management messages (such as TFPs or TFAs received from the SS7
network) MUST NOT be encapsulated as Data message Payload Data and
sent either from SG to ASP or from ASP to SG. The SG MUST terminate
these messages and generate M3UA messages as appropriate.
1.4.3.3 Application Server
A cluster of application servers is responsible for providing the
overall support for one or more SS7 upper layers. From an SS7
standpoint, a Signalling Point Management Cluster (SPMC) provides
complete support for the upper layer service for a given point code.
As an example, an SPMC providing MGC capabilities could provide
complete support for ISUP (and any other MTP3 user located at the
point code of the SPMC) for a given point code.
In the case where an ASP is connected to more than one SGP, the M3UA
layer must maintain the status of configured SS7 destinations and
route messages according to availability/congestion/restricted status
of the routes to these SS7 destinations.
1.4.3.4 IPSP Considerations
Since IPSPs use M3UA in a point-to-point fashion, there is no concept
of routing of messages beyond the remote end. Therefore, SS7 and
M3UA interworking is not necessary for this model.
1.4.4 Redundancy Models
1.4.4.1 Application Server Redundancy
All MTP3-User messages (e.g., ISUP, SCCP) which match a provisioned
Routing Key at an SGP are mapped to an Application Server.
The Application Server is the set of all ASPs associated with a
specific Routing Key. Each ASP in this set may be active, inactive or
unavailable. Active ASPs handle traffic; inactive ASPs might be used
when active ASPs become unavailable.
The failover model supports an "n+k" redundancy model, where "n" ASPs
is the minimum number of redundant ASPs required to handle traffic
and "k" ASPs are available to take over for a failed or unavailable
ASP. A "1+1" active/backup redundancy is a subset of this model. A
simplex "1+0" model is also supported as a subset, with no ASP
redundancy.
1.4.5 Flow Control
Local Management at an ASP may wish to stop traffic across an SCTP
association 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.
1.4.6 Congestion Management
The M3UA layer is informed of local and IP network congestion by
means of an implementation-dependent function (e.g., an
implementation dependent indication from the SCTP of IP network
congestion).
At an ASP or IPSP, the M3UA layer indicates congestion to local
MTP3-Users by means of an MTP-STATUS primitive, as per current MTP3
procedures, to invoke appropriate upper layer responses.
When an SG determines that the transport of SS7 messages to a
Signalling Point Management Cluster (SPMC) is encountering
congestion, the SG MAY trigger SS7 MTP3 Transfer Controlled
management messages to originating SS7 nodes, per the congestion
procedures of the relevant MTP3 standard. The triggering of SS7 MTP3
Management messages from an SG is an implementation-dependent
function.
The M3UA layer at an ASP or IPSP MAY indicate local congestion to an
M3UA peer with an SCON message. When an SG receives a congestion
message (SCON) from an ASP, and the SG determines that an SPMC is now
encountering congestion, it MAY trigger SS7 MTP3 Transfer Controlled
management messages to concerned SS7 destinations according to
congestion procedures of the relevant MTP3 standard.
1.4.7 SCTP Stream Mapping.
The M3UA layer at both the SGP and ASP also supports the assignment
of signalling traffic into streams within an SCTP association.
Traffic that requires sequencing SHOULD be assigned to the same
stream. To accomplish this, MTP3-User traffic may be assigned to
individual streams based on, for example, the SLS value in the MTP3
Routing Label or the ISUP CIC assignment, subject of course to the
maximum number of streams supported by the underlying SCTP
association.
1.4.8 Client/Server Model
It is recommended that the SGP and ASP be able to support both client
and server operation. The peer endpoints using M3UA SHOULD be
configured so that one always takes on the role of client and the
other the role of server for initiating SCTP associations. The
default orientation would be for the SGP to take on the role of
server while the ASP is the client. In this case, ASPs SHOULD
initiate the SCTP association to the SGP.
In the case of IPSP to IPSP communication, the peer endpoints using
M3UA SHOULD be configured so that one always takes on the role of
client and the other the role of server for initiating SCTP
associations.
The SCTP and TCP Registered User Port Number Assignment for M3UA is
2905.
1.5 Sample Configuration
1.5.1 Example 1: ISUP Message Transport
******** SS7 ***************** IP ********
* SEP *---------* SGP *--------* ASP *
******** ***************** ********
+------+ +---------------+ +------+
| ISUP | | (NIF) | | ISUP |
+------+ +------+ +------+ +------+
| MTP3 | | MTP3 | | M3UA | | M3UA |
+------| +------+-+------+ +------+
| MTP2 | | MTP2 | | SCTP | | SCTP |
+------+ +------+ +------+ +------+
| L1 | | L1 | | IP | | IP |
+------+ +------+ +------+ +------+
|_______________| |______________|
SEP - SS7 Signalling End Point
SCTP - Stream Control Transmission Protocol
NIF - Nodal Interworking Function
In this example, the SGP provides an implementation-dependent nodal
interworking function (NIF) that allows the MGC to exchange SS7
signalling messages with the SS7-based SEP. The NIF within the SGP
serves as the interface within the SGP between the MTP3 and M3UA.
This nodal interworking function has no visible peer protocol with
either the MGC or SEP. It also provides network status information
to one or both sides of the network.
For internal SGP modeling purposes, at the NIF level, SS7 signalling
messages that are destined to the MGC are received as MTP-TRANSFER
indication primitives from the MTP Level 3 upper layer interface,
translated to MTP-TRANSFER request primitives, and sent to the local
M3UA-resident message distribution function for ongoing routing to
the final IP destination. Messages received from the local M3UA
network address translation and mapping function as MTP-TRANSFER
indication primitives are sent to the MTP Level 3 upper layer
interface as MTP-TRANSFER request primitives for ongoing MTP Level 3
routing to an SS7 SEP. For the purposes of providing SS7 network
status information the NIF also delivers MTP-PAUSE, MTP-RESUME and
MTP-STATUS indication primitives received from the MTP Level 3 upper
layer interface to the local M3UA-resident management function. In
addition, as an implementation and network option, restricted
destinations are communicated from MTP network management to the
local M3UA-resident management function.
1.5.2 Example 2: SCCP Transport between IPSPs
******** IP ********
* IPSP * * IPSP *
******** ********
+------+ +------+
|SCCP- | |SCCP- |
| User | | User |
+------+ +------+
| SCCP | | SCCP |
+------+ +------+
| M3UA | | M3UA |
+------+ +------+
| SCTP | | SCTP |
+------+ +------+
| IP | | IP |
+------+ +------+
|________________|
This example shows an architecture where no Signalling Gateway is
used. In this example, SCCP messages are exchanged directly between
two IP-resident IPSPs with resident SCCP-User protocol instances,
such as RANAP or TCAP. SS7 network interworking is not required,
therefore there is no MTP3 network management status information for
the SCCP and SCCP-User protocols to consider. Any MTP-PAUSE, MTP-
RESUME or MTP-STATUS indications from the M3UA layer to the SCCP
layer should consider the status of the SCTP Association and
underlying IP network and any congestion information received from
the remote site.
1.5.3 Example 3: SGP Resident SCCP Layer, with Remote ASP
******** SS7 ***************** IP ********
* SEP *---------* *--------* *
* or * * SGP * * ASP *
* STP * * * * *
******** ***************** ********
+------+ +---------------+ +------+
| SCCP-| | SCCP | | SCCP-|
| User | +---------------+ | User |
+------+ | _____ | +------+
| SCCP | | | | | | SCCP |
+------+ +------+-+------+ +------+
| MTP3 | | MTP3 | | M3UA | | M3UA |
+------| +------+ +------+ +------+
| MTP2 | | MTP2 | | SCTP | | SCTP |
+------+ +------+ +------+ +------+
| L1 | | L1 | | IP | | IP |
+------+ +------+ +------+ +------+
|_______________| |______________|
STP - SS7 Signalling Transfer Point
In this example, the SGP contains an instance of the SS7 SCCP
protocol layer that may, for example, perform the SCCP Global Title
Translation (GTT) function for messages logically addressed to the SG
SCCP. If the result of a GTT for an SCCP message yields an SS7 DPC
or DPC/SSN address of an SCCP peer located in the IP domain, the
resulting MTP-TRANSFER request primitive is sent to the local M3UA-
resident network address translation and mapping function for ongoing
routing to the final IP destination.
Similarly, the SCCP instance in an SGP can perform the SCCP GTT
service for messages logically addressed to it from SCCP peers in the
IP domain. In this case, MTP-TRANSFER indication primitives are sent
from the local M3UA-resident network address translation and mapping
function to the SCCP for GTT. If the result of the GTT yields the
address of an SCCP peer in the SS7 network then the resulting MTP-
TRANSFER request primitive is given to the MTP3 for delivery to an
SS7-resident node.
It is possible that the above SCCP GTT at the SGP could yield the
address of an SCCP peer in the IP domain and the resulting MTP-
TRANSFER request primitive would be sent back to the M3UA layer for
delivery to an IP destination.
For internal SGP modeling purposes, this may be accomplished with the
use of an implementation-dependent nodal interworking function within
the SGP that effectively sits below the SCCP and routes MTP-TRANSFER
request/indication messages to/from both the MTP3 and the M3UA layer,
based on the SS7 DPC or DPC/SSN address information. This nodal
interworking function has no visible peer protocol with either the
ASP or SEP.
Note that the services and interface provided by the M3UA layer are
the same as in Example 1 and the functions taking place in the SCCP
entity are transparent to the M3UA layer. The SCCP protocol
functions are not reproduced in the M3UA protocol.
1.6 Definition of M3UA Boundaries
1.6.1 Definition of the Boundary between M3UA and an MTP3-User.
From ITU Q.701 [7]:
MTP-TRANSFER request
MTP-TRANSFER indication
MTP-PAUSE indication
MTP-RESUME indication
MTP-STATUS indication
1.6.2 Definition of the Boundary between M3UA and SCTP
An example of the upper layer primitives provided by the SCTP are
provided in Reference [17] Section 10.
1.6.3 Definition of the Boundary between M3UA and Layer Management
M-SCTP_ESTABLISH request
Direction: LM -> M3UA
Purpose: LM requests ASP to establish an SCTP association with its
peer.
M-STCP_ESTABLISH confirm
Direction: M3UA -> LM
Purpose: ASP confirms to LM that it has established an SCTP
association with its peer.
M-SCTP_ESTABLISH indication
Direction: M3UA -> LM
Purpose: M3UA informs LM that a remote ASP has established an SCTP
association.
M-SCTP_RELEASE request
Direction: LM -> M3UA
Purpose: LM requests ASP to release an SCTP association with its
peer.
M-SCTP_RELEASE confirm
Direction: M3UA -> LM
Purpose: ASP confirms to LM that it has released SCTP association
with its peer.
M-SCTP_RELEASE indication
Direction: M3UA -> LM
Purpose: M3UA informs LM that a remote ASP has released an SCTP
Association or the SCTP association has failed.
M-SCTP_RESTART indication
Direction: M3UA -> LM
Purpose: M3UA informs LM that an SCTP restart indication has been
received.
M-SCTP_STATUS request
Direction: LM -> M3UA
Purpose: LM requests M3UA to report the status of an SCTP
association.
M-SCTP_STATUS confirm
Direction: M3UA -> LM
Purpose: M3UA responds with the status of an SCTP association.
M-SCTP STATUS indication
Direction: M3UA -> LM
Purpose: M3UA reports the status of an SCTP association.
M-ASP_STATUS request
Direction: LM -> M3UA
Purpose: LM requests M3UA to report the status of a local or remote
ASP.
M-ASP_STATUS confirm
Direction: M3UA -> LM
Purpose: M3UA reports status of local or remote ASP.
M-AS_STATUS request
Direction: LM -> M3UA
Purpose: LM requests M3UA to report the status of an AS.
M-AS_STATUS confirm
Direction: M3UA -> LM
Purpose: M3UA reports the status of an AS.
M-NOTIFY indication
Direction: M3UA -> LM
Purpose: M3UA reports that it has received a Notify message
from its peer.
M-ERROR indication
Direction: M3UA -> LM
Purpose: M3UA reports that it has received an Error message from
its peer or that a local operation has been unsuccessful.
M-ASP_UP request
Direction: LM -> M3UA
Purpose: LM requests ASP to start its operation and send an ASP Up
message to its peer.
M-ASP_UP confirm
Direction: M3UA -> LM
Purpose: ASP reports that is has received an ASP UP Ack message from
its peer.
M-ASP_UP indication
Direction: M3UA -> LM
Purpose: M3UA reports it has successfully processed an incoming ASP
Up message from its peer.
M-ASP_DOWN request
Direction: LM -> M3UA
Purpose: LM requests ASP to stop its operation and send an ASP Down
message to its peer.
M-ASP_DOWN confirm
Direction: M3UA -> LM
Purpose: ASP reports that is has received an ASP Down Ack message
from its peer.
M-ASP_DOWN indication
Direction: M3UA -> LM
Purpose: M3UA reports it has successfully processed an incoming ASP
Down message from its peer, or the SCTP association has
been lost/reset.
M-ASP_ACTIVE request
Direction: LM -> M3UA
Purpose: LM requests ASP to send an ASP Active message to its peer.
M-ASP_ACTIVE confirm
Direction: M3UA -> LM
Purpose: ASP reports that is has received an ASP Active
Ack message from its peer.
M-ASP_ACTIVE indication
Direction: M3UA -> LM
Purpose: M3UA reports it has successfully processed an incoming ASP
Active message from its peer.
M-ASP_INACTIVE request
Direction: LM -> M3UA
Purpose: LM requests ASP to send an ASP Inactive message to its
peer.
M-ASP_INACTIVE confirm
Direction: LM -> M3UA
Purpose: ASP reports that is has received an ASP Inactive
Ack message from its peer.
M-ASP_INACTIVE indication
Direction: M3UA -> LM
Purpose: M3UA reports it has successfully processed an incoming ASP
Inactive message from its peer.
M-AS_ACTIVE indication
Direction: M3UA -> LM
Purpose: M3UA reports that an AS has moved to the AS-ACTIVE state.
M-AS_INACTIVE indication
Direction: M3UA -> LM
Purpose: M3UA reports that an AS has moved to the AS-INACTIVE state.
M-AS_DOWN indication
Direction: M3UA -> LM
Purpose: M3UA reports that an AS has moved to the AS-DOWN state.
If dynamic registration of RK is supported by the M3UA layer, the
layer MAY support the following additional primitives:
M-RK_REG request
Direction: LM -> M3UA
Purpose: LM requests ASP to register RK(s) with its peer by sending
REG REQ message
M-RK_REG confirm
Direction: M3UA -> LM
Purpose: ASP reports that it has received REG RSP message with
registration status as successful from its peer.
M-RK_REG indication
Direction: M3UA -> LM
Purpose: M3UA informs LM that it has successfully processed an
incoming REG REQ message.
M-RK_DEREG request
Direction: LM -> M3UA
Purpose: LM requests ASP to deregister RK(s) with its peer by
sending DEREG REQ message.
M-RK_DEREG confirm
Direction: M3UA -> LM
Purpose: ASP reports that it has received DEREG REQ message with
deregistration status as successful from its peer.
M-RK_DEREG indication
Direction: M3UA -> LM
Purpose: M3UA informs LM that it has successfully processed an
incoming DEREG REQ from its peer.
2. 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
[20].
3. M3UA Protocol Elements
The general M3UA message format includes a Common Message Header
followed by zero or more parameters as defined by the Message Type.
For forward compatibility, all Message Types may have attached
parameters even if none are specified in this version.
3.1 Common Message Header
The protocol messages for MTP3-User Adaptation require a message
header which contains the adaptation layer version, the message type,
and message length.
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 | Reserved | Message Class | Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+