[mpls] Fwd: Rtgdir early review of draft-ietf-mpls-bfd-directed-07

[Greg Mirsky <[e-mail]>]

Dear All,
as we've discussed at the MPLS WG meeting at IETF-108, I've been given an
action point to match updates to the draft-ietf-mpls-bfd-directed with the
RtgDir early review comments. Please find notes from the authors in-line
under GIM>> and Mach>> tags.
Welcome your questions, comments.

Regards,
Greg (on behalf of the authors)

---------- Forwarded message ---------
From: Carlos Pignataro <[e-mail]>
Date: Tue, Jul 11, 2017 at 7:00 AM
Subject: Rtgdir early review of draft-ietf-mpls-bfd-directed-07
To: <[e-mail]>
Cc: <[e-mail]>, <[e-mail]>, <
[e-mail]>


Reviewer: Carlos Pignataro
Review result: Has Issues

Hello

I have been selected to do a routing directorate “early” review of this
draft.
https://datatracker.ietf.org/doc/draft-ietf-mpls-bfd-directed/

The routing directorate will, on request from the working group chair,
perform
an “early” review of a draft before it is submitted for publication to the
IESG. The early review can be performed at any time during the draft’s
lifetime
as a working group document. The purpose of the early review depends on the
stage that the document has reached.

The MPLS chairs have requested an early review from the directorate with the
objective of improving document quality.  This document has had three
unsuccessful WG LCs.

For more information about the Routing Directorate, please see
http://trac.tools.ietf.org/area/rtg/trac/wiki/RtgDir

Document: draft-ietf-mpls-bfd-directed-07.txt
Reviewer: Carlos Pignataro
Review Date: Early July, 2017
Intended Status: Standards Track

Summary:
I have significant concerns about this document.
I also recommend a BFD WG Chair or appointee to review this document.

Comments:

First, I have a general concern about the architectural approach in this
document.

This document is modeled after RFC 7110. RFC 7110 describes the
specification
of a return path for MPLS LSP Ping. MPLS LSP Ping uses a request/reply
command/response paradigm, in which receipt of an Echo Request elicits the
generation of an Echo Reply.

BFD for MPLS, however, uses a different approach and paradigm (as per RFC
5884). An MPLS LSP Ping packet is used as a bootstrap, signaling
discriminator
value for a persistent BFD session. After the MPLS LSP Ping signals the
Discriminator (via MPLS LSP Ping TLV) to use, then BFD control messages are
sent back and forth.

However, while the BFD session is UP and  BFD control messages and being
sent
back and forth, and while no MPLS LSP Ping packets are sent after
bootstrapping
-- what happens if the return path changes (e.g., the return LSP goes down,
gets unconfigured, etc.)?
GIM>> Need to note that after bootstrapping a BFD session over MPLS LSP
with the MPLS LSP Ping that includes the BFD Discriminator TLV,
MPLS LSP Ping is periodically sent according to Section 3.2 RFC 5884:
   Hence, BFD is used in conjunction with LSP Ping for MPLS LSP fault
   detection:
...
    iii) LSP Ping is used to periodically verify the control plane
         against the data plane by ensuring that the LSP is mapped to
         the same FEC, at the egress, as the ingress.
[Mach] If the return LSP goes down, then the BFD session should go down as
well, this's one of the goals that this solution is trying to achieve. For
example, in some case, it want the forward and reverse paths share the same
route and fate.  The return path down normally means the forward path is
down as well.

In that case, not only this mechanism can actually make things worst,
because
it results in a false negative, but also the document does not specify how
the
system should recover.
[Mach] As above, this result is desired in the case that requires the
forward and reverse path share route and fate. It should be kept down until
the specified return path recovered.

The I-D seems to assume complete topological
invariability for it to work long-term, since it does not specify any
mechanism
to update or to deal with such a failure or change scenario.
GIM>> We've added the Operational Considerations section that describes how
the local BFD system using periodic MPLS LSP Ping can monitor the viability
of the Reverse path. Particularly, the text that describes this:
   If any of defined in
   [RFC7110] sub-TLVs used in BFD Reverse Path TLV, then the periodic
   verification of the control plane against the data plane, as
   recommended in Section 4 [RFC5884], MUST use the Return Path TLV, as
   per [RFC7110], with that sub-TLV.  By using the LSP Ping with Return
   Path TLV, an operator monitors whether at the egress BFD node the
   reverse LSP is mapped to the same FEC as the BFD session.  Selection
   and control of the rate of LSP Ping with Return Path TLV follows the
   recommendation of [RFC5884]: "The rate of generation of these LSP
   Ping Echo request messages SHOULD be significantly less than the rate
   of generation of the BFD Control packets.  An implementation MAY
   provide configuration options to control the rate of generation of
   the periodic LSP Ping Echo request messages."

On the other hand, there is already a BFD mechanism without the
bootstrapping
setup and with a command/response like behavior, that is S-BFD, RFC 7881.
That
one is notably missing from this draft.
GIM>> The goal of the proposed mechanism is not to avoid bootstrapping a
BFD over MPLS LSP session or simulate Echo Response/Reply behavior but to
control the path the remote BFD system uses to transmit BFD control packets
in BFD Asynchronous mode.
[Mach] The purpose of this draft is to specify a particular return path,
S-BFD cannot specify the return path, thus cannot satisfy the requirement.

Further, there seem to be a number of potentially erroneous assumptions
made,
see below.

Additional Comments:

     Bidirectional Forwarding Detection (BFD) Directed Return Path

The title should include that this is *only* for MPLS BFD.
GIM>> Thank you for the helpful editorial suggestion.

   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.

Scope: is this solution targeted only for "explicitly routed unidirectional
path", and the solution to have the reply come back the exact reverse
direction? That does not seem to be the case and the solution.
GIM>> The path from the remote BFD system may traverse the same nodes and
links traversed by the BFD control packets transmitted from the ingress
LER, i.e., be co-routed. But the proposed mechanism is more generic and
allows an operator to control the path traversed by a BFD control packet
transmitted by the egress LER.

   [RFC5880], [RFC5881], and [RFC5883] established the BFD protocol for
   IP networks.  [RFC5884] and [RFC7726] set rules of using BFD
   asynchronous mode over IP/MPLS LSPs.  These standards implicitly
   assume that the egress BFD peer will use the shortest path route
   regardless of route being used to send BFD control packets towards
   it.

Is "These standards" referring to the three former or the four latter?
GIM>> Two latter two. We've clarified that by the following update:
OLD TEXT:
   [RFC5884] and [RFC7726] set rules of using BFD
   asynchronous mode over IP/MPLS LSPs.  These standards implicitly
   assume that the egress BFD peer will use the shortest path route
   regardless of route being used to send BFD control packets towards
   it.
NEW TEXT:
[RFC5884] and [RFC7726] set rules for using BFD
   asynchronous mode over IP/MPLS LSPs, while not defining means to
   control the path an egress BFD system uses to send BFD control
   packets towards the ingress BFD system.

   For the case where a LSP is explicitly routed it is likely that the
   shortest return path to the ingress BFD peer would not follow the
   same path as 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 is a significant factor in
   producing false positive defect notifications, i.e. false alarms, if
   used by the ingress BFD peer to deduce the state of the forward
   direction.

There may be an implicit mis-assumption in this text and overall approach:
the
fact that traffic flows on one direction does not imply that the reverse
direction using the same interfaces and nodes would actually be consequently
properly programmed and working.
GIM>> If the objective of controlling the path from the egress to the
ingress system is to monitor such path, then detecting a defect in the
forwarding path fulfills the objective.

   This document defines the BFD Reverse Path TLV as an extension to LSP
   Ping [RFC8029] 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.

This text assumes that the BFD return path is MPLS. However, my
understanding
from RFC 5884 is that this is not necessarily the case, and the return can
be
IP.
GIM>> The document defines an optional mechanism to control the BFD Reverse
path. The default behavior, if the BFD Reverse Path TLV is not included in
the MPLS LSP Ping with the BFD Discriminator TLV, remains, as you've noted
- over IP.

   When BFD is used to monitor unidirectional explicitly routed path,
   e.g.  MPLS-TE LSP, BFD control packets in forward direction would be
   in-band using the mechanism defined in [RFC5884] and [RFC5586].

Which BFD uses RFC 5586? RFC5586 says that is not needed:
GIM>> We've removed references to RFC 5586.

   "Some of these functions can be supported using existing
   tools such as Virtual Circuit Connectivity Verification (VCCV)
   [RFC5085], Bidirectional Forwarding Detection for MPLS LSPs (BFD-
   MPLS) [BFD-MPLS], LSP-Ping [RFC4379], or BFD-VCCV [BFD-VCCV]."

And then:

   o  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 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.

But using a specific path for return cannot either imply "interpreted as
bi-directional failure unambiguously", so the scenario is not *addressed*.

   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 Figure 1.

What does the remote endpoint do with that "MUST" if the return FEC goes
away?
GIM>> The Operational Considerations section includes the following text:
   Suppose an operator planned network maintenance activity that
   possibly affects FEC used in the BFD Reverse Path TLV.  In that case,
   the operator MUST avoid the unnecessary disruption using the LSP Ping
   with a new FEC in the BFD Reverse Path TLV.  But in some scenarios,
   proactive measures cannot be taken.  Because the frequency of LSP
   Ping messages will be lower than the defect detection time provided
   by the BFD session.  As a result, a change in the reverse-path FEC
   will first be detected as the BFD session's failure.  In such a case,
   the ingress BFD node SHOULD immediately transmit the LSP Ping Echo
   request with Return Path TLV to verify whether the FEC is still
   valid.  If the failure was caused by the change in the FEC used for
   the reverse direction of the BFD session, the ingress BFD node SHOULD
   bootstrap a new BFD session using another FEC in BFD Reverse Path
   TLV.

There also seem to be some self-contradiction. This document says:

   LSP ping, defined in [RFC8029], uses BFD Discriminator TLV [RFC5884]
   to bootstrap a BFD session over an MPLS LSP.  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 value in BFD Discriminator TLV.

And then says:

   Reverse Path field contains a sub-TLV.

But then says:

   None, one or more sub-TLVs MAY be included in the BFD Reverse
   Path TLV.  If none sub-TLVs found in the BFD Reverse Path TLV, the
   egress BFD peer MUST revert to using the default, i.e., over IP
   network, reverse path.

So is it only one, or none/one/multiple?
GIM>> Thank you for pointing this. Below is the update in the working
version of the draft:
OLD TEXT:
   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.
NEW TEXT:
   This
   document defines a new TLV, BFD Reverse Path TLV, that MAY contain
   none, one or more sub-TLVs 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.

and one more:
OLD TEXT:
Reverse Path field contains a sub-TLV
NEW TEXT:
Reverse Path field contains none, one or more sub-TLVs.

I believe it needs to be multiple since then a Tunnel can be specified. But
the
document as-is seems self-contradicting.

Further, where has that "default" been defined as "over IP network"?
GIM>> The text was re-worked to avoid using "default":
   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 [RFC5884], i.e.,
   routed over IP network.

There's another contradiction here:

   If the egress LSR cannot find the path specified in the Reverse Path
   TLV it MUST send Echo Reply with the received Reverse Path TLV and
   set the Return Code to "Failed to establish the BFD session.  The
   specified reverse path was not found" Section 3.3.  The egress BFD
   peer MAY establish the BFD session over IP network as defined in
   [RFC5884].

So the response is "Failed to establish the BFD session." But then it MAY
establish the session?
GIM>> Echo Reply with the error code is to indicate that the reverse path
is not available at the egress LSR. The local policy at the egress BFD
system may command the transmission of a BFD control packet from egress to
the ingress if the Reverse Path is not available.
And, again, what if the path is found at bootstrap but
lost afterwards?
GIM>> Section 5 includes text that provides information on handling such a
scenario.

4.  Use Case Scenario

The fact that A-B-C-D-G-H works does not mean that the reverse, H-G-D-C-B-A,
will work.
GIM>> If the objective is to verify that the reverse path works, then
detecting a defect fulfills the task.
6.  Security Considerations

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

There seem to be additional security considerations with returns taking
explicit paths, and should be expanded in here.
GIM>> That scenario is discussed in the Security Considerations section in
RFC 7110. A reference to RFC 7110 was added:

   Security considerations discussed in [RFC5880], [RFC5884], [RFC7726],
   [RFC8029], and [RFC7110] apply to this document.

Net-net, I do have concerns about this document. I believe it is not ready
to
advance, and could use more whiteboard time as well as a review by BFD
experts.

Best,

Carlos Pignataro.