1. Mr. Mark Welton

2.  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. 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.  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.  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.  UDP – connectionless, no error checking, no notification of loss packets ◦ UDP-based applications can be sensitive to lost packets ◦ Examples???

7.  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.  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.  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.  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. CoSIP precedenceDSCPName 000-7Routing (Best Effort) 118-15Priority 2216-23Immediate 3324-31Flash 4432-39Flash-override 5540-47Critical 6648-55Internet 7756-63Network

12.  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.  Traffic policing vs. Traffic-shaping

14.  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.  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.  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

18.  If we send the traffic at DS3 speed to the T1 the queue will fill and packets will be dropped

19.  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.  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.  “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.  “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.  “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.  “QoS prevents packets from being dropped” ◦ WRONG – What QoS will 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