What is the purpose of the Time to Live (TTL) field?

• A. To carry the upper-layer protocol (e.g., TCP)
• B. To denote the packet's priority
• C. To encrypt the header
• D. To limit a packet's lifetime and prevent routing loops; each router decrements TTL

Answer: (D)

The time-to-live field is there to prevent a packet from circulating forever in the network. Each router that forwards the IPv4 packet subtracts one from this value. When the TTL reaches zero, the packet is discarded and the source can be informed (typically via an ICMP Time Exceeded message). This mechanism stops routing loops and avoids endless traffic in case of misconfigurations or path changes. TTL is not about carrying the upper-layer protocol (that’s indicated by another header field), nor about giving priority or encrypting the header, so it’s used solely to bound how long a packet can live as it traverses routers.