Chapter 40 - Quality of Service (QoS) Flashcards
1
Q
What is QoS?
A
- Quality of Service
- A suite of tools that networking devices use to decide how to treat packets based on certain parameters
- Allows you to manage the bandwidth, delay, jitter, and loss of traffic
2
Q
Define Bandwidth
A
- The capacity/speed of a link in bits per second
- QoS can be used to prioritise bandwidth to certain traffic so that network congestion does not affect this traffic. For example dedicating a certain amount of bandwidth to VoIP traffic.
3
Q
Define Delay
A
- One-Way Delay - The time between a packet being sent and it arriving at its destination
- Round Trip Delay - The One-Way Delay plus the time that it takes the receiver of the first packet to respond
4
Q
Define Jitter
A
- The variation in one-way delay between consecutive packets sent by a particular application.
- For example if the one-way delay of the first packet is 100 ms but the second is 110ms that is considered jitter.
- VoIP phones generally have a Jitter Buffer to provide a fixed delay to audio packets
5
Q
Define Loss
A
- Defined as the number of lost packets as a percentage of packets sent
6
Q
What are the guidelines set out by Cisco for how much Delay, Jitter, and Loss a VoIP media connection should be able to experience before it is considered too much?
A
- One-way Delay - 150 ms or less
- Jitter - 30ms or less
- Loss - 1% or less
7
Q
What are the guidelines set out by Cisco for how much Delay, Jitter, and Loss a video media connection should be able to experience before it is considered too much? How much Bandwidth should be dedicated?
A
- Bandwidth - 384Kbps to 20+ Mbps
- Delay (one-way) - 200-400ms
- Jitter - 30-50ms
- Loss - 0.1%-1%
8
Q
What is Classification and Marking?
A
- Type of QoS tool
- Organising packets into certain classes which can then be used to move certain packets into certain queues to ensure that some are prioritised above others. These can be marked using DSCP and CoS (PCP field)
- Methods of doing this could be ACLs that only action certain traffic that pass through them or NBAR.
9
Q
What is NBAR?
A
- Network Based Application Recognition
- NBAR2 or Next Generation NBAR is the most widely used nowadays
- Matches packets to be classified in many ways useful to QoS
- For example it can specifically match traffic from applications such as Facetime or ESPN video using Application Signatures so that this traffic can specifically have QoS applied
10
Q
What is the Type of Service (ToS) byte?
A
- Field in IPv4 header
- Includes the DSCP (Differentiated Services Code Point) and ECN (Explicit Congestion Notification) values that are used for QoS marking. There are 64 unique values.
- DSCP’s predecessor was IPP (IP Precedence) which only allowed 7 different values.
11
Q
What field can be used to mark QoS in the 802.1Q header?
A
- PCP - Priority Code Point
These can only be used on frames where there is an 802.1Q header. For this reason, 802.1Q trunking has to be enabled on the interface(s) these are used on.
12
Q
What QoS marking fields can be utilised over what type of link/technology?
A
- DSCP - Any packet - 6 bits
- IPP - Any packet - 3 bits
- PCP/CoS - Frame with 802.1Q header (trunk links and access links with voice VLANs) - 3 bits
- TID - Wifi - 3 bits
- EXP - MPLS - 3 bits
13
Q
What is a Trust Boundary?
A
- The point in the path of a traffic flow after which networking devices can trust QoS markings.
- If the markings aren’t on the Trusted side of the boundary, they will be changed according to the QoS policy.
- For example, if a PC is daisy chained from a phone, you could set the trust boundary at the phone since the PCs DSCP/PCP markings and shouldn’t be trusted could be changed but the phone’s likely wouldn’t be able to so could be trusted.
14
Q
What is DiffServ?
A
- Defines a series of standardised DSCP markings that can be used by multiple different vendors without compatibility issues.
15
Q
Common DSCP values
A
- DF/CS0 (Default Forwarding)
- Best effort - EF (Expedited Forwarding)
- Used for packets that need low Delay, low Jitter, and low Loss, (e.g. Voice traffic).
- DSCP value 46
- As an example, RTP packets from Cisco phones generally will be marked with EF, but SIP signalling
packets may be marked with another value such as CS3. - AF (Assured Forwarding)
- Defines a set of 12 DSCP values. These are each defined by the letters AF followed by two numbers.
These two numbers represent a point on a grid with 4 queue classes and 3 drop prescidence numbers.
The first being the queue and the second being the drop prescedence.
- The queue class axis goes 1 (worst) to 4 (best)
- The drop prescedence axis goes 3 (worst) to 1 (best)
- For example, AF41 is the best queue and drop prescedence
- Drop prescedence refers to Congestion Avoidance. The higher the number (3) the more likely it is that
the traffic will be dropped.
- In binary, the first 3 bits of the DSCP value are the queue, the next 2 bits are the drop prescedence, and
the last bit (1) is always 0. - CS (Class Selector)
- Defines a set of 8 DSCP values used for backwards compatibility with the original IPP values.
- The DSCP values have an extra 3 bits more than IPP values in binary. (Instead of just 4,2,1 for IPP you
have 32,16,8,4,2,1 for DSCP but like AF, the 1 bit is always 0)
- The first 3 bits of the DSCP value are the same as its corresponding IPP value. For example, IPP 0 = CS0 =
DSCP 0, IPP 1 = CS1 = DSCP 8, IPP 2 = CS2 = DSCP 16 etc.
16
Q
What plans does Cisco lay out in RFC4954 that advise what DSCP values should be used for what types of traffic?
A
- EF - Voice payload
- AF4x - Interactive video (e.g. Video conferencing)
- AF3x - Streaming video
- AF2x - High priority (low latency) data
- CS0 (DF) - Standard data
