Subject: [bess] Alvaro Retana's Discuss on draft-ietf-bess-evpn-igmp-mld-proxy-14: (with DISCUSS and COMMENT)

From: Alvaro Retana via Datatracker <[email-masked]>
Date: Wed, 27 Oct 2021 10:23:31 -0700

Alvaro Retana has entered the following ballot position for
draft-ietf-bess-evpn-igmp-mld-proxy-14: Discuss

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DISCUSS:
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First of all, I am surprised that a document related to IGMP/MLD was not sent
to the pim WG for review.  I can't find any mention of this draft in the pim
WG's archive.

§11 says this:

   This document does not provide any detail about IGMPv1 processing.
   Multicast working group are in process of deprecating uses of IGMPv1.
   Implementations MUST only use IGMPv2 and above for IPv4 and MLDv1 and
   above for IPv6.  IGMP V1 routes MUST be considered as invalid and the
   PE MUST apply the "treat-as-withdraw" procedure as per [RFC7606].
   Initial version of document did mention use of IGMPv1 and flag had
   provision to support IGMPv1.  There may be an implementation which is
   deployed as initial version of document, to interop flag has not been
   changed.

Note that the "Multicast working group" mentioned above is in fact the pim WG. 
There's no current WG to deprecate IGMPv1, but draft-ietf-pim-3376bis was
recently adopted with the intent to progress IGMPv3 to Internet Standard.  This
text is from draft-ietf-pim-3376bis (it is the same text as in rfc3376):

   IGMPv3 is backward compatible with previous versions of the IGMP
   protocol.  In order to remain backward compatible with older IGMP
   systems, IGMPv3 multicast routers MUST also implement versions 1 and
   2 of the protocol (see section Section 7).

(Section 7/draft-ietf-pim-3376bis talks about interoperability with older
versions.)

All this is to say that requiring that IGMPv1 not be used contradicts the
IGMPv3 specification, which requires the support.  The interoperation between
the different versions is already considered in rfc3376, so the extra
complexity added to this document (tracking the versions in the BGP updates) is
not needed from the router side.

I am balloting DISCUSS because this document is not in line with other
consensus documents (specifically the IGMP specification).  To clear, I will
want the document reviewed by the pim WG.

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COMMENT:
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(1) The terminology section should include IGMP/MLD-related terminology or at
least a pointer to the relevant RFCs.

Also, the messages are called Membership Reports, and not "Join" or "IGMP
Reports".  Similar comment related to "IGMP Queries" and "Membership Requests"
(should be Membership Query).

[I will not make other comments below about this same point.]

(2)
[Line numbers from idnits.]

260       1.  When the first hop PE receives several IGMP Membership Reports
261           (Joins), belonging to the same IGMP version, from different
262           attached hosts for the same (*,G) or (S,G), it SHOULD send a
263           single BGP message corresponding to the very first IGMP
264           Membership Request (BGP update as soon as possible) for that
265           (*,G) or (S,G).  This is because BGP is a stateful protocol and
266           no further transmission of the same report is needed.  If the

The behavior in this rule is not required.  Under what circumstances is it ok
for the PE to not wait for several Membership Reports from multiple hosts
before sending a BGP message?

Waiting for multiple messages can clearly result in a delay for an interested
host in receiving the multicast service.  Note that rfc3376 says that
"Multicast routers need to know only that *at least one* system on an attached
network is interested..."

(3)

269           (v2 or v3) set.  In case of IGMPv3, the exclude flag MUST also be
270           set to indicate that no source IP address must be excluded
271           (include all sources "*").  If the IGMP Join is for (S,G), then
272           besides setting multicast group address along with the version
273           flag v3, the source IP address and the IE flag MUST be set.  It

"the exclude flag MUST also be set"  I think you meant to reference the Exclude
Group and the IE field in the flags.  Note that the second part ("IE flag MUST
be set") also refers to the same field, but for a different condition.  Please
be consistent and call things (the IE field, in this case) by a single name.

The definitions in §9.* are not consistent either.

(4)

277       2.  When the first hop PE receives an IGMPv3 Join for (S,G) on a
278           given BD, it SHOULD advertise the corresponding EVPN Selective
279           Multicast Ethernet Tag (SMET) route regardless of whether the
280           source (S) is attached to itself or not in order to facilitate
281           the source move in the future.

When is it ok for the SMET route not to be advertised?  IOW, why is it a
recommendation and not a requirement?

(5)

283       3.  When the first hop PE receives an IGMP version-X Join first for
284           (*,G) and then later it receives an IGMP version-Y Join for the
285           same (*,G), then it MUST re-advertise the same EVPN SMET route
286           with flag for version-Y set in addition to any previously-set
287           version flag(s).  In other words, the first hop PE MUST NOT
288           withdraw the EVPN route before sending the new route because the
289           flag field is not part of BGP route key processing.

The requirement (MUST) to re-advertise the same SMET route assumes that there
was an advertisement done already, but rule 2 doesn’t require that.

(6)

291       4.  When the first hop PE receives an IGMP version-X Join first for
292           (*,G) and then later it receives an IGMPv3 Join for the same
293           multicast group address but for a specific source address S, then
294           the PE MUST advertise a new EVPN SMET route with v3 flag set (and
295           v2 reset).  The IE flag also need to be set accordingly.  Since
296           source IP address is used as part of BGP route key processing it
297           is considered as a new BGP route advertisement.  When different
298           version of IGMP join are received, final state MUST be as per
299           section 5.1 of [RFC3376].  At the end of route processing local
300           and remote group record state MUST be as per section 5.1 of
301           [RFC3376].

Receiving an IGMPv3 Membership Report for the first time, as described here, is
equivalent to the case in rule 2,  However, the normative language is
different: sending an SMET route is required here, but only recommended in rule
2.  I fail to see why the conditions are different.

Also, this rule mentions that the “ IE flag also need[s] to be set accordingly”
while rule 1 requires a specific setting.

This section is talking about the actions of a PE when it receives IGMP
messages — this is what rfc3376 refers to as a multicast router.  §5.1/rfc3376
refers to the host function (group members).  Both statements (which seems
redundant to me) requiring compliance with rfc3376 are misplaced.

BTW, these are the only references (in the specification part of the text) to
rfc3376.  Given that this document is about IGMP/MLD Proxy, there should be
other references that make clear the normative relationship.  The same comment
applies to the other versions of IGMP mentioned as well as MLD.

(7)

§4.1.1: The set of rules in this section (IGMP/MLD Membership Report
Advertisement in BGP) is preceded with:

256        When a PE wants to advertise an IGMP Membership Report (Join) using
257        the BGP EVPN route, it follows the following rules (BGP encoding
258        stated in Section 9):

But rules 5-7 are about the actions related to the SMET route being received,
not advertising it.  Perhaps divide the list of rules so that it is clear when
they apply.

(8)

§4.1.1: Rule 7 talks about listening to PIM Hellos.  Please add a Normative
reference to rfc7761.

(9)
§4.1.2: Rules 2-3 are about the actions after the SMET route is received, which
doesn't match with the preface to the rules.  Perhaps divide the list of
rules...

(10)

1269       IGMP MAY be configured with immediate leave option.  This allows the
1270       device to remove the group entry from the multicast routing table
1271       immediately upon receiving a IGMP leave message for (x,G).  In case
1272       of all active multi-homing while synchronizing the IGMP Leave state
1273       to redundancy peers, Maximum Response Time MAY be filled in as Zero.
1274       Implementations SHOULD have identical configuration across multi-
1275       homed peers.  In case IGMP Leave Synch route is received with Maximum
1276       Response Time Zero, irrespective of local IGMP configuration it MAY
1277       be processed as an immediate leave.

By "immediate leave" I assume you're referring to "low leave latency"
(rfc2236/rfc3376), is that right?   There is no "immediate leave" mentioned in
whose documents.

"IGMP MAY be configured with immediate leave option."  This "MAY" seems to just
be stating a fact. s/MAY/may

When is it ok for implementations to not have the same configuration?  IOW, why
is that a recommendation and not a requirement?