Internet DRAFT - draft-tjoens-pppext-l2tp-atmext
draft-tjoens-pppext-l2tp-atmext
Submitted to PPP Working Group Yves T'Joens
INTERNET DRAFT Laurent Hermans
Paolo Crivellari
<draft-tjoens-pppext-l2tp-atmext-01.txt> Bernard Sales
Alcatel
August 1998
Expires February, 1999
Layer Two Tunnelling Protocol : ATM access extensions.
Status of this memo
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Distribution of this memo is unlimited.
Abstract
The L2TP working draft describes a protocol which permits the
tunnelling of the link layer of PPP. This document augments the
procedures described in the L2TP working draft to support ATM SVC or
PVC based access networks. The necessary extensions to the L2TP
working draft are defined in order to allow for non-HDLC based
framing (and related link options negotiation according to "PPP over
AAL5" as defined in [3]) and asymmetric bi-directional call
establishment in the access network.
Table Of Contents
1. Introduction
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1.1 Conventions
2. Assumptions
2.1 Topology
2.2 Connection Establishment
3. ATM access enhanced procedures
3.1 ATM connectivity
3.2 Tunnel establishment
3.3 call establishment
3.4 Framing
4. Service model issues
4.1 Authentication
4.2 Authorization
5. New and extended AVPs
5.1 New AVP Summary
5.2 New AVP definition
5.3 Changed AVP Definition
6. References
7. Contacts
1. Introduction
L2TP as described in [1] is a tunnelling protocol that allows
tunnelling of PPP sessions between a LAC and LNS (see [1] for these
definitions). Currently it has been defined to support HDLC based
ISDN/PSTN access networks.
This document augments the procedures described in [1] to support ATM
SVC or PVC based access networks. The necessary extensions to [1] are
defined in order to allow for non-HDLC based framing (and related
link options negotiation such as with procedures described in PPP
over ATM [3]) and asymmetric bi-directional call establishment in the
access network.
1.1 Conventions
Throughout this document, the words that are used to define the
significance of particular requirements are capitalised. These words
are:
o "MUST"
This word or the adjective "REQUIRED" means that the item is an
absolute requirement of this specification.
o "MUST NOT"
This phrase means that the item is an absolute prohibition of this
specification.
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o "SHOULD"
This word or the adjective "RECOMMENDED" means that there may exist
valid reasons in particular circumstances to ignore this item, but
the full implications should be understood and the case carefully
weighed before choosing a different course.
o "SHOULD NOT"
This phrase means that there may exist valid reasons in particular
circumstances when the listed behaviour is acceptable or even useful,
but the full implications should be understood and the case carefully
weighed before implementing any behaviour described with this label.
o "MAY"
This word or the adjective "OPTIONAL" means that this item is truly
optional. One vendor may choose to include the item because a
particular marketplace requires it or because it enhances the
product, for example; another vendor may omit the same item.
2. Assumptions
In this section we describe some assumptions that have lead to the
extensions described in this draft.
2.1 Topology
The procedures as defined in [1] apply solely to access network
technology such as PSTN and ISDN, which may be respectively asynch.
HDLC and synch HDLC based. The aim of this document is to extend L2TP
support to allow for user to LAC communication based on ATM access
network technology which is inherently non-HDLC based. See also [3].
2.2 Connection Establishment
Due to the wide variety of existing signalling protocols and ATM
service categories, the signalling protocol specific information
elements are not negotiated during call establishment over the L2TP
tunnel.
3. ATM access enhanced procedures
When comparing HDLC based access network support to non-HDLC based
access network support, differences can be seen in the encapsulation
of the PPP packets, and the allowance of negotiation of certain LCP
features between client and LNS. These differences are translated in
protocol operation changes.
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In order to illustrate the procedures specified within this draft,
this section will provide a description of what might happen when a
Virtual dial-up client initiates access through a (switched) ATM
access network. Note that the emphasis is on the changes proposed
within this draft relative to [1].
3.1 ATM connectivity
Prior to initiating the PPP protocol layer, a Virtual Connection (VC)
MUST be established between the user and the Network Access Server
(LAC). This virtual connection MAY either be a preconfigured
Permanent VC(PVC), where the access network provider, NAS and user
agree beforehand on the characteristics of the PVC, or MAY be an on-
demand switched VC(SVC), where the negotiation between user, network
and NAS takes place by means of an ATM signalling protocol.
In both cases, the user is referred to as the Virtual dial-in user.
Prior to accepting the switched connection from the Virtual dial-in
user, the LAC MAY check with the LNS whether the call should be
accepted. In the latter situation, the LAC MAY determine based upon
parameters available within the call establishment message that this
concerns a virtual dial in user, or MAY undertake a partial
authentication of the end system/user, in order to bind the end
system/user with a specific LNS.
3.2 Tunnel establishment
If no tunnel connection currently exists to the desired LNS, one is
initiated. During the tunnel establishment, LNS and LAC indicate
bearer and framing capabilities to each other, according to normal
procedures. The AVPs that cover these capabilities are however
extended.
The bearer capability is extended to allow the identification of ATM
devices in the LAC. This allows the LNS to use the extensions as
defined within this draft to support ATM based outgoing calls.
The framing capability is extended to allow the identification of PPP
session transport over AAL5 between user and LAC (not based on HDLC-
like framing). This allows the LNS to negotiate link options
according to "PPP over AAL5" as defined in [3].
If no compatibility between LNS and LAC exists according to the
extensions defined within this draft, no tunnel establishment can
take place. This would be because the LAC does not support any bearer
capability which is expected by the LNS, or vice versa. It is however
encouraged that LAC or LNS implementations would allow for seamless
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interworking with peer devices which do not implement the extensions
defined within this draft.
3.3 call establishment
During incoming and outgoing broadband call establishment, the
following extensions are defined to existing procedures.
During OCRQ, the LNS MUST indicate to the LAC minimum and maximum
speeds for receive and transmit traffic (from the LAC perspective).
This is to allow for the bi-directional asymmetric nature of ATM
traffic contracts. Note that in order to support UBR connections
between LAC and user, the Minimum BPS MUST be set to zero.
Further the LNS MUST indicate to the LAC in OCRQ message the dialed
number according to the format described in this draft.
3.4 Framing
Within this document the PPP PDU refers to the concatenation of PPP
protocol ID, PPP Information and PPP padding fields.
In the direction of user to LNS, the PPP PDU will be carried on top
of an AAL5 connection between user and LAC. The LAC MUST strip off
the AAL5 specific fields based on the encapsulation mechanism in use
on the ATM connection, i.e. VC multiplexed or LLC encapsulated [3].
In the direction of LNS to user, the PPP PDU will be carried on top
of an AAL5 connection between LAC and user. The LAC MUST insert the
AAL5 specific fields based on the encapsulation mechanism in use on
the ATM connection, i.e. VC multiplexed or LLC encapsulated [3].
4. Service model issues
4.1 Authentication
In case of ATM switched VC establishment, calling party number
information may be used for first level authentication much in the
same way as for PSTN or ISDN access. In case of permanent VC
establishment, authentication may not be an issue from the LAC side,
because of the permanent character of the VC. Bilateral agreement
between LAC and LNS providers may eliminate the authentication phase
in the latter case.
4.2 Authorization
Because of the flexibility of establishing ATM connections with
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varying parameters, some authorization may be required prior to
accepting the establishment of an ATM connection from the user with
certain ATM traffic parameters. This authorization may be performed
against the ATM specific authentication information (e.g. calling
line id), or may be performed after partial authentication of the
user at the PPP level. Non authorized access requests result in
connection release.
5. New and extended AVPs
The Vendor ID field SHOULD be set to the Alcatel vendor ID. (note
here that adoption of this I-d as an RFC would lead to the IETF
supported vendor id which equals zero.)
5.1 New AVP Summary
The following table lists the extra AVPs that are defined within this
document. The "attr" column indicates the integer value assigned to
this attribute. Note that the attribute value is relative compared
to the vendor ID. The "M" column indicates the setting of the
"Mandatory" bit of the AVP header for each attribute. The "LEN"
column indicates the size of the AVP including the AVP header. A "+"
in this column indicates that the length varies depending upon the
length of the actual contents of the value field.
The usage list for each entry indicates the message types that
utilize each AVP. An abbreviation shown in mixed or upper case
letters indicates that the corresponding AVP MUST be present in this
message type. All lower case indicates that the AVP MAY optionally
appear in this message type. Some AVPs MAY be present only when a
corresponding optional AVP or specific setting within the AVP is
present, these AVPs are shown in lower case as well.
Attr M Len Attribute Name (usage)
1 0 32 Rx Minimum BPS (OCRQ)
32-bit integer indicating the lowest acceptable line speed for the
call in the receive direction. Rx indicates the user to LAC
direction.
2 0 32 Rx Maximum BPS (OCRQ)
32-bit integer indicating the highest acceptable line speed for
call in the receive direction. Rx indicates the user to LAC
direction.
3 0 9+ ATM Cause Code (cdn)
16-bit cause code, 1 octet cause message, and optional ASCII
advisory message.
5.2 New AVP definition
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The following lists the new AVPs defined within this draft, and
describes the expected behaviour when this AVP would be present
within a message.
Rx Minimum BPS
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|0|0| 10 | Alcatel id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | BPS (H) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BPS (L) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Rx Minimum BPS AVP encodes the lowest acceptable line speed
for this call in the receive direction, for these cases where
asymmetric transmission is required. This AVP MAY be included
within the OCRQ, and SHOULD only be included when the LAC
indicated ATM support in the bearer capabilities AVP during
tunnel establishment.
Rx Maximum BPS
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|0|0| 10 | Alcatel id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | BPS (H) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BPS (L) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Rx Maximum BPS AVP encodes the highest acceptable line speed
for this call in the receive direction, for these cases where
asymmetric transmission is required. This AVP MAY be included
within the OCRQ, and SHOULD only be included when the LAC
indicated ATM support in the bearer capabilities AVP during
tunnel establishment.
ATM Cause Code
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|0|0| 8 + len.Cause Code| Alcatel id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | Cause Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The ATM Cause code AVP indicates the Cause Code as specified in
[4], for ATM connection establishment failures. The ATM Cause
Code AVP MUST be present in the CDN message.
5.3 Changed AVP Definition
The following AVPs see their contents changed relative to [1] in
order to support ATM access awareness.
Framing Capabilities
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | 0x00 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 |0|0|0|0|0|N|A|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The framing Capabilities AVP within a SCCRQ or SCCRP indicates
the type of framing that the sender of this message can provide.
If bit A is set, asynchronous framing is supported. If bit S is
set, synchronous framing is supported.
If bit N is set, this indicates that LNS and LAC implement the
extensions described in this document, particularly the PPP on
the LNS supports link options negotiation for LCP phase
according to "PPP over AAL5" as described in [3].
Bearer Capabilities
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | 0x00 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 |0|0|0|0|0|B|A|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The bearer Capabilities AVP within a SCCRQ or SCCRP indicates
the bearer capabilities that the sender of this message can
provide for outgoing calls. If bit B is set, broadband access is
supported (ATM), if bit A is set, analogue access is supported.
If bit D is set, Digital access is supported.
This AVP provides the peer with an indication of the bearer
device types supported by the hardware interfaces of the sender
for outgoing calls. An LNS SHOULD NOT initiate an outgoing call
specifying a value in the Bearer Type AVP for a device type not
advertised in the Bearer Capabilities AVP it received from the
LAC during control connection establishment. Attempts to do so
will result in the call being rejected.
In these cases where the LAC only supports the B bit, and the
LNS would not recognize the B bit, no outgoing calls are
possible. Note that when the LAC only has ATM based devices, it
may still opt for fall back to digital bearer types.
(Tx) Minimum BPS
The (Tx) Minimum BPS AVP encodes the lowest acceptable line
speed for this call in the transmit direction. The (Tx) Minimum
BPS AVP MAY be used in OCRQ. If the Rx Minimum BPS is not
available in the message, then symmetric transmission is
implied, with both minimum receive and transmit bit-rates equal
to Minimum BPS.
(Tx) Minimum BPS is set in bits/second, and may be mapped to the
PCR in cells/second for ATM based traffic. Note that the PCR is
a mandatory parameter for all service capabilities in ATM
signalling. The exact mapping to signalling IEs is outside the
scope of this draft.
(Tx) Maximum BPS
(Tx) Maximum BPS AVP encodes the highest acceptable line speed
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for this call in the transmit direction. The (Tx) Maximum BPS
AVP MAY be used in OCRQ. If the Rx Maximum BPS AVP is not
available in the message, then symmetric transmission is
implied, with both maximum receive and transmit bitrates equal
to Maximum BPS.
(Tx) Maximum BPS is set in bits/second, and may be mapped to the
PCR in cells/second for ATM based traffic. Note that the PCR is
a mandatory parameter for all service capabilities in ATM
signalling. The exact mapping to signalling IEs is outside the
scope of this draft.
Bearer Type
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 18 | 0x00 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 |0|0|0|0|0|B|A|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The bearer type AVP encodes the bearer type for the requested
call. The bearer type AVP MUST be present in the OCRQ, and MAY
be present in the ICRQ. If bit B is set, broadband access is
requested (ATM), if bit A is set, analogue access is requested.
If bit D is set, Digital access is requested.
Note that in the OCRQ all 3 bits (B,A,D) may be set indicating
that the call may be of either type. The B bit SHOULD only be
set if the Broadband capability was indicated during tunnel
establishment.
If ONLY the B bit is set in the Bearer Type AVP in the OCRQ
message, the LNS MUST proceed LCP negotiation with the user
according to the procedures concerning LCP options negotiation
identified in [3] for PPP over AAL5.
Framing Type
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 19 | 0x00 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 |0|0|0|0|0|N|A|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The framing type AVP encodes the framing type for the requested
call. If bit A is set, asynchronous framing is requested. If bit
S is set, synchronous framing is requested.
If bit N is set, this indicates that LNS implement the
extensions described in this document, particularly the PPP on
the LNS supports link options negotiation for LCP phase
according to "PPP over AAL5" as described in [3]. The N bit
should only be set if tunnel negotiation indicated its support
by the LAC.
The framing type MAY be used as an indication to PPP on the LNS
as to what link options to use for LCP phase (refer to "PPP over
AAL5" [3]).
Dialed Number
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0|0|0| 26 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 21 | NSAP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSAP (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSAP (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSAP (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSAP (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSAP (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The Dialed Number AVP indicates the number that should be used
to address the virtual dial in user. The Dialed number AVP MUST
be present in OCRQ, and MAY be present in ICRQ. The Dialed
number AVP SHOULD be interpreted as binary encoded when the B
bit is set in the Bearer Type AVP. The NSAP binary encoded
address provides a broader range of address encapsulation
methods than an ASCII field. In the NSAP field, which MUST have
the length of 20 octets, the addressing protocol used (E.164,
ICD, DCC) is specified as defined in [4].
Sub-Address
see Dialed Number
6. References
[1] K. Hamzeh, T. Kolar, M. Littlewood, G. Singh Pall, J. Taarud, A.
Valencia, A. Rubens, W.M. Townsley, B. Palter, W. Verthein "Layer Two
Tunnelling Protocol (L2TP)", Internet Draft, May 1998. <draft-ietf-
pppext-l2tp-11.txt>
[2] W. Simpson, "The Point-to-Point Protocol (PPP)", STD 51, RFC
1661, July 1994
[3] G. Gross, M. Kaycee, A. Lin, A. Malis, J. Stephens "PPP over
AAL5", RFC 2364, July 1998
[4] User-Network Interface (UNI) Specification, Version 3.1, ATM
Forum, September, 1994
[5] W. Simpson, "PPP in HDLC-like Framing", STD 51, RFC 1662, July
1994
7. Contacts
Yves T'joens
Alcatel Corporate Research Center
Francis Wellesplein 1, 2018 Antwerp, Belgium
Phone : +32 3 240 7890
E-mail : tjoensy@rc.bel.alcatel.be
Paolo Crivellari
Alcatel Access Systems Division
Francis Wellesplein 1, 2018 Antwerp, Belgium
Phone : +32 3 240 8583
E-mail : pcri@sebb.bel.alcatel.be
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Laurent Hermans
Alcatel Corporate Research Center
Francis Wellesplein 1, 2018 Antwerp, Belgium
Phone : +32 3 240 8671
E-mail : hermanla@rc.bel.alcatel.be
Bernard Sales
Alcatel Corporate Research Center
Francis Wellesplein 1, 2018 Antwerp, Belgium
Phone : +32 3 240 9574
E-mail : bernard.sales@btmaa.bel.alcatel.be
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