Hi,
The TOS field in the IP header has meant different things at different times. You have to understand that IP was designed a long time ago and the concept of QoS that we have today is a lot different to what was envisaged at that time.
The original TOS field in IP incorporated two things:
- precedence values
- indication of requested delay, throughput and reliability
More recently, the first 6 bits of the TOS field were re-assigned to DiffServ Code Points.
The terminology CoS is typically used in data link layer headers e.g. the 802.1p field and the CoS field in the ISL header.
Hope that helps - pls rate the post if it does.
Paresh
