Tsvart last call review of draft-ietf-bfd-multipoint-16

Bob Briscoe <ietf@bobbriscoe.net> Tue, 22 May 2018 00:20 UTC

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Subject: Tsvart last call review of draft-ietf-bfd-multipoint-16
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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?


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

4.8.  Packet consumption on tails

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

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

If bfd.SessionType is PointToPoint, update the Detection Time as
      described in section 6.8.4 of [RFC5880].  If bfd.SessionType is
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".