Network Working Group P. Psenak, Ed.
Internet-Draft Cisco Systems
Intended status: Standards Track S. Hegde
Expires: May 16, 2019 Juniper Networks, Inc.
C. Filsfils
K. Talaulikar
Cisco Systems, Inc.
A. Gulko
Thomson Reuters
November 12, 2018
IGP Flexible Algorithm
draft-ietf-lsr-flex-algo-01.txt
Abstract
IGP protocols traditionally compute best paths over the network based
on the IGP metric assigned to the links. Many network deployments
use RSVP-TE based or Segment Routing based Traffic Engineering to
enforce traffic over a path that is computed using different metrics
or constraints than the shortest IGP path. This document proposes a
solution that allows IGPs themselves to compute constraint based
paths over the network. This document also specifies a way of using
Segment Routing Prefix-SIDs to steer packets along the constraint-
based paths.
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 May 16, 2019.
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Flexible Algorithm . . . . . . . . . . . . . . . . . . . . . 4
5. Flexible Algorithm Definition Advertisement . . . . . . . . . 5
5.1. ISIS Flexible Algorithm Definition Sub-TLV . . . . . . . 5
5.2. OSPF Flexible Algorithm Definition TLV . . . . . . . . . 7
5.3. Common Handling of Flexible Algorithm Definition TLV . . 8
6. Sub-TLVs of ISIS FAD Sub-TLV . . . . . . . . . . . . . . . . 9
6.1. ISIS Flexible Algorithm Exclude Admin Group Sub-TLV . . . 9
6.2. ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV . 10
6.3. ISIS Flexible Algorithm Include-All Admin Group Sub-TLV . 10
7. Sub-TLVs of OSPF FAD TLV . . . . . . . . . . . . . . . . . . 10
7.1. OSPF Flexible Algorithm Exclude Admin Group Sub-TLV . . . 11
7.2. OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV . 11
7.3. OSPF Flexible Algorithm Include-All Admin Group Sub-TLV . 11
8. Advertisement of Node Participation in a Flex-Algorithm . . . 12
8.1. Advertisement of Node Participation for Segment Routing . 12
8.2. Advertisement of Node Participation for Other
Applications . . . . . . . . . . . . . . . . . . . . . . 12
9. Advertisement of Link Attributes for Flex-Algorithm . . . . . 13
10. Calculation of Flexible Algorithm Paths . . . . . . . . . . . 13
11. Flex-Algorithm and Forwarding Plane . . . . . . . . . . . . . 15
11.1. Segment Routing MPLS Forwarding for Flex-Algorithm . . . 15
11.2. Other Applications' Forwarding for Flex-Algorithm . . . 15
12. Backward Compatibility . . . . . . . . . . . . . . . . . . . 16
13. Security Considerations . . . . . . . . . . . . . . . . . . . 16
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
14.1. IGP IANA Considerations . . . . . . . . . . . . . . . . 16
14.1.1. IGP Algorithm Types Registry . . . . . . . . . . . . 16
14.1.2. Flexible Algorithm Definition Metric-Type Registry . 16
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14.2. ISIS IANA Considerations . . . . . . . . . . . . . . . . 17
14.2.1. Sub TLVs for Type 242 . . . . . . . . . . . . . . . 17
14.2.2. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-
TLV . . . . . . . . . . . . . . . . . . . . . . . . 17
14.3. OSPF IANA Considerations . . . . . . . . . . . . . . . . 18
14.3.1. OSPF Router Information (RI) TLVs Registry . . . . . 18
14.3.2. OSPF Flexible Algorithm Definition TLV Sub-TLV
Registry . . . . . . . . . . . . . . . . . . . . . . 18
15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 19
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
16.1. Normative References . . . . . . . . . . . . . . . . . . 19
16.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
An IGP computed path based on the shortest IGP metric must often be
replaced by traffic engineered path due to the traffic requirements
which are not reflected by the IGP metric. Some networks engineer
the IGP metric assignments in a way that the IGP Metric reflects the
link bandwidth or delay. If, for example, the IGP metric is
reflecting the bandwidth on the link and the application traffic is
delay sensitive, the best IGP path may not reflect the best path from
such application's perspective.
To overcome this limitation, various sorts of traffic engineering
have been deployed, including RSVP-TE and SR-TE, in which case the TE
component is responsible for computing the path based on additional
metrics and/or constraints. Such paths need to be installed in the
forwarding tables in addition to, or as a replacement for the
original paths computed by IGPs. Tunnels are often used to represent
the engineered paths and mechanisms like one described in [RFC3906]
are used to replace the native IGP paths with such tunnel paths.
This document specifies a set of extensions to ISIS, OSPFv2 and
OSPFv3 that enable a router to send TLVs that (a) describe a set of
constraints on the topology, (b) identify calculation-type, and (c)
metric-type that are to be used to compute the best paths along the
constrained topology. A given combination of calculation-type,
metric-type and constraints is known as a "Flexible Algorithm
Definition". A router that sends such a set of TLVs also assigns a
specific value, Flex-Algorithm, to the specified combination of
calculation-type, metric-type and constraints.
This document also specifies a way for a router to use IGPs to
associate one or more Segment Routing Prefix-SIDs with a particular
Flex-Algorithm. Each such Prefix-SID then represents a path that is
computed according to the identified Flex-Algorithm.
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2. Requirements notation
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
[BCP14] [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Terminology
This section defines terms that are often used in this document.
Flexible Algorithm Definition - the set consisting of (a)
calculation-type, (b) metric-type and (c) a set of constraints.
Flexible Algorithm - a numeric identifier in the range 128-255 that
is associated via provisioning with the Flexible-Algorithm
Definition.
Local Flexible Algorithm Definition - Flexible Algorithm Definition
defined locally on the node.
Remote Flexible Algorithm Definition - Flexible Algorithm Definition
received from other nodes via IGP flooding.
Flexible Algorithm Participation - per application configuration
state that expresses whether the node is participating in a
particular Flexible Algorithm.
IGP Algorithm - value from the the "IGP Algorithm Types" registry
defined under "Interior Gateway Protocol (IGP) Parameters" IANA
registries. IGP Algorithms represents the triplet (Calculation Type,
Metric, Constraints), where the second and third elements of the
triple MAY not exist.
4. Flexible Algorithm
Many possible constraints may be used to compute a path over a
network. Some networks are deployed as multiple planes. A simple
form of constraint may be to use a particular plane. A more
sophisticated form of constraint can include some extended metric as
described in [RFC7810]. Constraints which restrict paths to links
with specific affinities or avoid links with specific affinities are
also possible. Combinations of these are also possible.
To provide maximum flexibility, we want to provide a mechanism that
allows a router to (a) identify a particular calculation-type, (b)
metric-type, (c) describe a particular set of constraints, and (d)
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assign a numeric identifier, referred to as Flex-Algorithm, to the
combination of that calculation-type, metric-type and those
constraints. We want the mapping between the Flex-Algorithm and it's
meaning to be flexible and defined by the user. As long as all
routers in the domain have a common understanding as to what a
particular Flex-Algorithm represents, the resulting routing
computation is consistent and traffic is not subject to any looping.
The set consisting of (a) calculation-type, (b) metric-type and (c) a
set of constraints is referred to as a Flexible-Algorithm Definition.
Flexible-Algorithm is a numeric identifier in the range 128-255 that
is associated via provisioning with the Flexible-Algorithm
Definition.
IANA "IGP Algorithm Types" registry defines the set of values for IGP
Algorithms. We propose to allocate the following values for Flex-
Algorithms from this registry:
128-255 - Flex-Algorithms
5. Flexible Algorithm Definition Advertisement
To guarantee the loop free forwarding for paths computed for a
particular Flex-Algorithm, all routers that (a) are configured to
participate in a particular Flex-Algorithm, and (b) are in the same
Flex-Algorithm definition advertisement scope MUST agree on the
definition of the Flex-Algorithm.
5.1. ISIS Flexible Algorithm Definition Sub-TLV
ISIS Flexible Algorithm Definition Sub-TLV (FAD Sub-TLV) is used to
advertise the definition of the Flex-Algorithm.
ISIS FAD Sub-TLV is advertised as a Sub-TLV of the ISIS Router
Capability TLV-242 that is defined in [RFC7981].
ISIS FAD Sub-TLV has the following format:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |Flex-Algorithm | Metric-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Calc-Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs |
+ +
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: TBD, suggested value 26
Length: variable, dependent on the included Sub-TLVs
Flex-Algorithm: Single octet value between 128 and 255 inclusive.
Metric-Type: Type of metric to be used during the calculation.
Following values are defined:
0: IGP Metric
1: Min Unidirectional Link Delay as defined in [RFC7810].
2: TE default metric as defined in [RFC5305].
Calc-Type: value from 0 to 127 inclusive from the "IGP Algorithm
Types" registry defined under "Interior Gateway Protocol (IGP)
Parameters" IANA registries. IGP algorithms in the range of 0-127
have a defined triplet (Calculation Type, Metric, Constraints).
When used to specify the Calc-Type in the FAD Sub-TLV, only the
Calculation Type defined for the specified IGP Algorithm is used.
The Metric/Constraints MUST NOT be inherited. If the required
calculation type is Shortest Path First, the value 0 SHOULD appear
in this field.
Priority: Value between 0 and 255 inclusive that specifies the
priority of the advertisement.
Sub-TLVs - optional sub-TLVs.
The ISIS FAD Sub-TLV MAY be flooded only in a given level or
throughout the domain. In the latter case the S-flag is set as
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described in [RFC7981]. It is recommended that domain-wide flooding
NOT be the default behavior.
5.2. OSPF Flexible Algorithm Definition TLV
OSPF FAD TLV is advertised as a top-level TLV of the RI LSA that is
defined in [RFC7770].
OSPF FAD TLV has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm | Metric-Type | Calc-Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs |
+ +
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: TBD, suggested value 16
Length: variable, dependent on the included Sub-TLVs
Flex-Algorithm:: Flex-Algorithm number. Value between 128 and 255
inclusive.
Metric-Type: as described in Section 5.1
Calc-Type: as described in Section 5.1
Priority: as described in Section 5.1
Sub-TLVs - optional sub-TLVs.
When multiple OPSF FAD TLVs, for the same Flexible-Algorithm, are
received from a given router, the receiver MUST use the first
occurrence of the TLV in the Router Information LSA. If the OSPF FAD
TLV, for the same Flex-Algorithm, appears in multiple Router
Information LSAs that have different flooding scopes, the OSPF FAD
TLV in the Router Information LSA with the area-scoped flooding scope
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MUST be used. If the OSPF FAD TLV, for the same algorithm, appears
in multiple Router Information LSAs that have the same flooding
scope, the OSPF FAD TLV in the Router Information (RI) LSA with the
numerically smallest Instance ID MUST be used and subsequent
instances of the OSPF FAD TLV MUST be ignored.
The RI LSA can be advertised at any of the defined opaque flooding
scopes (link, area, or Autonomous System (AS)). For the purpose of
OSPF FAD TLV advertisement, area-scoped flooding is REQUIRED. The
Autonomous System flooding scope SHOULD not be used by default unless
local configuration policy on the originating router indicates domain
wide flooding.
5.3. Common Handling of Flexible Algorithm Definition TLV
This section describes the protocol independent handling of the FAD
TLV (OSPF) or FAD Sub-TLV (ISIS). We will refer to it as FAD TLV in
this section, even though in case of ISIS it is a Sub-TLV.
The value of the Flex-Algorithm MUST be between 128 and 255
inclusive. If it is not, the FAD TLV MUST be ignored.
Not every router configured to participate in a particular Flex-
Algorithm need a local definition of such Flex-Algorithm. Only a
subset of the routers participating in the particular Flex-Algorithm
need the local definition of the Flex-Algorithm.
Every router, that is configured to participate in a particular Flex-
Algorithm, MUST select the Flex-Algorithm definition based on the
following ordered rules. This allows for the consistent Flex-
Algorithm definition selection in cases where different routers
advertise different definitions for a given Flex-Algorithm:
1. From the advertisements of the FAD in the area (including both
locally generated advertisements and received advertisements)
select the one(s) with the highest priority.
2. If there are multiple advertisements of the FAD with the same
highest priority, select the one that is originated from the
router with the highest System-ID in case of ISIS or Router ID in
case of OSPFv2 and OSPFv3. For ISIS the System-ID is described in
[ISO10589]. For OSPFv2 and OSPFv3 standard Router ID is described
in [RFC2328] and [RFC5340] respectively.
A router that is not configured to participate in a particular Flex-
Algorithm MUST ignore FAD Sub-TLVs advertisements for such Flex-
Algorithm.
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Any change in the Flex-Algorithm definition may result in temporary
disruption of traffic that is forwarded based on such Flex-Algorithm
paths. The impact is similar to any other event that requires
network wide convergence.
If a node is configured to participate in a particular Flexible-
Algorithm, but the selected Flex-Algorithm definition includes
calculation-type, metric-type or constraint that is not supported by
the node, it MUST stop participating in such Flexible-Algorithm.
That implies that it MUST NOT announce participation for such
Flexible-Algorithm and it MUST remove any forwarding state associated
with it.
Flex-Algorithm definition is topology independent. It applies to all
topologies that a router participates in.
6. Sub-TLVs of ISIS FAD Sub-TLV
6.1. ISIS Flexible Algorithm Exclude Admin Group Sub-TLV
The Flexible-Algorithm definition can specify 'colors' that are used
by the operator to exclude links during the Flex-Algorithm path
computation.
Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-TLV) is a
Sub-TLV of the ISIS FAD Sub-TLV. It has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group as
defined in [RFC7308].
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ISIS FAEAG Sub-TLV MAY NOT appear more then once in an ISIS FAD Sub-
TLV. If it appears more then once, the ISIS FAD Sub-TLV MUST be
ignored by the receiver.
6.2. ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV
The Flexible-Algorithm definition can specify 'colors' that are used
by the operator to include link during the Flex-Algorithm path
computation.
ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV is used to
advertise include-any rule that is used during the Flex-Algorithm
path calculation as specified in Section Section 10.
The format of the SIS Flexible Algorithm Include-Any Admin Group Sub-
TLV is identical to the format of the FAEAG Sub-TLV in Section 6.1.
Flexible Algorithm Include-Any Admin Group Sub-TLV Type is 2.
ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV MAY NOT
appear more then once in an ISIS FAD Sub-TLV. If it appears more
then once, the ISIS FAD Sub-TLV MUST be ignored by the receiver.
6.3. ISIS Flexible Algorithm Include-All Admin Group Sub-TLV
The Flexible-Algorithm definition can specify 'colors' that are used
by the operator to include link during the Flex-Algorithm path
computation.
ISIS Flexible Algorithm Include-All Admin Group Sub-TLV is used to
advertise include-all rule that is used during the Flex-Algorithm
path calculation as specified in Section Section 10.
The format of the SIS Flexible Algorithm Include-All Admin Group Sub-
TLV is identical to the format of the FAEAG Sub-TLV in Section 6.1.
ISIS Flexible Algorithm Include-All Admin Group Sub-TLV Type is 3.
ISIS Flexible Algorithm Include-All Admin Group Sub-TLV MAY NOT
appear more then once in an ISIS FAD Sub-TLV. If it appears more
then once, the ISIS FAD Sub-TLV MUST be ignored by the receiver.
7. Sub-TLVs of OSPF FAD TLV
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7.1. OSPF Flexible Algorithm Exclude Admin Group Sub-TLV
Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-TLV) is a
Sub-TLV of the OSPF FAD TLV. It's usage is described in Section 6.1.
It has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group as
defined in [RFC7308].
OSPF FAEAG Sub-TLV MAY NOT appear more then once in an OSPF FAD TLV.
If it appears more then once, the OSPF FAD TLV MUST be ignored by the
receiver.
7.2. OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV
The usage of this Sub-TLVs is described in Section 6.2.
The format of the OSPF Flexible Algorithm Include-Any Admin Group
Sub-TLV is identical to the format of the OSPF FAEAG Sub-TLV in
Section 7.1.
Flexible Algorithm Include-Any Admin Group Sub-TLV Type is 2.
OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV MAY NOT
appear more then once in an OPSF FAD TLV. If it appears more then
once, the OSPF FAD TLV MUST be ignored by the receiver.
7.3. OSPF Flexible Algorithm Include-All Admin Group Sub-TLV
The usage of this Sub-TLVs is described in Section 6.3.
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The format of the OSPF Flexible Algorithm Include-Any Admin Group
Sub-TLV is identical to the format of the OSPF FAEAG Sub-TLV in
Section 7.1.
Flexible Algorithm Include-Any Admin Group Sub-TLV Type is 3.
OSPF Flexible Algorithm Include-All Admin Group Sub-TLV MAY NOT
appear more then once in an OPSF FAD TLV. If it appears more then
once, the OSPF FAD TLV MUST be ignored by the receiver.
8. Advertisement of Node Participation in a Flex-Algorithm
When a router is configured to support a particular Flex-Algorithm,
we say it is participating in that Flex-Algorithm.
Paths computed for a specific Flex-Algorithm MAY be used by various
applications, each potentially using its own specific data plane for
forwarding the data over such paths. To guarantee the presence of
the application specific forwarding state associated with a
particular Flex-Algorithm, a router MUST advertise its participation
for a particular Flex-Algorithm for each application specifically.
8.1. Advertisement of Node Participation for Segment Routing
[I-D.ietf-isis-segment-routing-extensions],
[I-D.ietf-ospf-segment-routing-extensions] and
[I-D.ietf-ospf-ospfv3-segment-routing-extensions] (IGP Segment
Routing extensions) describe how SR-Algorithm is used to define how
the best path is computed by the IGP.
Routers advertise the support for the SR-Algorithm as a node
capability as described in the above mentioned IGP Segment Routing
extensions. To advertise participation for a particular Flex-
Algorithm for Segment Routing, the Flex-Algorithm value MUST be
advertised in the SR-Algorithm TLV (OSPF) or sub-TLV (ISIS).
Segment Routing Flex-Algorithm participation advertisement is
topology independent. When a router advertises participation in an
SR-Algorithm, the participation applies to all topologies in which
the advertising node participates.
8.2. Advertisement of Node Participation for Other Applications
This section describes considerations related to how other
applications can advertise its participation in a specific Flex-
Algorithm.
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Application specific Flex-Algorithm participation advertisements MAY
be topology specific or MAY be topology independent, depending on the
application itself.
Application specific advertisement for Flex-Algorithm participation
MUST be defined for each application and is outside of the scope of
this document.
9. Advertisement of Link Attributes for Flex-Algorithm
Various link include or exclude rules can be part of the Flex-
Algorithm definition. These rules use Admin Groups (AG) as defined
in [RFC7308] and [RFC5305], or Extended Administrative Groups (EAG)
as defined in [RFC7308].
To advertise a link affinity in a form of the AG or EAG that is used
during Flex-Algorithm calculation, an Application Specific Link
Attributes sub-TLV as described in [I-D.ietf-isis-te-app], or sub-TLV
of Extended Link TLV as described in
[I-D.ietf-ospf-te-link-attr-reuse] MUST be used. The advertisement
MUST indicate that it is usable by the Flex-Algorithm application.
10. Calculation of Flexible Algorithm Paths
A router MUST be configured to participate in a given Flex-Algorithm
K before it can compute any path for that Flex-Algorithm.
A router which participates in a given Flex Algorithm MUST use the
FAD selected based on the rules defined in Section Section 5.3.
As described in Section 8, participation for any particular Flex-
Algorithm MUST be advertised on a per application basis. Calculation
of the paths for any particular Flex-Algorithm MUST be application
specific.
The way applications handle nodes that do not participate in
Flexible-Algorithm is application specific. If the application only
wants to consider participating nodes during the Flex-Algorithm
calculation, then when computing paths for a given Flex-Algorithm,
all nodes that do not advertise participation for that Flex-Algorithm
in the application specific advertisements MUST be pruned from the
topology. MPLS Segment Routing is an application that MUST use such
pruning when computing Flex-Algorithm paths.
When computing the path for a give Flex-Algorithm, the metric-type
that is part of the Flex-Algorithm definition (Section 5) MUST be
used.
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When computing the path for a given Flex-Algorithm, the calculation-
type that is part of the Flex-Algorithm definition (Section 5) MUST
be used.
Various link include or exclude rules can be part of the Flex-
Algorithm definition. To refer to particular bit within an AG or EAG
we uses term 'color'.
Rules, in the order as specified below, MUST be used to prune link
from the topology during the Flex-Algorithm computation.
For all links in the topology:
1. Check if any exclude rule is part of the Flex-Algorithm
definition. If such exclude rule exists, check if any color that
is part of the exclude rule is also set on the link. If such a
color exist, the link MUST be pruned from the computation.
2. Check if any include-any rule is part of the Flex-Algorithm
definition. if such include-any rule exists, check if any color
that is part of the include-any rule is also set on the link. If
such color does not exist, the link MUST be pruned from the
computation.
3. Check if any include-all rule is part of the Flex-Algorithm
definition. If such include-all rule exists, check if all colors
that are part of the include-all rule are also set on the link.
If not all such colors are set on the link, the link MUST be
pruned from the computation.
4. If the Flex-Algorithm definition uses other than IGP metric
(Section 5), and such metric is not advertised for the particular
link in a topology for which the computation is done, such link
MUST be pruned from the computation. A metric of value 0 MUST NOT
be assumed in such case.
Any IGP Shortest Path Tree calculation is limited to a single area.
Same applies to Flex-Algorithm calculations. Given that the
computing router may not have the visibility to the topology of
remote areas, the Flex-Algorithm specific path to an inter-area
prefix will only be computed for the local area only. The egress L1/
L2 router (ABR in OSPF) will be selected based on the best path for
the given Flex-Algorithm in the local area and such egress L1/L2 (ABR
in OSPF) router will be responsible to compute the best Flex-
Algorithm specific path over the next area. This may produce an end-
to-end path, which is sub-optimal based on Flex-Algorithm
constraints. If the best end-to-end path for a given Flex-Algorithm
needs to be used for inter-area destinations, paths for such
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destinations need to be computed by the entity that has the
topological information about all areas.
11. Flex-Algorithm and Forwarding Plane
This section describes how Flex-Algorithm paths are used with
forwarding.
11.1. Segment Routing MPLS Forwarding for Flex-Algorithm
This section describes how Flex-Algorithm paths are used with SR MPLS
forwarding.
Prefix SID advertisements include an SR-Algorithm value and as such
are associated with the specified SR-Algorithm. Prefix-SIDs are also
associated with a specific topology which is inherited from the
associated prefix reachability advertisement. When the algorithm
value advertised is a Flex-Algorithm value, the Prefix SID is
associated with paths calculated using that Flex-Algorithm in the
associated topology.
A Flex-Algorithm path MUST be installed in the MPLS forwarding plane
using the MPLS label that corresponds to the Prefix-SID that was
advertised for that Flex-algorithm. If the Prefix SID for a given
Flex-algorithm is not known, the Flex-Algorithm specific path cannot
be installed in the MPLS forwarding plane.
Traffic that is supposed to be routed via Flex-Algorithm specific
paths, MUST be dropped where there are no such paths available.
Loop Free Alternate (LFA) paths for a given Flex-Algorithm MUST be
computed using the same constraints as the calculation of the primary
paths for that Flex-Algorithm. LFA paths MUST only use Prefix-SIDs
advertised specifically for the given algorithm. LFA paths MUST NOT
use an Adjacency-SID that belongs to a link that has been pruned from
the Flex-Algorithm computation.
If LFA protection is being used to protect a given Flex-Algorithm
paths, all routers in the area participating in the given Flex-
Algorithm SHOULD advertise at least one Flex-Algorithm specific Node-
SID. These Node-SIDs are used to enforce traffic over the LFA
computed backup path.
11.2. Other Applications' Forwarding for Flex-Algorithm
Any application that wants to use Flex-Algorithm specific forwarding
need to install some form of Flex-Algorithm specific forwarding
entries.
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Application specific forwarding for Flex-Algorithm MUST be defined
for each application and is outside of the scope of this document.
12. Backward Compatibility
This extension brings no new backward compatibility issues.
13. Security Considerations
This draft adds a two new ways to disrupt the IGP networks:
An attacker can hijack a particular Flex-Algorithm by advertising
a FAD with a priority of 255 (or any priority higher than that of
the legitimate nodes).
An attacker could make it look like a router supports a particular
Flex-Algorithm when it actually doesn't, or vice versa.
Both of these attacks can be addressed by the existing security
extensions as described in [RFC5304] and [RFC5310] for ISIS, in
[RFC2328] and [RFC7474] for OSPFv2 and in [RFC5340] and [RFC4552] for
OSPFv3.
14. IANA Considerations
14.1. IGP IANA Considerations
14.1.1. IGP Algorithm Types Registry
This document makes the following registrations in the "IGP Algorithm
Types" registry:
Type: 128-255.
Description: Flexible Algorithms.
Reference: This document (Section 4).
14.1.2. Flexible Algorithm Definition Metric-Type Registry
IANA is requested to set up a registry called "Flexible Algorithm
Definition Metric-Type Registry" under a "Interior Gateway Protocol
(IGP) Parameters" IANA registries. The registration policy for this
registry is "Standards Action" ([RFC8126] and [RFC7120]).
Values in this registry come from the range 0-255.
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This document registers following values in the "Flexible Algorithm
Definition Metric-Type Registry":
Type: 0
Description: IGP metric
Reference: This document (Section 5.1)
Type: 1
Description: Min Unidirectional Link Delay [RFC7810]
Reference: This document (Section 5.1)
Type: 2
Description: TE Default Metric [RFC5305]
Reference: This document (Section 5.1)
14.2. ISIS IANA Considerations
14.2.1. Sub TLVs for Type 242
This document makes the following registrations in the "sub-TLVs for
TLV 242" registry.
Type: TBD (suggested value 26).
Description: Flexible Algorithm Definition Sub-TLV.
Reference: This document (Section 5.1).
14.2.2. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
This document creates the following Sub-Sub-TLV Registry:
Registry: Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
Registration Procedure: Expert review
Reference: This document (Section 5.1)
This document defines the following Sub-Sub-TLVs in the "Sub-Sub-TLVs
for Flexible Algorithm Definition Sub-TLV" registry:
Type: 1
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Description: Flexible Algorithm Exclude Admin Group Sub-TLV
Reference: This document (Section 6.1).
Type: 2
Description: Flexible Algorithm Include-Any Admin Group Sub-TLV
Reference: This document (Section 6.2).
Type: 3
Description: Flexible Algorithm Include-All Admin Group Sub-TLV
Reference: This document (Section 6.3).
14.3. OSPF IANA Considerations
14.3.1. OSPF Router Information (RI) TLVs Registry
This specification updates the OSPF Router Information (RI) TLVs
Registry with the following value:
o TBD (suggested value 16) - Flexible Algorithm Definition TLV
14.3.2. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry
This document creates the following registry:
Registry: OSPF Flexible Algorithm Definition TLV sub-TLV
Registration Procedure: Expert review
Reference: This document (Section 5.2)
The "OSPF Flexible Algorithm Definition TLV sub-TLV" registry will
define sub-TLVs at any level of nesting for Flexible Algorithm TLV
and should be added to the "Open Shortest Path First (OSPF)
Parameters" registries group. New values can be allocated via IETF
Review or IESG Approval.
This document resisters following Sub-TLVs in the "TLVs for Flexible
Algorithm Definition TLV" registry:
Type: 1
Description: Flexible Algorithm Exclude Admin Group Sub-TLV
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Reference: This document (Section 7.1).
Type: 2
Description: Flexible Algorithm Include-Any Admin Group Sub-TLV
Reference: This document (Section 7.2).
Type: 3
Description: Flexible Algorithm Include-All Admin Group Sub-TLV
Reference: This document (Section 7.3).
Types in the range 32768-33023 are for experimental use; these will
not be registered with IANA, and MUST NOT be mentioned by RFCs.
Types in the range 33024-65535 are not to be assigned at this time.
Before any assignments can be made in the 33024-65535 range, there
MUST be an IETF specification that specifies IANA Considerations that
covers the range being assigned.
15. Contributors
This draft, among other things, is also addressing the problem that
the [I-D.gulkohegde-routing-planes-using-sr] was trying to solve.
All authors of that draft agreed to join this draft.
Thanks to Eric Rosen, Les Ginsberg and Tony Przygienda for their
detailed review and excellent comments.
Thanks to Cengiz Halit for his review and feedback during initial
phase of the solution definition.
Thanks to Kenji Kumaki for his comments.
16. References
16.1. Normative References
[BCP14] , <https://tools.ietf.org/html/bcp14>.
[I-D.ietf-isis-segment-routing-extensions]
Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
Gredler, H., and B. Decraene, "IS-IS Extensions for
Segment Routing", draft-ietf-isis-segment-routing-
extensions-20 (work in progress), November 2018.
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[I-D.ietf-isis-te-app]
Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and
J. Drake, "IS-IS TE Attributes per application", draft-
ietf-isis-te-app-05 (work in progress), October 2018.
[I-D.ietf-ospf-ospfv3-segment-routing-extensions]
Psenak, P. and S. Previdi, "OSPFv3 Extensions for Segment
Routing", draft-ietf-ospf-ospfv3-segment-routing-
extensions-17 (work in progress), November 2018.
[I-D.ietf-ospf-segment-routing-extensions]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", draft-ietf-ospf-segment-
routing-extensions-25 (work in progress), April 2018.
[I-D.ietf-ospf-te-link-attr-reuse]
Psenak, P., Lindem, A., Ginsberg, L., Henderickx, W.,
Tantsura, J., Gredler, H., and J. Drake, "OSPF Link
Traffic Engineering (TE) Attribute Reuse", draft-ietf-
ospf-te-link-attr-reuse-06 (work in progress), November
2018.
[ISO10589]
International Organization for Standardization,
"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)", ISO/
IEC 10589:2002, Second Edition, Nov 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>.
[RFC7308] Osborne, E., "Extended Administrative Groups in MPLS
Traffic Engineering (MPLS-TE)", RFC 7308,
DOI 10.17487/RFC7308, July 2014,
<https://www.rfc-editor.org/info/rfc7308>.
[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>.
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[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>.
16.2. Informative References
[I-D.gulkohegde-routing-planes-using-sr]
Hegde, S. and a. arkadiy.gulko@thomsonreuters.com,
"Separating Routing Planes using Segment Routing", draft-
gulkohegde-routing-planes-using-sr-00 (work in progress),
March 2017.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC3906] Shen, N. and H. Smit, "Calculating Interior Gateway
Protocol (IGP) Routes Over Traffic Engineering Tunnels",
RFC 3906, DOI 10.17487/RFC3906, October 2004,
<https://www.rfc-editor.org/info/rfc3906>.
[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>.
[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>.
[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, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
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[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <https://www.rfc-editor.org/info/rfc7120>.
[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>.
[RFC7810] Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and
Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions",
RFC 7810, DOI 10.17487/RFC7810, May 2016,
<https://www.rfc-editor.org/info/rfc7810>.
[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>.
Authors' Addresses
Peter Psenak (editor)
Cisco Systems
Apollo Business Center
Mlynske nivy 43
Bratislava, 82109
Slovakia
Email: ppsenak@cisco.com
Shraddha Hegde
Juniper Networks, Inc.
Embassy Business Park
Bangalore, KA, 560093
India
Email: shraddha@juniper.net
Clarence Filsfils
Cisco Systems, Inc.
Brussels
Belgium
Email: cfilsfil@cisco.com
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Ketan Talaulikar
Cisco Systems, Inc.
S.No. 154/6, Phase I, Hinjawadi
PUNE, MAHARASHTRA 411 057
India
Email: ketant@cisco.com
Arkadiy Gulko
Thomson Reuters
Email: arkadiy.gulko@thomsonreuters.com
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