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IETF RFC 7175
Last modified on Thursday, May 8th, 2014
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Internet Engineering Task Force (IETF) V. Manral
Request for Comments: 7175 Ionos Corp.
Category: Standards Track D. Eastlake 3rd
ISSN: 2070-1721 Huawei R&D USA
D. Ward
Cisco Systems
A. Banerjee
Cumulus Networks
May 2014
Transparent Interconnection of Lots of Links (TRILL):
Bidirectional Forwarding Detection (BFD) Support
 Abstract
This document specifies use of the Bidirectional Forwarding Detection
(BFD) protocol in Routing Bridge (RBridge) campuses based on the
RBridge Channel extension to the Transparent Interconnection of Lots
of Links (TRILL) protocol.
BFD is a widely deployed Operations, Administration, and Maintenance
(OAM) mechanism in IP and MPLS networks, using UDP and Associated
Channel Header (ACH) encapsulation respectively. This document
specifies the BFD encapsulation over TRILL.
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/RFC 7175.
Manral, et al. Standards Track PAGE 1 
RFC 7175 TRILL BFD Support May 2014
Copyright Notice
Copyright (c) 2014 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.
Table of Contents
1. Introduction ....................................................2
1.1. Terminology ................................................3
2. BFD over TRILL ..................................................3
2.1. Sessions and Initialization ................................4
3. TRILL BFD Control Protocol ......................................5
3.1. One-Hop TRILL BFD Control ..................................5
3.2. BFD Control Frame Processing ...............................5
4. TRILL BFD Echo Protocol .........................................6
4.1. BFD Echo Frame Processing ..................................6
5. Management and Operations Considerations ........................7
6. Default Authentication ..........................................7
7. Security Considerations .........................................8
8. IANA Considerations .............................................9
9. Acknowledgements ................................................9
10. References .....................................................9
10.1. Normative References ......................................9
10.2. Informative References ...................................10
1. Introduction
Faster convergence is a critical feature of Transparent
Interconnection of Lots of Links (TRILL) [RFC 6325] networks. The
TRILL IS-IS Hellos [RFC 7177] [IS-IS] used between RBridges provide a
basic neighbor and continuity check for TRILL links. However,
failure detection by non-receipt of such Hellos is based on the
Holding Time parameter that is commonly set to a value of tens of
seconds and, in any case, has a minimum expressible value of one
second.
Manral, et al. Standards Track PAGE 2
RFC 7175 TRILL BFD Support May 2014
Some applications, including Voice over IP, may wish, with high
probability, to detect interruptions in continuity within a much
shorter time period. In some cases, physical-layer failures can be
detected very rapidly, but this is not always possible, such as when
there is a failure between two bridges that are in turn between two
RBridges. There are also many subtle failures possible at higher
levels. For example, some forms of failure could affect unicast
frames while still letting multicast frames through; since all TRILL
IS-IS Hellos are multicast, such a failure cannot be detected with
Hellos. Thus, a low-overhead method for frequently testing
continuity for the TRILL Data between neighbor RBridges is necessary
for some applications. The BFD protocol [RFC 5880] provides a low-
overhead method for the rapid detection of connectivity failures.
BFD is a widely deployed OAM [RFC 6291] mechanism in IP and MPLS
networks, using UDP and ACH encapsulation, respectively. This
document describes a TRILL encapsulation for BFD packets for networks
that forward based on the TRILL Header.
1.1. Terminology
This document uses the acronyms defined in [RFC 6325] along with the
following:
BFD: Bidirectional Forwarding Detection
IP: Internet Protocol
IS-IS: Intermediate System to Intermediate System
MH: Multi-Hop
PPP: Point-to-Point Protocol
OAM: Operations, Administration, and Maintenance
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].
2. BFD over TRILL
TRILL supports unicast neighbor BFD Echo and one-hop and multi-hop
BFD Control, as specified below, over the RBridge Channel facility
[RFC 7178]. (Multi-destination BFD is a work in progress [MultiBFD].)
BFD-over-TRILL support is similar to BFD-over-IP support [RFC 5881],
except where differences are explicitly mentioned.
Manral, et al. Standards Track PAGE 3
RFC 7175 TRILL BFD Support May 2014
Asynchronous and demand modes MUST be supported [RFC 5880]. BFD over
TRILL supports the Echo function; however, implementation of TRILL
BFD Echo is optional, and it can only be used for single-hop
sessions.
The TRILL Header hop count in the BFD packets is sent out with the
maximum value of 0x3F. To prevent spoofing attacks, the TRILL hop
count of a received session is checked [RFC 5082]. For a single-hop
session, if the hop count is less than 0x3F and the RBridge Channel
Header MH flag is zero, the packet is discarded. For multi-hop
sessions, the hop count check can be disabled if the MH flag is one.
As in BFD for IP, the format of the Echo Packet content is not
defined.
New RBridge Channel code points for BFD TRILL Control and BFD Echo
packets are specified.
Authentication mechanisms as supported in BFD are also supported for
BFD running over TRILL.
2.1. Sessions and Initialization
Within an RBridge campus, there will be no more than one TRILL BFD
Control session from any RBridge RB1 to RBridge RB2 for each RB1
TRILL port. This 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 and interface.
Note that TRILL BFD provides OAM facilities for the TRILL data plane.
This is above whatever protocol is in use on a particular link, such
as a pseudowire [RFC 7173], Ethernet [RFC 6325], or PPP link [RFC 6361].
Link-technology-specific OAM protocols may be used on a link between
neighbor RBridges, for example, Continuity Fault Management [802.1Q]
if the link is Ethernet. But such link-layer OAM (and coordination
between it and OAM in the TRILL data-plane layer, such as TRILL BFD)
is beyond the scope of this document.
If lower-level mechanisms are in use, such as link aggregation
[802.1AX], that present a single logical interface to TRILL IS-IS,
then only a single TRILL BFD session can be established to any other
RBridge over this logical interface. However, lower-layer OAM could
be aware of and/or run separately on each of the components of an
aggregation.
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RFC 7175 TRILL BFD Support May 2014
3. TRILL BFD Control Protocol
TRILL BFD Control frames are unicast TRILL RBridge Channel frames
[RFC 7178]. The RBridge Channel Protocol value is given in Section 8.
The protocol-specific data associated with the TRILL BFD Control
protocol is as shown in Section 4.1 of [RFC 5880].
3.1. One-Hop TRILL BFD Control
One-hop TRILL BFD Control is typically used to rapidly detect link
and RBridge failures. TRILL BFD frames over one hop for such
purposes SHOULD be sent with high priority; that is, the Inner.VLAN
tag priority should be 7, they should be queued for transmission as
maximum priority frames, and, if they are being sent on an Ethernet
link where the output port is configured to include an Outer.VLAN
tag, that tag should specify priority 7.
For neighbor RBridges RB1 and RB2, each RBridge sends one-hop TRILL
BFD Control frames to the other only if TRILL IS-IS has detected
bidirectional connectivity; that is, the adjacency is in the 2-Way or
Report state [RFC 7177], and both RBridges indicate support of TRILL
BFD is enabled. The BFD-Enabled TLV is used to indicate this as
specified in [RFC 6213].
3.2. BFD Control Frame Processing
The following tests SHOULD be performed on received TRILL BFD Control
frames before generic BFD processing.
o Is the M bit in the TRILL Header non-zero? If so, discard the
frame. (Multi-destination BFD is a work in progress [MultiBFD].)
Failure to perform this test would make a denial-of-service attack
using bogus multi-destination BFD Control frames easier.
o If the Channel Header MH flag is zero, indicating one hop, test
that the TRILL Header hop count received was 0x3F (i.e., is 0x3E
if it has already been decremented); if it is any other value,
discard the frame. If the Channel Header MH flag is one,
indicating multi-hop, test that the TRILL Header hop count
received was not less than a configurable value that defaults to
0x30. If it is less, discard the frame. Failure to perform these
tests would make it easier to spoof BFD Control frames. However,
if forged BFD Control frames are a concern, then BFD
Authentication [RFC 5880] should be used.
Manral, et al. Standards Track PAGE 5
RFC 7175 TRILL BFD Support May 2014
4. TRILL BFD Echo Protocol
A TRILL BFD Echo frame is a unicast RBridge Channel frame, as
specified in [RFC 7178], which should be forwarded back by an
immediate neighbor because both the ingress and egress nicknames are
set to a nickname of the originating RBridge. Normal TRILL Data
frame forwarding will cause the frame to be returned unless micro-
loop suppression logic in the neighbor RBridge prohibits sending a
frame back out the port on which it was received or the like.
RBridges with such prohibitions cannot support BFD Echo. The TRILL
OAM protocol number for BFD Echo is given in Section 8.
TRILL BFD Echo frames SHOULD be sent on a link only if the following
conditions are met. An Echo originating under other circumstances
will consume bandwidth and CPU resources but is unlikely to be
returned.
- A TRILL BFD Control session has been established,
- TRILL BFD Echo support is indicated by the RBridge that would
potentially respond to the BFD Echo,
- The adjacency is in the Report state [RFC 7177], and
- The TRILL BFD Echo originating RBridge wishes to make use of this
optional feature.
Since the originating RBridge is the RBridge that will be processing
a returned Echo frame, the entire TRILL BFD Echo protocol-specific
data area is considered opaque and left to the discretion of the
originating RBridge. Nevertheless, it is suggested that this data
include information by which the originating RBridge can authenticate
the returned BFD Echo frame and confirm the neighbor that echoed the
frame back. For example, it could include its own System ID, the
neighbor's System ID, a session identifier, and a sequence count as
well as a Message Authentication Code.
4.1. BFD Echo Frame Processing
The following tests MUST be performed on returned TRILL BFD Echo
frames before other processing. The RBridge Channel document
[RFC 7178] requires that the information in the TRILL Header be given
to the BFD protocol.
o Is the M bit in the TRILL Header non-zero? If so, discard the
frame. (Multi-destination BFD is a work in progress [MultiBFD].)
Manral, et al. Standards Track PAGE 6
RFC 7175 TRILL BFD Support May 2014
o The TRILL BFD Echo frame should have gone exactly two hops, so
test that the TRILL Header hop count as received was 0x3E (i.e.,
0x3D if it has already been decremented), and if it is any other
value, discard the frame. The RBridge Channel Header in the frame
MUST have the MH bit equal to one, and if it is zero, discard the
frame.
5. Management and Operations Considerations
The TRILL BFD parameters on an RBridge are configurable. The default
values are the same as in the IP BFD case [RFC 5881], except where
specified in this document, such as for hop count.
It is up to the operator of an RBridge campus to configure the rates
at which TRILL BFD frames are transmitted on a link to avoid
congestion (e.g., link, input/output (I/O), CPU) and false failure
detection. See also the discussion of congestion in Section 2 of
[RFC 5881].
As stated in [RFC 5880]:
It is worth noting that a single BFD session does not consume a
large amount of bandwidth. An aggressive session that achieves a
detection time of 50 milliseconds, by using a transmit interval of
16.7 milliseconds and a detect multiplier of 3, will generate 60
packets per second. The maximum length of each packet on the wire
is on the order of 100 bytes, for a total of around 48 kilobits
per second of bandwidth consumption in each direction.
6. Default Authentication
Consistent with TRILL's goal of being able to operate with minimum
configuration, the default for BFD authentication between neighbor
RBridges is based on the state of the IS-IS shared secret
authentication for Hellos between those RBridges as detailed below.
The BFD authentication algorithm and methods in this section MUST be
implemented at an RBridge if TRILL IS-IS authentication and BFD are
implemented at that RBridge. If such BFD authentication is
configured, then its configuration is not restricted by the
configuration of IS-IS security.
If IS-IS authentication is not in effect between neighbor RBridges,
then, by default, TRILL BFD between those RBridges is also unsecured.
If such IS-IS authentication is in effect, then, unless configured
otherwise, TRILL BFD Control frames sent between those RBridges MUST
use BFD Meticulous Keyed SHA1 authentication [RFC 5880]. The BFD
authentication keys between neighbor RBridges by default are derived
Manral, et al. Standards Track PAGE 7
RFC 7175 TRILL BFD Support May 2014
from the IS-IS shared secret authentication keys for Hellos between
those RBridges as detailed below. However, such BFD authentication
keys MAY be configured to some other value.
HMAC-SHA256 ( ( "TRILL BFD Control" | originPortID | originSysID ),
IS-IS-shared-key )
In the above, "|" indicates concatenation; HMAC-SHA256 is as
described in [FIPS180] and [RFC 6234]; and "TRILL BFD Control" is the
17-byte US ASCII [ASCII] string indicated that is then concatenated
with the 2-byte Port ID of the originating port and the 6-byte IS-IS
System ID of the originating RBridge, the last two items being in
network byte order. The Port and System IDs are included to minimize
exposure of the same key to improve resistance to cryptanalysis.
IS-IS-shared-key is secret keying material being used for IS-IS
authentication on the link.
The use of the above derived key is accomplished by associating the
above default authentication type and key with the Key ID of the
IS-IS-shared-key used in the derivation and then using that Key ID in
the Authentication Section of the BFD Control frame OAM protocol-
specific data. Also, Auth Type would be 5, and Auth Len would be 28
in the Authentication Section. RBridges MAY be configured to use
other BFD security modes or keying material or configured to use no
security.
Authentication for TRILL BFD Echo is a local implementation issue as
BFD Echo frames are authenticated by their sender when returned by a
neighbor. However, if TRILL IS-IS and BFD Control are being
authenticated to a neighbor and BFD Echo is in use, BFD Echo frames
to be returned by that neighbor should be authenticated, and such
authentication should use different keying material from other types
of authentication. For example, it could use keying material derived
as follows, where "|" indicates concatenation:
HMAC-SHA256 ( ( "TRILL BFD Echo" | originPortID | originSysID ),
IS-IS-shared-key )
7. Security Considerations
BFD over TRILL utilizes the RBridge Channel extension to the TRILL
protocol and is generally analogous to BFD over IP. As such, the BFD
authentication facility is available to authenticate BFD-over-TRILL
packet payloads, but no encryption or other security features are
provided at the BFD-over-TRILL level. See the following:
- [RFC 5881] for general BFD security considerations,
Manral, et al. Standards Track PAGE 8
RFC 7175 TRILL BFD Support May 2014
- [RFC 7178] for general RBridge Channel security considerations, and
- [RFC 6325] for general TRILL protocol security considerations.
Section 3.2 describes security concerns with multi-hop BFD Control
packets and failure to check the TRILL Header M bit in BFD Control
packets.
8. IANA Considerations
IANA has allocated two RBridge Channel protocol numbers [RFC 7178]
from the Standards Action range, as follows:
Protocol Number
-------- ------
BFD Control 0x002
BFD Echo 0x003
9. Acknowledgements
The authors would like to specially thank Dave Katz, an author of
[RFC 5880] and [RFC 5881], from which some material herein has been
reproduced.
The following individuals are thanked for their comments and
suggestions: Scott Bradner, Stewart Bryant, Stephen Farrell, Eric
Gray, Brian Haberman, Barry Leiba, Erik Nordmark, John Scudder,
Robert Sparks, Martin Stiemerling, and Sean Turner.
10. References
10.1. Normative References
[ASCII] American National Standards Institute, "Coded Character
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[FIPS180] National Institute of Science and Technology, "Secure Hash
Standard (SHS)", Federal Information Processing Standard
(FIPS) 180-4, March 2012, <http://csrc.nist.gov/
publications/fips/fips180-4/fips-180-4.pdf>.
[IS-IS] International Organization for Standardization,
"Intermediate System to Intermediate System intra-domain
routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode network service (ISO 8473)", Second
Edition, November 2002.
Manral, et al. Standards Track PAGE 9
RFC 7175 TRILL BFD Support May 2014
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
[RFC 5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
2010.
[RFC 6213] Hopps, C. and L. Ginsberg, "IS-IS BFD-Enabled TLV", RFC
6213, April 2011.
[RFC 6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, July 2011.
[RFC 7177] Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H., and
V. Manral, "Transparent Interconnection of Lots of Links
(TRILL): Adjacency", RFC 7177, May 2014.
[RFC 7178] Eastlake 3rd, D., Manral, V., Li, Y., Aldrin, S., and D.
Ward, "Transparent Interconnection of Lots of Links
(TRILL): RBridge Channel Support", RFC 7178, May 2014.
10.2. Informative References
[802.1AX] IEEE, "IEEE Standard for Local and metropolitan area
networks -- Link Aggregation", IEEE Std 802.1AX-2008,
January 2008.
[802.1Q] IEEE, "IEEE Standard for Local and metropolitan area
networks -- Media Access Control (MAC) Bridges and Virtual
Bridged Local Area Networks", IEEE Std 802.1Q-2011, August
2011.
[MultiBFD] Katz, D. and D. Ward, "BFD for Multipoint Networks", Work
in Progress, February 2014.
[RFC 5082] Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, October 2007.
[RFC 6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.
Manral, et al. Standards Track PAGE 10
RFC 7175 TRILL BFD Support May 2014
[RFC 6291] Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
D., and S. Mansfield, "Guidelines for the Use of the "OAM"
Acronym in the IETF", BCP 161, RFC 6291, June 2011.
[RFC 6361] Carlson, J. and D. Eastlake 3rd, "PPP Transparent
Interconnection of Lots of Links (TRILL) Protocol Control
Protocol", RFC 6361, August 2011.
[RFC 7173] Yong, L., Eastlake 3rd, D., Aldrin, S., and J. Hudson,
"Transparent Interconnection of Lots of Links (TRILL)
Transport Using Pseudowires", RFC 7173, May 2014.
Manral, et al. Standards Track PAGE 11
RFC 7175 TRILL BFD Support May 2014
Authors' Addresses
Vishwas Manral
Ionos Corp.
4100 Moorpark Ave.
San Jose, CA 95117
USA
EMail: vishwas@ionosnetworks.com
Donald Eastlake 3rd
Huawei R&D USA
155 Beaver Street
Milford, MA 01757
USA
Phone: +1-508-333-2270
EMail: d3e3e3@gmail.com
Dave Ward
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95138
USA
EMail: dward@cisco.com
Ayan Banerjee
Cumulus Networks
1089 West Evelyn Avenue
Sunnyvale, CA 94086
USA
EMail: ayabaner@gmail.com
Manral, et al. Standards Track PAGE 12
RFC TOTAL SIZE: 24954 bytes
PUBLICATION DATE: Thursday, May 8th, 2014
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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