Network Working Group R. Mukundan

Network Working Group R. Mukundan
Request for Comments: 4129 Wipro Technologies
Category: Standards Track K. Morneault
Cisco Systems
N. Mangalpally
Nortel Networks
August 2005

Digital Private Network Signaling System (DPNSS)/
Digital Access Signaling System 2 (DASS 2)
Extensions to the IUA Protocol

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 (2005).

Abstract

This document defines a mechanism for backhauling Digital Private
Network Signaling System 1 (DPNSS 1) and Digital Access Signaling
System 2 (DASS 2) messages over IP by extending the ISDN User
Adaptation (IUA) Layer Protocol defined in RFC 3057. DPNSS 1,
specified in ND1301:2001/03 (formerly BTNR 188), is used to
interconnect Private Branch Exchanges (PBX) in a private network.
DASS 2, specified in BTNR 190, is used to connect PBXs to the PSTN.
This document aims to become an Appendix to IUA and to be the base
for a DPNSS 1/DASS 2 User Adaptation (DUA) implementation.

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

Table of Contents

1. Introduction ................................................. 2
1.1. Scope .................................................. 2
1.2. Terminology ............................................ 3
1.3. DPNSS Overview ......................................... 4
1.4. Proposed DPNSS Backhaul Architecture ................... 5
2. Changes from IUA ............................................. 5
2.1. New Message Class for DUA .............................. 5
2.2. Message Header ......................................... 6
2.3. Unit Data Message ...................................... 7
2.4. DLC Status Message ..................................... 7
2.5. Management (MGMT) Messages ............................. 9
3. IANA Considerations .......................................... 10
4. Use of SCTP Payload Protocol ID .............................. 10
5. Message Sequence in DUA ...................................... 11
5.1. Resetting of single DLC ................................ 11
5.2. Resetting all DLCs in a Link ........................... 11
5.3. Information Transfer on a DLC .......................... 12
5.4. Link Takedown (Single DLC) ............................. 12
5.5. Link Takedown (All DLCs) ............................... 12
5.6. Getting Link Status .................................... 12
5.7. Error Conditions ....................................... 12
6. Security Considerations ...................................... 13
7. References ................................................... 13
7.1. Normative References ................................... 13
8. Acknowledgements ............................................. 13

1. Introduction

This document describes a method of implementing Digital Private
Network Signaling System 1 (DPNSS 1) [2] (henceforth referred to as
just DPNSS) and Digital Access Signaling System 2 (DASS 2)[3]
backhaul messaging over IP using a modified version of the ISDN User
Adaptation Protocol (IUAP) [1]. The DPNSS/DASS 2 User Adaptation
(DUA) builds on top of IUA by defining the necessary extensions to
IUA for a DPNSS/DASS2 implementation.

1.1. Scope

There is a need for Switched Circuit Network (SCN) signaling protocol
delivery from a DPNSS Signaling Gateway (SG) to a Media Gateway
Controller (MGC). The delivery mechanism should support the
following protocols:

- DPNSS (Digital Private Network Signaling System) [2]
- DASS 2 (Digital Access Signaling System Number 2) [3]

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

Unless specifically mentioned, the details in this document are
applicable to both DPNSS and DASS 2.

1.2. Terminology

Data channel (D-channel) - A 64 kbit/s time slot that functions as a
common signaling channel on a 2048 kbits/s interface or a 1544
kbits/s interface that is provisioned to carry DPNSS signaling.

DPNSS channel - Time slots 1 to 15 and 17 to 31 on a 2048 kbits/s
interface or Time slots 1 to 23 on a 1544 kbits/s interface are
termed as DPNSS channels. These are the traffic channels that carry
voice or data traffic.

- DPNSS supports 60 Channels (30 Real and 30 Virtual)
- DASS2 supports 30 Channels (All Real)

Data Link Connection(DLC) - A DLC is the level 2 process that
controls the transfer of level 3 messages on behalf of one DPNSS
channel. A DLC uniquely identifies one DPNSS channel.

- DPNSS supports 60 DLCs (30 Real and 30 Virtual)
- DASSII supports 30 DLCs (All Real)

DPNSS Link - A logical collection of the D-channel and the
associated DPNSS channels in a 2048 kbits/s interface or a 1544
kbits/s interface is called a "DPNSS Link".

Real channel - A signalling channel with an associated traffic
channel (TS).

Virtual channel - A signalling channel with no associated traffic
channel.

NT1 - The DPNSS minimum retransmission period.

NT2 - The DPNSS minimum post retransmission acknowledgement delay.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [5].

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

1.3. DPNSS Overview

DPNSS is an industry standard interface (ref. ND1301:2001/03) [2],
which is defined between a PBX and an Access Network (AN). DPNSS
extends facilities that are normally only available between
extensions on a single PBX to all extensions on PBXs that are
connected in a private network. DPNSS was originally derived from
BT's Digital Access Signaling System I (DASS I), and was enhanced
where necessary to meet the private network requirements. Some of
these enhancements were incorporated in DASS 2 [3]. DPNSS uses a
2048 kbits/s or 1544 kbits/s Digital Transmission System Interface,
as shown in Figure 1 below.

---------- ---------- o--o
| | 2048 kbits/s | |------- /\
| |--------------| | --
| PBX | 1544 kbits/s | AN |
| |--------------| | o--o
| | | |------- /\
---------- ---------- --

Figure 1

Channel 16 is on a 2048 kbits/s (E1) interface and channel 24 is on a
1544 kbits/s (T1) interface and is reserved for data communication
between LE and AN. The channels reserved for data are called "Data
Channels" or "D-Channels."

The D-Channels are the physical media used to exchange data between
the DPNSS protocol peer entities. A logical collection of the
D-channel and the associated DPNSS channels is called a "DPNSS Link".

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

1.4. Proposed DPNSS Backhaul Architecture

****** DPNSS ****** IP *******
*PBX *---------------* SG *--------------* MGC *
****** ****** *******

+-----+ +-----+
|DPNSS| (NIF) |DPNSS|
| L3 | | L3 |
+-----+ +----------+ +-----+
| | | | DUA| | DUA |
|DPNSS| |DPNSS+----+ +-----+
| L2 | | L2 |SCTP| |SCTP |
| | | +----+ +-----+
| | | | IP + | IP |
+-----+ +-----+----+ +-----+

NIF - Nodal Interworking function
SCTP - Stream Control Transmission Protocol
DUA - DPNSS User Adaptation Layer Protocol

2. Changes from IUA

This section outlines the differences between DUA and IUA.

2.1. New Message Class for DUA

The DPNSS/DASS2 Layer 2 to Layer 3 primitives [2] [3] need to be
identifiable from IUA boundary primitive transport messages and the
boundary primitive transport messages of other IUA extensions (i.e.,
V5 or GR-303). Therefore, it is necessary to use a different message
class parameter for DUA messages.

For all DPNSS/DASS2 interface boundary primitives, a new Message
Class is introduced:

13 DPNSS/DASS2 Boundary Primitives Transport Messages
(DPTM)

Similar to IUA, other valid message classes for DUA are:

0 Management (MGMT) Message
3 ASP State Maintenance (ASPSM) Messages
4 ASP Traffic Maintenance (ASPTM) Messages

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

2.2. Message Header

The IUA Message Header [1] MUST be used with the DPTM messages, but
the DLCI field in the DLCI parameter is formatted differently.
Figure 2 below shows the IUA Message Header with integer-based
Interface Identifier.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x1) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Identifier (integer) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x5) | Length=8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DLCI | Spare |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2 IUA Message Header (integer-based Interface Identifier)

In DUA, the DLCI field has a different format, in accordance with the
ND1301:2001/03 (formerly BTNR 188) [2].

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |V|0|Channel No.|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Reserved: 7 bits

Should be set to all '0's and ignored by the receiver.

V-bit: 1 bit

The V-bit is used to determine if the message is for a particular
DLC or if it is applicable for all the DLCs in the carrier. The
possible values of the V-bit are listed below:

Value Description
0 Action is to be performed on all DLCs;
Channel number parameter is ignored.
1 Action is to be performed on a single
DLC specified by channel number.

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

This V-bit value is used only by the Establish and Release
messages. Data messages should ignore this value. This indicator
is provided so that a single command can be issued to establish or
release all the DLCs in one DPNSS Link.

For Channel Number (Channel No.), the valid values are 0 to 63 for
DPNSS and 0 to 31 for DASS 2. This is because DASS 2 does not
support virtual DLCs and, hence, has only 32 DLCs.

2.3. Unit Data Message

DPNSS layer 2 does not have a unit data primitive and, hence, the
Unit Data Messages (Request, Indication) are invalid for a DUA
application. The Data Request and Indication messages (message types
1 and 2, respectively) will be used with DUA.

2.4. DLC Status Message

For DUA, a new message is necessary to carry the status of the DLCs.
This message will be a Management message (i.e., its message class
will be a value of 0 for Management). The following message types
will be used for these messages:

5 DLC Status Request
6 DLC Status Confirm
7 DLC Status Indication

The DLC Status messages are exchanged between DUA layer peers to
request, confirm, and indicate the status of the DLCs. The DLC
Status messages contain the common message header, followed by IUA
message header, as described in section 2.2.

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

In addition, the DLC Status Confirm and Indication messages will
contain the new parameter, called the DLC Status parameter. This
parameter will have the following format for an E1 interface:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x12) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NA| D1| D2| D3| D4| D5| D6| D7| D8| D9|D10|D11|D12|D13|D14|D15|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NA|D17|D18|D19|D20|D21|D22|D23|D24|D25|D26|D27|D28|D29|D30|D31|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NA|D33|D34|D35|D36|D37|D38|D39|D40|D41|D42|D43|D44|D45|D46|D47|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NA|D49|D50|D51|D52|D53|D54|D55|D56|D57|D58|D59|D60|D61|D62|D63|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NA stands for Not Applicable. D0 and D16 are not applicable for an
E1 interface because timeslot 0 is used for E1 framing and
synchronization bits and timeslot 16 is used for signaling. For
DPNSS, there would be a total of max 60 DLCs (30 real + 30 virtual)
and in case of DASS2 there would be a total of 30 DLCs (no virtuals).

This parameter will have the following format for a T1 interface:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x12) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| D0| D1| D2| D3| D4| D5| D6| D7| D8| D9|D10|D11|D12|D13|D14|D15|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|D16|D17|D18|D19|D20|D21|D22| NA|D24|D25|D26|D27|D28|D29|D30|D31|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NA|D33|D34|D35|D36|D37|D38|D39|D40|D41|D42|D43|D44|D45|D46| NA|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

D23 is not applicable for a T1 interface because timeslot 23 is used
for signaling. For DPNSS, there would be a total of max 46 DLCs (23
real + 23 virtual) and in case of DASS2 there would be a total of 23
DLCs (no virtuals).

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

The parameter carries the status of DLCs using two bits for each DLC.
The possible values for the two bits are shown below:

Value Description
00 Out Of Service
01 Reset Attempted
10 Reset Completed
11 Information Transfer

For DASS 2, the value 00 (Out Of Service) is invalid because the DASS
2 DLC does not have this state. In addition, the Idle state is a
transient state local to the DLC, therefore, a value is not allocated
for it.

For DASS 2, there are no virtual DLCs and, hence, information about
only 32 DLCs need to be carried. Therefore, the status message will
have a length of 12 for a DASS 2 DLC Status message.

2.5. Management (MGMT) Messages

Only the Notify and Error messages are valid for DUA. The TEI Status
messages are not used.

2.5.1. Error Message

The ERR message is sent when an invalid value or unrecognized message
is found in an incoming message.

The Error Code parameter indicates the reason for the Error Message.
These are the supported values in IUA.

Invalid Version 0x01
Invalid Interface Identifier 0x02
Unsupported Message Class 0x03
Unsupported Message Type 0x04
Unsupported Traffic Handling Mode 0x05
Unexpected Message 0x06
Protocol Error 0x07
Unsupported Interface Identifier Type 0x08
Invalid Stream Identifier 0x09
Unassigned TEI 0x0a
Unrecognized SAPI 0x0b
Invalid TEI, SAPI combination 0x0c
Refused - Management Blocking 0x0d
ASP Identifier Required 0x0e
Invalid ASP Identifier 0x0f

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

In DUA, the error codes 0x0a, 0x0b, and 0x0c are invalid, as they are
specific to ISDN.

The following additional error codes are supported in DUA:

Channel Number out of range 0x1c
Channel Number not configured 0x1d

The "Channel Number out of range" error is sent if a message is
received with a channel number greater than 63 for DPNSS or 31 for
DASS 2.

The "Channel Number not configured" error is sent if a message is
received with a channel number that is not configured.

3. IANA Considerations

IANA has assigned a DUA value for the SCTP Payload Protocol
Identifier field that is used in SCTP Payload Data chunks. The
following value for the SCTP Payload Protocol Identifier field SHOULD
be used for DUA:

SCTP Payload Protocol ID = "10"

4. Use of SCTP Payload Protocol ID

As an option, the IUA value for SCTP Payload Protocol ID MAY also be
used for DUA, for instance, if one wanted to backhaul ISDN and DPNSS
over the same SCTP association. However, use of separate SCTP
Payload Protocol IDs (10 for DUA and 1 for IUA) is recommended as the
primary option, even in scenarios where ISDN and DPNSS are backhauled
over the same SCTP association.

SCTP Payload Protocol ID of "10" SHOULD be used for DUA if only DPNSS
is backhauled over an SCTP association (i.e., in scenarios where
simultaneous backhauling of ISDN and DPNSS over the same association
is NOT required).

The SCTP Payload Protocol Identifier is included in each SCTP Data
chunk, to indicate which protocol the SCTP is carrying. This Payload
Protocol Identifier is not directly used by SCTP but MAY be used by
certain network entities to identify the type of information being
carried in a Data chunk.

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

5. Message Sequence in DUA

An example of the message flows for establishing a data link on a
signaling channel, passing PDUs and releasing a data link on a DPNSS
channel is shown below. An active association between MGC and SG is
established prior to the following message flows.

5.1. Resetting of single DLC

i) Successful

PBX SG MGC
<----------- SABMR <----------- Est Req(Ind=1)
UA -----------> Est Cfm -----------> (DLC in RC State)
Ind=1)

ii) Unsuccessful(Link Failure)

PBX SG MGC
<----------- SABMR <----------- Est Req(Ind=1)
Retransmissions over
NT1 and NT2 expired
Rel Ind -----------> (DLC in RA state)
(RELEASE_OTHER,Ind=1)

5.2. Resetting all DLCs in a Link

PBX SG MGC
<----------- SABMR(1) <----------- Est Req(Ind=0)
<----------- SABMR(2)
<----------- SABMR(3)
.............
<----------- SABMR(N)
In each DLC either
UA is received or
NT1/NT2 is expired

Est Cfm -----------> (Status of DLCs
(Ind=0) are not updated)
<----------- Status Req
Status cfm ----------> (Mark DLC status
based on
status bits)

If one of more DLCs remains out-of-service after this procedure
(e.g., due to layer 2 management), the MGC can either retry this DLC
with an Est Req(Ind=1) indicating the specific DLC or with an

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

Est Req(Ind=0) and the SG will retry the appropriate DLC that is
out-of-service.

5.3. Information Transfer on a DLC

PBX SG MGC
<----------- UI(C) <----------- Data Req
UI(R)-----------> Data Ind ----------->

5.4. Link Takedown (Single DLC)

PBX SG MGC
(For DPNSS, mark DLC as OOS) <----------- Rel Req
(For DASSII, mark DLC as RA) (RELEASE_MGMT,
Ind=1)
Rel Cfm ---------->
(Ind=1)

5.5. Link Takedown (All DLCs)

PBX SG MGC
(For DPNSS, mark all DLCs as OOS) <-------- Rel Req
(For DASSII, mark DLC as RA) (RELEASE_MGMT,
Ind=0)
Rel Cfm ---------->
(Ind=0)

5.6. Getting Link Status

PBX SG MGC
<----------- Stat Req
Stat Cfm -----------> (Mark DLC status
based on
status bits)

5.7. Error Conditions

PBX SG MGC
Invalid Message <-----------Est/Rel/Data/-
Stat Req
Error Ind ----------->
(Error Code)

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

6. Security Considerations

The security considerations for the ISDN User Adaptation Protocol
(IUAP) [1] (Section 6) and the security considerations for SIGTRAN
Protocols document [4] apply to this document as well.

7. References

7.1. Normative References

[1] Morneault, K., Rengasami, S., Kalla, M., and G. Sidebottom,
"ISDN Q.921-User Adaptation Layer", RFC 3057, February 2001.

[2] Ofcom/NICC ND1301:2001/03, DPNSS [188], Digital Private
Signalling System No 1 (DPNSS 1) (Formerly BTNR 188).

[3] BTNR (British Telecom Network Requirements) 190 Issue 2 Digital
Access Signaling System No 2.

[4] Loughney, J., Tuexen, M., and J. Pastor-Balbas, "Security
Considerations for Signaling Transport (SIGTRAN) Protocols", RFC
3788, June 2004.

[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

8. Acknowledgements

The authors would like to thank Shashi Kumar and Venkatesh Seshasayee
of Wipro Technologies for their useful suggestions and comments.

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

Authors' Addresses

All correspondence regarding this document should be sent to the
following addresses:

Ranjith Mukundan
Wipro Technologies
72, Electronics City
Hosur Main Road
Bangalore 560100
India

Phone: +91-80-51195893
EMail: [email protected]

Ken Morneault
Cisco Systems Inc.
13615 Dulles Technology Drive
Herndon, VA. 20171
USA

Phone: +1-703-484-3323
EMail: [email protected]

Narsimuloo Mangalpally
Nortel Networks
250 Sidney Street
Belleville, Ontario K8P 3Z3
Canada

Phone: +1-613-967-5034
EMail: [email protected]

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RFC 4129 DPNSS/DASS 2 Extensions to the IUA Protocol August 2005

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