Add simple PMTU calculation with heartbeats

[kc5nra]

Calculate the maximum MTU size that can travel the entire path machine1<->machine2.

The easiest way is probably to craft heartbeat messages starting at the maximum value until they are successfully returned. Potentially listen to ICMP PacketTooBig

[Sean-Der]

@enobufs is this still an issue/something that needs to be done?

If there is anything I can do to help please tell me :) but you know way more about SCTP then me, so defering to the expert

[enobufs]

SCTP RFC actually mentions RFC4821 which recommends using the heartbeat. It is an interesting approach. The current implementation does not fully support heartbeat. In WebRTC context, we have heartbeat at ICE layer, and I am not sure how critical it is not having heartbeat at SCTP layer..

I think we need to investigate how other WebRTC libraries deal with path MTU discovery.