RFC 5641 PROPOSED STANDARD L2TP · PSEUDOWIRES · FAULT REPORTING
L2TPv3 circuit status: Layer 2 Tunneling Protocol Version 3 (L2TPv3) Extended Circuit Status Values
Seven status bits let an L2TPv3 endpoint say which direction of a link has failed, and whether a pseudowire is on standby.
RFC EDITOR
McGill, N. and C. Pignataro, "Layer 2 Tunneling Protocol Version 3 (L2TPv3) Extended Circuit Status Values", RFC 5641, DOI 10.17487/RFC5641, August 2009, <https://www.rfc-editor.org/info/rfc5641>.
BIBTEX
@misc{rfc5641,
series = {Request for Comments},
number = 5641,
howpublished = {RFC 5641},
publisher = {RFC Editor},
doi = {10.17487/RFC5641},
url = {https://www.rfc-editor.org/info/rfc5641},
author = {N. McGill and C. Pignataro},
title = {{Layer 2 Tunneling Protocol Version 3 (L2TPv3) Extended Circuit Status Values}},
pagetotal = 11,
year = 2009,
month = aug,
} RFC text
Contents
Network Working Group N. McGill
Request for Comments: 5641 C. Pignataro
Updates: 3931, 4349, 4454, 4591, 4719 Cisco Systems
Category: Standards Track August 2009
Layer 2 Tunneling Protocol Version 3 (L2TPv3)
Extended Circuit Status Values
Abstract
This document defines additional Layer 2 Tunneling Protocol Version 3
(L2TPv3) bit values to be used within the "Circuit Status" Attribute
Value Pair (AVP) to communicate finer-grained error states for
Attachment Circuits (ACs) and pseudowires (PWs). It also generalizes
the Active bit and deprecates the use of the New bit in the Circuit
Status AVP, updating RFC 3931, RFC 4349, RFC 4454, RFC 4591, and RFC
4719.
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) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Specification of Requirements . . . . . . . . . . . . . . 2
1.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 3
2. L2TPv3 Extended Circuit Status Values . . . . . . . . . . . . 3
3. Circuit Status Usage and Clarifications . . . . . . . . . . . 7
4. Updates to Existing RFCs . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
1. Introduction
Currently, the L2TPv3 Circuit Status AVP [RFC3931] is able to convey
the UP/DOWN status of an access circuit. However, a finer
granularity is often useful to determine the direction of the fault,
as has been added for MPLS-based pseudowires and is used in the
pseudowire control protocol using the Label Distribution Protocol
(LDP); see Section 3.5 of [RFC4446] and Section 5.4.2 of [RFC4447].
Additionally, it is useful (in session-level redundancy scenarios) to
be able to indicate if a pseudowire is in a standby state, where it
is fully established by signaling and allows Operations,
Administration, and Maintenance, but is not switching data. Again,
such functionality is available for MPLS-based pseudowires using LDP,
see [PREF-FWD].
This document provides extended circuit status bit values for L2TPv3
and adds them in a manner such that it is backwards compatible with
the current Circuit Status AVP. These new bits are applicable to all
pseudowire types that use the Circuit Status AVP.
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 [RFC2119].
1.2. Abbreviations
The following abbreviations are used in this document and in the
documents that it updates. L2TPv3 Control Message Types are listed
in Section 6 of [RFC3931].
AC Attachment Circuit
AVP Attribute Value Pair
LCCE L2TP Control Connection Endpoint
NNI Network-Network Interface
PE Provider Edge
PSN Packet Switched Network
PW Pseudowire
2. L2TPv3 Extended Circuit Status Values
The Circuit Status AVP (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, SLI),
Attribute Type 71, indicates the initial status of, or a status
change in, the circuit to which the session is bound.
The Attribute Value field for this AVP, currently defined in
[RFC3931], has the following format:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |N|A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Bit Bit-Value Name
----------------------------------------------------------------
(A) 15 0x0001 Active
(N) 14 0x0002 New
As currently defined in [RFC3931] and replicated in [RFC4349],
[RFC4454], [RFC4591], and [RFC4719], the two bits have the following
meanings:
o The A (Active) bit indicates whether the circuit is up/active/
ready (1) or down/inactive/not-ready (0).
o The N (New) bit indicates whether the circuit status indication is
for a new circuit (1) or an existing circuit (0).
This document updates the semantics of the A and N bits as follows
(see also Section 4):
The A (Active) bit indicates whether the local pseudowire endpoint
(including the local Attachment Circuit (AC) and local Packet
Switched Network (PSN)-facing pseudowire termination) has no faults
present and is up/active/ready (1) or has faults present and is down/
inactive/not-ready (0).
The N (New) bit indicates if the notification is for a new circuit
(1) or an existing circuit (0), and is provided to emulate Network-
Network Interface (NNI) signaling between Provider Edge (PE) routers,
e.g., Frame Relay NNI. It MAY be used to convey that a circuit has
been re-provisioned or newly provisioned at the PE, which can already
be inferred from the L2TP control message type. It is therefore
uncertain as to what use the receiving PE can make of this bit,
although it MAY include logging. This document deprecates this bit
as it is of little or no use, hence this bit SHOULD be ignored on
receipt and is OPTIONAL to set on sending. For reference, see
Section 3.4 of [RFC4591], which does not specify any additional usage
beyond the setting of the N bit in the ICRQ, ICRP (and OCRQ, OCRP)
and the clearing of it in all other control messages.
This document also extends this bitmap of values to allow for finer
granularity of local pseudowire (i.e., Attachment Circuit or PSN-
facing endpoint) status reporting.
The Attribute Value field for the Circuit Status AVP, including the
new values, has the following format:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |S|E|I|T|R|N|A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Bit Bit-Value Name
-----------------------------------------------------------------
(A) 15 0x0001 Active: Pseudowire has no faults
(N) 14 0x0002 New [use deprecated]
(R) 13 0x0004 Local Attachment Circuit (ingress) Receive Fault
(T) 12 0x0008 Local Attachment Circuit (egress) Transmit Fault
(I) 11 0x0010 Local PSN-facing PW (ingress) Receive Fault
(E) 10 0x0020 Local PSN-facing PW (egress) Transmit Fault
(S) 9 0x0040 Pseudowire is in Standby mode
The new bit values have the following meanings:
(R), Local Attachment Circuit (ingress) Receive Fault
Fault Here
|
|
| +----------------------+ +----------------------+
| Rx| LCCE |Egress | Peer LCCE |
--X-->| |-------->| |
| L2TPv3 | [PSN] | L2TPv3 |
Tx| Circuit Pseudowire |Ingress | Pseudowire Circuit |
<-----| |<--------| |
+----------------------+ +----------------------+
An alarm or fault has occurred at the local Attachment Circuit
such that it is unable to receive traffic. It can still transmit
traffic.
(T), Local Attachment Circuit (egress) Transmit Fault
+----------------------+ +----------------------+
Rx| LCCE |Egress | Peer LCCE |
----->| |-------->| |
| L2TPv3 | [PSN] | L2TPv3 |
Tx| Circuit Pseudowire |Ingress | Pseudowire Circuit |
<--X--| |<--------| |
| +----------------------+ +----------------------+
|
|
Fault Here
A fault has occurred at the local Attachment Circuit such that it
is unable to transmit traffic. It can still receive traffic.
(I), Local PSN-facing PW (ingress) Receive Fault
+----------------------+ +----------------------+
Rx| LCCE |Egress | Peer LCCE |
----->| |-------->| |
| L2TPv3 | [PSN] | L2TPv3 |
Tx| Circuit Pseudowire |Ingress | Pseudowire Circuit |
<-----| |<---X----| |
+----------------------+ | +----------------------+
|
|
Fault Here
A fault has occurred in the receive direction between the local
endpoint and the remote L2TP endpoint.
Note that a fault at the session level would not necessarily
trigger an L2TP control connection timeout. The means of
detecting this fault are outside the scope of this document; as an
example, detection may be via PW Type-specific means,
Bidirectional Forwarding Detection (BFD), or other methods.
(E), Local PSN-facing PW (egress) Transmit Fault
Fault Here
|
|
+----------------------+ | +----------------------+
Rx| LCCE |Egress| | Peer LCCE |
----->| |------X->| |
| L2TPv3 | [PSN] | L2TPv3 |
Tx| Circuit Pseudowire |Ingress | Pseudowire Circuit |
<-----| |<--------| |
+----------------------+ +----------------------+
A fault has occurred in the transmit direction between the local
endpoint and the remote L2TP endpoint.
Note that a fault at the session level would not necessarily
trigger an L2TP control connection timeout. The means of
detecting this fault are outside the scope of this document; as an
example, detection may be via PW Type-specific means, BFD, or
other methods.
(S), Pseudowire is in Standby mode
Standby
|
|
+----------------------+ | +----------------------+
Rx| LCCE |Egress | Peer LCCE |
----->| |---X---->| |
| L2TPv3 | [PSN] | L2TPv3 |
Tx| Circuit Pseudowire |Ingress | Pseudowire Circuit |
<-----| |<--X-----| |
+----------------------+ | +----------------------+
|
|
Standby
The pseudowire has been placed into a Standby mode, which means
that although it was signaled (during setup of the PW) and is
operational, it is NOT switching user traffic. Any received user
traffic SHOULD be dropped. User traffic MUST NOT be transmitted.
A standby pseudowire also allows for means to check its data plane
liveness in order to ensure its ability to switch data packets
end-to-end. This is achieved, for example, as detailed in
[RFC5085] or [VCCV-BFD]. However, data is not forwarded from an
Attachment Circuit (AC) into the L2TPv3 session, or from the
L2TPv3 session out to the AC.
3. Circuit Status Usage and Clarifications
In implementations prior to this specification, bits 0-13 MUST be set
to zero (see Section 5.4.5 of [RFC3931]). This allows for legacy
implementations to interwork properly with new implementations.
The following are clarifications regarding the usage of the Circuit
Status AVP bits as defined in this specification:
o The (R), (T), (I), and (E) bits are collectively referred to as
"fault status bits".
o [RFC3931] defined the (A) bit as pertaining to local access
circuit state only. This document redefines it as meaning that
"no faults are present on the local pseudowire endpoint."
o If multiple faults occur, all the fault status bits corresponding
to each fault MUST be set (i.e., they MUST be bitwise ORed
together).
o The (A) bit MUST NOT be set until all fault status bits are
cleared. This behavior allows an endpoint to be backwards
compatible with a remote endpoint that does not understand these
new status bits.
o If any of the fault status bits are set, then the (A) bit MUST be
cleared. That is, the fault status bits (R, T, I, E) are a more
granular definition of (A), such that ORing the bits provides an
inverted (A).
o If (A) is clear and the fault status bits (R, T, I, E) are clear,
it means that there is no extended circuit status. That is, the
circuit is down/inactive/not-ready (from the (A) bit), without a
more granular (extended) indication.
o The (S) bit can be set in conjunction with any other bit,
including (A). A pseudowire endpoint in Standby (S bit set) can
be up/active/ready (A bit set) or experiencing a fault (A bit
cleared and one or more of the fault status bits (R, T, I, E) set.
o Leaving Standby mode is indicated by the clearing of the (S) bit.
o The usage of the (N) bit has been deprecated.
4. Updates to Existing RFCs
This document updates existing RFCs that define (either generically
or in the context of a specific set of PW Types) the Active and New
bits of the Circuit Status AVP. The Active and New bits of the
Circuit Status AVP are specified in Section 5.4.5 of [RFC3931].
Those definitions are adapted to specific Attachment Circuits and
replicated in Section 3.4 of [RFC4349] (High-Level Data Link Control
Frames over L2TPv3), Section 8 of [RFC4454] (Asynchronous Transfer
Mode over L2TPv3), Section 3.4 of [RFC4591] (Frame Relay over
L2TPv3), and Section 2.3.3 of [RFC4719] (Ethernet Frames over
L2TPv3). This document updates the definitions in all five of these
references to say:
The A (Active) bit indicates whether the local pseudowire endpoint
(including the local Attachment Circuit and local PSN-facing
pseudowire termination) has no faults present and is up/active/
ready (1) or has faults present and is down/inactive/not-ready
(0).
The N (New) bit usage is deprecated; it SHOULD be ignored on
receipt and is OPTIONAL to set on sending.
This document also updates Section 2.2 (bullet c) of [RFC4719],
removing the following two sentences:
For ICRQ and ICRP, the Circuit Status AVP MUST indicate that the
circuit status is for a new circuit (refer to N bit in Section
2.3.3).
For ICCN and SLI (refer to Section 2.3.2), the Circuit Status AVP
MUST indicate that the circuit status is for an existing circuit
(refer to N bit in Section 2.3.3) and reflect the current status
of the link (refer to A bit in Section 2.3.3).
And finally, this document updates Section 3.1 of [RFC4349], Section
3.1 of [RFC4454], Section 3.1 of [RFC4591], and Section 2.2 of
[RFC4719] with the following paragraph addition:
The usage of the N bit in the Circuit Status AVP is deprecated.
Therefore, for ICRQ and ICRP, the Circuit Status AVP need not
indicate on sending (nor check on receipt) that the circuit status
is for a new circuit, and for ICCN and SLI, the Circuit Status AVP
need not indicate on sending (nor check on receipt) that the
circuit status is for an existing circuit.
5. Security Considerations
Security considerations for the Circuit Status AVP are covered in the
base L2TPv3 specification (see Section 8 of [RFC3931]). No
additional security considerations exist with extending this
attribute.
6. IANA Considerations
The Circuit Status Bits number space [IANA-l2tp] is managed by IANA
as per Section 10.7 of [RFC3931]. Five new bits (bits 9 through 13)
and one updated bit (bit 14) have been assigned as follows:
Circuit Status Bits - per [RFC3931]
-------------------
Bit 9 - S (Standby) bit
Bit 10 - E (Local PSN-facing PW (egress) Tx Fault) bit
Bit 11 - I (Local PSN-facing PW (ingress) Rx Fault) bit
Bit 12 - T (Local AC (egress) Tx Fault) bit
Bit 13 - R (Local AC (ingress) Rx Fault) bit
Bit 14 - N (New) bit [use deprecated]
7. Acknowledgements
The authors wish to thank Muhammad Yousuf, Mark Townsley, George
Wilkie, Prashant Jhingran, Pawel Sowinski, and Ignacio Goyret for
useful comments received.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two
Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931,
March 2005.
[RFC4349] Pignataro, C. and M. Townsley, "High-Level Data Link
Control (HDLC) Frames over Layer 2 Tunneling Protocol,
Version 3 (L2TPv3)", RFC 4349, February 2006.
[RFC4454] Singh, S., Townsley, M., and C. Pignataro, "Asynchronous
Transfer Mode (ATM) over Layer 2 Tunneling Protocol
Version 3 (L2TPv3)", RFC 4454, May 2006.
[RFC4591] Townsley, M., Wilkie, G., Booth, S., Bryant, S., and J.
Lau, "Frame Relay over Layer 2 Tunneling Protocol
Version 3 (L2TPv3)", RFC 4591, August 2006.
[RFC4719] Aggarwal, R., Townsley, M., and M. Dos Santos,
"Transport of Ethernet Frames over Layer 2 Tunneling
Protocol Version 3 (L2TPv3)", RFC 4719, November 2006.
8.2. Informative References
[IANA-l2tp] Internet Assigned Numbers Authority, "Layer Two
Tunneling Protocol 'L2TP'", <http://www.iana.org>.
[PREF-FWD] Muley, P., Bocci, M., and L. Martini, "Preferential
Forwarding Status bit definition", Work in Progress,
September 2008.
[RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to
Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
[RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
Heron, "Pseudowire Setup and Maintenance Using the Label
Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC5085] Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
Connectivity Verification (VCCV): A Control Channel for
Pseudowires", RFC 5085, December 2007.
[VCCV-BFD] Nadeau, T. and C. Pignataro, "Bidirectional Forwarding
Detection (BFD) for the Pseudowire Virtual Circuit
Connectivity Verification (VCCV)", Work in Progress,
July 2009.
Authors' Addresses
Neil McGill
Cisco Systems
7025-4 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
USA
EMail: nmcgill@cisco.com
Carlos Pignataro
Cisco Systems
7200-12 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
USA
EMail: cpignata@cisco.com
AI ELI5
On this page
PROBLEMThe L2TPv3 Circuit Status AVP could only say that a circuit was up or down. That does not show which direction failed, and it cannot show that a pseudowire is held in reserve.
MECHANISMRFC 5641 adds five bit values to the Circuit Status AVP: four fault bits (R, T, I, E) and a Standby bit (S). It also redefines the Active bit as 'no faults on the local pseudowire endpoint' and deprecates the New bit.
WHY IT MATTERSOld and new implementations can still interwork, because the Active bit stays set only when no fault bit is set. The RFC updates RFC 3931, RFC 4349, RFC 4454, RFC 4591 and RFC 4719.
§1Why a Finer Status Is Needed
L2TPv3 carries layer 2 traffic, such as Frame Relay or Ethernet, across a packet network. Each carried circuit is bound to a session, and the Circuit Status AVP from RFC 3931 tells the peer whether that circuit is up or down.
Up or down is often too coarse. A finer granularity helps find the direction of a fault, as was already added for MPLS-based pseudowires that use the Label Distribution Protocol (see RFC 4446 and RFC 4447). It is also useful, in session-level redundancy setups, to mark a pseudowire as standby: fully signaled and open to Operations, Administration, and Maintenance, but not switching data.
The new bits are added in a backwards compatible way and apply to every pseudowire type that uses the Circuit Status AVP.
A plain on/off light cannot tell you what is wrong. This RFC adds more lights so the two ends of a link can say what is broken and which way.
§2The Circuit Status AVP Value
The Circuit Status AVP is Attribute Type 71. It can appear in ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN and SLI messages, and it gives the initial status of the circuit bound to a session, or a status change.
Its value is a 16-bit field. Before this RFC only two bits were defined: A (Active, bit 15, 0x0001) and N (New, bit 14, 0x0002). RFC 5641 keeps them and adds five more, giving the layout Reserved | S | E | I | T | R | N | A.
Bit numbering counts from the left of the 16-bit field, so the Active bit is bit 15 and the Standby bit is bit 9.
The status is a row of 16 little switches. Two used to mean something. Now seven do, and the rest are saved for later.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
RESERVED | Reserved | 9 bits | Bits 0 to 8; unused and set to zero. | |
S | S | 1 bits | Bit 9: pseudowire is in Standby mode. | |
E | E | 1 bits | Bit 10: local PSN-facing PW egress transmit fault. | |
I | I | 1 bits | Bit 11: local PSN-facing PW ingress receive fault. | |
T | T | 1 bits | Bit 12: local AC egress transmit fault. | |
R | R | 1 bits | Bit 13: local AC ingress receive fault. | |
N | N | 1 bits | Bit 14: New; use deprecated. | |
A | A | 1 bits | Bit 15: Active; no faults on the local pseudowire endpoint. |
§3The New Fault and Standby Bits
The four fault status bits describe the local pseudowire endpoint, which covers the local Attachment Circuit (AC) and the endpoint that faces the packet switched network (PSN).
R: the AC cannot receive traffic, but can still transmit.T: the AC cannot transmit traffic, but can still receive.I: a fault in the receive direction between the local endpoint and the remote L2TP endpoint.E: a fault in the transmit direction between the local endpoint and the remote L2TP endpoint.
For I and E, the RFC notes that a session-level fault would not necessarily cause an L2TP control connection timeout. How the fault is detected is out of scope; examples given are PW Type-specific means and Bidirectional Forwarding Detection (BFD).
The S bit means the pseudowire is in Standby mode. It was signaled during setup and is operational, but it is NOT switching user traffic. Received user traffic SHOULD be dropped, and user traffic MUST NOT be transmitted. A standby pseudowire can still have its data plane liveness checked, for example as in RFC 5085.
Four of the switches say 'I can't hear' or 'I can't talk' for two different parts of the path. The fifth says 'I'm on the bench: ready to play, but not playing right now.'
- 0x0001A (Active) — Pseudowire has no faults.
- 0x0002N (New) — Use deprecated.
- 0x0004R — Local Attachment Circuit (ingress) Receive Fault.
- 0x0008T — Local Attachment Circuit (egress) Transmit Fault.
- 0x0010I — Local PSN-facing PW (ingress) Receive Fault.
- 0x0020E — Local PSN-facing PW (egress) Transmit Fault.
- 0x0040S — Pseudowire is in Standby mode.
§4Rules for Setting the Bits
Older implementations set bits 0 to 13 to zero, so they can interwork with new ones. The RFC then gives these rules:
- If several faults occur, every matching fault bit MUST be set (bitwise OR).
- The
Abit MUST NOT be set until all fault bits are cleared, and if any fault bit is set,AMUST be cleared. This lets an endpoint work with a remote endpoint that does not understand the new bits. - If
Aand all fault bits are clear, there is no extended status. The circuit is simply down. Scan be set together with any other bit, includingA. Leaving standby is shown by clearingS.
The A bit is redefined. RFC 3931 tied it to the local access circuit only. Now it means that no faults are present on the local pseudowire endpoint.
The N bit, which says whether a notification is for a new or an existing circuit, is deprecated. It SHOULD be ignored on receipt and is OPTIONAL to set on sending, because the control message type already tells the receiver most of what it conveys.
If anything is wrong, the 'all good' switch must be off. If nothing is wrong, it must be on. That way, an older device that only knows 'good or bad' still understands.
§5Changes to Existing RFCs
The Active and New bits were first defined in Section 5.4.5 of RFC 3931 and were repeated for specific circuit types. RFC 5641 updates the definitions in five documents: RFC 3931, RFC 4349 (HDLC), RFC 4454 (ATM), RFC 4591 (Frame Relay) and RFC 4719 (Ethernet).
It also removes two sentences from Section 2.2 of RFC 4719 that required the New bit to be set for ICRQ and ICRP and cleared for ICCN and SLI. In four of the documents it adds a paragraph saying that, since the N bit is deprecated, the sender need not indicate it and the receiver need not check it.
When you change a rule in one rulebook, you have to fix the copies in the other rulebooks too. This RFC lists which pages to fix.
§6Security Considerations
The RFC adds nothing new here. Security considerations for the Circuit Status AVP are covered in Section 8 of RFC 3931, and the RFC states that no additional security considerations exist from extending the attribute.
Adding more switches does not add new ways to break in, so the old safety rules still apply.
§7IANA Registrations
The Circuit Status Bits number space is managed by IANA under Section 10.7 of RFC 3931. Five new bits, 9 through 13, and one updated bit, 14, were assigned. Bit 9 is S, 10 is E, 11 is I, 12 is T, 13 is R, and bit 14 is N, marked as use deprecated.
A public list keeps track of what each switch means, so everyone agrees. These new switches were added to it.
§8References
[RFC3931]RFC 3931Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.March 2005[RFC4349]RFC 4349Pignataro, C. and M. Townsley, "High-Level Data Link Control (HDLC) Frames over Layer 2 Tunneling Protocol, Version 3 (L2TPv3)", RFC 4349, February 2006.February 2006[RFC4454]RFC 4454Singh, S., Townsley, M., and C. Pignataro, "Asynchronous Transfer Mode (ATM) over Layer 2 Tunneling Protocol Version 3 (L2TPv3)", RFC 4454, May 2006.May 2006[RFC4591]RFC 4591Townsley, M., Wilkie, G., Booth, S., Bryant, S., and J. Lau, "Frame Relay over Layer 2 Tunneling Protocol Version 3 (L2TPv3)", RFC 4591, August 2006.August 2006[RFC4719]RFC 4719Aggarwal, R., Townsley, M., and M. Dos Santos, "Transport of Ethernet Frames over Layer 2 Tunneling Protocol Version 3 (L2TPv3)", RFC 4719, November 2006.November 2006[RFC4447]RFC 4447Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. Heron, "Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)", RFC 4447, April 2006.April 2006[RFC5085]RFC 5085Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit Connectivity Verification (VCCV): A Control Channel for Pseudowires", RFC 5085, December 2007.December 2007