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.

Copyright Notice

Copyright © 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

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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.

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.

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.

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.

   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.

• 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.

• 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:

• T: Traffic statistics collection time interval.

• 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.

• 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.
   
   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).

• 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.

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

• 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 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 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>.
   
   [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>.

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.

• 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.

• 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

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