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Internet Engineering Task Force (IETF)                           D. KatzRequest for Comments: 5880                                       D. WardCategory: Standards Track                               Juniper NetworksISSN: 2070-1721                                                June 2010Bidirectional Forwarding Detection (BFD)Abstract   This document describes a protocol intended to detect faults in the   bidirectional path between two forwarding engines, including   interfaces, data link(s), and to the extent possible the forwarding   engines themselves, with potentially very low latency.  It operates   independently of media, data protocols, and routing protocols.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/rfc5880.Copyright Notice   Copyright (c) 2010 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.Katz & Ward                  Standards Track                    [Page 1]

RFC 5880           Bidirectional Forwarding Detection          June 2010Table of Contents1. Introduction ....................................................31.1. Conventions Used in This Document ..........................42. Design ..........................................................43. Protocol Overview ...............................................53.1. Addressing and Session Establishment .......................53.2. Operating Modes ............................................54. BFD Control Packet Format .......................................74.1. Generic BFD Control Packet Format ..........................74.2. Simple Password Authentication Section Format .............11      4.3. Keyed MD5 and Meticulous Keyed MD5 Authentication           Section Format ............................................11      4.4. Keyed SHA1 and Meticulous Keyed SHA1           Authentication Section Format .............................135. BFD Echo Packet Format .........................................146. Elements of Procedure ..........................................146.1. Overview ..................................................146.2. BFD State Machine .........................................166.3. Demultiplexing and the Discriminator Fields ...............176.4. The Echo Function and Asymmetry ...........................186.5. The Poll Sequence .........................................196.6. Demand Mode ...............................................196.7. Authentication ............................................216.7.1. Enabling and Disabling Authentication ..............216.7.2. Simple Password Authentication .....................226.7.3. Keyed MD5 and Meticulous Keyed MD5 Authentication ..23           6.7.4. Keyed SHA1 and Meticulous Keyed SHA1                  Authentication .....................................256.8. Functional Specifics ......................................276.8.1. State Variables ....................................276.8.2. Timer Negotiation ..................................306.8.3. Timer Manipulation .................................316.8.4. Calculating the Detection Time .....................326.8.5. Detecting Failures with the Echo Function ..........336.8.6. Reception of BFD Control Packets ...................336.8.7. Transmitting BFD Control Packets ...................366.8.8. Reception of BFD Echo Packets ......................396.8.9. Transmission of BFD Echo Packets ...................396.8.10. Min Rx Interval Change ............................406.8.11. Min Tx Interval Change ............................406.8.12. Detect Multiplier Change ..........................406.8.13. Enabling or Disabling The Echo Function ...........406.8.14. Enabling or Disabling Demand Mode .................406.8.15. Forwarding Plane Reset ............................416.8.16. Administrative Control ............................416.8.17. Concatenated Paths ................................416.8.18. Holding Down Sessions .............................42Katz & Ward                  Standards Track                    [Page 2]

RFC 5880           Bidirectional Forwarding Detection          June 20107. Operational Considerations .....................................438. IANA Considerations ............................................449. Security Considerations ........................................4510. References ....................................................4610.1. Normative References .....................................4610.2. Informative References ...................................47Appendix A. Backward Compatibility (Non-Normative) ................48Appendix B. Contributors ..........................................48Appendix C. Acknowledgments .......................................491.  Introduction   An increasingly important feature of networking equipment is the   rapid detection of communication failures between adjacent systems,   in order to more quickly establish alternative paths.  Detection can   come fairly quickly in certain circumstances when data link hardware   comes into play (such as Synchronous Optical Network (SONET) alarms).   However, there are media that do not provide this kind of signaling   (such as Ethernet), and some media may not detect certain kinds of   failures in the path, for example, failing interfaces or forwarding   engine components.   Networks use relatively slow "Hello" mechanisms, usually in routing   protocols, to detect failures when there is no hardware signaling to   help out.  The time to detect failures ("Detection Times") available   in the existing protocols are no better than a second, which is far   too long for some applications and represents a great deal of lost   data at gigabit rates.  Furthermore, routing protocol Hellos are of   no help when those routing protocols are not in use, and the   semantics of detection are subtly different -- they detect a failure   in the path between the two routing protocol engines.   The goal of Bidirectional Forwarding Detection (BFD) is to provide   low-overhead, short-duration detection of failures in the path   between adjacent forwarding engines, including the interfaces, data   link(s), and, to the extent possible, the forwarding engines   themselves.   An additional goal is to provide a single mechanism that can be used   for liveness detection over any media, at any protocol layer, with a   wide range of Detection Times and overhead, to avoid a proliferation   of different methods.   This document specifies the details of the base protocol.  The use of   some mechanisms are application dependent and are specified in a   separate series of application documents.  These issues are so noted.Katz & Ward                  Standards Track                    [Page 3]

RFC 5880           Bidirectional Forwarding Detection          June 2010   Note that many of the exact mechanisms are implementation dependent   and will not affect interoperability, and are thus outside the scope   of this specification.  Those issues are so noted.1.1.  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 inRFC 2119 [KEYWORDS].2.  Design   BFD is designed to detect failures in communication with a forwarding   plane next hop.  It is intended to be implemented in some component   of the forwarding engine of a system, in cases where the forwarding   and control engines are separated.  This not only binds the protocol   more to the forwarding plane, but decouples the protocol from the   fate of the routing protocol engine, making it useful in concert with   various "graceful restart" mechanisms for those protocols.  BFD may   also be implemented in the control engine, though doing so may   preclude the detection of some kinds of failures.   BFD operates on top of any data protocol (network layer, link layer,   tunnels, etc.)  being forwarded between two systems.  It is always   run in a unicast, point-to-point mode.  BFD packets are carried as   the payload of whatever encapsulating protocol is appropriate for the   medium and network.  BFD may be running at multiple layers in a   system.  The context of the operation of any particular BFD session   is bound to its encapsulation.   BFD can provide failure detection on any kind of path between   systems, including direct physical links, virtual circuits, tunnels,   MPLS Label Switched Paths (LSPs), multihop routed paths, and   unidirectional links (so long as there is some return path, of   course).  Multiple BFD sessions can be established between the same   pair of systems when multiple paths between them are present in at   least one direction, even if a lesser number of paths are available   in the other direction (multiple parallel unidirectional links or   MPLS LSPs, for example).   The BFD state machine implements a three-way handshake, both when   establishing a BFD session and when tearing it down for any reason,   to ensure that both systems are aware of the state change.Katz & Ward                  Standards Track                    [Page 4]

RFC 5880           Bidirectional Forwarding Detection          June 2010   BFD can be abstracted as a simple service.  The service primitives   provided by BFD are to create, destroy, and modify a session, given   the destination address and other parameters.  BFD in return provides   a signal to its clients indicating when the BFD session goes up or   down.3.  Protocol Overview   BFD is a simple Hello protocol that, in many respects, is similar to   the detection components of well-known routing protocols.  A pair of   systems transmit BFD packets periodically over each path between the   two systems, and if a system stops receiving BFD packets for long   enough, some component in that particular bidirectional path to the   neighboring system is assumed to have failed.  Under some conditions,   systems may negotiate not to send periodic BFD packets in order to   reduce overhead.   A path is only declared to be operational when two-way communication   has been established between systems, though this does not preclude   the use of unidirectional links.   A separate BFD session is created for each communications path and   data protocol in use between two systems.   Each system estimates how quickly it can send and receive BFD packets   in order to come to an agreement with its neighbor about how rapidly   detection of failure will take place.  These estimates can be   modified in real time in order to adapt to unusual situations.  This   design also allows for fast systems on a shared medium with a slow   system to be able to more rapidly detect failures between the fast   systems while allowing the slow system to participate to the best of   its ability.3.1.  Addressing and Session Establishment   A BFD session is established based on the needs of the application   that will be making use of it.  It is up to the application to   determine the need for BFD, and the addresses to use -- there is no   discovery mechanism in BFD.  For example, an OSPF [OSPF]   implementation may request a BFD session to be established to a   neighbor discovered using the OSPF Hello protocol.3.2.  Operating Modes   BFD has two operating modes that may be selected, as well as an   additional function that can be used in combination with the two   modes.Katz & Ward                  Standards Track                    [Page 5]

RFC 5880           Bidirectional Forwarding Detection          June 2010   The primary mode is known as Asynchronous mode.  In this mode, the   systems periodically send BFD Control packets to one another, and if   a number of those packets in a row are not received by the other   system, the session is declared to be down.   The second mode is known as Demand mode.  In this mode, it is assumed   that a system has an independent way of verifying that it has   connectivity to the other system.  Once a BFD session is established,   such a system may ask the other system to stop sending BFD Control   packets, except when the system feels the need to verify connectivity   explicitly, in which case a short sequence of BFD Control packets is   exchanged, and then the far system quiesces.  Demand mode may operate   independently in each direction, or simultaneously.   An adjunct to both modes is the Echo function.  When the Echo   function is active, a stream of BFD Echo packets is transmitted in   such a way as to have the other system loop them back through its   forwarding path.  If a number of packets of the echoed data stream   are not received, the session is declared to be down.  The Echo   function may be used with either Asynchronous or Demand mode.  Since   the Echo function is handling the task of detection, the rate of   periodic transmission of Control packets may be reduced (in the case   of Asynchronous mode) or eliminated completely (in the case of Demand   mode).   Pure Asynchronous mode is advantageous in that it requires half as   many packets to achieve a particular Detection Time as does the Echo   function.  It is also used when the Echo function cannot be supported   for some reason.   The Echo function has the advantage of truly testing only the   forwarding path on the remote system.  This may reduce round-trip   jitter and thus allow more aggressive Detection Times, as well as   potentially detecting some classes of failure that might not   otherwise be detected.   The Echo function may be enabled individually in each direction.  It   is enabled in a particular direction only when the system that loops   the Echo packets back signals that it will allow it, and when the   system that sends the Echo packets decides it wishes to.   Demand mode is useful in situations where the overhead of a periodic   protocol might prove onerous, such as a system with a very large   number of BFD sessions.  It is also useful when the Echo function is   being used symmetrically.  Demand mode has the disadvantage that   Detection Times are essentially driven by the heuristics of the   system implementation and are not known to the BFD protocol.  DemandKatz & Ward                  Standards Track                    [Page 6]

RFC 5880           Bidirectional Forwarding Detection          June 2010   mode may not be used when the path round-trip time is greater than   the desired Detection Time, or the protocol will fail to work   properly.  Seesection 6.6 for more details.4.  BFD Control Packet Format4.1.  Generic BFD Control Packet Format   BFD Control packets are sent in an encapsulation appropriate to the   environment.  The specific encapsulation is outside of the scope of   this specification.  See the appropriate application document for   encapsulation details.   The BFD Control packet has a Mandatory Section and an optional   Authentication Section.  The format of the Authentication Section, if   present, is dependent on the type of authentication in use.   The Mandatory Section of a BFD Control packet has the following   format:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |Vers |  Diag   |Sta|P|F|C|A|D|M|  Detect Mult  |    Length     |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                       My Discriminator                        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                      Your Discriminator                       |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                    Desired Min TX Interval                    |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                   Required Min RX Interval                    |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                 Required Min Echo RX Interval                 |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   An optional Authentication Section MAY be present:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |   Auth Type   |   Auth Len    |    Authentication Data...     |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Version (Vers)      The version number of the protocol.  This document defines      protocol version 1.Katz & Ward                  Standards Track                    [Page 7]

RFC 5880           Bidirectional Forwarding Detection          June 2010   Diagnostic (Diag)      A diagnostic code specifying the local system's reason for the      last change in session state.  Values are:         0 -- No Diagnostic         1 -- Control Detection Time Expired         2 -- Echo Function Failed         3 -- Neighbor Signaled Session Down         4 -- Forwarding Plane Reset         5 -- Path Down         6 -- Concatenated Path Down         7 -- Administratively Down         8 -- Reverse Concatenated Path Down      9-31 -- Reserved for future use      This field allows remote systems to determine the reason that the      previous session failed, for example.   State (Sta)      The current BFD session state as seen by the transmitting system.      Values are:         0 -- AdminDown         1 -- Down         2 -- Init         3 -- Up   Poll (P)      If set, the transmitting system is requesting verification of      connectivity, or of a parameter change, and is expecting a packet      with the Final (F) bit in reply.  If clear, the transmitting      system is not requesting verification.   Final (F)      If set, the transmitting system is responding to a received BFD      Control packet that had the Poll (P) bit set.  If clear, the      transmitting system is not responding to a Poll.Katz & Ward                  Standards Track                    [Page 8]

RFC 5880           Bidirectional Forwarding Detection          June 2010   Control Plane Independent (C)      If set, the transmitting system's BFD implementation does not      share fate with its control plane (in other words, BFD is      implemented in the forwarding plane and can continue to function      through disruptions in the control plane).  If clear, the      transmitting system's BFD implementation shares fate with its      control plane.      The use of this bit is application dependent and is outside the      scope of this specification.  See specific application      specifications for details.   Authentication Present (A)      If set, the Authentication Section is present and the session is      to be authenticated (seesection 6.7 for details).   Demand (D)      If set, Demand mode is active in the transmitting system (the      system wishes to operate in Demand mode, knows that the session is      Up in both directions, and is directing the remote system to cease      the periodic transmission of BFD Control packets).  If clear,      Demand mode is not active in the transmitting system.   Multipoint (M)      This bit is reserved for future point-to-multipoint extensions to      BFD.  It MUST be zero on both transmit and receipt.   Detect Mult      Detection time multiplier.  The negotiated transmit interval,      multiplied by this value, provides the Detection Time for the      receiving system in Asynchronous mode.   Length      Length of the BFD Control packet, in bytes.   My Discriminator      A unique, nonzero discriminator value generated by the      transmitting system, used to demultiplex multiple BFD sessions      between the same pair of systems.Katz & Ward                  Standards Track                    [Page 9]

RFC 5880           Bidirectional Forwarding Detection          June 2010   Your Discriminator      The discriminator received from the corresponding remote system.      This field reflects back the received value of My Discriminator,      or is zero if that value is unknown.   Desired Min TX Interval      This is the minimum interval, in microseconds, that the local      system would like to use when transmitting BFD Control packets,      less any jitter applied (seesection 6.8.2).  The value zero is      reserved.   Required Min RX Interval      This is the minimum interval, in microseconds, between received      BFD Control packets that this system is capable of supporting,      less any jitter applied by the sender (seesection 6.8.2).  If      this value is zero, the transmitting system does not want the      remote system to send any periodic BFD Control packets.   Required Min Echo RX Interval      This is the minimum interval, in microseconds, between received      BFD Echo packets that this system is capable of supporting, less      any jitter applied by the sender (seesection 6.8.9).  If this      value is zero, the transmitting system does not support the      receipt of BFD Echo packets.   Auth Type      The authentication type in use, if the Authentication Present (A)      bit is set.         0 - Reserved         1 - Simple Password         2 - Keyed MD5         3 - Meticulous Keyed MD5         4 - Keyed SHA1         5 - Meticulous Keyed SHA1     6-255 - Reserved for future use   Auth Len      The length, in bytes, of the authentication section, including the      Auth Type and Auth Len fields.Katz & Ward                  Standards Track                   [Page 10]

RFC 5880           Bidirectional Forwarding Detection          June 20104.2.  Simple Password Authentication Section Format   If the Authentication Present (A) bit is set in the header, and the   Authentication Type field contains 1 (Simple Password), the   Authentication Section has the following format:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |   Auth Type   |   Auth Len    |  Auth Key ID  |  Password...  |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                              ...                              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Auth Type      The Authentication Type, which in this case is 1 (Simple      Password).   Auth Len      The length of the Authentication Section, in bytes.  For Simple      Password authentication, the length is equal to the password      length plus three.   Auth Key ID      The authentication key ID in use for this packet.  This allows      multiple keys to be active simultaneously.   Password      The simple password in use on this session.  The password is a      binary string, and MUST be from 1 to 16 bytes in length.  The      password MUST be encoded and configured according tosection6.7.2.4.3.  Keyed MD5 and Meticulous Keyed MD5 Authentication Section Format   The use of MD5-based authentication is strongly discouraged.   However, it is documented here for compatibility with existing   implementations.   If the Authentication Present (A) bit is set in the header, and the   Authentication Type field contains 2 (Keyed MD5) or 3 (Meticulous   Keyed MD5), the Authentication Section has the following format:Katz & Ward                  Standards Track                   [Page 11]

RFC 5880           Bidirectional Forwarding Detection          June 2010    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |   Auth Type   |   Auth Len    |  Auth Key ID  |   Reserved    |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        Sequence Number                        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                      Auth Key/Digest...                       |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                              ...                              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Auth Type      The Authentication Type, which in this case is 2 (Keyed MD5) or 3      (Meticulous Keyed MD5).   Auth Len      The length of the Authentication Section, in bytes.  For Keyed MD5      and Meticulous Keyed MD5 authentication, the length is 24.   Auth Key ID      The authentication key ID in use for this packet.  This allows      multiple keys to be active simultaneously.   Reserved      This byte MUST be set to zero on transmit, and ignored on receipt.   Sequence Number      The sequence number for this packet.  For Keyed MD5      Authentication, this value is incremented occasionally.  For      Meticulous Keyed MD5 Authentication, this value is incremented for      each successive packet transmitted for a session.  This provides      protection against replay attacks.   Auth Key/Digest      This field carries the 16-byte MD5 digest for the packet.  When      the digest is calculated, the shared MD5 key is stored in this      field, padded to 16 bytes with trailing zero bytes if needed.  The      shared key MUST be encoded and configured tosection 6.7.3.Katz & Ward                  Standards Track                   [Page 12]

RFC 5880           Bidirectional Forwarding Detection          June 20104.4.  Keyed SHA1 and Meticulous Keyed SHA1 Authentication Section Format   If the Authentication Present (A) bit is set in the header, and the   Authentication Type field contains 4 (Keyed SHA1) or 5 (Meticulous   Keyed SHA1), the Authentication Section has the following format:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |   Auth Type   |   Auth Len    |  Auth Key ID  |   Reserved    |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        Sequence Number                        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                       Auth Key/Hash...                        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                              ...                              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Auth Type      The Authentication Type, which in this case is 4 (Keyed SHA1) or 5      (Meticulous Keyed SHA1).   Auth Len      The length of the Authentication Section, in bytes.  For Keyed      SHA1 and Meticulous Keyed SHA1 authentication, the length is 28.   Auth Key ID      The authentication key ID in use for this packet.  This allows      multiple keys to be active simultaneously.   Reserved      This byte MUST be set to zero on transmit and ignored on receipt.   Sequence Number      The sequence number for this packet.  For Keyed SHA1      Authentication, this value is incremented occasionally.  For      Meticulous Keyed SHA1 Authentication, this value is incremented      for each successive packet transmitted for a session.  This      provides protection against replay attacks.Katz & Ward                  Standards Track                   [Page 13]

RFC 5880           Bidirectional Forwarding Detection          June 2010   Auth Key/Hash      This field carries the 20-byte SHA1 hash for the packet.  When the      hash is calculated, the shared SHA1 key is stored in this field,      padded to a length of 20 bytes with trailing zero bytes if needed.      The shared key MUST be encoded and configured tosection 6.7.4.5.  BFD Echo Packet Format   BFD Echo packets are sent in an encapsulation appropriate to the   environment.  See the appropriate application documents for the   specifics of particular environments.   The payload of a BFD Echo packet is a local matter, since only the   sending system ever processes the content.  The only requirement is   that sufficient information is included to demultiplex the received   packet to the correct BFD session after it is looped back to the   sender.  The contents are otherwise outside the scope of this   specification.   Some form of authentication SHOULD be included, since Echo packets   may be spoofed.6.  Elements of Procedure   This section discusses the normative requirements of the protocol in   order to achieve interoperability.  It is important for implementors   to enforce only the requirements specified in this section, as   misguided pedantry has been proven by experience to affect   interoperability adversely.   Remember that all references of the form "bfd.Xx" refer to internal   state variables (defined insection 6.8.1), whereas all references to   "the Xxx field" refer to fields in the protocol packets themselves   (defined insection 4).6.1.  Overview   A system may take either an Active role or a Passive role in session   initialization.  A system taking the Active role MUST send BFD   Control packets for a particular session, regardless of whether it   has received any BFD packets for that session.  A system taking the   Passive role MUST NOT begin sending BFD packets for a particular   session until it has received a BFD packet for that session, and thus   has learned the remote system's discriminator value.  At least one   system MUST take the Active role (possibly both).  The role that a   system takes is specific to the application of BFD, and is outside   the scope of this specification.Katz & Ward                  Standards Track                   [Page 14]

RFC 5880           Bidirectional Forwarding Detection          June 2010   A session begins with the periodic, slow transmission of BFD Control   packets.  When bidirectional communication is achieved, the BFD   session becomes Up.   Once the BFD session is Up, a system can choose to start the Echo   function if it desires and the other system signals that it will   allow it.  The rate of transmission of Control packets is typically   kept low when the Echo function is active.   If the Echo function is not active, the transmission rate of Control   packets may be increased to a level necessary to achieve the   Detection Time requirements for the session.   Once the session is Up, a system may signal that it has entered   Demand mode, and the transmission of BFD Control packets by the   remote system ceases.  Other means of implying connectivity are used   to keep the session alive.  If either system wishes to verify   bidirectional connectivity, it can initiate a short exchange of BFD   Control packets (a "Poll Sequence"; seesection 6.5) to do so.   If Demand mode is not active, and no Control packets are received in   the calculated Detection Time (seesection 6.8.4), the session is   declared Down.  This is signaled to the remote end via the State   (Sta) field in outgoing packets.   If sufficient Echo packets are lost, the session is declared Down in   the same manner.  Seesection 6.8.5.   If Demand mode is active and no appropriate Control packets are   received in response to a Poll Sequence, the session is declared Down   in the same manner.  Seesection 6.6.   If the session goes Down, the transmission of Echo packets (if any)   ceases, and the transmission of Control packets goes back to the slow   rate.   Once a session has been declared Down, it cannot come back up until   the remote end first signals that it is down (by leaving the Up   state), thus implementing a three-way handshake.   A session MAY be kept administratively down by entering the AdminDown   state and sending an explanatory diagnostic code in the Diagnostic   field.Katz & Ward                  Standards Track                   [Page 15]

RFC 5880           Bidirectional Forwarding Detection          June 20106.2.  BFD State Machine   The BFD state machine is quite straightforward.  There are three   states through which a session normally proceeds: two for   establishing a session (Init and Up) and one for tearing down a   session (Down).  This allows a three-way handshake for both session   establishment and session teardown (assuring that both systems are   aware of all session state changes).  A fourth state (AdminDown)   exists so that a session can be administratively put down   indefinitely.   Each system communicates its session state in the State (Sta) field   in the BFD Control packet, and that received state, in combination   with the local session state, drives the state machine.   Down state means that the session is down (or has just been created).   A session remains in Down state until the remote system indicates   that it agrees that the session is down by sending a BFD Control   packet with the State field set to anything other than Up.  If that   packet signals Down state, the session advances to Init state; if   that packet signals Init state, the session advances to Up state.   Semantically, Down state indicates that the forwarding path is   unavailable, and that appropriate actions should be taken by the   applications monitoring the state of the BFD session.  A system MAY   hold a session in Down state indefinitely (by simply refusing to   advance the session state).  This may be done for operational or   administrative reasons, among others.   Init state means that the remote system is communicating, and the   local system desires to bring the session up, but the remote system   does not yet realize it.  A session will remain in Init state until   either a BFD Control Packet is received that is signaling Init or Up   state (in which case the session advances to Up state) or the   Detection Time expires, meaning that communication with the remote   system has been lost (in which case the session advances to Down   state).   Up state means that the BFD session has successfully been   established, and implies that connectivity between the systems is   working.  The session will remain in the Up state until either   connectivity fails or the session is taken down administratively.  If   either the remote system signals Down state or the Detection Time   expires, the session advances to Down state.Katz & Ward                  Standards Track                   [Page 16]

RFC 5880           Bidirectional Forwarding Detection          June 2010   AdminDown state means that the session is being held administratively   down.  This causes the remote system to enter Down state, and remain   there until the local system exits AdminDown state.  AdminDown state   has no semantic implications for the availability of the forwarding   path.   The following diagram provides an overview of the state machine.   Transitions involving AdminDown state are deleted for clarity (but   are fully specified in sections6.8.6 and6.8.16).  The notation on   each arc represents the state of the remote system (as received in   the State field in the BFD Control packet) or indicates the   expiration of the Detection Timer.                             +--+                             |  | UP, ADMIN DOWN, TIMER                             |  V                     DOWN  +------+  INIT              +------------|      |------------+              |            | DOWN |            |              |  +-------->|      |<--------+  |              |  |         +------+         |  |              |  |                          |  |              |  |               ADMIN DOWN,|  |              |  |ADMIN DOWN,          DOWN,|  |              |  |TIMER                TIMER|  |              V  |                          |  V            +------+                      +------+       +----|      |                      |      |----+   DOWN|    | INIT |--------------------->|  UP  |    |INIT, UP       +--->|      | INIT, UP             |      |<---+            +------+                      +------+6.3.  Demultiplexing and the Discriminator Fields   Since multiple BFD sessions may be running between two systems, there   needs to be a mechanism for demultiplexing received BFD packets to   the proper session.   Each system MUST choose an opaque discriminator value that identifies   each session, and which MUST be unique among all BFD sessions on the   system.  The local discriminator is sent in the My Discriminator   field in the BFD Control packet, and is echoed back in the Your   Discriminator field of packets sent from the remote end.Katz & Ward                  Standards Track                   [Page 17]

RFC 5880           Bidirectional Forwarding Detection          June 2010   Once the remote end echoes back the local discriminator, all further   received packets are demultiplexed based on the Your Discriminator   field only (which means that, among other things, the source address   field can change or the interface over which the packets are received   can change, but the packets will still be associated with the proper   session).   The method of demultiplexing the initial packets (in which Your   Discriminator is zero) is application dependent, and is thus outside   the scope of this specification.   Note that it is permissible for a system to change its discriminator   during a session without affecting the session state, since only that   system uses its discriminator for demultiplexing purposes (by having   the other system reflect it back).  The implications on an   implementation for changing the discriminator value is outside the   scope of this specification.6.4.  The Echo Function and Asymmetry   The Echo function can be run independently in each direction between   a pair of systems.  For whatever reason, a system may advertise that   it is willing to receive (and loop back) Echo packets, but may not   wish to ever send any.  The fact that a system is sending Echo   packets is not directly signaled to the system looping them back.   When a system is using the Echo function, it is advantageous to   choose a sedate reception rate for Control packets, since liveness   detection is being handled by the Echo packets.  This can be   controlled by manipulating the Required Min RX Interval field (seesection 6.8.3).   If the Echo function is only being run in one direction, the system   not running the Echo function will more likely wish to receive fairly   rapid Control packets in order to achieve its desired Detection Time.   Since BFD allows independent transmission rates in each direction,   this is easily accomplished.   A system SHOULD otherwise advertise the lowest value of Required Min   RX Interval and Required Min Echo RX Interval that it can under the   circumstances, to give the other system more freedom in choosing its   transmission rate.  Note that a system is committing to be able to   receive both streams of packets at the rate it advertises, so this   should be taken into account when choosing the values to advertise.Katz & Ward                  Standards Track                   [Page 18]

RFC 5880           Bidirectional Forwarding Detection          June 20106.5.  The Poll Sequence   A Poll Sequence is an exchange of BFD Control packets that is used in   some circumstances to ensure that the remote system is aware of   parameter changes.  It is also used in Demand mode (seesection 6.6)   to validate bidirectional connectivity.   A Poll Sequence consists of a system sending periodic BFD Control   packets with the Poll (P) bit set.  When the other system receives a   Poll, it immediately transmits a BFD Control packet with the Final   (F) bit set, independent of any periodic BFD Control packets it may   be sending (seesection 6.8.7).  When the system sending the Poll   sequence receives a packet with Final, the Poll Sequence is   terminated, and any subsequent BFD Control packets are sent with the   Poll bit cleared.  A BFD Control packet MUST NOT have both the Poll   (P) and Final (F) bits set.   If periodic BFD Control packets are already being sent (the remote   system is not in Demand mode), the Poll Sequence MUST be performed by   setting the Poll (P) bit on those scheduled periodic transmissions;   additional packets MUST NOT be sent.   After a Poll Sequence is terminated, the system requesting the Poll   Sequence will cease the periodic transmission of BFD Control packets   if the remote end is in Demand mode; otherwise, it will return to the   periodic transmission of BFD Control packets with the Poll (P) bit   clear.   Typically, the entire sequence consists of a single packet in each   direction, though packet losses or relatively long packet latencies   may result in multiple Poll packets to be sent before the sequence   terminates.6.6.  Demand Mode   Demand mode is requested independently in each direction by virtue of   a system setting the Demand (D) bit in its BFD Control packets.  The   system receiving the Demand bit ceases the periodic transmission of   BFD Control packets.  If both systems are operating in Demand mode,   no periodic BFD Control packets will flow in either direction.   Demand mode requires that some other mechanism is used to imply   continuing connectivity between the two systems.  The mechanism used   does not have to be the same in both directions, and is outside of   the scope of this specification.  One possible mechanism is the   receipt of traffic from the remote system; another is the use of the   Echo function.Katz & Ward                  Standards Track                   [Page 19]

RFC 5880           Bidirectional Forwarding Detection          June 2010   When a system in Demand mode wishes to verify bidirectional   connectivity, it initiates a Poll Sequence (seesection 6.5).  If no   response is received to a Poll, the Poll is repeated until the   Detection Time expires, at which point the session is declared to be   Down.  Note that if Demand mode is operating only on the local   system, the Poll Sequence is performed by simply setting the Poll (P)   bit in regular periodic BFD Control packets, as required bysection6.5.   The Detection Time in Demand mode is calculated differently than in   Asynchronous mode; it is based on the transmit rate of the local   system, rather than the transmit rate of the remote system.  This   ensures that the Poll Sequence mechanism works properly.  Seesection6.8.4 for more details.   Note that the Poll mechanism will always fail unless the negotiated   Detection Time is greater than the round-trip time between the two   systems.  Enforcement of this constraint is outside the scope of this   specification.   Demand mode MAY be enabled or disabled at any time, independently in   each direction, by setting or clearing the Demand (D) bit in the BFD   Control packet, without affecting the BFD session state.  Note that   the Demand bit MUST NOT be set unless both systems perceive the   session to be Up (the local system thinks the session is Up, and the   remote system last reported Up state in the State (Sta) field of the   BFD Control packet).   When the transmitted value of the Demand (D) bit is to be changed,   the transmitting system MUST initiate a Poll Sequence in conjunction   with changing the bit in order to ensure that both systems are aware   of the change.   If Demand mode is active on either or both systems, a Poll Sequence   MUST be initiated whenever the contents of the next BFD Control   packet to be sent would be different than the contents of the   previous packet, with the exception of the Poll (P) and Final (F)   bits.  This ensures that parameter changes are transmitted to the   remote system and that the remote system acknowledges these changes.   Because the underlying detection mechanism is unspecified, and may   differ between the two systems, the overall Detection Time   characteristics of the path will not be fully known to either system.   The total Detection Time for a particular system is the sum of the   time prior to the initiation of the Poll Sequence, plus the   calculated Detection Time.Katz & Ward                  Standards Track                   [Page 20]

RFC 5880           Bidirectional Forwarding Detection          June 2010   Note that if Demand mode is enabled in only one direction, continuous   bidirectional connectivity verification is lost (only connectivity in   the direction from the system in Demand mode to the other system will   be verified).  Resolving the issue of one system requesting Demand   mode while the other requires continuous bidirectional connectivity   verification is outside the scope of this specification.6.7.  Authentication   An optional Authentication Section MAY be present in the BFD Control   packet.  In its generic form, the purpose of the Authentication   Section is to carry all necessary information, based on the   authentication type in use, to allow the receiving system to   determine the validity of the received packet.  The exact mechanism   depends on the authentication type in use, but in general the   transmitting system will put information in the Authentication   Section that vouches for the packet's validity, and the receiving   system will examine the Authentication Section and either accept the   packet for further processing or discard it.   The same authentication type, and any keys or other necessary   information, obviously must be in use by the two systems.  The   negotiation of authentication type, key exchange, etc., are all   outside the scope of this specification and are expected to be   performed by means outside of the protocol.   Note that in the subsections below, to "accept" a packet means only   that the packet has passed authentication; it may in fact be   discarded for other reasons as described in the general packet   reception rules described insection 6.8.6.   Implementations supporting authentication MUST support both types of   SHA1 authentication.  Other forms of authentication are optional.6.7.1.  Enabling and Disabling Authentication   It may be desirable to enable or disable authentication on a session   without disturbing the session state.  The exact mechanism for doing   so is outside the scope of this specification.  However, it is useful   to point out some issues in supporting this mechanism.   In a simple implementation, a BFD session will fail when   authentication is either turned on or turned off, because the packet   acceptance rules essentially require the local and remote machines toKatz & Ward                  Standards Track                   [Page 21]

RFC 5880           Bidirectional Forwarding Detection          June 2010   do so in a more or less synchronized fashion (within the Detection   Time) -- a packet with authentication will only be accepted if   authentication is "in use" (and likewise packets without   authentication).   One possible approach is to build an implementation such that   authentication is configured, but not considered "in use" until the   first packet containing a matching authentication section is received   (providing the necessary synchronization).  Likewise, authentication   could be configured off, but still considered "in use" until the   receipt of the first packet without the authentication section.   In order to avoid security risks, implementations using this method   SHOULD only allow the authentication state to be changed at most once   without some form of intervention (so that authentication cannot be   turned on and off repeatedly simply based on the receipt of BFD   Control packets from remote systems).  Unless it is desired to enable   or disable authentication, an implementation SHOULD NOT allow the   authentication state to change based on the receipt of BFD Control   packets.6.7.2.  Simple Password Authentication   The most straightforward (and weakest) form of authentication is   Simple Password Authentication.  In this method of authentication,   one or more Passwords (with corresponding Key IDs) are configured in   each system and one of these Password/ID pairs is carried in each BFD   Control packet.  The receiving system accepts the packet if the   Password and Key ID matches one of the Password/ID pairs configured   in that system.   Transmission Using Simple Password Authentication      The currently selected password and Key ID for the session MUST be      stored in the Authentication Section of each outgoing BFD Control      packet.  The Auth Type field MUST be set to 1 (Simple Password).      The Auth Len field MUST be set to the proper length (4 to 19      bytes).      The password is a binary string, and MUST be 1 to 16 bytes in      length.  For interoperability, the management interface by which      the password is configured MUST accept ASCII strings, and SHOULD      also allow for the configuration of any arbitrary binary string in      hexadecimal form.  Other configuration methods MAY be supported.Katz & Ward                  Standards Track                   [Page 22]

RFC 5880           Bidirectional Forwarding Detection          June 2010   Reception Using Simple Password Authentication      If the received BFD Control packet does not contain an      Authentication Section, or the Auth Type is not 1 (Simple      Password), then the received packet MUST be discarded.      If the Auth Key ID field does not match the ID of a configured      password, the received packet MUST be discarded.      If the Auth Len field is not equal to the length of the password      selected by the key ID, plus three, the packet MUST be discarded.      If the Password field does not match the password selected by the      key ID, the packet MUST be discarded.      Otherwise, the packet MUST be accepted.6.7.3.  Keyed MD5 and Meticulous Keyed MD5 Authentication   The Keyed MD5 and Meticulous Keyed MD5 Authentication mechanisms are   very similar to those used in other protocols.  In these methods of   authentication, one or more secret keys (with corresponding key IDs)   are configured in each system.  One of the keys is included in an MD5   [MD5] digest calculated over the outgoing BFD Control packet, but the   Key itself is not carried in the packet.  To help avoid replay   attacks, a sequence number is also carried in each packet.  For Keyed   MD5, the sequence number is occasionally incremented.  For Meticulous   Keyed MD5, the sequence number is incremented on every packet.   The receiving system accepts the packet if the key ID matches one of   the configured Keys, an MD5 digest including the selected key matches   that carried in the packet, and the sequence number is greater than   or equal to the last sequence number received (for Keyed MD5), or   strictly greater than the last sequence number received (for   Meticulous Keyed MD5).   Transmission Using Keyed MD5 and Meticulous Keyed MD5 Authentication      The Auth Type field MUST be set to 2 (Keyed MD5) or 3 (Meticulous      Keyed MD5).  The Auth Len field MUST be set to 24.  The Auth Key      ID field MUST be set to the ID of the current authentication key.      The Sequence Number field MUST be set to bfd.XmitAuthSeq.      The authentication key value is a binary string of up to 16 bytes,      and MUST be placed into the Auth Key/Digest field, padded with      trailing zero bytes as necessary.  For interoperability, the      management interface by which the key is configured MUST acceptKatz & Ward                  Standards Track                   [Page 23]

RFC 5880           Bidirectional Forwarding Detection          June 2010      ASCII strings, and SHOULD also allow for the configuration of any      arbitrary binary string in hexadecimal form.  Other configuration      methods MAY be supported.      An MD5 digest MUST be calculated over the entire BFD Control      packet.  The resulting digest MUST be stored in the Auth      Key/Digest field prior to transmission (replacing the secret key,      which MUST NOT be carried in the packet).      For Keyed MD5, bfd.XmitAuthSeq MAY be incremented in a circular      fashion (when treated as an unsigned 32-bit value).      bfd.XmitAuthSeq SHOULD be incremented when the session state      changes, or when the transmitted BFD Control packet carries      different contents than the previously transmitted packet.  The      decision as to when to increment bfd.XmitAuthSeq is outside the      scope of this specification.  See the section titled "Security      Considerations" below for a discussion.      For Meticulous Keyed MD5, bfd.XmitAuthSeq MUST be incremented in a      circular fashion (when treated as an unsigned 32-bit value).   Receipt Using Keyed MD5 and Meticulous Keyed MD5 Authentication      If the received BFD Control packet does not contain an      Authentication Section, or the Auth Type is not correct (2 for      Keyed MD5 or 3 for Meticulous Keyed MD5), then the received packet      MUST be discarded.      If the Auth Key ID field does not match the ID of a configured      authentication key, the received packet MUST be discarded.      If the Auth Len field is not equal to 24, the packet MUST be      discarded.      If bfd.AuthSeqKnown is 1, examine the Sequence Number field.  For      Keyed MD5, if the sequence number lies outside of the range of      bfd.RcvAuthSeq to bfd.RcvAuthSeq+(3*Detect Mult) inclusive (when      treated as an unsigned 32-bit circular number space), the received      packet MUST be discarded.  For Meticulous Keyed MD5, if the      sequence number lies outside of the range of bfd.RcvAuthSeq+1 to      bfd.RcvAuthSeq+(3*Detect Mult) inclusive (when treated as an      unsigned 32-bit circular number space) the received packet MUST be      discarded.      Otherwise (bfd.AuthSeqKnown is 0), bfd.AuthSeqKnown MUST be set to      1, and bfd.RcvAuthSeq MUST be set to the value of the received      Sequence Number field.Katz & Ward                  Standards Track                   [Page 24]

RFC 5880           Bidirectional Forwarding Detection          June 2010      Replace the contents of the Auth Key/Digest field with the      authentication key selected by the received Auth Key ID field.  If      the MD5 digest of the entire BFD Control packet is equal to the      received value of the Auth Key/Digest field, the received packet      MUST be accepted.  Otherwise (the digest does not match the Auth      Key/Digest field), the received packet MUST be discarded.6.7.4.  Keyed SHA1 and Meticulous Keyed SHA1 Authentication   The Keyed SHA1 and Meticulous Keyed SHA1 Authentication mechanisms   are very similar to those used in other protocols.  In these methods   of authentication, one or more secret keys (with corresponding key   IDs) are configured in each system.  One of the keys is included in a   SHA1 [SHA1] hash calculated over the outgoing BFD Control packet, but   the key itself is not carried in the packet.  To help avoid replay   attacks, a sequence number is also carried in each packet.  For Keyed   SHA1, the sequence number is occasionally incremented.  For   Meticulous Keyed SHA1, the sequence number is incremented on every   packet.   The receiving system accepts the packet if the key ID matches one of   the configured keys, a SHA1 hash including the selected key matches   that carried in the packet, and if the sequence number is greater   than or equal to the last sequence number received (for Keyed SHA1),   or strictly greater than the last sequence number received (for   Meticulous Keyed SHA1).   Transmission Using Keyed SHA1 and Meticulous Keyed SHA1      Authentication      The Auth Type field MUST be set to 4 (Keyed SHA1) or 5 (Meticulous      Keyed SHA1).  The Auth Len field MUST be set to 28.  The Auth Key      ID field MUST be set to the ID of the current authentication key.      The Sequence Number field MUST be set to bfd.XmitAuthSeq.      The authentication key value is a binary string of up to 20 bytes,      and MUST be placed into the Auth Key/Hash field, padding with      trailing zero bytes as necessary.  For interoperability, the      management interface by which the key is configured MUST accept      ASCII strings, and SHOULD also allow for the configuration of any      arbitrary binary string in hexadecimal form.  Other configuration      methods MAY be supported.      A SHA1 hash MUST be calculated over the entire BFD control packet.      The resulting hash MUST be stored in the Auth Key/Hash field prior      to transmission (replacing the secret key, which MUST NOT be      carried in the packet).Katz & Ward                  Standards Track                   [Page 25]

RFC 5880           Bidirectional Forwarding Detection          June 2010      For Keyed SHA1, bfd.XmitAuthSeq MAY be incremented in a circular      fashion (when treated as an unsigned 32-bit value).      bfd.XmitAuthSeq SHOULD be incremented when the session state      changes, or when the transmitted BFD Control packet carries      different contents than the previously transmitted packet.  The      decision as to when to increment bfd.XmitAuthSeq is outside the      scope of this specification.  See the section titled "Security      Considerations" below for a discussion.      For Meticulous Keyed SHA1, bfd.XmitAuthSeq MUST be incremented in      a circular fashion (when treated as an unsigned 32-bit value).   Receipt Using Keyed SHA1 and Meticulous Keyed SHA1 Authentication      If the received BFD Control packet does not contain an      Authentication Section, or the Auth Type is not correct (4 for      Keyed SHA1 or 5 for Meticulous Keyed SHA1), then the received      packet MUST be discarded.      If the Auth Key ID field does not match the ID of a configured      authentication key, the received packet MUST be discarded.      If the Auth Len field is not equal to 28, the packet MUST be      discarded.      If bfd.AuthSeqKnown is 1, examine the Sequence Number field.  For      Keyed SHA1, if the sequence number lies outside of the range of      bfd.RcvAuthSeq to bfd.RcvAuthSeq+(3*Detect Mult) inclusive (when      treated as an unsigned 32-bit circular number space), the received      packet MUST be discarded.  For Meticulous Keyed SHA1, if the      sequence number lies outside of the range of bfd.RcvAuthSeq+1 to      bfd.RcvAuthSeq+(3*Detect Mult) inclusive (when treated as an      unsigned 32-bit circular number space, the received packet MUST be      discarded.      Otherwise (bfd.AuthSeqKnown is 0), bfd.AuthSeqKnown MUST be set to      1, bfd.RcvAuthSeq MUST be set to the value of the received      Sequence Number field, and the received packet MUST be accepted.      Replace the contents of the Auth Key/Hash field with the      authentication key selected by the received Auth Key ID field.  If      the SHA1 hash of the entire BFD Control packet is equal to the      received value of the Auth Key/Hash field, the received packet      MUST be accepted.  Otherwise (the hash does not match the Auth      Key/Hash field), the received packet MUST be discarded.Katz & Ward                  Standards Track                   [Page 26]

RFC 5880           Bidirectional Forwarding Detection          June 20106.8.  Functional Specifics   The following section of this specification is normative.  The means   by which this specification is achieved is outside the scope of this   specification.   When a system is said to have "the Echo function active" it means   that the system is sending BFD Echo packets, implying that the   session is Up and the other system has signaled its willingness to   loop back Echo packets.   When the local system is said to have "Demand mode active," it means   that bfd.DemandMode is 1 in the local system (seesection 6.8.1), the   session is Up, and the remote system is signaling that the session is   in state Up.   When the remote system is said to have "Demand mode active," it means   that bfd.RemoteDemandMode is 1 (the remote system set the Demand (D)   bit in the last received BFD Control packet), the session is Up, and   the remote system is signaling that the session is in state Up.6.8.1.  State Variables   A minimum amount of information about a session needs to be tracked   in order to achieve the elements of procedure described here.  The   following is a set of state variables that are helpful in describing   the mechanisms of BFD.  Any means of tracking this state may be used   so long as the protocol behaves as described.   When the text refers to initializing a state variable, this takes   place only at the time that the session (and the corresponding state   variables) is created.  The state variables are subsequently   manipulated by the state machine and are never reinitialized, even if   the session fails and is reestablished.   Once session state is created, and at least one BFD Control packet is   received from the remote end, it MUST be preserved for at least one   Detection Time (seesection 6.8.4) subsequent to the receipt of the   last BFD Control packet, regardless of the session state.  This   preserves timing parameters in case the session flaps.  A system MAY   preserve session state longer than this.  The preservation or   destruction of session state when no BFD Control packets for this   session have been received from the remote system is outside the   scope of this specification.Katz & Ward                  Standards Track                   [Page 27]

RFC 5880           Bidirectional Forwarding Detection          June 2010   All state variables in this specification are of the form "bfd.Xx"   and should not be confused with fields carried in the protocol   packets, which are always spelled out to match the names insection4.   bfd.SessionState      The perceived state of the session (Init, Up, Down, or AdminDown).      The exact action taken when the session state changes is outside      the scope of this specification, though it is expected that this      state change (particularly, to and from Up state) is reported to      other components of the system.  This variable MUST be initialized      to Down.   bfd.RemoteSessionState      The session state last reported by the remote system in the State      (Sta) field of the BFD Control packet.  This variable MUST be      initialized to Down.   bfd.LocalDiscr      The local discriminator for this BFD session, used to uniquely      identify it.  It MUST be unique across all BFD sessions on this      system, and nonzero.  It SHOULD be set to a random (but still      unique) value to improve security.  The value is otherwise outside      the scope of this specification.   bfd.RemoteDiscr      The remote discriminator for this BFD session.  This is the      discriminator chosen by the remote system, and is totally opaque      to the local system.  This MUST be initialized to zero.  If a      period of a Detection Time passes without the receipt of a valid,      authenticated BFD packet from the remote system, this variable      MUST be set to zero.   bfd.LocalDiag      The diagnostic code specifying the reason for the most recent      change in the local session state.  This MUST be initialized to      zero (No Diagnostic).Katz & Ward                  Standards Track                   [Page 28]

RFC 5880           Bidirectional Forwarding Detection          June 2010   bfd.DesiredMinTxInterval      The minimum interval, in microseconds, between transmitted BFD      Control packets that this system would like to use at the current      time, less any jitter applied (seesection 6.8.2).  The actual      interval is negotiated between the two systems.  This MUST be      initialized to a value of at least one second (1,000,000      microseconds) according to the rules described insection 6.8.3.      The setting of this variable is otherwise outside the scope of      this specification.   bfd.RequiredMinRxInterval      The minimum interval, in microseconds, between received BFD      Control packets that this system requires, less any jitter applied      by the sender (seesection 6.8.2).  The setting of this variable      is outside the scope of this specification.  A value of zero means      that this system does not want to receive any periodic BFD Control      packets.  Seesection 6.8.18 for details.   bfd.RemoteMinRxInterval      The last value of Required Min RX Interval received from the      remote system in a BFD Control packet.  This variable MUST be      initialized to 1.   bfd.DemandMode      Set to 1 if the local system wishes to use Demand mode, or 0 if      not.   bfd.RemoteDemandMode      Set to 1 if the remote system wishes to use Demand mode, or 0 if      not.  This is the value of the Demand (D) bit in the last received      BFD Control packet.  This variable MUST be initialized to zero.   bfd.DetectMult      The desired Detection Time multiplier for BFD Control packets on      the local system.  The negotiated Control packet transmission      interval, multiplied by this variable, will be the Detection Time      for this session (as seen by the remote system).  This variable      MUST be a nonzero integer, and is otherwise outside the scope of      this specification.  Seesection 6.8.4 for further information.Katz & Ward                  Standards Track                   [Page 29]

RFC 5880           Bidirectional Forwarding Detection          June 2010   bfd.AuthType      The authentication type in use for this session, as defined insection 4.1, or zero if no authentication is in use.   bfd.RcvAuthSeq      A 32-bit unsigned integer containing the last sequence number for      Keyed MD5 or SHA1 Authentication that was received.  The initial      value is unimportant.   bfd.XmitAuthSeq      A 32-bit unsigned integer containing the next sequence number for      Keyed MD5 or SHA1 Authentication to be transmitted.  This variable      MUST be initialized to a random 32-bit value.   bfd.AuthSeqKnown      Set to 1 if the next sequence number for Keyed MD5 or SHA1      authentication expected to be received is known, or 0 if it is not      known.  This variable MUST be initialized to zero.      This variable MUST be set to zero after no packets have been      received on this session for at least twice the Detection Time.      This ensures that the sequence number can be resynchronized if the      remote system restarts.6.8.2.  Timer Negotiation   The time values used to determine BFD packet transmission intervals   and the session Detection Time are continuously negotiated, and thus   may be changed at any time.  The negotiation and time values are   independent in each direction for each session.   Each system reports in the BFD Control packet how rapidly it would   like to transmit BFD packets, as well as how rapidly it is prepared   to receive them.  This allows either system to unilaterally determine   the maximum packet rate (minimum interval) in both directions.   Seesection 6.8.7 for the details of packet transmission timing and   negotiation.Katz & Ward                  Standards Track                   [Page 30]

RFC 5880           Bidirectional Forwarding Detection          June 20106.8.3.  Timer Manipulation   The time values used to determine BFD packet transmission intervals   and the session Detection Time may be modified at any time without   affecting the state of the session.  When the timer parameters are   changed for any reason, the requirements of this section apply.   If either bfd.DesiredMinTxInterval is changed or   bfd.RequiredMinRxInterval is changed, a Poll Sequence MUST be   initiated (seesection 6.5).  If the timing is such that a system   receiving a Poll Sequence wishes to change the parameters described   in this paragraph, the new parameter values MAY be carried in packets   with the Final (F) bit set, even if the Poll Sequence has not yet   been sent.   If bfd.DesiredMinTxInterval is increased and bfd.SessionState is Up,   the actual transmission interval used MUST NOT change until the Poll   Sequence described above has terminated.  This is to ensure that the   remote system updates its Detection Time before the transmission   interval increases.   If bfd.RequiredMinRxInterval is reduced and bfd.SessionState is Up,   the previous value of bfd.RequiredMinRxInterval MUST be used when   calculating the Detection Time for the remote system until the Poll   Sequence described above has terminated.  This is to ensure that the   remote system is transmitting packets at the higher rate (and those   packets are being received) prior to the Detection Time being   reduced.   When bfd.SessionState is not Up, the system MUST set   bfd.DesiredMinTxInterval to a value of not less than one second   (1,000,000 microseconds).  This is intended to ensure that the   bandwidth consumed by BFD sessions that are not Up is negligible,   particularly in the case where a neighbor may not be running BFD.   If the local system reduces its transmit interval due to   bfd.RemoteMinRxInterval being reduced (the remote system has   advertised a reduced value in Required Min RX Interval), and the   remote system is not in Demand mode, the local system MUST honor the   new interval immediately.  In other words, the local system cannot   wait longer than the new interval between the previous packet   transmission and the next one.  If this interval has already passed   since the last transmission (because the new interval is   significantly shorter), the local system MUST send the next periodic   BFD Control packet as soon as practicable.Katz & Ward                  Standards Track                   [Page 31]

RFC 5880           Bidirectional Forwarding Detection          June 2010   When the Echo function is active, a system SHOULD set   bfd.RequiredMinRxInterval to a value of not less than one second   (1,000,000 microseconds).  This is intended to keep received BFD   Control traffic at a negligible level, since the actual detection   function is being performed using BFD Echo packets.   In any case other than those explicitly called out above, timing   parameter changes MUST be effected immediately (changing the   transmission rate and/or the Detection Time).   Note that the Poll Sequence mechanism is ambiguous if more than one   parameter change is made that would require its use, and those   multiple changes are spread across multiple packets (since the   semantics of the returning Final are unclear).  Therefore, if   multiple changes are made that require the use of a Poll Sequence,   there are three choices: 1) they MUST be communicated in a single BFD   Control packet (so the semantics of the Final reply are clear), or 2)   sufficient time must have transpired since the Poll Sequence was   completed to disambiguate the situation (at least a round trip time   since the last Poll was transmitted) prior to the initiation of   another Poll Sequence, or 3) an additional BFD Control packet with   the Final (F) bit *clear* MUST be received after the Poll Sequence   has completed prior to the initiation of another Poll Sequence (this   option is not available when Demand mode is active).6.8.4.  Calculating the Detection Time   The Detection Time (the period of time without receiving BFD packets   after which the session is determined to have failed) is not carried   explicitly in the protocol.  Rather, it is calculated independently   in each direction by the receiving system based on the negotiated   transmit interval and the detection multiplier.  Note that there may   be different Detection Times in each direction.   The calculation of the Detection Time is slightly different when in   Demand mode versus Asynchronous mode.   In Asynchronous mode, the Detection Time calculated in the local   system is equal to the value of Detect Mult received from the remote   system, multiplied by the agreed transmit interval of the remote   system (the greater of bfd.RequiredMinRxInterval and the last   received Desired Min TX Interval).  The Detect Mult value is (roughly   speaking, due to jitter) the number of packets that have to be missed   in a row to declare the session to be down.Katz & Ward                  Standards Track                   [Page 32]

RFC 5880           Bidirectional Forwarding Detection          June 2010   If Demand mode is not active, and a period of time equal to the   Detection Time passes without receiving a BFD Control packet from the   remote system, and bfd.SessionState is Init or Up, the session has   gone down -- the local system MUST set bfd.SessionState to Down and   bfd.LocalDiag to 1 (Control Detection Time Expired).   In Demand mode, the Detection Time calculated in the local system is   equal to bfd.DetectMult, multiplied by the agreed transmit interval   of the local system (the greater of bfd.DesiredMinTxInterval and   bfd.RemoteMinRxInterval).  bfd.DetectMult is (roughly speaking, due   to jitter) the number of packets that have to be missed in a row to   declare the session to be down.   If Demand mode is active, and a period of time equal to the Detection   Time passes after the initiation of a Poll Sequence (the transmission   of the first BFD Control packet with the Poll bit set), the session   has gone down -- the local system MUST set bfd.SessionState to Down,   and bfd.LocalDiag to 1 (Control Detection Time Expired).   (Note that a packet is considered to have been received, for the   purposes of Detection Time expiration, only if it has not been   "discarded" according to the rules ofsection 6.8.6).6.8.5.  Detecting Failures with the Echo Function   When the Echo function is active and a sufficient number of Echo   packets have not arrived as they should, the session has gone down --   the local system MUST set bfd.SessionState to Down and bfd.LocalDiag   to 2 (Echo Function Failed).   The means by which the Echo function failures are detected is outside   of the scope of this specification.  Any means that will detect a   communication failure are acceptable.6.8.6.  Reception of BFD Control Packets   When a BFD Control packet is received, the following procedure MUST   be followed, in the order specified.  If the packet is discarded   according to these rules, processing of the packet MUST cease at that   point.      If the version number is not correct (1), the packet MUST be      discarded.      If the Length field is less than the minimum correct value (24 if      the A bit is clear, or 26 if the A bit is set), the packet MUST be      discarded.Katz & Ward                  Standards Track                   [Page 33]

RFC 5880           Bidirectional Forwarding Detection          June 2010      If the Length field is greater than the payload of the      encapsulating protocol, the packet MUST be discarded.      If the Detect Mult field is zero, the packet MUST be discarded.      If the Multipoint (M) bit is nonzero, the packet MUST be      discarded.      If the My Discriminator field is zero, the packet MUST be      discarded.      If the Your Discriminator field is nonzero, it MUST be used to      select the session with which this BFD packet is associated.  If      no session is found, the packet MUST be discarded.      If the Your Discriminator field is zero and the State field is not      Down or AdminDown, the packet MUST be discarded.      If the Your Discriminator field is zero, the session MUST be      selected based on some combination of other fields, possibly      including source addressing information, the My Discriminator      field, and the interface over which the packet was received.  The      exact method of selection is application specific and is thus      outside the scope of this specification.  If a matching session is      not found, a new session MAY be created, or the packet MAY be      discarded.  This choice is outside the scope of this      specification.      If the A bit is set and no authentication is in use (bfd.AuthType      is zero), the packet MUST be discarded.      If the A bit is clear and authentication is in use (bfd.AuthType      is nonzero), the packet MUST be discarded.      If the A bit is set, the packet MUST be authenticated under the      rules ofsection 6.7, based on the authentication type in use      (bfd.AuthType).  This may cause the packet to be discarded.      Set bfd.RemoteDiscr to the value of My Discriminator.      Set bfd.RemoteState to the value of the State (Sta) field.      Set bfd.RemoteDemandMode to the value of the Demand (D) bit.      Set bfd.RemoteMinRxInterval to the value of Required Min RX      Interval.Katz & Ward                  Standards Track                   [Page 34]

RFC 5880           Bidirectional Forwarding Detection          June 2010      If the Required Min Echo RX Interval field is zero, the      transmission of Echo packets, if any, MUST cease.      If a Poll Sequence is being transmitted by the local system and      the Final (F) bit in the received packet is set, the Poll Sequence      MUST be terminated.      Update the transmit interval as described insection 6.8.2.      Update the Detection Time as described insection 6.8.4.      If bfd.SessionState is AdminDown          Discard the packet      If received state is AdminDown          If bfd.SessionState is not Down              Set bfd.LocalDiag to 3 (Neighbor signaled                  session down)              Set bfd.SessionState to Down      Else          If bfd.SessionState is Down              If received State is Down                  Set bfd.SessionState to Init              Else if received State is Init                  Set bfd.SessionState to Up          Else if bfd.SessionState is Init              If received State is Init or Up                  Set bfd.SessionState to Up          Else (bfd.SessionState is Up)              If received State is Down                  Set bfd.LocalDiag to 3 (Neighbor signaled                      session down)                  Set bfd.SessionState to Down      Check to see if Demand mode should become active or not (seesection 6.6).      If bfd.RemoteDemandMode is 1, bfd.SessionState is Up, and      bfd.RemoteSessionState is Up, Demand mode is active on the remote      system and the local system MUST cease the periodic transmission      of BFD Control packets (seesection 6.8.7).Katz & Ward                  Standards Track                   [Page 35]

RFC 5880           Bidirectional Forwarding Detection          June 2010      If bfd.RemoteDemandMode is 0, or bfd.SessionState is not Up, or      bfd.RemoteSessionState is not Up, Demand mode is not active on the      remote system and the local system MUST send periodic BFD Control      packets (seesection 6.8.7).      If the Poll (P) bit is set, send a BFD Control packet to the      remote system with the Poll (P) bit clear, and the Final (F) bit      set (seesection 6.8.7).      If the packet was not discarded, it has been received for purposes      of the Detection Time expiration rules insection 6.8.4.6.8.7.  Transmitting BFD Control Packets   With the exceptions listed in the remainder of this section, a system   MUST NOT transmit BFD Control packets at an interval less than the   larger of bfd.DesiredMinTxInterval and bfd.RemoteMinRxInterval, less   applied jitter (see below).  In other words, the system reporting the   slower rate determines the transmission rate.   The periodic transmission of BFD Control packets MUST be jittered on   a per-packet basis by up to 25%, that is, the interval MUST be   reduced by a random value of 0 to 25%, in order to avoid self-   synchronization with other systems on the same subnetwork.  Thus, the   average interval between packets will be roughly 12.5% less than that   negotiated.   If bfd.DetectMult is equal to 1, the interval between transmitted BFD   Control packets MUST be no more than 90% of the negotiated   transmission interval, and MUST be no less than 75% of the negotiated   transmission interval.  This is to ensure that, on the remote system,   the calculated Detection Time does not pass prior to the receipt of   the next BFD Control packet.   The transmit interval MUST be recalculated whenever   bfd.DesiredMinTxInterval changes, or whenever bfd.RemoteMinRxInterval   changes, and is equal to the greater of those two values.  See   sections6.8.2 and6.8.3 for details on transmit timers.   A system MUST NOT transmit BFD Control packets if bfd.RemoteDiscr is   zero and the system is taking the Passive role.   A system MUST NOT periodically transmit BFD Control packets if   bfd.RemoteMinRxInterval is zero.Katz & Ward                  Standards Track                   [Page 36]

RFC 5880           Bidirectional Forwarding Detection          June 2010   A system MUST NOT periodically transmit BFD Control packets if Demand   mode is active on the remote system (bfd.RemoteDemandMode is 1,   bfd.SessionState is Up, and bfd.RemoteSessionState is Up) and a Poll   Sequence is not being transmitted.   If a BFD Control packet is received with the Poll (P) bit set to 1,   the receiving system MUST transmit a BFD Control packet with the Poll   (P) bit clear and the Final (F) bit set as soon as practicable,   without respect to the transmission timer or any other transmission   limitations, without respect to the session state, and without   respect to whether Demand mode is active on either system.  A system   MAY limit the rate at which such packets are transmitted.  If rate   limiting is in effect, the advertised value of Desired Min TX   Interval MUST be greater than or equal to the interval between   transmitted packets imposed by the rate limiting function.   A system MUST NOT set the Demand (D) bit unless bfd.DemandMode is 1,   bfd.SessionState is Up, and bfd.RemoteSessionState is Up.   A BFD Control packet SHOULD be transmitted during the interval   between periodic Control packet transmissions when the contents of   that packet would differ from that in the previously transmitted   packet (other than the Poll and Final bits) in order to more rapidly   communicate a change in state.   The contents of transmitted BFD Control packets MUST be set as   follows:   Version      Set to the current version number (1).   Diagnostic (Diag)      Set to bfd.LocalDiag.   State (Sta)      Set to the value indicated by bfd.SessionState.   Poll (P)      Set to 1 if the local system is sending a Poll Sequence, or 0 if      not.Katz & Ward                  Standards Track                   [Page 37]

RFC 5880           Bidirectional Forwarding Detection          June 2010   Final (F)      Set to 1 if the local system is responding to a Control packet      received with the Poll (P) bit set, or 0 if not.   Control Plane Independent (C)      Set to 1 if the local system's BFD implementation is independent      of the control plane (it can continue to function through a      disruption of the control plane).   Authentication Present (A)      Set to 1 if authentication is in use on this session (bfd.AuthType      is nonzero), or 0 if not.   Demand (D)      Set to bfd.DemandMode if bfd.SessionState is Up and      bfd.RemoteSessionState is Up.  Otherwise, it is set to 0.   Multipoint (M)      Set to 0.   Detect Mult      Set to bfd.DetectMult.   Length      Set to the appropriate length, based on the fixed header length      (24) plus any Authentication Section.   My Discriminator      Set to bfd.LocalDiscr.   Your Discriminator      Set to bfd.RemoteDiscr.   Desired Min TX Interval      Set to bfd.DesiredMinTxInterval.Katz & Ward                  Standards Track                   [Page 38]

RFC 5880           Bidirectional Forwarding Detection          June 2010   Required Min RX Interval      Set to bfd.RequiredMinRxInterval.   Required Min Echo RX Interval      Set to the minimum required Echo packet receive interval for this      session.  If this field is set to zero, the local system is      unwilling or unable to loop back BFD Echo packets to the remote      system, and the remote system will not send Echo packets.   Authentication Section      Included and set according to the rules insection 6.7 if      authentication is in use (bfd.AuthType is nonzero).  Otherwise,      this section is not present.6.8.8.  Reception of BFD Echo Packets   A received BFD Echo packet MUST be demultiplexed to the appropriate   session for processing.  A means of detecting missing Echo packets   MUST be implemented, which most likely involves processing of the   Echo packets that are received.  The processing of received Echo   packets is otherwise outside the scope of this specification.6.8.9.  Transmission of BFD Echo Packets   BFD Echo packets MUST NOT be transmitted when bfd.SessionState is not   Up.  BFD Echo packets MUST NOT be transmitted unless the last BFD   Control packet received from the remote system contains a nonzero   value in Required Min Echo RX Interval.   BFD Echo packets MAY be transmitted when bfd.SessionState is Up.  The   interval between transmitted BFD Echo packets MUST NOT be less than   the value advertised by the remote system in Required Min Echo RX   Interval, except as follows:      A 25% jitter MAY be applied to the rate of transmission, such that      the actual interval MAY be between 75% and 100% of the advertised      value.  A single BFD Echo packet MAY be transmitted between      normally scheduled Echo transmission intervals.   The transmission of BFD Echo packets is otherwise outside the scope   of this specification.Katz & Ward                  Standards Track                   [Page 39]

RFC 5880           Bidirectional Forwarding Detection          June 20106.8.10.  Min Rx Interval Change   When it is desired to change the rate at which BFD Control packets   arrive from the remote system, bfd.RequiredMinRxInterval can be   changed at any time to any value.  The new value will be transmitted   in the next outgoing Control packet, and the remote system will   adjust accordingly.  Seesection 6.8.3 for further requirements.6.8.11.  Min Tx Interval Change   When it is desired to change the rate at which BFD Control packets   are transmitted to the remote system (subject to the requirements of   the neighboring system), bfd.DesiredMinTxInterval can be changed at   any time to any value.  The rules insection 6.8.3 apply.6.8.12.  Detect Multiplier Change   When it is desired to change the detect multiplier, the value of   bfd.DetectMult can be changed to any nonzero value.  The new value   will be transmitted with the next BFD Control packet, and the use of   a Poll Sequence is not necessary.  Seesection 6.6 for additional   requirements.6.8.13.  Enabling or Disabling The Echo Function   If it is desired to start or stop the transmission of BFD Echo   packets, this MAY be done at any time (subject to the transmission   requirements detailed insection 6.8.9).   If it is desired to enable or disable the looping back of received   BFD Echo packets, this MAY be done at any time by changing the value   of Required Min Echo RX Interval to zero or nonzero in outgoing BFD   Control packets.6.8.14.  Enabling or Disabling Demand Mode   If it is desired to start or stop Demand mode, this MAY be done at   any time by setting bfd.DemandMode to the proper value.  Demand mode   will subsequently become active under the rules described insection6.6.   If Demand mode is no longer active on the remote system, the local   system MUST begin transmitting periodic BFD Control packets as   described insection 6.8.7.Katz & Ward                  Standards Track                   [Page 40]

RFC 5880           Bidirectional Forwarding Detection          June 20106.8.15.  Forwarding Plane Reset   When the forwarding plane in the local system is reset for some   reason, such that the remote system can no longer rely on the local   forwarding state, the local system MUST set bfd.LocalDiag to 4   (Forwarding Plane Reset), and set bfd.SessionState to Down.6.8.16.  Administrative Control   There may be circumstances where it is desirable to administratively   enable or disable a BFD session.  When this is desired, the following   procedure MUST be followed:      If enabling session         Set bfd.SessionState to Down      Else         Set bfd.SessionState to AdminDown         Set bfd.LocalDiag to an appropriate value         Cease the transmission of BFD Echo packets   If signaling is received from outside BFD that the underlying path   has failed, an implementation MAY administratively disable the   session with the diagnostic Path Down.   Other scenarios MAY use the diagnostic Administratively Down.   BFD Control packets SHOULD be transmitted for at least a Detection   Time after transitioning to AdminDown state in order to ensure that   the remote system is aware of the state change.  BFD Control packets   MAY be transmitted indefinitely after transitioning to AdminDown   state in order to maintain session state in each system (seesection6.8.18 below).6.8.17.  Concatenated Paths   If the path being monitored by BFD is concatenated with other paths   (connected end-to-end in series), it may be desirable to propagate   the indication of a failure of one of those paths across the BFD   session (providing an interworking function for liveness monitoring   between BFD and other technologies).   Two diagnostic codes are defined for this purpose: Concatenated Path   Down and Reverse Concatenated Path Down.  The first propagates   forward path failures (in which the concatenated path fails in the   direction toward the interworking system), and the second propagatesKatz & Ward                  Standards Track                   [Page 41]

RFC 5880           Bidirectional Forwarding Detection          June 2010   reverse path failures (in which the concatenated path fails in the   direction away from the interworking system, assuming a bidirectional   link).   A system MAY signal one of these failure states by simply setting   bfd.LocalDiag to the appropriate diagnostic code.  Note that the BFD   session is not taken down.  If Demand mode is not active on the   remote system, no other action is necessary, as the diagnostic code   will be carried via the periodic transmission of BFD Control packets.   If Demand mode is active on the remote system (the local system is   not transmitting periodic BFD Control packets), a Poll Sequence MUST   be initiated to ensure that the diagnostic code is transmitted.  Note   that if the BFD session subsequently fails, the diagnostic code will   be overwritten with a code detailing the cause of the failure.  It is   up to the interworking agent to perform the above procedure again,   once the BFD session reaches Up state, if the propagation of the   concatenated path failure is to resume.6.8.18.  Holding Down Sessions   A system MAY choose to prevent a BFD session from being established.   One possible reason might be to manage the rate at which sessions are   established.  This can be done by holding the session in Down or   AdminDown state, as appropriate.   There are two related mechanisms that are available to help with this   task.  First, a system is REQUIRED to maintain session state   (including timing parameters), even when a session is down, until a   Detection Time has passed without the receipt of any BFD Control   packets.  This means that a system may take down a session and   transmit an arbitrarily large value in the Required Min RX Interval   field to control the rate at which it receives packets.   Additionally, a system MAY transmit a value of zero for Required Min   RX Interval to indicate that the remote system should send no packets   whatsoever.   So long as the local system continues to transmit BFD Control   packets, the remote system is obligated to obey the value carried in   Required Min RX Interval.  If the remote system does not receive any   BFD Control packets for a Detection Time, it SHOULD reset   bfd.RemoteMinRxInterval to its initial value of 1 (persection 6.8.1,   since it is no longer required to maintain previous session state)   and then can transmit at its own rate.Katz & Ward                  Standards Track                   [Page 42]

RFC 5880           Bidirectional Forwarding Detection          June 20107.  Operational Considerations   BFD is likely to be deployed as a critical part of network   infrastructure.  As such, care should be taken to avoid disruption.   Obviously, any mechanism that blocks BFD packets, such as firewalls   or other policy processes, will cause BFD to fail.   Mechanisms that control packet scheduling, such as policers, traffic   shapers, priority queueing, etc., have the potential of impacting BFD   operations if the Detection Time is similar in scale to the scheduled   packet transmit or receive rate.  The delivery of BFD packets is   time-critical, relative to the magnitude of the Detection Time, so   this may need to be taken into account in implementation and   deployment, particularly when very short Detection Times are to be   used.   When BFD is used across multiple hops, a congestion control mechanism   MUST be implemented, and when congestion is detected, the BFD   implementation MUST reduce the amount of traffic it generates.  The   exact mechanism used is outside the scope of this specification, and   the requirements of this mechanism may differ depending on how BFD is   deployed, and how it interacts with other parts of the system (for   example, exponential backoff may not be appropriate in cases where   routing protocols are interacting closely with BFD).   Note that "congestion" is not only a traffic phenomenon, but also a   computational one.  It is possible for systems with a large number of   BFD sessions and/or very short packet intervals to become CPU-bound.   As such, a congestion control algorithm SHOULD be used even across   single hops in order to avoid the possibility of catastrophic system   collapse, as such failures have been seen repeatedly in other   periodic Hello-based protocols.   The mechanisms for detecting congestion are outside the scope of this   specification, but may include the detection of lost BFD Control   packets (by virtue of holes in the authentication sequence number   space, or by BFD session failure) or other means.   The mechanisms for reducing BFD's traffic load are the control of the   local and remote packet transmission rate via the Min RX Interval and   Min TX Interval fields.   Note that any mechanism that increases the transmit or receive   intervals will increase the Detection Time for the session.Katz & Ward                  Standards Track                   [Page 43]

RFC 5880           Bidirectional Forwarding Detection          June 2010   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.8.  IANA Considerations   This document defines two registries administered by IANA.  The first   is titled "BFD Diagnostic Codes" (seesection 4.1).  Initial values   for the BFD Diagnostic Code registry are given below.  Further   assignments are to be made through Expert Review   [IANA-CONSIDERATIONS].  Assignments consist of a BFD Diagnostic Code   name and its associated value.      Value    BFD Diagnostic Code Name      -----    ------------------------       0       No Diagnostic       1       Control Detection Time Expired       2       Echo Function Failed       3       Neighbor Signaled Session Down       4       Forwarding Plane Reset       5       Path Down       6       Concatenated Path Down       7       Administratively Down       8       Reverse Concatenated Path Down       9-31    Unassigned   The second registry is titled "BFD Authentication Types" (seesection4.1).  Initial values for the BFD Authentication Type registry are   given below.  Further assignments are to be made through Expert   Review [IANA-CONSIDERATIONS].  Assignments consist of a BFD   Authentication Type Code name and its associated value.      Value    BFD Authentication Type Name      -----    ----------------------------       0       Reserved       1       Simple Password       2       Keyed MD5       3       Meticulous Keyed MD5       4       Keyed SHA1       5       Meticulous Keyed SHA1       6-255   UnassignedKatz & Ward                  Standards Track                   [Page 44]

RFC 5880           Bidirectional Forwarding Detection          June 20109.  Security Considerations   As BFD may be tied into the stability of the network infrastructure   (such as routing protocols), the effects of an attack on a BFD   session may be very serious: a link may be falsely declared to be   down, or falsely declared to be up; in either case, the effect is   denial of service.   An attacker who is in complete control of the link between the   systems can easily drop all BFD packets but forward everything else   (causing the link to be falsely declared down), or forward only the   BFD packets but nothing else (causing the link to be falsely declared   up).  This attack cannot be prevented by BFD.   To mitigate threats from less capable attackers, BFD specifies two   mechanisms to prevent spoofing of BFD Control packets.  The   Generalized TTL Security Mechanism [GTSM] uses the time to live (TTL)   or Hop Count to prevent off-link attackers from spoofing packets.   The Authentication Section authenticates the BFD Control packets.   These mechanisms are described in more detail below.   When a BFD session is directly connected across a single link   (physical, or a secure tunnel such as IPsec), the TTL or Hop Count   MUST be set to the maximum on transmit, and checked to be equal to   the maximum value on reception (and the packet dropped if this is not   the case).  See [GTSM] for more information on this technique.  If   BFD is run across multiple hops or an insecure tunnel (such as   Generic Routing Encapsulation (GRE)), the Authentication Section   SHOULD be utilized.   The level of security provided by the Authentication Section varies   based on the authentication type used.  Simple Password   authentication is obviously only as secure as the secrecy of the   passwords used, and should be considered only if the BFD session is   guaranteed to be run over an infrastructure not subject to packet   interception.  Its chief advantage is that it minimizes the   computational effort required for authentication.   Keyed MD5 Authentication is much stronger than Simple Password   Authentication since the keys cannot be discerned by intercepting   packets.  It is vulnerable to replay attacks in between increments of   the sequence number.  The sequence number can be incremented as   seldom (or as often) as desired, trading off resistance to replay   attacks with the computational effort required for authentication.   Meticulous Keyed MD5 authentication is stronger yet, as it requires   the sequence number to be incremented for every packet.  Replay   attack vulnerability is reduced due to the requirement that theKatz & Ward                  Standards Track                   [Page 45]

RFC 5880           Bidirectional Forwarding Detection          June 2010   sequence number must be incremented on every packet, the window size   of acceptable packets is small, and the initial sequence number is   randomized.  There is still a window of attack at the beginning of   the session while the sequence number is being determined.  This   authentication scheme requires an MD5 calculation on every packet   transmitted and received.   Using SHA1 is believed to have stronger security properties than MD5.   All comments about MD5 in this section also apply to SHA1.   Both Keyed MD5/SHA1 and Meticulous Keyed MD5/SHA1 use the "secret   suffix" construction (also called "append only") in which the shared   secret key is appended to the data before calculating the hash,   instead of the more common Hashed Message Authentication Code (HMAC)   construction [HMAC].  This construction is believed to be appropriate   for BFD, but designers of any additional authentication mechanisms   for BFD are encouraged to read [HMAC] and its references.   If both systems randomize their Local Discriminator values at the   beginning of a session, replay attacks may be further mitigated,   regardless of the authentication type in use.  Since the Local   Discriminator may be changed at any time during a session, this   mechanism may also help mitigate attacks.   The security implications of the use of BFD Echo packets are   dependent on how those packets are defined, since their structure is   local to the transmitting system and outside the scope of this   specification.  However, since Echo packets are defined and processed   only by the transmitting system, the use of cryptographic   authentication does not guarantee that the other system is actually   alive; an attacker could loop the Echo packets back (without knowing   any secret keys) and cause the link to be falsely declared to be up.   This can be mitigated by using a suitable interval for BFD Control   packets.  [GTSM] could be applied to BFD Echo packets, though the   TTL/Hop Count will be decremented by 1 in the course of echoing the   packet, so spoofing is possible from one hop away.10.  References10.1.  Normative References   [GTSM]     Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.              Pignataro, "The Generalized TTL Security Mechanism              (GTSM)",RFC 5082, October 2007.   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.Katz & Ward                  Standards Track                   [Page 46]

RFC 5880           Bidirectional Forwarding Detection          June 2010   [MD5]      Rivest, R., "The MD5 Message-Digest Algorithm",RFC 1321,              April 1992.   [SHA1]     Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1              (SHA1)",RFC 3174, September 2001.10.2.  Informative References   [HMAC]     Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-              Hashing for Message Authentication",RFC 2104, February              1997.   [IANA-CONSIDERATIONS]              Narten, T. and H. Alvestrand, "Guidelines for Writing an              IANA Considerations Section in RFCs",BCP 26,RFC 5226,              May 2008.   [OSPF]     Moy, J., "OSPF Version 2", STD 54,RFC 2328, April 1998.Katz & Ward                  Standards Track                   [Page 47]

RFC 5880           Bidirectional Forwarding Detection          June 2010Appendix A.  Backward Compatibility (Non-Normative)   Although version 0 of this protocol (as defined in early versions of   the Internet-Draft that became this RFC) is unlikely to have been   deployed widely, some implementors may wish to have a backward   compatibility mechanism.  Note that any mechanism may be potentially   used that does not alter the protocol definition, so interoperability   should not be an issue.   The suggested mechanism described here has the property that it will   converge on version 1 if both systems implement it, even if one   system is upgraded from version 0 within a Detection Time.  It will   interoperate with a system that implements only one version (or is   configured to support only one version).  A system should obviously   not perform this function if it is configured to or is only capable   of using a single version.   A BFD session will enter a "negotiation holddown" if it is configured   for automatic versioning and either has just started up, or the   session has been manually cleared.  The session is set to AdminDown   state and version 1.  During the holddown period, which lasts for one   Detection Time, the system sends BFD Control packets as usual, but   ignores received packets.  After the holddown time is complete, the   state transitions to Down and normal operation resumes.   When a system is not in holddown, if it doing automatic versioning   and is currently using version 1, if any version 0 packet is received   for the session, it switches immediately to version 0.  If it is   currently using version 0 and a version 1 packet is received that   indicates that the neighbor is in state AdminDown, it switches to   version 1.  If using version 0 and a version 1 packet is received   indicating a state other than AdminDown, the packet is ignored (per   spec).   If the version being used is changed, the session goes down as   appropriate for the new version (Down state for version 1 or Failing   state for version 0).Appendix B.  Contributors   Kireeti Kompella and Yakov Rekhter of Juniper Networks were also   significant contributors to this document.Katz & Ward                  Standards Track                   [Page 48]

RFC 5880           Bidirectional Forwarding Detection          June 2010Appendix C.  Acknowledgments   This document was inspired by (and is intended to replace) the   Protocol Liveness Protocol document, written by Kireeti Kompella.   Demand mode was inspired by "A Traffic-Based Method of Detecting Dead   Internet Key Exchange (IKE) Peers", by G. Huang, et al.   The authors would also like to thank Mike Shand, John Scudder,   Stewart Bryant, Pekka Savola, Richard Spencer, and Pasi Eronen for   their substantive input.   The authors would also like to thank Owen Wheeler for hosting   teleconferences between the authors of this specification and   multiple vendors in order address implementation and clarity issues.Authors' Addresses   Dave Katz   Juniper Networks   1194 N. Mathilda Ave.   Sunnyvale, CA  94089-1206   USA   Phone: +1-408-745-2000   EMail: dkatz@juniper.net   Dave Ward   Juniper Networks   1194 N. Mathilda Ave.   Sunnyvale, CA  94089-1206   USA   Phone: +1-408-745-2000   EMail: dward@juniper.netKatz & Ward                  Standards Track                   [Page 49]