Number of hops a packet on a network can take before it is considered lost or in an infinite loop and should be dropped. See Also: TTL and TCP/IP

A field in a datagram, or IP packet header. It specifies how long, in seconds, the datagram is allowed to remain in the system. The computer sets a maximum time wheb injecting a datagram into the internet; the routers and hosts that process the datagram decrement this field (by 1) with each hop and remove it when the field reaches zero. If the packet is discarded, the router sends an error message back to the source. Acts as a hop cont. TTL was originally specified in seconds, but this is difficult to implement. The recommended default TTL is 64, though values like 15 and 32 are occasionally used.

The Time to Live (TTL) header field in IPv4 packets is an unsigned 8-bit integer, which means the value can range from 0 to 255. The default values range between 16 and 255 on most hosts and routers. Common values are (in approximately decreasing order of frequency): 255, 60, 128, 64, 30, 254, 32, 100, and 16. Less common values include: 192, 200, 180, and 160.

Generally: Most Windows boxes (including Windows NT 4.0, Windows 98, and Windows 2000) use 128. Cisco, Sun, and Cabletron machines defaults to 255. Lucent Cajun Switches default to 30. Most Hewlett Packard printers use 60. NCR's MP-RAS (remote access server) uses 64, as does Novell and some HP printers. Tektronic printers use 32. Xerox DocuPrint servers use 100, and BayStack uses 16.

The TTL field is used extensively by traceroute, although that was not the original purpose of the header field. While the original RFC calls for it to be decremented every second, later RFCs have added the constraint that it be decremented once for every router the packet goes through. This is a design problem, as this means the checksum must recomputed, as the TTL field is part of the checksum.

IPv6 has a similar field, still 8 bits, called Hop Limit (I guess the notion of dying disturbed the designers). It serves the same purpose, which is to keep packets from looping on the Internet forever. You might ask why does this occur, for which the answer is: the internet is big.

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