Two new bits are requested from IANA to indicate S-BFD operation.

2.1. Co-existence of S-BFD and BFD capabilites

Since the CV types for S-BFD and BFD are unique, BFD and S-BFD capabilities can be advertised concurrently.

2.2. S-BFD CV Operation

2.2.1. S-BFD Initiator Operation

The S-BFD Initiator SHOULD bootstrap S-BFD sessions after it learns the discriminator of the remote target identifier. This can be achieved, for example but not limited to, through one or more of the following methods:

1. Advertisements of S-BFD discriminators made through a PW signaling protocol, for example AVP/TLVs defined in L2TP/LDP.
2. Provisioning of S-BFD discriminators by manual configuration of the PE/LCCEs.
3. Assignment of S-BFD discriminators by a controller.
4. Probing remote S-BFD discriminators through a mechanism such as S-BFD Alert discriminators [I-D.akiya-bfd-seamless-alert-discrim]

S-BFD Initiator operation MUST be according to the specifications in Section 7.2 of [I-D.ietf-bfd-seamless-base].

2.2.2. S-BFD Reflector Operation

• When a pseudowire signalling protocol such as LDP or L2TPv3 is in use, the S-BFD Reflector can advertise its target discriminators using that signalling protocol. When static PWs are in use the target discriminator of S-BFD needs to be provisioned on the S-BFD Initiator nodes.
• All point to point pseudowires are bidirectional, the S-BFD Reflector therefore reflects the S-BFD packet back to the Initiator using the VCCV channel of the reverse direction of the PW on which it was received.
• It is observed that the reflector has enough information to reflect the S-BFD Async packet received by it back to the S-BFD initiator using the PW context (e.g., fields of the L2TPv3 headers).
• S-BFD Reflector operation for BFD protocol fields MUST be according to the specifications of [I-D.ietf-bfd-seamless-base].

2.2.2.1. Demultiplexing

Demultiplexing of S-BFD is achieved using the PW context, following the procedures in Section 7.1 of [I-D.ietf-bfd-seamless-base].

2.2.2.2. Transmission of Control Packets

The procedures of S-BFD Reflector described in [I-D.ietf-bfd-seamless-base] apply for S-BFD using VCCV.

2.2.2.3. Advertisement of Target Discriminators using LDP

The advertisement of the target discriminator using LDP is left for further study. It should be noted that S-BFD can still be used with signaled PWs over an MPLS PSN, by provisioning of the S-BFD discriminators or by learning the S-BFD discriminators by other means.

2.2.2.4. Advertisement of Target Discriminators Using L2TP

The S-BFD Reflector MUST use the AVP [I-D.ietf-l2tpext-sbfd-discriminator] defined for advertising its target discriminators using L2TP.

2.2.2.5. Provisioning of Target Discriminators

S-BFD target discriminators MAY be provisioned when static PWs are used.

2.3. S-BFD Encapsulation

Unless specified differently below, the encapsulation of S-BFD packets is identical to the method specified in Section 3.2 [RFC5885] and in [RFC5880] for the encapsulation of BFD packets.

• IP/UDP BFD Encapsulation (BFD with IP/UDP Headers)
  • The destination UDP port for the IP encapsulated S-BFD packet MUST be 7784 [I-D.ietf-bfd-seamless-ip].
  • The encapsulation of the S-BFD header fields MUST be according to Section 7.3.2 of [I-D.ietf-bfd-seamless-base].

• PW-ACH/ L2SS BFD Encapsulation (BFD without IP/UDP Headers)
  • The encapsulation of S-BFD packets using this format MUST be according to Section 3.2 of [RFC5885] with the exception of the value for the PW-ACH/L2SS type.
  • When VCCV carries PW-ACH/ L2SS-encapsulated S-BFD (i.e., "raw" S-BFD), the PW-ACH (pseudowire CW's) or L2SS' Channel Type MUST be set to TBD3 to indicate "S-BFD Control, PW-ACH/ L2SS-encapsulated" (i.e., S-BFD without IP/UDP headers; see Section 5.3). This is to allow the identification of the encased S-BFD payload when demultiplexing the VCCV control channel.

2.4. S-BFD CV Types

3. Capability Selection

When multiple S-BFD CV Types are advertised, and after applying the rules in [RFC5885], the set that both ends of the pseudowire have in common is determined. If the two ends have more than one S-BFD CV Type in common, the following list of S-BFD CV Types is considered in the order of the lowest list number CV Type to the highest list number CV Type, and the CV Type with the lowest list number is used:

1. TBD1 - S-BFD IP/UDP-encapsulated, for PW Fault Detection only.
2. TBD2 - S-BFD PW-ACH/ L2SS-encapsulated (without IP/UDP headers), for PW Fault Detection only.

The order of capability selection between S-BFD and BFD is defined as follows:

      MPLS Connectivity Verification (CV) Types:

   Bit (Value)  Description                       Reference
   ===========  ===========                       ==============
   TBD1(0xY)    S-BFD IP/UDP-encapsulated,        This document
                for PW Fault Detection only
   TBD2(0xZ)    S-BFD PW-ACH-encapsulated,        This document
                for PW Fault Detection only

   VCCV Capability AVP (Attribute Type 96) Values
   ----------------------------------------------

   L2TPv3 Connectivity Verification (CV) Types:

   Bit (Value)  Description                  Reference
   ===========  ===========                  ==============
   TBD1(0xY)    S-BFD IP/UDP-encapsulated,   This document
                for PW Fault Detection only
   TBD2(0xZ)    S-BFD L2SS-encapsulated,     This document
                for PW Fault Detection only

Registry:
                                             TLV
 Value   Description                         Follows  Reference
 ------  ----------------------------------  -------  ---------------
 TBD3    S-BFD Control, PW-ACH/L2SS          No       [This document]
         encapsulation
         (without IP/UDP Headers)