The ability for an ingress LSR to establish multiple BFD sessions for
an <MPLS LSP, FEC> tuple is useful in scenarios such as LSPs based on
Segment Routing [SEG-ROUTING] or LSPs having Equal-Cost Multipath
(ECMP). The process used by the ingress LSR to determine the number
of BFD session(s) to be bootstrapped for an <MPLS LSP, FEC> tuple and
the mechanism used to construct those session(s) are outside the
scope of this document.
1.1. 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
2. Theory of Operation
2.1. Procedures for Establishment of Multiple BFD Sessions
Section 4 of [RFC5884] specifies the procedure for bootstrapping BFD
sessions using LSP ping. It further states that "a BFD session
SHOULD be established for each alternate path that is discovered."
This requirement has been the source of some ambiguity as the
procedures of establishing concurrent, multiple sessions have not
been explicitly specified. This ambiguity can also be attributed in
part to the text in Section 7 of [RFC5884] forbidding either end to
change local discriminator values in BFD control packets after the
session reaches the UP state. The following procedures are described
to clarify the ambiguity based on the interpretation of the authors'
reading of the referenced sections:
At the ingress LSR:
MPLS LSP ping can be used to bootstrap multiple BFD sessions for a
given <MPLS LSP, FEC>. Each LSP ping MUST carry a different
discriminator value in the BFD discriminator TLV [RFC5884].
The egress LSR needs to perform the following:
If the validation of the Forwarding Equivalence Class (FEC) in the
MPLS Echo request message succeeds, check the discriminator
specified in the BFD discriminator TLV of the MPLS Echo request.
If there is no local session that corresponds to the (remote)
discriminator received in the MPLS Echo request, a new session is
bootstrapped and a local discriminator is allocated. The
validation of a FEC is a necessary condition to be satisfied to
create a new BFD session at the egress LSR. However, the policy
or procedure, if any, to be applied by the egress LSR before
allowing a new BFD session to be created is outside the scope of
this document. Such policies or procedures could consider
availability of system resources before allowing a session to be
created. When the egress LSR disallows the creation of a BFD
session due to policy, it MUST drop the MPLS Echo request message.
Ensure the uniqueness of the <MPLS LSP, FEC, Remote Discriminator>
Except for the clarification mentioned above, the remaining
procedures of BFD session establishment are as specified in
Sections 4-6 of [RFC5884].
2.2. Procedures for Maintenance of Multiple BFD Sessions
Both the ingress LSR and egress LSR use the Your Discriminator of the
received BFD packet to demultiplex BFD sessions.
2.3. Procedures for Removing BFD Sessions at the Egress LSR
[RFC5884] does not specify an explicit procedure for deleting BFD
sessions. The procedure for removing a BFD session established by an
out-of-band discriminator exchange using the MPLS LSP ping can
improve resource management (e.g., memory), especially in scenarios
involving thousands or more of such sessions. A few observations are
The BFD session MAY be removed in the egress LSR if the BFD
session transitions from UP to DOWN. This can either be done
immediately after the BFD session transitions from UP to DOWN or
after the expiry of a configurable timer started after the BFD
session state transitions from UP to DOWN at the egress LSR to
reduce flapping by adding hysteresis.
The BFD session on the egress LSR MAY be removed by the ingress
LSR by using the BFD diagnostic code AdminDown(7) as specified in
[RFC5880]. When the ingress LSR wants to remove a session without
triggering any state change at the egress, it MAY transmit BFD
packets indicating the State as Down(1), diagnostic code
AdminDown(7) detectMultiplier number of times. Upon receiving
such a packet, the egress LSR MAY remove the BFD session, without
triggering a change of state.
The procedures to be followed at the egress LSR when BFD
session(s) remain in the DOWN state for a significant amount of
time is a local matter. Such procedures are outside the scope of
All BFD sessions established with the FEC MUST be removed
automatically if the FEC is removed.
The egress MUST use the discriminators exchanged when the session
was brought UP to indicate any session state change to the
ingress. The egress SHOULD reset this to zero after transmitting
bfd.detectMult number of packets if the BFD session transitions to
2.4. Changing Discriminators for a BFD Session
The discriminators of a BFD session established over an MPLS LSP
cannot be changed when it is in UP state. The BFD session could be
removed after a graceful transition to AdminDown state using the BFD
diagnostic code AdminDown. A new session could be established with a
different discriminator. The initiation of the transition from the
UP to DOWN state can be done by either the ingress LSR or the egress
3. Backwards Compatibility
The procedures clarified by this document are fully backward
compatible with an existing implementation of [RFC5884]. While the
capability to bootstrap and maintain multiple BFD sessions may not be
present in current implementations, the procedures outlined by this
document can be implemented as a software upgrade without affecting
existing sessions. In particular, the egress LSR needs to support
multiple BFD sessions per <MPLS LSP, FEC> before the ingress LSR is
4. Security Considerations
This document clarifies the mechanism to bootstrap multiple BFD
sessions per <MPLS LSP, FEC>. BFD sessions, naturally, use system
and network resources. More BFD sessions means more resources will
be used. It is highly important to ensure that only a minimum number
of BFD sessions are provisioned per FEC and that bootstrapped BFD
sessions are properly deleted when they are no longer required.
Additionally, security measures described in [RFC4379] and [RFC5884]
are to be followed.