PCE Working Group X. Zhang, Ed. Internet-Draft Huawei Technologies Intended status: Informational I. Minei, Ed. Expires: March 29, 2014 Juniper Networks, Inc. September 25, 2013 Applicability of Stateful Path Computation Element (PCE) draft-ietf-pce-stateful-pce-app-01 Abstract A stateful Path Computation Element (PCE) maintains information about Label Switched Path (LSP) characteristics and resource usage within a network in order to provide traffic engineering calculations for its associated Path Computation Clients (PCCs). This document describes general considerations for a stateful PCE deployment and examines its applicability and benefits, as well as its challenges and limitations through a number of use cases. Path Computation Element Protocol (PCEP) extensions required for stateful PCE usage are covered in separate documents. 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on March 29, 2014. Copyright Notice Copyright (c) 2013 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents Zhang & Minei Expires March 29, 2014 [Page 1] Internet-Draft Applicability for Stateful PCE September 2013 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Overview of Stateful PCE . . . . . . . . . . . . . . . . . . . 4 4. Deployment Considerations . . . . . . . . . . . . . . . . . . 5 4.1. Multi-PCE Deployments . . . . . . . . . . . . . . . . . . 5 4.2. LSP State Synchronization . . . . . . . . . . . . . . . . 5 4.3. PCE Survivability . . . . . . . . . . . . . . . . . . . . 6 5. Application Scenarios . . . . . . . . . . . . . . . . . . . . 6 5.1. Optimization of LSP Placement . . . . . . . . . . . . . . 7 5.1.1. Throughput Maximization and Bin Packing . . . . . . . 8 5.1.2. Deadlock . . . . . . . . . . . . . . . . . . . . . . . 9 5.1.3. Minimum Perturbation . . . . . . . . . . . . . . . . . 11 5.1.4. Predictability . . . . . . . . . . . . . . . . . . . . 12 5.2. Auto-bandwidth Adjustment . . . . . . . . . . . . . . . . 13 5.3. Bandwidth Scheduling . . . . . . . . . . . . . . . . . . . 13 5.4. Recovery . . . . . . . . . . . . . . . . . . . . . . . . . 14 5.4.1. Protection . . . . . . . . . . . . . . . . . . . . . . 14 5.4.2. Restoration . . . . . . . . . . . . . . . . . . . . . 15 5.4.3. SRLG Diversity . . . . . . . . . . . . . . . . . . . . 16 5.5. Maintenance of Virtual Network Topology (VNT) . . . . . . 17 5.6. LSP Re-optimization . . . . . . . . . . . . . . . . . . . 17 5.7. Resource Defragmentation . . . . . . . . . . . . . . . . . 18 5.8. Impairment-Aware Routing and Wavelength Assignment (IA-RWA) . . . . . . . . . . . . . . . . . . . . . . . . . 19 6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 7. Contributing Authors . . . . . . . . . . . . . . . . . . . . . 20 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 9.1. Normative References . . . . . . . . . . . . . . . . . . . 22 9.2. Informative References . . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 Zhang & Minei Expires March 29, 2014 [Page 2] Internet-Draft Applicability for Stateful PCE September 2013 1. Introduction [RFC4655] defines the architecture for a Path Computation Element (PCE)-based model for the computation of Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) Traffic Engineering Label Switched Paths (TE LSPs). To perform such a constrained computation, a PCE stores the network topology (i.e., TE links and nodes) and resource information (i.e., TE attributes) in its TE Database (TED). [RFC5440] describes the Path Computation Element Protocol (PCEP) for interaction between a Path Computation Client (PCC) and a PCE, or between two PCEs, enabling computation of TE LSPs in the context of MPLS networks. Extensions for support of GMPLS in PCEP are defined in [I-D.ietf-pce-gmpls-pcep-extensions]. As per [RFC4655], a PCE can be either stateful or stateless. Stateless PCEs have been shown to be useful in many scenarios, including constraint-based path computation in multi-domain/ multi-layer networks. Compared to a stateless PCE, a stateful PCE has access to not only the network state, but also to the set of active paths and their reserved resources. Furthermore, a stateful PCE might also retain information regarding LSPs under construction in order to reduce churn and resource contention. This state allows the PCE to compute constrained paths while considering individual LSPs and their interactions. However, this requires reliable state synchronization mechanisms between the PCE and the network, PCE and PCC, and between cooperating PCEs, with potentially significant control plane overhead and maintenance of a large amount of state data, as explained in [RFC4655]. This document describes how a stateful PCE can be used to solve various problems for MPLS-TE and GMPLS networks, and the benefits it brings to such deployments. Note that alternative solutions relying on stateless PCEs may also be possible for some of these use cases, and will be mentioned for completeness where appropriate. 2. Terminology This document uses the following terms defined in [RFC5440]: PCC, PCE, PCEP peer. This document uses the following terms defined in [I-D.ietf-pce-stateful-pce]: Passive Stateful PCE, Active Stateful PCE, Delegation, Revocation, Delegation Timeout Interval, LSP State Report, LSP Update Request, LSP State Database. This document defines the following term: Zhang & Minei Expires March 29, 2014 [Page 3] Internet-Draft Applicability for Stateful PCE September 2013 Minimum Cut Set: the minimum set of links for a specific source destination pair which, when removed from the network, results in a specific source being completely isolated from specific destination. The summed capacity of these links is equivalent to the maximum capacity from the source to the destination by the max-flow min-cut theorem. 3. Overview of Stateful PCE This section is included for the convenience of the reader, please refer to the referenced documents for details of the operation. [I-D.ietf-pce-stateful-pce] specifies a set of extensions to PCEP to enable stateful control of tunnels within and across PCEP sessions in compliance with [RFC4657]. It includes mechanisms to effect tunnel state synchronization between PCCs and PCEs, delegation of control over tunnels to PCEs, and PCE control of timing and sequence of path computations within and across PCEP sessions. [I-D.ietf-pce-stateful-pce] applies equally to MPLS-TE and GMPLS LSPs. [I-D.ietf-pce-stateful-pce] distinguishes between an active and a passive stateful PCE. A passive stateful PCE uses LSP state information learned from PCCs to optimize path computations but does not actively update LSP state. In contrast, an active stateful PCE may issue recommendations to the network. For example, an active stateful PCE may utilize the delegation mechanism to update LSP parameters in those PCCs that delegated control over their LSPs to the PCE. Several new functions are added in PCEP to support both active and passive stateful PCEs. They are described in [I-D.ietf-pce-stateful-pce]. A function can be initiated either from a PCC towards a PCE (C-E) or from a PCE towards a PCC (E-C). The new functions are: Capability negotiation (E-C,C-E): both the PCC and the PCE must announce during PCEP session establishment that they support PCEP stateful PCE extensions. LSP state synchronization (C-E): after the session between the PCC and a stateful PCE is initialized, the PCE must learn the state of a PCC's LSPs before it can perform path computations or update LSP attributes in a PCC. Zhang & Minei Expires March 29, 2014 [Page 4] Internet-Draft Applicability for Stateful PCE September 2013 LSP update request (E-C): A PCE requests modification of attributes on a PCC's LSP. LSP state report (C-E): a PCC sends an LSP state report to a PCE whenever the state of an LSP changes. LSP control delegation (C-E,E-C): a PCC grants to a PCE the right to update LSP attributes on one or more LSPs; the PCE becomes the authoritative source of the LSP's attributes as long as the delegation is in effect; the PCC may withdraw the delegation or the PCE may give up the delegation. [I-D.sivabalan-pce-disco-stateful] defines the extensions needed to support auto-discovery of stateful PCEs when using IGP for PCE discovery. 4. Deployment Considerations This section discusses generic issues with stateful PCE deployments, and how specific protocol mechanisms can be used to address them. 4.1. Multi-PCE Deployments Stateless and stateful PCEs can co-exist in the same network and be in charge of path computation of different types. To solve the problem of distinguishing between the two types of PCEs, either discovery or configuration may be used. The capability negotiation in [I-D.ietf-pce-stateful-pce] ensures correct operation when the PCE address is configured on the PCC. Multiple stateful PCEs can co-exist in the same network. These PCEs may provide redundancy for load sharing, resilience, or partitioning of computation features. Regardless of the reason for multiple PCEs, an LSP is only delegated to one of the PCEs at any given point in time. [I-D.ietf-pce-stateful-pce] describes how LSPs can be re- delegated between PCEs, and the procedures on a PCE failure. [I-D.ietf-pce-questions] discusses various approaches for synchronizing state among the PCEs when multiple PCEs are used for load sharing and compute LSPs for the same network. 4.2. LSP State Synchronization A stateful PCE maintains two sets of information for use in path computation. The first is the Traffic Engineering Database (TED) which includes the topology and resource state in the network. This information can be obtained by a stateful PCE using the same mechanisms as a stateless PCE (see [RFC4655]). The second is the LSP Zhang & Minei Expires March 29, 2014 [Page 5] Internet-Draft Applicability for Stateful PCE September 2013 State Database (LSP-DB), in which a PCE stores attributes of all active LSPs in the network, such as their paths through the network, bandwidth/resource usage, switching types and LSP constraints. The stateful PCE extensions defined in [I-D.ietf-pce-stateful-pce] support population of this database using information received from PCCs via LSP state report messages. Population of the LSP database via other means is not precluded. Because the accuracy of the computations depends on the accuracy of the databases used, it is worth noting that the PCE view lags behind the true state of the network, because the updates must reach the PCE from the network. Thus, the use of stateful PCE reduces but cannot eliminate the possibility of crankbacks, nor can it guarantee optimal computations all the time. [I-D.ietf-pce-questions] discusses these limitations and potential ways to alleviate them. 4.3. PCE Survivability For a stateful PCE, an important issue is to get the LSP state information resynchronized after a restart. [I-D.ietf-pce-stateful-pce] includes support of a synchronization function, allowing the PCC to synchronize its LSP state with the PCE. This can be applied equally to a Label Edge Router (LER) client or another PCE, allowing for support of multiple ways of re-acquiring the LSP database on a restart. For example, the state can be retrieved from the network nodes, or from another stateful PCE. Because synchronization may also be skipped, if a PCE implementation has the means to retrieve its database in a different way (for example from a backup copy stored locally), the state can be restored without further overhead in the network. A hybrid approach where the bulk of the state is recovered locally, and a small amount of state is reacquired from the network is also possible. Note that locally recovering the state would still require some degree of resynchronization to ensure that the recovered state is indeed up-to- date. Depending on the resynchronization mechanism used, there may be additional load on the PCE, and there may be a delay in reaching the synchronized state, which may negatively affect survivabiliy. Different resynchronization methods are suited for different deployments and objectives. 5. Application Scenarios In the following sections, several use cases are described, showcasing scenarios that benefit from the deployment of a stateful PCE. Zhang & Minei Expires March 29, 2014 [Page 6] Internet-Draft Applicability for Stateful PCE September 2013 5.1. Optimization of LSP Placement The following use cases demonstrate a need for visibility into global LSP states in PCE path computations, and for a PCE control of sequence and timing in altering LSP path characteristics within and across PCEP sessions. Reference topologies for the use cases described later in this section are shown in Figures 1 and 2. Some of the use cases below are focused on MPLS-TE deployments, but may also apply to GMPLS. Unless otherwise cited, use cases assume that all LSPs listed exist at the same LSP priority. The main benefit in the cases below comes from moving away from an asynchronous PCC-driven mode of operation to a model that allows for central control over LSP computations and setup, and focuses specifically on the active stateful PCE model of operation. +-----+ | A | +-----+ \ +-----+ +-----+ | C |----------------------| E | +-----+ +-----+ / \ +-----+ / +-----+ +-----| D |-----+ | B | +-----+ +-----+ Figure 1: Reference topology 1 +-----+ +-----+ +-----+ | A | | B | | C | +--+--+ +--+--+ +--+--+ | | | | | | +--+--+ +--+--+ +--+--+ | E +--------+ F +--------+ G | +-----+ +-----+ +-----+ Figure 2: Reference topology 2 Zhang & Minei Expires March 29, 2014 [Page 7] Internet-Draft Applicability for Stateful PCE September 2013 5.1.1. Throughput Maximization and Bin Packing Because LSP attribute changes in [RFC5440] are driven by PCReq messages under control of a PCC's local timers, the sequence of resource reservation arrivals occurring in the network will be randomized. This, coupled with a lack of global LSP state visibility on the part of a stateless PCE may result in suboptimal throughput in a given network topology, as will be shown in the example below. Reference topology 2 in Figure 2 and Tables 1 and 2 show an example in which throughput is at 50% of optimal as a result of lack of visibility and synchronized control across PCC's. In this scenario, the decision must be made as to whether to route any portion of the E-G demand, as any demand routed for this source and destination will decrease system throughput. +------+--------+----------+ | Link | Metric | Capacity | +------+--------+----------+ | A-E | 1 | 10 | | B-F | 1 | 10 | | C-G | 1 | 10 | | E-F | 1 | 10 | | F-G | 1 | 10 | +------+--------+----------+ Table 1: Link parameters for Throughput use case +------+-----+-----+-----+--------+----------+-------+ | Time | LSP | Src | Dst | Demand | Routable | Path | +------+-----+-----+-----+--------+----------+-------+ | 1 | 1 | E | G | 10 | Yes | E-F-G | | 2 | 2 | A | B | 10 | No | --- | | 3 | 1 | F | C | 10 | No | --- | +------+-----+-----+-----+--------+----------+-------+ Table 2: Throughput use case demand time series In many cases throughput maximization becomes a bin packing problem. While bin packing itself is an NP-hard problem, a number of common heuristics which run in polynomial time can provide significant improvements in throughput over random reservation event distribution, especially when traversing links which are members of the minimum cut set for a large subset of source destination pairs. Tables 3 and 4 show a simple use case using Reference Topology 1 in Figure 1, where LSP state visibility and control of reservation order across PCCs would result in significant improvement in total Zhang & Minei Expires March 29, 2014 [Page 8] Internet-Draft Applicability for Stateful PCE September 2013 throughput. +------+--------+----------+ | Link | Metric | Capacity | +------+--------+----------+ | A-C | 1 | 10 | | B-C | 1 | 10 | | C-E | 10 | 5 | | C-D | 1 | 10 | | D-E | 1 | 10 | +------+--------+----------+ Table 3: Link parameters for Bin Packing use case +------+-----+-----+-----+--------+----------+---------+ | Time | LSP | Src | Dst | Demand | Routable | Path | +------+-----+-----+-----+--------+----------+---------+ | 1 | 1 | A | E | 5 | Yes | A-C-D-E | | 2 | 2 | B | E | 10 | No | --- | +------+-----+-----+-----+--------+----------+---------+ Table 4: Bin Packing use case demand time series 5.1.2. Deadlock This section discusses a use case of cross-LSP impact under degraded operation. Most existing RSVP-TE implementations will not tear down established LSPs in the event of the failure of the bandwidth increase procedure detailed in [RFC3209]. This behavior is directly implied to be correct in [RFC3209] and is often desirable from an operator's perspective, because either a) the destination prefixes are not reachable via any means other than MPLS or b) this would result in significant packet loss as demand is shifted to other LSPs in the overlay mesh. In addition, there are currently few implementations offering dynamic ingress admission control (policing of the traffic volume mapped onto an LSP) at the LER. Having ingress admission control on a per LSP basis is not necessarily desirable from an operational perspective, as a) one must over-provision tunnels significantly in order to avoid deleterious effects resulting from stacked transport and flow control systems (for example for tunnels that are dynamically resized based on current traffic) and b) there is currently no efficient commonly available northbound interface for dynamic configuration of per LSP ingress admission control. Lack of ingress admission control coupled with the behavior in [RFC3209] may result in LSPs operating out of profile for significant Zhang & Minei Expires March 29, 2014 [Page 9] Internet-Draft Applicability for Stateful PCE September 2013 periods of time. It is reasonable to expect that these out-of- profile LSPs will be operating in a degraded state and experience traffic loss, but because they end up sharing common network interfaces with other LSPs operating within their bandwidth reservations, they will end up impacting the operation of the in- profile LSPs, even when there is unused network capacity elsewhere in the network. Furthermore, this behavior will cause information loss in the TED with regards to the actual available bandwidth on the links used by the out-of-profile LSPs, as the reservations on the links no longer reflect the capacity used. Reference Topology 1 in Figure 1 and Tables 5 and 6 show a use case that demonstrates this behavior. Two LSPs, LSP 1 and LSP 2 are signaled with demand 2 and routed along paths A-C-D-E and B-C-D-E respectively. At a later time, the demand of LSP 1 increases to 20. Under such a demand, the LSP cannot be resignaled. However, the existing LSP will not be torn down. In the absence of ingress policing, traffic on LSP 1 will cause degradation for traffic of LSP 2 (due to oversubscription on the links C-D and D-E), as well as information loss in the TED with regard to the actual network state. The problem could be easily ameliorated by global visibility of LSP state coupled with PCC-external demand measurements and placement of two LSPs on disjoint links. Note that while the demand of 20 for LSP 1 could never be satisfied in the given topology, what could be achieved would be isolation from the ill-effects of the (unsatisfiable) increased demand. +------+--------+----------+ | Link | Metric | Capacity | +------+--------+----------+ | A-C | 1 | 10 | | B-C | 1 | 10 | | C-E | 10 | 5 | | C-D | 1 | 10 | | D-E | 1 | 10 | +------+--------+----------+ Table 5: Link parameters for the 'Degraded operation' example Zhang & Minei Expires March 29, 2014 [Page 10] Internet-Draft Applicability for Stateful PCE September 2013 +------+-----+-----+-----+--------+----------+---------+ | Time | LSP | Src | Dst | Demand | Routable | Path | +------+-----+-----+-----+--------+----------+---------+ | 1 | 1 | A | E | 2 | Yes | A-C-D-E | | 2 | 2 | B | E | 2 | Yes | B-C-D-E | | 3 | 1 | A | E | 20 | No | --- | +------+-----+-----+-----+--------+----------+---------+ Table 6: Degraded operation demand time series 5.1.3. Minimum Perturbation As a result of both the lack of visibility into global LSP state and the lack of control over event ordering across PCE sessions, unnecessary perturbations may be introduced into the network by a stateless PCE. Tables 7 and 8 show an example of an unnecessary network perturbation using Reference Topology 1 in Figure 1. In this case an unimportant (high LSP priority value) LSP (LSP1) is first set up along the shortest path. At time 2, which is assumed to be relatively close to time 1, a second more important (lower LSP- priority value) LSP (LSP2) is established, preempting LSP1, potentially causing traffic loss. LSP1 is then reestablished on the longer A-C-E path. +------+--------+----------+ | Link | Metric | Capacity | +------+--------+----------+ | A-C | 1 | 10 | | B-C | 1 | 10 | | C-E | 10 | 10 | | C-D | 1 | 10 | | D-E | 1 | 10 | +------+--------+----------+ Table 7: Link parameters for the 'Minimum-Perturbation' example +------+-----+-----+-----+--------+----------+----------+---------+ | Time | LSP | Src | Dst | Demand | LSP Prio | Routable | Path | +------+-----+-----+-----+--------+----------+----------+---------+ | 1 | 1 | A | E | 7 | 7 | Yes | A-C-D-E | | 2 | 2 | B | E | 7 | 0 | Yes | B-C-D-E | | 3 | 1 | A | E | 7 | 7 | Yes | A-C-E | +------+-----+-----+-----+--------+----------+----------+---------+ Table 8: Minimum-Perturbation LSP and demand time series A stateful PCE can help in this scenario by evaluating both requests at the same time (due to their proximity in time). This will ensure Zhang & Minei Expires March 29, 2014 [Page 11] Internet-Draft Applicability for Stateful PCE September 2013 placement of the more important LSP along the shortest path, avoiding the preemption of the lower priority LSP. Similarly, when a new higher priority LSP which requires preemption of existing lower priority LSP(s), a stateful PCE can determine the minimum number of lower priority LSP(s) to reroute using the make-before-break (MBB) mechanism without disrupting any service and then set up the higher priority LSP. 5.1.4. Predictability Randomization of reservation events caused by lack of control over event ordering across PCE sessions results in poor predictability in LSP routing. An offline system applying a consistent optimization method will produce predictable results to within either the boundary of forecast error when reservations are over-provisioned by reasonable margins or to the variability of the signal and the forecast error when applying some hysteresis in order to minimize churn. Predictable results are valuable for being able to simulate the network and reliably test it under various scenarios, especially under various failure modes and planned maintenances when predictable path characteristics are desired under contention for network resources. Reference Topology 1 and Tables 9, 10 and 11 show the impact of event ordering and predictability of LSP routing. +------+--------+----------+ | Link | Metric | Capacity | +------+--------+----------+ | A-C | 1 | 10 | | B-C | 1 | 10 | | C-E | 1 | 10 | | C-D | 1 | 10 | | D-E | 1 | 10 | +------+--------+----------+ Table 9: Link parameters for the 'Predictability' example +------+-----+-----+-----+--------+----------+---------+ | Time | LSP | Src | Dst | Demand | Routable | Path | +------+-----+-----+-----+--------+----------+---------+ | 1 | 1 | A | E | 7 | Yes | A-C-E | | 2 | 2 | B | E | 7 | Yes | B-C-D-E | +------+-----+-----+-----+--------+----------+---------+ Table 10: Predictability LSP and demand time series 1 Zhang & Minei Expires March 29, 2014 [Page 12] Internet-Draft Applicability for Stateful PCE September 2013 +------+-----+-----+-----+--------+----------+---------+ | Time | LSP | Src | Dst | Demand | Routable | Path | +------+-----+-----+-----+--------+----------+---------+ | 1 | 2 | B | E | 7 | Yes | B-C-E | | 2 | 1 | A | E | 7 | Yes | A-C-D-E | +------+-----+-----+-----+--------+----------+---------+ Table 11: Predictability LSP and demand time series 2 As can be shown in the example, both LSPs are routed in both cases, but along very different paths. This would be a challenge if reliable simulation of the network is attempted. A stateful PCE can solve this through control over LSP ordering. 5.2. Auto-bandwidth Adjustment The bandwidth requirement of LSPs often change over time, requiring resizing the LSP. Currently the head-end node performs this function by monitoring the actual bandwidth usage, triggering a recomputation and resignaling when a threshold is reached. This operation is referred as auto-bandwidth adjustment. The head-end node either recomputes the path locally, or it requests a recomputation from a PCE by sending a Path Computation Request (PCReq) message. In the latter case, the PCE computes a new path and provides the new route suggestion. Upon receiving the reply from the PCE, the PCC re- signals the LSP in Shared-Explicit (SE) mode along the newly computed path. If a passive stateful PCE is used, only the new bandwidth information is needed to trigger a path re-computation since the LSP information is already known to the PCE. Note that in this scenario, the head-end node is the one that drives the LSP resizing based on local information, and that the difference between using a stateless and a passive stateful PCE is in the level of optimization of the LSP placement as discussed in the previous section. A more interesting smart bandwidth adjustment case is one where the LSP resizing decision is done by an external entity, with access to additional information such as historical trending data, application- specific information about expected demands or policy information, as well as knowledge of the actual desired flow volumes. In this case an active stateful PCE provides an advantage in both the computation with knowledge of all LSPs in the domain and in the ability to trigger bandwidth modification of the LSP. 5.3. Bandwidth Scheduling Bandwidth scheduling allows network operators to reserve resources in advance according to the agreements with their customers, and allow them to transmit data with specified starting time and duration, for Zhang & Minei Expires March 29, 2014 [Page 13] Internet-Draft Applicability for Stateful PCE September 2013 example for a scheduled bulk data replication between data centers. Traditionally, this can be supported by NMS operation through path pre-establishment and activation on the agreed starting time. However, this does not provide efficient network usage since the established paths exclude the possibility of being used by other services even when they are not used for undertaking any service. It can also be accomplished through GMPLS protocol extensions by carrying the related request information (e.g., starting time and duration) across the network. Nevertheless, this method inevitably increases the complexity of signaling and routing process. A passive stateful PCE can support this application with better efficiency since it can alleviate the burden of processing on network elements. This requires the PCE to maintain the scheduled LSPs and their associated resource usage, as well as the ability of head-ends to trigger signaling for LSP setup/deletion at the correct time. This approach requires coarse time synchronization between PCEs and PCCs. If an active stateful PCE is available, the PCE can trigger the setup/deletion of scheduled requests in a centralized manner, without modification of existing head-end behaviors, by notifying the PCCs to set up or tear down the paths. 5.4. Recovery The recovery use cases discussed in the following sections show how leveraging a stateful PCE can simplify the computation of recovery path(s). In particular, two characteristics of a stateful PCE are used: 1) using information stored in the LSP-DB for determining shared protection resources and 2) performing computations with knowledge of all LSPs in a domain. 5.4.1. Protection For protection purposes, a PCC may send a request to a PCE for computing a set of paths for a given LSP. Alternatively, the PCC can send multiple requests to the PCE, asking for working and backup LSPs separately. Either way, the resources bound to backup paths can be shared by different LSPs to improve the overall network efficiency, such as m:n protection or pre-configured shared mesh recovery techniques as specified in [RFC4427]. If resource sharing is supported for LSP protection, the information relating to existing LSPs is required to avoid allocation of shared protection resources to two LSPs that might fail together and cause protection contention issues. A stateless PCE can accommodate this use case by having the PCC pass this information as a constraint in the path computation request. A passive stateful PCE can more easily accommodate this need using the information stored in its LSP-DB. Furthermore, an Zhang & Minei Expires March 29, 2014 [Page 14] Internet-Draft Applicability for Stateful PCE September 2013 active stateful PCE can help with (re)-optimizization of protection resource sharing as well as LSP maintenance operation with fewer impact on protection resources. +----+ |PCE | +----+ +------+ +------+ +------+ | A +----------+ B +----------+ C | +--+---+ +---+--+ +---+--+ | | | | +---------+ | | | | | +--+---+ +------+ | +-----+ E +----------+ D +-----+ +------+ +------+ Figure 3: Reference topology 3 For example, in the network depicted in Figure 3 , suppose there exists LSP1 with working path LSP1_working following A->E and with backup path LSP1_backup following A->B->E. A request arrives asking for a working and backup path pair to be computed for LSP2, for a request from B to E. If the PCE decides LSP2_working follows B->A->E, then the backup path LSP2_backup should not use the same protection resource with LSP1 since LSP2 shares part of its resource (specifically A->E) with LSP1 (i.e., these two LSPs are in the same shared risk group). Alternatively, there is no such constraint if B->C->D->E is chosen for LSP2_working. If a stateless PCE is used, the head node B needs to be aware of the existence of LSPs which share the route of LSP2_working and of the details of their protection resources. B must pass this information to the PCE as a constraint so as to request a path with diversity. On the other hand, a stateful PCE can get the LSPs information by itself and can achieve the goal of finding SRLG-diversified protection paths for both LSPs. This is made possible by comparing the LSP resource usage exploiting the LSP DB accessible by the stateful PCE. 5.4.2. Restoration In case of a link failure, such as fiber cut, multiple LSPs may fail at the same time. Thus, the source nodes of the affected LSPs will be informed of the failure by the nodes detecting the failure. These source nodes will send requests to a PCE for rerouting. In order to Zhang & Minei Expires March 29, 2014 [Page 15] Internet-Draft Applicability for Stateful PCE September 2013 reuse the resource taken by an existing LSP, the source node can send a PCReq message including the XRO object with F bit set, together with RRO object, as specified in [RFC5521]. If a stateless PCE is exploited, it might respond to the rerouting requests separately if they arrive at different times. Thus, it might result in sub-optimal resource usage. Even worse, it might unnecessarily block some of the rerouting requests due to insufficient resources for later-arrived rerouting messages. If a passive stateful PCE is used to fulfill this task, it can re-compute the affected LSPs concurrently while reusing part of the existing LSPs resources when it is informed of the failed link identifier provided by the first request. This is made possible since the passive stateful PCE can check what other LSPs are affected by the failed link and their route information by inspecting its LSP-DB. As a result, a better performance, such as better resource usage, minimal probability of blocking upcoming new rerouting requests sent as a result of the link failure, can be achieved. In order to further reduce the amount of LSP rerouting messages flow in the network, the notification can be performed at the node(s) which detect the link failure. For example, suppose there are two LSPs in the network as shown in Figure 3: (i) LSP1: A->E->D->C; (ii) LSP2: B->E->D. They traverse the failed link between E-D. When D detects the failure, it can send a notification message to a stateful PCE. Note that the stateful PCE stores the path information of the LSPs that are affected by the link failure, so it does not need to acquire this information from D. Moreover, it can make use of the bandwidth resources occupied by the affected LSPs when performing path recalculation. After D receives the new paths from the PCE, it notifies the ingress nodes of the LSPs, i.e., A and B, and specifies the new paths which should be used as the rerouting paths. To support this, it would require extensions to the existing signaling protocols. Alternatively, if the target is to avoid resource contention within the time-window of high LSP requests, a stateful PCE can retain the under-construction LSP resource usage information for a given time and exclude it from being used for forthcoming LSPs request. In this way, it can ensure that the resource will not be double-booked and thus the issue of resource contention and computation crank-backs can be resolved. 5.4.3. SRLG Diversity An alternative way to achieve efficient resilience is to maintain SRLG disjointness between LSPs, irrespective of whether these LSPs share the source and destination nodes or not. This can be achieved Zhang & Minei Expires March 29, 2014 [Page 16] Internet-Draft Applicability for Stateful PCE September 2013 at provisioning time, if the routes of all the LSPs are requested together, using a synchronized computation of the different LSPs with SRLG disjointness constraint. If the LSPs need to be provisioned at different times (more general, the routes are requested at different times, e.g. in the case of a restoration), the PCC can specify, as constraints to the path computation a set of Shared Risk Link Groups (SRLGs) using the Exclude Route Object [RFC5521]. However, for the latter to be effective, it is needed that the entity that requests the route to the PCE maintains updated SRLG information of all the LSPs to which it must maintain the disjointness. A stateless PCE can compute an SRLG-disjoint path by inspecting the TED and precluding the links with the same SRLG values specified in the PCReq message sent by a PCC. A passive stateful PCE maintains the updated SRLG information of the established LSPs in a centralized manner. Therefore, the PCC can specify as constraints to the path computation the SRLG disjointness of a set of already established LSPs by only providing the LSP identifiers. Similarly, a passive stateful PCE can also accommodate disjointness using other contraints, such as link, node or path segment etc. 5.5. Maintenance of Virtual Network Topology (VNT) In Multi-Layer Networks (MLN), a Virtual Network Topology (VNT) [RFC5212] consists of a set of one or more TE LSPs in the lower layer which provides TE links to the upper layer. In [RFC5623], the PCE- based architecture is proposed to support path computation in MLN networks in order to achieve inter-layer TE. The establishment/teardown of a TE link in VNT needs to take into consideration the state of existing LSPs and/or new LSP request(s) in the higher layer. Hence, when a stateless PCE cannot find the route for a request based on the upper layer topology information, it does not have enough information to decide whether to set up or remove a TE link or not, which then can result in non-optimal usage of resource. On the other hand, a passive stateful PCE can make a better decision of when and how to modify the VNT either to accommodate new LSP requests or to re-optimize resource usage across layers irrespective of the PCE models as described in [RFC5623]. Furthermore, given the active capability, the stateful PCE can issue VNT modification suggestions in order to accommodate path setup requests or re-optimize resource usage across layers. 5.6. LSP Re-optimization In order to make efficient usage of network resources, it is sometimes desirable to re-optimize one or more LSPs dynamically. In Zhang & Minei Expires March 29, 2014 [Page 17] Internet-Draft Applicability for Stateful PCE September 2013 the case of a stateless PCE, in order to optimize network resource usage dynamically through online planning, a PCC must send a request to the PCE together with detailed path/bandwidth information of the LSPs that need to be concurrently optimized. This means the PCC must be able to determine when and which LSPs should be optimized. In the case of a stateful PCE, given the LSP state information in the LSP database, the process of dynamic optimization of network resources can be automated without requiring the PCC to supply LSP state information or to trigger the request. Moreover, since a stateful PCE can maintain information for all LSPs that are in the process of being set up and since it may have the ability to control timing and sequence of LSP setup/deletion, the optimization procedures can be performed more intelligently and effectively. A stateful PCE can also determine which LSP should be re-optimized based on network events. For example, when a LSP is torn down, its resources are freed. This can trigger the stateful PCE to automatically determine which LSP should be reoptimized so that the recently freed resources may be allocated to it. A special case of LSP re-optimization is Global Concurrent Optimization (GCO) [RFC5557]. Global control of LSP operation sequence in [RFC5557] is predicated on the use of what is effectively a stateful (or semi-stateful) NMS. The NMS can be either not local to the network nodes, in which case another northbound interface is required for LSP attribute changes, or local/collocated, in which case there are significant issues with efficiency in resource usage. A stateful PCE adds a few features that: o Roll the NMS visibility into the PCE and remove the requirement for an additional northbound interface o Allow the PCE to determine when re-optimization is needed, with which level (GCO or a more incremental optimization) o Allow the PCE to determine which LSPs should be re-optimized o Allow a PCE to control the sequence of events across multiple PCCs, allowing for bulk (and truly global) optimization, LSP shuffling etc. 5.7. Resource Defragmentation In networks with link bundles, if LSPs are dynamically allocated and released over time, the resource becomes fragmented. The overall available resource on a (bundle) link might be sufficient for a new LSP request, but if the available resource is not continuous, the request is rejected. In order to perform the defragmentation procedure, stateful PCEs can be used, since global visibility of LSPs Zhang & Minei Expires March 29, 2014 [Page 18] Internet-Draft Applicability for Stateful PCE September 2013 in the network is required to accurately assess resources on the LSPs, and perform de-fragmentation while ensuring a minimal disruption of the network. This use case cannot be accommodated by a stateless PCE since it does not possess the detailed information of existing LSPs in the network. A case of particular interest to GMPLS-based transport networks is the frequency defragmentation in flexible grid. In Flexible grid networks [I-D.ogrcetal-ccamp-flexi-grid-fwk], LSPs with different slot widths (such as 12.5G, 25G etc.) can co-exist so as to accommodate the services with different bandwidth requests. Therefore, even if the overall spectrum can meet the service request, it may not be usable if it is not contiguous. Thus, with the help of existing LSP state information, a stateful PCE can make the resource grouped together to be usable. Moreover, a stateful PCE can proactively choose routes for upcoming path requests to reduce the chance of spectrum fragmentation. 5.8. Impairment-Aware Routing and Wavelength Assignment (IA-RWA) In WSONs [RFC6163], a wavelength-switched LSP traverses one or more fiber links. The bit rates of the client signals carried by the wavelength LSPs may be the same or different. Hence, a fiber link may transmit a number of wavelength LSPs with equal or mixed bit rate signals. For example, a fiber link may multiplex the wavelengths with only 10G signals, mixed 10G and 40G signals, or mixed 40G and 100G signals. IA-RWA in WSONs refers to the RWA process (i.e., lightpath computation) that takes into account the optical layer/transmission imperfections by considering as additional (i.e., physical layer) constraints. To be more specific, linear and non-linear effects associated with the optical network elements should be incorporated into the route and wavelength assignment procedure. For example, the physical imperfection can result in the interference of two adjacent lightpaths. Thus, a guard band should be reserved between them to alleviate these effects. The width of the guard band between two adjacent wavelengths depends on their characteristics, such as modulation formats and bit rates. Two adjacent wavelengths with different characteristics (e.g., different bit rates) may need a wider guard band and with same characteristics may need a narrower guard band. For example, 50GHz spacing may be acceptable for two adjacent wavelengths with 40G signals. But for two adjacent wavelengths with different bit rates (e.g., 10G and 40G), a larger spacing such as 300GHz spacing may be needed. Hence, the characteristics (states) of the existing wavelength LSPs should be considered for a new RWA request in WSON. Zhang & Minei Expires March 29, 2014 [Page 19] Internet-Draft Applicability for Stateful PCE September 2013 In summary, when stateful PCEs are used to perform the IA-RWA procedure, they need to know the characteristics of the existing wavelength LSPs. The impairment information relating to existing and to-be-established LSPs can be obtained by nodes in WSON networks via external configuration or other means such as monitoring or estimation based on a vendor-specific impair model. However, WSON related routing protocols, i.e., [I-D.ietf-ccamp-wson-signal-compatibility-ospf] and [I-D.ietf-ccamp-gmpls-general-constraints-ospf-te], only advertise limited information (i.e., availability) of the existing wavelengths, without defining the supported client bit rates. It will incur substantial amount of control plane overhead if routing protocols are extended to support dissemination of the new information relevant for the IA-RWA process. In this scenario, stateful PCE(s) would be a more appropriate mechanism to solve this problem. Stateful PCE(s) can exploit impairment information of LSPs stored in LSP-DB to provide accurate RWA calculation. 6. Security Considerations The PCEP extensions in support of stateful PCE and the delegation of path control, result in more information being available for a hypothetical adversary and a number of additional attack surfaces which must be protected. [I-D.ietf-pce-stateful-pce] discusses different attack vectors and defines protocol mechanisms to protect against them. It also lays out implementation requirements for configuration capabilities that allow the operator to control the PCC behavior when faced with an attack. This document does not introduce any new security considerations beyond those discussed in [I-D.ietf-pce-stateful-pce]. 7. Contributing Authors The following people all contributed significantly to this document and are listed below in alphabetical order: Ramon Casellas CTTC - Centre Tecnologic de Telecomunicacions de Catalunya Av. Carl Friedrich Gauss n7 Castelldefels, Barcelona 08860 Spain Email: ramon.casellas@cttc.es Edward Crabbe Google, Inc. 1600 Amphitheatre Parkway Zhang & Minei Expires March 29, 2014 [Page 20] Internet-Draft Applicability for Stateful PCE September 2013 Mountain View, CA 94043 US Email: edc@google.com Dhruv Dhody Huawei Technology Leela Palace Bangalore, Karnataka 560008 INDIA EMail: dhruv.dhody@huawei.com Oscar Gonzalez de Dios Telefonica Investigacion y Desarrollo Emilio Vargas 6 Madrid, 28045 Spain Phone: +34 913374013 Email: ogondio@tid.es Young Lee Huawei 1700 Alma Drive, Suite 100 Plano, TX 75075 US Phone: +1 972 509 5599 x2240 Fax: +1 469 229 5397 EMail: leeyoung@huawei.com Jan Medved Cisco Systems, Inc. 170 West Tasman Dr. San Jose, CA 95134 US Email: jmedved@cisco.com Robert Varga Pantheon Technologies LLC Mlynske Nivy 56 Bratislava 821 05 Slovakia Email: robert.varga@pantheon.sk Fatai Zhang Huawei Technologies F3-5-B R&D Center, Huawei Base Bantian, Longgang District Shenzhen 518129 P.R.China Phone: +86-755-28972912 Zhang & Minei Expires March 29, 2014 [Page 21] Internet-Draft Applicability for Stateful PCE September 2013 Email: zhangfatai@huawei.com Xiaobing Zi Email: unknown 8. Acknowledgements We would like to thank Cyril Margaria, Adrian Farrel, JP Vasseur and Ravi Torvi for the useful comments and discussions. 9. References 9.1. Normative References [I-D.ietf-pce-questions] Farrel, A. and D. King, "Unanswered Questions in the Path Computation Element Architecture", draft-ietf-pce-questions-00 (work in progress), July 2013. [I-D.ietf-pce-stateful-pce] Crabbe, E., Medved, J., Minei, I., and R. Varga, "PCEP Extensions for Stateful PCE", draft-ietf-pce-stateful-pce-06 (work in progress), August 2013. [RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation Element (PCE)-Based Architecture", RFC 4655, August 2006. [RFC5440] Vasseur, JP. and JL. Le Roux, "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, March 2009. 9.2. Informative References [I-D.crabbe-pce-stateful-pce-mpls-te] Crabbe, E., Medved, J., Minei, I., and R. Varga, "Stateful PCE extensions for MPLS-TE LSPs", draft-crabbe-pce-stateful-pce-mpls-te-01 (work in progress), May 2013. [I-D.ietf-ccamp-gmpls-general-constraints-ospf-te] Zhang, F., Lee, Y., Han, J., Bernstein, G., and Y. Xu, "OSPF-TE Extensions for General Network Element Constraints", draft-ietf-ccamp-gmpls-general-constraints-ospf-te-05 (work in progress), June 2013. Zhang & Minei Expires March 29, 2014 [Page 22] Internet-Draft Applicability for Stateful PCE September 2013 [I-D.ietf-ccamp-wson-signal-compatibility-ospf] Lee, Y. and G. Bernstein, "GMPLS OSPF Enhancement for Signal and Network Element Compatibility for Wavelength Switched Optical Networks", draft-ietf-ccamp-wson-signal-compatibility-ospf-11 (work in progress), February 2013. [I-D.ietf-pce-gmpls-pcep-extensions] Margaria, C., Dios, O., and F. Zhang, "PCEP extensions for GMPLS", draft-ietf-pce-gmpls-pcep-extensions-08 (work in progress), July 2013. [I-D.ogrcetal-ccamp-flexi-grid-fwk] Dios, O., Casellas, R., Zhang, F., Fu, X., Ceccarelli, D., and I. Hussain, "Framework and Requirements for GMPLS based control of Flexi-grid DWDM networks", draft-ogrcetal-ccamp-flexi-grid-fwk-03 (work in progress), June 2013. [I-D.sivabalan-pce-disco-stateful] Sivabalan, S., Medved, J., and X. Zhang, "IGP Extensions for Stateful PCE Discovery", draft-sivabalan-pce-disco-stateful-02 (work in progress), July 2013. [MPLS-PC] Chaieb, I., Le Roux, JL., and B. Cousin, "Improved MPLS-TE LSP Path Computation using Preemption", Global Information Infrastructure Symposium, July 2007. [MXMN-TE] Danna, E., Mandal, S., and A. Singh, "Practical linear programming algorithm for balancing the max-min fairness and throughput objectives in traffic engineering", pre- print, 2011. [NET-REC] Vasseur, JP., Pickavet, M., and P. Demeester, "Network Recovery: Protection and Restoration of Optical, SONET- SDH, IP, and MPLS", The Morgan Kaufmann Series in Networking, June 2004. [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209, December 2001. [RFC4427] Mannie, E. and D. Papadimitriou, "Recovery (Protection and Restoration) Terminology for Generalized Multi-Protocol Label Switching (GMPLS)", RFC 4427, March 2006. [RFC4657] Ash, J. and J. Le Roux, "Path Computation Element (PCE) Zhang & Minei Expires March 29, 2014 [Page 23] Internet-Draft Applicability for Stateful PCE September 2013 Communication Protocol Generic Requirements", RFC 4657, September 2006. [RFC5212] Shiomoto, K., Papadimitriou, D., Le Roux, JL., Vigoureux, M., and D. Brungard, "Requirements for GMPLS-Based Multi- Region and Multi-Layer Networks (MRN/MLN)", RFC 5212, July 2008. [RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash, "Policy-Enabled Path Computation Framework", RFC 5394, December 2008. [RFC5521] Oki, E., Takeda, T., and A. Farrel, "Extensions to the Path Computation Element Communication Protocol (PCEP) for Route Exclusions", RFC 5521, April 2009. [RFC5557] Lee, Y., Le Roux, JL., King, D., and E. Oki, "Path Computation Element Communication Protocol (PCEP) Requirements and Protocol Extensions in Support of Global Concurrent Optimization", RFC 5557, July 2009. [RFC5623] Oki, E., Takeda, T., Le Roux, JL., and A. Farrel, "Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic Engineering", RFC 5623, September 2009. [RFC6163] Lee, Y., Bernstein, G., and W. Imajuku, "Framework for GMPLS and Path Computation Element (PCE) Control of Wavelength Switched Optical Networks (WSONs)", RFC 6163, April 2011. Authors' Addresses Xian Zhang (editor) Huawei Technologies F3-5-B R&D Center, Huawei Industrial Base, Bantian, Longgang District Shenzhen, Guangdong 518129 P.R.China Email: zhang.xian@huawei.com Zhang & Minei Expires March 29, 2014 [Page 24] Internet-Draft Applicability for Stateful PCE September 2013 Ina Minei (editor) Juniper Networks, Inc. 1194 N. Mathilda Ave. Sunnyvale, CA 94089 US Email: ina@juniper.net Zhang & Minei Expires March 29, 2014 [Page 25]