Re: [homenet] Despair

[Eric Gray <[e-mail]>]

It strikes me as something of a mistake generally to assume that multicast is as reliable as
unicast.

Unicast reliability depends on the mechanism(s) used to ensure reliability.  Unicast traffic
tends to get lost every now and then.

All the same factors that affect unicast packet delivery also affect delivery of each packet
with multicast.  Hence multicast reliability should be worse than unicast reliability by an
amount roughly proportional to the amount of packet replication necessary to support it.

Each replicated packet is as likely to be lost as any unicast packet.  Loss of one or more
packets should be expected to be more likely with multiple packets than with a single
packet.

Multicast reliability, even when considered at the link level and assuming replication is not
required in transmission of multicast packets onto the link itself, is only slightly better.  As
full-duplex, point-to-point connectivity becomes increasingly likely (fat yellow cables are
relatively rare any more), data replication still occurs - just not at the level where a router
sending packets onto the link is likely to be aware of it.

Hence it is interesting in this discussion that we are talking about an assumption that seems
broken at the start.

Have I missed something?

--
Eric

-----Original Message-----
From: Mikael Abrahamsson [mailto:[e-mail]] 
Sent: Thursday, August 06, 2015 2:45 AM
To: Glenn Parsons
Cc: Alia Atlas; Acee Lindem (acee); Toerless Eckert (eckert); Homenet; Eric Gray; Dan Romascanu ([e-mail])
Subject: RE: [homenet] Despair

On Thu, 6 Aug 2015, Glenn Parsons wrote:

> As I indicated in another thread, the right place to start a discussion on this would be in the IETF-IEEE 802 coordination that Dan leads.
>
> While this issue may be solved be current work underway (and included in the coordination), perhaps a clearer problem statement would help us to ensure that is the case.

There are documents that talk about multicast from a power efficiency
standpoint:

https://tools.ietf.org/html/draft-desmouceaux-ipv6-mcast-wifi-power-usage-01
https://tools.ietf.org/html/draft-yourtchenko-colitti-nd-reduce-multicast-00
https://tools.ietf.org/html/draft-ietf-6man-rs-refresh-00

Slide 2 of
http://www.ipv6council.be/IMG/pdf/20141212-08_vyncke_-_ipv6_multicast_issues-pptx.pdf
pretty much sums it up, most of IETF protocols are designed around multicast being as reliable as unicast. IPv6 relies on this. On 802.11 this isn't the case. Slide 5 describes how this works in 802.11.

The fact that multicast and broadcast is unreliable (not ACKed) on 802.11 is from what I can see the major cause of the unreliability problem that the mesh wifi networking protocols are trying to solve by basically only using multicast for discovery.

The whole question is whether this should be fixed by 802.11 or if the IETF needs to (basically) abandon multicast/unicast, or if the IETF should develop a multicast->unicast replication mechanism for wifi (there is work in this area going on).

Personally, I think 802.11 needs to fix multicast/unicast so it's reliable, or get back the IETF and say it can't be fixed and then the IETF can continue the work on multicast reduction (or workaround) even harder.

I find the current approach of (basically) individuals within the IETF working on multicast reduction without (as far as I can see) any dialogue with 802.11 to be a non-optimal way of solving the problems we're seeing.

-- 
Mikael Abrahamsson    email: [e-mail]