LSR Working Group W. Britto
Internet-Draft S. Hegde
Intended status: Standards Track P. Kaneriya
Expires: November 14, 2022 R. Shetty
R. Bonica
Juniper Networks
P. Psenak
Cisco Systems
May 13, 2022
IGP Flexible Algorithms (Flex-Algorithm) In IP Networks
draft-ietf-lsr-ip-flexalgo-05
Abstract
An IGP Flexible Algorithm (Flex-Algorithm) allows IGP to compute
constraint-based paths. The base IGP Flex-Algorithm specification
describes how it is used with Segment Routing (SR) data planes - SR
MPLS and SRv6.
This document extends IGP Flex-Algorithm, so that it can be used with
regular IPv4 and IPv6 forwarding.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 14, 2022.
Copyright Notice
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
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(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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Use Case Example . . . . . . . . . . . . . . . . . . . . . . 3
4. Advertising Flex-Algorithm Definitions (FAD) . . . . . . . . 3
5. Advertising IP Flex-Algorithm Participation . . . . . . . . . 3
5.1. The ISIS IP Algorithm Sub-TLV . . . . . . . . . . . . . . 4
5.2. The OSPF IP Algorithm TLV . . . . . . . . . . . . . . . . 5
6. Advertising IP Flex-Algorthm Reachability . . . . . . . . . . 6
6.1. The ISIS IPv4 Algorithm Prefix Reachability TLV . . . . . 6
6.2. The ISIS IPv6 Algorithm Prefix Reachability TLV . . . . . 8
6.3. The OSPFv2 IP Algorithm Prefix Reachability Sub-TLV . . . 8
6.4. The OSPFv3 IP Algorithm Prefix Reachability Sub-TLV . . . 10
6.5. The OSPF IP Flexible Algorithm ASBR Metric Sub-TLV . . . 11
7. Calculating of IP Flex-Algorthm Paths . . . . . . . . . . . . 12
8. IP Flex-Algorthm Forwarding . . . . . . . . . . . . . . . . . 13
9. Deployment Considerations . . . . . . . . . . . . . . . . . . 13
10. Protection . . . . . . . . . . . . . . . . . . . . . . . . . 14
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
12. Security Considerations . . . . . . . . . . . . . . . . . . . 17
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
14.1. Normative References . . . . . . . . . . . . . . . . . . 17
14.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
An IGP Flex-Algorithm as specified in [I-D.ietf-lsr-flex-algo]
computes a constraint-based path to:
o All Flex-Algorithm specific Prefix Segment Identifiers (SIDs)
[RFC8402].
o All Flex-Algorityhm specific SRv6 Locators [RFC8986].
Therefore, Flex-Algorithm cannot be deployed in the absence of SR and
SRv6.
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This document extends Flex-Algorithm, allowing it to compute paths to
IPv4 and IPv6 prefixes.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Use Case Example
Mobile networks are becoming more and more IP centric. Each end user
session from gNB (gNodeB) can be destined to a specific UPFs (User
Plane Function) based on the session requirements. For example, some
sessions require high bandwidth, others need to be routed along the
shortest latency path. Each UPF is assigned a unique IP address. As
a result traffic for different sessions is destined to a different
destination IP address.
By associating the prefix that contains the UPF addresses with a
specific IP algorithm and routing the algorithm specific traffic
according to a certain constraints, e.g. low latency, a session
traffic is routed according to the SLA (Service Level Agreement)
appropriate for such session.
4. Advertising Flex-Algorithm Definitions (FAD)
To guarantee loop free forwarding, all routers that participate in a
Flex-Algorithm MUST agree on the Flex-Algorithm Definition (FAD).
Selected nodes within the IGP domain MUST advertise FADs as described
in Sections 5, 6 and 7 of [I-D.ietf-lsr-flex-algo].
5. Advertising IP Flex-Algorithm Participation
A node may use various algorithms when calculating paths to nodes and
prefixes. Algorithm values are defined in the IGP Algorithm Type
Registry [IANA-ALG].
A node MUST participate in a Flex-Algorithm to be:
o able to compute path for such Flex-Algorithm
o be part of the topology for such Flex-Algorithm
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Flex-Algorithm participation MUST be advertised for each Flex-
Algorithm data-plane independently, as specified in Section 10.2 of
[I-D.ietf-lsr-flex-algo]. Using Flex-Algorithm for regular IPv4 and
IPv6 prefixes represents an independent Flex-Algorithm data-plane,
and as such, the Flex-Algorithm participation for the IP Flex-
Algorithm data-plane MUST be signalled independently of any other
Flex-Algorithm data-plane (e.g. SR).
Advertisement of participation in IP Flex-Algorithm MUST NOT impact
the router participation in default algorithm 0.
Advertisement if participation in IP Flex-Algorithm MUST NOT impact
the router participation signalled for other data-planes.
Following sections describe how the IP Flex-Algorithm participation
is advertised in IGP protocols.
5.1. The ISIS IP Algorithm Sub-TLV
The ISIS [ISO10589] IP Algorithm Sub-TLV is a sub-TLV of the ISIS
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: ISIS IP Algorithm Sub-TLV
o Type: IP Algorithm Sub-TLV (Value 29)
o Length: Variable
o 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
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in the lowest-numbered LSP and subsequent instances of the IP
Algorithm Sub-TLV MUST be ignored.
The use of IP Algorithm Sub-TLV to advertise support for algorithms
outside the Flex-Algorithm range (128-255) is outside the scope of
this document.
The IP Flex-Algorithm participation advertised in ISIS IP Algorithm
Sub-TLV is topology independent. When a router advertises
participation in ISIS 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
o Type: IP Algorithm TLV (Value TBD by IANA)
o Length: Variable
o Algorithm (1 octet): value from 128 to 255.
The IP Algorithm TLV is optional. It MUST only be advertised once in
the Router Information Opaque LSA.
When multiple IP Algorithm TLVs are received from a given router, the
receiver MUST use the first occurrence of the TLV in the Router
Information Opaque LSA. If the IP Algorithm TLV appears in multiple
Router Information Opaque LSAs that have different flooding scopes,
the IP Algorithm TLV in the Router Information Opaque LSA with the
area-scoped flooding scope MUST be used. If the IP Algorithm TLV
appears in multiple Router Information Opaque LSAs that have the same
flooding scope, the IP Algorithm TLV in the Router Information (RI)
Opaque LSA with the numerically smallest Instance ID MUST be used and
subsequent instances of the IP Algorithm TLV MUST be ignored.
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The RI LSA can be advertised at any of the defined opaque flooding
scopes (link, area, or Autonomous System (AS)). For the purpose of
IP Algorithm TLV advertisement, area-scoped flooding is REQUIRED.
The IP Flex-Algorithm participation advertised in OSPF IP Algorithm
TLV is topology independent. When a router advertises participation
in OSPF IP Algorithm TLV, the participation applies to all topologies
in which the advertising node participates.
6. Advertising IP Flex-Algorthm Reachability
To be able to associate the prefix with the Flex-Algorithm, the
existing prefix reachability advertisements can not be used, because
they advertise the prefix reachability in default algorithm 0.
Instead, a new IP Flex-Algorithm reachability advertisements are
defined in ISIS and OSPF.
The M-flag in FAD is not applicable to IP Algorithm Prefixes. Any IP
Algorithm Prefix advertisement includes the Algorithm and Metric
fields. When 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 ISIS IPv4 Algorithm Prefix Reachability TLV
A new top level TLV is defined for advertising IPv4 Flex-Algorithm
Prefix Reachability in ISIS - IPv4 Algorithm Prefix Reachability TLV.
This new TLV shares the sub-TLV space defined for TLVs Advertising
Prefix Reachability.
The ISIS IPv4 Algorithm Prefix Reachability TLV has the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |R|R|R|R| MTID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: ISIS IPv4 Algorithm Prefix Reachability TLV
o Type: IPv4 Algorithm Prefix Reachability TLV (Value 126).
o Length: variable.
o R bits (4 bits): reserved for future use. They MUST be set to
zero on transmission and MUST be ignored on receipt.
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o 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: ISIS IPv4 Algorithm Prefix Reachability TLV
o Metric (4 octets): Metric information.
o 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.
o Algorithm (1 octet): Associated Algorithm from 128 to 255.
o Prefix Len (1 octet): Prefix length measured in bits.
o Prefix (variable length): Prefix mapped to Flex-Algorithm.
o Optional Sub-TLV-length (1 octet): Number of octets used by sub-
TLVs
o Optional sub-TLVs (variable length).
A router receiving multiple IPv4 Algorithm Prefix Reachability
advertisements for the same prefix, from the same originator, each
with a different Algorithm, MUST select the first advertisement in
the lowest-numbered LSP and ignore any subsequent IPv4 Algorithm
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Prefix Reachability advertisements for the same prefix for any other
Algorithm.
A router receiving multiple IPv4 Algorithm Prefix Reachability
advertisements for the same prefix, from different originators, each
with a different Algorithm, MUST ignore all of them and MUST NOT
install any forwarding entries based on these advertisements.
In cases where a prefix advertisement is received in both a IPv4
Prefix Reachability TLV and an IPv4 Algorithm Prefix Reachability
TLV, the IPv4 Prefix Reachability advertisement MUST be preferred
when installing entries in the forwarding plane.
6.2. The ISIS IPv6 Algorithm Prefix Reachability TLV
The ISIS IPv6 Algorithm Prefix Reachability TLV is identical to the
ISIS IPv4 Algorithm Prefix Reachability TLV, except that it has a
unique type. The type is 127.
A router receiving multiple IPv6 Algorithm Prefix Reachability
advertisements for the same prefix, from the same originator, each
with a different Algorithm, MUST select the first advertisement in
the lowest-numbered LSP and ignore any subsequent IPv6 Algorithm
Prefix Reachability advertisements for the same prefix for any other
Algorithm.
A router receiving multiple IPv6 Algorithm Prefix Reachability
advertisements for the same prefix, from different originators, each
with a different Algorithm, MUST ignore all of them and MUST NOT
install any forwarding entries based on these advertisements.
In cases where a prefix advertisement is received in both an IPv6
Prefix Reachability TLV and an IPv6 Algorithm Prefix Reachability
TLV, the IPv6 Prefix Reachability advertisement MUST be preferred
when installing entries in the forwarding plane.
In cases where a prefix advertisement is received in both ISIS SRv6
Locator TLV and in ISIS IPv6 Algorithm Prefix Reachability TLV, the
receiver MUST ignore both of them and MUST NOT install any forwarding
entries based on these advertisements.
6.3. The OSPFv2 IP Algorithm Prefix Reachability Sub-TLV
A new Sub-TLV of OSPFv2 Extended Prefix TLV is defined for
advertising IP Algorithm Prefix Reachability in OSPFv2 - OSPFv2 IP
Algorithm Prefix Reachability Sub-TLV.
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The OSPFv2 IP Algorithm Prefix Reachability Sub-TLV has the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | Algorithm | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: OSPFv2 IP Algorithm Prefix Reachability Sub-TLV
o Type (2 octets) : The value is TBD.
o Length (1 octet): 8
o MT-ID (1 octet): Multi-Topology ID as defined in [RFC4915]
o Algorithm (1 octet): Associated Algorithm from 128 to 255.
Algorithm values are defined in the IGP Algorithm Type registry.
If the value of Algorithm is 0 the TLV MUST be ignored.
o Reserved: (2 octets). SHOULD be set to 0 on transmission and MUST
be ignored on reception.
o Metric (3 octets): The algorithm specific metric value.
A 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.
A OSPFv2 router receiving multiple OSPFv2 IP Algorithm Prefix
Reachability TLVs for the same prefix, from different originators,
each with a different Algorithm, MUST ignore all of them and MUST NOT
install any forwarding entries based on these advertisements.
In cases where a prefix advertisement is received in any of the LSAs
advertising the prefix reachability for algorithm 0 and in an OSPFv2
IP Algorithm Prefix Reachability TLV, only the prefix reachability
advertisement for algorithm 0 MUST be used and all occurences of the
OSPFv2 IP Algorithm Prefix Reachability TLV MUST be ignored.
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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]:
o Intra-Area-Prefix TLV
o Inter-Area-Prefix TLV
o 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 6: OSPFv3 Algorithm Prefix Sub-TLV
Where:
Type (3 octets): The value is TBD.
Length (2 octets): 4.
Algorithm (1 octet): Associated Algorithm from 128 to 255.
Algorithm values are defined in the IGP Algorithm Type registry.
If the value of Algorithm is 0 the TLV MUST be ignored.
Reserved: (3 octets). SHOULD be set to 0 on transmission and MUST
be ignored on reception.
Metric (4 octets): The algorithm specific metric value.
When the OSPFv3 IP Algorithm Prefix Reachability Sub-TLV is present,
the metric value in its parent TLV MUST be set to LSInfinity
[RFC2328]. If the metric value in the parent TLV is not set to
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LSInfinity, the OSPFv3 IP Algorithm Prefix Sub-TLV MUST be ignored by
the receiver.
A 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.
A OSPFv3 router receiving multiple OSPFv3 IP Algorithm Prefix
Reachability TLVs for the same prefix, from different originators,
each with a different Algorithm, MUST ignore all of them and MUST NOT
install any forwarding entries based on these advertisements.
In cases where a prefix advertisement is received in any of the LSAs
advertising the prefix reachability for algorithm 0 and in an OSPFv3
OSPFv3 IP Algorithm Prefix Reachability Sub-TLV, only the prefix
reachability advertisement for algorithm 0 MUST be used and all
occurences of the OSPFv3 IP Algorithm Prefix Reachability TLV MUST be
ignored.
In cases where a prefix advertisement is received in both OSPFv3 SRv6
Locator TLV and in 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.
6.5. The OSPF IP Flexible Algorithm ASBR Metric Sub-TLV
Section 10.2 of the [I-D.ietf-lsr-flex-algo] defines the OSPF
Flexible Algorithm ASBR Metric Sub-TLV (FAAM) that is used by OSPFv2
and OSPFv3 to advertise Flex-Algorithm specific metric associated
with a given ASBR reachability advertisement by an ABR.
As described in section 11 of [I-D.ietf-lsr-flex-algo] each data-
plane signals the participation independently. IP Flex-Algorithm
participation is signalled independently of the Segment Routing (SR)
Flex-Algorithm participation. As a result, the calculated topologies
for SR and IP Flex-Algorithm could be different. Such difference
prevents the usage of FAAM for the purpose of the IP Flex-Algorithm.
The OSPF IP Flexible Algorithm ASBR Metric (IPFAAM) Sub-TLV is
defined for the advertisement of the IP Flex-Algorithm specific
metric associated with an ASBR by the ABR.
The IPFAAM Sub-TLV is a Sub-TLV of the:
- OSPFv2 Extended Inter-Area ASBR TLV as defined in
[I-D.ietf-lsr-flex-algo]
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- OSPFv3 Inter-Area-Router TLV defined in [RFC8362]
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 7: OSPF IP Flexible Algorithm ASBR Metric Sub-TLV
Type (2 octets): TBD for OSPFv2, TBD for OSPFv3.
Length (2 octets): 8.
Algorithm (1 octet): Associated Algorithm from 128 to 255.
Algorithm values are defined in the IGP Algorithm Type registry.
If the value of Algorithm is 0 the TLV MUST be ignored.
Reserved: (3 octets). SHOULD be set to 0 on transmission and MUST
be ignored on reception.
Metric (4 octets): The algorithm specific metric value.
The usage of the IPFAAM Sub-TLV is similar to the usage of the FAAM
Sub-TLV defined in [I-D.ietf-lsr-flex-algo], but it is used to
advertise IP Flex-Algorithm metric.
An OSPF ABR MUST include the OSPF IPFAAM Sub-TLVs as part of the ASBR
reachability advertisement between areas for every IP Flex-Algorithm
in which it participates and the ASBR is reachable in.
FAAM Sub-TLV as defined in [I-D.ietf-lsr-flex-algo] MUST NOT be used
during IP Flex-Algorithm path calculation, IPFAAM Sub-TLV MUST be
used instead.
7. Calculating of IP Flex-Algorthm Paths
IP Flex-Algorthm is considered as yet another data-plane of the Flex-
Algorithm as described in Section 10 and Section 12 of the
[I-D.ietf-lsr-flex-algo].
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Participation for 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 the Section 12 of
[I-D.ietf-lsr-flex-algo]. This computation uses the IP Flex-
Algorithm data-plane participation and is independent of the Flex-
Algorithm calculation done for any other Flex-Algorithm data-plane
(e.g. SR, SRv6).
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 IP Flex-Algorithm, as described in Section 5, MUST be pruned from
the topology.
8. IP Flex-Algorthm Forwarding
IP Algorithm Prefix Reachability advertisement as described in
Section 5 includes the MTID value that associates the prefix with a
specific topology. Algorithm Prefix Reachability advertisement also
includes an Algorithm value that explicitly associates the prefix
with a specific Flex-Algorithm. The paths to the prefix MUST be
calculated using the specified Flex-Algorithm in the associated
topology.
Forwarding entries for the IP Flex-Algorithm prefixes advertised in
IGPs MUST be installed in the forwarding plane of the receiving IP
Flex-Algorithm prefix capable routers when they participate in the
associated topology and algorithm. Forwarding entries for IP Flex-
Algorithm prefixes associated with Flex-Algorithms in which the node
is not participating MUST NOT be installed in the forwarding plane.
When the IP Flex-Algorithm prefix is associated with a Flex-
Algorithm, LFA paths to the prefix MUST be calculated using such
Flex-Algorithm in the associated topology, to guarantee that they
follow the same constraints as the calculation of the primary paths.
9. Deployment Considerations
IGP Flex-Algorithm can be used by many data-planes. 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 the IGP
Flex-Algorithm.
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Flex-Algorithm definition (FAD), as described in
[I-D.ietf-lsr-flex-algo], is data-plane independent and is used by
all Flex-Algorithm data-planes.
Participation in the Flex-Algorithm, as described in
[I-D.ietf-lsr-flex-algo], is data-plane specific.
Calculation of the flex-algo paths is data-plane specific and uses
data-plane specific participation advertisements.
Data-plane specific participation and calculation guarantee that the
forwarding of the traffic over the Flex-Algorithm data-plane specific
paths is consistent between all nodes over which the traffic is being
forwarded.
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 such 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 announcements. Note, that for a
particular Flex-Algorithm, for a particular IP prefix, there will be
path(s) calculated and installed for a single data-plane only.
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:
o Loop Free Alternates (LFA) [RFC5286]
o Remote Loop Free Alternates (R-LFA) [RFC7490]
LFA and R-LFA computations MUST be restricted to the flex-algo
topology and the computed backup nexthops should be programmed for
the IP flex-algo prefixes.
11. IANA Considerations
This specification updates the OSPF Router Information (RI) TLVs
Registry as follows:
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+-------+------------------+---------------------------+
| Value | TLV Name | Reference |
+-------+------------------+---------------------------+
| TBD | IP Algorithm TLV | This Document Section 5.2 |
+-------+------------------+---------------------------+
Table 1
This document also updates the "Sub-TLVs for TLV 242" registry as
follows:
+-------+----------------------+---------------------------+
| Value | TLV Name | Reference |
+-------+----------------------+---------------------------+
| 29 | IP Algorithm Sub-TLV | This Document Section 5.1 |
+-------+----------------------+---------------------------+
Table 2
This document also updates the "ISIS TLV Codepoints Registry"
registry as follows:
+-------+------------------+-----+-----+-----+-------+--------------+
| Value | TLV Name | IIH | LSP | SNP | Purge | Reference |
+-------+------------------+-----+-----+-----+-------+--------------+
| 126 | IPv4 Algorithm | N | Y | N | N | This |
| | Prefix | | | | | document, |
| | Reachability TLV | | | | | Section 6.1 |
| 127 | IPv6 Algorithm | N | Y | N | N | This |
| | Prefix | | | | | document, |
| | Reachability TLV | | | | | Section 6.2 |
+-------+------------------+-----+-----+-----+-------+--------------+
Table 3
Above new TLVs share the sub-TLV space defined in "IS-IS Sub-TLVs for
TLVs Advertising Prefix Reachability". This document updates the
description of that registry by including IPv4 Algorithm Prefix
Reachability TLV and IPv6 Algorithm Prefix Reachability TLV. It also
includes these TLVs in a table which lists the presence of Sub-TLVs
in a parent TLVs as follows:
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Type Description 126 127
---- ---------------------------------- --- ---
1 32-bit Administrative Tag Sub-TLV y y
2 64-bit Administrative Tag Sub-TLV y y
3 Prefix Segment Identifier n n
4 Prefix Attribute Flags y y
5 SRv6 End SID n n
6 Flex-Algorithm Prefix Metric n n
11 IPv4 Source Router ID y y
12 IPv6 Source Router ID y y
32 BIER Info n n
This document updates the "OSPFv2 Extended Prefix TLV Sub-TLVs"
registry as follows:
+-------+-----------------------------------+-----------------------+
| Value | TLV Name | Reference |
+-------+-----------------------------------+-----------------------+
| TBD | OSPFv2 IP Algorithm Prefix | This Document, |
| | Reachability TLV | Section 6.3 |
+-------+-----------------------------------+-----------------------+
Table 4
This document updates the "OSPFv3 Extended-LSA Sub-TLVs" registry as
follows:
+-------+-------------------------------------+---------------------+
| Value | TLV Name | Reference |
+-------+-------------------------------------+---------------------+
| TBD | OSPFv3 IP Algorithm Prefix | This Document, |
| | Reachability Sub-TLV | Section 6.4 |
| TBD | OSPFv3 IP Flexible Algorithm ASBR | This Document, |
| | Metric Sub-TLV | Section 6.5 |
+-------+-------------------------------------+---------------------+
Table 5
This document updates the "OSPFv2 Extended Inter-Area ASBR Sub-TLVs"
registry as follows:
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+-------+------------------------------------+----------------------+
| Value | TLV Name | Reference |
+-------+------------------------------------+----------------------+
| 2 | OSPF IP Flexible Algorithm ASBR | This Document, |
| | Metric Sub-TLV | Section 6.5 |
+-------+------------------------------------+----------------------+
Table 6
12. Security Considerations
This document inherits security considerations from
[I-D.ietf-lsr-flex-algo].
13. Acknowledgements
Thanks to Bruno Decraene for his contributions to this document.
Special thanks to Petr Bonbon Adamec of Cesnet for supporting
interoperability testing.
14. References
14.1. Normative References
[I-D.ietf-lsr-flex-algo]
Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
algo-19 (work in progress), April 2022.
[ISO10589]
IANA, "Intermediate system to Intermediate system routing
information exchange protocol for use in conjunction with
the Protocol for providing the Connectionless-mode Network
Service (ISO 8473)", August 1987, <ISO/IEC 10589:2002>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[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>.
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[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.
[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>.
[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>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.
[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>.
[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>.
[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>.
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[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[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>.
14.2. Informative References
[IANA-ALG]
IANA, "Sub-TLVs for TLV 242 (IS-IS Router CAPABILITY
TLV)", August 1987, <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, 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>.
[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>.
Authors' Addresses
William Britto
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore, Karnataka 560103
India
Email: bwilliam@juniper.net
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Shraddha Hegde
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore, Karnataka 560103
India
Email: shraddha@juniper.net
Parag Kaneriya
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore, Karnataka 560103
India
Email: pkaneria@juniper.net
Rejesh Shetty
Juniper Networks
Elnath-Exora Business Park Survey
Bangalore, Karnataka 560103
India
Email: mrajesh@juniper.net
Ron Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, Virginia 20171
USA
Email: rbonica@juniper.net
Peter Psenak
Cisco Systems
Apollo Business Center
Mlynske nivy 43, Bratislava 82109
Slovakia
Email: ppsenak@cisco.com
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