BFD Stability

The Bidirectional Forwarding Detection ( BFD) protocol operates by transmitting and receiving BFD control packets, generally at high frequency, over the datapath being monitored. In order to prevent significant data loss due to a datapath failure, BFD session detection time as defined in BFD is set to the smallest feasible value. This document proposes a mechanism to detect lost packets in a BFD session in addition to the datapath fault detection mechanisms of BFD. Such a mechanism presents significant value to measure the stability of BFD sessions and provides data to the operators for the cause of a BFD failure. This document does not propose any BFD extension to measure data traffic loss or delay on a link or tunnel and the scope is limited to BFD packets.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 and RFC 8174. The reader is expected to be familiar with the BFD , Optimizing BFD Authentication and BFD Secure Sequence Numbers .

Bidirectional Forwarding Detection as defined in BFD cannot detect any BFD packet loss if the loss does not last for detection time. This document proposes a method to detect a dropped packet on the receiver. For example, if the receiver receives BFD control packet k at time t but receives packet k+3 at time t+10ms, and never receives packet k+1 and/or k+2, then it has experienced a drop. This proposal enables BFD implementations to generate diagnostic information on the health of each BFD session that could be used to preempt a failure on a datapath that BFD was monitoring by allowing time for a corrective action to be taken. In a faulty datapath scenario, an operator can use BFD health information to trigger delay and loss measurement OAM protocol (Connectivity Fault Management (CFM) or Loss Measurement (LM)-Delay Measurement (DM)) to further isolate the issue.

The functionality proposed for BFD stability measurement is achieved by appending an authentication section with the NULL Authentication type (as defined in Optimizing BFD Authentication ) to the BFD control packets that do not have authentication enabled.

This mechanism allows operators to measure the loss of BFD control packets. When using MD5 or SHA authentication, BFD uses an authentication section that carries the Sequence Number. However, if non-meticulous authentication is being used, or no authentication is in use, then the non-authenticated BFD control packets MUST include an authentication section with the NULL Authentication type.

Loss measurement counts the number of BFD control packets missed at the receiver during any Detection Time period. The loss is detected by comparing the Sequence Number field in the Auth TLV (NULL or otherwise) in successive BFD control packets. The Sequence Number in each successive control packet generated on a BFD session by the transmitter is incremented by one. This loss count can then be exposed using the YANG module defined in the subsequent section. The first BFD authentication section with a non-zero sequence number, in a valid BFD control packet, processed by the receiver is used for bootstrapping the logic. When using secure sequence numbers, if the expected values are pre-calculated, the value must be matched to detect lost packets as defined in BFD secure sequence numbers .

This YANG module augments the "ietf-bfd" module to add the loss count to the per-session or lsp for BFD packets that are lost.

This YANG module imports Common YANG Types, A YANG Data Model for Routing, and YANG Data Model for Bidirectional Forwading Detection (BFD).

file "ietf-bfd-stability@2024-01-23.yang" module ietf-bfd-stability { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:ietf-bfd-stability"; prefix "bfds"; import ietf-yang-types { prefix "yang"; reference "RFC 6991: Common YANG Data Types"; } import ietf-routing { prefix "rt"; reference "RFC 8349: A YANG Data Model for Routing Management (NMDA version)"; } import ietf-bfd { prefix bfd; reference "RFC 9314: YANG Data Model for Bidirectional Forwarding Detection."; } import ietf-bfd-ip-sh { prefix bfd-ip-sh; reference "RFC 9314: YANG Data Model for Bidirectional Forwarding Detection."; } import ietf-bfd-ip-mh { prefix bfd-ip-mh; reference "RFC 9314: YANG Data Model for Bidirectional Forwarding Detection."; } import ietf-bfd-lag { prefix bfd-lag; reference "RFC 9314: YANG Data Model for Bidirectional Forwarding Detection."; } import ietf-bfd-mpls { prefix bfd-mpls; reference "RFC 9314: YANG Data Model for Bidirectional Forwarding Detection."; } organization "IETF BFD Working Group"; contact "WG Web: WG List: Authors: Mahesh Jethanandani (mjethanandani@gmail.com) Ashesh Mishra (mishra.ashesh@gmail.com) Ankur Saxena (ankurpsaxena@gmail.com) Santosh Pallagatti (santosh.pallagati@gmail.com) Mach Chen (mach.chen@huawei.com) Peng Fan (fanp08@gmail.com)."; description "This YANG module augments the base BFD YANG model to add attributes related to BFD Stability. In particular it adds a a per session count for BFD packets that are lost. Copyright (c) 2024 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Revised BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info). This version of this YANG module is part of RFC XXXX (https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself for full legal notices. The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document are to be interpreted as described in BCP 14 (RFC 2119) (RFC 8174) when, and only when, they appear in all capitals, as shown here."; revision "2024-01-23" { description "Initial Version."; reference "RFC XXXX, BFD Stability."; } augment "/rt:routing/rt:control-plane-protocols/" + "rt:control-plane-protocol/bfd:bfd/bfd-ip-sh:ip-sh/" + "bfd-ip-sh:sessions/bfd-ip-sh:session/" + "bfd-ip-sh:session-statistics" { leaf lost-packet-count { type yang:counter32; description "Number of BFD packets that were lost without bringing the session down."; } description "Augment the 'bfd' container to add attributes related to BFD stability."; } augment "/rt:routing/rt:control-plane-protocols/" + "rt:control-plane-protocol/bfd:bfd/bfd-ip-mh:ip-mh/" + "bfd-ip-mh:session-groups/bfd-ip-mh:session-group/" + "bfd-ip-mh:sessions/bfd-ip-mh:session-statistics" { leaf lost-packet-count { type yang:counter32; description "Number of BFD packets that were lost without bringing the session down."; } description "Augment the 'bfd' container to add attributes related to BFD stability."; } augment "/rt:routing/rt:control-plane-protocols/" + "rt:control-plane-protocol/bfd:bfd/bfd-lag:lag/" + "bfd-lag:sessions/bfd-lag:session/bfd-lag:member-links/" + "bfd-lag:micro-bfd-ipv4/bfd-lag:session-statistics" { leaf lost-packet-count { type yang:counter32; description "Number of BFD packets that were lost without bringing the session down."; } description "Augment the 'bfd' container to add attributes related to BFD stability."; } augment "/rt:routing/rt:control-plane-protocols/" + "rt:control-plane-protocol/bfd:bfd/bfd-lag:lag/" + "bfd-lag:sessions/bfd-lag:session/bfd-lag:member-links/" + "bfd-lag:micro-bfd-ipv6/bfd-lag:session-statistics" { leaf lost-packet-count { type yang:counter32; description "Number of BFD packets that were lost without bringing the session down."; } description "Augment the 'bfd' container to add attributes related to BFD stability."; } augment "/rt:routing/rt:control-plane-protocols/" + "rt:control-plane-protocol/bfd:bfd/bfd-mpls:mpls/" + "bfd-mpls:session-groups/bfd-mpls:session-group/" + "bfd-mpls:sessions/bfd-mpls:session-statistics" { leaf lost-packet-count { type yang:counter32; description "Number of BFD packets that were lost without bringing the session down."; } description "Augment the 'bfd' container to add attributes related to BFD stability."; } }

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      The YANG module specified in this document defines a schema for data
      that is designed to be accessed via network management protocols such as
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      Manav Bhatia
    

    
      Authors would like to thank Nobo Akiya, Jeffery Haas, Dileep Singh,
      Basil Saji, Sagar Soni, Albert Fu and Mallik Mudigonda who also
      contributed to this document.