Internet-Draft IP Flex-Algorithm July 2023
Britto, et al. Expires 25 January 2024 [Page]
Workgroup:
LSR Working Group
Internet-Draft:
draft-ietf-lsr-ip-flexalgo-17
Published:
Intended Status:
Standards Track
Expires:
Authors:
W. Britto
Juniper Networks
S. Hegde
Juniper Networks
P. Kaneriya
Juniper Networks
R. Shetty
Juniper Networks
R. Bonica
Juniper Networks
P. Psenak
Cisco Systems

IGP Flexible Algorithms (Flex-Algorithm) In IP Networks

Abstract

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 25 January 2024.

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Table of Contents

1. Introduction

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.

An IGP Flex-Algorithm as specified in [RFC9350] computes a constraint-based path to:

Therefore, Flex-Algorithm cannot be deployed in the absence of SR or 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

In this subsection, we illustrate one use case that motivates this specification: if a specific service can be identified by an IP address, traffic to it can use constraint-based paths computed according to this specification.

The System Architecture for the 5G System [TS.23.501-3GPP] describes the N3 interface between gNodeB and UPF (User Plane Function).

Mobile networks are becoming more and more IP centric. Each end-user session from a 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.

The IP address allocated to the UPF can be associated with an algorithm. The mobile user traffic is then forwarded along the path based on the algorithm-specific metric and constraints. As a result, traffic can be sent over a path that is optimized for minimal latency or highest bandwidth. This mechanism is used to achieve SLA (Service Level Agreement) appropriate for a user 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 [RFC9350].

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].

Only a node that is participating in a Flex-Algorithm is:

Flex-Algorithm participation MUST be advertised for each Flex-Algorithm data-plane independently, as specified in [RFC9350]. 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).

All routers in an IGP domain participate in default algorithm 0. Advertisement of participation in IP Flex-Algorithm does not impact the router participation in default algorithm 0.

Advertisement of participation in IP Flex-Algorithm does not impact the router participation signaled for other data-planes. For example, it is possible that a router participates in a particular flex-algo for the IP data-plane but does not participate in the same flex-algo for the SR data-plane.

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 IS-IS [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 (1 octet): IP Algorithm Sub-TLV (Value 29)
  • Length (1 octet): 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.

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.

Algorithms outside the Flex-Algorithm range (128-255) MUST be ignored by the receiver. This situation SHOULD be logged as an error.

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 (2 octets): IP Algorithm TLV (Value TBD1 by IANA)
  • Length( 2 octets): 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.

Algorithms outside the Flex-Algorithm range (128-255) MUST be ignored by the receiver. This situation SHOULD be logged as an error.

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, 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 or Autonomous System scoped flooding is REQUIRED. The AS flooding scope SHOULD NOT be used unless local configuration policy on the originating router indicates domain-wide flooding.

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-Algorithm Reachability

To be able to associate the prefix with the Flex-Algorithm, the existing prefix reachability advertisements cannot be used, because they advertise the prefix reachability in default algorithm 0. Instead, new IP Flex-Algorithm reachability advertisements are defined in IS-IS and OSPF.

The M-flag in the 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

The IPv4 Algorithm Prefix Reachability top-level TLV is defined for advertising IPv4 Flex-Algorithm Prefix Reachability in IS-IS.

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    |  Rsvd |    MTID               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: IS-IS IPv4 Algorithm Prefix Reachability TLV
  • Type (1 octet): IPv4 Algorithm Prefix Reachability TLV (Value 126).
  • Length (1 octet): Variable based on number of prefix entries encoded
  • Rsvd (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 as defined in [RFC5305].

  • 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.
  • Optional Sub-TLV-length (1 octet): Number of octets used by sub-TLVs
  • Optional sub-TLVs (variable length).

If the Algorithms in the IS-IS IPv4 Algorithm Prefix Reachability TLV is outside the Flex-Algorithm range (128-255), the IS-IS IPv4 Algorithm Prefix Reachability TLV MUST be ignored by the receiver. This situation SHOULD be logged as an error.

If a router receives multiple IPv4 Algorithm Prefix Reachability advertisements for the same prefix from the same originator, it MUST select the first advertisement in the lowest-numbered LSP and ignore any subsequent IPv4 Algorithm Prefix Reachability advertisements for the same prefix.

If a router receives multiple IPv4 Algorithm Prefix Reachability advertisements for the same prefix, from different originators, where all of them do not advertise the same algorithm, it 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 ([RFC5305], [RFC5120]) 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 distinct type. The type is 127.

If the Algorithms in the IS-IS IPv6 Algorithm Prefix Reachability TLV is outside the Flex-Algorithm range (128-255), the IS-IS IPv6 Algorithm Prefix Reachability TLV MUST be ignored by the receiver. This situation SHOULD be logged as an error.

If a router receives multiple IPv6 Algorithm Prefix Reachability advertisements for the same prefix from the same originator, it MUST select the first advertisement in the lowest-numbered LSP and ignore any subsequent IPv6 Algorithm Prefix Reachability advertisements for the same prefix.

If a router receives multiple IPv6 Algorithm Prefix Reachability advertisements for the same prefix, from different originators, where all of them do not advertise the same algorithm, it 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 ([RFC5308], [RFC5120]) and an IPv6 Algorithm Prefix Reachability TLV, the IPv6 Prefix Reachability advertisement MUST be preferred when installing entries in the forwarding plane.

In cases where a prefix advertisement is received in both an IS-IS SRv6 Locator TLV [RFC9352] 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    |     Flags     |     Reserved  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          Metric                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: OSPFv2 IP Algorithm Prefix Reachability Sub-TLV
  • Type (2 octets) : The value is TBD2.
  • Length (2 octets): 8
  • MT-ID (1 octet): Multi-Topology ID as defined in [RFC4915]
  • Algorithm (1 octet): Associated Algorithm from 128 to 255.

  • Flags: (1 octet): The following flags are defined:

    
       0 1 2 3 4 5 6 7 8
      +-+-+-+-+-+-+-+-+-+
      |E|   Reserved    |
      +-+-+-+-+-+-+-+-+-+

      where:
    • bit E: Same as bit E defined in section A.4.5 of [RFC2328].
    • The remaining bits, are reserved for future use. They MUST be set to zero on transmission and MUST be ignored on receipt.
  • Reserved: (1 octet). 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.

If the Algorithms in the OSPFv2 IP Algorithm Prefix Reachability Sub-TLV is outside the Flex-Algorithm range (128-255), the OSPFv2 IP Algorithm Prefix Reachability Sub-TLV MUST be ignored by the receiver. This situation SHOULD be logged as an error.

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.

An OSPFv2 router receiving multiple OSPFv2 IP Algorithm Prefix Reachability TLVs for the same prefix, from different originators, where all of them do not advertise the same 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 Sub-TLV, only the prefix reachability advertisement for algorithm 0 MUST be used and all occurences of the OSPFv2 IP Algorithm Prefix Reachability Sub-TLV MUST be ignored.

When computing the IP Algorithm Prefix reachability in OSPFv2, only information present in the OSPFv2 Extended Prefix TLV MUST be used. There will not be any information advertised for the IP Algorithm Prefix in any of the OSPFv2 LSAs that advertise prefix reachability for algorithm 0. For the IP Algorithm Prefix the OSPFv2 Extended Prefix TLV is used to advertise the prefix reachability, unlike for algorithm 0 prefixes, where the OSPFv2 Extended Prefix TLV is only used to advertise additional attributes, but not the reachability itself.

6.3.1. The OSPFv2 IP Forwarding Address Sub-TLV

A new Sub-TLV of the OSPFv2 Extended Prefix TLV is defined for advertising IP Forwarding Address, the OSPFv2 IP Forwarding Address Sub-TLV.

The OSPFv2 IP Forwarding Address 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            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Forwarding Address                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: OSPFv2 IP Forwarding Address Sub-TLV
  • Type (2 octets) : The value is TBD4.
  • Length (2 octets): 4
  • Forwarding Address (4 octets): Same as defined in section A.4.5 of [RFC2328].

The OSPFv2 IP Forwarding Address Sub-TLV MUST NOT be used for computing algorithm 0 prefix reachability and MUST be ignored for algorithm 0 prefixes.

The OSPFv2 IP Forwarding Address Sub-TLV is optional. If it is not present, the forwarding address for computing the IP Algorithm Prefix reachability is assumed to be equal to 0.0.0.0.

The OSPFv2 IP Forwarding Address Sub-TLV is only applicable to Autonomous System (AS) External and Not-So-Stubby Area (NSSA) External route types. If the OSPFv2 IP Forwarding Address Sub-TLV is advertised in the OSPFv2 Extended Prefix TLV that has the Route Type field set to any other type, the OSPFv2 IP Forwarding Address Sub-TLV MUST be ignored.

6.4. 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:

  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 7: OSPFv3 IP Algorithm Prefix Reachability Sub-TLV

Where:

  • Type (2 octets): The value is TBD3.
  • 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.

If the Algorithms in the OSPFv3 IP Algorithm Prefix Reachability Sub-TLV is outside the Flex-Algorithm range (128-255), the OSPFv3 IP Algorithm Prefix Reachability Sub-TLV MUST be ignored by the receiver. This situation SHOULD be logged as an error.

When the OSPFv3 IP Algorithm Prefix Reachability Sub-TLV is present, the NU-bit in the PrefixOptions field of the parent TLV MUST be set. This is needed to prevent the OSPFv3 IP Algorithm Prefix Reachability advertisement from contributing to the base algorithm reachability. If the NU-bit in the PrefixOptions field of the parent TLV is not set, the OSPFv3 IP Algorithm Prefix Sub-TLV MUST be ignored by the receiver.

The metric value in the parent TLV is RECOMMENDED to be set to LSInfinity [RFC2328]. This recommendation is provided as a network troubleshooting convenience; if it is not followed the protocol will still function correctly.

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, where all of them do not advertise the same 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 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 Sub-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.

6.5. The OSPF IP Flexible Algorithm ASBR Metric Sub-TLV

[RFC9350] 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 [RFC9350] each data-plane signals its participation independently. IP Flex-Algorithm participation is signaled independent of 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 [RFC9350]
- OSPFv3 Inter-Area-Router TLV defined in [RFC8362]

The OSPF IPFAAM 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            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Algorithm   |                   Reserved                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Metric                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where:
Figure 8: OSPF IP Flexible Algorithm ASBR Metric Sub-TLV
Type (2 octets): 2 (allocated by IANA) for OSPFv2, TBD5 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.

If the Algorithms in the OSPF IP Flexible Algorithm ASBR Metric Sub-TLV is outside the Flex-Algorithm range (128-255), the OSPF IP Flexible Algorithm ASBR Metric Sub-TLV MUST be ignored by the receiver. This situation SHOULD be logged as an error.

The usage of the IPFAAM Sub-TLV is similar to the usage of the FAAM Sub-TLV defined in [RFC9350], 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 [RFC9350] MUST NOT be used during IP Flex-Algorithm path calculation, the IPFAAM Sub-TLV MUST be used instead.

7. Calculating of IP Flex-Algorithm Paths

The IP Flex-Algorithm is considered as yet another data-plane of the Flex-Algorithm as described in [RFC9350].

Participation in the IP Flex-Algorithm is signalled as described in Section 5 and is specific to the IP Flex-Algorithm data-plane.

Calculation of IP Flex-Algorithm paths follows what is described in [RFC9350]. 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-Algorithm 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.

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 [RFC9350], is data-plane independent and is used by all Flex-Algorithm data-planes.

Participation in the Flex-Algorithm, as described in [RFC9350], 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:

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:

Table 1
Value TLV Name Reference
TBD1 IP Algorithm This Document Section 5.2

This document also updates the IS-IS "IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV" registry as follows:

Table 2
Value TLV Name Reference
29 IP Algorithm This Document Section 5.1

This document also updates the "IS-IS TLV Codepoints Registry" registry as follows:

Table 3
Value TLV Name IIH LSP SNP Purge Reference
126 IPv4 Algorithm Prefix Reachability N Y N N This document, Section 6.1
127 IPv6 Algorithm Prefix Reachability N Y N N This document, Section 6.2

Since the above TLVs share the sub-TLV space managed in the "IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability" registry, IANA is requested to add "IPv4 Algorithm Prefix Reachability TLV (126)" and "IPv6 Algorithm Prefix Reachability TLV (127)" to the list of TLVs in the description of that registry.

In addition, columns headed '126' and '127' are added to that registry, as follows:

     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:

Table 4
Value TLV Name Reference
TBD2 OSPFv2 IP Algorithm Prefix Reachability This Document, Section 6.3
TBD4 OSPFv2 IP Forwarding Address This Document, Section 6.3.1

This document creates a new registry under "Open Shortest Path First v2 (OSPFv2) Parameters" registry, called "IP Algorithm Prefix Reachability Sub-TLV Flags". The new registry defines the bits in the 8-bit Flags field in the OSPFv2 IP Algorithm Prefix Reachability Sub-TLV (Section 6.3). New bits can be allocated via IETF Review or IESG Approval [RFC8126]

Table 5
Bit # Name Reference
0 bit E This Document, Section 6.3
1-7 Unassigned This Document, Section 6.3

This document updates the "OSPFv3 Extended-LSA Sub-TLVs" registry as follows:

Table 6
Value TLV Name L2BM Reference
TBD3 OSPFv3 IP Algorithm Prefix Reachability X This Document, Section 6.4
TBD5 OSPFv3 IP Flexible Algorithm ASBR Metric X This Document, Section 6.5

This document updates the "OSPFv2 Extended Inter-Area ASBR Sub-TLVs" registry as follows:

Table 7
Value TLV Name Reference
2 OSPF IP Flexible Algorithm ASBR Metric This Document, Section 6.5

12. Security Considerations

This document inherits security considerations from [RFC9350].

This document adds one new way to disrupt IGP networks that are using Flex-Algorithm: an attacker can suppress reachability for a given prefix whose reachability is advertised by a legitimate node for a particular IP Flex-Algorithm X, by advertising the same prefix in Flex-Algorithm Y from another, malicious node. (To see why this is, consider, for example, the rule given in the second-last paragraph of Section 6.1).

This attack 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 [RFC4552] and [RFC5340] for OSPFv3.

If a node that is authenticated is taken over by an attacker, such a rogue node can perform the attack described above. Such an attack is not preventable through authentication, and it is not different from advertising any other incorrect information through IS-IS or OSPF.

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

[ISO10589]
ISO, "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)", , <ISO/IEC 10589:2002>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC2328]
Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, , <https://www.rfc-editor.org/info/rfc2328>.
[RFC4552]
Gupta, M. and N. Melam, "Authentication/Confidentiality for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, , <https://www.rfc-editor.org/info/rfc4552>.
[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, , <https://www.rfc-editor.org/info/rfc4915>.
[RFC5120]
Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs)", RFC 5120, DOI 10.17487/RFC5120, , <https://www.rfc-editor.org/info/rfc5120>.
[RFC5304]
Li, T. and R. Atkinson, "IS-IS Cryptographic Authentication", RFC 5304, DOI 10.17487/RFC5304, , <https://www.rfc-editor.org/info/rfc5304>.
[RFC5305]
Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, , <https://www.rfc-editor.org/info/rfc5305>.
[RFC5308]
Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, DOI 10.17487/RFC5308, , <https://www.rfc-editor.org/info/rfc5308>.
[RFC5310]
Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., and M. Fanto, "IS-IS Generic Cryptographic Authentication", RFC 5310, DOI 10.17487/RFC5310, , <https://www.rfc-editor.org/info/rfc5310>.
[RFC5340]
Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, , <https://www.rfc-editor.org/info/rfc5340>.
[RFC7474]
Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed., "Security Extension for OSPFv2 When Using Manual Key Management", RFC 7474, DOI 10.17487/RFC7474, , <https://www.rfc-editor.org/info/rfc7474>.
[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, , <https://www.rfc-editor.org/info/rfc7770>.
[RFC7981]
Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions for Advertising Router Information", RFC 7981, DOI 10.17487/RFC7981, , <https://www.rfc-editor.org/info/rfc7981>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[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, , <https://www.rfc-editor.org/info/rfc8362>.
[RFC9350]
Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm", RFC 9350, DOI 10.17487/RFC9350, , <https://www.rfc-editor.org/info/rfc9350>.
[RFC9352]
Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B., and Z. Hu, "IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352, , <https://www.rfc-editor.org/info/rfc9352>.

14.2. Informative References

[IANA-ALG]
IANA, "IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV", , <https://www.iana.org/assignments/igp-parameters/igp-parameters.xhtml#igp-algorithm-types>.
[RFC5286]
Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, DOI 10.17487/RFC5286, , <https://www.rfc-editor.org/info/rfc5286>.
[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, , <https://www.rfc-editor.org/info/rfc7490>.
[RFC8126]
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <https://www.rfc-editor.org/info/rfc8126>.
[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, , <https://www.rfc-editor.org/info/rfc8402>.
[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, , <https://www.rfc-editor.org/info/rfc8986>.
[TS.23.501-3GPP]
3rd Generation Partnership Project (3GPP), "System Architecture for 5G System; Stage 2, 3GPP TS 23.501 v16.4.0", .

Authors' Addresses

William Britto
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore 560103
Karnataka
India
Shraddha Hegde
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore 560103
Karnataka
India
Parag Kaneriya
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore 560103
Karnataka
India
Rejesh Shetty
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore 560103
Karnataka
India
Ron Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, Virginia 20171
United States of America
Peter Psenak
Cisco Systems
Apollo Business Center
Mlynske nivy 43
82109 Bratislava
Slovakia