On 20/08/16 17:49, Bruce Simpson wrote:
- Providing a mechanism for ip_input() to signal to udp_input() that the
packet was addressed to an L3 broadcast address would require
rototilling the pr_input interface, and I'd have to carefully ensure
that if anything might interpose itself between the two layers (IPSec?)
that the flag would have to be passed through correctly.
(We had to do something similar for ILNPv6, because of how IPv6 input
works, so it allocates an unused bit from the IPv6 flow label.)
In case anyone's interested, IPSEC is one big reason we had to do this.
Performance was another; without marking ILNPv6 traffic, demultiplexing
an ILNPv6 packet -- even 'just enough' to forward it -- would have
required processing all IPv6 extension headers, and additional state
lookups.
Using a bit from the flow label means ILNPv6 routers don't need to do
any of that, and it got translated into an M_ILNP mbuf flag internally.
I shudder to think what gymnastics the FreeBSD stack may have to endure
to accommodate modern Ethernet switching capabilities, though.
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