wdiff draft-ietf-l2tpext-v92-moh-04.txt draft-ietf-l2tpext-v92-moh-05.txt

Network Working Group I. Goyret
Internet Draft Lucent Technologies
Category: Standards Track February 2003
draft-ietf-l2tpext-v92-moh-04.txtA new Request for Comments is now available in online RFC libraries.

RFC 3573

Title: Signalling of Modem-On-Hold status
in Layer 2 Tunneling Protocol (L2TP)

Status of this Memo

This document is an Internet-Draft and is in full conformance with
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
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The distribution of this memo is unlimited. It is filed as "draft-
ietf-l2tpext-v92-moh-04.txt" and expires
Author(s): I. Goyret
Status: Standards Track
Date: July 31, 2003. Please send
comments to the L2TP mailing list (l2tpext@ietf.org).

Abstract 2003
Mailbox: igoyret@lucent.com
Pages: 13
Characters: 22758
Updates/Obsoletes/SeeAlso: None

I-D Tag: draft-ietf-l2tpext-v92-moh-05.txt

URL: ftp://ftp.rfc-editor.org/in-notes/rfc3573.txt

The Layer 2 Tunneling Protocol (L2TP) defines a mechanism for
tunneling Point-to-Point Protocol (PPP) sessions. It is common for
these PPP sessions to be established using modems connected over the
public switched telephone network.

One of the standards governing modem operation defines procedures
that enable a client modem to put the call on hold and later, re-
establish the modem link with minimal delay and without having to
redial. While the modem call is on hold, the client phone line can
be used to place or receive other calls.

This document describes a method to let the LNS know when a client
modem connected to a LAC has placed the call on hold.

Contents

Status of this Memo .......................................... 1

Abstract ..................................................... 1

1. Introduction .............................................. 4
1.1 Specification

This document is a product of Requirements ......................... 4
1.2 Terminology ........................................... 4

2. Protocol Operation ........................................ 5
2.1 Typical modem on hold usage scenario .................. 6
2.2 Capability negotiation ................................ 6
2.3 Modem On-Hold ......................................... 6
2.4 Modem Online .......................................... 6

3. New Control Messages ...................................... 6
3.1 Modem-Status (MDMST) .................................. 7

4. New Attribute Value Pairs ................................. 7
4.1 Modem On-Hold Capable AVP ............................. 8
4.2 Modem On-Hold Status AVP .............................. 8

5. Sample LNS actions ........................................ 9

6. IANA Considerations ....................................... 10

7. Security Considerations ................................... 10

8. References ................................................ 11

9. Acknowledgments ........................................... 11

10. Author's Address ......................................... 12
11. Full Copyright Statement ................................. 12

12. Expiration Date .......................................... 12

Appendix A: Vendor specific assignments ...................... 13

1. Introduction

The the Layer 2 Two Tunneling Protocol (L2TP) [RFC2661] defines a general
purpose mechanism for tunneling Point-to-Point Protocol (PPP) [STD51]
sessions over various media. By design, the operation of L2TP is
insulated from the details
Extensions Working Group of the media from which the PPP session
originated.

It is common for PPP sessions to be established using modems
connected over the public switched telephone network. The ITU-T
Recommendation V.92 [V92] IETF.

This is one of the standards governing modem
operation and it defines procedures that enable now a client modem to put
the call on hold and later, re-establish the modem link without
having to redial. While the modem call is on hold, the client phone
line can be used for another phone call.

The L2TP base protocol does not provide any means to signal these
events from the L2TP Access Controller (LAC), where the modem is
physically connected, to the L2TP Network Server (LNS), where the PPP
session is handled. It may be desirable for this information (which
is available only on the LAC) to be provided to the LNS. Proposed Standard Protocol.

This document provides additional L2TP AVPs and control messages that
may be used to communicate some modem information from the LAC to the
LNS. However, it does not define what, if anything, the LNS should
do with this information. A sample of the possible actions that specifies 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].

1.2 Terminology

Definition of terms used in this document may be found in the L2TP
Protocol document [L2TP].

2. Protocol Operation

The typical dial in topology looks like this:

+-----+ { } +----------+ [ IP ]
| |-[M]-----{ PSTN }-----[SM] |.....[ network ]
+-----+ { } +----------+ [ ]
Remote NAS
System

M is Internet standards track protocol for
the client modem Internet community, and it may be an integral part of the Remote
System. If this modem implements V.92, it can ask the server modem
SM (a part of the NAS) whether the call can be placed on-hold requests discussion and suggestions
for
some period of time.

If the server modem agrees, the client modem can signal the PSTN to
place the call on-hold (usually, a flash hook). The user at the
remote system can then use the same POTS line where the client modem
is connected improvements. Please refer to make or receive another call.

In the above scenario, the server modem module can notify the rest current edition of the NAS of these events via its usual signalling mechanism. The NAS
layers can then act on this information as desired. See section 5
"Internet Official Protocol Standards" (STD 1) for
a sample list of possible actions.

In the case of L2TP, this document looks at this particular topology:

+-----+ { } +-----+ [ packet ] +-----+ [ home ]
| |-[M]---{ PSTN }---[SM] |...[ network ]...| |...[ network ]
+-----+ { } +-----+ [ ] +-----+ [ ]
Remote LAC LNS
System

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
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
standardization state and status of the modem on-hold indications.

2.1 Typical modem on hold usage scenario

A user connects to his internet service provider with a V.92-capable
modem. He then starts downloading a big file which will take several
minutes to complete.

While the file is being downloaded, a friend calls him. If the user
has call waiting enabled, his modem can let him know this protocol. Distribution
of the incoming
call and he can choose to either pick up the incoming call or reject
it. Let's say he chooses to pick up the phone to talk to his friend,
for example because he recognized the caller's phone number.

Before the user picks up his phone, he tells his modem to go on hold
and switch to the incoming call (usually signalled with a "flash-
hook"). His modem will then notify the server modem (attached to the
LAC) that it this memo is about to go on hold. If the server modem agrees, the
client performs a flash hook so the user can talk to his friend.

After talking to his friend, the user let's his modem know that it
can return to the original call (where the server modem has been
patiently waiting). After another flash hook, the modems are
connected again and the download can continue.

2.2 Capability negotiation

A LAC MUST NOT send a Modem Status (MDMST) control message to an LNS
that has not indicated the capability of processing such control
messages. unlimited.

This capability announcement 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.

2.3 Modem On-Hold

When the client modem requests the LAC to go on-hold, the LAC SHOULD
send a MDMST control message to the LNS with the H (Hold) bit set to
1 IETF list and the negotiated maximum on-hold time.

2.4 Modem Online

When the client modem returns back online after having gone on-hold,
the LAC SHOULD send a MDMST control message RFC-DIST list.
Requests to the LNS with the H
(Hold) bit set to 0.

3. New Control Messages

The following control messages 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 added to 0 should be ignored by compliant L2TP peers [RFC2661].

3.1 Modem-Status (MDMST)

The Modem-Status (MDMST) control message is used by the LAC to notify
the LNS when the client modem on-hold status changes.

The MDMST control message MUST NOT be sent to peers that have not
included the "Modem On-Hold Capable" AVP in their Start-Control-
Connection-Request (SCCRQ) or Start-Control-Connection-Reply (SCCRP)
control messages.

Furthermore, the MDMST control message can only be sent after session
establishment is successful (i.e., after the LAC has sent either an
Incoming-Call-Connected (ICCN) or an Outgoing-Call-Connected (OCCN)
control message), and before the session ends deleted from the LAC's point of
view (i.e., before the LAC has sent or received a Call-Disconnect-
Notify (CDN) control message).

Note that due to protocol race conditions, it is possible for a LAC
to send a MDMST control message about the same time that the LNS is
sending a CDN. An LNS MUST ignore MDMST control messages received
after sending a CDN.

An LNS MUST ignore redundant modem status reports.

This control message is encoded as follows:

Vendor ID = 0 (IETF)
Attribute Type = TBDC1

The following AVPs MUST be present in the MDMST control message:

Message Type
Modem On-Hold Status

The M-bit on the Message Type AVP for this control message MUST be
set to 0.

4. New Attribute Value Pairs

The following sections contain a IETF distribution list of the new L2TP AVPs defined in
this document.

4.1 Modem On-Hold Capable AVP

The Modem On-Hold Capable AVP, Attribute Type TBDA1, indicates that
the sender (an LNS) is capable of receiving MDMST control messages.
This AVP MUST
should be included on the SCCRQ or SCCRP control messages sent to
indicate that the sender implements this specification.

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 IETF-REQUEST@IETF.ORG. Requests to 0. The Length is 6.

4.2 Modem On-Hold Status AVP

The Modem On-Hold Status AVP, Attribute Type TBDA2, indicates the
current on-hold status of the client modem. This AVP MUST be present
on the MDMST control message.

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 placed the call
on-hold and the maximum amount of time that the call is allowed
added to
remain on-hold.

The H field is a single bit that indicates whether the client modem
has placed the call 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 call can remain on-hold. It is only
valid if the H bit is 1 and it MUST be ignored if or deleted from 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
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
1111 15 Reserved

Bits 1 through 11 are reserved. These bits MUST RFC-DIST distribution list should
be set sent to 0 when
sending this AVP and MUST be ignored RFC-DIST-REQUEST@RFC-EDITOR.ORG.

Details on reception.

This AVP MAY be hidden (the H-bit on the AVP header MAY be 0 obtaining RFCs via FTP or 1).
The M-bit for this AVP MUST EMAIL may be set to 0. The Length is 8.

5. Sample LNS actions

The specific actions taken obtained by sending
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 EMAIL message to rfc-info@RFC-EDITOR.ORG with this information.

The choice of actions taken by the LNS may have an impact on higher
layer protocols. message body
help: ways_to_get_rfcs. 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 example:

To: rfc-info@RFC-EDITOR.ORG
Subject: getting rfcs

help: ways_to_get_rfcs

Requests 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 special distribution 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
channels. Dropping valid client packets may lead to
a slower recovery after returning online due to the
forced retries.

6. IANA Considerations

This document requires one new L2TP "Message Type" number to be
assigned by IANA:

TBDC1, Section 3.1, Modem Status

It also requires two new "AVP Attributes" to be assigned by IANA:

TBDA1, Section 4.1, Modem On-Hold Capable AVP
TBDA2, Section 4.2, Modem On-Hold Status AVP

The Modem On-Hold Status AVP 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
control message and AVPs are intended addressed to indicate when a client modem
has gone on-hold and cannot receive data. It does not define what,
if anything, either the LNS should do with this information. A sample
author 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
[RFC2661].

8. References

8.1. Normative references

[RFC2661] Townsley W., et al., "Layer Two Tunneling Protocol (L2TP)", RFC 2661, August 1999.

[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[V92] ITU-T Recommendation V.92, "Enhancements to Recommendation
V.90", November 2000

8.2. Informative references

[BCP9] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.

[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.

9. Acknowledgments

Josh Bailey of Lucent Technologies provided invaluable help in
reviewing this draft.

Mark Townsley of Cisco Systems provided helpful guidance.

Thomas Narten of IBM Corporation provided invaluable insights and
suggestions.

10. Author's Address

Ignacio Goyret
Lucent Technologies
1701 Harbor Bay Parkway
Alameda, CA 94502

Email: igoyret@lucent.com

11. Full Copyright Statement

This document and translations of it may be copied and furnished to
others, and derivative works that comment on question, or to RFC-Manager@RFC-EDITOR.ORG. Unless
specifically noted 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 RFC itself, all RFCs are for the purpose of develop-
ing Internet standards in which case the procedures
unlimited distribution.echo
Submissions for copyrights
defined in the Internet Standards process must Requests for Comments should be followed, or as
required sent 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.

12. Expiration Date

This memo is filed as "draft-ietf-l2tpext-v92-moh-04.txt" and expires
July 31, 2003.

Appendix A: Vendor specific assignments

THIS SECTION IS NOT NORMATIVE

Early implementations of this specification used vendor-specific
values for the new control message and AVPs. This appendix describes
those initial vendor-specific assignments
RFC-EDITOR@RFC-EDITOR.ORG. Please consult RFC 2223, Instructions to RFC
Authors, for historical reference
only.

The following table shows the vendor-specific assignments:

Vendor Attr Attr
ID Type Value Equivalent to
------ ---- ----- -------------
Control message:
Modem-Status 529 0 2 Section 3.1

AVP:
Modem On-Hold Capable 529 2 none Section 4.1
Modem On-Hold Status 529 3 [..] Section 4.2 further information.