CCAMP Working Group
Internet Draft
Zafar Ali
Jean-Philippe Vasseur
Anca Zamfir
Cisco Systems, Inc.
Jonathan Newton
Cable and Wireless
Category: Informational
Expires: July 19, 2010 January 20, 2010
draft-ietf-ccamp-mpls-graceful-shutdown-13.txt
Graceful Shutdown in MPLS and Generalized MPLS
Traffic Engineering Networks
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Abstract
MPLS-TE Graceful Shutdown is a method for explicitly notifying
the nodes in a Traffic Engineering (TE) enabled network that the
TE capability on a link or on an entire Label Switching Router
(LSR) is going to be disabled. MPLS-TE graceful shutdown
mechanisms are tailored toward addressing planned outage in the
network.
This document provides requirements and protocol mechanisms to
reduce/eliminate traffic disruption in the event of a planned
shutdown of a network resource. These operations are equally
applicable to both MPLS-TE and its Generalized MPLS (GMPLS)
extensions.
Table of Contents
1. Introduction....................................................2
2. Terminology.....................................................3
3. Requirements for Graceful Shutdown..............................4
4. Mechanisms for Graceful Shutdown................................5
4.1 OSPF/ ISIS Mechanisms for graceful shutdown...................5
4.2 RSVP-TE Signaling Mechanisms for graceful shutdown............6
5. Manageability Considerations....................................7
6. Security Considerations.........................................8
7. IANA Considerations.............................................8
8. Acknowledgments.................................................8
9. Reference.......................................................8
9.1 Normative Reference...........................................8
9.2 Informative Reference.........................................8
10. Authors' Address:..............................................9
1. Introduction
When outages in a network are planned (e.g., for maintenance
purposes), some mechanisms can be used to avoid traffic
disruption. This is in contrast with unplanned network element
failure, where traffic disruption can be minimized thanks to
recovery mechanisms, but may not be avoided. Therefore, a Service
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Provider may desire to gracefully (temporarily or indefinitely)
remove a TE Link, a group of TE Links or an entire node for
administrative reasons such as link maintenance,
software/hardware upgrade at a node or significant TE
configuration changes. In all these cases, the goal is to
minimize the impact on the traffic carried over TE LSPs in the
network by triggering notifications so as to gracefully reroute
such flows before the administrative procedures are started.
These operations are equally applicable to both MPLS-TE [RFC3209]
and its Generalized MPLS (GMPLS) extensions [RFC3471], [RFC3473].
This document describes the mechanisms that can be used to
gracefully shutdown MPLS-TE/ GMPLS Traffic Engineering on a
resource such as a TE link, a component link within a bundled TE
link, a label resource or an entire TE node.
Graceful shutdown of a resource may require several steps. These
steps can be broadly divided into two sets: disabling the
resource in the control plane and disabling the resource in the
data plane. The node initiating the graceful shutdown condition
introduces a delay between the two sets to allow the control
plane to gracefully divert the traffic away from the resource
being gracefully shutdown. The trigger for the graceful shutdown
event is a local matter at the node initiating the graceful
shutdown. Typically, graceful shutdown is triggered for
administrative reasons, such as link maintenance or
software/hardware upgrade.
2. Terminology
LSR: Label Switching Router. The terms node and LSR are used
interchangeably in this document.
GMPLS: The term GMPLS is used in this document to refer to packet
MPLS-TE, as well as GMPLS extensions to MPLS-TE.
TE Link: The term TE link refers to single or a bundle of
physical links or FA-LSPs (see below) on which traffic
engineering is enabled.
TE LSP: A Traffic Engineered Label Switched Path.
S-LSP: A segment of a TE LSP
FA-LSP (Forwarding Adjacency LSP): An LSP that is announced as a
TE link into the same instance of the GMPLS control plane as the
one that was used to create the LSP [RFC4206].
ISIS-LSP: Link State Packets generated by ISIS routers and that
contain routing information.
LSA: Link State Advertisements generated by OSPF routers and that
contain routing information.
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TE-LSA/ TE-ISIS-LSP: The traffic engineering extensions to OSPF/
ISIS.
Head-end node: Ingress LSR that initiated signaling for the Path.
Border node: Ingress LSR of a TE LSP segment (S-LSP).
PCE (Path Computation Element): An entity that computes the
routes on behalf of its clients (PCC) [RFC4655].
Last resort resource: If a path to a destination from a given
head-end node cannot be found upon removal of a resource (e.g.,
TE link, TE node), the resource is called last resort to reach
that destination from the given head-end node.
3. Requirements for Graceful Shutdown
This section lists the requirements for graceful shutdown in the
context of GMPLS.
- Graceful shutdown is required to address graceful removal of
one TE link, one component link within a bundled TE link, a set
of TE links, a set of component links, label resources, or an
entire node.
- Once an operator has initiated graceful shutdown of a network
resource, no new TE LSPs may be set up that use the resource.
Any signaling message for a new TE LSP that explicitly specifies
the resource, or that would require the use of the resource due
to local constraints, is required to be rejected as if the
resource were unavailable.
- It is desirable for new TE LSP setup attempts that would be
rejected because of graceful shutdown of a resource (as described
in the previous requirement) to avoid any attempt to use the
resource by selecting an alternate route or other resources.
- If the resource being shut down is a last resort resource,
based on a local decision, the node initiating the graceful
shutdown procedure can cancel the shutdown operation.
- It is required to give the ingress node the opportunity to take
actions in order to reduce/eliminate traffic disruption on the TE
LSPs that are using the network resources which are about to be
shut down.
- Graceful shutdown mechanisms are equally applicable to intra-
domain and TE LSPs spanning multiple domains, as defined in
[RFC4726]. Examples of such domains include IGP areas and
Autonomous Systems.
- Graceful shutdown is equally applicable to packet and non-
packet networks.
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- In order to make rerouting effective, it is required that when
a node initiates the graceful shutdown of a resource, it
identifies to all other network nodes the TE resource under
graceful shutdown.
- Depending on switching technology, it may be possible to shut
down a label resource, e.g., shutting down a lambda in a Lambda
Switch Capable (LSC) node.
4. Mechanisms for Graceful Shutdown
An IGP only solution based on [RFC3630], [RFC5305], [RFC4203] and
[RFC5307] is not applicable when dealing with inter-area and
inter-AS traffic engineering, as IGP flooding is restricted to
IGP areas/levels. An RSVP based solution is proposed in this
document to handle TE LSPs spanning multiple domains.
In addition, in order to prevent LSRs in a domain to use the
resource being shut down.
In addition, in order to discourage nodes from establishing new
TE LSPs through the resources being shutdown, existing IGP
mechanisms are used for the shutdown notification.
A node where a link or the whole node is being shutdown first
triggers the IGP updates as described in Section 4.1 and then,
with some delay to allow network convergence, uses the signaling
mechanism described in Section 4.2.
4.1 OSPF/ ISIS Mechanisms for graceful shutdown
This section describes the use of existing OSPF and ISIS
mechanisms for the graceful shutdown in GMPLS networks.
The OSPF and ISIS procedures for graceful shutdown of TE links
are similar to the graceful restart of OSPF and ISIS as described
in [RFC4203] and [RFC5307], respectively. Specifically, the node
where graceful shutdown of a link is desired originates the TE
LSA/ISIS-LSP containing a Link TLV for the link under graceful
shutdown with Traffic Engineering metric set to 0xffffffff, 0 as
unreserved bandwidth, and if the TE link has LSC or FSC as its
Switching Capability then also with 0 in the "Max LSP Bandwidth"
field of the Interface Switching Capability Descriptor (ISCD)
sub-TLV. A node may also specify a value which is greater than
the available bandwidth in the "Minimum LSP bandwidth" field of
the same ISCD sub-TLV. This would discourage new TE LSP
establishment through the link under graceful shutdown.
If graceful shutdown procedure is performed for a component link
within a TE Link bundle and it is not the last component link
available within the TE link, the link attributes associated with
the TE link are recomputed. Similarly, If graceful shutdown
procedure is performed on a label resource within a TE Link, the
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link attributes associated with the TE link are recomputed. If
the removal of the component link or label resource results in a
significant bandwidth change event, a new LSA is originated with
the new traffic parameters. If the last component link is being
shut down, the routing procedure related to TE link removal is
used.
Neighbors of the node where graceful shutdown procedure is in
progress continue to advertise the actual unreserved bandwidth of
the TE links from the neighbors to that node, without any routing
adjacency change.
When graceful shutdown at node level is desired, the node in
question follows the procedure specified in the previous section
for all TE Links.
4.2 RSVP-TE Signaling Mechanisms for graceful shutdown
As discussed in Section 3, one of the requirements for the
signaling mechanism for graceful shutdown is to carry information
about the resource under graceful shutdown. For this purpose the
Graceful Shutdown uses TE LSP rerouting mechanism as defined in
[RFC5710].
Specifically, the node where graceful shutdown of an unbundled TE
link or an entire bundled TE link is desired triggers a PathErr
message with the error code "Notify" and error value "Local link
maintenance required", for all affected TE LSPs. Similarly, the
node that is being gracefully shut down triggers a PathErr
message with the error code "Notify" and error value "Local node
maintenance required", for all TE LSPs. For graceful shutdown of
a node, an unbundled TE link or an entire bundled TE link, the
PathErr message may contain either an [RFC2205] format ERROR_SPEC
object, or an IF_ID [RFC3473] format ERROR_SPEC object. In either
case, it is the address and TLVs carried by the ERROR_SPEC object
and not the error value that indicates the resource that is to be
gracefully shut down.
MPLS TE Link Bundling [RFC4201] requires that an TE LSP is pinned
down to a component link. Consequently, graceful shutdown of a
component link in a bundled TE link differs from graceful
shutdown of unbundled TE link or entire bundled TE link.
Specifically, in the former case, when only a subset of component
links and not the entire bundled TE link is being shutdown, the
remaining component links of the bundled TE link may still be
able to admit new TE LSPs. The node where graceful shutdown of a
component link is desired triggers a PathErr message with the
error code "Notify" and error value of "Local link maintenance
required". The rest of the ERROR_SPEC object is constructed using
Component Reroute Request procedure defined in [RFC5710].
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If graceful shutdown of a label resource is desired, the node
initiating this action triggers a PathErr message with the error
codes and error values of "Notify/Local link maintenance
required". The rest of the ERROR_SPEC object is constructed using
Label Reroute Request procedure defined in [RFC5710].
When a head-end node, a transit node or a border node receives a
PathErr message with the error code "Notify" and error value
"Local link maintenance required" or "Local node maintenance
required", it follows the procedures defined in [RFC5710] to
reroute the traffic around the resource being gracefully
shutdown. When performing path computation for the new TE LSP,
the head-end node, or border node avoids using the TE resources
identified by the ERROR_SPEC object. If PCE is used for path
computation, head-end (or border) node acting as PCC specifies in
its requests to the PCE that path computation should avoid the
resource being gracefully shutdown. The amount of time the head-
end node, or border node avoids using the TE resources identified
by the IP address contained in the PathErr is based on a local
decision at head-end node or border node.
If the node initiating the graceful shutdown procedure receives a
path setup request for a new tunnel using resource being
gracefully shutdown, it sends a Path Error message with "Notify"
error code in the ERROR SPEC object and an error value consistent
with the type of resource being gracefully shut down. However,
based on a local decision, if an existing tunnel continues to use
the resource being gracefully shutdown, the node initiating the
graceful shutdown procedure may allow resource being gracefully
shutdown to be used as a "last resort". The node initiating the
graceful shutdown procedure can distinguish between new and
existing tunnels by inspecting the SENDER TEMPLATE and SESSION
objects.
If the resource being shut down is a last resort resource, it
can be used, i.e., based on a local decision the node initiating
the graceful shutdown procedure can cancel the shutdown operation.
Similarly, based on a local decision the node initiating
the graceful shutdown procedure can delay the actual removal of
resource for forwarding. This is to give time to network to move
traffic from the resource being shutdown. For this purpose, the
node initiating graceful shutdown procedure follows the Reroute
Request Timeout procedure defined in [RFC5710].
5. Manageability Considerations
When a TE link is being showdown, a linkDown trap as defined in
[RFC2863] should be generated for the TE link. Similarly, if a
bundled TE links is being showdown, a linkDown trap as defined
in [RFC2863] should be generated for the bundled TE link, as well
as for each of its component links. If a TE node is being
shutdown, a linkDown trap as defined in [RFC2863] should be
generated for all TE links at the node.
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6. Security Considerations
This document introduces no new security considerations as this
document describes usage of existing formats and mechanisms. This
document relies on existing procedures for advertisement of TE
LSA/ISIS-LSP containing Link TLV. Tampering with TE LSAs/ISIS-
LSPs may have an effect on traffic engineering computations, and
it is suggested that any mechanisms used for securing the
transmission of normal LSAs/ISIS-LSPs be applied equally to all
Opaque LSAs/ISIS-LSPs this document uses. Existing security
considerations specified in [RFC3630], [RFC5305], [RFC4203],
[RFC5307] and [MPLS-GMPLS-SECURITY] remain relevant and suffice.
Furthermore, security considerations section in [RFC5710] and
section 9 of [RFC4736] should be used for understanding the
security considerations related to the formats and mechanisms
used in this document.
7. IANA Considerations
This document has no IANA actions.
8. Acknowledgments
The authors would like to thank Adrian Farrel for his detailed
comments and suggestions. The authors would also like to
acknowledge useful comments from David Ward, Sami Boutros, and
Dimitri Papadimitriou.
9. Reference
9.1 Normative Reference
[RFC2205] Braden, R. Ed. et al, "Resource ReSerVation Protocol
(RSVP) Version 1, Functional Specification", RFC 2205.
[RFC5710] Berger, L., Papadimitriou, D., and J. Vasseur,
"PathErr Message Triggered MPLS and GMPLS LSP Reroute",
RFC5710.
9.2 Informative Reference
[RFC3209] Awduche D., Berger, L., Gan, D., Li T., Srinivasan, V.,
Swallow, G., "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC
3209.
[RFC4736] Jean-Philippe Vasseur, et al "Reoptimization of MPLS
Traffic Engineering loosely routed LSP paths", RFC 4736.
[RFC3630] Katz D., Kompella K., Yeung D., "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630.
[RFC5305] Smit, H. and T. Li, "Intermediate System to
Intermediate System (IS-IS) Extensions for Traffic Engineering
(TE)", RFC 5305.
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[RFC4203] Kompella, K., Ed., and Y. Rekhter, Ed., "OSPF
Extensions in Support of Generalized Multi-Protocol Label
Switching (GMPLS)", RFC 4203.
[RFC5307] Kompella, K., Ed., and Y. Rekhter, Ed., "Intermediate
System to Intermediate System (IS-IS) Extensions in Support of
Generalized Multi-Protocol Label Switching (GMPLS)", RFC 5307.
[RFC3471] Berger, L., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Functional Description", RFC 3471.
[RFC3473] Berger, L., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) Extensions", RFC 3473.
[RFC4726] Farrel A, Vasseur, J.-P., Ayyangar A., "A Framework for
Inter-Domain MPLS Traffic Engineering", RFC 4726, November 2006.
[RFC4201] Kompella, K., Rekhter, Y., Berger, L., "Link Bundling
in MPLS Traffic Engineering", RFC 4201.
[RFC4206] Kompella K., Rekhter Y., "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching (GMPLS)
Traffic Engineering (TE)", RFC 4206.
[RFC4655] A. Farrel, J.-P. Vasseur, J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655.
[RFC2863] McCloghrie K., Kastenholz F., "The Interfaces Group
MIB", RFC 2863.
[MPLS-GMPLS-SECURITY] Luyuan F., Ed. "Security Framework for
MPLS and GMPLS Networks", draft-ietf-mpls-mpls-and-gmpls-
security-framework, work in progress.
10. Authors' Address:
Zafar Ali
Cisco systems, Inc.,
2000 Innovation Drive
Kanata, Ontario, K2K 3E8
Canada.
Email: zali@cisco.com
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Jean Philippe Vasseur
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough , MA - 01719
USA
Email: jpv@cisco.com
Anca Zamfir
Cisco Systems, Inc.
2000 Innovation Drive
Kanata, Ontario, K2K 3E8
Canada
Email: ancaz@cisco.com
Jonathan Newton
Cable and Wireless
jonathan.newton@cw.com
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