Network Working Group R. Verma
Request for Comments: 3145 Deloitte Consulting
Category: Standards Track M. Verma
3Com Corporation
J. Carlson
Sun Microsystems
July 2001

L2TP Disconnect Cause Information

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 (2001). All Rights Reserved.

Abstract

This document provides an extension to the Layer 2 Tunneling Protocol
("L2TP"), a mechanism for tunneling Point-to-Point Protocol (PPP)
sessions. L2TP lacks a mechanism for a host to provide PPP-related
disconnect cause information to another host. This information,
provided by the extension described in this document, can be useful
for accounting and debugging purposes.

1. IntrodUCtion

L2TP [1] defines a general-purpose mechanism for tunneling PPP over
various media. By design, it insulates L2TP operation from the
details of the PPP session that is being encapsulated by L2TP. There
are, however, cases where it may be desirable for PPP-specific
disconnect information to be provided to an L2TP host (L2TP Access
Concentrator [LAC] or L2TP Network Server [LNS]) in a descriptive
format. The lack of this information is especially a problem when
the LAC and LNS are not owned or managed by the same entities.

The Result and Error Codes defined for L2TP specify only L2TP-
specific disconnect information. This document provides an
additional Attribute Value Pair (AVP), called PPP Disconnect Cause
Code, that MAY be used by an L2TP host to provide PPP-specific

disconnect information to its peer. This AVP should be used in
conjunction with, and not as a replacement for, the L2TP Result and
Error Code AVPs.

The PPP Disconnect Cause Code AVP can also be used to provide a
human-readable disconnect reason to the user. This AVP should not
have any effect on either the functioning of the tunnel or the
functioning of the PPP session; it is for informational and logging
purposes only.

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 [2].

2. PPP Disconnect Cause Code AVP

The AVP is valid in the L2TP Call-Disconnect-Notify (CDN) message
only, and it MUST NOT be marked Mandatory.

The PPP Disconnect Cause Code AVP is encoded with Vendor ID 0 and an
Attribute Type of PPP Disconnect Cause Code (46). The length of the
Value field MUST be at least 11 octets. If the length is more than
11 octets, the additional octets MUST contain a descriptive text in
UTF-8 [3] format that can be displayed to the user or in a log file.
The format of the AVP is shown below.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MH rsvd Length Vendor ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute Type Disconnect Code
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Control Protocol Number Direction Message...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Figure 1: PPP Disconnect Cause Code AVP

Mandatory (M) bit: MUST be 0.

Hidden (H) bit: MAY be 1 if the attribute is hidden.

Length: The length of the entire attribute in octets, eXPressed as a
single octet. The length MUST be at least 11.

Vendor ID: A two octet value in network byte order; set to 0 to
indicate that this is an IETF-assigned attribute.

Attribute Type: A two octet value in network byte order; set to 46
(PPP Disconnect Cause Code).

Disconnect Code: A two octet value in network byte order. (Described
in section 3 of this document.)

Control Protocol Number: The PPP Control Protocol number of the
primary protocol known to have caused the error, if any. This field
may be 0 unless mentioned otherwise in the description of the
Disconnect Codes in section 3.

Direction: A single octet value; specifies the direction in which the
Disconnect Code applies.

The valid values of this field are:

0: global error
1: at peer
2: at local
3-255: Reserved

This field SHOULD be 0 unless documented otherwise in the description
of the specific Disconnect Code.

3. Disconnect Codes

This section contains the list of well-known values of the Disconnect
Code field in the PPP Disconnect Cause Code AVP. The IANA will
maintain a registry of the up-to-date values (see section 5 of this
document). These values should be used in conjunction with the
Direction value and the Control Protocol Number field to interpret
the specific error condition.

Unless documented otherwise for a specific Disconnect Code, the
Direction value SHOULD be 0.

3.1. Global Errors

The global error codes, given in the list below, are Disconnect Codes
that do not relate to one particular PPP Control Protocol. The
Control Protocol Number for these errors thus MUST be set to 0.

0 No information available.

1 Administrative disconnect.

2 Link Control Protocol (LCP) renegotiation at LNS disabled; LNS
expects proxy LCP information, LAC did not send it.

3 Normal Disconnection, LCP Terminate-Request sent.

Valid Direction values are:

1: LCP Terminate-Request sent by peer
2: LCP Terminate-Request sent by local

4 Compulsory encryption required by a PPP peer was refused by the
other.

Valid Direction values are:

1: Required by local; refused by peer
2: Required by peer; refused by local

3.2. LCP Errors

The LCP error codes, listed below, are disconnect reasons that are
directly related to the failure of PPP peers to negotiate mutually
agreeable link parameters. The Control Protocol Number for these
errors MUST be set to C021 hexadecimal (LCP).

5 FSM (Finite State Machine) Timeout error. (PPP event "TO-".)

6 No recognizable LCP packets were received.

7 LCP failure: Magic Number error; link possibly looped back.

8 LCP link failure: Echo Request timeout.

9 Peer has unexpected Endpoint-Discriminator for existing
Multilink PPP (MP) bundle.

10 Peer has unexpected MRRU for existing MP bundle.

11 Peer has unexpected Short-Sequence-Number option for existing
MP bundle.

12 Compulsory call-back required by a PPP peer was refused by the
other.

Valid Direction values are:

1: Required by local; refused by peer
2: Required by peer; refused by local

3.3. Authentication Errors

The authentication error codes, listed below, are disconnect reasons
that are directly related to authentication failures between the PPP
peers. The Control Protocol Number for such errors MUST correspond
to the PPP Control Protocol number for the authentication protocol in
use.

13 FSM Timeout error.

14 Peer has unexpected authenticated name for existing MP bundle.

15 PPP authentication failure: Authentication protocol
unacceptable.

Valid Direction values are:

1: All local authentication protocols were rejected by the
peer.

2: All authentication protocols requested by peer were
unacceptable or unimplemented locally.

16 PPP authentication failure: Authentication failed (bad name,
password, or secret).

Valid Direction values are:

1: Authentication of peer identity by local system.
2: Authentication of local identity by peer system.

3.4. Network Control Protocol (NCP) Errors

NCP Errors are disconnect reasons that are directly related to the
failure of PPP peers to negotiate a mutually agreeable set of
parameters for the network protocols. The Control Protocol Number
for such errors SHOULD correspond to the PPP Network Control Protocol
number in use. Where multiple network protocols are in use, multiple
copies of this AVP MAY be given to indicate failure reasons for each
NCP. Otherwise, if only one copy of the AVP is given, the Control
Protocol Number SHOULD correspond to the last (most recent) failing
NCP.

17 FSM Timeout error.

18 No NCPs available (all disabled or rejected); no NCPs went to
Opened state. (Control Protocol Number may be zero only if
neither peer has enabled NCPs.)

19 NCP failure: failed to converge on acceptable addresses.

Valid Direction values are:

1: Too many Configure-Naks received from peer.
2: Too many Configure-Naks sent to peer.

20 NCP failure: user not permitted to use any addresses.

Valid Direction values are:

1: Local link address not acceptable to peer.
2: Remote link address not acceptable to local system.

4. Notes

This AVP MAY may be sent by either the LNS or LAC. It is generally
expected that this AVP will be most useful in sending notification
from the LNS to LAC in the compulsory tunneling case, although it is
not precluded from use in any other case.

A draft form of this AVP used Vendor ID 43 (3Com Corporation) and
vendor-specific Attribute Type 46. Implementations MAY accept AVPs
with these values as equivalent to the message described in this
document, but SHOULD NOT transmit an AVP using these values.

5. Security Considerations

The integrity and confidentiality of this AVP relies on the
underlying L2TP security mechanisms. It is intended for logging and
diagnostic purposes in the event of PPP link failure and should not
pose a threat to system security, but the AVP MAY be hidden as
described in section 4.3 of RFC2661.

The defined extension does not provide information that would be
useful to an attacker. Future extensions should not be defined to
lessen security. For instance, it is inappropriate to provide
distinguishing information that would inform the peer that a given
authentication name is correct, but the password/secret is incorrect.

6. IANA Considerations

IANA has assigned an L2TP Attribute Type value of 46 for the PPP
Disconnect Cause Code defined in Section 2.

This AVP includes an enumerated cause code value, called the
"Disconnect Code." Values 0 through 20 are described in this
document. Values 21 through 32767 (inclusive) are assigned by the
IANA subject to IESG Approval. Values 32768 through 65279
(inclusive) are assigned by the IANA on a First Come First Served
basis, and are intended for vendor-specific features. Values 65280
through 65535 (inclusive) are allocated for Private or Experimental
Use, and no assignment through the IANA is expected.

7. References

[1] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and B.
Palter, "Layer 2 Tunnel Protocol (L2TP)", RFC2661, August 1999.

[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

[3] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.

8. Acknowledgments

The authors thank W. Mark Townsley and Thomas Narten for their
comments and help.

9. Contacts

9.1. L2TP Working Group Chair

W. Mark Townsley
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709

EMail: townsley@cisco.com

9.2. Authors' Addresses

Rohit Verma
180 N. Stetson Avenue
Chicago IL 60601

Phone: +1 312 374 2475
Fax: +1 312 870 2475
EMail: rverma@dc.com

Madhvi Verma
3800 Golf Road
Rolling Meadows IL 60008

Phone: +1 847 262 2987
Fax: +1 847 262 2255
EMail: Madhvi_Verma@3com.com

James Carlson
Sun Microsystems
1 Network Drive MS UBUR02-212
Burlington MA 01803-2757

Phone: +1 781 442 2084
Fax: +1 781 442 1677
EMail: james.d.carlson@sun.com

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