1. Network Emulation using tc
Jeromy Fu

2. Agenda
• Why emulation
• What to be emulated
• How TC works
• Emulation Howto
• Compared with Nistnet/WANem
• Other references

3. Mathematic model
• Mathematic model analysis can provide
import insight on the behavior of a system
• But, sometimes difficult because of too many
factors combined

4. Network simulator
• Network simulator is a software program that
imitates the working of a computer network
• fast and inexpensive
• controlled and reproducible environment

5. Network emulator
• A network emulator emulates the network
which connects end-systems, not the end-
systems themselves
• transmit actual network traffic
• Can use real code

6. Real world test
• Impractical experiments
• Planet Lab helps
• not reproducible

7. Why emulator
• Complementary
More uncontrolled
Trade off
Risk Occurrence
Likelihood of
More reality
Consequence
of Risk Occurrence

8. Agenda
• Why emulation
• What to be emulated
• How TC works
• Emulation Howto
• Compared with Nistnet/WANem
• Other references

9. What to be emulated
• Bandwidth.
• Shaped(Policed) bandwidth more specifically.
• Capacity can’t be emulated.

10. What to be emulated
• RTT
• Jitter
• Queuing delay

11. What to be emulated
• Duplicate/Disorder/Corrupt
• Lossrate
• Loss burstiness, a longest sequence beginning
and ending with a loss, consecutive received
packets is less than some value Gmin

12. Agenda
• Why emulation
• What to be emulated
• How TC works
• Emulation Howto
• Compared with Nistnet/WANem
• Other references

13. What is TC
• TC is abbr. of Traffic Control
- Rate control
- Bandwidth management
- Active Queue Management(AQM)
- Network Emulator, pkt loss, pkt disorder, pkt duplication,
pkt delay
- QoS ( diffserv + rsvp )
- Many more …

14. How TC works

15. How TC works

16. TC basic concepts
• Classification(Filter)
- Used to distinguish among different classes of packets
and process each class in a specific way.
• Qdisc(Queue discipline)
- Decide which ones to send first, which ones to delay,
and which ones to drop
- class/classful Qdisc: Qdisc with/without configurable
internal subdivision

17. TC basic concepts
• Class
Classes either contain other Classes, or a Qdisc is attached
Qdiscs and Classes are intimately tied together
• Action
Actions get attached to classifiers and are invoked after a
successful classification. Common used actions includes
instantly drop, modify or redirect packets, etc.
Works on ingress only.

18. TC basic concepts

19. TC Commands
• OPTIONS: options are effective for all sub
commands
• OBJECTS: the object of the tc command
operates on
• COMMAND: the sub command for each object

20. TC Qdisc
• Operations on qdisc: add | del | replace |
change | show
• Handle: qdisc handle used to identify qdisc
• root|ingress|parent CLASSID(handle), specify
the parent node

21. qdisc handle
• Qdisc handle is used to identify Qdisc
- {none|major[:]}
- none, autogen by kernel
- major is 16bits HEX number(Without ‘0x’ prefix)
- : is optional
• Internally, qdisc_handle = major<<16

22. TC class
• Class’s parent can be class or qdisc, classid
should have the same major with parent
• classid, {[major]:minor} - major/minor are
both 16bits HEX numbers(Without ‘0x’ prefix),
major is optional
• Internally, classid = (major<<16)|minor

23. TC filter
• Perf(prio): priority of matching.
• Protocol: protocol on which the filter must
operate, ip/icmp etc, see /etc/protocols.
• root|classid CLASSID|handle FILTERID, specify
the class or qdisc attached.

24. Classful qdisc example

25. Classful qdisc example

26. Agenda
• Why emulation
• What to be emulated
• How TC works
• Emulation Howto
• Compared with Nistnet/WANem
• Other references

27. topology
• Client, Emulator and Server are in the same
subnet. Add route.

28. topology
• Client, Emulator in one subnet, server in
another subnet. Use NAT.

29. tc-tbf
• Tokens are added at a fixed rate
• Check if the bucket contains sufficient tokens

30. Bernoulli loss model
• Model uncorrelated loss events, “loss
probability” p.
• Two state, one independent parameter.

31. Simple Gilbert model
• A system with “consecutive loss events”, which
can be characterized by a “loss probability”.
(p)and a “burst duration” (1-r).
• Two state, two independent parameters.
• 1-r = p -> Bernoulli

32. Gilbert model
• Within the Bad state there is a probability h
that a packet is transmitted.
• “loss probability” (p), a “burst duration”(1-r)
and a “loss density”(1-h).
• Two state, three independent parameters.
• h=0->Simple

33. Gilbert-Elliot model
• k is the probability that the packet is
transmitted while the system is in Good state.
• In good state, loss events appear as “isolated”
and independent with each other
• Two state, four independent parameters.
• k=1-> Gilbert

34. 4-state Markov chain

35. difference

36. tc-netem
• Loss random(independent loss probability,
correlation can be added)| Loss state | Loss
gemodel | ecn

37. tc-netem
• crand(n) = corr*crand(n-1) + (1-corr)*rand()
• delay(n) = delay + distri(jitter, crand(n))
• duplicate, corrupt , loss, reorder aslo use
crand.
• Delay should specified if need reorder(packets
should be queued first)
• If gap not specified, gap = 1 will be used.

38. Distribution table

39. Why not loss correlation
• Correlation changed the distribution

40. Netem example
• tc qdisc add dev eth0 root netem delay 100ms
20ms 25% distribution normal
• tc qdisc add dev eth0 root netem loss 0.3%
25%
• tc qdisc add dev eth0 root netem duplicate 1%
corrupt 0.1%
• tc qdisc add dev eth0 root netem delay 10ms
reorder 25% 50% gap 5

41. Bandwidth emulation - tbf
• Tc-tbf
• bfifo is the default child qdisc of tbf,
• can be replace by other qdiscs such as pfifo.

42. Bandwidth emulation - tbf
• limit - limit is the size (in bytes) of bfifo, bfifo is
the queue which stores the packets.
• rate - the bandwidth cap we need to enforce
• burst/buffer/maxburst - this is the bucket size
of the first tbf. Its value should be larger than
rate/HZ to achieve the specified throughput,
the larger value means more burst when
traffic starts(tokens are accumulated in large
bucket).

43. Bandwidth emulation - tbf
• peakrate - if we only have one bucket, the
burst rate will be larger than rate we set, so
we need peakrate to limit the burst. the
peakrate should be no less than rate.
• mtu/minburst - most of the time, set this to
MTU of the interface, larger values means
larger burst.

44. Policing and shaping
• Policer: Rate limiting without buffering,
typically set at ingress, un-conformed packets
are dropped directly.
• Shaper: Rate limiting with buffering, typically
set at exgress, and can be buffered and then if
no extra buffer, then be dropped, will add
extra queuing delay.

45. Policing and shaping

46. Shape emulation
• No delay
Bandwidth cap 1mbit/s, don't allow burst
traffic, then burst = max(MTU, rate/8/HZ) =
(3000,1000000/8/100) = (3000,1250) = 3000,
if queuing delay 100ms, set latency 100ms or
we set limit = qdelay*rate/8/1000+burst =
100*1000000/8/1000+3000 = 13750

47. Shape emulation
• With delay
attach netem to the engress first, and then add tbf to
the child qdisc of netem.
use limit parameter for tbf here, if using latency, tbf
will not include the extra buffer needed for netem
limit = tbf_burst + netem_qsize + tbf_qsize =
max(rate/8/hz, MTU) + delay*rate/8000 +
qdelay*rate/8000.

48. Police emulation
• policer drops packets directly for the Non-
conformant packets, and it has no buffer
• tc-tbf with very small buffer.
• tc-tbf use bfifo as the default child qdisc,
queue length(in bytes) is set automatically by
specifying 'limit' or 'latency', which ensures
that the queue length is no less than token
bucket depth(introduce queuing delay).

49. Police emulation
• Workaround is replace the bfifo with pfifo
• You can also use police on ingress. tc-police
also use token bucket to do bandwidth cap,
but it don't own queue, so there's no qdelay
introduced.

50. Burst emulation
• Most adsl will allow some burst traffic, this
kind of burst is caused by the large token
bucket size, which accumulates many tokens
when transmission starts.
• To emulate the burst, we only need to turn
the 'burst' parameter.

51. Burst emulation
• For example, we allow for 2mbit/s in the first
second in the following case. 1m*t + burst =
2m*t => burst = 1m*t => burst=1m=125k

52. Burst emulation
• What if adding delay? Any problem?
• Extra burst will use the netem buffer and
cause extra queuing delay.
• Separate the buffer using ifb(Intermediate
Functional Block device)
• If traffic is redirect to ifb dev, it is returned
back to the original point when dequeueing
from ifb.
• Can add qdisc for ifb dev

53. Burst emulation
• Using ifb

54. Agenda
• Why emulation
• What to be emulated
• How TC works
• Emulation Howto
• Compared with Nistnet/WANem
• Other references

55. Compared with Nistnet/WANem

56. How Nistnet work

57. How Nistnet work
• Bandwidth limitation is implemented as
adding delay, just like a packet go through a
bottleneck link.
• Determine the amount of time to delay a
packet. This is the maximum of two
quantities:
1. Probabilistic packet delay time
2. Bandwidth-limitation delay time

58. How Nistnet work
• probdelay = correlatedtabledist(&tableme->ltEntry.lteIDelay);
if (hitme->hitreq.bandwidth) {
fixed_gettimeofday(&our_time);
//last queue delay
bandwidthdelay = timeval_diff(&hitme->next_packet, &our_time);
if (bandwidthdelay < 0) {
bandwidthdelay = 0;
hitme->next_packet = our_time;
}
//add transmission delay
packettime = (long)skb->len*(MILLION/hitme->hitreq.bandwidth)
+ ((long)skb->len*(MILLION%hitme->hitreq.bandwidth)
+ hitme->hitreq.bandwidth/2)/hitme->hitreq.bandwidth;
timeval_add(&hitme->next_packet, packettime);
bandwidthdelay += packettime;
}
delay = probdelay > bandwidthdelay ? probdelay : bandwidthdelay;

59. Nistnet drawbacks
• Bandwidth model not emulate the real one.
• Queuing delay and one way delay are
combined.
• Buffer size can only be tune by DRD.
• Only ip:port filter supported.
• Not support 4 state loss burst model
• Only DRD (Derivative Random Drop) AQM
supported.

60. WANem
• WANem is just a WEB UI which use tc
underneath.

61. WANem
• WEB UI, Easy to use
• Add connection disconnect
• Queue size need patches to work
• No burst settings
• No settings for GE or 4-state loss model
• Queuing delay can controlled directly

62. Agenda
• Why emulation
• What to be emulated
• How TC works
• Emulation Howto
• Compared with Nistnet/WANem
• Other references

63. Reference
• NEWT (Network Emulator for Windows
Toolkit) in vs2010
• Introducing True Network Emulation in Visual
Studio 2010
• Network Emulator Toolkit
• dummynet
• Nistnet FAQ