Bidirectional Forwarding Detection (BFD) for Multihop Paths
Using BFD to check paths that cross several routers, including links that carry traffic in one direction only.
Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883, June 2010, <https://www.rfc-editor.org/info/rfc5883>.
@misc{rfc5883,
series = {Request for Comments},
number = 5883,
howpublished = {RFC 5883},
publisher = {RFC Editor},
doi = {10.17487/RFC5883},
url = {https://www.rfc-editor.org/info/rfc5883},
author = {D. Katz and D. Ward},
title = {{Bidirectional Forwarding Detection (BFD) for Multihop Paths}},
pagetotal = 6,
year = 2010,
month = jun,
} RFC text
Contents
Internet Engineering Task Force (IETF) D. Katz
Request for Comments: 5883 D. Ward
Category: Standards Track Juniper Networks
ISSN: 2070-1721 June 2010
Bidirectional Forwarding Detection (BFD) for Multihop Paths
Abstract
This document describes the use of the Bidirectional Forwarding
Detection (BFD) protocol over multihop paths, including
unidirectional links.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5883.
Copyright Notice
Copyright (c) 2010 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.
1. Introduction
The Bidirectional Forwarding Detection (BFD) protocol [BFD] defines a
method for liveness detection of arbitrary paths between systems.
The BFD one-hop specification [BFD-1HOP] describes how to use BFD
across single hops of IPv4 and IPv6.
BFD can also be useful on arbitrary paths between systems, which may
span multiple network hops and follow unpredictable paths.
Furthermore, a pair of systems may have multiple paths between them
that may overlap. This document describes methods for using BFD in
such scenarios.
1.1. Conventions Used in This Document
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 [KEYWORDS].
2. Applicability
Please note that BFD is intended as an Operations, Administration,
and Maintenance (OAM) mechanism for connectivity check and connection
verification. It is applicable for network-based services (e.g.
router-to-router, subscriber-to-gateway, LSP/circuit endpoints, and
service appliance failure detection). In these scenarios it is
required that the operator correctly provision the rates at which BFD
is transmitted to avoid congestion (e.g link, I/O, CPU) and false
failure detection. It is not applicable for application-to-
application failure detection across the Internet because it does not
have sufficient capability to do necessary congestion detection and
avoidance and therefore cannot prevent congestion collapse. Host-to-
host or application-to-application deployment across the Internet
will require the encapsulation of BFD within a transport that
provides "TCP-friendly" [TFRC] behavior.
3. Issues
There are three primary issues in the use of BFD for multihop paths.
The first is security and spoofing; [BFD-1HOP] describes a
lightweight method of avoiding spoofing by requiring a Time to Live
(TTL)/Hop Limit of 255 on both transmit and receive, but this
obviously does not work across multiple hops. The utilization of BFD
authentication addresses this issue.
The second, more subtle, issue is that of demultiplexing multiple BFD
sessions between the same pair of systems to the proper BFD session.
In particular, the first BFD packet received for a session may carry
a Your Discriminator value of zero, resulting in ambiguity as to
which session the packet should be associated. Once the
discriminator values have been exchanged, all further packets are
demultiplexed to the proper BFD session solely by the contents of the
Your Discriminator field.
[BFD-1HOP] addresses this by requiring that multiple sessions
traverse independent physical or logical links -- the first packet is
demultiplexed based on the link over which it was received. In the
more general case, this scheme cannot work, as two paths over which
BFD is running may overlap to an arbitrary degree (including the
first and/or last hop).
Finally, the Echo function MUST NOT be used over multiple hops.
Intermediate hops would route the packets back to the sender, and
connectivity through the entire path would not be possible to verify.
4. Demultiplexing Packets
There are a number of possibilities for addressing the demultiplexing
issue that may be used, depending on the application.
4.1. Totally Arbitrary Paths
It may be desired to use BFD for liveness detection over paths for
which no part of the route is known (or if known, may not be stable).
A straightforward approach to this problem is to limit BFD deployment
to a single session between a source/destination address pair.
Multiple sessions between the same pair of systems must have at least
one endpoint address distinct from one another.
In this scenario, the initial packet is demultiplexed to the
appropriate BFD session based on the source/destination address pair
when Your Discriminator is set to zero.
This approach is appropriate for general connectivity detection
between systems over routed paths and is also useful for OSPF Virtual
Links [OSPFv2] [OSPFv3].
4.2. Out-of-Band Discriminator Signaling
Another approach to the demultiplexing problem is to signal the
discriminator values in each direction through an out-of-band
mechanism prior to establishing the BFD session. Once learned, the
discriminators are sent as usual in the BFD Control packets; no
packets with Your Discriminator set to zero are ever sent. This
method is used by the BFD MPLS specification [BFD-MPLS].
This approach is advantageous because it allows BFD to be directed by
other system components that have knowledge of the paths in use, and
from the perspective of BFD implementation it is very simple.
The disadvantage is that it requires at least some level of BFD-
specific knowledge in parts of the system outside of BFD.
4.3. Unidirectional Links
Unidirectional links are classified as multihop paths because the
return path (which should exist at some level in order to make the
link useful) may be arbitrary, and the return paths for BFD sessions
protecting parallel unidirectional links may overlap or even be
identical. (If two unidirectional links, one in each direction, are
to carry a single BFD session, this can be done using the single-hop
approach.)
Either of the two methods outlined earlier may be used in the
unidirectional link case, but a more general solution can be found
strictly within BFD and without addressing limitations.
The approach is similar to the one-hop specification, since the
unidirectional link is a single hop. Let's define the two systems as
the Unidirectional Sender and the Unidirectional Receiver. In this
approach, the Unidirectional Sender MUST operate in the Active role
(as defined in the base BFD specification), and the Unidirectional
Receiver MUST operate in the Passive role.
In the Passive role, by definition, the Unidirectional Receiver does
not transmit any BFD Control packets until it learns the
discriminator value in use by the other system (upon receipt of the
first BFD Control packet). The Unidirectional Receiver demultiplexes
the first packet to the proper BFD session based on the physical or
logical link over which it was received. This allows the receiver to
learn the remote discriminator value, which it then echoes back to
the sender in its own (arbitrarily routed) BFD Control packet, after
which time all packets are demultiplexed solely by discriminator.
5. Encapsulation
The encapsulation of BFD Control packets for multihop application in
IPv4 and IPv6 is identical to that defined in [BFD-1HOP], except that
the UDP destination port MUST have a value of 4784. This can aid in
the demultiplexing and internal routing of incoming BFD packets.
6. Authentication
By their nature, multihop paths expose BFD to spoofing. As the
number of hops increases, the exposure to attack grows. As such,
implementations of BFD SHOULD utilize cryptographic authentication
over multihop paths to help mitigate denial-of-service attacks.
7. IANA Considerations
Port 4784 has been assigned by IANA for use with BFD Multihop
Control.
8. Security Considerations
As the number of hops increases, BFD becomes further exposed to
attack. The use of strong forms of authentication is strongly
encouraged.
No additional security issues are raised in this document beyond
those that exist in the referenced BFD documents.
9. References
9.1. Normative References
[BFD] Katz, D. and D. Ward, "Bidirectional Forwarding
Detection", RFC 5880, June 2010.
[BFD-1HOP] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
2010.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[BFD-MPLS] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, June 2010.
[OSPFv2] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[OSPFv3] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
[TFRC] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", RFC
5348, September 2008.
Authors' Addresses
Dave Katz
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089-1206
USA
Phone: +1-408-745-2000
EMail: dkatz@juniper.net
Dave Ward
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089-1206
USA
Phone: +1-408-745-2000
EMail: dward@juniper.net
ISO Plain Language
On this page
§1Why BFD Over More Than One Hop
The Bidirectional Forwarding Detection protocol defines a method for liveness detection of arbitrary paths between systems. The BFD one-hop specification describes how to use BFD across single hops of IPv4 and IPv6.
BFD can also be useful on arbitrary paths between systems, which may span multiple network hops and follow unpredictable paths. A pair of systems may also have multiple paths between them that may overlap. This document describes methods for using BFD in such scenarios.
BFD is a way for two routers to check that the path between them still works, by sending small packets at regular intervals. This RFC extends it to paths that cross several routers, where the route taken may not even be known in advance.
§2Where BFD Multihop Applies
BFD is intended as an Operations, Administration and Maintenance mechanism for connectivity check and connection verification. It is applicable for network-based services, such as router-to-router, subscriber-to-gateway, LSP and circuit endpoints, and service appliance failure detection.
In these scenarios it is required that the operator correctly provision the rates at which BFD is transmitted, to avoid congestion of a link, of input and output, or of the CPU, and to avoid false failure detection. It is not applicable for application-to-application failure detection across the Internet, because it does not have sufficient capability to do the necessary congestion detection and avoidance and therefore cannot prevent congestion collapse. Host-to-host or application-to-application deployment across the Internet will require the encapsulation of BFD within a transport that provides TCP-friendly behaviour.
BFD is for checking that a network path works, for example between two routers or between a customer and their gateway. It is not for checking whether a web service on the other side of the Internet is answering, because BFD cannot tell congestion apart from failure.
§3Three Problems With Multihop BFD
There are three primary issues in the use of BFD for multihop paths.
The first is security and spoofing. The one-hop specification describes a lightweight method of avoiding spoofing by requiring a Time to Live, or Hop Limit, of 255 on both transmit and receive, but this obviously does not work across multiple hops, because every router along the path decrements it. The use of BFD authentication addresses this issue.
The second, more subtle, issue is that of demultiplexing multiple BFD sessions between the same pair of systems to the proper BFD session. In particular, the first BFD packet received for a session may carry a Your Discriminator value of zero, which leaves it ambiguous which session the packet should be associated with. Once the discriminator values have been exchanged, all further packets are demultiplexed to the proper BFD session solely by the contents of the Your Discriminator field.
The one-hop specification addresses this by requiring that multiple sessions traverse independent physical or logical links, so the first packet is demultiplexed based on the link over which it was received. In the more general case this scheme cannot work, as two paths over which BFD is running may overlap to an arbitrary degree, including the first or the last hop.
Finally, the Echo function MUST NOT be used over multiple hops. Intermediate hops would route the packets back to the sender, and connectivity through the entire path would not be possible to verify.
Three things break when the path is longer than one link. The trick that proves a packet came from the neighbour does not survive a router in the middle. Telling two BFD conversations apart is harder when the paths overlap. And the echo mode, where the far end sends packets back, cannot work, because the routers in between would answer first.
- SpoofingThe TTL of 255 used on a single hop does not survive a router in the middle, so authentication is needed.
- DemultiplexingThe first packet of a session may carry a zero discriminator, so the session is ambiguous.
- Echo functionIt must not be used over multiple hops, because intermediate routers would answer instead.
§4.1Demultiplexing Packets
There are a number of possibilities for addressing the demultiplexing issue that may be used, depending on the application.
For totally arbitrary paths, where no part of the route is known or the known part may not be stable, a straightforward approach is to limit BFD deployment to a single session between a source and destination address pair. Multiple sessions between the same pair of systems must have at least one endpoint address distinct from one another. In this scenario the initial packet is demultiplexed to the appropriate BFD session based on the source and destination address pair when the Your Discriminator is set to zero. The approach is appropriate for general connectivity detection between systems over routed paths, and is also useful for OSPF Virtual Links.
With out-of-band discriminator signaling, the discriminator values in each direction are signalled through an out-of-band mechanism before the BFD session is established. Once learned, the discriminators are sent as usual in the BFD Control packets, so no packets with the Your Discriminator set to zero are ever sent. This method is used by the BFD MPLS specification. It is advantageous because it allows BFD to be directed by other system components that have knowledge of the paths in use, and from the point of view of a BFD implementation it is very simple. The disadvantage is that it requires at least some level of BFD-specific knowledge in parts of the system outside of BFD.
There are two ways to sort the first packet of a session to the right conversation. Either each session has its own pair of addresses, or the discriminator numbers are agreed out of band before the session starts, so no packet ever arrives with a zero discriminator.
| Approach | How the first packet is matched |
|---|---|
| Totally arbitrary paths | By the source and destination address pair |
| Out-of-band discriminator signaling | No packet with a zero discriminator is ever sent |
| Unidirectional links | By the physical or logical link it arrived on |
§4.3Unidirectional Links
Unidirectional links are classified as multihop paths, because the return path, which should exist at some level in order to make the link useful, may be arbitrary, and the return paths for BFD sessions protecting parallel unidirectional links may overlap or even be identical. If two unidirectional links, one in each direction, are to carry a single BFD session, this can be done using the single-hop approach.
Either of the two methods above may be used in the unidirectional link case, but a more general solution can be found strictly within BFD and without addressing limitations. The approach is similar to the one-hop specification, since the unidirectional link is a single hop. The two systems are called the Unidirectional Sender and the Unidirectional Receiver. The Unidirectional Sender MUST operate in the Active role, as defined in the base BFD specification, and the Unidirectional Receiver MUST operate in the Passive role.
In the Passive role, by definition, the Unidirectional Receiver does not transmit any BFD Control packets until it learns the discriminator value in use by the other system, which happens on receipt of the first BFD Control packet. The receiver demultiplexes the first packet to the proper BFD session based on the physical or logical link over which it was received. This allows the receiver to learn the remote discriminator value, which it then echoes back to the sender in its own, arbitrarily routed, BFD Control packet, after which time all packets are demultiplexed solely by discriminator.
On a link that only carries traffic one way, the sending side always speaks first and the receiving side stays quiet until it has learned the other end's number. Because the link is a single hop, the receiver can sort the first packet by the link it arrived on.
- BFD CONTROLThe first packet. The receiver matches it by the link it arrived on and learns the remote discriminator.
- BFD CONTROLThe receiver echoes the sender's discriminator back in its own, arbitrarily routed, packet.
§5Encapsulation
The encapsulation of BFD Control packets for multihop application in IPv4 and IPv6 is identical to that defined in the one-hop specification, except that the UDP destination port MUST have a value of 4784. The RFC notes that this can aid in the demultiplexing and internal routing of incoming BFD packets.
Port 4784 has been assigned by IANA for use with BFD Multihop Control.
The packets look the same as in the single-hop case, but they go to a different UDP port, 4784, so that the router can tell multihop BFD traffic apart from other traffic on the same interface.
§6Authentication and Security Considerations
By their nature, multihop paths expose BFD to spoofing, and as the number of hops increases the exposure to attack grows. Implementations of BFD SHOULD therefore utilize cryptographic authentication over multihop paths, to help mitigate denial-of-service attacks.
The security considerations of the RFC say the same thing again: as the number of hops increases, BFD becomes further exposed to attack, and the use of strong forms of authentication is strongly encouraged. No additional security issues are raised in this document beyond those that exist in the referenced BFD documents.
The longer the path, the easier it is for someone to send false BFD packets and make the link look broken. The RFC strongly recommends that BFD packets be authenticated.