Network Working Group K. Shiomoto
Request for Comments: 5212 NTT
Category: Informational D. Papadimitriou
Alcatel-Lucent
JL. Le Roux
France Telecom
M. Vigoureux
Alcatel-Lucent
D. Brungard
AT&T
July 2008
Requirements for GMPLS-Based
Multi-Region and Multi-Layer Networks (MRN/MLN)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
Most of the initial efforts to utilize Generalized MPLS (GMPLS) have
been related to environments hosting devices with a single switching
capability. The complexity raised by the control of such data planes
is similar to that seen in classical IP/MPLS networks. By extending
MPLS to support multiple switching technologies, GMPLS provides a
comprehensive framework for the control of a multi-layered network of
either a single switching technology or multiple switching
technologies.
In GMPLS, a switching technology domain defines a region, and a
network of multiple switching types is referred to in this document
as a multi-region network (MRN). When referring in general to a
layered network, which may consist of either single or multiple
regions, this document uses the term multi-layer network (MLN). This
document defines a framework for GMPLS based multi-region / multi-
layer networks and lists a set of functional requirements.
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Table of Contents
1. Introduction ....................................................3
1.1. Scope ......................................................4
2. Conventions Used in This Document ...............................5
2.1. List of Acronyms ...........................................6
3. Positioning .....................................................6
3.1. Data Plane Layers and Control Plane Regions ................6
3.2. Service Layer Networks .....................................7
3.3. Vertical and Horizontal Interaction and Integration ........8
3.4. Motivation .................................................9
4. Key Concepts of GMPLS-Based MLNs and MRNs ......................10
4.1. Interface Switching Capability ............................10
4.2. Multiple Interface Switching Capabilities .................11
4.2.1. Networks with Multi-Switching-Type-Capable
Hybrid Nodes .......................................12
4.3. Integrated Traffic Engineering (TE) and Resource Control ..12
4.3.1. Triggered Signaling ................................13
4.3.2. FA-LSPs ............................................13
4.3.3. Virtual Network Topology (VNT) .....................14
5. Requirements ...................................................15
5.1. Handling Single-Switching and
Multi-Switching-Type-Capable Nodes ........................15
5.2. Advertisement of the Available Adjustment Resources .......15
5.3. Scalability ...............................................16
5.4. Stability .................................................17
5.5. Disruption Minimization ...................................17
5.6. LSP Attribute Inheritance .................................17
5.7. Computing Paths with and without Nested Signaling .........18
5.8. LSP Resource Utilization ..................................19
5.8.1. FA-LSP Release and Setup ...........................19
5.8.2. Virtual TE Links ...................................20
5.9. Verification of the LSPs ..................................21
5.10. Management ...............................................22
6. Security Considerations ........................................24
7. Acknowledgements ...............................................24
8. References .....................................................25
8.1. Normative References ......................................25
8.2. Informative References ....................................25
9. Contributors' Addresses ........................................26
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1. Introduction
Generalized MPLS (GMPLS) extends MPLS to handle multiple switching
technologies: packet switching, Layer-2 switching, TDM (Time-Division
Multiplexing) switching, wavelength switching, and fiber switching
(see [RFC3945]). The Interface Switching Capability (ISC) concept is
introduced for these switching technologies and is designated as
follows: PSC (packet switch capable), L2SC (Layer-2 switch capable),
TDM capable, LSC (lambda switch capable), and FSC (fiber switch
capable).
The representation, in a GMPLS control plane, of a switching
technology domain is referred to as a region [RFC4206]. A switching
type describes the ability of a node to forward data of a particular
data plane technology, and uniquely identifies a network region. A
layer describes a data plane switching granularity level (e.g., VC4,
VC-12). A data plane layer is associated with a region in the
control plane (e.g., VC4 is associated with TDM, MPLS is associated
with PSC). However, more than one data plane layer can be associated
with the same region (e.g., both VC4 and VC12 are associated with
TDM). Thus, a control plane region, identified by its switching type
value (e.g., TDM), can be sub-divided into smaller-granularity
component networks based on "data plane switching layers". The
Interface Switching Capability Descriptor (ISCD) [RFC4202],
identifying the interface switching capability (ISC), the encoding
type, and the switching bandwidth granularity, enables the
characterization of the associated layers.
In this document, we define a multi-layer network (MLN) to be a
Traffic Engineering (TE) domain comprising multiple data plane
switching layers either of the same ISC (e.g., TDM) or different ISC
(e.g., TDM and PSC) and controlled by a single GMPLS control plane
instance. We further define a particular case of MLNs. A multi-
region network (MRN) is defined as a TE domain supporting at least
two different switching types (e.g., PSC and TDM), either hosted on
the same device or on different ones, and under the control of a
single GMPLS control plane instance.
MLNs can be further categorized according to the distribution of the
ISCs among the Label Switching Routers (LSRs):
- Each LSR may support just one ISC.
Such LSRs are known as single-switching-type-capable LSRs. The MLN
may comprise a set of single-switching-type-capable LSRs some of
which support different ISCs.
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- Each LSR may support more than one ISC at the same time.
Such LSRs are known as multi-switching-type-capable LSRs, and can
be further classified as either "simplex" or "hybrid" nodes as
defined in Section 4.2.
- The MLN may be constructed from any combination of single-
switching-type-capable LSRs and multi-switching-type-capable LSRs.
Since GMPLS provides a comprehensive framework for the control of
different switching capabilities, a single GMPLS instance may be used
to control the MLN/MRN. This enables rapid service provisioning and
efficient traffic engineering across all switching capabilities. In
such networks, TE links are consolidated into a single Traffic
Engineering Database (TED). Since this TED contains the information
relative to all the different regions and layers existing in the
network, a path across multiple regions or layers can be computed
using this TED. Thus, optimization of network resources can be
achieved across the whole MLN/MRN.
Consider, for example, a MRN consisting of packet-switch-capable
routers and TDM cross-connects. Assume that a packet Label Switched
Path (LSP) is routed between source and destination packet-switch-
capable routers, and that the LSP can be routed across the PSC region
(i.e., utilizing only resources of the packet region topology). If
the performance objective for the packet LSP is not satisfied, new TE
links may be created between the packet-switch-capable routers across
the TDM-region (for example, VC-12 links) and the LSP can be routed
over those TE links. Furthermore, even if the LSP can be
successfully established across the PSC-region, TDM hierarchical LSPs
(across the TDM region between the packet-switch capable routers) may
be established and used if doing so is necessary to meet the
operator's objectives for network resource availability (e.g., link
bandwidth). The same considerations hold when VC4 LSPs are
provisioned to provide extra flexibility for the VC12 and/or VC11
layers in an MLN.
Sections 3 and 4 of this document provide further background
information of the concepts and motivation behind multi-region and
multi-layer networks. Section 5 presents detailed requirements for
protocols used to implement such networks.
1.1. Scope
Early sections of this document describe the motivations and
reasoning that require the development and deployment of MRN/MLN.
Later sections of this document set out the required features that
the GMPLS control plane must offer to support MRN/MLN. There is no
intention to specify solution-specific and/or protocol elements in
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this document. The applicability of existing GMPLS protocols and any
protocol extensions to the MRN/MLN is addressed in separate documents
[MRN-EVAL].
This document covers the elements of a single GMPLS control plane
instance controlling multiple layers within a given TE domain. A
control plane instance can serve one, two, or more layers. Other
possible approaches such as having multiple control plane instances
serving disjoint sets of layers are outside the scope of this
document. It is most probable that such a MLN or MRN would be
operated by a single service provider, but this document does not
exclude the possibility of two layers (or regions) being under
different administrative control (for example, by different Service
Providers that share a single control plane instance) where the
administrative domains are prepared to share a limited amount of
information.
For such a TE domain to interoperate with edge nodes/domains
supporting non-GMPLS interfaces (such as those defined by other
standards development organizations (SDOs)), an interworking function
may be needed. Location and specification of this function are
outside the scope of this document (because interworking aspects are
strictly under the responsibility of the interworking function).
This document assumes that the interconnection of adjacent MRN/MLN TE
domains makes use of [RFC4726] when their edges also support inter-
domain GMPLS RSVP-TE extensions.
2. Conventions Used in This Document
Although this is not a protocol specification, the key words "MUST",
"MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" are used in this document to
highlight requirements, and are to be interpreted as described in RFC
2119 [RFC2119].
In the context of this document, an end-to-end LSP is defined as an
LSP that starts in some client layer, ends in the same layer, and may
cross one or more lower layers. In terms of switching capabilities,
this means that if the outgoing interface on the head-end LSR has
interface switching capability X, then the incoming interface on the
tail-end LSR also has switching capability X. Further, for any
interface traversed by the LSP at any intermediate LSR, the switching
capability of that interface, Y, is such that Y >= X.
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2.1. List of Acronyms
ERO: Explicit Route Object
FA: Forwarding Adjacency
FA-LSP: Forwarding Adjacency Label Switched Path
FSC: Fiber Switching Capable
ISC: Interface Switching Capability
ISCD: Interface Switching Capability Descriptor
L2SC: Layer-2 Switching Capable
LSC: Lambda Switching Capable
LSP: Label Switched Path
LSR: Label Switching Router
MLN: Multi-Layer Network
MRN: Multi-Region Network
PSC: Packet Switching Capable
SRLG: Shared Risk Link Group
TDM: Time-Division Multiplexing
TE: Traffic Engineering
TED: Traffic Engineering Database
VNT: Virtual Network Topology
3. Positioning
A multi-region network (MRN) is always a multi-layer network (MLN)
since the network devices on region boundaries bring together
different ISCs. A MLN, however, is not necessarily a MRN since
multiple layers could be fully contained within a single region. For
example, VC12, VC4, and VC4-4c are different layers of the TDM
region.
3.1. Data Plane Layers and Control Plane Regions
A data plane layer is a collection of network resources capable of
terminating and/or switching data traffic of a particular format
[RFC4397]. These resources can be used for establishing LSPs for
traffic delivery. For example, VC-11 and VC4-64c represent two
different layers.
From the control plane viewpoint, an LSP region is defined as a set
of one or more data plane layers that share the same type of
switching technology, that is, the same switching type. For example,
VC-11, VC-4, and VC-4-7v layers are part of the same TDM region. The
regions that are currently defined are: PSC, L2SC, TDM, LSC, and FSC.
Hence, an LSP region is a technology domain (identified by the ISC
type) for which data plane resources (i.e., data links) are
represented into the control plane as an aggregate of TE information
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associated with a set of links (i.e., TE links). For example, VC-11
and VC4-64c capable TE links are part of the same TDM region.
Multiple layers can thus exist in a single region network.
Note also that the region may produce a distinction within the
control plane. Layers of the same region share the same switching
technology and, therefore, use the same set of technology-specific
signaling objects and technology-specific value setting of TE link
attributes within the control plane, but layers from different
regions may use different technology-specific objects and TE
attribute values. This means that it may not be possible to simply
forward the signaling message between LSRs that host different
switching technologies. This is due to changes in some of the
signaling objects (for example, the traffic parameters) when crossing
a region boundary even if a single control plane instance is used to
manage the whole MRN. We may solve this issue by using triggered
signaling (see Section 4.3.1).
3.2. Service Layer Networks
A service provider's network may be divided into different service
layers. The customer's network is considered from the provider's
perspective as the highest service layer. It interfaces to the
highest service layer of the service provider's network.
Connectivity across the highest service layer of the service
provider's network may be provided with support from successively
lower service layers. Service layers are realized via a hierarchy of
network layers located generally in several regions and commonly
arranged according to the switching capabilities of network devices.
For instance, some customers purchase Layer-1 (i.e., transport)
services from the service provider, some Layer 2 (e.g., ATM), while
others purchase Layer-3 (IP/MPLS) services. The service provider
realizes the services by a stack of network layers located within one
or more network regions. The network layers are commonly arranged
according to the switching capabilities of the devices in the
networks. Thus, a customer network may be provided on top of the
GMPLS-based multi-region/multi-layer network. For example, a Layer-1
service (realized via the network layers of TDM, and/or LSC, and/or
FSC regions) may support a Layer-2 network (realized via ATM Virtual
Path / Virtual Circuit (VP/VC)), which may itself support a Layer-3
network (IP/MPLS region). The supported data plane relationship is a
data plane client-server relationship where the lower layer provides
a service for the higher layer using the data links realized in the
lower layer.
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Services provided by a GMPLS-based multi-region/multi-layer network
are referred to as "multi-region/multi-layer network services". For
example, legacy IP and IP/MPLS networks can be supported on top of
multi-region/multi-layer networks. It has to be emphasized that
delivery of such diverse services is a strong motivator for the
deployment of multi-region/multi-layer networks.
A customer network may be provided on top of a server GMPLS-based
MRN/MLN which is operated by a service provider. For example, a pure
IP and/or an IP/MPLS network can be provided on top of GMPLS-based
packet-over-optical networks [RFC5146]. The relationship between the
networks is a client/server relationship and, such services are
referred to as "MRN/MLN services". In this case, the customer
network may form part of the MRN/MLN or may be partially separated,
for example, to maintain separate routing information but retain
common signaling.
3.3. Vertical and Horizontal Interaction and Integration
Vertical interaction is defined as the collaborative mechanisms
within a network element that is capable of supporting more than one
layer or region and of realizing the client/server relationships
between the layers or regions. Protocol exchanges between two
network controllers managing different regions or layers are also a
vertical interaction. Integration of these interactions as part of
the control plane is referred to as vertical integration. Thus, this
refers to the collaborative mechanisms within a single control plane
instance driving multiple network layers that are part of the same
region or not. Such a concept is useful in order to construct a
framework that facilitates efficient network resource usage and rapid
service provisioning in carrier networks that are based on multiple
layers, switching technologies, or ISCs.
Horizontal interaction is defined as the protocol exchange between
network controllers that manage transport nodes within a given layer
or region. For instance, the control plane interaction between two
TDM network elements switching at OC-48 is an example of horizontal
interaction. GMPLS protocol operations handle horizontal
interactions within the same routing area. The case where the
interaction takes place across a domain boundary, such as between two
routing areas within the same network layer, is evaluated as part of
the inter-domain work [RFC4726], and is referred to as horizontal
integration. Thus, horizontal integration refers to the
collaborative mechanisms between network partitions and/or
administrative divisions such as routing areas or autonomous systems.
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This distinction needs further clarification when administrative
domains match layer/region boundaries. Horizontal interaction is
extended to cover such cases. For example, the collaborative
mechanisms in place between two LSC areas relate to horizontal
integration. On the other hand, the collaborative mechanisms in
place between a PSC (e.g., IP/MPLS) domain and a separate TDM capable
(e.g., VC4 Synchronous Digital Hierarchy (SDH)) domain over which it
operates are part of the horizontal integration, while it can also be
seen as a first step towards vertical integration.
3.4. Motivation
The applicability of GMPLS to multiple switching technologies
provides a unified control and management approach for both LSP
provisioning and recovery. Indeed, one of the main motivations for
unifying the capabilities and operations of the GMPLS control plane
is the desire to support multi-LSP-region [RFC4206] routing and TE
capabilities. For instance, this enables effective network resource
utilization of both the Packet/Layer2 LSP regions and the TDM or
Lambda LSP regions in high-capacity networks.
The rationales for GMPLS-controlled multi-layer/multi-region networks
are summarized below:
- The maintenance of multiple instances of the control plane on
devices hosting more than one switching capability not only
increases the complexity of the interactions between control plane
instances, but also increases the total amount of processing each
individual control plane instance must handle.
- The unification of the addressing spaces helps in avoiding multiple
identifiers for the same object (a link, for instance, or more
generally, any network resource). On the other hand such
aggregation does not impact the separation between the control
plane and the data plane.
- By maintaining a single routing protocol instance and a single TE
database per LSR, a unified control plane model removes the
requirement to maintain a dedicated routing topology per layer and
therefore does not mandate a full mesh of routing adjacencies as is
the case with overlaid control planes.
- The collaboration between technology layers where the control
channel is associated with the data channel (e.g., packet/framed
data planes) and technology layers where the control channel is not
directly associated with the data channel (SONET/SDH, G.709, etc.)
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is facilitated by the capability within GMPLS to associate in-band
control plane signaling to the IP terminating interfaces of the
control plane.
- Resource management and policies to be applied at the edges of such
an MRN/MLN are made more simple (fewer control-to-management
interactions) and more scalable (through the use of aggregated
information).
- Multi-region/multi-layer traffic engineering is facilitated as TE
links from distinct regions/layers are stored within the same TE
Database.
4. Key Concepts of GMPLS-Based MLNs and MRNs
A network comprising transport nodes with multiple data plane layers
of either the same ISC or different ISCs, controlled by a single
GMPLS control plane instance, is called a multi-layer network (MLN).
A subset of MLNs consists of networks supporting LSPs of different
switching technologies (ISCs). A network supporting more than one
switching technology is called a multi-region network (MRN).
4.1. Interface Switching Capability
The Interface Switching Capability (ISC) is introduced in GMPLS to
support various kinds of switching technology in a unified way
[RFC4202]. An ISC is identified via a switching type.
A switching type (also referred to as the switching capability type)
describes the ability of a node to forward data of a particular data
plane technology, and uniquely identifies a network region. The
following ISC types (and, hence, regions) are defined: PSC, L2SC,
TDM capable, LSC, and FSC. Each end of a data link (more precisely,
each interface connecting a data link to a node) in a GMPLS network
is associated with an ISC.
The ISC value is advertised as a part of the Interface Switching
Capability Descriptor (ISCD) attribute (sub-TLV) of a TE link end
associated with a particular link interface [RFC4202]. Apart from
the ISC, the ISCD contains information including the encoding type,
the bandwidth granularity, and the unreserved bandwidth on each of
eight priorities at which LSPs can be established. The ISCD does not
"identify" network layers, it uniquely characterizes information
associated to one or more network layers.
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TE link end advertisements may contain multiple ISCDs. This can be
interpreted as advertising a multi-layer (or multi-switching-
capable) TE link end. That is, the TE link end (and therefore the TE
link) is present in multiple layers.
4.2. Multiple Interface Switching Capabilities
In an MLN, network elements may be single-switching-type-capable or
multi-switching-type-capable nodes. Single-switching-type-capable
nodes advertise the same ISC value as part of their ISCD sub-TLV(s)
to describe the termination capabilities of each of their TE link(s).
This case is described in [RFC4202].
Multi-switching-type-capable LSRs are classified as "simplex" or
"hybrid" nodes. Simplex and hybrid nodes are categorized according
to the way they advertise these multiple ISCs:
- A simplex node can terminate data links with different switching
capabilities where each data link is connected to the node by a
separate link interface. So, it advertises several TE links each
with a single ISC value carried in its ISCD sub-TLV (following the
rules defined in [RFC4206]). An example is an LSR with PSC and TDM
links each of which is connected to the LSR via a separate
interface.
- A hybrid node can terminate data links with different switching
capabilities where the data links are connected to the node by the
same interface. So, it advertises a single TE link containing more
than one ISCD each with a different ISC value. For example, a node
may terminate PSC and TDM data links and interconnect those
external data links via internal links. The external interfaces
connected to the node have both PSC and TDM capabilities.
Additionally, TE link advertisements issued by a simplex or a hybrid
node may need to provide information about the node's internal
adjustment capabilities between the switching technologies supported.
The term "adjustment" refers to the property of a hybrid node to
interconnect the different switching capabilities that it provides
through its external interfaces. The information about the
adjustment capabilities of the nodes in the network allows the path
computation process to select an end-to-end multi-layer or multi-
region path that includes links with different switching capabilities
joined by LSRs that can adapt (i.e., adjust) the signal between the
links.
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4.2.1. Networks with Multi-Switching-Type-Capable Hybrid Nodes
This type of network contains at least one hybrid node, zero or more
simplex nodes, and a set of single-switching-type-capable nodes.
Figure 1 shows an example hybrid node. The hybrid node has two
switching elements (matrices), which support, for instance, TDM and
PSC switching, respectively. The node terminates a PSC and a TDM
link (Link1 and Link2, respectively). It also has an internal link
connecting the two switching elements.
The two switching elements are internally interconnected in such a
way that it is possible to terminate some of the resources of, say,
Link2 and provide adjustment for PSC traffic received/sent over the
PSC interface (#b). This situation is modeled in GMPLS by connecting
the local end of Link2 to the TDM switching element via an additional
interface realizing the termination/adjustment function. There are
two possible ways to set up PSC LSPs through the hybrid node.
Available resource advertisement (i.e., Unreserved and Min/Max LSP
Bandwidth) should cover both of these methods.
.............................
: Network element :
: -------- :
: | PSC | :
Link1 -------------<->--|#a | :
: | | :
: +--<->---|#b | :
: | -------- :
: | ---------- :
TDM : +--<->--|#c TDM | :
+PSC : | | :
Link2 ------------<->--|#d | :
: ---------- :
:............................
Figure 1. Hybrid node.
4.3. Integrated Traffic Engineering (TE) and Resource Control
In GMPLS-based multi-region/multi-layer networks, TE links may be
consolidated into a single Traffic Engineering Database (TED) for use
by the single control plane instance. Since this TED contains the
information relative to all the layers of all regions in the network,
a path across multiple layers (possibly crossing multiple regions)
can be computed using the information in this TED. Thus,
optimization of network resources across the multiple layers of the
same region and across multiple regions can be achieved.
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These concepts allow for the operation of one network layer over the
topology (that is, TE links) provided by other network layers (for
example, the use of a lower-layer LSC LSP carrying PSC LSPs). In
turn, a greater degree of control and interworking can be achieved,
including (but not limited to):
- Dynamic establishment of Forwarding Adjacency (FA) LSPs [RFC4206]
(see Sections 4.3.2 and 4.3.3).
- Provisioning of end-to-end LSPs with dynamic triggering of FA LSPs.
Note that in a multi-layer/multi-region network that includes multi-
switching-type-capable nodes, an explicit route used to establish an
end-to-end LSP can specify nodes that belong to different layers or
regions. In this case, a mechanism to control the dynamic creation
of FA-LSPs may be required (see Sections 4.3.2 and 4.3.3).
There is a full spectrum of options to control how FA-LSPs are
dynamically established. The process can be subject to the control
of a policy, which may be set by a management component and which may
require that the management plane is consulted at the time that the
FA-LSP is established. Alternatively, the FA-LSP can be established
at the request of the control plane without any management control.
4.3.1. Triggered Signaling
When an LSP crosses the boundary from an upper to a lower layer, it
may be nested into a lower-layer FA-LSP that crosses the lower layer.
From a signaling perspective, there are two alternatives to establish
the lower-layer FA-LSP: static (pre-provisioned) and dynamic
(triggered). A pre-provisioned FA-LSP may be initiated either by the
operator or automatically using features like TE auto-mesh [RFC4972].
If such a lower-layer LSP does not already exist, the LSP may be
established dynamically. Such a mechanism is referred to as
"triggered signaling".
4.3.2. FA-LSPs
Once an LSP is created across a layer from one layer border node to
another, it can be used as a data link in an upper layer.
Furthermore, it can be advertised as a TE link, allowing other nodes
to consider the LSP as a TE link for their path computation
[RFC4206]. An LSP created either statically or dynamically by one
instance of the control plane and advertised as a TE link into the
same instance of the control plane is called a Forwarding Adjacency
LSP (FA-LSP). The FA-LSP is advertised as a TE link, and that TE
link is called a Forwarding Adjacency (FA). An FA has the special
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characteristic of not requiring a routing adjacency (peering) between
its end points yet still guaranteeing control plane connectivity
between the FA-LSP end points based on a signaling adjacency. An FA
is a useful and powerful tool for improving the scalability of
GMPLS-TE capable networks since multiple higher-layer LSPs may be
nested (aggregated) over a single FA-LSP.
The aggregation of LSPs enables the creation of a vertical (nested)
LSP hierarchy. A set of FA-LSPs across or within a lower layer can
be used during path selection by a higher-layer LSP. Likewise, the
higher-layer LSPs may be carried over dynamic data links realized via
LSPs (just as they are carried over any "regular" static data links).
This process requires the nesting of LSPs through a hierarchical
process [RFC4206]. The TED contains a set of LSP advertisements from
different layers that are identified by the ISCD contained within the
TE link advertisement associated with the LSP [RFC4202].
If a lower-layer LSP is not advertised as an FA, it can still be used
to carry higher-layer LSPs across the lower layer. For example, if
the LSP is set up using triggered signaling, it will be used to carry
the higher-layer LSP that caused the trigger. Further, the lower
layer remains available for use by other higher-layer LSPs arriving
at the boundary.
Under some circumstances, it may be useful to control the
advertisement of LSPs as FAs during the signaling establishment of
the LSPs [DYN-HIER].
4.3.3. Virtual Network Topology (VNT)
A set of one or more lower-layer LSPs provides information for
efficient path handling in upper layer(s) of the MLN, or, in other
words, provides a virtual network topology (VNT) to the upper layers.
For instance, a set of LSPs, each of which is supported by an LSC
LSP, provides a VNT to the layers of a PSC region, assuming that the
PSC region is connected to the LSC region. Note that a single
lower-layer LSP is a special case of the VNT. The VNT is configured
by setting up or tearing down the lower-layer LSPs. By using GMPLS
signaling and routing protocols, the VNT can be adapted to traffic
demands.
A lower-layer LSP appears as a TE link in the VNT. Whether the
diversely-routed lower-layer LSPs are used or not, the routes of
lower-layer LSPs are hidden from the upper layer in the VNT. Thus,
the VNT simplifies the upper-layer routing and traffic engineering
decisions by hiding the routes taken by the lower-layer LSPs.
However, hiding the routes of the lower-layer LSPs may lose important
information that is needed to make the higher-layer LSPs reliable.
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For instance, the routing and traffic engineering in the IP/MPLS
layer does not usually consider how the IP/MPLS TE links are formed
from optical paths that are routed in the fiber layer. Two optical
paths may share the same fiber link in the lower-layer and therefore
they may both fail if the fiber link is cut. Thus the shared risk
properties of the TE links in the VNT must be made available to the
higher layer during path computation. Further, the topology of the
VNT should be designed so that any single fiber cut does not bisect
the VNT. These issues are addressed later in this document.
Reconfiguration of the VNT may be triggered by traffic demand
changes, topology configuration changes, signaling requests from the
upper layer, and network failures. For instance, by reconfiguring
the VNT according to the traffic demand between source and
destination node pairs, network performance factors, such as maximum
link utilization and residual capacity of the network, can be
optimized. Reconfiguration is performed by computing the new VNT
from the traffic demand matrix and optionally from the current VNT.
Exact details are outside the scope of this document. However, this
method may be tailored according to the service provider's policy
regarding network performance and quality of service (delay,
loss/disruption, utilization, residual capacity, reliability).
5. Requirements
5.1. Handling Single-Switching and Multi-Switching-Type-Capable Nodes
The MRN/MLN can consist of single-switching-type-capable and multi-
switching-type-capable nodes. The path computation mechanism in the
MLN should be able to compute paths consisting of any combination of
such nodes.
Both single-switching-type-capable and multi-switching-type-capable
(simplex or hybrid) nodes could play the role of layer boundary.
MRN/MLN path computation should handle TE topologies built of any
combination of nodes.
5.2. Advertisement of the Available Adjustment Resources
A hybrid node should maintain resources on its internal links (the
links required for vertical integration between layers). Likewise,
path computation elements should be prepared to use information about
the availability of termination and adjustment resources as a
constraint in MRN/MLN path computations. This would reduce the
probability that the setup of the higher-layer LSP will be blocked by
the lack of necessary termination/adjustment resources in the lower
layers.
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The advertisement of a node's MRN adjustment capabilities (the
ability to terminate LSPs of lower regions and forward the traffic in
upper regions) is REQUIRED, as it provides critical information when
performing multi-region path computation.
The path computation mechanism should cover the case where the
upper-layer links that are directly connected to upper-layer
switching elements and the ones that are connected through internal
links between upper-layer element and lower-layer element coexist
(see Section 4.2.1).
5.3. Scalability
The MRN/MLN relies on unified routing and traffic engineering models.
- Unified routing model: By maintaining a single routing protocol
instance and a single TE database per LSR, a unified control plane
model removes the requirement to maintain a dedicated routing
topology per layer, and therefore does not mandate a full mesh of
routing adjacencies per layer.
- Unified TE model: The TED in each LSR is populated with TE links
from all layers of all regions (TE link interfaces on multiple-
switching-type-capable LSRs can be advertised with multiple ISCDs).
This may lead to an increase in the amount of information that has
to be flooded and stored within the network.
Furthermore, path computation times, which may be of great importance
during restoration, will depend on the size of the TED.
Thus, MRN/MLN routing mechanisms MUST be designed to scale well with
an increase of any of the following:
- Number of nodes
- Number of TE links (including FA-LSPs)
- Number of LSPs
- Number of regions and layers
- Number of ISCDs per TE link.
Further, design of the routing protocols MUST NOT prevent TE
information filtering based on ISCDs. The path computation mechanism
and the signaling protocol SHOULD be able to operate on partial TE
information.
Since TE links can advertise multiple Interface Switching
Capabilities (ISCs), the number of links can be limited (by
combination) by using specific topological maps referred to as VNTs
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(Virtual Network Topologies). The introduction of virtual
topological maps leads us to consider the concept of emulation of
data plane overlays.
5.4. Stability
Path computation is dependent on the network topology and associated
link state. The path computation stability of an upper layer may be
impaired if the VNT changes frequently and/or if the status and TE
parameters (the TE metric, for instance) of links in the VNT changes
frequently. In this context, robustness of the VNT is defined as the
capability to smooth changes that may occur and avoid their
propagation into higher layers. Changes to the VNT may be caused by
the creation, deletion, or modification of LSPs.
Protocol mechanisms MUST be provided to enable creation, deletion,
and modification of LSPs triggered through operational actions.
Protocol mechanisms SHOULD be provided to enable similar functions
triggered by adjacent layers. Protocol mechanisms MAY be provided to
enable similar functions to adapt to the environment changes such as
traffic demand changes, topology changes, and network failures.
Routing robustness should be traded with adaptability of those
changes.
5.5. Disruption Minimization
When reconfiguring the VNT according to a change in traffic demand,
the upper-layer LSP might be disrupted. Such disruption to the upper
layers must be minimized.
When residual resource decreases to a certain level, some lower-layer
LSPs may be released according to local or network policies. There
is a trade-off between minimizing the amount of resource reserved in
the lower layer and disrupting higher-layer traffic (i.e., moving the
traffic to other TE-LSPs so that some LSPs can be released). Such
traffic disruption may be allowed, but MUST be under the control of
policy that can be configured by the operator. Any repositioning of
traffic MUST be as non-disruptive as possible (for example, using
make-before-break).
5.6. LSP Attribute Inheritance
TE link parameters should be inherited from the parameters of the LSP
that provides the TE link, and so from the TE links in the lower
layer that are traversed by the LSP.
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These include:
- Interface Switching Capability
- TE metric
- Maximum LSP bandwidth per priority level
- Unreserved bandwidth for all priority levels
- Maximum reservable bandwidth
- Protection attribute
- Minimum LSP bandwidth (depending on the switching capability)
- SRLG
Inheritance rules must be applied based on specific policies.
Particular attention should be given to the inheritance of the TE
metric (which may be other than a strict sum of the metrics of the
component TE links at the lower layer), protection attributes, and
SRLG.
As described earlier, hiding the routes of the lower-layer LSPs may
lose important information necessary to make LSPs in the higher-layer
network reliable. SRLGs may be used to identify which lower-layer
LSPs share the same failure risk so that the potential risk of the
VNT becoming disjoint can be minimized, and so that resource-disjoint
protection paths can be set up in the higher layer. How to inherit
the SRLG information from the lower layer to the upper layer needs
more discussion and is out of scope of this document.
5.7. Computing Paths with and without Nested Signaling
Path computation can take into account LSP region and layer
boundaries when computing a path for an LSP. Path computation may
restrict the path taken by an LSP to only the links whose interface
switching capability is PSC. For example, suppose that a TDM-LSP is
routed over the topology composed of TE links of the same TDM layer.
In calculating the path for the LSP, the TED may be filtered to
include only links where both end include requested LSP switching
type. In this way hierarchical routing is done by using a TED
filtered with respect to switching capability (that is, with respect
to particular layer).
If triggered signaling is allowed, the path computation mechanism may
produce a route containing multiple layers/regions. The path is
computed over the multiple layers/regions even if the path is not
"connected" in the same layer as where the endpoints of the path
exist. Note that here we assume that triggered signaling will be
invoked to make the path "connected", when the upper-layer signaling
request arrives at the boundary node.
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The upper-layer signaling request MAY contain an ERO (Explicit Route
Object) that includes only hops in the upper layer; in which case,
the boundary node is responsible for triggered creation of the
lower-layer FA-LSP using a path of its choice, or for the selection
of any available lower-layer LSP as a data link for the higher layer.
This mechanism is appropriate for environments where the TED is
filtered in the higher layer, where separate routing instances are
used per layer, or where administrative policies prevent the higher
layer from specifying paths through the lower layer.
Obviously, if the lower-layer LSP has been advertised as a TE link
(virtual or real) into the higher layer, then the higher-layer
signaling request MAY contain the TE link identifier and so indicate
the lower-layer resources to be used. But in this case, the path of
the lower-layer LSP can be dynamically changed by the lower layer at
any time.
Alternatively, the upper-layer signaling request MAY contain an ERO
specifying the lower-layer FA-LSP route. In this case, the boundary
node MAY decide whether it should use the path contained in the
strict ERO or re-compute the path within the lower layer.
Even in the case that the lower-layer FA-LSPs are already
established, a signaling request may also be encoded as a loose ERO.
In this situation, it is up to the boundary node to decide whether it
should create a new lower-layer FA-LSP or it should use an existing
lower-layer FA-LSP.
The lower-layer FA-LSP can be advertised just as an FA-LSP in the
upper layer or an IGP adjacency can be brought up on the lower-layer
FA-LSP.
5.8. LSP Resource Utilization
Resource usage in all layers should be optimized as a whole (i.e.,
across all layers), in a coordinated manner (i.e., taking all layers
into account). The number of lower-layer LSPs carrying upper-layer
LSPs should be minimized (note that multiple LSPs may be used for
load balancing). Lower-layer LSPs that could have their traffic
re-routed onto other LSPs are unnecessary and should be avoided.
5.8.1. FA-LSP Release and Setup
If there is low traffic demand, some FA-LSPs that do not carry any
higher-layer LSP may be released so that lower-layer resources are
released and can be assigned to other uses. Note that if a small
fraction of the available bandwidth of an FA-LSP is still in use, the
nested LSPs can also be re-routed to other FA-LSPs (optionally using
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RFC 5212 MRN/MLN Requirements July 2008
the make-before-break technique) to completely free up the FA-LSP.
Alternatively, unused FA-LSPs may be retained for future use.
Release or retention of underutilized FA-LSPs is a policy decision.
As part of the re-optimization process, the solution MUST allow
rerouting of an FA-LSP while keeping interface identifiers of
corresponding TE links unchanged. Further, this process MUST be
possible while the FA-LSP is carrying traffic (higher-layer LSPs)
with minimal disruption to the traffic.
Additional FA-LSPs may also be created based on policy, which might
consider residual resources and the change of traffic demand across
the region. By creating the new FA-LSPs, the network performance
such as maximum residual capacity may increase.
As the number of FA-LSPs grows, the residual resources may decrease.
In this case, re-optimization of FA-LSPs may be invoked according to
policy.
Any solution MUST include measures to protect against network
destabilization caused by the rapid setup and teardown of LSPs as
traffic demand varies near a threshold.
Signaling of lower-layer LSPs SHOULD include a mechanism to rapidly
advertise the LSP as a TE link and to coordinate into which routing
instances the TE link should be advertised.
5.8.2. Virtual TE Links
It may be considered disadvantageous to fully instantiate (i.e.,
pre-provision) the set of lower-layer LSPs that provide the VNT since
this might reserve bandwidth that could be used for other LSPs in the
absence of upper-layer traffic.
However, in order to allow path computation of upper-layer LSPs
across the lower layer, the lower-layer LSPs may be advertised into
the upper layer as though they had been fully established, but
without actually establishing them. Such TE links that represent the
possibility of an underlying LSP are termed "virtual TE links". It
is an implementation choice at a layer boundary node whether to
create real or virtual TE links, and the choice (if available in an
implementation) MUST be under the control of operator policy. Note
that there is no requirement to support the creation of virtual TE
links, since real TE links (with established LSPs) may be used. Even
if there are no TE links (virtual or real) advertised to the higher
layer, it is possible to route a higher-layer LSP into a lower layer
on the assumption that proper hierarchical LSPs in the lower layer
will be dynamically created (triggered) as needed.
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If an upper-layer LSP that makes use of a virtual TE link is set up,
the underlying LSP MUST be immediately signaled in the lower layer.
If virtual TE links are used in place of pre-established LSPs, the TE
links across the upper layer can remain stable using pre-computed
paths while wastage of bandwidth within the lower layer and
unnecessary reservation of adaptation resources at the border nodes
can be avoided.
The solution SHOULD provide operations to facilitate the build-up of
such virtual TE links, taking into account the (forecast) traffic
demand and available resources in the lower layer.
Virtual TE links can be added, removed, or modified dynamically (by
changing their capacity) according to the change of the (forecast)
traffic demand and the available resources in the lower layer. It
MUST be possible to add, remove, and modify virtual TE links in a
dynamic way.
Any solution MUST include measures to protect against network
destabilization caused by the rapid changes in the VNT as traffic
demand varies near a threshold.
The concept of the VNT can be extended to allow the virtual TE links
to form part of the VNT. The combination of the fully provisioned TE
links and the virtual TE links defines the VNT provided by the lower
layer. The VNT can be changed by setting up and/or tearing down
virtual TE links as well as by modifying real links (i.e., the fully
provisioned LSPs). How to design the VNT and how to manage it are
out of scope of this document.
In some situations, selective advertisement of the preferred
connectivity among a set of border nodes between layers may be
appropriate. Further decreasing the number of advertisements of the
virtual connectivity can be achieved by abstracting the topology
(between border nodes) using models similar to those detailed in
[RFC4847].
5.9. Verification of the LSPs
When a lower-layer LSP is established for use as a data link by a
higher layer, the LSP may be verified for correct connectivity and
data integrity before it is made available for use. Such mechanisms
are data-technology-specific and are beyond the scope of this
document, but the GMPLS protocols SHOULD provide mechanisms for the
coordination of data link verification.
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5.10. Management
An MRN/MLN requires management capabilities. Operators need to have
the same level of control and management for switches and links in
the network that they would have in a single-layer or single-region
network.
We can consider two different operational models: (1) per-layer
management entities and (2) cross-layer management entities.
Regarding per-layer management entities, it is possible for the MLN
to be managed entirely as separate layers, although that somewhat
defeats the objective of defining a single multi-layer network. In
this case, separate management systems would be operated for each
layer, and those systems would be unaware of the fact that the layers
were closely coupled in the control plane. In such a deployment, as
LSPs were automatically set up as the result of control plane
requests from other layers (for example, triggered signaling), the
management applications would need to register the creation of the
new LSPs and the depletion of network resources. Emphasis would be
placed on the layer boundary nodes to report the activity to the
management applications.
A more likely scenario is to apply a closer coupling of layer
management systems with cross-layer management entities. This might
be achieved through a unified management system capable of operating
multiple layers, or by a meta-management system that coordinates the
operation of separate management systems each responsible for
individual layers. The former case might only be possible with the
development of new management systems, while the latter is feasible
through the coordination of existing network management tools.
Note that when a layer boundary also forms an administrative
boundary, it is highly unlikely that there will be unified multi-
layer management. In this case, the layers will be separately
managed by the separate administrative entities, but there may be
some "leakage" of information between the administrations in order to
facilitate the operation of the MLN. For example, the management
system in the lower-layer network might automatically issue reports
on resource availability (coincident with TE routing information) and
alarm events.
This discussion comes close to an examination of how a VNT might be
managed and operated. As noted in Section 5.8, issues of how to
design and manage a VNT are out of scope for this document, but it
should be understood that the VNT is a client-layer construct built
from server-layer resources. This means that the operation of a VNT
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is a collaborative activity between layers. This activity is
possible even if the layers are from separate administrations, but in
this case the activity may also have commercial implications.
MIB modules exist for the modeling and management of GMPLS networks
[RFC4802] [RFC4803]. Some deployments of GMPLS networks may choose
to use MIB modules to operate individual network layers. In these
cases, operators may desire to coordinate layers through a further
MIB module that could be developed. Multi-layer protocol solutions
(that is, solutions where a single control plane instance operates in
more than one layer) SHOULD be manageable through MIB modules. A
further MIB module to coordinate multiple network layers with this
control plane MIB module may be produced.
Operations and Management (OAM) tools are important to the successful
deployment of all networks.
OAM requirements for GMPLS networks are described in [GMPLS-OAM].
That document points out that protocol solutions for individual
network layers should include mechanisms for OAM or make use of OAM
features inherent in the physical media of the layers. Further
discussion of individual-layer OAM is out of scope of this document.
When operating OAM in a MLN, consideration must be given to how to
provide OAM for end-to-end LSPs that cross layer boundaries (that may
also be administrative boundaries) and how to coordinate errors and
alarms detected in a server layer that need to be reported to the
client layer. These operational choices MUST be left open to the
service provider and so MLN protocol solutions MUST include the
following features:
- Within the context and technology capabilities of the highest
technology layer of an LSP (i.e., the technology layer of the first
hop), it MUST be possible to enable end-to-end OAM on a MLN LSP.
This function appears to the ingress LSP as normal LSP-based OAM
[GMPLS-OAM], but at layer boundaries, depending on the technique
used to span the lower layers, client-layer OAM operations may need
to mapped to server-layer OAM operations. Most such requirements
are highly dependent on the OAM facilities of the data plane
technologies of client and server layers. However, control plane
mechanisms used in the client layer per [GMPLS-OAM] MUST map and
enable OAM in the server layer.
- OAM operation enabled per [GMPLS-OAM] in a client layer for an LSP
MUST operate for that LSP along its entire length. This means that
if an LSP crosses a domain of a lower-layer technology, the
client-layer OAM operation must operate seamlessly within the
client layer at both ends of the client-layer LSP.
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RFC 5212 MRN/MLN Requirements July 2008
- OAM functions operating within a server layer MUST be controllable
from the client layer such that the server-layer LSP(s) that
support a client-layer LSP have OAM enabled at the request of the
client layer. Such control SHOULD be subject to policy at the
layer boundary, just as automatic provisioning and LSP requests to
the server layer are subject to policy.
- The status including errors and alarms applicable to a server-layer
LSP MUST be available to the client layer. This information SHOULD
be configurable to be automatically notified to the client layer at
the layer boundary and SHOULD be subject to policy so that the
server layer may filter or hide information supplied to the client
layer. Furthermore, the client layer SHOULD be able to select to
not receive any or all such information.
Note that the interface between layers lies within network nodes and
is, therefore, not necessarily the subject of a protocol
specification. Implementations MAY use standardized techniques (such
as MIB modules) to convey status information (such as errors and
alarms) between layers, but that is out of scope for this document.
6. Security Considerations
The MLN/MRN architecture does not introduce any new security
requirements over the general GMPLS architecture described in
[RFC3945]. Additional security considerations form MPLS and GMPLS
networks are described in [MPLS-SEC].
However, where the separate layers of an MLN/MRN network are operated
as different administrative domains, additional security
considerations may be given to the mechanisms for allowing LSP setup
crossing one or more layer boundaries, for triggering lower-layer
LSPs, or for VNT management. Similarly, consideration may be given
to the amount of information shared between administrative domains,
and the trade-off between multi-layer TE and confidentiality of
information belonging to each administrative domain.
It is expected that solution documents will include a full analysis
of the security issues that any protocol extensions introduce.
7. Acknowledgements
The authors would like to thank Adrian Farrel and the participants of
ITU-T Study Group 15, Question 14 for their careful review. The
authors would like to thank the IESG review team for rigorous review:
special thanks to Tim Polk, Miguel Garcia, Jari Arkko, Dan Romascanu,
and Dave Ward.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3945] Mannie, E., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Architecture", RFC 3945, October 2004.
[RFC4202] Kompella, K., Ed., and Y. Rekhter, Ed., "Routing
Extensions in Support of Generalized Multi-Protocol Label
Switching (GMPLS)", RFC 4202, October 2005.
[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching
(GMPLS) Traffic Engineering (TE)", RFC 4206, October
2005.
[RFC4397] Bryskin, I. and A. Farrel, "A Lexicography for the
Interpretation of Generalized Multiprotocol Label
Switching (GMPLS) Terminology within the Context of the
ITU-T's Automatically Switched Optical Network (ASON)
Architecture", RFC 4397, February 2006.
[RFC4726] Farrel, A., Vasseur, J.-P., and A. Ayyangar, "A Framework
for Inter-Domain Multiprotocol Label Switching Traffic
Engineering", RFC 4726, November 2006.
8.2. Informative References
[DYN-HIER] Shiomoto, K., Rabbat, R., Ayyangar, A., Farrel, A. and
Z. Ali, "Procedures for Dynamically Signaled Hierarchical
Label Switched Paths", Work in Progress, February 2008.
[MRN-EVAL] Le Roux, J.L., Ed., and D. Papadimitriou, Ed.,
"Evaluation of existing GMPLS Protocols against Multi
Layer and Multi Region Networks (MLN/MRN)", Work in
Progress, December 2007.
[RFC5146] Kumaki, K., Ed., "Interworking Requirements to Support
Operation of MPLS-TE over GMPLS Networks", RFC 5146,
March 2008.
[MPLS-SEC] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", Work in Progress, February 2008.
Shiomoto, et al. Informational [Page 25]
RFC 5212 MRN/MLN Requirements July 2008
[RFC4802] Nadeau, T., Ed., and A. Farrel, Ed., "Generalized
Multiprotocol Label Switching (GMPLS) Traffic Engineering
Management Information Base", RFC 4802, February 2007.
[RFC4803] Nadeau, T., Ed., and A. Farrel, Ed., "Generalized
Multiprotocol Label Switching (GMPLS) Label Switching
Router (LSR) Management Information Base", RFC 4803,
February 2007.
[RFC4847] Takeda, T., Ed., "Framework and Requirements for Layer 1
Virtual Private Networks", RFC 4847, April 2007.
[RFC4972] Vasseur, JP., Ed., Leroux, JL., Ed., Yasukawa, S.,
Previdi, S., Psenak, P., and P. Mabbey, "Routing
Extensions for Discovery of Multiprotocol (MPLS) Label
Switch Router (LSR) Traffic Engineering (TE) Mesh
Membership", RFC 4972, July 2007.
[GMPLS-OAM] Nadeau, T., Otani, T. Brungard, D., and A. Farrel, "OAM
Requirements for Generalized Multi-Protocol Label
Switching (GMPLS) Networks", Work in Progress, October
2007.
9. Contributors' Addresses
Eiji Oki
NTT Network Service Systems Laboratories
3-9-11 Midori-cho, Musashino-shi
Tokyo 180-8585
Japan
Phone: +81 422 59 3441
EMail: oki.eiji@lab.ntt.co.jp
Ichiro Inoue
NTT Network Service Systems Laboratories
3-9-11 Midori-cho, Musashino-shi
Tokyo 180-8585
Japan
Phone: +81 422 59 3441
EMail: ichiro.inoue@lab.ntt.co.jp
Emmanuel Dotaro
Alcatel-Lucent
Route de Villejust
91620 Nozay
France
Phone: +33 1 3077 2670
EMail: emmanuel.dotaro@alcatel-lucent.fr
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RFC 5212 MRN/MLN Requirements July 2008
Authors' Addresses
Kohei Shiomoto
NTT Network Service Systems Laboratories
3-9-11 Midori-cho, Musashino-shi
Tokyo 180-8585
Japan
EMail: shiomoto.kohei@lab.ntt.co.jp
Dimitri Papadimitriou
Alcatel-Lucent
Copernicuslaan 50
B-2018 Antwerpen
Belgium
Phone : +32 3 240 8491
EMail: dimitri.papadimitriou@alcatel-lucent.be
Jean-Louis Le Roux
France Telecom R&D
Av Pierre Marzin
22300 Lannion
France
EMail: jeanlouis.leroux@orange-ftgroup.com
Martin Vigoureux
Alcatel-Lucent
Route de Villejust
91620 Nozay
France
Phone: +33 1 3077 2669
EMail: martin.vigoureux@alcatel-lucent.fr
Deborah Brungard
AT&T
Rm. D1-3C22 - 200
S. Laurel Ave.
Middletown, NJ 07748
USA
Phone: +1 732 420 1573
EMail: dbrungard@att.com
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RFC 5212 MRN/MLN Requirements July 2008
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Shiomoto, et al. Informational [Page 28]
RFC, FYI, BCP