One document matched: draft-muley-dutta-pwe3-redundancy-bit-02.txt
Differences from draft-muley-dutta-pwe3-redundancy-bit-01.txt
Network Working Group Praveen Muley
Internet Draft Mustapha Aissaoui
Expires: May 2008 Matthew Bocci
Pranjal Kumar Dutta
Marc Lasserre
Alcatel-Lucent
Jonathan Newton
Cable & Wireless
Olen Stokes
Extreme Networks
Hamid Ould-Brahim
Nortel
Luca Martini
Cisco Systems Inc.
November 19, 2007
Preferential Forwarding Status bit definition
draft-muley-dutta-pwe3-redundancy-bit-02.txt
Status of this Memo
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This Internet-Draft will expire on May 19, 2008.
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..................................................4
3. Terminology.................................................5
4. Modes of Operation..........................................5
4.1. Independent Mode:.......................................5
4.2. Master/Slave Mode:......................................6
5. Signaling procedures of PW State Transition..................7
5.1. PW Standby notification procedures in Independent mode...8
5.2. PW Standby notification procedures in Master/Slave mode..8
5.2.1. PW State Machine...................................9
5.3. Coordination of PW Path Switchover.....................11
5.3.1. Procedures at the requesting endpoint.............12
5.3.2. Procedures at the receiving endpoint..............13
6. Applicability and Backward Compatibility....................14
7. Security Considerations.....................................14
8. IANA Considerations........................................14
8.1. Status Code for PW Preferential Forwarding Status.......14
8.2. Status Code for PW Request Switchover Status...........15
9. Acknowledgments............................................15
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10. References................................................15
Author's Addresses............................................15
Full Copyright Statement.......................................16
Intellectual Property Statement................................17
Acknowledgment................................................17
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 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 [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 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.
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnels-->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | | PE1|==================| | | +-----+
| |----------|....|...PW1.(active)...|....|----------| |
| | | |==================| | | CE2 |
| CE1 | +----+ |PE2 | | |
| | +----+ | | +-----+
| | | |==================| |
| |----------|....|...PW2.(standby)..| |
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+-----+ | | 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, PW redundancy is important to provide
resilience in the event of failure of S-PE node since PSN protection
mechanisms cannot.
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
The PWE3 control protocol [2] defines the following status codes to
indicate the operational state for an AC and a PW:
0x00000000 - Pseudowire forwarding (clear all failures)
0x00000001 - Pseudowire Not Forwarding
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.
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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..
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 traffic.
Standby PW: An UP PW that is not used for forwarding user traffic.
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.
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
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operationally UP at both endpoints and that shows Active states at
both the local and remote endpoint.
In steady state, a PE will always find an Active PW. However, it is
possible that due to a misconfiguration, such a PW is not found. The
behavior of a PE in this case is outside the scope of this document.
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
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.
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 forward user
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traffic to any other of the PW's in the redundant 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.
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 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.
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.
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.
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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
application. They advertise the corresponding Active/Standby
forwarding state for each PW. 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
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status message contains either the PW FEC TLV with only the Group ID
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
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Receive PW preferential forwarding ACTIVE
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 LDP status 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 set in the requested PW, 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 set to the newly
active PW 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:
i. Abort the timer.
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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 set to the newly
active PW 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
one, and the requested PW is operationally Down, the receiving
endpoint must perform the following:
i. Ignore the request and do nothing.
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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
active".
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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 and Philippe Niger for their valuable comments and
suggestions.
10. 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] Martini, L., et al., "Segmented Pseudo Wire", draft-ietf-pwe3-
segmented-pw-05.txt, July 2007.
[4] Bryant, S., et al., " Pseudo Wire Emulation Edge-to-Edge (PWE3)
Architecture", RFC 3985, March 2005
[5] Praveen, Pranjal et al., "draft-muley-pwe3-redundancy-01.txt",
March 2007.
Author's Addresses
Praveen Muley
Alcatel-lucent
701 E. Middlefiled Road
Mountain View, CA, USA
Email: Praveen.muley@alcatel-lucent.com
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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
Full Copyright Statement
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