Network Working Group Kohei Shiomoto (NTT)
Internet-Draft Dimitri Papadimitriou (Alcatel-Lucent)
Intended Status: Informational Jean-Louis Le Roux (France Telecom)
Martin Vigoureux (Alcatel-Lucent)
Deborah Brungard (AT&T)
Expires: July 2008 January 2008
Requirements for GMPLS-Based Multi-Region and
Multi-Layer Networks (MRN/MLN)
draft-ietf-ccamp-gmpls-mln-reqs-08.txt
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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.
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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 a 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.
Table of Contents
1. Introduction ....................................................
1.1. Scope .........................................................
2. Conventions Used in this Document ...............................
2.1. List of Acronyms ..............................................
3. Positioning .....................................................
3.1. Data Plane Layers and Control Plane Regions ...................
3.2. Service Layer Networks .. .....................................
3.3. Vertical and Horizontal Interaction and Integration ...........
3.4. Motivation ....................................................
4. Key Concepts of GMPLS-Based MLNs and MRNs .......................
4.1. Interface Switching Capability ................................
4.2. Multiple Interface Switching Capabilities .....................
4.2.1. Networks with Multi-Switching-Type-Capable Hybrid Nodes .....
4.3. Integrated Traffic Engineering (TE) and Resource Control ......
4.3.1. Triggered Signaling .........................................
4.3.2. FA-LSPs .....................................................
4.3.3. Virtual Network Topology (VNT) ..............................
5. Requirements ....................................................
5.1. Handling Single-Switching and Multi-Switching-Type-Capable
Nodes .......................................................
5.2. Advertisement of the Available Adjustment Resource ............
5.3. Scalability ...................................................
5.4. Stability .....................................................
5.5. Disruption Minimization .......................................
5.6. LSP Attribute Inheritance .....................................
5.7. Computing Paths With and Without Nested Signaling .............
5.8. LSP Resource Utilization ......................................
5.8.1. FA-LSP Release and Setup ....................................
5.8.2. Virtual TE-Links ............................................
5.9. Verification of the LSPs ......................................
6. Security Considerations .........................................
7. IANA Considerations ............................................
8. Acknowledgements ................................................
9. References ......................................................
9.1. Normative Reference ...........................................
9.2. Informative References ........................................
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10. Authors' Addresses .............................................
11. Contributors' Addresses ........................................
12. Intellectual Property Considerations ...........................
13. Full Copyright Statement .......................................
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1. Introduction
Generalized MPLS (GMPLS) extends MPLS to handle multiple switching
technologies: packet switching, layer-2 switching, TDM 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 (Time
Division Multiplex 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 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 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.
- 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.
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-
- - 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 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 resources 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.
1.1. Scope
This document describes the requirements to support multi-region/
multi-layer networks. There is no intention to specify solution-
specific and/or protocol elements in 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
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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 TE domain to interoperate with edge nodes/domains
supporting non-GMPLS interfaces (such as those defined by other
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].
2.1. List of Acronyms
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 Ling Group
TDM: Time-Division Switch Capable
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
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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 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 LSR hosting different
switching technologies because change 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.
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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
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.
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 [MPLS-GMPLS]. 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 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.
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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.
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 lambda switching capable areas
relate to horizontal integration. On the other hand, the
collaborative mechanisms in place between a packet switching
capable (e.g., IP/MPLS) domain and a separate time division
switching capable (e.g., VC4 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
Traffic Engineering (TE) capabilities. For instance, this enables
effective network resource utilization of both the Packet/Layer2
LSP regions and the Time Division Multiplexing (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 their interactions but also increases
the total amount of processing individual instances would 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.
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- 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.) 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 a MRN/MLN is 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 sub-set 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, 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.
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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.
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]). For example, 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 capacity between the switching technologies
supported. The term "adjustment" capacity refers to the property of
an hybrid node to interconnect different switching capabilities it
provides through its external interfaces.. This information allows
path computation to select an end- to-end multi-layer or multi-
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region path that includes links of different switching capabilities
that are joined by LSRs that can adapt the signal between the links.
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,
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optimization of network resources across the multiple layers of the
same region and across multiple regions can be achieved.
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 inter-working can be
achieved, including (but not limited too):
- 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
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[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
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 Traffic Engineering (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 of 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 virtual network topology 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 virtual network topology is configured by setting up or
tearing down the lower layer LSPs. By using GMPLS signaling and
routing protocols, the virtual network topology can be adapted to
traffic demands.
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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. 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 virtual network topology may be triggered by
traffic demand changes, topology configuration changes, signaling
requests from the upper layer, and network failures. For instance,
by reconfiguring the virtual network topology 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 Resource
A hybrid node SHOULD maintain resources on its internal links (the
links required for vertical (layer) integration) and SHOULD
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advertise the resource information for those links. Likewise, path
computation elements SHOULD be prepared to use the availability of
termination/ adjustment resources as a constraint in MRN/MLN path
computations to reduce the higher layer LSP setup blocking
probability caused by the lack of necessary termination/adjustment
resources in the lower layer(s).
The advertisement of the adjustment capability to terminate LSPs of
lower-region and forward traffic in the upper-region is REQUIRED,
as it provides critical information when performing multi-region
path computation.
The mechanism SHOULD cover the case where the upper-layer links
which are directly connected to upper-layer switching element and
the ones which 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-capability 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.
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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
(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.
Creation, deletion, and modification of LSPs MAY be triggered by
adjacent layers or through operational actions to meet traffic
demand changes, topology changes, signaling requests from the upper
layer, and network failures. Routing robustness SHOULD be traded
with adaptability with respect to the change of incoming traffic
requests.
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.
These include:
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- 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 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 MAY take into account LSP region and layer
boundaries when computing a path for an LSP. For example, path
computation MAY restrict the path taken by an LSP to only the links
whose interface switching capability is PSC.
Interface switching capability is used as a constraint in path
computation. For example, 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 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.
The upper-layer signaling request may contain an ERO that includes
only hops in the upper layer, in which case the boundary node is
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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 is responsible for decision as to which it should use the path
contained in the strict ERO or it should re-compute the path within
in the lower-layer.
Even in case the lower-layer FA-LSPs are already established, a
signaling request may also be encoded as loose ERO. In this
situation, it is up to the boundary node to decide whether it
should a new lower-layer FA-LSP or it should use the existing
lower-layer FA-LSPs.
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
It MUST be possible to utilize network resources efficiently.
Particularly, 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
Statistical multiplexing can only be employed in PSC and L2SC
regions. A PSC or L2SC LSP may or may not consume the maximum
reservable bandwidth of the TE link (FA-LSP) that carries it. On
the other hand, a TDM, or LSC LSP always consumes a fixed amount of
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bandwidth as long as it exists (and is fully instantiated) because
statistical multiplexing is not available.
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 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 resource 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
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that there is no requirement to support the creation of virtual TE-
links, since real TE-links (with established LSPs) may be used, and
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 assumptions that proper hierarchical LSPs in the
lower layer will be dynamically created (triggered) as needed.
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 resource 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 resource in the lower-layer.
Virtual TE-links MAY be added, removed or modified dynamically (by
changing their capacity) according to the change of the (forecast)
traffic demand and the available resource in the lower-layer. The
maximum number of virtual TE links that can be defined SHOULD be
configurable.
Any solution MUST include measures to protect against network
destabilization caused by the rapid changes in the virtual network
topology 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 advertisement 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. Such mechanisms are data technology-specific and
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are beyond the scope of this document, but may be coordinated
through the GMPLS control plane.
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 a MLN/MRN network are
operated as different administrative domains, additional security
considerations may be given to the mechanisms for allowing inter-
layer LSP setup, 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. IANA Considerations
This informational document makes no requests to IANA for action.
8. Acknowledgements
The authors would like to thank Adrian Farrel and the participants
of ITU-T Study Group 15 Question 14 for their careful review.
9. References
9.1. Normative Reference
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3945] E. Mannie (Editor), "Generalized Multi-Protocol Label
Switching (GMPLS) Architecture", RFC 3945, October 2004.
[RFC4202] Kompella, K., and Rekhter, Y., "Routing Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)," RFC4202, October 2005.
[RFC4206] Kompella, K., and Rekhter, Y., "Label Switched Paths
(LSP) Hierarchy with Generalized Multi-Protocol Label
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Switching (GMPLS) Traffic Engineering (TE)," RFC4206,
Oct. 2005.
[RFC4397] Bryskin, I., and Farrel, A., "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, JP., and Ayyangar, A., "A
Framework for Inter-Domain Multiprotocol Label
Switching Traffic Engineering", RFC 4726, November 2006.
9.2. Informative References
[DYN-HIER] Shiomoto, K., Rabbat, R., Ayyangar, A., Farrel, A.
and Ali, Z., "Procedures for Dynamically Signaled
Hierarchical Label Switched Paths", draft-ietf-ccamp-
lsp-hierarchy-bis, work in progress.
[MRN-EVAL] Le Roux, J.L., Brungard, D., Oki, E., Papadimitriou,
D., Shiomoto, K., Vigoureux, M., "Evaluation of
Existing GMPLS Protocols Against Multi-Layer and
Multi-Region Network (MLN/MRN) Requirements", draft-
ietf-ccamp-gmpls- mln-eval, work in progress.
[MPLS-GMPLS]
K. Kumaki (Editor), "Interworking Requirements to
Support Operation of MPLS-TE over GMPLS Networks",
draft-ietf-ccamp-mpls-gmpls-interwork-reqts, work in
progress.
[MPLS-SEC] Fang, L., et al., "Security Framework for MPLS and
GMPLS Networks", draft-ietf-mpls-mpls-and-gmpls-
security-framework, work in progress.
[RFC4847] T. Takeda (Editor), " Framework and Requirements for
Layer 1 Virtual Private Networks", RFC 4847, April
2007.
[RFC4972] Vasseur, JP., Le Roux, JL., et al., "Routing
Extensions for Discovery of Multiprotocol (MPLS)
Label Switch Router (LSR) Traffic Engineering (TE)
Mesh Membership", RFC 4972, July 2007.
10. 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
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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-ft.com
Martin Vigoureux
Alcatel-Lucent
Route de Nozay, 91461 Marcoussis cedex, France
Phone: +33 (0)1 69 63 18 52
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
11. 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 Nozay,
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91461 Marcoussis cedex,
France
Phone : +33 1 6963 4723
Email: emmanuel.dotaro@alcatel-lucent.fr
12. Intellectual Property Considerations
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
13. Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on
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REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
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WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
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FOR A PARTICULAR PURPOSE.
Expires July 2008 [Page 25]
