Networking Working Group Matthew. Meyer, Ed.
Internet-Draft British Telecom
Intended status: Standards Track JP. Vasseur, Ed.
Expires: August 21, 2008 Cisco Systems, Inc
February 18, 2008
MPLS Traffic Engineering Soft Preemption
draft-ietf-mpls-soft-preemption-10.txt
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This document details Multiprotocol Label Switching (MPLS) Traffic
Engineering Soft Preemption, a suite of protocol modifications
extending the concept of preemption with the goal of reducing/
eliminating traffic disruption of preempted Traffic Engineering Label
Switched Paths (TE LSPs). Initially MPLS RSVP-TE was defined
supporting only immediate TE LSP displacement upon preemption. The
utilization of a preemption pending flag helps more gracefully
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mitigate the re-route 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 re-routed. For this reason, the feature is
primarily but not exclusively interesting in MPLS enabled IP networks
with Differentiated Services and Traffic Engineering capabilities.
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].
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Acronyms and Abbreviations . . . . . . . . . . . . . . . . 3
1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. RSVP Extensions . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. SESSION-ATTRIBUTE Flags . . . . . . . . . . . . . . . . . 5
4.2. RRO IPv4/IPv6 Sub-Object Flags . . . . . . . . . . . . . . 5
4.3. Use of the RRO IPv4/IPv6 Sub-Object in Path message . . . 5
5. Mode of Operation . . . . . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . . . . . . . 9
8. Management . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9.1. New Session Attribute Object Flag . . . . . . . . . . . . 11
9.2. New Flag of the RRO IPv4/IPv6 Subobject . . . . . . . . . 11
10. Security Considerations . . . . . . . . . . . . . . . . . . . 11
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
12. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
13.1. Normative References . . . . . . . . . . . . . . . . . . . 12
13.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Terminology
This document follows the nomenclature of the MPLS Architecture
defined in [RFC3031].
1.1. Acronyms and Abbreviations
CSPF Constraint-based Shortest Path First.
DS Differentiated Services.
LER Label Edge Router.
LSR Label Switching Router.
LSP Label Switched Path.
MPLS MultiProtocol Label Switching.
PPend Preemption Pending.
RSVP Resource ReSerVation Protocol.
TE Traffic Engineering TE LSP Traffic Engineering Label Switched
Path.
1.2. 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. Motivations
Initially Multiprotocol Label Switching (MPLS) RSVP-TE [RFC3209] was
defined supporting 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
forwarding path can be established. The Error Code and Error Values
carried within the RSVP Path Error message are documented in
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[I-D.ietf-mpls-3209-patherr]. Note that such preemption is also
referred to as a fatal error in [I-D.ietf-mpls-3209-patherr]. In
cases of actual resource contention this might be helpful, however
preemption may be triggered by mere reservation contention and
reservations may not reflect forwarding 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 Traffic Engineering Labeled
Switched Path (TE LSP) and the preempting TE LSP(s).
Hard preemption may be a requirement to protect numerically lower
preemption priority traffic in a non Diff-Serv enabled architecture,
but in a Diff-Serv 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 Diff-
Serv 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
forwarding plane congestion.
3. Introduction
In an MPLS RSVP-TE (see [RFC3209]) enabled IP network, hard
preemption is the default behavior. Hard preemption provides no
mechanism to allow preempted 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 ingress LER automated
TE reservation adjustments are less palatable when preemption is
intrusive and high network stability levels are 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 re-routed in a non disruptive fashion (make-
before-break) by the ingress LER. During the period that the tunnel
is being re-routed, 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. Optionally the downstream path to the egress LER may be
signaled as well to more efficiently deal with any near simultaneous
soft preemptions that may have been triggered downstream of the
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initial preemption.
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 so 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. RRO IPv4/IPv6 Sub-Object Flags
To report that a soft preemption is pending for an LSP, a new flag is
defined in the IPv4/IPv6 sub-object carried in the RRO object message
defined in [RFC3209]. This flag is called the preemption pending
(PPend) flag.
Several flags in the RRO IPv4 and IPv6 sub-object have been defined
in [RFC3209] and [RFC4090] : This documents defines a new flag for
the use of soft preemption named the 'Preemption pending' flag and
defined as below:
Bit Flag Name Flag Reference
0x10 Preemption pending This document
The preempting node sets this flag if a pending preemption is in
progress for the TE LSP. This indicates to the ingress LER of this
LSP that it SHOULD be re-routed.
4.3. Use of the RRO IPv4/IPv6 Sub-Object in Path message
An LSR MAY use the Preemption pending flag in the IPv4/IPv6 RRO
subobject carried in a PATH RRO message to simultaneously alert
downstream LSRs that the LSP was soft preempted upstream. This
information could be used by the downstream LSR to bias future soft
preemption candidates toward LSPs already soft preempted elsewhere in
their path.
5. Mode of Operation
Let's consider the following example:
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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 1Gb/sec.
o Reservable BW on R1-R2, R1-R4, R2-R3, R3-R5 is 155 Mb/sec.
o Bandwidths and costs are identical in both directions.
o Each circuit has an IGP metric of 10 and IGP metric is used by
CSPF.
o Two TE tunnels are defined: - LSP1: 155 Mb, setup/hold priority 0
tunnel, path R0-R1-R5. - LSP2: 155 Mb, 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
having 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
re-route of LSP1, and re-signals LSP1 along shortest path available
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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 path tear for LSP2 upon preemption as with hard
preemption (which would result in an immediate traffic disruption for
LSP2), R1s local bandwidth accounting for LSP2 is zeroed and a
preemption pending flagged Resv RRO for LSP2 is issued. Optionally,
R1 MAY simultaneously send a soft preemption flagged Path RRO
notifying downstream LSRs of LSP2's soft preemption.
Upon reception of the LSP2's Resv message with the preemption pending
flag set, R2 may update the working copy of the TE-DB before running
calculating a new path for the new LSP. In the case that Diff-Serv
[RFC3270] and TE [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. R2 may choose to
reduce or zero 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 which requires to preempt a set of TE
LSP(s) because not all TE LSPs can be accommodated on a specific
interface, a node triggers a preemption action which 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.
For each preempted TE LSP, instead of sending an RSVP Path Tear
message after the receipt of an RSVP message notifiying a fatal
action as documented in [I-D.ietf-mpls-3209-patherr] upon preemption
as with hard preemption (which would result in an immediate traffic
disruption for the preempted TE LSP), the preempting node's local
bandwidth accounting for the preempted TE LSP is zeroed and a
preemption pending flagged Resv RRO for that TE LSP is issued
upstream toward the head-end LSR.
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Optionally, the preempting node MAY simultaneously set the RRO
"Preemption pending" flag in the RSVP Path message notifying
downstream LSRs of soft preemption. If more than one soft preempted
TE LSP has the same head-end LSR, these soft preemption Resv (Path)
notification messages may be bundled together.
The preempting node MUST immediately send a Resv message with the
preemption pending RRO flag set 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.
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,
Resv messages with the RRO preemption pending flag set SHOULD be sent
in reliable mode.
In the case that reservation availability is restored at the point of
preemption, the point of preemption MAY issue a Resv message with the
preemption pending flag cleared to signal restoration to the head-end
LSR. This implies that a head-end LSR might have delayed or been
unsuccessful in re-signaling.
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.
Selection of the preempted TE LSP at a preempting mid-point: 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), 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 flagged
PPend 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 reducing the
temporarily elevated under-provisioning ratio on the links where soft
preemption occurs and the number of preempted TE LSPs. Optionally, a
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midpoint LSR upstream or downstream from a soft preempting node MAY
choose to flag the TE LSPs soft preempted state. In the event a
local preemption is needed, the relevant priority level LSPs from the
cache 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' ('Soft Preemption desired'
flag of the corresponding SESSION- ATTRIBUTE object set). Note that
such decision MAY also be taken for TE LSPs under soft preemption
state.
6.2. On Head-end LSR of a Soft Preempted TE LSP
Upon reception of an RSVP Resv message with the preemption pending
flag set, 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 Diff-Serv [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 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.
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 operators Diff-Serv (if Diff-Serv is deployed) and
queuing scheme.
7. Interoperability
Backward compatibility should be assured as long as the
implementation followed the recommendations set forth in [RFC3209].
When processing an RRO, unrecognized sub-objects SHOULD be ignored
and passed on. An LSR without soft preemption capabilities but that
followed the aforementioned recommendation will simply ignore the RRO
Preemption Pending flag and treat the Resv message as a regular Resv
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refresh message. As a consequence, the soft preempted TE LSP will
not be rerouted with make before break by the head-end LSR.
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 total local router (ingress, egress,
midpoint)
o List current LSPs and bandwidth in PPend status
o List current sum bandwidth and session count in PPend status per
observed ERO hops (ingress, egress views only).
o Cumulative PPend events per observed ERO hops.
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9. IANA Considerations
IANA will not need to create a new registry.
9.1. New Session Attribute Object Flag
A new flag of the Sessin Attribute object is defined (to be confirmed
by IANA)
Soft Preemption Desired bit
Bit Flag Name Flag
0x40 Soft Preemption Desired
9.2. New Flag of the RRO IPv4/IPv6 Subobject
A new flag of the RRO IPv4/IPv6 subobject is defined in this
document.
Bit Flag Name Flag Reference
0x10 Preemption pending 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.
11. Acknowledgements
The authors would like to thank Carol Iturralde, Dave Cooper, Loa
Andersson, Arthi Ayyangar, Ina Minei, George Swallow and Adrian
Farrel for their valuable comments.
12. Authors' Addresses
The content of this document was contributed by the editors and the
co-authors listed below:
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Denver Maddux
Limelight Networks
USA
email: denver@nitrous.net
Curtis Villamizar
AVICI
curtis@faster-light.net
Amir Birjandi
Juniper Networks
2251 corporate park dr ste
herndon, VA 20171
USA
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.
13.2. Informative References
[I-D.ietf-mpls-3209-patherr]
Vasseur, J., Swallow, G., Farrel, A., and I. Minei, "Node
behavior upon originating and receiving Resource
ReserVation Protocol (RSVP) Path Error message",
draft-ietf-mpls-3209-patherr-01 (work in progress),
February 2008.
[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.
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[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
May 2005.
[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.
Authors' Addresses
Matthew R. Meyer (editor)
British Telecom
Email: mrminc@gmail.com
JP Vasseur (editor)
Cisco Systems, Inc
1414 Massachusetts Avenue
Boxborough, MA 01719
USA
Email: jpv@cisco.com
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