Setting DSCP for IPv6 Neighbor Discovery packets

[Patrik Lahti <plahti%qnx.com@localhost>]

Hi,

I'm of the opinion that IPv6 Neighbor Discovery packets are network 
control packets and hence they should be prioritized accordingly. In 
fact, network control should probably be the highest priority.

Currently Neighbor Discovery traffic is sent using the default (Best 
Effort, i.e. zero) DSCP value, and e.g. if other higher traffic (lets 
say a lot of voice traffic) is competing for bandwidth then it may cease 
to function correctly potentially leading to all sorts of havoc.

Yes, ND is only used on the link, and no routers would look at the DSCP 
in ND messages. However, the network stack, e.g. using ALTQ may 
experience congestions locally, and link level devices (e.g. switches) 
may experience congestion too and the DSCP is often mapped into link 
layer QoS mechanisms.

What are your thoughts? Shouldn't be too hard to change right? I'm 
thinking it's simply a matter of a one line change in the various 
nd6_*_output() routines and setting IPV6_TCLASS in rtadvd/rtsold.

Should it be sysctl'able or hard coded to a particular value? What 
default? I'd lean towards 0xe0 at the moment, as it's compatible with IP 
Precedence usage and IIRC I think I've seen packet traces where it was 
used as well.

PS. The same could be said about ARP, but since ARP rides directly on 
Ethernet, it's kind of a different topic.

Cheers!
/P