Bidirectional Forwarding Detection (BFD) in Segment Routing Networks Using MPLS Dataplane

Introduction , , and define the operation of the Bidirectional Forwarding Detection (BFD) protocol between two systems over IP networks. and set rules for using BFD Asynchronous mode over point-to-point (p2p) Multiprotocol Label Switching (MPLS) Label Switched Path (LSP). These latter standards implicitly assume that the remote BFD system, which is at the egress Label Edge Router (LER), will use the shortest path route to periodically transmit its BFD Control messages regardless of the path the BFD system at the ingress LER uses to send BFD Control packets towards it. defines the SR Policy architecture. When analyzing the applicability of a BFD-based mechanism for detecting network failures in a Segment Routing domain, it is essential to identify the monitored SR Policy elements. This document describes the use of BFD for monitoring individual segment lists of candidate paths of an SR Policy. It documents the use of various BFD modes and features such as BFD Demand mode, Seamless BFD, and BFD Echo function with the BFD Control packet payload. in the Segment Routing over MPLS (SR-MPLS) domain. Also, this document defines the use of LSP Ping for Segment Routing networks over the MPLS data plane to bootstrap and control path of a BFD session from the egress LER to the ingress LER using Segment Routing segment list with MPLS data plane (SR-MPLS).

Conventions

Terminology and Abbreviations Throughout this document, references to ingress LER and egress LER are used, respectively, as a shortened version of the "BFD system at the ingress/egress LER". BFD: Bidirectional Forwarding Detection FEC: Forwarding Equivalence Class MPLS: Multiprotocol Label Switching SR-MPLS: Segment Routing over MPLS LSP: Label Switched Path LER: Label Edge Router p2p: Point-to-point p2mp: Point-to-multipoint SID: Segment Identifier SR: Segment Routing S-BFD: Seamless BFD

Requirements Language 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 when, and only when, they appear in all capitals, as shown here.

Initialization of a BFD Session Over a Segment List with MPLS Data Plane LSP Ping SHOULD be used to bootstrap the BFD sessions unless other means are available, e.g., using an extension to a dynamic routing protocol as described in and . The procedures specified in for using LSP Ping with an MPLS data plane SHOULD be used. In some scenarios, e.g., when a Binding SID is the last segment of the SR Policy, procedures using Egress TLV MAY be used to bootstrap a BFD session for the given SR Policy. To support a BFD session for each candidate path of the given SR Policy, ingress and egress LERs MUST conform to the procedures specified in Section 2 of . Encapsulation of a BFD Control packet in Segment Routing network with MPLS data plane MUST follow Section 7 when the IP/UDP header is used. The encapsulation MUST follow Section 3.4 if the IP/UDP header is not used.

Using BFD Reverse Path TLV over SR Policy's Segment List For the BFD over MPLS LSP case, the egress LER MUST send BFD Control packets to the ingress LER as specified in . For the case of BFD over a p2p SR-MPLS segment list, the egress LER MUST send BFD Control Packets to the ingress LER either using an IP encapsulation as specified in Section 5 of , or encapsulated in an MPLS label stack as specified in Section 7 of . The mechanisms mentioned above don't ensure that both directions of the BFD session use co-routed paths, which may contribute to false positive defect notifications . To instruct the egress BFD system to use an explicit path for the BFD Control Packets associated with a particular BFD session, the procedures defined in MUST be used.

Use of Non-FEC Path TLV For the case of MPLS data plane, Segment Routing Architecture explains that "a segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels." This document defines a new optional Non-FEC Path TLV. The format of the Non-FEC Path TLV is presented in .

Non-FEC Path TLV Format Non-FEC Path TLV Type field is two octets in length and has a value of TBD1. The Length field is two octets long and defines the length in octets of the Non-FEC Path field. The Non-FEC Path field MUST contain at most one Non-FEC Path sub-TLV. If no sub-TLV has been found in the Non-FEC Path TLV, the egress LER MUST revert to using the reverse path selected based on its local policy. If there is more than one sub-TLV, then the Return Code in an MPLS echo reply MUST be set to value TBD3 "Too Many TLVs Detected". If the receiver of the MPLS echo request doesn't recognize Non-FEC Path TLV, it MUST set the Return Code in an MPLS echo reply to 2 ("One or more of the TLVs was not understood").

SR Policy's Segment List sub-TLV This document defines the SR Policy's Segment List sub-TLV that MAY be used with the Non-FEC Path TLV. The format of the sub-TLV is presented in .

SR Policy's Segment List sub-TLV The SR Policy's Segment List sub-TLV Type is two octets in length, and has a value of 1. The Length field is two octets long and defines the length in octets of Label Stack Entries listed in that sub-TLV. The Label Stack Entry field is four octets long and is the label stack entry as defined in Section 2.1 of . Label Stack Entries MUST be in network order. The egress LER MUST use the Label fields of the Label Stack Entry field as label stack for BFD Control packets for the BFD session identified by the source IP address of the MPLS LSP Ping packet and the value in the BFD Discriminator TLV.

BFD Reverse Path TLV over SR Policy's Segment List with Dynamic Control Plane Target FEC Stack sub-TLVs defined in are applicable in SR domains that are in the scope of .

Applicability of BFD Demand Mode in SR-MPLS Domain Sections 6.6 and 6.18.4 of define how Demand mode of BFD can be used to monitor uni-directional MPLS LSP. Similar procedures can be followed in SR-MPLS to monitor uni-directional SR tunnels:

an ingress SR node bootstraps the BFD session over SR-MPLS in Asynchronous BFD mode;
once the BFD session is Up, the ingress SR node switches the egress LER into the Demand mode by setting D field in BFD Control packet it transmits;
if the egress LER detects the failure of the BFD session, it sends its BFD Control packet to the ingress SR node over the IP network with a Poll sequence (Section 6.5 of );
if the ingress SR node receives a BFD Control packet from the remote node in a Demand mode with Poll sequence and Diag field indicating the failure, the ingress SR node transmits BFD Control packet with Final over IP and switches the BFD over SR-MPLS back into Asynchronous mode, sending BFD Control packets one per second.

Using BFD to Monitor Point-to-Multipoint SR Policy Segment Routing Replication for Multipoint Service Delivery defined variants of SR Policy to deliver point-to-multipoint (p2mp) services. For the given segment list of an p2mp SR Policy, BFD for Multipoint Networks can be used if, for example, leaves have an alternative source of the multicast service flow to select. In such a scenario, a leaf may switch to using the alternative flow after p2mp BFD detects the failure in the working multicast path. For scenarios where it is required for the root to monitor the state of the multicast tree BFD Multipoint Active Tails can be used. The root may use the detection of the failure of the multicast tree to the particular leaf to restore the path for that leaf or re-instantiate the whole multicast tree. An essential part of using p2mp BFD is the bootstrapping the BFD session at all the leaves. The root, acting as the MultipointHead, MAY use LSP Ping and with the BFD Discriminator TLV. Also, the BGP-BFD Attribute MAY be used to bootstrap a multipoint BFD session on a tail. Furthermore, other extensions to routing protocols or the management plane could be defined in the future to serve similar purposes, but such work is out of the scope of this document.

Use of BFD Echo in SR-MPLS BFD Echo can be used to monitor a segment list of the particular SR Policy between the local and the remote BFD peers. As defined in , the remote BFD system does not process the payload of a BFD Echo packet. Thus, the local system demultiplexes the BFD Echo packet. matches it to the appropriate BFD session, and detects missing BFD Echo packets. A BFD Control packet MAY be used as the payload of the BFD Echo packet. This specification defines the use of the BFD Echo function in the SR-MPLS network with BFD Control packet as the payload. The use of the BFD Echo function in modes other than defined in , e.g., , and other types of BFD Echo payload are outside the scope of this document. Because the remote BFD system does not process Echo BFD, the value of the Your Discriminator field MUST be set to the discriminator of the local BFD system assigned to the given BFD session. My Discriminator field MUST be zeroed. To ensure that the BFD Echo packet is returned to the sender without being processed, the sender MAY use a Binding SID that has been bound with the SR Policy that ensures the return of a packet to that particular node.

Use of S-BFD in SR-MPLS Seamless BFD (S-BFD), defined in , maintains essential characteristics and elements of the base BFD mechanism described in with a lighter approach to instantiating a BFD session between BFD peers. Similar to the BFD Asynchronous mode, S-BFD is capable of monitoring a segment list of a p2p SR Policy. Considering that a particular SR Policy can include multiple candidate paths, which, in turn, have one or more segment lists, it could be beneficial to monitor each segment list independently. To achieve that, the S-BFD Reflector advertises My Discriminator value. Then, the S-BFD Initiator uses the advertised My Discriminator value as Your Discriminator value in the BFD Control messages transmitted over the segment list of the SR Policy. Furthermore, the S-BFD Initiator assigns a unique My Discriminator for each S-BFD session monitoring a segment list. S-BFD Reflector transmits BFD Control messages as IP/UDP packets, taking advantage of the available resilience mechanisms of the IP network. From that point, to minimize the detection of failures in the IP network that do not affect the monitored segment list, it is reasonable not to use defect detection intervals that are close to the IP network repair time. Instead, having an S-BFD a detection interval three times longer than the IP network repair time is practical.

IANA Considerations

Non-FEC Path TLV IANA is requested to assign a new TLV type from the 16384-31739 range of the registry "Sub-TLVs for TLV Types 1, 16, and 21" of the "Multiprotocol Label Switching Architecture (MPLS) Label Switched Paths (LSPs) Ping Parameters" group as defined in . New Non-FEC Path TLV

Value	TLV Name	Reference	Sub-TLV Registry
TBD1	Non-FEC Path TLV	This document	Path sub-TLV

IANA is requested to create new Non-FEC Path sub-TLV registry for the Non-FEC Path TLV, as described in . Non-FEC Path sub-TLV registry

Range	Registration Procedures	Note
0-16383	Standards Action	This range is for sub-TLVs that require an error message if not recognized (Section 4.1 of ).
16384-31739	RFC Required	This range is for sub-TLVs that require an error message if not recognized (Section 4.1 of ).
31740-31743	Experimental Use. Reserved, not to be assigned.	This range is for sub-TLVs that require an error message if not recognized (Section 4.1 of ).
31744-32767	First Come First Served	This range is for sub-TLVs that require an error message if not recognized (Section 4.1 of ).
32768-49161	Standards Action	This range is for optional TLVs that can be silently dropped if not recognized.
49162-64507	RFC Required	This range is for optional TLVs that can be silently dropped if not recognized.
64508-64511	Experimental Use. Reserved, not to be assigned.	This range is for optional TLVs that can be silently dropped if not recognized.
64512-65535	First Come First Served	This range is for optional TLVs that can be silently dropped if not recognized.

IANA is requested to allocate the following values from the Non-FEC Path sub-TLV registry as defined in . New SR Segment List sub-TLV

Value	Description	Reference
0	Reserved	This document
1	SR Policy's Segment List sub-TLV	This document
65535	Reserved	This document

Return Code IANA is requested to create Non-FEC Path sub-TLV sub-registry for the new Non-FEC Path TLV and assign a new Return Code value from the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry, "Return Codes" sub-registry, as follows using a value from the RFC Required range. New Return Code

Value	Description	Reference
TBD3	Too Many TLVs Detected.	This document

Implementation Status Note to RFC Editor: This section MUST be removed before publication of the document. This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in . The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to , "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit". - The organization responsible for the implementation: ZTE Corporation. - The implementation's name ROSng SW empowers traditional routers, e.g., ZXCTN 6000. - A brief general description: A list of SIDs can be specified as the Return Path for an SR-MPLS segment list. - The implementation's level of maturity: production. - Coverage: complete - Version compatibility: draft-mirsky-spring-bfd-06. - Licensing: proprietary. - Implementation experience: Appreciate Early Allocation of values for Non-FEC TLV and SR Policy's Segment List sub-TLV (using First Come First Served code points). - Contact information: Qian Xin qian.xin2@zte.com.cn - The date when information about this particular implementation was last updated: 12/16/2019

Security Considerations This document describes the specifics of using MPLS LSP Ping, BFD, and BFD for multipoint networks for the Segment Routing network with the MPLS data plane. Since all the discussed tools have been used in MPLS networks, there are no additional security risks. The security considerations discussed in , , , , , , , , , , , , and apply to this document.

Operational Considerations BFD provided

The Scope of the Experiment The experimental part included in this document is limited to the use of Non-FEC Path TLV in BFD Reverse Path TLV . The goal of the experiment with the Non-FEC Path TLV is validation that its use does not adversely affect the defect detection in the forward direction while reducing the number of BFD sessions between a pair of label switching routers without reporting additional false-negative events.

Contributors

Acknowledgments Authors express their sincere gratitude to Alexander "Sasha" Vainshtein for his helpful comments and thought-inspiring discussion of SR Policies and BFD-based mechanisms. Authors greatly appreciate the help of Qian Xin, who provided the information about the implementation of this specification.