Network Emulation using tc

Jeromy Fu

Agenda

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

Mathematic model

• Mathematic model analysis can provide import insight on the behavior of a system

• But, sometimes difficult because of too many factors combined

Network simulator

• Network simulator is a software program that imitates the working of a computer network

• fast and inexpensive• controlled and reproducible environment

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

Real world test

• Impractical experiments• Planet Lab helps• not reproducible

Why emulator

• Complementary

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Agenda

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

What to be emulated

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

What to be emulated

• RTT

• Jitter

• Queuing delay

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

Agenda

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

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 …

How TC works

How TC works

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

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.

TC basic concepts

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

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

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

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

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.

Classful qdisc example

Classful qdisc example

Agenda

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

topology

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

topology

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

tc-tbf

• Tokens are added at a fixed rate• Check if the bucket contains sufficient tokens

Bernoulli loss model

• Model uncorrelated loss events, “loss probability” p.

• Two state, one independent parameter.

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

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

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

4-state Markov chain

difference

tc-netem

• Loss random(independent loss probability, correlation can be added)| Loss state | Loss gemodel | ecn

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.

Distribution table

Why not loss correlation

• Correlation changed the distribution

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

Bandwidth emulation - tbf

• Tc-tbf

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

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).

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.

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.

Policing and shaping

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

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.

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).

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.

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.

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

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

Burst emulation

• Using ifb

Agenda

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

Compared with Nistnet/WANem

How Nistnet work

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

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;

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.

WANem

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

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

Agenda

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

Reference

• NEWT (Network Emulator for Windows Toolkit) in vs2010

• Introducing True Network Emulation in Visual Studio 2010

• Network Emulator Toolkit• dummynet • Nistnet FAQ