QoS Summary

There are 3 model types of QoS:

1. Best Effort. It's default. No QoS at all
2. Integrated Service (IntServ). The service in this type is dedicated so need bandwidth reservation in the path. The protocol used in IntServ is RSVP
3. Differentiated Service (DiffServ). In the DIffServ various packets can be differentiated. The typical using DiffDerv there are classification and marking and policy for what need to do with the packet.

Layer 2 QoS Marking:

1. Ethernet. The marking use Class of Service (CoS). It's use 3-bit 802.1p field in 802.1Q header
2. Frame Relay. The marking use Discard Eligibility (DE), 1-bit drop eligibility flag
3. ATM. The marking is Cell Loss Priority (CLP), 1-bit drop eligibility flag
4. MPLS, the marking use Traffic Class (TC), 3-bit field compatibe with 802.1p

Layer 3 QoS Marking:

1. IP Precedence. The first 3 bits of the IP TOS field. There are 8 IP Precedence but we can only use from IP precedence 0 to 5. IP precedence 0 is best effort and IP precedence 5 is the critical data such as video conference, voice, etc.
2. Differentiated Service Code Point (DSCP). It use 8 bit which the first 6 bit use for classication. It's backward compatible with IP precedence. There are BE for best effort, CS (Class Selector) which backward-compatible with IP precedence, Assured Forwarding (AF) which have 4 classes with variable drop preferences, and Expedited Forwarding (EF) which use for priority queuing for delay-sensitive traffic.

The default for ethernet (<2Mbps) is FIFO and default for serial is WFQ.