IP Time to Live (TTL) and Hop Limit Basics | Search for a title, author or keyword | ||||||||
IP Time to Live (TTL) and Hop Limit Basics by Steven Iveson. To ensure IP packets have a limited lifetime on the network all IP packets have an 8 bit Time to Live (IPv4) or Hop Limit (IPv6) header field and value which specifies the maximum number of layer three hops (typically routers) that can be traversed on the path to their destination. Each time the packet arrives at a layer three network device (a hop) the value is reduced by one before it is routed onward. When the value eventually reaches one the packet is discarded by the device that receives it (as the value will be reduced to zero). Whilst this won’t prevent network issues caused by a routing loop or similar, it reduces their impact and may help avoid router failures. As it is an 8 bit field, the maximum possible value is 255 (11111111 in binary). In the unexpected event that a router receives an IPv6 packet with a Hop Limit of zero and that packet is not destined for the router itself, it is discarded as you’d expect. If a router (or any IPv6 host) receives an IPv6 packet with a Hop Limit of zero or one and the host is the destination of that packet (it won’t be routed onwards), the packet is accepted. The IPv4 RFC is far less explicit and simply states that if this field is zero the ‘datagram’ must be destroyed.
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