Network Working Group X. Zhang, Ed.
Internet-Draft Huawei Technologies
Intended status: Informational I. Minei, Ed.
Expires: August 26, 2013 Juniper Networks, Inc.
February 22, 2013
Applicability of PCEP Extensions for Stateful PCE
draft-zhang-pce-stateful-pce-app-03
Abstract
A Stateful PCE maintains information about LSP characteristics and
resource usage within the network in order to provide traffic
engineering calculations for its associated PCCs. This document
describes general considerations for stateful PCEP and examines its
applicability and benefits through a number of use cases. 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 August 26, 2013.
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include Simplified BSD License text as described in Section 4.e of
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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 . . . . . . . . . . . . . . . . . . . . 5
5. Application scenarios . . . . . . . . . . . . . . . . . . . . 5
5.1. Optimization of LSP placement . . . . . . . . . . . . . . 6
5.1.1. Throughput Maximization and Bin Packing . . . . . . . 6
5.1.2. Deadlock . . . . . . . . . . . . . . . . . . . . . . . 8
5.1.3. Minimum Perturbation . . . . . . . . . . . . . . . . . 9
5.1.4. Predictability . . . . . . . . . . . . . . . . . . . . 10
5.2. Stateful PCE in SDN . . . . . . . . . . . . . . . . . . . 11
5.2.1. Smart Bandwidth Adjustment . . . . . . . . . . . . . . 11
5.2.2. Bandwidth Scheduling . . . . . . . . . . . . . . . . . 12
5.3. Recovery . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.3.1. Protection . . . . . . . . . . . . . . . . . . . . . . 12
5.3.2. Restoration . . . . . . . . . . . . . . . . . . . . . 13
5.3.3. SRLG Diversity . . . . . . . . . . . . . . . . . . . . 14
5.4. Maintenance of Virtual Network Topology (VNT) . . . . . . 15
5.5. LSP Re-optimization . . . . . . . . . . . . . . . . . . . 15
5.6. Resource defragmentation . . . . . . . . . . . . . . . . . 16
5.7. Future applications . . . . . . . . . . . . . . . . . . . 16
5.7.1. Impairment-Aware Routing and Wavelength Assignment
(IA-RWA) . . . . . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. Contributing authors . . . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Editorial notes and open issues . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
[RFC5440] describes the Path Computation Element Protocol (PCEP).
PCEP defines the communication between a Path Computation Client
(PCC) and a Path Control Element (PCE), or between PCE and PCE,
enabling computation of Multiprotocol Label Switching (MPLS) for
Traffic Engineering Label Switched Path (TE LSP) characteristics.
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.
Compared to a stateless PCE, a stateful PCE has access to not only
the network states, but also to the set of active paths and their
reserved resources in use in the network. In other words, the state
in a stateful PCE is determined not only by the TED but also by the
set of active LSPs and their corresponding reserved resources.
Furthermore, a stateful PCE might also retain the information of LSPs
under construction in order to reduce resource contention. Such
augmented state allows the PCE to compute constrained paths while
considering individual LSPs and their interaction.
This document describes describes how stateful PCE can solve various
problems for MPLS-TE and GMPLS deployment use cases, and the benefits
it brings to such deployments.
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 terms:
Minimum Cut Set: the minimum set of links for a specific source
destination pair which, when removed from the network, result 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.
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3. Overview of stateful PCE
This section is included for the convenience of the reader, please
refer to the specification 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 between 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.
Several new functions were added in PCEP to support stateful PCEs and
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.
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 autodiscovery of stateful PCEs when using the IGPs for PCE
discovery.
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4. Deployment considerations
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 can be used. The capability negotiation
in [I-D.ietf-pce-stateful-pce] ensures correct operation when the PCE
address is configured on the PCC.
4.2. LSP State Synchronization
A stateful PCE maintains two databases for path computation. The
first database 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 steateless PCE (see [RFC4655]. The second database
is the LSP state Database (LSP-DB), in which a PCE stores attributes
of all active LSPs in the network, such as their path through the
network, bandwidth/resource usage, switching types, and LSP
contraints etc. The stateful PCE extensions support population of
this database using information received from the network nodes via
LSP Report messages. Population of the LSP database via other means
is not precluded.
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 an LER client or another PCE, allowing
for support of multiple ways of re-aquiring 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.
5. Application scenarios
In the following sections, several use cases are described,
showcasing scenarios that benefit from the deployment of a stateful
PCE.
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5.1. Optimization of LSP placement
The following use cases demonstrate a need for visibility into global
inter-PCC LSP state 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.
Although the use cases are for MPLS-TE deployments, they are equally
applicable to GMPLS. Unless otherwise cited, use cases assume that
all LSPs listed exist at the same LSP priority.
+-------+
| 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 PCReq
messages under control of a PCC's local timers, the sequence of RSVP
reservation arrivals occurring in the network will be randomized.
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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.
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
throughput.
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+------+--------+----------+
| 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
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 ingress
admission control at the LSP level. Again, 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 and b) there is currently
no efficient commonly available northbound interface for dynamic
configuration of per LSP ingress admission control (such an interface
could easily be defined using the extensions present in this spec,
but it beyond the scope of the current document).
Lack of ingress admission control coupled with the behavior in
[RFC3209] effectively results in mis-signaled LSPs during periods of
contention for network capacity between LSPs in a given LSP priority.
This in turn causes information loss in the TED with regard to actual
network state, resulting in LSPs sharing common network interfaces
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with mis-signaled LSPs operating in a degraded state for significant
periods of time, even when unused network capacity may potentially be
available.
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 'Deadlock' 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: Deadlock LSP and 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
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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 is established, preempting LSP 1 and shifting it
to 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
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.
Reference Topology 1 and Tables 9, 10 and 11 show the impact of event
ordering and predictability of LSP routing.
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+------+--------+----------+
| 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
5.2. Stateful PCE in SDN
SDN promises to incorporate more intelligence into the network by
using smart centralized controlers. The use cases below show the
integration between a stateful PCE and such a controller. Note that
although from an implementation point of view, the SDN controller and
stateful PCE could be combined, in the discussion below they are
separate to show how stateful PCE enables the control-loop feedback
central to SDN.
5.2.1. Smart Bandwidth Adjustment
The bandwidth requirement of LSPs often change over time, requiring
resizing the LSP. Currenly router software performs this function by
monitoring the actual bandwidth usage, triggering a recomputation and
resignaling when a threshold is reached. A central controller can
use additional information (such as historical trending data,
information from specific applications or policy information) in
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order to make the determination of when and along which path an LSP
should be resized. The controller can rely on a stateful PCE to
perform the central function.
5.2.2. Bandwidth Scheduling
Bandwidth Scheduling allows network operators to reserve resources in
advance upon request from the customers to transmit large bulk of
data with specified starting time and duration, such as in support of
scheduled data transmission 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 stateful PCE can support this application with better efficiency
since it can alleviate the burden of processing on network elements
as well as enable the flexibility of resources usage by only
excluding the time slot(s) reserved for bandwidth scheduling
requests. The details of organizing bandwidth scheduling related
information as well as its impact on LSP-DB is subject to network
providers policy and administrative consideration and thus outside of
the scope of this document.
5.3. Recovery
5.3.1. 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. Stateful PCEs can easily accommodate this need using the
information stored in its LSP-DB.
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+----+
|PCE |
+----+
+------+ +------+ +------+
| N1 +----------+ N2 +----------+ N3 |
+--+---+ +---+--+ +---+--+
| | |
| +---------+ |
| | |
| +--+---+ +------+ |
+-----+ N5 +----------+ N4 +-----+
+------+ +------+
Figure 3: Reference topology 3
For example, in the network depicted in 3 , suppose there exists LSP1
(N1->N5) with backup route following N1->N2->N5. A request arrives
asking for a working and backup path pair to be computed for a
request from N2 to N5. If the PCE decides N2->N1->N5 to be the best
working route, then the backup path should not use the same
protection resource with LSP1 since the new LSP shares part of its
resource with LSP1 (i.e., these two LSPs are in the same shared risk
group). Alternatively, there is no such constraint if N2->N3->N4->N5
is chosen to be the right candidate for undertaking the request.
5.3.2. 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
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
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 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
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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: N1->N5->N4->N3;
(ii) LSP2: N2->N5->N4. They traverse the failed link between N5-N4.
When N4 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 N4. Moreover, it can make use
of the bandwidth resources occupied by the affected LSPs when
performing path recalculation. After N4 receives the new paths from
the PCE, it notifies the ingress nodes of the LSPs, i.e., N1 and N2,
and specifies the new paths which should be used as the rerouting
paths. To support this, it would require extensions to existing
signaling protocol.
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.
5.3.3. SRLG Diversity
An alternative way to achieve efficient recovery is to maintain SRLG
disjointness between LSPs. 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
(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 Explicit 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.
Using a stateful PCE allows the maintenance of the updated SRLG
information of the established LSPs in a centralized manner. Having
such information in the PCE facilitates the PCC to specify, as
constraint to the path computation, the SRLG disjointess of a set of
already established LSPs by only providing the LSP identifiers.
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5.4. 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. As specified in [RFC5623], a VNT manager (VNTM) is
in charge of the topology in the upper layer by connections in the
lower layer. Hence, when a stateless PCE is requested to compute a
new TE link, it will need interaction with VNTM for detailed TE link
information. To be more specific, without detailed LSP information,
this process would be inefficient or even infeasible for stateless
PCE(s), unless with cooperation with VNTM. On the other hand, a
stateful PCE seems more suitable to make the decision of when and how
to modify the VNT either to accommodate new LSP requests or to re-
optimize resource use across layers irrespective of PCE models. As
described in Section 2.2, path computation for a VNT change can be
performed by the PCE if a single PCE model is adopted. On the other
hand, if a per-layer PCE model is more appropriate, coordination
between PCEs is required.
5.5. LSP Re-optimization
In order to make efficient usage of network resource, re-optimization
of one or more LSPs dynamically through online planning is desirable.
In case of a stateless PCE, in order to optimize network resource
usage dynamically through online planning, PCC (e.g., NMS) should
send a request to PCE together with detailed path/bandwidth
information of the LSPs that need to be concurrently optimized. This
would require a PCC (e.g., NMS) to determine when and which LSPs
should be optimized. Given all of the existing LSP state information
kept at a stateful PCE, it allows automation of this process without
the PCC (e.g. NMS) to supply give the re-optimization commands and
the existing LSP state information. Moreover, since a stateful PCE
can maintain the information regarding to all LSPs that are currently
under signaling, it makes the optimization procedures be performed
more intelligently and effectively.
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 switch, in which case another northbound interface is required
for LSP attribute changes, or local/collocated, in which case there
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are significant issues with efficiency in resource usage. 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.
5.6. 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 would be rejected. In order to perform the defragmentation
procedure, stateful PCEs cam be used, since existing TE LSPs
information is required to accurately assess spectrum resources on
the LSPs, and perform de-fragmentation while ensuring a minimal
disruption of the network, e.g., based on active LSP priorities .
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 they are not contiguous. Thus, with the help
of existing LSP state information, stateful PCE can make the resource
grouped together to be usable. Moreover, stateful PCE can
proactively choose routes for upcoming path requests to reduce the
chance of spectrum defragmentation.
5.7. Future applications
5.7.1. Impairment-Aware Routing and Wavelength Assignment (IA-RWA)
In WSON networks [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
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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.
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
This document does not introduce any new security considerations.
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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
Mountain View, CA 94043
US
Email: edc@google.com
Dhruv Dhody
Huawei Technology
Leela Palace
Bangalore, Karnataka 560008
INDIA
EMail: dhruvd@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: ylee@huawei.com
Jan Medved
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
US
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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
Email: zhangfatai@huawei.com
Xiaobing Zi
Huawei Technologies
F3-5-B R&D Center, Huawei Base
Bantian, Longgang District
Shenzhen 518129 P.R.China
Phone: +86-755-28973229
Email: zixiaobing@huawei.com
8. Acknowledgements
We would like to thank Cyril Margaria for the useful comments and
discussions.
9. References
9.1. Normative References
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[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.
[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
May 2005.
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[RFC4427] Mannie, E. and D. Papadimitriou, "Recovery (Protection and
Restoration) Terminology for Generalized Multi-Protocol
Label Switching (GMPLS)", RFC 4427, March 2006.
[RFC5088] Le Roux, JL., Vasseur, JP., Ikejiri, Y., and R. Zhang,
"OSPF Protocol Extensions for Path Computation Element
(PCE) Discovery", RFC 5088, January 2008.
[RFC5089] Le Roux, JL., Vasseur, JP., Ikejiri, Y., and R. Zhang,
"IS-IS Protocol Extensions for Path Computation Element
(PCE) Discovery", RFC 5089, January 2008.
[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.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5440] Vasseur, JP. and JL. Le Roux, "Path Computation Element
(PCE) Communication Protocol (PCEP)", RFC 5440,
March 2009.
[RFC5511] Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax
Used to Form Encoding Rules in Various Routing Protocol
Specifications", RFC 5511, April 2009.
[RFC5521] Oki, E., Takeda, T., and A. Farrel, "Extensions to the
Path Computation Element Communication Protocol (PCEP) for
Route Exclusions", RFC 5521, April 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.
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-00 (work in
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progress), October 2012.
[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-04
(work in progress), July 2012.
[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-07 (work in
progress), October 2012.
[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-02 (work in progress),
October 2012.
[I-D.ogrcetal-ccamp-flexi-grid-fwk]
Dios, O., Casellas, R., Zhang, F., Fu, X., Ceccarelli, D.,
and I. Hussain, "Framework for GMPLS based control of
Flexi-grid DWDM networks",
draft-ogrcetal-ccamp-flexi-grid-fwk-01 (work in progress),
October 2012.
[I-D.sivabalan-pce-disco-stateful]
Sivabalan, S. and J. Medved, "IGP Extensions for Stateful
PCE Discovery", draft-sivabalan-pce-disco-stateful-00
(work in progress), January 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.
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[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.
[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
McManus, "Requirements for Traffic Engineering Over MPLS",
RFC 2702, September 1999.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3346] Boyle, J., Gill, V., Hannan, A., Cooper, D., Awduche, D.,
Christian, B., and W. Lai, "Applicability Statement for
Traffic Engineering with MPLS", RFC 3346, August 2002.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
September 2003.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
[RFC4657] Ash, J. and J. Le Roux, "Path Computation Element (PCE)
Communication Protocol Generic Requirements", RFC 4657,
September 2006.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
[RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
"Policy-Enabled Path Computation Framework", RFC 5394,
December 2008.
[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.
Appendix A. Editorial notes and open issues
This section will be removed prior to publication.
The following open issues remain:
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Use cases from draft-ietf-pce-stateful-pce To avoid loss of
information, the use cases will be removed from
[I-D.ietf-pce-stateful-pce] only after this document becomes a
working group document.
This document WILL NOT repeat terminology defined in other documents
or attempt to place any additional requirements on stateful PCE.
Authors' Addresses
Xian Zhang (editor)
Huawei Technologies
F3-5-B R&D Center, Huawei Base Bantian, Longgang District
Shenzhen, Guangdong 518129
P.R.China
Email: zhang.xian@huawei.com
Ina Minei (editor)
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: ina@juniper.net
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