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Versions: (draft-anil-sigtran-dua) 00 01 02 03 04 05 06 07 08 RFC 4129

Internet Engineering Task Force                               A. Vydyam
                                                            R. Mukundan
                                                         N. Mangalpally
                                                        Nortel Networks
                                                          Ken Morneault
                                                          Cisco Systems
Expires in 6 months                                       November 2001




                DPNSS/DASS 2 extensions to the IUA protocol
                      <draft-ietf-sigtran-dua-02.txt>



Status of this Memo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.

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Abstract


   This document defines a mechanism for backhauling of DPNSS [2] and
   DASS2 [3] messages over IP by extending the ISDN User Adaptation
   Layer Protocol [1]. This document aims to become an Appendix to
   IUA [1] and to be the base for a DPNSS/DASS User Adaptation (DUA)
   implementation.


Table of Contents

      1.0 Introduction ......................................... 2
        1.1  Scope ............................................. 2
        1.2  Terminology ....................................... 2
        1.3  DPNSS Overview .................................... 3
        1.4  Proposed DPNSS Backhaul Architecture .............. 3

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      2.0 Changes from IUA...................................... 4
        2.1  New Message Class for DUA.......................... 4
        2.2  Message Header..................................... 4
        2.3  Unit Data Message.................................. 5
        2.4  Status Message..................................... 5
        2.5  Error Message...................................... 6
      3.0 IANA Considerations................................... 7
      4.0 Message Sequence in DUA............................... 7
        4.1  Resetting of single DLC............................ 7
        4.2  Resetting all DLCs in a link....................... 8
        4.3  Information Transfer on a DLC...................... 8
        4.4  Link Takedown(Single DLC).......................... 8
        4.5  Link Takedown(All DLCs)............................ 8
        4.6  Getting link Status................................ 9
        4.7  Error conditions................................... 9
      5.0 Glossary of Terms .................................... 9
      6.0 Security Considerations............................... 9
      7.0 References............................................ 9
      8.0 Acknowledgements...................................... 10
      9.0 Author's Addresses.................................... 10

1.0  Introduction

   This document describes a method of implementing DPNSS [2] and
   DASS 2 [3] backhaul messaging over IP using a modified version
   of the ISDN User Adaptation Protocol (IUAP) [1].  The DUA builds
   on top of IUA 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 No 2) [3]

   Unless specifically mentioned, the details in this document are
   applicable to both DPNSS and DASS2.

1.2  Terminology

   Data channel (D-channel) - A 64 kbit/s time slot which functions
   as a common signaling channel on a 2048 kbits/s interface or a
   1544 kbits/s interface which 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 which carry voice or data traffic.

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       - 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".

1.3  DPNSS Overview

   DPNSS is an industry standard interface (reference BTNR 188) [2]
   defined between a PBX and an Access Network (AN).  DPNSS extends
   facilities normally only available between extensions on a single
   PBX to all extensions on PBXs that are connected together in a
   private network.  DPNSS was originally derived from BT's Digital
   Access Signaling System I (DASS I) 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 on a 2048 kbits/s interface and channel 24 on a
   1544 kbits/s interface 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 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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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

2.0  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
   handled in a different way from the IUA boundary primitive transport
   messages and the boundary primitive transport messages of other
   IUA extensions (i.e. V5 or GR-303).  Therefore, it is neccessary to
   distinguish  between these from other IUA-based boundary primitive
   transport message types [1] by means of the Message Class parameter.

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

          10     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


2.2  Message Header

   IUA Message header [1] has the format as shown in Figure 2 below.



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

                          Figure 2 IUA Message Header


   In DUA header DLCI field has a different format in accordance with
   the 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 whether the message is for
   a particular DLC or 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.

   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


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   Unit Data Messages (Request, Indication) are invalid for a DUA
   application.


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).  A message type of 0x5 will be
   used for this message.

   The DLC Status messages are exchanged between IUA 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 2.1.


    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 (0x11)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | D1| D2| D3| D4| D5| D6| D7| D8| D9|D10|D11|D12|D13|D14|D15|D16|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ....................               |Dxx|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   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 since the DASS 2
   DLC does not have this state.

   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.




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2.5 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

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

   The following additional error codes are supported in DUA:

           Channel Number out of range            0x16
           Channel Number not configured          0x17

   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.

3.0 IANA Considerations

   A request will be made to IANA to assign a DUA value for the SCTP
   Payload Protocol Identifier field used in SCTP Payload Data chunks.
   The following SCTP Payload Protocol Identifier will be registered:

         DUA     "??"

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

   The User Adaptation peer may use the Payload Protocol Identifier as
   a way of determining whether the message is for IUA or DUA.

4.0 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.



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4.1 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_PHYS,Ind=1)

4.2 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)

4.3 Information Transfer on a DLC

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

4.4 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)





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4.5 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)

4.6 Getting link Status

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

4.7 Error conditions

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

5.0 Glossary of terms

          Real channel   : The signalling channel with associated
                           traffic channel (TS).
          Virtual channel: The signalling channel with no
                           associated traffic channel.
          NT1            : Retransmission period of 500msec.
          NT2            : Recommended value is 64.

6.0 Security Considerations

   The security considerations discussed for the ISDN User Adaptation
   Protocol (IUAP) [1] Section 6.0 apply to this document as well.

7.0  References

   [1] Morneault, et al., "ISDN Q.921-User Adaptation Layer", RFC 3057,
   February 2001.

   [2]  BTNR (British Telecom Network Requirements) 188 Issue 6 Digital
   Private Network Signaling System 1.

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

   [4]  ETS 300 167 (08/1993) : Transmission and Multiplexing;
   Functional characteristic of 2048 kbits/s interfaces
   (Standard is based on G.704, G.706).


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Internet Draft        draft-ietf-sigtran-dua-02.txt       November 2001



8.0  Acknowledgments

   The authors would like to thank Sudarsan Naganathan, Subhas
   Mondal and Sivaram Subramanian of Nortel Networks for their useful
   suggestions/comments.

9.0  Author's Addresses

   Authors : Anil Vydyam, Ranjith Mukundan, Narsimuloo Mangalpally and
   Ken Morneault

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

   Mick Dragon                      Phone: +44 (0)1628434388
   Nortel Networks plc              EMail: mdragon@nortelnetworks.com
   Concorde Road
   Maidenhead
   Berkshire SL6 4AG
   United Kingdom

   Ken Morneault                    Phone: +1-703-484-3323
   Cisco Systems Inc.               EMail: kmorneau@cisco.com
   13615 Dulles Technology Drive
   Herndon, VA. 20171
   USA


























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