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Internet Engineering Task Force (IETF)                          C. HoppsRequest for Comments: 6213                                   L. GinsbergCategory: Standards Track                                  Cisco SystemsISSN: 2070-1721                                               April 2011IS-IS BFD-Enabled TLVAbstract   This document describes a type-length-value (TLV) for use in the IS-   IS routing protocol that allows for the proper use of the   Bidirectional Forwarding Detection (BFD) protocol.  There exist   certain scenarios in which IS-IS will not react appropriately to a   BFD-detected forwarding plane failure without use of either this TLV   or some other method.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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6213.Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Hopps & Ginsberg             Standards Track                    [Page 1]

RFC 6213                  IS-IS BFD-Enabled TLV               April 2011Table of Contents1. Introduction ....................................................21.1. Requirements Language ......................................22. The Problem .....................................................23. The Solution ....................................................33.1. State Definitions ..........................................33.2. Adjacency Establishment and Maintenance ....................43.3. Advertisement of Topology-Specific IS Neighbors ............44. Transition ......................................................45. Graceful Restart ................................................56. The BFD-Enabled TLV .............................................57. Security Considerations .........................................68. IANA Considerations .............................................69. Acknowledgements ................................................610. Normative References ...........................................71.  Introduction   The Bidirectional Forwarding Detection (BFD) protocol [RFC5880] is a   protocol that allows for detection of a forwarding plane failure   between two routers.  A router can use [RFC5880] to validate that a   peer router's forwarding ability is functioning.   One specific application of BFD as described in [RFC5882] is to   verify the forwarding ability of an IS-IS [RFC1195] router's   adjacencies; however, the method described in [RFC5882] does not   allow for certain failure scenarios.  We will define a TLV that will   allow for proper response to the detection of all forwarding failures   where the use of BFD is employed with IS-IS.1.1.  Requirements Language   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 inRFC 2119 [RFC2119].2.  The Problem   We observe that, in order to allow for mixed use (i.e., some routers   running BFD and some not), [RFC5882] does not require a BFD session   be established prior to the establishment of an IS-IS adjacency.   Thus, if a router A has neighbors B and C, and B does not support   BFD, A would still form adjacencies with B and C, and it would only   establish a BFD session with C.Hopps & Ginsberg             Standards Track                    [Page 2]

RFC 6213                  IS-IS BFD-Enabled TLV               April 2011   The problem with this solution is that it assumes that the   transmission and receipt of IS-IS Hellos (IIHs) shares fate with   forwarded data packets.  This is not a fair assumption to make given   that the primary use of BFD is to protect IPv4 (and IPv6) forwarding,   and IS-IS does not utilize IPv4 or IPv6 for sending or receiving its   hellos.   Thus, if we consider our previous example, and if C is currently   experiencing an IPv4 forwarding failure that allows for IIHs to be   sent and received, when A first starts (or restarts), A will assume   that C simply does not support BFD, will form an adjacency with C,   and may incorrectly forward IPv4 traffic through C.3.  The Solution   A simple solution to this problem is for an IS-IS router to advertise   that it has BFD enabled on a given interface.  It can do this through   the inclusion of a TLV in its IIHs as described in this document.   When sending an IIH on a BFD enabled interface, a router that   supports this extension MUST include the BFD-enabled TLV in its IIH.   The contents of the TLV MUST indicate what topologies/protocols   [RFC5120] have been enabled for BFD by including the appropriate   Multi-Topology Identifier (MTID)/ Network Layer Protocol Identifier   (NLPID) pairs.   When sending an IIH on an interface on which BFD is NOT enabled, a   router MUST NOT include the BFD-enabled TLV.3.1.  State Definitions   The following definitions apply to each IS-IS neighbor:   For each locally supported MTID/NLPID pair, an   "ISIS_TOPO_NLPID_BFD_REQUIRED" variable is assigned.  If BFD is   supported by both the local system and the neighbor of the MTID/   NLPID, this variable is set to "TRUE".  Otherwise, the variable is   set to "FALSE".   For each locally supported MTID, an "ISIS_TOPO_BFD_REQUIRED" variable   is set to the logical "OR" of all "ISIS_TOPO_NLPID_BFD_REQUIRED"   variables associated with that MTID.   An "ISIS_BFD_REQUIRED" variable is set to the logical "AND" of all   "ISIS_TOPO_BFD_REQUIRED" variables.Hopps & Ginsberg             Standards Track                    [Page 3]

RFC 6213                  IS-IS BFD-Enabled TLV               April 2011   For each locally supported MTID/NLPID pair, an   "ISIS_TOPO_NLPID_STATE" variable is assigned.  If   "ISIS_TOPO_NLPID_BFD_REQUIRED" is "TRUE", this variable follows the   BFD session state for that MTID/NLPID ("UP == TRUE").  Otherwise, the   variable is set to "TRUE".   For each locally supported topology (MTID), an "ISIS_TOPO_USEABLE"   variable is set to the logical "AND" of the set of   "ISIS_TOPO_NLPID_STATE" variables associated with that MTID.   An "ISIS_NEIGHBOR_USEABLE" variable is set to the logical "OR" of all   "ISIS_TOPO_USEABLE" variables.3.2.  Adjacency Establishment and Maintenance   Whenever "ISIS_BFD_REQUIRED" is "TRUE", the following extensions to   the rules for adjacency establishment and maintenance MUST apply:   o  "ISIS_NEIGHBOR_USEABLE" MUST be "TRUE" before the adjacency can      transition from "INIT" to "UP" state.   o  When the IS-IS adjacency is "UP" and "ISIS_NEIGHBOR_USEABLE"      becomes "FALSE", the IS-IS adjacency MUST transition to "DOWN".   o  On a Point-to-Point circuit whenever "ISIS_NEIGHBOR_USEABLE" is      "FALSE", the Three-Way adjacency state MUST be set to "DOWN" in      the Point-to-Point Three-Way Adjacency TLV [RFC5303] in all      transmitted IIHs.   o  On a LAN circuit whenever "ISIS_NEIGHBOR_USEABLE" is "FALSE", the      IS Neighbors TLV advertising the Media Access Control (MAC)      address of the neighbor MUST be omitted in all transmitted IIHs.3.3.  Advertisement of Topology-Specific IS Neighbors   The advertisement of a topology-specific IS neighbor (as well as the   use of the neighbor in the topology-specific decision process) is   determined by the value of "ISIS_TOPO_USEABLE" for each topology.  If   "ISIS_TOPO_USEABLE" is "TRUE", then the topology-specific neighbor is   advertised.  If "ISIS_TOPO_USEABLE" is "FALSE", then the topology-   specific neighbor is not advertised.4.  Transition   To allow for a non-disruptive transition to the use of BFD, some   amount of time should be allowed before bringing down an "UP"   adjacency on a BFD enabled interface when the value of   "ISIS_BFD_REQUIRED" becomes "TRUE" as a result of the introduction ofHopps & Ginsberg             Standards Track                    [Page 4]

RFC 6213                  IS-IS BFD-Enabled TLV               April 2011   the BFD TLV or the modification (by adding a new supported MTID/   NLPID) of an existing BFD TLV in a neighbor's IIH.  A simple way to   do this is to not update the adjacency hold time when receiving such   an IIH from a neighbor with whom we have an "UP" adjacency until   "ISIS_NEIGHBOR_USEABLE" becomes "TRUE".   If the value of "ISIS_BFD_REQUIRED" becomes "FALSE" as a result of   the removal the BFD TLV or the modification (by removing a supported   MTID/NLPID) of an existing BFD TLV in a neighbor's IIH, then BFD   session establishment is no longer required to maintain the adjacency   or transition the adjacency to the "UP" state.   If a BFD session is administratively shut down [RFC5880] and the BFD   session state change impacts the value of "ISIS_NEIGHBOR_USEABLE",   then IS-IS SHOULD allow time for the corresponding MTID/NLPID to be   removed from the neighbor's BFD TLV by not updating the adjacency   hold time until "ISIS_BFD_REQUIRED" becomes "FALSE".  Note that while   this allows a non-disruptive transition, it still enforces   consistency between the administrative state of the BFD session and   the MTID/NLPID(s) advertised in the BFD TLV.  This is necessary to   provide consistent behavior regardless of whether the BFD AdminDown   state is introduced before or after an IS-IS adjacency "UP" state has   been achieved.5.  Graceful Restart   This section describes IS-IS implementation considerations when both   IS-IS graceful restart [RFC5306] and BFD are co-deployed.   In cases where BFD shares fate with the control plane, it can be   expected that BFD session failure may occur in conjunction with the   control-plane restart.  In such cases, premature abort of IS-IS   graceful restart as a result of BFD session failure is undesirable.   Therefore, some mechanism to ignore the BFD session failure for a   limited period of time would be beneficial.  The issue of the   interaction between graceful restart and BFD is described at length   inRFC 5882.  The implementation of this interaction is outside the   scope of this document.6.  The BFD-Enabled TLV   The BFD-enabled TLV is formatted as shown below.  The TLV SHALL only   be included in an IIH and only when BFD is enabled for one or more   supported MTID/protocols on the interface over which the IIH is being   sent.  The NLPIDs encoded in the TLV are defined in [ISO9577].Hopps & Ginsberg             Standards Track                    [Page 5]

RFC 6213                  IS-IS BFD-Enabled TLV               April 2011     Type   148     Length # of octets in the value field (3 to 255)     Value  3 octets specifying the MTID/NLPID for each            topology/data protocol for which BFD support is enabled                                          No. of octets                +-----------------------+                |R|R|R|R|   MTID        |     2                +-----------------------+                |      NLPID            |     1                +-----------------------+                :                       :                :                       :                +-----------------------+                |R|R|R|R|   MTID        |     2                +-----------------------+                | NLPID                 |     1                +-----------------------+7.  Security Considerations   The TLV defined within this document describes an addition to the   IS-IS Hello protocol.  Inappropriate use of this TLV could prevent an   IS-IS adjacency from forming or lead to failure to detect   bidirectional forwarding failures -- each of which is a form of   denial of service.  However, a party who can manipulate the contents   of this TLV is already in a position to create such a denial of   service by disrupting IS-IS routing in other ways.   Note that the introduction of this TLV has no impact on the use/   non-use of authentication either by IS-IS or by BFD.8.  IANA Considerations   The following IS-IS TLV type is defined by this document.   Name                                  Value  IIH  LSP  SNP  Purge   ----------------------                -----  ---  ---  ---  -----   BFD-Enabled TLV                         148   y    n    n     n   The IS-IS TLV Codepoint registry has been updated accordingly.9.  Acknowledgements   The authors wish to thank Jeffrey Haas, Matthew Jones, Dave Katz,   Jonathan Moon, Stefano Previdi, Mike Shand, Michael Shiplett, and   David Ward for various input on this document.Hopps & Ginsberg             Standards Track                    [Page 6]

RFC 6213                  IS-IS BFD-Enabled TLV               April 201110.  Normative References   [ISO9577]  International Organization for Standardization, "Protocol              identification in the network layer(ISO/IEC 9577)", ISO/              IEC 9577:1999, Fourth Edition, December 1999.   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and              dual environments",RFC 1195, December 1990.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi              Topology (MT) Routing in Intermediate System to              Intermediate Systems (IS-ISs)",RFC 5120, February 2008.   [RFC5303]  Katz, D., Saluja, R., and D. Eastlake, "Three-Way              Handshake for IS-IS Point-to-Point Adjacencies",RFC 5303,              October 2008.   [RFC5306]  Shand, M. and L. Ginsberg, "Restart Signaling for IS-IS",RFC 5306, October 2008.   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection              (BFD)",RFC 5880, June 2010.   [RFC5882]  Katz, D. and D. Ward, "Generic Application of              Bidirectional Forwarding Detection (BFD)",RFC 5882,              June 2010.Authors' Addresses   Christian E. Hopps   Cisco Systems   170 W. Tasman Dr.   San Jose, California  95134   USA   EMail: chopps@cisco.com   Les Ginsberg   Cisco Systems   510 McCarthy Blvd.   Milpitas, California  95035   USA   EMail: ginsberg@cisco.comHopps & Ginsberg             Standards Track                    [Page 7]