What is one of the main difficulties when estimating a useful TTL value?

The main difficulty when estimating a useful Time to Live (TTL) value is that it depends on the number of routers between the source and each receiver. TTL is a value that is decremented by one each time a packet is forwarded by a router, and it serves as a safeguard against packets circulating indefinitely in case of routing loops. As a result, the appropriate TTL must take into account the expected maximum number of hops (routers) a packet may traverse before reaching its destination.

Estimating that maximum number can vary significantly based on the specific network topology, the paths taken by packets, and the geographical distance involved. Each route may involve a different number of hops due to various factors, such as network design, congestion, or even dynamic changes in routing protocols. Therefore, determining a single useful TTL value that accommodates all potential routes becomes challenging, as it must ensure packets are not dropped prematurely while still being high enough to safely traverse the most complex paths in the network.

The other options mentioned do not appropriately capture the dynamic nature and the routing aspects that influence TTL.