Hi Kevin, On Sun, Mar 24, 2019 at 02:23:45PM +0000, Kevin 'ldir' Darbyshire-Bryant wrote: > savedscp is a method of storing the DSCP of an ip packet into conntrack > mark. In combination with a suitable tc filter action (conndscp but may > end up being integrated into connmark) DSCP values are able to be stored > on egress and restored on ingress across links that otherwise alter or > bleach DSCP. > > This is useful for qdiscs such as CAKE which are able to shape according > to policies based on DSCP. > > Ingress classification is traditionally a challenging task since > iptables rules haven't yet run and tc filter/eBPF programs are pre-NAT > lookups, hence are unable to see internal IPv4 addresses as used on the > typical home masquerading gateway. > > The ingress problem is solved by the tc filter, but the tc people didn't > like the idea of tc setting conntrack mark values, though they are ok > with reading conntrack values and hence restoring DSCP from conntrack > marks. > > x_tables CONNMARK with the new savedscp action solves the problem of > storing the DSCP to the conntrack mark. > > It accepts 2 parameters. The mark is a 32bit value with usually one > bit set. This bit is set when savedscp saves the DSCP to the mark. > This is useful to implement a 'one shot' > iptables based classification where the 'complicated' iptables rules are > only run once to classify the connection on initial (egress) packet and > subsequent packets are all marked/restored with the same DSCP. A mark > of zero disables the setting of a status bit/s. > > The mask is a 32bit value of at least 6 contiguous bits and represents > the area where the DSCP will be stored. > > e.g. > > iptables -A QOS_MARK_eth0 -t mangle -j CONNMARK --savedscp-mark 0x01000000/0xfc000000 > > Would store the DSCP in the top 6 bits of the 32bit mark field, and use > the LSB of the top byte as the 'DSCP has been stored' marker. I'd prefer we explore how to express this in nftables, I think the main problem here is that we do not support for datatype compatibility / casting yet, hence dscp cannot be assign to the mark. But we only need to update userspace code to achieve this. Thanks.
