Network Working Group D. Katz
Internet Draft Juniper Networks
D. Ward
Cisco Systems
Expires: September, 2007 March, 2007
BFD for IPv4 and IPv6 (Single Hop)
draft-ietf-bfd-v4v6-1hop-06.txt
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Abstract
This document describes the use of the Bidirectional Forwarding
Detection protocol over IPv4 and IPv6 for single IP hops. Comments
on this draft should be directed to rtg-bfd@ietf.org.
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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
One very desirable application for BFD [BFD] is to track IPv4 and
IPv6 connectivity between directly-connected systems. This could be
used to supplement the detection mechanisms in routing protocols, or
to monitor router-host connectivity, among other applications.
This document describes the particulars necessary to use BFD in this
environment. Interactions between BFD and other protocols and system
functions are described in the BFD Generic Applications document
[BFD-GENERIC].
2. Applications and Limitations
This application of BFD can be used by any pair of systems
communicating via IPv4 and/or IPv6 across a single IP hop that can be
associated with an incoming interface. This includes, but is not
limited to, physical media, virtual circuits, and tunnels.
Each BFD session between a pair of systems MUST traverse a separate
path in both directions.
If BFD is to be used in conjunction with both IPv4 and IPv6 on a
particular link, a separate BFD session MUST be established for each
protocol (and thus encapsulated by that protocol) over that link.
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3. Initialization and Demultiplexing
In this application, there will be only a single BFD session between
two systems over a given interface (logical or physical) for a
particular protocol. The BFD session must be bound to this
interface. As such, both sides of a session MUST take the "Active"
role (sending initial BFD Control packets with a zero value of Your
Discriminator) and any BFD packet from the remote machine with a zero
value of Your Discriminator MUST be associated with the session bound
to the remote system, interface, and protocol.
4. Encapsulation
4.1. BFD for IPv4
In the case of IPv4, BFD Control packets MUST be transmitted in UDP
packets with destination port 3784, within an IPv4 packet. The
source port MUST be in the range 49152 through 65535. The same UDP
source port number MUST be used for all BFD Control packets
associated with a particular session. The source port number SHOULD
be unique among all BFD sessions on the system. If more than 16384
BFD sessions are simultaneously active, UDP source port numbers MAY
be reused on multiple sessions, but the number of distinct uses of
the same UDP source port number SHOULD be minimized. An
implementation MAY use the UDP port source number to aid in
demultiplexing incoming BFD Control packets, but ultimately the
mechanisms in [BFD] MUST be used to demultiplex incoming packets to
the proper session.
BFD Echo packets MUST be transmitted in UDP packets with destination
UDP port 3785 in an IPv4 packet. The setting of the UDP source port
is outside the scope of this specification. The destination address
MUST be chosen in such a way as to cause the remote system to forward
the packet back to the local system. The source address MUST be
chosen in such a way as to preclude the remote system from generating
ICMP Redirect messages. In particular, the source address MUST NOT
be part of the subnet bound to the interface over which the BFD Echo
packet is being transmitted.
4.2. BFD for IPv6
In the case of IPv6, BFD Control packets MUST be transmitted in UDP
packets with destination port 3784, within an IPv6 packet. The
source port MUST be in the range 49152 through 65535. The same UDP
source port number MUST be used for all BFD Control packets
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associated with a particular session. The source port number SHOULD
be unique among all BFD sessions on the system. If more than 16384
BFD sessions are simultaneously active, UDP source port numbers MAY
be reused on multiple sessions, but the number of distinct uses of
the same UDP source port number SHOULD be minimized. An
implementation MAY use the UDP port source number to aid in
demultiplexing incoming BFD Control packets, but ultimately the
mechanisms in [BFD] MUST be used to demultiplex incoming packets to
the proper session.
BFD Echo packets MUST be transmitted in UDP packets with destination
UDP port 3785 in an IPv6 packet. The setting of the UDP source port
is outside the scope of this specification. The source and
destination addresses MUST both be associated with the local system.
The destination address MUST be chosen in such a way as to cause the
remote system to forward the packet back to the local system.
5. TTL/Hop Count Issues
If BFD authentication is not in use on a session, all BFD Control
packets for the session MUST be sent with a TTL or Hop Count value of
255. All received BFD Control packets that are demultiplexed to the
session MUST be discarded if the received TTL or Hop Count is not
equal to 255. A discussion of this mechanism can be found in [GTSM].
If BFD authentication is in use on a session, all BFD Control packets
MUST be sent with a TTL or Hop Count value of 255. All received BFD
Control packets that are demultiplexed the session MAY be discarded
if the received TTL or Hop Count is not equal to 255.
In the context of this section, "authentication in use" means that
the system is sending BFD control packets with the Authentication bit
set and with the Authentication Section included, and that all
unauthenticated packets demultiplexed to the session are discarded,
per the BFD base specification.
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6. Addressing Issues
On a subnetted network, BFD Control packets MUST be transmitted with
source and destination addresses that are part of the subnet
(addressed from and to interfaces on the subnet.)
On an addressed but unsubnetted point-to-point link, BFD Control
packets MUST be transmitted with source and destination addresses
that match the addresses configured on that link.
On an unnumbered point-to-point link, the source address of a BFD
Control packet MUST NOT be used to identify the session. This means
that the initial BFD packet MUST be accepted with any source address,
and that subsequent BFD packets MUST be demultiplexed solely by the
My Discriminator field (as is always the case.) This allows the
source address to change if necessary. If the received source
address changes, the local system MUST NOT use that address as the
destination in outgoing BFD Control packets; rather it MUST continue
to use the address configured at session creation. An implementation
MAY notify the application that the neighbor's source address has
changed, so that the application might choose to change the
destination address or take some other action. Note that the TTL/Hop
Count check described in section 5 (or the use of authentication)
precludes the BFD packets from having come from any source other than
the immediate neighbor.
7. BFD for use with Tunnels
A number of mechanisms are available to tunnel IPv4 and IPv6 over
arbitrary topologies. If the tunnel mechanism does not decrement the
TTL or hop count of the network protocol carried within, the
mechanism described in this document may be used to provide liveness
detection for the tunnel. The BFD Authentication mechanism SHOULD be
used and is strongly encouraged.
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Normative References
[BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection",
draft-ietf-bfd-base-06.txt, March, 2007.
[BFD-GENERIC] Katz, D., and Ward, D., "Generic Application of BFD",
draft-ietf-bfd-generic-03.txt, March, 2007 .
[GTSM] Gill, V., et al, "The Generalized TTL Security Mechanism
(GTSM)", RFC 3682, February 2004.
[KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
Security Considerations
In this application, the use of TTL=255 on transmit and receive is
viewed as supplying equivalent security characteristics to other
protocols used in the infrastructure, as it is not trivially
spoofable. The security implications of this mechanism are further
discussed in [GTSM].
The security implications of the use of BFD Authentication are
discussed in [BFD].
The use of the TTL=255 check simultaneously with BFD Authentication
provides a low overhead mechanism for discarding a class of
unauthorized packets and may be useful in implementations in which
cryptographic checksum use is susceptable to denial of service
attacks. The use or non-use of this mechanism does not impact
interoperability.
IANA Considerations
This document has no actions for IANA.
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Authors' Addresses
Dave Katz
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, California 94089-1206 USA
Phone: +1-408-745-2000
Email: dkatz@juniper.net
Dave Ward
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134 USA
Phone: +1-408-526-4000
Email: dward@cisco.com
Changes from the previous draft
This is a reissue of the previous version. There are only minor
editorial changes.
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This document expires in September, 2007.
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