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Setting DSCP for IPv6 Neighbor Discovery packets


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.


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