Does Flannel support Multicast?

[Chris Stivers]

Using Docker for a while, played w/ CoreOS since LinuxCon'14 and yet to use Flannel.

Q: Will Flannel allow me to setup Multicast /UDP between Docker instances on separate CoreOS machines? Our software has an option to send heartbeats via multicast / UDP, so we would like to enable this on our CoreOS cluster.

[Paul Morgan]

flannel, no. https://github.com/coreos/flannel/issues/52

weave, yes.

```console
on 1st host

# 1st host launches weave
ip-192-168-254-86 ~ # ./weave launch
acedc5e08c3b3b43ed958a5feec48ff07386e5bb7a1f13030882b49782a31b89
ip-192-168-254-86 ~ # C=$(./weave run 10.2.1.1/24 -t -i
neilellis/weave-multicast-tutorial /bin/bash)
ip-192-168-254-86 ~ # docker attach $C
root@154842c39eca:/#
root@154842c39eca:/# /mc
mcreceive mcsend
root@154842c39eca:/# (/mcreceive 239.1.2.3 54321) &
[1] 18


on 2nd host

# 2nd and subsequent hosts must specify one host from the weave
network (doesn't matter which one)
ip-192-168-254-87 ~ # ./weave launch 192.168.254.86
fbc67d627ed381c6a997f1ffb91cd42fa47ad12f6cad54aae37886bc20d20a28
ip-192-168-254-87 ~ # C=$(./weave run 10.2.1.2/24 -t -i
neilellis/weave-multicast-tutorial /bin/bash)
ip-192-168-254-87 ~ # docker attach $C
root@912e84e042ac:/#
root@912e84e042ac:/# (/mcreceive 239.1.2.3 54321) &
[1] 20



back to 1st host

# send an mcast message
root@154842c39eca:/# echo booga booga booga | /mcsend 239.1.2.3 54321
Begin typing (return to send, ctrl-C to quit):
Received 18 bytes from 10.2.1.1: booga booga booga
root@154842c39eca:/#


lo and behold, what appears on 2nd host?

# the mcreceive process in background dumps this to screen
root@912e84e042ac:/# Received 18 bytes from 10.2.1.1: booga booga booga
```

hth,
- paul

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--
Paul Morgan

[eugene.y...@coreos.com]

It does not. Multicast is a great option for disseminating information to multiple receivers _efficiently_. Switches and routers do the replication in hardware (usually) and are aware of the network topology to build the distribution trees effectively. Unfortunately most cloud environments don't support multicast so it would have to be emulated by flannel and would be very inefficient. Additionally, flannel can use a backend such as VXLAN to perform the data path in the kernel; however kernel's vxlan does not support emulating multicast.