|Title||Pseudowire Status for Static Pseudowires
|Author||L. Martini, G. Swallow, G.
Heron, M. Bocci
Internet Engineering Task Force (IETF) L. Martini
Request for Comments: 6478 G. Swallow
Updates: 5885 G. Heron
Category: Standards Track Cisco
ISSN: 2070-1721 M. Bocci
Pseudowire Status for Static Pseudowires
This document specifies a mechanism to signal Pseudowire (PW) status
messages using a PW associated channel (ACh). Such a mechanism is
suitable for use where no PW dynamic control plane exits, known as
static PWs, or where a Terminating Provider Edge (T-PE) needs to send
a PW status message directly to a far-end T-PE. The mechanism allows
PW Operations, Administration, and Maintenance (OAM) message mapping
and PW redundancy to operate on static PWs. This document also
updates RFC 5885 in the case when Bi-directional Forwarding Detection
(BFD) is used to convey PW status-signaling information.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction ....................................................3
2. Specification of Requirements ...................................3
3. Terminology .....................................................3
4. Applicability ...................................................3
5. Pseudowire Status Operation .....................................4
5.1. PW OAM Message .............................................4
5.2. Sending a PW Status Message ................................5
5.3. PW OAM Status Message Transmit and Receive .................6
5.3.1. Acknowledgment of PW Status .........................7
5.4. MPLS Label Stack ...........................................7
5.4.1. Label Stack for a Message Destined to the Next PE ...8
5.4.2. Label Stack for a Message Destined to the Egress PE .8
5.5. S-PE Bypass Mode ...........................................8
5.5.1. S-PE Bypass Mode LDP Flag Bit .......................9
6. S-PE Operation .................................................10
6.1. Static PW to Another Static PW ............................10
6.2. Dynamic PW to Static PW or Vice Versa .....................10
7. Security Considerations ........................................11
8. IANA Considerations ............................................11
9. References .....................................................11
9.1. Normative References ......................................11
9.2. Informative References ....................................12
The default control plane for Pseudowire (PW) technology, as defined
in [RFC4447], is based on the Label Distribution Protocol (LDP).
However, that document also describes a static provisioning mode
without a control plane. When a static PW is used, there is no
method to transmit the status of the PW or attachment circuit (AC)
between the two Provider Edge (PE) devices at each end of the PW.
This document defines a method to transport the PW status codes
defined in Section 5.4.2 of [RFC4447] and elsewhere [REDUNDANCY] in-
band with the PW data using a generic associated channel [RFC5586].
2. 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].
ACh: Associated Channel
ACH: Associated Channel Header
FEC: Forwarding Equivalence Class
LDP: Label Distribution Protocol
LSP: Label Switching Path
MS-PW: Multi-Segment Pseudowire
PE: Provider Edge
SS-PW: Single-Segment Pseudowire
S-PE: Switching Provider Edge Node of MS-PW
T-PE: Terminating Provider Edge Node of MS-PW
As described in [RFC4447] and [RFC6310], a PE that establishes an
MPLS PW using means other than LDP, e.g., by static configuration,
MUST support some alternative method of status reporting. The
procedures described in this document are for use when PWs are
statically configured and an LDP control plane is not available.
As defined in [RFC4447], a PE that establishes a PW using LDP MUST
use the PW status TLV mechanism for AC and PW status and defect
notification on that PW. In order to avoid duplicate notifications
and potentially conflicting notifications, such PEs MUST NOT use the
mechanisms described in this document for those PWs, except that the
S-PE bypass mode described in Section 5.5 MAY be used when both T-PEs
at each end of the PW use LDP to establish the PW.
In order to protect against duplicate notifications and potentially
conflicting notifications, when the Pseudowire Status protocol for
Static Pseudowires described in this document is used, the BFD VCCV
(Virtual Circuit Connectivity Verification) status-signaling
mechanisms described in [RFC5885] (CV Types 0x08 and 0x20) MUST NOT
be used. BFD VCCV for fault detection (CV types 0x04 and 0x10) MAY
still be used.
5. Pseudowire Status Operation
5.1. PW OAM Message
The PW status TLV as defined in Section 5.4.2 of [RFC4447] is
transported in a PW OAM message using the PW ACH.
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
|0 0 0 1|Version| Reserved | 0x0027 PW OAM Message |
| Refresh Timer | TLV Length |A| Flags |
~ TLVs ~
Figure 1: ACh PW OAM Message Packet Header
The first 32 bits are the standard ACH header construct as defined in
The first nibble (0001b) indicates the ACH instead of PW data. The
version and the reserved values are both set to 0 as specified in
The refresh timer is an unsigned integer and specifies refresh time
in seconds with a range from 1 to 65535. The value 0 means that the
refresh timer is set to infinity, and the PW OAM message will never
be refreshed, and will never timeout.
The TLV length field indicates the length of all TLVs only. This
document defines only the transport of the PW status TLV, as defined
in Section 5.4.2, [RFC4447], in the TLV field. In the future,
additional TLVs may be defined to be used in this field with code
points allocated from the IANA registry called "LDP TLV Type Name
The A flag bit is used to indicate an acknowledgment of the PW status
TLV included. The rest of the flag bits are reserved and they MUST
be set to 0 on transmit, and ignored upon receipt. When the A bit is
set, the refresh timer value is a requested timer value.
The PW OAM Message code point value is 0x0027.
5.2. Sending a PW Status Message
The PW Status messages are sent in-band using the PW OAM message
containing the PW Status TLV for a particular PW, as defined in
[RFC4447]. The PW Status TLV format is almost as defined in
[RFC4447] and is repeated here for the reader's convenience:
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
|Res| PW Status (0x096A) | Length |
| Status Code |
Figure 2: PW Status TLV Format
Unlike the case in [RFC4447], here, the first 2 bits are reserved,
and MUST be set to zero on transmit and ignored on receipt.
The PW Status TLV is prepended with a PW OAM message header and sent
on the ACh of the PW to which the status update applies.
To clear a particular status indication, the PE needs to send a new
PW OAM message containing a PW Status TLV with the corresponding bit
cleared as defined in [RFC4447].
The procedures described in [RFC6073] that apply to an S-PE and PW
using an LDP control plane also apply when sending PW status using
the PW OAM channel. The OPTIONAL procedures using the SP-PE TLV
described in [RFC6073] can also be applied when sending PW status
using the PW OAM channel.
The detailed message transmit and message receive procedures are
specified in the next section. PW OAM status messages MUST NOT be
used as a connectivity verification method.
5.3. PW OAM Status Message Transmit and Receive
Unlike the PW status procedures defined in [RFC4447], with this
method there is no TCP/IP session or session management. Therefore,
unlike the TCP/IP case, where each message is sent only once, the PW
OAM message containing the PW status TLV needs to be transmitted
repeatedly to ensure reliable message delivery. If a malformed TLV
or an unknown TLV is received in a PW OAM status message, the TLV
MUST be ignored, and the PE SHOULD report the event to the operator.
A PW OAM message containing a PW status TLV with a new status bit set
or reset will be transmitted immediately by the PE. Unless the
message is acknowledged within a second, the PW OAM message will then
be repeated twice more at an initial interval of one second.
Subsequently, the PW OAM message will be transmitted with an interval
specified by the refresh timer value in the packet. Note that this
value MAY be updated in the new PW OAM message packet, in which case
the new refresh timer value becomes the new packet transmit interval.
The suggested default value for the refresh timer is 600 seconds.
This default is adequate for typical deployments, and PEs are
designed to take into account processing these messages at the
When a PW OAM message containing a status TLV is received, a timer is
started according to the refresh rate specified in the packet. If
another non-zero PW status message is not received within 3.5 times
the specified timer value, the status condition will timeout in 3.5
times the last refresh timer value received, and the default status
of zero is assumed on the PW. It is also a good practice to
introduce some jitter in the delay between refresh transmissions, as
long as the maximum jitter delay is within the prescribed maximum
refresh time of 3.5 times the specified timer value for 3 consecutive
To clear a particular status fault, the PE need only send an updated
message with the corresponding bit cleared. If the PW status code is
zero, the PW OAM message will be sent like any other PW OAM status
message using the procedures described above; however, transmission
will cease after 3 PW status messages have been sent at one second
intervals and before the refresh timer expires. A PW status message
of zero MAY be acknowledged using the procedures described in Section
5.3.1. If it is acknowledged, then a timer value of zero MUST be
used. This SHOULD cause the PE sending the PW status notification
message with a PW status code equal to zero to stop sending and to
continue normal operation.
5.3.1. Acknowledgment of PW Status
A PE receiving a PW OAM message containing a PW status message MAY
acknowledge the PW status message by simply building a reply packet
with the same format and status code as the received PW OAM message,
but with the A bit set, and transmitting it on the PW ACh back to the
source of the PW OAM message. The receiving PE MAY use the refresh
timer field in the acknowledgement packet to request a new refresh
interval from the originator of the PW OAM message. The timer value
set in the reply packet SHOULD then be used by the originator of the
PW OAM message as the new transmit interval. If the requested
refresh timer value is to be used, then, when the the current timer
expires, the PW OAM message transmission interval is set to the new
value and the new value is sent in the PW OAM message. If the
transmitting PE does not want to use the new timer value (for local
policy reasons, or because it simply cannot support it), it MUST
refresh the PW OAM message with the timer value it desires. The
receiving PE will then set its timeout timer according to the new
refresh timer value that is in the packet received, regardless of
what timer value it requested. The receiving PE MUST NOT request a
new refresh timer value more than once per refresh interval.
The suggested default value for the refresh timer value in the
acknowledgment packet is 600 seconds.
If the sender PE receives an acknowledgment message that does not
match the current active PW status message being sent, it simply
ignores the acknowledgment packet.
If a PE that has received a non-zero status code for a PW detects by
any means that the far end PE has become unreachable, it will follow
the standard defect entry procedures of [RFC6310], Section 6.2.
5.4. MPLS Label Stack
With one exception, all PW OAM status messages are sent to the
adjacent PE across the PSN tunnel. In many cases, the transmitting
PE has no way to determine whether the adjacent PE is an S-PE or a
T-PE. This is a necessary behavior to preserve backward
compatibility with PEs that do not understand MS-PWs. In the
procedures described in this document, there are two possible
destinations for the PW OAM status messages: the adjacent PE or the
T-PE. Sending a PW status message directly to the T-PE is an
enhanced method that is only applicable using PW OAM status messages
sent in the PW ACH.
5.4.1. Label Stack for a Message Destined to the Next PE
A PE that needs to forward a PW OAM status message to the adjacent PE
across the PSN tunnel MUST set the PW label TTL field to 1.
Furthermore, if the control word is not in use on the particular PW,
the PE MUST place the GAL reserved label [RFC5586] below the PW label
with the TTL field set to 1.
5.4.2. Label Stack for a Message Destined to the Egress PE
This is also known as "S-PE bypass mode"; see below. A T-PE that
requires sending a PW OAM status message directly to the
corresponding T-PE at the other end of the PW MUST set the TTL of the
PW label to a value that is sufficient to reach the corresponding
T-PE. This value will be greater than one, but will be set according
to the local policy on the transmitting T-PE. Furthermore, if the
control word is not in use on the particular PW, the PE MUST also
place the GAL reserved label [RFC5586] below the PW label with the
TTL field set to 1.
5.5. S-PE Bypass Mode
S-PE bypass mode enables a T-PE that uses LDP as the PW setup and
control protocol to bypass all S-PEs that might be present along the
MS-PW and to send a message directly to the remote T-PE. This is
used for very fast message transmission in-band with the PW PDUs.
This mode is OPTIONAL and MUST be supported by both T-PEs to be
enabled. This mode MUST NOT be used if the first PW segment
connected to each T-PE is not using LDP.
Note that this method MUST NOT be used to send messages that are
permitted to originate at an S-PE. Otherwise, race conditions could
occur between messages sent via the control plane by S-PEs and
messages sent via the data plane by T-PEs.
Status codes, except for those listed below, MUST NOT be sent using
the S-PE bypass procedure and MUST be ignored on reception.
0x00000002 - Local Attachment Circuit (ingress) Receive Fault
0x00000004 - Local Attachment Circuit (egress) Transmit Fault
0x00000020 - PW forwarding standby
0x00000040 - Request switchover to this PW
Note that since "clear all failures" may be sent by an S-PE, it MUST
NOT be sent using the S-PE bypass mode.
When S-PE bypass mode is enabled, all PW Status TLVs received using
this method have priority over PW Status TLVs sent via control
protocols such as LDP [RFC4447]. However, the same PW Status TLVs
MUST also be sent in LDP to keep the S-PEs state updated.
5.5.1. S-PE Bypass Mode LDP Flag Bit
When a PW Segment along an MS-PW is using the LDP control protocol
and wishes to request the use of the S-PE bypass status message mode,
it sets the B bit in the generic protocol flags interface parameters
sub-TLV as shown in Figure 3. This flag can only be set by a T-PE
using LDP as the PW configuration and management protocol. If the
S-PE bypass mode LDP flag bit in the generic protocol flags interface
parameter does not match in the FEC advertisement for directions of a
specific PW, that PW MUST NOT be enabled.
The interface parameter is defined as follows:
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
| Type=0x18 | Length=4 |R R R R R R R R R R R R R R R B|
Figure 3: PW Generic Protocol Flags Sub-TLV
Type 0x18 - PW Generic Protocol Flags.
TLV length is always 4 octets.
Bit B, in position 31 above, is set to request the S-PE bypass
mode. R bits are to be allocated by IANA as described in the IANA
section. If they are not allocated, they are to be considered as
reserved for future use and MUST be zero on transmission and
ignored on reception of this TLV.
If the T-PE receives an LDP label mapping message containing a
generic protocol flags interface parameter TLV with the bit B set,
then the T-PE receiving the label mapping message MAY send S-PE
bypass status messages in the PW ACh. If bit B of said TLV is not
set, or the TLV is not present, then the T-PE receiving the label
mapping message MUST NOT send S-PE bypass status messages in the
6. S-PE Operation
The S-PE will operate according to the procedures defined in
[RFC6073]. The following additional procedures apply to the case
where a static PW segment is switched to a dynamic PW segment that
uses LDP, and the case where a static PW segment is switched to
another static PW segment.
6.1. Static PW to Another Static PW
The procedures that are described in [RFC6073] Section 10 also apply
to the case of a static PW switched to another static PW. The LDP
header is simply replaced by the PW OAM header; otherwise, the packet
format will be identical. The information that is necessary to form
an SP-PE TLV MUST be configured in the S-PE, or no SP-PE TLV will be
sent. [RFC6073] defines the IANA "Pseudowire Switching Point PE TLV
Type" registry. In order to support the static PW configuration and
addressing scheme, the following new code point has been assigned:
Type Length Description
---- ------ -----------
0x07 24 Static PW/MPLS-TP PW segment ID of last
PW segment traversed
The format of this TLV is that of the "Static Pseudowire Sub-TLV"
defined in [RFC6426].
6.2. Dynamic PW to Static PW or Vice Versa
The procedures that are described in Section 10 of [RFC6073] also
apply to this situation. However, if the PW label of the LDP-
controlled PW segment is withdrawn by the adjacent PE, the S-PE will
set the PW status code "0x00000001 - Pseudowire Not Forwarding" to
the adjacent PW on the static PW segment.
The S-PE will only withdraw its label for the dynamic, LDP-controlled
PW segment if the S-PE is not provisioned.
7. Security Considerations
The security measures described in [RFC4447], [RFC5085], and
[RFC6073] are adequate for the proposed mechanism.
8. IANA Considerations
IANA has set up the registry of "PW Generic Protocol Flags". These
are bit strings of length 16. Bit 0 is defined in this document.
Bits 1 through 15 are to be assigned by IANA using the "IETF Review"
policy defined in [RFC5226].
Any requests for allocation from this registry require a description
of up to 65 characters.
Initial PW Generic Protocol Flags value allocations are as follows:
Bit Mask Description
0x0001 - S-PE bypass mode [RFC6478]
This document uses a new Associated Channel Type. IANA already
maintains the "Pseudowire Associated Channel Types" registry. The
value 0x0027 has been assigned with the description "PW OAM Message".
This document uses a new Pseudowire Switching Point PE TLV Type.
IANA already maintains the "Pseudowire Switching Point PE sub-TLV
Type" registry. A value of 0x07 has been assigned with the
description "Static PW/MPLS-TP PW segment ID of last PW segment
This document uses a new interface parameter type. IANA already
maintains the "Pseudowire Interface Parameters Sub-TLV type
Registry". A value of 0x18 has been assigned with the description
"PW Generic Protocol Flags".
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word
for Use over an MPLS PSN", RFC 4385, February 2006.
[RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T.,
and G. Heron, "Pseudowire Setup and Maintenance Using
the Label Distribution Protocol (LDP)", RFC 4447, April
[RFC5085] Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire
Virtual Circuit Connectivity Verification (VCCV): A
Control Channel for Pseudowires", RFC 5085, December
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
[RFC6073] Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
Aissaoui, "Segmented Pseudowire", RFC 6073, January
[RFC6310] Aissaoui, M., Busschbach, P., Martini, L., Morrow, M.,
Nadeau, T., and Y(J). Stein, "Pseudowire (PW)
Operations, Administration, and Maintenance (OAM)
Message Mapping", RFC 6310, July 2011.
[RFC6426] Gray, E., Bahadur, N., Boutros, S., and R. Aggarwal,
"MPLS On-Demand Connectivity Verification and Route
Tracing", RFC 6426, November 2011.
9.2. Informative References
[REDUNDANCY] Muley, P., Ed., and M. Aissaoui, Ed., "Pseudowire
Preferential Forwarding Status Bit", Work in Progress,
[RFC5885] Nadeau, T., Ed., and C. Pignataro, Ed., "Bidirectional
Forwarding Detection (BFD) for the Pseudowire Virtual
Circuit Connectivity Verification (VCCV)", RFC 5885,
[RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
"MPLS Generic Associated Channel", RFC 5586, June 2009.
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