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Versions: (draft-meyer-mpls-soft-preemption) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 5712

Networking Working Group                             Matthew. Meyer, Ed.
Internet-Draft                                           British Telecom
Intended status: Standards Track                        JP. Vasseur, Ed.
Expires: January 30, 2010                             Cisco Systems, Inc
                                                           July 29, 2009


                MPLS Traffic Engineering Soft Preemption
                 draft-ietf-mpls-soft-preemption-18.txt

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   Copyright (c) 2009 IETF Trust and the persons identified as the
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Abstract

   This document specifies Multiprotocol Label Switching (MPLS) Traffic
   Engineering Soft Preemption, a suite of protocol modifications



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   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 reroute request notification helps more gracefully
   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].


































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Acronyms and Abbreviations . . . . . . . . . . . . . . . .  4
     2.2.  Nomenclature . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Motivations  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  RSVP Extensions  . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  SESSION-ATTRIBUTE Flags  . . . . . . . . . . . . . . . . .  6
     4.2.  Path Error - "Reroute request Soft Preemption" Error
           Value  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Mode of Operation  . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Elements Of Procedures . . . . . . . . . . . . . . . . . . . .  8
     6.1.  On a Soft Preempting LSR . . . . . . . . . . . . . . . . .  8
     6.2.  On Head-end LSR of a Soft Preempted TE LSP . . . . . . . . 10
   7.  Interoperability . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Management . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     9.1.  New Session Attribute Object Flag  . . . . . . . . . . . . 12
     9.2.  New error sub-code value . . . . . . . . . . . . . . . . . 12
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   12. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 13
     13.2. Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
























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1.  Introduction

   In an 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 ingress (Label Edge Router)
   LER automated (Traffic Engineering (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.


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.



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   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.


3.  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 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 [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 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 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
   data plane congestion.





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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.  Path Error - "Reroute request Soft Preemption" Error Value

   [I-D.ietf-mpls-gmpls-lsp-reroute] specifies defines a new reroute-
   specific error code that allows a mid-point to report a TE LSP
   reroute request (Error-code=34 - Reroute).  This document specifies a
   new error sub-code value for the case of Soft Preemption (to be
   confirmed by IANA upon publication of this document).

   Error-value               Meaning                    Reference
     1            Reroute Request Soft Preemption     This document

   Upon (soft) preemption, the preemting node MUST issue a PathErr
   message with the error code=34 ("Reroute") and a value=1 ("Reroute
   request soft preemption"), to be confirmed by IANA.


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
   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 running calculating a new path for
   the new LSP.  In the case that Diff-Serv [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 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.

   With hard preemption, when a TE LSP is preempted, the preempting node
   sends an RSVP PathErr message notifiying a fatal action as documented
   in [I-D.ietf-mpls-3209-patherr].  Upon receiving the RSVP PathErr
   message, the head-end LSR sends an RSVP Path Tear message, which
   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.




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   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 error code "Reroute" and an error
   value "Reroute request soft preemption" SHOULD be sent in reliable
   mode.

   The preempting node MAY preempt TE LSPs which have a numerically
   higher Holding priority than the Setup priority of the newly admitted
   LSP.  Within the same priority, it SHOULD attempt to pre-empt LSPs
   with the "Soft Preemption Desired" bit of the SESSION ATTRIBUTE
   object cleared, i.e., TE LSP considered as Hard Preemptable, first.

   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
   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 midpoint



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   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 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 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.  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 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 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].

   As mentioned previously, to guard against a situation where bandwidth



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   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.


9.  IANA Considerations

   IANA will not need to create a new registry.






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9.1.  New Session Attribute Object Flag

   A new flag of the Session Attribute object is defined (to be
   confirmed by IANA)

   Soft Preemption Desired bit
   Bit Flag       Name Flag                      Reference
     0x40    Soft Preemption Desired             This document

9.2.  New error sub-code value

   [I-D.ietf-mpls-gmpls-lsp-reroute] defines a new reroute-specific
   error code that allows a mid-point to report a TE LSP reroute
   request.  This document specifies a new error sub-code value for the
   case of Soft Preemption (to be confirmed by IANA upon publication of
   this document).

   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.


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.  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

   [I-D.ietf-mpls-3209-patherr]
              Vasseur, J., Swallow, G., and I. Minei, "Node behavior
              upon originating and receiving Resource ReserVation
              Protocol  (RSVP) Path Error message",
              draft-ietf-mpls-3209-patherr-04 (work in progress),
              February 2009.

   [I-D.ietf-mpls-gmpls-lsp-reroute]
              Berger, L., Papadimitriou, D., and J. Vasseur, "PathErr
              Message Triggered MPLS and GMPLS LSP Reroute",
              draft-ietf-mpls-gmpls-lsp-reroute-04 (work in progress),
              January 2009.

   [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

   [RFC2961]  Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F.,
              and S. Molendini, "RSVP Refresh Overhead Reduction



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              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.


Authors' Addresses

   Matthew R. Meyer (editor)
   British Telecom
   matthew.meyer@bt.com

   Email:


   JP Vasseur (editor)
   Cisco Systems, Inc
   11, Rue Camille Desmoulins
   Issy Les Moulineaux,   92782
   France

   Email: jpv@cisco.com























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