OSPF Working Group J. Tantsura
Internet-Draft Nuage Networks
Intended status: Standards Track U. Chunduri
Expires: November 18, 2018 Huawei Technologies
S. Aldrin
Google, Inc
P. Psenak
Cisco Systems
May 17, 2018

Signaling MSD (Maximum SID Depth) using OSPF
draft-ietf-ospf-segment-routing-msd-13

Abstract

This document defines a way for an OSPF Router to advertise multiple types of supported Maximum SID Depths (MSDs) at node and/or link granularity. Such advertisements allow entities (e.g., centralized controllers) to determine whether a particular SID stack can be supported in a given network. This document defines only one type of MSD, but defines an encoding that can support other MSD types. Here the term OSPF means both OSPFv2 and OSPFv3.

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on November 18, 2018.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.


Table of Contents

1. Introduction

When Segment Routing(SR) paths are computed by a centralized controller, it is critical that the controller learns the Maximum SID Depth(MSD) that can be imposed at each node/link on a given SR path to insure that the SID stack depth of a computed path doesn't exceed the number of SIDs the node is capable of imposing.

The PCEP SR extensions draft [I-D.ietf-pce-segment-routing] signals MSD in SR PCE Capability TLV and METRIC Object. However, if PCEP is not supported/configured on the head-end of an SR tunnel or a Binding-SID anchor node and controller do not participate in IGP routing, it has no way to learn the MSD of nodes and links. BGP-LS [RFC7752] defines a way to expose topology and associated attributes and capabilities of the nodes in that topology to a centralized controller. MSD signaling by BGP-LS has been defined in [I-D.ietf-idr-bgp-ls-segment-routing-msd]. Typically, BGP-LS is configured on a small number of nodes that do not necessarily act as head-ends. In order for BGP-LS to signal MSD for all the nodes and links in the network MSD is relevant, MSD capabilites should be advertised by every OSPF router in the network.

Other types of MSD are known to be useful. For example, [I-D.ietf-ospf-mpls-elc] defines Readable Label Depth Capability (RLDC) that is used by a head-end to insert an Entropy Label (EL) at a depth that can be read by transit nodes.

This document defines an extension to OSPF used to advertise one or more types of MSD at node and/or link granularity. It also defines the Base MPLS Imposition MSD type. In the future it is expected, that new MSD types will be defined to signal additional capabilities e.g., entropy labels, SIDs that can be imposed through recirculation, or SIDs associated with another dataplane e.g., IPv6. Although MSD advertisements are associated with Segment Routing, the advertisements MAY be present even if Segment Routing itself is not enabled.

1.1. Conventions used in this document

1.1.1. Terminology

This memo makes use of the terms defined in [RFC7770]

BGP-LS: Distribution of Link-State and TE Information using Border Gateway Protocol

BMI: Base MPLS Imposition is the number of MPLS labels that can be imposed inclusive of all service/transport/special labels

OSPF: Open Shortest Path First

MSD: Maximum SID Depth - the number of SIDs a node or one of its links can support

PCC: Path Computation Client

PCE: Path Computation Element

PCEP: Path Computation Element Protocol

SR: Segment Routing

SID: Segment Identifier

LSA: Link state advertisement

RI: OSPF Router Information LSA

1.2. Requirements Language

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 BCP14 [RFC2119], [RFC8174] when, and only when they appear in all capitals, as shown here .

2. Node MSD Advertisement

The node MSD TLV within the body of the OSPF RI Opaque LSA is defined to carry the provisioned SID depth of the router originating the RI LSA. Node MSD is the smallest MSD supported by the node on the set of interfaces configured for use by the advertising IGP instance. MSD values may be learned via a hardware API or may be provisioned..

       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

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Type                       |         Length                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         MSD Type and Value ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

          

Figure 1: Node MSD TLV

The Type: TBD1

Length: variable (minimum of 2, multiple of 2 octets) and represents the total length of value field.

Value: consists of one or more pairs of a 1 octet MSD-type and 1 octet MSD-Value.

MSD-Type: one of the values defined in the MSD Types registry defined in [I-D.ietf-isis-segment-routing-msd].

MSD-Value: a number in the range of 0-255. For all MSD-Types, 0 represents lack of the ability to impose MSD stack of any depth; any other value represents that of the node. This value MUST represent the lowest value supported by any link configured for use by the advertising OSPF instance.

This TLV is applicable to OSPFv2 and to OSPFv3 [RFC5838] and is optional. The scope of the advertisement is specific to the deployment.

When multiple Node MSD TLVs are received from a given router, the receiver MUST use the first occurrence of the TLV in the Router Information LSA. If the Node MSD TLV appears in multiple Router Information LSAs that have different flooding scopes, the Node MSD TLV in the Router Information LSA with the area-scoped flooding scope MUST be used. If the Node MSD TLV appears in multiple Router Information LSAs that have the same flooding scope, the Node MSD TLV in the Router Information (RI) LSA with the numerically smallest Instance ID MUST be used and subsequent instances of the Node MSD TLV MUST be ignored. The RI LSA can be advertised at any of the defined opaque flooding scopes (link, area, or Autonomous System (AS)). For the purpose of Node MSD TLV advertisement, area-scoped flooding is REQUIRED.

3. Link MSD sub-TLV

The link sub-TLV is defined to carry the MSD of the interface associated with the link. MSD values may be learned via a hardware API or may be provisioned.

       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

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Type                       |         Length                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         MSD Type and Value ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

          

Figure 2: Link MSD Sub-TLV

Type:

For OSPFv2, the Link level MSD value is advertised as an optional Sub-TLV of the OSPFv2 Extended Link TLV as defined in [RFC7684], and has value of TBD2.

For OSPFv3, the Link level MSD value is advertised as an optional Sub-TLV of the E-Router-LSA TLV as defined in [RFC8362], and has value of TBD3.

Length: variable and similar to that, defined in Section 2.

Value: consists of one or more pairs of a 1 octet MSD-type and 1 octet MSD-Value.

MSD-Type: one of the values defined in the MSD Types registry defined in [I-D.ietf-isis-segment-routing-msd].

MSD-Value field contains Link MSD of the router originating the corresponding LSA as specified for OSPFv2 and OSPFv3. Link MSD is a number in the range of 0-255. For all MSD-Types, 0 represents lack of the ability to impose MSD stack of any depth; any other value represents that of the particular link when used as an outgoing interface.

Other MSD Types are reserved for future extensions.

If this TLV is advertised multiple times in the same OSPFv2 Extended Link Opaque LSA, only the first instance of the TLV is used by receiving OSPFv2 routers. This situation SHOULD be logged as an error.

If this TLV is advertised multiple times for the same link in different OSPFv2 Extended Link Opaque LSAs originated by the same OSPFv2 router, the OSPFv2 Extended Link TLV in the OSPFv2 Extended Link Opaque LSA with the smallest Opaque ID is used by receiving OSPFv2 routers. This situation may be logged as a warning.

4. Using Node and Link MSD Advertisements

When Link MSD is present for a given MSD type, the value of the Link MSD MUST take preference over the Node MSD. When a Link MSD type is not signalled but the Node MSD type is, then the value of that Link MSD type MUST be considered as the corresponding Node MSD type value. In order to increase flooding efficiency, it is RECOMMENDED, that routers with homogenous link MSD values advertise just the Node MSD value.

The meaning of the absence of both Node and Link MSD advertisements for a given MSD type is specific to the MSD type. Generally it can only be inferred that the advertising node does not support advertisement of that MSD type. However, in some cases the lack of advertisement might imply that the functionality associated with the MSD type is not supported. The correct interpretation MUST be specified when an MSD type is defined.

5. Base MPLS Imposition MSD

The Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS labels a node is capable of imposing, including all service/transport/special labels.

Absence of BMI-MSD advertisements indicates solely that the advertising node does not support advertisement of this capability. Assignment of MSD-Type for BMI-MSD is defined in [I-D.ietf-isis-segment-routing-msd].

6. IANA Considerations

This document requests IANA to allocate TLV type (TBD1) from the OSPF Router Information (RI) TLVs Registry as defined by [RFC4970]. IANA has allocated the value 12 through the early assignment process. Also, this document requests IANA to allocate a sub-TLV type (TBD2) from the OSPFv2 Extended Link TLV Sub-TLVs registry. IANA has allocated the the value 6 through the early assignment process. Finally, this document requests IANA to allocate a sub-TLV type (TBD3) from the OSPFv3 Extended-LSA Sub-TLV registry.

7. Security Considerations

Security concerns for OSPF are addressed in [RFC7474]. Further security analysis for OSPF protocol is done in [RFC6863] including analysis of both the above documents. Security considerations, as specified by [RFC7770], [RFC7684] and [RFC8362] are applicable to this document.

Advertisement of an incorrect MSD value may result: in a path computation failing and the service unavailable or instantiation of a path that can't be supported by the head-end (the node performing the imposition).

8. Contributors

The following people contributed to this document:

Les Ginsberg

Email: ginsberg@cisco.com

9. Acknowledgements

The authors would like to thank Acee Lindem, Ketan Talaulikar, Tal Mizrahi, Stephane Litkowski and Bruno Decraene for their reviews and valuable comments.

10. References

10.1. Normative References

[I-D.ietf-isis-segment-routing-msd] Tantsura, J., Chunduri, U., Aldrin, S. and L. Ginsberg, "Signaling MSD (Maximum SID Depth) using IS-IS", Internet-Draft draft-ietf-isis-segment-routing-msd-12, May 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4970] Lindem, A., Shen, N., Vasseur, JP., Aggarwal, R. and S. Shaffer, "Extensions to OSPF for Advertising Optional Router Capabilities", RFC 4970, DOI 10.17487/RFC4970, July 2007.
[RFC7474] Bhatia, M., Hartman, S., Zhang, D. and A. Lindem, "Security Extension for OSPFv2 When Using Manual Key Management", RFC 7474, DOI 10.17487/RFC7474, April 2015.
[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W., Tantsura, J. and A. Lindem, "OSPFv2 Prefix/Link Attribute Advertisement", RFC 7684, DOI 10.17487/RFC7684, November 2015.
[RFC7770] Lindem, A., Shen, N., Vasseur, JP., Aggarwal, R. and S. Shaffer, "Extensions to OSPF for Advertising Optional Router Capabilities", RFC 7770, DOI 10.17487/RFC7770, February 2016.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V. and F. Baker, "OSPFv3 Link State Advertisement (LSA) Extensibility", RFC 8362, DOI 10.17487/RFC8362, April 2018.

10.2. Informative References

[I-D.ietf-idr-bgp-ls-segment-routing-msd] Tantsura, J., Chunduri, U., Mirsky, G. and S. Sivabalan, "Signaling Maximum SID Depth using Border Gateway Protocol Link-State", Internet-Draft draft-ietf-idr-bgp-ls-segment-routing-msd-01, October 2017.
[I-D.ietf-ospf-mpls-elc] Xu, X., Kini, S., Sivabalan, S., Filsfils, C. and S. Litkowski, "Signaling Entropy Label Capability and Readable Label-stack Depth Using OSPF", Internet-Draft draft-ietf-ospf-mpls-elc-05, January 2018.
[I-D.ietf-pce-segment-routing] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W. and J. Hardwick, "PCEP Extensions for Segment Routing", Internet-Draft draft-ietf-pce-segment-routing-11, November 2017.
[RFC5838] Lindem, A., Mirtorabi, S., Roy, A., Barnes, M. and R. Aggarwal, "Support of Address Families in OSPFv3", RFC 5838, DOI 10.17487/RFC5838, April 2010.
[RFC6863] Hartman, S. and D. Zhang, "Analysis of OSPF Security According to the Keying and Authentication for Routing Protocols (KARP) Design Guide", RFC 6863, DOI 10.17487/RFC6863, March 2013.
[RFC7752] Gredler, H., 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, March 2016.
[RFC8126] Cotton, M., Leiba, B. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017.

Authors' Addresses

Jeff Tantsura Nuage Networks EMail: jefftant.ietf@gmail.com
Uma Chunduri Huawei Technologies EMail: uma.chunduri@huawei.com
Sam Aldrin Google, Inc EMail: aldrin.ietf@gmail.com
Peter Psenak Cisco Systems EMail: ppsenak@cisco.com