Before an exhaustive in-depth view of the architectures is taken into account a general background about the metrics of QoS and the Service Level Agreements is given below.
 
a) Background:-
  i.  Service availability- the reliability of the user's connection to Internet Service.
ii. Delay- the time interval between transmitting and receiving packets; also called latencyiii. Delay variation- refers to the variation in time duration between all packets in a stream taking the same route; also called jitter.iv. Throughput- it is the average or peak rate at which packets are transmitted.
v. Packet loss rate- this actually results from congestion; it is the maximum rate at which packets can be discarded during       transfer through a network.
  An SLA is a simple standard contract which defines end-to-end service and may consist of the following:
i.   Availability- guaranteed uptime and service latency.
ii.  Services offered- specifications of the service levels offered.
iii. Service guarantees- for each class, throughput, loss rate, delay and delay variation.
iv. Responsibilities- breach of contract rules, customer support and service.
v.  Auditing.
vi. Pricing.