Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools

Problem, Formatting or Query -  Send Feedback

Was this page helpful?-10+1


Internet Engineering Task Force (IETF) Y. Zhuang Request for Comments: 8413 Q. Wu Category: Informational H. Chen ISSN: 2070-1721 Huawei

                                                             A. Farrel
                                                      Juniper Networks
                                                             July 2018
              Framework for Scheduled Use of Resources


 Time-Scheduled (TS) reservation of Traffic Engineering (TE) resources
 can be used to provide resource booking for TE Label Switched Paths
 so as to better guarantee services for customers and to improve the
 efficiency of network resource usage at any moment in time, including
 network usage that is planned for the future.  This document provides
 a framework that describes and discusses the architecture for
 supporting scheduled reservation of TE resources.  This document does
 not describe specific protocols or protocol extensions needed to
 realize this service.

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

Zhuang, et al. Informational [Page 1] RFC 8413 Scheduled Use of Resources 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
 ( 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4
   2.1.  Provisioning TE-LSPs and TE Resources . . . . . . . . . .   4
   2.2.  Selecting the Path of an LSP  . . . . . . . . . . . . . .   4
   2.3.  Planning Future LSPs  . . . . . . . . . . . . . . . . . .   5
   2.4.  Looking at Future Demands on TE Resources . . . . . . . .   6
     2.4.1.  Interaction between Time-Scheduled and Ad Hoc
             Reservations  . . . . . . . . . . . . . . . . . . . .   6
   2.5.  Requisite State Information . . . . . . . . . . . . . . .   7
 3.  Architectural Concepts  . . . . . . . . . . . . . . . . . . .   8
   3.1.  Where is Scheduling State Held? . . . . . . . . . . . . .   8
   3.2.  What State is Held? . . . . . . . . . . . . . . . . . . .  10
   3.3.  Enforcement of Operator Policy  . . . . . . . . . . . . .  12
 4.  Architecture Overview . . . . . . . . . . . . . . . . . . . .  13
   4.1.  Service Request . . . . . . . . . . . . . . . . . . . . .  13
     4.1.1.  Reoptimization After TED Updates  . . . . . . . . . .  14
   4.2.  Initialization and Recovery . . . . . . . . . . . . . . .  15
   4.3.  Synchronization Between PCEs  . . . . . . . . . . . . . .  15
 5.  Multi-domain Considerations . . . . . . . . . . . . . . . . .  16
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
 7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
 8.  Informative References  . . . . . . . . . . . . . . . . . . .  19
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  21
 Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  21
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

Zhuang, et al. Informational [Page 2] RFC 8413 Scheduled Use of Resources July 2018

1. Introduction

 Traffic Engineering Label Switched Paths (TE-LSPs) are connection-
 oriented tunnels in packet and non-packet networks [RFC3209]
 [RFC3945].  TE-LSPs may reserve network resources for use by the
 traffic they carry, thus providing some guarantees of service
 delivery and allowing a network operator to plan the use of the
 resources across the whole network.
 In some technologies (such as wavelength switched optical networks)
 the resource is synonymous with the label that is switched on the
 path of the LSP so that it is not possible to establish an LSP that
 can carry traffic without assigning a physical resource to the LSP.
 In other technologies (such as packet switched networks), the
 resources assigned to an LSP are a measure of the capacity of a link
 that is dedicated for use by the traffic on the LSP.
 In all cases, network planning consists of selecting paths for LSPs
 through the network so that there will be no contention for
 resources.  LSP establishment is the act of setting up an LSP and
 reserving resources within the network.  Network optimization or
 reoptimization is the process of repositioning LSPs in the network to
 make the unreserved network resources more useful for potential
 future LSPs while ensuring that the established LSPs continue to
 fulfill their objectives.
 It is often the case that it is known that an LSP will be needed at
 some specific time in the future.  While a path for that LSP could be
 computed using knowledge of the currently established LSPs and the
 currently available resources, this does not give any degree of
 certainty that the necessary resources will be available when it is
 time to set up the new LSP.  Yet, setting up the LSP ahead of the
 time when it is needed (which would guarantee the availability of the
 resources) is wasteful since the network resources could be used for
 some other purpose in the meantime.
 Similarly, it may be known that an LSP will no longer be needed after
 some future time and that it will be torn down, which will release
 the network resources that were assigned to it.  This information can
 be helpful in planning how a future LSP is placed in the network.
 Time-Scheduled (TS) reservation of TE resources can be used to
 provide resource booking for TE-LSPs so as to better guarantee
 services for customers and to improve the efficiency of network
 resource usage into the future.  This document provides a framework
 that describes the problem and discusses the architecture for the

Zhuang, et al. Informational [Page 3] RFC 8413 Scheduled Use of Resources July 2018

 scheduled reservation of TE resources.  This document does not
 describe specific protocols or protocol extensions needed to realize
 this service.

2. Problem Statement

2.1. Provisioning TE-LSPs and TE Resources

 TE-LSPs in existing networks are provisioned using a variety of
 techniques.  They may be set up using RSVP-TE as a signaling protocol
 [RFC3209] [RFC3473].  Alternatively, they could be established by
 direct control of network elements such as in the Software-Defined
 Networking (SDN) paradigm.  They could also be provisioned using the
 PCE Communication Protocol (PCEP) [RFC5440] as a control protocol to
 communicate with the network elements.
 TE resources are reserved at the point of use.  That is, the
 resources (wavelengths, timeslots, bandwidth, etc.) are reserved for
 use on a specific link and are tracked by the Label Switching Routers
 (LSRs) at the end points of the link.  Those LSRs learn which
 resources to reserve during the LSP setup process.
 The use of TE resources can be varied by changing the parameters of
 the LSP that uses them, and the resources can be released by tearing
 down the LSP.
 Resources that have been reserved in the network for use by one LSP
 may be preempted for use by another LSP.  If RSVP-TE signaling is in
 use, a holding priority and a preemption priority are used to
 determine which LSPs may preempt the resources that are in use for
 which other LSPs.  If direct (central) control is in use, the
 controller is able to make preemption decisions.  In either case,
 operator policy forms a key part of preemption since there is a trade
 between disrupting existing LSPs and enabling new LSPs.

2.2. Selecting the Path of an LSP

 Although TE-LSPs can determine their paths hop by hop using the
 shortest path toward the destination to route the signaling protocol
 messages [RFC3209], in practice this option is not applied because it
 does not look far enough ahead into the network to verify that the
 desired resources are available.  Instead, the full length of the
 path of an LSP is usually computed ahead of time either by the head-
 end LSR of a signaled LSP or by Path Computation Element (PCE)
 functionality that is in a dedicated server or built into network
 management software [RFC4655].

Zhuang, et al. Informational [Page 4] RFC 8413 Scheduled Use of Resources July 2018

 Such full-path computation is applied in order that an end-to-end
 view of the available resources in the network can be used to
 determine the best likelihood of establishing a viable LSP that meets
 the service requirements.  Even in this situation, however, it is
 possible that two LSPs being set up at the same time will compete for
 scarce network resources, which means that one or both of them will
 fail to be established.  This situation is avoided by using a
 centralized PCE that is aware of the LSP setup requests that are in
 Path selection may make allowance for preemption as described in
 Section 2.1.  That is, when selecting a path, the decision may be
 made to choose a path that will result in the preemption of an
 existing LSP.  The trade-off between selecting a less optimal path,
 failing to select any path at all, and preempting an existing LSP
 must be subject to operator policy.
 Path computation is subject to "objective functions" that define what
 criteria are to be met when the LSP is placed [RFC4655].  These can
 be criteria that apply to the LSP itself (such as the shortest path
 to the destination) or to the network state after the LSP is set up
 (such as the maximized residual link bandwidth).  The objective
 functions may be requested by the application requesting the LSP and
 may be filtered and enhanced by the computation engine according to
 operator policy.

2.3. Planning Future LSPs

 LSPs may be established "on demand" when the requester determines
 that a new LSP is needed.  In this case, the path of the LSP is
 computed as described in Section 2.2.
 However, in many situations, the requester knows in advance that an
 LSP will be needed at a particular time in the future.  For example,
 the requester may be aware of a large traffic flow that will start at
 a well-known time, perhaps for a database synchronization or for the
 exchange of content between streaming sites.  Furthermore, the
 requester may also know for how long the LSP is required before it
 can be torn down.
 The set of requests for future LSPs could be collected and held in a
 central database (such as at a Network Management System (NMS)): when
 the time comes for each LSP to be set up, the NMS can ask the PCE to
 compute a path and can then request the LSP to be provisioned.  This
 approach has a number of drawbacks because it is not possible to
 determine in advance whether it will be possible to deliver the LSP
 since the resources it needs might be used by other LSPs in the

Zhuang, et al. Informational [Page 5] RFC 8413 Scheduled Use of Resources July 2018

 network.  Thus, at the time the requester asks for the future LSP,
 the NMS can only make a best-effort guarantee that the LSP will be
 set up at the desired time.
 A better solution, therefore, is for the requests for future LSPs to
 be serviced at once.  The paths of the LSPs can be computed ahead of
 time and converted into reservations of network resources during
 specific windows in the future.  That is, while the path of the LSP
 is computed and the network resources are reserved, the LSP is not
 established in the network until the time for which it is scheduled.
 There is a need to take into account items that need to be subject to
 operator policy, such as 1) the amount of capacity available for
 scheduling future reservations, 2) the operator preference for the
 measures that are used with respect to the use of scheduled resources
 during rapid changes in traffic demand events, or 3) a complex
 (multiple nodes/links) failure event so as to protect against network
 destabilization.  Operator policy is discussed further in
 Section 3.3.

2.4. Looking at Future Demands on TE Resources

 While path computation, as described in Section 2.2, takes account of
 the currently available network resources and can act to place LSPs
 in the network so that there is the best possibility of future LSPs
 being accommodated, it cannot handle all eventualities.  It is simple
 to construct scenarios where LSPs that are placed one at a time lead
 to future LSPs being blocked, but where foreknowledge of all of the
 LSPs would have made it possible for them all to be set up.
 If, therefore, we were able to know in advance what LSPs were going
 to be requested, we could plan for them and ensure resources were
 available.  Furthermore, such an approach enables a commitment to be
 made to a service user that an LSP will be set up and available at a
 specific time.
 A reservation service can be achieved by tracking the current use of
 network resources and also having a future view of the resource
 usage.  We call this Time-Scheduled TE (TS-TE) resource reservation.

2.4.1. Interaction between Time-Scheduled and Ad Hoc Reservations

 There will, of course, be a mixture of resource uses in a network.
 For example, normal unplanned LSPs may be requested alongside TS-TE
 LSPs.  When an unplanned LSP is requested, no prior accommodation can
 be made to arrange resource availability, so the LSP can be placed no
 better than would be the case without TS-TE.  However, the new LSP
 can be placed considering the future demands of TS-TE LSPs that have

Zhuang, et al. Informational [Page 6] RFC 8413 Scheduled Use of Resources July 2018

 already been requested.  Of course, the unplanned LSP has no known
 end time and so any network planning must assume that it will consume
 resources forever.

2.5. Requisite State Information

 In order to achieve the TS-TE resource reservation, the use of
 resources on the path needs to be scheduled.  The scheduling state is
 used to indicate when resources are reserved and when they are
 available for use.
 A simple information model for one piece of the scheduling state is
 as follows:
      link id;
      resource id or reserved capacity;
      reservation start time;
      reservation end time
 The resource that is scheduled could be link capacity, physical
 resources on a link, buffers on an interface, etc., and could include
 advanced considerations such as CPU utilization and the availability
 of memory at nodes within the network.  The resource-related
 information might also include the maximal unreserved bandwidth of
 the link over a time interval.  That is, the intention is to book
 (reserve) a percentage of the residual (unreserved) bandwidth of the
 link.  This could be used, for example, to reserve bandwidth for a
 particular class of traffic (such as IP) that doesn't have a
 provisioned LSP.
 For any one resource, there could be multiple pieces of the
 scheduling state, and for any one link, the timing windows might
 There are multiple ways to realize this information model and
 different ways to store the data.  The resource state could be
 expressed as a start time and an end time (as shown above), or it
 could be expressed as a start time and a duration.  Multiple
 reservation periods, possibly of different lengths, may need to be
 recorded for each resource.  Furthermore, the current state of
 network reservation could be kept separate from the scheduled usage,
 or everything could be merged into a single TS database.
 An application may make a reservation request for immediate resource
 usage or to book resources for future use so as to maximize the
 chance of services being delivered and to avoid contention for

Zhuang, et al. Informational [Page 7] RFC 8413 Scheduled Use of Resources July 2018

 resources in the future.  A single reservation request may book
 resources for multiple periods and might request a reservation that
 repeats on a regular cycle.
 A computation engine (that is, a PCE) may use the scheduling state
 information to help optimize the use of resources into the future and
 reduce contention or blocking when the resources are actually needed.
 Note that it is also necessary to store the information about future
 LSPs as distinct from the specific resource scheduling.  This
 information is held to allow the LSPs to be instantiated when they
 are due, and use the paths/resources that have been computed for
 them, and also to provide correlation with the TS-TE resource
 reservations so that it is clear why resources were reserved, thus
 allowing preemption and handling the release of reserved resources in
 the event of cancellation of future LSPs.  See Section 3.2 for
 further discussion of the distinction between scheduled resource
 state and scheduled LSP state.
 Network performance factors (such as maximum link utilization and the
 residual capacity of the network), with respect to supporting
 scheduled reservations, need to be supported and are subject to
 operator policy.

3. Architectural Concepts

 This section examines several important architectural concepts to
 understand the design decisions reached in this document to achieve
 TS-TE in a scalable and robust manner.

3.1. Where is Scheduling State Held?

 The scheduling state information described in Section 2.5 has to be
 held somewhere.  There are two places where this makes sense:
 o  in the network nodes where the resources exist; or,
 o  in a central scheduling controller where decisions about resource
    allocation are made.
 The first of these makes policing of resource allocation easier.  It
 means that many points in the network can request immediate or
 scheduled LSPs with the associated resource reservation, and that all
 such requests can be correlated at the point where the resources are

Zhuang, et al. Informational [Page 8] RFC 8413 Scheduled Use of Resources July 2018

 allocated.  However, this approach has some scaling and technical
 o  The most obvious issue is that each network node must retain the
    full time-based state for all of its resources.  In a busy network
    with a high arrival rate of new LSPs and a low hold time for each
    LSP, this could be a lot of state.  Network nodes are normally
    implemented with minimal spare memory.
 o  In order that path computation can be performed, the computing
    entity normally known as a Path Computation Element (PCE)
    [RFC4655] needs access to a database of available links and nodes
    in the network (as well as the TE properties of said links).  This
    database is known as the Traffic Engineering Database (TED) and is
    usually populated from information advertised in the IGP by each
    of the network nodes or exported using BGP Link State (BGP-LS)
    [RFC7752].  To be able to compute a path for a future LSP, the PCE
    needs to populate the TED with all of the future resource
    availability: if this information is held on the network nodes, it
    must also be advertised in the IGP.  This could be a significant
    scaling issue for the IGP and the network nodes, as all of the
    advertised information is held at every network node and must be
    periodically refreshed by the IGP.
 o  When a normal node restarts, it can recover the resource
    reservation state from the forwarding hardware, from Non-Volatile
    Random-Access Memory (NVRAM), or from adjacent nodes through the
    signaling protocol [RFC5063].  If the scheduling state is held at
    the network nodes, it must also be recovered after the restart of
    a network node.  This cannot be achieved from the forwarding
    hardware because the reservation will not have been made, could
    require additional expensive NVRAM, or might require that all
    adjacent nodes also have the scheduling state in order to
    reinstall it on the restarting node.  This is potentially complex
    processing with scaling and cost implications.
 Conversely, if the scheduling state is held centrally, it is easily
 available at the point of use.  That is, the PCE can utilize the
 state to plan future LSPs and can update that stored information with
 the scheduled reservation of resources for those future LSPs.  This
 approach also has several issues:
 o  If there are multiple controllers, then they must synchronize
    their stored scheduling state as they each plan future LSPs and
    they must have a mechanism to resolve resource contention.  This
    is relatively simple and is mitigated by the fact that there is
    ample processing time to replan future LSPs in the case of
    resource contention.

Zhuang, et al. Informational [Page 9] RFC 8413 Scheduled Use of Resources July 2018

 o  If other sources of immediate LSPs are allowed (for example, other
    controllers or autonomous action by head-end LSRs), then the
    changes in resource availability caused by the setup or tear down
    of these LSPs must be reflected in the TED (by use of the IGP as
    is already normally done) and may have an impact on planned future
    LSPs.  This impact can be mitigated by replanning future LSPs or
    through LSP preemption.
 o  If the scheduling state is held centrally at a PCE, the state must
    be held and restored after a system restart.  This is relatively
    easy to achieve on a central server that can have access to non-
    volatile storage.  The PCE could also synchronize the scheduling
    state with other PCEs after restart.  See Section 4.2 for details.
 o  Of course, a centralized system must store information about all
    of the resources in the network.  In a busy network with a high
    arrival rate of new LSPs and a low hold time for each LSP, this
    could be a lot of state.  This is multiplied by the size of the
    network measured both by the number of links and nodes and by the
    number of trackable resources on each link or at each node.  This
    challenge may be mitigated by the centralized server being
    dedicated hardware, but there remains the problem of collecting
    the information from the network in a timely way when there is
    potentially a very large amount of information to be collected and
    when the rate of change of that information is high.  This latter
    challenge is only solved if the central server has full control of
    the booking of resources and the establishment of new LSPs so that
    the information from the network only serves to confirm what the
    central server expected.
 Thus, considering these trade-offs, the architectural conclusion is
 that the scheduling state should be held centrally at the point of
 use and not in the network devices.

3.2. What State is Held?

 As already described, the PCE needs access to an enhanced, time-based
 TED.  It stores the Traffic Engineering (TE) information, such as
 bandwidth, for every link for a series of time intervals.  There are
 a few ways to store the TE information in the TED.  For example,
 suppose that the amount of the unreserved bandwidth at a priority
 level for a link is Bj in a time interval from time Tj to Tk (k =
 j+1), where j = 0, 1, 2, ....

Zhuang, et al. Informational [Page 10] RFC 8413 Scheduled Use of Resources July 2018

       |                                    B3
       |          B1                        ___________
       |          __________
       |B0                                             B4
       |__________          B2                         _________
       |                    ________________
      -+-------------------------------------------------------> Time
       |T0        T1        T2              T3         T4
           Figure 1: A Plot of Bandwidth Usage against Time
 The unreserved bandwidth for the link can be represented and stored
 in the TED as [T0, B0], [T1, B1], [T2, B2], [T3, B3], ... as shown in
 Figure 1.
 But it must be noted that service requests for future LSPs are known
 in terms of the LSPs whose paths are computed and for which resources
 are scheduled.  For example, if the requester of a future LSP decides
 to cancel the request or to modify the request, the PCE must be able
 to map this to the resources that were reserved.  When the LSP (or
 the request for the LSP with a number of time intervals) is canceled,
 the PCE must release the resources that were reserved on each of the
 links along the path of the LSP in every time interval from the TED.
 If the bandwidth that had been reserved for the LSP on a link was B
 from time T2 to T3 and the unreserved bandwidth on the link was B2
 from T2 to T3, then B is added back to the link for the time interval
 from T2 to T3 and the unreserved bandwidth on the link from T2 to T3
 will be seen to be B2 + B.
 This suggests that the PCE needs an LSP Database (LSP-DB) [RFC8231]
 that contains information not only about LSPs that are active in the
 network but also those that are planned.  For each time interval that
 applies to the LSP, the information for an LSP stored in the LSP-DB
 includes: the time interval, the paths computed for the LSP
 satisfying the constraints in the time interval, and the resources
 (such as bandwidth) reserved for the LSP in the time interval.  See
 also Section 2.3
 It is an implementation choice how the TED and LSP-DB are stored both
 for dynamic use and for recovery after failure or restart, but it may
 be noted that all of the information in the scheduled TED can be
 recovered from the active network state and from the scheduled LSP-

Zhuang, et al. Informational [Page 11] RFC 8413 Scheduled Use of Resources July 2018

3.3. Enforcement of Operator Policy

 Computation requests for LSPs are serviced according to operator
 policy.  For example, a PCE may refuse a computation request because
 the application making the request does not have sufficient
 permissions or because servicing the request might take specific
 resource usage over a given threshold.
 Furthermore, the preemption and holding priorities of any particular
 computation request may be subject to the operator's policies.  The
 request could be rejected if it does not conform to the operator's
 policies, or (possibly more likely) the priorities could be set/
 overwritten according to the operator's policies.
 Additionally, the Objective Functions (OFs) of computation request
 (such as maximizing residual bandwidth) are also subject to operator
 policies.  It is highly likely that the choice of OFs is not
 available to an application and is selected by the PCE or management
 system subject to operator policies and knowledge of the application.
 None of these statements is new to scheduled resources.  They apply
 to stateless, stateful, passive, and active PCEs, and they continue
 to apply to scheduling of resources.
 An operator may choose to configure special behavior for a PCE that
 handles resource scheduling.  For example, an operator might want
 only a certain percentage of any resource to be bookable.  And an
 operator might want the preemption of booked resources to be an
 inverse function of how far in the future the resources are needed
 for the first time.
 It is a general assumption about the architecture described in
 Section 4 that a PCE is under the operational control of the operator
 that owns the resources that the PCE manipulates.  Thus, the operator
 may configure any amount of (potentially complex) policy at the PCE.
 This configuration would also include policy points surrounding
 reoptimization of existing and planned LSPs in the event of changes
 in the current and future (planned) resource availability.
 The granularity of the timing window offered to an application will
 depend on an operator's policy as well as the implementation in the
 PCE and goes to define the operator' service offerings.  Different
 granularities and different lengths of prebooking may be offered to
 different applications.

Zhuang, et al. Informational [Page 12] RFC 8413 Scheduled Use of Resources July 2018

4. Architecture Overview

 The architectural considerations and conclusions described in the
 previous section lead to the architecture described in this section
 and illustrated in Figure 2.  The interfaces and interactions shown
 in the figure and labeled (a) through (f) are described in
 Section 4.1.
  1. ——————

| Service Requester |

  1. ——————


                -------   b   --------
               |       |<--->| LSP-DB |
               |       |      --------
               |  PCE  |
               |       |  c    -----
               |       |<---->| TED |
                -------        -----
                ^     ^
                |     |
               d|     |e
                |     |
                |     |          Network
                |     --------
                |    | Router |
                v     --------
              -----          -----
             | LSR |<------>| LSR |
              -----     f    -----
    Figure 2: Reference Architecture for Scheduled Use of Resources

4.1. Service Request

 As shown in Figure 2, some component in the network requests a
 service.  This may be an application, an NMS, an LSR, or any
 component that qualifies as a Path Computation Client (PCC).  We show
 this on the figure as the "Service Requester", and it sends a request
 to the PCE for an LSP to be set up at some time (either now or in the
 future).  The request, indicated on Figure 2 by the arrow (a),
 includes all of the parameters of the LSP that the requester wishes
 to supply, such as priority, bandwidth, start time, and end time.
 Note that the requester in this case may be the LSR shown in the
 figure or may be a distinct system.

Zhuang, et al. Informational [Page 13] RFC 8413 Scheduled Use of Resources July 2018

 The PCE enters the LSP request in its LSP-DB (b) and uses information
 from its TED (c) to compute a path that satisfies the constraints
 (such as bandwidth) for the LSP in the time interval from the start
 time to the end time.  It updates the future resource availability in
 the TED so that further path computations can take account of the
 scheduled resource usage.  It stores the path for the LSP into the
 LSP-DB (b).
 When it is time (i.e., at the start time) for the LSP to be set up,
 the PCE sends a PCEP Initiate request to the head-end LSR (d), which
 provides the path to be signaled as well as other parameters, such as
 the bandwidth of the LSP.
 As the LSP is signaled between LSRs (f), the use of resources in the
 network is updated and distributed using the IGP.  This information
 is shared with the PCE either through the IGP or using BGP-LS (e),
 and the PCE updates the information stored in its TED (c).
 After the LSP is set up, the head-end LSR sends a PCEP LSP State
 Report (PCRpt) message to the PCE (d).  The report contains the
 resources, such as bandwidth usage, for the LSP.  The PCE updates the
 status of the LSP in the LSP-DB according to the report.
 When an LSP is no longer required (either because the Service
 Requester has canceled the request or because the LSP's scheduled
 lifetime has expired), the PCE can remove it.  If the LSP is
 currently active, the PCE instructs the head-end LSR to tear it down
 (d), and the network resource usage will be updated by the IGP and
 advertised back to the PCE through the IGP or BGP-LS (e).  Once the
 LSP is no longer active, the PCE can remove it from the LSP-DB (b).

4.1.1. Reoptimization After TED Updates

 When the TED is updated as indicated in Section 4.1, depending on
 operator policy (so as to minimize network perturbations), the PCE
 may perform reoptimization of the LSPs for which it has computed
 paths.  These LSPs may be already provisioned, in which case the PCE
 issues PCEP Update request messages for the LSPs that should be
 adjusted.  Additionally, the LSPs being reoptimized may be scheduled
 LSPs that have not yet been provisioned, in which case reoptimization
 involves updating the store of scheduled LSPs and resources.
 In all cases, the purpose of reoptimization is to take account of the
 resource usage and availability in the network and to compute paths
 for the current and future LSPs that best satisfy the objectives of
 those LSPs while keeping the network as clear as possible to support
 further LSPs.  Since reoptimization may perturb established LSPs, it

Zhuang, et al. Informational [Page 14] RFC 8413 Scheduled Use of Resources July 2018

 is subject to operator oversight and policy.  As the stability of the
 network will be impacted by frequent changes, the extent and impact
 of any reoptimization needs to be subject to operator policy.
 Additionally, the status of the reserved resources (alarms) can
 enhance the computation and planning for future LSPs and may
 influence repair and reoptimization.  Control of recalculations based
 on failures and notifications to the operator is also subject to
 See Section 3.3 for further discussion of operator policy.

4.2. Initialization and Recovery

 When a PCE in the architecture shown in Figure 2 is initialized, it
 must learn the state from the network, from its stored databases, and
 potentially from other PCEs in the network.
 The first step is to get an accurate view of the topology and
 resource availability in the network.  This would normally involve
 reading the state directly from the network via the IGP or BGP-LS
 (e), but it might include receiving a copy of the TED from another
 PCE.  Note that a TED stored from a previous instantiation of the PCE
 is unlikely to be valid.
 Next, the PCE must construct a time-based TED to show scheduled
 resource usage.  How it does this is implementation specific, and
 this document does not dictate any particular mechanism: it may
 recover a time-based TED previously saved to non-volatile storage, or
 it may reconstruct the time-based TED from information retrieved from
 the LSP-DB previously saved to non-volatile storage.  If there is
 more than one PCE active in the network, the recovering PCE will need
 to synchronize the LSP-DB and time-based TED with other PCEs (see
 Section 4.3).
 Note that the stored LSP-DB needs to include the intended state and
 actual state of the LSPs so that when a PCE recovers, it is able to
 determine what actions are necessary.

4.3. Synchronization Between PCEs

 If there is active in the network more than one PCE that supports
 scheduling, it is important to achieve some consistency between the
 scheduled TED and scheduled LSP-DB held by the PCEs.
 [RFC7399] answers various questions around synchronization between
 the PCEs.  It should be noted that the time-based "scheduled"
 information adds another dimension to the issue of synchronization

Zhuang, et al. Informational [Page 15] RFC 8413 Scheduled Use of Resources July 2018

 between PCEs.  It should also be noted that a deployment may use a
 primary PCE and then have other PCEs as backup, where a backup PCE
 can take over only in the event of a failure of the primary PCE.
 Alternatively, the PCEs may share the load at all times.  The choice
 of the synchronization technique is largely dependent on the
 deployment of PCEs in the network.
 One option for ensuring that multiple PCEs use the same scheduled
 information is simply to have the PCEs driven from the same shared
 database, but it is likely to be inefficient, and interoperation
 between multiple implementations will be harder.
 Another option is for each PCE to be responsible for its own
 scheduled database and to utilize some distributed database
 synchronization mechanism to have consistent information.  Depending
 on the implementation, this could be efficient, but interoperation
 between heterogeneous implementations is still hard.
 A further approach is to utilize PCEP messages to synchronize the
 scheduled state between PCEs.  This approach would work well if the
 number of PCEs that support scheduling is small, but as the number
 increases, considerable message exchange needs to happen to keep the
 scheduled databases synchronized.  Future solutions could also
 utilize some synchronization optimization techniques for efficiency.
 Another variation would be to request information from other PCEs for
 a particular time slice, but this might have an impact on the
 optimization algorithm.

5. Multi-domain Considerations

 Multi-domain path computation usually requires some form of
 cooperation between PCEs, each of which has responsibility for
 determining a segment of the end-to-end path in the domain for which
 it has computational responsibility.  When computing a scheduled
 path, resources need to be booked in all of the domains that the path
 will cross so that they are available when the LSP is finally
 Per-domain path computation [RFC5152] is not an appropriate mechanism
 when a scheduled LSP is being computed because the computation
 requests at downstream PCEs are only triggered by signaling.
 However, a similar mechanism could be used where cooperating PCEs
 exchange Path Computation Request (PCReq) messages for a scheduled
 LSP, as shown in Figure 3.  In this case, the service requester asks
 for a scheduled LSP that will span two domains (a).  PCE1 computes a
 path across Domain 1 and reserves the resources and also asks PCE2 to
 compute and reserve in Domain 2 (b).  PCE2 may return a full path or
 could return a path key [RFC5520].  When it is time for LSP setup,

Zhuang, et al. Informational [Page 16] RFC 8413 Scheduled Use of Resources July 2018

 PCE1 triggers the head-end LSR (c), and the LSP is signaled (d).  If
 a path key is used, the entry LSR in Domain 2 will consult PCE2 for
 the path expansion (e) before completing signaling (f).
  1. ——————

| Service Requester |

  1. ——————


        ------         b          ------
       |      |<---------------->|      |
       | PCE1 |                  | PCE2 |
       |      |                  |      |
        ------                    ------
          ^                         ^
          |                         |
         c|                        e|
          |                         |
      ----+-----------------    ----+-----------------
     |    |        Domain 1 |  |    |        Domain 2 |
     |    v                 |  |    v                 |
     |  -----   d   -----   |  |   -----   f   -----  |
     | | LSR |<--->| LSR |<-+--+->| LSR |<--->| LSR | |
     |  -----       -----   |  |   -----       -----  |
      ----------------------    ----------------------
       Figure 3: Per-Domain Path Computation for Scheduled LSPs
 Another mechanism for PCE cooperation in multi-domain LSP setup is
 Backward Recursive PCE-Based Computation (BRPC) [RFC5441].  This
 approach relies on the downstream domain to supply a variety of
 potential paths to the upstream domain.  Although BRPC can arrive at
 a more optimal end-to-end path than per-domain path computation, it
 is not well suited to LSP scheduling because the downstream PCE would
 need to reserve resources on all of the potential paths and then
 release those that the upstream PCE announced it did not plan to use.
 Finally, we should consider hierarchical PCE (H-PCE) [RFC6805].  This
 mode of operation is similar to that shown in Figure 3, but a parent
 PCE is used to coordinate the requests to the child PCEs, which then
 results in better visibility of the end-to-end path and better
 coordination of the resource booking.  The sequenced flow of control
 is shown in Figure 4.

Zhuang, et al. Informational [Page 17] RFC 8413 Scheduled Use of Resources July 2018

  1. ——————

| Service Requester |

  1. ——————


       |        |
       | Parent |
       |  PCE   |
       |        |
           ^ ^         b
          b| |_______________________
           |                         |
           v                         v
        ------                    ------
       |      |                  |      |
       | PCE1 |                  | PCE2 |
       |      |                  |      |
        ------                    ------
          ^                         ^
          |                         |
         c|                        e|
          |                         |
      ----+-----------------    ----+-----------------
     |    |        Domain 1 |  |    |        Domain 2 |
     |    v                 |  |    v                 |
     |  -----   d   -----   |  |   -----   f   -----  |
     | | LSR |<--->| LSR |<-+--+->| LSR |<--->| LSR | |
     |  -----       -----   |  |   -----       -----  |
      ----------------------    ----------------------
  Figure 4: Hierarchical PCE for Path Computation for Scheduled LSPs

6. Security Considerations

 The protocol implications of scheduled resources are unchanged from
 "on demand" LSP computation and setup.  A discussion of securing PCEP
 is found in [RFC5440], and work to extend that security is provided
 in [RFC8253].  Furthermore, the path key mechanism described in
 [RFC5520] can be used to enhance privacy and security.
 Similarly, there is no change to the security implications for the
 signaling of scheduled LSPs.  A discussion of the security of the
 signaling protocols that would be used is found in [RFC5920].

Zhuang, et al. Informational [Page 18] RFC 8413 Scheduled Use of Resources July 2018

 However, the use of scheduled LSPs extends the attack surface for a
 PCE-enabled TE system by providing a larger (logically infinite)
 window during which an attack can be initiated or planned.  That is,
 if bogus scheduled LSPs can be requested and entered into the LSP-DB,
 then a large number of LSPs could be launched and significant network
 resources could be blocked.  Control of scheduling requests needs to
 be subject to operator policy, and additional authorization needs to
 be applied for access to LSP scheduling.  Diagnostic tools need to be
 provided to inspect the LSP-DB to spot attacks.

7. IANA Considerations

 This document has no IANA actions.

8. Informative References

 [AUTOBW]   Yong, L. and Y. Lee, "ASON/GMPLS Extension for Reservation
            and Time Based Automatic Bandwidth Service", Work in
            Progress, draft-yong-ccamp-ason-gmpls-autobw-service-00,
            October 2006.
 [DRAGON]   National Science Foundation, "The DRAGON Project: Dynamic
            Resource Allocation via GMPLS Optical Networks", Overview
            and Status Presentation at ONT3, September 2006,
            Chen, H., Toy, M., Liu, L., and K. Pithewan, "Framework
            for Temporal Tunnel Services", Work In Progress, draft-
            chen-teas-frmwk-tts-01, March 2016.
 [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,
 [RFC3473]  Berger, L., Ed., "Generalized Multi-Protocol Label
            Switching (GMPLS) Signaling Resource ReserVation Protocol-
            Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
            DOI 10.17487/RFC3473, January 2003,
 [RFC3945]  Mannie, E., Ed., "Generalized Multi-Protocol Label
            Switching (GMPLS) Architecture", RFC 3945,
            DOI 10.17487/RFC3945, October 2004,

Zhuang, et al. Informational [Page 19] RFC 8413 Scheduled Use of Resources July 2018

 [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
            Element (PCE)-Based Architecture", RFC 4655,
            DOI 10.17487/RFC4655, August 2006,
 [RFC5063]  Satyanarayana, A., Ed. and R. Rahman, Ed., "Extensions to
            GMPLS Resource Reservation Protocol (RSVP) Graceful
            Restart", RFC 5063, DOI 10.17487/RFC5063, October 2007,
 [RFC5152]  Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
            Per-Domain Path Computation Method for Establishing Inter-
            Domain Traffic Engineering (TE) Label Switched Paths
            (LSPs)", RFC 5152, DOI 10.17487/RFC5152, February 2008,
 [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
            Element (PCE) Communication Protocol (PCEP)", RFC 5440,
            DOI 10.17487/RFC5440, March 2009,
 [RFC5441]  Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
            "A Backward-Recursive PCE-Based Computation (BRPC)
            Procedure to Compute Shortest Constrained Inter-Domain
            Traffic Engineering Label Switched Paths", RFC 5441,
            DOI 10.17487/RFC5441, April 2009,
 [RFC5520]  Bradford, R., Ed., Vasseur, JP., and A. Farrel,
            "Preserving Topology Confidentiality in Inter-Domain Path
            Computation Using a Path-Key-Based Mechanism", RFC 5520,
            DOI 10.17487/RFC5520, April 2009,
 [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
            Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
 [RFC6805]  King, D., Ed. and A. Farrel, Ed., "The Application of the
            Path Computation Element Architecture to the Determination
            of a Sequence of Domains in MPLS and GMPLS", RFC 6805,
            DOI 10.17487/RFC6805, November 2012,
 [RFC7399]  Farrel, A. and D. King, "Unanswered Questions in the Path
            Computation Element Architecture", RFC 7399,
            DOI 10.17487/RFC7399, October 2014,

Zhuang, et al. Informational [Page 20] RFC 8413 Scheduled Use of Resources July 2018

 [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,
 [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
            Computation Element Communication Protocol (PCEP)
            Extensions for Stateful PCE", RFC 8231,
            DOI 10.17487/RFC8231, September 2017,
 [RFC8253]  Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
            "PCEPS: Usage of TLS to Provide a Secure Transport for the
            Path Computation Element Communication Protocol (PCEP)",
            RFC 8253, DOI 10.17487/RFC8253, October 2017,


 This work has benefited from the discussions of resource scheduling
 over the years.  In particular, the DRAGON project [DRAGON] and
 [AUTOBW], both of which provide approaches to auto-bandwidth services
 in GMPLS networks.
 Mehmet Toy, Lei Liu, and Khuzema Pithewan contributed to an earlier
 version of [FRAMEWORK-TTS].  We would like to thank the authors of
 that document on Temporal Tunnel Services for material that assisted
 in thinking about this document.
 Thanks to Michael Scharf and Daniele Ceccarelli for useful comments
 on this work.
 Jonathan Hardwick provided a helpful Routing Directorate review.
 Deborah Brungard, Mirja Kuehlewind, and Benjamin Kaduk suggested many
 changes during their Area Director reviews.


 The following person contributed to discussions that led to the
 development of this document:
 Dhruv Dhody

Zhuang, et al. Informational [Page 21] RFC 8413 Scheduled Use of Resources July 2018

Authors' Addresses

 Yan Zhuang
 101 Software Avenue, Yuhua District
 Nanjing, Jiangsu  210012
 Qin Wu
 101 Software Avenue, Yuhua District
 Nanjing, Jiangsu  210012
 Huaimo Chen
 Boston, MA
 United States of America
 Adrian Farrel
 Juniper Networks

Zhuang, et al. Informational [Page 22]

/data/webs/external/dokuwiki/data/pages/rfc/rfc8413.txt · Last modified: 2018/07/10 05:08 (external edit)