net: Introduce IPV6_DSCP(), IPV6_ECN() and IPV6_TRAFFIC_CLASS() macros

[kp]

Summary

Introduce convenience macros to retrieve the DSCP, ECN or traffic class
bits from an IPv6 header.

Use them where appropriate.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")

[ae]

LGTM.

[rscheff]

there are siimilar constructs in the tcp stack, when checking for ECN (extracting the "common, legacy" TOS byte first:

tcp_input.c:692: iptos = (ntohl(ip6->ip6_flow) >> 20) & 0xff;
tcp_lro.c:1549: iptos = (ntohl(ip6->ip6_flow) >> 20) & 0xff;

perhaps replace those with
iptos = IPV6_DSCP(ip6) | IPV6_ECN(ipv6);
in this patch too?

sys/netinet6/ip6_output.c

661

typo?

[kp]

In D29056#650734, @rscheff wrote:

perhaps replace those with
iptos = IPV6_DSCP(ip6) | IPV6_ECN(ipv6);
in this patch too?

We may as well, while we're at it.
I've introduced IPV6_TRAFFIC_CLASS() for that (IPV6_TCLASS already exists), and I found a few more places where it would be suitable.

sys/netinet6/ip6_output.c

661

Whoops. Good catch. I really should have caught that myself.