Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic Engineering
draft-ietf-pce-inter-layer-frwk-10
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 5623.
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|
|---|---|---|---|
| Authors | Jean-Louis Le Roux , Tomonori Takeda , Eiji Oki , Adrian Farrel | ||
| Last updated | 2018-12-20 (Latest revision 2009-03-26) | ||
| Replaces | draft-oki-pce-inter-layer-frwk | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
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| Stream | WG state | (None) | |
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draft-ietf-pce-inter-layer-frwk-10
Network Working Group E. Oki
Internet Draft University of Electro-Communications
Category: Informational Tomonori Takeda
Created: March, 2009 NTT
Expires: November, 2009 J-L Le Roux
France Telecom
A. Farrel
Old Dog Consulting
Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic Engineering
draft-ietf-pce-inter-layer-frwk-10.txt
Status of this Memo
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Abstract
A network may comprise multiple layers. It is important to globally
optimize network resource utilization, taking into account all
layers, rather than optimizing resource utilization at each layer
independently. This allows better network efficiency to be achieved
through a process that we call inter-layer traffic engineering. The
Path Computation Element (PCE) can be a powerful tool to achieve
inter-layer traffic engineering.
This document describes a framework for applying the PCE-based
architecture to inter-layer Multiprotocol Label Switching (MPLS) and
Generalized MPLS (GMPLS) traffic engineering. It provides
suggestions for the deployment of PCE in support of multi-layer
networks. This document also describes network models where PCE
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performs inter-layer traffic engineering, and the relationship
between PCE and a functional component called the Virtual Network
Topology Manager (VNTM).
Table of Contents
1. Introduction...................................................3
1.1. Terminology..................................................3
2. Inter-Layer Path Computation...................................4
3. Inter-Layer Path Computation Models............................6
3.1. Single PCE Inter-Layer Path Computation......................7
3.2. Multiple PCE Inter-Layer Path Computation....................7
3.3. General Observations.........................................9
4. Inter-Layer Path Control......................................10
4.1. VNT Management..............................................10
4.2. Inter-Layer Path Control Models.............................10
4.2.1. PCE-VNTM Cooperation Model................................10
4.2.2. Higher-Layer Signaling Trigger Model......................12
4.2.3. NMS-VNTM Cooperation Model................................15
4.2.4. Possible Combinations of Inter-Layer Path Computation and
Inter-Layer Path Control Models..................................20
5. Choosing Between Inter-Layer Path Control Models..............21
5.1. VNTM Functions..............................................21
5.2. Border LSR Functions........................................22
5.3. Complete Inter-Layer LSP Setup Time.........................22
5.4. Network Complexity..........................................23
5.5. Separation of Layer Management..............................24
6. Stability Considerations......................................24
7. IANA Considerations...........................................25
8. Manageability Considerations..................................25
8.1. Control of Function and Policy..............................25
8.1.1. Control of Inter-Layer Computation Function...............25
8.1.2. Control of Per-Layer Policy...............................26
8.1.3. Control of Inter-Layer Policy.............................26
8.2. Information and Data Models.................................27
8.3. Liveness Detection and Monitoring...........................27
8.4. Verifying Correct Operation.................................28
8.5. Requirements on Other Protocols and Functional Components...28
8.6. Impact on Network Operation.................................28
9. Security Considerations.......................................29
10. Acknowledgments..............................................30
11. References...................................................30
11.1. Normative Reference........................................30
11.2. Informative Reference......................................31
12. Authors' Addresses...........................................32
13. Intellectual Property Statement..............................32
14. Full Copyright Statement.....................................33
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1. Introduction
A network may comprise multiple layers. These layers may represent
separations of technologies (e.g., packet switch capable (PSC), time
division multiplex (TDM), or lambda switch capable (LSC)) [RFC3945],
separation of data plane switching granularity levels (e.g., PSC-1,
PSC-2, VC4, or VC12) [RFC5212], or a distinction between client and
server networking roles. In this multi-layer network, Label Switched
Paths (LSPs) in a lower layer are used to carry higher-layer LSPs
across the lower-layer network. The network topology formed by
lower-layer LSPs and advertised as traffic engineering links (TE
links) in the higher layer network is called the Virtual Network
Topology (VNT) [RFC5212].
It may be effective to optimize network resource utilization
globally, i.e., taking into account all layers, rather than
optimizing resource utilization at each layer independently. This
allows better network efficiency to be achieved and is what we call
inter-layer traffic engineering. This includes mechanisms allowing
the computation of end-to-end paths across layers (known as inter-
layer path computation), and mechanisms for control and management
of the Virtual Network Topology (VNT) by setting up and releasing
LSPs in the lower layers [RFC5212].
Inter-layer traffic engineering is included in the scope of the Path
Computation Element (PCE)-based architecture [RFC4655], and PCE can
provide a suitable mechanism for resolving inter-layer path
computation issues.
PCE Communication Protocol requirements for inter-layer traffic
engineering are set out in [PCE-INTER-LAYER-REQ].
This document describes a framework for applying the PCE-based
architecture to inter-layer traffic engineering. It provides
suggestions for the deployment of PCE in support of multi-layer
networks. This document also describes network models where PCE
performs inter-layer traffic engineering, and the relationship
between PCE and a functional component in charge of the control and
management of the VNT, called the Virtual Network Topology Manager
(VNTM).
1.1. Terminology
This document uses terminology from the PCE-based path computation
architecture [RFC4655] and also common terminology from Multi
Protocol Label Switching (MPLS) [RFC3031], Generalized MPLS (GMPLS)
[RFC3945], and Multi-Layer Networks [RFC5212].
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2. Inter-Layer Path Computation
This section describes key topics of inter-layer path computation in
MPLS and GMPLS networks.
[RFC4206] defines a way to signal a higher-layer LSP, which has an
explicit route that includes hops traversed by LSPs in lower layers.
The computation of end-to-end paths across layers is called Inter-
Layer Path Computation.
A Label Switching Router (LSR) in the higher-layer might not have
information on the topology of the lower-layer, particularly in an
overlay or augmented model deployment, and hence may not be able to
compute an end-to-end path across layers.
PCE-based Inter-Layer Path Computation consists of using one or more
PCEs to compute an end-to-end path across layers. This could be
achieved by a single PCE path computation where the PCE has topology
information about multiple layers and can directly compute an end-
to-end path across layers considering the topology of all of the
layers. Alternatively, the inter-layer path computation could be
performed as a multiple PCE computation where each member of a set
of PCEs has information about the topology of one or more layers
(but not all layers), and the PCEs collaborate to compute an end-to-
end path.
----- ----- ----- -----
| LSR |--| LSR |................| LSR |--| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ /----- -----
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 1 - A Simple Example of a Multi-Layer Network.
Consider, for instance, the two-layer network shown in Figure 1,
where the higher-layer network is a packet-based IP/MPLS or GMPLS
network (LSRs H1, H2, H3, and H4), and the lower-layer network
(LSRs, H2, L1, L2, and H3) is a GMPLS optical network. An ingress
LSR in the higher-layer network (H1) tries to set up an LSP to an
egress LSR (H4) also in the higher-layer network across the lower-
layer network, and needs a path in the higher-layer network. However,
suppose that there is no TE link in the higher-layer network between
the border LSRs located on the boundary between the higher-layer and
lower-layer networks (H2 and H3). Suppose also that the ingress LSR
does not have topology visibility into the lower layer. If a single-
layer path computation is applied in the higher-layer, the path
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computation fails because of the missing TE link. On the other hand,
inter-layer path computation is able to provide a route in the
higher-layer (H1-H2-H3-H4) and a suggestion that a lower-layer LSP
be set up between the border LSRs (H2-L1-L2-H3).
Lower-layer LSPs that are advertised as TE links into the higher-
layer network form a Virtual Network Topology (VNT) that can be used
for routing higher-layer LSPs. Inter-layer path computation for end-
to-end LSPs in the higher-layer network that span the lower-layer
network may utilize the VNT, and PCE is a candidate for computing
the paths of such higher-layer LSPs within the higher-layer network.
Alternatively, the PCE-based path computation model can:
- Perform a single computation on behalf of the ingress LSR using
information gathered from more than one layer. This mode is
referred to as Single PCE Computation in [RFC4655].
- Compute a path on behalf of the ingress LSR through cooperation
with PCEs responsible for each layer. This mode is referred to as
Multiple PCE Computation with inter-PCE communication in [RFC4655].
- Perform separate path computations on behalf of the TE-LSP head-
end and each transit border LSR that is the entry point to a new
layer. This mode is referred to as Multiple PCE Computation
(without inter-PCE communication) in [RFC4655]. This option
utilizes per-layer path computation performed independently by
successive PCEs.
Note that when a network consists of more than two layers (e.g., MPLS
over SONET over OTN), and a path traversing more than two layers
needs to be computed, it is possible to combine multiple PCE-based
path computation models. For example, the single PCE computation
model could be used for computing a path across the SONET layer and
the OTN layer, and the multiple PCE computation with inter-PCE
communication model could be used for computing a path across the
MPLS layer (computed by higher-layer PCE) and the SONET layer
(computed by lower-layer PCE).
The PCE invoked by the head-end LSR computes a path that the LSR can
use to signal an MPLS-TE or GMPLS LSP once the path information has
been converted to an Explicit Route Object (ERO) for use in RSVP-TE
signaling. There are two options.
- Option 1: Mono-layer path.
The PCE computes a "mono-layer" path, i.e., a path that includes
only TE links from the same layer. There are two cases for this
option. In the first case the PCE computes a path that includes
already established lower-layer LSPs or lower-layer LSPs to be
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established on demand. That is, the resulting ERO includes sub-
object(s) corresponding to lower-layer hierarchical LSPs expressed
as the TE link identifiers of the hierarchical LSPs when advertised
as TE links in the higher-layer network. The TE link may be a
regular TE link that is actually established, or a virtual TE link
that is not established yet (see [RFC5212]). If it is a virtual TE
link, this triggers a setup attempt for a new lower-layer LSP when
signaling reaches the head-end of the lower-layer LSP. Note that
the path of a virtual TE link is not necessarily known in advance,
and this may require a further (lower-layer) path computation.
The second case is that the PCE computes a path that includes a
loose hop that spans the lower-layer network. The higher layer path
computation selects which lower layer network to use, and selects
the entry and exit points of that lower-layer network, but does not
select the path across the lower-layer network. A transit LSR that
is the entry point to the lower-layer network is expected to expand
the loose hop (either itself or relying on the services of a PCE).
The path expansion process on the border LSR may result either in
the selection of an existing lower-layer LSP, or in the computation
and setup of a new lower-layer LSP.
Note that even if a PCE computes a path with a loose hop expecting
that the loose hop will be expanded across the lower-layer network,
the LSR (that is an entry point to the lower-layer network) may
simply expand the loose hop in the same layer. If more strict
control of how the LSR establishes the path is required, mechanisms
such as Path Key [PATH-KEY] could be applied.
- Option 2: Multi-layer path.
The PCE computes a "multi-layer" path, i.e., a path that includes
TE links from distinct layers [RFC4206]. Such a path can include
the complete path of one or more lower-layer LSPs that already
exist or are not yet established. In the latter case, the signaling
of the higher-layer LSP will trigger the establishment of the
lower-layer LSPs.
3. Inter-Layer Path Computation Models
In Section 2, three models are defined to perform PCE-based inter-
layer path computation, namely Single PCE Computation, Multiple PCE
Computation with inter-PCE communication, and Multiple PCE
Computation without inter-PCE communication. Single PCE Computation
is discussed in Section 3.1 below, and Multiple PCE Computation (with
or without inter-PCE communication) is discussed in Section 3.2
below.
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3.1. Single PCE Inter-Layer Path Computation
In this model inter-layer path computation is performed by a single
PCE that has topology visibility into all layers. Such a PCE is
called a multi-layer PCE.
In Figure 2, the network is comprised of two layers. LSRs H1, H2, H3,
and H4 belong to the higher layer, and LSRs H2, H3, L1, and L2
belong to the lower layer. The PCE is a multi-layer PCE that has
visibility into both layers. It can perform end-to-end path
computation across layers (single PCE path computation). For
instance, it can compute an optimal path H1-H2-L1-L2-H3-H4, for a
higher layer LSP from H1 to H4. This path includes the path of a
lower layer LSP from H2 to H3, already in existence or not yet
established.
-----
| PCE |
-----
----- ----- ----- -----
| LSR |--| LSR |................| LSR |--| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ /----- -----
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 2: Single PCE Inter-Layer Path Computation
3.2. Multiple PCE Inter-Layer Path Computation
In this model there is at least one PCE per layer, and each PCE has
topology visibility restricted to its own layer. Some providers may
want to keep the layer boundaries due to factors such as
organizational and/or service management issues. The choice for
multiple PCE computation instead of single PCE computation may also
be driven by scalability considerations, as in this mode a PCE only
needs to maintain topology information for one layer (resulting in a
size reduction for the Traffic Engineering Database (TED)).
These PCEs are called mono-layer PCEs. Mono-layer PCEs collaborate
to compute an end-to-end optimal path across layers.
Figure 3 shows multiple PCE inter-layer computation with inter-PCE
communication. There is one PCE in each layer. The PCEs from each
layer collaborate to compute an end-to-end path across layers. PCE
Hi is responsible for computations in the higher layer and may
"consult" with PCE Lo to compute paths across the lower layer. PCE
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Lo is responsible for path computation in the lower layer. A simple
example of cooperation between the PCEs could be as follows:
- LSR H1 sends a request for a path H1-H4 to PCE Hi
- PCE Hi selects H2 as the entry point to the lower layer, and H3 as
the exit point.
- PCE Hi requests a path H2-H3 from PCE Lo.
- PCE Lo returns H2-L1-L2-H3 to PCE Hi.
- PEC Hi is now able to compute the full path (H1-H2-L1-L2-H3-H4)
and return it to H1.
Of course, more complex cooperation may be required if an optimal
end-to-end path is desired.
-----
| PCE |
| Hi |
--+--
|
----- ----- | ----- -----
| LSR |--| LSR |............|...........| LSR |--| LSR |
| H1 | | H2 | | | H3 | | H4 |
----- -----\ --+-- /----- -----
\ | PCE | /
\ | Lo | /
\ ----- /
\ /
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 3: Multiple PCE Inter-Layer Path Computation with Inter-PCE
Communication
Figure 4 shows multiple PCE inter-layer path computation without
inter-PCE communication. As described in Section 2, separate path
computations are performed on behalf of the TE-LSP head-end and each
transit border LSR that is the entry point to a new layer.
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-----
| PCE |
| Hi |
-----
----- ----- ----- -----
| LSR |--| LSR |........................| LSR |--| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ ----- /----- -----
\ | PCE | /
\ | Lo | /
\ ----- /
\ /
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 4: Multiple PCE Inter-layer Path Computation Without Inter-
PCE Communication
3.3. General Observations
- Depending on implementation details, the time to perform inter-
layer path computation in the single PCE inter-layer path
computation model may be less than that of the multiple PCE model
with cooperating mono-layer PCEs, because there is no requirement
to exchange messages between cooperating PCEs.
- When TE topology for all layer networks is visible within one
routing domain, the single PCE inter-layer path computation model
may be adopted because a PCE is able to collect all layers' TE
topologies by participating in only one routing domain.
- As the single PCE inter-layer path computation model uses more TE
topology information in one computation than is used by PCEs in the
multiple PCE path computation model, it requires more computation
power and memory.
When there are multiple candidate layer border nodes (we may say
that the higher layer is multi-homed), optimal path computation
requires that all the possible paths transiting different layer
border nodes or links be examined. This is relatively simple in the
single PCE inter-layer path computation model because the PCE has
full visibility - the computation is similar to the computation
within a single domain of a single layer. In the multiple PCE inter-
layer path computation model, backward recursive techniques
described in [BRPC] could be used, by considering layers as separate
domains.
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4. Inter-Layer Path Control
4.1. VNT Management
As a result of mono-layer path computation, a PCE may determine that
there is insufficient bandwidth available in the higher-layer
network to support this or future higher-layer LSPs. The problem
might be resolved if new LSPs were provisioned across the lower-
layer network. Furthermore, the modification, re-organization and
new provisioning of lower-layer LSPs may enable better utilization
of lower-layer network resources given the demands of the higher-
layer network. In other words, the VNT needs to be controlled or
managed in cooperation with inter-layer path computation.
A VNT Manager (VNTM) is defined as a functional element that manages
and controls the VNT. PCE and VNT Manager are distinct functional
elements that may or may not be co-located.
4.2. Inter-Layer Path Control Models
4.2.1. PCE-VNTM Cooperation Model
----- ------
| PCE |--->| VNTM |
----- ------
^ :
: :
: :
v V
----- ----- ----- -----
| LSR |----| LSR |................| LSR |----| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ /----- -----
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 5: PCE-VNTM Cooperation Model
A multi-layer network consists of higher-layer and lower-layer
networks. LSRs H1, H2, H3, and H4 belong to the higher-layer network,
LSRs H2, L1, L2, and H3 belong to the lower-layer network, as shown
in Figure 5. The case of single PCE inter-layer path computation is
considered here to explain the cooperation model between PCE and
VNTM, but multiple PCE path computation with or without inter-PCE
communication can also be applied to this model.
Consider that H1 requests the PCE to compute an inter-layer path
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between H1 and H4. There is no TE link in the higher-layer between
H2 and H3 before the path computation request, so the request fails.
But the PCE may provide information to the VNT Manager responsible
for the lower layer network that may help resolve the situation for
future higher-layer LSP setup.
The roles of PCE and VNTM are as follows. PCE performs inter-layer
path computation and is unable to supply a path because there is no
TE link between H2 and H3. The computation fails, but PCE suggests
to VNTM that a lower-layer LSP (H2-H3) could be established to
support future LSP requests. Messages from PCE to VNTM contain
information about the higher-layer demand (from H2 to H3), and may
include a suggested path in the lower layer (if the PCE has
visibility into the lower layer network). VNTM uses local policy and
possibly management/configuration input to determine how to process
the suggestion from PCE, and may request an ingress LSR (e.g. H2) to
establish a lower-layer LSP. VNTM or the ingress LSR (H2) may
themselves use a PCE with visibility into the lower layer to compute
the path of this new LSP.
When the higher-layer PCE fails to compute a path and notifies VNTM,
it may wait for the lower-layer LSP to be set up and advertised as a
TE link. PCE may have a timer. After TED is updated within a
specified duration, PCE will know a new TE link. It could then
compute the complete end-to-end path for the higher-layer LSP and
return the result to the PCC. In this case, the PCC may be kept
waiting for some time, and it is important that the PCC understands
this. It is also important that the PCE and VNTM have an agreement
that the lower-layer LSP will be set up in a timely manner, or that
the PCE will be notified by VNTM that no new LSP will become
available. In any case, if the PCE decides to wait, it must operate
a timeout. An example of such a cooperative procedure between PCE
and VNTM is as follows using the example network in Figure 4.
Step 1: H1 (PCC) requests PCE to compute a path between H1 and H4.
Step 2: The path computation fails because there is no TE link
across the lower-layer network.
Step 3: PCE suggests to VNTM that a new TE link connecting H2 and H3
would be useful. The PCE notifies VNTM that it will be waiting for
the TE link to be created. VNTM considers whether lower-layer LSPs
should be established if necessary and if acceptable within VNTM's
policy constraints.
Step 4: VNTM requests an ingress LSR in the lower-layer network
(e.g., H2) to establish a lower-layer LSP. The request message may
include a lower-layer LSP route obtained from the PCE responsible
for the lower-layer network.
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Step 5: The ingress LSR signals to establish the lower-layer LSP.
Step 6: If the lower-layer LSP setup is successful, the ingress LSR
notifies VNTM that the LSP is complete and supplies the tunnel
information.
Step 7: The ingress LSR (H2) advertises the new LSP as a TE link in
the higher-layer network routing instance.
Step 8: PCE notices the new TE link advertisement and recomputes the
requested path.
Step 9: PCE replies to H1 (PCC) with a computed higher-layer LSP
route. The computed path is categorized as a mono-layer path that
includes the already-established lower layer-LSP as a single hop in
the higher layer. The higher-layer route is specified as H1-H2-H3-H4,
where all hops are strict.
Step 10: H1 initiates signaling with the computed path H2-H3-H4 to
establish the higher-layer LSP.
4.2.2. Higher-Layer Signaling Trigger Model
-----
| PCE |
-----
^
:
:
v
----- ----- ----- -----
| LSR |----| LSR |................| LSR |--| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ /----- -----
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 6: Higher-layer Signaling Trigger Model
Figure 6 shows the higher-layer signaling trigger model. The case of
single PCE path computation is considered to explain the higher-
layer signaling trigger model here, but multiple PCE path
computation with/without inter-PCE communication can also be applied
to this model.
As in the case described in Section 4.2.1, consider that H1 requests
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PCE to compute a path between H1 and H4. There is no TE link in the
higher-layer between H2 and H3 before the path computation request.
PCE is unable to compute a mono-layer path, but may judge that the
establishment of a lower-layer LSP between H2 and H3 would provide
adequate connectivity. If the PCE has inter-layer visibility it may
return a path that includes hops in the lower layer (H1-H2-L1-L2-H3-
H4), but if it has no visibility into the lower layer, it may return
a path with a loose hop from H2 to H3 (H1-H2-H3(loose)-H4). The
former is a multi-layer path, and the latter a mono-layer path that
includes loose hops.
In the higher-layer signaling trigger model with a multi-layer path,
the LSP route supplied by the PCE includes the route of a lower-
layer LSP that is not yet established. A border LSR that is located
at the boundary between the higher-layer and lower-layer networks
(H2 in this example) receives a higher-layer signaling message,
notices that the next hop is in the lower-layer network, starts to
setup the lower-layer LSP as described in [RFC4206]. Note that these
actions depend on a policy being applied at the border LSR. An
example procedure of the signaling trigger model with a multi-layer
path is as follows.
Step 1: H1 (PCC) requests PCE to compute a path between H1 and H4.
The request indicates that inter-layer path computation is allowed.
Step 2: As a result of the inter-layer path computation, PCE judges
that a new lower-layer LSP needs to be established.
Step 3: PCE replies to H1 (PCC) with a computed multi-layer route
including higher-layer and lower-layer LSP routes. The route may be
specified as H1-H2-L1-L2-H3-H4, where all hops are strict.
Step 4: H1 initiates higher-layer signaling using the computed
explicit router of H2-L1-L2-H3-H4.
Step 5: The border LSR (H2) that receives the higher-layer signaling
message starts lower-layer signaling to establish a lower-layer LSP
along the specified lower-layer route of H2-L1-L2-H3. That is, the
border LSR recognizes the hops within the explicit route that apply
to the lower-layer network, verifies with local policy that a new
LSP is acceptable, and establishes the required lower-layer LSP.
Note that it is possible that a suitable lower-layer LSP has already
been established (or become available) between the time that the
computation was performed and the moment when the higher-layer
signaling message reached the border LSR. In this case, the border
LSR may select such a lower-layer LSP without the need to signal a
new LSP provided that the lower-layer LSP satisfies the explicit
route in the higher-layer signaling request.
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Step 6: After the lower-layer LSP is established, the higher-layer
signaling continues along the specified higher-layer route of H2-H3-
H4 using hierarchical signaling [RFC4206].
On the other hand, in the signaling trigger model with a mono-layer
path, a higher-layer LSP route includes a loose hop to traverse the
lower-layer network between the two border LSRs. A border LSR that
receives a higher-layer signaling message needs to determine a path
for a new lower-layer LSP. It applies local policy to verify that a
new LSP is acceptable and then either consults a PCE with
responsibility for the lower-layer network or computes the path by
itself, and initiates signaling to establish the lower-layer LSP.
Again, it is possible that a suitable lower-layer LSP has already
been established (or become available). In this case, the border LSR
may select such a lower-layer LSP without the need to signal a new
LSP provided that the existing lower-layer LSP satisfies the
explicit route in the higher-layer signaling request. Since the
higher-layer signaling request used a loose hop without specifying
any specifics of the path within the lower-layer network, the border
LSR has greater freedom to choose a lower-layer LSP than in the
previous example.
The difference between procedures of the signaling trigger model
with a multi-layer path and a mono-layer path is Step 5. Step 5 of
the signaling trigger model with a mono-layer path is as follows:
Step 5': The border LSR (H2) that receives the higher-layer
signaling message applies local policy to verify that a new LSP is
acceptable and then initiates establishment of a lower-layer LSP. It
either consults a PCE with responsibility for the lower-layer
network or computes the route by itself to expand the loose hop
route in the higher-layer path.
Finally, note that a virtual TE link may have been advertised into
the higher-layer network. This causes the PCE to return a path H1-
H2-H3-H4 where all the hops are strict. But when the higher-layer
signaling message reaches the layer border node H2 (that was
responsible for advertising the virtual TE link) it realizes that
the TE link does not exist yet, and signals the necessary LSP across
the lower-layer network using its own path determination (just as
for a loose hop in the higher layer) before continuing with the
higher-layer signaling.
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PCE
^
:
:
V
H1--H2 H3--H4
\ /
L1==L2==L3--L4--L5
|
|
L6--L7
\
H5--H6
Figure 7: Example of a Multi-Layer Network
Examples of multi-layer EROs are explained using Figure 7. It is
described how lower-layer LSP setup is performed in the higher-layer
signaling trigger model using an ERO that can include subobjects in
both the higher and lower layers. It gives rise to several options
for the ERO when it reaches the last LSR in the higher layer network
(H2).
1. The next subobject is a loose hop to H3 (mono layer ERO).
2. The next subobject is a strict hop to L1 followed by a loose hop
to H3.
3. The next subobjects are a series of hops (strict or loose) in the
lower-layer network followed by H3. For example, {L1(strict),
L3(loose), L5(loose), H3(strict)}
In the first example, the lower layer can utilize any LSP tunnel
that will deliver the end-to-end LSP to H3. In the third case, the
lower layer must select an LSP tunnel that traverses L3 and L5.
However, this does not mean that the lower layer can or should use
an LSP from L1 to L3 and another from L3 to L5.
4.2.3. NMS-VNTM Cooperation Model
In this model, NMS and VNTM cooperate to establish a lower-layer LSP.
There are two flavors in this model. One is where interaction between
layers in path computation is performed at the PCE level. This is
called "integrated flavor". The other is where interaction between
layers in path computation is achieved through NMS and VNTM
cooperation, which could be a point of application of administrative,
billing, and security policy. This is called "separated flavor".
o NMS-VNTM Cooperation Model (integrated flavor)
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------ -----
| NMS |<-->| PCE |
| | -----
| ---- |
||VNTM||
| ---- |
------
: :
: ---------
: :
V V
----- ----- ----- -----
| LSR |----| LSR |................| LSR |----| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ /----- -----
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 8: NMS-VNTM Cooperation Model (integrated flavor)
Figure 8 shows NMS-VNTM cooperation model (integrated flavor). The
case of single PCE path computation is considered to explain the NMS-
VNTM cooperation model (integrated flavor) here, but multiple PCE
path computation with inter-PCE communication can also be applied to
this model. Note that multiple PCE path computation without inter-PCE
communication does not fit in with this model. For this model to have
meaning, the VNTM and NMS are closely coupled.
The NMS sends the path computation request to the PCE. The PCE
returns inter-layer path computation result. When the NMS receives
the path computation result, the NMS works with the VNTM and sends
the request to LSR H2 to set up the lower-layer LSP. VNTM uses local
policy and possibly management/configuration input to determine how
to process the computation result from PCE.
An example procedure of the NMS-VNTM cooperation model (integrated
flavor) is as follows.
Step 1: NMS requests PCE to compute a path between H1 and H4.
The request indicates that inter-layer path computation is allowed.
Step 2: PCE computes a path. The result (H1-H2-L1-L2-H3-H4) is sent
back to the NMS.
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Step 3: NMS discovers that a lower layer LSP is needed. NMS works
with VNTM to determine whether the new TE LSP H2-L1-L2-H3 is
permitted according to policy, etc.
Step 4: VNTM requests the ingress LSR in the lower-layer network (H2)
to establish a lower-layer LSP. The request message includes the
lower-layer LSP route obtained from PCE.
Step 5: H2 signals to establish the lower-layer LSP.
Step 6: If the lower-layer LSP setup is successful, H2 notifies VNTM
that the LSP is complete and supplies the tunnel information.
Step 7: H2 advertises the new LSP as a TE link in the higher-layer
network routing instance.
Step 8: VNTM notifies NMS that the underlying lower-layer LSP has
been set up, and NMS notices the new TE link advertisement.
Step 9: NMS requests H1 to set up a higher-layer LSP between H1 and
H4 with the path computed in Step 2. The lower layer links are
replaced by the corresponding higher layer TE link. Hence, the NMS
sends the path H1-H2-H3-H4 to H1.
Step 10: H1 initiates signaling with the path H2-H3-H4 to establish
the higher-layer LSP.
o NMS-VNTM Cooperation Model (separate flavor)
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-----
| NMS |
| | -----
----- | PCE |
^ ^ | Hi |
: : -----
: : ^
: : :
: : :
: v v
: ------ ----- ----- ------
: | LSR |--| LSR |........................| LSR |--| LSR |
: | H1 | | H2 | | H3 | | H4 |
: ------ -----\ /----- ------
: ^ \ /
: : \ /
: -------- \ /
v : \ /
------ ----- \----- -----/
| VNTM |<-->| PCE | | LSR |--| LSR |
| | | Lo | | L1 | | L2 |
------ ----- ----- -----
Figure 9: NMS-VNTM Cooperation Model (separate flavor)
Figure 9 shows the NMS-VNTM cooperation model (separate flavor). The
NMS manages the higher layer. The case of multiple PCE computation
without inter-PCE communication is used to explain the NMS-VNTM
cooperation model here, but single PCE path computation could also be
applied to this model. Note that multiple PCE path computation with
inter-PCE communication does not fit in with this model.
The NMS requests a head-end LSR (H1 in this example) to set up a
higher-layer LSP between head-end and tail-end LSRs without
specifying any route. The head-end LSR, which is a PCC, requests the
higher-layer PCE to compute a path between head-end and tail-end
LSRs. There is no TE link in the higher-layer between border LSRs
(H2 and H3 in this example). When the PCE fails to compute a path,
it informs the PCC (i.e., head-end LSR) that notifies the NMS. The
notification may include information about the reason for failure
(such as that there is no TE link between the border LSRs or that
computation constraints cannot be met).
Note that it is equally valid for the higher-layer PCE to be
consulted by the NMS rather than by the head-end LSR. In this case,
the result is the same - the NMS discovers that an end-to-end LSP
cannot be provisioned owing to the lack of a TE link between H2 and
H3.
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The NMS may now suggest (or request) to the VNTM that a lower-layer
LSP between the border LSRs could be established and could be
advertised as a TE link in the higher layer to support future
higher-layer LSP requests. The communication between the NMS and the
VNTM may be performed in an automatic manner or in a manual manner,
and is a key interaction between layers that may also be separate
administrative domains. Thus, this communication is potentially a
point of application of administrative, billing, and security policy.
The NMS may wait for the lower-layer LSP to be set up and advertised
as a TE link, or may reject the operator's request for the service
that requires the higher-layer LSP with a suggestion that the
operator tries again later.
The VNTM requests the lower-layer PCE to compute a path, and then
requests H2 to establish a lower-layer LSP. Alternatively, the VNTM
may make a direct request to H2 for the LSP, and H2 may consult the
lower-layer PCE. After the NMS is informed or notices that the
lower-layer LSP has been established, it can request the head-end
LSR (H1) to set up the higher-layer end-to-end LSP between H1 and H4.
Thus, cooperation between the high layer and lower layer is
performed though communication between NMS and VNTM. An example of
such a procedure of the NSM-VNTM cooperation model is as follows
using the example network in Figure 6.
Step 1: NMS requests a head-end LSR (H1) to set up a higher-layer
LSP between H1 and H4 without specifying any route.
Step 2: H1 (PCC) requests PCE to compute a path between H2 and H3.
Step 3: The path computation fails because there is no TE link
across the lower-layer network.
Step 4: H1 (PCC) notifies NMS. The notification may include an
indication that there is no TE link between H2 and H4.
Step 5: NMS suggests (or requests) to VNTM that a new TE link
connecting H2 and H3 would be useful. The NMS notifies VNTM that it
will be waiting for the TE link to be created. VNTM considers
whether lower-layer LSPs should be established if necessary and if
acceptable within VNTM's policy constraints.
Step 6: VNTM requests the lower-layer PCE for path computation.
Step 7: VNTM requests the ingress LSR in the lower-layer network
(H2) to establish a lower-layer LSP. The request message includes a
lower-layer LSP route obtained from the lower-layer PCE responsible
for the lower-layer network.
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Step 8: H2 signals the lower-layer LSP.
Step 9: If the lower-layer LSP setup is successful, H2 notifies VNTM
that the LSP is complete and supplies the tunnel information.
Step 10: H2 advertises the new LSP as a TE link in the higher-layer
network routing instance.
Step 11: VNTM notifies NMS that the underlying lower-layer LSP has
been set up, and NMS notices the new TE link advertisement.
Step 12: NMS again requests H1 to set up a higher-layer LSP between
H1 and H4.
Step 13: H1 requests the higher-layer PCE to compute a path and
obtains a successful result that includes the higher-layer route
that is specified as H1-H2-H3-H4, where all hops are strict.
Step 14: H1 initiates signaling with the computed path H2-H3-H4 to
establish the higher-layer LSP.
4.2.4. Possible Combinations of Inter-Layer Path Computation and Inter-
Layer Path Control Models
Table 1 summarizes the possible combinations of inter-layer path
computation and inter-layer path control models. There are three
inter-layer path computation models: the single PCE path computation
model; the multiple PCE path computation with inter-PCE
communication model; and the multiple PCE path computation without
inter-PCE communication model. There are also four inter-layer path
control models: the PCE-VNTM cooperation model; the higher-layer
signaling trigger model; the NMS-VNTM cooperation model (integrated
flavor); the NMS-VNTM cooperation model (separate flavor). All the
combinations between inter-layer path computation and path control
models, except for the combination of the multiple PCE path
computation with inter-layer PCE communication model and the NMS-
VNTM cooperation model are possible.
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Table 1: Possible Combinations of Inter-Layer Path Computation and
Inter-Layer Path Control Models.
------------------------------------------------------
| Path computation | Single | Multiple | Multiple |
| \ | PCE | PCE with | PCE w/o |
| Path control | | inter-PCE | inter-PCE |
|---------------------+--------------------------------|
| PCE-VNTM | Yes | Yes | Yes |
| cooperation | | | |
|---------------------+--------+-----------+-----------|
| Higher-layer | Yes | Yes | Yes |
| signaling trigger | | | |
|---------------------+--------+-----------+-----------|
| NMS-VNTM | Yes | Yes | No |
| cooperation | | | |
| (integrated flavor) | | | |
|---------------------+--------+-----------+-----------|
| NMS-VNTM | No* | No | Yes |
| cooperation | | | |
| (separate flavor) | | | |
---------------------+--------+-----------+-----------
*Note that, in case of NSM-VNTM cooperation (separate flavor) and
single PCE inter-layer path computation, the PCE function used by NMS
and VNTM may be collocated, but it will operate on separate TEDs.
5. Choosing Between Inter-Layer Path Control Models
This section compares the cooperation model between PCE and VNTM,
the higher-layer signaling trigger model, and NMS-VNTM cooperation
model, in terms of VNTM functions, border LSR functions, higher-layer
signaling time, and complexity (in terms of number of states and
messages). An appropriate model may be chosen by a network operator
in different deployment scenarios taking all these considerations
into account.
5.1. VNTM Functions
VNTM functions are required in both the PCE-VNTM cooperation model
and the NMS-VNTM model. In the PCE-VNTM cooperation model,
communications are required between PCE and VNTM, and between VNTM
and a border LSR. Communications between a higher-layer PCE and the
VNTM are event notifications and may use SNMP notifications from the
PCE MIB modules [PCE-MIB]. Note that communications from the PCE to
the VNTM do not have any acknowledgements. VNTM-LSR communication can
use existing GMPLS-TE MIB modules [RFC4802].
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In the NMS-VNTM cooperation model, communications are required
between NMS and VNTM, between VNTM and a lower-layer PCE, and between
VNTM and a border LSR. NMS-VNTM communications, which are out of
scope of this document, may use proprietary or standard interfaces,
some of which, for example, are standardized in TM Forum.
Communications between VNTM and a lower-layer PCE use PCEP [RFC5440].
VNTM-LSR communications are the same as in the PCE-VNTM cooperation
model.
In the higher-layer signaling trigger model, no VNTM functions are
required, and no such communications are required.
If VNTM functions are not supported in a multi-layer network, the
higher-layer signaling trigger model has to be chosen.
The inclusion of VNTM functionality allows better coordination of
cross-network LSP tunnels and application of network-wide policy
that is far harder to apply in the trigger model since it requires
the coordination of policy between multiple border LSRs.
Also, VNTM functions could be applied to establish LSPs (or
connections) in non-MPLS/GMPLS networks, which do not have signaling
capabilities, by configuring each node along the path from the VNTM.
5.2. Border LSR Functions
In the higher-layer signaling trigger model, a border LSR must have
some additional functions. It needs to trigger lower-layer signaling
when a higher-layer path message suggests that lower-layer LSP setup
is necessary. Note that, if virtual TE links are used, the border
LSRs must be capable of triggered signaling.
If the ERO in the higher-layer Path message uses a mono-layer path
or specifies a loose hop, the border LSR receiving the Path message
must obtain a lower-layer route either by consulting a PCE or by
using its own computation engine. If the ERO in the higher-layer
Path message uses a multi-layer path, the border LSR must judge
whether lower-layer signaling is needed.
In the PCE-VNTM cooperation model and the NMS-VNTM model, no
additional function for triggered signaling is required in border
LSRs except when virtual TE links are used. Therefore, if these
additional functions are not supported in border LSRs, where a
border LSR is controlled by VNTM to set up a lower-layer LSP, the
cooperation model has to be chosen.
5.3. Complete Inter-Layer LSP Setup Time
The complete inter-layer LSP setup time includes inter-layer path
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computation, signaling, and the communication time between PCC and
PCE, PCE and VNTM, NMS and VNTM, and VNTM and LSR. In the PCE-VNTM
cooperation model and the NMS-VNTM model, the additional
communication steps are required compared with the higher-layer
signaling trigger model. On the other hand, the cooperation model
provides better control at the cost of a longer service setup time.
Note that, in terms of higher-layer signaling time, in the higher-
layer signaling trigger model, the required time from when higher-
layer signaling starts to when it is completed, is more than that of
the cooperation model except when a virtual TE link is included.
This is because the former model requires lower-layer signaling to
take place during the higher-layer signaling. A higher-layer ingress
LSR has to wait for more time until the higher-layer signaling is
completed. A higher-layer ingress LSR is required to be tolerant of
longer path setup times.
5.4. Network Complexity
If the higher and lower layer networks have multiple interconnects
then optimal path computation for end-to-end LSPs that cross the
layer boundaries is non-trivial. The higher layer LSP must be routed
to the correct layer border nodes to achieve optimality in both
layers.
Where the lower layer LSPs are advertised into the higher layer
network as TE links, the computation can be resolved in the higher
layer network. Care needs to be taken in the allocation of TE
metrics (i.e., costs) to the lower layer LSPs as they are advertised
as TE links into the higher layer network, and this might be a
function for a VNT Manager component. Similarly, attention should be
given to the fact that the LSPs crossing the lower-layer network
might share points of common failure (e.g., they might traverse the
same link in the lower-layer network) and the shared risk link
groups (SRLGs) for the TE links advertised in the higher-layer must
be set accordingly.
In the single PCE model an end-to-end path can be found in a single
computation because there is full visibility into both layers and
all possible paths through all layer interconnects can be considered.
Where PCEs cooperate to determine a path, an iterative computation
model such as [BRPC] can be used to select an optimal path across
layers.
When non-cooperating mono-layer PCEs, each of which is in a separate
layer, are used with the triggered LSP model, it is not possible to
determine the best border LSRs, and connectivity cannot even be
guaranteed. In this case, signaling crankback techniques [RFC4920]
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can be used to eventually achieve connectivity, but optimality is
far harder to achieve. In this model, a PCE that is requested by an
ingress LSR to compute a path expects a border LSR to setup a lower-
layer path triggered by high-layer signaling when there is no TE
link between border LSRs.
5.5. Separation of Layer Management
Many network operators may want to provide a clear separation
between the management of the different layer networks. In some
cases, the lower layer network may come from a separate commercial
arm of an organization or from a different corporate body entirely.
In these cases, the policy applied to the establishment of LSPs in
the lower-layer network and to the advertisement of these LSPs as TE
links in the higher-layer network will reflect commercial agreements
and security concerns (see Section 9). Since the capacity of the
LSPs in the lower-layer network are likely to be significantly
larger than those in the client higher-layer network (multiplex-
server model), the administrator of the lower-layer network may want
to exercise caution before allowing a single small demand in the
higher layer to tie up valuable resources in the lower layer.
The necessary policy points for this separation of administration
and management are more easily achieved through the VNTM approach
than by using triggered signaling. In effect, the VNTM is the
coordination point for all lower layer LSPs and can be closely tied
to a human operator as well as to policy and billing. Such a model
can also be achieved using triggered signaling.
6. Stability Considerations
Inter-layer traffic engineering needs to be managed and operated
correctly to avoid introducing instability problems.
Lower-layer LSPs are likely, by the nature of the technologies used
in layered networks, to be of considerably higher capacity than the
higher-layer LSPs. This has the benefit of allowing multiple higher-
layer LSPs to be carried across the lower-layer network in a single
lower-layer LSP. However, when a new lower-layer LSP is set up to
support a request for a higher-layer LSP because there is no
suitable route in the higher-layer network, it may be the case that
a very large LSP is established in support of a very small traffic
demand. Further, if the higher-layer LSP is short-lived, the
requirement for the lower-layer LSP will go away leaving it either
in-place but unused, or requiring it to be torn down. This may cause
excessive tie-up of unused lower-layer network resources, or may
introduce instability into the lower-layer network. It is important
that appropriate policy controls or configuration features are
available so that demand-led establishment of lower-layer LSPs (the
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so-called "bandwidth on demand") is filtered according to the
requirements of the lower-layer network.
When a higher-layer LSP is requested to be set up, a new lower-layer
LSP may be established if there is no route with the requested
bandwidth for the higher-layer LSP. After the lower-layer LSP is
established, existing high-layer LSPs could be re-routed to use the
newly established lower-layer LSP if using the lower-layer LSP
provides a better route than that taken by the existing LSPs. This
re-routing may result in lower utilization of other lower-layer LSPs
that used to carry the existing higher-layer LSPs. When the
utilization of a lower-layer LSP drops below a threshold (or drops
to zero), the LSP is deleted according to lower-layer network policy.
But consider that some other new higher-layer LSP may be requested
at once requiring the establishment or re-establishment of a lower-
layer LSP. This, in turn, may cause higher-layer re-routing making
other lower-layer LSPs under-utilized, in a cyclic manner. This
behavior makes the higher-layer network unstable.
Inter-layer traffic engineering needs to avoid network instability
problems. To solve the problem, network operators may have some
constraints achieved through configuration or policy, where inter-
layer path control actions such as re-routing and deletion of lower-
layer LSPs are not easily allowed. For example, threshold parameters
for the actions are determined so that hysteresis control behavior
can be performed.
7. IANA Considerations
This informational document makes no requests for IANA action.
8. Manageability Considerations
Inter-layer MPLS or GMPLS traffic engineering must be considered in
the light of administrative and management boundaries that are
likely to coincide with the technology layer boundaries. That is,
each layer network may possibly be under separate management control
with different policies applied to the networks, and specific policy
rules applied at the boundaries between the layers.
Management mechanisms are required to make sure that inter-layer
traffic engineering can be applied without violating the policy and
administrative operational procedures used by the network operators.
8.1. Control of Function and Policy
8.1.1. Control of Inter-Layer Computation Function
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PCE implementations that are capable of supporting inter-layer
computations should provide a configuration switch to allow support
of inter-layer path computations to be enabled or disabled.
When a PCE is capable of, and configured for, inter-layer path
computation, it should advertise this capability as described in
[PCE-INTER-LAYER-REQ], but this advertisement may be suppressed
through a secondary configuration option.
8.1.2. Control of Per-Layer Policy
Where each layer is operated as a separate network, the operators
must have control over the policies applicable to each network, and
that control should be independent of the control of policies for
other networks.
Where multiple layers are operated as part of the same network, the
operator may have a single point of control for an integrated policy
across all layers, or may have control of separate policies for each
layer.
8.1.3. Control of Inter-Layer Policy
Probably the most important issue for inter-layer traffic
engineering is inter-layer policy. This may cover issues such as
under what circumstances a lower layer LSP may be established to
provide connectivity in the higher layer network. Inter-layer policy
may exist to protect the lower layer (high capacity) network from
very dynamic changes in micro-demand in the higher layer network
(see Section 6). It may also be used to ensure appropriate billing
for the lower layer LSPs.
Inter-layer policy should include the definition of the points of
connectivity between the network layers, the inter-layer TE model to
be applied (for example, the selection between the models described
in this document), and the rules for path computation and LSP setup.
Where inter-layer policy is defined, it must be used consistently
throughout the network, and should be made available to the PCEs
that perform inter-layer computation so that appropriate paths are
computed. Mechanisms for providing policy information to PCEs are
discussed in [RFC5394].
VNTM may provide a suitable functional component for the
implementation of inter-layer policy. Use of VNTM allows the
administrator of the lower layer network to apply inter-layer policy
without making that policy public to the operator of the higher
layer network. Similarly, a cooperative PCE model (with or without
inter-PCE communication) allows separate application of policy
during the selection of paths.
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8.2. Information and Data Models
Any protocol extensions to support inter-layer computations must be
accompanied by the definition of MIB objects for the control and
monitoring of the protocol extensions. These MIB object definitions
will conventionally be placed in a separate document from that which
defines the protocol extensions. The MIB objects may be provided in
the same MIB module as used for the management of the base protocol
that is being extended.
Note that inter-layer PCE functions should, themselves, be
manageable through MIB modules. In general, this means that the MIB
modules for managing PCEs should include objects that can be used to
select and report on the inter-layer behavior of each PCE. It may
also be appropriate to provide statistical information that reports
on the inter-layer PCE interactions.
Where there are communications between a PCE and VNTM, additional
MIB modules may be necessary to manage and model these
communications. On the other hand, if these communications are
provided through MIB notifications, then those notifications must
form part of a MIB module definition.
Policy Information Base (PIB) modules may also be appropriate to
meet the requirements as described in Section 6.1 and [RFC5394].
8.3. Liveness Detection and Monitoring
Liveness detection and monitoring is required between PCEs and PCCs,
and between cooperating PCEs as described in [RFC4657]. Inter-layer
traffic engineering does not change this requirement.
Where there are communications between a PCE and VNTM, additional
liveness detection and monitoring may be required to allow the PCE
to know whether the VNTM has received its information about failed
path computations and desired TE links.
When a lower layer LSP fails (perhaps because of the failure of a
lower layer network resource) or is torn down as a result of lower
layer network policy, the consequent change should be reported to
the higher layer as a change in the VNT, although inter-layer policy
may dictate that such a change is hidden from the higher layer. The
higher layer network may additionally operate data plane failure
techniques over the virtual TE links in the VNT in order to monitor
the liveness of the connections, but it should be noted that if the
virtual TE link is advertised but not yet established as an LSP in
the lower layer, such higher layer OAM techniques will report a
failure.
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8.4. Verifying Correct Operation
The correct operation of the PCE computations and interactions are
described in [RFC4657], [RFC5440], etc., and does not need further
discussion here.
The correct operation of inter-layer traffic engineering may be
measured in several ways. First, the failure rate of higher layer
path computations owing to an absence of connectivity across the
lower layer may be observed as a measure of the effectiveness of the
VNT and may be reported as part of the data model described in
Section 6.2. Second, the rate of change of the VNT (i.e., the rate
of establishment and removal of higher layer TE links based on lower
layer LSPs) may be seen as a measure of the correct planning of the
VNT and may also form part of the data model described in Section
6.2. Third, network resource utilization in the lower layer (both in
terms of resource congestion, and in consideration of under
utilization of LSPs set up to support virtual TE links) can indicate
whether effective inter-layer traffic engineering is being applied.
Management tools in the higher layer network should provide a view
of which TE links are provided using planned lower layer capacity
(that is, physical connectivity or permanent connections) and which
TE links are dynamic and achieved through inter-layer traffic
engineering. Management tools in the lower layer should provide a
view of the use to which lower layer LSPs are put including whether
they have been set up to support TE links in a VNT, and if so for
which client network.
8.5. Requirements on Other Protocols and Functional Components
There are no protocols or protocol extensions defined in this
document and so it is not appropriate to consider specific
interactions with other protocols. It should be noted, however, that
the objective of this document is to enable inter-layer traffic
engineering for MPLS-TE and GMPLS networks and so it is assumed that
the necessary features for inter-layer operation of routing and
signaling protocols are in existence or will be developed.
This document introduces roles for various network components (PCE,
LSR, NMS, and VNTM). Those components are all required to play their
part in order that inter-layer TE can be effective. That is, an
inter-layer TE model that assumes the presence and operation of any
of these functional components obviously depends on those components
to fulfill their roles as described in this document.
8.6. Impact on Network Operation
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The use of a PCE to compute inter-layer paths is expected to have a
significant and beneficial impact on network operations. Inter-layer
traffic engineering of itself may provide additional flexibility to
the higher layer network while allowing the lower layer network to
support more and varied client networks in a more efficient way.
Traffic engineering across network layers allows optimal use to be
made of network resources in all layers.
The use of PCE as described in this document may also have a
beneficial effect on the loading of PCEs responsible for performing
inter-layer path computation while facilitating a more independent
operation model for the network layers.
9. Security Considerations
Inter-layer traffic engineering with PCE raises new security issues
in all three inter-layer path control models.
In the cooperation model between PCE and VNTM, when the PCE
determines that a new lower-layer LSP is desirable, communications
are needed between the PCE and VNTM and between VNTM and a border
LSR. In this case, these communications should have security
mechanisms to ensure authenticity, privacy and integrity of the
information exchanged. In particular, it is important to protect
against false triggers for LSP setup in the lower-layer network
since such falsification could tie up lower-layer network resources
(achieving a denial of service attack on the lower-layer network and
on the higher layer network that is attempting to use it) and could
result in incorrect billing for services provided by the lower-layer
network. Where the PCE MIB modules are used to provide the
notification exchanges between the higher-layer PCE and the VNTM,
SNMP v3 should be used to ensure adequate security. Additionally,
the VNTM should provide configurable or dynamic policy functions so
that the VNTM behavior upon receiving notification from a higher-
layer PCE can be controlled.
The main security concern in the higher-layer signaling trigger
model is related to confidentiality. The PCE may inform a higher-
layer PCC about a multi-layer path that includes an ERO in the
lower-layer network, but the PCC may not have TE topology visibility
into the lower-layer network and might not be trusted with this
information. A loose hop across the lower-layer network could be
used, but this decreases the benefit of multi-layer traffic
engineering. A better alternative may be to mask the lower-layer
path using a path key [PATH-KEY] that can be expanded within the
lower-layer network. Consideration must also be given to filtering
the recorded path information from the lower-layer - see [RFC4208],
for example.
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Additionally, in the higher-layer signaling trigger model,
consideration must be given to the security of signaling at the
inter-layer interface since the layers may belong to different
administrative or trust domains.
The NMS-VNTM cooperation model introduces communication between the
NMS and the VNTM. Both of these components belong to the management
plane and the communication is out of scope for this PCE document.
Note that the NMS-VNTM cooperation model may be considered to
address many security and policy concerns because the control and
decision-making is placed within the sphere of influence of the
operator in contrast to the more dynamic mechanisms of the other
models. However, the security issues have simply moved, and will
require authentication of operators and of policy.
Security issues may also exist when a single PCE is granted full
visibility of TE information that applies to multiple layers. Any
access to the single PCE will immediately gain access to the
topology information for all network layers - effectively, a single
security breach can expose information that requires multiple
breaches in other models.
Note that, as described in Section 6, inter-layer TE can cause
network stability issues, and this could be leveraged to attack
either the higher or lower layer network. Precautionary measures,
such as those described in Section 8.1.3, can be applied through
policy or configuration to dampen any network oscillations.
10. Acknowledgments
We would like to thank Kohei Shiomoto, Ichiro Inoue, Julien Meuric,
Jean-Francois Peltier, Young Lee, Ina Minei, and Jean-Philippe
Vasseur, Franz Rambach for their useful comments.
11. References
11.1. Normative Reference
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3945] Mannie, E., "Generalized Multi-Protocol Label Switching
Architecture", RFC 3945, October 2004.
[RFC4206] K. Kompella and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching
(GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.
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11.2. Informative Reference
[RFC5212] K. Shiomoto et al., "Requirements for GMPLS-Based Multi-
Region and Multi-Layer Networks (MRN/MLN)", RFC 5212, July
2008.
[PCE-INTER-LAYER-REQ] E. Oki et al., "PCC-PCE Communication
Requirements for Inter-Layer Traffic Engineering", draft-
ietf-pce-inter-layer-req work in progress.
[BRPC] JP. Vasseur et al., "A Backward Recursive PCE-based
Computation (BRPC) procedure to compute shortest inter-
domain Traffic Engineering Label Switched Paths", draft-
ietf-pce-brpc, work in progress.
[RFC4920] A. Farrel et al., "Crankback Signaling Extensions for MPLS
and GMPLS RSVP-TE", RFC 4920, July 2007.
[PCE-MIB] E. Stephan, "Definitions of Textual Conventions for Path
Computation Element", draft-ietf-pce-tc-mib.txt, work in
progress.
[RFC4802] A. Farrel and T. Nadeau, "Generalized Multiprotocol Label
Switching (GMPLS) Traffic Engineering Management
Information Base", RFC 4802, February 2007.
[PATH-KEY] Bradford, R., Vasseur, JP., and Farrel, A., "Preserving
Topology Confidentiality in Inter-Domain Path Computation
Using a Key Based Mechanism", draft-ietf-pce-path-key, work
in progress.
[RFC4208] Swallow, G., Drake, J., Ishimatsu, H., and Rekhter, Y.,
"Generalized Multiprotocol Label Switching (GMPLS) User-
Network Interface (UNI): Resource ReserVation Protocol-
Traffic Engineering (RSVP-TE) Support for the Overlay
Model", RFC 4208, October 2005.
[RFC4655] A. Farrel, JP. Vasseur and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
[RFC4657] J. Ash and J.L. Le Roux (Ed.), "Path Computation Element
(PCE) Communication Protocol Generic Requirements", RFC
4657, September 2006.
[RFC5394] Bryskin, I., Papadimitriou, P., Berger, L., and Ash J,
"Policy-Enabled Path Computation Framework", RFC 5394,
December 2008.
[RFC5440] JP. Vasseur et al, "Path Computation Element (PCE)
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communication Protocol (PCEP)" RFC 5440, March 2009.
12. Authors' Addresses
Eiji Oki
University of Electro-Communications
Tokyo
Japan
Email: oki@ice.uec.ac.jp
Tomonori Takeda
NTT
3-9-11 Midori-cho,
Musashino-shi, Tokyo 180-8585, Japan
Email: takeda.tomonori@lab.ntt.co.jp
Jean-Louis Le Roux
France Telecom R&D,
Av Pierre Marzin,
22300 Lannion, France
Email: jeanlouis.leroux@orange-ftgroup.com
Adrian Farrel
Old Dog Consulting
Email: adrian@olddog.co.uk
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Oki, et al. [Page 33]