Network Working Group I. Goyret Internet Draft Lucent Technologies Category: Standards Track May 2002 draft-ietf-l2tpext-v92-moh-00.txt V.92 Modem-On-Hold signalling on L2TP Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 10 of RFC 2026 [BCP9]. 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 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". The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html The distribution of this memo is unlimited. It is filed as "draft- ietf-l2tpext-v92-moh-00.txt" and expires October 31, 2002. Please send comments to the L2TP mailing list (l2tp@l2tp.net). Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract The Layer 2 Tunneling Protocol (L2TP) [RFC2661] defines a mechanism for tunneling PPP [STD51] sessions. One of the features introduced by V.92 [V92] is the ability of the client modem to put the call on hold. The L2TP base protocol does not provide any means to signal this event. This document describes a method used to indicate when a client modem has gone on-hold. Goyret, I. draft-ietf-l2tpext-v92-moh-00.txt [Page 1] INTERNET DRAFT V.92 Modem-On-Hold signalling on L2TP May 2002 Contents Status of this Memo .......................................... 1 Abstract ..................................................... 1 1. Introduction .............................................. 3 1.1 Specification of Requirements ......................... 3 2. Protocol Operation ........................................ 3 2.1 Capability negotiation ................................ 4 2.2 Modem On-Hold ......................................... 4 2.3 Modem Online .......................................... 4 3. New Commands .............................................. 4 3.1 Modem-Status (MDMST) .................................. 4 4. New Attribute Value Pairs ................................. 5 4.1 Modem On-Hold Capable AVP ............................. 5 4.2 Modem On-Hold Status AVP .............................. 5 5. Sample LNS actions ........................................ 7 6. IANA Considerations ....................................... 8 7. Security Considerations ................................... 8 8. References ................................................ 8 9. Acknowledgments ........................................... 9 10. Author's Address ......................................... 9 11. Full Copyright Statement ................................. 9 12. Expiration Date .......................................... 10 Goyret, I. draft-ietf-l2tpext-v92-moh-00.txt [Page 2] INTERNET DRAFT V.92 Modem-On-Hold signalling on L2TP May 2002 1. Introduction L2TP defines a general purpose mechanism for tunneling PPP over various media. By design, the operation of L2TP is insulated from the details of the media from which the PPP session originated, including when a V.92 client modem requests to go on-hold. It may be desirable for this information to be provided to the LNS. There are no standard AVPs that can communicate this information. This document provides additional AVPs and commands that may be used to provide modem information. However, it does not define what, if anything, the LNS should do with this information. A sample of the possible actions that an LNS may consider are listed in section 5. 1.1 Specification of Requirements 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 RFC 2119 [BCP14]. 2. Protocol Operation L2TP can be used in many different topologies. This document looks at this particular topology: +-----+ { } +-----+ [ packet ] +-----+ [ home ] | |-[M]---{ PSTN }---| LAC |...[ network ]...| LNS |...[ network ] +-----+ { } +-----+ [ ] +-----+ [ ] Remote System M is the client modem and it may be an integral part of the Remote System. If this modem implements V.92, it can request the server modem (part of the LAC) to go on-hold for some period of time. If the server modem agrees, the client modem can signal the PSTN to go on-hold (usually, a flash hook). The user at the remote system may then use the same POTS line where the client modem is connected to make or receive another call. If the LAC implements the functionality described here, it can signal to the LNS when the client modem has gone on-hold and when it comes back online. This document does not define what, if anything, the LNS should do with this information. A sample of the possible actions that an LNS Goyret, I. draft-ietf-l2tpext-v92-moh-00.txt [Page 3] INTERNET DRAFT V.92 Modem-On-Hold signalling on L2TP May 2002 MAY consider are listed in section 5. However, the LNS MUST NOT stop processing otherwise valid data packets arriving from the LAC, regardless of the current state of the modem on-hold indications. 2.1 Capability negotiation A LAC MUST NOT send a Modem Status (MDMST) command to an LNS that has not indicated the capability of processing such commands. This capability is indicated by adding a "Modem On-Hold Capable" AVP on the SCCRQ or SCCRP sent to the LAC when the tunnel is brought up. 2.2 Modem On-Hold When the client modem requests the LAC to go on-hold, the LAC SHOULD send a MDMST command to the LNS with the H (Hold) bit set to 1 and the negotiated maximum on-hold time. 2.3 Modem Online When the client modem returns back online after having gone on-hold, the LAC SHOULD send a MDMST command to the LNS with the H (Hold) bit set to 0. 3. New Commands The following commands MUST be sent with the M-bit in the Message Type AVP set to 0 to prevent interoperability issues. Messages with unknown values in the Message Type AVP with the M-bit set to 0 should be ignored by compliant L2TP peers. 3.1 Modem-Status (MDMST) The Modem-Status (MDMST) command is a control message used by the LAC to notify the LNS when the client modem on-hold status changes. The MDMST message MUST NOT be sent to peers that have not included in their SCCRQ or SCCRP the Modem On-Hold Capable AVP. Furthermore, the MDMST message can only be sent after session establishment is successful (i.e., after the LAC has sent either an ICCN or an OCCN), and before the session ends from the LAC's point of view (i.e., before the LAC has sent or received a CDN). Note that due to protocol race conditions, it is possible for a LAC to send a MDMST command about the same time that the LNS is sending a CDN. An LNS MUST ignore MDMST commands received after sending a CDN. Goyret, I. draft-ietf-l2tpext-v92-moh-00.txt [Page 4] INTERNET DRAFT V.92 Modem-On-Hold signalling on L2TP May 2002 There is currently no number assigned to this command by the IANA. A number will be requested if this draft is approved by the L2TP WG. Currently, this command is encoded as follows: Vendor ID = 529 Attribute Type = 0 (Message Type AVP) Attribute Value = 2 (MDMST) The above encoding corresponds to a vendor-specific control message. Vendor ID 529 indicates Lucent Technologies, the initial developer of this specification. The following AVPs MUST be present in the MDMST message: Message Type Modem On-Hold Status The M-bit on the Message Type AVP for this command MUST be set to 0. 4. New Attribute Value Pairs The following sections contain a list of the new L2TP AVPs defined in this document. 4.1 Modem On-Hold Capable AVP The Modem On-Hold Capable AVP indicates that the sender is capable of sending or receiving MDMST commands. This AVP MUST be included on the SCCRQ or SCCRP commands to indicate that the sender implements this specification. It is encoded as Vendor ID 529 with an Attribute Type of 2. The Vendor ID 529 indicates Lucent Technologies, the initial developer of this specification. It SHOULD be changed to 0 and an official Attribute Type chosen if this specification advances on a standards track. This AVP has no Attribute Value field. This AVP MAY be hidden (the H-bit on the AVP header MAY be 0 or 1). The M-bit for this AVP MUST be set to 0. The Length is 6. 4.2 Modem On-Hold Status AVP The Modem On-Hold Status AVP indicates the current on-hold status of the client modem. This AVP MUST be present on the MDMST command. Goyret, I. draft-ietf-l2tpext-v92-moh-00.txt [Page 5] INTERNET DRAFT V.92 Modem-On-Hold signalling on L2TP May 2002 It is encoded as Vendor ID 529 with an Attribute Type of 3. The Vendor ID 529 indicates Lucent Technologies, the initial developer of this specification. It SHOULD be changed to 0 and an official Attribute Type chosen if this specification advances on a standards track. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |H| reserved |Timeout| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Modem On-Hold Status AVP is a 16-bit quantity, containing two fields that indicate whether the client modem has gone on-hold and the maximum amount of time that the modem is allowed to remain on- hold. The H field is a single bit that indicates whether the client modem has gone on-hold. If the H bit is 1, the client modem is on-hold. If the H bit is 0, the client modem is back online. The Timeout field is a 4 bits quantity that indicates the negotiated maximum amount of time that the client modem can remain on-hold. It is only valid if the H bit is 1 and it MUST be ignored if the H bit is 0. The value of this field is defined in [V92] and it is reproduced here for easy reference: Bits Decimal Meaning ---- ------- ------- 0000 0 Reserved for the ITU 0001 1 10 seconds 0010 2 20 seconds 0011 3 30 seconds 0100 4 40 seconds 0101 5 1 minute 0110 6 2 minutes 0111 7 3 minutes 1000 8 4 minutes 1001 9 6 minutes 1010 10 8 minutes 1011 11 12 minutes 1100 12 16 minutes 1101 13 No limit 1110 14 Reserved for the ITU 1111 15 Reserved for the ITU Goyret, I. draft-ietf-l2tpext-v92-moh-00.txt [Page 6] INTERNET DRAFT V.92 Modem-On-Hold signalling on L2TP May 2002 Bits 1 through 11 are reserved. These bits MUST be set to 0 when sending this AVP and MUST be ignored on reception. This AVP MAY be hidden (the H-bit on the AVP header MAY be 0 or 1). The M-bit for this AVP MUST be set to 0. The Length is 8. 5. Sample LNS actions The specific actions taken by an LNS upon receipt of a Modem On-Hold Status AVP are implementation dependent. This document does not mandate what, if anything, the LNS should do with this information. The choice of actions taken by the LNS may have an impact on higher layer protocols. For example, TCP connections and other connection- oriented applications may timeout or disconnect during the on-hold time. The impact that those choices may have on these or other protocols is not addressed by this document. The following list is a sample of possible actions that an LNS implementation might consider. Note that some of these actions are not really alternatives, as some of the possibilities preclude others. * Temporarily stop polling protocols such as LCP Echo Requests, Link Quality Monitoring (LQM), Multilink PPP (MP), etc. * Drop data packets directed to the now on-hold remote client. * Start a new accounting session, to account for the on-hold time. * Stop or hold accounting until the modem returns online again. * Start a separate time accounting for the time that the modem is on hold. Here are a few things that an LNS should probably NOT do: * Buffer data packets directed to the now on-hold remote client. Reason: How many data packets should be buffered? What would be the impact on higher layer protocols such as TCP? What would be the impact caused by the delay introduced when the client returns online again? * Answer TCP keepalives in lieu of the client. Reason: It may interfere with TCP's recovery once the client returns online. * Stop processing otherwise valid data packets from the client. Reason: There is a race condition between the notification of the modem returning online and the first packet from the client because they are delivered on independent Goyret, I. draft-ietf-l2tpext-v92-moh-00.txt [Page 7] INTERNET DRAFT V.92 Modem-On-Hold signalling on L2TP May 2002 channels. Dropping valid client packets may lead to a slower recovery after returning online due to the forced retries. 6. IANA Considerations The Modem On-Hold Status AVP includes an enumerated value, called "Timeout", whose value is defined by ITU in [V92]. This AVP also contains a set of reserved bits (bits 1 through 11) that are assigned by IANA through IETF Consensus [BCP26]. 7. Security Considerations The integrity and confidentiality of the method described in this document relies on the underlying L2TP security mechanisms. The new command and AVPs are intended to indicate when a client modem has gone on-hold and cannot receive data. It does not define what, if anything, the LNS should do with this information. A sample of possible actions that an LNS may consider are listed in section 5. It is believed that the defined extension does not provide information that would be useful to an attacker, and as such, it should not pose a threat to system security. If desired, the new AVPs MAY be hidden as described in section 4.3 of [L2TP]. 8. References [RFC2661] Townsley W., et al., "Layer Two Tunneling Protocol (L2TP)", RFC 2661, August 1999. [BCP9] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. [BCP14] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [BCP26] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [STD51] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. Goyret, I. draft-ietf-l2tpext-v92-moh-00.txt [Page 8] INTERNET DRAFT V.92 Modem-On-Hold signalling on L2TP May 2002 [V92] ITU-T Recommendation V.92, "Enhancements to Recommendation V.90", November 2000 9. Acknowledgments Josh Bailey of Lucent Technologies provided invaluable help in reviewing this draft. Mark Townsley of Cisco Systems provided helpful guidance. 10. Author's Address Ignacio Goyret Lucent Technologies 1701 Harbor Bay Parkway Alameda, CA 94502 Email: igoyret@lucent.com 11. Full Copyright Statement Copyright (C) The Internet Society 2002. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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 develop- ing 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. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Goyret, I. draft-ietf-l2tpext-v92-moh-00.txt [Page 9] INTERNET DRAFT V.92 Modem-On-Hold signalling on L2TP May 2002 12. Expiration Date This memo is filed as "draft-ietf-l2tpext-v92-moh-00.txt" and expires October 31, 2002. Goyret, I. draft-ietf-l2tpext-v92-moh-00.txt [Page 10]