 However, a problem persists in that some caching DNS nameservers set their own TTLs regardless of the authoritative records, thus it cannot be guaranteed that all downstream DNS servers have the new records after the TTL has expired.Time to live may also be expressed as a date and time on which a record expires.The codice_1 header in HTTP responses, the codice_2 header field in both requests and responses and the codice_3 field in HTTP cookies express time-to-live in this way.