Hi, Not able to understand 16 byetes in ESP packet present after sequence no and before Original IP header while doing tunnel mode Ipsec with ESP. Details are as below. I am trying to achieve Ipsec functionality using fast-path application which will do encryption/decryption using some hardware(Cavium) specific API. This application will by-pass the IP layer of kernel.. Keys for start-up are pre-shared. Communication is done between two machine A and B. On Machine A running i386 linux, SA/SP database are updated using setkey utility and packets is encrypted/decrypted using kernel Ipsec. On Machine B Cavium h/w, keys are pre-shared to application performing Ipsec functionlity... Example: M/c A configuration: add 50.50.50.51 50.50.50.53 esp 15701 -E aes-cbc "0123456789abcdef"; spdadd 10.10.10.20 10.10.10.21 any -P out ipsec esp/tunnel/50.50.50.51 50.50.50.53/require I am able to decrypt received packets on machine B send by M/c A and send encrypted packet to M/c A. Issue: 1. Not able to find what are 16 bytes present after sequence no in ESP header and before original IP header representing... Decrypted Packet on machine B is like below Ethernet header 14 bytes Outer Ip header 20 bytes ESP header SPI 4 bytes Seq no 4 bytes Some data 16 bytes ??????? Original IP header 20 bytes UDP header Payload data Padding Pad lenght Next Ip header 2. Packets send from machine B are encrypted and received as ESP packet on machine A.. Not sure if decryption is happening fine...Seems packets are dropped at IP layer.. Is there way to confirm if packet are decrypted fine by kernel IPSEC... Encrypted packet send by Machine B is having encrypted payload(of original IP header plus data) after Sequence number of ESP header... Seems 16 bytes mentioned above play role for successful decryption at machine A running Linux IPSEC Any Inputs for same will be appreciated for same Cheers Mukesh