We know from the 5-Day QoS bootcamp that Differentiated Services is one of the three major overall approaches to providing Quality of Service in an enterprise. The other options are Integrated Services and Best Effort.

When we studied Differentiated Services, we saw that the primary marking technology approach was the Differentiated Services Code Point (DSCP) concept. These are the high order 6 bits in the IP packet ToS Byte. But how can MPLS use these markings in order to provide QoS treatment (Per Hop Behaviors (PHBs)) to various traffic forms?

The first major issue to solve is the fact that Label Switch Routers (LSRs) rely solely on the MPLS header when making forwarding decisions. These devices will no longer analyze the IP Header information, thus negating the use of the ToS Byte. This was solved through the creation of the Experimental Bits field  in the MPLS header. The IETF has now renamed the field to the Traffic Class field.  See RFC 5462.

But now there is another issue. There are 6 bits used for DSCP (providing 64 classifications), while there are only 3 Traffic Class bits (providing a mere 8 classifications).

It turns out there are two approaches to dealing with this issue. If you should happen to require less than 8 Per Hop Behaviours, just use the EXP Bits (Traffic Class). In fact, these bits are mapped to IP Precedence by default in Cisco's implementation, so they should be populated appropriately for QoS classification by default. This approach is called E-LSPs in official MPLS terminology. E stands for EXP-inferred in this case.

The second option is when we need more than 8 classifications in our network. Obviously, the three EXP bits fall far short of providing the necessary markings. In this case, the label itself is used to help mark traffic! In this approach, both the EXP bits and the label are used for the PHB. Typically the marking in the label will assign the congestion management treatment, while the EXP bits will control drop priority. This approach is called L-LSP. Here the L stands for label-inferred.

Thanks for reading this blog supplement to the QoS course, and you can expect many more over the coming months. Happy studies!

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