LSR Working Group W. Britto Internet-Draft S. Hegde Intended status: Standards Track P. Kaneriya Expires: 24 April 2023 R. Shetty R. Bonica Juniper Networks P. Psenak Cisco Systems 21 October 2022 IGP Flexible Algorithms (Flex-Algorithm) In IP Networks draft-ietf-lsr-ip-flexalgo-07 Abstract An IGP Flexible Algorithm (Flex-Algorithm) allows IGPs to compute constraint-based paths. The base IGP Flex-Algorithm specification describes how it is used with Segment Routing (SR) data planes - SR MPLS and SRv6. This document extends IGP Flex-Algorithm, so that it can be used with regular IPv4 and IPv6 forwarding. 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 24 April 2023. Copyright Notice Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved. Britto, et al. Expires 24 April 2023 [Page 1] Internet-Draft IP Flex-Algorithm October 2022 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 3. Use Case Example . . . . . . . . . . . . . . . . . . . . . . 3 4. Advertising Flex-Algorithm Definitions (FAD) . . . . . . . . 3 5. Advertising IP Flex-Algorithm Participation . . . . . . . . . 3 5.1. The IS-IS IP Algorithm Sub-TLV . . . . . . . . . . . . . 4 5.2. The OSPF IP Algorithm TLV . . . . . . . . . . . . . . . . 5 6. Advertising IP Flex-Algorthm Reachability . . . . . . . . . . 6 6.1. The IS-IS IPv4 Algorithm Prefix Reachability TLV . . . . 6 6.2. The IS-IS IPv6 Algorithm Prefix Reachability TLV . . . . 8 6.3. The OSPFv2 IP Algorithm Prefix Reachability Sub-TLV . . . 9 6.4. LSInfinity metric for TLVs defined in RFC8362 . . . . . . 10 6.5. The OSPFv3 IP Algorithm Prefix Reachability Sub-TLV . . . 10 6.6. The OSPF IP Flexible Algorithm ASBR Metric Sub-TLV . . . 12 7. Calculating of IP Flex-Algorthm Paths . . . . . . . . . . . . 13 8. IP Flex-Algorthm Forwarding . . . . . . . . . . . . . . . . . 14 9. Deployment Considerations . . . . . . . . . . . . . . . . . . 14 10. Protection . . . . . . . . . . . . . . . . . . . . . . . . . 15 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 12. Security Considerations . . . . . . . . . . . . . . . . . . . 18 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 14.1. Normative References . . . . . . . . . . . . . . . . . . 18 14.2. Informative References . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction An IGP Flex-Algorithm as specified in [I-D.ietf-lsr-flex-algo] computes a constraint-based path to: * All Flex-Algorithm specific Prefix Segment Identifiers (SIDs) [RFC8402]. * All Flex-Algorithm specific SRv6 Locators [RFC8986]. Britto, et al. Expires 24 April 2023 [Page 2] Internet-Draft IP Flex-Algorithm October 2022 Therefore, Flex-Algorithm cannot be deployed in the absence of SR and SRv6. This document extends Flex-Algorithm, allowing it to compute paths to IPv4 and IPv6 prefixes. 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. 3. Use Case Example Mobile networks are becoming more and more IP centric. Each end-user session from a gNB (gNodeB) can be destined to a specific UPFs (User Plane Function) based on the session requirements. For example, some sessions require high bandwidth, others need to be routed along the lowest latency path. Each UPF is assigned a unique IP address. As a result, traffic for different sessions is destined to a different destination IP address. By associating the prefix that contains the UPF addresses with a specific IP algorithm and routing the algorithm specific traffic according to a certain constraints, e.g., low latency, a session traffic is routed according to the SLA (Service Level Agreement) appropriate for such session. 4. Advertising Flex-Algorithm Definitions (FAD) To guarantee loop free forwarding, all routers that participate in a Flex-Algorithm MUST agree on the Flex-Algorithm Definition (FAD). Selected nodes within the IGP domain MUST advertise FADs as described in Sections 5, 6, and 7 of [I-D.ietf-lsr-flex-algo]. 5. Advertising IP Flex-Algorithm Participation A node may use various algorithms when calculating paths to nodes and prefixes. Algorithm values are defined in the IGP Algorithm Type Registry [IANA-ALG]. A node MUST participate in a Flex-Algorithm to be: * Able to compute path for such Flex-Algorithm Britto, et al. Expires 24 April 2023 [Page 3] Internet-Draft IP Flex-Algorithm October 2022 * Part of the topology for such Flex-Algorithm Flex-Algorithm participation MUST be advertised for each Flex- Algorithm data-plane independently, as specified in [I-D.ietf-lsr-flex-algo]. Using Flex-Algorithm for regular IPv4 and IPv6 prefixes represents an independent Flex-Algorithm data-plane, and as such, the Flex-Algorithm participation for the IP Flex- Algorithm data-plane MUST be signalled independently of any other Flex-Algorithm data-plane (e.g., SR). Advertisement of participation in IP Flex-Algorithm MUST NOT impact the router participation in default algorithm 0. Advertisement of participation in IP Flex-Algorithm MUST NOT impact the router participation signaled for other data-planes. The following sections describe how the IP Flex-Algorithm participation is advertised in IGP protocols. 5.1. The IS-IS IP Algorithm Sub-TLV The ISIS [ISO10589] IP Algorithm Sub-TLV is a sub-TLV of the IS-IS Router Capability TLV [RFC7981] and has 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Algorithm 1 | Algorithm 2 | Algorithm ... | Algorithm n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: IS-IS IP Algorithm Sub-TLV * Type: IP Algorithm Sub-TLV (Value 29) * Length: Variable * Algorithm (1 octet): Value from 128 to 255. The IP Algorithm Sub-TLV MUST be propagated throughout the level and MUST NOT be advertised across level boundaries. Therefore, the S bit in the Router Capability TLV, in which the IP Algorithm Sub-TLV is advertised, MUST NOT be set. Britto, et al. Expires 24 April 2023 [Page 4] Internet-Draft IP Flex-Algorithm October 2022 The IP Algorithm Sub-TLV is optional. It MUST NOT be advertised more than once at a given level. A router receiving multiple IP Algorithm sub-TLVs from the same originator MUST select the first advertisement in the lowest-numbered LSP and subsequent instances of the IP Algorithm Sub-TLV MUST be ignored. The use of IP Algorithm Sub-TLV to advertise support for algorithms outside the Flex-Algorithm range (128-255) is outside the scope of this document. The IP Flex-Algorithm participation advertised in the IS-IS IP Algorithm Sub-TLV is topology independent. When a router advertises participation in the IS-IS IP Algorithm Sub-TLV, the participation applies to all topologies in which the advertising node participates. 5.2. The OSPF IP Algorithm TLV The OSPF [RFC2328] IP Algorithm TLV is a top-level TLV of the Router Information Opaque LSA [RFC7770] and has 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Algorithm 1 | Algorithm... | Algorithm n | | +- -+ | | + + Figure 2: OSPF IP Algorithm TLV * Type: IP Algorithm TLV (Value TBD by IANA) * Length: Variable * Algorithm (1 octet): Value from 128 to 255. The IP Algorithm TLV is optional. It MUST only be advertised once in the Router Information LSA. When multiple IP Algorithm TLVs are received from a given router, the receiver MUST use the first occurrence of the TLV in the Router Information LSA. If the IP Algorithm TLV appears in multiple Router Information LSAs that have different flooding scopes, the IP Algorithm TLV in the Router Information LSA with the area-scoped flooding scope MUST be used. If the IP Algorithm TLV appears in multiple Router Information LSAs that have the same flooding scope, Britto, et al. Expires 24 April 2023 [Page 5] Internet-Draft IP Flex-Algorithm October 2022 the IP Algorithm TLV in the Router Information LSA with the numerically smallest Instance ID (Opaque ID for OSPFv2 or Link State ID for OSPFv3) MUST be used and subsequent instances of the IP Algorithm TLV MUST be ignored. The Router Information LSA can be advertised at any of the defined flooding scopes (link, area, or Autonomous System (AS)). For the purpose of IP Algorithm TLV advertisement, area-scoped flooding is REQUIRED. The IP Flex-Algorithm participation advertised in the OSPF IP Algorithm TLV is topology independent. When a router advertises participation in OSPF IP Algorithm TLV, the participation applies to all topologies in which the advertising node participates. 6. Advertising IP Flex-Algorthm Reachability To be able to associate the prefix with the Flex-Algorithm, the existing prefix reachability advertisements can not be used, because they advertise the prefix reachability in default algorithm 0. Instead, a new IP Flex-Algorithm reachability advertisements are defined in IS-IS and OSPF. The M-flag in FAD is not applicable to IP Algorithm Prefixes. Any IP Algorithm Prefix advertisement includes the Algorithm and Metric fields. When an IP Algorithm Prefix is advertised between areas or domains, the metric field in the IP Algorithm Prefix advertisement MUST be used irrespective of the M-flag in the FAD advertisement. 6.1. The IS-IS IPv4 Algorithm Prefix Reachability TLV A top-level TLV is defined for advertising IPv4 Flex-Algorithm Prefix Reachability in IS-IS - IPv4 Algorithm Prefix Reachability TLV. This new TLV shares the sub-TLV space defined for TLVs Advertising Prefix Reachability. The IS-IS IPv4 Algorithm Prefix Reachability TLV has 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 |R|R|R|R| MTID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: IS-IS IPv4 Algorithm Prefix Reachability TLV Britto, et al. Expires 24 April 2023 [Page 6] Internet-Draft IP Flex-Algorithm October 2022 * Type: IPv4 Algorithm Prefix Reachability TLV (Value 126). * Length: Variable. * R bits (4 bits): Reserved for future use. They MUST be set to zero on transmission and MUST be ignored on receipt. * MTID (12 bits): Multitopology Identifier as defined in [RFC5120]. Note that the value 0 is legal. Followed by one or more prefix entries of the form: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Pfx Length | Prefix (variable)... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-tlv-len | Sub-TLVs (variable) . . . | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: IS-IS IPv4 Algorithm Prefix Reachability TLV * Metric (4 octets): Metric information. * Flags (1 octet): 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |D| Reserved | +-+-+-+-+-+-+-+-+ D-flag: When the Prefix is leaked from level-2 to level-1, the D bit MUST be set. Otherwise, this bit MUST be clear. Prefixes with the D bit set MUST NOT be leaked from level-1 to level-2. This is to prevent looping. * Algorithm (1 octet): Associated Algorithm from 128 to 255. * Prefix Len (1 octet): Prefix length measured in bits. * Prefix (variable length): Prefix mapped to Flex-Algorithm. Britto, et al. Expires 24 April 2023 [Page 7] Internet-Draft IP Flex-Algorithm October 2022 * Optional Sub-TLV-length (1 octet): Number of octets used by sub- TLVs * Optional sub-TLVs (variable length). A router receiving multiple IPv4 Algorithm Prefix Reachability advertisements for the same prefix, from the same originator, each with a different Algorithm, MUST select the first advertisement in the lowest-numbered LSP and ignore any subsequent IPv4 Algorithm Prefix Reachability advertisements for the same prefix for any other Algorithm. A router receiving multiple IPv4 Algorithm Prefix Reachability advertisements for the same prefix, from different originators, each with a different Algorithm, MUST ignore all of them and MUST NOT install any forwarding entries based on these advertisements. This situation SHOULD be logged as an error. In cases where a prefix advertisement is received in both a IPv4 Prefix Reachability TLV and an IPv4 Algorithm Prefix Reachability TLV, the IPv4 Prefix Reachability advertisement MUST be preferred when installing entries in the forwarding plane. 6.2. The IS-IS IPv6 Algorithm Prefix Reachability TLV The IS-IS IPv6 Algorithm Prefix Reachability TLV is identical to the IS-IS IPv4 Algorithm Prefix Reachability TLV, except that it has a unique type. The type is 127. A router receiving multiple IPv6 Algorithm Prefix Reachability advertisements for the same prefix, from the same originator, each with a different Algorithm, MUST select the first advertisement in the lowest-numbered LSP and ignore any subsequent IPv6 Algorithm Prefix Reachability advertisements for the same prefix for any other Algorithm. A router receiving multiple IPv6 Algorithm Prefix Reachability advertisements for the same prefix, from different originators, each with a different Algorithm, MUST ignore all of them and MUST NOT install any forwarding entries based on these advertisements. This situation SHOULD be logged as an error. In cases where a prefix advertisement is received in both an IPv6 Prefix Reachability TLV and an IPv6 Algorithm Prefix Reachability TLV, the IPv6 Prefix Reachability advertisement MUST be preferred when installing entries in the forwarding plane. Britto, et al. Expires 24 April 2023 [Page 8] Internet-Draft IP Flex-Algorithm October 2022 In cases where a prefix advertisement is received in both IS-IS SRv6 Locator TLV and in IS-IS IPv6 Algorithm Prefix Reachability TLV, the receiver MUST ignore both of them and MUST NOT install any forwarding entries based on these advertisements. This situation SHOULD be logged as an error. 6.3. The OSPFv2 IP Algorithm Prefix Reachability Sub-TLV A new Sub-TLV of the OSPFv2 Extended Prefix TLV is defined for advertising IP Algorithm Prefix Reachability in OSPFv2, the OSPFv2 IP Algorithm Prefix Reachability Sub-TLV. The OSPFv2 IP Algorithm Prefix Reachability Sub-TLV has 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MT-ID | Algorithm | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: OSPFv2 IP Algorithm Prefix Reachability Sub-TLV * Type (2 octets) : The value is TBD. * Length (2 octet): 8 * MT-ID (1 octet): Multi-Topology ID as defined in [RFC4915] * Algorithm (1 octet): Associated Algorithm from 128 to 255. * Reserved: (2 octets). SHOULD be set to 0 on transmission and MUST be ignored on reception. * Metric (4 octets): The algorithm specific metric value. An OSPFv2 router receiving multiple OSPFv2 IP Algorithm Prefix Reachability Sub-TLVs in the same OSPFv2 Extended Prefix TLV, MUST select the first advertisement of this Sub-TLV and MUST ignore all remaining occurences of this Sub-TLV in the OSPFv2 Extended Prefix TLV. Britto, et al. Expires 24 April 2023 [Page 9] Internet-Draft IP Flex-Algorithm October 2022 An OSPFv2 router receiving multiple OSPFv2 IP Algorithm Prefix Reachability TLVs for the same prefix, from different originators, each with a different Algorithm, MUST ignore all of them and MUST NOT install any forwarding entries based on these advertisements. This situation SHOULD be logged as an error. In cases where a prefix advertisement is received in any of the LSAs advertising the prefix reachability for algorithm 0 and in an OSPFv2 IP Algorithm Prefix Reachability TLV, only the prefix reachability advertisement for algorithm 0 MUST be used and all occurences of the OSPFv2 IP Algorithm Prefix Reachability TLV MUST be ignored. 6.4. LSInfinity metric for TLVs defined in RFC8362 [RFC8362] defines three TLVs for advertising prefix reachability: * Intra-Area-Prefix TLV * Inter-Area-Prefix TLV * External-Prefix TLV All of these TLVs have 24-bits metric field. If the metric in any of these TLVs is set to LSInfinity, i.e., 0xFFFFFF, the advertised prefix MUST be considered unreachable. The unreachability applies to the prefix advertisement as described in the context of the [RFC8362]. 6.5. The OSPFv3 IP Algorithm Prefix Reachability Sub-TLV The OSPFv3 [RFC5340] IP Algorithm Prefix Reachability Sub-TLV is defined for advertisement of the IP Algorithm Prefix Reachability in OSPFv3. The OSPFv3 IP Algorithm Prefix Reachability Sub-TLV is a sub-TLV of the following OSPFv3 TLVs defined in [RFC8362]: * Intra-Area-Prefix TLV * Inter-Area-Prefix TLV * External-Prefix TLV The format of OSPFv3 IP Algorithm Prefix Reachability Sub-TLV is shown below: Britto, et al. Expires 24 April 2023 [Page 10] Internet-Draft IP Flex-Algorithm October 2022 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Algorithm | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: OSPFv3 IP Algorithm Prefix Reachability Sub-TLV Where: Type (2 octets): The value is TBD. Length (2 octets): 8. Algorithm (1 octet): Associated Algorithm from 128 to 255. Reserved: (3 octets). SHOULD be set to 0 on transmission and MUST be ignored on reception. Metric (4 octets): The algorithm specific metric value. The metric value of 0XFFFFFFFF MUST be considered as unreachable. When the OSPFv3 IP Algorithm Prefix Reachability Sub-TLV is present, the metric value in its parent TLV MUST be set to LSInfinity [RFC2328]. If the metric value in the parent TLV is not set to LSInfinity, the OSPFv3 IP Algorithm Prefix Sub-TLV MUST be ignored by the receiver. An OSPFv3 router receiving multiple OSPFv3 IP Algorithm Prefix Reachability Sub-TLVs in the same parent TLV, MUST select the first advertisement of this Sub-TLV and MUST ignore all remaining occurences of this Sub-TLV in the parent TLV. An OSPFv3 router receiving multiple OSPFv3 IP Algorithm Prefix Reachability TLVs for the same prefix, from different originators, each with a different Algorithm, MUST ignore all of them and MUST NOT install any forwarding entries based on these advertisements. This situation SHOULD be logged as an error. Britto, et al. Expires 24 April 2023 [Page 11] Internet-Draft IP Flex-Algorithm October 2022 In cases where a prefix advertisement is received in any of the LSAs advertising the prefix reachability for algorithm 0 and in an OSPFv3 OSPFv3 IP Algorithm Prefix Reachability Sub-TLV, only the prefix reachability advertisement for algorithm 0 MUST be used and all occurences of the OSPFv3 IP Algorithm Prefix Reachability TLV MUST be ignored. In cases where a prefix advertisement is received in both an OSPFv3 SRv6 Locator TLV and in an OSPFv3 IP Algorithm Prefix Reachability Sub-TLV, the receiver MUST ignore both of them and MUST NOT install any forwarding entries based on these advertisements. This situation SHOULD be logged as an error. 6.6. The OSPF IP Flexible Algorithm ASBR Metric Sub-TLV [I-D.ietf-lsr-flex-algo] defines the OSPF Flexible Algorithm ASBR Metric Sub-TLV (FAAM) that is used by an OSPFv2 or an OSPFv3 ABR to advertise a Flex-Algorithm specific metric associated with the corresponding ASBR LSA. As described in [I-D.ietf-lsr-flex-algo] each data-plane signals the participation independently. IP Flex-Algorithm participation is signaled independent of the Segment Routing (SR) Flex-Algorithm participation. As a result, the calculated topologies for SR and IP Flex-Algorithm could be different. Such difference prevents the usage of FAAM for the purpose of the IP Flex-Algorithm. The OSPF IP Flexible Algorithm ASBR Metric (IPFAAM) Sub-TLV is defined for the advertisement of the IP Flex-Algorithm specific metric associated with an ASBR by the ABR. The IPFAAM Sub-TLV is a Sub-TLV of the: - OSPFv2 Extended Inter-Area ASBR TLV as defined in [I-D.ietf-lsr-flex-algo] - OSPFv3 Inter-Area-Router TLV defined in [RFC8362] The OSPF IPFAAM Sub-TLV has the following format: Britto, et al. Expires 24 April 2023 [Page 12] Internet-Draft IP Flex-Algorithm October 2022 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Algorithm | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where: Figure 7: OSPF IP Flexible Algorithm ASBR Metric Sub-TLV Type (2 octets): TBD for OSPFv2, TBD for OSPFv3. Length (2 octets): 8. Algorithm (1 octet): Associated Algorithm from 128 to 255. Reserved: (3 octets). SHOULD be set to 0 on transmission and MUST be ignored on reception. Metric (4 octets): The algorithm specific metric value. The usage of the IPFAAM Sub-TLV is similar to the usage of the FAAM Sub-TLV defined in [I-D.ietf-lsr-flex-algo], but it is used to advertise IP Flex-Algorithm metric. An OSPF ABR MUST include the OSPF IPFAAM Sub-TLVs as part of the ASBR reachability advertisement between areas for every IP Flex-Algorithm in which it participates and the ASBR is reachable in. The FAAM Sub-TLV as defined in [I-D.ietf-lsr-flex-algo] MUST NOT be used during IP Flex-Algorithm path calculation, the IPFAAM Sub-TLV MUST be used instead. 7. Calculating of IP Flex-Algorthm Paths The IP Flex-Algorthm is considered as yet another data-plane of the Flex-Algorithm as described [I-D.ietf-lsr-flex-algo]. Participation for the IP Flex-Algorithm is signalled as described in Section 5 and is specific to the IP Flex-Algorithm data-plane. Britto, et al. Expires 24 April 2023 [Page 13] Internet-Draft IP Flex-Algorithm October 2022 Calculation of IP Flex-Algorithm paths follows what is described in [I-D.ietf-lsr-flex-algo]. This computation uses the IP Flex- Algorithm data-plane participation and is independent of the Flex- Algorithm calculation done for any other Flex-Algorithm data-plane (e.g., SR, SRv6). The IP Flex-Algorithm data-plane only considers participating nodes during the Flex-Algorithm calculation. When computing paths for a given Flex-Algorithm, all nodes that do not advertise participation for the IP Flex-Algorithm, as described in Section 5, MUST be pruned from the topology. 8. IP Flex-Algorthm Forwarding The IP Algorithm Prefix Reachability advertisement as described in Section 5 includes the MTID value that associates the prefix with a specific topology. Algorithm Prefix Reachability advertisement also includes an Algorithm value that explicitly associates the prefix with a specific Flex-Algorithm. The paths to the prefix MUST be calculated using the specified Flex-Algorithm in the associated topology. Forwarding entries for the IP Flex-Algorithm prefixes advertised in IGPs MUST be installed in the forwarding plane of the receiving IP Flex-Algorithm prefix capable routers when they participate in the associated topology and algorithm. Forwarding entries for IP Flex- Algorithm prefixes associated with Flex-Algorithms in which the node is not participating MUST NOT be installed in the forwarding plane. When the IP Flex-Algorithm prefix is associated with a Flex- Algorithm, LFA paths to the prefix MUST be calculated using such Flex-Algorithm in the associated topology, to guarantee that they follow the same constraints as the calculation of the primary paths. 9. Deployment Considerations IGP Flex-Algorithm can be used by many data-planes. The original specification was done for SR and SRv6, this specification adds IP as another data-plane that can use IGP Flex-Algorithm. Other data- planes may be defined in the future. This section provides some details about the coexistence of the various data-planes of an IGP Flex-Algorithm. Flex-Algorithm definition (FAD), as described in [I-D.ietf-lsr-flex-algo], is data-plane independent and is used by all Flex-Algorithm data-planes. Britto, et al. Expires 24 April 2023 [Page 14] Internet-Draft IP Flex-Algorithm October 2022 Participation in the Flex-Algorithm, as described in [I-D.ietf-lsr-flex-algo], is data-plane specific. Calculation of the flex-algo paths is data-plane specific and uses data-plane specific participation advertisements. Data-plane specific participation and calculation guarantee that the forwarding of the traffic over the Flex-Algorithm data-plane specific paths is consistent between all nodes that apply the IGP Flex- Algorithm to the data-plane. Multiple data-planes can use the same Flex-Algorithm value at the same time and, and as such, share the FAD for it. For example, SR- MPLS and IP can both use a common Flex-Algorithm. Traffic for SR- MPLS will be forwarded based on Flex-algorithm specific SR SIDs. Traffic for IP Flex-Algorithm will be forwarded based on Flex- Algorithm specific prefix reachability advertisements. Note that for a particular Flex-Algorithm, for a particular IP prefix, there will only be path(s) calculated and installed for a single data-plane. 10. Protection In many networks where IGP Flexible Algorithms are deployed, IGP restoration will be fast and additional protection mechanisms will not be required. IGP restoration may be enhanced by Equal Cost Multipath (ECMP). In other networks, operators can deploy additional protection mechanisms. The following are examples: * Loop Free Alternates (LFA) [RFC5286] * Remote Loop Free Alternates (R-LFA) [RFC7490] LFA and R-LFA computations MUST be restricted to the flex-algo topology and the computed backup nexthops should be programmed for the IP flex-algo prefixes. 11. IANA Considerations This specification updates the OSPF Router Information (RI) TLVs Registry as follows: Britto, et al. Expires 24 April 2023 [Page 15] Internet-Draft IP Flex-Algorithm October 2022 +=======+==================+===========================+ | Value | TLV Name | Reference | +=======+==================+===========================+ | TBD | IP Algorithm TLV | This Document Section 5.2 | +-------+------------------+---------------------------+ Table 1 This document also updates the ISIS "Sub-TLVs for TLV 242" registry as follows: +=======+======================+===========================+ | Value | TLV Name | Reference | +=======+======================+===========================+ | 29 | IP Algorithm Sub-TLV | This Document Section 5.1 | +-------+----------------------+---------------------------+ Table 2 This document also updates the "IS-IS TLV Codepoints Registry" registry as follows: +=======+==================+=====+=====+=====+=======+=============+ | Value | TLV Name | IIH | LSP | SNP | Purge | Reference | +=======+==================+=====+=====+=====+=======+=============+ | 126 | IPv4 Algorithm | N | Y | N | N | This | | | Prefix | | | | | document, | | | Reachability TLV | | | | | Section 6.1 | +-------+------------------+-----+-----+-----+-------+-------------+ | 127 | IPv6 Algorithm | N | Y | N | N | This | | | Prefix | | | | | document, | | | Reachability TLV | | | | | Section 6.2 | +-------+------------------+-----+-----+-----+-------+-------------+ Table 3 The above TLVs share the sub-TLV space defined in "IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability". This document updates the description of that registry by including IPv4 Algorithm Prefix Reachability TLV and IPv6 Algorithm Prefix Reachability TLV. It also includes these TLVs in a table which lists the presence of Sub-TLVs in a parent TLVs as follows: Britto, et al. Expires 24 April 2023 [Page 16] Internet-Draft IP Flex-Algorithm October 2022 Type Description 126 127 ---- ---------------------------------- --- --- 1 32-bit Administrative Tag Sub-TLV y y 2 64-bit Administrative Tag Sub-TLV y y 3 Prefix Segment Identifier n n 4 Prefix Attribute Flags y y 5 SRv6 End SID n n 6 Flex-Algorithm Prefix Metric n n 11 IPv4 Source Router ID y y 12 IPv6 Source Router ID y y 32 BIER Info n n This document updates the "OSPFv2 Extended Prefix TLV Sub-TLVs" registry as follows: +=======+=========================+================+ | Value | TLV Name | Reference | +=======+=========================+================+ | TBD | OSPFv2 IP Algorithm | This Document, | | | Prefix Reachability TLV | Section 6.3 | +-------+-------------------------+----------------+ Table 4 This document updates the "OSPFv3 Extended-LSA Sub-TLVs" registry as follows: +=======+==============================+================+ | Value | TLV Name | Reference | +=======+==============================+================+ | TBD | OSPFv3 IP Algorithm Prefix | This Document, | | | Reachability Sub-TLV | Section 6.5 | +-------+------------------------------+----------------+ | TBD | OSPFv3 IP Flexible Algorithm | This Document, | | | ASBR Metric Sub-TLV | Section 6.6 | +-------+------------------------------+----------------+ Table 5 This document updates the "OSPFv2 Extended Inter-Area ASBR Sub-TLVs" registry as follows: Britto, et al. Expires 24 April 2023 [Page 17] Internet-Draft IP Flex-Algorithm October 2022 +=======+============================+================+ | Value | TLV Name | Reference | +=======+============================+================+ | 2 | OSPF IP Flexible Algorithm | This Document, | | | ASBR Metric Sub-TLV | Section 6.6 | +-------+----------------------------+----------------+ Table 6 12. Security Considerations This document inherits security considerations from [I-D.ietf-lsr-flex-algo]. 13. Acknowledgements Thanks to Bruno Decraene for his contributions to this document. Special thanks to Petr Bonbon Adamec of Cesnet for supporting interoperability testing. 14. References 14.1. Normative References [I-D.ietf-lsr-flex-algo] Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm", Work in Progress, Internet-Draft, draft-ietf-lsr-flex-algo-25, 6 October 2022, . [ISO10589] IANA, "Intermediate system to Intermediate system routing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473)", August 1987, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, April 1998, . Britto, et al. Expires 24 April 2023 [Page 18] Internet-Draft IP Flex-Algorithm October 2022 [RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", RFC 4915, DOI 10.17487/RFC4915, June 2007, . [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, . [RFC7770] Lindem, A., Ed., 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, . [RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions for Advertising Router Information", RFC 7981, DOI 10.17487/RFC7981, October 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, . 14.2. Informative References [IANA-ALG] IANA, "Sub-TLVs for TLV 242 (IS-IS Router CAPABILITY TLV)", August 1987, . [RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, DOI 10.17487/RFC5286, September 2008, . [RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N. So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)", RFC 7490, DOI 10.17487/RFC7490, April 2015, . [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, . Britto, et al. Expires 24 April 2023 [Page 19] Internet-Draft IP Flex-Algorithm October 2022 [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, . Authors' Addresses William Britto Juniper Networks Elnath-Exora Business Park Survey Bangalore 560103 Karnataka India Email: bwilliam@juniper.net Shraddha Hegde Juniper Networks Elnath-Exora Business Park Survey Bangalore 560103 Karnataka India Email: shraddha@juniper.net Parag Kaneriya Juniper Networks Elnath-Exora Business Park Survey Bangalore 560103 Karnataka India Email: pkaneria@juniper.net Rejesh Shetty Juniper Networks Elnath-Exora Business Park Survey Bangalore 560103 Karnataka India Email: mrajesh@juniper.net Britto, et al. Expires 24 April 2023 [Page 20] Internet-Draft IP Flex-Algorithm October 2022 Ron Bonica Juniper Networks 2251 Corporate Park Drive Herndon, Virginia 20171 United States of America Email: rbonica@juniper.net Peter Psenak Cisco Systems Apollo Business Center Mlynske nivy 43 82109 Bratislava Slovakia Email: ppsenak@cisco.com Britto, et al. Expires 24 April 2023 [Page 21]