Add DSCP support for network QoS to iscsi target.

[rscheff]

Summary

In order to prioritize iSCSI traffic across a network,
DSCP can be used. In order not to rely on "ipfw setdscp"
or in-network reclassification, this adds the dscp value
directly to the portal group (where TCP sessions are accepted).

As the an incoming iSCSI session is first handled by ctld for
CHAP authentication and the socket - once found to match all
prerequisites - is then handed off to the in-kernel iscsi
driver without modification of the socket parameters, simply
setting up the socket here is sufficient to keep sending
outgoing iSCSI related traffic with the correct DSCP.

Test Plan

verify the IP Header DSCP value for both IPv4 and
IPv6 iSCSI traffic during CHAP authentication, and subsequent
iSCSI IOs (handled directly by the kernel driver).

[rscheff]

• prevent the ECN bits to be set.

[mav]

I haven't written that part of the code, but I think you should add its support to the uclparse.c.

Otherwise it looks like nice functionality.

usr.sbin/ctld/ctld.c

2193

It looks weird to modify the configuration while applying it. I'd say either modify it while parsing or use local variable here.

[rscheff]

• adding uclparse and using temporary dscp variable when doing setsockopt

[rscheff]

• uclparse hex values for dscp too

[rscheff]

• shift TOS predefinitions and dscp field correctly around

[trasz]

Looks good!