Applicability of GMPLS for Beyond 100G Optical Transport Network

Applicability of GMPLS for Beyond 100G Optical Transport Network ZTE Corporation

Nanjing China wang.qilei@zte.com.cn

Infinera Corp

Sunnyvale USA rvaliveti@infinera.com

Huawei

China zhenghaomian@huawei.com

Hai Gaoming B.V

Almere Netherlands huubatwork@gmail.com

Nokia

sergio.belotti@nokia.com

Internet Engineering Task Force This document examines the applicability of using existing GMPLS routing and signalling 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 G.709.

Introduction The current GMPLS routing and signalling extensions support the control of Optical Transport Network (OTN) signals and capabilities that were defined in the 2012 version of G.709 . In 2016 a further version of G.709 was published: . This version introduced higher rate Optical Transport Unit (OTU) and Optical Data Unit (ODU) signals, termed OTUCn and ODUCn respectively, which have a nominal rate of n x 100 Gbit/s. According to the definition in , OTUCn and ODUCn perform only the digital section layer role and ODUCn supports only ODUk clients. This document focuses on the use of existing GMPLS mechanisms to set up ODUk (e.g., ODUflex) Label Switched Paths (LSPs) over ODUCn links, independently from how these links have been set up. Because does not introduce any new features to OTUCn and ODUCn compared to , this document starts with by first presenting an overview of the OTUCn and ODUCn signals, and then analyzing how the current GMPLS routing and signalling mechanisms can be utilized to set up ODUk (e.g., ODUflex) LSPs over ODUCn links. This document assumes that the reader is familiar with OTN, GMPLS, and how GMPLS is applied in OTN networks. As such, this document doesn't provide any background pertaining to OTN networks that included links with capacities of 100G or less; this background could be found in documents such as and . This document provides an overview of the dataplane primitives that enable links with capacities greater than 100G, and analyses the extensions that would be required in the current GMPLS routing & signaling mechanisms to support the evolution in OTN networks.

OTN terminology used in this document

FlexO: Flexible OTN information structure. This information structure is usually with a specific bit rate and frame format, consisting of overhead and payload, which is used as a group for the transport of an OTUCn signal.
LSP: Label Switched Path.
ODU: Optical Data Unit. An ODU has the frame structure and overhead, as defined in Figure 12-1 of . ODUs can be formed in two ways: a) by encapsulating a single non-OTN client (such as SONET/SDH, Ethernet) b) multiplexing lower-rate ODUs. In general, the ODU layer represents the path layer in OTN networks. 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 ODU(s), called "ODUFlex" have a rate that is 239/238 times the bit rate of the client signal it encapsulates.
ODUk: Optical Data Unit-k, where k is one of {0, 1, 2, 2e, 3, 4}. The term ODUk references to an ODU whose bit rate is fully specified by the index k. The bit rates of the ODUk signal for k = {0, 1, 2, 2e, 3, 4} are approximately 1.25G, 2.5G, 10G, 10.3G, 40G, 100G respectively.
ODUflex: Optical Data Unit - flexible rate. An ODUflex has the same frame structure as a "generic" ODU, but with 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.
ODUC: Optical Data Unit -C; this signal has a bandwidth of approximately 100G, and is of a slightly higher bit rate than the fixed rate ODU4 signal. This signal has the format defined in Figure 12-1 of . This signal represents the building block for constructing a higher rate signal called ODUCn (defined below).
ODUCn: Optical Data Unit-Cn; Cn indicates the bit rate of approximately n*100G. This frame structure consists of "n" interleaved, frame and multi-frame synchronous instances of the ODUC signal, each of which has the format defined in Figure 12-1 of .
OPUC: Optical Payload Unit -C; with a payload of approximately 100G. This structure represents the payload area of the ODUC signal.
OPUCn: Optical Payload Unit-Cn. Where Cn indicates that the bit rate is approximately n*100G. This structure represents the payload area of the ODUCn signal.
OTUC: Optical Transport Unit -C; with a bandwidth of approximately 100G. This signal forms the building block of the OTUCn signal defined below, which has a bandwidth of approximately n*100G.
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 11-1 of . Note that the term "fully standardized" is defined by ITU-T in :Section 6.1.1.
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 5 Gbit/s tributary slots - where M is less than 20*n, which is the number of tributary slots in the OTUCn signal.
OTN: Optical Transport Network.
PSI: OPU 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.
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).
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 of these terms can be found in .

Overview of the OTUCn/ODUCn in G.709 This section provides an overview of OTUCn/ODUCn signals defined in . The text in this section is purely descriptive and is not normative. For a full description of OTUCn/ODUCn signals please refer to . In the event of any discrepancy between this text and , that other document is definitive.

OTUCn In order to carry client signals with rates greater than 100 Gbit/s, takes a general and scalable approach that decouples the rates of OTU signals from the client rate. The new OTU signal is called OTUCn, and this signal is defined to have a rate of (approximately) n*100G. The following are the key characteristics of the OTUCn signal:

The OTUCn signal contains one ODUCn. The OTUCn and ODUCn signals perform digital section roles only (see :Section 6.1.1)
The OTUCn signals can be viewed as being 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 . Note that the OTUC signal doesn't include the FEC columns illustrated in :Figure 11-1. The OTUC signal includes an ODUC.
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 independent manner, by means of n OTUC, ODUC and OPUC instances that are marked #1 to #n. 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 networks. ITU-T Recommendation 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.
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.

OTUCn-M The standard OTUCn signal has the same rate as that of the ODUCn signal. This implies that the OTUCn signal can only be transported over wavelength groups which have a total capacity of multiples of (approximately) 100G. Modern optical interfaces support a variety of bit rates 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*100G, it is possible to "crunch" the OTUCn not to transmit the unused tributary slots. ITU-T supports the notion of a reduced rate OTUCn signal, termed the OTUCn-M. The OTUCn-M signal is derived from the OTUCn signal by retaining all the n instances of overhead (one per OTUC instance) but with only M (M is less than 20*n) OPUCn tributary slots available to carry ODUk LSPs.

ODUCn The ODUCn signal defined in can be viewed as being formed by the appropriate interleaving of content from n ODUC signal instances. The ODUC frames have the same structure as a standard ODU in the sense that it has the same overhead and payload areas, but has a higher rate since its payload area can embed an ODU4 signal. 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 co-terminated, i.e. they will have identical source/sink locations (see ). In this figure, the term "OTN Switch" has the same meaning as that used in :Section 3. allows for the ODUCn signal to pass through one or more digital regenerator nodes (shown as Nodes B, C in ) which will terminate the OTUCn layer, but will pass the regenerated (but otherwise untouched) ODUCn towards a different OTUCn interface where a fresh OTUCn layer will be initiated. This process is termed as "ODUCn regeneration" in :Section 7.1. In this example, the ODUCn is carried by 3 OTUCn segments. Specifically, the OPUCn signal flows through these regenerators unchanged. That is, the set of client signals, their TPNs, tributary-slot allocation remains unchanged.

ODUCn signal <-------OTUCn------> ]]>

ODUCn signal - multihop <---------------><-----------------><------------------> OTUCn OTUCn OTUCn ]]>

Tributary Slot Granularity introduced the support for 1.25 Gbit/s granular tributary slots in OPU2, OPU3, and OPU4 signals. defined the OPUC with a 5 Gbit/s tributary slot granularity. This means that the ODUCn signal has 20*n tributary slots (of 5 Gbit/s capacity). The range of tributary port number (TPN) is 10*n instead of 20*n, which restricts the maximum client signals that could be carried over one single ODUC1.

Structure of OPUCn MSI with Payload type 0x22 As mentioned above, the OPUCn signal has 20*n 5 Gbit/s tributary slots (TSs). 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 (:Section 20.4.1):

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) 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.

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 clause 17 of .
The terms ODUj & ODUk are used in a multiplexing scenario, with ODUj being a low-order ODU which is multiplexed into ODUk, a high-order ODU. As 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 100G or less, they can be transported over either an ODUk (k=1..4) or an ODUCn. For ODUflex connections with rates greater than 100G, ODUCn is required.
ODU Virtual Concatenation 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-100G clients using ODU VCAT shall continue to be supported.

Digital Structure of OTN interfaces (from G.709:Figure 6-1)

GMPLS Implications and Applicability

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. below provides an illustration of a one-hop OTUCn TE link.

OTUCn TE-Links | |<---------TE-Link------------->| ]]> It is possible to create TE-links that span more than one hop by creating forward adjacencies (FA) between non-adjacent nodes (see ). In this illustration, the nodes B and C are performing the ODUCn regeneration function described in :Section 7.1, and are not electrically switching the ODUCn signal from one interface to another. As in the one-hop case, Multiple-hop TE-links advertise the ODU switching capability.

Implications and Applicability for GMPLS Signalling Once the ODUCn TE-Link is configured, the GMPLS mechanisms defined in can be reused to set up ODUk/ODUflex LSPs with no changes. As the resource on the ODUCn link which can be seen by the client ODUk/ODUflex is a set of 5 Gbit/s slots, the label defined in is able to accommodate the requirement of the setup of ODUk/ODUflex over ODUCn link. In , the OTN-TDM GENERALIZED_LABEL object is used to indicate how the lower order (LO) ODUj signal is multiplexed into the higher order (HO) ODUk link. In a similar manner, the OTN-TDM GENERALIZED_LABEL object is used to indicate how the ODUk signal is multiplexed into the ODUCn link. The ODUk Signal Type is indicated by Traffic Parameters. The IF_ID RSVP_HOP object provides a pointer to the interface associated with TE-Link and therefore the two nodes terminating the TE-link know (by internal/local configuration) the attributes of the ODUCn TE Link. Since the TPN defined in for an ODUCn link has 14 bits, while this field in only has 12 bits, some extension work will eventually be needed. Given that a 12-bit TPN field can support ODUCn links with up to n=400 (i.e. 40Tbit/s links), this need is not urgent. An example is given in to illustrate the label format defined in for multiplexing ODU4 onto ODUC10. One ODUC10 has 200 5 Gbit/s slots, and twenty of them are allocated to the ODU4. With this label encoding, only 20 out of the 200 bits mask are non-zero, and is very inefficient. The inefficiency grows for larger values of "n" and an optimized label format may be desirable.

Label format

Implications and Applicability for GMPLS Routing For routing, it is deemed that no extension to current mechanisms defined in is needed. Because, once an ODUCn link is up, the resources that need to be advertised are the resources that are exposed by this ODUCn link and the multiplexing hierarchy on this link. Since the ODUCn link is the lowest layer of the ODU multiplexing hierarchy involving multiple ODU layers, and there is a 1:1 correspondence with 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. The OSPF-TE extension defined in section 4 of can be reused to advertise the resource information on the ODUCn link to help finish the setup of ODUk/ODUflex.

Authors (Full List)

: Qilei Wang (editor)

: ZTE

: Nanjing, China

: Email: wang.qilei@zte.com.cn

: Radha Valiveti (editor)

: Infinera Corp

: Sunnyvale, CA, USA

: Email: rvaliveti@infinera.com

: Haomian Zheng (editor)

: Huawei

: CN

: EMail: zhenghaomian@huawei.com

: Huub van Helvoort

: Hai Gaoming B.V

: EMail: huubatwork@gmail.com

: Sergio Belotti

: Nokia

: EMail: sergio.belotti@nokia.com

Contributors

: Iftekhar Hussain, Infinera Corp, Sunnyvale, CA, USA, IHussain@infinera.com

: Daniele Ceccarelli, Ericsson, daniele.ceccarelli@ericsson.com

: Rajan Rao, Infinera Corp, Sunnyvale, USA, rrao@infinera.com

: Fatai Zhang, Huawei,zhangfatai@huawei.com

: Italo Busi, Huawei,italo.busi@huawei.com

: Dieter Beller, Nokia, Dieter.Beller@nokia.com

: Yuanbin Zhang, ZTE, Beiing, zhang.yuanbin@zte.com.cn

: Zafar Ali, Cisco Systems, zali@cisco.com

: Daniel King, d.king@lancaster.ac.uk

: Manoj Kumar, Cisco Systems, manojk2@cisco.com

: Antonello Bonfanti, Cisco Systems, abonfant@cisco.com

: Yuji Tochio, Fujitsu, tochio@fujitsu.com

IANA Considerations This memo includes no request to IANA.

Security Considerations This document analyses and reuses the protocol extensions in and without introducing any new extensions. Therefore, this document introduces no new security considerations to the existing signalling protocol and routing protocol comparing to and . Please refer to and for further details of the specific security measures. Additionally, addresses the security aspects that are relevant in the context of GMPLS.

References Normative References ITU-T G.709: Optical Transport Network Interfaces; 06/2020 ITU-T Security Framework for MPLS and GMPLS Networks This document provides a security framework for Multiprotocol Label Switching (MPLS) and Generalized Multiprotocol Label Switching (GMPLS) Networks. This document addresses the security aspects that are relevant in the context of MPLS and GMPLS. It describes the security threats, the related defensive techniques, and the mechanisms for detection and reporting. This document emphasizes RSVP-TE and LDP security considerations, as well as inter-AS and inter-provider security considerations for building and maintaining MPLS and GMPLS networks across different domains or different Service Providers. This document is not an Internet Standards Track specification; it is published for informational purposes. Informative References ITU-T G.709.1: Flexible OTN short-reach interface; 2018 ITU-T ITU-T G.709: Optical Transport Network Interfaces; 02/2012 ITU-T ITU-T G.709: Optical Transport Network Interfaces; 07/2016 ITU-T ITU-T G.872: Architecture of Optical Transport Networks; 12/2019 ITU-T

Possible Future Work As noted in Section , the GMPLS TPN field in Section 6.1 of 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 compatible way. Section 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 LSP Encoding Type / G-PID pairing to ensure correct interoperability.