RFC 6864 imposes various restrictions on the uniqueness of the IPv4
Identification field for non-atomic datagrams, defined as an IP datagram that
either can be fragmented (DF=0) or is already a fragment (MF=1 or positive
fragment offset). In order to be compliant, the ID field is assigned for all
non-atomic datagrams.
Add a TCP unit test that induces retransmissions and checks that the IPv4
ID field is unique every time. Add basic handling of the IP_MTU_DISCOVER
socket option so that the option can be used to disable PMTU discovery,
effectively setting DF=0. Attempting to set the sockopt to anything other
than disabled will fail because PMTU discovery is currently not implemented,
and the default behavior matches that of disabled.
PiperOrigin-RevId: 320081842
76c7bc51b7