398 lines
9.6 KiB
Plaintext
398 lines
9.6 KiB
Plaintext
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Quality of Service in IP Networks
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by
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Harald Welte <laforge@gnumonks.org>
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QoS in IP Networks
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Contents
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Definition of QoS
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Why QoS
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IP Networks are not designed for QoS
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How to do the impossible
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What can Linux based systems help
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Advanced Concepts (DiffServ, IntServ, RSVP, ...)
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References / Further Reading
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QoS in IP Networks
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Definiton of QoS
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Provide Service Differentiation
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Performance Assurance by
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Bandwitdh guarantees
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for streaming multimedia traffic
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priorizing certain important applications
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Latency guarantees
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for voice over IP
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for interactive character-oriented applications (ssh,telnet)
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Packet-loss guarantees
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for unreliable layer-4 protocols
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to avoid retransmits
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QoS in IP Networks
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Why QoS
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Decide how and who available bandwidth is devided
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Limit available bandwidth for certain users / applications
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Guarantee bandwidth for certain users / applications
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Divide bandwidth more equally between users / applications
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QoS in IP Networks
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IP networks not designed for QoS
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Properties of IP-based networks:
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offer a "best-effort" service
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make NO guarantees about
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bandwidth
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latency
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packet loss
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provide a non-reliable packet transport
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Conclusion: IP networks are not suitable for QoS
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QoS in IP Networks
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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.
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There's no real solution to control latency
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latency widely dependent on routing, which may be dynamic
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There's no real solution to control packet loss
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packet loss may occurr on any intermediate router
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But we can control bandwidth usage!
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The sender can limit bandwidth for outgoing streams
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Intermediate routers BEFORE a bottleneck can control bandwidth usage
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QoS in IP Networks
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What can Linux systems do?
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Bandwidth limiting at the sender application
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not many applications support it
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server often out of control (on Internet, ...)
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server doesn't know what's between him and the client
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Bandwidth control on intermediate router before bottleneck
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Ideal case because this is where packet loss would occurr
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Sophisticated queue scheduling on the outgoing queue
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Variety of different queue scheduling algorithms
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Flow throttling at the Receiver
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Worst case, because influence is limited
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Theoretically possible for TCP, no implementation yet.
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Ingress qdisc might help
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QoS in IP Networks
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Bandwidth limiting at server
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Some Internet Servers support bandwidth limiting
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ProFTPd (builtin support)
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Apache (using contributed mod_bandwidth)
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Using those features it is easy to limit
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maximum bandwidth used per connection
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maximum bandwidth used per client (IP/network)
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maximum bandwidth used by one virtual host (webserver/ftpserver)
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QoS in IP Networks
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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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%image "qos-1.png" 0 100 30
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The idea is to change this queue, thus decide
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which packets get enqueued in which order
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how many packets get queued
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which packets get dropped in case of a filling queue
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%page
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QoS in IP Networks
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The Linux 2.2 / 2.4 Solution
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Packet Scheduling algorithms in the Kernel
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CBQ - Class Based Queue
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RED - Random Early Drop
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SFQ - Stochastic Fairness Queueing
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TEQL - True Link Equalizer
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TBF - Token Bucket Filter
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tc command of iproute2 package for configuration
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almost no documentation
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very few examples on the internet
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Packet Classification
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tc builtin classes (route, u23, ...)
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all iptables/netfilter matches by using fwmark
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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
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Available queuing algorithms
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CBQ - Class Based Queue
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hierarchical bandwidth classes
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used as basis in almost all cases
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TBF - Token Bucket Filter
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really accurate algorithm
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uses a lot of CPU
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not possible for high bandwidth links (>1MBit)
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SFQ - Stochastic Fairness Queueing
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less accurate algorithm
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tries to distinguish between individual streams
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does round robin between those streams
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TEQL - True Link Equalizer
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allows to 'bundle' interfaces
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RED - Random Early Detect / Drop
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simulates congested link by statistic packet dropping
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uses almost no CPU
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recommended for high-bandwidth backbones
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others (WRR, TCINDEX, DSMARK, ..)
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WRR not officially included in kernel, similar to CBQ
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others mostly used for DiffServ
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QoS in IP Networks
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The big picture
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Overview of the a packet's journey
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Incoming Packets
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V
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Packet Classification classify
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(ipchains/iptables) set nfmark
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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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Different Bandwidth classes bandwidth classes (CBQ)
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Enqueuing output queue discipline
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Outgoing packets
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QoS in IP Networks
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Example scenario usin CBQ
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Let's assume we have a link with 10 MBit maximum available bandwidth.
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We offer two major services to the outside world: Anonymous FTP and a Webserver offering important Information.
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FTP Bulk data transfers are using up almost all available bandwidth, thus slowing down accesses to our website :(
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We want to have FTP transfers use up to 8MBit and reserve 2MBit for WWW.
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Implementation uses CBQ for bandwidth divisions.
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QoS in IP Networks
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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
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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
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bandwidth 10MBit rate 10MBit allot 1514
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weight 1Mbit prio 8 maxburst 20 avpkt 1000
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tc class add dev eth0 parent 10:1 classid 10:100 cbq
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bandwidth 10MBit rate 8MBit allot 1514
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weight 800kbit prio 5 maxburst 20 avpkt 1000 bounded
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tc class add dev eth0 parent 10:1 classid 10:200 cbq
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bandwidth 10MBit rate 2MBit allot 1514
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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
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tc filter add dev eth0 parent 10:1 protocol ip handle 7 fw classid 10:200
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iptables -t mangle -A PREROUTING -j MARK -p tcp --sport 20 --set-mark 6
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iptables -t mangle -A PREROUTING -j MARK -p tcp ! --sport 20 --set-mark 7
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QoS in IP Networks
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Further optimization
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Now we have achieved bandwidth division between two services.
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Within one service, however, one individual user with a high bandwith link can still use up most of our bandwidth, slowing down other user.
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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
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tc qdisc add dev eth0 parent 10:200 sfq quantum 1514b perturb 15
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%page
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QoS in IP Networks
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Further reading / Links
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Bandwidth limiting on Servers
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ProFTPd
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http://www.proftpd.net/
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Apache mod_bandwidth / mod_bwshare
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ftp://ftp.cohprog.com/pub/apache/module/mod_bandwidth.c
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http://www.topology.org/src/bwshare/
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Queue scheduling
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Advanced Routing HOWTO
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http://www.ds9a.nl/2.4Routing/
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Linux QoS HOWTO
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http://www.ittc.ukans.edu/~rsarav/howto/
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iproute2+tc
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This presentation
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Authors Homepage
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http://www.gnumonks.org/
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