Hop Count Sample Clauses

Hop Count. ‌ The Hop Count is the number of IP links traversed by a packet between its source s and destination d at the IP layer. Since each crossed router will decrement the IP Time- To-Live (TTL) field, by comparing the initial TTL value with the received one, the destination node can easily count the number of routers along the path. Hop(s, d) = TTL(s) − TTL(d) (2.1) In more details, the TTL field is set by the sender of the IP datagram, and reduced by every router on the route to its destination. If the TTL field reaches zero before the datagram arrives at its destination, then the datagram is discarded and an ICMP error datagram is sent back to the sender. The purpose of the TTL field is to avoid a situation in which an undeliverable datagram keeps circulating on an Internet system, and such a system eventually becoming swamped by such immortal datagrams. In practice, the TTL field is reduced by one on every hop. To reflect this practice, the field is named hop limit in IPv6. It is therefore easy to determine the hop count of a path from a source node s to a destination node d, provided that the initial TTL value is known. Unfortunately, no standard value is provided by any RFC, being it selection left open to any value the implementation may like. In addition, the OS usually offer the chance to the user to modify the initial TTL value, both as a system wide option, and as a single socket option. Notice also that some middle boxes (e.g., NAT boxes, Firewalls, etc.) may actually change the TTL set in the original IP packet. Given this, it is much better to explicitly communicate the TTL set in a packet to the destination d, so that no error can arise that will affect the hop count errors.