IDR Working Group J. Tantsura Internet-Draft Apstra, Inc. Intended status: Standards Track U. Chunduri Expires: October 28, 2020 Futurewei Technologies K. Talaulikar Cisco Systems G. Mirsky ZTE Corp. N. Triantafillis Amazon Web Services April 26, 2020 Signaling MSD (Maximum SID Depth) using Border Gateway Protocol - Link State draft-ietf-idr-bgp-ls-segment-routing-msd-17 Abstract This document defines a way for a Border Gateway Protocol - Link State (BGP-LS) speaker 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 Segment Identifier (SID) stack can be supported in a given network. 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 October 28, 2020. Tantsura, et al. Expires October 28, 2020 [Page 1] Internet-Draft Signaling MSD using BGP-LS April 2020 Copyright Notice Copyright (c) 2020 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. Conventions used in this document . . . . . . . . . . . . 3 1.1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 3 1.1.2. Requirements Language . . . . . . . . . . . . . . . . 4 2. Advertisement of MSD via BGP-LS . . . . . . . . . . . . . . . 4 3. Node MSD TLV . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Link MSD TLV . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Procedures for Defining and Using Node and Link MSD Advertisements . . . . . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7. Manageability Considerations . . . . . . . . . . . . . . . . 6 8. Security Considerations . . . . . . . . . . . . . . . . . . . 7 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 11.1. Normative References . . . . . . . . . . . . . . . . . . 8 11.2. Informative References . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction When Segment Routing (SR) [RFC8402] 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. This ensures that the Segment Identifier (SID) stack depth of a computed path doesn't exceed the number of SIDs the node is capable of imposing. [RFC8664] defines how to signal MSD in the Path Computation Element Protocol (PCEP). The OSPF and IS-IS extensions for signaling of MSD are defined in [RFC8476] and [RFC8491] respectively. Tantsura, et al. Expires October 28, 2020 [Page 2] Internet-Draft Signaling MSD using BGP-LS April 2020 However, if PCEP is not supported/configured on the head-end of a SR tunnel or a Binding-SID anchor node, and the controller does not participate in IGP routing, it has no way of learning 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. This document defines extensions to BGP-LS to advertise one or more types of MSDs at node and/or link granularity. Other types of MSD are known to be useful. For example, [I-D.ietf-ospf-mpls-elc] and [I-D.ietf-isis-mpls-elc] define 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. In the future, it is expected that new MSD-Types will be defined to signal additional capabilities, e.g., ELs, SIDs that can be imposed through recirculation, or SIDs associated with another data plane such as IPv6. MSD advertisements may be useful even if SR itself is not enabled. For example, in a non-SR MPLS network, MSD defines the maximum label depth. 1.1. Conventions used in this document 1.1.1. Terminology MSD: Maximum SID Depth - the number of SIDs supported by a node or a link on a node PCE: Path Computation Element PCEP: Path Computation Element Protocol SID: Segment Identifier as defined in [RFC8402] SR: Segment Routing 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. o The number of labels imposed is then the sum of the number of labels that are replaced and the number of labels that are pushed. See [RFC3031] for further details. Tantsura, et al. Expires October 28, 2020 [Page 3] Internet-Draft Signaling MSD using BGP-LS April 2020 1.1.2. 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. Advertisement of MSD via BGP-LS This document describes extensions that enable BGP-LS speakers to signal the MSD capabilities ([RFC8491] ) of nodes and their links in a network to a BGP-LS consumer of network topology such as a centralized controller. The centralized controller can leverage this information in computation of SR paths based on their MSD capabilities. When a BGP-LS speaker is originating the topology learnt via link-state routing protocols such as OSPF or IS-IS, the MSD information for the nodes and their links is sourced from the underlying extensions as defined in [RFC8476] and [RFC8491] respectively. The extensions introduced in this document allow for advertisement of different MSD-Types, which are defined elsewhere and were introduced in [RFC8491]. This enables sharing of MSD-Types that may be defined in the future by the IGPs in BGP-LS. 3. Node MSD TLV The Node MSD ([RFC8476] [RFC8491]) is encoded in a new Node Attribute TLV [RFC7752] to carry the provisioned SID depth of the router identified by the corresponding Router-ID. Node MSD is the smallest MSD supported by the node on the set of interfaces configured for use. MSD values may be learned via a hardware API or may be provisioned. The following format is used: 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 | MSD-Value | MSD-Type... | MSD-Value... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Node MSD TLV Format Where: o Type: 266 Tantsura, et al. Expires October 28, 2020 [Page 4] Internet-Draft Signaling MSD using BGP-LS April 2020 o Length: variable (multiple of 2); represents the total length of the value field in octets. o 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 "IGP MSD-Types" registry defined in [RFC8491]. * MSD-Value : a number in the range of 0-255. For all MSD-Types, 0 represents the lack of ability to impose an 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 protocol instance. 4. Link MSD TLV The Link MSD ([RFC8476] [RFC8491]) is defined to carry the MSD of the interface associated with the link. It is encoded in a new Link Attribute TLV [RFC7752] using the following format: 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 | MSD-Value | MSD-Type... | MSD-Value... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Link MSD TLV Format Where: o Type: 267 o Length: variable (multiple of 2); represents the total length of the value field in octets. o Value : consists of one or more pairs of a 1-octet MSD-Type and 1-octet MSD-Value. * MSD-Type : MSD-Type : one of the values defined in the "IGP MSD-Types" registry defined in [RFC8491]. * MSD-Value : a number in the range of 0-255. For all MSD-Types, 0 represents the lack of ability to impose an MSD stack of any depth; any other value represents that of the link when used as an outgoing interface. Tantsura, et al. Expires October 28, 2020 [Page 5] Internet-Draft Signaling MSD using BGP-LS April 2020 5. 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 in [RFC8491]. 6. IANA Considerations This document requests assigning code-points from the registry "BGP- LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs" based on table below. Early allocation for these code-points have been done by IANA. +------------+-----------------+---------------------------+-------------------+ | Code Point | Description | IS-IS TLV/Sub-TLV | Reference | +------------+-----------------+---------------------------+-------------------+ | 266 | Node MSD | 242/23 | This document | | 267 | Link MSD | (22,23,25,141,222,223)/15 | This document | +------------+-----------------+---------------------------+-------------------+ 7. Manageability Considerations The new protocol extensions introduced in this document augment the existing IGP topology information that is distributed via [RFC7752]. Procedures and protocol extensions defined in this document do not affect the BGP protocol operations and management other than as discussed in the Manageability Considerations section of [RFC7752]. Specifically, the malformed attribute tests for syntactic checks in the Fault Management section of [RFC7752] now encompass the new BGP- LS Attribute TLVs defined in this document. The semantic or content checking for the TLVs specified in this document and their association with the BGP-LS NLRI types or their BGP-LS Attribute is left to the consumer of the BGP-LS information (e.g. an application or a controller) and not the BGP protocol. Tantsura, et al. Expires October 28, 2020 [Page 6] Internet-Draft Signaling MSD using BGP-LS April 2020 A consumer of the BGP-LS information retrieves this information over a BGP-LS session (refer Section 1 and 2 of [RFC7752]). This document only introduces new Attribute TLVs and any syntactic error in them would result in the BGP-LS Attribute being discarded [RFC7752]. The MSD information introduced in BGP-LS by this specification, may be used by BGP-LS consumer applications like a SR path computation engine (PCE) to learn the SR SID stack handling capabilities of the nodes in the topology. This can enable the SR PCE to perform path computations taking into consideration the size of SID stack that the specific head-end node may be able to impose. Errors in the encoding or decoding of the MSD information may result in the unavailability of such information to the SR PCE or incorrect information being made available to it. This may result in the head- end node not being able to instantiate the desired SR path in its forwarding and provide the SR based optimization functionality. The handling of such errors by applications like SR PCE may be implementation specific and out of scope of this document. The extensions specified in this document do not specify any new configuration or monitoring aspects in BGP or BGP-LS. The specification of BGP models is an ongoing work based on the [I-D.ietf-idr-bgp-model]. 8. Security Considerations The 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 may also inform an attacker of how to induce any of the aforementioned conditions. The procedures and protocol extensions defined in this document do not affect the BGP security model. See the "Security Considerations" section of [RFC4271] for a discussion of BGP security. Also, refer to [RFC4272] and [RFC6952] for analyses of security issues for BGP. Security considerations for acquiring and distributing BGP-LS information are discussed in [RFC7752]. The TLVs introduced in this document are used to propagate the MSD IGP extensions defined in [RFC8476] [RFC8491]. It is assumed that the IGP instances originating these TLVs will support all the required security (as described in [RFC8476] [RFC8491]) in order to prevent any security issues when propagating the TLVs into BGP-LS. The advertisement of the node and link attribute information defined in this document presents no additional risk beyond that associated with the existing node and link attribute information already supported in [RFC7752]. Tantsura, et al. Expires October 28, 2020 [Page 7] Internet-Draft Signaling MSD using BGP-LS April 2020 9. Contributors Siva Sivabalan Cisco Systems Inc. Canada Email: msiva@cisco.com 10. Acknowledgements We like to thank Acee Lindem, Stephane Litkowski, Bruno Decraene and Alvaro Retana for their reviews and valuable comments. 11. References 11.1. 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, . [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak, "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, DOI 10.17487/RFC8476, December 2018, . [RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, DOI 10.17487/RFC8491, November 2018, . 11.2. Informative References [I-D.ietf-idr-bgp-model] Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP YANG Model for Service Provider Networks", draft-ietf-idr- bgp-model-08 (work in progress), February 2020. Tantsura, et al. Expires October 28, 2020 [Page 8] Internet-Draft Signaling MSD using BGP-LS April 2020 [I-D.ietf-isis-mpls-elc] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., and M. Bocci, "Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS", draft-ietf- isis-mpls-elc-11 (work in progress), March 2020. [I-D.ietf-ospf-mpls-elc] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., and M. Bocci, "Signaling Entropy Label Capability and Entropy Readable Label Depth Using OSPF", draft-ietf-ospf- mpls-elc-13 (work in progress), April 2020. [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, DOI 10.17487/RFC3031, January 2001, . [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006, . [RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", RFC 4272, DOI 10.17487/RFC4272, January 2006, . [RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of BGP, LDP, PCEP, and MSDP Issues According to the Keying and Authentication for Routing Protocols (KARP) Design Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013, . [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, . [RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., and J. Hardwick, "Path Computation Element Communication Protocol (PCEP) Extensions for Segment Routing", RFC 8664, DOI 10.17487/RFC8664, December 2019, . Authors' Addresses Tantsura, et al. Expires October 28, 2020 [Page 9] Internet-Draft Signaling MSD using BGP-LS April 2020 Jeff Tantsura Apstra, Inc. Email: jefftant.ietf@gmail.com Uma Chunduri Futurewei Technologies Email: umac.ietf@gmail.com Ketan Talaulikar Cisco Systems Email: ketant@cisco.com Greg Mirsky ZTE Corp. Email: gregimirsky@gmail.com Nikos Triantafillis Amazon Web Services Email: nikost@amazon.com Tantsura, et al. Expires October 28, 2020 [Page 10]