RFC 5712 PROPOSED STANDARD MPLS · TRAFFIC ENGINEERING · RSVP
MPLS Soft Preemption: MPLS Traffic Engineering Soft Preemption
How MPLS traffic engineering can warn a path that it is being bumped, so traffic moves over without being dropped.
RFC EDITOR
Meyer, M., Ed. and JP. Vasseur, Ed., "MPLS Traffic Engineering Soft Preemption", RFC 5712, DOI 10.17487/RFC5712, January 2010, <https://www.rfc-editor.org/info/rfc5712>.
BIBTEX
@misc{rfc5712,
series = {Request for Comments},
number = 5712,
howpublished = {RFC 5712},
publisher = {RFC Editor},
doi = {10.17487/RFC5712},
url = {https://www.rfc-editor.org/info/rfc5712},
author = {M. Meyer and JP. Vasseur},
title = {{MPLS Traffic Engineering Soft Preemption}},
pagetotal = 13,
year = 2010,
month = jan,
} RFC text
Contents
Internet Engineering Task Force (IETF) M. Meyer, Ed.
Request for Comments: 5712 British Telecom
Category: Standards Track JP. Vasseur, Ed.
ISSN: 2070-1721 Cisco Systems, Inc.
January 2010
MPLS Traffic Engineering Soft Preemption
Abstract
This document specifies Multiprotocol Label Switching (MPLS) Traffic
Engineering Soft Preemption, a suite of protocol modifications
extending the concept of preemption with the goal of reducing or
eliminating traffic disruption of preempted Traffic Engineering Label
Switched Paths (TE LSPs). Initially, MPLS RSVP-TE was defined with
support for only immediate TE LSP displacement upon preemption. The
utilization of a reroute request notification helps more gracefully
mitigate the reroute process of preempted TE LSP. For the brief
period soft preemption is activated, reservations (though not
necessarily traffic levels) are in effect under-provisioned until the
TE LSP(s) can be rerouted. For this reason, the feature is
primarily, but not exclusively, interesting in MPLS-enabled IP
networks with Differentiated Services and Traffic Engineering
capabilities.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5712.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
2.1. Acronyms and Abbreviations .................................3
2.2. Nomenclature ...............................................4
2.3. Requirements Language ......................................4
3. Motivations .....................................................4
4. RSVP Extensions .................................................5
4.1. SESSION-ATTRIBUTE Flags ....................................5
4.2. Path Error - "Reroute Request Soft Preemption"
Error Value ................................................5
5. Mode of Operation ...............................................6
6. Elements Of Procedures ..........................................7
6.1. On a Soft Preempting LSR ...................................7
6.2. On Head-end LSR of a Soft Preempted TE LSP .................9
7. Interoperability ...............................................10
8. Management .....................................................10
9. IANA Considerations ............................................11
9.1. New Session Attribute Object Flag .........................11
9.2. New Error Sub-Code Value ..................................11
10. Security Considerations .......................................11
11. Acknowledgements ..............................................12
12. Contributors ..................................................12
13. References ....................................................12
13.1. Normative References .....................................12
13.2. Informative References ...................................13
1. Introduction
In a Multiprotocol Label Switching (MPLS) Resource Reservation
Protocol Traffic Engineering (RSVP-TE) (see [RFC3209]) enabled IP
network, hard preemption is the default behavior. Hard preemption
provides no mechanism to allow preempted Traffic Engineering Label
Switched Paths (TE LSPs) to be handled in a make-before-break
fashion: the hard preemption scheme instead utilizes a very intrusive
method that can cause traffic disruption for a potentially large
amount of TE LSPs. Without an alternative, network operators either
accept this limitation, or remove functionality by using only one
preemption priority or using invalid bandwidth reservation values.
Understandably desirable features like TE reservation adjustments
that are automated by the ingress Label Edge Router (LER) are less
palatable when preemption is intrusive and maintaining high levels of
network stability levels is a concern.
This document defines the use of additional signaling and maintenance
mechanisms to alert the ingress LER of the preemption that is pending
and allow for temporary control-plane under-provisioning while the
preempted tunnel is rerouted in a non-disruptive fashion (make-
before-break) by the ingress LER. During the period that the tunnel
is being rerouted, link capacity is under-provisioned on the midpoint
where preemption initiated and potentially one or more links upstream
along the path where other soft preemptions may have occurred.
2. Terminology
This document follows the nomenclature of the MPLS Architecture
defined in [RFC3031].
2.1. Acronyms and Abbreviations
CSPF: Constrained Shortest Path First.
DS: Differentiated Services.
LER: Label Edge Router.
LSR: Label Switching Router.
LSP: Label Switched Path.
MPLS: MultiProtocol Label Switching.
RSVP: Resource ReSerVation Protocol.
TE LSP: Traffic Engineering Label Switched Path.
2.2. Nomenclature
Point of Preemption - the midpoint or ingress LSR which due to RSVP
provisioning levels is forced to either hard preempt or under-
provision and signal soft preemption.
Hard Preemption - The (typically default) preemption process in which
higher numeric priority TE LSPs are intrusively displaced at the
point of preemption by lower numeric priority TE LSPs. In hard
preemption, the TE LSP is torn down before reestablishment.
2.3. Requirements Language
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 [RFC2119].
3. Motivations
Initially, MPLS RSVP-TE [RFC3209] was defined with support for only
one method of TE LSP preemption, which immediately tears down TE
LSPs, disregarding the preempted in-transit traffic. This simple but
abrupt process nearly guarantees preempted traffic will be discarded,
if only briefly, until the RSVP Path Error message reaches and is
processed by the ingress LER and a new data path can be established.
The Error Code and Error Values carried within the RSVP Path Error
message to report a preemption action are documented in [RFC5711].
Note that such preemption is also referred to as a fatal error in
[RFC5711]. In cases of actual resource contention this might be
helpful; however, preemption may be triggered by mere reservation
contention, and reservations may not reflect data-plane contention up
to the moment. The result is that when conditions that promote
preemption exist and hard preemption is the default behavior,
inferior priority preempted traffic may be needlessly discarded when
sufficient bandwidth exists for both the preempted TE LSP and the
preempting TE LSP(s).
Hard preemption may be a requirement to protect numerically lower
preemption priority traffic in a non-Diffserv-enabled architecture,
but in a Diffserv-enabled-architecture, one need not rely exclusively
upon preemption to enforce a preference for the most valued traffic
since the marking and queuing disciplines should already be aligned
for those purposes. Moreover, even in non-Diffserv-aware networks,
depending on the TE LSP sizing rules (imagine all LSPs are sized at
double their observed traffic level), reservation contention may not
accurately reflect the potential for data-plane congestion.
4. RSVP Extensions
4.1. SESSION-ATTRIBUTE Flags
To explicitly signal the desire for a TE LSP to benefit from the soft
preemption mechanism (and thus not to be hard preempted if the soft
preemption mechanism is available), the following flag of the
SESSION-ATTRIBUTE object (for both the C-Type 1 and 7) is defined:
Soft Preemption Desired bit
Bit Flag Name Flag
0x40 Soft Preemption Desired
4.2. Path Error - "Reroute Request Soft Preemption" Error Value
[RFC5710] specifies defines a new reroute-specific error code that
allows a midpoint to report a TE LSP reroute request (Error Code=34 -
Reroute). This document specifies a new Error Value sub-code for the
case of soft preemption.
Error-value Meaning Reference
1 Reroute Request Soft Preemption This document
Upon (soft) preemption, the preempting node MUST issue a PathErr
message with the Error Code=34 ("Reroute") and a value=1 ("Reroute
Request Soft Preemption").
5. Mode of Operation
Let's consider the following example:
R0--1G--R1---155----R2
| \ |
| \ 155
| \ |
155 1G R3
| \ |
| \ 155
| \|
R4----1G----R5
LSP1: LSP2:
R0-->R1 R1<--R2
\ |
V V
R5 R4
Figure 1: Example of Soft Preemption Operation
In the network depicted above in Figure 1, consider the following
conditions:
o Reservable BW on R0-R1, R1-R5, and R4-R5 is 1 Gbit/s.
o Reservable BW on R1-R2, R1-R4, R2-R3, and R3-R5 is 155 Mbit/s.
o Bandwidths and costs are identical in both directions.
o Each circuit has an IGP metric of 10, and the IGP metric is used
by CSPF.
o Two TE tunnels are defined:
* LSP1: 155 Mbit/s, setup/hold priority 0 tunnel, path R0-R1-R5.
* LSP2: 155 Mbit/s, setup/hold priority 7 tunnel, path R2-R1-R4.
Both TE LSPs are signaled with the "Soft Preemption Desired" bit
of their SESSION-ATTRIBUTE object set.
o Circuit R1-R5 fails.
o Soft Preemption is functional.
When the circuit R1-R5 fails, R1 detects the failure and sends an
updated IGP LSA/LSP and Path Error message to all the head-end LSRs
that have a TE LSP traversing the failed link (R0 in the example
above). Either form of notification may arrive at the head-end LSRs
first. Upon receiving the link failure notification, R0 triggers a
TE LSP reroute of LSP1, and re-signals LSP1 along shortest path
available satisfying the TE LSP constraints: R0-R1-R4-R5 path. The
Resv messages for LSP1 travel in the upstream direction (from the
destination to the head-end LSR -- R5 to R0 in this example). LSP2
is soft preempted at R1 as it has a numerically lower priority value,
and both bandwidth reservations cannot be satisfied on the R1-R4
link.
Instead of sending a PathTear message for LSP2 upon preemption as
with hard preemption (which would result in an immediate traffic
disruption for LSP2), R1's local bandwidth accounting for LSP2 is
zeroed, and a PathErr message with error code "Reroute" and a value
"Reroute Request Soft Preemption" for LSP2 is issued.
Upon reception of the PathErr message for LSP2, R2 may update the
working copy of the TE-DB before calculating a new path for the new
LSP. In the case that Diffserv [RFC3270] and TE [RFC3209] are
deployed, receiving a "preemption pending" notification may imply to
a head-end LSR that the available bandwidth for the affected priority
level and numerically greater priority levels has been exhausted for
the indicated node interface. R2 may choose to reduce or zero the
available bandwidth for the implied priority range until more
accurate information is available (i.e., a new IGP TE update is
received). It follows that R2 re-computes a new path and performs a
non-traffic-disruptive rerouting of the new TE LSP T2 by means of the
make-before-break procedure. The old path is then torn down.
6. Elements Of Procedures
6.1. On a Soft Preempting LSR
When a new TE LSP is signaled that requires a set of TE LSP(s) to be
preempted because not all TE LSPs can be accommodated on a specific
interface, a node triggers a preemption action that consists of
selecting the set of TE LSPs that must be preempted so as to free up
some bandwidth in order to satisfy the newly signaled numerically
lower preemption TE LSP.
With hard preemption, when a TE LSP is preempted, the preempting node
sends an RSVP PathErr message that serves as notification of a fatal
action as documented in [RFC5711]. Upon receiving the RSVP PathErr
message, the head-end LSR sends an RSVP PathTear message, that would
result in an immediate traffic disruption for the preempted TE LSP.
By contrast, the mode of operation with soft preemption is as
follows: the preempting node's local bandwidth accounting for the
preempted TE LSP is zeroed and a PathErr with error code "Reroute",
and a error value "Reroute Request Soft Preemption" for that TE LSP
is issued upstream toward the head-end LSR.
If more than one soft preempted TE LSP has the same head-end LSR,
these soft preemption PathErr notification messages may be bundled
together.
The preempting node MUST immediately send a PathErr with error code
"Reroute" and a error value "Reroute Request Soft Preemption" for
each soft preempted TE LSP. The node MAY use the occurrence of soft
preemption to trigger an immediate IGP update or influence the
scheduling of an IGP update.
To guard against a situation where bandwidth under-provisioning will
last forever, a local timer (named the "Soft preemption timer") MUST
be started on the preemption node upon soft preemption. If this
timer expires, the preempting node SHOULD send an RSVP PathTear and
either a ResvTear message or a PathErr with the 'Path_State_Removed'
flag set.
Should a refresh event for a soft preempted TE LSP arrive before the
soft preemption timer expires, the soft preempting node MUST continue
to refresh the TE LSP.
When the MESSAGE-ID extensions defined in [RFC2961] are available and
enabled, PathErr messages with the error code "Reroute" and error
value "Reroute Request Soft Preemption" SHOULD be sent in reliable
mode.
The preempting node MAY preempt TE LSPs that have a numerically
higher Holding priority than the Setup priority of the newly admitted
LSP. Within the same priority, first it SHOULD attempt to preempt
LSPs with the "Soft Preemption Desired" bit of the SESSION ATTRIBUTE
object cleared, i.e., the TE LSPs that are considered as Hard
Preemptable.
Selection of the preempted TE LSP at a preempting midpoint: when a
numerically lower priority TE LSP is signaled that requires the
preemption of a set of numerically higher priority LSPs, the node
where preemption is to occur has to make a decision on the set of TE
LSP(s) that are candidates for preemption. This decision is a local
decision and various algorithms can be used, depending on the
objective (e.g, see [RFC4829]). As already mentioned, soft
preemption causes a temporary link under-provisioning condition while
the soft preempted TE LSPs are rerouted by their respective head-end
LSRs. In order to reduce this under-provisioning exposure, a soft
preempting LSR MAY check first if there exists soft preemptable TE
LSP bandwidth that is flagged by another node but still available for
soft preemption locally. If sufficient overlap bandwidth exists, the
LSR MAY attempt to soft preempt the same TE LSP. This would help
reduce the temporarily elevated under-provisioning ratio on the links
where soft preemption occurs and reduce the number of preempted TE
LSPs. Optionally, a midpoint LSR upstream or downstream from a soft
preempting node MAY choose to flag the TE LSPs in soft preempted
state. In the event a local preemption is needed, the LSPs that are
in the cache and of the relevant priority level are soft preempted
first, followed by the normal soft and hard preemption selection
process for the given priority.
Under specific circumstances such as unacceptable link congestion, a
node MAY decide to hard preempt a TE LSP (by sending a fatal Path
Error message, a PathTear, and either a ResvTear or a Path Error
message with the 'Path_State_Removed' flag set) even if its head-end
LSR explicitly requested soft preemption (by setting the "Soft
Preemption Desired" flag of the corresponding SESSION-ATTRIBUTE
object). Note that such a decision MAY also be made for TE LSPs
under soft preemption state.
6.2. On Head-end LSR of a Soft Preempted TE LSP
Upon reception of a PathErr message with error code "Reroute" and an
error value "Reroute request soft preemption", the head-end LSR MAY
first update the working copy of the TE-DB before computing a new
path (e.g., by running CSPF) for the new LSP. In the case that
Diffserv [RFC3270] and MPLS Traffic Engineering [RFC3209] are
deployed, receiving "preemption pending" may imply to a head-end LSR
that the available bandwidth for the affected priority level and
numerically greater priority levels has been exhausted for the
indicated node interface. A head-end LSR MAY choose to reduce or
zero the available bandwidth for the implied priority range until
more accurate information is available (i.e., a new IGP TE update is
received).
Once a new path has been computed, the soft preempted TE LSP is
rerouted using the non-traffic-disruptive make-before-break
procedure. The amount of time the head-end node avoids using the
node interface identified by the IP address contained in the PathErr
is based on a local decision at the head-end node.
As a result of soft preemption, no traffic will be needlessly black-
holed due to mere reservation contention. If loss is to occur, it
will be due only to an actual traffic congestion scenario and
according to the operator's Diffserv (if Diffserv is deployed) and
queuing scheme.
7. Interoperability
Backward compatibility should be assured as long as the
implementation followed the recommendations set forth in [RFC3209].
As mentioned previously, to guard against a situation where bandwidth
under-provisioning will last forever, a local timer (soft preemption
timer) MUST be started on the preemption node upon soft preemption.
When this timer expires, the soft preempted TE LSP SHOULD be hard
preempted by sending a fatal Path Error message, a PathTear message,
and either a ResvTear message or a PathErr message with the
'Path_State_Removed' flag set. This timer SHOULD be configurable,
and a default value of 30 seconds is RECOMMENDED.
It is RECOMMENDED that configuring the default preemption timer to 0
will cause the implementation to use hard-preemption.
Soft preemption as defined in this document is designed for use in
MPLS RSVP-TE enabled IP networks and may not functionally translate
to some GMPLS technologies. As with backward compatibility, if a
device does not recognize a flag, it should pass the subobject
transparently.
8. Management
Both the point of preemption and the ingress LER SHOULD provide some
form of accounting internally and to the network operator interface
with regard to which TE LSPs and how much capacity is under-
provisioned due to soft preemption. Displays of under-provisioning
are recommended for the following midpoint, ingress, and egress
views:
o Sum of current bandwidth per preemption priority per local
interface
o Sum of current bandwidth total per local interface
o Sum of current bandwidth per local router (ingress, egress,
midpoint)
o List of current LSPs and bandwidth in PPend (preemption pending)
status
o List of current sum bandwidth and session count in PPend status
per observed Explicit Route Object (ERO) hops (ingress and egress
views only).
o Cumulative PPend events per observed ERO hop.
9. IANA Considerations
9.1. New Session Attribute Object Flag
A new flag of the Session Attribute Object has been registered by
IANA.
Soft Preemption Desired bit
Bit Flag Name Reference
0x40 Soft Preemption Desired This document
9.2. New Error Sub-Code Value
[RFC5710] defines a new reroute-specific error code that allows a
midpoint to report a TE LSP reroute request. This document specifies
a new error sub-code value for the case of Soft Preemption.
Error-value Meaning Reference
1 Reroute Request Soft Preemption This document
10. Security Considerations
This document does not introduce new security issues. The security
considerations pertaining to the original RSVP protocol [RFC3209]
remain relevant. Further details about MPLS security considerations
can be found in [SEC_FMWK].
As noted in Section 6.1, soft preemption may result in temporary link
under provisioning condition while the soft preempted TE LSPs are
rerouted by their respective head-end LSRs. Although this is a less
serious condition than false hard preemption, and despite the
mitigation procedures described in Section 6.1, network operators
should be aware of the risk to their network in the case that the
soft preemption processes are subverted, and should apply the
relevant MPLS control plane security techniques to protect against
attacks.
11. Acknowledgements
The authors would like to thank Carol Iturralde, Dave Cooper, Loa
Andersson, Arthi Ayyangar, Ina Minei, George Swallow, Adrian Farrel,
and Mustapha Aissaoui for their valuable comments.
12. Contributors
Denver Maddux
Limelight Networks
USA
EMail: denver@nitrous.net
Curtis Villamizar
AVICI
EMail:curtis@faster-light.net
Amir Birjandi
Juniper Networks
2251 Corporate Park Dr., Ste. 100
Herndon, VA 20171
USA
EMail: abirjandi@juniper.net
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC5710] Berger, L., Papadimitriou, D., and JP. Vasseur, "PathErr
Message Triggered MPLS and GMPLS LSP Reroutes", RFC 5710,
January 2010.
[RFC5711] Vasseur, JP., Swallow, G., and I. Minei, "Node Behavior
upon Originating and Receiving Resource Reservation
Protocol (RSVP) Path Error Messages", RFC 5711, January
2010.
13.2. Informative References
[RFC2961] Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F.,
and S. Molendini, "RSVP Refresh Overhead Reduction
Extensions", RFC 2961, April 2001.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, May 2002.
[RFC4829] de Oliveira, J., Vasseur, JP., Chen, L., and C. Scoglio,
"Label Switched Path (LSP) Preemption Policies for MPLS
Traffic Engineering", RFC 4829, April 2007.
[SEC_FMWK] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", Work in Progress, October 2009.
Authors' Addresses
Matthew R. Meyer (editor)
British Telecom
EMail: matthew.meyer@bt.com
JP Vasseur (editor)
Cisco Systems, Inc.
11, Rue Camille Desmoulins
Issy Les Moulineaux, 92782
France
EMail: jpv@cisco.com
AI ELI5
On this page
PROBLEMIn MPLS RSVP-TE, preemption normally tears down the preempted path at once, so its traffic is likely discarded even when the link could carry it.
MECHANISMA preempting node zeroes its bandwidth accounting for the path and sends a PathErr with Error Code 34 and value 1, so the head-end can reroute make-before-break. A soft preemption timer guards against open-ended under-provisioning.
WHY IT MATTERSTraffic need not be needlessly black-holed by mere reservation contention. The cost is brief under-provisioning of reservations while the path is rerouted.
§1Hard Preemption and the Goal
In an MPLS RSVP-TE network (RFC 3209), hard preemption is the default. A higher-priority path displaces a lower-priority one, and the displaced Traffic Engineering Label Switched Path (TE LSP) is torn down before it is re-established. It is not handled make-before-break, and it can disrupt a potentially large number of TE LSPs.
Without an alternative, operators either accept this or give up features, for example by using only one preemption priority. RFC 5712 adds signaling that warns the ingress router (the head-end) of a pending preemption, so it can reroute the tunnel without disruption while the preempting node is temporarily under-provisioned.
Hard preemption is like being told to leave your seat right now. This RFC lets you be told early, so you can find another seat first.
§3Why Hard Preemption Hurts
Hard preemption immediately tears down LSPs and disregards the traffic in transit. That nearly guarantees preempted traffic is discarded, if only briefly, until the RSVP Path Error message reaches the ingress router and a new data path is built.
Preemption can be triggered by reservation contention alone, and reservations may not reflect actual data-plane contention. The result is that lower-priority traffic can be discarded even when enough bandwidth exists for both paths. The RFC adds that in a Diffserv-enabled network one need not rely exclusively on preemption to protect the most valued traffic, since marking and queuing should already serve that purpose.
Kicking someone out before checking whether there is room for both of you wastes a lot of good seats.
§4RSVP Extensions
Two small protocol additions carry the feature.
First, a new flag in the SESSION-ATTRIBUTE object (for C-Type 1 and 7), Soft Preemption Desired, with value 0x40. A head-end sets it to ask that its LSP be soft preempted rather than hard preempted, when soft preemption is available.
Second, a new error value under the reroute error code defined in RFC 5710. When it soft preempts, the preempting node MUST send a PathErr with Error Code 34 ("Reroute") and value 1 ("Reroute Request Soft Preemption").
A tunnel raises a little flag saying "please warn me first", and the warning is a special note sent back to its owner.
- 0x40Soft Preemption Desired — Bit flag in the SESSION-ATTRIBUTE object (C-Type 1 and 7).
- 34Error Code: Reroute — Reroute error code from RFC 5710.
- 1Reroute Request Soft Preemption — New error value defined by this document.
§5The Worked Example
The RFC uses a six-router network. Two tunnels, LSP1 (R0-R1-R5, priority 0) and LSP2 (R2-R1-R4, priority 7), each reserve 155 Mbit/s and both set the Soft Preemption Desired bit. The R1-R4 link has 155 Mbit/s reservable.
When circuit R1-R5 fails, R1 notifies the head-end of LSP1, R0, which re-signals LSP1 along R0-R1-R4-R5. That cannot fit beside LSP2 on R1-R4, so LSP2 is soft preempted at R1 because it has the numerically higher priority value.
Instead of a PathTear for LSP2, R1 zeroes its local bandwidth accounting for LSP2 and sends a PathErr with "Reroute" and "Reroute Request Soft Preemption". Head-end R2 may update its view of the network, computes a new path, reroutes make-before-break, and then the old path is torn down.
When a road closes and the cars from one route need the space, the car already there is asked to move over after a new lane is ready, not pushed off the road.
R0 (head-end of LSP1)R1 (point of preemption)R2 (head-end of LSP2)
- IGP update and Path ErrorCircuit R1-R5 failed; R0 re-signals LSP1 over R0-R1-R4-R5.
- PathErr: Reroute, Soft PreemptionSent instead of a PathTear for LSP2.
- Compute new path, make-before-breakThe old path is torn down afterwards.
§6What the Preempting Router Does
The preempting node zeroes its local bandwidth accounting for the preempted TE LSP and MUST immediately send the soft-preemption PathErr toward the head-end for each one. Messages for LSPs with the same head-end may be bundled. When RSVP MESSAGE-ID extensions (RFC 2961) are in use, the message SHOULD be sent reliably.
A local soft preemption timer MUST start on soft preemption, so under-provisioning does not last forever. If it expires, the node SHOULD send a PathTear and either a ResvTear or a PathErr with the Path_State_Removed flag. If a refresh arrives before expiry, the node MUST keep refreshing the LSP.
For choosing victims, the node SHOULD first try LSPs with the Soft Preemption Desired bit cleared. Which LSPs to preempt is a local decision, and the RFC points to RFC 4829 for algorithms. A node MAY also hard preempt, even an LSP that asked for soft preemption, under circumstances such as unacceptable link congestion.
The router tells the owner to move, sets a stopwatch, and if the owner takes too long, it finally removes the path.
§6.2What the Head-End Does
On receiving the soft-preemption PathErr, the head-end MAY first update its working copy of the traffic engineering database before computing a new path, for example with CSPF. With Diffserv and MPLS-TE together, the notice may imply that available bandwidth at that priority and numerically greater priorities is exhausted on that interface, and the head-end MAY reduce or zero it until a fresh IGP update arrives.
It then reroutes the LSP with the non-traffic-disruptive make-before-break procedure. How long it avoids the interface named in the PathErr is a local decision. The RFC concludes that no traffic will be needlessly black-holed due to mere reservation contention; any loss would come from actual congestion.
The owner builds the new route first, moves over, and only then closes the old one.
§7Interoperability, Timer and Management
Backward compatibility should hold if implementations followed RFC 3209. The soft preemption timer SHOULD be configurable, and a default of 30 seconds is RECOMMENDED. Configuring it to 0 is RECOMMENDED to make an implementation use hard preemption. The RFC says soft preemption is designed for MPLS RSVP-TE IP networks and may not translate to some GMPLS technologies.
For management, the preempting node and the ingress SHOULD keep accounting of which LSPs and how much capacity are under-provisioned, such as bandwidth per priority per interface, and the list of LSPs in preemption-pending status.
The stopwatch usually runs for thirty seconds, and setting it to zero turns the gentle method off.
§10Security Considerations
The RFC says it introduces no new security issues, and the considerations of RSVP-TE in RFC 3209 still apply. It notes that temporary link under-provisioning is a less serious condition than false hard preemption, but operators should be aware of the risk if soft preemption is subverted and should apply relevant MPLS control-plane security techniques.
If a trickster could fake the warnings, the roads could get crowded, so the control messages need protecting.
§9References
[RFC3031]RFC 3031Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, January 2001.January 2001[RFC3209]RFC 3209Awduche, D., et al., "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209, December 2001.December 2001[RFC5710]RFC 5710Berger, L., Papadimitriou, D., and JP. Vasseur, "PathErr Message Triggered MPLS and GMPLS LSP Reroutes", RFC 5710, January 2010.January 2010[RFC5711]RFC 5711Vasseur, JP., Swallow, G., and I. Minei, "Node Behavior upon Originating and Receiving Resource Reservation Protocol (RSVP) Path Error Messages", RFC 5711, January 2010.January 2010[RFC2961]RFC 2961Berger, L., et al., "RSVP Refresh Overhead Reduction Extensions", RFC 2961, April 2001.April 2001[RFC4829]RFC 4829de Oliveira, J., et al., "Label Switched Path (LSP) Preemption Policies for MPLS Traffic Engineering", RFC 4829, April 2007.April 2007