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Quality of Service in IP Networks

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by

Harald Welte <laforge@conectiva.com>


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QoS in IP Networks
Contents

	Definition of QoS

	Why QoS

	IP Networks are not designed for QoS

	How to do the impossible

	What can Linux based systems help

	Advanced Concepts (DiffServ, IntServ, RSVP, ...)

	References / Further Reading

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QoS in IP Networks
Definiton of QoS

	Provide Service Differentiation

	Performance Assurance by

		Bandwitdh guarantees
			for streaming multimedia traffic
			priorizing certain important applications

		Latency guarantees
			for voice over IP
			for interactive character-oriented applications (ssh,telnet)

		Packet-loss guarantees
			for unreliable layer-4 protocols
			to avoid retransmits


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QoS in IP Networks
Why QoS


	Decide how and who available bandwidth is devided

	Limit available bandwidth for certain users / applications

	Guarantee bandwidth for certain users / applications

	Divide bandwidth more equally between users / applications



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QoS in IP Networks
IP networks not designed for QoS


Properties of IP-based networks:

	offer a "best-effort" service

	make NO guarantees about 
		bandwidth
		latency
		packet loss

	provide a non-reliable packet transport

Conclusion: IP networks are not suitable for QoS

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QoS in IP Networks
How to do the Impossible

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As IP Networks including Hardware (Routers, ...) are widely deployed, all QoS efforts have to layer on top of the existing technology.

	There's no real solution to control latency
		latency widely dependent on routing, which may be dynamic

	There's no real solution to control packet loss
		packet loss may occurr on any intermediate router

	But we can control bandwidth usage!
		The sender can limit bandwidth for outgoing streams
		Intermediate routers BEFORE a bottleneck can control bandwidth usage

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QoS in IP Networks
What can Linux systems do?

	Bandwidth limiting at the sender application
		not many applications support it
		server often out of control (on Internet, ...)
		server doesn't know what's between him and the client

	Bandwidth control on intermediate router before bottleneck
		Ideal case because this is where packet loss would occurr
		Sophisticated queue scheduling on the outgoing queue
		Variety of different queue scheduling algorithms

	Flow throttling at the Receiver
		Worst case, because influence is limited
		Theoretically possible for TCP, no implementation yet.
		Ingress qdisc might help

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QoS in IP Networks
Bandwidth limiting at server

	Some Internet Servers support bandwidth limiting

		ProFTPd (builtin support)

		Apache (using contributed mod_bandwidth)


	Using those features it is easy to limit

		maximum bandwidth used per connection 

		maximum bandwidth used per client (IP/network)

		maximum bandwidth used by one virtual host (webserver/ftpserver)


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QoS in IP Networks
Router before bottleneck

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The router receives more packets on his incoming interface(s) than it can send out on the outgoing interface. It has to build a queue of packets (usually a FIFO one) and starts dropping packets as soon as the queue is full

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The idea is to change this queue, thus decide
	which packets get enqueued in which order
	how many packets get queued
	which packets get dropped in case of a filling queue


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QoS in IP Networks
The Linux 2.2 / 2.4 Solution

	Packet Scheduling algorithms in the Kernel
		CBQ - Class Based Queue
		RED - Random Early Drop
		SFQ - Stochastic Fairness Queueing
		TEQL - True Link Equalizer
		TBF - Token Bucket Filter

	tc command of iproute2 package for configuration
		almost no documentation
		very few examples on the internet

	Packet Classification
		tc builtin classes (route, u23, ...)
		all iptables/netfilter matches by using fwmark

Conclusion: Linux is the best suited general-purpose operating system for QoS, but almost nobody is using it because lack of knowledge.

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QoS in IP Networks
Available queuing algorithms

	CBQ - Class Based Queue
		hierarchical bandwidth classes
		used as basis in almost all cases
	TBF - Token Bucket Filter
		really accurate algorithm
		uses a lot of CPU
		not possible for high bandwidth links (>1MBit)
	SFQ - Stochastic Fairness Queueing
		less accurate algorithm
		tries to distinguish between individual streams
		does round robin between those streams
	TEQL - True Link Equalizer
		allows to 'bundle' interfaces
	RED - Random Early Detect / Drop
		simulates congested link by statistic packet dropping
		uses almost no CPU
		recommended for high-bandwidth backbones
	others (WRR, TCINDEX, DSMARK, ..)
		WRR not officially included in kernel, similar to CBQ	
		others mostly used for DiffServ


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QoS in IP Networks
The big picture

Overview of the a packet's journey

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    Incoming Packets 
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           |
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           V
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 Packet Classification          classify
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  (ipchains/iptables)           set nfmark
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           |
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           V
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    Routing decision
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           V
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       TC filter                select classes based on nfmark
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      /    |    \ 
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     /     |     \ 
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    /      |      \ 
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Different Bandwidth classes     bandwidth classes (CBQ)
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    \      |      /
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     \     |     /
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      \    |    /
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       Enqueuing                output queue discipline
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           |
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           V
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    Outgoing packets

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QoS in IP Networks
Example scenario usin CBQ

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Let's assume we have a link with 10 MBit maximum available bandwidth.
We offer two major services to the outside world: Anonymous FTP and a Webserver offering important Information. 

FTP Bulk data transfers are using up almost all available bandwidth, thus slowing down accesses to our website :(

We want to have FTP transfers use up to 8MBit and reserve 2MBit for WWW.

Implementation uses CBQ for bandwidth divisions.


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QoS in IP Networks
Example scenario

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	attach a CBQ to the device 
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tc qdisc add dev eth0 root handle 10: cbq 
   bandwidth 10Mbit avpkt 1000

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	create CBQ classes
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tc class add dev eth0 parent 10:0 classid 10:1 cbq 
   bandwidth 10MBit rate 10MBit allot 1514 
   weight 1Mbit prio 8 maxburst 20 avpkt 1000

tc class add dev eth0 parent 10:1 classid 10:100 cbq 
   bandwidth 10MBit rate 8MBit allot 1514 
   weight 800kbit prio 5 maxburst 20 avpkt 1000 bounded

tc class add dev eth0 parent 10:1 classid 10:200 cbq 
   bandwidth 10MBit rate 2MBit allot 1514 
   weight 200kbit prio 5 maxburst 20 avpkt 1000 bounded

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	add filter rules
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tc filter add dev eth0 parent 10:1 protocol ip handle 6 fw classid 10:100

iptables -t mangle -A PREROUTING -j MARK -p tcp --sport 20 --set-mark 6

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QoS in IP Networks
Further optimization

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Now we have achieved bandwidth division between two services. 

Within one service, however, one individual user with a high bandwith link can still use up most of our bandwidth, slowing down other user.

We can improve this behaviour of changing the scheduling algorithm from it's default (fifo)

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tc qdisc add dev eth0 parent 10:100 sfq quantum 1514b perturb 15
tc qdisc add dev eth0 parent 10:200 sfq quantum 1514b perturb 15


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QoS in IP Networks
Further reading / Links

	Bandwidth limiting on Servers
		ProFTPd 
			http://www.proftpd.net/
		Apache mod_bandwidth / mod_bwshare
			ftp://ftp.cohprog.com/pub/apache/module/mod_bandwidth.c
			http://www.topology.org/src/bwshare/

	Queue scheduling 
		Advanced Routing HOWTO
			http://www.ds9a.nl/2.4Routing/
		Linux QoS HOWTO
			http://www.ittc.ukans.edu/~rsarav/howto/
		iproute2+tc

	This presentation
		Authors Homepage
			http://www.gnumonks.org/