Subject: [manet] manet: draft-gerla-manet-odmrp-03

From: Toerless Eckert <[email-masked]>
Date: Wed, 23 Jul 2014 03:34:14 -0700

Sorry, resending with correct recipients.

Some comments on subject draft, and maybe take this mail
also as helpfull to decide if/how PIM WG could help with Manet 
groups multicast interest:

(A) Nice simple looking IP multicast protocol. This is how
simple i remember the first versions of PIM to be, 20 years ago,
before proliferation, success and generations of implementers
eyeballs beat all necessary bug-fixes, oversight amendments and
optimizations into the current PIM RFCs. Now having said this:
The current PIM RFC 4601 is like C++ vs. Java: Remove 80%
from it and you have PIM-SSM, which is relevant here.

(B) Fundamentally, ODMRP is what PIM-SSM + draft-ietf-pim-source-discovery-bsr (SD-Draft)
with IMHO three main differences:

1) ODMRP does rely on some pre-existing flooding mechanism (eg: SMF)
for messages indicating what (S,G) are actively sending traffic.
(called "join request" in ODMRP, called SG mapping in SD-draft).

2) PIM-SSM (like all other variants of PIM) maintains an OIF-list
(Outgoing Interface list) to which received multicast packets are
replicated, and it eliminates loops/duplicate packets via
RPF-check. ODMRP proposes to transmit the packet to all interfaces,
(even though it could build an OIF list from received "join reply"),
and it uses duplicate packet detection instead of RPF check.

3) ODMRP uses "join reply" acknowledgements sent from the up
to the downstream router to detect unidirectional adjacencies,
and proposes to then use an ad-on draft (gerla-manet-odmrp-asym)
to make that work.

I think that these differences hardly justify inventing a new protocol
and going through all the trouble of (A). Instead i would strongly
suggest to consider adopting the solution worked on in PIM and
modify it accordingly:

- If existing MANET networks have working flooding solutions,
  they should of course be useable by the solution. SD-Draft
  was brought into IETF for networks that do not have any (like
  pretty much all non-MANET networks i know of).

- RPF-check vs duplicate-elimination and OIF-list vs
  all-interface-forwarding might be justified differences
  based on whether the interfaces is MANET or not. Aka: This
  doesn't have to be a network wide protocol behavior, but could
  be IIF/OIF-link properties of PIM - therefore also allowing very
  easily a mixed network with MANET and non-MANET parts. Instead of
  having to figure out how to interconnect PIM with a MANET
  specific protocol (interconnecting multicast protocols is always
  a pain).

  And of course, i think there should be more review if these two
  choices are actually the best for MANET. I understand some reasons
  why they are good, but i also have some more concerns there. I think 
  this shouldn't be too tough to work out.

- Unidirectional link support seems like an extension anyhow,
  we had done some work in products for non-MANET networks, i
  don't quite remember what made it through the IETF and what
  stayed in proprietary features, so i'll comment on this later.

Cheers
    Toerless

-- 
---
Toerless Eckert, [email-masked]
Cisco NSSTG Systems & Technology Architecture
SDN: Let me play with the network, mommy!