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

Greg Mirsky <gregimirsky@gmail.com> Thu, 13 July 2017 02:19 UTC

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From: Greg Mirsky <gregimirsky@gmail.com>
Date: Wed, 12 Jul 2017 19:19:04 -0700
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To: Carlos Pignataro <cpignata@cisco.com>
Cc: rtg-dir@ietf.org, "mpls@ietf.org" <mpls@ietf.org>, draft-ietf-mpls-bfd-directed.all@ietf.org, bfd-chairs@ietf.org
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Subject: Re: [mpls] Rtgdir early review of draft-ietf-mpls-bfd-directed-07
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Dear Carlos,
thank you for sharing your detailed comments to the draft. Please find
response in-line and tagged GIM>>.

Regards,
Greg

On Tue, Jul 11, 2017 at 6:59 AM, Carlos Pignataro <cpignata@cisco.com>;
wrote:

> 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>> Proposed in the draft mechanism to direct the reverse direction of
the BFD session onto specific MPLS LSP effectively puts the reverse
direction in the same environment as the forward direction of the BFD
session. What happens to the forward direction of the BFD session over MPLS
LSP if the LSP goes down? That must be detected as LSP failure and clients
of the BFD session, I'd expect, will be notified (that is rather outside of
BFD RFCs but we understand why and how proactive OAM is used). If the LSP
gets unconfigured before the BFD session, then, I assume, that will be
detected as LSP failure too. I think that these are very fundamental
scenarios but they are not discussed in RFC 5884 either.

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. 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>> I disagree with your conclusion that  the draft does not include
mechanism to switch the reverse path of the BFD session in Up state.
Firstly, the BFD Reverse Path TLV MAY be included in LSP Ping at any moment
in time, not only when bootstrapping BFD session. Secondly, the draft
states that if BFD Reverse Path TLV contains none sub-TLVs, then the
reverse path MUST be switched to IP network, which is the default behavior
per RFC 5884.

>
> 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>> Frankly, I fail to find relevance of S-BFD to the proposed mechanism.
But if you insist on discussing S-BFD, then I see that its echo-like
mechanism that does not create state on the far end of S-BFD session rather
complicates potential mechanism to control the return path.

>
> 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>> Accept this editorial change.

>
>    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 mechanism is applicable to BFD over MPLS LSP. The quote
emphasizes one of use cases. Will prepend it , "without loss of generality
..." in the next version.

>
>    [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>> Thank you for pointing to the ambiguity. The reference to the latter
two. Will clarify in the next version.

>
>    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>> The proposed mechanism is optional and, of course, it has been
assumed that the operator will verify availability and liveliness of the
LSP to be used for the reverse direction of the given BFD session.

>
>    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>> You're absolutely correct in interpretation of RFC 5884, the reverse
direction of BFD session over MPLS LSP is always over IP. The proposed
mechanism provides option to change the default reverse path if and when
network operator sees that beneficial.

>
>    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>> Can you please clarify what is not needed according to RFC 5586? LSP
Ping? BFD over MPLS-TP LSP?

>
>    "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*.
>
GIM>> The proposed mechanism to control the reverse path of the BFD session
may improve determinism of defect detection in certain use cases.

>
>    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>> Please see my response to the very first question. This is the same
situation as for the ingress LER per RFC 5884.

>
> 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 to this. In the next version will change to
explicitly state:

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>> As per RFC 5884, the BFD control packets in reverse direction of the
BFD over MPLS LSP session are sent over IP.

>
> 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? And, again, what if the path is found at bootstrap
> but
> lost afterwards?
>
GIM>> Please see the response to the first question.

>
> 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.
>
> 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.
>
> 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.
>
>