Feature/pmtu discovery #64
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- only discover pmtu for reachable hosts - pick a starting test point closer to the more probable MTU - tune the interval and timeout based on discovered RTT
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Thanks for implementing this feature. I'm currently looking into adding PMTUD to MeshPing, a little network monitoring tool I'm building. I'm currently working on a somewhat-hacky PMTUD implementation, but I'd also love to contribute back to icmplib, hence I found your PR. Your implementation looks a lot cleaner than mine 😄 I'd just like to point out that, rather than guessing and trying new MTU values when you get an error back, the new MTU is actually included in the downstream gateways' ICMP messages, and you can query them from the socket:
def get_mtu(self):
return self._sock.getsockopt(socket.IPPROTO_IP, IP_MTU)
Similar for IPv6:
def get_mtu(self):
return self._sock.getsockopt(socket.IPPROTO_IPV6, IPV6_MTU)
Maybe this would be a nice addition? 🙂
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Thanks for the comment and apologies for the delayed response.
I agree, a brute-force PMTU discovery mechanism like this is not often needed these days.
However, the reason I had for implementing this feature is that there are some network devices, like fibre media converters, that operate as bridges on an Ethernet segment, and are entirely transparent at the IP layer - except when they're dropping large Ethernet frames.
In that scenario, there are no ICMP messages to signal the MTU constraint, and that's the scenario I needed this tool for.
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Here is a potential implementation of path MTU discovery. It has recently been mentioned in issue #54 and #71
Some test results here:
https://github.com/acooks/icmplib/wiki
The results show that interfaces conform to the Robustness principle in that they may accept packets somewhat larger than the configured MTU, but won't send anything larger than the configured MTU.
Characterizing that asymmetric behavior was what motivated me to implement the pmtu discovery.