MTU probing

[rhclopes]

Should perfSONAR automatically configure net.ipv4.tcp_mtu_probing equal to 1 or 2.

ESnet fasterdata also recommends setting net.core.optmem_max to 1024K. I have values of 824K (Red Hat default), 1024K and 4096K without noticing any difference.

[arlake228]

Part of general tuning mentioned in #511