Network Working Group A. Tveretin
Internet-Draft March 15, 2016
Intended status: Standards Track
Expires: September 16, 2016

Session Description Protocol Support for Tunnels (L2TP)


This document registeres new payload type (application/l2tp) to be used with SDP, and clarifies procedure to be used by peers for L2TP tunnels.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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."

This Internet-Draft will expire on September 16, 2016.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

1. Usage Scenario

Consider the interworking between plain old analog telephone and a SIP softphone. Alice might use her analog telephone with a fax modem to call Bob, who is in an IP network with a softphone. Alice starts with a voice call, a SIP gateway handles all interworking (gateway operation is outside the scope of this specification) and routes this call to Bob. Alice sends a fax and switches back to voice; Bob receives the fax. Alice switches to data mode, uses text chat, sends a file using Zmodem, and Bob receives it. Or Bob sends a file. Now Alice starts a PPP [RFC1661] session. To maintain connectivity with Bob, a tunnel (as of L2TP [RFC2661], PPTP [RFC2637], IPsec [RFC2401], GPRS Tunneling Protocol) is needed.

The last thing is currently not standartised. This document is to fill the gap.

Of course, capabilites depend on actual hardware and software at both sides (analog telephony or ISDN, and IP) and the gateway.

The scenario is not the only possible. I think that "IP phones at both sides" will be even more common. This will allow them to use SIP infrastructure (including P2P) to establish virtual private networks (VPNs).

This specification intentionally treats gateways exactly as native IP phones.

2. Requirements notation

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].

3. Opening a Connection: Normal Procedure

This specification will refer to parties as an offerer and an answerer, as in [RFC6665].

3.1. The Offerer Makes an Initial Offer

The offerer sends an initial offer or a re-offer with an "m=" line defined here. To introduce L2TP, the line is

The port number MUST be nonzero, although its actual value need not be honoured.

3.2. The Answerer Accepts the Offer

The answerer replies with a similar "m=" line, using a port number 1701. It MAY choose another port.

3.3. Opening a Tunnel

The offerer uses known port (from the answer), and initiates a tunnel with a usual SCCRQ message. The answerer confirmes with an SCCRP. The offerer does SCCCN, the answerer does ZLB ACK. Parties SHOULD open only one tunnel.

3.4. Opening a Session

The offerer starts with ICRQ, assuming the role of LAC, whether it is a native IP terminal or a gateway. Parties SHOULD open only one session.

4. Opening a Connection: Failure

If the answerer does not recognize or does not support "application/l2tp", it replies as in [RFC6665].

5. Terminating Procedures

SDP dialog and L2TP tunnel and session are loosely coupled. Thus, terminating a session does not imply closing a tunnel nor L2TP session. Parties MUST continue to operate L2TP even if SIP (or other) session terminates. They may tear down the tunnel, of course. Indeed, the party that wishes to terminate the session will do this almost simultaneously.

Likewise, if L2TP tunnel stops (with StopCCN), parties are not obliged to stop the dialog. Next re-offer may trigger re-opening.

6. Re-opening Procedures

In some cases e.g. roaming, or when dynamic IP address changes, a tunnel must be reestablished. Then, a party SHOULD use the following. Break existing L2TP tunnel, then re-offer SDP with new IP address.

7. IANA Considerations

This memo registers an MIME or SDP media type:

8. Security Considerations

Due to intended usage, parties of L2TP tunnels may not have each other credentials, and so rely on external (e.g. SIP) authentication. Also, a description may be intercepted by third party.

All security will be limited by a gateway, if any, rather than end-to-end.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661, DOI 10.17487/RFC2661, August 1999.
[RFC6665] Roach, A., "SIP-Specific Event Notification", RFC 6665, DOI 10.17487/RFC6665, July 2012.

9.2. Informative References

[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, DOI 10.17487/RFC2401, November 1998.
[RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W. and G. Zorn, "Point-to-Point Tunneling Protocol (PPTP)", RFC 2637, DOI 10.17487/RFC2637, July 1999.

Author's Address

Anton Tveretin ul.Dzerzhinskogo, d. 13/1, kv.34 Surgut, HMAO-Yugra 628416 RU Phone: +79224149328 EMail: URI: