RFC 9747 PROPOSED STANDARD BFD · ROUTING · FAULT DETECTION
Unaffiliated BFD Echo: Unaffiliated Bidirectional Forwarding Detection (BFD) Echo
Use BFD Echo on its own: one side runs BFD and the other just loops packets back.
RFC EDITOR
Cheng, W., Wang, R., Min, X., Ed., Rahman, R., and R. Boddireddy, "Unaffiliated Bidirectional Forwarding Detection (BFD) Echo", RFC 9747, DOI 10.17487/RFC9747, March 2025, <https://www.rfc-editor.org/info/rfc9747>.
BIBTEX
@misc{rfc9747,
series = {Request for Comments},
number = 9747,
howpublished = {RFC 9747},
publisher = {RFC Editor},
doi = {10.17487/RFC9747},
url = {https://www.rfc-editor.org/info/rfc9747},
author = {W. Cheng and R. Wang and X. Min and R. Rahman and R. Boddireddy},
title = {{Unaffiliated Bidirectional Forwarding Detection (BFD) Echo}},
pagetotal = 12,
year = 2025,
month = mar,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Internet Engineering Task Force (IETF) W. Cheng
Request for Comments: 9747 R. Wang
Updates: 5880 China Mobile
Category: Standards Track X. Min, Ed.
ISSN: 2070-1721 ZTE Corp.
R. Rahman
Equinix
R. Boddireddy
Juniper Networks
March 2025
Unaffiliated Bidirectional Forwarding Detection (BFD) Echo
Abstract
This document specifies an extension to the Bidirectional Forwarding
Detection (BFD) protocol that enables the use of the BFD Echo
function without the need for an associated BFD control session.
This "Unaffiliated BFD Echo" mechanism allows rapid detection of
forwarding path failures in networks where establishing BFD control
sessions is impractical or undesirable. By decoupling the Echo
function from the control plane, network devices can utilize BFD's
fast failure detection capabilities in a simplified manner, enhancing
network resiliency and operational efficiency.
This document updates RFC 5880 by defining a new Unaffiliated BFD
Echo mechanism.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for 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 this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9747.
Copyright Notice
Copyright (c) 2025 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
(https://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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Conventions Used in This Document
2. Unaffiliated BFD Echo Procedures
3. Updates to RFC 5880
4. Operational Considerations
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
To minimize the impact of device and link faults on services and to
improve network availability in single-hop scenarios, a network
device needs the capability to quickly detect communication faults
with adjacent devices. Prompt detection allows for timely remedial
actions to ensure service continuity.
BFD [RFC5880] provides a low-overhead, short-interval method for
detecting faults on the communication path between adjacent
forwarding engines, which may include interfaces, data links, and the
forwarding engines themselves. BFD offers a unified mechanism to
monitor any media and protocol layers in real time.
BFD defines two primary modes -- Asynchronous mode and Demand mode --
to accommodate various deployment scenarios. Additionally, it
supports an Echo function that reduces the level of BFD support
required in device implementations, as described in Section 3.2 of
[RFC5880]. When the Echo function is activated, the local system
sends BFD Echo packets, and the remote system loops back the received
Echo packets through the forwarding path, as described in Section 5
of [RFC5880] and Section 4 of [RFC5881]. If several consecutive BFD
Echo packets are not received by the local system, the BFD session is
declared Down.
There are two typical scenarios when using the BFD Echo function:
* Full BFD protocol capability with adjunct Echo function
(Affiliated BFD Echo): This scenario requires both the local
device and the adjacent device to support the full BFD protocol.
This operation remains unchanged from [RFC5880].
* BFD Echo-Only method without full BFD protocol capability
(Unaffiliated BFD Echo): This scenario requires only the local
device to support sending and demultiplexing BFD Control packets.
In this case, BFD Control packets are sent over the BFD Echo port,
and the processing procedures for Asynchronous mode are used with
the modifications specified in this document. Note that this
method requires the local device to send packets with one of its
own IP addresses as the destination address, upon receipt of which
the adjacent device loops them back to the local device. Also
note that this method monitors the connectivity to a device over a
specific interface and does not verify the availability of a
specific IP address at that device.
This document specifies the Unaffiliated BFD Echo scenario.
Section 5 of [RFC5880] indicates that the payload of an Affiliated
BFD Echo packet is a local matter; therefore, its contents are
outside the scope of that specification. This document, however,
specifies the contents of the Unaffiliated BFD Echo packet and the
procedures for handling them. While this may appear to contravene
Section 5 of [RFC5880], the core behavior in that RFC states that the
contents of BFD Echo packets are a local matter; this document is
defining that "local matter". Regarding the selection of IP
addresses, the rules stated in Section 4 of [RFC5881] are applicable
to the encapsulation of an Unaffiliated BFD Echo packet.
Section 6.2.2 of [BBF-TR-146] describes a use case for the
Unaffiliated BFD Echo.
This document updates [RFC5880] by defining a new method of BFD Echo-
only operation which only impacts the sender of BFD Echo packets
without requiring an implementation to support the BFD protocol at
the loopback device, such that any IP forwarder can loop back the BFD
Echo packets. It specifies the use of the Unaffiliated BFD Echo over
IPv4 and IPv6 for a single IP hop. The reason why it cannot be used
for multihop paths is that the Unaffiliated BFD Echo packets would be
looped back by the first hop. A full description of the updates to
[RFC5880] is provided in Section 3.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Unaffiliated BFD Echo Procedures
This section specifies the Unaffiliated BFD Echo procedures.
Device A Device B
+----------------+ +----------------+
| | | |
| |------------| | |
| |Unaffiliated| | |
| | BFD Echo ---> | |
| | Session | | |
| | | Unaffiliated BFD Echo | |
| | -------------------------------| BFD |
| | | | packets |
| | <-------------------------------| looped |
| |------------| | |
| | | |
| | | |
+----------------+ +----------------+
BFD supported BFD not supported
Figure 1: Unaffiliated BFD Echo
As shown in Figure 1, device A supports BFD, whereas device B is a
regular IP forwarder that does not support BFD. Device A would send
Unaffiliated BFD Echo packets, and after receiving the Unaffiliated
BFD Echo packets sent from device A, the one-hop-away BFD peer device
B immediately loops them back by normal IP forwarding. This allows
device A to rapidly detect a connectivity loss to device B. Note
that device B would not intercept any received Unaffiliated BFD Echo
packet or parse any BFD protocol field within the Unaffiliated BFD
Echo packet.
An Unaffiliated BFD Echo session is not actually a BFD session
because there is no coordination of BFD protocol state between the
two link ends: the remote end does not support BFD and so cannot
engage in a BFD session. From the standpoint of the local end (as an
initiator), the Unaffiliated BFD Echo session may be regarded as a
BFD session.
For the Unaffiliated Echo procedure, an Unaffiliated BFD Echo session
is established on device A. The session MUST adhere to the BFD state
machine specified in Section 6.2 of [RFC5880], with the exception
that the received state is not derived from BFD Control packets
originating from the remote system, but rather from packets that are
generated by the local system and looped back from the remote system.
Consequently, the AdminDown state is not utilized in Unaffiliated BFD
Echo.
BFD Control packets are transmitted and received as Unaffiliated BFD
Echo packets, using UDP destination port 3785, as defined in
[RFC5881]. The standard procedures for BFD Asynchronous sessions are
applied to the looped BFD Control packets, including packet
validation and authentication, in accordance with [RFC5880].
Once an Unaffiliated BFD Echo session is created on device A, it
starts sending Unaffiliated BFD Echo packets. Unaffiliated BFD Echo
packets with zeroed "Your Discriminator" field are demultiplexed to
the proper session based on the source IP address or UDP source port.
After the remote system loops back the local discriminator, all
further received packets are demultiplexed based on the "Your
Discriminator" field only, which conforms to the procedure specified
in Section 6.3 of [RFC5880]. An Unaffiliated BFD Echo packet follows
the same encapsulation rules as for a BFD Echo packet as specified in
Section 4 of [RFC5881]. All Unaffiliated BFD Echo packets for the
session MUST be sent with a TTL or Hop Limit value of 255. Received
packets MUST have a TTL or Hop Limit value of 254 (similar to
Appendix A of [RFC5082] to verify against a configured number of
hops); otherwise, the received packets MUST be dropped.
In the context of an Unaffiliated BFD Echo packet, the "Desired Min
TX Interval" and "Required Min RX Interval" fields, as defined in
[RFC5880], MUST be populated with a specific value to prevent the
potential exposure of uninitialized memory. It is RECOMMENDED that
these fields be set to a value of 1 second (1,000,000 microseconds).
However, upon receipt, these values MUST be ignored and MUST NOT be
used in the calculation of the Detection Time.
The "Required Min Echo RX Interval" field, as defined in [RFC5880],
MUST be populated with a specific value to prevent the potential
exposure of uninitialized memory. It is RECOMMENDED that this field
be set to 0. However, this value MUST be ignored upon receipt. The
transmission interval for Unaffiliated BFD Echo packets when in the
Up state MUST be provisioned on device A.
The functionality of the Unaffiliated BFD Echo feature is dependent
on device B performing IP forwarding. While this capability is
typically expected to be supported on routers, it may not be enabled
by default on hosts. The method for provisioning device B to loop
back Unaffiliated BFD Echo packets is outside the scope of this
document.
Similar to what's specified in [RFC5880], the Unaffiliated BFD Echo
session begins with the periodic, slow transmission of Unaffiliated
BFD Echo packets. The slow transmission rate should be no greater
than one packet per second, until the session on device A is Up.
After the session is Up, the provisioned transmission interval is
used. When the Unaffiliated BFD Echo session on device A goes Down,
the slow transmission rate is resumed. The "Detect Mult" field
defined in [RFC5880] MUST be set to a value provisioned on device A.
When the bfd.SessionState is Up and a "Detect Mult" number of
Unaffiliated BFD Echo packets have not arrived at device A as they
should, the device A "MUST set bfd.SessionState to Down and
bfd.LocalDiag to 2 (Echo Function Failed)", as specified in
Section 6.8.5 of [RFC5880].
In summary, the Unaffiliated BFD Echo packet reuses the format of the
BFD Control packet defined in [RFC5880], and the fields within the
Unaffiliated BFD Echo packet are populated as follows:
* My Discriminator: MUST be set to the provisioned local
discriminator.
* Your Discriminator: MUST initially be set to 0, and then MUST be
set to the value of "My Discriminator" looped back from the remote
system.
* Desired Min TX Interval: MUST be set to a specific value, with a
suggested value of 1 second (1,000,000 microseconds).
* Required Min RX Interval: MUST be set to a specific value, with a
suggested value of 1 second (1,000,000 microseconds).
* Required Min Echo RX Interval: MUST be set to a specific value,
with a suggested value of 0.
* Detect Mult: MUST be set to the provisioned maximum allowable
number of consecutively lost Unaffiliated BFD Echo packets.
3. Updates to RFC 5880
The Unaffiliated BFD Echo described in this document reuses the BFD
Echo function as described in [RFC5880] and [RFC5881], but does not
require BFD Asynchronous or Demand mode. In the Unaffiliated BFD
Echo operation, only the local system has the BFD protocol enabled,
while the remote system simply loops back the received BFD Echo
packets as ordinary data packets, without engaging in the BFD
protocol.
This document updates [RFC5880] with respect to its descriptions on
the BFD Echo function as follows.
The fourth paragraph of Section 3.2 of [RFC5880] is updated as below:
OLD TEXT
| An adjunct to both modes is the Echo function.
NEW TEXT
| An adjunct to both modes is the Echo function, which can also be
| running independently.
OLD TEXT
| Since the Echo function is handling the task of detection, the
| rate of periodic transmission of Control packets may be reduced
| (in the case of Asynchronous mode) or eliminated completely (in
| the case of Demand mode).
NEW TEXT
| Since the Echo function is handling the task of detection, the
| rate of periodic transmission of Control packets may be reduced
| (in the case of Asynchronous mode) or eliminated completely (in
| the case of Demand mode). The Echo function may also be used
| independently, with neither Asynchronous nor Demand mode.
The third and ninth paragraphs of Section 6.1 of [RFC5880] are
updated as below:
OLD TEXT
| Once the BFD session is Up, a system can choose to start the Echo
| function if it desires and the other system signals that it will
| allow it. The rate of transmission of Control packets is
| typically kept low when the Echo function is active.
NEW TEXT
| When a system is running with Asynchronous or Demand mode, once
| the BFD session is Up, it can choose to start the Echo function if
| it desires and the other system signals that it will allow it.
| The rate of transmission of Control packets is typically kept low
| for Asynchronous mode or eliminated completely for Demand mode
| when the Echo function is active.
OLD TEXT
| If the session goes Down, the transmission of Echo packets (if
| any) ceases, and the transmission of Control packets goes back to
| the slow rate.
NEW TEXT
| In Asynchronous mode or Demand mode, if the session goes Down, the
| transmission of Echo packets (if any) ceases, and the transmission
| of Control packets goes back to the slow rate.
The second paragraph of Section 6.4 of [RFC5880] is updated as below:
OLD TEXT
| When a system is using the Echo function, it is advantageous to
| choose a sedate reception rate for Control packets, since liveness
| detection is being handled by the Echo packets. This can be
| controlled by manipulating the Required Min RX Interval field (see
| section 6.8.3).
NEW TEXT
| When a system is using the Echo function with Asynchronous mode,
| it is advantageous to choose a sedate reception rate for Control
| packets, since liveness detection is being handled by the Echo
| packets. This can be controlled by manipulating the Required Min
| RX Interval field (see section 6.8.3). Note that a system
| operating in Demand mode would direct the remote system to cease
| the periodic transmission of BFD Control packets, by setting the
| Demand (D) bit in its BFD Control packets.
The second paragraph of Section 6.8 of [RFC5880] is updated as below:
OLD TEXT
| When a system is said to have "the Echo function active" it means
| that the system is sending BFD Echo packets, implying that the
| session is Up and the other system has signaled its willingness to
| loop back Echo packets.
NEW TEXT
| When a system in Asynchronous or Demand mode is said to have "the
| Echo function active" it means that the system is sending BFD Echo
| packets, implying that the session is Up and the other system has
| signaled its willingness to loop back Echo packets.
The seventh paragraph of Section 6.8.3 of [RFC5880] is updated as
below:
OLD TEXT
| When the Echo function is active, a system SHOULD set
| bfd.RequiredMinRxInterval to a value of not less than one second
| (1,000,000 microseconds). This is intended to keep received BFD
| Control traffic at a negligible level, since the actual detection
| function is being performed using BFD Echo packets.
NEW TEXT
| When the Echo function is active with Asynchronous mode, a system
| SHOULD set bfd.RequiredMinRxInterval to a value of not less than
| one second (1,000,000 microseconds). This is intended to keep
| received BFD Control traffic at a negligible level, since the
| actual detection function is being performed using BFD Echo
| packets. A system operating in Demand mode would not receive BFD
| Control traffic.
The first and second paragraphs of Section 6.8.9 of [RFC5880] are
updated as below:
OLD TEXT
| BFD Echo packets MUST NOT be transmitted when bfd.SessionState is
| not Up. BFD Echo packets MUST NOT be transmitted unless the last
| BFD Control packet received from the remote system contains a
| nonzero value in Required Min Echo RX Interval.
|
| BFD Echo packets MAY be transmitted when bfd.SessionState is Up.
| The interval between transmitted BFD Echo packets MUST NOT be less
| than the value advertised by the remote system in Required Min
| Echo RX Interval, except as follows: [...]
NEW TEXT
| When a system is using the Echo function with either Asynchronous
| or Demand mode, BFD Echo packets MUST NOT be transmitted when
| bfd.SessionState is not Up, and BFD Echo packets MUST NOT be
| transmitted unless the last BFD Control packet received from the
| remote system contains a nonzero value in Required Min Echo RX
| Interval.
|
| When a system is using the Echo function with either Asynchronous
| or Demand mode, BFD Echo packets MAY be transmitted when
| bfd.SessionState is Up, and the interval between transmitted BFD
| Echo packets MUST NOT be less than the value advertised by the
| remote system in Required Min Echo RX Interval, except as follows:
| [...]
4. Operational Considerations
All operational considerations from [RFC5880] apply. Since this
mechanism leverages existing BFD machinery, particularly periodic
pacing of traffic based on configuration, there's no real possibility
to create congestion. Moreover, creating congestion would be
counterproductive to checking the bidirectional connectivity.
Some devices that would benefit from the use of BFD may be unable to
support the full BFD protocol. Examples of such devices include
servers running virtual machines, or Internet of Things (IoT)
devices. By using Unaffiliated BFD Echo, these devices only need to
support a basic loopback function.
As specified in Section 2 of this document, some configuration is
needed to make the Unaffiliated BFD Echo work, although the
configuration won't go beyond the scope of [RFC5880]. At a BFD-
enabled local system, the Unaffiliated BFD Echo session can coexist
with other types of BFD sessions. In that scenario, the remote
system for the Unaffiliated BFD Echo session must be different from
the remote system for any other type of BFD session, and the local
system's discriminators for different BFD sessions must be different.
At the same time, it's not necessary for the local system to
differentiate the Unaffiliated BFD Echo session from the other types
of BFD sessions.
5. Security Considerations
All security considerations from [RFC5880] and [RFC5881] apply.
Unaffiliated BFD Echo requires the remote device to loop Unaffiliated
BFD Echo packets. In order to provide this service, the remote
device cannot make use of Unicast Strict Reverse Path Forwarding
(RPF) [RFC3704], otherwise the Unaffiliated BFD Echo packets might
not pass the RPF check at the remote device.
As described in Section 5 of [RFC5880], BFD Echo packets may be
spoofed. Specifically for Unaffiliated BFD Echo, a DoS attacker may
send spoofed Unaffiliated BFD Echo packets to the loopback device, so
some form of authentication SHOULD be included. Considering the
Unaffiliated BFD Echo packets in this document are also BFD Control
packets, the "Authentication Section" as defined in [RFC5880] for a
BFD Control packet is RECOMMENDED to be included within the
Unaffiliated BFD Echo packet.
As stated in Section 2, in order to avoid unset values being a
potential vector for disclosure of uninitialized memory, all fields
of the Unaffiliated BFD Echo packet MUST be populated with a certain
value, even if some of the fields are ignored on receipt.
6. IANA Considerations
This document has no IANA actions.
7. References
7.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
DOI 10.17487/RFC5881, June 2010,
<https://www.rfc-editor.org/info/rfc5881>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[BBF-TR-146]
Broadband Forum, "TR-146: Subscriber Sessions", Broadband
Forum Technical Report, TR-146, Issue 1, May 2013,
<https://www.broadband-forum.org/pdfs/tr-146-1-0-0.pdf>.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
2004, <https://www.rfc-editor.org/info/rfc3704>.
[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
<https://www.rfc-editor.org/info/rfc5082>.
Acknowledgements
The authors would like to acknowledge Ketan Talaulikar, Greg Mirsky,
Santosh Pallagatti, Aijun Wang, Éric Vyncke, Adrian Farrel, Tim
Wicinski, Dhruv Dhody, Stephen Farrell, Gunter Van de Velde, Gyan
Mishra, Brian Trammell, Gorry Fairhurst, Mahesh Jethanandani, John
Scudder, Murray Kucherawy, and Zaheduzzaman Sarker for their careful
reviews and very helpful comments.
The authors would like to acknowledge Jeff Haas for his guidance,
insightful review, and very helpful comments.
The authors would like to acknowledge Erik Auerswald for his
insightful comments during the discussion of this document.
The authors would like to acknowledge Detao Zhao for the very helpful
discussion.
Contributors
Liu Aihua
ZTE
Email: liu.aihua@zte.com.cn
Qian Xin
ZTE
Email: qian.xin2@zte.com.cn
Zhao Yanhua
ZTE
Email: zhao.yanhua3@zte.com.cn
Authors' Addresses
Weiqiang Cheng
China Mobile
Beijing
China
Email: chengweiqiang@chinamobile.com
Ruixue Wang
China Mobile
Beijing
China
Email: wangruixue@chinamobile.com
Xiao Min (editor)
ZTE Corp.
Nanjing
China
Email: xiao.min2@zte.com.cn
Reshad Rahman
Equinix
Ottawa
Canada
Email: reshad@yahoo.com
Raj Chetan Boddireddy
Juniper Networks
Email: rchetan@juniper.net
AI ELI5
On this page
PROBLEMBFD normally needs both ends of a link to run the protocol, but some devices, such as servers running virtual machines or IoT devices, cannot support full BFD.
MECHANISMThe local device sends BFD Control packets to the BFD Echo port with its own IP address as destination, and the neighbor loops them back through ordinary IP forwarding. The local device runs the BFD state machine on the looped packets.
WHY IT MATTERSA device can quickly detect loss of connectivity to a one-hop neighbor that has no BFD support, but only over a single IP hop. The RFC updates RFC 5880.
§1Affiliated and Unaffiliated Echo
Bidirectional Forwarding Detection (BFD, RFC 5880) detects faults on the path between adjacent forwarding engines. It has Asynchronous and Demand modes, plus an optional Echo function where the remote system loops the sender's packets back. If several consecutive Echo packets do not return, the session is declared Down.
The RFC names two ways to use Echo. Affiliated BFD Echo needs full BFD on both devices and is unchanged. Unaffiliated BFD Echo needs only the local device to support BFD. It sends BFD Control packets over the Echo port, using Asynchronous mode procedures with modifications, with one of its own IP addresses as the destination, so the neighbor returns them.
This method checks connectivity to a device over a specific interface. It does not verify that a specific IP address at that device is available. It works for a single IP hop over IPv4 and IPv6, because over multiple hops the first hop would loop the packets back.
Normally two players both need to know the game. Here only one player needs to know the game, and the other just bounces the ball back.
§2How Unaffiliated Echo Works
Device A supports BFD. Device B is a regular IP forwarder. A sends Unaffiliated BFD Echo packets and B loops them back by normal IP forwarding, without intercepting them or parsing any BFD field. A can then rapidly detect a connectivity loss to B. No BFD state is coordinated between the two ends, so strictly this is not a BFD session, though A can treat it as one.
The session on A MUST follow the BFD state machine of RFC 5880 Section 6.2, except that the received state comes from looped-back packets A itself generated. The AdminDown state is not used. Packets use UDP destination port 3785 and the encapsulation of RFC 5881. Standard Asynchronous procedures apply, including validation and authentication.
Packets with a zeroed Your Discriminator are matched to a session by source IP address or UDP source port. Once the remote system loops back the local discriminator, matching uses Your Discriminator only. Packets MUST be sent with TTL or Hop Limit 255, and received packets MUST have 254 or be dropped.
A starts with slow transmission, no greater than one packet per second, until the session is Up, then uses the provisioned interval. If a Detect Mult number of packets fail to arrive while Up, A sets bfd.SessionState to Down and bfd.LocalDiag to 2 (Echo Function Failed). Going Down resumes slow transmission.
You send yourself a message through a friend who just passes it along unopened. If your messages stop coming back several times in a row, you know the road to your friend is broken.
Device A (BFD supported)Device B (BFD not supported)
- Unaffiliated BFD EchoDestination address is one of A's own IP addresses.
- Looped back packetB forwards it normally and does not parse BFD fields.
§2Packet Field Values
An Unaffiliated BFD Echo packet reuses the BFD Control packet format. Every field MUST be filled with a definite value, even if the receiver ignores it, so that uninitialized memory is not exposed.
My Discriminator: the provisioned local discriminator.Your Discriminator: 0 at first, then the My Discriminator value looped back.Desired Min TX IntervalandRequired Min RX Interval: a specific value, suggested 1 second (1,000,000 microseconds). They MUST be ignored on receipt and not used for Detection Time.Required Min Echo RX Interval: a specific value, suggested 0, ignored on receipt.Detect Mult: the provisioned maximum number of consecutively lost packets.
The transmit interval while Up MUST be provisioned on device A. How device B is set up to loop packets back is outside the scope of the document.
Every blank on the form must be filled in with something, even blanks nobody will read, so no old scribbles leak out.
§3Updates to RFC 5880
Unaffiliated Echo reuses the Echo function without Asynchronous or Demand mode. Only the local system runs BFD, and the remote system treats Echo packets as ordinary data. To allow this, the RFC changes several passages of RFC 5880 with OLD TEXT and NEW TEXT blocks.
The changes are mostly clarifications. Section 3.2 now says the Echo function can run independently, with neither Asynchronous nor Demand mode. Sections 6.1, 6.4, 6.8, 6.8.3 and 6.8.9 now say that their existing Echo rules apply when a system is using Echo with Asynchronous or Demand mode. For instance, the rule that Echo packets MUST NOT be sent unless the remote system advertises a nonzero Required Min Echo RX Interval applies to those modes, not to Unaffiliated Echo.
The old rulebook assumed both players always sat at the table. A few lines were reworded to say the bounce-back game can also be played alone.
§4Operational Notes
All operational considerations from RFC 5880 apply. Because traffic is paced by configuration, there is no real possibility of congestion, and congesting the path would defeat the check anyway.
The device that loops packets back must perform IP forwarding. This is typical for routers but may not be on by default for hosts. An Unaffiliated Echo session can coexist with other BFD sessions on the local system, but its remote system must differ from the remote system of any other BFD session, and discriminators must differ across sessions.
The friend who bounces your messages must be willing to pass them along. Many routers do this by default, but a plain computer may need to be told.
§5Security Considerations
The security considerations of RFC 5880 and RFC 5881 apply. The remote device cannot use Unicast Strict Reverse Path Forwarding (RFC 3704), or the looped packets might fail the check.
Echo packets can be spoofed, and a DoS attacker could send spoofed Unaffiliated BFD Echo packets to the loopback device, so some form of authentication SHOULD be used. Since the packets are also BFD Control packets, including the Authentication Section is RECOMMENDED. The RFC has no IANA actions.
A prankster could throw fake balls at the wall. A secret stamp on your own balls lets you tell yours from fakes.
§7References
[RFC5880]RFC 5880Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, June 2010.June 2010[RFC5881]RFC 5881Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June 2010.June 2010[RFC3704]RFC 3704Baker, F. and P. Savola, "Ingress Filtering for Multihomed Networks", BCP 84, RFC 3704, March 2004.March 2004[RFC5082]RFC 5082Gill, V., et al., "The Generalized TTL Security Mechanism (GTSM)", RFC 5082, October 2007.October 2007