TCP payload persistence (convert Cisco ACE l4-payload stickiness)

Digging deeper into the issue, I thought about something like that...

when CLIENT_ACCEPTED {
  TCP::collect 50
}

when CLIENT_DATA {
  set payload_first_50 [TCP::payload]
  set persist_string [substr $payload_first_50 29 20]
   log local0. "Fetched $persist_string as conn ID for client [IP::client_addr]."
  persist uie $persist_string 60
  TCP::release
}

... but I'm not sure if the data returned by [TCP::payload] is to be considered a string, thus being unsure whether using the substr method on it will return valid data.