MTU

Maximum Transmission Unit - of packets, normally for ethernet 1500
Links: linux/PacketOffloadingMtu
Ping with dont fragment to test mtu
```
ping -M do -s 1472 example.com
```

2018 Ubuntu 14.04 (old) could not connect to another machine, doing a tcpdump found

22:31:17.504887 IP 10.128.155.77.ssh > 10.128.145.23.46488: Flags [.], seq 45:1493, ack 2013, win 257, options [nop,nop,TS val 2677742 ecr 60427352], length 1448
22:31:17.505381 IP 172.16.6.2 > 10.128.155.77: ICMP 10.128.145.23 unreachable - need to frag (mtu 1436), length 556

Ubuntu was ignoring the ICMP and thus the ssh just hanged.

Fix1 - static route with mtu

# ip route add 10.118.145.0/27 via 10.118.155.65 mtu 1400

Fix2 Root Cause - uRPF explained here http://mellowd.co.uk/ccie/?p=5662 a security feature.
- set sysctl failed
```
net.ipv4.conf.default.rp_filter = 0
 net.ipv4.conf.all.rp_filter = 0
```

Fix3 - Worked ( from https://blog.cloudflare.com/path-mtu-discovery-in-practice/ )

sudo sysctl -w net.ipv4.tcp_mtu_probing=1
- make permanent by adding to /etc/sysctl.d/10-network-security.conf

3) Enable smart MTU black hole detection.

RFC4821 proposes a mechanism to detect ICMP black holes and tries to adjust the path MTU in a smart way. To enable this on Linux type:

echo 1 > /proc/sys/net/ipv4/tcp_mtu_probing
echo 1024 > /proc/sys/net/ipv4/tcp_base_mss
The second setting bumps the starting MSS used in discovery from a miserable default of 512 bytes to an RFC4821 suggested 1,024.