Internet DRAFT - draft-ietf-ccamp-wson-impairments
draft-ietf-ccamp-wson-impairments
Network Working Group Y. Lee
Huawei
G. Bernstein
Grotto Networking
D. Li
Huawei
G. Martinelli
Cisco
Internet Draft
Intended status: Informational January 5, 2012
Expires: July 2012
A Framework for the Control of Wavelength Switched Optical Networks
(WSON) with Impairments
draft-ietf-ccamp-wson-impairments-10.txt
Abstract
As an optical signal progresses along its path, it may be altered by
the various physical processes in the optical fibers and devices it
encounters. When such alterations result in signal degradation,
these processes are usually referred to as "impairments". These
physical characteristics may be important constraints to consider
when using a GMPLS control plane to support path setup and
maintenance in wavelength switched optical networks.
This document provides a framework for applying GMPLS protocols and
the PCE architecture to support Impairment Aware Routing and
Wavelength Assignment (IA-RWA) in wavelength switched optical
networks. Specifically, this document discusses key computing
constraints, scenarios and architectural processes: Routing,
Wavelength Assignment, and Impairment Validation. This document does
not define optical data plane aspects; impairment parameters,
measurement of, or assessment and qualification of a route, but
rather it describes the architectural and information components for
protocol solutions.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction...................................................3
2. Terminology....................................................4
3. Applicability..................................................6
4. Impairment Aware Optical Path Computation......................7
4.1. Optical Network Requirements and Constraints..............8
4.1.1. Impairment Aware Computation Scenarios...............8
4.1.2. Impairment Computation and Information Sharing
Constraints.................................................9
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4.1.3. Impairment Estimation Process.......................11
4.2. IA-RWA Computation and Control Plane Architectures.......12
4.2.1. Combined Routing, WA, and IV........................14
4.2.2. Separate Routing, WA, or IV.........................14
4.2.3. Distributed WA and/or IV............................15
4.3. Mapping Network Requirements to Architectures............16
5. Protocol Implications.........................................18
5.1. Information Model for Impairments........................19
5.2. Routing..................................................20
5.3. Signaling................................................20
5.4. PCE......................................................21
5.4.1. Combined IV & RWA...................................21
5.4.2. IV-Candidates + RWA.................................21
5.4.3. Approximate IA-RWA + Separate Detailed IV...........23
6. Manageability and Operations..................................25
7. Security Considerations.......................................26
8. IANA Considerations...........................................27
9. References....................................................27
9.1. Normative References.....................................27
9.2. Informative References...................................27
10. Acknowledgments..............................................28
1. Introduction
Wavelength Switched Optical Networks (WSONs) are constructed from
subsystems that may include Wavelength Division Multiplexed (WDM)
links, tunable transmitters and receivers, Reconfigurable Optical
Add/Drop Multiplexers (ROADM), wavelength converters, and electro-
optical network elements. A WSON is a wavelength division
multiplexed (WDM)-based optical network in which switching is
performed selectively based on the center wavelength of an optical
signal.
As an optical signal progresses along its path, it may be altered by
the various physical processes in the optical fibers and devices it
encounters. When such alterations result in signal degradation,
these processes are usually referred to as "impairments". Optical
impairments accumulate along the path (without 3R regeneration)
traversed by the signal. They are influenced by the type of fiber
used, the types and placement of various optical devices, and the
presence of other optical signals that may share a fiber segment
along the signal's path. The degradation of the optical signals due
to impairments can result in unacceptable bit error rates or even a
complete failure to demodulate and/or detect the received signal.
In order to provision an optical connection (an optical path)
through a WSON, a combination of path continuity, resource
availability, and impairments constraints must be met to determine
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viable and optimal paths through the network. The determination of
appropriate paths is known as Impairment Aware Routing and
Wavelength Assignment (IA-RWA).
Generalized Multi-Protocol Label Switching (GMPLS) [RFC3945]
provides a set of control plane protocols that can be used to
operate networks ranging from packet switch capable networks,
through those networks that use time division multiplexing and WDM.
The Path Computation Element (PCE) architecture [RFC4655] defines
functional computation components that can be used in cooperation
with the GMPLS control plane to compute and suggest appropriate
paths. [RFC4054] provides an overview of optical impairments and
their routing (path selection) implications for GMPLS. This document
uses as reference [G.680] and other ITU-T Recommendations for the
optical data plane aspects.
This document provides a framework for applying GMPLS protocols and
the PCE architecture to the control and operation of IA-RWA for
WSONs. To aid in this evaluation, this document provides an
overview of the subsystems and processes that comprise WSONs and
describes IA-RWA models based on the corresponding ITU-T
Recommendations, so that the information requirements for use by
GMPLS and PCE systems can be identified. This work will facilitate
the development of protocol extensions in support of IA-RWA within
the GMPLS and PCE protocol families.
2. Terminology
ADM: Add/Drop Multiplexers - An optical device used in WDM networks
composed of one or more line side ports and typically many tributary
ports.
Black links: Black links refer to tributary interfaces where only
link characteristics are defined. This approach enables transverse
compatibility at the single-channel point using a direct wavelength-
multiplexing configuration.
CWDM: Coarse Wavelength Division Multiplexing
DGD: Differential Group Delay
DWDM: Dense Wavelength Division Multiplexing
FOADM: Fixed Optical Add/Drop Multiplexer
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GMPLS: Generalized Multi-Protocol Label Switching
IA-RWA: Impairment Aware Routing and Wavelength Assignment
Line side: In WDM system line side ports and links typically can
carry the full multiplex of wavelength signals, as compared to
tributary (add or drop ports) that typically carry a few (typically
one) wavelength signals.
NEs: Network Elements
OADMs: Optical Add Drop Multiplexers
OSNR: Optical Signal to Noise Ratio
OXC: Optical cross connect - An optical switching element in which a
signal on any input port can reach any output port.
PCC: Path Computation Client - Any client application requesting a
path computation to be performed by the Path Computation Element.
PCE: Path Computation Element - An entity (component, application,
or network node) that is capable of computing a network path or
route based on a network graph and applying computational
constraints.
PCEP: PCE Communication Protocol - The communication protocol
between a Path Computation Client and Path Computation Element.
PXC: Photonic Cross Connects
Q-factor: The Q-factor provides a qualitative description of the
receiver performance. It is a function of the signal to optical
noise ratio. The Q-factor suggests the minimum SNR (Signal Noise
Ratio) required to obtain a specific BER for a given signal.
ROADM: Reconfigurable Optical Add/Drop Multiplexer - A wavelength
selective switching element featuring input and output line side
ports as well as add/drop tributary ports.
RWA: Routing and Wavelength Assignment
Transparent Network: A wavelength switched optical network that does
not contain regenerators or wavelength converters.
Translucent Network: A wavelength switched optical network that is
predominantly transparent but may also contain limited numbers of
regenerators and/or wavelength converters.
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Tributary: A link or port on a WDM system that can carry
significantly less than the full multiplex of wavelength signals
found on the line side links/ports. Typical tributary ports are the
add and drop ports on an ADM and these support only a single
wavelength channel.
Wavelength Conversion/Converters: The process of converting
information bearing optical signal centered at a given wavelength to
one with "equivalent" content centered at a different wavelength.
Wavelength conversion can be implemented via an optical-electronic-
optical (OEO) process or via a strictly optical process.
WDM: Wavelength Division Multiplexing
Wavelength Switched Optical Networks (WSONs): WDM based optical
networks in which switching is performed selectively based on the
center wavelength of an optical signal.
3. Applicability
There are deployment scenarios for WSON networks where not all
possible paths will yield suitable signal quality. There are
multiple reasons; below is a non-exhaustive list of examples:
o WSON is evolving using multi-degree optical cross connects in a
way that network topologies are changing from rings (and
interconnected rings) to general mesh. Adding network equipment
such as amplifiers or regenerators, to ensure all paths are
feasible, leads to an over-provisioned network. Indeed, even with
over provisioning, the network could still have some infeasible
paths.
o Within a given network, the optical physical interface may change
over the network life, e.g., the optical interfaces might be
upgraded to higher bit-rates. Such changes could result in paths
being unsuitable for the optical signal. Moreover, the optical
physical interfaces are typically provisioned at various stages
of the network's life span as needed by traffic demands.
o There are cases where a network is upgraded by adding new optical
cross connects to increase network flexibility. In such cases,
existing paths will have their feasibility modified while new
paths will need to have their feasibility assessed.
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o With the recent bit rate increases from 10G to 40G and 100G over
a single wavelength, WSON networks will likely be operated with a
mix of wavelengths at different bit rates. This operational
scenario will impose impairment constraints due to different
physical behavior of different bit rates and associated
modulation formats.
Not having an impairment aware control plane for such networks will
require a more complex network design phase that needs to take into
account the evolving network status in term of equipments and
traffic at the beginning stage. In addition, network operations such
as path establishment, will require significant pre-design via non-
control plane processes resulting in significantly slower network
provisioning.
It should be highlighted that the impact of impairments and use in
determination of path viability is not sufficiently well established
for general applicability [G.680]; it will depend on network
implementations. The use of an impairment aware control plane and
set of information distributed will need to be evaluated on a case
by case scenario.
4. Impairment Aware Optical Path Computation
The basic criteria for path selection is whether one can
successfully transmit the signal from a transmitter to a receiver
within a prescribed error tolerance, usually specified as a maximum
permissible bit error ratio (BER). This generally depends on the
nature of the signal transmitted between the sender and receiver and
the nature of the communications channel between the sender and
receiver. The optical path utilized (along with the wavelength)
determines the communications channel.
The optical impairments incurred by the signal along the fiber and
at each optical network element along the path determine whether the
BER performance or any other measure of signal quality can be met
for a signal on a particular end-to-end path.
Impairment-aware path calculation also needs to take into account
when regeneration is used along the path. [RFC6163] provides
background on the concept of optical translucent networks which
contains transparent elements and electro-optical elements such as
OEO regenerations. In such networks, a generic light path can go
through a number of regeneration points.
Regeneration points could happen for two reasons:
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(i) Due to wavelength conversion to assist RWA to avoid wavelength
blocking. This is the impairment free case covered by [RFC6163].
(ii) The optical signal without regeneration would be too degraded
to meet end to end BER requirements. This is the case when RWA
takes into consideration impairment estimation covered by this
document.
In the latter case, an optical path can be seen as a set of
transparent segments. The optical impairments calculation needs to
be reset at each regeneration point so each transparent segment will
have its own impairment evaluation.
+---+ +----+ +----+ +-----+ +----+ +---+
| I |----| N1 |---| N2 |-----| REG |-----| N3 |----| E |
+---+ +----+ +----+ +-----+ +----+ +---+
|<----------------------------->|<-------------------->|
Segment 1 Segment 2
Figure 1 Optical path as a set of transparent segments
For example, Figure 1 represents an optical path from node I to node
E with a regeneration point REG in between. It is feasible from an
impairment validation perspective if both segments (I, N1, N2, REG)
and (REG, N3, E) are feasible.
4.1. Optical Network Requirements and Constraints
This section examines the various optical network requirements and
constraints under which an impairment aware optical control plane
may have to operate under. These requirements and constraints
motivate the IA-RWA architectural alternatives to be presented in
the following section. Different optical networks contexts can be
broken into two main criteria: (a) the accuracy required in the
estimation of impairment effects, and (b) the constraints on the
impairment estimation computation and/or sharing of impairment
information.
4.1.1. Impairment Aware Computation Scenarios
A. No concern for impairments or Wavelength Continuity Constraints
This situation is covered by existing GMPLS with local wavelength
(label) assignment.
B. No concern for impairments but Wavelength Continuity Constraints
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This situation is applicable to networks designed such that every
possible path is valid for the signal types permitted on the
network. In this case, impairments are only taken into account
during network design and after that, for example during optical
path computation, they can be ignored. This is the case discussed in
[RFC6163] where impairments may be ignored by the control plane and
only optical parameters related to signal compatibility are
considered.
C. Approximated Impairment Estimation
This situation is applicable to networks in which impairment effects
need to be considered but there is sufficient margin such that they
can be estimated via approximation techniques such as link budgets
and dispersion [G.680],[G.sup39]. The viability of optical paths for
a particular class of signals can be estimated using well defined
approximation techniques [G.680], [G.sup39]. This is the generally
known as linear case where only linear effects are taken into
account. Note that adding or removing an optical signal on the path
should not render any of the existing signals in the network as non-
viable. For example, one form of non-viability is the occurrence of
transients in existing links of sufficient magnitude to impact the
BER of existing signals.
Much work at ITU-T has gone into developing impairment models at
this and more detailed levels. Impairment characterization of
network elements may be used to calculate which paths are conformant
with a specified BER for a particular signal type. In such a case,
the impairment aware (IA) path computation can be combined with the
RWA process to permit more optimal IA-RWA computations. Note that
the IA path computation may also take place in a separate entity,
i.e., a PCE.
D. Accurate Impairment Computation
This situation is applicable to networks in which impairment effects
must be more accurately computed. For these networks, a full
computation and evaluation of the impact to any existing paths needs
to be performed prior to the addition of a new path. Currently no
impairment models are available from ITU-T and this scenario is
outside the scope of this document.
4.1.2. Impairment Computation and Information Sharing Constraints
In GMPLS, information used for path computation is standardized for
distribution amongst the elements participating in the control plane
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and any appropriately equipped PCE can perform path computation. For
optical systems this may not be possible. This is typically due to
only portions of an optical system being subject to standardization.
In ITU-T recommendations [G.698.1] and [G.698.2] which specify
single channel interfaces to multi-channel DWDM systems, only the
single channel interfaces (transmit and receive) are specified while
the multi-channel links are not standardized. These DWDM links are
referred to as "black links" since their details are not generally
available. However, note that the overall impact of a black link at
the single channel interface points is limited by [G.698.1] and
[G.698.2].
Typically a vendor might use proprietary impairment models for DWDM
spans in order to estimate the validity of optical paths. For
example, models of optical nonlinearities are not currently
standardized. Vendors may also choose not to publish impairment
details for links or a set of network elements in order not to
divulge their optical system designs.
In general, the impairment estimation/validation of an optical path
for optical networks with "black links" in the path could not be
performed by a general purpose impairment aware (IA) computation
entity since it would not have access to or understand the "black
link" impairment parameters. However, impairment estimation (optical
path validation) could be performed by a vendor specific impairment
aware computation entity. Such a vendor specific IA computation
could utilize standardized impairment information imported from
other network elements in these proprietary computations.
In the following, the term "black links" will be used to describe
these computation and information sharing constraints in optical
networks. From the control plane perspective the following options
are considered:
1. The authority in control of the "black links" can furnish a list
of all viable paths between all viable node pairs to a
computational entity. This information would be particularly
useful as an input to RWA optimization to be performed by another
computation entity. The difficulty here is that such a list of
paths along with any wavelength constraints could get
unmanageably large as the size of the network increases.
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2. The authority in control of the "black links" could provide a
PCE-like entity a list of viable paths/wavelengths between two
requested nodes. This is useful as an input to RWA optimizations
and can reduce the scaling issue previously mentioned. Such a
PCE-like entity would not need to perform a full RWA computation,
i.e., it would not need to take into account current wavelength
availability on links. Such an approach may require PCEP
extensions for both the request and response information.
3. The authority in control of the "black links" provides a PCE that
performs full IA-RWA services. The difficulty is this requires
the one authority to also become the sole source of all RWA
optimization algorithms.
In all the above cases it would be the responsibility of the
authority in control of the "black links" to import the shared
impairment information from the other NEs via the control plane or
other means as necessary.
4.1.3. Impairment Estimation Process
The Impairment Estimation Process can be modeled through the
following functional blocks. These blocks are independent of any
Control Plane architecture, that is, they can be implemented by the
same or by different control plane functions as detailed in
following sections.
+-----------------+
+------------+ +-----------+ | +------------+ |
| | | | | | | |
| Optical | | Optical | | | Optical | |
| Interface |------->| Impairment|--->| | Channel | |
| (Transmit/ | | Path | | | Estimation | |
| Receive) | | | | | | |
+------------+ +-----------+ | +------------+ |
| || |
| || |
| Estimation |
| || |
| \/ |
| +------------+ |
| | BER / | |
| | Q Factor | |
| +------------+ |
+-----------------+
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Starting from functional block on the left, the Optical Interface
represents where the optical signal is transmitted or received and
defines the properties at the path end points. Even the impairment-
free case, like scenario B in section 4.1.1, needs to consider a
minimum set of interface characteristics. In such case, only a few
parameters used to assess the signal compatibility will be taken
into account (see [RFC6163]). For the impairment-aware case, these
parameters may be sufficient or not depending on the accepted level
of approximation (scenarios C and D). This functional block
highlights the need to consider a set of interface parameters during
the Impairment Validation Process.
The block "Optical Impairment Path" represents the types of
impairments affecting a wavelength as it traverses the networks
through links and nodes. In the case of a network where there are no
impairments (Scenario A), this block will not be present. Otherwise,
this function must be implemented in some way via the control plane.
Architectural alternatives to accomplish this are provided in
section 4.2. This block implementation (e.g., through routing,
signaling, or PCE) may influence the way the control plane
distributes impairment information within the network.
The last block implements the decision function for path
feasibility. Depending on the IA level of approximation, this
function can be more or less complex. For example in case of no IA
only the signal class compatibility will be verified. In addition to
feasible/not-feasible result, it may be worthwhile for decision
functions to consider the case in which paths can be likely-to-be-
feasible within some degree of confidence. The optical impairments
are usually not fixed values as they may vary within ranges of
values according to the approach taken in the physical modeling
(worst-case, statistical, or based on typical values). For example,
the utilization of the worst-case value for each parameter within
impairment validation process may lead to marking some paths as not-
feasible while they are very likely to be, in reality, feasible.
4.2. IA-RWA Computation and Control Plane Architectures
From a control plane point of view, optical impairments are
additional constraints to the impairment-free RWA process described
in [RFC6163]. In impairment aware routing and wavelength assignment
(IA-RWA), there are conceptually three general classes of processes
to be considered: Routing (R), Wavelength Assignment (WA), and
Impairment Validation (IV), i.e., estimation.
Impairment validation may come in many forms, and may be invoked at
different levels of detail in the IA-RWA process. All the variations
of impairment validation discussed in this section is based on
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Scenario C (Approximated Impairment Estimation) as discussed in
Section 4.1.1. From a process point of view, the following three
forms of impairment validation will be considered:
o IV-Candidates
In this case, an Impairment Validation (IV) process furnishes a set
of paths between two nodes along with any wavelength restrictions
such that the paths are valid with respect to optical impairments.
These paths and wavelengths may not be actually available in the
network due to its current usage state. This set of paths could be
returned in response to a request for a set of at most K valid paths
between two specified nodes. Note that such a process never directly
discloses optical impairment information. Note that that this case
includes any paths between source and destination that may have been
"pre-validated".
In this case, the control plane simply makes use of candidate paths
but does not know any optical impairment information. Another option
is when the path validity is assessed within the control plane. The
following cases highlight this situation.
o IV-Approximate Verification
Here approximation methods are used to estimate the impairments
experienced by a signal. Impairments are typically approximated by
linear and/or statistical characteristics of individual or combined
components and fibers along the signal path.
o IV-Detailed Verification
In this case, an IV process is given a particular path and
wavelength through an optical network and is asked to verify whether
the overall quality objectives for the signal over this path can be
met. Note that such a process never directly discloses optical
impairment information.
The next two cases refer to the way an impairment validation
computation can be performed.
o IV-Centralized
In this case, impairments to a path are computed at a single entity.
The information concerning impairments, however, may still be
gathered from network elements. Depending how information is
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gathered, this may put additional requirements on routing protocols.
This will be detailed in later sections.
o IV-Distributed
In the distributed IV process, approximate degradation measures such
as OSNR, dispersion, DGD, etc., may be accumulated along the path
via signaling. Each node on the path may already perform some part
of the impairment computation (i.e. distributed). When the
accumulated measures reach the destination node, a decision on the
impairment validity of the path can be made. Note that such a
process would entail revealing an individual network element's
impairment information but it does not generally require
distributing optical parameters to the entire network.
The Control Plane must not preclude the possibility to concurrently
perform one or all the above cases in the same network. For example,
there could be cases where a certain number of paths are already
pre-validated (IV-Candidates) so the control plane may setup one of
those paths without requesting any impairment validation procedure.
On the same network, however, the control plane may compute a path
outside the set of IV-Candidates for which an impairment evaluation
can be necessary.
The following subsections present three major classes of IA-RWA path
computation architectures and reviews some of their respective
advantages and disadvantages.
4.2.1. Combined Routing, WA, and IV
From the point of view of optimality, reasonably good IA-RWA
solutions can be achieved if the path computation entity (PCE) can
conceptually/algorithmically combine the processes of routing,
wavelength assignment and impairment validation.
Such a combination can take place if the PCE is given: (a) the
impairment-free WSON network information as discussed in [RFC6163]
and (b) impairment information to validate potential paths.
4.2.2. Separate Routing, WA, or IV
Separating the processes of routing, WA, and/or IV can reduce the
need for sharing of different types of information used in path
computation. This was discussed for routing separate from WA in
[RFC6163]. In addition, as was discussed, some impairment
information may not be shared and this may lead to the need to
separate IV from RWA. In addition, if IV needs to be done at a high
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level of precision, it may be advantageous to offload this
computation to a specialized server.
The following conceptual architectures belong in this general
category:
o R+WA+IV -- separate routing, wavelength assignment, and
impairment validation.
o R + (WA & IV) -- routing separate from a combined wavelength
assignment and impairment validation process. Note that
impairment validation is typically wavelength dependent. Hence
combining WA with IV can lead to efficiencies.
o (RWA)+IV - combined routing and wavelength assignment with a
separate impairment validation process.
Note that the IV process may come before or after the RWA processes.
If RWA comes first, then IV is just rendering a yes/no decision on
the selected path and wavelength. If IV comes first it would need to
furnish a list of possible (valid with respect to impairments)
routes and wavelengths to the RWA processes.
4.2.3. Distributed WA and/or IV
In the non-impairment RWA situation [RFC6163], it was shown that a
distributed wavelength assignment (WA) process carried out via
signaling can eliminate the need to distribute wavelength
availability information via an interior gateway protocol (IGP). A
similar approach can allow for the distributed computation of
impairment effects and avoid the need to distribute impairment
characteristics of network elements and links by routing protocols
or by other means. So the following conceptual options belong to
this category:
o RWA + D(IV) - Combined routing and wavelength assignment and
distributed impairment validation.
o R + D(WA & IV) -- routing separate from a distributed wavelength
assignment and impairment validation process.
Distributed impairment validation for a prescribed network path
requires that the effects of impairments be calculated by
approximate models with cumulative quality measures such as those
given in [G.680]. The protocol encoding of the impairment related
information from [G.680] would need to be agreed upon.
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If distributed WA is being done at the same time as distributed IV
then it is necessary to accumulate impairment related information
for all wavelengths that could be used. The amount of information is
reduced somewhat as potential wavelengths are discovered to be in
use, but could be a significant burden for lightly loaded high
channel count networks.
4.3. Mapping Network Requirements to Architectures
Figure 2 shows process flows for the three main architectural
alternatives to IA-RWA when approximate impairment validation is
sufficient. Figure 3 shows process flows for the two main
architectural alternatives when detailed impairment verification is
required.
+-----------------------------------+
| +--+ +-------+ +--+ |
| |IV| |Routing| |WA| |
| +--+ +-------+ +--+ |
| |
| Combined Processes |
+-----------------------------------+
(a)
+--------------+ +----------------------+
| +----------+ | | +-------+ +--+ |
| | IV | | | |Routing| |WA| |
| |candidates| |----->| +-------+ +--+ |
| +----------+ | | Combined Processes |
+--------------+ +----------------------+
(b)
+-----------+ +----------------------+
| +-------+ | | +--+ +--+ |
| |Routing| |------->| |WA| |IV| |
| +-------+ | | +--+ +--+ |
+-----------+ | Distributed Processes|
+----------------------+
(c)
Figure 2 Process flows for the three main approximate impairment
architectural alternatives.
The advantages, requirements, and suitability of these options are
as follows:
o Combined IV & RWA process
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This alternative combines RWA and IV within a single computation
entity enabling highest potential optimality and efficiency in IA-
RWA. This alternative requires that the computational entity knows
impairment information as well as non-impairment RWA information.
This alternative can be used with "black links", but would then need
to be provided by the authority controlling the "black links".
o IV-Candidates + RWA process
This alternative allows separation of impairment information into
two computational entities while still maintaining a high degree of
potential optimality and efficiency in IA-RWA. The candidates IV
process needs to know impairment information from all optical
network elements, while the RWA process needs to know non-impairment
RWA information from the network elements. This alternative can be
used with "black links", but the authority in control of the "black
links" would need to provide the functionality of the IV-candidates
process. Note that this is still very useful since the algorithmic
areas of IV and RWA are very different and conducive to
specialization.
o Routing + Distributed WA and IV
In this alternative, a signaling protocol may be extended and
leveraged in the wavelength assignment and impairment validation
processes. Although this doesn't enable as high a potential degree
of optimality as (a) or (b), it does not require distribution of
either link wavelength usage or link/node impairment information.
Note that this is most likely not suitable for "black links".
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+-----------------------------------+ +------------+
| +-----------+ +-------+ +--+ | | +--------+ |
| | IV | |Routing| |WA| | | | IV | |
| |approximate| +-------+ +--+ |---->| |Detailed| |
| +-----------+ | | +--------+ |
| Combined Processes | | |
+-----------------------------------+ +------------+
(a)
+--------------+ +----------------------+ +------------+
| +----------+ | | +-------+ +--+ | | +--------+ |
| | IV | | | |Routing| |WA| |---->| | IV | |
| |candidates| |----->| +-------+ +--+ | | |Detailed| |
| +----------+ | | Combined Processes | | +--------+ |
+--------------+ +----------------------+ | |
(b) +------------+
Figure 3 Process flows for the two main detailed impairment
validation architectural options.
The advantages, requirements, and suitability of these detailed
validation options are as follows:
o Combined Approximate IV & RWA + Detailed-IV
This alternative combines RWA and approximate IV within a single
computation entity enabling the highest potential optimality and
efficiency in IA-RWA while keeping a separate entity performing
detailed impairment validation. In the case of "black links" the
authority controlling the "black links" would need to provide all
functionality.
o Candidates-IV + RWA + Detailed-IV
This alternative allows separation of approximate impairment
information into a computational entity while still maintaining a
high degree of potential optimality and efficiency in IA-RWA; then a
separate computation entity performs detailed impairment validation.
Note that detailed impairment estimation is not standardized.
5. Protocol Implications
The previous IA-RWA architectural alternatives and process flows
make differing demands on a GMPLS/PCE based control plane. This
section discusses the use of (a) an impairment information model,
(b) PCE as computational entity assuming the various process roles
and consequences for PCEP, (c) possible extensions to signaling, and
(d) possible extensions to routing. This document is providing this
evaluation to aid protocol solutions work. The protocol
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specifications may deviate from this assessment. The assessment of
the impacts to the control plane for IA-RWA is summarized in Figure
4.
+-------------------+----+----+----------+--------+
| IA-RWA Option |PCE |Sig |Info Model| Routing|
+-------------------+----+----+----------+--------+
| Combined |Yes | No | Yes | Yes |
| IV & RWA | | | | |
+-------------------+----+----+----------+--------+-
| IV-Candidates |Yes | No | Yes | Yes |
| + RWA | | | | |
+-------------------+----+----+----------+--------+
| Routing + |No | Yes| Yes | No |
|Distributed IV, RWA| | | | |
+-------------------+----+----+----------+--------+
Figure 4 IA-RWA architectural options and control plane impacts.
5.1. Information Model for Impairments
As previously discussed, most IA-RWA scenarios rely, to a greater or
lesser extent, on a common impairment information model. A number of
ITU-T recommendations cover detailed, as well as, approximate
impairment characteristics of fibers, and a variety of devices, and
subsystems. An impairment model which can be used as a guideline for
optical network elements and assessment of path viability is given
in [G.680].
It should be noted that the current version of [G.680] is limited to
networks composed of a single WDM line system vendor combined with
OADMs and/or PXCs from potentially multiple other vendors. This is
known as situation 1 and is shown in Figure 1-1 of [G.680]. It is
planned in the future that [G.680] will include networks
incorporating line systems from multiple vendors, as well as, OADMs
and/or PXCs from potentially multiple other vendors. This is known
as situation 2 and is shown in Figure 1-2 of [G.680].
For the case of distributed impairment validation (distributed IV),
this would require more than an impairment information model. It
would need a common impairment "computation" model. In the
distributed IV case, one needs to standardize the accumulated
impairment measures that will be conveyed and updated at each node.
Section 9 of [G.680] provides guidance in this area with specific
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formulas given for OSNR, residual dispersion, polarization mode
dispersion/polarization dependent loss, and effects of channel
uniformity. However, specifics of what intermediate results are kept
and in what form would need to be standardized for interoperability.
As noted in [G.680], this information may possibly not be
sufficient, and in such case the applicability would be network
dependent.
5.2. Routing
Different approaches to path/wavelength impairment validation give
rise to different demands placed on GMPLS routing protocols. In the
case where approximate impairment information is used to validate
paths, GMPLS routing may be used to distribute the impairment
characteristics of the network elements and links based on the
impairment information model previously discussed.
Depending on the computational alternative, the routing protocol may
need to advertise information necessary to the impairment validation
process. This can potentially cause scalability issues due to the
high volume of data that need to be advertised. Such issue can be
addressed separating data that need to be advertised rarely from
data that need to be advertised more frequently or adopting other
form of awareness solutions described in previous sections (e.g.,
centralized and/or external IV entity).
In term of approximated scenario (see Section 4.1.1.), the model
defined by [G.680] will apply and the routing protocol will need to
gather information required for such computation.
In the case of distributed-IV, no new demands would be placed on the
routing protocol.
5.3. Signaling
The largest impacts on signaling occur in the cases where
distributed impairment validation is performed. In this case, it is
necessary to accumulate impairment information as previously
discussed. In addition, since the characteristics of the signal
itself, such as modulation type, can play a major role in the
tolerance of impairments, this type of information will need to be
implicitly or explicitly signaled so that an impairment validation
decision can be made at the destination node.
It remains for further study if it may be beneficial to include
additional information to a connection request such as desired
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egress signal quality (defined in some appropriate sense) in non-
distributed IV scenarios.
5.4. PCE
In section 4.3. a number of computation architectural alternatives
were given that could be used to meet the various requirements and
constraints of section 4.1. Here the focus is how these
alternatives could be implemented via either a single PCE or a set
of two or more cooperating PCEs, and the impacts on the PCEP. This
document provides this evaluation to aid solutions work. The
protocol specifications may deviate from this assessment.
5.4.1. Combined IV & RWA
In this situation, shown in Figure 2(a), a single PCE performs all
the computations needed for IA-RWA.
o TE Database Requirements: WSON Topology and switching
capabilities, WSON WDM link wavelength utilization, and WSON
impairment information
o PCC to PCE Request Information: Signal characteristics/type,
required quality, source node, destination node
o PCE to PCC Reply Information: If the computations completed
successfully then the PCE returns the path and its assigned
wavelength. If the computations could not complete successfully,
it would be potentially useful to know the reason why. At a
minimum, it is of interest to know if this was due to lack of
wavelength availability, impairment considerations, or both. The
information to be conveyed is for further study.
5.4.2. IV-Candidates + RWA
In this situation, as shown in Figure 2(b), two separate processes
are involved in the IA-RWA computation. This requires two
cooperating path computation entities: one for the Candidates-IV
process and another for the RWA process. In addition, the overall
process needs to be coordinated. This could be done with yet another
PCE or this functionality could be added to one of previously
defined entities. This later option requires the RWA entity to also
act as the overall process coordinator. The roles, responsibilities,
and information requirements for these two entities when
instantiated as PCEs are given below.
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RWA and Coordinator PCE (RWA-Coord-PCE):
Responsible for interacting with PCC and for utilizing Candidates-
PCE as needed during RWA computations. In particular, it needs to
know to use the Candidates-PCE to obtain potential set of routes and
wavelengths.
o TE Database Requirements: WSON Topology and switching
capabilities and WSON WDM link wavelength utilization (no
impairment information).
o PCC to RWA-PCE request: same as in the combined case.
o RWA-PCE to PCC reply: same as in the combined case.
o RWA-PCE to IV-Candidates-PCE request: The RWA-PCE asks for a set
of at most K routes along with acceptable wavelengths between
nodes specified in the original PCC request.
o IV-Candidates-PCE reply to RWA-PCE: The Candidates-PCE returns a
set of at most K routes along with acceptable wavelengths between
nodes specified in the RWA-PCE request.
IV-Candidates-PCE:
The IV-Candidates PCE is responsible for impairment aware path
computation. It need not take into account current link wavelength
utilization, but this is not prohibited. The Candidates-PCE is only
required to interact with the RWA-PCE as indicated above and not the
initiating PCC. Note: RWA-Coord PCE is also a PCC with respect to
the IV-Candidate.
o TE Database Requirements: WSON Topology and switching
capabilities and WSON impairment information (no information link
wavelength utilization required).
Figure 5 shows a sequence diagram for the possible interactions
between the PCC, RWA-Coord PCE, and IV-Candidates PCE.
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+---+ +-------------+ +-----------------+
|PCC| |RWA-Coord PCE| |IV-Candidates PCE|
+-+-+ +------+------+ +---------+-------+
...___ (a) | |
| ````---...____ | |
| ```-->| |
| | |
| |--..___ (b) |
| | ```---...___ |
| | ```---->|
| | |
| | |
| | (c) ___...|
| | ___....---'''' |
| |<--'''' |
| | |
| | |
| (d) ___...| |
| ___....---''' | |
|<--''' | |
| | |
| | |
Figure 5 Sequence diagram for the interactions between PCC, RWA-
Coordinating-PCE, and the IV-Candidates-PCE.
In step (a), the PCC requests a path meeting specified quality
constraints between two nodes (A and Z) for a given signal
represented either by a specific type or a general class with
associated parameters. In step (b), the RWA-Coordinating-PCE
requests up to K candidate paths between nodes A and Z and
associated acceptable wavelengths. The term "K candidate paths" is
associated with K-shortest path algorithm. It refers to an algorithm
that finds multiple K short paths connecting the source and the
destination in a graph allowing repeated vertices and edges in the
paths. See details in [Eppstein].
In step (c), The IV-Candidates PCE returns this list to the RWA-
Coordinating PCE which then uses this set of paths and wavelengths
as input (e.g., a constraint) to its RWA computation. In step (d)
the RWA-Coordinating PCE returns the overall IA-RWA computation
results to the PCC.
5.4.3. Approximate IA-RWA + Separate Detailed IV
Previously, Figure 3 showed two cases where a separate detailed
impairment validation process could be utilized. It is possible to
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place the detailed validation process into a separate PCE. Assuming
that a different PCE assumes a coordinating role and interacts with
the PCC, it is possible to keep the interactions with this separate
IV-Detailed-PCE very simple. Please note that there is some
inefficiency by separating the IV-Candidates-PCE from the IV-
Detailed-PCE from a message flow perspective in order to achieve a
high degree of potential optimality.
IV-Detailed-PCE:
o TE Database Requirements: The IV-Detailed-PCE will need optical
impairment information, WSON topology, and possibly WDM link
wavelength usage information. This document puts no restrictions
on the type of information that may be used in these
computations.
o Coordinating-PCE to IV-Detailed-PCE request: The coordinating-PCE
will furnish signal characteristics, quality requirements, path,
and wavelength to the IV-Detailed-PCE.
o IV-Detailed-PCE to Coordinating-PCE reply: The reply is
essentially a yes/no decision as to whether the requirements
could actually be met. In the case where the impairment
validation fails, it would be helpful to convey information
related to cause or quantify the failure, e.g., so that a
judgment can be made whether to try a different signal or adjust
signal parameters.
Figure 6 shows a sequence diagram for the interactions corresponding
to the process shown in Figure 3(b). This involves interactions
between the PCC, RWA-PCE (acting as coordinator), IV-Candidates-PCE,
and the IV-Detailed-PCE.
In step (a), the PCC requests a path meeting specified quality
constraints between two nodes (A and Z) for a given signal
represented either by a specific type or a general class with
associated parameters. In step (b), the RWA-Coordinating-PCE
requests up to K candidate paths between nodes A and Z and
associated acceptable wavelengths. In step (c), The IV-Candidates-
PCE returns this list to the RWA-Coordinating PCE which then uses
this set of paths and wavelengths as input (e.g., a constraint) to
its RWA computation. In step (d), the RWA-Coordinating-PCE request a
detailed verification of the path and wavelength that it has
computed. In step (e), the IV-Detailed-PCE returns the results of
the validation to the RWA-Coordinating-PCE. Finally in step (f), the
IA-RWA-Coordinating PCE returns the final results (either a path and
wavelength or cause for the failure to compute a path and
wavelength) to the PCC.
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+----------+ +--------------+ +------------+
+---+ |RWA-Coord | |IV-Candidates | |IV-Detailed |
|PCC| | PCE | | PCE | | PCE |
+-+-+ +----+-----+ +------+-------+ +-----+------+
|.._ (a) | | |
| ``--.__ | | |
| `-->| | |
| | (b) | |
| |--....____ | |
| | ````---.>| |
| | | |
| | (c) __..-| |
| | __..---'' | |
| |<--'' | |
| | |
| |...._____ (d) |
| | `````-----....._____ |
| | `````----->|
| | |
| | (e) _____.....+
| | _____.....-----''''' |
| |<----''''' |
| (f) __.| |
| __.--'' |
|<-'' |
| |
Figure 6 Sequence diagram for the interactions between PCC, RWA-
Coordinating-PCE, IV-Candidates-PCE, and IV-Detailed-PCE.
6. Manageability and Operations
The issues concerning manageability and operations are beyond the
scope of this document. The details of manageability and operational
issues will have to be deferred to future protocol implementation.
On a high-level, the GMPLS-routing based architecture discussed in
Section 5.2. may have to deal with how to resolve potential scaling
issues associated with disseminating a large amount of impairment
characteristics of the network elements and links.
From a scaling point of view, the GMPLS-signaling based architecture
discussed in Section 5.3. would be more scalable than other
alternatives as this architecture would avoid the dissemination of a
large amount of data to the networks. This benefit may come,
however, at the expense of potentially inefficient use of network
resources.
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The PCE-based architectures discussed in Section 5.4. would have to
consider operational complexity when implementing options that
require the use of multiple PCE servers. The most serious case is
the option discussed in Section 5.4.3., namely, "Approximate IA-RWA
+ Separate Detailed IV". The combined IV & RWA option (which was
discussed on Section 5.4.1.), on the other hand, is simpler than
other alternatives to operate as one PCE server handles all
functionality; however, this option may suffer from a heavy
computation and processing burden compared to other alternatives.
Interoperability may be a hurdle to overcome when trying to agree on
some impairment parameters especially those which are associated
with the black links. This work has been in progress in ITU-T and
needs some more time to mature.
7. Security Considerations
This document discusses a number of control plane architectures that
incorporate knowledge of impairments in optical networks. If such
architecture is put into use within a network, it will by its nature
contain details of the physical characteristics of an optical
network. Such information would need to be protected from
intentional or unintentional disclosure similar to other network
information used within intra-domain protocols.
This document does not require changes to the security models within
GMPLS and associated protocols. That is, the OSPF-TE, RSVP-TE, and
PCEP security models could be operated unchanged. However,
satisfying the requirements for impairment information dissemination
using the existing protocols may significantly affect the loading of
those protocols.
This may make the operation of the network more vulnerable to active
attacks such as injections, impersonation, and MITMs. Therefore,
additional care may be required to ensure that the protocols are
secure in the impairment-aware WSON environment.
Furthermore, the additional information distributed in order to
address impairment information represents a disclosure of network
capabilities that an operator may wish to keep private.
Consideration should be given to securing this information. For a
general discussion on MPLS- and GMPLS-related security issues, see
the MPLS/GMPLS security framework [RFC5920] and, in particular, text
detailing security issues when Control Plane is physically separated
from Data Plane.
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8. IANA Considerations
This draft does not currently require any consideration from IANA.
9. References
9.1. Normative References
[G.680] ITU-T Recommendation G.680, Physical transfer functions of
optical network elements, July 2007.
[RFC3945] Mannie, E., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Architecture", RFC 3945, October 2004.
[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
August 2006.
9.2. Informative References
[G.Sup39] ITU-T Series G Supplement 39, Optical system design and
engineering considerations, February 2006.
[G.698.1] ITU-T Recommendation G.698.1, Multichannel DWDM
applications with Single-Channel optical interface,
December 2006.
[G.698.2] ITU-T Recommendation G.698.2, Amplified multichannel DWDM
applications with Single-Channel optical interface, July
2007.
[RFC4054] Strand, J., Ed., and A. Chiu, Ed., "Impairments and Other
Constraints on Optical Layer Routing", RFC 4054, May 2005.
[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
[RFC6163] Lee, Y., Ed., G. Bernstein, Ed., and W. Imajuku,
"Framework for GMPLS and PCE Control of Wavelength
Switched Optical Networks", RFC 6163, April 2011.
[Eppstein] Eppstein, D., "Finding the k shortest paths", 35th IEEE
Symp. Foundations of Comp. Sci., Santa Fe, pp. 154-165,
1994.
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10. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
Copyright (c) 2012 IETF Trust and the persons identified as authors
of the code. All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
o Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
o Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in
the documentation and/or other materials provided with the
distribution.
o Neither the name of Internet Society, IETF or IETF Trust, nor the
names of specific contributors, may be used to endorse or promote
products derived from this software without specific prior
written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.
Lee & Bernstein Expires July 5, 2012 [Page 28]
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Authors' Addresses
Young Lee (ed.)
Huawei Technologies
1700 Alma Drive, Suite 100
Plano, TX 75075
USA
Phone: (972) 509-5599 (x2240)
Email: ylee@huawei.com
Greg M. Bernstein (ed.)
Grotto Networking
Fremont California, USA
Phone: (510) 573-2237
Email: gregb@grotto-networking.com
Dan Li
Huawei Technologies Co., Ltd.
F3-5-B R&D Center, Huawei Base,
Bantian, Longgang District
Shenzhen 518129 P.R.China
Phone: +86-755-28973237
Email: danli@huawei.com
Giovanni Martinelli
Cisco
Via Philips 12
20052 Monza, Italy
Phone: +39 039 2092044
Email: giomarti@cisco.com
Contributor's Addresses
Ming Chen
Huawei Technologies Co., Ltd.
F3-5-B R&D Center, Huawei Base,
Bantian, Longgang District
Shenzhen 518129 P.R.China
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Phone: +86-755-28973237
Email: mchen@huawei.com
Rebecca Han
Huawei Technologies Co., Ltd.
F3-5-B R&D Center, Huawei Base,
Bantian, Longgang District
Shenzhen 518129 P.R.China
Phone: +86-755-28973237
Email: hanjianrui@huawei.com
Gabriele Galimberti
Cisco
Via Philips 12,
20052 Monza, Italy
Phone: +39 039 2091462
Email: ggalimbe@cisco.com
Alberto Tanzi
Cisco
Via Philips 12,
20052 Monza, Italy
Phone: +39 039 2091469
Email: altanzi@cisco.com
David Bianchi
Cisco
Via Philips 12,
20052 Monza, Italy
Email: davbianc@cisco.com
Moustafa Kattan
Cisco
Dubai 500321
United Arab Emirates
Email: mkattan@cisco.com
Dirk Schroetter
Cisco
Email: dschroet@cisco.com
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Daniele Ceccarelli
Ericsson
Via A. Negrone 1/A
Genova - Sestri Ponente
Italy
Email: daniele.ceccarelli@ericsson.com
Elisa Bellagamba
Ericsson
Farogatan 6,
Kista 164 40
Sweeden
Email: elisa.bellagamba@ericcson.com
Diego Caviglia
Ericsson
Via A. negrone 1/A
Genova - Sestri Ponente
Italy
Email: diego.caviglia@ericcson.com
Lee & Bernstein Expires July 5, 2012 [Page 31]
