CSIS 4823 Data Communications Networking – QoS

1. CSIS 4823Data Communications Networking – QoS Mr. Mark Welton

2. Quality of Service • Quality of Service is deployed to prevent data from saturating a link to the point that other data cannot gain access to it • QoS allows certain types of traffic to be given a higher priority than other traffic • Fundamental purpose of QoS is to determine which traffic should be given priority

3. Quality of Service Recall: • When the rate of bits being sent to a link exceeds the line rate (called?) • The interface on the router buffers as many of the packets as possible • Then the interface begins dropping packets (called?)

4. Quality of Service • QoS helps to deal with congestion • QoS can not deal with continued saturation • It can get important traffic through a saturated link • It will never turn a T1 into a DS3

5. Quality of Service • Transport Layer protocols behave differently on congested networks • TCP - connection-oriented, sender/receiver adapt to network conditions and can resend lost packets • TCP-based applications are generally not sensitive to lost packets • What are some examples?

6. Quality of Service • UDP – connectionless, no error checking, no notification of loss packets • UDP-based applications can be sensitive to lost packets • Examples???

7. Quality of Service • Queues • FIFO – first in first out • WFQ - Cisco proprietary, Weighted Fair Queuing • CBWFQ – Class Based WFQ, creates classes of traffic that each have a priority and each class queue uses WFQ • Priority - queues created and traffic is serviced in highest priority queue first • Low Latency queuing – a CBWFQ with strict priority queue. Preferred method for voice networks • Traffic Shaping – monitors traffic, and when a configured threshold is met, packets are queued until a point where we want them to be sent

8. Quality of Service • QoS Mechanics • Marking - deciding what priority a packet should be and labeling it accordingly • Policing – the actions the router takes based on how the packets are marked • Scheduling – the interface actually serving the packets in the order determined by how the marked packets are policed

9. Quality of Service • Two primary types of IP prioritization used at Layer 3 (what device is implementing this?) • IP precedence • Defined in RFC 795 • Differential services • Defined in RFC 2474 • Both use the type of service (TOS) field in a IP packet

10. Quality of Service • Class of service (CoS) is a Layer-2 form of QoS • CoS works under the same principles as IP precedence • The values are stored in the 802.1P frame header not the IP header • What does this mean???

11. Quality of Service

12. Quality of Service • Traffic policing vs. Traffic-shaping • Traffic policy deals with what type of traffic should given priority over other traffic • If the traffic is not given priority and the link is congested then it will be dropped • Traffic-shaping will “delay” traffic and deliver it “late” • It will attempt to queue the traffic that would be dropped and deliver it as the link is not congested NOTE: I have over simplified this description to help make it easier to understand

13. Quality of Service • Traffic policing vs. Traffic-shaping

14. Quality of Service • So what happens if I have a DS3 in my data center and my branch office haves a T1on a frame relay network? • So what happens if I have a DS3 in my data center and my branch office haves a T1on a MPLS network?

15. Quality of Service • A DS3 is 28 T1s • So a DS3 gives you more bandwidth? • In Frame relay each end of a DLCI is defines with a bandwidth setting (throughput) • What does MPLS do?

16. Quality of Service • We increase the “frequency” that a bit can be sent which increases the throughput • This is also true for other media • CAT 5, CAT 5E, CAT 6, Fiber

17. Quality of Service

18. Quality of Service • If we send the traffic at DS3 speed to the T1 the queue will fill and packets will be dropped

19. Quality of Service • If we “hold” the packets back at the DS3 and send them at a speed the T1 can handle we will not fill the queue and not drop packets

20. Quality of Service • Problems???? • Possibly • As long as we do not hold the packets too long • Bufferbloat • Time for you to do some work • What is bufferbloat tech talk • http://en.wikipedia.org/wiki/Bufferbloat • http://packetpushers.net/show-125-bufferbloat-what-can-you-do-today-to-suffer-less-2/

21. Common QoS Misconceptions • “QoS“Carves Up” a Link into Smaller Logical Links” • “If I set a policy to give 10% of a link to voice then I have reserved this to only be used by voice traffic” • WRONG – it will use this policy only during times of congestion • When congestion is not present the link will work as normal

22. Common QoS Misconceptions • “QoS Limits bandwidth” • WRONG - QoS guarantees a queue will get priority of some traffic over others during time of congestion • If a QoS setting sets FTP to 10% of the link if congestion does not exist it will receive all the link it asks for

23. Common QoS Misconceptions • “QoS resolves a need for more bandwidth” • WRONG - the best QoS can do is to prioritize which packets should be sent first, which also means it’s determining which packets can be dropped! • If you have a T1 and you’re trying to shove 20 Mbps through it, it doesn’t matter how you prioritize and queue your packets, you’re going to drop a lot of them

24. Common QoS Misconceptions • “QoS prevents packets from being dropped” • WRONG – What QoSwill do for you is help you get the important packets through so that only the less important packets get dropped • Traffic shaping can prevent packets from being dropped (assuming certain buffers are not similarly saturated), though this gets complicated