Layer Two Tunnelling Protocol (L2TP): ATM access network extensions
Extracted elements (30)
Detailed ATM signalling protocol information elements (specific to SVC establishment) are explicitly not negotiated over the L2TP tunnel; instead, flexible AVPs describe ATM connection characteristics. This avoids coupling L2TP to specific ATM signalling protocol variants.
NSAP binary encoding was chosen for Called Number, Calling Number, Sub-Address, and Calling Sub-Address AVPs instead of ASCII because it supports a broader range of address encapsulation methods (E.164, ICD, DCC), matching ATM Forum AESA addressing.
Separate Rx Minimum/Maximum BPS AVPs were introduced alongside the existing Tx BPS AVPs because ATM traffic contracts are inherently asymmetric; absence of the Rx AVPs signals symmetric operation, preserving backward compatibility with RFC 2661 implementations.
The Service Name AVP (inspired by ATM Forum Auto-PVC procedures) was introduced to allow PVC binding via a textual name when no NSAP address is available (Called Number all zeros), accommodating deployments where PVCs are identified administratively rather than by address.
A non-compliant LNS receiving a Bearer Capabilities AVP with the B bit set will treat it as an unrecognized AVP and fail tunnel establishment with Result Code 2, Error Code 3 ('Reserved field was non-zero'), preventing silent misoperation.
If the LAC only supports ATM-based (B bit) bearer and the LNS does not recognize it, no outgoing calls are possible; however implementations are encouraged to allow graceful fallback to digital bearer types for seamless interworking with non-ATM peers.
The Q.931 Cause Code AVP is unchanged from RFC 2661 in encoding, but its cause codes SHOULD now be interpreted relative to the Bearer Type in use; ATM-specific cause codes per ATM Forum UNI 4.0 are permitted.
Prior to initiating the PPP protocol layer, a Virtual Connection (VC) MUST be established between the user and the Network Access Server (LAC), either as a preconfigured PVC or an on-demand SVC via ATM signalling.
The Called Number, Calling Number, Sub-Address, and Calling Sub-Address AVPs MUST each be encoded as a 20-octet binary NSAP address when the B bit is set in the Bearer Type AVP.
The LAC MUST strip AAL5-specific fields (VC-multiplexed or LLC-encapsulated per RFC 2364) and encapsulate the PPP PDU with address and control fields per HDLC procedures on the L2TP tunnel in the user-to-LNS direction; the reverse applies LNS-to-user.
The LNS MUST indicate to the LAC in the OCRQ the called number in NSAP format. When the Called Number carries an all-zero payload, the LAC SHOULD use the Service Name AVP to bind the tunnel call to an ATM VC connection.
The M-bit for all new ATM extension AVPs (Rx Minimum BPS, Rx Maximum BPS, Service Category, Service Name, Calling Sub-Address, VPI/VCI Identifier) MUST be set to 0; the B Bearer bit in a non-compliant LNS will cause tunnel establishment failure with Result Code 2, Error Code 3.
Within an OCRQ, the LNS MUST indicate to the LAC minimum and maximum speeds for both receive and transmit traffic (from the LAC perspective) to support asymmetric ATM traffic contracts. For UBR connections, Minimum BPS MUST be set to zero.
Within the ICCN, both Tx Connect Speed AVP and Rx Connect Speed SHOULD be used if an asymmetric ATM connection has been established.
Within the ICRQ, the LAC MUST indicate a broadband bearer in the Bearer Type AVP (B bit set to 1). If the LNS does not support the B Bearer bit, it returns an error; the implementation MAY fall back to digital bearer and SHOULD refrain from using ATM extensions.
The Bearer Capabilities AVP is extended with a B bit: if set, the sender supports broadband (ATM) access. Outgoing call attempts with Bearer Type B while Bearer Capabilities lacks B are rejected with Result Code 5.
The Bearer Type AVP is extended with a B bit indicating broadband (ATM) access is requested; in OCRQ all three bits (B, A, D) may be set together, permitting any bearer type. The B bit SHOULD only be set if broadband capability was indicated at tunnel establishment.
The Calling Sub-Address AVP (Attribute Type 44) encodes additional Calling Party subaddress information as a 20-octet binary NSAP address; it MAY appear in ICRQ and SHOULD only be present if the Calling Party number is also in the message.
The Rx Maximum BPS AVP (Attribute Type 41) encodes the highest acceptable line speed in the receive (user-to-LAC) direction for asymmetric calls. It is optional in OCRQ; absence implies symmetric transmission with Max BPS equal to the Tx Maximum BPS.
The Rx Minimum BPS AVP (Attribute Type 40) encodes the lowest acceptable line speed for an outgoing call in the receive (user-to-LAC) direction, enabling asymmetric ATM bandwidth allocation. It is optional in OCRQ and SHOULD only be included when broadband bearer was indicated.
The Service Category AVP (Attribute Type 42) is a 16-bit mask with one defined bit (S): S=0 indicates non-real-time, S=1 indicates real-time ATM service. It MAY appear in OCRQ and ICRQ to select CBR/rt-VBR vs. UBR/nrt-VBR/ABR/GFR service categories.
The Service Name AVP (Attribute Type 43) provides a UTF-8 encoded textual name for referring to an ATM PVC connection; it MUST only be present in OCRQ when the Called Number field is all zeros, and MAY appear in ICRQ.
The VPI/VCI Identifier AVP (Attribute Type 45) encodes the ATM VPI (12 bits) and VCI (16 bits) at the LAC interface in a 32-bit field (4 bits reserved); it MAY appear in ICRQ and OCRP for management purposes alongside or instead of Physical Channel ID AVP.
IANA is required to allocate 6 new L2TP AVP type values: Rx Minimum BPS (40), Rx Maximum BPS (41), Service Category (42), Service Name (43), Calling Sub-Address (44), and VPI/VCI Identifier (45). Further Service Category flag bits require 'Specification Required' process.
The document states that no extra security risks beyond those specified in RFC 2661 are foreseen for the ATM access network extensions. For ATM SVC establishment, calling party number information MAY be used for first-level authentication analogously to PSTN/ISDN calling line ID.
Called Number, Calling Number, Sub-Address, and Calling Sub-Address AVPs each encode a 20-octet binary NSAP address (supporting E.164, ICD, DCC structures per AESA); total AVP length is 26 octets.
Rx Minimum BPS and Rx Maximum BPS AVPs each carry a single 32-bit unsigned integer field indicating speed in bits per second; total AVP length (including 6-byte header) is 10 octets.
Service Category AVP carries a 16-bit mask (2-octet value field); only bit 0 (S) is defined: S=1 means real-time. Total AVP length is 8 octets.
The (Tx) Minimum BPS and (Tx) Maximum BPS AVPs from RFC 2661 are reinterpreted: if the corresponding Rx BPS AVPs are absent, symmetric transmission is implied with identical Tx and Rx rates.
VPI/VCI Identifier AVP value field is a 32-bit word: 4 reserved bits, 12-bit VPI, and 16-bit VCI. Total AVP length is 10 octets.