Internet DRAFT - draft-goyret-l2tpext-v92-moh
draft-goyret-l2tpext-v92-moh
Network Working Group I. Goyret
Internet Draft Lucent Technologies
Category: Experimental March 2002
draft-goyret-l2tpext-v92-moh-01.txt
V.92 Modem-On-Hold signalling on L2TP
Status of this Memo
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Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
The Layer 2 Tunneling Protocol [L2TP] defines a mechanism for
tunneling PPP [PPP] 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.
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1. Introduction
L2TP defines a general purpose mechanism for tunneling PPP over
various media. By design, it insulates L2TP operation 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) will the LNS do with this information.
1.1 Specification of Requirements
This document uses the terms "MUST", "SHOULD", "MAY" and their
negatives, as described in [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) will the LNS do with
this information. A sample of the possible actions taken by an LNS
are listed in section 5.
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2.1 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.
2.2 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. 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 indicated
in the SCCRQ or SCCRP that they implement this command with 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.
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.
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.
The following AVPs MUST be present in the MDMST message:
Message Type
Modem On-Hold Status
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The M-bit on the Message Type AVP for this command MUST be set to 0.
4. Control Message 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 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.
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 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
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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
Since V.92 has not been approved yet by the ITU, it is possible for
this mapping to change.
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. Possible actions by the LNS
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) will the LNS 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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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,
LQM (Link Quality Monitoring), MP (Multilink PPP), 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.
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 will
the LNS do with this information (if anything). A sample of possible
actions taken by an LNS 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
RFC 2661.
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7. References
[L2TP] Townsley W., et al., "Layer Two Tunneling Layer Two Tunneling
Protocol (L2TP)", RFC 2661, August 1999.
[PPP] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[V92] ITU-T Recommendation V.92, "Enhancements to Recommendation V.90",
November 2000
[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.
8. Acknowledgments
Josh Bailey of Lucent Technologies provided invaluable help in
reviewing this draft.
Mark Townsley of Cisco Systems provided helpful guidance.
9. Author's Address
Ignacio Goyret
Lucent Technologies
1701 Harbor Bay Parkway
Alameda, CA 94502
Email: igoyret@lucent.com
10. Full Copyright Statement
Copyright (C) The Internet Society 2002. All Rights Reserved.
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ing Internet standards in which case the procedures for copyrights
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defined in the Internet Standards process must be followed, or as
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Goyret, I. Internet Draft [Page 8]
