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- RFC 9376
RFC 9376: Applicability of GMPLS for beyond 100 Gbit/s Optical Transport Network
- Q. Wang, Ed.,
- R. Valiveti, Ed.,
- H. Zheng, Ed.,
- H. van Helvoort,
- S. Belotti
Abstract
This document examines the applicability of using existing GMPLS routing and signaling mechanisms to set up Optical Data Unit-k (ODUk) Label Switched Paths (LSPs) over Optical Data Unit-Cn (ODUCn) links as defined in the 2020 version of ITU-T Recommendation G.709.¶
Status of This Memo
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1. Introduction
The current GMPLS routing [RFC7138]
and signaling [RFC7139]
extensions support the control of the Optical Transport Network (OTN) signals and capabilities that
were defined in the 2012 version of ITU-T
Recommendation G.709 [ITU
In 2016, a new version of ITU-T
Recommendation G.709 was published: [ITU
Because [ITU
This document assumes that readers are familiar with OTN, GMPLS, and how GMPLS is applied in OTN. As such, this document doesn't provide any background pertaining to OTN that include links with capacities of 100 Gbit/s or less; this background could be found in documents such as [RFC7062] and [RFC7096]. This document provides an overview of the data plane primitives that enable links with capacities greater than 100 Gbit/s and analyzes the extensions that would be required in the current GMPLS routing and signaling mechanisms to support evolution in OTN.¶
2. OTN Terminology Used in This Document
- FlexO:
- Flexible OTN information structure. This information structure usually has a specific bitrate and frame format that consists of overhead and payload, which are used as a group for the transport of an OTUCn signal.¶
- LSP:
- Label Switched Path¶
- MSI:
- Multiplex Structure Indicator. This structure indicates the grouping of the tributary slots in an OPU payload area that realizes a client signal, which is multiplexed into an OPU. The individual clients multiplexed into the OPU payload area are distinguished by the Tributary Port Number (TPN).¶
- ODU:
- Optical Data Unit. An ODU has the frame structure and overhead, as defined in
Figure 12-1 of [ITU
-T_ ]. ODUs can be formed in two ways: a) by encapsulating a single non-OTN client, such as SONET/SDH (Synchronous Optical Network / Synchronous Digital Hierarchy) or Ethernet, or b) by multiplexing lower-rate ODUs. In general, the ODU layer represents the path layer in OTN. The only exception is the ODUCn signal (defined below), which is defined to be a section-layer signal. In the classification based on bitrates of the ODU signals, ODUs are of two types: fixed rate and flexible rate. Flexible-rate ODUs, called "ODUflex", have a rate that is 239/238 times the bitrate of the client signal they encapsulate.¶G709_ 2020 - ODUC:
- Optical Data Unit-C. This signal has a bandwidth of approximately 100 Gbit/s and
is of a slightly higher bitrate than the fixed rate ODU4 signal. This signal
has the format defined in Figure 12-1 of [ITU
-T_ ]. This signal represents the building block for constructing a higher-rate signal called "ODUCn" (defined below).¶G709_ 2020 - ODUCn:
- Optical Data Unit-Cn, where Cn indicates the bitrate of approximately n*100 Gbit/s.
This frame structure consists of "n" interleaved frame and multiframe
synchronous instances of the ODUC signal, each of which has the format defined in
Figure 12-1 of [ITU
-T_ ].¶G709_ 2020 - ODUflex:
- Optical Data Unit - flexible rate. An ODUflex has the same frame structure
as a "generic" ODU but with a rate that is a fixed multiple of the bitrate of the client signal it
encapsulates. [ITU
-T_ ] defines specific ODUflex containers that are required to transport specific clients such as 50GE, 200GE, 400GE, etc.¶G709_ 2020 - ODUk:
- Optical Data Unit-k, where k is one of {0, 1, 2, 2e, 3, 4}. The term "ODUk" refers to an ODU whose bitrate is fully specified by the index k. The bitrates of the ODUk signal for k = {0, 1, 2, 2e, 3, 4} are approximately 1.25 Gbit/s, 2.5 Gbit/s, 10 Gbit/s, 10.3 Gbit/s, 40 Gbit/s, and 100 Gbit/s, respectively.¶
- OPUC:
- Optical Payload Unit-C. This signal has a payload of approximately 100 Gbit/s. This structure represents the payload area of the ODUC signal.¶
- OPUCn:
- Optical Payload Unit-Cn, where Cn indicates that the bitrate is approximately n*100 Gbit/s. This structure represents the payload area of the ODUCn signal.¶
- OTN:
- Optical Transport Network¶
- OTUC:
- Optical Transport Unit-C. This signal has a bandwidth of approximately 100 Gbit/s. This signal forms the building block of the OTUCn signal defined below, which has a bandwidth of approximately n*100 Gbit/s.¶
- OTUCn:
- Fully standardized Optical Transport Unit-Cn. This frame structure is
realized by extending the ODUCn signal with the OTU layer overhead.
The structure of this signal is illustrated in Figure 11-4 of
[ITU
-T_ ]. Note that the term "fully standardized" is defined by ITU-T in Section 6.1.1 of [ITUG709_ 2020 -T_ ].¶G709_ 2020 - OTUCn-M:
- This signal is an extension of the OTUCn signal introduced above. This signal contains the same amount of overhead as the OTUCn signal but contains a reduced amount of payload area. Specifically, the payload area consists of M tributary slots (each 5 Gbit/s), where M is less than 20*n, which is the number of tributary slots in the OTUCn signal.¶
- PSI:
- Payload Structure Indicator. This is a 256-byte signal that describes the composition of the OPU signal. This field is a concatenation of the payload type (PT) and the Multiplex Structure Indicator (MSI) defined below.¶
- TPN:
- Tributary Port Number. The tributary port number is used to indicate the port number of the client signal that is being transported in one specific tributary slot.¶
Detailed descriptions for some of these terms can be found in [ITU
3. Overview of OTUCn/ODUCn in G.709
This section provides an overview of the OTUCn/ODUCn signals defined in
[ITU
3.1. OTUCn
In order to carry client signals with rates greater than 100 Gbit/s,
[ITU
- The OTUCn signal contains one ODUCn. The OTUCn and ODUCn signals
perform digital section-layer roles only (see Section 6.1.1 of
[ITU
-T_ ])¶G709_ 2020 - The OTUCn signals are formed by interleaving n synchronous OTUC signals (which are labeled 1, 2, ..., n).¶
- Each of the OTUC instances has the same overhead as the standard
OTUk signal in [ITU
-T_ ]. Note that the OTUC signal doesn't include the Forward Error Correction (FEC) columns illustrated in Figure 11-1 of [ITUG709_ 2020 -T_ ]. The OTUC signal includes an ODUC.¶G709_ 2020 - The OTUC signal has a slightly higher rate compared to the OTU4 signal (without FEC); this is to ensure that the OPUC payload area can carry an ODU4 signal.¶
- The combined signal OTUCn has n instances of OTUC overhead and n instances of ODUC overhead.¶
The OTUCn, ODUCn, and OPUCn signal structures are presented in a
(physical) interface
OTUCn interfaces can be categorized as follows, based on the type of peer network element:¶
- inter-domain interfaces:
- These types of interfaces are used for
connecting OTN edge nodes to (a) client equipment (e.g., routers)
or (b) hand-off points from other OTN. ITU-T
Recommendation G709.1 [ITU
-T_ ] specifies a flexible interoperable short-reach OTN interface over which an OTUCn (n >=1) is transferred, using bonded Flexible OTN information structure (FlexO) interfaces, which belong to a FlexO group.¶G709 .1 - intra-domain interfaces:
- In these cases, the OTUCn is transported
using a proprietary
(vendor -specific ) encapsulation, FEC, etc. It is also possible to transport OTUCn for intra-domain links using FlexO.¶
3.1.1. OTUCn-M
The standard OTUCn signal has the same rate as the ODUCn signal. This
implies that the OTUCn signal can only be transported over wavelength
groups that have a total capacity of multiples of (approximately) 100 Gbit/s.
Modern optical interfaces support a variety of bitrates per wavelength,
depending on the reach requirements for the optical path. If the total
rate of the ODUk LSPs planned to be carried over an ODUCn link is smaller
than n*100 Gbit/s, it is possible to "crunch" the OTUCn, and the unused
tributary slots are thus not transmitted. [ITU
3.2. ODUCn
The ODUCn signal defined in [ITU
The ODUCn is a multiplex section ODU signal and is mapped into an
OTUCn signal, which provides the regenerator section layer. In some
scenarios, the ODUCn and OTUCn signals will be coterminated, i.e.,
they will have identical source/sink locations (see
Figure 1). In Figure 1,
the term "OTN Switch" has the same meaning as that used in
Section 3 of [RFC7138].
[ITU
Specifically, the OPUCn signal flows through these regenerators unchanged. That is, the set of client signals, their TPNs, and tributary-slot allocations remains unchanged.¶
+--------+ +--------+
| +-----------+ |
| OTN |-----------| OTN |
| Switch +-----------+ Switch |
| A | | B |
| +-----------+ |
+--------+ +--------+
<--------ODUCn------->
<-------OTUCn------>
+---------+ +--------+ +--------+ +--------+
| +--------+ | | +----------+ |
| OTN |--------| OTN | | OTN |----------| OTN |
| Switch +--------+ Regen +--------+ Regen +----------+ Switch |
| A | | B | | C | | D |
| +--------+ | | +----------+ |
+---------+ +--------+ +--------+ +--------+
<-------------------------ODUCn-------------------------->
<---------------><-----------------><------------------>
OTUCn OTUCn OTUCn
3.3. Tributary Slot Granularity
[ITU
3.4. Structure of OPUCn MSI with Payload Type 0x22
As mentioned above, the OPUCn signal has 20*n tributary slots (TSs) (each 5
Gbit/s). The OPUCn MSI field has a fixed length of 40*n bytes and
indicates the availability and occupation of each TS. Two bytes are used
for each of the 20*n tributary slots, and each such information structure
has the following format (see Section 20.4.1 of [ITU
- The TS availability bit indicates if the tributary slot is available or unavailable.¶
- The TS occupation bit indicates if the tributary slot is allocated or unallocated.¶
- The tributary port number (14 bits) indicates the port number of the client signal that is being carried in this specific TS. A flexible assignment of tributary port to tributary slots is possible. Numbering of tributary ports is from 1 to 10*n.¶
The concatenation of the OPUCn payload type (PT) and the MSI field is carried over
the overhead byte designated as PSI in Figure 15-6 of [ITU
3.5. Client Signal Mappings
The approach taken by the ITU-T to map non-OTN client signals to the appropriate ODU containers is as follows:¶
- All client signals are mapped into an ODUj or ODUk (e.g., ODUflex) as
specified in Section 17 of [ITU
-T_ ].¶G709_ 2020 - The terms "ODUj" and "ODUk" are used in a multiplexing scenario, with ODUj being a low-order ODU that is multiplexed into ODUk, a high-order ODU. As Figure 3 illustrates, the ODUCn is also a high-order ODU into which other ODUs can be multiplexed. The ODUCn itself cannot be multiplexed into any higher-rate ODU signal; it is defined to be a section-level signal.¶
- ODUflex signals are low-order signals only. If the ODUflex entities have rates of 100 Gbit/s or less, they can be transported over either an ODUk (k=1..4) or an ODUCn. For ODUflex connections with rates greater than 100 Gbit/s, ODUCn is required.¶
- ODU Virtual Concatenation (VCAT) has been deprecated. This simplifies the network and the supporting hardware since multiple different mappings for the same client are no longer necessary. Note that legacy implementations that transported sub-100 Gbit/s clients using ODU VCAT shall continue to be supported.¶
Clients (e.g., SONET/SDH and Ethernet)
| | | | | |
| | | | | |
| | | | | |
+---+---+---+----+ | | |
| OPUj | | | |
+----------------+ | | |
| ODUj | | | |
+----------------+----------------------+---+---+----------+
| |
| OPUk |
+----------------------------------------------------------+
| |
| ODUk k in {0,1,2,2e,3,4,flex}|
+-------------------------+-----+--------------------------+
| | | |
| OTUk, OTUk-SC, OTUk-V | | OPUCn |
+-------------------------+ +--------------------------+
| |
| ODUCn |
+--------------------------+
| |
| OTUCn |
+--------------------------+
4. GMPLS Implications and Applicability
4.1. TE Link Representation
Section 3 of [RFC7138] describes how to represent G.709 OTUk/ODUk with TE links in GMPLS. In the same manner, OTUCn links can also be represented as TE links. Figure 4 provides an illustration of a one-hop OTUCn TE link.¶
+----------+ +---------+
| OTN | | OTN |
| Switch +-------------------+ Switch |
| A | | B |
+----------+ +---------+
|<---------OTUCn Link---------->|
|<---------TE Link------------->|
It is possible to create TE links that span more than one hop by creating
forward adjacencies (FAs) between non-adjacent nodes
(see Figure 5). In Figure 5,
nodes B and C are performing the ODUCn regeneration
function described in Section 7.1 of [ITU
+--------+ +--------+ +--------+ +---------+
| OTN | | OTN | | OTN | | OTN |
| Switch |<------->| Regen |<-------->| Regen |<------->| Switch |
| A | OTUCn | B | OTUCn | C | OTUCn | D |
+--------+ Link +--------+ Link +--------+ Link +---------+
|<-------------------- ODUCn Link -------------------->|
|<---------------------- TE Link --------------------->|
The two endpoints of a TE link are configured with the supported resource information (which may include whether the TE link is supported by an ODUCn, ODUk, or OTUk), as well as the link attribute information (e.g., slot granularity and list of available tributary slot).¶
4.2. GMPLS Signaling
Once the ODUCn TE link is configured, the GMPLS mechanisms defined in
[RFC7139]
can be reused to set up ODUk/ODUflex LSPs with no changes.
As the resource on the ODUCn link that can be seen by the ODUk/ODUflex
client signal is a set of 5 Gbit/s slots, the label defined in
[RFC7139]
is able to accommodate the requirement of the setup of an ODUk/ODUflex client
signal over an ODUCn link. In [RFC7139], the
OTN-TDM GENERALIZED_
The TPN defined in [ITU
The example in Figure 6 illustrates the label format defined in [RFC7139] for multiplexing ODU4 onto ODUC10. One ODUC10 has 200 slots (each 5 Gbit/s), and twenty of them are allocated to the ODU4. With this label encoding, only 20 out of the 200 bits mask are non-zero, which is very inefficient. The inefficiency grows for larger values of "n", and an optimized label format may be desirable.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TPN = 3 | Reserved | Length = 200 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 1 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0| Padding Bits(0) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.3. GMPLS Routing
For routing, it is deemed that no extension to the current mechanisms defined in [RFC7138] is needed.¶
The ODUCn link, which is the lowest layer of the ODU multiplexing hierarchy involving multiple ODU layers, is assumed to have been already configured when GMPLS is used to set up ODUk over ODUCn; therefore, the resources that need to be advertised are the resources that are exposed by this ODUCn link and the ODUk multiplexing hierarchy on it. The 5 Gbit/s OPUCn time slots do not need to be advertised, while the 1.25 Gbit/s and 2.5 Gbit/s OPUk time slots need to be advertised using the mechanisms already defined in [RFC7138].¶
Since there is a 1:1 correspondence between the ODUCn and the OTUCn signal, there is no need to explicitly define a new value to represent the ODUCn signal type in the OSPF-TE routing protocol.¶
5. IANA Considerations
This document has no IANA actions.¶
6. Security Considerations
This document analyzes the applicability of protocol extensions in
[RFC7138]
and [RFC7139] for use in the 2020 version of
ITU-T Recommendation G.709 [ITU
7. References
7.1. Normative References
- [ITU
-T_ G709_ 2020] - ITU-T, "Interfaces for the optical transport network", ITU-T Recommendation G.709, .
- [RFC5920]
-
Fang, L., Ed., "Security Framework for MPLS and GMPLS Networks", RFC 5920, DOI 10
.17487 , , <https:///RFC5920 www >..rfc -editor .org /info /rfc5920 - [RFC7138]
-
Ceccarelli, D., Ed., Zhang, F., Belotti, S., Rao, R., and J. Drake, "Traffic Engineering Extensions to OSPF for GMPLS Control of Evolving G.709 Optical Transport Networks", RFC 7138, DOI 10
.17487 , , <https:///RFC7138 www >..rfc -editor .org /info /rfc7138 - [RFC7139]
-
Zhang, F., Ed., Zhang, G., Belotti, S., Ceccarelli, D., and K. Pithewan, "GMPLS Signaling Extensions for Control of Evolving G.709 Optical Transport Networks", RFC 7139, DOI 10
.17487 , , <https:///RFC7139 www >..rfc -editor .org /info /rfc7139
7.2. Informative References
- [ITU
-T_ G709 .1] - ITU-T, "Flexible OTN short-reach interfaces", ITU-T Recommendation G.709.1, .
- [ITU
-T_ G709_ 2012] - ITU-T, "Interfaces for the optical transport network", ITU-T Recommendation G.709, .
- [ITU
-T_ G709_ 2016] - ITU-T, "Interfaces for the optical transport network", ITU-T Recommendation G.709, .
- [ITU
-T_ G872] - ITU-T, "Architecture of optical transport networks", ITU-T Recommendation G.872, .
- [RFC7062]
-
Zhang, F., Ed., Li, D., Li, H., Belotti, S., and D. Ceccarelli, "Framework for GMPLS and PCE Control of G.709 Optical Transport Networks", RFC 7062, DOI 10
.17487 , , <https:///RFC7062 www >..rfc -editor .org /info /rfc7062 - [RFC7096]
-
Belotti, S., Ed., Grandi, P., Ceccarelli, D., Ed., Caviglia, D., Zhang, F., and D. Li, "Evaluation of Existing GMPLS Encoding against G.709v3 Optical Transport Networks (OTNs)", RFC 7096, DOI 10
.17487 , , <https:///RFC7096 www >..rfc -editor .org /info /rfc7096
Appendix A. Possible Future Work
As noted in Section 4.2,
the GMPLS TPN field defined in
Section 6.1 of [RFC7139] is only 12 bits, whereas
an ODUCn link could require up to 14 bits.
Although the need is not urgent, future work could extend the TPN field
in GMPLS to use the Reserved bits immediately adjacent.
This would need to be done in a backward
Section 4.2 further notes that the current encoding of
GMPLS labels can be inefficient for larger values of n in ODUCn.
Future work might examine a more compact, yet generalized, label encoding
to address this issue should it be felt, after analysis of the operational
aspects, that the current encoding is causing problems.
Introduction of a new label encoding would need to be done using a
new pairing of LSP encoding type and Generalized Payload Identifier (G-PID) to ensure correct interoperabilit