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


   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

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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
   ( in effect on the date of
   publication of this document.  Please review these documents
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   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) [RFC6325] networks.  The
   TRILL IS-IS Hellos [RFC7177] [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

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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 [RFC5880] provides a low-
   overhead method for the rapid detection of connectivity failures.

   BFD is a widely deployed OAM [RFC6291] 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 [RFC6325] along with the

      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",
   document are to be interpreted as described in [RFC2119].

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
   [RFC7178].  (Multi-destination BFD is a work in progress [MultiBFD].)
   BFD-over-TRILL support is similar to BFD-over-IP support [RFC5881],
   except where differences are explicitly mentioned.

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   Asynchronous and demand modes MUST be supported [RFC5880].  BFD over
   TRILL supports the Echo function; however, implementation of TRILL
   BFD Echo is optional, and it can only be used for single-hop

   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 [RFC5082].  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

   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 [RFC7173], Ethernet [RFC6325], or PPP link [RFC6361].
   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

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3.  TRILL BFD Control Protocol

   TRILL BFD Control frames are unicast TRILL RBridge Channel frames
   [RFC7178].  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 [RFC5880].

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 [RFC7177], and both RBridges indicate support of TRILL
   BFD is enabled.  The BFD-Enabled TLV is used to indicate this as
   specified in [RFC6213].

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 [RFC5880] should be used.

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4.  TRILL BFD Echo Protocol

   A TRILL BFD Echo frame is a unicast RBridge Channel frame, as
   specified in [RFC7178], 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

   -  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 [RFC7177], 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
   [RFC7178] 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].)

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   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

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 [RFC5881], 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

   As stated in [RFC5880]:

      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 [RFC5880].  The BFD
   authentication keys between neighbor RBridges by default are derived

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   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 [RFC6234]; 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

   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:

   -  [RFC5881] for general BFD security considerations,

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RFC 7175                    TRILL BFD Support                   May 2014

   -  [RFC7178] for general RBridge Channel security considerations, and

   -  [RFC6325] 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

8.  IANA Considerations

   IANA has allocated two RBridge Channel protocol numbers [RFC7178]
   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
   [RFC5880] and [RFC5881], from which some material herein has been

   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, <

   [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.

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RFC 7175                    TRILL BFD Support                   May 2014

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June

   [RFC6213]  Hopps, C. and L. Ginsberg, "IS-IS BFD-Enabled TLV", RFC
              6213, April 2011.

   [RFC6325]  Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, July 2011.

   [RFC7177]  Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H., and
              V. Manral, "Transparent Interconnection of Lots of Links
              (TRILL): Adjacency", RFC 7177, May 2014.

   [RFC7178]  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

   [MultiBFD] Katz, D. and D. Ward, "BFD for Multipoint Networks", Work
              in Progress, February 2014.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, October 2007.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.

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RFC 7175                    TRILL BFD Support                   May 2014

   [RFC6291]  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.

   [RFC6361]  Carlson, J. and D. Eastlake 3rd, "PPP Transparent
              Interconnection of Lots of Links (TRILL) Protocol Control
              Protocol", RFC 6361, August 2011.

   [RFC7173]  Yong, L., Eastlake 3rd, D., Aldrin, S., and J. Hudson,
              "Transparent Interconnection of Lots of Links (TRILL)
              Transport Using Pseudowires", RFC 7173, May 2014.

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Authors' Addresses

   Vishwas Manral
   Ionos Corp.
   4100 Moorpark Ave.
   San Jose, CA  95117


   Donald Eastlake 3rd
   Huawei R&D USA
   155 Beaver Street
   Milford, MA  01757

   Phone: +1-508-333-2270

   Dave Ward
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95138


   Ayan Banerjee
   Cumulus Networks
   1089 West Evelyn Avenue
   Sunnyvale, CA 94086


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