CCAMP Working Group Dimitri Papadimitriou (Alcatel)
Category: Internet Draft Fabrice Poppe (Alcatel)
Expires: December 2002 Sudheer Dharanikota (Nayna)
Riad Hartani (Caspian)
Raj Jain (Nayna)
Jim Jones (Alcatel)
Senthil Venkatachalam (Alcatel)
Yong Xue (WorldCom)
June 2002
Shared Risk Link Groups Encoding and Processing
draft-papadimitriou-ccamp-srlg-processing-00.txt (*)
Status of this Memo
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(*) Formerly draft-many-inference-srlg-02.txt
Abstract
The Shared Risk Link Group (SRLG) concept introduced in [IPO-Frame]
and extended in [CCAMP-SRG] is considered as one of the most
important criteria concerning the constraint-based path computation
of optical channel routes. By applying the SRLG disjointness
criteria to the constraint-based path computation, one can select a
route taking into account both resource and logical structure
disjointness. That implies a lower probability of simultaneous
optical channel failure.
This document describes the various physical and logical resource
types considered in the SRLG concept. The proposed method focuses on
the inference of SRLG information between the network physical and
logical layers and the logical structures representing geographical
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locations. The main applications of the proposed model are related
to the Constraint-based Shortest Path First (CSPF) algorithm for
optical channel route computation and the aggregation of the SRLG
information flooded throughout traffic engineering extensions of the
IGP routing protocols (such as OSPF and IS-IS).
1. Introduction
Many proposals include the Shared Risk Link Group (SRLG) concept
when considering the disjointness for the constraint-based path
computation of optical channel routes. In the optical domain, the
SRLG concept is used for deriving a path, which is disjoint with
respect to physical resources and logical (topological) structures.
The corresponding requirements have already been described in [IPO-
IMP] while considering physical network topology and associated
risks. In the scope of this document, these requirements can be
summarized as follows:
1. The SRLG encoding mechanism should reduce the path computation
complexity.
2. The SRLG information flooding should be scoped to reduce the
amount of information that is exchanged across domains.
3. The SRLG encoding should accommodate the physical and logical
restrictions imposed on the diversity requirements.
However, the definition of SRLG in the current format described in
[GMPLS-OSPF] and [GMPLS-ISIS] does not provide:
1. The relationship between physical (or logical) resources and
logical structures. For example, a fiber could be part of a
sequence of fiber segments that are included in a given
geographical region.
2. The risk assessment during path computation implying the
assignment of a conditional failure probability to the SRLGs.
3. The analysis of the specifics aspects of constraint-based path
computation and path re-optimization taking SRLG information into
account.
Therefore, this document proposes among others a technique to
compute the SRLG with respect to a given risk type. This is achieved
by identifying for a given physical layer the resources belonging to
an SRLG. The proposed model also permits to compute the dependencies
of these resources on the resources belonging to lower physical
layers. The result of the computation also enables to determine the
risk associated to each of the SRLGs, that is, the probability that
two entities that both use network resources belonging to a given
SRLG simultaneously go down if one of them goes down (thus defining
a conditional failure probability).
The remainder of this memo is organized as follows. In section 3, we
present the hierarchical model of the network resources and the
corresponding SRLG encoding. In section 4, we discuss the use of the
proposed approach for risk assessment during path computation and
selection. Considerations on topology summarization and path
disjointness with respect to SRLG are proposed in section 5.
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2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119 [1].
3. Hierarchical Model
The proposed approach is based on a hierarchical representation of
the network resources and geography:
- Network resource hierarchy, which is related to the fiber topology
(more generally the physical resources) of the optical network
and the wavelengths built on top of this physical topology.
- Logical structure hierarchy, which is related (upon network
administration policy) to the geographical topology of the optical
network.
Between these two hierarchies, the nodes such as Optical Cross-
Connect (OXC) and Photonic Cross-Connect (PXC) constitute the
boundary layer.
The encoding of the SRLG could be either mapped on this hierarchical
model or simply use a flat encoding scheme. Difference between both
encoding relies on the extended usage of the SRLG identifiers in the
context of diverse route computation (i.e. path disjointness). Since
a link can belong to more than one SRLG, an SRLG identifier list (in
the SRLG Sub-TLV), as described in [GMPLS-OSPF] and [GMPLS-ISIS] is
associated with each link (i.e. the SRLG Sub-TLV is defined as a
Sub-TLV of the TE Link TLV). This results in a linear, unordered and
non-structured information from which the underlying structure
cannot be deduced.
Consequently, either a type field indicating the type of network
resource (or logical structure) to which this SRLG identifier refers
extends the flat encoding scheme or the encoding itself translates
the underlying hierarchical structure. Worth mentioning here is that
a hierarchical encoding (since depending on the physical layer which
is static by definition) needs an additional mapping structure in
order to keep the relationship with link identifiers.
3.1 Network Resource Hierarchy
The network (physical and logical) resource model considered in the
inference of the Shared Risk Link Groups (SRLGs) is based on
premises detailed in [IPO-FRAME] and [IPO-IMP]. The network resource
hierarchy includes the logical resources built on top of the
physical resource hierarchy belonging to the optical network.
The concepts around network resource hierarchy are based on the
following logical resource definitions:
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- Sub-Channel: a dedicated container included within a given optical
channel uniquely identifies a sub-channel.
- Channel(*): an optical channel is uniquely identified by its
corresponding and dedicated wavelength (sub-)interface identifier.
(*) when conversion or amplification is non-collocated with the
corresponding interface capability, the passive components are
logically embedded into the fiber link (see below definition).
These resources are built on top of the physical resource hierarchy
composed by the following physical resources considered as a common
denominator of most Optical Transport Network (OTN) environments:
- Fiber Link: a fiber connects two node ports communicating through
one or more optical channel if their interfaces support (Dense)
Wavelength Division Multiplexing û (D)WDM, also a fiber link can
be decomposed as a sequence of fiber spans separated by optical
amplifiers
- Fiber Sub-segment: a group of fiber links, any fiber link of a
sub-segment start and terminates at the same location
- Fiber Segment: a group of fiber sub-segments, any fiber sub-
segment of segment start and terminates at the same location
Moreover, one can consider sequences of fiber (sub-)segment starting
and terminating at the same node and defined them as fiber trunks.
Thus, the approach developed here extends the definition and
concepts given in [IPO-FRAME] and [IPO-IMP] by enabling fiber
topologies not strictly limited to point-to-point physical inter-
connections between nodes.
As represented in Figure 1, the fiber trunk going from location N[1]
to location N[3] is composed by the fiber segments A and B and the
fiber trunk from location N[1] to location N[2] includes the fiber
segments A, C and D.
Location N[1] Location N[3]
Sub-Segment A[1] Sub-Segment B[1]
---------------------------------------------------------------
=== . . . ====== Fiber Fiber ====== . . . ====
=== . . . ====== Fiber Fiber ====== . . . ====
--------------------------------------------------------------
Segment A Segment B
------------------------------ -----------------------------
=== . . . ====== Fiber | | Fiber ====== . . . ====
=== . . . ====== Fiber | | Fiber ====== . . . ====
------------------------- | | -------------------------
Sub-Segment A[n] | | | | Sub-Segment B[n]
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| | | |
| | | | Segment C
| | | |
Sub-Segment D[1] | | | | Sub-Segment E[1]
------------------------- | | -------------------------
=== . . . ====== Fiber | | Fiber ====== . . . ====
=== . . . ====== Fiber | | Fiber ====== . . . ====
------------------------------ ------------------------------
Segment D Segment E
---------------------------------------------------------------
=== . . . ====== Fiber Fiber ====== . . . ====
=== . . . ====== Fiber Fiber ====== . . . ====
---------------------------------------------------------------
Sub-Segment D[n] Sub-Segment E[n]
Location N[2] Location N[4]
Figure 1. Example of Physical Topology
In this figure, the Segment A is composed by the fiber sub-segments
A[1], A[2], ..., A[i], ..., A[n]. The same terminology applies for
the segments B, C, D and E.
Consequently, the fiber trunk from location N[2] to location N[4]
includes the sub-segments D[2] to D[n] and their corresponding sub-
segments within the segment E: E[2] to E[n]. The fiber trunk from
location N[1] to location N[2] includes the fiber sub-segments A[n],
C[1] and D[1].
3.2 Geographical Hierarchy
Concerning the geographical hierarchy, in order to encompass from
the least to the most extended topological partitioning of the
locations that are covered by the optical network, one defines the
following logical structure:
- Region: a region includes one or more network resources (as
defined in Section 3.1) whose location is limited to a confined
area for the sake of maintainability. Regions have a fixed number
of exit points and are recursively definable. This means that a
region may include one or more non-overlapping sub-regions each of
them encompassing generally more than one resource. For instance,
the points of presence of each cities of a given state.
Note: A region may correspond to an IGP area such as an OSPF area,
and a sub-region to an OSPF Autonomous System (or BGP Autonomous
System). However, the current approach does not exclude situations
where the logical structure hierarchy does not map the hierarchical
routing topology.
3.3 SRLG Definition and Properties
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A SRLG is defined as the set of links (or optical lines) sharing a
common physical resource (including fiber links, sub-segment and
segment) i.e. sharing a common risk. For instance, a set of links L
belongs to the same SRLG s, if established over the same fiber link
l.
3.3.1 SRLG Properties
The SRLG properties can be summarized as follows:
1) A link belongs to more than one SRLG if and only if it crosses
(at least) one of the resources covered by each of these SRLGs
For instance: fiber link l belongs to the SRLG s1 and s2, if and
only if it crosses the fiber sub-segment A[1] and B[1]
2) Two links belonging to the same SRLG can belong individually to
other (one or more) SRLGs
For instance: fiber links l1 and l2 belong to SRLG s3 (segment A)
while l1 belongs to SRLG s1 (since it covers sub-segment A[1]) and
l2 to SRLG s4 (since it covers sub-segment D[1])
3.4.2 SRLG Disjointness
To define the LSP SRLG disjointness notion, we first introduce the
following definition: an LSP (i.e. sequence of links) covers an SRLG
if and only if it crosses one of the links belonging to that SRLG.
For instance: LSP p1 covers SRLG s1 (since it crosses link l1)
LSP SRLG disjointness can then be defined as follows.
1) Two LSPs are disjoint with respect to an SRLG s1 if and only if
at most one of them covers this SRLG.
For instance: LSPs p1 and p2 are disjoint with respect to SRLG s1
since only p1 covers SRLG s1.
2) Two LSPs are disjoint with respect to a set of SRLG S if and only
if the sets of SRLGs they individually cover are completely disjoint
For instance: LSP p1 and p3 are disjoint with respect to set of SRLG
S = {s1, s2, s3} since only p1 covers SRLG set S.
3.5 Computational Model Capabilities
This section briefly describes guidelines for an SRLG computational
model described in appendix A and based on the above definitions.
The main features of this model are:
- Support Constraint-based Shortest Path First (CSPF) algorithm for
explicit route (or path) computation by considering SRLG
disjointness with respect to one or more risk types
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- Encompass hierarchical dependencies between physical resources
(inference of SRLG sets using bottom-up computation)
- CSPF computation including the relationship between physical
resources and topological structures. For instance:
. a fiber link can be part of trunk included in a specific
and confined geographical region
. a fiber cable passing through an earthquake region
- Provide risk assessment during path computation implying
allocation of conditional failure probabilities with SRLGs
- SRLG information flooding well-scoped to reduce the amount of
link-state advertisements by using summarization
Consequently, the above features suggest an SRLG encoding mechanism
that enables:
- Accommodation of the physical and topological hierarchies
- Reduction of the path (i.e. explicit route) optimality
3.6 SRLG Encoding
Using the above definitions and properties, the objective of the
hierarchical encoding is to achieve aggregation (i.e. summarization)
of the SRLG Identifiers at the boundary of geographical structures
defined logically on top of the optical network topology. For this
purpose, we propose a linear encoding scheme including a type field.
This provides abstraction of the physical layer structure and should
facilitate the management of the SRLG Identifiers.
Consequently, the SRLG sub-TLV of the TE Link top level TLV includes
the following:
1. SRLG Location (32-bit field)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Region ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The SRLG Location identifies the logical structure into which the
common resources defining the SRLG are included.
The Location field includes the Region ID (32 bits) field which
identifies a given region. For simplicity, we say that the SRLG
Location field identifies the location of the SRLG in terms of its
region identifier.
2. SRLG Identifier (64-bit field)
0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Weight |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Within the SRLG Identifier field (64 bits), the Type field (8 bits)
defines the resource type (also generically referred to as the link
type) to which the Identifier (32 bits) integer value refers. The
weight field (24 bits) assigned on a per-SRLG basis along with the
Identifier is defined in Section 4.
The following Type field values are currently defined for physical
resources, any other value being reserved:
Type Value
-------------------- ------
Reserved 0x00
Fiber Trunk 0x10
Fiber Segment 0x20
Fiber Sub-segment 0x30
Fiber Link 0x40
Node(*) 0xFF
(*) represents a non-GMPLS capable switching elements
Logical resources such as optical channels and TDM circuits (or
optical sub-channels) can take one of the following values:
Type Value
------------------- ------
Optical Channel 0x50
Optical Sub-Channel 0x60
Since a given physical or logical resource (for instance a fiber
link) can belong to more than one SRLG, the SRLG Identifier
structure is defined in the general case as a list of SRLG
Identifier (n x 64-bit):
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Weight |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Weight |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// ... //
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| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Weight |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Therefore, though we propose a linear encoding, the summarization of
the SRLG (taking into account the logical structure boundaries) is
still possible since the SRLG information encoding is structured as
follows:
- SRLG Location (32 bits) : Region (32 bits)
- SRLG Identifier (32 bits): Type (8 bits) û Weight (24 bits)
Identifier (32 bits)
This encoding enables to perform summarization at the boundaries of
logical structures defining the spatial coverage of an SRLG
Identifier list while overcoming the drawbacks of full hierarchical
encoding schemes. In this context, the weight value follows the
rules defined in the Section 4.
4. Risk Assessment
Risk assessment is defined as the quantification process of the
potential risk associated to the inclusion of a given resource
(belonging to a given resource type) located within a given logical
structure such as a geographical location for a given logical
network resource such as an optical channel.
4.1 Rationale for Risk Assessment
Consider the following example, where the client device makes the
following connection requests to the optical network:
- Request for a persistent connection with 99.999 % percent of
availability or equivalently a downtime less than X minutes per
year.
- Request for protection for a portion of the traffic (at the
expense of more charging) compared to other portion of the
traffic left unprotected and considered as extra traffic by the
server.
Such requirements will be translated into path specific requests.
Such path specific requests can be grouped into path selection
requirements and path characterization requirements.
1. Path selection requirements
These typically dictate which physical path should be taken to
achieve the availability requirements of the client request. These
requirements are typically related to the logical and physical
diversity as mentioned in Section 3.
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2. Path characterization requirements
Path characterization requirements typically dictate the recovery
mechanisms as specified by the client connection request. This can
be achieved in the form of section and/or path recovery but also
depending on the ring or meshed topology of the optical network.
However, these considerations are out of the scope of this document.
The components that need formalization in this example are:
- Step 1. Specification of the client requirements (service level).
- Step 2. Configuration of the network that helps in assessing the
service specifics such as the availability.
- Step 3. Propagation of the above-configured information.
- Step 4. Processing of the above-propagated information.
Step 1 of specifying the requirements is not in the scope of this
document. Steps 2 to 4 are discussed in the following sections where
we elaborate on the risk assessment during path selection.
4.2 Risk Assessment
Risk (the complementary of availability) assessment is defined as
the evaluation of the potential risk associated to the inclusion for
a given path of one specific resource (belonging to a given resource
type) located within a given logical structure.
Given that an SRLG Identifier list is used to encode the group of
logical and physical resources, if a mechanism is devised to assign
the risk of failure associated with the corresponding resource, the
availability of the path using these resources can be computed.
This, in order to meet the connection availability as requested by
the client request.
A simple approach is to assign the conditional failure probability
to each of the SRLG Identifiers. This information is encoded in the
weight (24 bits) field along with the SRLG information as defined in
Section 3.3. The associated weights can be used to either increase
or decrease the potential usage of the resource (i.e. inclusion into
the selected route). The Weight field is defined as a 24-bit length
encoded integer number (maximum value 2^24 - 1) to be divided by
(2^24 û 1) and multiplied by 100 to get the corresponding value in
percents. For instance a conditional failure probability of 0.99999
corresponds to a weight of 16.777.047 and 0.00005 to 839.
For instance, we can associate a conditional failure probability of
25% (weight = 4.194.304) to any fiber sub-segment located within the
same region. It means that by selecting two (or more than two)
different optical channel paths including the same SRLG identifier
with respect to fiber sub-segment failure, if one of these
connections fails, then the probability that the other connection
fails is 25%.
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The failure probability of a fiber can also depend on the region
into which the fiber is located (as well as the length of the
fiber). In addition, a fiber can pass across different regions with
different failure probabilities. In this case, we need to consider
an aggregated failure probability per fiber taking into account each
of the failure probabilities of its sub-components.
For instance, if we refer to our previous example and by considering
1. a conditional failure probability of 5% is associated to any
fiber link (located within the same region)
2. a conditional failure probability of 1% is associated to any
fiber segment (located within the same region)
Then by selecting two different optical channels included within the
same SRLG with respect to fiber segment failure, we obtain a
simultaneous connection failure probability of 1%. Consequently,
when a protected connection service is requested, by choosing fiber
segment path disjointness, the simultaneous connection failure
probability is also of 1%. However, if two optical channels flowing
through the same fiber link are selected, then we have a probability
of 5% that both optical channels fail simultaneously.
More generically, if one assigns a conditional failure probability
c[i] to each SRLG Identifier i belonging to a given region, the
resulting conditional failure probability C for a logical resource
(a connection, for instance) using these N independent physical
resources, is given by:
C = 1 - (1 û c[1]) x (1 û c[2]) x . . . x (1 û c[N])
When all the conditional failure probabilities c[i] are small (i.e.
c[i] << 1) C can be approximated by Sum[i]c[i].
4.3 Risk Assessment Application
The relationship between the availability of the service requested
by the client (i.e. a working and a protection connection) and the
conditional failure probabilities of the physical resource elements
included within the corresponding paths can be described as follows.
If we consider the following example:
1. a connection whose ingress node is located in region 1 and whose
destination is located in region 2
2. a conditional failure probability of 1% if fiber links are
selected within the same fiber trunk (and located in region 1)
3. a conditional failure probability of 1% if fiber links are
selected within the same fiber trunk (and located in region 2)
4. the corresponding failure are independent events
Then the availability of a connection that has a backup connection
running over fiber links (in region 1 and region 2) within the same
fiber trunks over the whole path as the connection itself is
approximately 98% (by using the above formula).
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Note that the initial value of the conditional failure probability
needs to be statically encoded; however, based on the history of the
failures these values could be dynamically re-evaluated. The
corresponding mechanism needs to be specified and is left for
further study.
5. Applications
The SRLG method developed here results in applications related to
the CSPF explicit route computation and the SRLG identifier sets
summarization enabling, in turn, intra- and inter-area diverse path
computation. For that purpose we first extend the SRLG concept for
logical resources such as optical channels and optical sub-channels
(i.e. TDM connections).
5.1 Extension to Logical Structures and Resources
The SRLG concept can be extended to logical structures and resources
by taking into account the following:
1. Given the physical and geographical decomposition of the optical
network topology, the SRLG encoding can be hierarchically
structured. The hierarchical encoding helps in constructing the
logical topology abstraction, which in turn can be used in SRLG
summarization and loose-path computation. The link semantic
may also be extended to accommodate the inter-region links.
2. Propagate the SRLG information related to these logical
(structures and resources) links, for instance (a set of) optical
channels using IGP flooding for intra- and inter-area routing
purposes.
3. Reduce the amount of the flooded information and hence explicit
route computation optimality and extend the flooding scope of the
propagated information to accommodate logical structures (i.e.
regions) and logical resources (i.e. optical channels and TDM
circuits).
5.2 SRLG Information Flooding and Summarization
The SRLG set of each link (i.e. physical and logical resources) is
encoded as described in Section 3.3. This information is propagated
once at initial configuration between the various nodes using the
GMPLS Traffic Engineering extensions to IGPs (see [GMPLS-OSPF] and
IS-IS [GMPLS-ISIS]). After this initial SRLG identifier exchange,
corresponding values do not change over time.
Nevertheless, the propagation of SRLG information will be necessary
whenever a new link is added or an existing link is removed.
Initially, it is assumed that the failure probabilities of the
various resources are (statically) configured. However, it is
envisioned that after a certain running period, the failure
probability associated to the SRLG and propagated along with the
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SRLG sub-TLV itself (as described in Section 3.3) will change over
time and thus give rise to a dynamic metric.
5.3 Bottom-Up Computation of SRLG Relationships
Once the traffic engineering (topology-related) link information is
received by the node, the corresponding SRLG relationships can be
computed on a regular basis.
The fiber trunk SRLG relationships are used to compute the fiber
segment SRLG relationships, which in turn are used to compute the
fiber sub-segment SRLG relationships and finally the fiber SRLG
relationships. To each SRLG relationship (at each level), which
defines the membership of a resource to a particular SRLG, we
associate the conditional failure probability between two resources
belonging to this level (for instance, between two fibers that have
a common fiber sub-segment).
5.4 Topology Summarization and Route Computation
A direct application of this model is the Constraint-based Shortest
Path First (CSPF) algorithm used for explicit route computation
(i.e. traffic-engineered LSP creation) to maximize the connection
disjointness and so decrease their common failure probability. Given
an existing set of connections across the network, the objective is
to compute, for a newly requested connection, an explicit route
across the optical network topology such that this connection is
diversely routed from an existing given set of connections.
The diversity requirement is a routing constraint, that can be
expressed in terms of a conditional failure probability with respect
to an existing (set of) connections and more generally any kind of
resources. Hence, in addition to the other traffic-engineering
constraints, the diversity constraint requires that the conditional
failure probability does not exceed a given threshold. The CSPF
algorithm needs to be updated to take the routing diversity
constraint into account.
The SRLG concept generates another dimension to the existing
constraint-based path computation methods traditionally used in
hierarchical networks. The SRLG constraints comes in addition to the
common traffic-engineering constraints such as bandwidth
availability, link metrics and TE attributes (see [OSPF-TE]). The
specificity of the routing diversity constraint requires the use of
a path computation algorithm that provides not only complete multi-
path disjointness but also partial multi-path disjointness with
respect to various risk factors. In a similar way, appropriate
mechanisms should also be used in order to perform path re-
optimization following various recovery strategies.
The specific aspect of complete and partial multi-path disjointness
is related to the fact that a path may with respect to SRLG be fully
or partially disjoint from a given set of path. In brief, one speaks
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about SRLG partial or full multi-path disjointness. This means that
a connection may be disjoint from another but only to some extent
with respect to some risk factors. Thus when referring to path
disjointness with respect to SRLG one may also include the ratio of
disjointness with respect to various risk type assigned to their
component resources. Consider for instance the LSPs p1 and p2 using
j1 and j2 resources respectively, m of these resources having an
SRLG in common, with m =< j1 and m =< j2, then [(j1-m) + (j2-
m)]/(j1+j2) provides the disjointness ratio. For instance, if j1=13,
j2=7 and m=1, this ratio equals 0.90, thus corresponding path p1 and
p2 are SRLG disjoint at 90%.
As such, this feature differentiates path disjointness constraint
from any other constraint commonly considered in path computation.
6. Security Considerations
Security considerations related to SRLG and related applications are
left for further study.
7. References
[IEEE-ORL] J.Strand et al., æIssues for Routing in the Optical
LayerÆ, IEEE Communication Magazine, Volume 39, Number
2, FebÆ01.
[GMPLS-OSPF] K.Kompella et al., æOSPF Extensions in Support of
Generalized MPLSÆ, Internet Draft, Work in Progress,
draft-ietf-ccamp-ospf-gmpls-extensions-06.txt, MayÆ02.
[GMPLS-ISIS] K.Kompella et al., æISIS Extensions in Support of
Generalized MPLSÆ, Internet Draft, Work in Progress,
draft-ietf-isis-gmpls-extensions-12.txt, MayÆ02.
[IPO-FRAME] J.Luciani et al., æIP over Optical Networks A
FrameworkÆ, Internet Draft, Work in progress, draft-
ietf-ipo-framework-01.txt, FebÆ02.
[IPO-IMP] J.Strand et al., æImpairments And Other Constraints On
Optical Layer RoutingÆ, Internet Draft, Work in
progress, draft-ietf-ipo-impairments-02.txt, FebÆ02.
[MPLS-BUNDLE K.Kompella et al., æLink Bundling in MPLS Traffic
EngineeringÆ, Internet Draft, Work in progress, draft-
ietf-mpls-bundle-03.txt, MayÆ02.
8. Acknowledgments
The authors would like to thank Bernard Sales, Emmanuel Desmet and
Gert Grammel for their constructive comments, David Griffith for its
active contribution and Bart Rousseau for its revision efforts.
9. Author's Addresses
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Dimitri Papadimitriou (Editor)
Francis Wellesplein, 1
B-2018 Antwerpen, Belgium
Phone: +32 3 240-8491
Email: dimitri.papadimitriou@alcatel.be
Fabrice Poppe (Alcatel)
Francis Wellesplein, 1
B-2018 Antwerpen, Belgium
Phone: +32 3 240-8006
Email: fabrice.poppe@alcatel.be
Jim Jones (Alcatel)
3400 W. Plano Parkway,
Plano, TX 75075, USA
Phone: +1 972 519-2744
Email: jim.d.jones1@usa.alcatel.com
Senthil Venkatachalam (Alcatel)
45195 Business Court, Suite 400
Dulles, VA 20166, USA
Phone: +1 703 654-8635
Email: senthil.venkatachalam@usa.alcatel.com
Sudheer Dharanikota (Nayna Networks)
157 Topaz St.,
Milpitas, CA 95035, USA
Phone: +1 408 956-8000X357
Email: sudheer@nayna.com
Raj Jain (Nayna Networks)
157 Topaz St.,
Milpitas, CA 95035, USA
Phone: +1 408 956-8000X309
Email: raj@nayna.com
Riad Hartani (Caspian Networks)
170 Baytech Drive,
San Jose, CA 95134, USA
Phone: +1 408 382-5216
Email: riad@caspiannetworks.com
Yong Xue (WorldCom)
Ashburn, VA, USA
Phone: +1 703 886-5358
Email: yong.xue@wcom.com
Appendix A: SRLG Computational Model
A.1 SRLG Concept and Example
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The present model is intended to be used to automate the discovery
of the Shared Risk Link Groups (SRLGs) at a given layer for a given
physical resource type. This resource type may be located within a
given region.
Note that a typical resource type can be a fiber, a fiber sub-
segment, a fiber segment or a fiber trunk and a typical resource
location can be a zone, a region or a node. For a given resource
type, when the resource location is not specified, the resource
location is limited to the nodes. SRLG definition and properties
were described in Section 3.
Example:
The following example referring to Figure 5 (for the physical
network topology) offers some clarification. Let assume that
- N1, N2, N3, and N4 represent locations that are linked by the
fiber sub-segments,
- A, B, C, D and E be fiber segments,
- and F1 (ACD), F2 (AB), F3 (BCD) and F4 (DE) are fiber links routed
over the fiber segment topology.
N1 N2
| |
| | F1
|A |D N1 ------------ N2
| | | |
| | | |
| C | | |
x-------------x |F2 |F4
| | | |
| | | |
| | | F3 |
|B |E N3 ------------ N4
| |
| |
N3 N4
Figure 4. Correlation between Fiber segment and Fiber link topology
In such a physical topology the obvious SRLGs are the following:
- {F1, F2} both going down when segment A breaks
- {F1, F3} both going down when segment C breaks
- {F1, F4} both going down when segment D breaks
- {F2, F3} both going down when segment B breaks
- {F3, F4} both going down when segment E breaks
These five SRLGs can be replaced by two SRLGs, S1 = {F1, F2, F3} and
S2 = {F1, F3, F4}, where S1 and S2 constitute the minimum edge
covering with cliques of the Shared Risk Relationship (SRR) graph
that can be drawn between F1, F2, F3, F4 (see Figure 5). A clique of
a graph G is a sub-graph of G in which every two nodes are connected
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by an edge. This decomposition is unique. If there was an additional
dependency between F2 and F4, there would be a unique SRLG, S = {F1,
F2, F3, F4}.
F1 ------- F4
| \ |
| \ |
| \ |
| \ |
| \ |
| \ |
| \ |
F2 ------- F3
Figure 5. SRR Graph between fiber link and segment
Although R1 = F1-F2-F3 and R2 = F4 are diverse path between
locations N2 and N4 in the fiber topology (link and node
disjointness), they are not diverse with respect to SRLGs. This is
because both R1 and R2 cover SRLG S2, which contains F1, F3 (part of
R1) and F4 (part of R2). SRLGs are thus a way of formalizing the
propagation of (link-based) risk dependencies from server to client
layers.
The rules guiding the definition of minimum set of SRLGs for more
complex physical network topologies will be addressed in a future
version of this document.
A.2 Risk Type
As specified up to now, the current approach considers that each of
the network resource may experience one or more failure type(s). The
same applies to geographical locations: a given location might
experience one or more failure types. Moreover, by applying the SRLG
properties, a network resource failure can potentially cover more
than one geographical location. Consequently, some heuristics must
be introduced to keep the SRLG computational complexity limited.
In order to limit the computational complexity, we define the
following heuristics when considering the SRLG computation with
respect to the type of risk:
1. The set of risk types associated to network resources corresponds
exactly to the set of resource type failure.
- So, for instance, the risk type associated to a fiber segment
is a fiber segment failure. The same principle applies for
other network resources such as fiber link, fiber sub-segment
and fiber trunk. Consequently, the current approach does not
consider a finest granularity for the network resource failure
than the one referred by their type.
2. A risk type associated to a geographical structure covers exactly
the region where it is defined. Moreover, a geographical failure
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is limited to a given location and does not contaminate the
neighboring locations or generate another failure
- For instance, we consider that an earthquake covers exactly
one region or one area and that such a failure does not
generate a hurricane impacting the neighboring locations. So,
there is no correlation between geographical failures.
3. Each of the network resources covers exactly one geographical
logical structure (identified by a region ID).
- Consequently, when a geographical failure occurs, it
generates a failure impacting the entire network resources
included within the corresponding location. Hence, there is
an ON/OFF relationship between geographical and network
resource failures.
Consequently, when considering network resources, the risk type
associated to an SRLG is defined as the potential failure of one (or
more than one) instance belonging to a given resource type or the
potential failure of any of its hierarchical resource dependence.
Thus, we define the concept of SRLG with respect to a given resource
type and subsequently to the Risk Type (RT) to which this resource
type refers. Moreover, for a given resource type (or class), each of
the resources are identified by a Resource Identifier (RID). Since
each of these resource classes corresponds to an SRLG class, we can
assign an identifier to each of the SRLG classes members and define
their value as the SRLG identifier.
Moreover, by applying the defined heuristics above, the SRLG
identifiers can be grouped together by taking into account their
geographical location. The latter is encoded by identifying the
region identifier (region ID) including the resource identifiers to
which the SRLG refers.
A.3 Calculation of Shared Risk Link Groups
In the calculation method, shared-risk(RID i, RID j, RT) is TRUE
only if the resource identifiers RID i and RID j belong to the same
SRLG with respect to the risk type RT. The risk types considered
here are related the fiber segment, the fiber sub-segment and the
fiber link risk failure.
A recursive calculation of shared-risk proceeds as follows:
shared-risk(RID i, RID j, RT) =
at-risk(RID i, RT)
and at-risk(RID j, RT)
and (RID i = RID j
or (exists RID k, RID l
such that
depends-on(RID i, RID k)
and depends-on(RID j, RID l)
and shared-risk(RID k, RID l, RT)))
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In this calculation:
- at-risk(RID i, RT) is TRUE only if RID is susceptible to a risk of
type RT, either directly, or indirectly, through the failure of
one of the resources it depends on.
- depends-on(RID i, RID j) is TRUE only if RID i fails as soon as
RID j fails.
If we refer to the example detailed in section 1.1, then shared-
risk(F1, F2, [fiber segment failure]) = TRUE because depends-on(F1,
A) = TRUE, depends-on(F2, A) = TRUE and at-risk(A, [fiber segment
failure]) = TRUE, the latter simply because A is a fiber segment.
A.4 Practical Method for SRLG Calculation
The recursive formula presented in the previous section does not
directly lead to an efficient algorithm. ItÆs top-down nature
illustrates nicely the recursive nature of the SRLG concept, but the
calculation of the SRLGs in a top-down fashion would be totally
inefficient, entailing the calculation of the same SRLGs in lower
network layers over and over again.
A more efficient algorithm can be obtained from a bottom-up
calculation. Figure 6 illustrates this by using the example we
introduced in the Section A.1 and in by introducing the concept of
Shared Risk Relationship Graph (SRR) which defines the membership of
a resource belonging to the same SRLG.
F1 ---------- F4
| \ ^ |
| \ | |
| \ | | Fiber SRR Graph
--->| \ | |<---
| | \ | | | where F1=ACD, F2=AB, F3=BCE, F4=DE
| | ^\ | | |
| | | \| | |
| | | | | |
| | | |\ | |
| | | | \ | |
| F2 ----|--|-- F3 |
| ^ | | |
| | | | |
+++++++++++++++++++++++++++++
| | | | |
| | | | |
--- A | | -- D |
| | |
| | |
| C | Fiber Segment SRR Graph
| |
| |
B -- E ---
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Figure 6. Bottom-up calculation of Shared Risk Relationships
For the calculation of a set of SRLGs, we need to calculate a Shared
Risk Relationship (SRR) graph. The bottom-up calculation of the
fiber SRR graph proceeds as follows:
- Step 1. For each fiber segment, there is an SRR between every two
fibers contained in that segment (vertical arrows in Figure 6.)
- Step 2. For every SRR between two fiber segments, there is an SRR
between every two fibers contained in either of the two fiber
segments.
In the previous example, there are no SRRs between fiber segments,
and the calculation stops after Step 1.
A.5 Application of the Model
The model is intended to be used to automate the discovery of the
SRLGs at a given layer for a given risk type (RT).
The dependencies may be confined to one layer, e.g. the dependency
of an optical link on a node (for instance, a DWDM system) to which
it is connected, when the RT = [Node failure]. Dependencies may also
extend over layer boundaries, e.g. the dependency of a TDM circuit
(or sub-channel) in an SDH network established by using an optical
channel (or wavelength) through the optical network that is the
server layer of the SDH network, when RT = [fiber failure].
Let two optical network resources RID i and RID j within the same
layer share a common risk of type RT. Let this risk type be tied to
a lower layer, referred to as the risk layer. To enable this layer
to infer shared-risk(RID i, RID j, RT), its server layer should
advertise the following information:
shared-risk(component_1, component_2, RT)
where component_1 are services of the server layer on which RID i
relies and component_2 are services of the server layer on which
RID j relies, respectively.
If the server layer is not the risk layer, the latter has to infer
this knowledge itself from what its server layer is advertising. If
shared risk relationships are not advertised, client layers should
at least be able to query from their server layer the shared risk
relationships between the services they receive.
Some dependencies do not lend themselves easily to automatic
discovery. For instance, it is hardly imaginable that the process of
finding out through which fiber segments a fiber goes can be
automated. This means that part of the image of depends-on (RID i,
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RID j) will have to be provided æmanuallyÆ by the operator or be at
least statically configured into a centralized repository.
More formally, an efficient calculation of shared risk link
relationships relies on two things:
- In the lowest network layer with elements susceptible to the risk
type RT that is considered, every network element RID j
susceptible to the risk RT constitutes an SRR on its own, that is,
(RID j, RID j) satisfies the recursive formula;
- Every SRR that has been discovered in one network layer leads to
SRRs in the next higher network layer. In particular, two next
higher layer network elements (RID i, RID j) depending on lower
layer network elements that have an SRR satisfy the recursive
formula. In order to allow an efficient calculation of the shared
risk relationships in the next higher layer (e.g. the fiber
layer), the shared risk relationships that were discovered in
lower layers (e.g. the fiber segment layer) are stored in SRR
graphs. This way, the recalculation of lower layer shared risk
relationships can be avoided.
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