A host MUST NOT send a datagram with a Time-to-Live (TTL) value of zero.
A host MUST NOT discard a datagram just because it was received with TTL less than 2.
The IP layer MUST provide a means for the transport layer to set the TTL field of every datagram that is sent. When a fixed TTL value is used, it MUST be configurable. The current suggested value will be published in the "Assigned Numbers" RFC.
The TTL field has two functions: limit the lifetime of TCP segments (see RFC-793 [TCP:1], p. 28), and terminate Internet routing loops. Although TTL is a time in seconds, it also has some attributes of a hop- count, since each gateway is required to reduce the TTL field by at least one.
The intent is that TTL expiration will cause a datagram to be discarded by a gateway but not by the destination host; however, hosts that act as gateways by forwarding datagrams must follow the gateway rules for TTL.
A higher-layer protocol may want to set the TTL in order to implement an "expanding scope" search for some Internet resource. This is used by some diagnostic tools, and is expected to be useful for locating the "nearest" server of a given class using IP multicasting, for example. A particular transport protocol may also want to specify its own TTL bound on maximum datagram lifetime.
A fixed value must be at least big enough for the Internet "diameter," i.e., the longest possible path. A reasonable value is about twice the diameter, to allow for continued Internet growth.