This document has been reviewed as part of the transport area review team'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 and WG to allow them to address any issues raised and also to the IETF
discussion list for information.

When done at the time of IETF Last Call, the authors should consider this
review as part of the last-call comments they receive. Please always CC
tsv-art@ietf.org if you reply to or forward this review.

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There are no transport issues in this document.

However, the following two key issues appear to be significant and should be addressed.

The document appears to be about selecting multicast IP addresses to avoid link address collisions, rather than IP address collisions. That should be made more clear in the title, abstract, intro, and throughout. The current text is easily misread as focusing on IP address collisions until section 2. 

It also appears that the limited number of hardware address filters has been overlooked; i.e., if a device is capable of filtering only 4 multicast addresses in hardware, the device would more efficiently support 7 addresses if at least 3 DID collide, rather than ensuring that none collide.  When no addresses collide and the number of filters is exceeded, the network interface is driven into promiscuous mode that requires software filtering – i.e., it completely undermines the goal of this doc.

The follow comments are also provided, if useful:

The document jumps between link and network issues without enough context for the reader to distinguish between the two. Collisions should be referred to throughout as link address collisions, not just address collisions, and address assignment should be referred to as IP address assignment. 

The protocol requirements need some refinement. Resistance to failure is NOT the same as robustness to a single point of failure; the tag phrase and the definition need to be adjusted to align. Item 3 is “multicast self-assignment protocol coexistence”, i.e., this isn’t about interoperation with other protocols in general. Item 7 (collision detection and resolution) is not a requirement; it is an approach to satisfying other requirements, such as 1, 2, and 5. It would help if the tag phrases were similar (they seem to be phrases defining properties, except for #6, which should be “supports host-level multiplexing” – check this throughout for all lists).

The Note needs revision; the collisions arise after *rejoining disconnected networks caused by* a temporary network partition. I.e., it is the rejoining that causes the problem, not the partitioning.
The second list should describe goals of a design; it seems odd to include “cross-platform availability”, as this seems to conflate deployment with OS capabilities; it is difficult to appreciate what OS properties could ever interfere with these mechanisms. An example would help as would rephrasing this as “OS independence”.

Sec 5 should more clearly relate the way 32 IP addresses could map to the same MAC address, i.e., this is because only the lower 23 bits are used of the 28 bits in the IP multicast address range. It would be very useful if the reason for this mapping were discussed in this doc – i.e., why did we not just use all 28 bits?

Sec 5 seems to suggest a revision of RFC 5771 is warranted. Simply saying “that doesn’t work, so you should use IPv6” is not realistic. Further, using addresses from /8 blocks (admin scoped) still requires care; it would be useful to remind designers that they should ensure that the bottom 23 bits are unique, i.e., to NOT rely on the top bit there (or in other multicast addressing) for independence.

Sec 6 should reconsider the advice to not revise RFC 5771 assignments. I.e., new systems could select addresses in ways that avoid collisions, which could relieve even legacy systems from the effect of collisions in switches.