Internet DRAFT - draft-ietf-pce-stateful-pce-app
draft-ietf-pce-stateful-pce-app
PCE Working Group X. Zhang, Ed.
Internet-Draft Huawei Technologies
Intended status: Informational I. Minei, Ed.
Expires: April 21, 2016 Google, Inc.
October 19, 2015
Applicability of a Stateful Path Computation Element (PCE)
draft-ietf-pce-stateful-pce-app-05
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. PCE Communication Protocol (PCEP)
extensions required for stateful PCE usage are covered in separate
documents.
Status of This Memo
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This Internet-Draft will expire on April 21, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview of the Stateful PCE Protocol Extensions . . . . . . 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 . . . . . . . . . . . . . . 6
5.1.1. Throughput Maximization and Bin Packing . . . . . . . 7
5.1.2. Deadlock . . . . . . . . . . . . . . . . . . . . . . 9
5.1.3. Minimum Perturbation . . . . . . . . . . . . . . . . 10
5.1.4. Predictability . . . . . . . . . . . . . . . . . . . 11
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. Point-to-Multi-Point Applications . . . . . . . . . . . . 19
5.9. Impairment-Aware Routing and Wavelength Assignment (IA-
RWA) . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 20
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8. Contributing Authors . . . . . . . . . . . . . . . . . . . . 20
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
10.1. Normative References . . . . . . . . . . . . . . . . . . 22
10.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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
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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. 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. 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 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:
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
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the maximum capacity from the source to the destination by the
max-flow min-cut theorem.
3. Overview of the Stateful PCE Protocol Extensions
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 LSPs within and across PCEP sessions in
compliance with [RFC4657]. It includes mechanisms to effect LSP
state synchronization between PCCs and PCEs, delegation of control
over LSPs 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
and distinguishes between an active and a passive stateful PCE. A
passive stateful PCE uses LSP state information 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 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:
Stateful Capability negotiation (E-C,C-E): both the PCC and the PCE
must announce during PCEP session establishment that they support
stateful PCE PCEP extensions.
LSP state synchronization (C-E): after the session between a PCC and
a stateful PCE is initialized, the PCE can perform path
computation and update attributes in a PCC. However, if the goal
of the PCE is to provide accurate path information based on the
most up-to-date state of the network, the PCE should wait until it
learns the state of the PCC's LSP states before doing so.
LSP update request (E-C): A PCE requests the modification of one or
more attributes (e.g., route) 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
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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 or backup and compute LSPs for the same network.
4.2. LSP State Synchronization
The population of the LSP-DB using information received from PCCs is
supported by the stateful PCE extensions defined in
[I-D.ietf-pce-stateful-pce] , i.e., 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.
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In case of multiple PCEs with different capabilities, co-existing in
the same network, such as a passive stateful PCE and an active
stateful PCE, it is useful to refer to a LSP, be it delegated or not,
by a unique identifier instead of providing detailed information
(e.g., route, bandwidth etc.) associated with it, when these PCEs
cooperate on path computation, such as for loading sharing.
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] defines a synchronization function and
procedure, allowing a PCC to synchronize its LSP state with the PCE
and [I-D.ietf-pce-stateful-sync-optimizations] specifies
optimizations to the synchronizations procedures. LSP state
synchronization procedures can be applied equally to a network nodes
or another PCE, allowing multiple ways of re-acquiring the LSP
database on a restart. 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 an additional load on the PCE, and there may be a
delay in reaching the synchronized state, which may negatively affect
survivability. 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.
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.
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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
5.1.1. 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.
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+------+--------+----------+
| 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
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
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+------+-----+-----+-----+--------+----------+---------+
| 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 label edge router (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
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, thus 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
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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
+------+-----+-----+-----+--------+----------+---------+
| 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,
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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 computing both routes at
the same time. The advantages of using a stateful PCE over
exploiting a stateless PCE via Global Concurrent Optimization(GCO)
are three folds. First is the ability to accommodate concurrent path
computation from different PCCs. Second is the reduction of control
plane overhead since the stateful PCE has the route information of
the affected LSPs. Thirdly, the stateful PCE can use the LSP-DB to
further optimize the placement of LSPs. This will ensure placement
of the more important LSP along the shortest path, avoiding the setup
and subsequent 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
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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
+------+-----+-----+-----+--------+----------+---------+
| 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.
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5.2. Auto-bandwidth Adjustment
The bandwidth requirement of LSPs often change over time, requiring
resizing the LSP. In most implementations available today, 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 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. With a stateless PCE, the head-end node needs to provide the
current used bandwidth and the route information via path computation
request messages. 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
example for a scheduled bulk data replication between data centers.
Traditionally, this can be supported by network management system
(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.
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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
If a PCC can specify in a request whether the computation is for a
working or for protection, and a PCC can report the resource by a
working or protection path, then the following text applies. A PCC
can send multiple requests to the PCE, asking for two LSPs and use
them as working and backup paths 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 active stateful PCE can help with (re)-
optimizization of protection resource sharing as well as LSP
maintenance operation with fewer impact on protection resources.
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+----+
|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 from B to
E. If the PCE decides LSP2_working follows B->A->E, then the backup
path LSP2_backup should not share 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). 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.
Alternatively, a stateless PCE may able to compute carry out Shared
Risk Link Group (SRLG)-diversified paths if TED is extended so that
it includes the SRLG information that are protected by a given backup
resource, but at the expense of a high complexity in routing. On the
other hand, a stateful PCE can get the LSPs information by itself
given that the LSP identifier(s) 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 a 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 reuse the resource taken by an existing LSP, the source node
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can send a PCReq message including the Exclude Route Object (XRO)
with Fail (F) bit set, together with the record route object (RRO)
containing the current route information, 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, the procedure can
be simplified. The PCCs reporting the failures can include LSP
identifiers instead of detailed information and the PCE can find
relevant LSP information by inspecting the LSP-DB. Moreover, the PCE
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 can be achieved, such as
better resource usage or minimal probability of blocking upcoming new
rerouting requests sent as a result of the link failure.
If the target is to avoid resource contention within the time-window
of high number of LSP rerouting 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 alleviated.
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
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, the PCC can specify, as constraints to the path
computation a set of 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.
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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 constraints, 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
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 passive stateful PCE, given the LSP state information in
the LSP database, the process of dynamic optimization of network
resources can be simplified without requiring the PCC to supply
detailed LSP state information. Moreover, an active stateful PCE can
even make the process automated by triggering the request since a
stateful PCE can maintain information for all LSPs that are in the
process of being set up and 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
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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 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
If LSPs are dynamically allocated and released over time, the
resource becomes fragmented. In networks with link bundle, 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
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.
Another case of particular interest is the optical spectrum
defragmentation in flexible grid networks. In Flexible grid networks
[I-D.ietf-ccamp-flexi-grid-fwk], LSPs with different optical spectrum
sizes (such as 12.5GHz, 25GHz etc.) can co-exist so as to accommodate
the services with different bandwidth requests. Therefore, even if
the overall spectrum size can meet the service request, it may not be
usable if the available spectrum resource is not contiguous, but
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rather fragmented into smaller pieces. 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. Point-to-Multi-Point Applications
PCE has been identified as an appropriate technology for the
determination of the paths of point-to-multipoint (P2MP) TE LSPs
[RFC5671]. The application scenarios and use-cases described in
Section 5.1, Section 5.4 and Section 5.6 are also applicable to P2MP
TE LSPs.
In addition to these, the stateful nature of a PCE simplifies the
information conveyed in PCEP messages since it is possible to refer
to the LSPs via an identifier. For P2MP, this is an added advantage,
where the size of the PCEP message is much larger. In case of
stateless PCEs, modification of a P2MP tree requires encoding of all
leaves along with the paths in PCReq message. But using a stateful
PCE with P2MP capability, the PCEP message can be used to convey only
the modifications (the other information can be retrieved from the
identifier via the LSP-DB).
5.9. Impairment-Aware Routing and Wavelength Assignment (IA-RWA)
In Wavelength Switched Optical Networks (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 10Gb/s signals,
mixed 10Gb/s and 40Gb/s signals, or mixed 40Gb/s and 100Gb/s signals.
IA-RWA in WSONs refers to the 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
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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.
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. IANA Considerations
This document does not require any IANA action.
8. 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
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Av. Carl Friedrich Gauss n7
Castelldefels, Barcelona 08860
Spain
Email: ramon.casellas@cttc.es
Edward Crabbe
Email: edward.crabbe@gmail.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
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F3-5-B R&D Center, Huawei Base
Bantian, Longgang District
Shenzhen 518129 P.R.China
Phone: +86-755-28972912
Email: zhangfatai@huawei.com
Xiaobing Zi
Email: unknown
9. Acknowledgements
We would like to thank Cyril Margaria, Adrian Farrel, JP Vasseur and
Ravi Torvi for the useful comments and discussions.
10. References
10.1. Normative References
[I-D.ietf-pce-questions]
Farrel, A. and D. King, "Unanswered Questions in the Path
Computation Element Architecture", draft-ietf-pce-
questions-08 (work in progress), October 2014.
[I-D.ietf-pce-stateful-pce]
Crabbe, E., Minei, I., Medved, J., and R. Varga, "PCEP
Extensions for Stateful PCE", draft-ietf-pce-stateful-
pce-11 (work in progress), April 2015.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<http://www.rfc-editor.org/info/rfc4655>.
[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<http://www.rfc-editor.org/info/rfc5440>.
10.2. Informative References
[I-D.ietf-ccamp-flexi-grid-fwk]
Dios, O. and R. Casellas, "Framework and Requirements for
GMPLS-based control of Flexi-grid DWDM networks", draft-
ietf-ccamp-flexi-grid-fwk-07 (work in progress), August
2015.
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[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-10 (work in progress), March 2015.
[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-17 (work in progress), August 2015.
[I-D.ietf-pce-gmpls-pcep-extensions]
Margaria, C., Dios, O., and F. Zhang, "PCEP extensions for
GMPLS", draft-ietf-pce-gmpls-pcep-extensions-11 (work in
progress), October 2015.
[I-D.ietf-pce-stateful-sync-optimizations]
Crabbe, E., Minei, I., Medved, J., Varga, R., Zhang, X.,
and D. Dhody, "Optimizations of Label Switched Path State
Synchronization Procedures for a Stateful PCE", draft-
ietf-pce-stateful-sync-optimizations-03 (work in
progress), October 2015.
[I-D.sivabalan-pce-disco-stateful]
Sivabalan, S., Medved, J., and X. Zhang, "IGP Extensions
for Stateful PCE Discovery", draft-sivabalan-pce-disco-
stateful-03 (work in progress), January 2014.
[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,
<http://www.rfc-editor.org/info/rfc3209>.
[RFC4427] Mannie, E., Ed. and D. Papadimitriou, Ed., "Recovery
(Protection and Restoration) Terminology for Generalized
Multi-Protocol Label Switching (GMPLS)", RFC 4427,
DOI 10.17487/RFC4427, March 2006,
<http://www.rfc-editor.org/info/rfc4427>.
[RFC4657] Ash, J., Ed. and J. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol Generic
Requirements", RFC 4657, DOI 10.17487/RFC4657, September
2006, <http://www.rfc-editor.org/info/rfc4657>.
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[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,
DOI 10.17487/RFC5212, July 2008,
<http://www.rfc-editor.org/info/rfc5212>.
[RFC5521] Oki, E., Takeda, T., and A. Farrel, "Extensions to the
Path Computation Element Communication Protocol (PCEP) for
Route Exclusions", RFC 5521, DOI 10.17487/RFC5521, April
2009, <http://www.rfc-editor.org/info/rfc5521>.
[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, DOI 10.17487/RFC5557,
July 2009, <http://www.rfc-editor.org/info/rfc5557>.
[RFC5623] Oki, E., Takeda, T., Le Roux, JL., and A. Farrel,
"Framework for PCE-Based Inter-Layer MPLS and GMPLS
Traffic Engineering", RFC 5623, DOI 10.17487/RFC5623,
September 2009, <http://www.rfc-editor.org/info/rfc5623>.
[RFC5671] Yasukawa, S. and A. Farrel, Ed., "Applicability of the
Path Computation Element (PCE) to Point-to-Multipoint
(P2MP) MPLS and GMPLS Traffic Engineering (TE)", RFC 5671,
DOI 10.17487/RFC5671, October 2009,
<http://www.rfc-editor.org/info/rfc5671>.
[RFC6163] Lee, Y., Ed., Bernstein, G., Ed., and W. Imajuku,
"Framework for GMPLS and Path Computation Element (PCE)
Control of Wavelength Switched Optical Networks (WSONs)",
RFC 6163, DOI 10.17487/RFC6163, April 2011,
<http://www.rfc-editor.org/info/rfc6163>.
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
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Ina Minei (editor)
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: inaminei@google.com
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