When discarding a packet with a TTL or Hop Limit of one or zero the router in question may (it’s not mandatory) send an ICMP error message to the source as follows: Here’s what the Time to Live and Hop Limit field look like in a standard IPv4 and IPv6 header respectively: Hence, the TTL is described as a “self destruct time limit”. In any case, based on this logic, in theory the maximum time a packet can exist in the network is 4.25 minutes (255 seconds). There is no way of knowing how long any particular host will take to process a packet and most will do so in far less than a second. The IPv4 RFC states that TTL is measured in seconds but acknowledges this is an imperfect measure. The IPv4 RFC is far less explicit and simply states that if this field is zero the ‘datagram’ must be destroyed. 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.
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. As it is an 8 bit field, the maximum possible value is 255 (11111111 in binary). Whilst this won’t prevent network issues caused by a routing loop or similar, it reduces their impact and may help avoid router failures. When the value eventually reaches one the packet is discarded by the device that receives it (as the value will be reduced to zero). Each time the packet arrives at a layer three network device (a hop) the value is reduced by one before it is routed onward. 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.
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