TCP MD5 signature:
This MD5 signature is a MD5 hash function of following values.

1. TCP Pseudo-header (which consist of some properties from tcp header and ipv4-header)
2. TCP  header (excluding options and checksum)
3. TCP data
4. Connection key 

MD5 hash is made and kept in the option 19th field of tcp header when the segment is received by the receiver then receiving device recalculates that segment's MD5 hash using the above values and if recalculated value matches with the received hash then only authentication will be passed, else authentication fails.