Network Working Group P. Psenak, Ed.
Internet-Draft Cisco Systems
Intended status: Standards Track S. Hegde, Ed.
Expires: April 26, 2018 Juniper Networks, Inc.
C. Filsfils
Cisco Systems, Inc.
A. Gulko
Thomson Reuters
October 23, 2017
ISIS Segment Routing Flexible Algorithm
draft-hegdeppsenak-isis-sr-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 based or Segment Routing based Traffic Engineering to
enforce traffic over a path that is computed using different metrics
or constrains then IGP path. Various mechanisms are used to steer
the traffic towards such traffic engineered paths. This document
proposes a solution that allows IGPs itself to compute constrained
based path over the network without the usage of the traffic
engineering.
Status of This Memo
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This Internet-Draft will expire on April 26, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Flexible Algorithm . . . . . . . . . . . . . . . . . . . . . 3
3. Flexible Algorithm Advertisement . . . . . . . . . . . . . . 3
4. Flexible Algorithm Definition Advertisement . . . . . . . . . 4
4.1. Flexible Algorithm Definition TLV . . . . . . . . . . . . 4
4.2. Flexible Algorithm Exclude Admin Group Sub-TLV . . . . . 7
5. Calculation of Flexible Algorithm Paths . . . . . . . . . . . 7
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8.1. Sub TLVs for Type 242 . . . . . . . . . . . . . . . . . . 9
8.2. New TLV Codepoint and Sub-TLV registry . . . . . . . . . 9
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
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 in 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 such IGP limitation, various sorts of traffic engineering
has been deployed, including RSVP-TE or SR-TE, in which case the TE
component is responsible for computing the path based on some other
or additional metrics and/or constrains. Such paths need to be
installed in the forwarding and replace the original paths computed
by IGPs. Tunnels are often used to represent the engineered paths
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and mechanisms like one described in [RFC3906] are used to replace
the native IGP paths with such tunnel paths.
Segment Routing (SR) allows a flexible definition of end-to-end paths
within IGP topologies by encoding paths as sequences of topological
sub-paths, called segments. It also defines an algorithm that
defines how the path is computed. It also provides a way to
associate Prefix-SID with an algorithm. This allows IGPs to compute
the path based on various algorithms and forward the traffic on a
such path using the algorithm specific segments.
This document describes the IS-IS extension to support Segment
Routing Flexible Algorithm on an MPLS data-plane.
1.1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Flexible Algorithm
Many possible constrains may be used to compute a path over a
network. Some networks are deployed as multiple planes. A simple
form of constrain may be to use a particular plane. A more
sophisticated form of constrain can include some extended metric as
described in [RFC7810]. Even more advanced case could be to restrict
the path and avoid links with certain affinities. Combinations of
these are also possible.
To provide a maximum flexibility we do not want to provide a strict
mapping between the set of constrains and the algorithm that is
associated with it. We want the mapping between the algorithm value
and it's meaning to be flexible and defined by the user. As far as
all routers in the domain has the common understanding what the
particular algorithm value represents, the computation for such
algorithm is consistent and traffic is not subject to the looping.
Because the meaning of the algorithm is not defined by any standard,
but is defined by the user, we call it Flex-Algorithm.
3. Flexible Algorithm Advertisement
[I-D.ietf-isis-segment-routing-extensions] defines an SR-Algorithm.
This algorithm defines how the best path is computed by IGP. Routers
advertise the support for the algorithm as a node capability. Prefix
SIDs are also advertised with an algorithm value and as such are
tightly coupled with the algorithm.
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Existing advertisement of the SR-Algorithm is used for the Flex-
Algorithm advertisements as defined in
[I-D.ietf-isis-segment-routing-extensions].
SR-Algorithm is a one octet value. We propose to split the range of
values as follows:
0-127 - standardised values provided by IANA
128-255 - user defined values.
4. Flexible Algorithm Definition Advertisement
To guarantee the loop free forwarding for paths computed for a
particular Flex-Algorithm, all routers in the network MUST share the
same definition of the Flex-Algorithm. This can be achieved by each
router advertising its definition of each Flex-Algorithm that is
locally defined and detect any conflicts in the Flex-Algorithm
definition between routers.
Alternatively, the central entity in the network can advertise the
definition of the Flex-Algorithm and let all routers to use it.
Two definitions of the Flex-Algorithm are considered to match if all
of the following conditions are met:
Metric Type for both definitions is the same.
The set of Admin Groups that are excluded is exactly the same in
both definitions.
4.1. Flexible Algorithm Definition TLV
Flexible Algorithm Definition TLV (FAD TLV) is used to advertise the
definition of the Flex-Algorithm.
FAD TLV can be advertised as:
Sub-TLV of the IS-IS Router Capability TLV-242 that is defined in
[RFC7981]. When advertised as Sub-TLV of the IS-IS Router
Capability TLV-242, it is used to advertise the local definition
of the Flex-Algorithm on the originating router.
ISIS top-level TLV. When advertised as top-level TLV, it is used
to inform routers in entire domain about the definition of the
Flex-Algorithm.
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When the definition of the Flex Algorithm is advertised, ether in the
TLV-242 or in the new top-level TLV, it is is applicable to all
topologies supported on the receiving node.
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 | Flags | Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flex-Algorithm Exclude sub-TLVs |
+- -+
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type:
When advertised as Sub-TLV of the IS-IS Router Capability TLV-
242: TBD1
When advertised as ISIS top-level TLV: TBD2
Length: variable, dependent on the number of Sub-TLVs
Flags: Single octet field containing the following flags:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|S|D| |
+-+-+-+-+-+-+-+-+
where:
S-Flag: If set, the FAD top-level TLV SHOULD be flooded across
the entire routing domain. If the S flag is not set, the FAD
TLV MUST NOT be leaked between levels. This bit MUST NOT be
altered during the TLV leaking. This bit MUST be ignored in
the FAD Sub-TLV of the IS-IS Router Capability TLV-242.
D-Flag: when the FAD top-level TLV is leaked from level-2 to
level-1, the D bit MUST be set. Otherwise, this bit MUST be
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clear. FAD top-level TLVs with the D bit set MUST NOT be
leaked from level-1 to level-2. This is to prevent TLV looping
across levels. This bit MUST be ignored in the FAD Sub-TLV of
the IS-IS Router Capability TLV-242.
Algorithm: Flex-Algorithm number. 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 metric as defined in [RFC5305].
Flex-Algorithm Exclude sub-TLVs - optional sub-TLVs as described
in section Section 4.2.
When the router is configured with the local definition of the Flex-
Algorithm, the router SHOULD advertise its local definition in the
FAD Sub-TLV of the IS-IS Router Capability TLV-242. If the local
definition of the Flex-Algorithm is not advertised, the inconsistency
in the configuration of the Flex-Algorithm on various nodes can not
be detected and traffic routed based on a Flex-Algorithm path may
loop permanently.
When the router receives the FAD TLV as top-level TLV, it uses it as
a definition of the Flex-Algorithm. If the local definition of the
same Flex-Algorithm exists on the router and is in conflict with the
definition received over top-level FAD TLV, the router MUST NOT
compute any path for such Flex-Algorithm and MUST stop advertising
support for such Flex-Algorithm in its SR-Algorithm Sub-TLV
([I-D.ietf-isis-segment-routing-extensions]).
When router receives the FAD Sub-TLV of the IS-IS Router Capability
TLV-242 from multiple sources and the Flex-Algorithm definition in
these advertisements are conflicting, it MUST NOT compute any path
for such Flex-Algorithm and MUST stop advertising support for such
Flex-Algorithm in its SR-Algorithm Sub-TLV
([I-D.ietf-isis-segment-routing-extensions]).
When router receives the FAD Sub-TLV of the IS-IS Router Capability
TLV-242 from another router and the definition is in conflict with
either the local definition of the Flex-Algorithm OR the definition
received in the FAD top-level TLV, it MUST NOT compute any path for
such Flex-Algorithm.
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The FAD Sub-TLV of the IS-IS Router Capability TLV-242 MUST be
propagated throughout the level and MUST be advertised across level
boundaries. Therefore Router Capability TLV distribution flags
SHOULD be set accordingly, i.e., the S flag in the Router Capability
TLV MUST be set.
4.2. Flexible Algorithm Exclude Admin Group Sub-TLV
To provide even more granularity, the Flexible-Algorithm can include
link 'colors' that the operator wants to exclude from the
computation. This provides a per link granularity for the Flex-
Algorithm definition.
Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-TLV) is a
Sub-TLV of the FAD TLV. It has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: TBD3
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].
FAEAG Sub-TLV SHOULD only appear once in FAD TLV. If it appears more
then once, FAD TLV MUST be ignored by the receiver.
5. Calculation of Flexible Algorithm Paths
A router may compute path for multiple Flex-Algorithms.
A router MUST be configured to support Flex-Algorithm K before it can
compute any path for Flex-Algorithm K.
A router MUST either be configured with a local definition of Flex-
Algorithm K or receive the definition via the FAD top-level TLV as
described in Section 4.1 from the central entity that acts as the
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Flex-Algorithm definition holder before it can compute any path for
Flex-Algorithm K.
If any conflicts in the Flex-Algorithm K definition exists, as
described in Section 4.1, the router MUST NOT compute any path for
Flex-Algorithm K.
When computing the Shortest Path Tree for Flex-Algorithm K, all nodes
that do not advertise support for Flex-Algorithm K in SR-Algorithm
Sub-TLV ([I-D.ietf-isis-segment-routing-extensions]), MUST be pruned
from the topology.
When computing the Shortest Path Tree for Flex-Algorithm K, any link
advertised with any of the corresponding bits in both (Extended)
Administrative Groups sub-TLV and FAEAG Sub-TLV set to 1, MUST be
pruned from the topology.
When computing the Shortest Path Tree for Flex-Algorithm K, router
MUST use the metric that is part of the Flex-Algorithm definition.
If the metric is not advertised for the particular link, such link
MUST be pruned from the topology. A metric of value 0 MUST NOT be
assumed in such case.
Flex-Algorithm K path to any prefix MUST be installed in the
forwarding using the Prefix-SID that was advertised for algorithm K.
If the Prefix SID for algorithm K is not known, Flex-Algorithm K path
to such prefix MUST NOT be installed in the forwarding.
Loop Free Alternate (LFA) paths for Flex-Algorithm K path MUST be
computed using the same constrains as the calculation of the primary
paths for Flex-Algorithm K. LFA path MUST only use Prefix-SIDs
advertised specifically for algorithm K to enforce the traffic over
such path.
Any 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 K path to inter-area prefix will only be computed
for the local area. The 'exit' L1/L2 router will be selected based
on the best path for the Flex-Algorithm K in the local area and such
'exit' L1/L2 router will be responsible to compute the best Flex-
Algorithm K path over the next area. This may produce end-to-end
path, which is not the best from the Flex-Algorithm K perspective.
If the best end-to-end path for Flex-Algorithm K needs to be used for
inter-area destinations, paths for such destinations need to be
computed by the entity that has the topological information about all
areas.
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6. Backward Compatibility
This extension brings no new backward compatibility issues.
7. Security Considerations
This extension adds no new security considerations.
8. IANA Considerations
This documents request allocation for the following TLVs and subTLVs.
8.1. Sub TLVs for Type 242
This document makes the following registrations in the "sub-TLVs for
TLV 242" registry.
Type: TBD1 (suggested value 24).
Description: Flexible Algorithm Definition Sub-TLV.
Reference: This document (Section 4.1).
8.2. New TLV Codepoint and Sub-TLV registry
This document registers the following TLV:
Type: TBD2 (suggested value 151)
name: Flexible Algorithm Definition TLV.
IIH: no
LSP: yes
SNP: no
Purge: no
Reference: This document (Section 4.1).
This document creates the following sub-TLV Registry:
Registry: sub-TLVs for TLV 151
Registration Procedure: Expert review
Reference: This document (Section 4.1)
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This document resisters following TLV in the "sub-TLVs for TLV 151"
registry
Type: TBD3, suggested value 1.
Description: Flexible Algorithm Exclude Admin Group Sub-TLV.
Reference: This document (Section 4.2).
9. Acknowledgments
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 Les Ginsberg for review and useful comments on the initial
version of the draft.
Thanks to Cengiz Halit for his review and feedback during initial
phase of the solution definition.
10. References
10.1. Normative References
[I-D.ietf-isis-segment-routing-extensions]
Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
Litkowski, S., Decraene, B., and j. jefftant@gmail.com,
"IS-IS Extensions for Segment Routing", draft-ietf-isis-
segment-routing-extensions-13 (work in progress), June
2017.
[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>.
[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>.
[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>.
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[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>.
[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>.
10.2. 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.
[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>.
Authors' Addresses
Peter Psenak (editor)
Cisco Systems
Apollo Business Center
Mlynske nivy 43
Bratislava, 82109
Slovakia
Email: ppsenak@cisco.com
Shraddha Hegde (editor)
Juniper Networks, Inc.
Embassy Business Park
Bangalore, KA, 560093
India
Email: shraddha@juniper.net
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Clarence Filsfils
Cisco Systems, Inc.
Brussels
Belgium
Email: cfilsfil@cisco.com
Arkadiy Gulko
Thomson Reuters
Email: arkadiy.gulko@thomsonreuters.com
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