IS-IS Working Group J. Tantsura Internet-Draft Apstra, Inc. Intended status: Standards Track U. Chunduri Expires: April 12, 2019 Huawei Technologies S. Aldrin Google, Inc L. Ginsberg Cisco Systems October 9, 2018 Signaling MSD (Maximum SID Depth) using IS-IS draft-ietf-isis-segment-routing-msd-19 Abstract This document defines a way for an Intermediate System to Intermediate System (IS-IS) 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 (Segment ID) stack can be supported in a given network. This document only defines one type of MSD (Base MPLS Imposition), but defines an encoding that can support other MSD types. This document focuses on MSD use in a Segment Routing enabled network, but MSD may also be useful when Segment Routing is not enabled. 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 April 12, 2019. Tantsura, et al. Expires April 12, 2019 [Page 1]
Internet-Draft draft-ietf-isis-segment-routing-msd October 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4 2. Node MSD Advertisement . . . . . . . . . . . . . . . . . . . 4 3. Link MSD Advertisement . . . . . . . . . . . . . . . . . . . 5 4. Procedures for Defining and Using Node and Link MSD Advertisements . . . . . . . . . . . . . . . . . . . . . . . 6 5. Base MPLS Imposition MSD . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . 9 10.2. Informative References . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 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 of a given SR path to ensure that the Segment Identifier (SID) stack depth of a computed path does not exceed the number of SIDs the node is capable of imposing. [I-D.ietf-pce-segment-routing] defines how to signal MSD in the Path Computation Element communication Protocol (PCEP). However, if PCEP is not supported/configured on the head-end of an SR tunnel or a Binding-SID anchor node and controller does not participate in IGP routing, it has no way to learn the MSD of nodes and links. BGP-LS Tantsura, et al. Expires April 12, 2019 [Page 2]
Internet-Draft draft-ietf-isis-segment-routing-msd October 2018 (Distribution of Link-State and TE Information using Border Gateway Protocol) [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 capabilities SHOULD be advertised by every Intermediate System to Intermediate System(IS-IS) router in the network. Other types of MSD are known to be useful. For example, [I-D.ietf-isis-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 could be read by transit nodes. This document defines an extension to IS-IS used to advertise one or more types of MSD at node and/or link granularity. It also creates an IANA registry for assigning MSD-type identifiers. 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. MSD advertisements MAY be useful even if Segment Routing itself is not enabled. For example, in a non-SR MPLS network, MSD defines the maximum label depth. 1.1. Terminology BMI: Base MPLS Imposition is the number of MPLS labels which can be imposed inclusive of all service/transport/special labels MSD: Maximum SID Depth - the number of SIDs supported by a node or a link on a node SID: Segment Identifier as defined in [RFC8402] Label Imposition: Imposition is the act of modifying and/or adding labels to the outgoing label stack associated with a packet. This includes: o replacing the label at the top of the label stack with a new label o pushing one or more new labels onto the label stack Tantsura, et al. Expires April 12, 2019 [Page 3]
Internet-Draft draft-ietf-isis-segment-routing-msd October 2018 The number of labels imposed is then the sum of the number of labels which are replaced and the number of labels which are pushed. See [RFC3031] for further details. 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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here . 2. Node MSD Advertisement The node MSD sub-TLV is defined within the body of the IS-IS Router Capability TLV [RFC7981], to carry the provisioned SID depth of the router originating the Router Capability TLV. 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 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MSD-Type | MSD-Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // ................... // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MSD-Type | MSD-Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Node MSD Sub-TLV Type: 23 (allocated by IANA via the early assignment process) Length: variable (multiple of 2 octets) and represents the total length of value field. Value: field consists of one or more pairs of a 1 octet MSD-Type and 1 octet MSD-Value. MSD-Type is a value defined in the IGP MSD-Types registry created by the IANA Section of this document. Tantsura, et al. Expires April 12, 2019 [Page 4]
Internet-Draft draft-ietf-isis-segment-routing-msd October 2018 MSD-Value is a number in the range of 0-255. For all MSD-Types, 0 represents lack of the ability to support SID 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 IS-IS instance. This sub-TLV is optional. The scope of the advertisement is specific to the deployment. If there exist multiple Node MSD advertisements for the same MSD-Type originated by the same router, the procedures defined in [RFC7981] apply. These procedures may result in different MSD values being used by (for example) different controllers - but this does not create any interoperability issue. 3. Link MSD Advertisement The link MSD sub-TLV is defined for TLVs 22, 23, 25, 141, 222, and 223 to carry the MSD of the interface associated with the link. MSD values may be signaled by the forwarding plane or may be provisioned. 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MSD-Type | MSD-Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // ................... // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MSD-Type | MSD-Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Link MSD Sub-TLV Type: 15 (allocated by IANA via the early assignment process) Length: variable (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 is a value defined in the MSD-Types registry created by the IANA Section of this document. Tantsura, et al. Expires April 12, 2019 [Page 5]
Internet-Draft draft-ietf-isis-segment-routing-msd October 2018 MSD-Value is a number in the range of 0-255. For all MSD-Types, 0 represents lack of the ability to support SID stack of any depth; any other value represents that of the particular link when used as an outgoing interface. This sub-TLV is optional. If multiple Link MSD advertisements for the same MSD-Type and the same link are received, the procedure used to select which copy is used is undefined. If the advertising router performs label imposition in the context of the ingress interface, it is not possible to meaningfully advertise per link values. In such a case only the Node MSD SHOULD be advertised. 4. Procedures for Defining and Using Node and Link MSD Advertisements When Link MSD is present for a given MSD-type, the value of the Link MSD MUST take precedence over the Node MSD. When a Link MSD-type is not signaled but the Node MSD-type is, then the Node MSD-type value MUST be considered as the MSD value for that link. 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 Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS labels which can be imposed, including all service/transport/special labels. Absence of BMI-MSD advertisements indicates solely that the advertising node does not support advertisement of this capability. Tantsura, et al. Expires April 12, 2019 [Page 6]
Internet-Draft draft-ietf-isis-segment-routing-msd October 2018 6. IANA Considerations This document requests IANA to allocate a sub-TLV type for the new sub TLV proposed in Section 2 of this document from IS-IS Router Capability TLV Registry as defined by [RFC7981]. IANA has allocated the following value through the early assignment process: Value Description Reference ----- --------------- ------------- 23 Node MSD This document Figure 3: Node MSD This document requests IANA to allocate a sub-TLV type as defined in Section 3 from Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223 registry. IANA has allocated the following value through the early assignment process: Value Description Reference ----- --------------- ------------- 15 Link MSD This document Figure 4: Link MSD Per TLV information where Link MSD sub-TLV can be part of: TLV 22 23 25 141 222 223 --- -------------------- y y y y y y Figure 5: TLVs where LINK MSD Sub-TLV can be present This document requests creation of an IANA managed registry under the category of "Interior Gateway Protocol (IGP) Parameters" IANA registries to identify MSD-types as proposed in Section 2 and Section 3. The registration procedure is "Expert Review" as defined in [RFC8126]. Suggested registry name is "IGP MSD-Types". Types are an unsigned 8 bit number. The following values are defined by this document: Tantsura, et al. Expires April 12, 2019 [Page 7]
Internet-Draft draft-ietf-isis-segment-routing-msd October 2018 Value Name Reference ----- --------------------- ------------- 0 Reserved This document 1 Base MPLS Imposition MSD This document 2-250 Unassigned This document 251-254 Experimental Use This document 255 Reserved This document Figure 6: MSD-Types Codepoints Registry General guidance for the Designated Experts is as defined in [RFC7370] 7. Security Considerations Security considerations as specified by [RFC7981] are applicable to this document. Advertisement of an incorrect MSD value may have negative consequences. If the value is smaller than supported, path computation may fail to compute a viable path. If the value is larger than supported, an attempt to instantiate a path that can't be supported by the head-end (the node performing the SID imposition) may occur. The presence of this information also may inform an attacker of how to induce any of the aforementioned conditions. 8. Contributors The following people contributed to this document: Peter Psenak Email: ppsenak@cisco.com 9. Acknowledgements The authors would like to thank Acee Lindem, Ketan Talaulikar, Stephane Litkowski and Bruno Decraene for their reviews and valuable comments. 10. References Tantsura, et al. Expires April 12, 2019 [Page 8]
Internet-Draft draft-ietf-isis-segment-routing-msd October 2018 10.1. 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>. [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, DOI 10.17487/RFC3031, January 2001, <https://www.rfc-editor.org/info/rfc3031>. [RFC7370] Ginsberg, L., "Updates to the IS-IS TLV Codepoints Registry", RFC 7370, DOI 10.17487/RFC7370, September 2014, <https://www.rfc-editor.org/info/rfc7370>. [RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions for Advertising Router Information", RFC 7981, DOI 10.17487/RFC7981, October 2016, <https://www.rfc-editor.org/info/rfc7981>. [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, <https://www.rfc-editor.org/info/rfc8126>. [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>. [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, July 2018, <https://www.rfc-editor.org/info/rfc8402>. 10.2. Informative References [I-D.ietf-idr-bgp-ls-segment-routing-msd] Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan, "Signaling MSD (Maximum SID Depth) using Border Gateway Protocol Link-State", draft-ietf-idr-bgp-ls-segment- routing-msd-02 (work in progress), August 2018. [I-D.ietf-isis-mpls-elc] Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S. Litkowski, "Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS", draft-ietf-isis-mpls- elc-06 (work in progress), September 2018. Tantsura, et al. Expires April 12, 2019 [Page 9]
Internet-Draft draft-ietf-isis-segment-routing-msd October 2018 [I-D.ietf-pce-segment-routing] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., and J. Hardwick, "PCEP Extensions for Segment Routing", draft-ietf-pce-segment-routing-12 (work in progress), June 2018. [RFC7752] 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, March 2016, <https://www.rfc-editor.org/info/rfc7752>. Authors' Addresses Jeff Tantsura Apstra, Inc. Email: jefftant.ietf@gmail.com Uma Chunduri Huawei Technologies Email: uma.chunduri@huawei.com Sam Aldrin Google, Inc Email: aldrin.ietf@gmail.com Les Ginsberg Cisco Systems Email: ginsberg@cisco.com Tantsura, et al. Expires April 12, 2019 [Page 10]