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Versions: 00 01

Network Working Group                                      GH. Hellstrom
Internet-Draft                                                   Omnitor
Updates: RFC 4102, RFC 4103 (if                        February 26, 2020
         approved)
Intended status: Standards Track
Expires: August 29, 2020


            Indicating source of multi-party Real-time text
           draft-hellstrom-avtcore-multi-party-rtt-source-00

Abstract

   Real-time text mixers need to identify the source of each transmitted
   text chunk so that it can be presented in suitable grouping with
   other text from the same source.  An enhancement for RFC 4103 real-
   time text is provided, suitable for a centralized conference model
   that enables source identification, and provision of text mixers and
   conference-enabled participants.  The mechanism builds on use of the
   CSRC list in the RTP packet.  A capability exchange is specified so
   that it can be verified that a participant can handle the multi-party
   coded real-time text stream.  The capability is indicated by a format
   parameter "rtp-mix".

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 29, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Nomenclature  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Intended application  . . . . . . . . . . . . . . . . . . . .   3
   4.  Use of fields in the RTP packets  . . . . . . . . . . . . . .   3
   5.  Actions at transmission by a mixer  . . . . . . . . . . . . .   4
   6.  Actions at reception  . . . . . . . . . . . . . . . . . . . .   5
   7.  RTCP considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Chained operation . . . . . . . . . . . . . . . . . . . . . .   6
   9.  Use with SIP centralized conferencing framework . . . . . . .   6
   10. Usage without redundancy  . . . . . . . . . . . . . . . . . .   7
   11. SDP considerations  . . . . . . . . . . . . . . . . . . . . .   7
   12. Capability negotiation  . . . . . . . . . . . . . . . . . . .   7
   13. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   14. Performance considerations  . . . . . . . . . . . . . . . . .   9
   15. Presentation level considerations . . . . . . . . . . . . . .   9
   16. Congestion considerations . . . . . . . . . . . . . . . . . .  10
   17. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   18. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     18.1.  Addition of the rtp-mix fmtp-parameter to the text/red
            format . . . . . . . . . . . . . . . . . . . . . . . . .  10
     18.2.  Addition of the rtp-mix fmtp-parameter to the text/t140
            format . . . . . . . . . . . . . . . . . . . . . . . . .  11
   19. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   20. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     20.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     20.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550] for
   transmission of RTT and the text/t140 format.  It also specifies a
   redundancy scheme text/red for increased robustness.  RFC 4102
   [RFC4102] registers the text/red format.  The redundancy scheme
   enables very efficient transmission of redundant text in packets
   together with new text.  However the redundant header format has no
   source indicators for the redundant transmissions.  An assumption has
   had to be made that the redundant parts in a packet are from the same



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   source as the new text.  The recommended transmission is one new and
   two redundant generations of text (T140blocks) in each packet and the
   recommended transmission interval is 300 ms.  A mixer, selecting
   between text input from different sources and transmitting it in a
   common stream need to make sure that the receiver can assign the
   received text to the proper sources for presentation.  Therefore, in
   order to switch source, the mixer needs to stop sending new text from
   that source and then make sure that all text so far has been sent
   with all intended redundancy levels (usually two).  That causes the
   very long time of one second to switch between transmission of text
   from one source to text from another source.  Both the total
   throughput and the switching performance in the mixer is too low for
   most applications.  A more efficient source identification scheme
   requires that each redundant T140block has its source individually
   preserved.  The present specification introduces a source indicator
   by specific rules for populating the CSRC-list in the RTP-packet.

2.  Nomenclature

   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 [RFC2119].

   The terms SDES, CNAME, NAME, SSRC, CSRC, CSRC list, CC are explained
   in [RFC3550]

   The term "T140block" is defined in RFC 4103 [RFC4103] to contain one
   or more T.140 code elements.

3.  Intended application

   The scheme for identification of source of redundant transmissions is
   intended for transmission from entities taking the mixer role in
   centralised mixing configurations for RTT.  It is intended for
   reception by both participants and mixers.

4.  Use of fields in the RTP packets

   RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a
   redundancy scheme text/red for increased robustness.  This
   specification updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by
   introducing a rule for populating and using the CSRC-list in the RTP
   packet to enhance the use in multi-party RTT sessions.

   The first member in the CSRC-list shall contain the SSRC of the
   source of the primary T140block in the packet.  The second and
   further members in the CSRC-list shall contain the SSRC of the source
   of the first, second, etc generation T140blocks included in the



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   packet. ( the recommended level of redundancy is to use one primary
   and two redundant generations of T140blocks.)  In some cases, the
   primary or redundant T140block is empty, but is still represented by
   a member in the redundancy header.  For such cases, the corresponding
   CSRC-list member MUST also be included.

   The CC field shall show the number of members in the CSRC list.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=2|P|X| CC=3  |M|  "RED" PT   |   sequence number of primary  | Note CC=3 for three sources in this packet.
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               timestamp of primary encoding "P"               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           synchronization source (SSRC) identifier            |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |  CSRC list member 1 = SSRC of source of "P"                   | This is the CSRC list.
   |  CSRC list member 2 = SSRC of source of "R1"                  |
   |  CSRC list member 3 = SSRC of source of "R2"                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|   T140 PT   |  timestamp offset of "R2" | "R2" block length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|   T140 PT   |  timestamp offset of "R1" | "R1" block length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|   T140 PT   | "R2" T.140 encoded redundant data             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
   |   |  "R1" T.140 encoded redundant data        |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         +-+-+-+
   |              "P" T.140 encoded primary data             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 1: text/red packet with sources indicated in the CSRC-list.

5.  Actions at transmission by a mixer

   A text/red transmitter is usually sending packets at a regular
   transmission interval as long as there is something (new or redundant
   T140blocks) to transmit. 300 ms is the default transmission interval,
   but shorter intervals may be considered for specific cases.  The
   transmitter has its own SSRC, and its own RTP sequence number series.
   At time of transmission, the RTP packet SHALL be populated with next
   T140block queued for transmission from any of the active sources.
   This T140block shall be placed in the primary area of the packet.
   When performing as a mixer, the SSRC of its source shall be placed as
   the first member in the CSRC-list.  The current time is inserted and
   the timestamp.





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   If no unsent T140blocks were available, at this time, but T140blocks
   available which have not been yet been sent the full intended number
   of redundant transmissions, then The primary T140block is created by
   an empty T140block, and populated in a packet for transmission.  The
   SSRC of the transmitter is included in the first place of the CSRC-
   list.

   The primary T140block, in the latest transmission is used to populate
   the first redundant T140block, and its source is placed as the second
   member of the CSRC-list.  The first redundant T140block from the
   latest transmission is now placed as the second redundant T140block,
   and the corresponding CSRC placed in its place in the CSRC-list.
   Usually this is the level of redundancy used.  If a higher number of
   redundancy is used, then the procedure is maintained until all
   available redundant levels of T140blocks and their sources are placed
   in the packet.  If a receiver has negotiated a lower number of text/
   red generations, then that level shall be the maximum used by the
   transmitter.

   The timer offset values are inserted in the redundancy header, with
   the time offset from when the corresponding T140block was sent as
   original. (usually one or two times the transmission interval).

   The number of members in the CSRC list shall be place in the CC
   header field.

   When there is no new T140block to transmit, and no redundant
   T140block that has not been retransmitted the intended number of
   times, the transmission process can stop until either new T140blocks
   arrive, or if a keep-alive method calls for transmission of keep-
   alive packets.

6.  Actions at reception

   The enhanced text/red receiver shall receive RTP packets in the
   single stream from the transmitter, and distribute the T140blocks for
   presentation in presentation areas for each source.  Other tasks for
   receivers, such as gateways or chained mixers are also feasible.

   The CC field of the received packets indicate the used level of
   redundancy for the current packet.

   The RTP sequence numbers of the received packets are monitored for
   gaps or packets out of order.

   As long as the sequence is correct, each packet is unpacked in order.
   The T140blocks are extracted from the primary area, and the
   corresponding SSRC is extracted from the first position in the CSRC



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   list and used for assigning the new T140block to the correct input
   area (or correspondingly).

   If a sequence number gap appears and is still there after some short
   defined time for jitter resolution, T140data needs to be recovered
   from redundant data.  If the gap is wider than the number of
   generations of redundant T140blocks in the packet, then a t140block
   is created with a marker for text loss [T140ad1] and assigned to the
   SSRC of the transmitter as a general input from the mixer.  Then, the
   T140blocks in the received packet are retrieved beginning with the
   highest redundant generation, grouping them with the corresponding
   SSRC from the CSRC-list and assigning them to further processing per
   source.  Finally the primary T140block is retrieved from the packet
   and similarly its source retrieved from the first position in the
   CSRC-list, and then assigned to the corresponding input handling for
   that source.

   If the sequence number gap was equal or less than the number of
   redundancy generations in the received packet, no missing text marker
   shall be inserted, and instead the T140blocks and their SSRCs
   recovered from the redundancy information and the CSRC-list in the
   way indicated above.

   Unicode character BOM is sometimes used as a filler or keep alive by
   transmission implementations.  These should be deleted on reception.

   Note that empty T140blocks are sometimes included in the packets.
   They just do not provide any contents.

7.  RTCP considerations

   A mixer should send RTCP reports with SDES, CNAME and NAME
   information about the sources in the conference.  This makes it
   possible for participants to compose a suitable label for text from
   each source.  This is as an alternative to what may provided in the
   conference notification.

8.  Chained operation

   By strictly applying the rules for CSRC-list population by all
   conforming devices, mixers can be arranged in chains.

9.  Use with SIP centralized conferencing framework

   The SIP conferencing framework, mainly specified in RFC
   4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable
   for coordinating sessions including multi-party RTT.  Even if the RTT
   stream is one and the same during the conference, the participants



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   get announced by notifications when participants are joining or
   leaving, and further user information may be provided.  The SSRC of
   the text to expect from joined users can be included in a
   notification.  This can be used both for security purposes and for
   preparation of an SSRC to label translation for presentation to other
   users.

10.  Usage without redundancy

   The CSRC list member should be used as source indicator also for
   cases when the text/t140 format is used.  That may be the case when
   robustness in transmission is provided by some other means than by
   redundancy and the text/red format.  All aspects of this memo applies
   except the redundant generations in transmission.

   For the use case without redundancy, the CC field in the RTP packet
   shall have the value 1, and the CSRC list contain one member.

11.  SDP considerations

   A new FMTP parameter for the text/red and text/t140 formats is
   defined here, named "rtp-mix", without any parameter.

   Syntax, shown together with the other fmtp parameters of text/red.

   a=fmtp:<format> cps=<integer>;rtp-mix

   The "cps" and "rtp-mix" parameters may appear in any order.

12.  Capability negotiation

   There are RTT implementations which implement RFC 4103 [RFC4103] but
   not the present specification.  Sending mixed text according to the
   present specification to a device implementing only RFC 4103
   [RFC4103] would lead to unreadable output.  Therefore, in order to
   negotiate RTT Mixing capability according to the present
   specification, All devices supporting the present specification shall
   include this fmtp parameter in offers and answers.  Multi-party
   streams using the coding in this specification must not be sent to
   devices who have not negotiated the rtp-mix capability.

   Format, as added to the format for the text/red fmtp parameters.

   a=fmtp:<format> cps=<integer>;rtp-mix

   Implementations not understanding this parameter should ignore it
   according to common SDP rules.




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13.  Examples

   A symbolic flow of packets from a mixer is shown with loss and
   recovery.

   ----------------
   Seq no 1
   CSRC list A,M,B
   P: A1
   R1: Empty
   R2 B99
   ----------------

   Assuming that earlier packets were received in sequence, text A1 is
   received and assigned to reception area A.

   ----------------
   Seq no 2
   CC=3
   CSRC list C,A,M
   P: C1
   R1: A1
   R2 Empty
   ----------------
   Text C1 is received and assigned to reception area C.

   ----------------
   X Seq no 3
   X CC=3
   X CSRC list A,C,A
   X P: A2
   X R1: C1
   X R2: A1
   ----------------
   Assumed to be dropped in network problems

   ----------------
   X Seq no 4
   X CC=3
   X CSRC list B,A,C
   X P: B1
   X R1: A2
   X R2: C1
   ----------------
   Assumed to be dropped in network problems






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   ----------------
   X Seq no 5
   X CC=3
   X CSRC list A,B,A
   X P: A3
   X R1: B1
   X R2: A2
   ----------------
   Assumed to be dropped in network problems

----------------
Seq no 6
CC=3
CSRC list C,A,B
P: C2
R1: A3
R2: B1
----------------
Latest received sequence number was 2. Recovery is tried for 3,4,5.
But there is no coverage for seq no 3. A missing text mark is created and assigned to the mixer reception area.
For seqno 4, text B1 is recovered and assigned to reception area B.
For seqno 5, text A3 is recovered and assigned to reception area A.
Primary text C2 is received and assigned to reception area C.

   With only one or two packets lost, there would not be any need to
   create a missing text marker, and all text would be recovered.

   It will be a design decision how to present the missing text markers
   assigned to the mixer as a source.

14.  Performance considerations

   This specification allows new text from one source per packet.
   Packets are transmitted with timed intervals.  The default
   transmission interval is 300 ms for RFC 4103[RFC4103], and is
   suitable for transmission from single sources.  However when more
   sources contribute to the flow, a shorter transmission interval may
   be applicable.  The transmission interval is therefore recommended to
   be 100 ms for mixers.  This interval provides for smooth flow of text
   from 5 sources simultaneously.

15.  Presentation level considerations

   ITU-T T.140 [T140] provides the presentation level requirements for
   the RFC 4103 [RFC4103] transport.  T.140 [T140] has functions for
   erasure and other formatting and this general statement for the
   presentation:




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   "The display of text from the members of the conversation should be
   arranged so that the text from each participant is clearly readable,
   and its source and the relative timing of entered text is visualized
   in the display.  Mechanisms for looking back in the contents from the
   current session should be provided.  The text should be displayed as
   soon as it is received."

   There is no strict "message" concept in real-time text.  Line
   separator is used as a separator allowing a part of received text to
   be grouped in presentation.  The receiving party may separate
   presentation of parts of text from a source in readable groups based
   on other criteria when it benefits the user to most easily find new
   text or correlated text from different parties.

   Further presentation level considerations are out of scope for this
   document.

16.  Congestion considerations

   The congestion considerations and recommended actions from RFC 4103
   [RFC4103] are valid also in multi-party situations.

17.  Acknowledgements

18.  IANA Considerations

   [RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the
   RFC number of this document.]

18.1.  Addition of the rtp-mix fmtp-parameter to the text/red format

   This document adds the format parameter "rtp-mix" to the parameters
   for the a=fmtp attribute of the text/red format.  The parameter has
   no values.  It is an indication of the capability to use the rtp-
   mixer format in a real-time text session.  The parameter can occur
   with other fmtp parameters in any order.  The usage is specified in
   section "negotiation" above.  The usage adds to the usage of the fmtp
   attribute defined in RFC 4102 [RFC4102] and RFC 4103 [RFC4103].













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   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | fmtp                                      |
   | Usage level:          | media                                     |
   | Added parameter       | rtp-mix                                   |
   | Purpose:              | Indicate support for the rtp-mixer format |
   |                       | for real-time text transmission with      |
   |                       | redundancy                                |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

18.2.  Addition of the rtp-mix fmtp-parameter to the text/t140 format

   This document adds the format parameter "rtp-mix" to the parameters
   for the a=fmtp attribute of the text/t140 format.  The parameter has
   no values.  It is an indication of the capability to use the rtp-
   mixer format in a real-time text session.  The parameter can occur
   with other fmtp parameters in any order.  The usage is specified in
   section "negotiation" above.  The usage adds to the usage of the fmtp
   attribute defined in RFC 4102 and RFC 4103.

   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | fmtp                                      |
   | Usage level:          | media                                     |
   | Added parameter       | rtp-mix                                   |
   | Purpose:              | Indicate support for the rtp-mixer format |
   |                       | for real-time text transmission without   |
   |                       | redundancy                                |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

19.  Security Considerations

   The RTP-mixer model requires the mixer to be allowed to decrypt, pack
   and encrypt secured text from the conference participants.  Therefore
   the mixer needs to be trusted.  This is similar to the situation for
   central mixers of audio and video.

   The requirement to transfer information about the user in RTCP
   reports in SDES, CNAME and NAME fields for creation of labels may
   have privacy concerns as already stated in RFC 3550 [RFC3550], and
   may be restricted of privacy reasons.  The receiving user will then
   get a more symbolic label for the source.





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20.  References

20.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC4102]  Jones, P., "Registration of the text/red MIME Sub-Type",
              RFC 4102, DOI 10.17487/RFC4102, June 2005,
              <https://www.rfc-editor.org/info/rfc4102>.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
              <https://www.rfc-editor.org/info/rfc4103>.

   [T140]     ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for
              multimedia application text conversation", February 1998.

   [T140ad1]  ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000),
              Protocol for multimedia application text conversation",
              February 2000.

20.2.  Informative References

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353,
              DOI 10.17487/RFC4353, February 2006,
              <https://www.rfc-editor.org/info/rfc4353>.

   [RFC4575]  Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
              Session Initiation Protocol (SIP) Event Package for
              Conference State", RFC 4575, DOI 10.17487/RFC4575, August
              2006, <https://www.rfc-editor.org/info/rfc4575>.

   [RFC4579]  Johnston, A. and O. Levin, "Session Initiation Protocol
              (SIP) Call Control - Conferencing for User Agents",
              BCP 119, RFC 4579, DOI 10.17487/RFC4579, August 2006,
              <https://www.rfc-editor.org/info/rfc4579>.






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Author's Address

   Gunnar Hellstrom
   Omnitor
   Esplanaden 30
   Vendelso  13670
   Sweden

   Phone: +46708204288
   Email: gunnar.hellstrom@omnitor.se









































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