Subject: [Bier] draft-ietf-bier-path-mtu-discovery-03 comments

From: Toerless Eckert <[email-masked]>
Date: Wed, 4 Apr 2018 00:38:02 +0200

Quite nice. Almost makes me not hate PMTUD ;-)

Would suggest to add two explanatory things

1) Abstract

Sentence like "PMTUD for BIER as described here is significantly easier
than PMTUD for IP multicast, because it leverages unique properties of BIER"

2) Add section "comparison with PMTUD for IP multicast"

Summarize the claim from abstract, e.g.:

a) The network/transport stack of sender in IP multicast has usually no clear
   understanding who their receivers are, so it is more difficult to figure out when to re-probe.

b) Senders in IP multicast can not target any subset of receivers to probe their
   MTU. In BIER this is at the core of the BIER forwarding mechnism.

c) Midpoints in IP multicast with an MTU issue have even less chance to know
   what subset of receivers is south of the adjacenc with MTU issues (the sender
   could have application layer mechanisms to know all receivers). In BIER this
   knowledge is immediately available from the bitstring of the packet causing the issue.

d) IP multicast trees can be arbitrarily large, which together with b) and c)
   has resulted in so many IP Multicast PMTUD that there is no standards IP multicast
   PMTUD discovery mechanism.

   In addition to the solutions to a)..c) that BIER provides, the number of receivers
   for every packet is also limited to the BSL.

Or something similar fixing whatever might be wrong or missing in above.

Cheers
    Toerless

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