wdiff rfc7880.original rfc7880.txt

Internet Engineering Task Force (IETF) C. Pignataro
Internet-Draft
Request for Comments: 7880 D. Ward
Updates: 5880 (if approved) Cisco
Intended status:
Category: Standards Track N. Akiya
Expires: November 7, 2016
ISSN: 2070-1721 Big Switch Networks
M. Bhatia
Ionos Networks
S. Pallagatti
May 6,
July 2016

Seamless Bidirectional Forwarding Detection (S-BFD)
draft-ietf-bfd-seamless-base-11

Abstract

This document defines a simplified mechanism to use Seamless Bidirectional Forwarding Detection (BFD) (S-
BFD), a simplified mechanism for using BFD with a large portions proportion of
negotiation aspects eliminated, thus providing benefits such as quick provisioning
provisioning, as well as improved control and flexibility to for network
nodes initiating
the path monitoring.

This document updates RFC5880.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. 5880.

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 an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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 November 7, 2016.
http://www.rfc-editor.org/info/rfc7880.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 ....................................................4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 .....................................................4
3. Seamless BFD Overview . . . . . . . . . . . . . . . . . . . . 5 ...........................................6
4. S-BFD Discriminators . . . . . . . . . . . . . . . . . . . . 6 ............................................7
4.1. S-BFD Discriminator Uniqueness . . . . . . . . . . . . . 6 .............................7
4.2. Discriminator Pools . . . . . . . . . . . . . . . . . . . 7 ........................................7
5. Reflector BFD Session . . . . . . . . . . . . . . . . . . . . 7 ...........................................8
6. State Variables . . . . . . . . . . . . . . . . . . . . . . . 8 .................................................9
6.1. New State Variables . . . . . . . . . . . . . . . . . . . 8 ........................................9
6.2. State Variable Initialization and Maintenance . . . . . . 9 ..............9
7. S-BFD Procedures . . . . . . . . . . . . . . . . . . . . . . 9 ...............................................10
7.1. Demultiplexing of S-BFD Control Packet . . . . . . . . . 9 ....................10
7.2. Responder Procedures . . . . . . . . . . . . . . . . . . 10 ......................................11
7.2.1. Responder Demultiplexing . . . . . . . . . . . . . . 10 ...........................11
7.2.2. Transmission of S-BFD Control Packet by
SBFDReflector 10 ......................................11
7.2.3. Additional SBFDReflector Behaviors . . . . . . . . . 11 .................12
7.3. Initiator Procedures . . . . . . . . . . . . . . . . . . 12 ......................................13
7.3.1. SBFDInitiator State Machine . . . . . . . . . . . . . 12 ........................14
7.3.2. Transmission of S-BFD Control Packet by
SBFDInitiator 13 ......................................15
7.3.3. Additional SBFDInitiator Behaviors . . . . . . . . . 14 .................15
7.4. Diagnostic Values . . . . . . . . . . . . . . . . . . . . 14 .........................................16
7.5. The Poll Sequence . . . . . . . . . . . . . . . . . . . . 15 .........................................16
8. Operational Considerations . . . . . . . . . . . . . . . . . 15 .....................................16
8.1. Scaling Aspect . . . . . . . . . . . . . . . . . . . . . 15 ............................................17
8.2. Congestion Considerations . . . . . . . . . . . . . . . . 16 .................................17
9. Co-existence with Classical BFD Sessions . . . . . . . . . . 16 .......................17
10. S-BFD Echo Function . . . . . . . . . . . . . . . . . . . . . 16 ...........................................18
11. Security Considerations . . . . . . . . . . . . . . . . . . . 17 .......................................19
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 19
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
15.1. ....................................................20
12.1. Normative References . . . . . . . . . . . . . . . . . . 19
15.2. .....................................20
12.2. Informative References . . . . . . . . . . . . . . . . . 19 ...................................20
Appendix A. Loop Problem and Solution . . . . . . . . . . . . . 20 .............................22
Acknowledgements ..................................................23
Contributors ......................................................23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 ................................................24

1. Introduction

Bidirectional Forwarding Detection (BFD), as described in [RFC5880]
and related documents, has efficiently generalized the failure
detection mechanism for multiple protocols and applications. There
are some improvements that can be made to better fit existing
technologies. There is a possibility of evolving BFD to better fit
new technologies. This document focuses on several aspects of BFD in
order to further improve efficiency, to expand failure detection
coverage
coverage, and to allow BFD usage for wider scenarios. Additional use
cases are listed in [I-D.ietf-bfd-seamless-use-case]. [RFC7882].

Specifically, this document defines Seamless Bidirectional Forwarding
Detection (S-BFD) (S-BFD), a simplified mechanism to use Bidirectional
Forwarding Detection (BFD) for using BFD with a large portions
proportion of negotiation aspects eliminated, thus providing benefits
such as quick provisioning provisioning, as well as improved control and
flexibility to for network nodes initiating
the path monitoring. S-BFD
enables cases benefiting from the use of core BFD technologies in a
fashion that leverages existing implementations and protocol
machinery while providing a rather simplified and largely stateless
infrastructure for continuity testing.

One key aspect of the mechanism described in this document eliminates
the time between a network node wanting to perform a continuity test
and completing the continuity test. In traditional BFD terms, the
initial state changes from DOWN to UP are virtually nonexistent.
Removal of this seam "seam" (i.e., time delay) in BFD provides applications a smooth
and continuous operational experience. experience for applications. Therefore,
"Seamless BFD" (S-BFD) has been chosen as the name for this
mechanism.

2. Terminology

The reader is expected to be familiar with the BFD [RFC5880], IP
[RFC0791] [RFC2460]
[RFC791] [RFC2460], and MPLS [RFC3031] terminologies terms and protocol constructs. This
The remainder of this section describes several new terminologies terms introduced
by S-BFD.

o Classical BFD - BFD session types based on [RFC5880].

o S-BFD - Seamless BFD.

o S-BFD control Control packet - a BFD control Control packet for the S-BFD
mechanism.

o S-BFD echo Echo packet - a BFD echo Echo packet for the S-BFD mechanism.

o S-BFD packet - a BFD control Control packet or a BFD echo Echo packet.

o Entity - a function on a network node that to which the S-BFD mechanism
allows remote network nodes to perform continuity test to. tests. An
entity can be abstract (e.g., reachability) or specific (e.g., IP
addresses,
router-IDs, Router-IDs, functions).

o SBFDInitiator - an S-BFD session on a network node that performs a
continuity test to a remote entity by sending S-BFD packets.

o SBFDReflector - an S-BFD session on a network node that listens
for incoming S-BFD control Control packets to local entities and generates
response S-BFD control Control packets.

o Reflector BFD session - synonymous with SBFDReflector.

o S-BFD discriminator Discriminator - a BFD discriminator Discriminator allocated for a local
entity and is being listened by an SBFDReflector.
entity. An SBFDReflector listens for S-BFD Discriminators.

o BFD discriminator Discriminator - a BFD discriminator Discriminator allocated for an
SBFDInitiator.

o Initiator - a network node hosting an SBFDInitiator.

o Responder - a network node hosting an SBFDReflector.

Figure 1 describes the relationship between S-BFD terminologies. terms.

+---------------------+ +------------------------+
| Initiator | | Responder |
| +-----------------+ | | +-----------------+ |
| | SBFDInitiator |---S-BFD ctrl Ctrl pkt----->| SBFDReflector | |
| | +-------------+ |<--S-BFD ctrl Ctrl pkt------| +-------------+ | |
| | | BFD discrim Discrim | | | | | |S-BFD discrim| Discrim| | |
| | | | |---S-BFD echo Echo pkt---+ | | | | |
| | +-------------+ | | | | | +----------^--+ | |
| +-----------------+<-------------------+ +------------|----+ |
| | | | |
| | | +---v----+ |
| | | | Entity | |
| | | +--------+ |
+---------------------+ +------------------------+

Figure 1: S-BFD Terminology Relationship

3. Seamless BFD Overview

An S-BFD module on each network node allocates one or more S-BFD
discriminators
Discriminators for local entities, entities and creates a reflector Reflector BFD
session. Allocated S-BFD discriminators Discriminators may be advertised by
applications (e.g., OSPF/IS-IS). Required result The required result is that
applications,
applications on other network nodes, possess the knowledge of nodes will know about the S-BFD discriminators
Discriminators allocated by a remote node to remote entities. The reflector
Reflector BFD session is to, session, upon receiving an S-BFD control Control packet
targeted to one of the local S-BFD discriminator Discriminator values, is to
transmit a response S-BFD control Control packet back to the initiator.

Once the above setup is complete, any network node, having the
knowledge of node that knows about
the S-BFD discriminator Discriminator allocated by a remote node to a remote entity/entities, entity
or entities can quickly perform a continuity test to the remote
entity by simply sending S-BFD control Control packets with a corresponding
S-BFD discriminator Discriminator value in the "your discriminator" Your Discriminator field.

This is exemplified in Figure 2.

<------- IS-IS Network ------->

+---------+
| |
A---------B---------C---------D
^ ^
| |
SystemID SystemID
System-ID System-ID
xxx yyy
BFD Discrim BFD Discrim
123 456

Figure 2: S-BFD for IS-IS Network

An S-BFD module in a system with IS-IS SystemID System-ID xxx (node (Node A)
allocates an S-BFD discriminator Discriminator 123, and IS-IS advertises the S-BFD discriminator
Discriminator 123 in an IS-IS TLV. An S-BFD module in a system with
IS-IS SystemID System-ID yyy (node (Node D) allocates an S-BFD discriminator Discriminator 456,
and IS-IS advertises the S-BFD discriminator Discriminator 456 in an IS-IS TLV. A reflector
Reflector BFD session is created on both network nodes (node (Node A and node
Node D). When network node Node A wants to check the reachability to network node of Node D, node
Node A can send an S-BFD control packet, Control packet destined to node D, Node D with
"your discriminator" the
Your Discriminator field set to 456. When the reflector Reflector BFD session
on node Node D receives this S-BFD control Control packet, then a response S-BFD control
Control packet is sent back to node Node A, which allows node Node A to
complete the continuity test.

When a node allocates multiple S-BFD discriminators, Discriminators, how remote nodes
determine which of the discriminators is associated with a specific
entity is currently unspecified. The use of multiple S-BFD
discriminators
Discriminators by a single network node is therefore discouraged
until a means of learning the mapping is defined.

4. S-BFD Discriminators

4.1. S-BFD Discriminator Uniqueness

One important characteristic of an S-BFD discriminator Discriminator is that it
MUST be unique within an administrative domain. If multiple network
nodes allocated allocate the same S-BFD discriminator Discriminator value, then S-BFD
control Control
packets falsely terminating on a wrong network node can result in a reflector
Reflector BFD session to generate generating a response back, due to
"your discriminator" matching. back because of a
matching Your Discriminator value. This is clearly not desirable.

4.2. Discriminator Pools

This subsection describes a discriminator pool implementation
technique to minimize S-BFD discriminator Discriminator collisions. The result This technique
will allow an implementation to better satisfy the S-BFD
discriminator
Discriminator uniqueness requirement defined in Section 4.1.

o An SBFDInitiator is to allocate a discriminator from the BFD
discriminator
Discriminator pool. If the system also supports classical BFD
that runs on [RFC5880],
(i.e., implements [RFC5880]), then the BFD discriminator Discriminator pool
SHOULD be shared by SBFDInitiator sessions and classical BFD
sessions.

o An SBFDReflector is to allocate a discriminator from the S-BFD
discriminator
Discriminator pool. The S-BFD discriminator Discriminator pool SHOULD be a
separate pool than from the BFD discriminator Discriminator pool.

The remainder of this subsection describes the reasons for the
suggestions above.

Locally allocated S-BFD discriminator Discriminator values for entities, listened
by entities that
SBFDReflector sessions, sessions are listening for may be arbitrary arbitrarily allocated
or derived from values provided by applications. These values may be
protocol IDs (e.g., System-ID, Router-ID) or network targets (e.g.,
IP address). To avoid derived S-BFD discriminator Discriminator values already
being assigned to other BFD sessions (i.e., SBFDInitiator sessions
and classical BFD sessions), it is RECOMMENDED that the discriminator
pool for SBFDReflector sessions be separate from other BFD sessions.

Even when following the separate "separate discriminator pool pool" approach, a
collision is still possible between one S-BFD application to another different S-BFD application, applications that
may be using different values and algorithms to derive S-BFD discriminator
Discriminator values. If the two applications are using S-BFD for the
same purpose (e.g., network reachability), then the colliding S-BFD discriminator
Discriminator value can be shared. If the two applications are using
S-BFD for a different purpose, then the collision must be addressed.
The use of multiple S-BFD
discriminators Discriminators by a single network node,
however, is discouraged (see Section 3).

5. Reflector BFD Session

Each network node creates one or more reflector Reflector BFD sessions. This
reflector
Reflector BFD session is a session that transmits S-BFD control Control
packets in response to received S-BFD control Control packets with "your
discriminator" the
Your Discriminator field having S-BFD discriminators Discriminators allocated for
local entities. Specifically, this reflector Reflector BFD session has the
following characteristics:

o MUST NOT transmit any S-BFD packets based on local timer expiry.

o MUST transmit an S-BFD control Control packet in response to a received
S-BFD control Control packet having a valid S-BFD discriminator Discriminator in the
"your discriminator"
Your Discriminator field, unless prohibited by local policies
(e.g., administrative, security, rate-limiter, etc.) rate-limiter).

o MUST be capable of sending only two states: UP and ADMINDOWN. AdminDown.

One reflector Reflector BFD session may be responsible for handling received
S-BFD control Control packets targeted to all locally allocated S-BFD
discriminators,
Discriminators, or a few reflector Reflector BFD sessions may each be
responsible for a subset of locally allocated S-BFD discriminators. Discriminators.
This policy is a local matter, matter and is outside the scope of this
document.

Note that incoming S-BFD control Control packets may be based on IPv4, IPv6 IPv6,
or MPLS
based [I-D.ietf-bfd-seamless-ip], and [RFC7881]. Note also that other options are possible and
can may
be defined in future documents. How such S-BFD control Control packets reach
an appropriate reflector Reflector BFD session is also a local matter, matter and is
outside the scope of this document.

6. State Variables

S-BFD introduces new state variables, variables and modifies the usage of
existing ones.

6.1. New State Variables

A new state variable is added to the base specification in support
of S-BFD.

o bfd.SessionType: This is a new state variable that describes
the type of this a particular session. Allowable values for S-BFD
sessions are:

* SBFDInitiator - an S-BFD session on a network node that
performs a continuity test to a target entity by sending S-BFD
packets.

* SBFDReflector - an S-BFD session on a network node that listens
for incoming S-BFD control Control packets to local entities and
generates response S-BFD control Control packets.

The bfd.SessionType variable MUST be initialized to the appropriate
type when an S-BFD session is created.

6.2. State Variable Initialization and Maintenance

A state variable defined

State variables (defined in Section 6.8.1 of [RFC5880] [RFC5880]) need to
be initialized or manipulated differently differently, depending on the
session type.

o bfd.DemandMode: This variable MUST be initialized to 1 for session
type SBFDInitiator, SBFDInitiator and MUST be initialized to 0 for session type
SBFDReflector. This is done to prevent loops (see Appendix A).

7. S-BFD Procedures

7.1. Demultiplexing of S-BFD Control Packet

An S-BFD packet MUST be demultiplexed with lower layer lower-layer information
(e.g., dedicated destination UDP port [I-D.ietf-bfd-seamless-ip], [RFC7881], associated channel type [I-D.ietf-pals-seamless-vccv]). Channel
Type [RFC7885]). The following procedure SHOULD be executed on both
initiator and
reflector. reflector:

If the packet is an S-BFD packet

If the S-BFD packet is for an SBFDReflector

Packet

The packet MUST be looked up to locate a corresponding
SBFDReflector session based on the value from the "your
discriminator"
Your Discriminator field in the table describing S-BFD
discriminators.
Discriminators.

Else

Packet

The packet MUST be looked up to locate a corresponding
SBFDInitiator session or classical BFD session based on the
value from the "your discriminator" Your Discriminator field in the table
describing BFD discriminators. Discriminators. If no match match, then the
received packet MUST be discarded.

If the session is an SBFDInitiator

Destination session

The destination of the packet (i.e., the destination IP
address) SHOULD be validated to be verified as being for self. itself.

Else

Packet

The packet MUST be discarded discarded.

Else

Procedure

The procedure described in Section 6.8.6 of [RFC5880] MUST be
applied.

More details on S-BFD control Control packet demultiplexing are described provided in
relevant S-BFD data plane data-plane documents.

7.2. Responder Procedures

A network node that receives S-BFD control Control packets transmitted by an
initiator is referred to as the responder. The responder, upon
reception of S-BFD control Control packets, is to perform necessary relevant validations verify the validity of the
packets, as described in [RFC5880].

7.2.1. Responder Demultiplexing

An S-BFD packet MUST be demultiplexed with lower layer lower-layer information.
The following procedure SHOULD be executed by the responder:

If "your discriminator" the Your Discriminator field is not one of the entry entries
allocated for local entities

Packet

The packet MUST be discarded.

Else

Packet

The packet is determined to be handled by a reflector Reflector BFD
session responsible for that S-BFD discriminator. Discriminator.

If allowable per local policy allows (e.g., administrative, security, rate-
limiter, etc.)

Chosen reflector
rate-limiter)

The chosen Reflector BFD session SHOULD transmit a response
BFD
control Control packet using the procedures described in
Section 7.2.2.

7.2.2. Transmission of S-BFD Control Packet by SBFDReflector

Contents

The contents of S-BFD control Control packets sent by an SBFDReflector MUST
be set as per Section 6.8.7 of [RFC5880]. There are a few fields
that
needs need to be set differently from [RFC5880] [RFC5880], as follows:

State (Sta)

Set to bfd.SessionState (either UP or ADMINDOWN AdminDown only).
Clarification of reflector Reflector BFD session state is described in
Section 7.2.3.

Demand (D)

Set to 0, to identify indicate that the S-BFD packet is sent by the
SBFDReflector.

Detect Mult

Value to be copied from "Detection Multiplier" filed the Detection Multiplier field of the
received BFD packet.

My Discriminator

Value to be copied from "your discriminator" filed the Your Discriminator field of the
received BFD packet.

Your Discriminator

Value to be copied from "my discriminator" filed the My Discriminator field of the
received BFD packet.

Desired Min TX Interval

Value to be copied from "Desired the Desired Min TX Interval" filed Interval field of
the received BFD packet.

Required Min RX Interval

Set to a bfd.RequiredMinRxInterval, value describing bfd.RequiredMinRxInterval. Value indicating the minimum
interval, in microseconds microseconds, between received SBFD S-BFD Control
packets. Further details are described provided in Section 7.2.3.

Required Min Echo RX Interval

If the device supports looping back S-BFD echo Echo packets

Set to the minimum required S-BFD Echo packet receive
interval for this session.

Else

Set to 0.

7.2.3. Additional SBFDReflector Behaviors

o S-BFD control Control packets transmitted by the SBFDReflector MUST have
"Required
Required Min RX Interval" Interval set to a value that expresses, in
microseconds, the minimum interval between incoming S-BFD control Control
packets that this SBFDReflector can handle. The SBFDReflector can
control how fast SBFInitiators SBFDInitiators will be sending S-BFD control Control
packets to self themselves by ensuring "Required that Required Min RX Interval" Interval
indicates a value based on the current load.

o When the SBFDReflector receives an S-BFD control Control packet from an
SBFDInitiator, then the SBFDReflector needs to determine what
"state" to send in the response S-BFD control Control packet. If the
monitored local entity is in service, then the "state" state MUST be set
to UP. If the monitored local entity is "temporarily out of
service", then the "state" state SHOULD be set to ADMINDOWN. AdminDown.

o If an SBFDReflector receives an S-BFD control Control packet with the
Demand (D) bit cleared, the packet MUST be discarded (see
Appendix A).

7.3. Initiator Procedures

S-BFD control Control packets transmitted by an SBFDInitiator MUST set "your
discriminator" the
Your Discriminator field to an S-BFD discriminator Discriminator corresponding to
the remote entity.

Every SBFDInitiator MUST have a locally unique "my discriminator" My Discriminator value
allocated from the BFD discriminator Discriminator pool.

Figure 3 describes the high-level concept of continuity test testing using
S-BFD. R2 allocates XX as the S-BFD discriminator Discriminator for its network
reachability purpose, purposes and advertises XX to neighbors. ASCII art Figure 3 shows
R1 and R4 performing a continuity test to R2.

+--- md=50/yd=XX (ping) ----+
| |
|+-- md=XX/yd=50 (pong) --+ |
|| | |
|v | v
R1 ==================== R2[*] ========= R3 ========= R4
| ^ |^
| | ||
| +-- md=60/yd=XX (ping) --+|
| |
+---- md=XX/yd=60 (pong) ---+

[*] Reflector BFD session on R2.
=== Links connecting network nodes.
--- S-BFD control Control packet traversal.

Figure 3: S-BFD Continuity Test

7.3.1. SBFDInitiator State Machine

An SBFDInitiator may be a persistent "persistent" session on the initiator with
a timer for S-BFD control Control packet transmissions (stateful
SBFDInitiator). An SBFDInitiator may also be a module, a script script, or
a tool on the initiator that transmits one or more S-BFD control Control
packets "when needed" (stateless SBFDInitiator). For stateless
SBFDInitiators, a complete BFD state machine may not be applicable.
For stateful SBFDInitiators, the states and the state machine
described in [RFC5880] will not function due to the SBFDReflector
session only sending the UP and ADMINDOWN AdminDown states (i.e., the
SBFDReflector session does not send the INIT state). The following
diagram provides the RECOMMENDED state machine for stateful
SBFDInitiators. The notation on each arc represents the state of the
SBFDInitiator (as received in the State field in the S-BFD control Control
packet) or indicates the expiration of the Detection Timer. See
Figure 4.

+--+
ADMIN DOWN, | |
TIMER | V
+------+ UP +------+
| |-------------------->| |----+
| DOWN | | UP | | UP
| |<--------------------| |<---+
+------+ ADMIN DOWN, +------+
TIMER

Figure 4: SBFDInitiator FSM Finite State Machine

Note that the above state machine is different from the base BFD
specification [RFC5880]. This is because the INIT state is no longer
applicable for the SBFDInitiator. Another important difference is
the transition of the state machine from the DOWN state to the UP
state when a packet with State an UP state setting is received by the
SBFDInitiator. The definitions of the states and the events have the
same meaning meanings as those defined in the base BFD specification
[RFC5880].

7.3.2. Transmission of S-BFD Control Packet by SBFDInitiator

Contents

The contents of S-BFD control Control packets sent by an SBFDInitiator MUST
be set as per Section 6.8.7 of [RFC5880]. There are a few fields which
needs
that need to be set differently from [RFC5880] [RFC5880], as follows:

Demand (D)

D bit is used

Used to identify indicate that the S-BFD packet originated from
SBFDInitiator and is always the
SBFDInitiator. Always set to 1.

Your Discriminator

Set to bfd.RemoteDiscr. bfd.RemoteDiscr is set to discriminator the
Discriminator value of the remote entity. It MAY be learnt
from routing protocols or configured locally.

Required Min RX Interval

Set to 0.

Required Min Echo RX Interval

Set to 0.

7.3.3. Additional SBFDInitiator Behaviors

o If the SBFDInitiator receives a valid S-BFD control Control packet in
response to a transmitted S-BFD control Control packet to a remote entity,
then the SBFDInitiator SHOULD conclude that the S-BFD control Control
packet reached the intended remote entity.

o When an SBFDInitiator receives a response S-BFD control Control packet, if
the state specified is ADMINDOWN, AdminDown, the SBFDInitiator MUST NOT
conclude loss of that the reachability to of the corresponding remote entity, entity
is lost and MUST back off the packet transmission interval for the
remote entity to an interval no faster than 1 second.

o When a sufficient number of S-BFD packets have not arrived as they
should, the SBFDInitiator SHOULD declare loss of reachability to
the remote entity. The criteria for declaring loss of
reachability and the action that would be triggered as a result
are outside the scope of this document; the action MAY include
logging an error.

o Relating to above Regarding the third bullet item, it is critical for an
implementation to understand the latency to/from the reflector Reflector BFD
session on the responder. In other words, for the very first
S-BFD packet transmitted by the SBFDInitiator, an implementation
MUST NOT expect a response S-BFD packet to be received for a time
equivalent to the sum of the latencies: initiator to responder and
responder back to initiator.

o If the SBFDInitiator receives an S-BFD control Control packet with the
Demand (D) bit set, the packet MUST be discarded (see Appendix A).

7.4. Diagnostic Values

Diagnostic

The diagnostic value in both directions MAY be set to a certain
value, to attempt to communicate further information to both ends.
Implementation
Implementations MAY use already existing the already-existing diagnostic values
defined in Section 4.1 of [RFC5880]. However, details of such regarding this
topic are outside the scope of this specification.

7.5. The Poll Sequence

The Poll sequence Sequence MAY be used in both directions. The Poll sequence Sequence
MUST operate in accordance with [RFC5880]. An SBFDReflector MAY use
the Poll sequence Sequence to slow down that the rate at which S-BFD control Control
packets are generated from an SBFDInitiator. This is done by the
SBFDReflector
SBFDReflector, using the procedures described in Section 7.2.3 and
setting the Poll (P) bit in the reflected S-BFD control Control packet. The
SBFDInitiator is to then send the next S-BFD control Control packet with the
Final (F) bit set. If an SBFDReflector receives an S-BFD control Control
packet with Poll (P) the P bit set, then the SBFDReflector MUST respond with
an S-BFD control Control packet with Poll (P) the P bit cleared and Final (F) the F bit set.

8. Operational Considerations

S-BFD provides a smooth and continuous (i.e., seamless) operational
experience as an Operations, Administration, and Maintenance (OAM)
mechanism for connectivity check checking and connection verification.
This is achieved by providing a simplified mechanism with a large portions
proportion of negotiation aspects eliminated, resulting in a faster and
simpler provisioning.

Because of this simplified mechanism, due to a misconfiguration, misconfiguration an
SBFDInitiator could send S-BFD control Control packets to a target that does
not exist or that is outside the S-BFD administrative domain. As
explained in Section 7.3.1, an SBFDInitiator can be a "persistent" persistent
initiator or a "when needed" one. When an S-BFD "persistent" persistent
SBFDInitiator is used, it a deployment SHOULD be controlled ensure that S-BFD control
packet Control
packets do not propagate for an extended period of time outside of
the administrative domain that uses it. Further, operational
measures SHOULD be taken to identify determine if responses to S-BFD packets
are not responded to sent for an extended period of time, time and then remediate the
situation. These potential concerns are largely mitigated by dynamic
advertisement mechanisms for S-BFD, S-BFD and with automation checks before
applying configurations.

8.1. Scaling Aspect

This mechanism brings forth one noticeable difference in terms of the
scaling aspect: the number of SBFDReflector. SBFDReflectors. This specification
eliminates the need for egress nodes to have fully active BFD
sessions when only one side desires to perform continuity tests.
With the introduction of reflector the Reflector BFD concept, egress is no
longer is required to create any active BFD session per path/LSP/function sessions on a per-path/LSP/
function basis. Due to Because of this, the total number of BFD sessions in
a network is reduced.

8.2. Congestion Considerations

When S-BFD performs failure detection by consuming detection, it consumes resources,
including bandwidth and CPU processing. It To avoid congestion, it is
therefore imperative that operators correctly provision the rates at
which S-BFD is transmitted
to avoid congestion. packets are transmitted. When BFD is used across
multiple hops, a congestion control mechanism MUST be implemented,
and when congestion is detected, the BFD implementation MUST reduce
the amount of traffic it generates. The exact mechanism used to
detect congestion is outside the scope of this specification, specification but may
include the detection of lost BFD control Control packets or other means.
The SBFDReflector can limit the rate at which an SBFInitiators SBFDInitiators will be
sending S-BFD control Control packets by utilizing the "Required Required Min RX Interval", Interval,
but at the expense of
increasing the detection time. time (i.e., detection time will
increase).

9. Co-existence with Classical BFD Sessions

Initial

Demultiplexing requirements for the initial packet demultiplexing requirement is are described in
Section 7.1. Because of this, the S-BFD mechanism can co-exist with
classical BFD sessions.

10. S-BFD Echo Function

The concept of the S-BFD Echo function is similar to the BFD Echo
function described in [RFC5880]. S-BFD echo Echo packets have the
destination of self, thus "self"; thus, S-BFD echo Echo packets are self-generated
and self-terminated after traversing a link/path. S-BFD echo Echo packets
are expected to u-turn U-turn on the target node in the data plane and
MUST NOT be processed by any reflector Reflector BFD sessions on the
target node.

When using the S-BFD Echo function, it is RECOMMENDED that:

o Both S-BFD control Control packets and S-BFD echo Echo packets be sent.

o Both S-BFD control Control packets and S-BFD echo Echo packets have the same
semantics in the forward direction to reach the target node.

In other words, it is not preferable to send just S-BFD echo Echo packets
without also sending S-BFD control Control packets. There are two reasons
behind this suggestion:

o S-BFD control Control packets can verify the reachability to of the intended
target node, which node; this allows one to have confidence that S-BFD echo Echo
packets are u-turning U-turning on the expected target node.

o S-BFD control Control packets can detect when the target node is going out
of service (i.e., via by receiving back ADMINDOWN AdminDown state).

S-BFD Echo packets can be spoofed, spoofed and can u-turn U-turn in a transit node
before reaching the expected target node. When the S-BFD Echo
function is used, it is RECOMMENDED in this specification that both
S-BFD control Control packets and S-BFD echo Echo packets be sent. While the
additional use of S-BFD control Control packets alleviates these two
concerns, some form of authentication MAY still be included.

The usage of the "Required Required Min Echo RX Interval" Interval field is described in Section 7.3.2
Sections 7.2.2 and Section 7.2.2. 7.3.2. Because of the stateless nature of
SBFDReflector sessions, a value specified in the "Required Required Min Echo RX Interval"
Interval field is not very meaningful at to the SBFDReflector. Thus Thus, it
is RECOMMENDED that the "Required Required Min Echo RX Interval" Interval field simply be
set to zero from by the SBFDInitiator. The SBFDReflector MAY set the
Required Min Echo RX Interval field to an appropriate value to
control the rate at which it wants to receives
SBFD echo receive S-BFD Echo packets.

The following aspects of S-BFD Echo functions are left as
implementation details, details and are outside the scope of this document:

o Format of the S-BFD echo Echo packet (e.g., data beyond UDP header).

o Procedures on when and how to use the S-BFD Echo function.

11. Security Considerations

Same

The same security considerations as those described in [RFC5880]
apply to this document. Additionally, implementing the following
measures will strengthen security aspects of the mechanism described
by this document:

o The SBFDInitiator MAY pick a sequence number to be set in
"sequence
Number" number" in authentication section the Authentication Section, based on the
configured authentication mode
configured. mode.

o The SBFDReflector MUST NOT use the crypto sequence number to make
a decision about accepting the packet. This is because the
SBFDReflector does not maintain S-BFD peer state, state and because the
SBFDReflector can receive S-BFD packets from multiple
SBFDInitiators. Consequently, BFD authentication can be used used, but
not the sequence number.

o The SBFDReflector MAY use the Auth Key ID in the incoming packet
to verify the authentication data. Authentication Data.

o The SBFDReflector MUST accept the packet if authentication is
successful.

o The SBFDReflector MUST compute the Authentication data Data and MUST
use the same sequence number that it received in the S-BFD control Control
packet that to which it is responding to. responding.

o The SBFDInitiator SHOULD accept an S-BFD control Control packet with a
sequence number within the permissible window. range. One potential
approach is the procedure explained in [I-D.ietf-bfd-generic-crypto-auth]. [BFD-GEN-AUTH].

Using the above method,

o SBFDReflector SBFDReflectors continue to remain stateless stateless, despite using
security.

o SBFDReflector SBFDReflectors are not susceptible to replay attacks attacks, as they
always respond to S-BFD control Control packets irrespective of the
sequence number carried.

o An attacker cannot impersonate the responder responder, since the
SBFDInitiator will only accept S-BFD control Control packets that come
with the sequence number that it had originally used when sending
the S-BFD control Control packet.

Additionally, the use of strong forms of authentication is strongly
encouraged for S-BFD. The use of Simple Password authentication
[RFC5880] potentially puts other services at risk, risk if S-BFD packets
can be intercepted and if those password values are reused for other
services.

Considerations about related to loop problems are covered in Appendix A.

12. IANA Considerations

No action is required by IANA References

12.1. Normative References

[RFC2119] Bradner, S., "Key words for this document.

13. Acknowledgements

The authors would like use in RFCs to thank Jeffrey Haas, Greg Mirsky, Marc
Binderberger, and Alvaro Retana for performing thorough reviews and
providing number of suggestions. The authors would also like to
thank Girija Raghavendra Rao, Les Ginsberg, Srihari Raghavan, Vanitha
Neelamegam, and Vengada Prasad Govindan from Cisco Systems for
providing valuable comments. Finally, the authors would also like to
thank John E. Drake and Pablo Frank for providing comments and
suggestions.

14. Contributors

The following are key contributors to this document:

Tarek Saad, Cisco Systems, Inc.
Siva Sivabalan, Cisco Systems, Inc.
Nagendra Kumar, Cisco Systems, Inc.
Mallik Mudigonda, Cisco Systems, Inc.
Sam Aldrin, Google

15. References

15.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC5880] Katz, D. Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<http://www.rfc-editor.org/info/rfc5880>.

15.2.

12.2. Informative References

[I-D.ietf-bfd-generic-crypto-auth]

[BFD-GEN-AUTH]
Bhatia, M., Manral, V., Zhang, D., and M. Jethanandani,
"BFD Generic Cryptographic Authentication", draft-ietf-
bfd-generic-crypto-auth-06 (work Work in progress),
Progress, draft-ietf-bfd-generic-crypto-auth-06,
April 2014.

[I-D.ietf-bfd-seamless-ip]
Akiya, N., Pignataro, C., and D. Ward, "Seamless
Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6
and MPLS", draft-ietf-bfd-seamless-ip-04 (work in
progress), April 2016.

[I-D.ietf-bfd-seamless-use-case]
Aldrin, S., Pignataro, C., Mirsky, G., and N. Kumar,
"Seamless Bidirectional Forwarding Detection (S-BFD) Use
Cases", draft-ietf-bfd-seamless-use-case-06 (work in
progress), April 2016.

[I-D.ietf-pals-seamless-vccv]
Govindan, V. and C. Pignataro, "Seamless BFD for VCCV",
draft-ietf-pals-seamless-vccv-03 (work in progress), April
2016.

[RFC0791]

[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, 10.17487/RFC791, September 1981,
<http://www.rfc-editor.org/info/rfc791>.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.

[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<http://www.rfc-editor.org/info/rfc3031>.

[RFC7881] Pignataro, C., Ward, D., and N. Akiya, "Seamless
Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6,
and MPLS", RFC 7881, DOI 10.17487/RFC7881, July 2016,
<http://www.rfc-editor.org/info/rfc7881>.

[RFC7882] Aldrin, S., Pignataro, C., Mirsky, G., and N. Kumar,
"Seamless Bidirectional Forwarding Detection (S-BFD) Use
Cases", RFC 7882, DOI 10.17487/RFC7882, July 2016,
<http://www.rfc-editor.org/info/rfc7882>.

[RFC7885] Govindan, V. and C. Pignataro, "Seamless Bidirectional
Forwarding Detection (S-BFD) for Virtual Circuit
Connectivity Verification (VCCV)", RFC 7885,
DOI 10.17487/RFC7885, July 2016,
<http://www.rfc-editor.org/info/rfc7885>.

Appendix A. Loop Problem and Solution

Consider a scenario where we have two nodes and both are S-BFD
capable.

Node A (IP 2001:db8::1) ----------------- Node B (IP 2001:db8::2)
|
|
Man in the Middle (MiM) (MITM)

Assume node that Node A reserved a discriminator 0x01010101 for target
identifier 2001:db8::1 and has a reflector session in listening mode.
Similarly node
Similarly, Node B reserved a discriminator 0x02020202 for its target
identifier 2001:db8::2 and also has a reflector session in
listening mode.

Suppose MiM that a MITM sends a spoofed packet with MyDisc My Discriminator =
0x01010101, YourDisc Your Discriminator = 0x02020202, source IP as 2001:db8::1
2001:db8::1, and dest destination IP as 2001:db8::2. When this packet
reaches Node B, the reflector session on Node B will swap the
discriminators and IP addresses of the received packet and reflect it
back, since YourDisc the Your Discriminator value of the received packet matched with
matches the reserved discriminator of Node B. The reflected packet
that reached Node A will have MyDdisc=0x02020202 My Discriminator = 0x02020202 and YourDisc=0x01010101.
Your Discriminator = 0x01010101. Since
YourDisc the Your Discriminator value
of the received packet matched matches the reserved discriminator of Node A,
Node A will swap the discriminators and reflects reflect the packet back to
Node B. Since the reflectors must set the TTL of the reflected
packets to 255, the above scenario will result in an infinite loop
with
because of just one malicious packet injected from MiM. the MITM.

The solution is to avoid the loop problem uses by using the "D" D bit (Demand
mode bit). The Initiator initiator always sets the 'D' bit D bit, and the reflector
always clears it. This way way, we can identify determine if a received packet
was a reflected packet and avoid reflecting it back.

Acknowledgements

The authors would like to thank Jeffrey Haas, Greg Mirsky, Marc
Binderberger, and Alvaro Retana for performing thorough reviews and
providing a number of suggestions. The authors would also like to
thank Girija Raghavendra Rao, Les Ginsberg, Srihari Raghavan, Vanitha
Neelamegam, and Vengada Prasad Govindan from Cisco Systems for
providing valuable comments. Finally, the authors would also like to
thank John E. Drake and Pablo Frank for providing comments and
suggestions.

Contributors

The following are key contributors to this document:

Tarek Saad, Cisco Systems, Inc.
Siva Sivabalan, Cisco Systems, Inc.
Nagendra Kumar, Cisco Systems, Inc.
Mallik Mudigonda, Cisco Systems, Inc.
Sam Aldrin, Google

Authors' Addresses

Carlos Pignataro
Cisco Systems, Inc.

Email: cpignata@cisco.com

Dave Ward
Cisco Systems, Inc.

Email: wardd@cisco.com

Nobo Akiya
Big Switch Networks

Email: nobo.akiya.dev@gmail.com

Manav Bhatia
Ionos Networks

Email: manav@ionosnetworks.com

Santosh Pallagatti

Email: santosh.pallagatti@gmail.com