Internet-Draft SR over FA Links February 2022
Saad, et al. Expires 26 August 2022 [Page]
SPRING Working Group
Intended Status:
T. Saad
Juniper Networks, Inc.
V.P. Beeram
Juniper Networks, Inc.
C. Barth
Juniper Networks, Inc.
S. Sivabalan
Ciena Corporation.

Segment-Routing over Forwarding Adjacency Links


Label Switched Paths (LSPs) set up in Multiprotocol Label Switching (MPLS) networks can be used to form Forwarding Adjacency (FA) links that carry traffic in those networks. An FA link can be assigned Traffic Engineering (TE) parameters that allow other LSR(s) to include it in their constrained path computation. FA link(s) can be also assigned Segment-Routing (SR) segments that enable the steering of traffic on to the associated FA link(s). The TE and SR attributes of an FA link can be advertised using known protocols that carry link state information. This document elaborates on the usage of FA link(s) and their attributes in SR enabled networks.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 26 August 2022.

Table of Contents

1. Introduction

To improve scalability in Multi-Protocol Label Switching (MPLS) networks, it may be useful to create a hierarchy of LSPs as Forwarding Adjacencies (FA). The concept of FA link(s) and FA-LSP(s) was introduced in [RFC4206].

In Segment-Routing (SR), this is particularly useful for two main reasons.

First, it allows the stitching of sub-path(s) so as to realize an end-to-end SR path. Each sub-path can be represented by a FA link that is supported by one or more underlying LSP(s). The underlying LSP(s) that support an FA link can be setup using different technologies- including RSVP-TE, LDP, and SR. The sub-path(s), or FA link(s) in this case, can possibly interconnect multiple administrative domains, allowing each FA link within a domain to use a different technology to setup the underlying LSP(s).

Second, it allows shortening of a large SR Segment-List by compressing one or more slice(s) of the list into a corresponding FA TE link that each can be represented by a single segment- see Section 4. Effectively, it reduces the number of segments that an ingress router has to impose to realize an end-to-end path.

The FA links are treated as normal link(s) in the network and hence it can leverage existing link state protocol extensions to advertise properties associated with the FA link. For example, Traffic-Engineering (TE) link parameters and Segment-Routing (SR) segments parameters can be associated with the FA link and advertised throughout the network.

Once advertised in the network using a suitable protocols that support carrying link state information, such as OSPF, ISIS or BGP Link State (LS)), other LSR(s) in the network can use the FA TE link(s) as well as possibly other normal TE link(s) when performing path computation and/or when specifying the desired explicit path.

Though the concepts discussed in this document are specific to MPLS technology, these are also extensible to other dataplane technologies - e.g. SRv6.

2. Terminology

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.

5. IANA Considerations

This document has no IANA actions.

6. Security Considerations


7. Acknowledgement

The authors would like to thank Peter Psenak for reviewing and providing valuable feedback on this document.

8. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links in Resource ReSerVation Protocol - Traffic Engineering (RSVP-TE)", RFC 3477, DOI 10.17487/RFC3477, , <>.
Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering (TE) Extensions to OSPF Version 2", RFC 3630, DOI 10.17487/RFC3630, , <>.
Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP) Hierarchy with Generalized Multi-Protocol Label Switching (GMPLS) Traffic Engineering (TE)", RFC 4206, DOI 10.17487/RFC4206, , <>.
Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, , <>.
Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, , <>.
Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and S. Ray, "North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP", RFC 7752, DOI 10.17487/RFC7752, , <>.
Farrel, A., Ed., Drake, J., Bitar, N., Swallow, G., Ceccarelli, D., and X. Zhang, "Problem Statement and Architecture for Information Exchange between Interconnected Traffic-Engineered Networks", BCP 206, RFC 7926, DOI 10.17487/RFC7926, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, , <>.
Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of IGP Traffic Engineering Performance Metric Extensions", RFC 8571, DOI 10.17487/RFC8571, , <>.
Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler, H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF Extensions for Segment Routing", RFC 8665, DOI 10.17487/RFC8665, , <>.
Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C., Bashandy, A., Gredler, H., and B. Decraene, "IS-IS Extensions for Segment Routing", RFC 8667, DOI 10.17487/RFC8667, , <>.

Authors' Addresses

Tarek Saad
Juniper Networks, Inc.
Vishnu Pavan Beeram
Juniper Networks, Inc.
Colby Barth
Juniper Networks, Inc.
Siva Sivabalan
Ciena Corporation.