Calculating a two byte header in message payload

Good catch.

Your modification look great. Only a couple suggestions:

Here is what it would look like:

when CLIENT_ACCEPTED {
    TCP::collect 2
    set header_collected "false"
    }
}
when CLIENT_DATA {

    if { not ($header_collected) } {
     Comment out one of the two binary scan lines below.
     use this form if lower order byte is last (big endian order)
    binary scan [TCP::payload] S message_length
     use this form if lower order byte is first (little endian order)
     binary scan [TCP::payload] s message_length

     as the message length collected does not include itself add 2 more onto the result
    

    if { $message_length > 0 } {
        set header_collected "true"

         check if we already have the required data in the buffer if so 
         capture the payload in vars and release it
        if {[TCP::payload length] == [expr { $message_length + 2 }] } {
             set vars with payload info for HSL logging later
            set my_req_msg_payload [TCP::payload]
            set my_req_msg_length [TCP::payload length]
             release the data already collected
            TCP::release
        } else {
             collect data with the required length
            TCP::collect $message_length            
        }
       }
    } else {
       if we are here the actual message is collected.
        set my_req_msg_payload [TCP::payload]
        set my_req_msg_length [TCP::payload length]
        TCP::release
    }
}