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RFC 3573
Network Working Group I. Goyret
Request for Comments: 3573 Lucent Technologies
Category: Standards Track July 2003
Signaling of Modem-On-Hold status
in Layer 2 Tunneling Protocol (L2TP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
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.
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.
This document describes a method to let the LNS know when a client
modem connected to a LAC has placed the call on hold.
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RFC 3573 Signaling of Modem-On-Hold status in L2TP July 2003
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Specification of Requirements. . . . . . . . . . . . . . 3
1.2. Terminology. . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Operation . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Typical Modem on Hold Usage Scenario . . . . . . . . . . 4
2.2. Capability Negotiation . . . . . . . . . . . . . . . . . 4
2.3. Modem On-Hold. . . . . . . . . . . . . . . . . . . . . . 5
2.4. Modem Online . . . . . . . . . . . . . . . . . . . . . . 5
3. New Control Messages . . . . . . . . . . . . . . . . . . . . . 5
3.1. Modem-Status (MDMST) . . . . . . . . . . . . . . . . . . 5
4. New Attribute Value Pairs. . . . . . . . . . . . . . . . . . . 6
4.1. Modem On-Hold Capable AVP. . . . . . . . . . . . . . . . 6
4.2. Modem On-Hold Status AVP . . . . . . . . . . . . . . . . 6
5. Sample LNS Actions . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations. . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 9
9. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix A: Vendor Specific Assignments. . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
The Layer 2 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 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] is one of the standards governing modem
operation and it defines procedures that enable 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.
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RFC 3573 Signaling of Modem-On-Hold status in L2TP July 2003
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 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 BCP 14, 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 the client modem and 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 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 to make or receive another call.
In the above scenario, the server modem module can notify the rest of
the NAS of these events via its usual signaling mechanism. The NAS
layers can then act on this information as desired. See section 5.
for a sample list of possible actions.
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RFC 3573 Signaling of Modem-On-Hold status in L2TP July 2003
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 state 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 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 signaled with a "flash-
hook"). His modem will then notify the server modem (attached to the
LAC) that it 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. 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.
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RFC 3573 Signaling of Modem-On-Hold status in L2TP July 2003
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) field set
to 1 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 to the LNS with the H
(Hold) field set to 0. The LAC MUST send this message if it has
previously sent a MDMST message with the H (Hold) field set to 1.
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 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 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.
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RFC 3573 Signaling of Modem-On-Hold status in L2TP July 2003
This control message is encoded as follows:
Vendor ID = 0 (IETF)
Attribute Type = 17
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 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 53, indicates that the
sender (an LNS) is capable of receiving MDMST control messages. This
AVP MUST be included on the SCCRQ or SCCRP control messages 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 to 0. The Length is 6.
4.2. Modem On-Hold Status AVP
The Modem On-Hold Status AVP, Attribute Type 54, 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 to
remain on-hold.
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RFC 3573 Signaling of Modem-On-Hold status in L2TP July 2003
The H (Hold) field is a single bit that indicates whether the client
modem has placed the call on-hold. If the H (Hold) field is 1, the
client modem is on-hold. If the H (Hold) field 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 valid
only if the H (Hold) field is 1 and MUST be ignored if the H (Hold)
field is 0. The values for the Timeout field are defined in [V92]
and they are 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 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.
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RFC 3573 Signaling of Modem-On-Hold status in L2TP July 2003
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 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:
17, Section 3.1., Modem Status
It also requires two new "AVP Attributes" to be assigned by IANA:
53, Section 4.1., Modem On-Hold Capable AVP
54, 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].
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RFC 3573 Signaling of Modem-On-Hold status in L2TP July 2003
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 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
[RFC2661].
8. References
8.1. Normative References
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.
and B. Peter, "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.
[BCP26] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[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.
[STD51] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
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RFC 3573 Signaling of Modem-On-Hold status in L2TP July 2003
9. Acknowledgments
Josh Bailey, Emmanuel Hislen and Marc Bongartz of Lucent Technologies
provided invaluable help in reviewing this document and its
implementation.
Mark Townsley of Cisco Systems provided helpful guidance.
Thomas Narten of IBM Corporation provided invaluable insights and
suggestions.
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RFC 3573 Signaling of Modem-On-Hold status in L2TP July 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 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.
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RFC 3573 Signaling of Modem-On-Hold status in L2TP July 2003
Author's Address
Ignacio Goyret
Lucent Technologies
1801 Harbor Bay Parkway
Alameda, CA 94502
EMail: igoyret@lucent.com
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RFC 3573 Signaling of Modem-On-Hold status in L2TP July 2003
Full Copyright Statement
Copyright (C) The Internet Society (2003). 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
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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
developing Internet standards in which case the procedures for
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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
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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