PCE Working Group X. Zhang, Ed.
Internet-Draft Huawei Technologies
Intended status: Informational I. Minei, Ed.
Expires: May 4, 2017 Google, Inc.
October 31, 2016

Applicability of a Stateful Path Computation Element (PCE)


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. PCE Communication Protocol (PCEP) extensions required for stateful PCE usage are covered in separate documents.

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Table of Contents

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.

As per [RFC4655], a PCE can be either stateful or stateless. 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 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. This state information allows the PCE to compute constrained paths while considering individual LSPs and their inter-dependency. However, this requires reliable state synchronization mechanisms between the PCE and the network, between the PCE and the PCCs, 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 defines the following terms:

Stateful PCE:
a PCE that has access to not only the network state, but also to the set of active paths and their reserved resources for its computations. A stateful PCE might also retain information regarding LSPs under construction in order to reduce churn and resource contention. The additional state allows the PCE to compute constrained paths while considering individual LSPs and their interactions. Note that this requires reliable state synchronization mechanisms between the PCE and the network, PCE and PCC, and between cooperating PCEs.
Passive Stateful PCE:
a PCE that uses LSP state information learned from PCCs to optimize path computations. It does not actively update LSP state. A PCC maintains synchronization with the PCE.
Active Stateful PCE::
a PCE that 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.
an operation to grant a PCE temporary rights to modify a subset of LSP parameters on one or more PCC's LSPs. LSPs are delegated from a PCC to a PCE, and are referred to as delegated LSPs. The PCC that owns the PCE state for the LSP has the right to delegate it. An LSP is owned by a single PCC at any given point in time. For intra-domain LSPs, this PCC should be the LSP head end.
LSP State Database:
information about all LSPs and their attributes.
PCE Initiation:
a PCE, assuming LSP delegation granted by default, can issue recommendations to the network.
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. Application Scenarios

In the following sections, several use cases are described, showcasing scenarios that benefit from the deployment of a stateful PCE.

3.1. 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 maintenance, 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

3.1.1. Throughput Maximization and Bin Packing

Because LSP attribute changes in [RFC5440] are driven by Path Computation Request (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.