Layer 3 marking allows a more persistent method of marking packets over an end-to-end transmission. Layer 2 marking has the disadvantage that class of service information may be lost if a packet travels a non-802.1Q link.
Quality of Service marking in a Layer 3 packet uses the Type of Service field, 8 bits in length. Only the first 3 bits of the Type of Service field are used for marking. The marking is known as IP Precedence, and follow the same priority rules as Class of Service. Giving a packet a IP Precedence value from 0 to 7. Values 6 and 7 are reserved for internal network use.
Newer standards have come into play changing the Type of Service and IPv6 Traffic Class fields into an 8-bit differentiated services field.
The differentiated services, or DiffServ field, uses the same 8 bits that were used for the IPv4 Type of Service field. It is designed to be backwards compatible with IP Precedence. The DiffServ field consists of a 6-bit Differentiated Services Code Point (DSCP) field that allows for classification of packets of up to 64 different values, along with a 2-bit Explicit Congestion Notification (ECN) field.
Differentiated Services Code Point
As packets are classified and marked, they will be treated according to their marking per hop along the path towards their destination. A packet could be expedited, delayed in a queue, or dropped based on its marking.
The DiffServ field is used to mark packets into groups of classifications called DiffServ Behaviour Aggregates.
The DiffServ Behaviour Aggregate is a collection of packets with the same DiffServ value crossing a link in the same direction. The per-hop behaviour is an observation of the forwarding treatment applied at a network hop supporting DiffServ features to a collection of packets.
The network hop supporting DiffServ will expediate, delay, or drop groups of packets in a differentiated services behaviour aggregate group based on their differentiated services code point value. The group can include multiple applications but are all marked with the same differentiated services code point value.
The groups of packets being manipulated based on their DSCP value makes it easy for network devices providing transit to optimise their bandwidth with simple configuration. The classification of individual applications is left to the network edge devices.
They are four different per hop behaviours characterised for general use:
Class Selector uses the first 3 bits of the Differentiated Services Code Point to be used as Class Selector bits. The Class Selector bits allow DSCP to be backwards compatible with IP Precedence as the same 3 bit are used by IP Precedence.
Default Forwarding is used for best effort service.
Assured Forwarding uses a guaranteed bandwidth service
Expedited Forwarding is used to provide a low-delay service