[Tsv-art] Tsvart telechat review of draft-ietf-pim-source-discovery-bsr-08

[David Black <[e-mail]>]

Reviewer: David Black
Review result: Ready with Issues

I've reviewed this document as part of TSV-ART's ongoing effort to review key
IETF documents. These comments were written primarily for the transport area
directors, but are copied to the document's authors for their information and
to allow them to address any issues raised.  When done at the time of IETF Last
Call, the authors should consider this review together with any other last-call
comments they receive. Please always CC [e-mail] if you reply to or
forward this review.

This draft describes an experimental PFM (PIM Flooding Mechanism) mechanism for
flooding PIM information among multicast routers that is a generalized form of
the RFC 5059 PIM BSR (BootStrap Router) mechanism, and applies this mechanism
to distribution of source group mappings (PFM-SD).

Early implementation experience with PFM-SD on low bandwidth radio links
(described Section 2) suggests that the mechanism is able to work better than
PIM-SM without starving other traffic in the fashion that PIM-DM may.  This is
promising and (in this reviewer's opinion) justifies experimentation at larger
scale and in other network environments.  In general, this is a well-written
document and the authors should be commended for including the "running code"
implementation experience report in Section 2.

Flooding mechanisms are very useful, but the time periods that govern sending
of flooding messages are crucial to avoid excessive consumption of network
resources.  Section 5 of RFC 5059 has a solid discussion of the time periods
that apply to use of flooding by the BSR mechanism.   The discussion in this
draft is somewhat weaker, raising a couple of minor issues:

1) For PFM-SD, Section 4.2 provides a reasonable discussion of time periods
that apply, but appears to be missing a minimum time period between sending
messages.   Section 5 of RFC 5059 recommends a default of 10 seconds for that
minimum time period by comparison to a default PIM BSR sending interval of 60
seconds.  That 10 second minimum default should be added to this draft, as the
same default sending interval of 60 seconds is used.

2) For future use of PFM for other purposes, Section 3.3 provides the following
guidance:

   Each TLV definition will need to define when a triggered PFM message needs
   to be originated, and also whether to send periodic messages, and how
   frequent.

That guidance is correct as far as it goes, but it's not particularly helpful
to future protocol designers.   Text should be added to at least point to the
examples in section 4.2 of this draft and/or part of Section 5 of RFC 5059 to
suggest the sorts of values that have proven to be workable, and perhaps also
strongly encourage (SHOULD use) a default minimum time between messages of at
least 10 seconds.

Understanding this draft requires that the reader be familiar with multicast
and PIM, which is reasonable.  In addition, an understanding of PIM BSR is also
required, which is perhaps somewhat less reasonable.  An example that this
reviewer tripped over is that Section 3 of this draft states that "Like BSR,
messages are forwarded hop by hop."  There is no further explanation or
definition of "forwarded hop by hop," making it necessary to consult RFC 5059
to understand that term, e.g., this has nothing to do with IPv6 hop-by-hop
options.  A sentence or two of explanation of this hop by hop forwarding
concept ought to be copied and adapted from RFC 5059, and it would be good to
check for other concepts that rely on RFC 5059 for definitions.