| Internet-Draft | B100G Extensions | May 2022 |
| Wang, et al. | Expires 9 November 2022 | [Page] |
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.¶
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The current GMPLS routing [RFC7138] and signalling [RFC7139] extensions support the control of Optical Transport Network (OTN) signals and capabilities that were defined in the 2012 version of G.709 [ITU-T_G709_2012].¶
In 2016 a further version of G.709 was published: [ITU-T_G709_2016]. 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 [ITU-T_G709_2016], OTUCn and ODUCn perform only 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 [ITU-T_G709_2020] does not introduce any new features to OTUCn and ODUCn compared to [ITU-T_G709_2016], this document starts with [ITU-T_G709_2020] by first presenting an overview of the OTUCn and ODUCn signals, and then analysing how the current GMPLS routing and signalling mechanisms can be utilized to set up ODUk (e.g., ODUflex) LSPs over ODUCn links.¶
Detailed descriptions of these terms can be found in [ITU-T_G709_2020].¶
This section provides an overview of OTUCn/ODUCn signals defined in [ITU-T_G709_2020]. The text in this section is purely descriptive and is not normative. For a full description of OTUCn/ODUCn signals please refer to [ITU-T_G709_2020]. In the event of any discrepancy between this text and [ITU-T_G709_2020], that other document is definitive.¶
In order to carry client signals with rates greater than 100 Gbit/s, [ITU-T_G709_2020] 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, 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:¶
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 DSPs 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 some of 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.¶
The ODUCn signal defined in [ITU-T_G709_2020] 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 area, and the payload area, 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. [ITU-T_G709_2020] allows for the ODUCn signal to pass through a digital regenerator node 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 (see Figure 1). In this case, 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, trib-slot allocation remains unchanged.¶
+--------+ +--------+
| +-----------+ |
| OTN |-----------| OTN |
| DXC +-----------+ DXC |
| | | |
+--------+ +--------+
<--------ODUCn------->
<-------OTUCn------>
+--------+ +--------+ +--------+ +--------+
| +--------+ | | +----------+ |
| OTN |--------| OTN | | OTN |----------| OTN |
| DXC +--------+ WXC +--------+ WXC +----------+ DXC |
| | | 3R | | 3R | | |
+--------+ +--------+ +--------+ +--------+
<-------------------------ODUCn-------------------------->
<---------------> <---------------> <------------------>
OTUCn OTUCn OTUCn
[ITU-T_G709_2012] introduced the support for 1.25 Gbit/s granular tributary slots in OPU2, OPU3, and OPU4 signals. [ITU-T_G709_2020] 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.¶
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 ([ITU-T_G709_2020]:Section 20.4.1):¶
The approach taken by the ITU-T to map non-OTN client signals to the appropriate ODU containers is as follows:¶
Clients (e.g. SONET/SDH, Ethernet)
+ + +
| | |
+------------------+-------+------+------------------------+
| OPUk |
+----------------------------------------------------------+
| ODUk |
+-----------------------+---------------------------+------+
| OTUk, OTUk.V, OTUkV | OPUk | |
+----------+----------------------------------------+ |
| OTLk.n | | ODUk | |
+----------+ +---------------------+-----+ |
| OTUk, OTUk.V, OTUkV | OPUCn |
+----------+-----------------------+
| OTLk.n | | ODUCn |
+----------+ +------------+
| OTUCn |
+------------+
Section 3 of RFC7138 describes how to represent G.709 OTUk/ODUk with TE-Links in GMPLS. Similar to that, ODUCn links can also be represented as TE-Links, which can be seen in the Figure 3.¶
+-----+ +-----+
| | | |
| A |<-OTUCn Link->| B |
| | | |
+-----+ +-----+
|<--- ODUCn Link -->|
|<---- TE-Link ---->|
3R 3R
+-----+ +-----+ +-----+ +-----+
| | | | | | | |
| A |<-OTUCn Link->| B |<-OTUCn Link->| C |<-OTUCn Link->| D |
| | | | | | | |
+-----+ +-----+ +-----+ +-----+
|<----------------------- 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 or an ODUk or an OTUk, as well as the link attribute information (e.g., slot granularity, list of available tributary slot).¶
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 which can be seen by the client ODUk/ODUflex is a set of 5 Gbit/s slots, the label defined in [RFC7139] is able to accommodate the requirement of the setup of ODUk/ODUflex over ODUCn link. In [RFC7139], 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 [ITU-T_G709_2020] for an ODUCn link has 14 bits, while this field in [RFC7139] only has 12 bits, some extension work is needed. Given that a 12-bit TPN field can support ODUCn links with up to n=400 (i.e. 40Tbit/s links), this extension is not urgently needed.¶
An example is given in Figure 4 to illustrate the label format defined in [RFC7139] for multiplexing ODU4 onto ODUC10. One ODUC10 has 200 5 Gbit/s slots, and twenty of them are allocated to the ODU4. Along with the increase of "n", the label may become lengthy, an optimized label format may be needed.¶
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) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For routing, it is deemed that no extension to current mechanisms defined in [RFC7138] are needed. Because, once an ODUCn link is up, the resources that need to be advertised are the resources that 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, 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 [RFC7138] can be reused to advertise the resource information on the ODUCn link to help finish the setup of ODUk/ODUflex.¶
This memo includes no request to IANA.¶
This document analyses and reuses the protocol extensions in [RFC7138] and [RFC7139] without introducing any new extensions. Therefore, this document introduces no new security considerations to the existing signalling protocol and routing protocol comparing to [RFC7138] and [RFC7139]. Please refer to [RFC7138] and [RFC7139] for further details of the specific security measures. Additionally, [RFC5920] addresses the security aspects that are relevant in the context of GMPLS.¶