QoS

Quality of Service in IP networks

 QoS aims at prioritization of critical traffic over non-critical traffic (e.g. giving RTP=voice
higher priority than email/HTTP or guaranteeing a certain maximum end-to-end delay).

 The internet is „best-effort“ service (fire and forget). Packets may be dropped by routers in
case of congestion or be unduly delayed (which is bad for real-time applications).

 QoS is not widely implemented and available today (only limited applications, e.g. in LANs 
or on leased lines). Real-time applications (VoIP, Video over IP) work reasonably well since
there is sufficient bandwidth available. But during peak hours (office hours) the quality of
real-time applications may be impaired by increased packet loss and delay.

Basic difference between layer-2 and layer-3 QoS

Explanation:

layer-2 has frames while packets are at layer-3, if that is clear, read on :-)

Layer-2 802.1Q frames have a 2-byte field called Tag Control Information. The three most significant bits of this 2-byte field represents the CoS (Class of Service) value. Layer-2 QoS is represented by this CoS value which is from 0 to 7 (thus 8 values). In addition to 802.1Q and Cisco proprietary ISL (Inter-Switch Link), no other layer-2 frames can have CoS (and hence QoS) values!

IEEE 802.1Q (also known as VLAN tagging) defines a 3-bit field called Class of Service (CoS), which can be used in order to differentiate traffic. Table 1 shows the 8 possible values of the CoS field and their original purpose.

CoS	Acronym	Purpose
0	BE	Best effort
1	BK	Background
2	-	Spare
3	EE	Excellent Effort
4	CL	Controlled Load
5	VI	"Video" < 100 ms latency and jitter
6	VO	"Voice" < 10 ms latency and jitter
7	NC	Network control

Layer-3 IP packets can carry either an IP precedence (IPP) value or a Differentiated Services Code Point (DSCP) value. QoS supports the use of either value because DSCP values are backward-compatible with IP precedence values.

IP precedence values range from 0 to 7.
DSCP values range from 0 to 63
Both IPP and DSCP use the bits in the ToS (Type of Service) byte in the IPv4 header. IPP uses the 3 Most Significant Bits (MSB) while DSCP uses the 6 MSBs of the ToS byte

Following picture shall be helpful in understanding:

So, remember: layer-2 QoS deals with CoS, while at layer-3 we deal with IPP and|or DSCP

Scenario:

Once the Layer 3 configuration is applied to router (packet flow over Layer 3 Network) and the Layer 2 configuration is applied to the switch, the customer is given end-to-end QoS.

There are two options concerning the marking of packets or frames performed by the border router at the egress:

• CoS marking: For each packet with a given DSCP value, mark the frame with the corresponding CoS value as indicated in Table 2.

• DSCP	CoS	Description
  46,47,40	5	IP Premium
  0	0	Best Effort
  8	1	Less than Best Effort

  TABLE 2.

  DSCP-to-CoS mapping.
• DSCP marking: For each packet with a given DSCP value keep this value intact.
• CoS classification is preferred as it can provide backwards compatibility with some no-multilayer capable switches.