Internet DRAFT - draft-ietf-lsr-ospf-reverse-metric
draft-ietf-lsr-ospf-reverse-metric
Link State Routing K. Talaulikar, Ed.
Internet-Draft P. Psenak
Intended status: Standards Track Cisco Systems, Inc.
Expires: 13 April 2023 H. Johnston
AT&T Labs
10 October 2022
OSPF Reverse Metric
draft-ietf-lsr-ospf-reverse-metric-13
Abstract
This document specifies the extensions to OSPF that enable a router
to use link-local signaling to signal the metric that receiving OSPF
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 metrics on both sides of a link between them.
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
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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 13 April 2023.
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document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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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 . . . . . . . . . . . . . . . . . . . 9
8. Backward Compatibility . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Normative References . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
A router running the Open Shortest Path First (OSPFv2) [RFC2328] or
OSPFv3 [RFC5340] routing protocols originates a Router-LSA (Link-
State Advertisement) that describes all its links to its neighbors
and includes a metric that indicates its "cost" to reach the neighbor
over that link. Consider two routers R1 and R2 that are connected
via a link. The metric for this link in the 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
may result in the desired change in the traffic distribution that R1
wanted to achieve towards itself over the link from R2.
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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 the OSPF reverse metric
helps address. The usage of the OSPF reverse metric need not be
limited to these cases; it 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, 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 shift traffic onto alternate
paths (where available) with limited disruption (depending on the
network topology) to in-flight communications by applications or end-
users. When performed successfully, this allows the operator to
perform disruptive augmentation, fault diagnosis, or repairs on a
link in a production 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 many 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) to a "maintenance mode". This causes the
router to advertise the maximum metric for that link and 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 stops the
signaling of reverse metric on that link resulting in its neighbor
also reverting to its provisioned metric for that link.
2.2. Adaptive Metric Signaling
In Figure 1 above, consider that at some point in time T, AGGR1 loses
some of its capacity towards the core. This may result in a
congestion issue towards the core on AGGR1 that it needs to mitigate
by redirecting some of its traffic load to transit via 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.
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In such a scenario, the AGGR1 router could signal an incremental OSPF
reverse metric to some or all the R1-RN routers. When the R1-RN
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 be similarly applied between spine
and leaf nodes in a Clos network [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 for 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
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Type: 19
Length: 4 octets
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: unsigned integer of 2 octets that carries 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 octets
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. MUST be set to 0 on transmission and MUST
be ignored on receipt.
Reverse TE Metric: unsigned integer of 4 octets that carries 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 packets 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 MUST use the RM value to derive the
metric for the link to the advertising router in its Router-LSA when
the reverse metric feature is enabled (refer Section 7 for details on
enablement of RM). 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. The metric value 0xffff (maximum
interface cost) is advertised when the sum exceeds the maximum
interface cost. When the O flag is clear, the metric value to be
advertised is copied 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 copied directly from the value in the TLV only when the
RM value signaled is higher than the provisioned metric for the link.
The H and O flags are mutually exclusive and the H flag is ignored
when the O flag is set.
A router stops including the Reverse Metric TLV in its Hello packets
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 MUST revert to using its provisioned metric value.
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In certain scenarios, two or more routers may start the RM signaling
on the same link. This could create collision scenarios. The
following guidelines are RECOMMENDED for adoption to ensure that
there is no instability in the network due to churn in their metric
caused by the signaling of RM:
* The RM value that is signaled by a router to its neighbor should
not be derived from the reverse metric being signaled by any of
its neighbors on any of its links.
* The RM value that is signaled by a router should 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 guidelines, a router would not start, stop, or change
its RM metric signaling based on the RM metric signaling initiated by
some other routers. 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 packet - one for each of the topologies for which it
desires to signal the reverse metric. A router MUST NOT include more
than one instance of this TLV per MTID. If more than a single
instance of this TLV per MTID is present, the receiving router MUST
only use the value from the first instance and ignore the others.
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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 to those described above, MAY be used to
signal the reverse TE metric for router links. The neighbor MUST 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] when
the reverse metric feature is enabled (refer Section 7 for details on
enablement of RM). The rules for doing so are analogous to those
given above for the Router-LSA.
7. Operational Guidelines
The signaled reverse metric does not alter the OSPF metric parameters
stored in a receiving router's persistent provisioning database.
Routers that receive a reverse metric advertisement SHOULD log an
event to notify system administration. 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.
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.
An implementation MUST NOT signal reverse metric to neighbors by
default and MUST provide a configuration option to enable the
signaling of reverse metric on specific links. An implementation
MUST NOT accept the RM from its neighbors by default. An
implementation MAY provide configuration to accept the RM globally on
the device, or per area, but an implementation MUST support
configuration to enable/disable acceptance of the RM from neighbors
on specific links. This is to safeguard against inadvertent use of
RM.
For the use case in Section 2.1, it is RECOMMENDED that the network
operator limits the period of enablement of the reverse metric
mechanism to be only the duration of a network maintenance window.
[I-D.ietf-ospf-yang] specifies the base OSPF YANG model. The
required configuration and operational elements for this feature are
expected to be introduced as an augmentation to this base OSPF YANG
model.
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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 document allocates code points from the "Link Local Signalling
TLV Identifiers" registry in the "Open Shortest Path First (OSPF)
Link Local Signalling (LLS) - Type/Length/Value Identifiers (TLV)"
registry group for the TLVs introduced.
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
Metric TLVs may result in misrouting of traffic. If authentication
is being used in the OSPFv2 routing domain [RFC5709][RFC7474], then
the Cryptographic Authentication TLV [RFC5613] MUST also be used to
protect the contents of the LLS block.
A router that is misbehaving or misconfigured, may end up signaling
varying values of reverse metrics or toggle the state of reverse
metric. This can result in a neighbor router having to frequently
update its Router LSA causing network churn and instability despite
existing OSPF protocol mechanisms (e.g., MinLSInterval, and
[RFC8405]). It is RECOMMENDED that implementations support the
detection of frequent changes in reverse metric signaling and ignore
the reverse metric (i.e., revert to using their provisioned metric
value) during such conditions.
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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. Acknowledgements
The authors would like to thanks Jay Karthik for his contributions to
the use cases and the review of the solution.
The authors would like to thank Les Ginsberg, Aijun Wang, Gyan
Mishra, Matthew Bocci, Thomas Fossati, and Steve Hanna 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 authors would also like to thank John Scudder for
his detailed AD review and suggestions to improve this document.
The document leverages the concept of Reverse Metric for IS-IS, its
related use cases, and applicability aspects from [RFC8500].
12. References
12.1. 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>.
[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>.
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[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>.
12.2. Informative References
[CLOS] Clos, C., "A Study of Non-Blocking Switching Networks:
Bell System Technical Journal Vol. 32(2)", March 1953.
[I-D.ietf-ospf-yang]
Yeung, D., Qu, Y., Zhang, J., Chen, I., and A. Lindem,
"YANG Data Model for OSPF Protocol", Work in Progress,
Internet-Draft, draft-ietf-ospf-yang-29, 17 October 2019,
<https://www.ietf.org/archive/id/draft-ietf-ospf-yang-
29.txt>.
[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>.
[RFC8405] Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
Francois, P., and C. Bowers, "Shortest Path First (SPF)
Back-Off Delay Algorithm for Link-State IGPs", RFC 8405,
DOI 10.17487/RFC8405, June 2018,
<https://www.rfc-editor.org/info/rfc8405>.
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[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 (editor)
Cisco Systems, Inc.
India
Email: ketant.ietf@gmail.com
Peter Psenak
Cisco Systems, Inc.
Apollo Business Center
Mlynske nivy 43
821 09 Bratislava
Slovakia
Email: ppsenak@cisco.com
Hugh Johnston
AT&T Labs
United States of America
Email: hugh_johnston@labs.att.com
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