Hi, Can someone from Microsoft who manages the network for the Skype for Business please contact me to help resolve what I believe is a problem with the S4B network? I am experiencing PMTUD issues whereby connections with an IPv4 TCP MSS above 1492 and an IPv6 TCP MSS of above 1432 do not function. I can run a clean 1500 byte MTU on both IPv4 and IPv6 from the connection I am testing with. External validation (thus ruling out my own link) from https://wand.net.nz/pmtud/ reports that the following URLs used by Lync/S4B are failing PMTUD: meet.lync.com webdir0b.online.lync.com ---------- tbit from 2001:df0:4:4000::1:115 to 2603:1047:0:a::12 server-mss 1440, result: pmtud-fail app: http, url: https://meet.lync.com/ [ 0.010] TX SYN 64 seq = 0:0 [ 0.049] RX SYN/ACK 64 seq = 0:1 [ 0.049] TX 60 seq = 1:1 [ 0.049] TX 373 seq = 1:1(313) [ 0.097] RX 1500 seq = 1:314(1440) [ 0.097] RX 1500 seq = 1441:314(1440) [ 0.097] RX 868 seq = 2881:314(808) [ 0.097] TX PTB 1280 mtu = 1280 [ 0.097] TX 60 seq = 314:1 [ 0.399] RX 1500 seq = 1:314(1440) [ 0.399] TX PTB 1280 mtu = 1280 [ 0.994] RX 1500 seq = 1:314(1440) [ 0.994] TX PTB 1280 mtu = 1280 [ 2.197] RX 1500 seq = 1:314(1440) [ 2.197] TX PTB 1280 mtu = 1280 [ 4.603] RX 1500 seq = 1:314(1440) tbit from 130.217.250.115 to 52.113.65.78 server-mss 1460, result: pmtud-fail app: http, url: https://meet.lync.com/ [ 0.009] TX SYN 44 seq = 0:0 c5c9 [ 0.152] RX SYN/ACK 44 seq = 0:1 56ef [ 0.152] TX 40 seq = 1:1 c5ca [ 0.153] TX 353 seq = 1:1(313) c5cb DF [ 0.299] RX 808 seq = 2921:314(768) 56f7 DF [ 0.299] RX 1500 seq = 1:314(1460) 56f5 DF [ 0.299] RX 1500 seq = 1461:314(1460) 56f6 DF [ 0.299] TX 40 seq = 314:1 c5cc [ 0.299] TX PTB 56 mtu = 1280 [ 0.763] RX 1500 seq = 1:314(1460) 5720 DF [ 0.764] TX PTB 56 mtu = 1280 [ 1.592] RX 1500 seq = 1:314(1460) 5750 DF [ 1.592] TX PTB 56 mtu = 1280 [ 3.232] RX 1500 seq = 1:314(1460) 57dc DF [ 3.232] TX PTB 56 mtu = 1280 [ 6.514] RX 1500 seq = 1:314(1460) 5910 DF --------- This is in the Asia Pacific region. Thanks, Reuben Farrelly