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1
2This documented is slightly dated but should give you idea of how things
3work.
4
5What is it?
6-----------
7
8An extension to the filtering/classification architecture of Linux Traffic
9Control.
10Up to 2.6.8 the only action that could be "attached" to a filter was policing.
11i.e you could say something like:
12
13-----
14tc filter add dev lo parent ffff: protocol ip prio 10 u32 match ip src \
15127.0.0.1/32 flowid 1:1 police mtu 4000 rate 1500kbit burst 90k
16-----
17
18which implies "if a packet is seen on the ingress of the lo device with
19a source IP address of 127.0.0.1/32 we give it a classification id  of 1:1 and
20we execute a policing action which rate limits its bandwidth utilization
21to 1.5Mbps".
22
23The new extensions allow for more than just policing actions to be added.
24They are also fully backward compatible. If you have a kernel that doesnt
25understand them, then the effect is null i.e if you have a newer tc
26but older kernel, the actions are not installed. Likewise if you
27have a newer kernel but older tc, obviously the tc will use current
28syntax which will work fine. Of course to get the required effect you need
29both newer tc and kernel. If you are reading this you have the
30right tc ;->
31
32A side effect is that we can now get stateless firewalling to work with tc.
33Essentially this is now an alternative to iptables.
34I wont go into details of my dislike for iptables at times, but
35scalability is one of the main issues; however, if you need stateful
36classification - use netfilter (for now).
37
38This stuff works on both ingress and egress qdiscs.
39
40Features
41--------
42
431) new additional syntax and actions enabled. Note old syntax is still valid.
44
45Essentially this is still the same syntax as tc with a new construct
46"action". The syntax is of the form:
47tc filter add <DEVICE> parent 1:0 protocol ip prio 10 <Filter description>
48flowid 1:1 action <ACTION description>*
49
50You can have as many actions as you want (within sensible reasoning).
51
52In the past the only real action was the policer; i.e you could do something
53along the lines of:
54tc filter add dev lo parent ffff: protocol ip prio 10 u32 \
55match ip src 127.0.0.1/32 flowid 1:1 \
56police mtu 4000 rate 1500kbit burst 90k
57
58Although you can still use the same syntax, now you can say:
59
60tc filter add dev lo parent 1:0 protocol ip prio 10 u32 \
61match ip src 127.0.0.1/32 flowid 1:1 \
62action police mtu 4000 rate 1500kbit burst 90k
63
64" generic Actions" (gact) at the moment are:
65{ drop, pass, reclassify, continue}
66(If you have others, no listed here give me a reason and we will add them)
67+drop says to drop the packet
68+pass and ok (are equivalent) says to accept it
69+reclassify requests for reclassification of the packet
70+continue requests for next lookup to match
71
722)In order to take advantage of some of the targets written by the
73iptables people, a classifier can have a packet being massaged by an
74iptable target. I have only tested with mangler targets up to now.
75(infact anything that is not in the mangling table is disabled right now)
76
77In terms of hooks:
78*ingress is mapped to pre-routing hook
79*egress is mapped to post-routing hook
80I dont see much value in the other hooks, if you see it and email me good
81reasons, the addition is trivial.
82
83Example syntax for iptables targets usage becomes:
84tc filter add ..... u32 <u32 syntax> action ipt -j <iptables target syntax>
85
86example:
87tc filter add dev lo parent ffff: protocol ip prio 8 u32 \
88match ip dst 127.0.0.8/32 flowid 1:12 \
89action ipt -j mark --set-mark 2
90
91NOTE: flowid 1:12 is parsed flowid 0x1:0x12.  Make sure if you want flowid
92decimal 12, then use flowid 1:c.
93
943) A feature i call pipe
95The motivation is derived from Unix pipe mechanism but applied to packets.
96Essentially take a matching packet and pass it through
97action1 | action2 | action3 etc.
98You could do something similar to this with the tc policer and the "continue"
99operator but this rather restricts it to just the policer and requires
100multiple rules (and lookups, hence quiet inefficient);
101
102as an example -- and please note that this is just an example _not_ The
103Word Youve Been Waiting For (yes i have had problems giving examples
104which ended becoming dogma in documents and people modifying them a little
105to look clever);
106
107i selected the metering rates to be small so that i can show better how
108things work.
109
110The script below does the following:
111- an incoming packet from 10.0.0.21 is first given a firewall mark of 1.
112
113- It is then metered to make sure it does not exceed its allocated rate of
1141Kbps. If it doesnt exceed rate, this is where we terminate action execution.
115
116- If it does exceed its rate, its "color" changes to a mark of 2 and it is
117then passed through a second meter.
118
119-The second meter is shared across all flows on that device [i am suprised
120that this seems to be not a well know feature of the policer; Bert was telling
121me that someone was writing a qdisc just to do sharing across multiple devices;
122it must be the summer heat again; weve had someone doing that every year around
123summer  -- the key to sharing is to use a operator "index" in your policer
124rules (example "index 20"). All your rules have to use the same index to
125share.]
126
127-If the second meter is exceeded the color of the flow changes further to 3.
128
129-We then pass the packet to another meter which is shared across all devices
130in the system. If this meter is exceeded we drop the packet.
131
132Note the mark can be used further up the system to do things like policy
133or more interesting things on the egress.
134
135------------------ cut here -------------------------------
136#
137# Add an ingress qdisc on eth0
138tc qdisc add dev eth0 ingress
139#
140#if you see an incoming packet from 10.0.0.21
141tc filter add dev eth0 parent ffff: protocol ip prio 1 \
142u32 match ip src 10.0.0.21/32 flowid 1:15 \
143#
144# first give it a mark of 1
145action ipt -j mark --set-mark 1 index 2 \
146#
147# then pass it through a policer which allows 1kbps; if the flow
148# doesnt exceed that rate, this is where we stop, if it exceeds we
149# pipe the packet to the next action
150action police rate 1kbit burst 9k pipe \
151#
152# which marks the packet fwmark as 2 and pipes
153action ipt -j mark --set-mark 2 \
154#
155# next attempt to borrow b/width from a meter
156# used across all flows incoming on eth0("index 30")
157# and if that is exceeded we pipe to the next action
158action police index 30 mtu 5000 rate 1kbit burst 10k pipe \
159# mark it as fwmark 3 if exceeded
160action ipt -j mark --set-mark 3 \
161# and then attempt to borrow from a meter used by all devices in the
162# system. Should this be exceeded, drop the packet on the floor.
163action police index 20 mtu 5000 rate 1kbit burst 90k drop
164---------------------------------
165
166Now lets see the actions installed with
167"tc filter show parent ffff: dev eth0"
168
169-------- output -----------
170jroot# tc filter show parent ffff: dev eth0
171filter protocol ip pref 1 u32
172filter protocol ip pref 1 u32 fh 800: ht divisor 1
173filter protocol ip pref 1 u32 fh 800::800 order 2048 key ht 800 bkt 0 flowid 1:15
174
175   action order 1: tablename: mangle  hook: NF_IP_PRE_ROUTING
176        target MARK set 0x1  index 2
177
178   action order 2: police 1 action pipe rate 1Kbit burst 9Kb mtu 2Kb
179
180   action order 3: tablename: mangle  hook: NF_IP_PRE_ROUTING
181        target MARK set 0x2  index 1
182
183   action order 4: police 30 action pipe rate 1Kbit burst 10Kb mtu 5000b
184
185   action order 5: tablename: mangle  hook: NF_IP_PRE_ROUTING
186        target MARK set 0x3  index 3
187
188   action order 6: police 20 action drop rate 1Kbit burst 90Kb mtu 5000b
189
190  match 0a000015/ffffffff at 12
191-------------------------------
192
193Note the ordering of the actions is based on the order in which we entered
194them. In the future i will add explicit priorities.
195
196Now lets run a ping -f from 10.0.0.21 to this host; stop the ping after
197you see a few lines of dots
198
199----
200[root@jzny hadi]# ping -f  10.0.0.22
201PING 10.0.0.22 (10.0.0.22): 56 data bytes
202....................................................................................................................................................................................................................................................................................................................................................................................................................................................
203--- 10.0.0.22 ping statistics ---
2042248 packets transmitted, 1811 packets received, 19% packet loss
205round-trip min/avg/max = 0.7/9.3/20.1 ms
206-----------------------------
207
208Now lets take a look at the stats with "tc -s filter show parent ffff: dev eth0"
209
210--------------
211jroot# tc -s filter show parent ffff: dev eth0
212filter protocol ip pref 1 u32
213filter protocol ip pref 1 u32 fh 800: ht divisor 1
214filter protocol ip pref 1 u32 fh 800::800 order 2048 key ht 800 bkt 0 flowid 1:1
2155
216
217   action order 1: tablename: mangle  hook: NF_IP_PRE_ROUTING
218        target MARK set 0x1  index 2
219         Sent 188832 bytes 2248 pkts (dropped 0, overlimits 0)
220
221   action order 2: police 1 action pipe rate 1Kbit burst 9Kb mtu 2Kb
222         Sent 188832 bytes 2248 pkts (dropped 0, overlimits 2122)
223
224   action order 3: tablename: mangle  hook: NF_IP_PRE_ROUTING
225        target MARK set 0x2  index 1
226         Sent 178248 bytes 2122 pkts (dropped 0, overlimits 0)
227
228   action order 4: police 30 action pipe rate 1Kbit burst 10Kb mtu 5000b
229         Sent 178248 bytes 2122 pkts (dropped 0, overlimits 1945)
230
231   action order 5: tablename: mangle  hook: NF_IP_PRE_ROUTING
232        target MARK set 0x3  index 3
233         Sent 163380 bytes 1945 pkts (dropped 0, overlimits 0)
234
235   action order 6: police 20 action drop rate 1Kbit burst 90Kb mtu 5000b
236         Sent 163380 bytes 1945 pkts (dropped 0, overlimits 437)
237
238  match 0a000015/ffffffff at 12
239-------------------------------
240
241Neat, eh?
242
243
244Wanna write an action module?
245------------------------------
246Its easy. Either look at the code or send me email. I will document at
247some point; will also accept documentation.
248
249TODO
250----
251
252Lotsa goodies/features coming. Requests also being accepted.
253At the moment the focus has been on getting the architecture in place.
254Expect new things in the spurious time i have to work on this
255(particularly around end of year when i have typically get time off
256from work).
257
258