I am currently using the ultimate-tc script from http://lartc.org/howto/lartc.cookbook.ultimate-tc.html
and I want to make sure that internet radio packets (mp3 streaming audio)
will always get through no matter what. I have added some iptables commands
like this:
iptables -A OUTPUT -t mangle -p tcp --dport 8000 -j TOS --set-tos Minimize-Delay
iptables -A OUTPUT -t mangle -p tcp --sport 8000 -j TOS --set-tos Minimize-Delay
OUTPUT only sees locally generated packets.
with the aim of marking the streaming audio packets so that they will get a higher priority: but I'm not sure if this is needed or exactly how it works!
Some audio streams come in with the incoming packets marked [tos 0x40] and the outgoing packets marked [tos 0x10] (according to tcpdump) but not all.
I would use MARK as other traffic may have TOS set, see below.
The ultimate-tc script ends with these ingress rules:
########## downlink #############
# slow downloads down to somewhat less than the real speed to prevent # queuing at our ISP. Tune to see how high you can set it.
# ISPs tend to have *huge* queues to make sure big downloads are fast
#
# attach ingress policer:
tc qdisc add dev $DEV handle ffff: ingress
# filter *everything* to it (0.0.0.0/0), drop everything that's # coming in too fast:
tc filter add dev $DEV parent ffff: protocol ip prio 50 u32 match ip src \ 0.0.0.0/0 police rate ${DOWNLINK}kbit burst 10k drop flowid :1
This will drop packets to keep the download rate just below the maximum capacity of the link: which will keep the ISP's queue empty and improve latency. But I am concerned that if there are a *lot* of other download streams going at the same time as my audio stream, then these rules may drop lots of packets from the audio stream and cause it to skip.
Should I add rules to drop audio stream packets at ${DOWNLINK}kbit rate and drop all other traffic at $[9*$DOWNLINK/10]kbit rate, in the same way that ultimate-tc does for outgoing traffic? If so, what should the rules look like?
There are lots of complicated things you can do with policers/u32 but I have no experience.
First thoughts are to mark all that aren't -sport 8000 and change the police rule to police to police marked.
iptables -A PREROUTING -t mangle -p tcp ! --sport 8000 -j MARK --set-mark 1
tc qdisc add dev $DEV handle ffff: ingress
tc filter add dev $DEV parent ffff: protocol ip prio 1 handle 1 fw police rate ${DOWNLINK}kbit burst 10k drop flowid :1
I haven't tested that.
Something else I don't understand about ultimate-tc is that the high priority
class gets a rate of ${UPLINK}kbit and the low priority class gets
$[9*$UPLINK/10]kbit: but doesn't the rate refer to traffic *in that class*.
Traffic-Control-HOWTO Section 7.1.5. (Rules) says:
"Ideally, the sum of the rates of the children classes would match the rate of
the parent class, allowing the parent class to distribute leftover bandwidth
(ceil - rate) among the children classes." but this isn't the case for the
ultimate-tc script.
I don't do it like that - I use ceil and like my rates to add up - but I suppose it works OK.
Andy.
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