Bidirectional Forwarding Detection (BFD) in Segment Routing Networks Using MPLS Dataplane
draft-mirsky-spring-bfd-07

Abstract

Segment Routing (SR) architecture leverages the paradigm of source routing. It can be realized in the Multiprotocol Label Switching (MPLS) network without any change to the data plane. A segment is encoded as an MPLS label, and an ordered list of segments is encoded as a stack of labels. Bidirectional Forwarding Detection (BFD) is expected to monitor any existing path between systems. This document defines how to use Label Switched Path Ping to bootstrap a BFD session, control path in reverse direction of the SR-MPLS tunnel and applicability of BFD Demand mode in the SR-MPLS domain.

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Table of Contents

• 1. Introduction
• 1.1. Conventions
• 1.1.1. Terminology
• 1.1.2. Requirements Language
• 2. Bootstrapping BFD Session over Segment Routed Tunnel with MPLS Data Plane
• 3. Use BFD Reverse Path TLV over Segment Routed MPLS Tunnel
• 4. Use Non-FEC Path TLV
• 5. BFD Reverse Path TLV over Segment Routed MPLS Tunnel with Dynamic Control Plane
• 6. Applicability of BFD Demand Mode in SR-MPLS Domain
• 7. IANA Considerations
• 7.1. Non-FEC Path TLV
• 7.2. Return Code
• 8. Security Considerations
• 9. Acknowledgments
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Authors' Addresses

1. Introduction

[RFC5880], [RFC5881], and [RFC5883] defined the operation of Bidirectional Forwarding Detection (BFD) protocol between the two systems over IP networks. [RFC5884] and [RFC7726] 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 egress BFD peer, which is the egress Label Edge Router (LER), will use the shortest path route regardless of the path the ingress LER uses to send BFD Control packets towards it.

This document defines the use of LSP Ping for Segment Routing networks over MPLS data plane [RFC8287] to bootstrap and control path of a BFD session from the egress to ingress LER using Segment Routing tunnel with MPLS data plane (SR-MPLS).

1.1. Conventions

1.1.1. Terminology

BFD: Bidirectional Forwarding Detection

FEC: Forwarding Equivalence Class

MPLS: Multiprotocol Label Switching

SR-MPLS Segment Routing with MPLS data plane

LSP: Label Switched Path

LSR Label Switching Router

LER Label Edge Router

p2p Point-to-point

SID Segment Identifier

SR Segment Routing

1.1.2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. Bootstrapping BFD Session over Segment Routed Tunnel with MPLS Data Plane

Use of an LSP Ping to bootstrap BFD over MPLS LSP is required, as documented in [RFC5884], to establish an association between a fault detection message, i.e., BFD Control message, and the Forwarding Equivalency Class (FEC) of a single label stack LSP in case of Penultimate Hop Popping or when the egress Label Switching Router (LSR) distributes the Explicit NULL label to the penultimate hop router. The Explicit NULL label is not advertised as a Segment Identifier (SID) by an SR node but, as demonstrated in section 3.1 [I-D.ietf-spring-segment-routing-mpls] if the operation at the penultimate hop is NEXT; then the egress SR node will receive an IP encapsulated packet. Thus the conclusion is that LSP Ping MUST be used to bootstrap a BFD session in SR-MPLS domain.

As demonstrated in [RFC8287], the introduction of Segment Routing network domains with an MPLS data plane requires three new sub-TLVs that MAY be used with Target FEC TLV. Section 6.1 addresses use of the new sub-TLVs in Target FEC TLV in LSP ping and LSP traceroute. For the case of LSP ping, the [RFC8287] states that:

• The initiator, i.e., ingress LSR, MUST include FEC(s) corresponding to the destination segment.
• The initiator MAY include FECs corresponding to some or all of segments imposed in the label stack by the ingress LSR to communicate the segments traversed.

It has been noted in [RFC5884] that a BFD session monitors for defects particular <MPLS LSP, FEC> tuple. [RFC7726] clarified how to establish and operate multiple BFD sessions for the same <MPLS LSP, FEC> tuple. Because only ingress edge router is aware of the SR-based explicit route, the egress edge router can associate the LSP ping with BFD Discriminator TLV with only one of the FECs it advertised for the particular segment. Thus this document clarifies that:

• When LSP Ping is used to bootstraping a BFD session for SR-MPLS tunnel the FEC corresponding to the segment to be associated with the BFD session MUST be as the very last sub-TLV in the Target FEC TLV.

If the target segment is an anycast prefix segment ([I-D.ietf-spring-mpls-anycast-segments]) the corresponding Anycast SID MUST be included in the Target TLV as the very last sub-TLV. Also, for BFD control packet the ingress SR node MUST use precisely the same label stack encapsulation, especially Entropy Label ([RFC6790]), as for the LSP ping with the BFD Discriminator TLV that bootstrapped the BFD session. Other operational aspects of using BFD to monitor the continuity of the path to the particular Anycast SID, advertised by a group of SR-MPLS capable nodes, will be considered in the future versions of the document.

Encapsulation of a BFD Control packet in Segment Routing network with MPLS data plane MUST follow Section 7 [RFC5884] when the IP/UDP header used and MUST follow Section 3.4 [RFC6428] without IP/UDP header being used.

3. Use BFD Reverse Path TLV over Segment Routed MPLS Tunnel

For BFD over MPLS LSP case, per [RFC5884], egress LER MAY send BFD control packet to the ingress LER either over IP network or an MPLS LSP. Similarly, for the case of BFD over p2p SR-MPLS tunnel, the egress LER MAY route BFD control packet over the IP network, as described in [RFC5883], or transmit over a segment tunnel, as described in Section 7 [RFC5884]. In some cases, there may be a need to direct egress BFD peer to use specific path for the reverse direction of the BFD session by using the BFD Reverse Path TLV and following all procedures as defined in [I-D.ietf-mpls-bfd-directed].

4. Use Non-FEC Path TLV

For the case of MPLS data plane, Segment Routing Architecture [RFC8402] explains that "a segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels." YANG Data Model for MPLS Static LSPs [I-D.ietf-mpls-static-yang] models outgoing MPLS labels to be imposed as leaf-list [RFC6020], i.e., as array of rt-types:mpls-label [RFC8294].

    
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Non-FEC Path TLV Type      |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                          Non-FEC Path                         ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Non-FEC Path TLV Format

This document defines new optional Non-FEC Path TLV. The format of the Non-FEC Path TLV is presented in Figure 1

Non-FEC Path TLV Type is two octets in length and has a value of TBD1 (to be assigned by IANA as requested in Section 7.1).

Length field is two octets long and defines the length in octets of the Non-FEC Path field.

Non-FEC Path field contains a sub-TLV. Any Non-FEC Path sub-TLV (defined in this document or to be defined in the future) for Non-FEC Path TLV type MAY be used in this field. None or one sub-TLV MAY be included in the Non-FEC Path TLV. If no sub-TLV has been found in the Non-FEC Path TLV, the egress BFD peer 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 echo reply MUST be set to value TBD3 "Too Many TLVs Detected" (to be assigned by IANA as requested in Table 4).

Non-FEC Path TLV MAY be used to specify the reverse path of the BFD session identified in the BFD Discriminator TLV. If the Non-FEC Path TLV is present in the echo request message the BFD Discriminator TLV MUST be present as well. If the BFD Discriminator TLV is absent when the Non-FEC Path TLV is included, then it MUST be treated as malformed Echo Request, as described in [RFC8029].

This document defines Static Routing MPLS Tunnel sub-TLV that MAY be used with the Non-FEC Path TLV. The format of the sub-TLV is presented in Figure 2.

    
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  SR MPLS Tunnel sub-TLV Type  |           Length              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Label Entry 1 (Top Label)                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         Label Entry 2                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 ~                                                               ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Label Entry N (Bottom Label)               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Segment Routing MPLS Tunnel sub-TLV

The Segment Routing MPLS Tunnel sub-TLV Type is two octets in length, and has a value of TBD2 (to be assigned by IANA as requested in Section 7.1).

The egress LSR MUST use the Value 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. Label Entries MUST be in network order.

5. BFD Reverse Path TLV over Segment Routed MPLS Tunnel with Dynamic Control Plane

When Segment Routed domain with MPLS data plane uses distributed tunnel computation BFD Reverse Path TLV MAY use Target FEC sub-TLVs defined in [RFC8287].

6. Applicability of BFD Demand Mode in SR-MPLS Domain

[I-D.mirsky-bfd-mpls-demand] defines how Demand mode of BFD, specified in sections 6.6 and 6.18.4 of [RFC5880], can be used to monitor uni-directional MPLS LSP. Similar procedures can be following in SR-MPLS to monitor uni-directional SR tunnels:

• ingress SR node bootstraps BFD session over SR-MPLS in Async BFD mode;
• once BFD session is Up, the ingress node switches the egress BFD node into the Demand mode by setting D field in BFD Control packet it transmits;
• if the egress BFD node detects the failure of the BFD session, it sends its BFD control packet to the ingress over the IP network with Poll sequence;
• if the ingress 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 transmits BFD control packet with Final over IP and switches the BFD over SR-MPLS back into Async mode, sending BFD Control packets one per second.

7. IANA Considerations

7.1. Non-FEC Path TLV

IANA is requested to assign new TLV type from the from Standards Action range of the registry "Multiprotocol Label Switching Architecture (MPLS) Label Switched Paths (LSPs) Ping Parameters - TLVs" as defined in Table 1.

IANA is requested to create new Non-FEC Path sub-TLV registry for the Non-FEC Path TLV as described in Table 2.

IANA is requested to allocate the following values from the Non-FEC Path sub-TLV registry as defined in Table 3.

7.2. 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 Standards Action value.

8. Security Considerations

Security considerations discussed in [RFC5880], [RFC5884], [RFC7726], and [RFC8029] apply to this document.

9. Acknowledgments

TBD

10. References

10.1. Normative References

10.2. Informative References

Authors' Addresses