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 post