Link State Routing K. Talaulikar
Internet-Draft Arrcus Inc
Intended status: Standards Track P. Psenak
Expires: October 30, 2022 Cisco Systems, Inc.
H. Johnston
AT&T Labs
April 28, 2022
OSPF Reverse Metric
draft-ietf-lsr-ospf-reverse-metric-05
Abstract
This document specifies the extensions to OSPF that enable a router
to use link-local signaling to signal the metric that receiving
neighbor(s) should use for a link to the signaling router. The
signaling of this reverse metric, to be used on the link to the
signaling router, allows a router to influence the amount of traffic
flowing towards itself and in certain use cases enables routers to
maintain symmetric metric on both sides of a link between them.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 30, 2022.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Link Maintenance . . . . . . . . . . . . . . . . . . . . 4
2.2. Adaptive Metric Signaling . . . . . . . . . . . . . . . . 4
3. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. LLS Reverse Metric TLV . . . . . . . . . . . . . . . . . . . 5
5. LLS Reverse TE Metric TLV . . . . . . . . . . . . . . . . . . 6
6. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Operational Guidelines . . . . . . . . . . . . . . . . . . . 8
8. Backward Compatibility . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Security Considerations . . . . . . . . . . . . . . . . . . . 9
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
13.1. Normative References . . . . . . . . . . . . . . . . . . 10
13.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Routers running the Open Shortest Path First (OSPFv2) [RFC2328] and
OSPFv3 [RFC5340] routing protocols originate a Router-LSA (Link State
Advertisement) that describes all its links to its neighbors and
includes a metric that indicates its "cost" of reaching the neighbor
over that link. Consider two routers R1 and R2 that are connected
via a link. The metric for this link in direction R1->R2 is
configured on R1 and in the direction R2->R1 is configured on R2.
Thus the configuration on R1 influences the traffic that it forwards
towards R2 but does not influence the traffic that it may receive
from R2 on that same link.
This document describes certain use cases where a router is required
to signal what we call the "reverse metric" (RM) to its neighbor to
adjust the routing metric in the inbound direction. When R1 signals
its reverse metric on its link to R2, then R2 advertises this value
as its metric to R1 in its Router-LSA instead of its locally
configured value. Once this information is part of the topology,
then all other routers do their computation using this value which
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results in the desired change in the traffic distribution that R1
wanted to achieve towards itself over the link from R2.
This document describes extensions to OSPF Link-Local Signaling (LLS)
[RFC5613] to signal OSPF reverse metrics. Section 4 specifies the
LLS Reverse Metric TLV and Section 5 specifies the LLS Reverse TE
Metric TLV. The related procedures are specified in Section 6.
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. Use Cases
This section describes certain use cases that OSPF reverse metric
helps address. The usage of the OSPF reverse metric need not be
limited to these cases and is intended to be a generic mechanism.
Core Network
^ ^
| |
V v
+----------+ +----------+
| AGGR1 | | AGGR2 |
+----------+ +----------+
^ ^ ^ ^
| | | |
| +-----------+ |
| | | |
| +--------+ | |
v v v v
+-----------+ +-----------+
| R1 | | RN |
| Router | ... | Router |
+-----------+ +-----------+
Figure 1: Reference Dual Hub and Spoke Topology
Consider a deployment scenario where, as shown in Figure 1, a bunch
of routers R1 through RN, are dual-home connected to AGGR1 and AGGR2
that are aggregating their traffic towards a core network.
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2.1. Link Maintenance
Before network maintenance events are performed on individual links,
operators substantially increase (to maximum value) the OSPF metric
simultaneously on both routers attached to the same link. In doing
so, the routers generate new Router LSAs that are flooded throughout
the network and cause all routers to gradually shift traffic onto
alternate paths with very little or no disruption to in-flight
communications by applications or end-users. When performed
successfully, this allows the operator to confidently perform
disruptive augmentation, fault diagnosis, or repairs on a link
without disturbing ongoing communications in the network.
In deployments such as a hub and spoke topology as shown in Figure 1,
it is quite common to have routers with several hundred interfaces
and individual interfaces that move anywhere from several hundred
gigabits/second to terabits/second of traffic. The challenge in such
conditions is that the operator must accurately identify the same
point-to-point link on two separate devices to increase (and
afterward decrease) the OSPF metric appropriately and to do so in a
coordinated manner. When considering maintenance for PE-CE links
when a large number of CE routers connect to a PE router, an
additional challenge related to coordinating access to the CE routers
may arise when the CEs are not managed by the provider.
The OSPF reverse metric mechanism helps address these challenges.
The operator can set the link on one of the routers (generally the
hub like AGGR1 or a PE) in a "maintenance mode". This causes the
router to advertise the maximum metric for that link and also to
signal its neighbor on the same link to advertise maximum metric via
the reverse metric signaling mechanism. Once the link maintenance is
completed and the "maintenance mode" is turned off, the router
returns to using its provisioned metric for the link and also stops
the signaling of reverse metric on that link resulting in its
neighbor to also revert to its provisioned metric for that link.
2.2. Adaptive Metric Signaling
In Figure 1 above, consider that at some point T, AGGR1 loses some of
its capacity towards the core that may result in a congestion issue
towards the core and it needs to reduce the traffic towards the core
by redirecting some of the load to transit AGGR2 which is not
experiencing a similar issue. Altering its link metric towards the
R1-RN routers would influence the traffic from the core towards R1-RN
but not the other way around as desired.
In such a scenario, the AGGR1 router could signal an incremental OSPF
reverse metric to some or all of the R1-RN routers. When the R1-RN
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routers add this signaled reverse metric offset to the provisioned
metric on their links towards AGGR1, then the path via AGGR2 becomes
a better path causing traffic towards the core to be diverted away
from AGGR1. Note that the reverse metric mechanism allows such
adaptive metric changes to be applied on the AGGR1 as opposed to
being provisioned on a possibly large number of R1-RN routers.
The reverse metric mechanism may also be similarly applied between
spine and leaf nodes in a CLOS topology deployment.
3. Solution
To address the use cases described earlier and to allow an OSPF
router to indicate its reverse metric for a specific link to its
neighbor(s), this document proposes to extend OSPF link-local
signaling to signal the Reverse Metric TLV in OSPF Hello packets.
This ensures that the RM signaling is scoped ONLY to each specific
link individually. The router continues to include the Reverse
Metric TLV in its Hello packets on the link as long as it needs its
neighbor to use that metric value towards itself. Further details of
the procedures involved are specified in Section 6.
The reverse metric mechanism specified in this document applies only
for point-to-point, point-to-multipoint, and hybrid broadcast point-
to-multipoint ( [RFC6845]) links. It is not applicable for broadcast
or non-broadcast-multi-access (NBMA) links since the same objective
is achieved there using the OSPF Two-Part Metric mechanism [RFC8042]
for OSPFv2. The OSPFv3 solution for broadcast or NBMA links is
outside the scope of this document.
4. LLS Reverse Metric TLV
The Reverse Metric TLV is a new LLS TLV. It has 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTID | Flags |O|H| Reverse Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Figure 2: Reverse Metric TLV
Type: 19
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Length: 4 octet
MTID : the multi-topology identifier of the link ([RFC4915])
Flags: 1 octet, the following flags are defined currently and the
rest MUST be set to 0 on transmission and ignored on reception.
* H (0x1) : Indicates that the neighbor should use the value only
if it is higher than its provisioned metric value for the link.
* O (0x2) : Indicates that the reverse metric value provided is
an offset that is to be added to the provisioned metric.
Reverse Metric: 2 octets, the value or offset of reverse metric to
replace or be added to the provisioned link metric.
5. LLS Reverse TE Metric TLV
The Reverse TE Metric TLV is a new LLS 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |O|H| RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse TE Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Figure 3: Reverse TE Metric TLV
Type: 20
Length: 4 octet
Flags: 1 octet, the following flags are defined currently and the
rest MUST be set to 0 on transmission and ignored on reception.
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* H (0x1) : Indicates that the neighbor should use the value only
if it is higher than its provisioned TE metric value for the
link.
* O (0x2) : Indicates that the reverse TE metric value provided
is an offset that is to be added to the provisioned TE metric.
RESERVED: 24-bit field. SHOULD be set to 0 on transmission and
MUST be ignored on receipt.
Reverse TE Metric: 4 octets, the value or offset of reverse
traffic engineering metric to replace or to be added to the
provisioned TE metric of the link.
6. Procedures
When a router needs to signal an RM value that its neighbor(s) should
use for a link towards the router, it includes the Reverse Metric TLV
in the LLS block of its hello messages sent on that link and
continues to include this TLV for as long as it needs its neighbor to
use this value. The mechanisms used to determine the value to be
used for the RM is specific to the implementation and use case and is
outside the scope of this document. For example, the RM value may be
derived based on the router's link bandwidth with respect to a
reference bandwidth.
A router receiving a hello packet from its neighbor that contains the
Reverse Metric TLV on a link SHOULD use the RM value to derive the
metric for the link to the advertising router in its Router-LSA.
When the O flag is set, the metric value to be advertised is derived
by adding the value in the TLV to the provisioned metric for the
link. When the O flag is clear, the metric value to be advertised is
derived directly from the value in the TLV. When the H flag is set
and the O flag is clear, the metric value to be advertised is derived
directly from the value in the TLV only when the RM value signaled is
higher than the provisioned metric for the link.
A router stops including the Reverse Metric TLV in its hello messages
when it needs its neighbors to go back to using their own provisioned
metric values. When this happens, a router that had modified its
metric in response to receiving a Reverse Metric TLV from its
neighbor should revert to using its provisioned metric value.
In certain scenarios, two or more routers may start the RM signaling
on the same link. This could create collision scenarios. The
following rules MUST be adopted by routers to ensure that there is no
instability in the network due to churn in their metric due to
signaling of RM:
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o The RM value that is signaled by a router to its neighbor MUST NOT
be derived from the reverse metric being signaled by any of its
neighbors on any of its links.
o The RM value that is signaled by a router MUST NOT be derived from
its metric which has been modified on account of an RM signaled
from any of its neighbors on any of its links. RM signaling from
other routers can affect the router's metric advertised in its
Router-LSA. When deriving the RM values that a router signals to
its neighbors, it should use its provisioned local metric values
not influenced by any RM signaling.
Based on these rules, a router MUST never start, stop, or change its
RM metric signaling based on the RM metric signaling initiated by
some other router. Based on the local configuration policy, each
router would end up accepting the RM value signaled by its neighbor
and there would be no churn of metrics on the link or the network on
account of RM signaling.
In certain use cases when symmetrical metrics are desired (e.g., when
metrics are derived based on link bandwidth), the RM signaling can be
enabled on routers on either end of a link. In other use cases (as
described in Section 2.1), RM signaling may need to be enabled only
on the router at one end of a link.
When using multi-topology routing with OSPF [RFC4915], a router MAY
include multiple instances of the Reverse Metric TLV in the LLS block
of its hello message - one for each of the topologies for which it
desires to signal the reserve metric.
In certain scenarios, the OSPF router may also require the
modification of the TE metric being advertised by its neighbor router
towards itself in the inbound direction. The Reverse TE Metric TLV,
using similar procedures as described above, MAY be used to signal
the reverse TE metric for router links. The neighbor SHOULD use the
reverse TE metric value to derive the TE metric advertised in the TE
Metric sub-TLV of the Link TLV in its TE Opaque LSA [RFC3630].
7. Operational Guidelines
The use of reverse metric signaling does not alter the OSPF metric
parameters stored in a router's persistent provisioning database.
If routers that receive a reverse metric advertisement send a syslog
message, this will assist in rapidly identifying the node in the
network that is advertising an OSPF metric or TE metric different
from that which is configured locally on the device.
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When the link TE metric is raised to the maximum value, either due to
the reverse metric mechanism or by explicit user configuration, this
SHOULD immediately trigger the CSPF (Constrained Shortest Path First)
recalculation to move the TE traffic away from that link.
Implementations SHOULD provide a configuration option to enable the
signaling of reverse metric from a router to its neighbors and are
RECOMMENDED to provide a configuration option to disable the
acceptance of the RM from its neighbors.
If an implementation enables this mechanism by default, it is
RECOMMENDED that it be disabled by the operators when not explicitly
using it.
For the use case in Section 2.1, a router SHOULD limit the period of
advertising reverse metric towards a neighbor only for the duration
of a network maintenance window.
8. Backward Compatibility
The signaling specified in this document happens at a link-local
level between routers on that link. A router that does not support
this specification would ignore the Reverse Metric and Reverse TE
Metric LLS TLVs and not update its metric(s) in the other LSAs. As a
result, the behavior would be the same as prior to this
specification. Therefore, there are no backward compatibility
related issues or considerations that need to be taken care of when
implementing this specification.
9. IANA Considerations
This specification updates Link Local Signalling TLV Identifiers
registry.
IANA is requested to make permanent the following code points that
have been assigned via early allocation
o 19 - Reverse Metric TLV
o 20 - Reverse TE Metric TLV
10. Security Considerations
The security considerations for "OSPF Link-Local Signaling" [RFC5613]
also apply to the extension described in this document. The usage of
the reverse metric TLVs is to alter the metrics used by routers on
the link and influence the flow and routing of traffic over the
network. Hence, modification of the Reverse Metric and Reverse TE
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Metric TLVs may result in misrouting of traffic. If authentication
is being used in the OSPF routing domain [RFC5709][RFC7474], then the
Cryptographic Authentication TLV [RFC5613] SHOULD also be used to
protect the contents of the LLS block.
Receiving a malformed LLS Reverse Metric or Reverse TE Metric TLVs
MUST NOT result in a hard router or OSPF process failure. The
reception of malformed LLS TLVs or sub-TLVs SHOULD be logged, but
such logging MUST be rate-limited to prevent denial-of-service (DoS)
attacks.
11. Contributors
Thanks to Jay Karthik for his contributions to the use cases and the
review of the solution.
12. Acknowledgements
The authors would like to thank Les Ginsberg, Aijun Wang, Gyan
Mishra, and Matthew Bocci for their review and feedback on this
document. The authors would also like to thank Acee Lindem for this
detailed shepherd's review and comments on this document.
The document leverages the concept of Reverse Metric for IS-IS, its
related use cases, and applicability aspects from [RFC8500].
13. References
13.1. Normative References
[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>.
[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>.
[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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[RFC5613] Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D.
Yeung, "OSPF Link-Local Signaling", RFC 5613,
DOI 10.17487/RFC5613, August 2009,
<https://www.rfc-editor.org/info/rfc5613>.
[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>.
13.2. Informative References
[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>.
[RFC5709] Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
Authentication", RFC 5709, DOI 10.17487/RFC5709, October
2009, <https://www.rfc-editor.org/info/rfc5709>.
[RFC6845] Sheth, N., Wang, L., and J. Zhang, "OSPF Hybrid Broadcast
and Point-to-Multipoint Interface Type", RFC 6845,
DOI 10.17487/RFC6845, January 2013,
<https://www.rfc-editor.org/info/rfc6845>.
[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>.
[RFC8042] Zhang, Z., Wang, L., and A. Lindem, "OSPF Two-Part
Metric", RFC 8042, DOI 10.17487/RFC8042, December 2016,
<https://www.rfc-editor.org/info/rfc8042>.
[RFC8500] Shen, N., Amante, S., and M. Abrahamsson, "IS-IS Routing
with Reverse Metric", RFC 8500, DOI 10.17487/RFC8500,
February 2019, <https://www.rfc-editor.org/info/rfc8500>.
Authors' Addresses
Ketan Talaulikar
Arrcus Inc
India
Email: ketant.ietf@gmail.com
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Peter Psenak
Cisco Systems, Inc.
Apollo Business Center
Mlynske nivy 43
Bratislava 821 09
Slovakia
Email: ppsenak@cisco.com
Hugh Johnston
AT&T Labs
USA
Email: hugh_johnston@labs.att.com
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