Internet Engineering Task Force (IETF) W. Britto
Request for Comments: 9502 S. Hegde
Category: Standards Track P. Kaneriya
ISSN: 2070-1721 R. Shetty
R. Bonica
Juniper Networks
P. Psenak
Cisco Systems
November 2023
IGP Flexible Algorithm in IP Networks
Abstract
This document extends IGP Flexible Algorithm so that it can be used
with regular IPv4 and IPv6 forwarding.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9502.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
2. Requirements Language
3. Use Case Example
4. Advertising Flexible Algorithm Definitions (FADs)
5. Advertising IP Flexible Algorithm Participation
5.1. The IS-IS IP Algorithm Sub-TLV
5.2. The OSPF IP Algorithm TLV
6. Advertising IP Flexible Algorithm Reachability
6.1. The IS-IS IPv4 Algorithm Prefix Reachability TLV
6.2. The IS-IS IPv6 Algorithm Prefix Reachability TLV
6.3. The OSPFv2 IP Algorithm Prefix Reachability Sub-TLV
6.3.1. The OSPFv2 IP Forwarding Address Sub-TLV
6.4. The OSPFv3 IP Algorithm Prefix Reachability Sub-TLV
6.5. The OSPF IP Flexible Algorithm ASBR Metric Sub-TLV
7. Calculating of IP Flexible Algorithm Paths
8. IP Flexible Algorithm Forwarding
9. Deployment Considerations
10. Protection
11. IANA Considerations
12. Security Considerations
13. References
13.1. Normative References
13.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
An IGP Flexible Algorithm allows IGPs to compute constraint-based
paths. The base IGP Flexible Algorithm specification describes how
it is used with Segment Routing (SR) data planes: SR MPLS and SRv6.
An IGP Flexible Algorithm as specified in [RFC9350] computes a
constraint-based path to:
* All Flexible-Algorithm-specific Prefix Segment Identifiers (SIDs)
[RFC8402].
* All Flexible-Algorithm-specific SRv6 Locators [RFC8986].
Therefore, Flexible Algorithm cannot be deployed in the absence of SR
or SRv6.
This document extends Flexible 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 section, 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 UPF based on the
session requirements. For example, some sessions require high
bandwidth, while 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 Service
Level Agreement (SLA) appropriate for a user session.
4. Advertising Flexible Algorithm Definitions (FADs)
To guarantee loop-free forwarding, all routers that participate in a
Flex-Algorithm MUST agree on the Flexible 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 Flexible Algorithm Participation
A node may use various algorithms when calculating paths to nodes and
prefixes. Algorithm values are defined in the "IGP Algorithm Types"
registry [IANA-ALG].
Only a node that is participating in a Flex-Algorithm is:
* Able to compute a path for such Flex-Algorithm
* Part of the topology for such Flex-Algorithm
Flexible Algorithm participation MUST be advertised for each Flexible
Algorithm data plane independently, as specified in [RFC9350]. Using
Flexible Algorithm for regular IPv4 and IPv6 prefixes represents an
independent Flexible Algorithm data plane; as such, the Flexible
Algorithm participation for the IP Flexible Algorithm data plane MUST
be signaled independently of any other Flexible Algorithm data plane
(e.g., SR).
All routers in an IGP domain participate in default algorithm 0.
Advertisement of participation in IP Flexible Algorithm does not
impact the router participation in default algorithm 0.
Advertisement of participation in IP Flexible 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-Algorithm for the IP data plane but does not participate in the
same Flex-Algorithm for the SR data plane.
The following sections describe how the IP Flexible 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 Link State PDU (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 Link State Advertisement (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 (21)
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 AS-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 Flexible 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 Flexible 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 Flexible 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 Flexible 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: The D-flag is described as the "up/down bit" in
Section 4.1 of [RFC5305]. 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.
The remaining bits: Reserved for future use. They MUST be set to
zero on transmission and MUST be ignored on receipt.
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
are 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 an 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
are 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 6
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:
E bit: The same as the E bit defined in Appendix A.4.5 of
[RFC2328].
The remaining bits: 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 unreachable.
If the Algorithms in the OSPFv2 IP Algorithm Prefix Reachability Sub-
TLV are 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 occurrences 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
occurrences 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 7
Length (2 octets): 4
Forwarding Address (4 octets): The same as defined in Appendix 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 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 35
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 unreachable.
If the Algorithms in the OSPFv3 IP Algorithm Prefix Reachability Sub-
TLV are 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
occurrences 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
occurrences 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 (FAAM) Sub-
TLV that is used by an OSPFv2 or an OSPFv3 Area Border Router (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 Flexible Algorithm participation is signaled
independent of SR Flexible Algorithm participation. As a result, the
calculated topologies for SR and IP Flexible Algorithm could be
different. Such a difference prevents the usage of FAAM for the
purpose of the IP Flexible 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, as 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: OSPF IP Flexible Algorithm ASBR Metric Sub-TLV
Where:
Type (2 octets): 2 (allocated by IANA) for OSPFv2, 36 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 are 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 Flexible
Algorithm metric.
An OSPF ABR MUST include the OSPF IPFAAM Sub-TLVs as part of any IP
Flexible Algorithm ASBR reachability advertisement between areas.
The FAAM Sub-TLV as defined in [RFC9350] MUST NOT be used during IP
Flexible Algorithm path calculation; the IPFAAM Sub-TLV MUST be used
instead.
7. Calculating of IP Flexible Algorithm Paths
The IP Flexible Algorithm is considered as yet another data plane of
the Flexible Algorithm as described in [RFC9350].
Participation in the IP Flexible Algorithm is signaled as described
in Section 5 and is specific to the IP Flexible Algorithm data plane.
Calculation of IP Flexible Algorithm paths follows what is described
in [RFC9350]. This computation uses the IP Flexible Algorithm data
plane participation and is independent of the Flexible Algorithm
calculation done for any other Flexible Algorithm data plane (e.g.,
SR, SRv6).
The IP Flexible Algorithm data plane only considers participating
nodes during the Flexible Algorithm calculation. When computing
paths for a given Flex-Algorithm, all nodes that do not advertise
participation for such IP Flex-Algorithm, as described in Section 5,
MUST be pruned from the topology.
8. IP Flexible 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 Flexible 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 Flexible 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
Flexible Algorithm.
Flexible Algorithm Definition (FAD), as described in [RFC9350], is
data plane independent and is used by all Flexible Algorithm data
planes.
Participation in the Flexible Algorithm, as described in [RFC9350],
is data plane specific.
Calculation of the Flexible Algorithm paths is data plane specific
and uses data-plane-specific participation advertisements.
Data-plane-specific participation and calculation guarantee that the
forwarding of the traffic over the Flex-Algorithm data-plane-specific
paths is consistent between all nodes that apply the IGP Flex-
Algorithm to the data plane.
Multiple data planes can use the same Flex-Algorithm value at the
same time and, and as such, share the FAD for it. For example, SR-
MPLS and IP can both use a common Flex-Algorithm. Traffic for SR-
MPLS will be forwarded based on Flex-Algorithm-specific SR SIDs.
Traffic for IP Flex-Algorithm will be forwarded based on Flex-
Algorithm-specific prefix reachability advertisements. Note that for
a particular Flex-Algorithm, for a particular IP prefix, there will
only be path(s) calculated and installed for a single data plane.
10. Protection
In many networks where IGP Flexible Algorithms are deployed, IGP
restoration will be fast and additional protection mechanisms will
not be required. IGP restoration may be enhanced by Equal Cost
Multipath (ECMP).
In other networks, operators can deploy additional protection
mechanisms. The following are examples:
* Loop-Free Alternates (LFAs) [RFC5286]
* Remote Loop-Free Alternates (R-LFAs) [RFC7490]
LFA and R-LFA computations MUST be restricted to the Flex-Algorithm
topology and the computed backup next hops should be programmed for
the IP Flex-Algorithm prefixes.
11. IANA Considerations
This specification updates the "OSPF Router Information (RI) TLVs"
registry as follows:
+=======+==============+=======================+
| Value | TLV Name | Reference |
+=======+==============+=======================+
| 21 | IP Algorithm | RFC 9502, Section 5.2 |
+-------+--------------+-----------------------+
Table 1
This document also updates the "IS-IS Sub-TLVs for IS-IS Router
CAPABILITY TLV" registry as follows:
+=======+==============+=======================+
| Value | TLV Name | Reference |
+=======+==============+=======================+
| 29 | IP Algorithm | RFC 9502, Section 5.1 |
+-------+--------------+-----------------------+
Table 2
This document also updates the "IS-IS Top-Level TLV Codepoints"
registry as follows:
+=======+=====================+=====+=====+=====+=======+===========+
| Value | TLV Name | IIH | LSP | SNP | Purge | Reference |
+=======+=====================+=====+=====+=====+=======+===========+
| 126 | IPv4 Algorithm | n | y | n | n | RFC 9502, |
| | Prefix | | | | | Section |
| | Reachability | | | | | 6.1 |
+-------+---------------------+-----+-----+-----+-------+-----------+
| 127 | IPv6 Algorithm | n | y | n | n | RFC 9502, |
| | Prefix | | | | | Section |
| | Reachability | | | | | 6.2 |
+-------+---------------------+-----+-----+-----+-------+-----------+
Table 3
Since the above TLVs share the sub-TLV space managed in the "IS-IS
Sub-TLVs for TLVs Advertising Prefix Reachability" registry, IANA has
added "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" have been 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 | Flexible Algorithm Prefix Metric (FAPM) | n | n |
+------+-----------------------------------------+-----+-----+
| 11 | IPv4 Source Router ID | y | y |
+------+-----------------------------------------+-----+-----+
| 12 | IPv6 Source Router ID | y | y |
+------+-----------------------------------------+-----+-----+
| 32 | BIER Info | n | n |
+------+-----------------------------------------+-----+-----+
Table 4
This document registers the following in the "OSPFv2 Extended Prefix
TLV Sub-TLVs" registry:
+=======+=========================================+===============+
| Value | TLV Name | Reference |
+=======+=========================================+===============+
| 6 | OSPFv2 IP Algorithm Prefix Reachability | RFC 9502, |
| | | Section 6.3 |
+-------+-----------------------------------------+---------------+
| 7 | OSPFv2 IP Forwarding Address | RFC 9502, |
| | | Section 6.3.1 |
+-------+-----------------------------------------+---------------+
Table 5
IANA has created the "IP Algorithm Prefix Reachability Sub-TLV Flags"
registry within the "Open Shortest Path First v2 (OSPFv2) Parameters"
group of registries. 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]
+=====+============+=======================+
| Bit | Name | Reference |
+=====+============+=======================+
| 0 | E bit | RFC 9502, Section 6.3 |
+-----+------------+-----------------------+
| 1-7 | Unassigned | |
+-----+------------+-----------------------+
Table 6
This document registers the following in the "OSPFv3 Extended-LSA
Sub-TLVs" registry:
+=======+=======================+======+=============+
| Value | Description | L2BM | Reference |
+=======+=======================+======+=============+
| 35 | OSPFv3 IP Algorithm | X | RFC 9502, |
| | Prefix Reachability | | Section 6.4 |
+-------+-----------------------+------+-------------+
| 36 | OSPFv3 IP Flexible | X | RFC 9502, |
| | Algorithm ASBR Metric | | Section 6.5 |
+-------+-----------------------+------+-------------+
Table 7
This document registers the following in the "OSPFv2 Extended Inter-
Area ASBR Sub-TLVs" registry:
+=======+========================================+=============+
| Value | Description | Reference |
+=======+========================================+=============+
| 2 | OSPF IP Flexible Algorithm ASBR Metric | RFC 9502, |
| | | Section 6.5 |
+-------+----------------------------------------+-------------+
Table 8
12. Security Considerations
This document inherits security considerations from [RFC9350].
This document adds one new way to disrupt IGP networks that are using
Flexible 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-to-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. References
13.1. Normative References
[ISO10589] ISO, "Information technology - Telecommunications and
information exchange between systems - Intermediate System
to Intermediate System intra-domain routeing information
exchange protocol for use in conjunction with the protocol
for providing the connectionless-mode network service (ISO
8473)", Second Edition, ISO/IEC 10589:2002, November 2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
<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, June 2007,
<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, February 2008,
<https://www.rfc-editor.org/info/rfc5120>.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <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, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
<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, February
2009, <https://www.rfc-editor.org/info/rfc5310>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, Ed.,
"OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, July
2008, <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, April 2015,
<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,
February 2016, <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, October 2016,
<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,
May 2017, <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, April
2018, <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, February 2023,
<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,
February 2023, <https://www.rfc-editor.org/info/rfc9352>.
13.2. Informative References
[IANA-ALG] IANA, "IGP Algorithm Types",
<https://www.iana.org/assignments/igp-parameters>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<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, April 2015,
<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, June 2017,
<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,
July 2018, <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, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
[TS.23.501-3GPP]
3GPP, "System architecture for 5G System (5GS)", Release
18.3.0, 3GPP TS 23.501, September 2023.
Acknowledgements
Thanks to Bruno Decraene for his contributions to this document.
Special thanks to Petr Bonbon Adamec of Cesnet for supporting
interoperability testing.
Authors' Addresses
William Britto
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore 560103
Karnataka
India
Email: bwilliam@juniper.net
Shraddha Hegde
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore 560103
Karnataka
India
Email: shraddha@juniper.net
Parag Kaneriya
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore 560103
Karnataka
India
Email: pkaneria@juniper.net
Rejesh Shetty
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore 560103
Karnataka
India
Email: mrajesh@juniper.net
Ron Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, Virginia 20171
United States of America
Email: rbonica@juniper.net
Peter Psenak
Cisco Systems
Apollo Business Center
Mlynske nivy 43
82109 Bratislava
Slovakia
Email: ppsenak@cisco.com
