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Network Working Group A. Tveretin
Internet-Draft March 13, 2016
Intended status: Standards Track
Expires: September 14, 2016


Session Description Protocol Support for Tunnels (L2TP)
draft-tveretin-dispatch-l2tp-sdp-01.txt

Abstract

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
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This Internet-Draft will expire on September 14, 2016.

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Table of Contents

1. Usage Scenario . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
3. Opening a Connection: Normal Procedure . . . . . . . . . . . 3
3.1. The Offerer Makes an Initial Offer . . . . . . . . . . . 3
3.2. The Answerer Accepts the Offer . . . . . . . . . . . . . 3
3.3. Opening a Tunnel . . . . . . . . . . . . . . . . . . . . 3
3.4. Opening a Session . . . . . . . . . . . . . . . . . . . . 3
4. Opening a Connection: Failure . . . . . . . . . . . . . . . . 3
5. Terminating Procedures . . . . . . . . . . . . . . . . . . . 4
6. Re-opening Procedures . . . . . . . . . . . . . . . . . . . . 4
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6

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.





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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

m=application 1 UDP l2tp

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].








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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:

o Media type name: application

o Subtype name: l2tp

o Required parameters: none

o Optional parameter: none

o Encoding considerations: This type is only defined for transfer
via RTP

o Security considerations: see RFC, section 8

o Interoperability considerations: none

o Pubished specification: RFC

o Applications which use this media type: IP telephones and
softphones, and IP-PSTN gateways

o Additional Information: Not expected to be stored in files.

o Person & e-mail address to contact for further information: Anton
Tveretin, fas_vm@surguttel.ru




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o Intended usage: COMMON

o Restrictions on usage: none

o Author/Change controller: The IETF

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,
.







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Author's Address

Anton Tveretin
ul.Dzerzhinskogo, d. 13/1, kv.34
Surgut, HMAO-Yugra 628416
RU

Phone: +79224149328
Email: fas_vm@surguttel.ru
URI: http://www.fit-rulez.narod.ru









































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