Internet Draft

Internet Draft
Submitted to L2TP Working Group Yves T'Joens
INTERNET DRAFT Paolo Crivellari
<draft-ietf-l2tpext-atmext-03.txt> Bernard Sales
Alcatel
July 2000
Expires December, 2000

Layer Two Tunnelling Protocol : ATM access network extensions.

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

L2TP [RFC2661] specifies a protocol which permits the tunnelling of
the link layer of PPP over packet based networks, to support remote
access (mainly) by ISDN and PSTN networks. This document augments the
procedures described in [RFC2661] to further support ATM SVC or PVC
based access networks. The extensions defined within this document
allow for asymmetric bi-directional call establishment and service
selection in the ATM access network.

Table Of Contents

1. Introduction
1.1 Conventions

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2. Assumptions
2.1 Topology
2.2 Connection Establishment
2.3 LCP Negotiation
3. ATM access enhanced procedures
3.1 ATM connectivity
3.2 Tunnel establishment
3.3 Call establishment
3.3.1 Incoming Call Establishment
3.3.2 Outgoing 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. IANA considerations
7. Security considerations
8. Acknowledgements
9. References
10. Contacts
11. Full copyright statement

1. Introduction

L2TP [RFC2661] is a tunnelling protocol that allows tunnelling of PPP
sessions between a so called L2TP Access Concentrator (LAC) and an
L2TP Network Server (LNS). The main focus of [RFC2661] is on
supporting HDLC based ISDN/PSTN access networks.

This document augments the procedures described in [RFC2661] to
further support ATM SVC or PVC based access networks. Support for ATM
access networks requires extensions to the present L2TP procedures
along the following lines :

(a) the traffic management aspects of ATM connections (e.g.
assymetric bandwidth allocation and service category selection
capabilities),

(b) the addressing format to be used in switched ATM networks [AESA]
and

(c) the limititations imposed on LCP negotiation by transporting PPP
over AAL5 over the access network segment of the PPP connection
[RFC2364].

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Within this draft, the necessary extensions to [RFC2661] are defined
to cope with issues (a) and (b), issue (c) which is not specific to
ATM may be solved as described in [L2TP_link].

1.1 Conventions

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].

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 [RFC2661] apply mainly to access network
technology such as PSTN and ISDN, which may be respectively
asynchronous HDLC and synchronous HDLC based. The aim of this
document is to extend L2TP support to allow for user / LAC
communication based on ATM access network technology.

2.2 Connection Establishment

Due to the wide variety of existing signalling protocols and ATM
service categories, and their support or non-support within ATM based
access networks, this document takes as approach to provide for a
flexible identification of ATM connection characteristics while
establishing outgoing and incoming L2TP calls. The procedures as
defined within this document allow the allocation of asymmetric
bandwidth and service category selection in terms of real or non-real
time requirements on the ATM portion of the access network.

As such, the detailed signalling protocol specific information
elements that are necessary for switched VC service, are explicitly
not negotiated during call establishment over the L2TP tunnel.

In order to identify the endpoint of the ATM connection within the
ATM access network, SVCs can be established on the basis of the ATM
end system addressing format [AESA]. For PVC based services, the PVC
can either be referred to by using the ATM end system addressing
procedure (Called/Calling Number), or by making use of a textual name
(Service Name). The latter is inspired by the procedures defined
within [Auto_PVC].

2.3 LCP negotiation

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The procedures described within this draft may be combined with the
procedures described in [L2TP_link] to limit LCP negotiation between
LNS and user, so as to enforce PPP over AAL5 specific LCP negotiation
[RFC2364].

3. ATM access enhanced procedures

In order to illustrate the procedures specified within this draft,
this section will provide an operational description of Virtual
dial-up access through an ATM based access network (e.g., ADSL). Note
that the emphasis is on the changes proposed within this draft
relative to [RFC2661].

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. Note that
for establishing PVCs, alternative use may be made of the procedures
as described in [Auto_PVC].

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.

For PVC based users, the LAC MAY be triggered by the arrival of an
LCP Configure Request, or PPP Authentication request message from the
virtual dial-in user to initiate conversation with the LNS. Note that
the exact timing of triggering communication between LAC and LNS is
outside the scope of this document.

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.

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The bearer capability is extended to allow the LAC to indicate its
support of ATM bearer devices. Positive receipt of this indication,
allows both LAC and LNS to use the extensions as defined within this
draft to support ATM based incoming and outgoing calls.

If no compatibility between LNS and LAC exists according to the
extensions defined within this document, no tunnel establishment can
take place. This would be because the LAC does not support any bearer
capability which is expected by the LNS (e.g., an ATM based LAC, that
only signals the "Broadband" Bearer Capability), or vice versa. It is
however encouraged that LAC or LNS implementations would allow for
seamless interworking with peer devices which do not implement the
extensions defined within this draft. This could be implemented by
allowing a gracefull fallback to digital bearer capability.

3.3 call establishment

During incoming and outgoing broadband call establishment, the
following extensions are defined to existing procedures.

3.3.1 Incoming Call Establishment

The ATM connection between the virtual Dial-in user and LAC MAY
either be dynamically or statically established. When the VC
connection is dynamically established (Switched VC), the LAC will
receive a SETUP message over the interface that connects it to the
ATM network. This specification does not assume any specific
interface type (UNI or NNI). Permanent VC connections MAY either be
manually configured, or configured by use of the extensions to the
ILMI procedures as defined by [Auto_PVC].

For switched VC connections, the LAC MAY select the peer LNS on the
basis of connection establishment information, or by allowing partial
PPP authentication of the virtual Dial-in user. The connection
establishment information that can be used by the LAC include Called
Party AESA, Called Party AESA Subaddress, Calling Party AESA or
Calling Party AESA Subaddress.

For Permanent VC connections, the LAC MAY be triggered by (a) the
establishment of the PVC, (b) by an LCP configure request, (c) by
partially authenticating the virtual Dial-in user, or (d) by means
outside the scope of this specification.

Within the ICRQ, the LAC MUST indicate a broadband bearer in the
Bearer Type AVP (B bit set to 1), MAY include the Service Category
AVP, and MAY include the Service Name AVP. If the LNS would not
support the B Bearer bit, it will return an error on the ICRQ
message. In such a case, the implementation MAY decide to fall back

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to digital bearer capability, and SHOULD refrain from using the
extensions defined within this document. Further, the ICRQ message
MAY contain the VPI/VCI identifier AVP. This identifier can further
be used at the LNS for management purposes next to or alternative to
the Physical Channel ID AVP.

Within the ICCN, both Tx Connect Speed AVP and Rx Connect Speed
SHOULD be used if an assymetric connection has been established.

3.3.2 Outgoing Call Establishment

Within an 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 during OCRQ, the LNS MAY include the required Service
Category AVP, i.e., indicating real time (rt) or non-real time (nrt)
transport services. The combination of minimum and maximum receive
and transmit speed, and the indication of the required service
category allows the LAC to establish an ATM connection according to
its own capabilities, and the ATM access network capabilities,
however within the service requirement for the PPP layer.

Real time connectivity can be provided by either CBR or rt-VBR ATM
service categories, non-real time connectivity can be provided by
UBR, nrt-VBR, ABR or GFR ATM service categories.

Further the LNS MUST indicate to the LAC in OCRQ message the called
number according to the format described in this document (NSAP
format). When the called number carries an all zero payload, the LAC
SHOULD look at the Service Name AVP to bind the tunnel call to an ATM
VC connection.

Next to the normal AVPs, the OCRP message MAY contain the VPI/VCI
identifier AVP. This identifier can further be used at the LNS for
management purposes next to or alternative to the Physical Channel ID
AVP.

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

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on the ATM connection, i.e. VC multiplexed or LLC encapsulated
[RFC2364], and MUST encapsulate the PPP PDU with address and control
field, as per HDLC procedures, on the L2TP tunnel.

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 strip the
PPP PDU from the address and control field on the L2TP tunnel, and
insert the AAL5 specific fields based on the encapsulation mechanism
in use on the ATM connection, i.e. VC multiplexed or LLC
encapsulated.

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
varying parameters, some authorization may be required prior to
accepting the establishment of a switched 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

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.

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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)

tbd 0 10 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.
tbd 0 10 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.
tbd 0 8 Service Category (ocrq, icrq)
The Service Category indicates the service expected for the call,
e.g., real time or non-real time.
tbd 0 6+ Service Name (ocrq, icrq)
The Service Name indicates the service name linked to a
preestablished PVC.
tbd 0 26 Calling Sub-Address(icrq)
20 octet binary encoded NSAP subaddress indicatin the Calling
Party Sub-Address.
tbd 0 10 VPI/VCI identifier (icrq, ocrp)
10 octet binary encoded identification of VPI/VCI values used for
incoming calls.

5.2 New AVP definition

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 (OCRQ)

The Rx Minimum BPS, Attribute Type tbd, encodes the lowest
acceptable line speed for this call in the receive direction,
for these cases where asymmetric transmission is required.

The Attribute Value field for this AVP has the following format:

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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rx Minimum BPS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Rx Minimum BPS is a 32 bit value indicating the speed in
bits per second.

This AVP MAY be included within the OCRQ, and SHOULD only be
included when the LAC indicated broadband bearer support in the
bearer capabilities AVP during tunnel establishment.

This AVP may be hidden (the H-bit may be set to 0 or 1). The M-
bit for this AVP must be set to 0. The Length (before hiding) of
this AVP is 10.

Rx Maximum BPS

The Rx Maximum BPS, Attribute Type tbd, encodes the highest
acceptable line speed for this call in the receive direction,
for these cases where asymmetric transmission is required.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rx Maximum BPS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Rx Maximum BPS is a 32 bit value indicating the speed in
bits per second.

This AVP MAY be included within the OCRQ, and SHOULD only be
included when the LAC indicated broadband bearer support in the
bearer capabilities AVP during tunnel establishment.

This AVP may be hidden (the H-bit may be set to 0 or 1). The M-
bit for this AVP must be set to 0. The Length (before hiding) of
this AVP is 10.

Service Category

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The Service Category AVP, Attribute type tbd, indicates optional
extra information on the Quality of Service expected for the
call establishment on the broadband bearer medium.

The Attribute Value field for this AVP has the following format:

0 1

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resvd for future QoS ind. |S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Attribute Value field is a 16-bit mask, with one bit
defined. The S bit indicates either non real time (S bit set to
0) or real time (S bit set to 1) service requirement. The other
bit fields are reserved for future use.

The Service Category AVP MAY be present in OCRQ and ICRQ
messages, and SHOULD only be included when the LAC indicated
broadband bearer support in the bearer capabilities AVP during
tunnel establishment.

This AVP may be hidden (the H-bit may be set to 0 or 1). The M-
bit for this AVP must be set to 0. The Length (before hiding) of
this AVP is 8.

Service Name

The Service Name AVP, Attribute Type tbd, provides the peer with
an textual name for referring to an ATM VC connection.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Name (arbitrary number of octets) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Service Name is of arbitrary length, but must be at least 1
octet. The Service Name is UTF-8 encoded. [10646]

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The Service Name should be unique at least to the LNS/LAC
combination.

The Service Name AVP MAY only be provided when the Called Number
field is encoded as all zeros in OCRQ. The Service Name AVP MAY
be present in OCRQ and ICRQ messages, and SHOULD only be
included when the LAC indicated broadband bearer support in the
bearer capabilities AVP during tunnel establishment.

This AVP may be hidden (the H-bit may be set to 0 or 1). The M-
bit for this AVP must be set to 0. The length of this attribute
is arbitrary, however at least 7.

Calling Sub-Address (ICRQ)

The Calling Sub-Address AVP, Attribute Type tbd, encodes
additional Calling Party subaddress information.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSAP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSAP (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSAP (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSAP (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSAP (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Calling Sub-Address AVP MUST be encoded as a 20 octet binary
encoded NSAP address 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. The structure
of the NSAP address (e.g., E.164, ICD, DCC) is defined in
[AESA].

The Calling Sub-Address number AVP MAY be present in ICRQ, and
SHOULD only be available if the Calling Party number is also
within the message.

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This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this
AVP is 26.

VPI/VCI identifier(icrq, ocrp)

The VPI/VCI identifier, Attribute Type tbd, encodes the VPI/VCI
value used at the ATM interface at the LAC.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|resvd | VPI | VCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The VPI/VCI identifier is a 32 bit value encoding the VPI(12
bits) and VCI (16 bits) value.

This AVP MAY be included within the ICRQ and OCRP, and SHOULD
only be included when the LAC indicated broadband bearer support
in the bearer capabilities AVP during tunnel establishment.

This AVP may be hidden (the H-bit may be set to 0 or 1). The M-
bit for this AVP must be set to 0. The Length (before hiding) of
this AVP is 10.

5.3 Changed AVP Definition

The following AVPs see their contents changed relative to [RFC2661]
in order to support the procedures described in this document.

Bearer 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resvd for future bearer capability definitions |B|A|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The bearer Capabilities AVP within a SCCRQ or SCCRP indicates
the bearer capabilities that the sender of this message can

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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 with Result Code 5 'Call
failed due to lack of appropriate facilities being available
(permanent condition)'.

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 seamless fall back to digital bearer types.

This specification assumes a non-compliant LNS to categorize a
Bearer Capabilities AVP where the B bit is set as unrecognized
AVP, upon which the tunnel establishment will fail. This is to
be indicated by a Result Code '2-General error - Error Code
indicates the problem', Error Code '3- Reserved field was non-
zero'.

(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 AVP, as
defined within this document, is not available in the message,
then symmetric transmission is implied, with both minimum
receive and transmit bit-rates equal to Minimum BPS.

(Tx) Maximum BPS

(Tx) Maximum BPS AVP encodes the highest acceptable line speed
for this call in the transmit direction. The (Tx) Maximum BPS
AVP MAY be used in OCRQ. If the Rx Maximum BPS AVP, as defined
within this document, is not available in the message, then
symmetric transmission is implied, with both maximum receive and
transmit bitrates equal to Maximum BPS.

Bearer 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resvd for future bearer types definitions |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.

Q.931 Cause Code

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code | Cause Msg | Advisory Msg...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Cause code is not changed from [RFC2661], except for the
fact that it can also carry Cause Codes specific to ATM
signalling messages, i.e. ATM Forum UNI 4.0 [UNI]. The Cause
code should be interpreted relative to the Bearer Type in use
for the specific call.

Called Number

The Called Number AVP, Attribute Type 21, encodes the AESA
number to be called for an OCRQ, and the Called number at the
LAC for an ICRQ.

The Attribute Value field for this AVP has a changed encoding
from [RFC2661]:

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The Called Number AVP MUST be encoded as a 20 octet binary
encoded NSAP address 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. The structure
of the NSAP address (e.g., E.164, ICD, DCC) is defined in
[AESA].

The Called number AVP MUST be present in OCRQ, and MAY be
present in ICRQ.

If the Called Number AVP in an OCRQ carries an all zero NSAP
address, the Service Name AVP SHOULD provide further information
to bind the L2TP call to a specific VC connection. See also
[Auto_PVC].

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this
AVP is 26.

Calling Number

The Calling Number AVP, Attribute Type 22, encodes the Calling
Party AESA as received from the Virtual Dial-in User.

The Attribute Value field for this AVP has a changed encoding
from [RFC2661]:

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The Calling Number AVP MUST be encoded as a 20 octet binary
encoded NSAP address 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. The structure
of the NSAP address (e.g., E.164, ICD, DCC) is defined in
[AESA].

The Calling number AVP MAY be present in ICRQ.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this
AVP is 26.

Sub-Address

The Sub-Address AVP, Attribute Type 23, encodes additional
Called Party subaddress information.

The Attribute Value field for this AVP has a changed encoding
from [RFC2661]:

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The Sub-Address AVP MUST be encoded as a 20 octet binary encoded
NSAP address 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. The structure of the
NSAP address (e.g., E.164, ICD, DCC) is defined in [AESA].

The Sub-Address number AVP MAY be present in ICRQ and OCRQ, and
SHOULD only be available if the Called Party number is also
within the message.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this
AVP is 26.

6. IANA Considerations

This document requires 6 new type values for the following AVPs :

- Rx Minimum BPS

- Rx Maximum BPS

- Service Category

- Service Name

- Calling Sub-Address

- VPI/VCI Identifier

This document further defines a new bit (B) in the bearer
capabilities and bearer type AVPs.

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This document defines a flag field in the Service Category AVP, only
one bit in this flag has been assigned within this document (S).
Further assignments fall under the rule of "Specification Required",
i.e. Values and their meaning must be documented in an RFC or other
permanent and readily available reference, in sufficient detail so
that interoperability between independent implementations is
possible.

7. Security Considerations

No extra security risk outside these specified by [RFC2661] are
foreseen.

8. Acknowledgements

The authors would like to thank Laurent Hermans for his work on
earlier versions of this draft, Juha Heinanen (Telia) and David Allen
(Nortel Networks) for their constructive discussion on the draft
during the Minneapolis IETF meeting, Mark Townsley (cisco) for his
hint on the use of the VPI/VCI identifier AVP.

9. References

[RFC2661] W.M. Townsley, A. Valencia, A. Rubens, G. Singh Pall,
G. Zorn, B. Palter, "Layer Two Tunnelling Protocol (L2TP)", RFC 2661,
August 1999

[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2364] G. Gross, M. Kaycee, A. Lin, A. Malis, J. Stephens,
"PPP over AAL5", RFC 2364, July 1998

[UNI] User-Network Interface (UNI) Specification, Version 4.0,
ATM Forum, July, 1996

[AESA] ATM Forum Addressing : Reference Guide, version 1.0, ATM
Forum, Final Ballot, January 1999

[L2TP_link] M. Townsley, W. Palter, "L2TP Link Extensions", Internet
Draft, November 1998,

[Auto_PVC] ATM Forum, "Auto-configuration of PVCs", af-nm-0122.000,
March 1999

[10646] ISO/IEC, Information Technology - Universal Multiple-Octet
Coded Character Set (UCS) - Part 1: Architecture and Basic
Multilingual Plane, May 1993, with amendments

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10. Contacts

Yves T'joens
Alcatel Network Strategy Group
Francis Wellesplein 1, 2018 Antwerp, Belgium
Phone : +32 3 240 7890
E-mail : yves.tjoens@alcatel.be

Paolo Crivellari
Alcatel Access Systems Division
Francis Wellesplein 1, 2018 Antwerp, Belgium
Phone : +32 3 240 3319
E-mail : paolo.crivellari@alcatel.be

Bernard Sales
Alcatel Network Strategy Group
Francis Wellesplein 1, 2018 Antwerp, Belgium
Phone : +32 3 240 9574
E-mail : bernard.sales@alcatel.be

11. Full copyright statement

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However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English.

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