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

[Stig Venaas <[e-mail]>]

Hi, thanks for the great comments.

I agree with all you wrote and will update the document. However,
there is one slight issue with the minimum time between origination of
each message. When a new source is detected, we would like to
originate a message ASAP so that receivers can start receiving the
multicast without much delay. A 10s delay would be a rather long time
if a source was detected right after the previous message was
originated. I think some delay would be warranted though, in
particular in a case where perhaps a router starts up and a large
number of directly connected sources could be detected within a short
time frame. I think an exponential back-off could make sense here.
E.g., if it is just one new source, maybe trigger a message ASAP. If a
new source is detected right after the previous one, wait a bit
longer, which also allows for aggregation of multiple sources in one
messages if several are detected later. In extreme cases one could
over time keep increasing the delay until the next update.
If sufficient we could maybe have a fixed minimum delay of 1s or not,
but that is probably too short in those extreme cases. Hence maybe an
exponential back-off.

I would appreciate some further guidance what you think is reasonable
here, and perhaps whether I can borrow something here from other
protocols/drafts. Part of the experiment here might be to find out
what minimum values, or how rapid back-off, is needed based on the
size of the network, the amount of sources, the types of links etc.

Also note that the general mechanism can be used for many types of
information. It depends on the information how urgent it is to
distribute it. Source discovery is particular is fairly urgent.

Stig


On Tue, Jan 23, 2018 at 3:40 PM, David Black <[e-mail]> wrote:
> 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.
>
>