On Thu, 6 Aug 2015, Eric Gray wrote:
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.
Nobody doubts that packets get lost, but the general tendency since IP
networking was invented, was that multicast delivery of packets wasn't
especially worse than unicast. Packets get lost, but generally less than
1% get lost, and multicast and unicast are affected equally.
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.
Hm, care to elaborate? That seems a lot worse than my experience in
deploying networks would tell me.
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.
But it's still only a single packet per link.
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.
Correct, as of 20 years ago or something we do not use 10base5 so L2
devices do L2 replication.
Hence it is interesting in this discussion that we are talking about an
assumption that seems
broken at the start.
Have I missed something?
Well, 802.11 treats multicast (and broadcast) packets as a second rate
citizen, I am not aware of any other L1/L2 technology that does this. 3GPP
uses basically a point-to-point tunnel, so unicast and multicast is
treated in very similar fashion without multicast being at a disadvantage.
So IETF needs to sit down and work out a strategy on how its protocols
should work going forward, if everybody who designs protocols in the IETF
should be told that multicast and broadcast "doesn't work properly", and
act accordingly.
What probably needs to happen is that over time, the IETF should try to
use less multicast, but on the other hand, 802.11 really needs to make
sure that multicast works a lot better than it does today.
--
Mikael Abrahamsson email: [email protected]
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