The IPv4 Identification (ID) field enables fragmentation and
reassembly and, as currently specified, is required to be unique
within the maximum lifetime for all datagrams with a given
source address/destination address/protocol tuple. If enforced,
this uniqueness requirement would limit all connections to 6.4 Mbps
for typical datagram sizes. Because individual connections
commonly exceed this speed, it is clear that existing systems
violate the current specification. This document updates
the specification of the IPv4 ID field in RFCs 791, 1122,
and 2003 to more closely reflect current practice and to more
closely match IPv6 so that the field's value is defined only when a
datagram is actually fragmented. It also discusses the impact of
these changes on how datagrams are used. [STANDARDS-TRACK]