Submitted to PPP Working Group Yves T'Joens INTERNET DRAFT Laurent Hermans Paolo Crivellari Bernard Sales Alcatel August 1998 Expires February, 1999 Layer Two Tunnelling Protocol : ATM access extensions. Status of this memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months. Internet-Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet- Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au(Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). 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 asymmetric bi-directional call establishment. Table Of Contents 1. Introduction 1.1 Conventions 2. Assumptions T'Joens, et al. Expires February 1999 [Page 1] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 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 asymmetric bi-directional call establishment such as with procedures described in PPP over ATM [3]. 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. o "SHOULD" T'Joens, et al. Expires February 1999 [Page 2] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 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 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. In order to illustrate the procedures specified within this draft, T'Joens, et al. Expires February 1999 [Page 3] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 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. In case of non-HDLC based access network connectivity, ATM/AAL5 frames transport the PPP protocol and information field without the usage of Address and Control fields over the ATM part of the connection. Within this draft, such a framing structure is extended to the L2TP tunnel itself, thereby improving on transmission efficiency and avoiding translation between L2TP tunnel and ATM connection within the LAC. The support of non-HDLC compliant framing on the L2TP tunnel is indicated by means of an extension to the framing capabilities AVP. If no compatibility between LNS and LAC exists according to the T'Joens, et al. Expires February 1999 [Page 4] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 extensions defined within this draft, no tunnel establishment can take place. This would be because the LNS does not support any framing capability which is expected by the LAC, 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. 3.3 call establishment During incoming and outgoing call establishment, the following extensions are defined to existing procedures. During OCRQ, the LNS MUST indicate to the LAC minimum and maximum BPSs for up- and downstream traffic. 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 SHOULD be set to zero. During both OCCN and ICCN, the LAC MAY indicate the MRU it supports on its interface at the user side. The minimum of the MRU values of both LNS and LAC SHOULD apply to subsequent PPP LCP negotiation between LNS and user. Upon completion of the negotiation with the user, the LNS may indicate the final agreed MRU to the LAC by means of an SLI message. 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 of 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 LAC to LNS, the PPP PDU MUST be encapsulated depending on the setting of the framing type indicated for the call. The PPP PDU MUST be encapsulated with Address and Control field when the framing type indicates either asynchronous or synchronous (HDLC) framing, or the PPP PDU must be carried directly in the L2TP payload message, if non-HDLC framing has been indicated. In the other direction, the LAC MUST strip L2TP header and Address and Control fields from the payload packet if the framing type indicates either asynchronous or synchronous (HDLC) framing, and forward the PPP PDU on the AAL5 connection. If the framing type indicates non-HDLC framing, the PPP PDU should be extracted from the L2TP payload message by stripping the L2TP header, prior to T'Joens, et al. Expires February 1999 [Page 5] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 forwarding the PPP PDU on the AAL5 connection. 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 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 T'Joens, et al. Expires February 1999 [Page 6] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 present, these AVPs are shown in lower case as well. Attr M Len Attribute Name (usage) 1 0 32 Minimum BPS upstream (OCRQ) 32-bit integer indicating the lowest acceptable line speed for the call in the upstream direction. Upstream indicates the user to LAC direction. 2 0 32 Maximum BPS upstream (OCRQ) 32-bit integer indicating the highest acceptable line speed for call in the upstream direction. Upstream indicates the user to LAC direction. 3 0 16 Maximum Receive Unit (occn, iccn, sli) 16-bit integer indicating the maximum receive unit acceptable by the device. 4 0 9+ ATM Cause Code (cdn) 16-bit cause code, 1 octet cause message, and optional ASCII advisory message. 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. Minimum BPS upstream 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 Minimum BPS upstream AVP encodes the lowest acceptable line speed for this call in the upstream 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. Maximum BPS upstream T'Joens, et al. Expires February 1999 [Page 7] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 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 Maximum BPS upstream AVP encodes the highest acceptable line speed for this call in the upstream 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. Maximum Receive Unit 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 | Alcatel id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 3 | MRU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Maximum Receive Unit AVP indicates the maximum receive unit the sender of this AVP is able to accept. This AVP MAY be included in the OCCN, ICCN and SLI messages. When present in an OCCN or ICCN message, it is used to indicate to the LNS the MRU capabilities of a device acting as LAC. The LNS SHOULD not negotiate a MRU with the user which exceeds this value. The negotiated MRU value may be indicated by the LNS to the LAC upon completion of LCP negotiation by means of a SLI message, since this would enable optimized LAC implementation. It SHOULD however be avoided to increase this MRU again in later negotiations, since this might not be supported anymore by the LAC. ATM Cause Code T'Joens, et al. Expires February 1999 [Page 8] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 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 N is set, Non-HDLC framing is supported. If bit A is set, asynchronous framing is supported. If bit S is set, synchronous framing is supported. Bearer Capabilities T'Joens, et al. Expires February 1999 [Page 9] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 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. Minimum BPS The Minimum BPS AVP encodes the lowest acceptable line speed for this call. The Minimum BPS AVP MAY be used in OCRQ. If the minimum BPS upstream is not available in the message, then symmetric transmission is implied, with both minimum upstream and minimum downstream bit-rates equal to the Minimum BPS. 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. Maximum BPS Maximum BPS AVP encodes the highest acceptable line speed for this call. The Maximum BPS AVP MAY be used in OCRQ. If the T'Joens, et al. Expires February 1999 [Page 10] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 Maximum BPS upstream AVP is not available in the message, then symmetric transmission is implied, with both maximum upstream and maximum downstream bitrates equal to the Maximum BPS. 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 the B bit is set, 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 T'Joens, et al. Expires February 1999 [Page 11] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 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 N is set, Non-HDLC framing is requested. If bit A is set, asynchronous framing is requested. If bit S is set, synchronous framing is requested. The N bit should only be set if tunnel negotiation indicated support of non-HDLC framing by the LAC. 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) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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, T'Joens, et al. Expires February 1999 [Page 12] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 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. [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 Laurent Hermans Alcatel Corporate Research Center Francis Wellesplein 1, 2018 Antwerp, Belgium Phone : +32 3 240 8671 E-mail : hermanla@rc.bel.alcatel.be T'Joens, et al. Expires February 1999 [Page 13] Internet Draft draft-alcatel-pppext-l2tp-atmext-00.txt August 1998 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 T'Joens, et al. Expires February 1999 [Page 14]