TEAS Working Group                                     H. Sitaraman, Ed.
Internet-Draft                                                 V. Beeram
Intended status: Informational                          Juniper Networks
Expires: December 30, 2017                                      I. Minei
                                                            Google, Inc.
                                                            S. Sivabalan
                                                     Cisco Systems, Inc.
                                                           June 28, 2017

    Recommendations for RSVP-TE and Segment Routing LSP co-existence


   Operators are looking to introduce services over Segment Routing (SR)
   LSPs in networks running Resource Reservation Protocol (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 co-exist
   with RSVP-TE in the same network.  In other cases, services running
   on RSVP-TE might be migrated to run over SR.  Such introduction or
   migration of traffic to SR might require co-existence 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 (TED) consistent across
   the network, accounting for the different bandwidth utilization
   between SR and RSVP-TE.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 30, 2017.

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Copyright Notice

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   document authors.  All rights reserved.

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   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  . . . . . . . . . . . .   3
     3.2.  Centralized management of available capacity  . . . . . .   4
     3.3.  Flooding SR utilization in IGP  . . . . . . . . . . . . .   4
     3.4.  Running SR over RSVP-TE . . . . . . . . . . . . . . . . .   5
     3.5.  TED consistency by reflecting SR traffic  . . . . . . . .   5
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Multiplier value range . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Introduction of SR [I-D.ietf-spring-segment-routing] 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 and 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.

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   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 TED.  The general problem is
   management of dark bandwidth pools and can be generalized 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 TED.  In most practical
   instances given the static nature of the traffic demands, limiting
   the available 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 or assumptions 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 co-existence 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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Solution options

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 and each can operate
   within that bandwidth allocation and SHOULD NOT preempt each other.

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   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

   The downside of this approach is the inability to use the reservable
   bandwidth effectively and 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 the 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.  This does not fit with assumption 2
   mentioned earlier.

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 (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.

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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

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.

   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

   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 to decipher 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

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      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

   o  T: Traffic statistics collection time interval

   o  N: Traffic averaging calculation (adjustment) interval such that N
      = k * T, where k is a constant integer multiplier greater or equal
      to 1.  Its purpose is to provide a smoothing function to the
      statistics collection.

   o  Maximum-Reservable-Bandwidth: The maximum available bandwidth for

      If Differentiated-Service (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.

   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

   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

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   At every interval T, each node SHOULD collect the SR traffic
   statistics for each of its TE interfaces.  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.

   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 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
   only BC0 may be updated.  Whereas, when Maximum Allocation Model
   (MAM) [RFC4125] is in use, then all 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.  It is RECOMMENDED that the IGP-TE update
   threshold SHOULD be lower in order to flood unreserved bandwidth
   updates often.  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.

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   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

4.  Acknowledgements

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

5.  Contributors

   The following individuals contributed to this document:

   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
   Email: martin.vigoureux@nokia.com

6.  IANA Considerations

   This draft does not have any request for IANA.

7.  Security Considerations

   No new security issues are introduced in this document beyond is
   already part of RSVP-TE and Segment routing architectures.

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8.  References

8.1.  Normative References

              Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
              and R. Shakir, "Segment Routing Architecture", draft-ietf-
              spring-segment-routing-12 (work in progress), June 2017.

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

   [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,

8.2.  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,

   [RFC4124]  Le Faucheur, F., Ed., "Protocol Extensions for Support of
              Diffserv-aware MPLS Traffic Engineering", RFC 4124,
              DOI 10.17487/RFC4124, June 2005,

   [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,

   [RFC4127]  Le Faucheur, F., Ed., "Russian Dolls Bandwidth Constraints
              Model for Diffserv-aware MPLS Traffic Engineering",
              RFC 4127, DOI 10.17487/RFC4127, June 2005,

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <http://www.rfc-editor.org/info/rfc5305>.

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   [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,

   [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,

   [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,

   [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,

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.

Authors' Addresses

   Harish Sitaraman (editor)
   Juniper Networks
   1133 Innovation Way
   Sunnyvale, CA  94089

   Email: hsitaraman@juniper.net

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   Vishnu Pavan Beeram
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886

   Email: vbeeram@juniper.net

   Ina Minei
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   Email: inaminei@google.com

   Siva Sivabalan
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata, Ontario  K2K 3E8

   Email: msiva@cisco.com

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