Advertising Exclusive Links for Flex-Algorithm in IGP
draft-gong-lsr-exclusive-link-for-flex-algo-07
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Liyan Gong , Weiqiang Cheng , Changwang Lin , Mengxiao Chen , Ran Chen , Yanrong Liang | ||
| Last updated | 2024-03-03 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-gong-lsr-exclusive-link-for-flex-algo-07
Network Working Group L. Gong
Internet Draft W. Cheng
Intended status: Standards Track China Mobile
Expires: September 1, 2024 C. Lin
M. Chen
New H3C Technologies
R. Chen
ZTE Corporation
Y. Liang
Ruijie Networks Co., Ltd.
March 1, 2024
Advertising Exclusive Links for Flex-Algorithm in IGP
draft-gong-lsr-exclusive-link-for-flex-algo-07
Abstract
This document proposes the method to advertise exclusive links for
Flex-Algorithm in IGP.
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), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire on September 1 2024.
Gong, et al. Expire September 1, 2024 [Page 1]
Internet-Draft Advertise Exclusive Link for Flex-Algo March 2024
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
(http://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 include Simplified BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction ................................................ 2
1.1. Requirements Language .................................. 3
2. Problem Statement ........................................... 3
3. Solution A: Maximum Link Metric ............................. 4
3.1. Advertising Maximum Link Metric in IS-IS ............... 4
3.2. Advertising Maximum Link Metric in OSPF ................ 4
3.3. Considerations for Flex-Algorithm Using IGP Metric ..... 4
4. Solution B: Unreachable Link Flag ........................... 5
4.1. Advertising Unreachable Link Flag in IS-IS ............. 5
4.2. Advertising Unreachable Link Flag in OSPF .............. 6
5. Backward Compatibility ...................................... 7
6. Security Considerations ..................................... 7
7. IANA Considerations ......................................... 8
8. References .................................................. 8
8.1. Normative References ................................... 8
8.2. Informative References ................................. 8
9. Acknowledgments ............................................. 9
Authors' Addresses ............................................ 10
1. Introduction
Flexible Algorithm (Flex-Algorithm) allows IGP to compute
constraint-based paths. [I-D.ietf-lsr-flex-algo] specifies the usage
of Flex-Algorithm in Segment Routing (SR) data planes - SR MPLS and
SRv6. [I-D.ietf-lsr-ip-flexalgo] extends the Flex-Algorithm for
native IPv4 and IPv6 data planes.
In some scenarios, exclusive links may be deployed for Flex-
Algorithm, but not for best-effort service. However, these links
Gong, et al. Expires September 1, 2024 [Page 2]
Internet-Draft Advertise Exclusive Link for Flex-Algo March 2024
cannot be pruned in normal SPF calculation, and unexpected flows may
be steered into these links.
This document proposes the method to advertise exclusive links for
Flex-Algorithm in IGP.
1.1. 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.
2. Problem Statement
Flex-Algorithm allows IGP to compute the best paths along the
constrained topology.
A network topology is shown in Figure 1. Node A, B, C and D have an
extra link between each other. These links have EAG attribute of
"red" color.
Flex-Algorithm 128 are enable on Node A, B, C and D, with metric
type of IGP cost and EAG rule of including "red". The topology used
by Flex-Algorithm 128 is shown in Figure 2.
Flex-Algorithm 128 are used to transmit particular flows, such as
network slice. The links used by Flex-Algorithm 128 are sub-
interfaces with dedicated queues for bandwidth guarantee. So it is
expected that only the particular flows are transmitted on these
links using Flex-Algorithm 128. However, these links are also
contained in the default topology used by normal SPF calculation,
and unexpected flows of best-effort service may be steered into
these links. Therefore, it is a problem that exclusive links for
Flex-Algorithm cannot be pruned in normal SPF calculation.
Gong, et al. Expires September 1, 2024 [Page 3]
Internet-Draft Advertise Exclusive Link for Flex-Algo March 2024
A======C------E
|| || |
|| || |
|| || |
B======D------F
Figure 1
A------C
| |
| |
| |
B------D
Figure 2
3. Solution A: Maximum Link Metric
3.1. Advertising Maximum Link Metric in IS-IS
As specified in [RFC5305], if a link is advertised with the maximum
link metric (2^24 - 1), this link MUST NOT be considered during the
normal SPF computation in IS-IS.
The exclusive links for Flex-Algorithm may be advertised with the
maximum link metric, so that they will be the pruned in normal SPF
computation.
3.2. Advertising Maximum Link Metric in OSPF
In OSPF protocol, if a link is advertised with the maximum link
metric (2^16 - 1), it may be still reachable. [RFC1247] specifies
that, if the cost of the link is (2^16 - 1), the link should not be
used for data traffic. However, if a router performs an intra-area
Dijkstra calculation as specified in [RFC1583] and higher, it do not
treat links with maximum link metric as unreachable.
If an exclusive link for Flex-Algorithm is advertised with the
maximum link metric, OSPF routers will prefer alternate paths in the
network, rather than the path through that link. However, if there
is no alternate path, the path through the exclusive link will still
be used.
3.3. Considerations for Flex-Algorithm Using IGP Metric
If the associated Flex-Algorithm needs to use IGP Metric in path
calculation, a user defined metric type (128-255) may be assigned to
substitute IGP Metric, and the Generic Metric sub-TLV may be
Gong, et al. Expires September 1, 2024 [Page 4]
Internet-Draft Advertise Exclusive Link for Flex-Algo March 2024
advertised to carry the metric value, as specified in [I-D.ietf-lsr-
flex-algo-bw-con].
4. Solution B: Unreachable Link Flag
4.1. Advertising Unreachable Link Flag in IS-IS
A new ISIS Link Flags sub-TLV is defined in IS-IS. The format is as
the following:
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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: TBD.
o Length: Variable, dependent on the size of the Flags field. MUST
be a multiple of 4 octets.
o Flags: Following flags are currently defined.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|U| ~
+-+-+-+-+-+-+-+-+
o U-Flag: Unreachable Link Flag. The associated link MUST be
treated as unreachable during SPF calculation.
The ISIS Link Flags sub-TLV is advertised in the TLVs/sub-TLVs
below:
o TLV-22 (Extended IS reachability) [RFC5305]
o TLV-222 (MT-ISN) [RFC5120]
o TLV-23 (IS Neighbor Attribute) [RFC5311]
o TLV-223 (MT IS Neighbor Attribute) [RFC5311]
The ISIS Link Flags sub-TLV with U-Flag can be advertised for the
exclusive links used by Flex-Algorithm, so that these links will be
pruned during normal SPF calculation.
Gong, et al. Expires September 1, 2024 [Page 5]
Internet-Draft Advertise Exclusive Link for Flex-Algo March 2024
Due to the change of procedures in the SPF calculation, all routers
in a level must support the changes specified in this section. To
ensure that, if a level is provisioned to support Unreachable Link
Flag, all routers supporting this capability must advertise an IS-IS
Router Capability TLV-242 that includes the following Unreachable
Link Flag Sub-TLV:
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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Upon detecting the presence of a reachable TLV-242 without an
Unreachable Link Flag Sub-TLV, all routers MUST recalculate routes
without considering any Unreachable Link Flag.
4.2. Advertising Unreachable Link Flag in OSPF
A new OSPF Link Flags sub-TLV is defined in OSPF. The format is as
the following:
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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: TBD.
o Length: Variable, dependent on the size of the Flags field. MUST
be a multiple of 4 octets.
o Flags: Following flags are currently defined.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|U| ~
+-+-+-+-+-+-+-+-+
o U-Flag: Unreachable Link Flag. The associated link MUST be
treated as unreachable during SPF calculation.
The OSPF Link Flags sub-TLV is advertised in the TLVs/sub-TLVs
below:
Gong, et al. Expires September 1, 2024 [Page 6]
Internet-Draft Advertise Exclusive Link for Flex-Algo March 2024
o OSPFv2 Extended Link TLV of OSPFv2 Extended Link Opaque LSA
[RFC7684]
o Router-Link TLV of OSPFv3 E-Router-LSA [RFC8362]
The OSPF Link Flags sub-TLV with U-Flag can be advertised for the
exclusive links used by Flex-Algorithm, so that these links will be
pruned during normal SPF calculation.
Due to the change of procedures in the SPF calculation, all routers
in an area must support the changes specified in this section. To
ensure that, if an area is provisioned to support Unreachable Link
Flag, all routers supporting this capability must advertise a Router
Information (RI) LSA with a Router Functional Capabilities TLV
[RFC7770] that includes the following Router Functional Capability
Bit:
Bit Capabilities
TBD Unreachable Link Flag support
Upon detecting the presence of a reachable Router-LSA without a
companion RI LSA that has the bit set, all routers MUST recalculate
routes without considering any Unreachable Link Flag.
5. Backward Compatibility
An obvious benefit of solution A is that using maximum link metric
is backward compatible. However, in OSPF, it may not work as well as
in ISIS, since the links with maximum link metric are not always
treated as unreachable by OSPF routers. Besides, additional
mechanisms are required for the Flex-Algorithm using IGP Metric in
path calculation.
When using the Link Flags sub-TLV with U-Flag in solution B, all
nodes in the same area or level must support this feature. To avoid
topology inconsistence and achieve backward compatibility, routers
supporting the Unreachable Link Flag MUST advertise that capability.
Upon detecting the absence of that capability from any router in the
same area or level, all routers MUST recalculate routes without
considering any Unreachable Link Flag. The backward-compatibility
procedures described in [RFC8042] should be followed to ensure loop-
free routing.
6. Security Considerations
TBD
Gong, et al. Expires September 1, 2024 [Page 7]
Internet-Draft Advertise Exclusive Link for Flex-Algo March 2024
7. IANA Considerations
Link Flags sub-TLV (TBD)
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017
[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-20 (work in progress), May 2022.
8.2. Informative References
[RFC1247] Moy, J., "OSPF Version 2", RFC 1247, July 1991.
[RFC1583] Moy, J., "OSPF Version 2", RFC 1583, March 1994.
[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>.
[RFC5311] McPherson, D., Ed., Ginsberg, L., Previdi, S., and M.
Shand, "Simplified Extension of Link State PDU (LSP) Space
for IS-IS", RFC 5311, DOI 10.17487/RFC5311, February 2009,
<https://www.rfc-editor.org/info/rfc5311>.
[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, <http://www.rfc-editor.org/info/rfc7770>.
Gong, et al. Expires September 1, 2024 [Page 8]
Internet-Draft Advertise Exclusive Link for Flex-Algo March 2024
[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", DOI 10.17487/RFC8362, RFC 8362, April
2018, <https://www.rfc-editor.org/info/rfc8362>.
[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", draft-ietf-
lsr-ip-flexalgo-06 (work in progress), May 2022.
[I-D.ietf-lsr-flex-algo-bw-con] Hegde, S., J, W. B. A., Shetty, R.,
Decraene, B., Psenak, P., and T. Li, "Flexible Algorithms:
Bandwidth, Delay, Metrics and Constraints", draft-ietf-
lsr-flex-algo-bw-con-03 (work in progress), July 2022.
9. Acknowledgments
The authors would like to thank the following for their valuable
contributions of this document:
TBD
Gong, et al. Expires September 1, 2024 [Page 9]
Internet-Draft Advertise Exclusive Link for Flex-Algo March 2024
Authors' Addresses
Liyan Gong
China Mobile
Email: gongliyan@chinamobile.com
Weiqiang Cheng
China Mobile
Email: chengweiqiang@chinamobile.com
Changwang Lin
New H3C Technologies
Email: linchangwang.04414@h3c.com
Mengxiao Chen
New H3C Technologies
Email: chen.mengxiao@h3c.com
Ran Chen
ZTE Corporation
Email: chen.ran@zte.com.cn
Yanrong Liang
Ruijie Networks Co., Ltd.
Email: liangyanrong@ruijie.com.cn
Gong, et al. Expires September 1, 2024 [Page 10]