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MPLS FRR bypass path calculation with bandwidth awareness without reservation

Document Type Active Internet-Draft (individual)
Authors Rafal Jan Szarecki , Jon Mitchell
Last updated 2024-01-23
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Traffic Engineering Architecture and Signaling             R.J. Szarecki
Internet-Draft                                               J. Mitchell
Intended status: Informational                                Google LLC
Expires: 26 July 2024                                    23 January 2024

   MPLS FRR bypass path calculation with bandwidth awareness without


   RFC4090 documents facility backup FRR in MPLS-TE networks.  This
   document describes methods that allow the Point of Local Repair (PLR)
   to find a path with sufficient available bandwidth to accommodate
   protected traffic, while not making undesired reservations that would
   require additional capacity.  Below aspects are covered:

   *  Automatic determination of bypass required bandwidth by the PLR

   *  Calculation of path based on this information using constrained
      shortest path algorithm (CSPF)

   *  Determination of RSVP SENDER-TSPEC rate value for bypass tunnel

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 26 July 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   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 Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Finding of bypass required bandwidth  . . . . . . . . . . . .   3
     2.1.  Static Protected Bandwidth  . . . . . . . . . . . . . . .   3
     2.2.  Dynamically Computed Protected Bandwidth  . . . . . . . .   4
     2.3.  Hybrid Approach . . . . . . . . . . . . . . . . . . . . .   4
   3.  Bypass path computation . . . . . . . . . . . . . . . . . . .   5
   4.  Bypass tunnel signaling . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Network operators use MPLS-TE technology, as described in [RFC3209],
   to optimize network resources (bandwidth over network graph) while
   providing prioritized treatment - high bandwidth and low loss.  This
   is often done by combination of bandwidth reservation for LSPs and
   protection (of some more critical) of them by facility backup
   [RFC4090] technique.  The facility backup technique uses a pre-
   signaled bypass tunnel (instantiated as MPLS LSP) to temporally carry
   impacted LSP around a faulty resource, such as a link or node,
   immediately after failure.  In many networks, operators have not
   configured the LSPs to have bandwidth protection desired, and
   therefore the bypass tunnels are provisioned without any matching
   bandwidth reservation constraint, to prevent the booking of bandwidth
   for these tunnels which only carry traffic briefly during failure
   events until global convergence.  Without a bandwidth reservation
   constraint that matches the protected LSPs required bandwidth, the
   bypass tunnels are typically routed on the shortest path between the
   Point of Local Repair (PLR) and Merge Point (MP) based on a CSPF
   satisfying any other constraints such as shared risk link groups

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   (SRLG).  This practice has the limitation that the bypass tunnel
   could be routed over interfaces that have available bandwidth much
   lower than protected resource's reservations.  This limitation is
   specifically common in networks that utilize IGP metric values based
   on distance or latency rather than bandwidth.  Such networks may have
   a large difference between the highest and lowest capacity of
   adjacencies between various locations.

   This document provides procedures that the headend routing node of
   the bypass tunnel (PLR) can utilize to optimize bypass tunnel path
   computation, so it is routed over links in a network that are more
   likely to have sufficient available bandwidth, while still not
   utilizing bandwidth reservations.  Procedures described are local to
   PLR, hence do not impose any special requirements on other nodes in
   the network, and could be deployed incrementally.  These procedures
   could be deployed whether bypass tunnel's Merge Point (MP) is
   explicitly provisioned via local configuration or when the PLR
   automatically determines the necessary MP form Traffic Engineering
   Database (TED) and inspection of RRO of protected LSPs.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Finding of bypass required bandwidth

   In order for the PLR to perform bypass path computation that has
   sufficient available bandwidth, the volume of traffic expected to be
   protected by a given bypass tunnel needs to be determined even if it
   will not be utilized in RSVP signaling.  In this document the
   terminology protected bandwidth (PBW) refers to this value.

2.1.  Static Protected Bandwidth

   The PBW for the bypass tunnel can be computed by an offline tool and
   configured on the PLR either on per bypass tunnel basis, globally or
   on per protected resource basis.  The accuracy of utilization of this
   method is limited by the offline modeling and the current network
   state, so it may not be as reactive as the other approach this
   document describes.  The exact procedure on how the offline tool
   calculates PBW is out of scope of this document.

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2.2.  Dynamically Computed Protected Bandwidth

   Since the PLR is aware of the bandwidth reservations by the LSPs per
   protected resource, such as a link or node, the PLR can compute
   protected bandwidth per bypass tunnel to match the sum of these
   protected LSPs bandwidth reservations, and can update the required
   bandwidth utilized for CSPF on a bypass tunnel on regular intervals.
   By setting the interval by which the value is reset, referred to as
   the PBW timer, the operator can balance accuracy versus computational
   resources and churn included by making this calculation.  To reduce
   the error rate from a single measurement between intervals, multiple
   samples of the aggregate value of PBW can be stored and only the
   largest sample over several sample periods utilized for the next PBW
   timer induced CSPF.

   Optionally, a percentage scaled value of the top sample could be
   applied to optimize the trade off between how much of the sum of
   protected LSPs bandwidth reservations the PBW should account for.
   For instance, a Implementation should allow the application of a
   scaling factor in the range from 0% to at least 200% of PBW to derive
   the value, however the default value should be 100%.

   Implementations may apply some logic to prevent negligible changes
   requiring churn induced by re-routing of the tunnel by comparing the
   value to be utilized versus the previous PBW timer initiated value if
   that value is retained by the implementation.  The periodic
   computation interval is expected to be shorter or equal to
   optimization timer LSPs implementing bypass tunnel.

   This overall approach and implementation of this technology is
   conceptually very similar to many existing vendor implementations of
   the "auto-bandwidth" feature, a generalized summary of which is
   described in Section 4 of [RFC8733].  Because the number of bypass
   tunnels in many networks is relatively low compared to protected
   LSPs, the cost of re-computation and associated churn is likely to be
   relatively low in comparison to those incurred if the network
   utilizes auto-bandwidth for their protected LSPs.

2.3.  Hybrid Approach

   The initial PBW value utilized for a bypass tunnel using this
   approach even when using the dynamically computed PBW maybe incorrect
   for a long period of time after initial bypass tunnel creation if the
   PBW timer period is long as the number of protected LSPs over a newly
   created bypass tunnel will likely rapidly increase in a number of
   network events such as when new protected resource, such as a new
   interface, becomes operational.  Initially there will be just very
   few LSP (or just one) LSP that require protection, hence PBW may be

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   initially very low.  Implementations SHOULD allow for configuration
   of minimum PBW value to be used when a bypass tunnel is initialized
   and in place of the dynamically calculated PBW if the dynamically
   calculated PBW is lower than the minimum PBW configured.

3.  Bypass path computation

   The network path for the bypass tunnel is computed using the same
   logic as described in Section 6.2 of [RFC4090].  There are four
   classes of events that trigger the computation of a bypass path:

   1.  Expiration of existing bypass lsp reoptimization timer

   2.  Receipt of PathErr due to network failure along existing bypass
       path or failure of existing bypass egress interface on PLR

   3.  Initialization of new bypass tunnel

   4.  Change of reserved bandwidth of protected LSP or signaling of a
       new LSP that is to be protected by an existing bypass tunnel

   The last event can occur quite frequently, especially in networks
   that utilize automatic bandwidth determination for protected LSPs
   with aggressive intervals.  As such, these events SHOULD NOT trigger
   bypass path re-computation.  This is because including these events
   would lead to never converging network and adversely impact
   computational resources - control plane CPU of network device.

   For the events of the first three classes, the PBW value determined
   as per Section 2 should be used to derive path computation bandwidth
   constraints utilized by CSPF.  This value together with other
   constraining attributes configured, (e.g.  SRLG) are used as
   arguments for path computation by CSPF procedure.. The result of this
   process is an ordered list of network interfaces in the form of
   Explicit Route Object (ERO) for bypass tunnel, that is used for
   signaling of LSP instantiating bypass tunnel.

4.  Bypass tunnel signaling

   Bypass tunnels are instantiated as an LSP that have head-end on PLR
   and tail-end on MP.  They are signaled by RSVP just like any other
   MPLS LSP.  Specifically PLR uses and inserts ERO objects into PATH
   messages to enforce network path given bypass traverses.  The other
   important RSVP PATH's object is Traffic-Specification (T-SPEC), which
   encodes bandwidth that needs to be reserved for signaled LSP.

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   Under this procedure, T-SPEC bandwidth is independent of the
   bandwidth utilized by the CSPF algorithm as described in Section 2.
   Since the purpose of the procedures in this document are to provide a
   less likely to be congested path for the backup tunnel, the LSP
   bandwidth for the bypass tunnels should be configured to a value
   smaller than the value of PBW (Section 2) and value utilized for CSPF
   (Section 3) procedures, otherwise reservations are more likely to
   fail due to lack of available bandwidth on the computed path.
   Implementation SHOULD support static configuration of signaled
   bandwidth independent from the PBW, including signaled bandwidth of
   zero bps value.

   PLR MUST insert The ERO object in RSVP PATH.  Its value MUST be the
   ERO computed as per Section 3.

   Note: If ERO returned by path computation described in Section 3 is
   equal to network path bypass tunnel currently traverses, and the
   signaled bandwidth did not changed (e.g. because it has a statically
   configured value), there is no need for signaling a new LSP -
   existing one can be refreshed and utilized.

5.  IANA Considerations

   This memo includes no request to IANA.

6.  Security Considerations

   This document does not introduce new security issues.  The security
   considerations listed in Section 9 of [RFC4090] still remain

7.  References

7.1.  Normative References

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,

   [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
              Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              DOI 10.17487/RFC4090, May 2005,

7.2.  Informative References

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8733]  Dhody, D., Ed., Gandhi, R., Ed., Palle, U., Singh, R., and
              L. Fang, "Path Computation Element Communication Protocol
              (PCEP) Extensions for MPLS-TE Label Switched Path (LSP)
              Auto-Bandwidth Adjustment with Stateful PCE", RFC 8733,
              DOI 10.17487/RFC8733, February 2020,





Authors' Addresses

   Rafal Jan Szarecki
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, California 94043
   United States of America

   Jon Mitchell
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, California 94043
   United States of America

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