Final answer:
In IPv4 datagrams containing UDP segments, the 'upper layer protocol' field remains constant, indicating that UDP is being used.
Step-by-step explanation:
The field in the sequence of IPv4 datagrams containing UDP segments that stays constant is the upper layer protocol.
When UDP segments are sent as part of an IPv4 datagram, the UDP protocol number in the IPv4 header is constant because it indicates the transport layer protocol being used - in this case, UDP, which has a protocol number of 17. Therefore, the IPv4 field 'Protocol' remains unchanged for all the datagrams carrying UDP segments.
Other fields such as time to live (TTL) decreases with each router hop, identification may change if IP fragmentation occurs, and the header checksum changes as each router must recompute it after modifying the TTL.
In IPv4 datagrams containing UDP segments, the 'upper layer protocol' field remains constant, indicating that UDP is being used.