Seamless Bidirectional Forwarding Detection (S-BFD)
draft-ietf-bfd-seamless-base-02
The information below is for an old version of the document.
| Document | Type |
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|---|---|---|---|
| Authors | Nobo Akiya , Carlos Pignataro , David Ward , Manav Bhatia , Santosh Pallagatti | ||
| Last updated | 2014-08-01 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-bfd-seamless-base-02
Internet Engineering Task Force N. Akiya
Internet-Draft C. Pignataro
Updates: 5880 (if approved) D. Ward
Intended status: Standards Track Cisco Systems
Expires: February 2, 2015 M. Bhatia
Ionos Networks
P. K. Santosh
Juniper Networks
August 1, 2014
Seamless Bidirectional Forwarding Detection (S-BFD)
draft-ietf-bfd-seamless-base-02
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 February 2, 2015.
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Copyright Notice
Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . 4
4. S-BFD Discriminators . . . . . . . . . . . . . . . . . . . . 5
4.1. Discriminator Pools . . . . . . . . . . . . . . . . . . . 5
4.2. S-BFD Discriminator Uniqueness . . . . . . . . . . . . . 6
5. Reflector BFD Session . . . . . . . . . . . . . . . . . . . . 7
6. State Variables . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. New State Variables . . . . . . . . . . . . . . . . . . . 7
6.2. State Variable Initialization and Maintenance . . . . . . 8
7. S-BFD Procedures . . . . . . . . . . . . . . . . . . . . . . 8
7.1. S-BFD Packet Demultiplexing . . . . . . . . . . . . . . . 8
7.2. Initiator Procedures . . . . . . . . . . . . . . . . . . 8
7.2.1. SBFDInitiator State Machine . . . . . . . . . . . . . 9
7.2.2. Details of S-BFD Packet Sent by SBFDInitiator . . . . 10
7.3. Responder Procedures . . . . . . . . . . . . . . . . . . 10
7.3.1. Responder Demultiplexing . . . . . . . . . . . . . . 10
7.3.2. Details of S-BFD Packet Sent by SBFDReflector . . . . 11
7.4. Diagnostic Values . . . . . . . . . . . . . . . . . . . . 11
7.5. The Poll Sequence . . . . . . . . . . . . . . . . . . . . 11
7.6. Control Plane Independent (C) . . . . . . . . . . . . . . 11
7.7. Additional SBFDInitiator Behaviors . . . . . . . . . . . 12
7.8. Additional SBFDReflector Behaviors . . . . . . . . . . . 12
8. Scaling Aspect . . . . . . . . . . . . . . . . . . . . . . . 13
9. Co-existence with Classical BFD Sessions . . . . . . . . . . 13
10. S-BFD Echo Function . . . . . . . . . . . . . . . . . . . . . 13
11. Security Considerations . . . . . . . . . . . . . . . . . . . 14
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
14. Contributing Authors . . . . . . . . . . . . . . . . . . . . 15
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
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15.1. Normative References . . . . . . . . . . . . . . . . . . 16
15.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Loop Problem . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
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 which 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. This document
extends BFD to provide solutions to use cases listed in
[I-D.ietf-bfd-seamless-use-case].
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, IP and MPLS
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.
o S-BFD packet - a BFD control packet destined to or sourced from
the well-known S-BFD port.
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 (ex: reachability) or specific (ex: 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.
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o SBFDReflector - an S-BFD session on a network node that listens
for incoming S-BFD packets to local entities and generates
response S-BFD 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.
Below figure describes the relationship between S-BFD terminologies.
+---------------------+ +---------------------+
| Initiator | | Responder |
| +-----------------+ | | +-----------------+ |
| | SBFDInitiator |--- S-BFD packet -->| SBFDReflector | |
| | +-------------+ | | | | +-------------+ | |
| | | BFD discrim | | | | | |S-BFD discrim| | |
| | +-------------+ |<-- S-BFD packet ---| +----------^--+ | |
| +-----------------+ | | +------------|----+ |
| | | | |
| | | +---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 (ex: OSPF/IS-IS). Required result is that applications,
on other network nodes, possess the knowledge of the mapping from
remote entities to S-BFD discriminators. The reflector BFD session
is to, upon receiving an S-BFD packet targeted to one of local S-BFD
discriminator values, transmit a response S-BFD packet back to the
initiator.
Once above setup is complete, any network nodes, having the knowledge
of the mapping from a remote entity to an S-BFD discriminator, can
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quickly perform a continuity test to the remote entity by simply
sending S-BFD packets with corresponding S-BFD discriminator value in
the "your discriminator" field.
For example:
<------- 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
The IS-IS with SystemID xxx (node A) allocates an S-BFD discriminator
123, and advertises the S-BFD discriminator 123 in an IS-IS TLV. The
IS-IS with SystemID yyy (node D) allocates an S-BFD discriminator
456, and 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 packet, destined to node D,
with "your discriminator" field set to 456. When the reflector BFD
session on node D receives this S-BFD packet, then response S-BFD
packet is sent back to node A, which allows node A to complete the
continuity test.
4. S-BFD Discriminators
4.1. Discriminator Pools
This document defines following suggestions for discriminator
management on SBFDInitiator and SBFDReflector sessions, to minimize
the collision between required S-BFD discriminators on a local
device.
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.
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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.
Remainder of this subsection describes the reasons for above
suggestions.
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
(ex: System-ID, Router-ID) or network targets (ex: 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 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 a same purpose (ex: 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. How such collisions are addressed is
outside the scope of this document.
4.2. S-BFD Discriminator Uniqueness
One important characteristics of an S-BFD discriminator is that it
MUST be unique within an administrative domain. If multiple network
nodes allocated a same S-BFD discriminator value, then S-BFD 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. If only IP based S-BFD is
considered, then it is possible for the reflector BFD session to
require demultiplexing of incoming S-BFD packets with combination of
destination IP address and "your discriminator". Then S-BFD
discriminator only has to be unique within a local node. However,
S-BFD is a generic mechanism defined to run on wide range of
environments: IP, MPLS, etc. For other transports like MPLS, because
of the need to use non-routable IP destination address, it is not
possible for reflector BFD session to demultiplex using IP
destination address. With PHP, there may not be any incoming label
stack to aid in demultiplexing either. Thus, S-BFD imposes a
requirement that S-BFD discriminators MUST be unique within an
administrative domain.
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5. Reflector BFD Session
Each network node creates one or more reflector BFD sessions. This
reflector BFD session is a session which transmits S-BFD packets in
response to received S-BFD packets with "your discriminator" having
S-BFD discriminators allocated for local entities. Specifically,
this reflector BFD session is to have following characteristics:
o MUST NOT transmit any S-BFD packets based on local timer expiry.
o MUST transmit an S-BFD packet in response to a received S-BFD
packet having a valid S-BFD discriminator in the "your
discriminator" field, unless prohibited by local policies (ex:
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 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 packets may be IPv4, IPv6 or MPLS based.
How such S-BFD 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: The type of this session. Allowable values 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 packets to local entities and generates
response S-BFD packets.
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bfd.SessionType variable MUST be initialized to the appropriate type
when an S-BFD session is created.
6.2. State Variable Initialization and Maintenance
Some state variables defined in section 6.8.1 of the BFD base
specification 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.
7. S-BFD Procedures
7.1. S-BFD Packet Demultiplexing
Received BFD control packet MUST first be demultiplexed with
information from the lower layer (ex: destination UDP port,
associated channel type). If the packet is determined to be for an
SBFDReflector, then the 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.
If the packet is determined not to be for SBFDReflector, then the
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
the located session is a SBFDInitiator, then destination of the
packet (i.e. destination IP address) SHOULD be validated to be for
self.
Details of the initial BFD control packet demultiplexing are
described in relevant S-BFD data plane documents.
7.2. Initiator Procedures
S-BFD 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.
Below ASCII art describes 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.
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+--- 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 packet traversal.
Figure 3: S-BFD Continuity Test
7.2.1. SBFDInitiator State Machine
An SBFDInitiator may be a persistent session on the initiator with a
timer for S-BFD 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 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 packet) or indicates the expiration of the
Detection Timer.
+--+
ADMIN DOWN, | |
TIMER | V
+------+ UP +------+
| |-------------------->| |----+
| DOWN | | UP | | UP
| |<--------------------| |<---+
+------+ ADMIN DOWN, +------+
TIMER
Figure 4: SBFDInitiator FSM
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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].
7.2.2. Details of S-BFD Packet Sent by SBFDInitiator
S-BFD packets sent by an SBFDInitiator is to have following contents:
o "my discriminator" assigned by local node.
o "your discriminator" corresponding to a remote entity.
o "State" MUST be set to a value describing local state.
o "Desired Min TX Interval" MUST be set to a value describing local
desired minimum transmit interval.
o "Required Min RX Interval" MUST be zero.
o "Required Min Echo RX Interval" SHOULD be zero.
o "Detection Multiplier" MUST be set to a value describing locally
used multiplier value.
o Demand (D) bit MUST be set.
7.3. Responder Procedures
A network node which receives S-BFD packets transmitted by an
initiator is referred as responder. The responder, upon reception of
S-BFD packets, is to perform necessary relevant validations described
in [RFC5880], [RFC5881], [RFC5883], [RFC5884] and [RFC5885].
7.3.1. Responder Demultiplexing
When a responder receives an S-BFD packet, if the value in the "your
discriminator" field is not one of S-BFD discriminators allocated for
local entities, then this packet MUST NOT be considered for this
mechanism. If the value in the "your discriminator" field is one of
S-BFD discriminators allocated for local entities, then the packet is
determined to be handled by a reflector BFD session responsible for
the S-BFD discriminator. If the packet was determined to be
processed further for this mechanism, then chosen reflector BFD
session is to transmit a response BFD control packet using procedures
described in Section 7.3.2, unless prohibited by local policies (ex:
administrative, security, rate-limiter, etc).
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7.3.2. Details of S-BFD Packet Sent by SBFDReflector
S-BFD packets sent by an SBFDReflector is to have following contents:
o "my discriminator" MUST be copied from received "your
discriminator".
o "your discriminator" MUST be copied from received "my
discriminator".
o "State" MUST be UP or ADMINDOWN. Clarification of reflector BFD
session state is described in Section 7.8.
o "Desired Min TX Interval" MUST be copied from received "Desired
Min TX Interval".
o "Required Min RX Interval" MUST be set to a value describing how
many incoming control packets this reflector BFD session can
handle. Further details are described in Section 7.8.
o "Required Min Echo RX Interval" SHOULD be set to zero.
o "Detection Multiplier" MUST be copied from received "Detection
Multiplier".
o Demand (D) bit MUST be cleared.
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. However,
details of such are outside the scope of this specification.
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 packets are
generated from an SBFDInitiator. This is done by the SBFDReflector
using procedures described in Section 7.8 and setting the Poll (P)
bit in the reflected S-BFD packet. The SBFDInitiator is to then send
the next S-BFD packet with the Final (F) bit set. If an
SBFDReflector receives an S-BFD packet with Poll (P) bit set, then
the SBFDReflector MUST respond with an S-BFD packet with Poll (P) bit
cleared and Final (F) bit set.
7.6. Control Plane Independent (C)
Control plane independent (C) bit for an SBFDInitiator sending S-BFD
packets to a reflector BFD session MUST work according to [RFC5880].
Reflector BFD session also MUST work according to [RFC5880].
Specifically, if reflector BFD session implementation does not share
fate with control plane, then response S-BFD packets transmitted MUST
have control plane independent (C) bit set. If reflector BFD session
implementation shares fate with control plane, then response S-BFD
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packets transmitted MUST NOT have control plane independent (C) bit
set.
7.7. Additional SBFDInitiator Behaviors
o If the SBFDInitiator receives a valid S-BFD packet in response to
transmitted S-BFD packet to a remote entity, then the
SBFDInitiator SHOULD conclude that S-BFD packet reached the
intended remote entity.
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.
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 packet with Demand (D) bit
set, the packet MUST be discarded.
7.8. Additional SBFDReflector Behaviors
o S-BFD packets transmitted by the SBFDReflector MUST have "Required
Min RX Interval" set to a value which expresses how many incoming
S-BFD packets this SBFDReflector can handle. The SBFDReflector
can control how fast SBFInitiators will be sending S-BFD packets
to self by ensuring "Required Min RX Interval" indicates a value
based on the current load.
o If the SBFDReflector wishes to communicate to some or all
SBFDInitiators that monitored local entity is "temporarily out of
service", then S-BFD packets with "state" set to ADMINDOWN are
sent to those SBFDInitiators. The SBFDInitiators, upon reception
of such packets, MUST NOT conclude loss of reachability to
corresponding remote entity, and MUST back off packet transmission
interval for the remote entity to an interval no faster than 1
second. If the SBFDReflector is generating a response S-BFD
packet for a local entity that is in service, then "state" in
response BFD control packets MUST be set to UP.
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o If an SBFDReflector receives an S-BFD packet with Demand (D) bit
cleared, the packet MUST be discarded.
8. 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.
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], 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 packets (with BFD control header) and S-BFD echo
packets (implementation specific) be sent.
o Both S-BFD 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.
There are two reason behind this suggestion:
o S-BFD packets can verify reachability to intended target node,
which allows one to conclude that S-BFD echo packets are u-turning
on the expected target node.
o S-BFD packets can detect when the target node is going out of
service (i.e. via receiving back ADMINDOWN state).
Implementations MAY set "Required Min Echo RX Interval" field to
indicate the rate which SBFDInitiator is sending S-BFD Echo packets
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(in ping) or the rate which SBFDReflector wants SBFDInitiators to
send S-BFD Echo packets (in pong). However, this is likely more than
necessary for the S-BFD Echo function to operate. Therefore, it is
RECOMMENDED that "Required Min Echo RX Interval" field simply be set
to zero in both directions.
Additionally, following aspects are left as implementation details,
and are outside the scope of this document:
o Format of the S-BFD Echo packet (ex: 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], [RFC5881], [RFC5883],
[RFC5884] and [RFC5885] apply to this document. Additionally,
implementing the following measures will strengthen security aspects
of the mechanism described by this document:
o SBFDInitiator MAY pick crypto sequence number based on
authentication mode configured.
o SBFDReflector MUST NOT look at the crypto sequence number before
accepting the packet.
o SBFDReflector MAY look at the Key ID
[I-D.ietf-bfd-generic-crypto-auth] in the incoming packet and
verify the authentication data.
o SBFDReflector MUST accept the packet if authentication is
successful.
o SBFDReflector MUST compute the Authentication data and MUST use
the same sequence number that it received in the S-BFD packet that
it is responding to.
o SBFDInitiator MUST accept the S-BFD packet if it either comes with
the same sequence number as it had sent or it's within the window
that it finds acceptable (described in detail in
[I-D.ietf-bfd-generic-crypto-auth])
Using the above method,
o SBFDReflector continue to remain stateless despite using security.
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o SBFDReflector are not susceptible to replay attacks as they always
respond to S-BFD packets irrespective of the sequence number
carried.
o An attacker cannot impersonate the responder since the
SBFDInitiator will only accept S-BFD packets that come with the
sequence number that it had originally used when sending the S-BFD
packet.
12. IANA Considerations
No action is required by IANA for this document.
13. Acknowledgements
Authors would like to thank Jeffrey Haas, Greg Mirsky and Marc
Binderberger for performing thorough reviews and providing number of
suggestions. Authors would like to thank Girija Raghavendra Rao, Les
Ginsberg, Srihari Raghavan, Vanitha Neelamegam and Vengada Prasad
Govindan from Cisco Systems for providing valuable comments. Authors
would also like to thank John E. Drake and Pablo Frank for providing
comments and suggestions.
14. Contributing Authors
Tarek Saad
Cisco Systems
Email: tsaad@cisco.com
Siva Sivabalan
Cisco Systems
Email: msiva@cisco.com
Nagendra Kumar
Cisco Systems
Email: naikumar@cisco.com
Mallik Mudigonda
Cisco Systems
Email: mmudigon@cisco.com
Sam Aldrin
Huawei Technologies
Email: aldrin.ietf@gmail.com
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15. References
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
2010.
[RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for Multihop Paths", RFC 5883, June 2010.
[RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, June 2010.
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-use-case]
Aldrin, S., Bhatia, M., Mirsky, G., Kumar, N., and S.
Matsushima, "Seamless Bidirectional Forwarding Detection
(BFD) Use Case", draft-ietf-bfd-seamless-use-case-00 (work
in progress), June 2014.
[RFC5885] Nadeau, T. and C. Pignataro, "Bidirectional Forwarding
Detection (BFD) for the Pseudowire Virtual Circuit
Connectivity Verification (VCCV)", RFC 5885, June 2010.
Appendix A. Loop Problem
Consider a scenario where we have two nodes and both are S-BFD
capable.
Node A (IP 192.0.2.1) ----------------- Node B (IP 192.0.2.2)
|
|
Man in the Middle (MiM)
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Assume node A reserved a discriminator 0x01010101 for target
identifier 192.0.2.1 and has a reflector session in listening mode.
Similarly node B reserved a discriminator 0x02020202 for its target
identifier 192.0.2.2 and also has a reflector session in listening
mode.
Suppose MiM sends a spoofed packet with MyDisc = 0x01010101, YourDisc
= 0x02020202, source IP as 192.0.2.1 and dest IP as 192.0.2.2. 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.
FYI: Packet fields do not carry any direction information, i.e., if
this is Ping packet or reply packet.
Solutions
The current proposals to avoid the loop problem are:
o Overload "D" bit (Demand mode bit): Initiator always sets the 'D'
bit and reflector clears it. This way we can identify if a
received packet was a reflected packet and avoid reflecting it
back. However this changes the interpretation of 'D' bit.
o Use of State field in the BFD control packets: Initiator will
always send packets with State set to DOWN and reflector will send
back packets with state field set to UP. Reflectors will never
reflect any received packets with state as UP. However the only
issue is the use of state field differently i.e. state in the
S-BFD control packet from initiator does not reflect the local
state which is anyway not significant at reflector.
o Use of local discriminator as My Disc at reflector: Reflector will
always fill in My Discriminator with a locally allocated
discriminator value (not reserved discriminators) and will not
copy it from the received packet.
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Authors' Addresses
Nobo Akiya
Cisco Systems
Email: nobo@cisco.com
Carlos Pignataro
Cisco Systems
Email: cpignata@cisco.com
Dave Ward
Cisco Systems
Email: wardd@cisco.com
Manav Bhatia
Ionos Networks
Email: manav@ionosnetworks.com
Santosh
Juniper Networks
Email: santoshpk@juniper.net
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