draft-ietf-lsr-ospf-bfd-strict-mode-00.txt   draft-ietf-lsr-ospf-bfd-strict-mode-01.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 9, 2020 A. Fu Expires: January 1, 2021 A. Fu
Bloomberg Bloomberg
M. Rajesh M. Rajesh
Juniper Networks Juniper Networks
January 6, 2020 June 30, 2020
OSPF Strict-Mode for BFD OSPF Strict-Mode for BFD
draft-ietf-lsr-ospf-bfd-strict-mode-00 draft-ietf-lsr-ospf-bfd-strict-mode-01
Abstract Abstract
This document specifies the extensions to OSPF that enables a router This document specifies the extensions to OSPF that enable an OSPF
and its neighbor to signal their intention to use Bidirectional router to signal the requirement for a Bidirectional Forwarding
Forwarding Detection (BFD) for their adjacency using link-local Detection (BFD) session prior to adjacency formation. Link-Local
advertisement between them. The signaling of this BFD enablement, Signaling (LLS) is used to advertise this requirement of "strict-
allows the router to block and not allow the establishment of mode" of BFD session establishment for OSPF adjacency. If both OSPF
adjacency with its neighbor router until a BFD session is neighbors advertise the "strict-mode" of BFD, adjacency formation
successfully established between them. The document describes this will be blocked until a BFD session has been successfully
OSPF "strict-mode" of BFD establishment as a prerequisite to established.
adjacency formation.
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.
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 9, 2020.
This Internet-Draft will expire on January 1, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 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
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. LLS B-bit Flag . . . . . . . . . . . . . . . . . . . . . . . 3 2. LLS B-bit Flag . . . . . . . . . . . . . . . . . . . . . . . 3
3. Local Interface IPv4 Address TLV . . . . . . . . . . . . . . 4 3. Local Interface IPv4 Address TLV . . . . . . . . . . . . . . 3
4. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. OSPFv3 IPv4 Address-Family Specifics . . . . . . . . . . 6 4.1. OSPFv3 IPv4 Address-Family Specifics . . . . . . . . . . 6
4.2. Graceful Restart Considerations . . . . . . . . . . . . . 6 4.2. Graceful Restart Considerations . . . . . . . . . . . . . 6
5. Operations & Management Considerations . . . . . . . . . . . 6 5. Operations & Management Considerations . . . . . . . . . . . 6
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . 7 6. Backward Compatibility . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9 10.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction 1. Introduction
Bidirectional Forwarding Detection (BFD) [RFC5880] enables routers to Bidirectional Forwarding Detection (BFD) [RFC5880] enables routers to
monitor dataplane connectivity over links between them and to detect monitor dataplane connectivity and to detect faults in the
faults in the bidirectional path between them. This capability is bidirectional path between them. BFD is leveraged by routing
leveraged by routing protocols like Open Shortest Path First (OSPFv2) protocols like OSPFv2[RFC2328] and OSPFv3 [RFC5340] to detect
[RFC2328] and OSPFv3 [RFC5340] to detect connectivity failures for connectivity failures for established adjacencies and trigger the
their adjacencies and trigger the rerouting of traffic around this rerouting of traffic around the failure more quickly than with OSPF
failure more quickly than their periodic hello messaging based hello packet monitoring.
detection mechanism.
The use of BFD for monitoring routing protocols adjacencies is The use of BFD for monitoring routing protocols adjacencies is
described in [RFC5882]. When BFD monitoring is enabled for OSPF described in [RFC5882]. When BFD monitoring is enabled for OSPF
adjacencies, the BFD session is bootstrapped based on the neighbor adjacencies, the BFD session is bootstrapped based on the neighbor
address information discovered by the exchange of OSPF hello address information discovered by the exchange of OSPF hello packets.
messages. Faults in the bidirectional forwarding detected via BFD Faults in the bidirectional forwarding detected via BFD then result
then result in the bringing down of the OSPF adjacency. Note that it in the OSPF adjacency being brought down. Note that it is possible
is possible in some failure scenarios for the network to be in a in some failure scenarios for the network to be in a state such that
state such that the OSPF adjacency is capable of coming up, but the an OSPF adjacency can be established but a BFD session cannot be
BFD session cannot be established, and, more particularly, data established and maintained. In certain other scenarios, a degraded
cannot be forwarded. In certain other scenarios, a degraded or poor or poor quality link may result in OSPF adjacency formation to
quality link may result in OSPF adjacency formation to succeed only succeed only to result in BFD session establishment not being
to result in BFD session establishment not being successful or the successful or flapping of the BFD session.
BFD session going down frequently due to its faster detection
mechanism.
To avoid such situations which result in routing churn in the To avoid the routing churn associated with these scenarios, it would
network, it would be beneficial not to allow OSPF to establish a be beneficial to not allow OSPF to establish an adjacency until a BFD
neighbor adjacency until the BFD session is successfully established session is successfully established and has stabilized. However,
and stabilized. However, this would preclude the OSPF operation in this would preclude the OSPF operation in an environment in which not
an environment in which not all OSPF routers support BFD and are all OSPF routers support BFD and are enabled for BFD on the link. A
enabled for BFD monitoring. A solution would be to block the solution is to block OSPF adjacency establishment until a BFD session
establishment of OSPF adjacencies if both systems are willing to is established as long as both neighbors advertise such a
establish a BFD session but a BFD session cannot be established. requirement. Such a mode of OSPF BFD usage is referred to as
Such a mode of BFD use by OSPF is referred to as "strict-mode" "strict-mode".
wherein BFD session establishment becomes a prerequisite for OSPF
adjacency coming up.
This document specifies the OSPF protocol extensions using link-local This document specifies the OSPF protocol extensions using link-local
signaling (LLS) [RFC5613] for a router to indicate to its neighbor signaling (LLS) [RFC5613] for a router to indicate to its neighbor
the willingness to establish a BFD session in the "strict-mode". It the willingness to establish a BFD session in the "strict-mode". It
also introduces an extension for OSPFv3 link-local signaling of also introduces an extension for OSPFv3 link-local signaling of
interface IPv4 address when used for IPv4 address-family (AF) interface IPv4 address when used for IPv4 address-family (AF)
instance to enable discovery of the IPv4 addresses for BFD session instance to enable discovery of the IPv4 addresses for BFD session
setup. setup.
A similar functionality for IS-IS is specified [RFC6213]. A similar functionality for IS-IS is specified [RFC6213].
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. LLS B-bit Flag 2. LLS B-bit Flag
A new B-bit is defined in the LLS Type 1 Extended Options and Flags This document defines the B-bit in the LLS Type 1 Extended Options
field. This bit is defined for the LLS block included in Hello and Flags field. This bit is defined for the LLS block included in
packets and indicates that BFD is enabled on the link and that the Hello packets and indicates that BFD is enabled on the link and that
router supports BFD strict-mode. Section 7 describes the position of the router requests BFD strict-mode. Section 7 describes the
this new B-bit. position of the B-bit.
A router MUST include the LLS block with the LLS Type 1 Extended A router MUST include the LLS block with the LLS Type 1 Extended
Options and Flags TLV with the B-bit set its Hello messages when BFD Options and Flags TLV with the B-bit set its Hello messages when BFD
is enabled on the link. is enabled on the link.
3. Local Interface IPv4 Address TLV 3. Local Interface IPv4 Address TLV
The Local Interface IPv4 Address TLV is a new LLS TLV meant for The Local Interface IPv4 Address TLV is an LLS TLV meant for OSPFv3
OSPFv3 protocol operations for IPv4 AF instances [RFC5838]. It has protocol operations for IPv4 AF instances [RFC5838]. It has
following format: 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Interface IPv4 Address | | Local Interface IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 4, line 31 skipping to change at page 4, line 25
Type: TBD, suggested value 21 Type: TBD, suggested value 21
Length: 4 octet Length: 4 octet
Local Interface IPv4 Address: The primary IPv4 address of the Local Interface IPv4 Address: The primary IPv4 address of the
local interface. local interface.
4. Procedures 4. Procedures
A router supporting BFD strict-mode advertises this capability A router supporting BFD strict-mode advertises this capability
through its hello messages as described in Section 2 above. When a through its hello messages as described in Section 2. When a router
router supporting BFD strict-mode, detects a new neighbor router that supporting BFD strict-mode discovers a new neighbor router that also
also supports BFD strict-mode, then it proceeds to establish supports BFD strict-mode, then it will establish a BFD session first
adjacency with that neighbor as described further in this section. with that neighbor before bringing up the OSPF adjacency as described
further in this section.
This document updates the OSPF neighbor state machine as described in This document updates the OSPF neighbor state machine as described in
[RFC2328] specifically the operations related to the Init state as [RFC2328]. Specifically, the operations related to the Init state as
below when BFD strict-mode is used: below when BFD strict-mode is used:
Init (without BFD strict-mode) Init (without BFD strict-mode)
In this state, an Hello packet has recently been seen from the In this state, a Hello packet has recently been received from the
neighbor. However, bidirectional communication has not yet been neighbor. However, bidirectional communication has not yet been
established with the neighbor (i.e., the router itself did not established with the neighbor (i.e., the router itself did not
appear in the neighbor's Hello packet). All neighbors in this appear in the neighbor's Hello packet). All neighbors in this
state (or higher) are listed in the Hello packets sent from the state (or higher) are listed in the Hello packets sent from the
associated interface. associated interface.
Init (with BFD strict-mode) Init (with BFD strict-mode)
In this state, an Hello packet has recently been seen from the In this state, an Hello packet has recently been received from the
neighbor. However, bidirectional communication has not yet been neighbor. However, bidirectional communication has not yet been
established with the neighbor (i.e., the router itself did not established with the neighbor (i.e., the router itself did not
appear in the neighbor's Hello packet). A BFD session appear in the neighbor's Hello packet). A BFD session
establishment to the neighbor is requested, if not already done establishment to the neighbor is requested, if not already done
(e.g. in the event of transition from 2-way state). All neighbors (e.g. in the event of transition from 2-way state). Neighbors in
in higher than Init state and those in Init state with BFD session Init state or higher will be listed in the Hello packets
up are listed in the Hello packets sent from the associated associated with the interface if they either have a corresponding
interface. BFD session established or have not advertised "strict-mode" BFD
in the Hello packet LLS Extended Options and Flags.
Whenever the neighbor state transitions to Down state, the removal of Whenever the neighbor state transitions to Down state, the removal of
the BFD session associated with that neighbor SHOULD be requested by the BFD session associated with that neighbor SHOULD be requested by
OSPF and the session re-setup SHOULD similarly be requested by OSPF OSPF and subsequent BFD session establishement SHOULD similarly be
after transitioning into Init state. This may result in the deletion requested by OSPF upon transitioning into Init state. This may
and creation of BFD session respectively when OSPF is the only client result in the deletion and creation of the BFD session respectively
interested in BFD session to the neighbor address. when OSPF is the only client interested in the BFD session to the
neighbor address.
An implementation MUST NOT wait for BFD session establishment in Init An implementation MUST NOT wait for BFD session establishment in Init
state unless BFD strict-mode is enabled on the router and the state unless BFD strict-mode is enabled on the router and the
specific neighbor indicates BFD strict-mode capability via its Hello specific neighbor indicates BFD strict-mode capability via its Hello
messages. When BFD is enabled, but the strict-mode of operation LLS options. When BFD is enabled, but the strict-mode of operation
cannot be used, then an implementation SHOULD start the BFD session has not be signaled by both neighbors, then an implementation SHOULD
establishment only in 2-Way or higher state. This makes it possible start the BFD session establishment only in 2-Way state or higher
for router to operate a mix of BFD operation in strict-mode or normal state. This makes it possible for an OSPF router to operate a mix of
mode across different interfaces or even different neighbors on the BFD operation in strict-mode or normal mode across different
same multi-access LAN interface. interfaces or even different neighbors on the same multi-access LAN
interface.
Once the OSPF state machine has moved beyond the Init state, any Once the OSPF state machine has moved beyond the Init state, any
change in the B-bit advertised in subsequent Hello messages MUST NOT change in the B-bit advertised in subsequent Hello messages MUST NOT
result in any trigger in either the OSPF adjacency or the BFD session result in any trigger in either the OSPF adjacency or the BFD session
management (i.e. the B-bit is considered only when in the Init management (i.e., the B-bit is considered only when in the Init
state). The disabling of BFD (or BFD strict-mode) on a router would state). Disabling BFD (or BFD strict-mode) on an OSPF router would
result in its not setting the B-bit in its subsequent Hello messages. result in it not setting the B-bit in its subsequent Hello LLS
The disabling of BFD strict-mode has no change on the BFD operations options. Disabling BFD strict-mode has no effect on the BFD
and would not result in bringing down of any established BFD session. operations and would not result in bringing down of any established
The disabling of BFD would result in the BFD session brought down due BFD session. Disabling BFD would result in the BFD session brought
to Admin reason and hence would not bring down the OSPF adjacency. down due to Admin reason and hence would not bring down the OSPF
adjacency.
When BFD is enabled on an interface over which we already have an When BFD is enabled on an interface over which we already have an
existing OSPF adjacency, it would result in the router setting the existing OSPF adjacency, it would result in the router setting the
B-bit in its subsequent Hello messages. If the adjacency is already B-bit in its subsequent Hello messages. If the adjacency is already
up (i.e. in its terminal state of Full or 2-way with non-DR routers up (i.e., in its terminal state of Full or 2-way with non-DR routers
on a LAN) with a neighbor that also support BFD strict-mode, then an on a LAN) with a neighbor that also supports BFD strict-mode, then an
implemantion SHOULD NOT bring this adjacency down and instead use the implemantion SHOULD NOT bring this adjacency down but instead use the
BFD strict-mode of operations after the next transition into Init BFD strict-mode of operation after the next transition into Init
state. However, if the adjacency is not up, then an implementation state. However, if the adjacency is not up, then an implementation
MAY bring such an adjacency down so it can use the BFD strict-mode MAY bring such an adjacency down so it can use the BFD strict-mode
for its bring up. for its bring up.
4.1. OSPFv3 IPv4 Address-Family Specifics 4.1. OSPFv3 IPv4 Address-Family Specifics
The multiple AF support in OSPFv3 [RFC5838] requires the use of IPv6 Multiple AF support in OSPFv3 [RFC5838] requires the use of an IPv6
link-local address as source address for hello packets even when link-local address as the source address for hello packets even when
forming adjacencies for IPv4 AF instances. In most deployments of forming adjacencies for IPv4 AF instances. In most deployments of
OSPFv3 IPv4 AF, it is required that BFD be used to monitor and verify OSPFv3 IPv4 AF, it is required that BFD is used to monitor and verify
the IPv4 data plane connectivity between the routers on the link and the IPv4 data plane connectivity between the routers on the link and,
hence the BFD session is setup using IPv4 neighbor addresses. The hence, the BFD session is setup using IPv4 neighbor addresses. The
IPv4 neighbor address on the interface is learnt only later in the IPv4 neighbor address on the interface is learnt only later in the
adjacency formation phase when the neighbor's Link-LSA is received. adjacency formation process when the neighbor's Link-LSA is received.
This results in the setup of the BFD session either after the This results in the setup of the BFD session either after the
adjacency is established or much later in the adjacency formation adjacency is established or later in the adjacency formation
sequence. sequence.
To enable the BFD operations in strict-mode, it is necessary for a To enable BFD operation in strict-mode, it is necessary for an OSPF
router to learn it's neighbor's IPv4 link address during the Init router to learn it's neighbor's IPv4 link address during the Init
state of adjacency formation (ideally when it receives the first state of adjacency formation (ideally when it receives the first
hello). The use of the Local Interface IPv4 Address TLV (as defined hello). The use of the Local Interface IPv4 Address TLV (as defined
in Section 3) in the LLS block of the OSPFv3 Hello messages for IPv4 in Section 3) in the LLS block of the OSPFv3 Hello messages for IPv4
AF instances makes this possible. Implementations that support AF instances makes this possible. Implementations that support
strict-mode of BFD operations for OSPFv3 IPv4 AF instances MUST strict-mode of BFD operation for OSPFv3 IPv4 AF instances MUST
include the Local Interface IPv4 Address TLV in the LLS block of include the Local Interface IPv4 Address TLV in the LLS block of
their hello messages whenever the B-bit is set. A receiver MUST their hello messages whenever the B-bit is also set in the LLS
ignore the B-bit (i.e. not operate in BFD strict mode) unless the Options and Flags field. A receiver MUST ignore the B-bit (i.e., not
Local Interface IPv4 Address TLV is present in OSPFv3 Hello message operate in BFD strict mode) when the Local Interface IPv4 Address TLV
for IPv4 AF instances. is not present in OSPFv3 Hello message for IPv4 AF OSPFv3 instances.
4.2. Graceful Restart Considerations 4.2. Graceful Restart Considerations
An implementation needs to handle scenarios where both graceful An implementation needs to handle scenarios where both graceful
restart (GR) and the strict-mode of BFD operations are deployed restart (GR) and the strict-mode of BFD operation are deployed
together. The GR aspects discussed in [RFC5882] also apply with together. The GR aspects discussed in [RFC5882] also apply with
strict-mode of operations. In addition to that, since the OSPF strict-mode of BFD operation. Additionally, in strict-mode of BFD
adjacency formation is held up until the BFD session establishment in operation, since the OSPF adjacency formation is delayed until the
the strict-mode of operation, the resultant delay in adajcency BFD session establishment, the resultant delay in adajcency formation
formation may affect or break the GR based recovery. In such cases, may affect or break the GR-based recovery. In such cases, it is
it is RECOMMENDED that the GR timers are setup such that they provide RECOMMENDED that the GR timers are set such that they provide
sufficient time to cover for normal BFD session establishment delays. sufficient time to allow for normal BFD session establishment delays.
5. Operations & Management Considerations 5. Operations & Management Considerations
An implementation SHOULD report the BFD session status along with the An implementation SHOULD report the BFD session status along with the
OSPF Init adjacency state when operating in BFD strict-mode and OSPF Init adjacency state when operating in BFD strict-mode and
perform logging operations on state transitions to include the BFD perform logging operations on state transitions to include the BFD
events. This allows an operator to detect scenarios where an OSPF events. This allows an operator to detect scenarios where an OSPF
adjacency may be stuck waiting for BFD session establishment. adjacency may be stuck waiting for BFD session establishment.
In network deployments with noisy links or those with packet loss, In network deployments with noisy links or those with packet loss,
BFD sessions may flap frequently. In such scenarions, OSPF strict- BFD sessions may flap frequently. In such scenarions, OSPF strict-
mode for BFD may be deployed in conjunction with an BFD dampening or mode for BFD may be deployed in conjunction with a BFD dampening or
hold-down mechanism to help avoid frequent adjacency flaps due BFD hold-down mechanism to help avoid frequent adjacency flaps that cause
causing routing churn. routing churn.
6. Backward Compatibility 6. Backward Compatibility
An implementation MUST support OSPF adjacency formation and An implementation MUST support OSPF adjacency formation and
operations with a neighbor router that does not advertise the BFD operations with a neighbor router that does not advertise the BFD
strict-mode capability - both when that neighbor router does not strict-mode capability - both when that neighbor router does not
support BFD and when it does support BFD but not in the strict-mode support BFD and when it does support BFD but not in the strict-mode
of operation as described in this document. Implementations MAY of operation as described in this document. Implementations MAY
provide an option to specifically enable BFD operations only in the provide an option to specifically enable BFD operations only in the
strict-mode in which case, OSPF adjacency with a neighbor that does strict-mode. In this case, an OSPF adjacency with a neighbor that
not support BFD strict-mode would not be established successfully. does not support BFD strict-mode would not be established
Implementations MAY provide an option to disable BFD strict-mode successfully. Implementations MAY provide an option to disable BFD
which results in the router not advertising the B-bit and BFD strict-mode which results in the router not advertising the B-bit and
operations being performed in the same way as before this BFD operations being performed in the same way as prior to this
specification. specification.
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 B-bit in the LLS block of hello this specification would ignore the B-bit in the LLS block of hello
messages from its neighbors and continue to bootstrap BFD sessions, messages from its neighbors and continue to establish BFD sessions,
if enabled, without holding back the OSPF adjacency formation. Since if enabled, without delaying the OSPF adjacency formation. Since the
the router which does not support this specification would not have router that does not support this specification would not have set
set the B-bit in the LLS block of its own hello messages, its the B-bit in the LLS block of its own hello messages, its neighbor
neighbor routers that support this specification would not use BFD routers that support this specification would not use BFD strict-mode
strict-mode with it. As a result, the behavior would be the same as with such OSPF routers. As a result, the behavior would be the same
before this specification. Therefore, there are no backward as before this specification. Therefore, there are no backward
compatibility related issues or considerations that need to be taken compatibility issues or implementations considerations beyond what is
care of when implementing this specification. specified herein.
7. IANA Considerations 7. IANA Considerations
This specification updates Link Local Signaling TLV Identifiers This specification updates Link Local Signaling TLV Identifiers
registry. registry.
Following values are requested for allocation: Following values are requested for allocation:
o B-bit from "LLS Type 1 Extended Options and Flags" registry at bit o B-bit from "LLS Type 1 Extended Options and Flags" registry at bit
position 0x00000010. position 0x00000010.
skipping to change at page 8, line 20 skipping to change at page 8, line 20
message could prevent an OSPF adjacency from forming or lead to message could prevent an OSPF adjacency from forming or lead to
failure to detect bidirectional forwarding failures. If failure to detect bidirectional forwarding failures. If
authentication is being used in the OSPF routing domain authentication is being used in the OSPF routing domain
[RFC5709][RFC7474], then the Cryptographic Authentication TLV [RFC5709][RFC7474], then the Cryptographic Authentication TLV
[RFC5613] SHOULD also be used to protect the contents of the LLS [RFC5613] SHOULD also be used to protect the contents of the LLS
block. block.
9. Acknowledgements 9. Acknowledgements
The authors would like to acknowledge the review and inputs from Acee The authors would like to acknowledge the review and inputs from Acee
Lindem, Manish Gupta, Balaji Ganesh and Rajesh M. Lindem, Manish Gupta and Balaji Ganesh.
The authors would like to acknowledge Dylan van Oudheusden for The authors would like to acknowledge Dylan van Oudheusden for
highlighting the problems in using strict-mode for BFD session for highlighting the problems in using strict-mode for BFD session for
IPv4 AF instance with OSPFv3 and Baalajee S for his suggestions on IPv4 AF instance with OSPFv3 and Baalajee S for his suggestions on
the approach to address it. the approach to address it.
10. References 10. References
10.1. Normative References 10.1. Normative References
 End of changes. 36 change blocks. 
129 lines changed or deleted 130 lines changed or added

This html diff was produced by rfcdiff 1.47. The latest version is available from http://tools.ietf.org/tools/rfcdiff/