TEAS Working Group                                     H. Sitaraman, Ed.
Internet-Draft                                                 V. Beeram
Intended status: Informational                          Juniper Networks
Expires: January 19, 2017                                       I. Minei
                                                            Google, Inc.
                                                           July 18, 2016

    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

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   This Internet-Draft will expire on January 19, 2017.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
   3.  Solution options  . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Partitioning of static 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 . . . . . . . . . . . . . . . . .   4
     3.5.  TED consistency by reducing RSVP-TE subscription  . . . .   5
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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.  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 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 MUST be avoided or 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.  Partitioning of static 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.

   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.

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3.2.  Centralized management of available capacity

   In this model, a central controller performs path placement for both
   RSVP-TE signaled 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 signaled
   LSPs as a prerequisite to the deployment of any SR-signaled LSPs.
   Therefore, this approach is not practical for networks where
   distributed TE with RSVP-TE signaled 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.

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, steps must
   be taken to prevent non-SR traffic from taking 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-

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   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 reducing RSVP-TE subscription

   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 is higher priority than RSVP-
   TE and there can be different priority RSVP-TE LSPs.  The following
   methodology can be used at every TE node for this solution:

   o  T: Traffic statistics collection interval

   o  N: Traffic averaging calculation (adjustment) interval such that N
      = k * T, where k is a constant multiplier

   o  RSVP-TE subscription percentage: The percentage of static
      bandwidth of an interface that is usable by RSVP-TE

   o  SR traffic threshold percentage: The percentage difference of
      traffic demand that when exceeded can result in a change to the
      RSVP-TE subscription percentage

   o  IGP-TE update threshold: Specifies the frequency at which IGP-TE
      updates should be triggered based on TE bandwidth updates on a

   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 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 the
   RSVP-TE subscription percentage for that interface MUST be adjusted.
   This MAY result in updates to maximum reservable link bandwidth in

   As SR traffic is considered higher priority compared to RSVP-TE, the
   reduction in RSVP-TE subscription percentage can 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.

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   If LSP preemption is not acceptable, then the RSVP-TE subscription
   percentage 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 subscription percentages is
   an implementation choice, and all changes are local in nature.

   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

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

6.  IANA Considerations

   This draft does not have any request for IANA.

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7.  Security Considerations

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

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-09 (work in progress), July 2016.

   [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

   [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,

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Authors' Addresses

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

   Email: hsitaraman@juniper.net

   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

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