The TTL can be found in which protocol header?

UDP headers contain only source port, destination port, length, and checksum fields - there is no TTL field defined in the UDP header specification.

TCP headers contain sequence numbers, acknowledgment numbers, control flags, window size, and checksum fields, but no TTL field.

TTL is a dedicated 8-bit field in the IPv4 header as defined in RFC 791, set by the source host and decremented by each forwarding router; in IPv6, this field is renamed 'Hop Limit' but serves the same purpose. Because TTL is a network-layer concept controlling packet lifetime across internetworks, it belongs in the IP header rather than any higher-layer or encapsulated protocol header.

ICMP messages are encapsulated inside IP packets and rely on the outer IP header for TTL; the ICMP header itself does not include or define a TTL field.