IFB is intended to replace IMQ.
Advantage over current IMQ; cleaner in particular in in SMP;
with a _lot_ less code.

Known IMQ/IFB USES
------------------

As far as i know the reasons listed below is why people use IMQ.
It would be nice to know of anything else that i missed.

1) qdiscs/policies that are per device as opposed to system wide.
IFB allows for sharing.

2) Allows for queueing incoming traffic for shaping instead of
dropping. I am not aware of any study that shows policing is
worse than shaping in achieving the end goal of rate control.
I would be interested if anyone is experimenting.

3) Very interesting use: if you are serving p2p you may wanna give
preference to your own localy originated traffic (when responses come back)
vs someone using your system to do bittorent. So QoSing based on state
comes in as the solution. What people did to achive this was stick
the IMQ somewhere prelocal hook.
I think this is a pretty neat feature to have in Linux in general.
(i.e not just for IMQ).
But i wont go back to putting netfilter hooks in the device to satisfy
this.  I also dont think its worth it hacking ifb some more to be
aware of say L3 info and play ip rule tricks to achieve this.
--> Instead the plan is to have a contrack related action. This action will
selectively either query/create contrack state on incoming packets.
Packets could then be redirected to ifb based on what happens -> eg
on incoming packets; if we find they are of known state we could send to
a different queue than one which didnt have existing state. This
all however is dependent on whatever rules the admin enters.

At the moment this 3rd function does not exist yet. I have decided that
instead of sitting on the patch for another year, to release it and then
if theres pressure i will add this feature.

An example, to provide functionality that most people use IMQ for below:

--------
export TC="/sbin/tc"

$TC qdisc add dev ifb0 root handle 1: prio
$TC qdisc add dev ifb0 parent 1:1 handle 10: sfq
$TC qdisc add dev ifb0 parent 1:2 handle 20: tbf rate 20kbit buffer 1600 limit 3000
$TC qdisc add dev ifb0 parent 1:3 handle 30: sfq
$TC filter add dev ifb0 protocol ip pref 1 parent 1: handle 1 fw classid 1:1
$TC filter add dev ifb0 protocol ip pref 2 parent 1: handle 2 fw classid 1:2

ifconfig ifb0 up

$TC qdisc add dev eth0 ingress

# redirect all IP packets arriving in eth0 to ifb0
# use mark 1 --> puts them onto class 1:1
$TC filter add dev eth0 parent ffff: protocol ip prio 10 u32 \
match u32 0 0 flowid 1:1 \
action ipt -j MARK --set-mark 1 \
action mirred egress redirect dev ifb0

--------


Run A Little test:

from another machine ping so that you have packets going into the box:
-----
[root@jzny action-tests]# ping 10.22
PING 10.22 (10.0.0.22): 56 data bytes
64 bytes from 10.0.0.22: icmp_seq=0 ttl=64 time=2.8 ms
64 bytes from 10.0.0.22: icmp_seq=1 ttl=64 time=0.6 ms
64 bytes from 10.0.0.22: icmp_seq=2 ttl=64 time=0.6 ms

--- 10.22 ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.6/1.3/2.8 ms
[root@jzny action-tests]#
-----
Now look at some stats:

---
[root@jmandrake]:~# $TC -s filter show parent ffff: dev eth0
filter protocol ip pref 10 u32
filter protocol ip pref 10 u32 fh 800: ht divisor 1
filter protocol ip pref 10 u32 fh 800::800 order 2048 key ht 800 bkt 0 flowid 1:1
  match 00000000/00000000 at 0
        action order 1: tablename: mangle  hook: NF_IP_PRE_ROUTING
        target MARK set 0x1
        index 1 ref 1 bind 1 installed 4195sec  used 27sec
         Sent 252 bytes 3 pkts (dropped 0, overlimits 0)

        action order 2: mirred (Egress Redirect to device ifb0) stolen
        index 1 ref 1 bind 1 installed 165 sec used 27 sec
         Sent 252 bytes 3 pkts (dropped 0, overlimits 0)

[root@jmandrake]:~# $TC -s qdisc
qdisc sfq 30: dev ifb0 limit 128p quantum 1514b
 Sent 0 bytes 0 pkts (dropped 0, overlimits 0)
qdisc tbf 20: dev ifb0 rate 20Kbit burst 1575b lat 2147.5s
 Sent 210 bytes 3 pkts (dropped 0, overlimits 0)
qdisc sfq 10: dev ifb0 limit 128p quantum 1514b
 Sent 294 bytes 3 pkts (dropped 0, overlimits 0)
qdisc prio 1: dev ifb0 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 504 bytes 6 pkts (dropped 0, overlimits 0)
qdisc ingress ffff: dev eth0 ----------------
 Sent 308 bytes 5 pkts (dropped 0, overlimits 0)

[root@jmandrake]:~# ifconfig ifb0
ifb0    Link encap:Ethernet  HWaddr 00:00:00:00:00:00
          inet6 addr: fe80::200:ff:fe00:0/64 Scope:Link
          UP BROADCAST RUNNING NOARP  MTU:1500  Metric:1
          RX packets:6 errors:0 dropped:3 overruns:0 frame:0
          TX packets:3 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:32
          RX bytes:504 (504.0 b)  TX bytes:252 (252.0 b)
-----

You send it any packet not originating from the actions it will drop them.
[In this case the three dropped packets were ipv6 ndisc].

cheers,
jamal