| Internet-Draft | IGP Flexible Algorithm | October 2022 |
| Psenak, et al. | Expires 20 April 2023 | [Page] |
IGP protocols historically compute best paths over the network based on the IGP metric assigned to the links. Many network deployments use RSVP-TE based or Segment Routing based Traffic Engineering to steer traffic over a path that is computed using different metrics or constraints than the shortest IGP path. This document specifies a solution that allows IGPs themselves to compute constraint-based paths over the network. This document also specifies a way of using Segment Routing (SR) Prefix-SIDs and SRv6 locators to steer packets along the constraint-based paths.¶
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 20 April 2023.¶
Copyright (c) 2022 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 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.¶
An IGP-computed path based on the shortest IGP metric is often replaced by a traffic-engineered path due to requirements which are not reflected by the IGP metric. Some networks engineer the IGP metric assignments in a way that the IGP metric reflects the link bandwidth or delay. If, for example, the IGP metric reflects the bandwidth on the link and user traffic is delay sensitive, the best IGP path may not reflect the best path from such a user's perspective.¶
To overcome this limitation, various sorts of traffic engineering have been deployed, including RSVP-TE and SR-TE, in which case the TE component is responsible for computing paths based on additional metrics and/or constraints. Such paths need to be installed in the forwarding tables in addition to, or as a replacement for, the original paths computed by IGPs. Tunnels are often used to represent the engineered paths and mechanisms like the one described in [RFC3906] are used to replace the original IGP paths with such tunnel paths.¶
This document specifies a set of extensions to IS-IS, OSPFv2, and OSPFv3 that enable a router to advertise TLVs that (a) identify calculation-type, (b) specify a metric-type, and (c) describe a set of constraints on the topology, that are to be used to compute the best paths along the constrained topology. A given combination of calculation-type, metric-type, and constraints is known as a "Flexible Algorithm Definition". A router that sends such a set of TLVs also assigns a Flex-Algorithm value to the specified combination of calculation-type, metric-type, and constraints.¶
This document also specifies a way for a router to use IGPs to associate one or more "Segment Routing with the MPLS Data Plane (SR-MPLS)" Prefix-SIDs [RFC8660], or "Segment Routing over IPv6 (SRv6)" locators [RFC8986] with a particular Flex-Algorithm. Each such Prefix-SID or SRv6 locator then represents a path that is computed according to the identified Flex-Algorithm. In SRv6 it is the locator, not the SID, that holds the binding to the algorithm.¶
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.¶
This section defines terms that are often used in this document.¶
Flexible Algorithm Definition (FAD) - the set consisting of (a) calculation-type, (b) metric-type, and (c) a set of constraints.¶
Flex-Algorithm - a numeric identifier in the range 128-255 that is associated via configuration with the Flexible Algorithm Definition.¶
Local Flexible Algorithm Definition - Flexible Algorithm Definition defined locally on the node.¶
Remote Flexible Algorithm Definition - Flexible Algorithm Definition received from other nodes via IGP flooding.¶
Flexible Algorithm Participation - per data-plane configuration state that expresses whether the node is participating in a particular Flexible Algorithm. Not all routers in a given network need to participate in a given Flexible Algorithm. The Flexible Algorithm(s) a given router participates in is determined by configuration.¶
IGP Algorithm - value from the "IGP Algorithm Types" registry defined under "Interior Gateway Protocol (IGP) Parameters" IANA registry grouping. IGP Algorithms represents the triplet (calculation-type, metric-type, constraints), where the second and third elements of the triple MAY be unspecified.¶
ABR - Area Border Router. In IS-IS terminology it is also known as L1/L2 router.¶
ASBR - Autonomous System Border Router.¶
Many possible constraints may be used to compute a path over a network. Some networks are deployed as multiple planes. A simple form of constraint may be to use a particular plane. A more sophisticated form of constraint can include some extended metric as described in [RFC8570]. Constraints which restrict paths to links with specific affinities or avoid links with specific affinities are also possible. Combinations of these are also possible.¶
To provide maximum flexibility, a mechanism is provided that allows a router to (a) identify a particular calculation-type and (b) metric-type, (c) describe a particular set of constraints, and (d) assign a numeric identifier, referred to as Flex-Algorithm, to the combination of that calculation-type, metric-type, and those constraints. The mapping between the Flex-Algorithm and its meaning is flexible and defined by the user. As long as all routers in the domain have a common understanding as to what a particular Flex-Algorithm represents, the resulting routing computation is consistent and traffic is not subject to any looping.¶
The set consisting of (a) calculation-type, (b) metric-type, and (c) a set of constraints is referred to as a Flexible Algorithm Definition.¶
Flex-Algorithm is a numeric identifier in the range 128-255 that is associated via configuration with the Flexible Algorithm Definition.¶
The IANA "IGP Algorithm Types" registry defines the set of values for IGP Algorithms. The following values area allocated by IANA from this registry for Flex-Algorithms:¶
To guarantee loop-free forwarding for paths computed for a particular Flex-Algorithm, all routers that (a) are configured to participate in a particular Flex-Algorithm, and (b) are in the same Flex-Algorithm definition advertisement scope MUST agree on the definition of the Flex-Algorithm. The following procedures ensure this condition is fulfilled.¶
The IS-IS Flexible Algorithm Definition Sub-TLV (FAD Sub-TLV) is used to advertise the definition of the Flex-Algorithm.¶
The IS-IS FAD Sub-TLV is advertised as a Sub-TLV of the IS-IS Router Capability TLV-242 that is defined in [RFC7981].¶
IS-IS FAD 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 |Flex-Algorithm | Metric-Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Calc-Type | Priority | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs | + + | ... | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
Metric-Type: Type of metric from the "IGP Metric-Type Registry" (Section 18.1.2) to be used during the calculation. The following values are defined:¶
The IS-IS FAD Sub-TLV MAY be advertised in an LSP of any number. IS-IS router MAY advertise more than one IS-IS FAD Sub-TLV for a given Flexible Algorithm (see Section 6).¶
The IS-IS FAD Sub-TLV has an area scope. The Router Capability TLV in which the FAD Sub-TLV is present MUST have the S-bit clear.¶
An IS-IS L1/L2 router MAY be configured to re-generate the winning FAD from level 2, without any modification to it, to the level 1 area. The re-generation of the FAD Sub-TLV from level 2 to level 1 is determined by the L1/L2 router, not by the originator of the FAD advertisement in the level 2. In such a case, the re-generated FAD Sub-TLV will be advertised in the level 1 Router Capability TLV originated by the L1/L2 router.¶
An L1/L2 router MUST NOT re-generate any FAD Sub-TLV from level 1 to level 2.¶
The OSPF FAD TLV is advertised as a top-level TLV of the Router Information (RI) LSA that is defined in [RFC7770].¶
The OSPF FAD 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Flex-Algorithm | Metric-Type | Calc-Type | Priority | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs | + + | ... | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
Metric-Type: Type of metric from the "IGP Metric-Type Registry" (Section 18.1.2) to be used during the calculation. The following values are defined:¶
When multiple OSPF FAD TLVs, for the same Flexible Algorithm, are received from a given router, the receiver MUST use the first occurrence of the TLV in the Router Information LSA. If the OSPF FAD TLV, for the same Flex-Algorithm, appears in multiple Router Information LSAs that have different flooding scopes, the OSPF FAD TLV in the Router Information LSA with the area-scoped flooding scope MUST be used. If the OSPF FAD TLV, for the same algorithm, appears in multiple Router Information LSAs that have the same flooding scope, the OSPF FAD TLV in the Router Information (RI) LSA with the numerically smallest Instance ID MUST be used and subsequent instances of the OSPF FAD 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 OSPF FAD TLV advertisement, area-scoped flooding is REQUIRED. The Autonomous System flooding scope SHOULD NOT be used unless local configuration policy on the originating router indicates domain wide flooding.¶
This section describes the protocol-independent handling of the FAD TLV (OSPF) or FAD Sub-TLV (IS-IS). We will refer to it as FAD TLV in this section, even though in the case of IS-IS it is a Sub-TLV.¶
The value of the Flex-Algorithm MUST be between 128 and 255 inclusive. If it is not, the FAD TLV MUST be ignored.¶
Only a subset of the routers participating in the particular Flex-Algorithm need to advertise the definition of the Flex-Algorithm.¶
Every router, that is configured to participate in a particular Flex-Algorithm, MUST select the Flex-Algorithm definition based on the following ordered rules. This allows for the consistent Flex-Algorithm definition selection in cases where different routers advertise different definitions for a given Flex-Algorithm:¶
The FAD selected according to these rules is also known as the "winning FAD".¶
A router that is not configured to participate in a particular Flex-Algorithm MUST ignore FAD Sub-TLVs advertisements for such Flex-Algorithm.¶
A router that is not participating in a particular Flex-Algorithm MAY advertise FAD for such Flex-Algorithm. Receiving routers MUST consider a received FAD advertisement regardless of the Flex-Algorithm participation of that FAD advertisement's originator.¶
Any change in the Flex-Algorithm definition may result in temporary disruption of traffic that is forwarded based on such Flex-Algorithm paths. The impact is similar to any other event that requires network-wide convergence.¶
If a node is configured to participate in a particular Flexible Algorithm, but there is no valid Flex-Algorithm definition available for it, or the selected Flex-Algorithm definition includes calculation-type, metric-type, constraint, flag, or Sub-TLV that is not supported by the node, it MUST stop participating in such Flexible Algorithm. That implies that it MUST NOT announce participation for such Flexible Algorithm as specified in Section 11 and it MUST remove any forwarding state associated with it.¶
Flex-Algorithm definition is topology independent. It applies to all topologies that a router participates in.¶
One of the limitations of IS-IS [ISO10589] is that the length of a TLV/sub-TLV is limited to a maximum of 255 octets. For the FAD sub-TLV, there are a number of sub-sub-TLVs (defined below) which are supported. For a given Flex-Algorithm, it is possible that the total number of octets required to completely define a FAD exceeds the maximum length supported by a single FAD sub-TLV. In such cases, the FAD MAY be split into multiple such sub-TLVs and the content of the multiple FAD sub-TLVs combined to provide a complete FAD for the Flex-Algorithm. In such a case, the fixed portion of the FAD (see Section 5.1) MUST be identical in all FAD sub-TLVs for a given Flex-Algorithm from a given IS. In case the fixed portion of such FAD Sub-TLVs differ, the values in the fixed portion in the FAD sub-TLV in the first occurrence in the lowest numbered LSP from a given IS MUST be used.¶
Any specification that introduces a new IS-IS FAD sub-sub-TLV MUST specify whether the FAD sub-TLV may appear multiple times in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS and how to handle them if multiple are allowed.¶
The Flexible Algorithm definition can specify 'colors' that are used by the operator to exclude links during the Flex-Algorithm path computation.¶
The IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV is used to advertise the exclude rule that is used during the Flex-Algorithm path calculation as specified in Section 13.¶
The IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-TLV) is a Sub-TLV of the IS-IS FAD Sub-TLV. It 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extended Admin Group | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
The IS-IS FAEAG Sub-TLV MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver.¶
The IS-IS FAEAG Sub-TLV MUST NOT appear more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it appears more than once in such a set, the IS-IS FAEAG Sub-TLV in the first occurrence in the lowest numbered LSP from a given IS MUST be used and any other occurrences MUST be ignored.¶
The Flexible Algorithm definition can specify 'colors' that are used by the operator to include links during the Flex-Algorithm path computation.¶
The IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV is used to advertise the include-any rule that is used during the Flex-Algorithm path calculation as specified in Section 13.¶
The IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV is a Sub-TLV of the IS-IS FAD Sub-TLV. It 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extended Admin Group | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
The IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver.¶
The IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT appear more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it appears more than once in such a set, the IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV in the first occurrence in the lowest numbered LSP from a given IS MUST be used and any other occurrences MUST be ignored.¶
The Flexible Algorithm definition can specify 'colors' that are used by the operator to include links during the Flex-Algorithm path computation.¶
The IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV is used to advertise the include-all rule that is used during the Flex-Algorithm path calculation as specified in Section 13.¶
The IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV is is a Sub-TLV of the IS-IS FAD Sub-TLV. It 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extended Admin Group | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
The IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver.¶
The IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT appear more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it appears more than once in such a set, the IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV in the first occurrence in the lowest numbered LSP from a given IS MUST be used and any other occurrences MUST be ignored.¶
The IS-IS Flexible Algorithm Definition Flags Sub-TLV (FADF Sub-TLV) is a Sub-TLV of the IS-IS FAD Sub-TLV. It 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
Flags:¶
0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|M| | | ...
+-+-+-+-+-+-+-+-+...
¶
A new IANA "IGP Flexible Algorithm Definition Flags Registry" is defined for allocation of bits in the Flags field - see Section 18.2.¶
Bits are defined/sent starting with Bit 0 defined above. Additional bit definitions that may be defined in the future SHOULD be assigned in ascending bit order so as to minimize the number of bits that will need to be transmitted.¶
Undefined bits MUST be transmitted as 0.¶
Bits that are not transmitted MUST be treated as if they are set to 0 on receipt.¶
The IS-IS FADF Sub-TLV MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver.¶
The IS-IS FADF Sub-TLV MUST NOT appear more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it appears more than once in such a set, the IS-IS FADF Sub-TLV in the first occurrence in the lowest numbered LSP from a given IS MUST be used and any other occurrences MUST be ignored.¶
If the IS-IS FADF Sub-TLV is not present inside the IS-IS FAD Sub-TLV, all the bits are assumed to be set to 0.¶
If a node is configured to participate in a particular Flexible Algorithm, but the selected Flex-Algorithm definition includes a bit in the IS-IS FADF Sub-TLV that is not supported by the node, it MUST stop participating in such Flexible Algorithm.¶
New flag bits may be defined in the future. Implementations MUST check all advertised flag bits in the received IS-IS FADF Sub-TLV - not just the subset currently defined.¶
M-flag MUST not be used when calculating prefix reachability for SRv6 Locator prefix.¶
The Flexible Algorithm definition can specify Shared Risk Link Groups (SRLGs) that the operator wants to exclude during the Flex-Algorithm path computation.¶
The IS-IS Flexible Algorithm Exclude SRLG Sub-TLV (FAESRLG) is used to advertise the exclude rule that is used during the Flex-Algorithm path calculation as specified in Section 13.¶
The IS-IS FAESRLG Sub-TLV is a Sub-TLV of the IS-IS FAD Sub-TLV. It 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Shared Risk Link Group Value | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
The IS-IS FAESRLG Sub-TLV MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver.¶
The IS-IS FAESRLG Sub-TLV MAY appear more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. This may be necessary in cases where the total number of SRLG values which are specified cause the FAD sub-TLV to exceed the maximum length of a single FAD sub-TLV. In such a case the receiver MUST use the union of all values across all IS-IS FAESRLG Sub-TLVs from such set.¶
The Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-TLV) is a Sub-TLV of the OSPF FAD TLV. Its usage is described in Section 6.1. It 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extended Admin Group | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
The OSPF FAEAG Sub-TLV MUST NOT appear more than once in an OSPF FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored by the receiver.¶
The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV is a Sub-TLV of the OSPF FAD TLV. The usage of this Sub-TLVs is described in Section 6.2. It 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extended Admin Group | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT appear more than once in an OSPF FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored by the receiver.¶
The OSPF Flexible Algorithm Include-All Admin Group Sub-TLV is a Sub-TLV of the OSPF FAD TLV. The usage of this Sub-TLVs is described in Section 6.3. It 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extended Admin Group | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
The OSPF Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT appear more than once in an OSPF FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored by the receiver.¶
The OSPF Flexible Algorithm Definition Flags Sub-TLV (FADF Sub-TLV) is a Sub-TLV of the OSPF FAD TLV. It 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
Flags:¶
0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|M| | | ...
+-+-+-+-+-+-+-+-+...
¶
A new IANA "IGP Flexible Algorithm Definition Flags Registry" is defined for allocation of bits in the Flags field - see Section 18.2.¶
Bits are defined/sent starting with Bit 0 defined above. Additional bit definitions that may be defined in the future SHOULD be assigned in ascending bit order so as to minimize the number of bits that will need to be transmitted.¶
Undefined bits MUST be transmitted as 0.¶
Bits that are not transmitted MUST be treated as if they are set to 0 on receipt.¶
The OSPF FADF Sub-TLV MUST NOT appear more than once in an OSPF FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored by the receiver.¶
If the OSPF FADF Sub-TLV is not present inside the OSPF FAD TLV, all the bits are assumed to be set to 0.¶
If a node is configured to participate in a particular Flexible Algorithm, but the selected Flex-Algorithm definition includes a bit in the OSPF FADF Sub-TLV that is not supported by the node, it MUST stop participating in such Flexible Algorithm.¶
New flag bits may be defined in the future. Implementations MUST check all advertised flag bits in the received OSPF FADF Sub-TLV - not just the subset currently defined.¶
M-flag MUST not be used when calculating prefix reachability for SRv6 Locator prefix.¶
The OSPF Flexible Algorithm Exclude SRLG Sub-TLV (FAESRLG Sub-TLV) is a Sub-TLV of the OSPF FAD TLV. Its usage is described in Section 6.5. It 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Shared Risk Link Group Value | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
The OSPF FAESRLG Sub-TLV MUST NOT appear more than once in an OSPF FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored by the receiver.¶
The IS-IS Flexible Algorithm Prefix Metric (FAPM) Sub-TLV supports the advertisement of a Flex-Algorithm specific prefix metric associated with a given prefix advertisement.¶
The IS-IS FAPM Sub-TLV is a sub-TLV of TLVs 135, 235, 236, and 237 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 |Flex-Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
The IS-IS FAPM Sub-TLV MAY appear multiple times in its parent TLV. If it appears more than once with the same Flex-Algorithm value, the first instance MUST be used and any subsequent instances MUST be ignored.¶
If a prefix is advertised with a Flex-Algorithm prefix metric larger than MAX_PATH_METRIC as defined in [RFC5305] this prefix MUST NOT be considered during the Flexible Algorithm computation.¶
The usage of the Flex-Algorithm prefix metric is described in Section 13.¶
The IS-IS FAPM Sub-TLV MUST NOT be advertised as a sub-TLV of the IS-IS SRv6 Locator TLV [I-D.ietf-lsr-isis-srv6-extensions]. The IS-IS SRv6 Locator TLV includes the Algorithm and Metric fields which MUST be used instead. If the FAPM Sub-TLV is present as a sub-TLV of the IS-IS SRv6 Locator TLV in the received LSP, such FAPM Sub-TLV MUST be ignored.¶
The OSPF Flexible Algorithm Prefix Metric (FAPM) Sub-TLV supports the advertisement of a Flex-Algorithm specific prefix metric associated with a given prefix advertisement.¶
The OSPF Flex-Algorithm Prefix Metric (FAPM) Sub-TLV is a Sub-TLV of the:¶
OSPF FAPM 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Flex-Algorithm | Flags | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
Flags: One octet value¶
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|E| |
+-+-+-+-+-+-+-+-+
¶
The OSPF FAPM Sub-TLV MAY appear multiple times in its parent TLV. If it appears more than once with the same Flex-Algorithm value, the first instance MUST be used and any subsequent instances MUST be ignored.¶
The usage of the Flex-Algorithm prefix metric is described in Section 13.¶
An OSPF ABR advertises the reachability of ASBRs in its attached areas to enable routers within those areas to perform route calculations for external prefixes advertised by the ASBRs. OSPF extensions for advertisement of Flex-Algorithm specific reachability and metric for ASBRs is similarly required for Flex-Algorithm external prefix computations as described further in Section 13.1.¶
The OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) LSA is an OSPF Opaque LSA [RFC5250] that is used to advertise additional attributes related to the reachability of the OSPFv2 ASBR that is external to the area yet internal to the OSPF domain. Semantically, the OSPFv2 EIA-ASBR LSA is equivalent to the fixed format Type 4 Summary LSA [RFC2328]. Unlike the Type 4 Summary LSA, the LSID of the EIA-ASBR LSA does not carry the ASBR Router-ID - the ASBR Router-ID is carried in the body of the LSA. The OSPFv2 EIA-ASBR LSA is advertised by an OSPFv2 ABR and its flooding is defined to be area-scoped only.¶
An OSPFv2 ABR generates the EIA-ASBR LSA for an ASBR when it is advertising the Type-4 Summary LSA for it and has the need for advertising additional attributes for that ASBR beyond what is conveyed in the fixed format Type-4 Summary LSA. An OSPFv2 ABR MUST NOT advertise the EIA-ASBR LSA for an ASBR for which it is not advertising the Type 4 Summary LSA. This ensures that the ABR does not generate the EIA-ASBR LSA for an ASBR to which it does not have reachability in the base OSPFv2 topology calculation. The OSPFv2 ABR SHOULD NOT advertise the EIA-ASBR LSA for an ASBR when it does not have additional attributes to advertise for that ASBR.¶
The OSPFv2 EIA-ASBR LSA 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | LS Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opaque Type | Opaque ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +- TLVs -+ | ... |¶
LS age and Options fields are as defined in Section A.4.1. of [RFC2328].¶
The LS Type MUST be 10, indicating that the Opaque LSA flooding scope is area-local [RFC5250].¶
The Opaque Type used by the OSPFv2 EIA-ASBR LSA is 11. The Opaque Type is used to differentiate the various types of OSPFv2 Opaque LSAs and is described in Section 3 of [RFC5250].¶
The Opaque ID field is an arbitrary value used to maintain multiple OSPFv2 EIA-ASBR LSAs. For OSPFv2 EIA-ASBR LSAs, the Opaque ID has no semantic significance other than to differentiate OSPFv2 EIA-ASBR LSAs originated by the same OSPFv2 ABR. If multiple OSPFv2 EIA-ASBR LSAs specify the same ASBR, the attributes from the Opaque LSA with the lowest Opaque ID SHOULD be used.¶
Advertising Router, LS sequence number, and LS checksum fields are as defined in Section A.4.1. of [RFC2328].¶
The Length field is as defined in Section A.4.1. of [RFC5250]. It represents the total length (in octets) of the Opaque LSA, including the LSA header and all TLVs (including padding).¶
The format of the TLVs within the body of the OSPFv2 EIA-ASBR LSA is the same as the format used by the Traffic Engineering Extensions to OSPFv2 [RFC3630]. The variable TLV section consists of one or more nested TLV tuples. Nested TLVs are also referred to as sub- TLVs. The TLV Length field defines the length of the value portion in octets (thus, a TLV with no value portion would have a length of 0). The TLV is padded to 4-octet alignment; padding is not included in the Length field (so a 3-octet value would have a length of 3, but the total size of the TLV would be 8 octets). Nested TLVs are also 32-bit aligned. For example, a 1-octet value would have the Length field set to 1, and 3 octets of padding would be added to the end of the value portion of the TLV. The padding is composed of zeros.¶
The OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) TLV is a top-level TLV of the OSPFv2 EIA-ASBR LSA and is used to advertise additional attributes associated with the reachability of an ASBR.¶
The OSPFv2 EIA-ASBR 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ASBR Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . Sub-TLVs . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
Only a single OSPFv2 EIA-ASBR TLV MUST be advertised in each OSPFv2 EIA-ASBR LSA and the receiver MUST ignore all instances of this TLV other than the first one in an LSA.¶
OSPFv2 EIA-ASBR TLV MUST be present inside an OSPFv2 EIA-ASBR LSA and MUST include at least a single sub-TLV, otherwise the OSPFv2 EIA-ASBR LSA MUST be ignored by the receiver.¶
The OSPF Flexible Algorithm ASBR Metric (FAAM) Sub-TLV supports the advertisement of a Flex-Algorithm specific metric associated with a given ASBR reachability advertisement by an ABR.¶
The OSPF Flex-Algorithm ASBR Metric (FAAM) Sub-TLV is a Sub-TLV of the:¶
OSPF FAAM 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Flex-Algorithm | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:¶
The OSPF FAAM Sub-TLV MAY appear multiple times in its parent TLV. If it appears more than once with the same Flex-Algorithm value, the first instance MUST be used and any subsequent instances MUST be ignored.¶
The advertisement of the ASBR reachability using the OSPF FAAM Sub-TLV inside the OSPFv2 EIA-ASBR LSA follows Section 12.4.3 of [RFC2328] and inside the OSPFv3 E-Inter-Area-Router LSA follows Section 4.8.5 of [RFC5340]. The reachability of the ASBR is evaluated in the context of the specific Flex-Algorithm.¶
The FAAM computed by the ABR will be equal to the metric to reach the ASBR for a given Flex-Algorithm in a source area or the cumulative metric via other ABR(s) when the ASBR is in a remote area. This is similar in nature to how the metric is set when the ASBR reachability metric is computed in the default algorithm for the metric in the OSPFv2 Type 4 ASBR Summary LSA and the OSPFv3 Inter-Area-Router LSA.¶
An OSPF ABR MUST NOT include the OSPF FAAM Sub-TLV with a specific Flex-Algorithm in its reachability advertisement for an ASBR between areas unless that ASBR is reachable for it in the context of that specific Flex-Algorithm.¶
An OSPF ABR MUST include the OSPF FAAM Sub-TLVs as part of the ASBR reachability advertisement between areas for any Flex-Algorithm for which the winning FAD includes the M-flag and the ASBR is reachable in the context of that specific Flex-Algorithm.¶
OSPF routers MUST use the OSPF FAAM Sub-TLV to calculate the reachability of the ASBRs if the winning FAD for the specific Flex- Algorithm includes the M-flag. OSPF routers MUST NOT use the OSPF FAAM Sub-TLV to calculate the reachability of the ASBRs for the specific Flex-Algorithm if the winning FAD for such Flex-Algorithm does not include the M-flag. Instead, the OSPFv2 Type 4 Summary LSAs or the OSPFv3 Inter-Area-Router-LSAs MUST be used instead as specified in section 16.2 of [RFC2328] and section 4.8.5 of [RFC5340] for OSPFv2 and OSPFv3 respectively.¶
The processing of a new or changed OSPF FAAM Sub-TLV triggers the processing of External routes similar to what is described in section 16.5 of the [RFC2328] for OSPFv2 and section 4.8.5 of [RFC5340] for OSPFv3 for the specific Flex-Algorithm. The External and NSSA External route calculation should be limited to Flex-Algorithm(s) for which the winning FAD(s) includes the M-flag.¶
Processing of the OSPF FAAM Sub-TLV does not require the existence of the equivalent OSPFv2 Type 4 Summary LSA or the OSPFv3 Inter-Area-Router-LSA that is advertised by the same ABR inside the area. The presence of the base LSA is not mandatory for the usage of the extended LSA with the OSPF FAAM Sub-TLV.¶
When a router is configured to participate in a particular Flex-Algorithm and is advertising such participation, it is participating in that Flex-Algorithm.¶
Paths for various data-planes MAY be computed for a specific Flex-Algorithm. Each data-plane uses its own specific forwarding over such Flex-Algorithm paths. To guarantee the presence of the data-plane specific forwarding, associated with a particular Flex-Algorithm, a router MUST advertise its participation for a particular Flex-Algorithm for each data-plane. Some data-planes may share a common participation advertisement (e.g. SR-MPLS and SRv6).¶
Advertisement of the participation for any particular Flex-Algorithm in any data-plane is subject to the condition specified in Section 5.3.¶
[RFC8667], [RFC8665], and [RFC8666] (IGP Segment Routing extensions) describe how the SR-Algorithm is used to compute the IGP best path.¶
Routers advertise support for the SR-Algorithm as a node capability as described in the above-mentioned IGP Segment Routing extensions. To advertise participation for a particular Flex-Algorithm for Segment Routing, including both SR-MPLS and SRv6, the Flex-Algorithm value MUST be advertised in the SR-Algorithm TLV (OSPF) or sub-TLV (IS-IS).¶
Segment Routing Flex-Algorithm participation advertisement is topology independent. When a router advertises participation in an SR-Algorithm, the participation applies to all topologies in which the advertising node participates.¶
This section describes considerations related to how other data-planes can advertise their participation in a specific Flex-Algorithm.¶
Data-plane specific Flex-Algorithm participation advertisements MAY be topology specific or MAY be topology independent, depending on the data-plane itself.¶
Data-plane specific advertisement for Flex-Algorithm participation MUST be defined for each data-plane and is outside the scope of this document.¶
Various link attributes may be used during the Flex-Algorithm path calculation. For example, include or exclude rules based on link affinities can be part of the Flex-Algorithm definition as defined in Section 6 and Section 7.¶
Application-specific link attributes, as specified in [RFC8919] or [RFC8920], that are to be used during Flex-Algorithm calculation MUST use the Application-Specific Link Attribute (ASLA) advertisements defined in [RFC8919] or [RFC8920], unless, in the case of IS-IS, the L-Flag is set in the ASLA advertisement. When the L-Flag is set, then legacy advertisements MUST be used, subject to the procedures and constraints defined in [[RFC8919] Section 4.2 and Section 6.¶
The mandatory use of ASLA advertisements applies to link attributes specifically mentioned in this document (Min Unidirectional Link Delay, TE Default Metric, Administrative Group, Extended Administrative Group and Shared Risk Link Group) and any other link attributes that may be used in support of Flex-Algorithm in the future.¶
A new Application Identifier Bit is defined to indicate that the ASLA advertisement is associated with the Flex-Algorithm application. This bit is set in the Standard Application Bit Mask (SABM) defined in [RFC8919] or [RFC8920]:¶
ASLA Admin Group Advertisements to be used by the Flexible Algorithm application MAY use either the Administrative Group or Extended Administrative Group encodings.¶
A receiver supporting this specification MUST accept both ASLA Administrative Group and Extended Administrative Group TLVs as defined in [RFC8919] or [RFC8920]. In the case of IS-IS, if the L-Flag is set in ASLA advertisement, as defined in [RFC8919] Section 4.2, then the receiver MUST be able to accept both Administrative Group TLV as defined in [RFC5305] and Extended Administrative Group TLV as defined in [RFC7308].¶
A router MUST be configured to participate in a given Flex-Algorithm K and MUST select the FAD based on the rules defined in Section 5.3 before it can compute any path for that Flex-Algorithm.¶
No specific two-way connectivity check is performed during the Flex-Algorithm path computation. The result of the existing, Flex-Algorithm agnostic, two-way connectivity check is used during the Flex-Algorithm path computation.¶
As described in Section 11, participation for any particular Flex-Algorithm MUST be advertised on a per data-plane basis. Calculation of the paths for any particular Flex-Algorithm is data-plane specific.¶
Multiple data-planes MAY use the same Flex-Algorithm value at the same time, and as such, share the FAD for it. Traffic for each data-plane will be forwarded based on the data-plane specific forwarding entries.¶
Flex-Algorithm definition is data-plane independent and is used by all Flex-Algorithm data-planes.¶
The way various data-planes handle nodes that do not participate in Flexible Algorithm is data-plane specific. If the data-plane only wants to consider participating nodes during the Flex-Algorithm calculation, then when computing paths for a given Flex-Algorithm, all nodes that do not advertise participation for that Flex-Algorithm in their data-plane specific advertisements MUST be pruned from the topology. Segment Routing, including both SR-MPLS and SRv6, are data-planes that MUST use such pruning when computing Flex-Algorithm paths.¶
When computing the path for a given Flex-Algorithm, the metric-type that is part of the Flex-Algorithm definition (Section 5) MUST be used.¶
When computing the path for a given Flex-Algorithm, the calculation-type that is part of the Flex-Algorithm definition (Section 5) MUST be used.¶
Various link include or exclude rules can be part of the Flex-Algorithm definition. To refer to a particular bit within an Admin Group or Extended Admin Group we use the term 'color'.¶
Rules, in the order as specified below, MUST be used to prune links from the topology during the Flex-Algorithm computation.¶
For all links in the topology:¶
Any IGP Shortest Path Tree calculation is limited to a single area. This applies to Flex-Algorithm calculations as well. Given that the computing router does not have visibility of the topology of the next areas or domain, the Flex-Algorithm specific path to an inter-area or inter-domain prefix will be computed for the local area only. The egress L1/L2 router (ABR in OSPF), or ASBR for inter-domain case, will be selected based on the best path for the given Flex-Algorithm in the local area and such egress ABR or ASBR router will be responsible to compute the best Flex-Algorithm specific path over the next area or domain. This may produce an end-to-end path, which is suboptimal based on Flex-Algorithm constraints. In cases where the ABR or ASBR has no reachability to a prefix for a given Flex-Algorithm in the next area or domain, the traffic could be dropped by the ABR/ASBR.¶
To allow the optimal end-to-end path for an inter-area or inter-domain prefix for any Flex-Algorithm to be computed, the FAPM has been defined in Section 8 and Section 9. For external route calculation for prefixes originated by ASBRs in remote areas in OSPF, the FAAM has been defined in Section 10.2 for the ABR to indicate its ASBR reachability along with the metric for the specific Flex-Algorithm.¶
If the FAD selected based on the rules defined in Section 5.3 includes the M-flag, an ABR or ASBR MUST include the FAPM (Section 8, Section 9) when advertising the prefix, that is reachable in a given Flex-Algorithm, between areas or domains. Such metric will be equal to the metric to reach the prefix for that Flex-Algorithm in its source area or domain. This is similar in nature to how the metric is set when prefixes are advertised between areas or domains for the default algorithm. When a prefix is unreachable in its source area or domain in a specific Flex-Algorithm, then an ABR or ASBR MUST NOT include the FAPM for that Flex-Algorithm when advertising the prefix between areas or domains.¶
If the FAD selected based on the rules defined in Section 5.3 includes the M-flag, the FAPM MUST be used during the calculation of prefix reachability for the inter-area and external prefixes. If the FAPM for the Flex-Algorithm is not advertised with the inter-area or external prefix reachability advertisement, the prefix MUST be considered as unreachable for that Flex-Algorithm. Similarly, in the case of OSPF, for ASBRs in remote areas, if the FAAM is not advertised by the local ABR(s), the ASBR MUST be considered as unreachable for that Flex-Algorithm and the external prefix advertisements from such an ASBR are not considered for that Flex-Algorithm.¶
Flex-Algorithm prefix metrics and the OSPF Flex-Algorithm ASBR metrics MUST NOT be used during the Flex-Algorithm computation unless the FAD selected based on the rules defined in Section 5.3 includes the M-Flag, as described in (Section 6.4 or Section 7.4).¶
In the case of OSPF, when calculating external routes in a Flex-Algorithm, if the winning FAD includes the M-Flag, and where the advertising ASBR is in a remote area, the metric will be the sum of the following:¶
This is similar in nature to how the metric is calculated for routes learned from remote ASBRs in the default algorithm using the OSPFv2 Type 4 ASBR Summary LSA and the OSPFv3 Inter-Area-Router LSA.¶
If the FAD selected based on the rules defined in Section 5.3 does not include the M-flag, then the IGP metrics associated with the prefix reachability advertisements used by the base IS-IS and OSPF protocol MUST be used for the Flex-Algorithm route computation. Similarly, in the case of external route calculations in OSPF, the ASBR reachability is determined based on the base OSPFv2 Type 4 Summary LSA and the OSFPv3 Inter-Area-Router LSA.¶
It is NOT RECOMMENDED to use the Flex-Algorithm for inter-area or inter-domain prefix reachability without the M-flag set. The reason is that without the explicit Flex-Algorithm Prefix Metric advertisement (and the Flex-Algorithm ASBR metric advertisement in the case of OSPF external route calculation), it is not possible to conclude whether the ABR or ASBR has reachability to the inter-area or inter-domain prefix for a given Flex-Algorithm in the next area or domain. Sending the Flex-Algorithm traffic for such a prefix towards the ABR or ASBR may result in traffic looping or persistent traffic drop.¶
During the route computation, it is possible for the Flex-Algorithm specific metric to exceed the maximum value that can be stored in an unsigned 32-bit variable. In such scenarios, the value MUST be considered to be of value 0xFFFFFFFF during the computation and advertised as such.¶
The FAPM MUST NOT be advertised with IS-IS L1 or L2 intra-area, OSPFv2 intra-area, or OSPFv3 intra-area routes. If the FAPM is advertised for these route-types, it MUST be ignored during the prefix reachability calculation.¶
The M-flag in the FAD is not applicable to prefixes advertised as SRv6 locators. The IS-IS SRv6 Locator TLV [I-D.ietf-lsr-isis-srv6-extensions] includes the Algorithm and Metric fields. When the SRv6 Locator is advertised between areas or domains, the metric field in the Locator TLV of IS-IS MUST be used irrespective of the M-flag in the FAD advertisement.¶
OSPF external and NSSA external prefix advertisements MAY include a non-zero forwarding address in the prefix advertisements in the base protocol. In such a scenario, the Flex-Algorithm specific reachability of the external prefix is determined by Flex-Algorithm specific reachability of the forwarding address.¶
In OSPF, the procedures for translation of NSSA external prefix advertisements into external prefix advertisements performed by an NSSA ABR [RFC3101] remain unchanged for Flex-Algorithm. An NSSA translator MUST include the OSPF FAPM Sub-TLVs for all Flex-Algorithms that are in the original NSSA external prefix advertisement from the NSSA ASBR in the translated external prefix advertisement generated by it regardless of its participation in those Flex-Algorithms or its having reachability to the NSSA ASBR in those Flex-Algorithms.¶
An area could become partitioned from the perspective of the Flex-Algorithm due to the constraints and/or metric being used for it, while maintaining the continuity in the base algorithm. When that happens, some destinations inside that area could become unreachable in that Flex-Algorithm. These destinations will not be able to use an inter-area path. This is the consequence of the fact that the inter-area prefix reachability advertisement would not be available for these intra-area destinations within the area. It is RECOMMENDED to minimize the risk of such partitioning by providing enough redundancy inside the area for each Flex-Algorithm being used.¶
This section describes how Flex-Algorithm paths are used in forwarding.¶
This section describes how Flex-Algorithm paths are used with SR MPLS forwarding.¶
Prefix SID advertisements include an SR-Algorithm value and, as such, are associated with the specified SR-Algorithm. Prefix-SIDs are also associated with a specific topology which is inherited from the associated prefix reachability advertisement. When the algorithm value advertised is a Flex-Algorithm value, the Prefix SID is associated with paths calculated using that Flex-Algorithm in the associated topology.¶
A Flex-Algorithm path MUST be installed in the MPLS forwarding plane using the MPLS label that corresponds to the Prefix-SID that was advertised for that Flex-algorithm. If the Prefix SID for a given Flex-algorithm is not known, the Flex-Algorithm specific path cannot be installed in the MPLS forwarding plane.¶
Traffic that is supposed to be routed via Flex-Algorithm specific paths MUST be dropped when there are no such paths available.¶
Loop Free Alternate (LFA) paths ([RFC6571] or its variants) for a given Flex-Algorithm MUST be computed using the same constraints as the calculation of the primary paths for that Flex-Algorithm. LFA paths MUST only use Prefix-SIDs advertised specifically for the given algorithm. LFA paths MUST NOT use an Adjacency-SID that belongs to a link that has been pruned from the Flex-Algorithm computation.¶
If LFA protection is being used to protect a given Flex-Algorithm paths, all routers in the area participating in the given Flex-Algorithm SHOULD advertise at least one Flex-Algorithm specific Node-SID. These Node-SIDs are used to steer traffic over the LFA computed backup path.¶
This section describes how Flex-Algorithm paths are used with SRv6 forwarding.¶
In SRv6 a node is provisioned with a (topology, algorithm) specific locator for each of the topology/algorithm pairs supported by that node. Each locator is an aggregate prefix for all SIDs provisioned on that node which have the matching topology/algorithm.¶
The SRv6 locator advertisement in IS-IS [I-D.ietf-lsr-isis-srv6-extensions] includes the MTID value that associates the locator with a specific topology. SRv6 locator advertisements also includes an Algorithm value that explicitly associates the locator with a specific algorithm. When the algorithm value advertised with a locator represents a Flex-Algorithm, the paths to the locator prefix MUST be calculated using the specified Flex-Algorithm in the associated topology.¶
Forwarding entries for the locator prefixes advertised in IS-IS MUST be installed in the forwarding plane of the receiving SRv6 capable routers when the associated topology/algorithm is participating in them. Forwarding entries for locators associated with Flex-Algorithms in which the node is not participating MUST NOT be installed in the forwarding plane.¶
When the locator is associated with a Flex-Algorithm, LFA paths to the locator 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. LFA paths MUST only use SRv6 SIDs advertised specifically for the given Flex-Algorithm.¶
If LFA protection is being used to protect locators associated with a given Flex-Algorithm, all routers in the area participating in the given Flex-Algorithm SHOULD advertise at least one Flex-Algorithm specific locator and END SID per node and one END.X SID for every link that has not been pruned from such Flex-Algorithm computation. These locators and SIDs are used to steer traffic over the LFA-computed backup path.¶
Any data-plane that wants to use Flex-Algorithm specific forwarding needs to install some form of Flex-Algorithm specific forwarding entries.¶
Data-plane specific forwarding for Flex-Algorithm MUST be defined for each data-plane and is outside the scope of this document.¶
The scope of the Flex-Algorithm computation is an area, so is the scope of the FAD. In IS-IS, the Router Capability TLV in which the FAD Sub-TLV is advertised MUST have the S-bit clear, which prevents it from being flooded outside the level in which it was originated. Even though in OSPF the FAD Sub-TLV can be flooded in an RI LSA that has AS flooding scope, the FAD selection is performed for each individual area in which it is being used.¶
There is no requirement for the FAD for a particular Flex-Algorithm to be identical in all areas in the network. For example, traffic for the same Flex-Algorithm may be optimized for minimal delay (e.g., using delay metric) in one area or level, while being optimized for available bandwidth (e.g., using IGP metric) in another area or level.¶
As described in Section 5.1, IS-IS allows the re-generation of the winning FAD from level 2, without any modification to it, into a level 1 area. This allows the operator to configure the FAD in one or multiple routers in the level 2, without the need to repeat the same task in each level 1 area, if the intent is to have the same FAD for the particular Flex-Algorithm across all levels. This can similarly be achieved in OSPF by using the AS flooding scope of the RI LSA in which the FAD Sub-TLV for the particular Flex-Algoritm is advertised.¶
Re-generation of the FAD from a level 1 area to the level 2 area is not supported in IS-IS, so if the intent is to regenerate the FAD between IS-IS levels, the FAD MUST be defined on router(s) that are in level 2. In OSPF, the FAD definition can be done in any area and be propagated to all routers in the OSPF routing domain by using the AS flooding scope of the RI LSA.¶
There are two different ways in which SRLG information can be used with Flex-Algorithm:¶
In the context of multiple Flex-Algorithms, it can be used for creating disjoint sets of paths by pruning the links belonging to a specific SRLG from the topology on which a specific Flex-Algorithm computes its paths. This usage:¶
The two usages mentioned above are orthogonal.¶
Both IS-IS and OSPF have a mechanism to set the IGP metric on a link to a value that would make the link either non-reachable or to serve as the link of last resort. Similar functionality would be needed for the Min Unidirectional Link Delay and TE metric, as these can be used to compute Flex-Algorithm paths.¶
The link can be made un-reachable for all Flex-Algorithms that use Min Unidirectional Link Delay as metric, as described in Section 5.1, by removing the Flex-Algorithm ASLA Min Unidirectional Link Delay advertisement for the link. The link can be made the link of last resort by setting the delay value in the Flex-Algorithm ASLA delay advertisement for the link to the value of 16,777,215 (2^24 - 1).¶
The link can be made un-reachable for all Flex-Algorithms that use TE metric, as described in Section 5.1, by removing the Flex-Algorithm ASLA TE metric advertisement for the link. The link can be made the link of last resort by setting the TE metric value in the Flex-Algorithm ASLA delay advertisement for the link to the value of (2^24 - 1) in IS-IS and (2^32 - 1) in OSPF.¶
When configuring a node to participate in a specific Flex-Algorithm, the components of the FAD (calculation-type, metric-type, constraints) should be considered carefully. The configuration of participation in a particular Flex-Algorithm doesn't guarantee that the node will actively participate in it, because it may not support the calculation-type, metric type or some constraint advertised by the winning FAD (see Section 5.3). Changes in the FAD configuration should also be considered in light of the capabilities of the participating routers in the scope of the FAD advertisement.¶
As Section 5.3 notes, a change in the Flex-Algorithm definition may require network-wide SPF re-computation and network re-convergence. This potential for disruption should be taken into consideration when planning and making changes to the FAD.¶
The maximum number of Flex-Algorithms is determined by the algorithm range that is (128-255), as specified in Section 4. Although possible, it is not expected that all of them will be used simultaneously. Typically, only a limited subset of Flex-Algorithms is expected to be deployed in the network.¶
This extension brings no new backward compatibility issues. IS-IS, OSPFv2 and OSPFv3 all have well-defined handling of unrecognized TLVs and sub-TLVs that allows the introduction of new extensions, similar to those defined here, without introducing any interoperability issues.¶
This draft adds two new ways to disrupt IGP networks:¶
Both of these attacks can be addressed by the existing security extensions as described in [RFC5304] and [RFC5310] for IS-IS, in [RFC2328] and [RFC7474] for OSPFv2, and in [RFC5340] and [RFC4552] for OSPFv3.¶
If the node that is authenticated is taken over by an attacker, such rogue node can advertise the FAD for any Flex-Algorithm. Doing so may result in traffic for such Flex-Algorithm to be misrouted, or not being delivered at all, for example, by using an unsupported metric-type, calculation-type, or constraint. Such attack is not preventable through authentication, and it is not different from advertising any other incorrect information through IS-IS or OSPF.¶
This document makes the following registrations in the "IGP Algorithm Types" registry:¶
IANA is requested to set up a registry called "IGP Metric-Type Registry" under the "Interior Gateway Protocol (IGP) Parameters" IANA grouping. The registration policy for this registry is "Standards Action" ([RFC8126] and [RFC7120]).¶
Values in this registry come from the range 0-255.¶
This document registers following values in the "IGP Metric-Type Registry":¶
IANA is requested to set up a registry called "IGP Flexible Algorithm Definition Flags Registry" under the "Interior Gateway Protocol (IGP) Parameters" IANA grouping. The registration policy for this registry is "Standards Action" ([RFC8126] and [RFC7120]). New registrations should be assigned in ascending bit order (see Section 6.4).¶
This document defines the following single bit in Flexible Algorithm Definition Flags registry:¶
Bit # Name
----- ------------------------------
0 Prefix Metric Flag (M-flag)
¶
Reference: This document (Section 6.4, Section 7.4).¶
This document makes the following registrations in the "IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV" registry.¶
This document makes the following registrations in the "IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability" registry.¶
This document creates the following Sub-Sub-TLV Registry, under the IS-IS TLV Codepoints grouping.¶
This document defines the following Sub-Sub-TLVs in the "Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV" registry:¶
This specification makes the following registration in the OSPF Router Information (RI) TLVs Registry.¶
This document makes the following registrations in the "OSPFv2 Extended Prefix TLV Sub-TLVs" registry.¶
This document makes the following registrations in the "OSPFv3 Extended-LSA Sub-TLVs" registry.¶
For both of these sub-TLVs the column L2BN in the registry is set to "X" - meaning "sub-TLV is not a Router Link sub-TLV; it MUST NOT appear in L2 Bundle Member sub-TLV".¶
This specification requests creation of the "OSPF Flex-Algorithm Prefix Metric Bits" registry under the "Open Shortest Path First (OSPF) Parameters" with the following initial values:¶
The bits 1-7 are unassigned and the registration procedure to be followed for this registry is IETF Review.¶
This document makes the following registrations in the "Opaque Link-State Advertisements (LSA) Option Types" registry under the "Open Shortest Path First (OSPF) Opaque Link-State Advertisements (LSA) Option Types" grouping.¶
This specification requests creation of "OSPFv2 Extended Inter-Area ASBR TLVs" registry under the OSPFv2 Parameters Registry with the following initial values.¶
The values 2 to 32767 are unassigned, values 32768 to 33023 are reserved for experimental use while the values 0 and 33024 to 65535 are reserved. The registration procedure to be followed for this registry is IETF Review or IESG Approval.¶
This specification requests creation of "OSPFv2 Extended Inter-Area ASBR Sub-TLVs" registry under the "Open Shortest Path First v2 (OSPFv2) Parameters" grouping, with the following initial values.¶
The values 2 to 32767 are unassigned, values 32768 to 33023 are reserved for experimental use while the values 0 and 33024 to 65535 are reserved. The registration procedure to be followed for this registry is IETF Review or IESG Approval.¶
This document creates the following registry under the "Open Shortest Path First (OSPF) Parameters" grouping:¶
The "OSPF Flexible Algorithm Definition TLV sub-TLV" registry will define sub-TLVs at any level of nesting for the Flexible Algorithm TLV New values can be allocated via IETF Review or IESG Approval.¶
This document registers following Sub-TLVs in the "OSPF Flexible Algorithm Definition TLV sub-TLV" registry:¶
The values 6 to 32767 are unassigned, values 32768-33023 are for experimental use; these will not be registered with IANA.¶
Types in the range 33024-65535 are not to be assigned at this time. Before any assignments can be made in the 33024-65535 range, there MUST be an IETF specification that specifies IANA Considerations that covers the range being assigned.¶
This document registers following bit in the Link Attribute Applications Registry:¶
This draft, among other things, is also addressing the problem that the [I-D.gulkohegde-routing-planes-using-sr] was trying to solve. All authors of that draft agreed to join this draft.¶
Thanks to Eric Rosen, Tony Przygienda, William Britto A J, Gunter Van De Velde, Dirk Goethals, Manju Sivaji and, Baalajee S for their detailed review and excellent comments.¶
Thanks to Cengiz Halit for his review and feedback during initial phase of the solution definition.¶
Thanks to Kenji Kumaki for his comments.¶
Thanks to Acee Lindem for editorial comments.¶