Network Working Group Praveen Muley
Internet Draft Mustapha Aissaoui
Intended Status: Standards Track Matthew Bocci
Expires: March 2009 Pranjal Kumar Dutta
Marc Lasserre
Alcatel-Lucent
Jonathan Newton
Cable & Wireless
Olen Stokes
Extreme Networks
Hamid Ould-Brahim
Nortel
Luca Martini
Cisco Systems Inc.
Giles Heron
Thomas Nadeau
BT
September 30, 2008
Preferential Forwarding Status bit definition
draft-ietf-pwe3-redundancy-bit-01.txt
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Abstract
This document describes a mechanism for standby status signaling of
redundant PWs between their termination points. A set of redundant
PWs is configured between PE nodes in SS-PW applications, or between
T-PE nodes in MS-PW applications.
In order for the PE/T-PE nodes to indicate the preferred PW path to
forward to one another, a new status bit is needed to indicate a
preferential forwarding status of active or standby for each PW in
the redundancy set.
In addition, a second status bit is defined to allow peer PE/T-PE
nodes to coordinate a switchover operation of the PW/MS-PW path.
Conventions used in this document
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 RFC-2119 [1].
Table of Contents
1. Introduction...................................................3
2. Motivation and Scope...........................................4
3. Terminology....................................................7
4. Modes of Operation.............................................7
4.1. Independent Mode:.........................................8
4.2. Master/Slave Mode:........................................9
5. Signaling procedures of PW State Transition...................10
5.1. PW Standby notification procedures in Independent mode...10
5.2. PW Standby notification procedures in Master/Slave mode..11
5.2.1. PW State Machine....................................12
5.3. Coordination of PW Path Switchover.......................14
5.3.1. Procedures at the requesting endpoint...............15
5.3.2. Procedures at the receiving endpoint................16
6. Applicability and Backward Compatibility......................17
7. Security Considerations.......................................17
8. IANA Considerations...........................................17
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8.1. Status Code for PW Preferential Forwarding Status........17
8.2. Status Code for PW Request Switchover Status.............18
9. Acknowledgments...............................................18
10. References...................................................18
10.1. Normative References....................................18
10.2. Informative References..................................18
11. Appendix A - Applications of PW Redundancy Procedures........19
11.1. One Multi-homed CE with single SS-PW redundancy.........19
11.2. Multiple Multi-homed CEs with single SS-PW redundancy...21
11.3. Multi-homed CE with MS-PW redundancy....................22
11.4. Single Homed CE with MS-PW redundancy...................24
11.5. PW redundancy between H-VPLS MTU-s and PE-rs............25
Author's Addresses...............................................27
Full Copyright Statement.........................................28
Intellectual Property Statement..................................28
Acknowledgment...................................................29
1. Introduction
In single-segment PW (SS-PW) services such as VPWS and VPLS,
protection for the PW is provided by the PSN layer. This may be an
RSVP-TE LSP with a FRR backup and/or an end-to-end backup LSP. There
are however applications where PSN protection is insufficient to
fully protect the PWE3 service and pseudowire redundancy is required.
These scenarios are described in the PW redundancy and framework
document [5].
In a VPWS service, this is used to provide access AC redundancy to a
CE device which is dual-homed to target PE nodes. In a HVPLS service,
this is used to provide access PW redundancy to the MTU device which
is dual-homed to two PE-r devices. PSN protection mechanisms cannot
protect against failure of the target PE node or the failure of the
remote AC. These scenarios rely on a set of two or more pseudowires
to protect a given PWE3 service. Only one of these pseudowires is
used by the PEs to forward user traffic on at any given time. This is
the active PW. The other PWs in the set are considered standby and
are not used for forwarding unless they become active. In essence,
this provides a 1:1 or N:1 PW protection with the possibility of
multi-homing.
In order to support access AC or access PW redundancy, at least one
of the PEs on which a PW terminates must be different from that on
which the primary PW terminates, as described in [5]. Figure 1
illustrates an application of Active and Standby PWs.
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|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnels-->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | | PE1|==================| | | +-----+
| |----------|....|...PW1.(active)...|....|----------| |
| | | |==================| | | CE2 |
| CE1 | +----+ |PE2 | | |
| | +----+ | | +-----+
| | | |==================| |
| |----------|....|...PW2.(standby)..| |
+-----+ | | PE3|==================| |
AC +----+ +----+
Figure 1: Reference Model for PW Redundancy
In multi-segment PW (MS-PW) applications, multiple MS-PWs are
configured between a pair of T-PE nodes. The paths of these MS-PWs
are diverse and are switched at different S-PE nodes. Only one of
these MS-PWs is active at any given time. The others are put in
standby. In these applications, 1:1 or N:1 PW redundancy is important
to provide resilience in the event of failure of S-PE node since PSN
protection mechanisms cannot, and the desire is that MS-PW based
services have resiliency similar to that of SS-PW based services.
This document specifies a new PW status bit to indicate the
preferential forwarding status of the PW for the purpose of notifying
the remote PE of the active/standby state of each PW in the
redundancy set. This status bit is different from the operational
status bits already defined in the PWE3 control protocol [2]. In
addition, a second status bit is defined to allow peer PE/T-PE nodes
to coordinate a switchover operation of the PW/MS-PW path.
2. Motivation and Scope
The PWE3 control protocol [2] defines the following status codes in
PW the status TLV to indicate the operational state for an AC and a
PW:
0x00000000 - Pseudowire forwarding (clear all failures)
0x00000001 - Pseudowire Not Forwarding
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0x00000002 - Local Attachment Circuit (ingress) Receive Fault
0x00000004 - Local Attachment Circuit (egress) Transmit Fault
0x00000008 - Local PSN-facing PW (ingress) Receive Fault
0x00000010 - Local PSN-facing PW (egress) Transmit Fault
The scenarios defined in [5] allow the provisioning of a primary PW
and one or many secondary PWs in the same VPWS or VPLS service.
A PE node makes a selection of which PW to activate at any given time
for the purpose of forwarding user packets. This selection takes into
account the local operational state of the PW as well as the remote
operational state of the PW as indicated in the status bits of the PW
it received from the peer PE node.
In the absence of faults, all PWs are operationally UP both locally
and remotely and a PE node needs to select a single PW to forward
user packets to. This is referred to as the active PW. All other PWs
will be in standby and must not be used to forward user packets.
In order for both ends of the service to select the same PW for
forwarding user packets, it is proposed that a PE node communicates a
new status bit to indicate the forwarding state of a PW to its peer
PE node.
In addition, a second status bit is defined to allow peer PE/T-PE
nodes to coordinate a switchover operation of the PW/MS-PW path if
required by the application.
Together, the mechanisms described in this document achieve the
following PW protection capabilities:
a. A mandatory 1:1 PW protection with a single active PW and one
standby PW. An active PW can forward data traffic and control
plane traffic, such as OAM packets. A standby PW does not
carry data traffic.
b. An optional N:1 PW protection scheme with a single active PW
and N standby PWs.
c. An independent mode of operation in which each PW endpoint
node uses its own local rule to select which PW it intends to
activate at any given time and advertises it to the remote
endpoints. Only a PW which is operationally UP and which
indicated Active status bit locally and remotely is in the
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Active state and can be used to forward data packets. This is
described in Section 4.1. .
d. An optional mechanism to allow PW endpoints to coordinate the
switchover to a given PW path by using an explicit
request/acknowledgment switchover procedure. This mechanism is
complementary to the Independent mode of operation and is
described in Section 5.3. . This mechanism can be invoked
manually by the user, effectively providing a manual
switchover capability. It can also be invoked automatically to
resolve a situation where the PW endpoints could not match the
two directions of the PW.
e. A Master/Slave mode in which one PW endpoint, the Master
endpoint, selects and dictates to the other endpoint(s), the
Slave endpoint(s), which PW to activate. This is described in
Section 4.2. .
f. Multi-homing of a CE device to two or more PE/T-PE nodes.
g. Multi-homing of a PE/T-PE node to two or more PE/T-PE nodes.
h. Optionally, implementations can add support for precedence
parameter to govern the selection of a PW when more than one
PW qualify for the active state, as defined in sections 4.1.
and 4.2. The PW with the lowest precedence value has the
highest priority. This is a local configuration parameter to
the PW endpoint.
i. Optionally, implementations can designate by configuration one
PW in the 1:1 or N:1 set as a primary PW and the remaining as
secondary PWs. If more than one PW qualify for the active
state, as defined in sections 4.1. and 4.2. , a PE/T-PE node
selects the primary PW in preference to a secondary PW. In
other words, the primary PW has implicitly the lowest
precedence value. Furthermore, implementations can provide the
option to revert to the primary PW immediately after it comes
back up or after the expiration of a delay. The PE/T-PE node
can use the PW precedence parameter to select a secondary PW
among many that qualify for active state.
Appendix A shows how the mechanisms described in this document are
used to achieve the desired protection behavior for the scenarios
described in the PW redundancy and framework document [5].
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3. Terminology
UP PW: A PW which has been configured (label mapping exchanged
between PEs) and is not in any of the PW defect states
specified in [2]. Such a PW is available for forwarding
traffic.
DOWN PW: A PW that has either not been fully configured or has been
and is in any of the PW defect states specified in [2].
Such a PW is not available for forwarding traffic.
Active PW: An UP PW used for forwarding user and OAM traffic.
Standby PW: An UP PW that is not used for forwarding user traffic,
but may forward OAM traffic.
Primary PW: the PW which a PW endpoint activates in preference to any
other PW when more than one PW qualify for active state.
When the primary PW comes back up after a failure and
qualifies for active state, the PW endpoint always reverts
to it. The designation of Primary is performed by local
configuration at the PW endpoint.
Secondary PW: when it qualifies for active state, a Secondary PW is
only selected if no Primary PW is configured or if the
configured primary PW does not qualify for active state
(e.g., is operationally Down). By default, a PW in a
redundancy PW set is considered secondary. There is no
Revertive mechanism among secondary PWs.
PW Precedence: this is a configuration parameter that dictates the
order in which a PW endpoint activates a PW among multiple
PWs which qualify for active state. The lowest the
precedence value, the highest is the priority of selecting
the PW.
PW Endpoint: A PE where a PW terminates on an NSP e.g. A SS-PW PE, an
MS-PW T-PE, or a VPLS MTU or PE-r.
4. Modes of Operation
There are two modes of operation for the use of the PW preferential
forwarding status bits:
o Independent mode
o Master/Slave mode.
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4.1. Independent Mode:
PW endpoint nodes independently select which PW they intend to make
active and which PWs they intend to make standby. They advertise the
corresponding Active/Standby forwarding state for each PW. Each PW
endpoint compares local and remote status and uses the PW that is
operationally UP at both endpoints and that shows Active states at
both the local and remote endpoint.
If more than one PW qualify for the Active state, and the PW endpoint
implements the precedence parameter, it must use the PW with the
lowest precedence value. The precedence parameter is optional.
If more than one PW qualify for the Active state, and the PW endpoint
implements the primary/secondary procedures, it must use the primary
PW in preference to all secondary PWs. If a primary PW is not
available, it must use the secondary PW with the lowest precedence
value. If the primary PW becomes available, a PW endpoint may revert
to it immediately or after the expiration of a configurable delay.
These primary/secondary procedures are optional.
In steady state with consistent configuration, a PE will always find
an Active PW. However, it is possible that due to a misconfiguration,
such a PW is not found. In the event that an active PW is not found,
a management indication should be generated. If a management
indication for failure to find an active PW was generated and an
active PW is subsequently found, a management indication should be
generated, effectively clearing the previous failure indication.
Additionally, a PE may use the optional request switchover procedures
described in Section 5.3. to have both PE nodes switch to a common
PW path.
There may also be transient conditions where endpoints do not share a
common view of the active/standby state of the PWs. This could be
caused by propagation delay of the T-LDP status messages between
endpoints. In this case, the behavior of the receiving endpoint is
outside the scope of this document.
Thus, in this mode of operation, the following definition of Active
and Standby PW states apply:
o Active State
A PW is considered to be in Active state when the PW labels are
exchanged between its two endpoints in control plane, and the status
bits exchanged between the endpoints indicate the PW is UP and Active
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at both endpoints. In this state user traffic can flow over the PW in
both directions.
o Standby State
A PW is considered to be in Standby state when the PW labels are
exchanged between its two endpoints in the control plane, but the
status bits exchanged indicate the PW is in Standby state at one or
both endpoints. In this state the endpoints MUST NOT forward data
traffic over the PW but MAY allow PW OAM packets, e.g., VCCV, to be
sent and received in order to test the liveliness of standby PWs. If
the PW is a spoke in H-VPLS, any MAC addresses learned via the PW
should be flushed when it transitions to Standby state.
4.2. Master/Slave Mode:
One endpoint node of the redundant set of PWs is designated the
Master and is responsible for selecting which PW both endpoints must
use to forward user traffic.
The Master indicates the forwarding state in the Active/Standby
status bit. The other endpoint node, the Slave, MUST follow the
decision of the Master node based on the received status bits.
One endpoint of the PW, the Master, actively selects which PW to
activate and uses it for forwarding user traffic. This status is
indicated to the Slave node by setting the preferential forwarding
status bit in the status bit TLV to Active. It does not forward user
traffic to any other of the PW's in the redundancy set to the slave
node and indicates this by setting the preferential forwarding status
bit in the status bit TLV to Standby for those PWs. The master node
MUST ignore any Active/Standby status bits received from the Slave
nodes.
If more than one PW qualify for the Active state, and the PW endpoint
implements the precedence parameter, it must use the PW with the
lowest precedence value. The precedence parameter is optional.
If more than one PW qualify for the Active state, and the PW endpoint
implements the primary/secondary procedures, it must use the primary
PW in preference to all secondary PWs. If a primary PW is not
available, it must use the secondary PW with the lowest precedence
value. If the primary PW becomes available, a PW endpoint may revert
to it immediately or after the expiration of a configurable delay.
These primary/secondary procedures are optional. The Slave
endpoint(s) are required to act on the status bits received from the
Master. When the received status bit transitions from Active to
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Standby, a Slave node MUST stop forwarding over the previously active
PW. When the received status bit transitions from Standby to Active
for a given PW, the Slave node MUST start forwarding user traffic
over this PW.
There is a single PE/T-PE Master PW endpoint node and one or many
PE/T-PE PW endpoint Slave nodes. The assignment of Master/Slave roles
to the PW endpoints is performed by local configuration. Note that
the above behavior assumes correct configuration of the Master and
Slave endpoints. This document does not define a mechanism to detect
errors in the configuration.
In this mode of operation, the following definition of Active and
Standby PW states apply:
o Active State
A PW is considered to be in Active state when the PW labels are
exchanged between its two endpoints in control plane, and the status
bits exchanged between the endpoints indicate the PW is UP at both
endpoints, and the forwarding status sent by the Master endpoint
indicates Active state. In this state user traffic can flow over the
PW in both directions.
o Standby State
A PW is considered to be in Standby state when the PW labels are
exchanged between its two endpoints in the control plane, but the
status bits sent by the Master endpoint indicate the PW is in Standby
state. In this state the endpoints MUST NOT forward data traffic over
the PW but MAY allow PW OAM packets, e.g., VCCV, to be sent and
received. If the PW is a spoke in H-VPLS, any MAC addresses learned
via the PW should be flushed when it transitions to Standby state.
5. Signaling procedures of PW State Transition
This section describes the extensions proposed and the processing
rules for the extensions. It defines a new "PW preferential
forwarding" bit in Status Code that is to be used with PW Status TLV
proposed in RFC 4447 [2]. The PW preferential forwarding bit when set
is used to signal Standby state of PW by one terminating point to the
other end.
5.1. PW Standby notification procedures in Independent mode
PW endpoint nodes independently select which PW they intend to use
for forwarding, and which PWs they do not, based on the specific
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application. They advertise the corresponding Active/Standby
forwarding state for each PW. An active forwarding state is indicated
by clearing the Active/Standby status bit in the PW status TLV. A
standby forwarding state is indicated by setting the Active/Standby
status bit in the PW status TLV. This advertisement occurs in both
the initial label mapping message and in a subsequent notification
message when the forwarding state transitions as a result of a state
change in the specific application.
Each endpoint compares the updated local and remote status and
effectively activates the PW which is operationally UP at both
endpoints and which shows both local Active and remote Active states.
When a PW is in active state, the endpoints can forward both user
packets and OAM packets.
When a PW is in standby state, the endpoints MUST NOT forward user
packets over the PW but MAY forward PW OAM packets.
For MS-PWs, S-PEs MUST relay the PW status notification containing
both the operational and preferential forwarding status bits between
ingress and egress PWs.
5.2. PW Standby notification procedures in Master/Slave mode
Whenever the Master PW endpoint "actively" selects or deselects a PW
for forwarding user traffic at its end, it explicitly notifies the
event to the remote Slave endpoint. The slave endpoint carries out
the corresponding action on receiving the PW state change
notification.
If the PW preferential forwarding bit in PW Status TLV received by
the slave is set, it indicates that the PW at the Master end is not
used for forwarding and is thus kept in the Standby state, the PW
MUST also not be used for forwarding at Slave endpoint. Clearance of
the PW Preferential Forwarding bit in PW Status TLV indicates that
the PW at the Master endpoint is used for forwarding and is in Active
state, and the receiving Slave endpoint MUST activate the PW if it
was previously not used for forwarding.
This mechanism is RECOMMENDED to be used with PWs signaled in groups
with common group-id in PWid FEC Element or Grouping TLV in
Generalized PWid FEC Element defined in [2]. When PWs are provisioned
with such grouping a termination point sends a single "wildcard"
Notification message using a PW FEC TLV with only the group ID set,
to denote this change in status for all affected PW connections. This
status message contains either the PW FEC TLV with only the Group ID
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set, or else it contains the PW Generalized FEC TLV with only the PW
Grouping ID TLV. As mentioned in [2], the Group ID field of the PWid
FEC Element, or the PW Grouping TLV used with the Generalized ID FEC
Element, can be used to send status notification for all arbitrary
set of PWs. For example, Group-ID in PWiD may be used to send
wildcard status notification message for a group of PWs when PWid FEC
element is used for PW state signaling. When Generalized PWiD FEC
Element defined is used in PW state signaling, PW Grouping TLV may be
used for wildcard status notification for a group of PWs.
For MS-PWs, S-PEs MUST relay the PW status notification containing
both the operational and preferential forwarding status bits between
ingress and egress PW segments.
5.2.1. PW State Machine
It is convenient to describe the PW state change behavior in terms of
a state machine. The PW state machine is explained in detail in the
two defined states and the behavior is presented as a state
transition table. The same state machine is seamlessly applicable to
PW Groups.
PW State Transition State Table
STATE EVENT NEW STATE
ACTIVE PW put in Standby (master) STANDBY
Action: Transmit PW preferential forwarding bit set
Receive PW preferential forwarding STANDBY
bit set
Action: Stop forwarding over PW
Receive PW preferential forwarding ACTIVE
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bit set but bit not supported
Action: None
Receive PW preferential forwarding ACTIVE
bit clear
Action: None.
STANDBY PW activated (master) ACTIVE
Action: Transmit PW preferential forwarding bit
clear
Receive PW preferential forwarding ACTIVE
bit clear
Action: Activate PW
Receive PW preferential forwarding STANDBY
bit clear but bit not supported
Action: None
Receive PW preferential forwarding STANDBY
bit set
Action: No action
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5.3. Coordination of PW Path Switchover
There are PW redundancy applications which require that PE/T-PE nodes
coordinate the switchover to a PW/MS-PW path such that both endpoints
will be forwarding over the same path at any given time. One such
application of redundant MS-PW paths is identified in [5]. Multiple
MS-PWs are configured between a pair of T-PE nodes. The paths of
these MS-PWs are diverse and are switched at different S-PE nodes.
Only one of these MS-PWs is active at any given time. The others are
put in standby. The endpoints follow the Independent Mode procedures
to activate the PW which is UP and advertised Active 'preferential
forwarding' status bit by both endpoints.
The trigger for sending a request to switchover of the path of the
MS-PW by one endpoint can be due to an operational event, example a
failure, which caused the endpoints to not be able to match the
Active 'preferential forwarding' status bit. The other trigger is the
execution of an administrative maintenance operation by the network
operator in order to move the traffic away from the node/links to be
serviced.
Unlike the case of a Master/Slave mode of operation, the endpoint
requesting the switchover requires explicit acknowledgement from the
peer endpoint that the request is honored before it switches the path
of the PW. Furthermore, any of the endpoints can make the request to
switchover.
A new status bit is proposed to have a PE/T-PE node request the
switchover to its peer. This bit will be referred to as 'request PW
switchover' status bit. The 'preferential forwarding' status bit
continues to be used by each endpoint to indicate its current local
settings of the active/standby state of each PW in the redundancy
set. In other words, like in the Independent mode, it indicates to
the far-end which of the PWs is being used to forward packets and
which is being put in standby. It can thus be used as a way for the
far-end to acknowledge the requested switchover operation.
The following procedures must be followed by both endpoints of a
PW/MS-PW to coordinate the switchover of the PW/MS-PW path. These
procedures are enabled only when the user configured the use of the
'request switchover' status bit at both endpoints.
S-PEs nodes MUST relay the PW status notification containing the
operational status bits, as well as the 'preferential forwarding' and
'request switchover' status bits between ingress and egress PW
segments.
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5.3.1. Procedures at the requesting endpoint
a. The requesting endpoint sends a Status TLV in the LDP
notification message with the 'request switchover' bit set on the
PW it desires to switch to.
b. The endpoint does not activate forwarding on that PW/MS-PW at
this point in time. It may however enable receiving on that
PW/MS-PW. Thus the 'preferential forwarding' status bit still
reflects the currently used PW path.
c. The requesting endpoint starts a timer while waiting the remote
endpoint to acknowledge the request.
d. If while waiting for the acknowledgment, the requesting endpoint
receives a request from its peer to switchover to the same or a
different PW path, it must perform the following:
i. If its system IP address is higher than that of the peer,
this endpoint ignores the request and continues to wait
for the acknowledgement from its peer.
ii. If its system IP address is lower than that of its peer,
it aborts the timer and immediately starts the
procedures of the receiving endpoint in Section 5.3.2.
e. If while waiting for the acknowledgment, the requesting
endpoint receives a status notification message from its peer
with the 'preferential forwarding' status bit cleared in the
requested PW and set in all other PWs, it must treat this as an
explicit acknowledgment of the request and must perform the
following:
i. Abort the timer.
ii. Activate the PW path.
iii. Send an update status notification message with the
'preferential forwarding' status bit clear on the newly
active PW and set in all other PWs in the redudancy set,
and the 'request switchover' bit reset in all PWs in the
redundancy set..
f. If while waiting for the acknowledgment, the requesting endpoint
detects that the requested PW went into operational Down state
locally, and could use an alternate PW which is operationally UP,
it must perform the following:
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i. Abort the timer.
ii. Issue a new request to switchover to the alternate PW.
iii. Re-start the timer.
g. If while waiting for the acknowledgment, the requesting endpoint
detects that the requested PW went into operational Down state
locally, and could not use an alternate PW which is operationally
UP, it must perform the following:
i. Abort the timer.
ii. Send an update status notification message with the
'preferential forwarding' status bit unchanged and the
'request switchover' bit reset in all PWs in the
redundancy set.
h. If while waiting for the acknowledgment, the timer expired, the
requesting endpoint assumes the request is rejected and will
either issue a new request or do nothing.
i. If the requesting node receives the acknowledgment after the
request expired, it will treat it as if the remote endpoint
unilaterally switched the path of the PW without issuing a
request. In that case, it may issue a new request and follow the
requesting endpoint procedures to synchronize transmit and
receive paths of the PW.
5.3.2. Procedures at the receiving endpoint
a. Upon receiving a status notification message with the 'request
switchover' bit set on a PW different from the currently active
one, and the requested PW is operationally UP, the receiving
endpoint must perform the following:
i. Activate the PW.
ii. Send an update status notification message with the
'preferential forwarding' status bit clear on the newly
active PW and set in all other PWs in the redudancy set,
and the 'request switchover' bit reset in all PWs in the
redundancy set.
b. Upon receiving a status notification message with the 'request
switchover' bit set on a PW different from the currently active
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one, and the requested PW is operationally Down, the receiving
endpoint must perform the following:
i. Ignore the request and do nothing.
6. Applicability and Backward Compatibility
The mechanism defined in this document is OPTIONAL and is applicable
to PWE3 applications where standby state signaling of PW or PW group
is required.
A PE implementation that uses the mechanisms described in this
document MUST negotiate the use of PW status TLV between its T-LDP
peers as per RFC 4447 [2]. If PW Status TLV is found to be not
supported by either of its endpoint after status negotiation
procedures, then the mechanisms specified in this document cannot be
used.
A PE implementation compliant to RFC 4447 [2], and which does not
support the generation or processing of the 'preferential forwarding'
status bit or of the 'request switchover'status bit, will not set
these bits in the status bits transmitted to a peer PE and will not
examine them in the received status bits from a peer PE. The
mechanisms specified in this document cannot be used.
7. Security Considerations
This document uses the LDP extensions that are needed for protecting
pseudo-wires. It will have the same security properties as in the
PWE3 control protocol [2].
8. IANA Considerations
We have defined the following codes for the pseudo-wire redundancy
application.
8.1. Status Code for PW Preferential Forwarding Status
0x00000020 When the bit is set, it indicates "PW forwarding
standby".
When the bit is cleared, it indicates "PW forwarding
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active".
8.2. Status Code for PW Request Switchover Status
0x00000040 When the bit is set, it represents "Request switchover to
this PW".
When the bit is cleared, it represents no specific
action.
9. Acknowledgments
The authors would like to thank Vach Kompella, Kendall Harvey,
Tiberiu Grigoriu, John Rigby, Prashanth Ishwar, Neil Hart, Kajal
Saha, Florin Balus, Philippe Niger, Dave McDysan, and Roman
Krzanowski for their valuable comments and suggestions.
10. References
10.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Martini, L., et al., "Pseudowire Setup and Maintenance using
LDP", RFC 4447, April 2006.
[3] Kompella,V., Lasserrre, M. , et al., "Virtual Private LAN
Service (VPLS) Using LDP Signalling", RFC 4762, January 2007.
10.2. Informative References
[4] Martini, L., et al., "Segmented Pseudo Wire", draft-ietf-pwe3-
segmented-pw-09.txt, January 2009.
[5] Muley, P., et al., "Pseudowire (PW) Redundancy", draft-ietf-
pwe3-redundancy-01.txt", March 2009.
[6] Bryant, S., et al., " Pseudo Wire Emulation Edge-to-Edge (PWE3)
Architecture", RFC 3985, March 2005
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11. Appendix A - Applications of PW Redundancy Procedures
This section shows how the mechanisms described in this document are
used to achieve the desired protection behavior for the scenarios
described in the PW redundancy requirements and framework document
[5].
11.1. One Multi-homed CE with single SS-PW redundancy
The following figure illustrates an application of single segment
pseudo-wire redundancy.
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnels-->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | | PE1|==================| | | +-----+
| |----------|....|...PW1.(active)...|....|----------| |
| | | |==================| | | CE2 |
| CE1 | +----+ |PE2 | | |
| | +----+ | | +-----+
| | | |==================| |
| |----------|....|...PW2.(standby)..| |
+-----+ | | PE3|==================| |
AC +----+ +----+
Figure 2 Multi-homed CE with single SS-PW redundancy
The application in Figure 2 makes use of the Independent mode of
operation.
CE1 is dual homed to PE1 and to PE3 by attachment circuits. The
method for dual-homing of CE1 to PE1 and to PE3 nodes, and the used
protocols such as Multi-chassis Link Aggregation Group (MC-LAG), is
outside the scope of this document.
In normal operation, PE1 and PE3 will advertise "Active" and
"Standby" 'preferential forwarding' status bit respectively to PE2.
This status reflects the forwarding state of the two AC's to CE1 as
determined by the AC dual-homing protocol used. PE2 advertises
'preferential forwarding' status bit of "Active" on both PW1 and PW2
since the AC to CE2 is single homed. As both the local and remote
operational and preferential forwarding status for PW1 are UP and
Active, traffic is forwarded over PW1 in both directions.
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On failure of the AC between CE1 and PE1, the forwarding state of the
AC on PE3 transitions to Active. For example the MC-LAG control
protocol changes the link state on PE3 to active. PE3 then announces
the newly changed 'preferential forwarding' status bit of "active" to
PE2. PE1 will advertise a PW status notification message indicating
that the AC between CE1 and PE1 is operationally down. PE2 matches
the local and remote preferential forwarding status of "active" and
operational status "Up" and select PW2 as the new active pseudo-wire
to send traffic to.
On failure of PE1 node, PE3 will detect it and will transition the
forwarding state of its AC to Active. The method by which PE3 detects
that PE1 is down is outside the scope of this document. PE3 then
announces the newly changed 'preferential forwarding' status bit of
"active" to PE2. PE3 and PE2 match the local and remote preferential
forwarding status of "active" and operational status "Up" and select
PW2 as the new active pseudo-wire to send traffic to. Note that PE2
may have detected that the PW to PE1 went down via T-LDP Hello
timeout or via other means. However, it will not be able to forward
user traffic until it received the updated status bit from PE3.
Note in this example, the receipt of the operational status of the AC
on the CE1-PE1 link is normally sufficient to have PE2 switch the
path to PW2. However, the operator may want to trigger the switchover
of the path of the PW for administrative reasons, i.e., maintenance,
and thus the use of the 'preferential forwarding' status bit is
required to notify PE2 to trigger the switchover.
Note that the primary/secondary procedures do not apply in this case
as the PW Active/Standby status is driven by the AC forwarding state
as determined by the AC dual-homing protocol used.
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11.2. Multiple Multi-homed CEs with single SS-PW redundancy
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnels-->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | |....|.......PW1........|....| | +-----+
| |----------| PE1|...... .........| PE3|----------| |
| CE1 | +----+ \ / PW3 +----+ | CE2 |
| | +----+ X +----+ | |
| | | |....../ \..PW4....| | | |
| |----------| PE2| | PE4|--------- | |
+-----+ | |....|.....PW2..........|....| | +-----+
AC +----+ +----+ AC
Figure 3 Multiple Multi-homed CEs with single SS-PW redundancy
The application in Figure 3 makes use of the Independent mode of
operation.
CE1 is dual-homed to PE1 and PE2. CE2 is dual-homed PE3 and PE4. The
method for dual-homing and the used protocols such as Multi-chassis
Link Aggregation Group (MC-LAG) are outside the scope of this
document. Note that the PSN tunnels are not shown in this figure for
clarity. However, it can be assumed that each of the PWs shown is
encapsulated in a separate PSN tunnel.
PE1 advertises the preferential status "active" and operational
status "UP" for pseudo-wires PW1 and PW4 connected to PE3 and PE4.
This status reflects the forwarding state of the AC attached to PE1.
PE2 advertises preferential status "standby" where as operational
status "UP" for pseudo-wires PW2 and PW3 to PE3 and PE4. PE3
advertises preferential status "standby" where as operational status
"UP" for pseudo-wires PW1 and PW3 to PE1 and PE2. PE4 advertise the
preferential status "active" and operational status "UP" for pseudo-
wires PW2 and PW4 to PE2 and PE1 respectively. The method of
deriving Active/Standby status of the AC is outside the scope of
this document. In case of MC-LAG it is derived by the Link
Aggregation Control protocol (LACP) negotiation. Thus by matching
the local and remote preferential forwarding status of "active" and
operational status "Up" of pseudo-wire, the PE nodes determine which
PW should be in the Active state. In this case it is PW4 that will
be selected.
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On failure of the AC between CE1 and PE1, the forwarding state of
the AC on PE2 is changed to Active. For example the MC-LAG control
protocol changes the link state on PE2 to active. PE2 then announces
the newly changed 'preferential forwarding' status bit of "active"
to PE3 and PE4. PE1 will advertise a PW status notification message
indicating that the AC between CE1 and PE1 is operationally down.
PE2 and PE4 match the local and remote preferential forwarding
status of "active" and operational status "Up" and select PW2 as the
new active pseudo-wire to send traffic to.
On failure of PE1 node, PE2 will detect it and will transition the
forwarding state of its AC to Active. The method by which PE2
detects that PE1 is down is outside the scope of this document. PE2
then announces the newly changed 'preferential forwarding' status
bit of "active" to PE3 and PE4. PE2 and PE4 match the local and
remote preferential forwarding status of "active" and operational
status "Up" and select PW2 as the new active pseudo-wire to send
traffic to. Note that PE3 and PE4 may have detected that the PW to
PE1 went down via T-LDP Hello timeout or via other means. However,
they will not be able to forward user traffic until they received
the updated status bit from PE2.
Because each dual-homing algorithm running on the two node sets,
i.e., {CE1, PE1, PE2} and {CE2, PE3, PE4}, selects the active AC
independently, there is a need to signal the active status of the AC
such that the PE nodes can select a common active PW path for end-to-
end forwarding between CE1 and CE2 as per the procedures in the
independent mode.
Note that the primary/secondary procedures do not apply in this use
case as the Active/Standby status is driven by the AC forwarding
state as determined by the AC dual-homing protocol used.
11.3. Multi-homed CE with MS-PW redundancy
The following figure illustrates an application of multi-segment
pseudo-wire redundancy.
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Native |<-----------Pseudo Wire----------->| Native
Service | | Service
(AC) | |<-PSN1-->| |<-PSN2-->| | (AC)
| V V V V V V |
| +-----+ +-----+ +-----+
+----+ | |T-PE1|=========|S-PE1|=========|T-PE2| | +----+
| |-------|......PW1-Seg1.......|PW1-Seg2.......|-------| |
| | | |=========| |=========| | | |
| CE1| +-----+ +-----+ +-----+ | |
| | |.| +-----+ +-----+ | CE2|
| | |.|===========| |=========| | | |
| | |.....PW2-Seg1......|.PW2-Seg2......|-------| |
+----+ |=============|S-PE2|=========|T-PE4| | +----+
+-----+ +-----+ AC
Figure 4 Multi-homed CE with MS-PW redundancy
The application in Figure 4 makes use of the Independent mode of
operation.
CE2 is dual-homed to T-PE2 and T-PE4. PW1 and PW2 are used to extend
the resilient connectivity all the way to T-PE1. PW1 has two segments
and is active pseudo-wire while PW2 has two segments and is a standby
pseudo-wire. This application requires support for MS-PW with
segments of the same type as described in [4].
The operation in this case is the same as in the case of SS-PW as
described in Section 11.1. . The only difference is that the S-PE
nodes need to relay the PW status notification containing both the
operational and forwarding status to the T-PE nodes.
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11.4. Single Homed CE with MS-PW redundancy
The following is an application of the independent mode of operation
along with the optional request switchover procedures in order to
provide N:1 PW protection. A revertive behavior to a primary PW is
shown as an example of configuring and using the primary/secondary
procedures.
Native |<------------Pseudo Wire------------>| Native
Service | | Service
(AC) | |<-PSN1-->| |<-PSN2-->| | (AC)
| V V V V V V |
| +-----+ +-----+ +-----+ |
+----+ | |T-PE1|=========|S-PE1|=========|T-PE2| | +----+
| |-------|......PW1-Seg1.......|.PW1-Seg2......|-------| |
| CE1| | |=========| |=========| | | CE2|
| | +-----+ +-----+ +-----+ | |
+----+ |.||.| |.||.| +----+
|.||.| +-----+ |.||.|
|.||.|=========| |========== .||.|
|.||...PW2-Seg1......|.PW2-Seg2...||.|
|.| ===========|S-PE2|============ |.|
|.| +-----+ |.|
|.|============+-----+============= .|
|.....PW3-Seg1.| | PW3-Seg2......|
==============|S-PE3|===============
| |
+-----+
Figure 5 Single homed CE with multi-segment pseudo-wire redundancy
CE1 is connected to PE1 in provider Edge 1 and CE2 to PE2 in provider
edge 2 respectively. There are three segmented PWs. A primary PW,
PW1, is switched at S-PE1. A primary PW has the highest precedence. A
secondary PW, PW2, which is switched at S-PE2 and has a precedence of
1. Finally, another secondary PW, PW3, is switched at S-PE3 and has a
precedence of 2. The precedence is locally configured at the
endpoints of the PW, i.e., T-PE1 and T-PE2.
T-PE1 and T-PE2 will select the PW they intend to activate based on
their local and remote operational state as well as the local
primary/precedence configuration. In this case, they will both
advertise preferential forwarding' status bit of "active" on PW1 and
of "standby" on PW2 and PW3. Assuming all PWs are operationally UP,
T-PE1 and T-PE2 will use PW1 to forward user packets.
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If PW1 fails, then the T-PE detecting the failure will send a status
notification to the remote T-PE with a "PW Down" bit set as well as
the 'preferential forwarding' status bit set on PW1. It will also
clear 'preferential forwarding' status bit on PW2 as it is the next
it has the next lowest precedence value. T-PE2 will also perform the
same steps as soon as it is informed of the failure of PW1. Both T-PE
nodes will perform a match on the 'preferential forwarding' status
of "active" and operational status of "Up" and will use PW2 to
forward user packets.
However this does not guarantee that paths of the PW are synchronized
at all times. This may be due to a mismatch of the configuration of
the PW priority in each T-PE. This may also be due to a failure which
caused the endpoints to not be able to match the Active 'preferential
forwarding' status bit and operational status bits. In this case, T-
PE1 and/or T-PE2 can invoke the optional request
switchover/acknowledgement procedures to synchronize the PW paths in
both directions.
The trigger for sending a request to switchover can also be the
execution of an administrative maintenance operation by the network
operator in order to move the traffic away from the T-PE/S-PE nodes
/links to be serviced.
In case the request switchover is sent by both endpoints
simultaneously, both T-PEs send status notification with the newly
selected PW with 'request switchover' bit set, waiting for response
from the other endpoint. In such situation, the T-PE with greater
system address request is given precedence. This helps in
synchronizing paths in event of mismatch of priority configuration as
well.
11.5. PW redundancy between H-VPLS MTU-s and PE-rs
Following figure illustrates the application of use of PW redundancy
in H-VPLS for the purpose of dual-homing an MTU-s node to PE nodes
using PW spokes. This application makes use of the Master/Slave mode
of operation.
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|<-PSN1-->| |<-PSN2-->|
V V V V
+-----+ +--------+
|MTU-s|=========|PE1-rs |========
|..Active PW group.... | H-VPLS-core
| |=========| |=========
+-----+ +--------+
|.|
|.| +--------+
|.|===========| |==========
|...Standby PW group |.H-VPLS-core
=============| PE2-rs|==========
+--------+
Figure 6 Multi-homed MTU-s in H-VPLS core
MTU-s is dual homed to PE1-rs and PE2-rs. The primary spoke PWs from
MTU-s are connected to PE1-rs while the secondary PWs are connected
to PE2. PE1-rs and PE2-rs are connected to H-VPLS core on the other
side of network. MTU-s communicates to PE1-rs and PE2-rs the
forwarding status of its member PWs for a set of VSIs having common
status Active/Standby. It may be signaled using PW grouping with
common group-id in PWid FEC Element or Grouping TLV in Generalized
PWid FEC Element as defined in [2] to scale better. MTU-s derives
the status of the PWs based on local policy configuration. In this
example, the primary/secondary procedures are used but this can be
based on any other policy.
Whenever MTU-s performs a switchover, it sends a wildcard
Notification Message to PE2-rs for the Standby PW group containing PW
Status TLV with PW Standby bit cleared. On receiving the notification
PE-2rs unblocks all member PWs identified by the PW group and state
of PW group changes from Standby to Active. All procedures described
in Section 5.2. are applicable.
The use of the 'preferential forwarding' status bit in Master/Slave
mode is similar to Topology Change Notification in RSTP controlled
IEEE Ethernet Bridges but is restricted over a single hop. When these
procedures are implemented, PE-rs devices are aware of switchovers at
MTU-s and could generate MAC Withdraw Messages to trigger MAC
flushing within the H-VPLS full mesh. By default, MTU-s devices
should still trigger MAC Withdraw messages as currently defined in
[6] to prevent two copies of MAC withdraws to be sent, one by MTU-s
and another one by PE-rs nodes. Mechanisms to disable MAC Withdraw
trigger in certain devices is out of the scope of this document
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Author's Addresses
Praveen Muley
Alcatel-lucent
701 E. Middlefiled Road
Mountain View, CA, USA
Email: Praveen.muley@alcatel-lucent.com
Mustapha Aissaoui
Alcatel-lucent
600 March Rd
Kanata, ON, Canada K2K 2E6
Email: mustapha.aissaoui@alcatel-lucent.com
Matthew Bocci
Alcatel-Lucent
Voyager Place, Shoppenhangers Rd
Maidenhead, Berks, UK SL6 2PJ
Email: matthew.bocci@alcatel-lucent.co.uk
Pranjal Kumar Dutta
Alcatel-Lucent
Email: pdutta@alcatel-lucent.com
Marc Lasserre
Alcatel-Lucent
Email: mlasserre@alcatel-lucent.com
Jonathan Newton
Cable & Wireless
Email: Jonathan.Newton@cw.com
Olen Stokes
Extreme Networks
Email: ostokes@extremenetworks.com
Hamid Ould-Brahim
Nortel
Email: hbrahim@nortel.com
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO, 80112
Email: lmartini@cisco.com
Thomas Nadeau
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BT
tnadeau@lucidvision.com
Giles Heron
BT
giles.heron@gmail.com
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