Re: [PATCH net-next] ipv4: use zero IPID for atomic datagrams on connected sockets

[David Ahern <dsahern@kernel.org>]

On 9/29/26 10:38 AM, Eric Dumazet wrote:
> ip_select_ident_segs() uses the per-socket private generator
> for connected sockets, even for packets with IP_DF set.
> 
> This was historically done to work around buggy Windows95/2000
> VJ header compression implementations, dropping every other
> packet in a TCP stream when the IP ID field did not change.
> 
> RFC 6864 section 4.2 states that "Originating sources MAY set the
> IPv4 ID field of atomic datagrams to any value".
> 
> Packets with IP_DF set and skb->ignore_df cleared can not be
> fragmented, neither locally (ip_fragment() refuses to do so)
> nor by routers on the path.
> 
> Set their IPID to zero, like we already do for unconnected sockets
> and in ip_build_and_send_pkt(). FreeBSD also does the same by
> default (net.inet.ip.rfc6864 = 1).
> 
> Connected sockets still use their private generator for packets
> without IP_DF, or with skb->ignore_df set.
> 
> This avoids an atomic operation on a shared cache line for
> connected UDP sockets using IP_PMTUDISC_DO/IP_PMTUDISC_PROBE,
> and TCP no longer touches inet->inet_id in the fast path.
> 
> Minor side effects: IP IDs can no longer be used to distinguish
> network duplicates from TCP retransmits, or to correlate packet
> captures taken at different points. OS fingerprints will also change.
> 
> Signed-off-by: Eric Dumazet <edumazet@kernel.org>
> ---
>  include/net/ip.h | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 

Reviewed-by: David Ahern <dsahern@kernel.org>