Network Working Group Ken Morneault
INTERNET-DRAFT Cisco Systems
Mallesh Kalla
Telcordia
Greg Sidebottom
Nortel Networks
Ram Dantu
Tom George
Alcatel
Expires in six months December 1999
SS7 MTP2-User Adaptation Layer
<draft-ietf-sigtran-m2ua-02.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). One application of this protocol would be to use it
between a Signaling Gateway (SG) and Media Gateway Controller (MGC).
In this case, the Signaling Gateway would be acting as a Signaling
Link Terminal. Another application of this protocol would be to use
it between a SG and a SCP. In either application, it is assumed that
the SG receives SS7 signaling over a standard SS7 interface using the
SS7 Message Transfer Part (MTP) to provide transport.
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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..........4
1.5 Function Provided by the M2UA Layer....................6
1.6 Definition of the M2UA Boundaries......................7
2. Protocol Elements.........................................8
2.1 Common Message Header..................................8
2.2 M2UA Message Header....................................9
2.3 M2UA Messages.........................................10
3. Procedures...............................................17
3.1 Procedures to Support Service in Section 1.4.1........17
3.2 Procedures to Support Service in Section 1.4.2........17
3.3 Procedures to Support Service in Section 1.4.3........17
4. Examples of MTP2 User Adaptation (M2UA) Procedures.......22
4.1 Establishment of associations between SG and MGC......22
examples
4.2 MTP Level 2 / MTP Level 3 Boundary Examples...........23
4.3 Layer Management Communication Examples................25
5. Security.................................................31
6. Acknowledgements.........................................31
7. References...............................................31
8. Author's Addresses.......................................32
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, 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.
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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.
Stream - A stream refers to a SCTP stream.
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, à).
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 a next-preferred Application Server Process within an Application
Server in the event of failure or unavailability of the currently used
Application Server Process (e.g., from primary MGC to back-up MGC).
Fail-over may also apply upon the return to service of a previously
unavailable Application Server Process.
Signaling Link Terminal (SLT) - Refers to the means of performing all
of the functions defined at MTP level 2 regardless of their
implementation [2].
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1.3 Signaling Transport Architecture
The architecture that has been defined [4] for SCN signaling transport
over IP uses multiple components, including an IP transport
protocol, a signaling common transport protocol (SCTP) and an adaptation
module to support the functions expected by a particular SCN signaling
protocol from its underlying protocol layer.
In reference to the SIGTRAN framework architecture [4], this document
defines a SCN adaptation module that is suitable for the transport of
SS7 MTP2 User. The only SS7 MTP2 User is MTP3.
1.3.1 Case 1: 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:
****** SS7 ****** IP *******
*SEP *-----------* SG *-------------* MGC *
****** ****** *******
+----+
|S7UP|
+----+
+----+ |MTP |
|S7UP| |L3 |
+----+ +----+----+ +----+
|MTP | |MTP |M2UA| |M2UA|
|L3 | | +----+ +----+
|L2 | |L2 |SCTP| |SCTP|
|L1 | |L1 +----+ +----+
| | | |UDP | |UDP |
+----+ +---------+ +----+
SEP - SS7 Signaling Endpoint
UDP - User Datagram 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.
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1.3.2 Case 2: SG to IPSP
The following figure shows the seamless interworking at the MTP3
layer. MTP3 is adapted to the SCTP layer using MTP2 User Adaptation
Layer (M2UA). In this example, the Signaling Gateway could be an STP.
All the primitives between MTP3 and MTP2 are supported by M2UA. Any
of the nodes in the diagram could have SCCP or other SS7 user parts.
****** SS7 ****** IP ************
*SEP *-------* SG *-------------* IPSP *
****** ****** ************
+----+ +---------+
|TCAP| | TCAP |
+----+ +---------+
|SCCP| | SCCP |
+----+ +---------+ +---------+
|MTP | | MTP | | MTP |
|L3 | | L3 | | L3 |
|L2 | +----+----+ +----+----+
|L1 | |MTP |M2UA| |M2UA|MTP |
| | |L2 +----+ +----+L2 |
| | |L1 |SCTP| |SCTP|L1 |
| | | |----| |----| |
| | | |UDP | |UDP | |
+----+ +----+----+ +----+----+
SEP - SS7 Signaling Endpoint
IPSP - IP Signaling Point
UDP - User Datagram Protocol
SCTP - Simple Control Transmission Protocol
(Refer to Reference [5])
Figure 2: M2UA in the SG to IPSP Application
In this case, the SCTP association acts as an SS7 link between the SG
and the IPSP. The association contains two streams, one in each
direction. The IPSP may or may not have a termination to the SS7
network.
1.3.3 UDP port
A request will be made to IANA to assign a UDP port for M2UA.
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.
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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).
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 asychronous notification of link state changes to
be reported to the upper layer (MTP Level 3). An examples 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:
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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.
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 ID to SCTP association and to a
stream within the association.
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1.5.2 Status of ASPs
The M2UA layer on the Signaling Gateway must maintain the state of one or
more Application Server Process(es) it is associated with. The state of
an ASP changes because of reception of peer-to-peer messages or reception
of indications from the local SCTP association. ASP state transition
procedures are described in Section Section 3.3.
1.5.3 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.
1.5.4 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 will 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).
1.5.5 Seamless SS7 Network Management Interworking
If the SG loses the SCTP association to the MGC, it should follow
MTP 2 processor outage procedures [2].
1.5.6 Management Inhibit/Uninhibit
Local Management may wish to stop traffic across an SCTP association in
order to temporarily remove the association from service or to perform
testing and maintenance activity. The function could optionally be used
to manage the start of traffic on to a newly-available SCTP association.
1.6 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
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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.
2.1 Common Message Header
The protocol messages for MTP2 User Adaptation require a message header
structure which contains a version, message type and message length. This
message header is common among all SCN adaptation layers.
0 7 8 15 16 31
+---------------+---------------+
| Vers | Spare | Msg Type |
+---------------+---------------+
| Message Length |
+-------------------------------+
Figure 2 Common Message Header
2.1.1 Version
The version field (vers) contains the version of the M2UA adapation
layer. The supported versions are:
01 Release 1.0 of M2UA adaptation protocol
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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 Request 0601
Data Indication 0602
Establish Request 0603
Establish Confirm 0604
Release Request 0605
Release Confirm 0606
Release Indication 0607
State Request 0608
State Confirm 0609
State Indication 060a
Data Retrieval Request 060b
Data Retrieval Confirm 060c
Data Retrieval Indication 060d
Data Retrieval Complete Indication 060e
Application Server Process Maintenance (ASPM) Messages
ASP Up (ASPUP) 0301
ASP Down (ASPDN) 0302
ASP Active (ASPAC) 0401
ASP Inactive (ASPIA) 0402
Management (MGMT) Messages
Error 0001
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. Or, the Interface Identifier
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can be left empty (a null string of length zero). The Interface Identifier
follows the same endpoint naming scheme provided in MGCP [7]. For example,
if a Signaling Gateway terminates a E1 and the SS7 signaling link is one
timeslot 16, the interface identifier could be the following:
e1/16@sgw1.example.net
The use of wildcards is not acceptable.
Ed's Note: The Interface Identifier string should be padded to 32-bit
boundaries. The length field indicates the end of the string.
0 15 16 31
+---------------+---------------+
| Tag (0x1) | Length |
+-------------------------------+
| Interface Identifier |
+-------------------------------+
Figure 3 M2UA Message Header
The Tag value for Interface Identifier is 0x1. The length provides the
length of the Interface Identifier string in bytes.
2.3 M2UA Messages
The following section defines the messages and parameter contents. The
M2UA messages will use the command header and the M2UA specific header.
2.3.1 MTP2 User Adaptation Messages
2.3.1.1 Data (Request, Indication)
The Data message contains an SS7 MTP2-User Protocol Data Unit (PDU). The
Data message contains the protocol data.
The format for the Data Message parameters is as follows:
0 15 16 31
+-------------------------------+
...
| Protocol Data |
...
+---------------+---------------+
The Protocol Data field contains the MTP2-User application message.
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2.3.1.2 Establish (Request, Confirmation)
The Establish Request message is used to establish the channel or to
indicate that the channel has been established. Note that the gateway
may already have the channel established at its layer. If so, upon
receipt of an Establish Request, the gateway takes no action except to
send an Establish Confirm.
0 15 16 31
+---------------+---------------+
| State |
+---------------+---------------+
The valid values for State are shown in the following table.
Define Value Description
ESTABLISH_NORMAL 0x0 Follow normal procedure for
establishing a SS7 link
ESTABLISH_EMERGENCY 0x1 Follow emergency procedure for
establishing a SS7 link
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 15 16 31
+---------------+---------------+
| 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_OTHER 0x3 Other reason SS7 link out-of-service
(should we keep it simple, or provide list of reasons that would
enable debugging)
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 15 16 31
+---------------+---------------+
| State |
+---------------+---------------+
The valid values for State are shown in the following table.
Define Value Description
STATUS_LOC_PROC_SET 0x0 Request local processor outage.
STATUS_LOC_PROC_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 a call
agent to indicate a condition on a channel.
0 15 16 31
+---------------+---------------+
| 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 15 16 31
+---------------+---------------+
| 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.
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 15 16 31
+---------------+---------------+
| |
. 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 header.
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2.3.2.1 ASP UP (ASPUP)
The ASPUP message is used to indicate to a remote M2UA peer that the
layer is ready to receive traffic or maintenance messages.
The ASPUP message contains the following parameters:
Adaptation Layer Identifer (optional)
SCN Protocol Identifier (optional)
The Adaptation Layer Identifier is a string that identifies the
adaptation layer. This string must be set to "M2UA" which means
the length will be 4.
The Protocol Identifier field contains the identity of the specific SCN
signaling protocol being transported. The Protocol ID defines the
protocol type, variant, and version, and thereby specifies the components
and encoding of the PROTOCOL DATA field. The Protocol Identifier also
defines what SCN protocol message components are included in the PROTOCOL
DATA.
(Ed. Note: Need encoding of mime-type value or OID or fixed
string/integer that will be administered outside of this document
(IANA). Also, perhaps bring in text from Christian's mime document - See
"draft-ietf-sigtran-mime-isup.txt" for an example of an application/ISUP
media type defined according to the rules defined in RFC 2048.?)
The format for the ASPUP message is as follows:
0 15 16 31
+---------------+---------------+
| Tag (0x2) | Length |
+---------------+---------------+
| Adaptation Layer Identifier |
+---------------+---------------+
| Tag (0x3) | Length |
+---------------+---------------+
| Protocol Identifier |
+---------------+---------------+
| Tag (0x4) | Length |
+---------------+---------------+
| INFO String |
+---------------+---------------+
Note: Strings are padded to 32-bit boundaries. The length field
indicates the end of the string.
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2.3.2.2 ASP Down (ASPDN)
The ASPDN message is used to indicate to a remote M2UA peer that the
layer is not ready to receive traffic or maintenance messages.
The ASPDN message contains the following parameters:
INFO String
The format for the ASPDN message is as follows:
0 15 16 31
+---------------+---------------+
| Tag (0x4) | Length |
+---------------+---------------+
| INFO String |
+---------------+---------------+
#####
We discussed adding a reason code. Reason could be failure or
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
the active ASP to be used from within a list of primary and back-up ASPs
for a particular signaling mapping relationship.
The ASPAC message contains the following parameters:
Controlled/Forced flag (C/F flag)
INFO String
The format for the ASPAC message is as follows:
0 15 16 31
+---------------+---------------+
| C/F flag |
+---------------+---------------+
| Tag (0x4) | Length |
+---------------+---------------+
| INFO String |
+---------------+---------------+
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The valid values for C/F flag are shown in the following table.
Define Value Description
FORCED 0x0 Force sending of all messages to ASP
CONTROLLED 0x1 Only send "new work" to ASP
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 the the active ASP to be used from within a list of primary
and back-up ASP for a particular signaling mapping relationship. The
SG will respond with an ASPIA message and either buffer or discard
incoming messages for a timed period and then discard.
The ASPIA message contains the following parameters:
INFO String
The format for the ASPIA message is as follows:
0 15 16 31
+---------------+---------------+
| Tag (0x4) | Length |
+---------------+---------------+
| INFO String |
+---------------+---------------+
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
messages.
The ERR message contains the following parameters:
Error Code
The format for the ERR message is as follows:
0 7 8 15 16 31
+---------------+---------------+
| Error Code |
+---------------+---------------+
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The Error Code can be one of the following values:
Invalid Version 0x1
Invalid Interface Identifier 0x2
Invalid SCN Version 0x3
Invalid Adaptation Layer Identifier 0x4
Invalid Stream Identifier 0x5
Invalid Message Type 0x6
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 primitives from the local 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.
3.2 Procedures to Support Service in Section 1.4.2
These procedures achieve the IUA layer's "Support for Communication
between Layer Managements" service.
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.
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3.3.1 State Maintenance
3.3.1.1 ASP States
The state of the each ASP is maintained in the M2UA layer on the SG. The
state of an ASP changes due to events. The events include:
* Reception messages from peer M2UA layer
* Reception of indications from layers below
The ASP state transition diagram is shown in Figure 4. The possible states
of an ASP are:
ASP-DOWN: Application Server Process is unavailable. Initially all ASPs
are in this state.
ASP-UP: Application Server Process is available but application traffic is
stopped.
ASP-ACTIVE: Application Server Process is available and application traffic
is active. At most one ASP per AS can be in the active state.
Figure 4: ASP State Transition Diagram
+-------------+
|-------->| |
| | ASP-ACTIVE |
| | |
| | |
| +-------------+
| ^ |
| ASP | | ASP
| Active | | Inactive
| | v
| +-------------+
| | |
ASP Down / | | |
SCTP CDI | | ASP-UP |
| | |
| | |
| +-------------+
| ^ |
| ASP | | ASP Down /
| Up | | SCTP CDI
| | v
| +-------------+
| | |
|-------->| |
| ASP-DOWN |
| |
| |
+-------------+
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SCTP CDI: Local SCTP layer's Communication Down Indication to the Upper Layer
Protocol (M2UA) on an SG. SCTP will send this indication when it
detects the loss of connectivity to ASP's SCTP layer.
3.3.1.2 AS States
The state of the AS is maintained in the ITUN 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: Application Server is unavailable. This state implies that all
ASPs are in the ASP-DOWN state for this AS.
AS-UP: One or more ASPs are in the ASP-UP state.
AS-ACTIVE: Application Server is available and application traffic is
active. This state implies that one ASP is in the ASP-ACTIVE state.
AS-PENDING: Currently Active ASP became inactive or SCTP association with
it is lost. A Recovery timer will be started and in coming SCN messages
will be queuedby the SG. If an ASP becomes Active before the recovery
timer (Tr) expires, the AS will move to AS-ACTIVE state and all the
queued messages will be sent to the active ASP. If the recovery timer
expires before an ASP becomes active, SG stops queuing messages and
discards all queued messages. AS will move to AS-UP if at least one
ASP is in ASP-UP state, otherwise it will move to AS-DOWN state.
If Tr 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.
Ed's Note: If AS moves from AS-PENDING state to AS-UP or AS-DOWN
states, the Layer Management on MG may take appropriate SCN
notification actions.
Morneault, Kalla & Sidebottom [Page 20]
Internet Draft SS7 MTP2 User Adaptation Layer Dec 1999
+----------+ 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 CDI
| | / \ | |
| | / \ | |
| | /all ASP \ | |
| v / trans to \ | v
+----------+ / DOWN \ +-------------+
| |<--/ -| |
| AS-DOWN | | AS-PENDING |
| | | (queueing) |
| |<------------------------| |
+----------+ Tr Trigger no ASP +-------------+
in UP state or
prev ACTIVE ASP trans
to DOWN state
Tr = Recovery Timer
Figure 5: AS State Transition Diagram
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".
When the M2UA layer 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. 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.
Alternatively, if the remote M2UA-peer establishes the SCTP association
first, 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.
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Internet Draft SS7 MTP2 User Adaptation Layer Dec 1999
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 ITUN 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
3.3.3.1 ASP UP
The SG will mark the path as up if an explicit ASP UP (ASPUP) message is
received and internally the path is allowed to come up (i.e., not in a
locked local maintenance state). An ASP UP (ASPUP) message will be sent
to acknowledge the received ASPUP. The SG will respond to a ASPUP with a
ASPDN message if the path is in a locked maintenance state.
The SG will send a ASPUP message in response to a received ASPUP message
from the MGC even if that path was already marked as UP at the SG.
The paths are controlled by the MGC. The SG will only send ASPUP in
response to the reception of a ASPUP message.
The MGC will send ASPUP messages every 2 (add text regarding this being
a configurable timer) seconds until the path comes up (i.e. until it
receives a ASPUP message from the SG for that path). The MGC may decide
to reduce the frequency (say to every 5 seconds) if the an acknowledge-
ment is not received after a few tries.
The MGC should wait for the ASPUP message from the SG before transmitting
ASP maintenance messages (ASPIA or ASPAC) or M2UA messages or it will
risk message loss. The ASPUP message received from the SG is not
acknowledged by the MGC.
3.3.3.2 ASP Down
The SG will mark the ASP as down and send a ASPDN message to the MGC if
one of the following events occur:
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Internet Draft SS7 MTP2 User Adaptation Layer Dec 1999
- a ASP Down(ASPDN) message is received from the MGC,
- the ASP is locked by local maintenance.
The SG will also send a ASPDN message when the ASP is already down and
a ASPDN) message is received from the MGC.
The MGC will send ASPDN whenever it wants to take down a ASP. Since the
ASPDN messages to the SG or the ASPDN responses from the SG can be lost
(for example, during a MGC node failover), the MGC can send ASPDN messages
every 2 seconds until the path comes down (i.e. until it receives a ASPDN
message from the SG for that path).
3.3.4 ASP Version Control
If a ASP Up message with an unknown version is received, the receiving
end will respond with an Error message. This will indicate to the
sender which version the receiving node supports.
This is useful when protocol version upgrades are being performed. A
node with the newer version should support the older versions used on
other nodes it is communicating with.
The version field in the Error message header associated with the will
indicate the version supported by the node.
3.3.5 ASP Inactive
When a ASPIA message is received, message transmission to that ASP ceases.
The SG will either discard all incoming messages or start buffering the
incoming messages for N seconds after which messages will be discarded.
If the ASP is down, all of the Paths that were supported by that ASP
are, by default, down.
3.3.6 ASP Active
When a ASP Active (ASPAC) message is received, the SG will start routing
to that ASP. Reception of a ASPAC message overrides any previous ASPAC
messages and results in the ASP associated with the ASPAC message to
become the newly active ASP.
4.0 Examples of MTP2 User Adaptation (M2UA) Procedures
4.1 Establishment of associations between SG and MGC examples
An example of the message flows for establishing active associations between
SG and MGC is shown below.
SG ASP1
<----------- ASP Up
ASP Up ---------->
(ACK)
<----------- ASP Active
ASP Active ---------->
(ACK)
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An example of message flows for establishment of associations with two
ASPs and the message flows for take-over of the primary (ASP1) by the
secondary (ASP2).
SG ASP1 ASP2
<----------- ASP Up
ASP Up ---------->
(ACK)
<------------------------------ ASP Up
ASP Up ------------------------------>
(ACK)
<----------- ASP Active
ASP Active ---------->
(ACK)
...
<----------- ASP Inactive
ASP Inactive ---------->
(ACK)
(this message is optional)
ASP Inactive ------------------------------>
<------------------------------ ASP Active
ASP Active ------------------------------>
(ACK)
An example of message flows for establishment of associations with two
ASPs and the message flows for controlled take-over of the primary (ASP1)
by the secondary (ASP2). In this case, the SG sends new work to ASP2.
SG ASP1 ASP2
<----------- ASP Up
ASP Up ---------->
(ACK)
<------------------------------ ASP Up
ASP Up ------------------------------>
(ACK)
<----------- ASP Active
ASP Active ---------->
(ACK)
...
<------------------------------ ASP Active
(New Work)
ASP Active ------------------------------>
(ACK)
<----------- ASP Inactive
ASP Inactive ---------->
(ACK)
4.2 Case 1: SG to MGC, MTP Level 2 to MTP Level 3 Boundary Procedures
4.2.1 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 MGC
<----------- Establish Request (ESTABLISH_NORMAL)
Establish Response ---------->
An example of the message flow to bring a SS7 link in-service using the
emergency alignment procedure.
SG MGC
<----------- Establish Request (ESTABLISH_EMER)
Establish Response ---------->
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Internet Draft SS7 MTP2 User Adaptation Layer Dec 1999
4.2.2 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 MGC
<------------ Release Request (RELEASE_MGMT)
Release Response ------------>
The SG can autonomously indicate that a SS7 link has gone out-of-service
as shown below.
SG MGC
Release Indication ------------>
(RELEASE_PHYS)
4.2.3 Set and Clear Local Processor Outage
to be added
4.2.4 Notification of Processor Outage (local or remote)
to be added
4.2.5 Flush Buffers or Continue
to be added
4.2.6 SS7 Link Changeover
An example of the message flow for a changeover is shown below.
SG MGC
<---------- Retrieval Request (MTP2_RTRV_BSN)
Retrieval Confirm ---------->
(with BSN)
<---------- Retrieval Request (MTP2_RTRV_MSGS
with FSN)
Retrieval Confirm ---------->
Retrieval Ind ----------->
Retrieval Ind ----------->
Rtrvl 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.
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Internet Draft SS7 MTP2 User Adaptation Layer Dec 1999
4.3 Case 2: SG to IPSP, MTP Level 2 to MTP Level 3 Boundary Procedures
In general, messages passed between MTP3 and M2UA are the same as
those passed between MTP3 and MTP2. M2UA interprets messages from MTP3
and sends the appropriate message to SCTP. Likewise, messages from
SCTP are used to generate a meaningful message to MTP3.
4.3.1 Link Initialization (Alignment)
The MTP3 layer can request that an SS7 link be brought into alignment
using the normal or emergency procedure. An example of the message
flow to bring an SS7 link in-service is shown below.
There are two alignment procedures normal alignment and emergency
alignment. During normal alignment, communication to the other end is
tested for a period of time to make sure that the communication link
satisfies the performance requirements of the application. The
examples are RTT and packet loss. Normal alignment is used when there
are other links available to the same destination. Emergency alignment
is used when there are no other links to the same destination. During
emergency, the link is not tested for a long period of time. Instead,
an indication from the SCTP layer is used to bring the link in
service.
The procedure for beginning an Association is described in the SCTP
standard [5].
MTP3 M2UA SCTP SCTP M2UA MTP3
---- ---- ---- ---- ---- ----
Power On
------------>
Out of Service
<------------
Emergency OR
Emergency Ceases
------------>
Start
------------>
Associate
------------>
(SCTP Association
procedure)
Communication Up Communication Up
<------------ ------------>
Indication Indication
(Link In Service) (Link In Service)
<------------ ------------>
For the Emergency Ceases case, proving begins at this time. See the
section on Link Proving below.
4.3.2 Link Proving
One function of the adaptation layer is to make sure that the link
meets the performance requirements of the application. This is
usually done by proving the link. For example, for proving the link,
we need the adaptation layer to issue an heartbeat/RTT to its peer.
This is done before declaring link is in service to its application.
For this purpose, the existing "status" command is used. Note how the
link meets performance requirements is implementation dependent.
Also, the proving period can be configurable.
Proving is done by both ends of the link. To simplify the diagram,
proving is shown on one end only.
In the following diagram the Link has just completed the alignment
procedure.
The Status primitive is sent to determine if the Heartbeats were
delivered successfully within the desired time period.
MTP3 M2UA SCTP SCTP M2UA MTP3
---- ---- ---- ---- ---- ----
Request Heartbeat
------------>
(Heartbeat sent
and acknowledged)
Request Heartbeat
------------>
(Heartbeat sent
and acknowledged)
Request Heartbeat
------------>
(Heartbeat sent
and acknowledged)
Heartbeats are sent for M seconds (Note A).
Status
------------>
Indication
(Link In Service) (After checking that link is sane)
<------------
Note A M is implementation-dependent.
4.3.3 Message Transmission and Reception
Messages are transmitted using the Data Request primitive from MTP3 to
M2UA. The diagram shows the case where the Link is In Service. The
message is passed from MTP3 of the source to MTP3 of the destination.
MTP3 M2UA SCTP SCTP M2UA MTP3
---- ---- ---- ---- ---- ----
Request
(Message for
transmission)
------------>
Send
------------>
(SCTP sends message)
Receive
------------>
Indication
(Received message)
------------>
4.3.4 Link Status Indication
The M2UA layer sends an indication that the Link is In Service or Out
of Service after receiving a Communication indication from the SCTP
layer. In either case, MTP3 responds in its usual way.
MTP3 M2UA SCTP SCTP M2UA MTP3
---- ---- ---- ---- ---- ----
Communication Up
<------------
(If Emergency Ceases,
Link proving is done
by M2UA now.)
Indication
(Link In Service)
<------------
MTP3 M2UA SCTP SCTP M2UA MTP3
---- ---- ---- ---- ---- ----
Communication Lost
<------------
Indication
(Link Out of Service)
<------------
4.3.5 Congestion Notification to Upper layer
MTP3 layer expects notification of the link congestion. For example,
this is accomplished by two messages 1) Link Congestion Onset 2) Link
Congestion Abated. Congestion is assumed if M2UA layer notices
repeated failures to send requests to SCTP (this is implementation
dependent and it is assumed that the SEND Failure has an error code
"life time expired"). Subsequently M2UA can start polling status of
SCTP. If all the messages are successfully transmitted over a period
of time (implementation dependent) then it is assumed that the
congestion is abated. If the congestion condition should continue,
the link will be taken out of service. In this case, it is possible
to start the link changeover procedure.
The US National version of SS7 has congestion levels. For US National
SS7, the Indication primitive for Congestion Onset should report the
congestion level.
In the example below, M2UA has sent a message to SCTP.
MTP3 M2UA SCTP SCTP M2UA MTP3
---- ---- ---- ---- ---- ----
Send Failure
<------------
Send Failure
<------------
Send Failure
<------------
"N" consecutive fails -
implementation specific
Indication
(Congestion Onset)
<------------
Status
------------>
Status
------------>
Status
------------>
polled for certain time until
congestion ceases -
implementation specific
Indication
(Congestion Abatement)
<------------
4.3.6 Link Deactivation
The MTP3 can request that a SS7-IP link be taken out-of-service. It
uses the Release Request message as shown below.
MTP3 M2UA SCTP SCTP M2UA MTP3
---- ---- ---- ---- ---- ----
Request
(Deactivate Link)
------------>
Terminate
------------>
Terminate Successful
<------------
Communication Lost
<------------
Indication
(Link Out of Service)
<------------
4.3.7 Link Changeover
The objective of the changeover is to ensure that signaling traffic
carried by the unavailable signaling link is diverted to the
alternative signaling link as quickly as possible while avoiding
message loss, duplication, or mis-sequencing. For this purpose, the
changeover procedure includes data retrieval, which is performed
before reopening the alternative signaling links to the diverted
traffic. Data retrieval consists of identifying all those messages in
the retransmission buffer of the unavailable signaling link which have
not been received by the far end. Retrieval includes transferring the
concerned messages to the transmission buffers of the alternative
links. In order to support changeover, the SCTP SSN must be used in
place of the FSN/BSN of SS7.
Stream Sequence Numbers used by SCTP (Signaling Control Transport
Protocol) are sixteen bits long. MTP2's forward and backward sequence
numbers are only seven bits long. Hence it is necessary to modify
MTP3 to accomodate the larger SSNs. Reference [7] can be used as a guide
for the MTP3 changes.
For data retrieval, MTP3 requests Backward Sequence Number (BSN) from
M2UA. This is the sequence number of the last message received by the
local end. During normal period, SCTP delivers ordered messages to
the application. However, during congestion or failure condition, the
sequence numbers of the acknowledged messages can have gaps. In
particular, the SACK (selective acknowledgement message) message can
have several of these gaps. Hence, it is important to scan through
these gaps and find the sequence number before first gap. This is the
number from which the remote end has to transmit the messages. So,
this is the number considered as the Backward Sequence Number and
communicated to the remote end. In a similar way, the remote end also
detects the BSN and indicates to local end. As soon as the MTP3 of the
local end receives this BSN, MTP3 retrieves all the unacknowledged
messages starting from BSN. This is accomplished through "Retrieve
FSN" message. After all the messages are sent from M2UA to MTP3, a
retrieval complete message is sent.
Note that the sequence numbers and messages requested by MTP3 are sent
from SCTP to M2UA in the Communication Lost primitive.
MTP3 M2UA SCTP SCTP M2UA MTP3
---- ---- ---- ---- ---- ----
Communication Lost
<------------
Indication
(Link Out of Service)
<------------
Request
(Retrieve BSN)
------------>
(M2UA locates
first gap in
received messages)
Indication
(Indicate BSN)
<------------
Request - COO (BSN) on another link
------------------------------------------------------------>
Request
(Retrieve BSN)
<------------
Indication
(Indicate BSN)
------------>
Request - COA (BSN)
<------------------------------------------------------------
Request
(Retrieve FSN)
------------>
(M2UA locates
first gap in
acknowledgements)
Indication
(FSB Not retrievable) (in case)
<------------
Indication
(Retrieved Message)
<------------
Indication
(Retrieved Message)
<------------
Indication
(Retrieval Complete)
<------------
Send messages on another link.
4.4 Layer Management Communication Examples
An example of the message flows for communication between Layer Manage-
ment modules between SG and MGC is shown below. An active association
between MGC and SG is established (section 4.1) prior to the following
message flows.
SG MGC
<----------- Establish Request
Error ---------->
(Invalid Interface Id)
Morneault, Kalla & Sidebottom [Page 31]
Internet Draft SS7 MTP2 User Adaptation Layer Dec 1999
5.0 Security
SCN adaptation layers rely on SCTP to provide security.
6.0 Acknowledgements
The authors would like to thank Ian Rytina for his valuable comments and
suggestions.
7.0 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] Bellcore GR-246-CORE 'Bell Communications Research Specification
of Signaling System Number 7', Volume 1, December 1995
[4] Framework Architecture for Signaling Transport, draft-ietf-sigtran-
framework-arch-03.txt, June 1999
[5] Simple Control Transmission Protocol, draft-ietf-sigtran-sctp-00.txt,
August 1999
[6] Media Gateway Control Protocol (MGCP), draft-huitema-megaco-mgcp-
v1-03.txt, August 1999
[7] ITU-T Recommendation Q.2210, 'Message transfer part level 3
functions and messages using the services of ITU-T
Recommendation Q.2140'
Morneault, Kalla & Sidebottom [Page 32]
Internet Draft SS7 MTP2 User Adaptation Layer Dec 1999
8.0 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: +1-972-477-8446
Alcatel USA EMail: ram.dantu@usa.alcatel.com
1000 Coit Road
Plano, TX 74075
Tom George Tel: +1-972-519-3168
Alcatel USA EMail: tom.george@usa.alcatel.com
1000 Coit Road
Plano, TX 74075
This Internet Draft expires June 2000.
