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draft-meyer-mpls-soft-preemption-00.txt                  February, 2003

                                                        Matthew R. Meyer
                                                         Global Crossing
                                                           Denver Maddux
                                                         Global Crossing
                                                   Jean-Philippe Vasseur
                                                     Cisco Systems, Inc.

IETF Internet Draft
Expires: August, 2003
February, 2003


                MPLS Traffic Engineering Soft preemption

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026. Internet-Drafts are
Working documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups.  Note that other groups may also
distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
The list of Internet-Draft Shadow Directories can be accessed at

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draft-meyer-mpls-soft-preemption-00.txt                  February, 2003


This draft documents MPLS TE Soft Preemption, a suite of protocol
modifications extending the current concept of preemption with the goal
of reducing/eliminating traffic disruption of preempted TE LSPs.  Under
present RSVP-TE signaling methods, LSPs are immediately displaced upon
preemption.  The introduction of a new preemption pending flag helps
more gracefully mitigate the re-route process of displaced LSPs.  For
the brief period soft preemption is activated, reservations (though not
necessarily traffic levels) are in effect overbooked until the LSP can
be re-routed.  For this reason, the feature is primarily interesting in
packet oriented MPLS networks with Diffserv and TE capabilities.

1. Terminology

LSR - Label Switch Router

HE LSR - Head-End Label Switch Router

LSP - An MPLS Label Switched Path

TE LSP - Traffic Engineering Label Switched Path

Local Repair - Techniques used to repair TE LSP tunnels quickly
when a node or link along the TE LSPs path fails.

Preemption Pending flag - This flag is set on an IPv4 or Ipv6 RSVP Resv
RRO sub-object  to signal to the TE LSP head-end LSR that the TE LSP is
about to be preempted and must be re-signaled (in a non disruptive
fashion, with make before break) along another path.

PLR - Point of Local Repair. The head-end of a backup tunnel or a
detour LSP.

PHB - Per Hop Behavior

PHP - Penultimate Hop Popping

CSPF - Constraint-based Shortest Path First.

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2. Motivations

Present MPLS RSVP-TE implementations only support a method of TE LSP
preemption which immediately tears down TE LSPs, disregarding the
preempted in-transit traffic, in an effort to make way for a higher
priority TE LSP if not enough bandwidth is available on the link to
accommodate the newly signaled high priority TE LSPs.  This process
nearly guarantees preempted traffic will be discarded, if only briefly,
until the RSVP Path Error message reaches and is processed by the
head-end (HE) and a new forwarding path can be established.  In cases
of actual resource contention this might be helpful, however preemption
is triggered by mere reservation contention and reservations may not be
entirely accurate up to the moment.  The result is that the traffic is
often needlessly being discarded.

The current intrusive or 'hard' preemption may be a requirement to
protect traffic in a network without Diffserv, 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.

3. Introduction

In an MPLS RSVP-TE and Diffserv enabled network there are currently no
defined mechanisms to allow preempted TE LSPs to be handled in a
make- before-break fashion: the currently defined preemption scheme
proposes a very intrusive method that provokes traffic disruption for
potentially a large amount of TE LSPs. Typically, this makes TE LSP
dynamic resizing mechanisms less palatable when high network stability
is sought. This draft proposes the use of additional signaling and
accounting mechanisms to alert the HE LSR of the preemption that is
pending and allow for temporary overbooking while the tunnel is
re- routed in a non disruptive fashion (make-before-break) by the HE
LSR. During the period that the tunnel is being re-routed, link capacity
is effectively overbooked on links where soft preemption has occurred.

4. RSVP extensions


To explicitly signal the desire for a TE LSP to benefit from the soft
preemption mechanism (and so not to be 'hard' preempted), the following
new flag of the SESSION-ATTRIBUTE object (for both the C-Type 1 and 7)
is defined:

Soft preempted desired:  0x40

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4.2. RRO IPv4/IPv6 Sub-Object Flags

To report that a soft preemption is pending for an LSP, a new flag is
needed for the RSVP Resv RRO object message defined in RFC3209.  The
RRO is augmented with a preemption pending (PPend) flag. Any LSR
compliant with this draft must support the RRO object, as defined in
RFC 3209.

RRO IPv4 and IPv6 sub-object address

These two sub-objects currently have the following flags defined in RFC
3209 and [FAST-REROUTE]:

  Local protection available: 0x01
        Indicates that the link downstream of this node is protected
        via a local repair mechanism, which can be either one-to-one or
        facility backup.

  Local protection in use: 0x02
        Indicates that a local repair mechanism is in use to maintain
        this tunnel (usually in the face of an outage of the link it
        was previously routed over, or an outage of the neighboring

  Bandwidth protection: 0x04
        The PLR will set this when the protected LSP has a backup path
        which is guaranteed to provide the desired bandwidth specified
        in the FAST_REROUTE object or the bandwidth of the protected
        LSP, if no FAST_REROUTE object was included. The PLR may set
        this whenever the desired bandwidth is guaranteed; the PLR MUST
        set this flag when the desired bandwidth is guaranteed and the
        "bandwidth protection desired" flag was set in the
        SESSION_ATTRIBUTE object. If the requested bandwidth is not
        guaranteed, the PLR MUST NOT set this flag.

  Node protection: 0x08
        The PLR will set this when the protected LSP has a backup path
        which provides protection against a failure of the next LSR
        along the protected LSP. The PLR may set this whenever node
        protection is provided by the protected LSP's backup path; the
        PLR MUST set this flag when the node protection is provided and
        the "node protection desired" flag was set in the
        SESSION_ATTRIBUTE object. If node protection is not provided,
        the PLR MUST NOT set this flag. Thus, if a PLR could only setup
        a link-protection backup path, the "Local protection available"
        bit will be set but the "Node protection" bit will
        be cleared.

A new flag is added:

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  Preemption pending: 0x10
        The preempting node sets this flag if a pending preemption is
        in progress for the TE LSP. This indicates to the HE of this
        LSP that it must be re-routed as soon as possible using a make
        before break.

5. Mode of operation

       | \   |
       |  \  R3
       |   \ |
       |    \|

Fig 1.

In the network depicted above in figure 1, let suppose:

-Reservable BW on R0-R1, R1-R5 and R4-R5 is 1Gb/sec
-Reservable BW on R1-R2, R1-R4, R2-R3, R3-R5 is 155 Mb/sec.
Bandwidths are identical in both directions
-Each circuit has an IGP metric of 10 and IGP metric is used by CSPF
-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' bit of their
-Circuit R1-R5 fails.
-Soft Preemption is functional.

When the circuit R1-R5 fails, R1 detects the failure and sends a Path
Error message to all the Head-end having a TE LSP traversing the R1-R5
failed link (R0 is the example above). Upon receiving the link failure
notification (RSVP Path Error and/or IGP LSA/LSP update), R0 triggers a
TE LSP re-route 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
penultimate hop to the HE LSR - R4 to R1 in this example).
LSP2 is soft preempted at R1 as it has a higher preemption (lower
priority) and both bandwidth reservations cannot be satisfied on the
R1-R4 link.

Instead of sending a path tear for LSP2 upon preemption as with the
current preemption (which would result in an immediate traffic
disruption for LSP2), R1s local BW accounting for LSP2 is zeroed and a
preemption pending flagged RRO Resv for LSP2 is issued upstream toward
the HE LSR, R2. If more than one preempted candidate TE LSP has the
same HE, these soft preemption Resv messages MAY be bundled together
(see RFC2961)

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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 choose for soft preemption to impact the pacing of IGP
update and re-flood of the TE-TLV. An implementation MAY provide user
configurable exponential back off timers if preemption paced IGP
triggering is used.

Should a refresh event for LSP2 arrive before LSP2 is re-routed, soft
preempting nodes such as R1 MUST continue to refresh the LSP however
the RRO Soft Pending flag MUST be set.  This assures that if the
initial soft preemption Resv message is somehow dropped, the HE will
still receive notification. Resv messages with the RRO 'Preemption
pending' flag set should be sent in reliable mode (RFC 2961).

Upon reception of the Resv with the 'Preemption pending' flag set, the
HE (of LSP2 in this case) MAY update the working copy of the TE-DB
before running CSPF for the new LSP.  The preemption pending implies
exhausted bandwidth in the affected priority level and greater for the
indicated node interface.  An implementation MAY choose to reduce or
zero available BW for that range until more accurate information is
available (i.e. a new IGP TE update is received).

In the case that reservation availability is restored at the point of
preemption (R1) the point of preemption MAY issue a Resv message with
the 'Preemption pending' flag unset to signal restoral to the HE.  This
implies that a HE might have delayed or been unsuccessful in

After the HE has successfully established a new LSP, the old path MUST
be torn down.

As a result of this '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
(if deployed) according to the operators Diffserv and queuing scheme.

6. Selection of the preempted TE LSP at a preempting mid-point

When a lower preemption (higher priority) TE LSP is signaled that
requires the preemption of a set of lower priority the TE LSPs in order
to accommodate the newly signaled high priority TE LSP, the node has to
make a decision on the set of TE LSP, candidate for preemption. This
decision is a local decision and various algorithms can be used,
depending on the objective (minimize the number of preempted LSPs,

As already mentioned, a temporary link overbooking results from the
soft preemption, until all the soft preempted TE LSPs are effectively
re-routed by their respective HE LSR. In order to reduce this
overbooking period of time, the preempting LSR can limit the number of
soft preempted TE LSP to the TE LSP that have explicitly requested soft

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preemption via signaling, setting their 'Soft Preemption desired' bit
in the SESSION-ATTRIBUTE of their RSVP Path messages. This way, the
preemption could apply the current 'hard' preemption scheme to the TE
LSPs that have not explicitly requested soft preemption, sending a Path
Error message to their HE LSR and immediately removing the
corresponding local states. This would help reducing the overbooking
ratio on the related links. This TE LSP capability (Soft preemption
desired) could be reserved to the TE LSP, for which a traffic
disruption upon preemption is not unacceptable.

Optionally, a midpoint LSR upstream from a soft preempting node MAY
choose to cache the soft preempted LSPs downstream 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 preemption
selection process for the given priority.

7. Interoperability

Backward compatibility is assured since any HE LSR not compliant with
this draft that receives a Resv message with the RRO 'Preemption
Pending' bit set will simply ignore the flag and treat the Resv message
as a regular Resv refresh message. As a consequence, the soft preempted
TE LSP will not be re-routed with make before break by the HE LSR. To
guard against a situation where bandwidth overbooking will last
forever, a local timer (soft preemption expiration timer) MUST be
started on the preemption node, upon soft preemption. When this timer
expires, the soft preempted TE LSP will be torn down and the preempting
node will send a Path Error. This timer should be configurable. The
current 'hard' preemption scheme can be emulated with a soft preemption
expiration timer set to zero.

8. Management

Both the point of preemption and the HE LSR should provide some form
of accounting internally and to the user with regard to what TE LSPs
and how much capacity is over-booked due to soft preemption.

9. Security Considerations

The practice described in this draft does not raise specific security
issues beyond those of existing TE.

10. Acknowledgment

The authors would like to thank Carol Iturralde, Dave Cooper for
their valuable comments.

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11. Intellectual Property

The contributor represents that he has disclosed the existence of any
proprietary or intellectual property rights in the contribution that
are reasonably and personally known to the contributor.  The
contributor does not represent that he personally knows of all
potentially pertinent proprietary and intellectual property rights
owned or claimed by the organization he represents (if any) or third


[TE-REQ] Awduche et al, Requirements for Traffic Engineering over MPLS,
RFC2702, September 1999.

[OSPF-TE] Katz, Yeung, Traffic Engineering Extensions to OSPF, draft-
katz-yeung-ospf-traffic-09.txt, October 2002.

[ISIS-TE] Smit, Li, IS-IS extensions for Traffic Engineering, draft-
ietf-isis-traffic-04.txt, December 2002.

[RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP Tunnels",
RFC3209, December 2001.

[DS-TE] Le Faucheur et al, "Requirements for support of Diff-Serv-aware
MPLS Traffic Engineering", draft-ietf-tewg-diff-te-reqts-06.txt,
September  2002.

[DS-TE-PROT] Le Faucheur et al, "Protocol extensions for support of
Diff-Serv-aware MPLS Traffic Engineering", draft-ietf-tewg-diff-te-
proto-02.txt, October 2002

[FAST-REROUTE] Pan, P. et al., "Fast Reroute Techniques in
RSVP-TE", Internet Draft, draft-ietf-mpls-rsvp-lsp-fastreroute-01.txt
, May, 2003

[REFRESH-REDUCTION] Berger et al, "RSVP Refresh Overhead Reduction
Extensions", RFC 2961, April 2001.

Matthew R. Meyer
Global Crossing
14605 S. 50th
Phoenix, AZ 85044
email: mrm@gblx.net

Denver Maddux
Global Crossing
14605 S. 50th
Phoenix, AZ 85044

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draft-meyer-mpls-soft-preemption-00.txt                  February, 2003

email: denver@gblx.net

Jean Philippe Vasseur
Cisco Systems, Inc.
300 Apollo Drive
Chelmsford, MA 01824
Email: jpv@cisco.com

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