| CCAMP Working Group | O. Gonzalez de Dios, Ed. |
| Internet-Draft | Telefonica I+D |
| Intended status: Informational | R. Casellas, Ed. |
| Expires: February 26, 2016 | CTTC |
| August 25, 2015 |
Framework and Requirements for GMPLS-based control of Flexi-grid DWDM networks
draft-ietf-ccamp-flexi-grid-fwk-06
To allow efficient allocation of optical spectral bandwidth for high bit-rate systems, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) has extended its Recommendations G.694.1 and G.872 to include a new dense wavelength division multiplexing (DWDM) grid by defining a set of nominal central frequencies, channel spacings and the concept of "frequency slot". In such an environment, a data plane connection is switched based on allocated, variable-sized frequency ranges within the optical spectrum creating what is known as a flexible grid (flexi-grid).
Given the specific characteristics of flexi-grid optical networks and their associated technology, this document defines a framework and the associated control plane requirements for the application of the existing GMPLS architecture and control plane protocols to the control of flexi-grid DWDM networks. The actual extensions to the GMPLS protocols will be defined in companion documents.
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The term "Flexible grid" (flexi-grid for short) as defined by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) Study Group 15 in the latest version of [G.694.1], refers to the updated set of nominal central frequencies (a frequency grid), channel spacing and optical spectrum management/allocation considerations that have been defined in order to allow an efficient and flexible allocation and configuration of optical spectral bandwidth for high bit-rate systems.
A key concept of flexi-grid is the "frequency slot"; a variable-sized optical frequency range that can be allocated to a data connection. As detailed later in the document, a frequency slot is characterized by its nominal central frequency and its slot width which, as per [G.694.1], is constrained to be a multiple of a given slot width granularity.
Compared to a traditional fixed grid network, which uses fixed size optical spectrum frequency ranges or frequency slots with typical channel separations of 50 GHz, a flexible grid network can select its media channels with a more flexible choice of slot widths, allocating as much optical spectrum as required.
From a networking perspective, a flexible grid network is assumed to be a layered network [G.872][G.800] in which the media layer is the server layer and the optical signal layer is the client layer. In the media layer, switching is based on a frequency slot, and the size of a media channel is given by the properties of the associated frequency slot. In this layered network, a media channel can transport more than one Optical Tributary Signals (OTSi), as defined later in this document.
A Wavelength Switched Optical Network (WSON), addressed in [RFC6163], is a term commonly used to refer to the application/deployment of a GMPLS-based control plane for the control (provisioning/recovery, etc.) of a fixed grid wavelength division multiplexing (WDM) network in which media (spectrum) and signal are jointly considered.
This document defines the framework for a GMPLS-based control of flexi-grid enabled dense wavelength division multiplexing (DWDM) networks (in the scope defined by ITU-T layered Optical Transport Networks [G.872]), as well as a set of associated control plane requirements. An important design consideration relates to the decoupling of the management of the optical spectrum resource and the client signals to be transported.
Further terminology specific to flexi-grid networks can be found in Section 3.2.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
While [RFC2119] describes interpretations of these key words in terms of protocol specifications and implementations, they are used in this document to describe design requirements for protocol extensions.
FS: Frequency Slot
FSC: Fiber-Switch Capable
LSR: Label Switching Router
NCF: Nominal Central Frequency
OCh: Optical Channel
OCh-P: Optical Channel Payload
OTN: Optical Transport Network
OTSi: Optical Tributary Signal
OTSiG: OTSi Group is a set of OTSi
OCC: Optical Channel Carrier
PCE: Path Computation Element
ROADM: Reconfigurable Optical Add-Drop Multiplexer
SSON: Spectrum-Switched Optical Network
SWG: Slot Width Granularity
[G.872] describes, from a network level, the functional architecture of an OTN. It is decomposed into independent layer networks with client/layer relationships among them. A simplified view of the OTN layers is shown in Figure 1.
+----------------+
| Digital Layer |
+----------------+
| Signal Layer |
+----------------+
| Media Layer |
+----------------+
Figure 1: Generic OTN Overview
In the OTN layering context, the media layer is the server layer of the optical signal layer. The optical signal is guided to its destination by the media layer by means of a network media channel. In the media layer, switching is based on a frequency slot.
In this scope, this document uses the term flexi-grid enabled DWDM network to refer to a network in which switching is based on frequency slots defined using the flexible grid, and covers mainly the Media Layer as well as the required adaptations from the Signal layer. The present document is thus focused on the control and management of the media layer.
This section presents the definition of the terms used in flexi-grid networks. More detail about these terms can be found in the ITU-T Recommendations [G.694.1], [G.872]), [G.870], [G.8080], and [G.959.1-2013].
Where appropriate, this documents also uses terminology and lexicography from [RFC4397].
f = 193.1 THz + n x 0.00625 THz
-5 -4 -3 -2 -1 0 1 2 3 4 5 <- values of n
...+--+--+--+--+--+--+--+--+--+--+-
^
193.1 THz <- anchor frequency
Figure 2: Anchor Frequency and Set of Nominal Central Frequencies
Frequency Slot 1 Frequency Slot 2
------------- -------------------
| | | |
-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
...--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--...
------------- -------------------
^ ^
Slot NCF = 193.1THz Slot NCF = 193.14375 THz
Slot width = 25 GHz Slot width = 37.5 GHz
n=0, m=2 n=7, m=3
Figure 3: Example Frequency Slots
Frequency Slot 1
-------------
| |
-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
..--+--+--+--+--X--+--+--+--+--+--+--+--+--+--+--+--...
Frequency Slot 2
-------------------
| |
-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
..--+--+--+--+--X--+--+--+--+--+--+--+--+--+--+--+--...
===============================================
Effective Frequency Slot
-------------
| |
-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
..--+--+--+--+--X--+--+--+--+--+--+--+--+--+--+--+--...
Figure 4: Effective Frequency Slot
This subsection is focused on the frequency slot and related terms.
This section defines concepts such as (Network) Media Channel; the mapping to GMPLS constructs (i.e., LSP) is detailed in Section 4.
In summary, the concept of frequency slot is a logical abstraction that represents a frequency range, while the media layer represents the underlying media support. Media Channels are media associations, characterized by their (effective) frequency slot, respectively; and media channels are switched in media channel matrixes. From the control and management perspective, a media channel can be logically split into network media channels.
In Figure 5, a media channel has been configured and dimensioned to support two network media channels, each of them carrying one OTSi.
Media Channel Frequency Slot
+-------------------------------X------------------------------+
| |
| Frequency Slot Frequency Slot |
| +-----------X-----------+ +----------X-----------+ |
| | OTSi | | OTSi | |
| | o | | o | |
| | | | | | | |
-4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12
--+---+---+---+---+---+---+---+---+---+---+---+--+---+---+---+---+--
<- Network Media Channel-> <- Network Media Channel->
<------------------------ Media Channel ----------------------->
X - Frequency Slot Central Frequency
o - Signal Central Frequency
Figure 5: Example of Media Channel / Network Media Channels and Associated Frequency Slots
In the OTN layered network, the network media channel transports a single OTSi (see Figure 6)
| OTSi |
O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - O
| |
| Channel Port Network Media Channel Channel Port |
O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - O
| |
+--------+ +-----------+ +--------+
| \ (1) | | (1) | | (1) / |
| \----|-----------------|-----------|-------------------|-----/ |
+--------+ Link Channel +-----------+ Link Channel +--------+
Media Channel Media Channel Media Channel
Matrix Matrix Matrix
The symbol (1) indicates a Matrix Channel
Figure 6: Simplified Layered Network Model
Note that a particular example of OTSi is the OCh-P. Figure 7 shows this specific example as defined in G.805 [G.805].
OCh AP Trail (OCh) OCh AP
O- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - O
| |
--- OCh-P OCh-P ---
\ / source sink \ /
+ +
| OCh-P OCh-P Network Connection OCh-P |
O TCP - - - - - - - - - - - - - - - - - - - - - - - - - - -TCP O
| |
|Channel Port Network Media Channel Channel Port |
O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - O
| |
+--------+ +-----------+ +---------+
| \ (1) | OCh-P LC | (1) | OCh-P LC | (1) / |
| \----|-----------------|-----------|-----------------|------/ |
+--------+ Link Channel +-----------+ Link Channel +---------+
Media Channel Media Channel Media Channel
Matrix Matrix Matrix
The symbol (1) indicates a Matrix Channel
Figure 7: Layered Network Model According to G.805
A flexible grid network is constructed from subsystems that include WDM links, tunable transmitters, and receivers, (i.e, media elements including media layer switching elements that are media matrices) as well as electro-optical network elements. This is just the same as in a fixed grid network except that each element has flexible grid characteristics.
As stated in Clause 7 of [G.694.1] the flexible DWDM grid has a nominal central frequency granularity of 6.25 GHz and a slot width granularity of 12.5 GHz. However, devices or applications that make use of the flexible grid might not be capable of supporting every possible slot width or position. In other words, applications may be defined where only a subset of the possible slot widths and positions are required to be supported. For example, an application could be defined where the nominal central frequency granularity is 12.5 GHz (by only requiring values of n that are even) and that only requires slot widths as a multiple of 25 GHz (by only requiring values of m that are even).
The goal of this section is to provide an insight into the application of GMPLS as a control mechanism in flexi-grid networks. Specific control plane requirements for the support of flexi-grid networks are covered in Section 5. This framework is aimed at controlling the media layer within the OTN hierarchy, and controlling the required adaptations of the signal layer. This document also defines the term Spectrum-Switched Optical Network (SSON) to refer to a Flexi-grid enabled DWDM network that is controlled by a GMPLS/PCE control plane.
This section provides a mapping of the ITU-T G.872 architectural aspects to GMPLS/Control plane terms, and considers the relationship between the architectural concept/construct of media channel and its control plane representations (e.g., as a TE link, as defined in [RFC3945]).
The GMPLS control of the media layer deals with the establishment of media channels that are switched in media channel matrices. GMPLS labels are used to locally represent the media channel and its associated frequency slot. Network media channels are considered a particular case of media channels when the end points are transceivers (that is, source and destination of an OTSi).
From a theoretical / abstract point of view, a fiber can be modeled as having a frequency slot that ranges from minus infinity to plus infinity. This representation helps understand the relationship between frequency slots and ranges.
The frequency slot is a local concept that applies within a component or element. When applied to a media channel, we are referring to its effective frequency slot as defined in [G.872].
The association sequence of the three components (i.e., a filter, a fiber, and a filter), is a media channel in its most basic form. From the control plane perspective this may modeled as a (physical) TE-link with a contiguous optical spectrum. This can be represented by saying that the portion of spectrum available at time t0 depends on which filters are placed at the ends of the fiber and how they have been configured. Once filters are placed we have a one-hop media channel. In practical terms, associating a fiber with the terminating filters determines the usable optical spectrum.
---------------+ +-----------------+
| |
+--------+ +--------+
| | | | +---------
---o| =============================== o--|
| | Fiber | | | --\ /--
---o| | | o--| \/
| | | | | /\
---o| =============================== o--| --/ \--
| Filter | | Filter | |
| | | | +---------
+--------+ +--------+
| |
|------- Basic Media Channel ---------|
---------------+ +-----------------+
--------+ +--------
|--------------------------------------|
LSR | TE link | LSR
|--------------------------------------|
+--------+ +--------
Figure 8: (Basic) Media Channel and TE Link
Additionally, when a cross-connect for a specific frequency slot is considered, the resulting media support of joining basic media channels is still a media channel, i.e., a longer association sequence of media elements and its effective frequency slot. In other words, It is possible to "concatenate" several media channels (e.g., patch on intermediate nodes) to create a single media channel.
The architectural construct resulting of the association sequence of basic media channels and media layer matrix cross-connects can be represented as (i.e., corresponds to) a Label Switched Path (LSP) from a control plane perspective.
----------+ +------------------------------+ +---------
| | | |
+------+ +------+ +------+ +------+
| | | | +----------+ | | | |
--o| ========= o--| |--o ========= o--
| | Fiber | | | --\ /-- | | | Fiber | |
--o| | | o--| \/ |--o | | o--
| | | | | /\ | | | | |
--o| ========= o--***********|--o ========= o--
|Filter| |Filter| | | |Filter| |Filter|
| | | | | | | |
+------+ +------+ +------+ +------+
| | | |
<- Basic Media -> <- Matrix -> <- Basic Media->
|Channel| Channel |Channel|
----------+ +------------------------------+ +---------
<-------------------- Media Channel ---------------->
------+ +---------------+ +------
|------------------| |------------------|
LSR | TE link | LSR | TE link | LSR
|------------------| |------------------|
------+ +---------------+ +------
Figure 9: Extended Media Channel
Furthermore, if appropriate, the media channel can also be represented as a TE link or Forwarding Adjacency (FA) [RFC4206], augmenting the control plane network model.
----------+ +------------------------------+ +---------
| | | |
+------+ +------+ +------+ +------+
| | | | +----------+ | | | |
--o| ========= o--| |--o ========= o--
| | Fiber | | | --\ /-- | | | Fiber | |
--o| | | o--| \/ |--o | | o--
| | | | | /\ | | | | |
--o| ========= o--***********|--o ========= o--
|Filter| |Filter| | | |Filter| |Filter|
| | | | | | | |
+------+ +------+ +------+ +------+
| | | |
----------+ +------------------------------+ +---------
<------------------------ Media Channel ----------->
------+ +-----
|------------------------------------------------------|
LSR | TE link | LSR
|------------------------------------------------------|
------+ +-----
Figure 10: Extended Media Channel / TE Link / FA
The flexi-grid LSP is a control plane representation of a media channel. Since network media channels are media channels, an LSP may also be the control plane representation of a network media channel (without considering the adaptation functions). From a control plane perspective, the main difference (regardless of the actual effective frequency slot which may be dimensioned arbitrarily) is that the LSP that represents a network media channel also includes the endpoints (transceivers), including the cross-connects at the ingress and egress nodes. The ports towards the client can still be represented as interfaces from the control plane perspective.
Figure 11 shows an LSP routed between 3 nodes. The LSP is terminated before the optical matrix of the ingress and egress nodes and can represent a media channel. This case does not (and cannot) represent a network media channel because it does not include (and cannot include) the transceivers.
---------+ +--------------------------------+ +--------
| | | |
+------+ +------+ +------+ +------+
| | | | +----------+ | | | |
-o| ========= o---| |---o ========= o-
| | Fiber | | | --\ /-- | | | Fiber | |
-o|>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>o-
| | | | | /\ | | | | |
-o| ========= o---***********|---o ========= o-
|Filter| |Filter| | | |Filter| |Filter|
| | | | | | | |
+------+ +------+ +------+ +------+
| | | |
---------+ +--------------------------------+ +--------
>>>>>>>>>>>>>>>>>>>>>>>>>>>> LSP >>>>>>>>>>>>>>>>>>>>>>>>
-----+ +---------------+ +-----
|------------------| |----------------|
LSR | TE link | LSR | TE link | LSR
|------------------| |----------------|
-----+ +---------------+ +-----
Figure 11: Flex-grid LSP Representing a Media Channel that Starts at the Filter of the Outgoing Interface of the Ingress LSR and ends at the Filter of the Incoming Interface of the Egress LSR
In Figure 12 a Network Media Channel is represented as terminated at the network side of the transceivers. This is commonly named an OTSi-trail connection.
|--------------------- Network Media Channel ----------------------|
+----------------------+ +----------------------+
| | |
+------+ +------+ +------+ +------+
| | +----+ | | | | +----+ | |OTSi
OTSi| o-| |-o | +-----+ | o-| |-o |sink
src | | | | | ===+-+ +-+==| | | | | O---|R
T|***o******o********************************************************
| | |\ /| | | | | | | | |\ /| | |
| o-| \/ |-o ===| | | |==| o-| \/ |-o |
| | | /\ | | | +-+ +-+ | | | /\ | | |
| o-|/ \|-o | | \/ | | o-|/ \|-o |
|Filter| | | |Filter| | /\ | |Filter| | | |Filter|
+------+ | | +------+ +-----+ +------+ | | +------+
| | | | | | | |
+----------------------+ +----------------------+
LSP
<------------------------------------------------------------------->
LSP
<------------------------------------------------------------------>
+-----+ +--------+ +-----+
o--- | |-------------------| |----------------| |---o
| LSR | TE link | LSR | TE link | LSR |
| |-------------------| |----------------| |
+-----+ +--------+ +-----+
Figure 12: LSP Representing a Network Media Channel (OTSi Trail)
In a third case, a Network Media Channel is terminated on the Filter ports of the Ingress and Egress nodes. This is named in G.872 as OTSi Network Connection. As can be seen from the figures, there is no difference from a GMPLS modelling perspective between these cases, but they are shown as distinct examples to highlight the differences in the data plane.
|--------------------- Network Media Channel --------------------|
+------------------------+ +------------------------+
+------+ +------+ +------+ +------+
| | +----+ | | | | +----+ | |
| o-| |-o | +------+ | o-| |-o |
| | | | | =====+-+ +-+=====| | | | | |
T-o******o********************************************************O-R
| | |\ /| | | | | | | | |\ /| | |
| o-| \/ |-o =====| | | |=====| o-| \/ |-o |
| | | /\ | | | +-+ +-+ | | | /\ | | |
| o-|/ \|-o | | \/ | | o-|/ \|-o |
|Filter| | | |Filter| | /\ | |Filter| | | |Filter|
+------+ | | +------+ +------+ +------+ | | +------+
| | | | | | | |
+----------------------+ +----------------------+
<----------------------------------------------------------------->
LSP
LSP
<-------------------------------------------------------------->
+-----+ +--------+ +-----+
o--| |--------------------| |-------------------| |--o
| LSR | TE link | LSR | TE link | LSR |
| |--------------------| |-------------------| |
+-----+ +--------+ +-----+
Figure 13: LSP Representing a Network Media Channel (OTSi Network Connection)
Applying the notion of hierarchy at the media layer, by using the LSP as an FA (i.e., by using hierarchical LSPs), the media channel created can support multiple (sub-)media channels.
+--------------+ +--------------+
| Media Channel| TE | Media Channel| Virtual TE
| | link | | link
| Matrix |o- - - - - - - - - - o| Matrix |o- - - - - -
+--------------+ +--------------+
| +---------+ |
| | Media | |
|o----| Channel |-----o|
| |
| Matrix |
+---------+
Figure 14: Topology View with TE Link / FA
Note that there is only one media layer switch matrix (one implementation is a FlexGrid ROADM) in SSON, while a signal layer LSP (Network Media Channel) is established mainly for the purpose of management and control of individual optical signals. Signal layer LSPs with the same attributes (such as source and destination) can be grouped into one media-layer LSP (media channel): this has advantages in spectral efficiency (reduce guard band between adjacent OChs in one FSC channel) and LSP management. However, assuming some network elements perform signal layer switching in an SSON, there must be enough guard band between adjacent OTSis in any media channel to compensate for the filter concatenation effects and other effects caused by signal layer switching elements. In such a situation, the separation of the signal layer from the media layer does not bring any benefit in spectral efficiency or in other aspects, but makes the network switch and control more complex. If two OTSis must be switched to different ports, it is better to carry them by diferent FSC channels, and the media layer switch is enough in this scenario.
As discussed in Section 3.2.5, a media channel may be constructed from a compsite of network media channels. This may be achieved in two ways using LSPs. These mechanisms may be compared to the techniques used in GMPLS to support inverse multiplexing in Time Division Multiplexing (TDM) networks and in OTN [RFC4606], [RFC6344], and [RFC7139].
Optical transmitters and receivers may have different tunability constraints, and media channel matrixes may have switching restrictions. Additionally, a key feature of their implementation is their highly asymmetric switching capability which is described in detail in [RFC6163]. Media matrices include line side ports that are connected to DWDM links, and tributary side input/output ports that can be connected to transmitters/receivers.
A set of common constraints can be defined:
A media channel has an associated effective frequency slot. From the perspective of network control and management, this effective slot is seen as the "usable" end-to-end frequency slot. The establishment of an LSP is related to the establishment of the media channel and the configuration of the effective frequency slot.
A "service request" is characterized (at a minimum) by its required effective frequency slot width. This does not preclude that the request may add additional constraints such as also imposing the nominal central frequency. A given effective frequency slot may be requested for the media channel in the control plane LSP setup messages, and a specific frequency slot can be requested on any specific hop of the LSP setup. Regardless of the actual encoding, the LSP setup message specifies a minimum effective frequency slot width that needs to be fulfilled in order to successful establish the requsted LSP.
An effective frequency slot must equally be described in terms of a central nominal frequency and its slot width (in terms of usable spectrum of the effective frequency slot). That is, it must be possible to determine the end-to-end values of the n and m parameters. We refer to this by saying that the "effective frequency slot of the media channel/LSP must be valid".
In GMPLS the requested effective frequency slot is represented to the TSpec present in the RSVP-TE Path message, and the effective frequency slot is mapped to the FlowSpec carried in the RSVP-TE Resv message.
In GMPLS-controlled systems, the switched element corresponds to the 'label'. In flexi-grid where the switched element is a frequency slot, the label represents a frequency slot. In consequence, the label in flexi-grid conveys the necessary information to obtain the frequency slot characteristics (i.e, central frequency and slot width: the n and m parameters). The frequency slot is locally identified by the label.
The local frequency slot may change at each hop, given hardware constraints and capabilities (e.g., a given node might not support the finest granularity). This means that the values of n and m may change at each hop. As long as a given downstream node allocates enough optical spectrum, m can be different along the path. This covers the issue where media matrices can have different slot width granularities. Such variations in the local value of m will appear in the allocated label that encodes the frequency slot as well as the in the FlowSpec that describes the flow.
Different operational modes can be considered. For Routing and Spectrum Assignment (RSA) with explicit label control, and for Routing and Distributed Spectrum Assignment (R+DSA), the GMPLS signaling procedures are similar to those described in section 4.1.3 of [RFC6163] for Routing and Wavelength Assignment (RWA) and for Routing and Distributed Wavelength Assignment (R+DWA). The main difference is that the label set specifies the available nominal central frequencies that meet the slot width requirements of the LSP. The intermediate nodes use the control plane to collect the acceptable central frequencies that meet the slot width requirement hop by hop. The tail-end node also needs to know the slot width of an LSP to assign the proper frequency resource. Except for identifying the resource (i.e., fixed wavelength for WSON, and frequency resource for flexible grids), the other signaling requirements (e.g., unidirectional or bidirectional, with or without converters) are the same as for WSON as described in section 6.1 of [RFC6163].
Regarding how a GMPLS control plane can assign n and m hop-by-hop along the path of an LSP, different cases can apply:
The selection of a path that ensures n and m continuity can be delegated to a dedicated entity such as a Path Computation Element (PCE). Any constraint (including frequency slot and width granularities) can be taken into account during path computation. Alternatively, A PCE can compute a path leaving the actual frequency slot assignment to be done, for example, with a distributed (signaling) procedure:
For Figure 15 the effective slot is made valid by ensuring that the minimum m is greater than the requested m. The effective slot (intersection) is the lowest m (bottleneck).
For Figure 16 the effective slot is made valid by ensuring that it is valid at each hop in the upstream direction. The intersection needs to be computed because invalid slots could result otherwise.
C B A
|Path(m_req) | ^ |
|---------> | # |
| | # ^
-^--------------^----------------#----------------#--
Effective # # # #
FS n, m # . . . . . . .#. . . . . . . . # . . . . . . . .# <-fixed
# # # # n
-v--------------v----------------#----------------#---
| | # v
| | # Resv |
| | v <------ |
| | |FlowSpec(n, m_a)|
| | <--------| |
| | FlowSpec (n, |
<--------| min(m_a, m_b))
FlowSpec (n, |
min(m_a, m_b, m_c))
m_a, m_b, m_c: Selected frequency slot widths
Figure 15: Distributed Allocation with Different m and Same n
C B A
|Path(m_req) ^ | |
|---------> # | |
| # ^ ^
-^-------------#----------------#-----------------#--------
Effective # # # #
FS n, m # # # #
# # # #
-v-------------v----------------#-----------------#--------
| | # v
| | # Resv |
| | v <------ |
| | |FlowSpec(n_a, m_a)
| | <--------| |
| | FlowSpec (FSb [intersect] FSa)
<--------|
FlowSpec ([intersect] FSa,FSb,FSc)
n_a: Selected nominal central frequencyfr by node A
m_a: Selected frequency slot widths by node A
FSa, FSb, FSc: Frequency slot at each hop A, B, C
Figure 16: Distributed Allocation with Different m and Different n
Note, when a media channel is bound to one OTSi (i.e., is a network media channel), the effective FS must be the one of the OTSi. The media channel setup by the LSP may contain the effective FS of the network media channel effective FS. This is an endpoint property: the egress and ingress have to constrain the Effective FS to be the OTSi Effective FS.
There are potential interworking problems between fixed-grid DWDM and flexi-grid DWDM nodes. Additionally, even two flexi-grid nodes may have different grid properties, leading to link property conflict with resulting limited interworking.
Devices or applications that make use of the flexi-grid might not be able to support every possible slot width. In other words, different applications may be defined where each supports a different grid granularity. In this case the link between two optical nodes with different grid granularities must be configured to align with the larger of both granularities. Furthermore, different nodes may have different slot-width tuning ranges.
In summary, in a DWDM Link between two nodes, at least the following properties need to be negotiated:
In WSON, if there is no (available) wavelength converter in an optical network, an LSP is subject to the "wavelength continuity constraint" (see section 4 of [RFC6163]). Similarly in flexi-grid, if the capability to shift or convert an allocated frequency slot is absent, the LSP is subject to the "Spectrum Continuity Constraint".
Because of the limited availability of wavelength/spectrum converters (in what is called a "sparse translucent optical network") the wavelength/spectrum continuity constraint always has to be considered. When available, information regarding spectrum conversion capabilities at the optical nodes may be used by RSA mechanisms.
The RSA process determines a route and frequency slot for an LSP. Hence, when a route is computed the spectrum assignment process (SA) determines the central frequency and slot width based on the slot width and available central frequencies information of the transmitter and receiver, and utilizing the available frequency ranges information and available slot width ranges of the links that the route traverses.
Similar to RWA for fixed grids [RFC6163], different ways of performing RSA in conjunction with the control plane can be considered. The approaches included in this document are provided for reference purposes only: other possible options could also be deployed.
Note that all of these models allow the concept of a composite media channel supported by a single control plane LSP or by a set of associated LSPs.
In this case, a computation entity performs both routing and frequency slot assignment. The computation entity needs access to detailed network information, e.g., the connectivity topology of the nodes and links, the available frequency ranges on each link, the node capabilities, etc.
The computation entity could reside on a dedicated PCE server, in the provisioning application that requests the service, or on the ingress node.
In this case, routing computation and frequency slot assignment are performed by different entities. The first entity computes the routes and provides them to the second entity. The second entity assigns the frequency slot.
The first entity needs the connectivity topology to compute the proper routes. The second entity needs information about the available frequency ranges of the links and the capabilities of the nodes in order to assign the spectrum.
In this case an entity computes the route, but the frequency slot assignment is performed hop-by-hop in a distributed way along the route. The available central frequencies which meet the spectrum continuity constraint need to be collected hop-by-hop along the route. This procedure can be implemented by the GMPLS signaling protocol.
In the case of the combined RSA architecture, the computation entity needs the detailed network information, i.e., connectivity topology, node capabilities, and available frequency ranges of the links. Route computation is performed based on the connectivity topology and node capabilities, while spectrum assignment is performed based on the available frequency ranges of the links. The computation entity may get the detailed network information via the GMPLS routing protocol.
For WSON, the connectivity topology and node capabilities can be advertised by the GMPLS routing protocol (refer to section 6.2 of [RFC6163]. Except for wavelength-specific availability information, the information for flexi-grid is the same as for WSON and can equally be distributed by the GMPLS routing protocol.
This section analyses the necessary changes on link information brought by flexible grids.
In the case of flexible grids, channel central frequencies span from 193.1 THz towards both ends of the C band spectrum with 6.25 GHz granularity. Different LSPs could make use of different slot widths on the same link. Hence, the available frequency ranges need to be advertised.
The available slot width ranges need to be advertised in combination with the available frequency ranges, in order that the computing entity can verify whether an LSP with a given slot width can be set up or not. This is constrained by the available slot width ranges of the media matrix. Depending on the availability of the slot width ranges, it is possible to allocate more spectrum than strictly needed by the LSP.
The total available spectrum on a fiber can be described as a resource that can be partitioned. For example, a part of the spectrum could be assigned to a third party to manage, or parts of the spectrum could be assigned by the operator for different classes of traffic. This partitioning creates the impression that spectrum is a hierarchy in view of Management and Control Plane: each partition could be itself be partitioned. However, the hierarchy is created purely within a management system: it defines a hierarchy of access or management rights, but there is no corresponding resource hierarchy within the fiber.
The end of fiber is a link end and presents a fiber port which represents all of spectrum available on the fiber. Each spectrum allocation appears as Link Channel Port (i.e., frequency slot port) within fiber. Thus, while there is a hierarchy of ownership (the Link Channel Port and corresponding LSP are located on a fiber and so associated with a fiber port) there is no continued nesting hierarchy of frequency slots within larger frequency slots. In its way, this mirrors the fixed grid behavior where a wavelength is associated with a port/fiber, but cannot be subdivided even though it is a partition of the total spectrum available on the fiber.
This section defines an information model to describe the data that represents the capabilities and resources available in an flexi-grid network. It is not a data model and is not intended to limit any protocol solution such as an encoding for an IGP. For example, information required for routing/path selection may be the set of available nominal central frequencies from which a frequency slot of the required width can be allocated. A convenient encoding for this information is for further study in an IGP encoding document.
Fixed DWDM grids can also be described via suitable choices of slots in a flexible DWDM grid. However, devices or applications that make use of the flexible grid may not be capable of supporting every possible slot width or central frequency position. Thus, the information model needs to enable:
The information model is represented using Routing Backus-Naur Format (RBNF) as defined in [RFC5511].
<Available Spectrum> ::=
<Available Frequency Range-List>
<Available NCFs>
<Available Slot Widths>
where
<Available Frequency Range-List> ::=
<Available Frequency Range> [<Available Frequency Range-List>]
<Available Frequency Range> ::=
( <Start NCF> <End NCF> ) |
<FS defined by (n, m) containing contiguous available NCFs>
and
<Available NCFs> ::=
<Available NCF Granularity> [<Offset>]
-- Subset of supported n values given by p x n + q
-- where p is a positive integer
-- and q (offset) belongs to 0,..,p-1.
and
<Available Slot Widths> ::=
<Available Slot Width Granularity>
<Min Slot Width>
-- given by j x 12.5GHz, with j a positive integer
<Max Slot Width>
-- given by k x 12.5GHz, with k a positive integer (k >= j)
Figure 17: Routing Information Model
The control of a flexi-grid networks places additional requirements on the GMPLS protocols. This section summarizes those requirements for signaling and routing.
The control plane SHALL be able to support Media Channels, characterized by a single frequency slot. The representation of the Media Channel in the GMPLS control plane is the so-called flexi-grid LSP. Since network media channels are media channels, an LSP may also be the control plane representation of a network media channel. Consequently, the control plane will also be able to support Network Media Channels.
The signaling procedure SHALL be able to configure the nominal central frequency (n) of a flexi-grid LSP.
The signaling procedure SHALL allow a flexible range of values for the frequency slot width (m) parameter. Specifically, the control plane SHALL allow setting up a media channel with frequency slot width (m) ranging from a minimum of m=1 (12.5GHz) to a maximum of the entire C-band (the wavelength range 1530 nm to 1565 nm, which corresponds to the amplification range of erbium doped fiber amplifiers) with a slot width granularity of 12.5GHz.
The signaling procedure SHALL be able to configure the minimum width (m) of a flexi-grid LSP. In addition, the signaling procedure SHALL be able to configure local frequency slots.
The control plane architecture SHOULD allow for the support of L-band (the wavelength range 1565 nm to 1625 nm) and S-band (the wavelength range 1460 nm to 1530 nm).
The signalling process SHALL be able to collect the local frequency slot assigned at each link along the path.
The signaling procedures SHALL support all of the RSA architectural models (R&SA, R+SA, and R+DSA) within a single set of protocol objects although some objects may only be applicable within one of the models.
The routing protocol will support all functions as described in [RFC4202] and extend them to a flexi-grid data plane.
The routing protocol SHALL distribute sufficient information to compute paths to enable the signaling procedure to establish LSPs as described in the previous sections. This includes, at a minimum the data described by the Information Model in Figure 17.
The routing protocol SHALL update its advertisements of available resources and capabilities as the usage of resources in the network varies with the establishment or tear-down of LSPs. These updates SHOULD be amenable to damping and thresholds as in other traffic engineering routing advertisements.
The routing protocol SHALL support all of the RSA architectural models (R&SA, R+SA, and R+DSA) without any configuration or change of behavior. Thus, the routing protocols SHALL be agnostic to the computation and signaling model that is in use.
The signaling procedures SHALL allow resizing (grow or shrink) the frequency slot width of a media channel/network media channel. The resizing MAY imply resizing the local frequency slots along the path of the flexi-grid LSP.
The routing protocol SHALL update its advertisements of available resources and capabilities as the usage of resources in the network varies with the resizing of LSP. These updates SHOULD be amenable to damping and thresholds as in other traffic engineering routing advertisements.
A set of media channels can be used to transport signals that have a logical association between them. The control plane architecture SHOULD allow multiple media channels to be logically associated. The control plane SHOULD allow the co-routing of a set of media channels that are logically associated.
As described in Section 3.2.5 and Section 4.3, a media channel may be composed of multiple network media channels.
The signaling procedures SHOULD include support for signaling a single control plane LSP that includes information about multiple network media channels that will comprise the single compound media channel.
The signaling procedures SHOULD include a mechanism to associate separately signaled control plane LSPs so that the end points may correlate them into a single compound media channel.
The signaling procedures MAY include a mechanism to dynamically vary the composition of a composite media channel by allowing network media channels to be added to or removed from the whole.
The routing protocols MUST provide sufficient information for the computation of paths and slots for composite media channels using any of the three RSA architectural models (R&SA, R+SA, and R+DSA).
The control plane MAY include support for neighbor discovery such that an flexi-grid network can be constructed in a "plug-and-play" manner. Note, however, that in common operational practice validation processes are used rather than automatic discovery.
The control plane SHOULD allow the nodes at opposite ends of a link to correlate the properties that they will apply to the link. Such correlation SHOULD include at least the identities of the node and the identities they apply to the link. Other properties such as the link characteristics described for the routing information model in Figure 17 SHOULD also be correlated.
Such neighbor discovery and link property correlation, if provided, MUST be able to operate in both an out-of-band and an out-of-fiber control channel.
This framework document makes no requests for IANA action.
The control plane and data plane aspects of a flexi-grid system are fundamentally the same as a fixed grid system and there is no substantial reason to expect the security considerations to be any different.
A good overview of the security considerations for a GMPLS-based control plane can be found in [RFC5920].
[RFC6163] includes a section describing security considerations for WSON, and it is reasonable to infer that these considerations apply and may be exacerbated in a flexi-grid SSON system. In particular, the detailed and granular information describing a flexi- grid network and the capabilities of nodes in that network could put stress on the routing protocol or the out-of-band control channel used by the protocol. An attacker might be able to cause small variations in the use of the network or the available resources (perhaps by modifying the environment of a fiber) and so trigger the routing protocol to make new flooding announcements. This situation is explicitly mitigated in the requirements for the routing protocol extensions where it is noted that the protocol must include damping and configurable thresholds as already exist in the core GMPLS routing protocols.
GMPLS systems already contain a number of management tools.
The authors would like to thank Pete Anslow for his insights and clarifications, and to Matt Hartley and Jonas Mårtensson for their reviews.
This work was supported in part by the FP-7 IDEALIST project under grant agreement number 317999.