draft-ietf-bfd-multihop-06.txt   draft-ietf-bfd-multihop-07.txt 
Network Working Group D. Katz Network Working Group D. Katz
Internet Draft Juniper Networks Internet Draft Juniper Networks
D. Ward Intended status: Proposed Standard D. Ward
Cisco Systems Cisco Systems
Expires: July, 2008 January, 2008 Expires: August, 2009 February 5, 2009
BFD for Multihop Paths BFD for Multihop Paths
draft-ietf-bfd-multihop-06.txt draft-ietf-bfd-multihop-07.txt
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Abstract Abstract
This document describes the use of the Bidirectional Forwarding This document describes the use of the Bidirectional Forwarding
Detection protocol (BFD) over multihop paths, including Detection protocol (BFD) over multihop paths, including
unidirectional links. unidirectional links.
Conventions used in this document Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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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. Issues 2. Issues
There are two 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 TTL/hop limit
of 255 on both transmit and receive, but this obviously does not work of 255 on both transmit and receive, but this obviously does not work
across multiple hops. The utilization of BFD authentication across multiple hops. The utilization of BFD authentication
addresses this issue. addresses this issue.
The more subtle issue is that of demultiplexing multiple BFD sessions The second, more subtle issue is that of demultiplexing multiple BFD
between the same pair of systems to the proper BFD session. In sessions between the same pair of systems to the proper BFD session.
particular, the first BFD packet received for a session may carry a In particular, the first BFD packet received for a session may carry
Your Discriminator value of zero, resulting in ambiguity as to which a Your Discriminator value of zero, resulting in ambiguity as to
session the packet should be associated. Once the discriminator which session the packet should be associated. Once the
values have been exchanged, all further packets are demultiplexed to discriminator values have been exchanged, all further packets are
the proper BFD session solely by the contents of the Your demultiplexed to the proper BFD session solely by the contents of the
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.
Intermediate hops would route the packets back to the sender, and
connectivity through the entire path would not be possible to verify.
3. Demultiplexing Packets 3. 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 which may be used, depending on the application.
3.1. Totally Arbitrary Paths 3.1. 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
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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 Links [OSPFv2] [OSPFv3]. Virtual Links [OSPFv2] [OSPFv3].
3.2. Out-of-band Discriminator Signalling 3.2. 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 BFD's perspective 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.
3.3. Unidirectional Links 3.3. Unidirectional Links
Unidirectional links are classified as multihop paths because the Unidirectional links are classified as multihop paths because the
return path (which must exist at some level in order to make the link return path (which should exist at some level in order to make the
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 unidirection links, one in each direction, are to identical. (If two unidirectional links, one in each direction, are
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 done
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
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4. Encapsulation 4. 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.
5. Authentication 5. Authentication
By their nature, multihop paths expose BFD to spoofing. By their nature, multihop paths expose BFD to spoofing. As the
Implementations of BFD SHOULD utilize authentication over multihop number of hops increase, the exposure to attack grows. As such,
paths to help mitigate denial-of-service attacks. implementations of BFD SHOULD utilize cryptographic authentication
over multihop paths to help mitigate denial-of-service attacks.
Normative References Normative References
[BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection", [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection",
draft-ietf-bfd-base-07.txt, January, 2008. draft-ietf-bfd-base-09.txt, February, 2009.
[BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single [BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single
Hop)", draft-ietf-bfd-v4v6-1hop-07.txt, January, 2008. Hop)", draft-ietf-bfd-v4v6-1hop-09.txt, February, 2009.
[BFD-MPLS] Aggarwal, R., and Kompella, K., "BFD for MPLS LSPs",
draft-ietf-bfd-mpls-04.txt, March, 2007.
[KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate [KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997. Requirement Levels", RFC 2119, March 1997.
Informative References
[BFD-MPLS] Aggarwal, R., Kompella, K., et al, "BFD for MPLS LSPs",
draft-ietf-bfd-mpls-07.txt, June, 2008.
[OSPFv2] Moy, J., "OSPF Version 2", RFC 2328, April 1998. [OSPFv2] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[OSPFv3] Coltun, R., et al, "OSPF for IPv6", RFC 2740, December 1999. [OSPFv3] Coltun, R., et al, "OSPF for IPv6", RFC 2740, December 1999.
Security Considerations 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 No additional security issues are raised in this document beyond
those that exist in the referenced BFD documents. those that exist in the referenced BFD documents.
IANA Considerations IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
Authors' Addresses Authors' Addresses
Dave Katz Dave Katz
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Dave Ward Dave Ward
Cisco Systems Cisco Systems
170 W. Tasman Dr. 170 W. Tasman Dr.
San Jose, CA 95134 USA San Jose, CA 95134 USA
Phone: +1-408-526-4000 Phone: +1-408-526-4000
Email: dward@cisco.com Email: dward@cisco.com
Changes from the previous draft Changes from the previous draft
This is a reissue of the previous version of the draft. There are A note was added that the Echo function is not available for multihop
only minor editorial changes. BFD. All other changes are editorial in nature.
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This document expires in July, 2008. This document expires in August, 2009.
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