| Network Working Group | O. Gonzalez de Dios, Ed. |
| Internet-Draft | Telefónica I+D |
| Intended status: Standards Track | R. Casellas, Ed. |
| Expires: January 05, 2013 | CTTC |
| F. Zhang | |
| Huawei | |
| X. Fu | |
| ZTE | |
| D. Ceccarelli | |
| Ericsson | |
| I. Hussain | |
| Infinera | |
| July 06, 2012 |
Framework for GMPLS based control of Flexi-grid DWDM networks
draft-ogrcetal-ccamp-flexi-grid-fwk-00
This document defines a framework and the associated control plane requirements for the GMPLS based control of flexi-grid DWDM networks. To allow efficient allocation of optical spectral bandwidth for high bit-rate systems, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is extending the standard [G.694.1] to include the concept of flexible grid: a new DWDM grid has been developed within the ITU-T Study Group 15, by defining a set of nominal central frequencies, smaller channel spacings and the concept of "frequency slot". In such environment, a data plane connection is switched based on the allocated, variable-width optical spectrum frequency slot.
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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].
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 spacings 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, selected from the set of reference frequencies, 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 100 or 50 GHz, a flexible grid network can select its data channels with with a more flexible choice of slot widths, allocating as much optical spectrum as required, and allowing higher bitrates (e.g., 100G or 400G or higher).
From a networking perspective, a flexible grid network is assumed to be a layered network [G.872][G.805], extending the OTN architecture and interfaces [G.709], in which the flexi-grid layer (also referred to as the media layer) is the server layer and the OCh Layer (also referred to as the 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, the media channel itself can be dimensioned to contain one or more Optical Channels.
As described in [RFC3945], GMPLS extends MPLS from supporting only Packet Switching Capable (PSC) interfaces and switching to also support four new classes of interfaces and switching that include Lambda Switch Capable (LSC).
A Wavelength Switched Optical Network (WSON), addressed in [RFC6163], is a term commonly used to refer to the application/deployment of a Generalized Multi-Protocol Label Switching (GMPLS)-based control plane for the control (provisioning/recovery, etc) of a fixed grid WDM network. [editors' note: we need to think of the relationship of WSON and OCh switching. Are they equivalent? WSON includes regeneration, OCh does not? decoupling of lambda/OCh/OCC]
This document defines the framework for a GMPLS-based control of flexi-grid enabled 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. [Editor's note: a point was raised during the meeting that WSON has not made separation between Och and Lambda (spectrum and signal are bundled). This needs to be confirmed.] A direct consequence of this "separation of concerns" is that, although not in scope of the present document, single-carrier / multi-carrier and related modulation formats, etc. could be supported. [Editor's note: the concept of frequency slot channel supporting multiple OCHs is defined in an ITU contribution. It is not a standard document yet.]
[Editors' note: this document will track changes and evolutions of [G.694.1] [G.872] documents until their final publication. This document is not expected to become RFC until then. Likewise, as agreed during IETF83, the consideration of the concepts of Super-channel (a collection of one or more frequency slots to be treated as unified entity for management and control plane) and consequently Contiguous Spectrum Super-channel (a super-channel with a single frequency slot) and Split-Spectrum super-channel (a super-channel with multiple frequency slots) is postponed until the ITU-T data plane includes such physical layer entities, e.g., an ITU-T contribution exists]
FS: Frequency Slot
FSCh: Frequency Slot Channel
NCF: Nominal Central Frequency
OCG: Optical Carrier Group
OCh: Optical Channel
OCC: Optical Channel Carrier
OTUk: Optical channel Transport Unit level k
ODUk: Optical channel Data Unit Level k
ODUj: Optical channel Data Unit Level j
SWG: Slot Width Granularity
The following is a list of terms (see [G.694.1] and [G.872]) reproduced here for completeness. [Editors' note: regarding wavebands, we agreed NOT to use the term in flexigrid. The term has been used inconsistently in fixed-grid networks and overlaps with the definition of frequency slot. If need be, a question will be sent to ITU-T asking for clarification regarding wavebands.]
[Editors' note: *important* these terms are not yet final and they may change / be replaced or obsoleted at any time.]
-5 -4 -3 -2 -1 0 1 2 3 4 5 <- values of n
...+--+--+--+--+--+--+--+--+--+--+-
^
193.1 THz <- anchor frequency
Frequency Slot 1 Frequency Slot 2
------------- -------------------
| | | |
-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
..--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--...
------------- -------------------
^ ^
Central F = 193.1THz Central F = 193.14375 THz
Slot width = 25 GHz Slot width = 37.5 GHz
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--+--+--+--+--+--+--+--+--+--+--+--...
=============================================== Common
Common Frequency Slot (valid?, CF?)
----------
| |
-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
..--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--...
Frequency Slot
-----------------------------------+
| Optical Optical |
| Channel Channel |
| Signal Signal |
| +-----+ +-----------+ |
| | | | | |
| | o | | o | |
-4 -3 -2 -1 0 1 2 3 4 5 6 7 8
... +--+--+--+--+--X--+--+--+--+--+--+-+--...
^
+-- Frequency Slot
Central Frequency
o - signal central frequency
]
The following terms are defined in the scope of a GMPLS control plane. [Editors' note: the following ones were *not* agreed during IETF83 but are put here to be discussed.]
Similar to fixed grid networks, a flexible grid network is also constructed from subsystems that include Wavelength Division Multiplexing (WDM) links, tunable transmitters and receivers, Reconfigurable Optical Add/Drop Multiplexers (ROADMs), wavelength converters, and electro-optical network elements, all of them with flexible grid characteristics.
As stated in [G.694.1] the flexible DWDM grid defined in Clause 7 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 may 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).
As per [G.872] [Editor's note/CM: we need to better distinguish between G.872 and contributions, it would help to see what is agreed and what is still open, the list below contains items as per MB/XF slides]:
Optical transmitters/receivers may have different restrictions on the following properties:
Tributary Side: E5 I5 E6 I6
O | O |
| | | |
| O | O
+-----------------------+
|+-----+ +-----+|
Line side-1 --->||Split| |WSS-2||---> Line side-2
Input (I1) |+-----+ +-----+| Output (E2)
Line side-1 <---||WSS-1| |Split||<--- Line side-2
Output (E1) |+-----+ +-----+| Input (I2)
| ROADM |
|+-----+ +-----+|
Line side-3 --->||Split| |WSS-4||---> Line side-4
Input (I3) |+-----+ +-----+| Output (E4)
Line side-3 <---||WSS-3| |Split||<--- Line side-4
Output (E3) |+-----+ +-----+| Input (I4)
+-----------------------+
| O | O
| | | |
O | O |
Tributary Side: E7 I7 E8 I8
[Editor's note: different ROADM configuration such as C/CD/CDC will be added later.]
A Frequency slot matrix may have switching restrictions, for example , when it is realized using flexi-grid enabled ROADMs. A key feature of ROADMs is their highly asymmetric switching capability which is described in [RFC6163] in detail. The ports on ROADM include line side ports which are connected to DWDM links and tributary side input/output ports which can be connected to transmitters/receivers. The capability of ports on ROADM, which are characterized as follows:
AP Trail (OCh) AP
O- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - O
TCP Link Connection (OCh) TCP
o------------o-------------------------------------------o---------o
Subnetwork Subnetwork
Connection Connection
| Media Path |
AP O- - - - - - - - - - - - - - - - - - - - - -O AP
| |
| Link (Fiber) |
TCP o---------------o-----------o---------------o
Subnet. channel Link channel Subnet. chan
(freq slot) (freq slot) (freq slot)
[Editors' note: OTN hierarchy is not fully covered. It is important to understand, where the FSC sits in the OTN hierarchy. This is also important from control plane perspective as this layer becomes the connection end points of optical layer service]. OCh / flexi-grid layered model.
[Editors' note: we are replicating the figure here for reference, until the ITU-T document is official.
The media path is a piece of spectrum that has been allocated to a path between two ports of a media device. [Editors'note/CM/IH: it seems the media path is equivalent to the FSC (freq.slot channel is between the AP?. Why use a new term media path?]
+--------------+ +--------------+
| Signal (OCh) | TE | Signal (OCh) | Virtual TE
| | link | | link
| Matrix |o- - - - - - - - - - - - - o| Matrix |o- - - - - -
| | | |
+--------------+ +--------------+
| +---------+ |
| |Freq Slot| |
|o------| Matrix |---------o|
| |
+---------+
[Note: the frequency slot matrix connection may interconnect one or more frequency slot channels which in turn may carry one or more Och signals.]
[Editor's note: The considered topology view is a layered network, in which the media layer corresponds to the server layer (flexigrid) and the signal layer corresponds to the client layer (Och). This data plane modeling considers the flexigrid and the OCh as separate layers, especially considering both the single and multi-channel frequency slots. However, this has implications on the interop/interworking with WSON and OCh switching. We need to manage a MRN for OCh and stitching for WSON? In other words, a key part of the fwk is to define how can we have MRN/MLN hierarchical relationship with Och/FS and yet stitching 1:1 between WSON and SSON? In this line: how does OCh switching and WSON relate, actually?]
[Editor's note: formal requirements such as noted in the comments will be added in a later version of the document].
Hierarchy spectrum management decouples media and signal, but from the point of view of the control plane, such separation of concerns implies the management of a MRN/MLN network. So Control Plane needs to differentiate signal LSP and media LSP. It should also need to support Hierarchy-LSP [RFC4206] The central frequency of each hop should be same along end-to-end media or signal LSP because of Spectrum Continuity Constraint. Otherwise some nodes need to convert the central frequency along media or signal LSP.
[Editors' note: text from draft-li-ccamp-grid-property-lmp-01]
During the practical deployment procedure, fixed-grid optical nodes will be gradually replaced by flexible nodes. This will lead to an interworking problem between fixed-grid DWDM and flexible-grid DWDM nodes. Additionally, even two flexible-grid optical nodes may have different grid properties, leading to link property conflict.
Devices or applications that make use of the flexible-grid may not be able to support every possible slot width. In other words, applications may be defined where different grid granularity can be supported. Taking node F as an example, an application could be defined where the nominal central frequency granularity is 12.5 GHz requiring slot widths being multiple of 25 GHz. Therefore the link between two optical nodes with different grid granularity must be configured to align with the larger of both granularities. Besides, different nodes may have different slot width tuning ranges.
In summary, in a DWDM Link between two nodes, at least the following properties should be negotiated:
Much like in WSON, in which if there is no (available) wavelength converters in an optical network, an LSP is subject to the ''wavelength continuity constraint'' (see section 4 of [RFC6163]), if the capability of shifting or converting an allocated frequency slot, the LSP is subject to the Optical ''Spectrum Continuity Constraint''.
Because of the limited availability of wavelength/spectrum converters (sparse translucent optical network) the wavelength/spectrum continuity constraint should always 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 a LSP. Hence, when a route is computed the spectrum assignment process (SA) should determine the central frequency and slot width based on the slot width and available central frequencies information of the transmitter and receiver, and the available frequency ranges information and available slot width ranges of the links that the route traverses.
Similar to RWA for fixed grids, 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.
In this case, a computation entity performs both routing and frequency slot assignment. The computation entity should have the detailed network information, e.g. connectivity topology constructed by nodes/links information, available frequency ranges on each link, node capabilities, etc.
The computation entity could reside either on a PCE or 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 should get the connectivity topology to compute the proper routes; the second entity should get the available frequency ranges of the links and nodes' capabilities information to assign the spectrum.
In this case, one 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 should be collected hop by hop along the route. This procedure can be implemented by the GMPLS signaling protocol.
In the case of combined RSA architecture, the computation entity needs to get 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; spectrum assignment is performed based on the available frequency ranges of the links. The computation entity may get the detailed network information by the GMPLS routing protocol. Compared with [RFC6163], except wavelength-specific availability information, the connectivity topology and node capabilities are the same as WSON, which can be advertised by GMPLS routing protocol (refer to section 6.2 of [RFC6163]. 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 should be advertised.
The available slot width ranges needs to be advertised, in combination with the Available frequency ranges, in order to verify whether a LSP with a given slot width can be set up or not; this is is constrained by the available slot width ranges of the flexi-grid enabled ROADMs (the flexi-grid Frequency slot matrix). Depending on the availability of the slot width ranges, it is possible to allocate more spectrum than strictly needed by the LSP.
The slot width of a LSP is determined by the transmitter and receiver that could be mapped to ADD/DROP interfaces in WSON. Moreover their central frequency could be fixed or tunable, hence, both the slot width of an ADD/DROP interface and the available central frequencies should be advertised.
[Editors' note: the part on the hierarchy of the optical spectrum could be confusing, we can discuss it]. The total available spectrum on a fiber could be described as a resource that can be divided by a media device into a set of Frequency Slots. In terms of managing spectrum, it is necessary to be able to speak about different granularities of managed spectrum. For example, a part of the spectrum could be assigned to a third party to manage. This need to partition creates the impression that spectrum is a hierarchy in view of Management and Control Plane. The hierarchy is created within a management system, and it is an access right hierarchy only. It is a management hierarchy without any actual resource hierarchy within 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.
Fixed DM 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. Following is the definition of information model, not intended to limit any IGP encoding implementation. 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 (may be as a frequency slot or sets of contiguous slices) is further study in IGP encoding document.
<Available Spectrum in Fiber for frequency slot> ::=
<Available Frequency Range-List>
<Available Central Frequency Granularity >
<Available Slot Width Granularity>
<Minimal Slot Width>
<Maximal Slot Width>
<Available Frequency Range-List> ::=
<Available Frequency Range >[< Available Frequency Range-List>]
<Available Frequency Range >::=
<Start Spectrum Position><End Spectrum Position> |
<Sets of contiguous slices>
<Available Central Frequency Granularity> ::= n × 6.25GHz,
where n is positive integer, such as 6.25GHz, 12.5GHz, 25GHz, 50GHz
or 100GHz
<Available Slot Width Granularity> ::= m × 12.5GHz,
where m is positive integer
<Minimal Slot Width> ::= j x 12.5GHz,
j is a positive integer
<Maximal Slot Width> ::= k x 12.5GHz,
k is a positive integer (k >= j)
[Editor's note: to be discussed]
Note on explicit label control
Compared with [RFC6163], except 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 WSON described in the section 6.1 of [RFC6163]. In the case of routing and distributed SA, GMPLS signaling can be used to allocate the frequency slot to a LSP.
For R+DSA, the GMPLS signaling procedure is similar to the one described in section 4.1.3 of [RFC6163] except that the label set should specify the available nominal central frequencies that meet the slot width requirement of the LSP.
[Editors' note: the signaling requirements need to be discussed. This is just preliminary text].
In order to allocate a proper frequency slot for a LSP, the signaling should specify its slot width requirement. The intermediate nodes can 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 a LSP to assign the proper frequency resource. Hence, the slot width requirement should be specified in the signaling message when a LSP is being set up. [Note: other methods may not need to collect availability]
The frequency slot can be determined by the central frequency (n value) and slot width (m value). Such parameters should be able to be specified by the signaling protocol.
Resizing existing LSP(s) without deletion: refers to increase or decrease of slot width value 'm' without changing the value of 'n'
[Editor's note: Restoration / Resizing a single LSP without deletion as well as timing constraints. As per ITU-T clarification on service affecting or non-service affecting (i.e., hitless) restoration, at present no hitless resizing protocol has been defined for OCh. Hitless resizing is defined for an ODU entity only.]
Arguments for LSC switching capability
[QW] A LSP for an optical signal which has a bandwidth of 50GHz passes through both a fixed grid network and a flexible grid network. We assume that no OEOs exist in the LSP, so both the fixed grid path and flexible grid path occupy 50GHz. From the perspective of data plane, there is no change of the signal and no multiplexing when the fixed grid path interconnects with flexible grid path. From this scenario we can conclude that both fixed grid network path and flexible grid network path belong to the same layer. No notion of hierarchy exists between them.
[QW] stitching LSP which is described in [RFC5150] can be applied in one layer. LSP hierarchy allows more than one LSP to be mapped to an H-LSP, but in case of S-LSP, at most one LSP may be associated with an S-LSP. This is similar to the scenario of interconnection between fixed grid LSP and flexible grid LSP. Similar to an H-LSP, an S-LSP could be managed and advertised, although it is not required, as a TE link, either in the same TE domain as it was provisioned or a different one. Path setup procedure of stitching LSP can be applied in the scenario of interconnection between fixed grid path and flexible grid path.
e2e LSP
+++++++++++++++++++++++++++++++++++> (LSP1-2)
LSP segment (flexi-LSP)
====================> (LSP-AB)
C --- E --- G
/|\ | / |\
/ | \ | / | \
R1 ---- A \ | \ | / | / B --- R2
\| \ |/ |/
D --- F --- H
fixed grid --A-- flexi-grid --B-- fixed grid
TBD
The authors would like to thank Pete Anslow for his insights and clarifications.