draft-ietf-lsr-ospf-reverse-metric-02.txt   draft-ietf-lsr-ospf-reverse-metric-03.txt 
Link State Routing K. Talaulikar Link State Routing K. Talaulikar
Internet-Draft P. Psenak Internet-Draft P. Psenak
Intended status: Standards Track Cisco Systems, Inc. Intended status: Standards Track Cisco Systems, Inc.
Expires: July 3, 2021 H. Johnston Expires: October 29, 2021 H. Johnston
AT&T Labs AT&T Labs
December 30, 2020 April 27, 2021
OSPF Reverse Metric OSPF Reverse Metric
draft-ietf-lsr-ospf-reverse-metric-02 draft-ietf-lsr-ospf-reverse-metric-03
Abstract Abstract
This document specifies the extensions to OSPF that enables a router This document specifies the extensions to OSPF that enables a router
to signal to its neighbor the metric that the neighbor should use to signal to its neighbor the metric that the neighbor should use
towards itself using link-local advertisement between them. The towards itself using link-local advertisement between them. The
signalling of this reverse metric, to be used on link(s) towards signaling of this reverse metric, to be used on the links towards
itself, allows a router to influence the amount of traffic flowing itself, allows a router to influence the amount of traffic flowing
towards itself and in certain use-cases enables routers to maintain towards itself and in certain use-cases enables routers to maintain
symmetric metric on both sides of a link between them. symmetric metric on both sides of a link between them.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 3, 2021. This Internet-Draft will expire on October 29, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
skipping to change at page 2, line 35 skipping to change at page 2, line 35
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . 10 12.1. Normative References . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . 11 12.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction 1. Introduction
Routers running the Open Shortest Path First (OSPFv2) [RFC2328] and Routers running the Open Shortest Path First (OSPFv2) [RFC2328] and
OSPFv3 [RFC5340] routing protocols originate a Router-LSA (Link State OSPFv3 [RFC5340] routing protocols originate a Router-LSA (Link State
Advertisement) that describes all its links to its neighbors and Advertisement) that describes all its links to its neighbors and
includes a metric which indicates its "cost" of reaching the neighbor includes a metric that indicates its "cost" of reaching the neighbor
over that link. Consider two routers R1 and R2 that are connected 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 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. configured on R1 and in the direction R2->R1 is configured on R2.
Thus the configuration on R1 influences the traffic that it forwards Thus the configuration on R1 influences the traffic that it forwards
towards R2 but does not influence the traffic that it may receive towards R2 but does not influence the traffic that it may receive
from R2 on that same link. from R2 on that same link.
This document describes certain use-cases where it is desirable for a This document describes certain use-cases where a router is required
router to be able to signal what we call as the "reverse metric" (RM) to signal what we call the "reverse metric" (RM) to its neighbor to
to its neighbor to adjust the routing metric on the inbound adjust the routing metric in the inbound direction. When R1 signals
direction. When R1 signals its reverse metric on its link to R2, its reverse metric on its link to R2, then R2 advertises this value
then R2 advertises this value as its metric to R1 in its Router-LSA as its metric to R1 in its Router-LSA instead of its locally
instead of its locally configured value. Once this information is configured value. Once this information is part of the topology then
part of the topology then all other routers do their computation all other routers do their computation using this value which results
using this value which results in the desired change in traffic in the desired change in the traffic distribution that R1 wanted to
distribution that R1 wanted to achieve towards itself over the link achieve towards itself over the link from R2.
from R2.
This document proposes an extension to OSPF link-local signaling This document proposes an extension to OSPF link-local signaling
(LLS) [RFC5613] for signalling the OSPF reverse metric using the LLS (LLS) [RFC5613] for signaling the OSPF reverse metric using the LLS
Reverse Metric TLV in Section 4, the reverse Traffic Engineering (TE) Reverse Metric TLV in Section 4, the reverse Traffic Engineering (TE)
metric [RFC3630] using the LLS Reverse TE Metric TLV in Section 5 and metric [RFC3630] using the LLS Reverse TE Metric TLV in Section 5 and
describes the related procedures in section Section 6. describes the related procedures in section Section 6.
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. Use Cases 2. Use Cases
This section describes certain use-cases that OSPF reverse metric This section describes certain use-cases that OSPF reverse metric
helps to address. The usage of OSPF reverse metric need not be helps to address. The usage of the OSPF reverse metric need not be
limited to these cases and is intended to be a generic mechanism. limited to these cases and is intended to be a generic mechanism.
2.1. Symmetrical Metric Based on Reference Bandwidth 2.1. Symmetrical Metric Based on Reference Bandwidth
Certain OSPF implementations and deployments deduce the metric of Certain OSPF implementations and deployments deduce the metric of
links based on their bandwidth using a reference bandwidth. The OSPF links based on their bandwidth using a reference bandwidth. The OSPF
MIB [RFC4750] has ospfReferenceBandwidth that is used by entries in MIB [RFC4750] has ospfReferenceBandwidth that is used by entries in
the ospfIfMetricTable. This mechanism is leveraged in deployments the ospfIfMetricTable. This mechanism is leveraged in deployments
where the link metrics get lowered or increased as bandwidth capacity where the link metrics get lowered or increased as bandwidth capacity
is removed or added e.g. consider layer-2 links bundled as a layer-3 is removed or added e.g. consider layer-2 links bundled as a layer-3
interface on which OSPF is enabled. In the situations where these interface on which OSPF is enabled. In the situations where these
layer-2 links are directly connected to the two routers, the link and layer-2 links are directly connected to the two routers, the link and
bandwidth availability is detected and updated on both sides. This bandwidth availability are detected and updated on both sides. This
allows for schemes where the metric is maintained to be symmetric in allows for schemes where the metric is maintained to be symmetric in
both directions based on the bandwidth. both directions based on the bandwidth.
Now consider variation of the same deployment where the links between Now consider a variation of the same deployment where the links
routers are not directly connected and instead are provisioned over a between routers are not directly connected and instead are
layer-2 network consisting of switches or other mechanisms for a provisioned over a layer-2 network consisting of switches or other
layer-2 emulation. In such scenarios, as show in Figure 1, the mechanisms for a layer-2 emulation. In such scenarios, as shown in
router on one side of the link would not detect when the neighboring Figure 1, the router on one side of the link would not detect when
router has lost one of its layer-2 link and has reduced capacity to the neighboring router has lost one of its layer-2 links and has
its layer-2 switch. Note that the number of links and their reduced capacity to its layer-2 switch. Note that the number of
capacities on the router R0 may not be the same as those on R1, R2 links and their capacities on the router R0 may not be the same as
and R3. The left hand side diagram shows the actual physical those on R1, R2 and R3. The left-hand side diagram shows the actual
topology in terms of the layer-2 links while the right hand side physical topology in terms of the layer-2 links while the right-hand
diagram shows the logical layer-3 link topology between the routers. side diagram shows the logical layer-3 link topology between the
routers.
+--------+ +--------+
| R0 | | R0 |
| Router | | Router |
+--------+ +--------+ +--------+ +--------+
(a) Physical ^ ^ ^ (b) Layer-3 | R0 | (a) Physical ^ ^ ^ (b) Layer-3 | R0 |
Topology | | | Topology +--------+ Topology | | | Topology +--------+
v v v ^ ^ ^ v v v ^ ^ ^
+----------------+ | | | +----------------+ | | |
| Layer 2 Switch | | | | | Layer 2 Switch | | | |
skipping to change at page 4, line 33 skipping to change at page 4, line 33
| Router | | Router | | Router | +--------+ | Router | | Router | | Router | +--------+
+-- -----+ +--------+ +--------+ +-- -----+ +--------+ +--------+
Figure 1: Routers Interconnected over Layer-2 Network Figure 1: Routers Interconnected over Layer-2 Network
In such a scenario, the amount of traffic that can be forwarded in In such a scenario, the amount of traffic that can be forwarded in
bidirectional manner between say R0 and R1 is dictated by the lower bidirectional manner between say R0 and R1 is dictated by the lower
of the link capacity of R0 and R1 to the layer-2 transport network. of the link capacity of R0 and R1 to the layer-2 transport network.
In this scenario, when one of the link from R1 to the switch goes In this scenario, when one of the link from R1 to the switch goes
down, it would increase its link metric to R0 from say 20 to 40. down, it would increase its link metric to R0 from say 20 to 40.
However, similarly R0 also needs to increase its link metric to R1 as However, similarly, R0 also needs to increase its link metric to R1
well from 20 to 40 as otherwise, the traffic will hit congestion and as well from 20 to 40 as otherwise, the traffic will hit congestion
get dropped. and get dropped.
When R1 has the ability to signal the OSPF reverse metric of 40 When R1 can signal the OSPF reverse metric of 40 towards itself to
towards itself to R0, then R0 can also update its metric without any R0, then R0 also updates its metric without any manual intervention
manual intervention to ensure the correct traffic distribution. to ensure the correct traffic distribution. Consider if some
Consider if some destinations were reachable from R0 via R1 destinations were reachable from R0 via R1 previously and this
previously and this automatic metric adjustment now makes some of automatic metric adjustment now makes some of those destinations
those destinations reachable from R0 via R3. This allows some reachable from R0 via R3. This allows some of the traffic load on
traffic load on the link R0 to R1 to now flow via R3 to these the link R0 to R1 to now flow via R3 to these destinations.
destinations.
2.2. Adaptive Metric Signaling 2.2. Adaptive Metric Signaling
Now consider another deployment scenario where, as show in Figure 2, Now consider another deployment scenario where, as shown in Figure 2,
two routers AGGR1 and AGGR2 are connected to a bunch of routers R1 two routers AGGR1 and AGGR2 are connected to a bunch of routers R1
thru RN that are dual homed to them and aggregating the traffic from thru RN that are dual-homed to them and aggregating the traffic from
them towards a core network. At some point T, AGGR1 loses some of them towards a core network. At some point T, AGGR1 loses some of
its capacity towards the core or is facing some congestion issue its capacity towards the core or is facing some congestion issue
towards the core and it needs to reduce the traffic going through it towards the core and it needs to reduce the traffic going through it
and perhaps redirect some of that load via AGGR2 which is not facing and perhaps redirect some of that load via AGGR2 which is not facing
a similar issue. Altering its own metric towards Rx routers would a similar issue. Altering its metric towards Rx routers would
influence the traffic flowing through it in the direction from core influence the traffic flowing through it in the direction from the
to the Rx but not the other way around as desired. core to the Rx but not the other way around as desired.
Core Network Core Network
^ ^ ^ ^
| | | |
V v V v
+----------+ +----------+ +----------+ +----------+
| AGGR1 | | AGGR2 | | AGGR1 | | AGGR2 |
+----------+ +----------+ +----------+ +----------+
^ ^ ^ ^ ^ ^ ^ ^
| | | | | | | |
skipping to change at page 6, line 24 skipping to change at page 6, line 24
0 1 2 3 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 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 | | Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTID | Flags |O|H| Reverse Metric | | MTID | Flags |O|H| Reverse Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where: where:
Type: TBD, suggested value 19 Type: 19
Length: 4 octet Length: 4 octet
MTID : the multi-topology identifier of the link ([RFC4915]) MTID : the multi-topology identifier of the link ([RFC4915])
Flags: 1 octet, following are defined currently and the rest MUST Flags: 1 octet, following are defined currently and the rest MUST
be set to 0 and ignored on reception. be set to 0 and ignored on reception.
* H (0x1) : Indicates that neighbor should use value only if * H (0x1) : Indicates that neighbor should use value only if
higher than its current metric value in use higher than its current metric value in use
* O (0x2) : Indicates that the reverse metric value provided is * O (0x2) : Indicates that the reverse metric value provided is
an offset that is to be added to the original metric an offset that is to be added to the original metric
Reverse Metric: 2 octets, the value or offset of reverse metric to Reverse Metric: 2 octets, the value or offset of reverse metric to
be used be used
5. LLS Reverse TE Metric TLV 5. LLS Reverse TE Metric TLV
The Reverse TE Metric TLV is a new LLS TLV. It has following format: The Reverse TE Metric TLV is a new LLS TLV. It has the following
format:
0 1 2 3 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 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 | | Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |O|H| RESERVED | | Flags |O|H| RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse TE Metric | | Reverse TE Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where: where:
Type: TBD, suggested value 20 Type: 20
Length: 4 octet Length: 4 octet
Flags: 1 octet, following are defined currently and the rest MUST Flags: 1 octet, following are defined currently and the rest MUST
be set to 0 and ignored on reception. be set to 0 and ignored on reception.
* H (0x1) : Indicates that neighbor should use value only if * H (0x1) : Indicates that neighbor should use value only if
higher than its current TE metric value in use higher than its current TE metric value in use
* O (0x2) : Indicates that the reverse TE metric value provided * O (0x2) : Indicates that the reverse TE metric value provided
is an offset that is to be added to the original TE metric is an offset that is to be added to the original TE metric
RESERVED: 24-bit field. SHOULD be set to 0 on transmission and RESERVED: 24-bit field. SHOULD be set to 0 on transmission and
MUST be ignored on receipt. MUST be ignored on receipt.
Reverse TE Metric: 4 octets, the value or offset of reverse Reverse TE Metric: 4 octets, the value or offset of reverse
traffic engineering metric to be used traffic engineering metric to be used
6. Procedures 6. Procedures
When a router needs to signal a RM value that its neigbhor(s) should When a router needs to signal an RM value that its neighbor(s) should
use towards itself, it includes the Reverse Metric TLV in the LLS use towards itself, it includes the Reverse Metric TLV in the LLS
block of its hello messages sent on the link and continues to include block of its hello messages sent on the link and continues to include
this TLV for as long as it needs it's neighbor to use this value. this TLV for as long as it needs its neighbor to use this value. The
The mechanisms used to determine the value to be used for the RM is 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 specific to the implementation and use-case and is outside the scope
of this document. e.g. in the use-case related to symmetric metric of this document. e.g. in the use-case related to symmetric metric
described in Section 2.1, the RM value may be derived based on the described in Section 2.1, the RM value may be derived based on the
router's link's bandwidth with respect to the reference bandwidth. router's link's bandwidth with respect to the reference bandwidth.
A router receiving a hello packet from its neighbor that contains the A router receiving a hello packet from its neighbor that contains the
Reverse Metric TLV on its link SHOULD use the RM value to derive the Reverse Metric TLV on its link SHOULD use the RM value to derive the
metric for the link in its Router-LSA to the advertising router. metric for the link in its Router-LSA to the advertising router.
When the O flag is set, the value in the TLV needs to be added to the When the O flag is set, the value in the TLV needs to be added to the
existing original metric provisioned on the link to derive the new existing original metric provisioned on the link to derive the new
metric value to be used. When the O flag is clear, the value in the metric value to be used. When the O flag is clear, the value in the
TLV should be directly used as the metric to be used. When H flag is TLV should be directly used as the metric to be used. When the H
set and O flag is clear, this is done only when the RM value signaled flag is set and the O flag is clear, this is done only when the RM
is higher than the provisioned metric value being used already. This value signaled is higher than the provisioned metric value being used
mechanism applies only for point-to-point, point-to-multipoint and already. This mechanism applies only for point-to-point, point-to-
hybrid broadcast point-to-multipoint ( [RFC6845]) links. For multipoint, and hybrid broadcast point-to-multipoint ( [RFC6845])
broadcast and NBMA links the OSPF Two-Part Metric mechanism [RFC8042] links. For broadcast and NBMA links the OSPF Two-Part Metric
should be used in similar use-cases. mechanism [RFC8042] should be used in similar use-cases.
Implementations SHOULD provide a configuration option to enable the Implementations SHOULD provide a configuration option to enable the
signaling of RM from a router to its neighbors and MAY provide a signaling of RM from a router to its neighbors and MAY provide a
configuration option to disable the acceptance of the RM from its configuration option to disable the acceptance of the RM from its
neighbors. neighbors.
A router stops including the Reverse Metric TLV in its hello messages 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 when it needs its neighbors to go back to using their own provisioned
metric values. When that happens, a router which had modified its metric values. When this happens, a router that had modified its
metric in response to receiving a Reverse Metric TLV from its metric in response to receiving a Reverse Metric TLV from its
neighbor should revert back to using its original provisioned metric neighbor should revert to using its original provisioned metric
value. value.
In certain scenarios, it is possible that two or more routers start In certain scenarios, two or more routers may start the RM signaling
the RM signaling on the same link. This could create collision on the same link. This could create collision scenarios. The
scenarios. The following rules MUST be adopted by routers to ensure following rules MUST be adopted by routers to ensure that there is no
that there is no instability in the network due to churn in their instability in the network due to churn in their metric due to
metric due to signaling of RM: signaling of RM:
o The RM value that is signaled by a router to its neighbor MUST NOT 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 be derived from the reverse metric being signaled by any of its
neighbor on any of its links. neighbor on any of its links.
o The RM value that is signaled by a router MUST NOT be derived from o The RM value that is signaled by a router MUST NOT be derived from
its own metric which has been modified on account of a RM signaled 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 from any of its neighbors on any of its links. RM signaling from
other routers can affect the router's own metric advertised in its other routers can affect the router's metric advertised in its
Router-LSA. When deriving the RM values that a router signals to Router-LSA. When deriving the RM values that a router signals to
its neighbors, it should use its "original" local metric values its neighbors, it should use its "original" local metric values
not influenced by any RM signaling. not influenced by any RM signaling.
Based on these rules, a router MUST never start or stop or change its Based on these rules, a router MUST never start or stop or change its
RM metric signaling based on the RM metric signaling initiated by RM metric signaling based on the RM metric signaling initiated by
some other router. Based on the local configuration policy, each some other router. Based on the local configuration policy, each
router would end up accepting the RM value signaled by its neighbor 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 and there would be no churn of metrics on the link or the network on
account of RM signaling. account of RM signaling.
In certain use-case as described in Section 2.1 when symmetrical In certain use-case as described in Section 2.1 when symmetrical
metrics are desired, the RM signaling can be enabled on routers on metrics are desired, the RM signaling can be enabled on routers on
either ends of a link. In other use-cases as described in either ends of a link. In other use-cases as described in
Section 2.2 RM signaling may need to be enabled on only router at one Section 2.2 RM signaling may need to be enabled only on the router at
end of a link. one end of a link.
When using multi-topology routing with OSPF [RFC4915] a router MAY When using multi-topology routing with OSPF [RFC4915] a router MAY
include multiple instances of the Reverse Metric TLV in the LLS block include multiple instances of the Reverse Metric TLV in the LLS block
of its hello message - one for each of the topology for which it of its hello message - one for each of the topology for which it
desires to signal the reserve metric for. desires to signal the reserve metric for.
In certain scenarios, the OSPF router may also require the In certain scenarios, the OSPF router may also require the
modification of the TE metric being advertised by its neighbor router modification of the TE metric being advertised by its neighbor router
towards itself in the inbound direction. The Reverse TE Metric TLV, towards itself in the inbound direction. The Reverse TE Metric TLV,
using similar procedures as described above, MAY be used to signal using similar procedures as described above, MAY be used to signal
the reverse TE metric by a router. The neighbor SHOULD use the the reverse TE metric by a router. The neighbor SHOULD use the
reverse TE metric value to derive the TE metric to be used in the TE reverse TE metric value to derive the TE metric to be used in the TE
Metric sub-TLV of the Link TLV in its TE Opaque LSA [RFC3630]. Metric sub-TLV of the Link TLV in its TE Opaque LSA [RFC3630].
7. Backward Compatibility 7. Backward Compatibility
The signaling specified in this document happens at a link-local The signaling specified in this document happens at a link-local
level between routers on that link. A router which does not support level between routers on that link. A router that does not support
this specification would ignore the Reverse Metric and Reverse TE this specification would ignore the Reverse Metric and Reverse TE
Metric LLS TLVs and take no actions to updates its metric in the Metric LLS TLVs and take no actions to updates its metric in the
other LSAs. As a result, the behavior would be the same as before other LSAs. As a result, the behavior would be the same as before
this specification. Therefore, there are no backward compatibility this specification. Therefore, there are no backward compatibility
related issues or considerations that need to be taken care of when related issues or considerations that need to be taken care of when
implementing this specification. implementing this specification.
8. IANA Considerations 8. IANA Considerations
This specification updates Link Local Signalling TLV Identifiers This specification updates Link Local Signalling TLV Identifiers
registry. registry.
Following values are requested for allocation: Following values have been assigned via early allocation:
o TBD (Suggested value 19) - Reverse Metric TLV o 19 - Reverse Metric TLV
o TBD (Suggested value 20) - Reverse TE Metric TLV o 20 - Reverse TE Metric TLV
9. Security Considerations 9. Security Considerations
The security considerations for "OSPF Link-Local Signaling" [RFC5613] The security considerations for "OSPF Link-Local Signaling" [RFC5613]
also apply to the extension described in this document. The usage of 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 reverse metric TLVs is to alter the metrics used by routers on
the link and influence the flow and routing of traffic over the the link and influence the flow and routing of traffic over the
network. Hence, modification of the Reverse Metric and Reverse TE network. Hence, modification of the Reverse Metric and Reverse TE
Metric TLVs may result in misrouting of traffic. If authentication Metric TLVs may result in misrouting of traffic. If authentication
is being used in the OSPF routing domain [RFC5709][RFC7474], then the is being used in the OSPF routing domain [RFC5709][RFC7474], then the
Cryptographic Authentication TLV [RFC5613] SHOULD also be used to Cryptographic Authentication TLV [RFC5613] SHOULD also be used to
protect the contents of the LLS block. protect the contents of the LLS block.
Receiving a malformed LLS Reverse Metric or Reverse TE Metric TLVs Receiving a malformed LLS Reverse Metric or Reverse TE Metric TLVs
MUST NOT result in a hard router or OSPF process failure. The MUST NOT result in a hard router or OSPF process failure. The
reception of malformed LLS TLVs or sub-TLVs SHOULD be logged, but reception of malformed LLS TLVs or sub-TLVs SHOULD be logged, but
such logging MUST be rate- limited to prevent denial-of-service (DoS) such logging MUST be rate-limited to prevent denial-of-service (DoS)
attacks. attacks.
10. Contributors 10. Contributors
Thanks to Jay Karthik for his contributions on the use-cases related Thanks to Jay Karthik for his contributions on the use-cases related
to symmetric metric and the review of the solution. to symmetric metric and the review of the solution.
11. Acknowledgements 11. Acknowledgements
12. References 12. References
 End of changes. 34 change blocks. 
76 lines changed or deleted 76 lines changed or added

This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/