Subject: [Tsv-art] Tsvart last call review of draft-ietf-bfd-multipoint-16

From: Bob Briscoe <[email-masked]>
Date: Mon, 21 May 2018 17:20:00 -0700

Reviewer: Bob Briscoe
Review result: Not Ready

Altho this is a TSV-ART review, I did not find many transport-related issues to
focus on, except a need to justify why lack of information for adapting the
transmit interval is not an issue.

Nonetheless, I did find a few other non-trivial technical issues, including 2
security issues, enumerated below (I mis-spent some of my early research career
working on a multicast session control and security, for which we used
beaconing control channels). However, I only have passing prior knowledge of
BFD, so my critique might be off-beam.

==Main Technical Concerns===

1/ Mandatory return path?
RFC5880 is the base RFC that this draft updates. RFC5880 says that
"unidirectional links" are in scope, but only as long as there is a return path.

The introduction of this bfd-multipoint draft seems to contradict that, making
a return path optional: "
   As an option, the tail may notify the head of the lack of multipoint
   connectivity.  Details of tail notification to the head are outside
   the scope of this document.
"
It's allowable for irrelevant details to be outside the scope, but surely it
needs to be clear whether at least the existence of a return path is mandatory.

2/ Mechanism for verifying connectivity, or not?
The introduction seems to contradict itself:
"
   As multipoint transmissions are inherently unidirectional, this
   mechanism purports only to verify this unidirectional connectivity.
"
"
   Term "connectivity" in this document is not being used in the context
   of connectivity verification in transport network but as an
   alternative to "continuity", i.e. existence of a forwarding path
   between the sender and the receiver.
"
How can this mechanism verify connectivity, but not be used in the context of
connectivity verification in the transport network?

3/ Use case
The introduction seems to be written rather academically. Surely, in cases
where there is never a return path, only the tails will ever be able to verify
connectivity. The head could continue transmitting BFD packets (and data
packets) for years without ever knowing whether it is connected to anything.
Knowledge of connectivity is surely of little use if it excludes the link
sender, which is the node that always controls routing.

If there are scenarios where it is useful for tails but not the head to be able
to verify connectivity, can you please give a concrete example?

4/ Interval adaptation
Text is needed to describe why it is not an issue for the head to be unaware
whether it needs to adapt its transmit interval. Otherwise, this seems
potentially problematic.

5/ Inability to authenticate the sender with symmetric keys
In unicast scenarios, symmetric keys can be used for message authentication,
because each end knows there is only one other node with the shared key. But,
in multipoint scenarios, all the tails would share the key, so a shared key
does not authenticate who sent the message - any tail can spoof the head from
the viewpoint of the other tails.

Therefore text is needed to say that:
* multipoint message authentication is limited to cases where all tails are
trusted not to spoof the head, if shared keys are used. * otherwise asymmetric
message authentication would be needed, e.g. TESLA [RFC4082]

A related nit: Section 5 says all tails are assumed to have a common
authentication key. In cases with symmetric message authentication, surely the
head also needs the same key.

6/ Source address spoofing
A 3-way handshake makes a protocol robust against simple source address
spoofing. Without a 3WHS, surely the spec. needs to highlight this
vulnerability or discuss ways to address it or why it is not an issue.

7/ Scope
On eight occasions an issue is raised, but resolution is stated as outside the
scope of this document. It is OK to limit the scope of a spec, for example to
allow for multiple solutions to each issue. But at least one solution must
already exist for each of these eight issues. So, at least one example solution
ought to be cited in each case. If any issues are open, then this should not be
on the standards track.

It would be more useful to state why each issue is out of scope. This would be
helped by stating from the start what the scope of the document is.

There is also one issue that is "for further discussion". Does this mean the
document is not ready yet?

8/ Incremental deployment

Section 4.4.1.  "New State Variable Values" defines bfd.SessionType =
PointToPoint as well as a couple of Multipoint alternatives. Presumably this
spec does not require all existing PointToPoint systems to support this state
value. Is the implication that only Multipoint systems that happen to be in
PointToPoint mode should use this state?

==Nits==

* Sometimes 'tree' is used to mean a multipoint path in general. I suspect
'path' was intended.

4.8.  Packet consumption on tails
s/Node/Nodes/
s/packet/packets/
s/demultiplex/demultiplexed/

4.9.  Bringing Up and Shutting Down Multipoint BFD Service
"
   a newly
   started head (that does not have any previous state information
   available) SHOULD start with...
"
...
"
   ... (so long as the restarted head
   is using the same or a larger value of bfd.DesiredMinTxInterval than
   it did previously).
"
If it has no state available, how can it know whether a value is larger than
previously?

4.9.  Bringing Up and Shutting Down Multipoint BFD Service
There are a number of "SHOULD"s and "SHOULD NOT"s that do not state or give
examples of circumstances in which the "SHOULD" would not be appropriate. If
there are none, "MUST" would be more appropriate.

4.10.  Timer Manipulation
"
   Because of the one-to-many mapping, a session of type MultipointHead
   SHOULD NOT initiate a Poll Sequence in conjunction with timer value
   changes.  However, to indicate a change in the packets,
   MultipointHead session MUST send packets with the P bit set.
   MultipointTail session MUST NOT reply if the packet has M and P bits
   set and bfd.RequiredMinRxInterval set to 0.
"
The initial "SHOULD NOT" needs to be written another way. Perhaps
"
   ...a session of type MultipointHead
   does not initiate a Poll Sequence
"
The head's normative action is defined by the following "MUST", then the tail's
"MUST NOT reply" is what prevents the poll sequence happening.

4.11.  Detection Times
Delete "in the calculation" (repetition).

4.13.1.  Reception of BFD Control Packets
Some actions seem to be only relevant to PointToPoint sessions, but they are
stated for all session types. Specifically: "the transmission of Echo packets,
if any, MUST cease." "the Poll Sequence MUST be terminated." "MUST cease the
periodic transmission of BFD Control packets" "MUST send periodic BFD Control
packets"

"
If bfd.SessionType is PointToPoint, update the Detection Time as
      described in section 6.8.4 of [RFC5880].  If bfd.SessionType is
      MultipointTail,
"
The second sentence above ought to start on a new line as an Else if.

4.13.2.  Demultiplexing BFD Control Packets
"
   This section is part of the replacement for [RFC5880] section 6.8.6,
   separated for clarity.
"
Do you mean "This section replaces the sentence: "If the Multipoint (M) bit is
nonzero, the packet MUST be discarded." in [RFC5880] section 6.8.6?

The statements under "If the Multipoint (M) bit is set" are not formatted like
the rest of the if-else logic, and I think an Else is missing at the start of
the statement after the nested "If".

---

From: Greg Mirsky <[email-masked]>
Date: Sat, 26 May 2018 12:49:29 -0700

Hi Bob,
thank you for the thorough review, detailed questions and helpful comments.
Please find my answers in-line and tagged GIM>>.
I've updated the working version of the draft based on your comments and
suggestions. Appreciate your feedback whether all questions have been
addressed.
Attached please find the diff of -16 and the working version and the copy
of the working version of the draft.

Regards,
Greg

> needs to be clear whether at least the existence of a return path is
GIM>> Thank you for highlighting this issue. I think that the second
paragraph of Introduction is the appropriate place to note that this
mechanism does not require existence of a return path from tails to the
head. Would the following be acceptable:
NEW TEXT:
   Use of BFD in
   Demand mode enables a tail monitor availability of a multipoint path
   even without the existence of some kind of a return path to the head.

> connectivity verification in the transport network?
GIM>> This draft defines the base specification for multipoint BFD. In
order for multipoint BFD to support the transport-like connectivity
verification we need to do work similar to described in RFC 6428.

> to verify connectivity, can you please give a concrete example?
GIM>> One example could be a multicast system with 1+1 protection. Without
multipoint BFD tails would not be able to detect the failure of the
muticast path from the head. Other examples discussed in several drafts:

   - BESS WG draft MVPN fast upstream failover
   <https://tools.ietf.org/html/draft-ietf-bess-mvpn-fast-failover-03>
   - Individual draft BFD for Multipoint Networks and VRRP Use Case
   <https://tools.ietf.org/html/draft-mirsky-bfd-p2mp-vrrp-use-case-01>
   - Individual draft BFD for Multipoint Networks and PIM-SM Use Case
   <https://tools.ietf.org/html/draft-mirsky-pim-bfd-p2mp-use-case-00>

I am not sure how references to non-WG drafts affect the progress of this
document. Appreciate your suggestion.

> whether it needs to adapt its transmit interval. Otherwise, this seems
GIM>> Very interesting, thank you. I wouldn't say that the case when a tail
cannot process incoming mpBFD control packets at the offered rate is
entirely non-issue. Such scenario must be handled by the implementation and
may be controlled by local policy, e.g., close the MultipointTail session.
Where would you suggest to add the text?

> message authentication would be needed, e.g. TESLA [RFC4082]
GIM>> Thank you for the suggested text. Would the Security Considerations
section be appropriate place:
NEW TEXT:
   Use of shared keys to authenticate BFD Control packet in multipoint
   scenarios is limited because tail can spoof the head from the
   viewpoint of the other tails.  Thus, if shared keys are used, all
   tails MUST be trusted not to spoof the head.  Otherwise, asymmetric
   message authentication would be needed, e.g., Timed Efficient Stream
   Loss-Tolerant Authentication (TESLA) as described in [RFC4082].

> head also needs the same key.
GIM>> Thank you. Please check the updated text:
NEW TEXT:
   If authentication is in use, the head and all tails must be
   configured to have a common authentication key in order for the tails
   to validate received the multipoint BFD Control packets.

> vulnerability or discuss ways to address it or why it is not an issue.
GIM>> Because mpBFD control packets cannot be demultiplexed by  tail based
on the value of Your Discriminator field as per RFC 5880,
the new procedure outlined in Section 4.7:
   IP and MPLS multipoint tails MUST demultiplex BFD packets based on a
   combination of the source address, My Discriminator and the identity
   of the multipoint tree which the Multipoint BFD Control packet was
   received from.  Together they uniquely identify the head of the
   multipoint path.
and described in details in Section 4.13.2:
      If the Multipoint (M) bit is set

         If the Your Discriminator field is nonzero, the packet MUST be
         discarded.

         Select a session as based on source address, My Discriminator
         and the identity of the multipoint tree which the Multipoint
         BFD Control packet was received.  If a session is found, and
         bfd.SessionType is not MultipointTail, the packet MUST be
         discarded.  If a session is not found, a new session of type
         MultipointTail MAY be created, or the packet MAY be discarded.
         This choice is outside the scope of this specification.

Would you suggest additional text to a use case where the new
demultiplexing is not sufficent to protect from source address spoofing?

> helped by stating from the start what the scope of the document is.
GIM>> I've listed all eight occasions with the explanation for each one:

   1. Details of tail notification to the head are outside the scope of
   this document. - Notifications by tails addressed in
   draft-ietf-bfd-multipoint-active-tail
   <https://datatracker.ietf.org/doc/draft-ietf-bfd-multipoint-active-tail/>.
   Will add as informational reference.
   2. Details of how the head keeps track of tails and how tails alert
   their connectivity to the head are outside scope of this document. - Same
   as #1.
   3. Bootstrapping BFD session to multipoint MPLS LSP in case of
   penultimate hop popping is outside the scope of this document. - It may use
   control plane as in MVPN draft. Will add as informational reference.
   4. Use of other types of encapsulation of the BFD control message over
   multipoint LSP is outside the scope of this document. - This in reference
   to ACH encapsulation that is discussed in draft-mirsky-mpls-p2mp-bfd
   <https://tools.ietf.org/html/draft-mirsky-mpls-p2mp-bfd-03>. Should it
   be added as informational reference? What would be the imacpt of
   progressing this specification?
   5. Change in the value of bfd.RequiredMinRxInterval is outside the scope
   of this document. - Same as #1.
   6. If a session is not found, a new session of type MultipointTail MAY
   be created, or the packet MAY be discarded. This choice is outside the
   scope of this specification. - I propose to add "based on local policy" to
   the last sentence.
   7. The exact method of selection is application-specific and is thus
   outside the scope of this specification. - This is copied from RFC 5880:
   "The exact method of selection is application specific and is thus outside
   the scope of this specification." as the section is to replace Section
   6.8.6.
   8. If a matching session is not found, a new session of type
   PointToPoint MAY be created, or the packet MAY be discarded. This choice is
   outside the scope of this specification. - Same as #6.

> document is not ready yet?
GIM>> I think that the question left for further discussion is
non-technical:
   The semantic difference between Down and AdminDown state is for
   further discussion.
I propose to remove the sentence altogether.

> PointToPoint mode should use this state?
GIM>> You're aboultely right, existing implementations of BFD don't need to
support bfd.SessionType variable. Only implementations that support the
base BFD, single-hop or multi-hop, and this specification, mpBFD, should
support bfd.SessionType and set it to PointToPoint value when BFD is in
single-hop or multi-hop mode. When in mpBFD mode, bfd.SessionType will be
set to either MultipointHead or MultipointClient.

> * Sometimes 'tree' is used to mean a multipoint path in general. I suspect
GIM>> I've found six occasions of "tree" and s/tree/path/

> s/demultiplex/demultiplexed/
GIM>> Accepted all three.

> If it has no state available, how can it know whether a value is larger
GIM>> You are right, the BFD system at the head would not know the previous
value of bfd.DesiredMinTxInterval. This text is to caution operator from
decreasing  bfd.DesiredMinTxInterval upon restart of the BFD system.

> there are none, "MUST" would be more appropriate.
GIM>> In the first paragraph SHOULD may not be followed if the
implementation can differentiate between the very first start and restarts
of BFD system. If it is the first start of BFD system, the head MAY
directly progress to Up state skipping Down state.
The last paragraph describes graceful shuttdown. The head MAY shut the BFD
mp session abruptly by just stopping transmission of BFD Control packets.

> "MUST NOT reply" is what prevents the poll sequence happening.
GIM>> A Poll Sequence starts with the initiator setting Poll bit. Unless
the peer sends BFD Control packet with Finl bit set the poll sequence would
continue indefinetely. The initial SHOULD NOT, in my opinion, correctly
points that the mechanism of Poll Sequence not to be used by MultipointHead
when changing transmission interval. I think that MUST in the first
paragraph can be downgraded to MAY because the MultipointHead doesn't need
to use transition period when changing the transmission interval to lower
level, i.e., decreasing frequency. May I propose the following:
OLD TEXT:
   Because of the one-to-many mapping, a session of type MultipointHead
   SHOULD NOT initiate a Poll Sequence in conjunction with timer value
   changes.  However, to indicate a change in the packets,
   MultipointHead session MUST send packets with the P bit set.
NEW TEXT:
   Because of the one-to-many mapping, a session of type MultipointHead
   SHOULD NOT initiate a Poll Sequence in conjunction with timer value
   changes.  However, to indicate a change in the packets,
   MultipointHead session MAY send packets with the P bit set during
transition period.

> Delete "in the calculation" (repetition).
GIM>> Done.

> The second sentence above ought to start on a new line as an Else if.
GIM>> Hope I got it right:
      If bfd.SessionType is PointToPoint, update the Detection Time as
      described in section 6.8.4 of [RFC5880].

      Else

         If bfd.SessionType is MultipointTail, then update the Detection
         Time as the product of the last received values of Desired Min
         TX Interval and Detect Mult, as described in Section 5.11 of
         this specification.

> the statement after the nested "If".
GIM>> Agree, the paragraph is not structured properly. How about this
formating:
      If the Multipoint (M) bit is set

         If the Your Discriminator field is nonzero, the packet MUST be
         discarded.

         Select a session as based on source address, My Discriminator
         and the identity of the multipoint path which the Multipoint
         BFD Control packet was received.

         If a session is found, and bfd.SessionType is not
         MultipointTail, the packet MUST be discarded.

         Else

            If a session is not found, a new session of type
            MultipointTail MAY be created, or the packet MAY be
            discarded.  This choice is outside the scope of this
            specification.

---

From: Bob Briscoe <[email-masked]>
Date: Tue, 29 May 2018 11:22:52 +0100

Greg,

On 26/05/18 20:49, Greg Mirsky wrote:
> Hi Bob,
> thank you for the thorough review, detailed questions and helpful 
> comments. Please find my answers in-line and tagged GIM>>.
> I've updated the working version of the draft based on your comments 
> and suggestions. Appreciate your feedback whether all questions have 
> been addressed.
> Attached please find the diff of -16 and the working version and the 
> copy of the working version of the draft.
>
> Regards,
> Greg
>
> On Mon, May 21, 2018 at 5:20 PM, Bob Briscoe <[email-masked] 
> <mailto:[email-masked]>> wrote:
>
>     Reviewer: Bob Briscoe
>     Review result: Not Ready
>
> mechanism does not require existence of a return path from tails to 
> the head. Would the following be acceptable:
> NEW TEXT:
>    Use of BFD in
[BB]: Caveat: I am having to talk in generalizations, cos I don't 
actually know how you are going to get this protocol to work in a wide 
range of circumstances, given inherent problems like multipoint feedback 
implosion {Note 1}.

My point was that, having broken up the drafts in this way, this draft 
on its own no longer defined a workable protocol. Therefore, it needed 
some references to other drafts (even if they are placeholders), so that 
the extent of the pre-requisite collection of work is clear. The refs 
you give later go a long way to fixing this issue.

If each pre-requisite protocol is intended to only represent one 
example, the citation can say that and the ref can be informative. But 
with zero examples for all the prerequisite parts, all the reader sees 
is a dismembered octopus, not a protocol.

> GIM>> One example could be a multicast system with 1+1 protection. 
> Without multipoint BFD tails would not be able to detect the failure 
> of the muticast path from the head. Other examples discussed in 
> several drafts:
>
>   * BESS WG draft MVPN fast upstream failover
>   * Individual draft BFD for Multipoint Networks and VRRP Use Case
>   * Individual draft BFD for Multipoint Networks and PIM-SM Use Case
> I am not sure how references to non-WG drafts affect the progress of 
> this document. Appreciate your suggestion.
[BB]: In my experience, informative refs to non-WG drafts as use-cases 
would be OK during doc development. However, if a non-WG draft fails to 
proceed, its citation has to be removed later. So choose those that are 
most likely to proceed.

Nonetheless, if you cite some specs that turn this into a workable 
protocol (see previous issue) use-cases might not be necessary.

> GIM>> Very interesting, thank you. I wouldn't say that the case when a 
> tail cannot process incoming mpBFD control packets at the offered rate 
> is entirely non-issue. Such scenario must be handled by the 
> implementation and may be controlled by local policy, e.g., close the 
> MultipointTail session.
[BB]: Fair enough.

In some scenarios, this issue will not necessarily be so unlikely tho:
* If asymmetric crypto is used to solve the group message authentication 
problem (see later), the processing burden on any lightweight endpoints 
might lead to message verification leaving less available processor 
resource than needed for the host's other tasks.
* Each tail might be joined to a very large number of multipoint sessions.

> Where would you suggest to add the text?
I would suggest a new section listing potential issues when there is no 
back channel.

> GIM>> Thank you for the suggested text. Would the Security 
> Considerations section be appropriate place:
[BB]: Well, the point limits the applicability of the assumption about 
security in 5. 'Assumptions', so this would fit well there.
Then "Security Considerations" should point to everywhere in the doc 
that discusses security, such as this (to save time for security reviewers).

>    tails MUST be trusted not to spoof the head.
[BB]: Normally a MUST is applied to implementations. It would be rather 
odd to require users/operators to satisfy a spec requirement, 
particularly requiring them to trust each other. I think this should be 
written as an applicability statement not a normative requirement.

> Otherwise, asymmetric

[BB]: If you are going to allow for cases where all tails are trusted 
not to spoof the head, then the assumption written in section 5 is no 
longer correct.

[FYI, RFC4082 is only a generic description. Many RFCs have been written 
to authenticate specific protocols along TESLA lines.]

>    to validate received the multipoint BFD Control packets.
[BB]: Yup. Except delete "received the".
Also see above about whether this is now a correct assumption.

> GIM>> Because mpBFD control packets cannot be demultiplexed by  tail 
> based on the value of Your Discriminator field as per RFC 5880,

[BB]: I seem to have become co-opted into redesigning this protocol. I'd 
prefer to limit my involvement to reviewing :)

>     Will add as informational reference.
[BB]: Good.

Nonetheless, given you have confirmed that a reverse path is optional, 
the doc still needs to address the case where there is no reverse path.

{Note 1} For the active tail draft, you might find the following ideas 
for scaling multipoint feedback useful:

*Statistical feedback:*
Nonnenmacher, Jö. & Biersack, E.W., "Scalable Feedback for Large Groups 
<https://dl.acm.org/citation.cfm?id=312251>," IEEE/ACM Transactions on 
Networking 7(3):375--386 (June 1999)

FUHRMANN, T., AND WIDMER, J. "On the scaling of feedback algorithms for 
very large multicast groups 
<https://dl.acm.org/citation.cfm?id=2294709>," Computer Communications 
24, 5-6 (March 2001), 539 547;
WIDMER, J., AND FUHRMANN, T. Extremum feedback for very large multicast 
groups. Tech. Rep. TR 12-2001, Prakfische Informatik IV, University of 
Mannheim, Germany, May 2001.

Also, anycast can be used to select different representative feedback 
tails, e.g. for a certain time, which might overlap with the periods for 
which a few other tails are selected using subsequent anycasts.

*Logical 'AND' feedback:*
Burbridge, T., Soppera, A., Briscoe, R. and Jacquet, A. "Method and 
device for co-ordinating networked group members 
<https://worldwide.espacenet.com/publicationDetails/biblio?II=0&ND=3&adjacent=true&locale=en_EP&FT=D&date=20060406&CC=US&NR=2006075022A1&KC=A1#>" 
Patent WO2004059479, (Jul 2004; Priority 24 Dec 2002)
[AFAICT this patent is still being maintained, so use of it in a 
protocol would require an IPR declaration.]

>  1. Details of how the head keeps track of tails and how tails alert
>     their connectivity to the head are outside scope of this document.
>     - Same as #1.
>
[BB]: And my response is same as #1.
>
>  1. Bootstrapping BFD session to multipoint MPLS LSP in case of
>     penultimate hop popping is outside the scope of this document. -
>     It may use control plane as in MVPN draft. Will add as
>     informational reference.
>
[BB]: Good.
>
>  1. Use of other types of encapsulation of the BFD control message
>     over multipoint LSP is outside the scope of this document. - This
>     in reference to ACH encapsulation that is discussed in
>     draft-mirsky-mpls-p2mp-bfd
>     <https://tools.ietf.org/html/draft-mirsky-mpls-p2mp-bfd-03>.
>     Should it be added as informational reference? What would be the
>     imacpt of progressing this specification?
>
[BB]: See earlier comment about citing individual drafts (I don't have 
enough BFD knowledge to give a BFD-specific answer).

Also, in my review I should also have said: when creating new 
encapsulations, pls see the common transport issues related to 
encapsulation:
https://trac.ietf.org/trac/tsv/wiki/tsvdir-common-issues#TunnelingprotocolsandTransportRelatedIssues

>  1. Change in the value of bfd.RequiredMinRxInterval is outside the
>     scope of this document. - Same as #1.
>
[BB]: And my response is same as #1.
>
>  1. If a session is not found, a new session of type MultipointTail
>     MAY be created, or the packet MAY be discarded. This choice is
>     outside the scope of this specification. - I propose to add "based
>     on local policy" to the last sentence.
>
[BB]: On what basis will local policy decide? It's my job as a reviewer 
to ensure that this spec does not contain any loose ends (open issues).

>  1. The exact method of selection is application-specific and is thus
>     outside the scope of this specification. - This is copied from RFC
>     5880: "The exact method of selection is application specific and
>     is thus outside the scope of this specification." as the section
>     is to replace Section 6.8.6.
>
[BB]: OK.
>
>  1. If a matching session is not found, a new session of type
>     PointToPoint MAY be created, or the packet MAY be discarded. This
>     choice is outside the scope of this specification. - Same as #6.
>

[BB]: And my response is same as #6.
[Sry, my embedded comments have broken your numbered list.]

> I propose to remove the sentence altogether.
[BB]: OK.

> GIM>> You're aboultely right, existing implementations of BFD don't 
> need to support bfd.SessionType variable. Only implementations that 
> support the base BFD, single-hop or multi-hop, and this specification, 
> mpBFD, should support bfd.SessionType and set it to PointToPoint value 
> when BFD is in single-hop or multi-hop mode. When in mpBFD mode, 
> bfd.SessionType will be set to either MultipointHead or MultipointClient.
[BB]: Doesn't something need to be written (or referenced) to clarify 
all this? AFAIR, this spec. never made clear that multipoint is a fork 
in implementations.

> GIM>> You are right, the BFD system at the head would not know the 
> previous value of bfd.DesiredMinTxInterval. This text is to caution 
> operator from decreasing bfd.DesiredMinTxInterval upon restart of the 
> BFD system.
>
>
> implementation can differentiate between the very first start and 
> restarts of BFD system. If it is the first start of BFD system, the 
> head MAY directly progress to Up state skipping Down state.
> The last paragraph describes graceful shuttdown. The head MAY shut the 
> BFD mp session abruptly by just stopping transmission of BFD Control 
> packets.
[BB]: I assume you will say all this in the next rev, not just in this 
email.

> GIM>> A Poll Sequence starts with the initiator setting Poll bit. 
> Unless the peer sends BFD Control packet with Finl bit set the poll 
> sequence would continue indefinetely. The initial SHOULD NOT, in my 
> opinion, correctly points that the mechanism of Poll Sequence not to 
> be used by MultipointHead when changing transmission interval. I think 
> that MUST in the first paragraph can be downgraded to MAY because the 
> MultipointHead doesn't need to use transition period when changing the 
> transmission interval to lower level, i.e., decreasing frequency. May 
> I propose the following:
> OLD TEXT:
[BB]: If I were an implementer, I would not know what this is saying I 
ought to implement. The spec needs to answer this question: If the head 
changes the packets what happens differently if it sets the P bit vs. if 
it doesn't?

>             discarded.  This choice is outside the scope of this
[BB]: As long as this represents the logic you want, fine. The point is 
that the indentation is the only clue to whether one 'if' is conditional 
on a previous 'if', or not.

HTH

Bob

-- 
________________________________________________________________
Bob Briscoe                               http://bobbriscoe.net/

---

From: Spencer Dawkins at IETF <[email-masked]>
Date: Tue, 29 May 2018 12:05:57 -0500

Mirja and I do read these reviews, but don't usually comment on them while
the authors and reviewers are still chatting. But, on one point ...

> On 26/05/18 20:49, Greg Mirsky wrote:

[snip]

> odd to require users/operators to satisfy a spec requirement, particularly
> requiring them to trust each other. I think this should be written as an
> applicability statement not a normative requirement.
>

Bob is formally correct here, but it may be useful for me to say that I do
see "requirements" language used to provide guidance about security and
about operational considerations (as here).

If I understand Bob's suggestion to be something like

NEW

   Shared keys in multipath scenarios allow any tail to spoof
   the head from the viewpoint of any other tail. For this reason,
   using shared keys to authenticate BFD Control packets in multipoint
   scenarios is a significant security exposure unless all tails can
   be trusted not to spoof the head.

that would also work.

"Do the right thing", of course.

Spencer