Forum Discussion

kridsana's avatar
kridsana
Icon for Cirrocumulus rankCirrocumulus
Dec 26, 2012

How to clear Don't Fragment (DF) bit

there is some virtual server that have a problem that     packet segment lost when MTU = 1500     so i want to clear DF bit to fix this problem , and how to clear it?     than...
config
design
kridsana's avatar
kridsana
Icon for Cirrocumulus rankCirrocumulus
Jan 21, 2013
Posted By nitass on 01/18/2013 11:53 PM

after F5 reassemble TCP , it send packet 1500 to client but checkpoint drop it and return ICMP fragmentation needed to F5 with MTU next hop 1476. don't you see bigip resend packet with lower length?

e.g.

in mt-0110-1000.pcap, packet 133 has 1460 tcp length. packet 136 and 137 have 1396 and 64 tcp length respectively. 1460 is 1396 + 64. 117.176.110.113 mac address belongs to f5.

No. Time Delta Time Source Src port Destination Protocol Dst port Window BiF Vlan id Length Info 133 2012-10-01 10:38:48.570136 0.000000 10.153.1.13 80 117.176.110.113 TCP 4394 6848 1460 109 1518 [TCP segment of a reassembled PDU] 135 2012-10-01 10:38:48.570136 0.000000 10.151.25.1 80 10.153.1.13 ICMP 4394 109 74 Destination unreachable (Fragmentation needed) 136 2012-10-01 10:38:48.570136 0.000000 10.153.1.13 80 117.176.110.113 TCP 4394 6848 1621 109 1454 [TCP Out-Of-Order] 80 > 4394 [ACK] Seq=3441923333 Ack=2282044611 Win=6848 Len=1396 137 2012-10-01 10:38:48.570136 0.000000 10.153.1.13 80 117.176.110.113 TCP 4394 6848 1621 109 122 [TCP Out-Of-Order] [TCP segment of a reassembled PDU]

I see, packet seem appear out-of-order and it's confused.

e.g. packet 136 is retransmit in 1436 byte but packet 134 is HTTP/200 OK reassemble of packet 133,137,134 (not packet 136??) but this seem fine.

thank you