draft-ietf-bfd-multihop-09.txt   rfc5883.txt 
Network Working Group D. Katz Internet Engineering Task Force (IETF) D. Katz
Internet Draft Juniper Networks Request for Comments: 5883 D. Ward
Intended status: Proposed Standard D. Ward Category: Standards Track Juniper Networks
Juniper Networks ISSN: 2070-1721 June 2010
Expires: July, 2010 January 5, 2010
BFD for Multihop Paths Bidirectional Forwarding Detection (BFD) for Multihop Paths
draft-ietf-bfd-multihop-09.txt
Status of this Memo Abstract
This Internet-Draft is submitted to IETF in full conformance with the This document describes the use of the Bidirectional Forwarding
provisions of BCP 78 and BCP 79. Detection (BFD) protocol over multihop paths, including
unidirectional links.
Internet-Drafts are working documents of the Internet Engineering Status of This Memo
Task Force (IETF), its areas, and its working groups. Note that
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Internet-Drafts are draft documents valid for a maximum of six months This is an Internet Standards Track document.
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Abstract
This document describes the use of the Bidirectional Forwarding
Detection protocol (BFD) over multihop paths, including
unidirectional links.
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].
1. Introduction 1. Introduction
The Bidirectional Forwarding Detection (BFD) protocol [BFD] defines a The Bidirectional Forwarding Detection (BFD) protocol [BFD] defines a
method for liveness detection of arbitrary paths between systems. method for liveness detection of arbitrary paths between systems.
The BFD one-hop specification [BFD-1HOP] describes how to use BFD The BFD one-hop specification [BFD-1HOP] describes how to use BFD
across single hops of IPv4 and IPv6. across single hops of IPv4 and IPv6.
BFD can also be useful on arbitrary paths between systems, which may BFD can also be useful on arbitrary paths between systems, which may
span multiple network hops and follow unpredictable paths. span multiple network hops and follow unpredictable paths.
Furthermore, a pair of systems may have multiple paths between them Furthermore, a pair of systems may have multiple paths between them
that may overlap. This document describes methods for using BFD in that may overlap. This document describes methods for using BFD in
such scenarios. such scenarios.
2. Applicability 1.1. Conventions Used in This Document
Please note that BFD is intended as a connectivity check/connection The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
verification OAM mechanism. It is applicable for network-based "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
services (e.g. router-to-router, subscriber-to-gateway, LSP/circuit document are to be interpreted as described in RFC 2119 [KEYWORDS].
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 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. There are three primary issues in the use of BFD for multihop paths.
The first is security and spoofing; [BFD-1HOP] describes a The first is security and spoofing; [BFD-1HOP] describes a
lightweight method of avoiding spoofing by requiring a TTL/hop limit lightweight method of avoiding spoofing by requiring a Time to Live
of 255 on both transmit and receive, but this obviously does not work (TTL)/Hop Limit of 255 on both transmit and receive, but this
across multiple hops. The utilization of BFD authentication obviously does not work across multiple hops. The utilization of BFD
addresses this issue. authentication addresses this issue.
The second, more subtle issue is that of demultiplexing multiple BFD The second, more subtle, issue is that of demultiplexing multiple BFD
sessions between the same pair of systems to the proper BFD session. sessions between the same pair of systems to the proper BFD session.
In particular, the first BFD packet received for a session may carry In particular, the first BFD packet received for a session may carry
a Your Discriminator value of zero, resulting in ambiguity as to a Your Discriminator value of zero, resulting in ambiguity as to
which session the packet should be associated. Once the which session the packet should be associated. Once the
discriminator values have been exchanged, all further packets are discriminator values have been exchanged, all further packets are
demultiplexed to the proper BFD session solely by the contents of the demultiplexed to the proper BFD session solely by the contents of the
Your Discriminator field. Your Discriminator field.
[BFD-1HOP] addresses this by requiring that multiple sessions [BFD-1HOP] addresses this by requiring that multiple sessions
traverse independent physical or logical links--the first packet is traverse independent physical or logical links -- the first packet is
demultiplexed based on the link over which it was received. In the demultiplexed based on the link over which it was received. In the
more general case, this scheme cannot work, as two paths over which more general case, this scheme cannot work, as two paths over which
BFD is running may overlap to an arbitrary degree (including the BFD is running may overlap to an arbitrary degree (including the
first and/or last hop.) first and/or last hop).
Finally, the Echo function MUST NOT be used over multiple hops. Finally, the Echo function MUST NOT be used over multiple hops.
Intermediate hops would route the packets back to the sender, and Intermediate hops would route the packets back to the sender, and
connectivity through the entire path would not be possible to verify. connectivity through the entire path would not be possible to verify.
4. Demultiplexing Packets 4. Demultiplexing Packets
There are a number of possibilities for addressing the demultiplexing There are a number of possibilities for addressing the demultiplexing
issue which may be used, depending on the application. issue that may be used, depending on the application.
4.1. Totally Arbitrary Paths 4.1. Totally Arbitrary Paths
It may be desired to use BFD for liveness detection over paths for 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.) 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 A straightforward approach to this problem is to limit BFD deployment
to a single session between a source/destination address pair. to a single session between a source/destination address pair.
Multiple sessions between the same pair of systems must have at least Multiple sessions between the same pair of systems must have at least
one endpoint address distinct from one another. one endpoint address distinct from one another.
In this scenario, the initial packet is demultiplexed to the In this scenario, the initial packet is demultiplexed to the
appropriate BFD session based on the source/destination address pair appropriate BFD session based on the source/destination address pair
when Your Discriminator is set to zero. when Your Discriminator is set to zero.
This approach is appropriate for general connectivity detection This approach is appropriate for general connectivity detection
between systems over routed paths, and is also useful for OSPF between systems over routed paths and is also useful for OSPF Virtual
Virtual Links [OSPFv2] [OSPFv3]. Links [OSPFv2] [OSPFv3].
4.2. Out-of-band Discriminator Signaling 4.2. Out-of-Band Discriminator Signaling
Another approach to the demultiplexing problem is to signal the Another approach to the demultiplexing problem is to signal the
discriminator values in each direction through an out-of-band discriminator values in each direction through an out-of-band
mechanism prior to establishing the BFD session. Once learned, the mechanism prior to establishing the BFD session. Once learned, the
discriminators are sent as usual in the BFD Control packets; no discriminators are sent as usual in the BFD Control packets; no
packets with Your Discriminator set to zero are ever sent. This packets with Your Discriminator set to zero are ever sent. This
method is used by the BFD MPLS specification [BFD-MPLS]. method is used by the BFD MPLS specification [BFD-MPLS].
This approach is advantageous because it allows BFD to be directed by This approach is advantageous because it allows BFD to be directed by
other system components that have knowledge of the paths in use, and other system components that have knowledge of the paths in use, and
from the perspective of BFD implementation it is very simple. from the perspective of BFD implementation it is very simple.
The disadvantage is that it requires at least some level of BFD- The disadvantage is that it requires at least some level of BFD-
specific knowledge in parts of the system outside of BFD. specific knowledge in parts of the system outside of BFD.
4.3. Unidirectional Links 4.3. Unidirectional Links
Unidirectional links are classified as multihop paths because the Unidirectional links are classified as multihop paths because the
return path (which should exist at some level in order to make 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 link useful) may be arbitrary, and the return paths for BFD sessions
protecting parallel unidirectional links may overlap or even be protecting parallel unidirectional links may overlap or even be
identical. (If two unidirectional links, one in each direction, are identical. (If two unidirectional links, one in each direction, are
to carry a single BFD session, this can be done using the single-hop to carry a single BFD session, this can be done using the single-hop
approach.) approach.)
Either of the two methods outlined earlier may be used in the Either of the two methods outlined earlier may be used in the
Unidirectional link case, but a more general solution can be done unidirectional link case, but a more general solution can be found
strictly within BFD and without addressing limitations. strictly within BFD and without addressing limitations.
The approach is similar to the one-hop specification, since the The approach is similar to the one-hop specification, since the
unidirectional link is a single hop. Let's define the two systems as unidirectional link is a single hop. Let's define the two systems as
the Unidirectional Sender and the Unidirectional Receiver. In this the Unidirectional Sender and the Unidirectional Receiver. In this
approach the Unidirectional Sender MUST operate in the Active role approach, the Unidirectional Sender MUST operate in the Active role
(as defined in the base BFD specification), and the Unidirectional (as defined in the base BFD specification), and the Unidirectional
Receiver MUST operate in the Passive role. Receiver MUST operate in the Passive role.
In the Passive role, by definition, the Unidirectional Receiver does In the Passive role, by definition, the Unidirectional Receiver does
not transmit any BFD Control packets until it learns the not transmit any BFD Control packets until it learns the
discriminator value in use by the other system (upon receipt of the discriminator value in use by the other system (upon receipt of the
first BFD Control packet.) The Unidirectional Receiver demultiplexes first BFD Control packet). The Unidirectional Receiver demultiplexes
the first packet to the proper BFD session based on the physical or the first packet to the proper BFD session based on the physical or
logical link over which was received. This allows the receiver to logical link over which it was received. This allows the receiver to
learn the remote discriminator value, which it then echoes back to learn the remote discriminator value, which it then echoes back to
the sender in its own (arbitrarily routed) BFD Control packet, after the sender in its own (arbitrarily routed) BFD Control packet, after
which time all packets are demultiplexed solely by discriminator. which time all packets are demultiplexed solely by discriminator.
5. Encapsulation 5. Encapsulation
The encapsulation of BFD Control packets for multihop application in The encapsulation of BFD Control packets for multihop application in
IPv4 and IPv6 is identical to that defined in [BFD-1HOP], except that 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 UDP destination port MUST have a value of 4784. This can aid in
the demultiplexing and internal routing of incoming BFD packets. the demultiplexing and internal routing of incoming BFD packets.
6. Authentication 6. Authentication
By their nature, multihop paths expose BFD to spoofing. As the By their nature, multihop paths expose BFD to spoofing. As the
number of hops increase, the exposure to attack grows. As such, number of hops increases, the exposure to attack grows. As such,
implementations of BFD SHOULD utilize cryptographic authentication implementations of BFD SHOULD utilize cryptographic authentication
over multihop paths to help mitigate denial-of-service attacks. over multihop paths to help mitigate denial-of-service attacks.
Normative References 7. IANA Considerations
[BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection", Port 4784 has been assigned by IANA for use with BFD Multihop
draft-ietf-bfd-base-10.txt, January, 2010. Control.
[BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single 8. Security Considerations
Hop)", draft-ietf-bfd-v4v6-1hop-11.txt, January, 2010.
[KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate As the number of hops increases, BFD becomes further exposed to
Requirement Levels", RFC 2119, March 1997. attack. The use of strong forms of authentication is strongly
encouraged.
Informative References No additional security issues are raised in this document beyond
those that exist in the referenced BFD documents.
[BFD-MPLS] Aggarwal, R., Kompella, K., et al, "BFD for MPLS LSPs", 9. References
draft-ietf-bfd-mpls-07.txt, June, 2008.
[OSPFv2] Moy, J., "OSPF Version 2", RFC 2328, April 1998. 9.1. Normative References
[OSPFv3] Coltun, R., et al, "OSPF for IPv6", RFC 2740, December 1999. [BFD] Katz, D. and D. Ward, "Bidirectional Forwarding
Detection", RFC 5880, June 2010.
[TFRC] Floyd, S., et al, "TCP Friendly Rate Control (TFRC): Protocol [BFD-1HOP] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
Specification", RFC 5348, September, 2008. (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
2010.
Security Considerations [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
As the number of hops increases, BFD becomes further exposed to 9.2. Informative References
attack. The use of strong forms of authentication is strongly
encouraged.
No additional security issues are raised in this document beyond [BFD-MPLS] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
those that exist in the referenced BFD documents. "Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, June 2010.
IANA Considerations [OSPFv2] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
Port 4784 has been assigned by IANA for use with this protocol. [OSPFv3] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
Authors' Addresses [TFRC] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", RFC
5348, September 2008.
Dave Katz Authors' Addresses
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, California 94089-1206 USA
Phone: +1-408-745-2000
Email: dkatz@juniper.net
Dave Ward Dave Katz
Juniper Networks Juniper Networks
1194 N. Mathilda Ave. 1194 N. Mathilda Ave.
Sunnyvale, California 94089-1206 USA Sunnyvale, CA 94089-1206
Phone: +1-408-745-2000 USA
Email: dward@juniper.net
Changes from the previous draft Phone: +1-408-745-2000
EMail: dkatz@juniper.net
An applicability section was added. All other changes are editorial Dave Ward
in nature. Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089-1206
USA
This document expires in July, 2010. Phone: +1-408-745-2000
EMail: dward@juniper.net
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