Internet DRAFT - draft-ietf-bfd-seamless-base
draft-ietf-bfd-seamless-base
Internet Engineering Task Force C. Pignataro
Internet-Draft D. Ward
Updates: 5880 (if approved) Cisco
Intended status: Standards Track N. Akiya
Expires: November 7, 2016 Big Switch Networks
M. Bhatia
Ionos Networks
S. Pallagatti
May 6, 2016
Seamless Bidirectional Forwarding Detection (S-BFD)
draft-ietf-bfd-seamless-base-11
Abstract
This document defines a simplified mechanism to use Bidirectional
Forwarding Detection (BFD) with large portions of negotiation aspects
eliminated, thus providing benefits such as quick provisioning as
well as improved control and flexibility to 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].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 7, 2016.
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Seamless BFD Overview . . . . . . . . . . . . . . . . . . . . 5
4. S-BFD Discriminators . . . . . . . . . . . . . . . . . . . . 6
4.1. S-BFD Discriminator Uniqueness . . . . . . . . . . . . . 6
4.2. Discriminator Pools . . . . . . . . . . . . . . . . . . . 7
5. Reflector BFD Session . . . . . . . . . . . . . . . . . . . . 7
6. State Variables . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. New State Variables . . . . . . . . . . . . . . . . . . . 8
6.2. State Variable Initialization and Maintenance . . . . . . 9
7. S-BFD Procedures . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Demultiplexing of S-BFD Control Packet . . . . . . . . . 9
7.2. Responder Procedures . . . . . . . . . . . . . . . . . . 10
7.2.1. Responder Demultiplexing . . . . . . . . . . . . . . 10
7.2.2. Transmission of S-BFD Control Packet by SBFDReflector 10
7.2.3. Additional SBFDReflector Behaviors . . . . . . . . . 11
7.3. Initiator Procedures . . . . . . . . . . . . . . . . . . 12
7.3.1. SBFDInitiator State Machine . . . . . . . . . . . . . 12
7.3.2. Transmission of S-BFD Control Packet by SBFDInitiator 13
7.3.3. Additional SBFDInitiator Behaviors . . . . . . . . . 14
7.4. Diagnostic Values . . . . . . . . . . . . . . . . . . . . 14
7.5. The Poll Sequence . . . . . . . . . . . . . . . . . . . . 15
8. Operational Considerations . . . . . . . . . . . . . . . . . 15
8.1. Scaling Aspect . . . . . . . . . . . . . . . . . . . . . 15
8.2. Congestion Considerations . . . . . . . . . . . . . . . . 16
9. Co-existence with Classical BFD Sessions . . . . . . . . . . 16
10. S-BFD Echo Function . . . . . . . . . . . . . . . . . . . . . 16
11. Security Considerations . . . . . . . . . . . . . . . . . . . 17
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 19
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15. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
15.1. Normative References . . . . . . . . . . . . . . . . . . 19
15.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Loop Problem and Solution . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
Bidirectional Forwarding Detection (BFD), [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 and to allow BFD usage for wider scenarios. Additional use
cases are listed in [I-D.ietf-bfd-seamless-use-case].
Specifically, this document defines Seamless Bidirectional Forwarding
Detection (S-BFD) a simplified mechanism to use Bidirectional
Forwarding Detection (BFD) with large portions of negotiation aspects
eliminated, thus providing benefits such as quick provisioning as
well as improved control and flexibility to 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 (i.e., time delay) in BFD provides applications
a smooth and continuous operational experience. 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] and MPLS [RFC3031] terminologies and protocol
constructs. This section describes several new terminologies
introduced by S-BFD.
o Classical BFD - BFD session types based on [RFC5880].
o S-BFD - Seamless BFD.
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o S-BFD control packet - a BFD control packet for the S-BFD
mechanism.
o S-BFD echo packet - a BFD echo packet for the S-BFD mechanism.
o S-BFD packet - a BFD control packet or a BFD echo packet.
o Entity - a function on a network node that S-BFD mechanism allows
remote network nodes to perform continuity test to. An entity can
be abstract (e.g., reachability) or specific (e.g., IP addresses,
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 packets to local entities and generates
response S-BFD control packets.
o Reflector BFD session - synonymous with SBFDReflector.
o S-BFD discriminator - a BFD discriminator allocated for a local
entity and is being listened by an SBFDReflector.
o BFD discriminator - a BFD 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.
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+---------------------+ +------------------------+
| Initiator | | Responder |
| +-----------------+ | | +-----------------+ |
| | SBFDInitiator |---S-BFD ctrl pkt----->| SBFDReflector | |
| | +-------------+ |<--S-BFD ctrl pkt------| +-------------+ | |
| | | BFD discrim | | | | | |S-BFD discrim| | |
| | | | |---S-BFD 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 for local entities, and creates a reflector BFD
session. Allocated S-BFD discriminators may be advertised by
applications (e.g., OSPF/IS-IS). Required result is that
applications, on other network nodes, possess the knowledge of the
S-BFD discriminators allocated by a remote node to remote entities.
The reflector BFD session is to, upon receiving an S-BFD control
packet targeted to one of local S-BFD discriminator values, transmit
a response S-BFD control packet back to the initiator.
Once the above setup is complete, any network node, having the
knowledge of the S-BFD discriminator allocated by a remote node to
remote entity/entities, can quickly perform a continuity test to the
remote entity by simply sending S-BFD control packets with
corresponding S-BFD discriminator value in the "your discriminator"
field.
This is exemplified in Figure 2.
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<------- IS-IS Network ------->
+---------+
| |
A---------B---------C---------D
^ ^
| |
SystemID SystemID
xxx yyy
BFD Discrim BFD Discrim
123 456
Figure 2: S-BFD for IS-IS Network
S-BFD module in a system IS-IS SystemID xxx (node A) allocates an
S-BFD discriminator 123, and IS-IS advertises the S-BFD discriminator
123 in an IS-IS TLV. S-BFD module in a system with IS-IS SystemID
yyy (node D) allocates an S-BFD discriminator 456, and IS-IS
advertises the S-BFD discriminator 456 in an IS-IS TLV. A reflector
BFD session is created on both network nodes (node A and node D).
When network node A wants to check the reachability to network node
D, node A can send an S-BFD control packet, destined to node D, with
"your discriminator" field set to 456. When the reflector BFD
session on node D receives this S-BFD control packet, then a response
S-BFD control packet is sent back to node A, which allows node A to
complete the continuity test.
When a node allocates multiple S-BFD 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 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 is that it
MUST be unique within an administrative domain. If multiple network
nodes allocated the same S-BFD discriminator value, then S-BFD
control packets falsely terminating on a wrong network node can
result in a reflector BFD session to generate a response back, due to
"your discriminator" matching. This is clearly not desirable.
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4.2. Discriminator Pools
This subsection describes a discriminator pool implementation
technique to minimize S-BFD discriminator collisions. The result
will allow an implementation to better satisfy the S-BFD
discriminator uniqueness requirement defined in Section 4.1.
o SBFDInitiator is to allocate a discriminator from the BFD
discriminator pool. If the system also supports classical BFD
that runs on [RFC5880], then the BFD discriminator pool SHOULD be
shared by SBFDInitiator sessions and classical BFD sessions.
o SBFDReflector is to allocate a discriminator from the S-BFD
discriminator pool. The S-BFD discriminator pool SHOULD be a
separate pool than the BFD discriminator pool.
The remainder of this subsection describes the reasons for the
suggestions above.
Locally allocated S-BFD discriminator values for entities, listened
by SBFDReflector sessions, may be arbitrary 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 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 discriminator pool approach,
collision is still possible between one S-BFD application to another
S-BFD application, that may be using different values and algorithms
to derive S-BFD discriminator values. If the two applications are
using S-BFD for the same purpose (e.g., network reachability), then
the colliding S-BFD 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 by a single network node, however, is discouraged (see
Section 3).
5. Reflector BFD Session
Each network node creates one or more reflector BFD sessions. This
reflector BFD session is a session that transmits S-BFD control
packets in response to received S-BFD control packets with "your
discriminator" having S-BFD discriminators allocated for local
entities. Specifically, this reflector BFD session has the following
characteristics:
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o MUST NOT transmit any S-BFD packets based on local timer expiry.
o MUST transmit an S-BFD control packet in response to a received
S-BFD control packet having a valid S-BFD discriminator in the
"your discriminator" field, unless prohibited by local policies
(e.g., administrative, security, rate-limiter, etc.)
o MUST be capable of sending only two states: UP and ADMINDOWN.
One reflector BFD session may be responsible for handling received
S-BFD control packets targeted to all locally allocated S-BFD
discriminators, or few reflector BFD sessions may each be responsible
for subset of locally allocated S-BFD discriminators. This policy is
a local matter, and is outside the scope of this document.
Note that incoming S-BFD control packets may be IPv4, IPv6 or MPLS
based [I-D.ietf-bfd-seamless-ip], and other options are possible and
can be defined in future documents. How such S-BFD control packets
reach an appropriate reflector BFD session is also a local matter,
and is outside the scope of this document.
6. State Variables
S-BFD introduces new state 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 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 packets to local entities and
generates response S-BFD control packets.
bfd.SessionType variable MUST be initialized to the appropriate type
when an S-BFD session is created.
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6.2. State Variable Initialization and Maintenance
A state variable defined in Section 6.8.1 of [RFC5880] need to be
initialized or manipulated differently depending on the session type.
o bfd.DemandMode: This variable MUST be initialized to 1 for session
type 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
S-BFD packet MUST be demultiplexed with lower layer information
(e.g., dedicated destination UDP port [I-D.ietf-bfd-seamless-ip],
associated channel type [I-D.ietf-pals-seamless-vccv]). The
following procedure SHOULD be executed on both initiator and
reflector.
If S-BFD packet
If S-BFD packet is for SBFDReflector
Packet MUST be looked up to locate a corresponding
SBFDReflector session based on the value from the "your
discriminator" field in the table describing S-BFD
discriminators.
Else
Packet MUST be looked up to locate a corresponding
SBFDInitiator session or classical BFD session based on the
value from the "your discriminator" field in the table
describing BFD discriminators. If no match then received
packet MUST be discarded.
If session is SBFDInitiator
Destination of the packet (i.e., destination IP address)
SHOULD be validated to be for self.
Else
Packet MUST be discarded
Else
Procedure described in [RFC5880] MUST be applied.
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More details on S-BFD control packet demultiplexing are described in
relevant S-BFD data plane documents.
7.2. Responder Procedures
A network node that receives S-BFD control packets transmitted by an
initiator is referred as responder. The responder, upon reception of
S-BFD control packets, is to perform necessary relevant validations
described in [RFC5880].
7.2.1. Responder Demultiplexing
S-BFD packet MUST be demultiplexed with lower layer information. The
following procedure SHOULD be executed by the responder:
If "your discriminator" not one of the entry allocated for local
entities
Packet MUST be discarded.
Else
Packet is determined to be handled by a reflector BFD session
responsible for that S-BFD discriminator.
If local policy allows (e.g., administrative, security, rate-
limiter, etc.)
Chosen reflector BFD session SHOULD transmit a response BFD
control packet using procedures described in Section 7.2.2.
7.2.2. Transmission of S-BFD Control Packet by SBFDReflector
Contents of S-BFD control packets sent by an SBFDReflector MUST be
set as per Section 6.8.7 of [RFC5880]. There are a few fields that
needs to be set differently from [RFC5880] as follows:
State (Sta)
Set to bfd.SessionState (either UP or ADMINDOWN only).
Clarification of reflector BFD session state is described in
Section 7.2.3.
Demand (D)
Set to 0, to identify the S-BFD packet is sent by the
SBFDReflector.
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Detect Mult
Value to be copied from "Detection Multiplier" filed of
received BFD packet.
My Discriminator
Value be copied from "your discriminator" filed of received BFD
packet.
Your Discriminator
Value be copied from "my discriminator" filed of received BFD
packet.
Desired Min TX Interval
Value be copied from "Desired Min TX Interval" filed of
received BFD packet.
Required Min RX Interval
Set to a bfd.RequiredMinRxInterval, value describing minimum
interval, in microseconds between received SBFD Control
packets. Further details are described in Section 7.2.3.
Required Min Echo RX Interval
If device supports looping back S-BFD echo packets
Set to the minimum required Echo packet receive interval for
this session.
Else
Set to 0.
7.2.3. Additional SBFDReflector Behaviors
o S-BFD control packets transmitted by the SBFDReflector MUST have
"Required Min RX Interval" set to a value that expresses, in
microseconds, the minimum interval between incoming S-BFD control
packets this SBFDReflector can handle. The SBFDReflector can
control how fast SBFInitiators will be sending S-BFD control
packets to self by ensuring "Required Min RX Interval" indicates a
value based on the current load.
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o When the SBFDReflector receives an S-BFD control packet from an
SBFDInitiator, then the SBFDReflector needs to determine what
"state" to send in the response S-BFD control packet. If the
monitored local entity is in service, then the "state" MUST be set
to UP. If the monitored local entity is "temporarily out of
service", then the "state" SHOULD be set to ADMINDOWN.
o If an SBFDReflector receives an S-BFD control packet with Demand
(D) bit cleared, the packet MUST be discarded (see Appendix A).
7.3. Initiator Procedures
S-BFD control packets transmitted by an SBFDInitiator MUST set "your
discriminator" field to an S-BFD discriminator corresponding to the
remote entity.
Every SBFDInitiator MUST have a locally unique "my discriminator"
allocated from the BFD discriminator pool.
Figure 3 describes the high-level concept of continuity test using
S-BFD. R2 allocates XX as the S-BFD discriminator for its network
reachability purpose, and advertises XX to neighbors. ASCII art
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 packet traversal.
Figure 3: S-BFD Continuity Test
7.3.1. SBFDInitiator State Machine
An SBFDInitiator may be a persistent session on the initiator with a
timer for S-BFD control packet transmissions (stateful
SBFDInitiator). An SBFDInitiator may also be a module, a script or a
tool on the initiator that transmits one or more S-BFD control
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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 SBFDReflector session
only sending UP and ADMINDOWN states (i.e., SBFDReflector session
does not send 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 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
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 UP is received by the SBFDInitiator.
The definitions of the states and the events have the same meaning as
in the base BFD specification [RFC5880].
7.3.2. Transmission of S-BFD Control Packet by SBFDInitiator
Contents of S-BFD control packets sent by an SBFDInitiator MUST be
set as per Section 6.8.7 of [RFC5880]. There are few fields which
needs to be set differently from [RFC5880] as follows:
Demand (D)
D bit is used to identify S-BFD packet originated from
SBFDInitiator and is always set to 1.
Your Discriminator
Set to bfd.RemoteDiscr. bfd.RemoteDiscr is set to discriminator
value of remote entity. It MAY be learnt from routing
protocols or configured locally.
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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 packet in
response to transmitted S-BFD control packet to a remote entity,
then the SBFDInitiator SHOULD conclude that S-BFD control packet
reached the intended remote entity.
o When an SBFDInitiator receives a response S-BFD control packet, if
the state specified is ADMINDOWN, the SBFDInitiator MUST NOT
conclude loss of reachability to the corresponding remote entity,
and MUST back off 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 bullet item, it is critical for an
implementation to understand the latency to/from the reflector BFD
session on the responder. In other words, for very first S-BFD
packet transmitted by the SBFDInitiator, an implementation MUST
NOT expect response S-BFD packet to be received for time
equivalent to sum of latencies: initiator to responder and
responder back to initiator.
o If the SBFDInitiator receives an S-BFD control packet with Demand
(D) bit set, the packet MUST be discarded (see Appendix A).
7.4. Diagnostic Values
Diagnostic value in both directions MAY be set to a certain value, to
attempt to communicate further information to both ends.
Implementation MAY use already existing diagnostic values defined in
Section 4.1 of [RFC5880]. However, details of such are outside the
scope of this specification.
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7.5. The Poll Sequence
Poll sequence MAY be used in both directions. The Poll sequence MUST
operate in accordance with [RFC5880]. An SBFDReflector MAY use the
Poll sequence to slow down that rate at which S-BFD control packets
are generated from an SBFDInitiator. This is done by the
SBFDReflector using procedures described in Section 7.2.3 and setting
the Poll (P) bit in the reflected S-BFD control packet. The
SBFDInitiator is to then send the next S-BFD control packet with the
Final (F) bit set. If an SBFDReflector receives an S-BFD control
packet with Poll (P) bit set, then the SBFDReflector MUST respond
with an S-BFD control packet with Poll (P) bit cleared and Final (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 and connection verification. This
is achieved by providing a simplified mechanism with large portions
of negotiation aspects eliminated, resulting in a faster and simpler
provisioning.
Because of this simplified mechanism, due to a misconfiguration, an
SBFDInitiator could send S-BFD 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"
initiator or a "when needed" one. When an S-BFD "persistent"
SBFDInitiator is used, it SHOULD be controlled that S-BFD control
packet 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 if S-BFD packets are not responded to for
an extended period of time, and remediate the situation. These
potential concerns are largely mitigated by dynamic advertisement
mechanisms for S-BFD, and with automation checks before applying
configurations.
8.1. Scaling Aspect
This mechanism brings forth one noticeable difference in terms of
scaling aspect: number of SBFDReflector. This specification
eliminates the need for egress nodes to have fully active BFD
sessions when only one side desires to perform continuity tests.
With introduction of reflector BFD concept, egress no longer is
required to create any active BFD session per path/LSP/function
basis. Due to this, total number of BFD sessions in a network is
reduced.
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8.2. Congestion Considerations
S-BFD performs failure detection by consuming resources, including
bandwidth and CPU processing. It is therefore imperative that
operators correctly provision the rates at which S-BFD is transmitted
to avoid congestion. 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, but may include detection of
lost BFD control packets or other means. The SBFDReflector can limit
the rate at which an SBFInitiators will be sending S-BFD control
packets utilizing the "Required Min RX Interval", at the expense of
increasing the detection time.
9. Co-existence with Classical BFD Sessions
Initial packet demultiplexing requirement is described in
Section 7.1. Because of this, 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 packets have the
destination of self, thus S-BFD echo packets are self-generated and
self-terminated after traversing a link/path. S-BFD echo packets are
expected to u-turn on the target node in the data plane and MUST NOT
be processed by any reflector BFD sessions on the target node.
When using the S-BFD Echo function, it is RECOMMENDED that:
o Both S-BFD control packets and S-BFD echo packets be sent.
o Both S-BFD control packets and S-BFD 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 packets
without also sending S-BFD control packets. There are two reasons
behind this suggestion:
o S-BFD control packets can verify the reachability to intended
target node, which allows one to have confidence that S-BFD echo
packets are u-turning on the expected target node.
o S-BFD control packets can detect when the target node is going out
of service (i.e., via receiving back ADMINDOWN state).
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S-BFD Echo packets can be spoofed, and can 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 packets and S-BFD echo packets be sent. While the
additional use of S-BFD control packets alleviates these two
concerns, some form of authentication MAY still be included.
The usage of the "Required Min Echo RX Interval" field is described
in Section 7.3.2 and Section 7.2.2. Because of the stateless nature
of SBFDReflector sessions, a value specified the "Required Min Echo
RX Interval" field is not very meaningful at SBFDReflector. Thus it
is RECOMMENDED that the "Required Min Echo RX Interval" field simply
be set to zero from SBFDInitiator. SBFDReflector MAY set to
appropriate value to control the rate at which it wants to receives
SBFD echo packets.
The following aspects of S-BFD Echo functions are left as
implementation details, and are outside the scope of this document:
o Format of the S-BFD 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 security considerations as [RFC5880] apply to this document.
Additionally, implementing the following measures will strengthen
security aspects of the mechanism described by this document:
o SBFDInitiator MAY pick a sequence number to be set in "sequence
Number" in authentication section based on authentication mode
configured.
o 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, and because the
SBFDReflector can receive S-BFD packets from multiple
SBFDInitiators. Consequently, BFD authentication can be used but
not the sequence number.
o SBFDReflector MAY use the Auth Key ID in the incoming packet to
verify the authentication data.
o SBFDReflector MUST accept the packet if authentication is
successful.
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o SBFDReflector MUST compute the Authentication data and MUST use
the same sequence number that it received in the S-BFD control
packet that it is responding to.
o SBFDInitiator SHOULD accept S-BFD control packet with sequence
number within permissible window. One potential approach is the
procedure explained in [I-D.ietf-bfd-generic-crypto-auth].
Using the above method,
o SBFDReflector continue to remain stateless despite using security.
o SBFDReflector are not susceptible to replay attacks as they always
respond to S-BFD control packets irrespective of the sequence
number carried.
o An attacker cannot impersonate the responder since the
SBFDInitiator will only accept S-BFD control packets that come
with the sequence number that it had originally used when sending
the S-BFD control packet.
Additionally, the use of strong forms of authentication is strongly
encouraged for S-BFD. The use of Simple Password authentication
potentially puts other services at risk, if S-BFD packets can be
intercepted and if those password values are reused for other
services.
Considerations about loop problems are covered in Appendix A.
12. IANA Considerations
No action is required by IANA for this document.
13. Acknowledgements
The authors would like 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.
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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. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<http://www.rfc-editor.org/info/rfc5880>.
15.2. Informative References
[I-D.ietf-bfd-generic-crypto-auth]
Bhatia, M., Manral, V., Zhang, D., and M. Jethanandani,
"BFD Generic Cryptographic Authentication", draft-ietf-
bfd-generic-crypto-auth-06 (work in progress), 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.
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[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, 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>.
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)
Assume node A reserved a discriminator 0x01010101 for target
identifier 2001:db8::1 and has a reflector session in listening mode.
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 sends a spoofed packet with MyDisc = 0x01010101, YourDisc
= 0x02020202, source IP as 2001:db8::1 and dest 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 of the received packet matched with
reserved discriminator of Node B. The reflected packet that reached
Node A will have MyDdisc=0x02020202 and YourDisc=0x01010101. Since
YourDisc of the received packet matched the reserved discriminator of
Node A, Node A will swap the discriminators and reflects the packet
back to Node B. Since reflectors must set the TTL of the reflected
packets to 255, the above scenario will result in an infinite loop
with just one malicious packet injected from MiM.
The solution to avoid the loop problem uses the "D" bit (Demand mode
bit). The Initiator always sets the 'D' bit and the reflector always
clears it. This way we can identify if a received packet was a
reflected packet and avoid reflecting it back.
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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
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