SRv6 for Inter-Layer Network Programming

2.1. IP and Optical Inter-layer Programming

In many network scenarios, the underlay of the IP network is an optical network. The IP network and optical network are usually managed separately, the optical network works as an underlay which is normally invisible to the IP network. In some cases, the optical path resources and the IP path resources may not be one-to-one mapping, which makes the redundant optical paths not fully used by the IP layer. In some other cases, there may be optical paths between non-adjacent IP nodes thus they are not visible in the L3 topology, thus they can not be used for carrying traffic based on IP routing. However, such optical paths may be used for inter-layer traffic engineering.¶

2.2. IP and MTN Inter-layer Programming

The architecture of Metro Transport Network (MTN) is defined in [ITU-T_G.8310]. In an MTN based network, network nodes can support two forwarding modes: per-hop IP packet forwarding and the MTN Path (MTNP) layer cross-connect. An MTN path is a multi-hop underlay transport path which may be established between any two nodes in the MTN network, and the intermediate nodes on the MTN path will forward the traffic based on the pre-established MTN cross-connect without IP table lookup. Thus an MTN path is considered as an underlay connection between two remote MTN nodes. Although in some cases it is possible to set up a layer-3 adjacency between the two endpoints of the MTN path, it will make the provisioning of MTN path complicated. Moreover, in some cases the two endpoints may reside in different IGP areas or ASes, which makes a layer-3 adjacency between them more challenging. Last but not the least, the MTN path may be provisioned unidirectionally, which cannot pass the bidirectional connectivity check required for a layer-3 link. Since the MTN paths are usually not visible in the L3 topology, it is difficult to compute and establish an end-to-end inter-layer path which consists of both the layer-3 network segments and the MTN paths.¶

4.2. IP and MTN Integration

Assuming that an operator owns both an MTN network domain and an IP network domain. In the MTN network, each MTN node has both the layer-3 functionality and the MTN Path layer functionality. In layer-3, all the MTN nodes are in a layer-3 network topology, which connects to the IP network domain. In the MTN Path Layer, a set of MTN paths are provisioned between the selected pairs of MTN nodes for traffic engineering. In the MTN network, different types of services may be carried using either a layer-3 path, an end-to-end MTN path, or an inter-layer path comprising of both the layer-3 links and the MTN paths as segments. In addition, For some type of services, end-to-end paths across the IP domain and the MTN domain are needed, which is comprised of both the layer-3 paths and the MTN path as different segments.¶

 .......................................... ...........................
 .                                        . .                         .
 .          +----+     +----+     +----+  . . +----+     +----+       .
 .          | M1 |-----| M2 |-----| M3 |------| P1 |-----| P2 |       .
 .          +----+     +----+     +----+  . . +----+     +----+       .
 .         /  |          |          |     . .   |          |  \       .
 . +----+ /   |          |          |     . .   |          |   \+----+.
 . | M7 |/    |          |          |     . .   |          |    | P5 |.
 . +----+\    |          |          |     . .   |          |   /+----+.
 .        \   |          |          |     . .   |          |  /       .
 .         \+----+     +----+     +----+  . . +----+     +----+       .
 .          | M4 |-----| M5 |-----| M6 |------| P3 |-----| P4 |       .
 .          +----+     +----+     +----+  . . +----+     +----+       .
 .                                        . .                         .
 . Layer-3 Topology    MTN Network        . .        IP Network       .
 .                                        . ...........................
 ----------------------------------------------------------------------
 . MTN Path Layer Topology                .
 .                                        .
 .          +----+     +----+     +----+  .
 .          | M1'|################| M3'|  .
 .          +----+ ##  +----+  ## +----+  .
 .                   ##      ##           .
 . +----+              ##  ##             .
 . | M7'|                ##               .
 . +----+              ##  ##             .
 .                   ##      ##           .
 .          +----+ ##  +----+  ## +----+  .
 .          | M4'|################| M6'|  .
 .          +----+     +----+     +----+  .
 .                                        .
 .                                        .
 ..........................................
         .
      Figure 2. A network with MTN Domain and IP Domain

Figure 2 gives an example of a network with a MTN domain and an IP domain. M1 to M7 are MTN nodes, and P1 to P4 are IP nodes. The same set of MTN nodes builds two separate network layers. The topology in the IP layer shows the layer-3 connectivity between the MTN nodes and the connectivity with the IP network domain, while the topology in the MTN Path layer shows the MTN paths between the selected pair of MTN nodes. An end-to-end path from M7 to P5 can be established in layer-3 using an SRv6 SID list representing the layer-3 path {M7, M1, M2, M3, P1, P2, P5}. While for services which require low latency, an end-to-end path consisting of both the layer-3 segments and MTN paths could be established using an SRv6 SID list representing the inter-layer path {M7, M1::C3, P1, P2, P5}, where the End.XU SID M1::C3 represents the MTN path M1'-M3'.¶

This shows that it is convenient to use integrated SRv6 SID lists to program inter-layer TE paths both within the MTN domain, and across the IP and MTN domain using the combination of L3 SRv6 SIDs and the End.XU SIDs.¶

8.1. Normative References

[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>.

[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>.

[RFC8986]

Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 (SRv6) Network Programming", RFC 8986, DOI 10.17487/RFC8986, February 2021, <https://www.rfc-editor.org/info/rfc8986>.

8.2. Informative References

[I-D.ietf-teas-actn-poi-applicability]

Peruzzini, F., Bouquier, J., Busi, I., King, D., and D. Ceccarelli, "Applicability of Abstraction and Control of Traffic Engineered Networks (ACTN) to Packet Optical Integration (POI)", Work in Progress, Internet-Draft, draft-ietf-teas-actn-poi-applicability-11, 22 February 2024, <https://datatracker.ietf.org/doc/html/draft-ietf-teas-actn-poi-applicability-11>.

[ITU-T_G.8310]

ITU-T, "ITU-T G.8310: Architecture of the metro transport network", https://www.itu.int/rec/T-REC-G.8310-202012-I/en, December 2020.