CSIS 4823Data Communications

Networking – QoSMr. Mark Welton

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

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?)

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

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?

Quality of Service

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

packets◦ Examples???

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

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

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

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???

Quality of Service

CoS IP precedence DSCP Name0 0 0-7 Routing (Best

Effort)1 1 8-15 Priority2 2 16-23 Immediate3 3 24-31 Flash4 4 32-39 Flash-override5 5 40-47 Critical6 6 48-55 Internet7 7 56-63 Network

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

Quality of Service

Traffic policing vs. Traffic-shaping

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?

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?

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

Quality of Service

Quality of Service

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

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

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/

Quality of Service

“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

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

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

Common QoS Misconceptions

“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

Common QoS Misconceptions