RFC 8363 PROPOSED STANDARD ROUTING · OPTICAL · OSPF
GMPLS OSPF-TE flexi-grid: GMPLS OSPF-TE Extensions in Support of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks
How OSPF-TE tells GMPLS routers which slices of an optical fiber's spectrum are still free.
RFC EDITOR
Zhang, X., Zheng, H., Casellas, R., Gonzalez de Dios, O., and D. Ceccarelli, "GMPLS OSPF-TE Extensions in Support of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks", RFC 8363, DOI 10.17487/RFC8363, May 2018, <https://www.rfc-editor.org/info/rfc8363>.
BIBTEX
@misc{rfc8363,
series = {Request for Comments},
number = 8363,
howpublished = {RFC 8363},
publisher = {RFC Editor},
doi = {10.17487/RFC8363},
url = {https://www.rfc-editor.org/info/rfc8363},
author = {X. Zhang and H. Zheng and R. Casellas and O. Gonzalez de Dios and D. Ceccarelli},
title = {{GMPLS OSPF-TE Extensions in Support of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks}},
pagetotal = 17,
year = 2018,
month = may,
} RFC text
Contents
Internet Engineering Task Force (IETF) X. Zhang
Request for Comments: 8363 H. Zheng
Category: Standards Track Huawei
ISSN: 2070-1721 R. Casellas
CTTC
O. Gonzalez de Dios
Telefonica
D. Ceccarelli
Ericsson
May 2018
GMPLS OSPF-TE Extensions in Support of Flexi-Grid
Dense Wavelength Division Multiplexing (DWDM) Networks
Abstract
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 channel spacings, a set of
nominal central frequencies, and the concept of the "frequency slot".
Corresponding techniques for data-plane connections are known as
"flexi-grid".
Based on the characteristics of flexi-grid defined in G.694.1 and in
RFCs 7698 and 7699, this document describes the Open Shortest Path
First - Traffic Engineering (OSPF-TE) extensions in support of GMPLS
control of networks that include devices that use the new flexible
optical grid.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8363.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Conventions Used in This Document . . . . . . . . . . . . 4
3. Requirements for Flexi-Grid Routing . . . . . . . . . . . . . 4
3.1. Available Frequency Ranges . . . . . . . . . . . . . . . 4
3.2. Application Compliance Considerations . . . . . . . . . . 5
3.3. Comparison with Fixed-Grid DWDM Links . . . . . . . . . . 6
4. Extensions . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Interface Switching Capability Descriptor (ISCD)
Extensions for Flexi-Grid . . . . . . . . . . . . . . . . 7
4.1.1. Switching Capability Specific Information (SCSI) . . 8
4.1.2. An SCSI Example . . . . . . . . . . . . . . . . . . . 10
4.2. Extensions to the Port Label Restrictions Field . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
5.1. New ISCD Switching Type . . . . . . . . . . . . . . . . . 13
5.2. New SCSI Type . . . . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . 15
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 16
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
[G.694.1] defines the Dense Wavelength Division Multiplexing (DWDM)
frequency grids for Wavelength Division Multiplexing (WDM)
applications. A frequency grid is a reference set of frequencies
used to denote allowed nominal central frequencies that may be used
for defining applications. The channel spacing is the frequency
spacing between two allowed nominal central frequencies. All of the
wavelengths on a fiber should use different central frequencies and
occupy a fixed bandwidth of frequency.
Fixed-grid channel spacing ranges from one of 12.5 GHz, 25 GHz, 50
GHz, or 100 GHz to integer multiples of 100 GHz. But [G.694.1] also
defines a "flexible grid", also known as "flexi-grid". The terms
"frequency slot" (i.e., the frequency range allocated to a specific
channel and unavailable to other channels within a flexible grid) and
"slot width" (i.e., the full width of a frequency slot in a flexible
grid) are used to define a flexible grid.
[RFC7698] defines a framework and the associated control-plane
requirements for the GMPLS-based control of flexi-grid DWDM networks.
[RFC6163] provides a framework for GMPLS and Path Computation Element
(PCE) control of Wavelength Switched Optical Networks (WSONs).
[RFC7688] defines the requirements and OSPF-TE extensions in support
of GMPLS control of a WSON.
[RFC7792] describes requirements and protocol extensions for
signaling to set up Label Switched Paths (LSPs) in networks that
support the flexi-grid. This document complements [RFC7792] by
describing the requirement and extensions for OSPF-TE routing in a
flexi-grid network.
This document complements the efforts to provide extensions to the
OSPF-TE protocol so as to support GMPLS control of flexi-grid
networks.
2. Terminology
For terminology related to flexi-grid, please consult [RFC7698] and
[G.694.1].
2.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Requirements for Flexi-Grid Routing
The architecture for establishing LSPs in a Spectrum Switched Optical
Network (SSON) is described in [RFC7698].
A flexi-grid LSP occupies one or multiple specific frequency slots.
The process of computing a route and the allocation of a frequency
slot is referred to as "RSA" (Routing and Spectrum Assignment).
[RFC7698] describes three types of architectural approaches to RSA:
combined RSA, separated RSA, and routing and distributed SA. The
first two approaches could be called "centralized SA" because the
spectrum (frequency slot) assignment is performed by a single entity
before the signaling procedure.
In the case of centralized SA, the assigned frequency slot is
specified in the RSVP-TE Path message during the signaling process.
In the case of routing and distributed SA, only the requested slot
width of the flexi-grid LSP is specified in the Path message,
allowing the involved network elements to select the frequency slot
to be used.
If the capability of switching or converting the whole optical
spectrum allocated to an optical spectrum LSP is not available at
nodes along the path of the LSP, the LSP is subject to the Optical
"Spectrum Continuity Constraint", as described in [RFC7698].
The remainder of this section states the additional extensions on the
routing protocols in a flexi-grid network.
3.1. Available Frequency Ranges
In the case of flexi-grids, the central frequency steps from 193.1
THz with 6.25 GHz granularity. The calculation method of central
frequency and the frequency slot width of a frequency slot are
defined in [G.694.1], i.e., by using nominal central frequency n and
the slot width m.
On a DWDM link, the allocated or in-use frequency slots do not
overlap with each other. However, the border frequencies of two
frequency slots may be the same frequency, i.e., the upper bound of a
frequency slot and the lower bound of the directly adjacent frequency
slot are the same.
Frequency Slot 1 Frequency Slot 2
+-----------+-----------------------+
| | |
-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
...+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--...
------------ ------------------------
^ ^
Central F = 193.1 THz Central F = 193.1375 THz
Slot width = 25 GHz Slot width = 50 GHz
Figure 1: Two Frequency Slots on a Link
Figure 1 shows two adjacent frequency slots on a link. The highest
frequency of frequency slot 1 denoted by n=2 is the lowest frequency
of slot 2. In this example, it means that the frequency range from
n=-2 to n=10 is unavailable to other flexi-grid LSPs. Available
central frequencies are advertised for m=1, which means that for an
available central frequency n, the frequency slot from central
frequency n-1 to central frequency n+1 is available.
Hence, in order to clearly show which frequency slots are available
and can be used for LSP establishment and which frequency slots are
unavailable, the availability of frequency slots is advertised by the
routing protocol for the flexi-grid DWDM links. A set of non-
overlapping available frequency ranges is disseminated in order to
allow efficient resource management of flexi-grid DWDM links and RSA
procedures, which are described in Section 4.8 of [RFC7698].
3.2. Application Compliance Considerations
As described in [G.694.1], devices or applications that make use of
the flexi-grid may not be capable of supporting every possible slot
width or position (i.e., central frequency). In other words,
applications or implementations 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 the same application only requires slot
widths as a multiple of 25 GHz (by only requiring values of m that
are even).
Hence, in order to support all possible applications and
implementations, the following information SHOULD be advertised for a
flexi-grid DWDM link:
o Channel Spacing (C.S.): as defined in [RFC7699] for flexi-grid, is
set to 5 to denote 6.25 GHz.
o Central frequency granularity: a multiplier of C.S.
o Slot width granularity: a multiplier of 2*C.S.
o Slot width range: two multipliers of the slot width granularity,
each indicating the minimal and maximal slot width supported by a
port, respectively.
The combination of slot width range and slot width granularity can be
used to determine the slot widths set supported by a port.
3.3. Comparison with Fixed-Grid DWDM Links
In the case of fixed-grid DWDM links, each wavelength has a
predefined central frequency. Each wavelength maps to a predefined
central frequency, and the usable frequency range is implicit by the
channel spacing. All the wavelengths on a DWDM link can be
identified with an identifier that mainly conveys its central
frequency as the label defined in [RFC6205]; the status of the
wavelengths (available or not) can be advertised through a routing
protocol.
Figure 2 shows a link that supports a fixed-grid with 50 GHz channel
spacing. The central frequencies of the wavelengths are predefined
by values of "n", and each wavelength occupies a fixed 50 GHz
frequency range as described in [G.694.1].
W(-2) | W(-1) | W(0) | W(1) | W(2) |
...---------+-----------+-----------+-----------+-----------+----...
| 50 GHz | 50 GHz | 50 GHz | 50 GHz |
n=-2 n=-1 n=0 n=1 n=2
...---+-----------+-----------+-----------+-----------+----------...
^
Central F = 193.1 THz
Figure 2: A Link Supports Fixed Wavelengths
with 50 GHz Channel Spacing
Unlike the fixed-grid DWDM links, on a flexi-grid DWDM link, the slot
width of the frequency slot is flexible, as described in Section 3.1.
That is, the value of m in the following formula from [G.694.1] is
uncertain before a frequency slot is actually allocated for a flexi-
grid LSP.
Slot Width (in GHz) = 12.5GHz * m
For this reason, the available frequency slots (or ranges) are
advertised for a flexi-grid DWDM link instead of the specific
"wavelength" points that are sufficient for a fixed-grid link.
Moreover, this advertisement is represented by the combination of
central frequency granularity and slot width granularity.
4. Extensions
The network-connectivity topology constructed by the links and/or
nodes and node capabilities are the same as for WSON, as described in
[RFC7698], and they can be advertised by the GMPLS routing protocols
using Opaque Link State Advertisements (LSAs) [RFC3630] in the case
of OSPF-TE [RFC4203] (refer to Section 6.2 of [RFC6163]). In the
flexi-grid case, the available frequency ranges, instead of the
specific "wavelengths", are advertised for the link. This section
defines the GMPLS OSPF-TE extensions in support of advertising the
available frequency ranges for flexi-grid DWDM links.
4.1. Interface Switching Capability Descriptor (ISCD) Extensions for
Flexi-Grid
This section defines a new value for the Switching Capability field
of the ISCD with a value of 152 and type name Flexi-Grid-LSC.
Value Name
----- --------------
152 Flexi-Grid-LSC
Switching Capability and Encoding values MUST be used as follows:
Switching Capability = Flexi-Grid-LSC
Encoding Type = lambda (as defined in [RFC3471])
When the Switching Capability and Encoding fields are set to values
as stated above, the ISCD is interpreted as in [RFC4203] with the
optional inclusion of one or more Switching Capability Specific
Information (SCSI) sub-TLVs.
As the "Max LSP Bandwidth at priority x" (x from 0 to 7) fields in
the generic part of the ISCD [RFC4203] are not meaningful for flexi-
grid DWDM links, the values of these fields MUST be set to zero and
MUST be ignored. The SCSI as defined below provides the
corresponding information for flexi-grid DWDM links.
4.1.1. Switching Capability Specific Information (SCSI)
[RFC8258] defines a Generalized SCSI for the ISCD. This document
defines the Frequency Availability Bitmap as a new type of the
Generalized SCSI TLV. The technology-specific part of the flexi-grid
ISCD includes the available frequency-spectrum resource as well as
the information regarding max slot widths per priority. The format
of this flexi-grid SCSI, the Frequency Availability Bitmap sub-TLV,
is depicted in the following figure:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 11 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Priority | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Max Slot Width at Priority k | Unreserved Padding ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| C.S. | Starting n | No. of Effective Bits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bitmap ... ~
~ ... | padding bits ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (16 bits): The type of this sub-TLV (11).
Length (16 bits): The length of the value field of this sub-TLV in
octets.
Priority (8 bits): A bitmap used to indicate which priorities are
being advertised. The bitmap is in ascending order, with the
leftmost bit representing priority level 0 (i.e., the highest) and
the rightmost bit representing priority level 7 (i.e., the lowest).
A bit is set (1) corresponding to each priority represented in the
sub-TLV and clear (0) for each priority not represented in the sub-
TLV. At least one priority level MUST be advertised. If only one
priority level is advertised, it MUST be at priority level 0.
Reserved: The Reserved field MUST be set to zero on transmission and
MUST be ignored on receipt.
Max Slot Width at Priority k (16 bits): This field indicates maximal
frequency slot width supported at a particular priority level, up to
8. This field is set to max frequency slot width supported in the
unit of 2*C.S., for a particular priority level. One field MUST be
present for each bit set in the Priority field, and each present
field is ordered to match the Priority field. Fields MUST be present
for priority levels that are indicated in the Priority field.
Unreserved Padding (16 bits): The Padding field is used to ensure the
32-bit alignment of Max Slot Width at Priority k. When k is an odd
number, the Unreserved Padding field MUST be included. When k is an
even number, the Unreserved Padding field MUST be omitted. This
field MUST be set to 0 and MUST be ignored on receipt.
C.S. (4 bits): As defined in [RFC7699]; it is currently set to 5.
Starting n (16 bits): As defined in [RFC7699]. This value denotes
the starting point of the nominal central frequency of the frequency
availability bitmap sub-TLV.
No. of Effective Bits (12 bits): Indicates the number of effective
bits in the Bitmap field.
Bitmap (variable): Indicates whether or not a basic frequency slot,
characterized by a nominal central frequency and a fixed m value of
1, is available for flexi-grid LSP setup. The first nominal central
frequency is the value of starting n; subsequent nominal central
frequencies are implied by the position in the bitmap. Note that
setting to 1 indicates that the corresponding central frequency is
available for a flexi-grid LSP with m=1 and setting to 0 indicates
the corresponding central frequency is unavailable. Note that a
centralized SA process will need to extend this to high values of m
by checking a sufficiently large number of consecutive basic
frequency slots that are available.
padding bits (variable): Padded after the Bitmap to make it a
multiple of four bytes, if necessary. Padding bits MUST be set to 0
and MUST be ignored on receipt.
An example is provided in Section 4.1.2.
4.1.2. An SCSI Example
Figure 3 shows an example of the available frequency spectrum
resource of a flexi-grid DWDM link.
-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
...+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--...
|--Available Frequency Range--|
Figure 3: Flexi-Grid DWDM Link Example
The symbol "+" represents the allowed nominal central frequency. The
symbol "--" represents a central frequency granularity of 6.25 GHz,
which is currently standardized in [G.694.1]. The number on the top
of the line represents the "n" in the frequency calculation formula
(193.1 + n * 0.00625). The nominal central frequency is 193.1 THz
when n equals zero.
In this example, it is assumed that the lowest nominal central
frequency supported is n=-9 and the highest is n=11. Note they
cannot be used as a nominal central frequency for setting up an LSP,
but merely as the way to express the supported frequency range.
Using the encoding defined in Section 4.1.1, the relevant fields to
express the frequency resource availability can be filled as below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 11 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Priority | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Max Slot Width at Priority k | Unreserved Padding ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | Starting n (-9) | No. of Effec. Bits(21)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|0|0|0|0|1|1|1|1|1|1|1|1|1|0|0|0|0| padding bits (0s) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the above example, the starting n is selected to be the lowest
nominal central frequency, i.e., -9. It is observed from the bitmap
that n=-1 to 7 can be used to set up LSPs. Note other starting n
values can be chosen to represent the bitmap; for example, the first
available nominal central frequency (a.k.a., the first available
basic frequency slot) can be chosen, and the SCSI will be expressed
as the following:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 11 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Priority | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Max Slot Width at Priority k | Unreserved Padding ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | Starting n (-1) | No. of Effec. Bits(9)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1|1|1|1|1|1|1|1| padding bits (0s) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This encoding denotes that, other than the advertised available
nominal central frequencies, the other nominal central frequencies
within the whole frequency range supported by the link are not
available for flexi-grid LSP setup.
If an LSP with slot width m equal to 1 is set up using this link, say
using n=-1, then the SCSI information is updated to be the following:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 11 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Priority | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Max Slot Width at Priority k | Unreserved Padding ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | Starting n (-1) | No. of Effec. Bits(9)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|1|1|1|1|1|1|1| padding bits (0s) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.2. Extensions to the Port Label Restrictions Field
As described in Section 3.2, a port that supports flexi-grid may
support only a restricted subset of the full flexible grid. The Port
Label Restrictions field is defined in [RFC7579]. It can be used to
describe the label restrictions on a port and is carried in the top-
level Link TLV as specified in [RFC7580]. A new restriction type,
the flexi-grid Restriction Type, is defined here to specify the
restrictions on a port to support flexi-grid.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MatrixID | RstType = 5 | Switching Cap | Encoding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| C.S. | C.F.G | S.W.G | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Min Slot Width | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MatrixID (8 bits): As defined in [RFC7579].
RstType (Restriction Type, 8 bits): Takes the value of 5 to indicate
the restrictions on a port to support flexi-grid.
Switching Cap (Switching Capability, 8 bits): As defined in
[RFC7579], MUST be consistent with the one specified in ISCD as
described in Section 4.1.
Encoding (8 bits): As defined in [RFC7579], MUST be consistent with
the one specified in ISCD as described in Section 4.1.
C.S. (4 bits): As defined in [RFC7699]. For flexi-grid, it is 5 to
denote 6.25 GHz.
C.F.G (Central Frequency Granularity, 8 bits): A positive integer.
Its value indicates the multiple of C.S., in terms of central
frequency granularity.
S.W.G (Slot Width Granularity, 8 bits): A positive integer. Its
value indicates the multiple of 2*C.S., in terms of slot width
granularity.
Min Slot Width (16 bits): A positive integer. Its value indicates
the multiple of 2*C.S. (in GHz), in terms of the supported minimal
slot width.
Reserved: The Reserved field MUST be set to zero on transmission and
SHOULD be ignored on receipt.
5. IANA Considerations
5.1. New ISCD Switching Type
IANA has made the following assignment in the "Switching Types" sub-
registry of the "Generalized Multi-Protocol Label Switching (GMPLS)
Signaling Parameters" registry located at
<https://www.iana.org/assignments/gmpls-sig-parameters>:
Value Name Reference
------- ---------------- ----------
152 Flexi-Grid-LSC RFC 8363
5.2. New SCSI Type
This document defines a new generalized SCSI sub-TLV that is carried
in the Interface Switching Capability Descriptors [RFC4203] when the
Switching Type is set to Flexi-Grid-LSC.
IANA has made the following assignment in the "Generalized SCSI
(Switching Capability Specific Information) TLV Types" sub-registry
[RFC8258] of the "Generalized Multi-Protocol Label Switching (GMPLS)
Signaling Parameters" registry located at
<https://www.iana.org/assignments/gmpls-sig-parameters>:
Value SCSI-TLV Switching Type Reference
----- ----------------------------- -------------- ---------
11 Frequency Availability Bitmap 152 RFC 8363
6. Security Considerations
This document extends [RFC4203] and [RFC7580] to carry flexi-grid-
specific information in OSPF Opaque LSAs. This document does not
introduce any further security issues other than those discussed in
[RFC3630] and [RFC4203]. To be more specific, the security
mechanisms described in [RFC2328], which apply to Opaque LSAs carried
in OSPF, still apply. An analysis of the OSPF security is provided
in [RFC6863] and applies to the extensions to OSPF in this document
as well.
7. References
7.1. Normative References
[G.694.1] International Telecommunication Union, "Spectral grids for
WDM applications: DWDM frequency grid", ITU-T
Recommendation G.694.1, February 2012,
<https://www.itu.int/rec/T-REC-G.694.1/en>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3471] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Functional Description",
RFC 3471, DOI 10.17487/RFC3471, January 2003,
<https://www.rfc-editor.org/info/rfc3471>.
[RFC4203] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
<https://www.rfc-editor.org/info/rfc4203>.
[RFC6205] Otani, T., Ed. and D. Li, Ed., "Generalized Labels for
Lambda-Switch-Capable (LSC) Label Switching Routers",
RFC 6205, DOI 10.17487/RFC6205, March 2011,
<https://www.rfc-editor.org/info/rfc6205>.
[RFC7579] Bernstein, G., Ed., Lee, Y., Ed., Li, D., Imajuku, W., and
J. Han, "General Network Element Constraint Encoding for
GMPLS-Controlled Networks", RFC 7579,
DOI 10.17487/RFC7579, June 2015,
<https://www.rfc-editor.org/info/rfc7579>.
[RFC7580] Zhang, F., Lee, Y., Han, J., Bernstein, G., and Y. Xu,
"OSPF-TE Extensions for General Network Element
Constraints", RFC 7580, DOI 10.17487/RFC7580, June 2015,
<https://www.rfc-editor.org/info/rfc7580>.
[RFC7699] Farrel, A., King, D., Li, Y., and F. Zhang, "Generalized
Labels for the Flexi-Grid in Lambda Switch Capable (LSC)
Label Switching Routers", RFC 7699, DOI 10.17487/RFC7699,
November 2015, <https://www.rfc-editor.org/info/rfc7699>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8258] Ceccarelli, D. and L. Berger, "Generalized SCSI: A Generic
Structure for Interface Switching Capability Descriptor
(ISCD) Switching Capability Specific Information (SCSI)",
RFC 8258, DOI 10.17487/RFC8258, October 2017,
<https://www.rfc-editor.org/info/rfc8258>.
7.2. Informative References
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.
[RFC6163] Lee, Y., Ed., Bernstein, G., Ed., and W. Imajuku,
"Framework for GMPLS and Path Computation Element (PCE)
Control of Wavelength Switched Optical Networks (WSONs)",
RFC 6163, DOI 10.17487/RFC6163, April 2011,
<https://www.rfc-editor.org/info/rfc6163>.
[RFC6863] Hartman, S. and D. Zhang, "Analysis of OSPF Security
According to the Keying and Authentication for Routing
Protocols (KARP) Design Guide", RFC 6863,
DOI 10.17487/RFC6863, March 2013,
<https://www.rfc-editor.org/info/rfc6863>.
[RFC7688] Lee, Y., Ed. and G. Bernstein, Ed., "GMPLS OSPF
Enhancement for Signal and Network Element Compatibility
for Wavelength Switched Optical Networks", RFC 7688,
DOI 10.17487/RFC7688, November 2015,
<https://www.rfc-editor.org/info/rfc7688>.
[RFC7698] Gonzalez de Dios, O., Ed., Casellas, R., Ed., Zhang, F.,
Fu, X., Ceccarelli, D., and I. Hussain, "Framework and
Requirements for GMPLS-Based Control of Flexi-Grid Dense
Wavelength Division Multiplexing (DWDM) Networks",
RFC 7698, DOI 10.17487/RFC7698, November 2015,
<https://www.rfc-editor.org/info/rfc7698>.
[RFC7792] Zhang, F., Zhang, X., Farrel, A., Gonzalez de Dios, O.,
and D. Ceccarelli, "RSVP-TE Signaling Extensions in
Support of Flexi-Grid Dense Wavelength Division
Multiplexing (DWDM) Networks", RFC 7792,
DOI 10.17487/RFC7792, March 2016,
<https://www.rfc-editor.org/info/rfc7792>.
Acknowledgments
This work was supported in part by the FP-7 IDEALIST project under
grant agreement number 317999.
This work was supported in part by NSFC Project 61201260.
Contributors
Adrian Farrel
Juniper Networks
Email: afarrel@juniper.net
Fatai Zhang
Huawei Technologies
Email: zhangfatai@huawei.com
Lei Wang
Beijing University of Posts and Telecommunications
Email: wang.lei@bupt.edu.cn
Guoying Zhang
China Academy of Information and Communication Technology
Email: zhangguoying@ritt.cn
Authors' Addresses
Xian Zhang
Huawei Technologies
Email: zhang.xian@huawei.com
Haomian Zheng
Huawei Technologies
Email: zhenghaomian@huawei.com
Ramon Casellas, Ph.D.
CTTC
Spain
Phone: +34 936452916
Email: ramon.casellas@cttc.es
Oscar Gonzalez de Dios
Telefonica Investigacion y Desarrollo
Emilio Vargas 6
Madrid, 28045
Spain
Phone: +34 913374013
Email: oscar.gonzalezdedios@telefonica.com
Daniele Ceccarelli
Ericsson
Via A. Negrone 1/A
Genova - Sestri Ponente
Italy
Email: daniele.ceccarelli@ericsson.com
AI ELI5
On this page
PROBLEMFlexi-grid optical links let each channel take a variable-width slice of frequency, so advertising a list of fixed wavelengths no longer describes what is free on a link.
MECHANISMThe RFC adds a new ISCD switching type (Flexi-Grid-LSC, 152) with a Frequency Availability Bitmap sub-TLV, and a flexi-grid restriction type for port label restrictions, all carried in OSPF-TE.
WHY IT MATTERSPath computation can learn which frequency ranges are available on each flexi-grid DWDM link and what slot widths each port supports.
§1Why Flexi-Grid Needs New Routing Extensions
ITU-T Recommendation G.694.1 defines frequency grids for Dense Wavelength Division Multiplexing (DWDM). A fixed grid uses channel spacings from 12.5 GHz, 25 GHz, 50 GHz or 100 GHz up to integer multiples of 100 GHz. G.694.1 also defines a flexible grid, or flexi-grid, built on the ideas of a frequency slot (the range of frequency allocated to one channel) and a slot width.
RFC 7698 gives the framework for GMPLS control of flexi-grid networks, and RFC 7792 covers the RSVP-TE signaling. This RFC complements them by defining the OSPF-TE routing side.
Old fiber systems gave every color of light the same fixed lane. Flexi-grid lets each color pick a lane of the width it needs, so the routers need a new way to describe what is free.
§3What Routing Must Advertise
A flexi-grid LSP occupies one or more frequency slots. Computing the route and assigning the slot is called Routing and Spectrum Assignment (RSA). With centralized spectrum assignment the chosen slot is named in the RSVP-TE Path message; with distributed assignment only the requested slot width is named and the nodes along the path pick the slot.
On a flexi-grid link, central frequencies step from 193.1 THz with 6.25 GHz granularity. Allocated slots on a link do not overlap, although the upper bound of one slot may equal the lower bound of the next. Because the slot width is not known until a slot is allocated, the routing protocol advertises available frequency ranges instead of individual wavelengths.
Devices may support only a subset of the possible positions and widths, so a flexi-grid link SHOULD also advertise the channel spacing (C.S., set to 5 to denote 6.25 GHz), the central frequency granularity, the slot width granularity and the slot width range.
Because lane widths vary, you cannot just say which numbered lanes are free. You have to say which stretches of the shelf are empty and how finely they can be cut.
§3.3Fixed Grid Versus Flexi-Grid
On a fixed-grid link, each wavelength has a predefined central frequency and a usable range implied by the channel spacing. The wavelengths are identified by labels as defined in RFC 6205, and routing just advertises which are available.
On a flexi-grid link the slot width follows Slot Width (in GHz) = 12.5GHz * m, and m is uncertain until a slot is actually allocated. That is why the advertisement is built from the central frequency granularity and the slot width granularity rather than a list of fixed points.
A fixed grid is like a parking lot with painted spaces. Flexi-grid is a lot where the cars choose how much room they take.
| Aspect | Fixed grid | Flexi-grid |
|---|---|---|
| Channel width | Fixed by the channel spacing | Slot width 12.5 GHz * m |
| What is advertised THIS RFC | Which wavelengths are available | Which frequency ranges are available |
§4.1The ISCD and the Frequency Availability Bitmap
The RFC defines a new Switching Capability value for the Interface Switching Capability Descriptor (ISCD): 152, named Flexi-Grid-LSC, used with Encoding Type lambda. The ISCD is otherwise read as in RFC 4203, with optional Switching Capability Specific Information (SCSI) sub-TLVs. The generic Max LSP Bandwidth at priority x fields are not meaningful here, so they MUST be set to zero and MUST be ignored.
The flexi-grid SCSI is the Frequency Availability Bitmap sub-TLV, a new type of the Generalized SCSI TLV from RFC 8258. Its fields are:
Type(16 bits): 11.Length(16 bits): length of the value in octets.Priority(8 bits): which of priorities 0 to 7 are advertised; at least one MUST be, and a single one MUST be priority 0.Max Slot Width at Priority k(16 bits each): largest slot width at that priority, in units of 2*C.S., one per set priority bit.Unreserved Padding(16 bits): included when k is odd, omitted when even, to keep 32-bit alignment.C.S.(4 bits): currently 5.Starting n(16 bits): the first nominal central frequency in the bitmap.No. of Effective Bits(12 bits): how many bitmap bits are meaningful.Bitmap: one bit per basic slot (mof 1), 1 for available and 0 for unavailable.
A centralized spectrum assignment process must check enough consecutive available basic slots to build wider slots.
It is a row of tiny checkboxes, one per smallest slice of spectrum. A tick means the slice is free, and a header says where the row starts.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 2 bytes | 11 | Sub-TLV type. |
LENGTH | Length | 2 bytes | Length of the value field in octets. | |
PRIORITY | Priority | 1 byte | Bitmap of advertised priority levels 0 to 7. | |
RESERVED | Reserved | 3 bytes | Zero on transmission, ignored on receipt. | |
CS | C.S. | 4 bits | 5 | Channel spacing; currently 5. |
STARTING-N | Starting n | 2 bytes | Nominal central frequency of the first bitmap bit. | |
EFFECTIVE-BITS | No. of Effective Bits | 12 bits | Number of meaningful bits in the bitmap. |
§4.1.2A Worked Bitmap Example
The RFC uses a link whose supported range runs from n=-9 to n=11, with nominal central frequency 193.1 + n * 0.00625 THz. With Starting n set to -9 and 21 effective bits, the bitmap marks n=-1 to 7 as available.
The same availability can be expressed with Starting n set to -1 and 9 effective bits, all ones. If an LSP with m=1 is then set up at n=-1, the first bit flips to 0 and the advertisement is updated.
When someone takes one slice of the shelf, the router just flips that checkbox from tick to empty and tells its neighbors.
§4.2Port Label Restrictions
A port may support only a subset of the full flexible grid. RFC 7579 defines the Port Label Restrictions field, carried in the top-level Link TLV as specified in RFC 7580. This RFC adds a flexi-grid Restriction Type, RstType = 5.
Besides MatrixID, the Switching Cap and Encoding (which MUST match the ISCD), it carries C.S. (5 for 6.25 GHz), C.F.G (central frequency granularity, a multiple of C.S.), S.W.G (slot width granularity, a multiple of 2*C.S.) and Min Slot Width (in multiples of 2*C.S.). Reserved fields MUST be zero on transmission.
Some ports can only handle certain slice sizes, like a toaster that fits only some bread. This field lists what each port can take.
§5IANA Assignments
IANA assigned value 152, Flexi-Grid-LSC, in the Switching Types sub-registry of the GMPLS Signaling Parameters registry. It also assigned Generalized SCSI TLV type 11, Frequency Availability Bitmap, for Switching Type 152.
The official number keeper wrote the new names and numbers into its list so everyone uses the same ones.
§6Security Considerations
The extensions carry flexi-grid information in OSPF Opaque LSAs and introduce no security issues beyond those in RFC 3630 and RFC 4203. The security mechanisms of RFC 2328 for Opaque LSAs still apply, and the OSPF security analysis in RFC 6863 applies to these extensions too.
The new messages ride in the same envelopes as before, so the old locks still protect them.
§9References
[RFC4203]RFC 4203Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 4203, October 2005.October 2005[RFC7698]RFC 7698Gonzalez de Dios, O., Ed., et al., "Framework and Requirements for GMPLS-Based Control of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks", RFC 7698, November 2015.November 2015[RFC7699]RFC 7699Farrel, A., King, D., Li, Y., and F. Zhang, "Generalized Labels for the Flexi-Grid in Lambda Switch Capable (LSC) Label Switching Routers", RFC 7699, November 2015.November 2015[RFC7792]RFC 7792Zhang, F., et al., "RSVP-TE Signaling Extensions in Support of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks", RFC 7792, March 2016.March 2016[RFC8258]RFC 8258Ceccarelli, D. and L. Berger, "Generalized SCSI: A Generic Structure for Interface Switching Capability Descriptor (ISCD) Switching Capability Specific Information (SCSI)", RFC 8258, October 2017.October 2017[RFC7579]RFC 7579Bernstein, G., Ed., et al., "General Network Element Constraint Encoding for GMPLS-Controlled Networks", RFC 7579, June 2015.June 2015
[G.694.1]International Telecommunication Union, "Spectral grids for WDM applications: DWDM frequency grid", ITU-T Recommendation G.694.1, February 2012.