Network Working Group Y. Lee
Huawei
G. Bernstein
Grotto Networking
D. Li
Huawei
G. Martinelli
Cisco
Internet Draft
Intended status: Informational April 29, 2011
Expires: October 2011
A Framework for the Control of Wavelength Switched Optical Networks
(WSON) with Impairments
draft-ietf-ccamp-wson-impairments-07.txt
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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. 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.
Table of Contents
1. Introduction...................................................3
2. Terminology....................................................3
3. Applicability..................................................5
4. Impairment Aware Optical Path Computation......................6
4.1. Optical Network Requirements and Constraints..............7
4.1.1. Impairment Aware Computation Scenarios...............8
4.1.2. Impairment Computation and Information Sharing
Constraints.................................................9
4.1.3. Impairment Estimation Process.......................10
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............................14
4.3. Mapping Network Requirements to Architectures............15
5. Protocol Implications.........................................18
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5.1. Information Model for Impairments........................18
5.2. Routing..................................................19
5.3. Signaling................................................20
5.4. PCE......................................................20
5.4.1. Combined IV & RWA...................................20
5.4.2. IV-Candidates + RWA.................................21
5.4.3. Approximate IA-RWA + Separate Detailed IV...........23
6. Security Considerations.......................................24
7. IANA Considerations...........................................25
8. References....................................................25
8.1. Normative References.....................................25
8.2. Informative References...................................25
9. Acknowledgments...............................................25
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 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
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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
Add/Drop Multiplexers (ADM): An optical device used in WDM networks
composed of one or more line side ports and typically many tributary
ports.
CWDM: Coarse Wavelength Division Multiplexing.
DWDM: Dense Wavelength Division Multiplexing.
FOADM: Fixed Optical Add/Drop Multiplexer.
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.
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.
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PCEP: PCE Communication Protocol. The communication protocol between
a Path Computation Client and Path Computation Element.
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.
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.
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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.
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.
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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:
(i) 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 that 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
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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
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. Detailed Impairment Computation
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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
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. Note however 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 and 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" (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:
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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 for larger networks
such a list of paths along with any wavelength constraints could
get unmanageably large.
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.
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+-----------------+
+------------+ +-----------+ | +------------+ |
| | | | | | | |
| Optical | | Optical | | | Optical | |
| Interface |------->| Impairment|--->| | Channel | |
| (Transmit/ | | Path | | | Estimation | |
| Receive) | | | | | | |
+------------+ +-----------+ | +------------+ |
| || |
| || |
| Estimation |
| || |
| \/ |
| +------------+ |
| | BER / | |
| | Q Factor | |
| +------------+ |
+-----------------+
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 no-impairment
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 an 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.
Options for this will be given in the next section on architectural
alternatives. 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
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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
feasible in reality.
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 (estimation) (IV).
Impairment validation may come in many forms, and may be invoked at
different levels of detail in the IA-RWA process. 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
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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 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 operate one or
all the above cases concurrently 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.
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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 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
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similar approach can allow for the distributed computation of
impairment effects and avoid the need to distribute impairment
characteristics of network elements and links via 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.
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. This is somewhat windowed down 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 three main architectural
alternatives to IA-RWA when approximate impairment validation is
sufficient. Figure 3 shows process flows for two main architectural
alternatives when detailed impairment verification is required.
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+-----------------------------------+
| +--+ +-------+ +--+ |
| |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
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
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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 prone 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 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".
+-----------------------------------+ +------------+
| +-----------+ +-------+ +--+ | | +--------+ |
| | 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 highest potential optimality and
efficiency in IA-RWA; then has 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.
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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
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 to a greater or lesser
extent rely 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
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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
the 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
planed 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
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 for the protocol 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 gives
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 impairment validation
process. This can potentially cause scalability issues due to the
high amount of data that need to be advertised. Such issue can be
addressed separating data that need to be advertised rarely and 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).
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In term of approximated scenario (see Section 4.1.1.) the model
defined by [G.680] will apply and 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 ie 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 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 protocol. This document
is providing 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
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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 or 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 can 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.
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 needs not take into account current link wavelength
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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.
+---+ +-------------+ +-----------------+
|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. 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
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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
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.
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 a judgment can be made whether to
try a different signal or adjust signal parameters.
Figure 6 shows a sequence diagram for the interactions for 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)IA-RWA-Coordinating PCE
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returns the final results (either a path and wavelength or cause for
the failure to compute a path and wavelength) to the PCC.
+----------+ +--------------+ +------------+
+---+ |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. 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. It is expected that protocol solutions
work will address any issues on the use of impairment information.
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7. IANA Considerations
This draft does not currently require any consideration from IANA.
8. References
8.1. 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.
8.2. 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.
[RFC6163] Y. Lee, G. Bernstein, W. Imajuku, "Framework for GMPLS and
PCE Control of Wavelength Switched Optical Networks", RFC
6163, April 2011.
9. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
Copyright (c) 2011 IETF Trust and the persons identified as authors
of the code. All rights reserved.
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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.
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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 October 29, 2011 [Page 29]
