Bidirectional Forwarding Detection (BFD) Directed Return PathZTEgregimirsky@gmail.comNuage Networksjefftant.ietf@gmail.comGoogleIlya@nobulus.comHuaweimach.chen@huawei.com
Routing
MPLS Working GroupInternet-DraftLSP PingBFD
Bidirectional Forwarding Detection (BFD) is expected to be able to
monitor a wide variety of encapsulations of paths between systems.
When a BFD session monitors an explicitly routed unidirectional path there may be a need to direct
egress BFD peer to use a specific path for the reverse direction of the BFD session.
, , and established the BFD
protocol for IP networks. and
set rules for using BFD asynchronous mode over IP/MPLS LSPs.
These standards do not define means to control the path selection at the egress BFD peer to send BFD
control packets towards the ingress BFD system.
For the case when BFD is used to detect defects of the traffic engineered LSP the path
the BFD control packets transmitted by the egress BFD system toward the ingress may be disjoint
from the LSP in the forward direction.
The fact that BFD
control packets are not guaranteed to follow the same links and nodes
in both forward and reverse directions contributes to
producing false positive defect notifications, i.e., false alarms, at
the ingress BFD peer.
This document defines the BFD Reverse Path TLV as an extension to LSP Ping
and proposes
that it is to be used to instruct the egress BFD peer to use an explicit
path for its BFD control packets associated with a particular BFD session.
The TLV will be allocated from the
TLV and sub-TLV registry defined in .
As a special case, forward and reverse
directions of the BFD session can form a bi-directional co-routed associated channel.
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.
When BFD is used to monitor explicitly routed unidirectional path, e.g., MPLS-TE LSP, BFD control
packets in forward direction would be in-band using the mechanism defined in and .
But the reverse direction of the BFD session would follow the shortest path route and that might lead to the
problem in detecting failures on an explicit unidirectional path as described below:
a failure detection by ingress node on the reverse path cannot be
interpreted as bi-directional failure unambiguously and thus trigger, for
example, protection switchover of the forward direction without the possibility of
being a false positive.
To address this scenario, the egress BFD peer would be instructed to use a specific path for BFD control packets.
To bootstrap a BFD session over an MPLS LSP, LSP ping, defined in , MUST be used with
BFD Discriminator TLV .
This document defines a new TLV, BFD Reverse Path TLV, that MUST contain a single sub-TLV
that can be used to carry information about the reverse path for
the BFD session that is specified by the value in BFD Discriminator TLV.
The BFD Reverse Path TLV is an optional TLV within the LSP ping .
However, if used, the BFD Discriminator TLV MUST be included in an Echo Request message
as well. If the BFD Discriminator TLV is not present when the BFD Reverse
Path TLV is included; then it MUST be treated as malformed Echo Request, as described in .
The BFD Reverse Path TLV carries information about the path onto which the egress BFD peer of the BFD session referenced by the BFD
Discriminator TLV MUST transmit BFD control packets. The format of the BFD Reverse Path TLV is as presented in .
BFD Reverse Path TLV Type is two octets in length and has a value of TBD1 (to be assigned by IANA
as requested in ).
Length field is two octets long and defines the length in octets of the Reverse Path field.
Reverse Path field contains a sub-TLV. Any non-multicast Target FEC Stack
sub-TLV (already defined, or to be defined in the future)
for TLV Types 1, 16, and 21 of MPLS LSP Ping Parameters registry MAY be
used in this field. Multicast Target FEC Stack sub-TLVs, i.e., p2mp and mp2mp,
SHOULD NOT be included in Reverse Path field. If the egress LSR finds multicast
Target Stack sub-TLV, it MUST send echo reply with the received Reverse Path TLV,
BFD Discriminator TLV and set the Return Code to "Inappropriate Target FEC Stack
sub-TLV present" .
None, one or more sub-TLVs MAY be included in the BFD Reverse Path TLV.
If no sub-TLVs are found in the BFD Reverse Path TLV, the egress BFD peer MUST revert to
using the local policy based decision as described in Section 7 , i.e., routed over IP network.
If the egress LSR cannot find the path specified in the Reverse Path TLV it MUST send Echo
Reply with the received BFD Discriminator TLV, Reverse Path TLV
and set the Return Code to "Failed to establish the
BFD session. The specified reverse path was not found" .
The egress BFD peer MAY establish the BFD session over IP network as defined in .
When an explicit path on an MPLS data plane is set either as Static or RSVP-TE LSP,
corresponding sub-TLVs, defined in , MAY be used
to identify the explicit reverse path for the BFD session. If any of defined in
sub-TLVs used in BFD Reverse Path TLV, then the periodic verification of the control plane
against the data plane, as recommended in Section 3.2 , MUST use
the Return Path TLV, as per , with that sub-TLV.
By using the LSP Ping with Return Path TLV an operator will be able to verify
that the forward LSP and the reverse LSP are mapped to the same FECs as BFD session
both at the ingress and the egress systems.
This document defines the following Return Codes for MPLS LSP Echo Reply:
"Inappropriate Target FEC Stack sub-TLV present", (TBD3). When multicast Target FEC Stack sub-TLV found in
the received Echo Request by the egress BFD peer, an Echo Reply with the return code set to
"Inappropriate Target FEC Stack sub-TLV present" MUST be sent to the ingress BFD peer .
"Failed to establish the BFD session. The specified reverse path was not found", (TBD4).
When a specified reverse path is not available at the egress BFD peer, an Echo Reply with the return
code set to "Failed to establish the BFD session. The specified reverse path was not found"
MUST be sent back to the ingress BFD peer .
In the network presented in node A monitors two
tunnels to node H: A-B-C-D-G-H and A-B-E-F-G-H.
To bootstrap a BFD session to monitor the first tunnel, node A MUST include
a BFD Discriminator TLV with Discriminator value (e.g., foobar-1) and MAY include
a BFD Reverse Path TLV that references H-G-D-C-B-A tunnel. To bootstrap
a BFD session to monitor the second tunnel, node A MUST include
a BFD Discriminator TLV with a different Discriminator value (e.g., foobar-2)
and MAY include
a BFD Reverse Path TLV that references H-G-F-E-B-A tunnel.
If an operator needs node H to monitor a path to node A, e.g.
H-G-D-C-B-A tunnel, then by looking up the list of known Reverse Paths
it MAY find and use the existing BFD session.
The IANA is requested to assign a new value for BFD Reverse Path TLV from the "Multiprotocol Label
Switching Architecture (MPLS) Label Switched Paths (LSPs) Ping Parameters - TLVs" registry, "TLVs and
sub-TLVs" sub-registry.
ValueDescriptionReference (TBD1)BFD Reverse Path TLVThis document
The IANA is requested to 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.
ValueDescriptionReference (TBD3)Inappropriate Target FEC Stack sub-TLV present.This document (TBD4)Failed to establish the BFD session. The specified reverse path was not found.This document
Security considerations discussed in , , ,
and , apply to this document.
Authors greatly appreciate thorough review and the most helpful comments from Eric Gray
and Carlos Pignataro.