Internet DRAFT - draft-hu-lsr-igp-ca-flex-algorithm
draft-hu-lsr-igp-ca-flex-algorithm
Network Working Group Z. Hu
Internet-Draft G. Xu
Intended status: Standards Track J. Dong
Expires: 8 January 2023 Huawei
7 July 2022
IGP Flexible Algorithm with Common Address
draft-hu-lsr-igp-ca-flex-algorithm-00
Abstract
An IGP Flexible Algorithm (Flex-Algorithm) allows IGPs to compute
constraint-based paths. IGP Flex-Algorithm can be used with Segment
Routing (SR) or IP data plane. When used with SR data plane, Flex-
Algorithm requires to allocate algorithm specific Prefix Segment
Identifiers (SIDs) or algorithm specific SRv6 Locators. When used
with IP data plane, Flex-Algorithm requires to allocate algorithm
specfic IPv4 or IPv6 prefixes. This increases the complexity and
overhead of managing, advertising and maintaining additional SR SIDs,
SRv6 Locators and IPv4 or IPv6 prefixes, which may not be affordable
to some networks and network devices.
This document extends IGP Flex-Algorithm to allow the use of common
SR SIDs, SRv6 Locators and IP prefixes among multiple Flex-
Algorithms.
Requirements Language
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 BCP 14 [RFC2119]
[RFC8174] when, and only when, they appear in all capitals, as shown
here.
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/.
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This Internet-Draft will expire on 8 January 2023.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Advertising Flex-Algorithm Definitions . . . . . . . . . . . 3
3. Advertising CA Flex-Algorithm Participation . . . . . . . . . 4
3.1. The IS-IS CA Algorithm Sub-TLV . . . . . . . . . . . . . 4
3.2. The OSPFv3 CA Algorithm Sub-TLV . . . . . . . . . . . . . 5
4. SRv6 Locators for CA Flex-Algorithm . . . . . . . . . . . . . 5
5. SRv6 Adjacency SIDs for CA Flex-Algorithm . . . . . . . . . . 6
6. Calculating paths with CA Flex-Algorithm . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The IGP Flexible Algorithm (Flex-Algorithm) [I-D.ietf-lsr-flex-algo]
specifies a way of computing a constraint-based path using IGP. IGP
Flex-Algorithm can be used with Segment Routing (SR) or IP data plane
. When used with SR data plane, Flex-Algorithm requires to allocate
algorithm specific Prefix Segment Identifiers (SIDs) [RFC8402] or
algorithm specific SRv6 Locators [RFC8986]. When used with IP data
plane, Flex-Algorithm requires to allocate algorithm specfic IPv4 or
IPv6 prefixes [I-D.ietf-lsr-ip-flexalgo]. The current Flex-Algorithm
mechanism increases the complexity and overhead of managing,
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advertising and maintaining additional SR SIDs, SRv6 Locators and
IPv4 prefixes, which may not be affordable to some networks and
devices.
For example, [I-D.ietf-lsr-isis-srv6-extensions] and
[I-D.ietf-lsr-ospfv3-srv6-extensions] define the IGP Extensions to
support SRv6. For each Flex-Algorithm, separate SRv6 locators, End
SIDs and End.X SIDs needs to be allocated and advertised. As the
number of Flex-Algorithm increases, it the pressure on the IGP would
increase accordingly.
In addition, SInce the SRv6 VPN SIDs are always associated with the
Flex-Algorithm to which the SRv6 Locator is allocated, it is
impossible to steer different service flows of an SRv6 VPN using
different Flex-Algorithm.
To overcome the above issues and limitation with IGP Flex-Algo, this
document defines the Common Address (CA) Flex-Algorithm extensions to
allow the use of the same set of SR SIDs, SRv6 Locators, or IP
prefixes among multiple Flex-Algorithms. Here the term Common
Address (CA) refers to the common set of SR SIDs, SRv6 Locators or IP
prefixes respectively for SR or IP data plane.
Based on the CA Flex-Algo mechanism, constrainet-based path
computation is done for each Flex-Algorithm, and the routing entries
for each Flex-Algorithm is maintained in a separate Routing
Information Base (RIB). For packet forwarding, some additional data
plane field in packet header is needed to select the correct RIB for
the incoming packet lookup to make the forwarding decision.
[I-D.li-6man-topology-id] proposes the IPv6 extensions to carry the
topology and/or Algorithm ID in the data plane. The extensions to
other data plane are for further study. Such data plane mechanisms
together with the control plane extensions in this document provides
a CA Flex-Algo solution.
2. Advertising Flex-Algorithm Definitions
To guarantee loop free forwarding, all routers that participate in a
Flex-Algorithm MUST agree on the Flex-Algorithm Definition (FAD).
The advertisement of FAD for CA Flex-Algorithm is not changed.
Selected nodes within the IGP domain MUST advertise FADs as described
in Sections 5, 6, and 7 of [I-D.ietf-lsr-flex-algo].
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3. Advertising CA Flex-Algorithm Participation
When a router is configured to support a particular CA Flex-
Algorithm, we say it is participating in that CA Flex-Algorithm.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.
The following sections describe how the CA Flex-Algorithm
participation is advertised in IGP protocols.
3.1. The IS-IS CA Algorithm Sub-TLV
The ISIS [ISO10589] CA Algorithm Sub-TLV is a sub-TLV of the IS-IS
Router Capability TLV [RFC7981] and has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 1 | Algorithm 2 | Algorithm ... | Algorithm n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IS-IS CA Algorithm Sub-TLV
Type: TBD
Length: Variable
Algorithm (1 octet): Value from 128 to 255.
The CA 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 CA Algorithm Sub-TLV is
advertised, MUST NOT be set.
The CA Algorithm Sub-TLV is optional. It MUST NOT be advertised more
than once at a given level. A router receiving multiple CA Algorithm
sub-TLVs from the same originator MUST select the first advertisement
in the lowest-numbered LSP and subsequent instances of the CA
Algorithm Sub-TLV MUST be ignored.
The CA Flex-Algorithm participation advertised in the IS-IS CA
Algorithm Sub-TLV is topology independent. When a router advertises
participation in the IS-IS CA Algorithm Sub-TLV, the participation
applies to all topologies in which the advertising node participates.
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3.2. The OSPFv3 CA Algorithm Sub-TLV
The CA Algorithm TLV is a top-level TLV of the OSPFv3 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 2 | Algorithm ... | Algorithm n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: OSPFv3 CA Algorithm TLV
Type: TBD
Length: Variable
Algorithm (1 octet): Value from 128 to 255.
When multiple of the CA Algorithm TLVs are received from a given
router, the receiver MUST use the first occurrence of the TLV in the
OSPFv3 Router Information Opaque LSA. If the CA Algorithm TLV
appears in multiple Router Information Opaque LSAs that have
different flooding scopes, the CA Algorithm TLV in the Router
Information Opaque LSA with the area-scoped flooding scope MUST be
used. If the CA Algorithm TLV appears in multiple Router Information
Opaque LSAs that have the same flooding scope, the CA Algorithm TLV
in the Router Information (RI) Opaque LSA with the numerically
smallest Instance ID MUST be used and subsequent instances of the CA
Algorithm 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
CA Algorithm TLV advertisement, area-scoped flooding is REQUIRED.
4. SRv6 Locators for CA Flex-Algorithm
The SRv6 locator TLV is defined in
[I-D.ietf-lsr-isis-srv6-extensions] and
[I-D.ietf-lsr-ospfv3-srv6-extensions], which contains an algorithm
and indicates that the locator only performs path calculation in this
algorithm plane.
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This document defines a new flag in the SRv6 Locator TLV to indicate
that the locator has the ability to be used for path calculations in
multiple CA Flex-Algorithms planes. The position of this flag is
BIT(1) in IS-IS and BIT(2) in OSPFv3.
This bit is set in the Flags in the Locator TLV in which the value of
the algorithm field is 0. When the algorithm is non-zero, the flag
MUST NOT be set.
C-Flag: CA Flex-Algorithm Flag.
A receiver do not support machanisms defined in this document MUST
ignore this flag.
5. SRv6 Adjacency SIDs for CA Flex-Algorithm
The SRv6 SIDs associated with Adjacencies is defined in [I-D.ietf-
lsr-isis-srv6-extensions] and [I-D.ietf-lsr-ospfv3-srv6-extensions]
and is explicitly allocated by the locator.
A new Flag calle is defined to specify that the locator associated
with the adjacent SID has the C flag.
C-Flag: CA Flex-Algorithm Flag.
The position of this flag is BIT(4) in IS-IS and OSPFv3.
This bit is set in Flags in SRv6 END.X and LAN END.X SID TLV in which
the value of the algorithm field is 0. When the Locator
corresponding to the End.X SID does not have the C flag set, the C
flag in the END.X and LAN END.X SID TLV MUST NOT be set.
A receiver do not support machanisms defined in this document MUST
ignore this flag.
6. Calculating paths with CA Flex-Algorithm
The CA Flex-Algorthm can considered as yet another data-plane of the
Flex-Algorithm as described [I-D.ietf-lsr-flex-algo].
Participation for the CA Flex-Algorithm is signalled as described in
Section 3 and is specific to the CA Flex-Algorithm data-plane.
The calculation of CA Flex-Algorithm paths follows what is described
in [I-D.ietf-lsr-flex-algo]. This computation uses the CA Flex-
Algorithm data-plane participation and is independent of the Flex-
Algorithm calculation done for native SR or IP data plane.
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The CA Flex-Algorithm data-plane only considers the participating
nodes during the Flex-Algorithm calculation. When computing paths
for a given CA Flex-Algorithm, all nodes that do not advertise
participation for the CA Flex-Algorithm, as described in Section 3,
MUST be pruned from the topology.
7. IANA Considerations
This document updates the ISIS "Sub-TLVs for TLV 242" registry as
follows:
+-------+--------------------------------+---------------------------+
| Value | TLV Name | Reference |
+-------+--------------------------------+---------------------------+
| TBD | CA Algorithm Sub-TLV | This Document Section 3.1 |
+-------+----------------------+-------------------------------------+
Figure 3: ISIS CA Algorithm Sub-TLV
This document updates the OSPFv3 Router Information (RI) TLVs
registry as follows:
+-------+--------------------------------+---------------------------+
| Value | TLV Name | Reference |
+-------+--------------------------------+---------------------------+
| TBD | CA Algorithm Sub-TLV | This Document Section 3.2 |
+-------+--------------------------------+---------------------------+
Figure 4: OSPFv3 CA Algorithm TLV
This document updates the ISIS Locator Flags registry as follows:
Bit # Name
------------------ ------------------------------
TBD(suggest bit 1) CA Flex-Algorithm Flag (C-flag)
Figure 5: CA Flex-Algorithm Flag
This document updates the OSPFv3 Locator Flags registry as follows:
Bit # Name
------------------ ------------------------------
TBD(suggest bit 2) CA Flex-Algorithm Flag (C-flag)
Figure 6: CA Flex-Algorithm Flag
This document also updates the ISIS and OSPFv3 SRv6 END.X and LAN
END.X SID TLV Flags registry as follows:
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Bit # Name
------------------ ------------------------------
TBD(suggest bit 4) CA Flex-Algorithm Flag (C-flag)
Figure 7: END.X CA Flex-Algorithm Flag
8. References
8.1. Normative References
[I-D.ietf-6man-enhanced-vpn-vtn-id]
Dong, J., Li, Z., Xie, C., Ma, C., and G. Mishra,
"Carrying Virtual Transport Network (VTN) Identifier in
IPv6 Extension Header", Work in Progress, Internet-Draft,
draft-ietf-6man-enhanced-vpn-vtn-id-00, 5 March 2022,
<https://www.ietf.org/archive/id/draft-ietf-6man-enhanced-
vpn-vtn-id-00.txt>.
[I-D.ietf-lsr-flex-algo]
Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
A. Gulko, "IGP Flexible Algorithm", Work in Progress,
Internet-Draft, draft-ietf-lsr-flex-algo-20, 18 May 2022,
<https://www.ietf.org/archive/id/draft-ietf-lsr-flex-algo-
20.txt>.
[I-D.ietf-lsr-isis-srv6-extensions]
Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
Z. Hu, "IS-IS Extensions to Support Segment Routing over
IPv6 Dataplane", Work in Progress, Internet-Draft, draft-
ietf-lsr-isis-srv6-extensions-18, 20 October 2021,
<https://www.ietf.org/archive/id/draft-ietf-lsr-isis-srv6-
extensions-18.txt>.
[I-D.ietf-lsr-ospfv3-srv6-extensions]
Li, Z., Hu, Z., Talaulikar, K., and P. Psenak, "OSPFv3
Extensions for SRv6", Work in Progress, Internet-Draft,
draft-ietf-lsr-ospfv3-srv6-extensions-05, 1 July 2022,
<https://www.ietf.org/archive/id/draft-ietf-lsr-
ospfv3-srv6-extensions-05.txt>.
[I-D.li-6man-topology-id]
Li, Z., Hu, Z., and J. Dong, "Topology Identifier in IPv6
Extension Header", Work in Progress, Internet-Draft,
draft-li-6man-topology-id-00, 20 March 2022,
<https://www.ietf.org/archive/id/draft-li-6man-topology-
id-00.txt>.
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[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>.
8.2. Informative References
[I-D.ietf-lsr-ip-flexalgo]
Britto, W., Hegde, S., Kaneriya, P., Shetty, R., Bonica,
R., and P. Psenak, "IGP Flexible Algorithms (Flex-
Algorithm) In IP Networks", Work in Progress, Internet-
Draft, draft-ietf-lsr-ip-flexalgo-06, 16 May 2022,
<https://www.ietf.org/archive/id/draft-ietf-lsr-ip-
flexalgo-06.txt>.
[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
Zhibo Hu
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing
100095
China
Email: huzhibo@huawei.com
Guoqi Xu
Huawei
Huawei Bld., No. 156 Beiqing Rd.
Beijing
100095
China
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Email: xuguoqi@huawei.com
Jie Dong
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing
100095
China
Email: jie.dong@huawei.com
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