Here you are our most popular NetContractor’s post about QoS fields mystery.
QoS Classification is done mainly based on two fields: in Ethernet it’s CoS field and in IP header it’s ToS. Naming convention for specific fields in IP header has developed over years from the CS and IPP to DSCP. The main reason for that was not enough naming class to classify traffic. BTW, today once we classify traffic and would send it over provider’s MPLS cloud we have to properly map our classes to provider classes to take advantage from the QoS features that have been purchased. What is interesting that MPLS frame uses 3-bits long EXP field that can only address up tp 8 classes of traffic so marking more classes (from the client perspective) have no sense when we would push it over MPLS.
But let’s back to the naming. Due to demand for more classes naming has changed. At the begining just first 3 bits of 8-bits ToS was used to name and mark traffic, it would be enough even until now. Then QoS fetures and class naming has changed due to fast grow of VoIP. QoS has been popular and key significant to achieve better voice quality. Engineers tried to involve more bits to mark more classes. Finally we have still 8-bits long ToS field with few class names depends on what part of the field we take. For someone that is just starting with QoS it maybe confusing so I thought to share with you the QoS Values Calculator that I have created and used during my CCIE study.
I’ve added ToS in HEX to the QoS Values Calculator v2 . These values are useful when you would like to generate IP traffic with specific ToS/DSCP value by ping command from the IOS CLI. Ping with ToS is very helpful during QoS configuration test. You can easily generate test ICMP traffic with specific value in ToS field and see if it matches rigth QoS class.
Be aware that during extended ping from IOS CLI, TOS HEX value has to be defined in the 0xHH format where HH is HEX value.
To be honest this is the most popular post of this blog.
Please take a look, any feedbacks are more then welcome. Enjoy.
QoS Values Calculator v3 in PDF format here.