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Usually one of releases in particular year is chosen to be a Long Term Support release (LTS) which means that community will maintain it way longer than till next release (3-6 years). This version is typically used as a base for commercial products or Linux Distributions.
DPDK
Development
The DPDK development process is modeled (loosely) on the Linux Kernel development model. The process is described in more details here.
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Last month of every release is considered a bug fixing period and no new functionality will enter the tree within this period.
Maintenance
Stable and LTS releases are described here.
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