3. Use of the TC field

Due to the limited number of bits in the TC field, their use for QoS and ECN (Explicit Congestion Notification) functions is intended to be flexible. These functions may rewrite all or some of the bits in the TC field.

Current implementations look at the TC field with and without label context, and the TC field may be copied to the label stack entries that are pushed onto the label stack. This is done to avoid label stack entries that are pushed onto an existing label stack having different TC fields from the rest of the label stack entries.