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rfc:rfc8426

Internet Engineering Task Force (IETF) H. Sitaraman, Ed. Request for Comments: 8426 V. Beeram Category: Informational Juniper Networks ISSN: 2070-1721 I. Minei

                                                          Google, Inc.
                                                          S. Sivabalan
                                                   Cisco Systems, Inc.
                                                             July 2018
        Recommendations for RSVP-TE and Segment Routing (SR)
               Label Switched Path (LSP) Coexistence

Abstract

 Operators are looking to introduce services over Segment Routing (SR)
 Label Switched Paths (LSPs) in networks running Resource Reservation
 Protocol - Traffic Engineering (RSVP-TE) LSPs.  In some instances,
 operators are also migrating existing services from RSVP-TE to SR
 LSPs.  For example, there might be certain services that are well
 suited for SR and need to coexist with RSVP-TE in the same network.
 Such introduction or migration of traffic to SR might require
 coexistence with RSVP-TE in the same network for an extended period
 of time, depending on the operator's intent.  The following document
 provides solution options for keeping the traffic engineering
 database consistent across the network, accounting for the different
 bandwidth utilization between SR and RSVP-TE.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 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).  Not all documents
 approved by the IESG are candidates for any level of Internet
 Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8426.

Sitaraman, et al. Informational [Page 1] RFC 8426 RSVP-TE and SR LSP Coexistence July 2018

Copyright Notice

 Copyright (c) 2018 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
 (https://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  . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
 3.  Solution Options  . . . . . . . . . . . . . . . . . . . . . .   3
   3.1.  Static Partitioning of Bandwidth  . . . . . . . . . . . .   4
   3.2.  Centralized Management of Available Capacity  . . . . . .   4
   3.3.  Flooding SR Utilization in IGP  . . . . . . . . . . . . .   5
   3.4.  Running SR over RSVP-TE . . . . . . . . . . . . . . . . .   5
   3.5.  TED Consistency by Reflecting SR Traffic  . . . . . . . .   5
 4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
 5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
 6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
   6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
 Appendix A.  Multiplier Value Range . . . . . . . . . . . . . . .  11
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
 Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1. Introduction

 Introduction of SR [RFC8402] in the same network domain as RSVP-TE
 [RFC3209] presents the problem of accounting for SR traffic and
 making RSVP-TE aware of the actual available bandwidth on the network
 links.  RSVP-TE is not aware of how much bandwidth is being consumed
 by SR services on the network links; hence, both at computation time
 (for a distributed computation) and at signaling time, RSVP-TE LSPs
 will incorrectly place loads.  This is true where RSVP-TE paths are
 distributed or centrally computed without a common entity managing
 both SR and RSVP-TE computation for the entire network domain.

Sitaraman, et al. Informational [Page 2] RFC 8426 RSVP-TE and SR LSP Coexistence July 2018

 The problem space can be generalized as a dark bandwidth problem to
 cases where any other service exists in the network that runs in
 parallel across common links and whose bandwidth is not reflected in
 the available and reserved values in the Traffic Engineering Database
 (TED).  In most practical instances, given the static nature of the
 traffic demands, limiting the reservable bandwidth available to RSVP-
 TE has been an acceptable solution.  However, in the case of SR
 traffic, there is assumed to be very dynamic traffic demands, and
 there is considerable risk associated with stranding capacity or
 overbooking service traffic resulting in traffic drops.
 The high-level requirements to consider are:
 1.  Placement of SR LSPs in the same domain as RSVP-TE LSPs must not
     introduce inaccuracies in the TED used by distributed or
     centralized path computation engines.
 2.  Engines that compute RSVP-TE paths may have no knowledge of the
     existence of the SR paths in the same domain.
 3.  Engines that compute RSVP-TE paths should not require a software
     upgrade or change to their path-computation logic.
 4.  Protocol extensions should be avoided or be minimal as, in many
     cases, this coexistence of RSVP-TE and SR may be needed only
     during a transition phase.
 5.  Placement of SR LSPs in the same domain as RSVP-TE LSPs that are
     computed in a distributed fashion must not require migration to a
     central controller architecture for the RSVP-TE LSPs.

2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "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.

3. Solution Options

 The following section lists SR and RSVP coexistence solution options.
 A specific solution is not recommended as all solutions are valid,
 even though some may not satisfy all the requirements.  If a solution
 is acceptable for an operator based on their deployment model, then
 such a solution can be chosen.

Sitaraman, et al. Informational [Page 3] RFC 8426 RSVP-TE and SR LSP Coexistence July 2018

3.1. Static Partitioning of Bandwidth

 In this model, the static reservable bandwidth of an interface can be
 statically partitioned between SR and RSVP-TE; each one can operate
 within that bandwidth allocation and SHOULD NOT preempt the other.
 While it is possible to configure RSVP-TE to only reserve up to a
 certain maximum link bandwidth and manage the remaining link
 bandwidth for other services, this is a deployment where SR and RSVP-
 TE are separated in the same network (ships in the night) and can
 lead to suboptimal link bandwidth utilization not allowing each to
 consume more, if required and constraining the respective
 deployments.
 The downside of this approach is the inability to use the reservable
 bandwidth effectively and the inability to use bandwidth left unused
 by the other protocol.

3.2. Centralized Management of Available Capacity

 In this model, a central controller performs path placement for both
 RSVP-TE and SR LSPs.  The controller manages and updates its own view
 of the in-use and available capacity.  As the controller is a single
 common entity managing the network it can have a unified and
 consistent view of the available capacity at all times.
 A practical drawback of this model is that it requires the
 introduction of a central controller managing the RSVP-TE LSPs as a
 prerequisite to the deployment of any SR LSPs.  Therefore, this
 approach is not practical for networks where distributed TE with
 RSVP-TE LSPs is already deployed, as it requires a redesign of the
 network and is not backwards compatible.  This does not satisfy
 requirement 5.
 Note that it is not enough for the controller to just maintain the
 unified view of the available capacity, it must also perform the path
 computation for the RSVP-TE LSPs, as the reservations for the SR LSPs
 are not reflected in the TED.

Sitaraman, et al. Informational [Page 4] RFC 8426 RSVP-TE and SR LSP Coexistence July 2018

3.3. Flooding SR Utilization in IGP

 Using techniques in [RFC7810], [RFC7471], and [RFC7823], the SR
 utilization information can be flooded in IGP-TE, and the RSVP-TE
 path computation engine (Constrained Shortest Path First (CSPF)) can
 be changed to consider this information.  This requires changes to
 the RSVP-TE path computation logic and would require upgrades in
 deployments where distributed computation is done across the network.
 This does not fit with requirements 3 and 4 mentioned earlier.

3.4. Running SR over RSVP-TE

 SR can run over dedicated RSVP-TE LSPs that carry only SR traffic.
 In this model, the LSPs can be one-hop or multi-hop and can provide
 bandwidth reservation for the SR traffic based on functionality such
 as auto-bandwidth.  The model of deployment would be similar in
 nature to running LDP over RSVP-TE.  This would allow the TED to stay
 consistent across the network and any other RSVP-TE LSPs will also be
 aware of the SR traffic reservations.  In this approach, non-SR
 traffic MUST NOT take the SR-dedicated RSVP-TE LSPs, unless required
 by policy.
 The drawback of this solution is that it requires SR to rely on RSVP-
 TE for deployment.  Furthermore, the accounting accuracy/frequency of
 this method is dependent on performance of auto-bandwidth for RSVP-
 TE.  Note that, for this method to work, the SR-dedicated RSVP-TE
 LSPs must be set up with the best setup and hold priorities in the
 network.

3.5. TED Consistency by Reflecting SR Traffic

 The solution relies on dynamically measuring SR traffic utilization
 on each TE interface and reducing the bandwidth allowed for use by
 RSVP-TE.  It is assumed that SR traffic receives precedence in terms
 of the placement on the path over RSVP traffic (that is, RSVP traffic
 can be preempted from the path in case of insufficient resources).
 This is logically equivalent to SR traffic having the best preemption
 priority in the network.  Note that this does not necessarily mean
 that SR traffic has higher QoS priority; in fact, SR and RSVP traffic
 may be in the same QoS class.

Sitaraman, et al. Informational [Page 5] RFC 8426 RSVP-TE and SR LSP Coexistence July 2018

 Reducing the bandwidth allowed for use by RSVP-TE can be explored
 using the three parameters available in IGP-TE ([RFC5305] [RFC3630]),
 namely Maximum-Link-Bandwidth, Maximum-Reservable-Bandwidth, and
 Unreserved-Bandwidth.
 o  Maximum-Link-Bandwidth: This parameter can be adjusted to
    accommodate the bandwidth required for SR traffic with cascading
    impacts on Maximum-Reservable-Bandwidth and Unreserved-Bandwidth.
    However, changing the maximum bandwidth for the TE link will
    prevent any compute engine for SR or RSVP from determining the
    real static bandwidth of the TE link.  Further, when the Maximum-
    Reservable-Bandwidth is derived from the Maximum-Link-Bandwidth,
    its definition changes since Maximum-Link-Bandwidth will account
    for the SR traffic.
 o  Unreserved-Bandwidth: SR traffic could directly adjust the
    Unreserved-Bandwidth, without impacting Maximum-Link-Bandwidth or
    Maximum-Reservable-Bandwidth.  This model is equivalent to the
    option described in Section 3.4.  Furthermore this would result in
    overloading IGP-TE advertisements to directly reflect both RSVP-TE
    bandwidth bookings and SR bandwidth measurements.
 o  Maximum-Reservable-Bandwidth: As the preferred option, SR traffic
    could adjust the Maximum-Reservable-Bandwidth, with cascading
    impact on the Unreserved-Bandwidth.
 The following methodology can be used at every TE node for this
 solution, using the following parameters:
 o  T: Traffic statistics collection time interval.
 o  k: The number of traffic statistics samples that can provide a
    smoothing function to the statistics collection.  The value of k
    is a constant integer multiplier greater or equal to 1.
 o  N: Traffic averaging calculation (adjustment) interval such that N
    = k * T.
 o  Maximum-Reservable-Bandwidth: The maximum available bandwidth for
    RSVP-TE.
 o  If Diffserv-aware MPLS Traffic Engineering (DS-TE) [RFC4124] is
    enabled, the Maximum-Reservable-Bandwidth SHOULD be interpreted as
    the aggregate bandwidth constraint across all Class-Types
    independent of the Bandwidth Constraints model.

Sitaraman, et al. Informational [Page 6] RFC 8426 RSVP-TE and SR LSP Coexistence July 2018

 o  Initial Maximum-Reservable-Bandwidth: The Maximum-reservable-
    bandwidth for TE when no SR traffic or RSVP-TE reservations exist
    on the interface.
 o  RSVP-unreserved-bandwidth-at-priority-X: Maximum-Reservable-
    Bandwidth - sum of (existing reservations at priority X and all
    priorities better than X).
 o  SR traffic threshold percentage: The percentage difference of
    traffic demand that, when exceeded, can result in a change to the
    RSVP-TE Maximum-Reservable-Bandwidth.
 o  IGP-TE update threshold: Specifies the frequency at which IGP-TE
    updates should be triggered based on TE bandwidth updates on a
    link.
 o  M: An optional multiplier that can be applied to the SR traffic
    average.  This multiplier provides the ability to grow or shrink
    the bandwidth used by SR.  Appendix A offers further guidance on
    M.
 At every interval T, each node SHOULD collect the SR traffic
 statistics for each of its TE interfaces.  The measured SR traffic
 includes all labeled SR traffic and any traffic entering the SR
 network over that TE interface.  Further, at every interval N, given
 a configured SR traffic threshold percentage and a set of collected
 SR traffic statistics samples across the interval N, the SR traffic
 average (or any other traffic metric depending on the algorithm used)
 over this period is calculated.  This method of sampling traffic
 statistics and adjusting bandwidth reservation accordingly is similar
 to how bandwidth gets adjusted for auto-bandwidth RSVP-TE LSPs.
 If the difference between the new calculated SR traffic average and
 the current SR traffic average (that was computed in the prior
 adjustment) is at least SR traffic threshold percentage, then two
 values MUST be updated:
 o  New Maximum-Reservable-Bandwidth = Initial Maximum-Reservable-
    Bandwidth - (new SR traffic average * M)
 o  New RSVP-unreserved-bandwidth-at-priority-X = New Maximum-
    Reservable-Bandwidth - sum of (existing reservations at priority X
    and all priorities better than X)
 A DS-TE LSR that advertises a Bandwidth Constraints TLV should update
 the bandwidth constraints for class-types based on operator policy.
 For example, when Russian Dolls Model (RDM) [RFC4127] is in use, then

Sitaraman, et al. Informational [Page 7] RFC 8426 RSVP-TE and SR LSP Coexistence July 2018

 only BC0 may be updated.  Whereas, when Maximum Allocation Model
 (MAM) [RFC4125] is in use, then all Bandwidth Constraints (BCs) may
 be updated equally such that the total value updated is equal to the
 newly calculated SR traffic average.
 Note that the computation of the new RSVP-unreserved-bandwidth-at-
 priority-X MAY result in RSVP-TE LSPs being hard or soft preempted.
 Such preemption will be based on relative priority (e.g., low to
 high) between RSVP-TE LSPs.  The IGP-TE update threshold SHOULD allow
 for more frequent flooding of unreserved bandwidth.  From an
 operational point of view, an implementation SHOULD be able to expose
 both the configured and the actual values of the Maximum-Reservable-
 Bandwidth.
 If LSP preemption is not acceptable, then the RSVP-TE Maximum-
 Reservable-Bandwidth cannot be reduced below what is currently
 reserved by RSVP-TE on that interface.  This may result in bandwidth
 not being available for SR traffic.  Thus, it is required that any
 external controller managing SR LSPs SHOULD be able to detect this
 situation (for example, by subscribing to TED updates [RFC7752]) and
 SHOULD take action to reroute existing SR paths.
 Generically, SR traffic (or any non-RSVP-TE traffic) should have its
 own priority allocated from the available priorities.  This would
 allow SR to preempt other traffic according to the preemption
 priority order.
 In this solution, the logic to retrieve the statistics, calculating
 averages and taking action to change the Maximum-Reservable-Bandwidth
 is an implementation choice, and all changes are local in nature.
 However, note that this is a new network trigger for RSVP-TE
 preemption and thus is a consideration for the operator.
 The above solution offers the advantage of not introducing new
 network-wide mechanisms especially during scenarios of migrating to
 SR in an existing RSVP-TE network and reusing existing protocol
 mechanisms.

4. IANA Considerations

 This document has no IANA actions.

5. Security Considerations

 This document describes solution options for the coexistence of RSVP-
 TE and SR LSPs in the same administrative domain.  The security
 considerations for SR are described in [RFC8402].  The security
 considerations pertaining to RSVP-TE are described in [RFC5920].  The

Sitaraman, et al. Informational [Page 8] RFC 8426 RSVP-TE and SR LSP Coexistence July 2018

 security considerations of each architecture are typically unaffected
 by the presence of the other.  However, when RSVP-TE and SR LSPs
 coexist, it is possible for a hijacked SR traffic stream to
 maliciously consume sufficient bandwidth and cause disruption to
 RSVP-TE LSPs.  With the solution option specified in Section 3.5, the
 impact to RSVP-TE traffic can be controlled and paths re-routed.
 Some latent risk of disruption still remains because this solution
 option relies on taking statistics samples and adopting to new
 traffic flows only after the adjustment period.  The defensive
 mechanisms described in the base SR security framework should be
 employed to guard against situations that result in SR traffic
 hijacking or denial of service.

6. References

6.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [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,
            <https://www.rfc-editor.org/info/rfc3209>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.
 [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
            Decraene, B., Litkowski, S., and R. Shakir, "Segment
            Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
            July 2018, <https://www.rfc-editor.org/info/rfc8402>.

6.2. Informative References

 [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
            (TE) Extensions to OSPF Version 2", RFC 3630,
            DOI 10.17487/RFC3630, September 2003,
            <https://www.rfc-editor.org/info/rfc3630>.
 [RFC4124]  Le Faucheur, F., Ed., "Protocol Extensions for Support of
            Diffserv-aware MPLS Traffic Engineering", RFC 4124,
            DOI 10.17487/RFC4124, June 2005,
            <https://www.rfc-editor.org/info/rfc4124>.

Sitaraman, et al. Informational [Page 9] RFC 8426 RSVP-TE and SR LSP Coexistence July 2018

 [RFC4125]  Le Faucheur, F. and W. Lai, "Maximum Allocation Bandwidth
            Constraints Model for Diffserv-aware MPLS Traffic
            Engineering", RFC 4125, DOI 10.17487/RFC4125, June 2005,
            <https://www.rfc-editor.org/info/rfc4125>.
 [RFC4127]  Le Faucheur, F., Ed., "Russian Dolls Bandwidth Constraints
            Model for Diffserv-aware MPLS Traffic Engineering",
            RFC 4127, DOI 10.17487/RFC4127, June 2005,
            <https://www.rfc-editor.org/info/rfc4127>.
 [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
            Engineering", RFC 5305, DOI 10.17487/RFC5305, October
            2008, <https://www.rfc-editor.org/info/rfc5305>.
 [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
            Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
            <https://www.rfc-editor.org/info/rfc5920>.
 [RFC7471]  Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
            Previdi, "OSPF Traffic Engineering (TE) Metric
            Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
            <https://www.rfc-editor.org/info/rfc7471>.
 [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
            S. Ray, "North-Bound Distribution of Link-State and
            Traffic Engineering (TE) Information Using BGP", RFC 7752,
            DOI 10.17487/RFC7752, March 2016,
            <https://www.rfc-editor.org/info/rfc7752>.
 [RFC7810]  Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and
            Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions",
            RFC 7810, DOI 10.17487/RFC7810, May 2016,
            <https://www.rfc-editor.org/info/rfc7810>.
 [RFC7823]  Atlas, A., Drake, J., Giacalone, S., and S. Previdi,
            "Performance-Based Path Selection for Explicitly Routed
            Label Switched Paths (LSPs) Using TE Metric Extensions",
            RFC 7823, DOI 10.17487/RFC7823, May 2016,
            <https://www.rfc-editor.org/info/rfc7823>.

Sitaraman, et al. Informational [Page 10] RFC 8426 RSVP-TE and SR LSP Coexistence July 2018

Appendix A. Multiplier Value Range

 The following is a suggestion for the range of values for M:
 M is a per-node positive real number that ranges from 0 to 2 with a
 default of 1 and may be expressed as a percentage.
 o  If M < 1, then the SR traffic average is being understated, which
    can result in the link getting full even though Maximum-
    Reservable-Bandwidth does not reach zero.
 o  If M > 1, then the SR traffic average is overstated, thereby
    resulting in the Maximum-Reservable-Bandwidth reaching zero before
    the link gets full.  If the reduction of Maximum-Reservable-
    Bandwidth becomes a negative value, then a value of zero SHOULD be
    used and advertised.

Acknowledgements

 The authors would like to thank Steve Ulrich for his detailed review
 and comments.

Contributors

 Chandra Ramachandran
 Juniper Networks
 Email: csekar@juniper.net
 Raveendra Torvi
 Juniper Networks
 Email: rtorvi@juniper.net
 Sudharsana Venkataraman
 Juniper Networks
 Email: sudharsana@juniper.net
 Martin Vigoureux
 Nokia
 Email: martin.vigoureux@nokia.com

Sitaraman, et al. Informational [Page 11] RFC 8426 RSVP-TE and SR LSP Coexistence July 2018

Authors' Addresses

 Harish Sitaraman (editor)
 Juniper Networks
 1133 Innovation Way
 Sunnyvale, CA  94089
 United States of America
 Email: hsitaraman@juniper.net
 Vishnu Pavan Beeram
 Juniper Networks
 10 Technology Park Drive
 Westford, MA  01886
 United States of America
 Email: vbeeram@juniper.net
 Ina Minei
 Google, Inc.
 1600 Amphitheatre Parkway
 Mountain View, CA  94043
 United States of America
 Email: inaminei@google.com
 Siva Sivabalan
 Cisco Systems, Inc.
 2000 Innovation Drive
 Kanata, Ontario  K2K 3E8
 Canada
 Email: msiva@cisco.com

Sitaraman, et al. Informational [Page 12]

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