Which field in the IPv4 header is used to prevent a packet from traversing a network endlessly?
The field in the IPv4 header that is used to prevent a packet from traversing a network endlessly is called the Time to Live (TTL) field.
The TTL field is a 1-byte (8-bit) field that is initially set by the sender of the packet. Each time the packet passes through a router, the router decrements the TTL value by 1. If the TTL value reaches zero, the router discards the packet and sends an Internet Control Message Protocol (ICMP) Time Exceeded message back to the original sender.
This mechanism helps to prevent packets from being stuck in a routing loop, where they continuously traverse a network without ever reaching their destination. By setting a finite TTL value, the IP protocol ensures that packets have a limited lifespan and are eventually dropped if they don’t reach their destination within a certain number of hops.
In addition to preventing endless traversal, the TTL field is also used for other purposes, such as measuring network round-trip times using the Internet Control Message Protocol (ICMP) Echo Request and Echo Reply messages (commonly known as “ping”), as well as for path MTU (Maximum Transmission Unit) discovery.
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