4.2.2.9 Time to Live: RFC 791 Section 3.2

Time to Live (TTL) handling for packets originated or received by the router is governed by [INTRO:2]; this section changes none of its stipulations. However, since the remainder of the IP Protocol section of [INTRO:2] is rewritten, this section is as well.

Note in particular that a router MUST NOT check the TTL of a packet except when forwarding it.

A router MUST NOT originate or forward a datagram with a Time-to-Live (TTL) value of zero.

A router MUST NOT discard a datagram just because it was received with TTL equal to zero or one; if it is to the router and otherwise valid, the router MUST attempt to receive it.

On messages the router originates, 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 number SHOULD exceed the typical internet diameter, and current wisdom suggests that it should exceed twice the internet diameter to allow for growth. Current suggested values are normally posted 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 router is required to reduce the TTL field by at least one.

TTL expiration is intended to cause datagrams to be discarded by routers, but not by the destination host. Hosts that act as routers by forwarding datagrams must therefore follow the router's 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 default 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. As of this writing, messages crossing the United States frequently traverse 15 to 20 routers; this argues for a default TTL value in excess of 40, and 64 is a common value.