+
+RSS specifics
+-------------
+Packet distribution in TAP is done by the kernel which has a default
+distribution. This feature is adding RSS distribution based on eBPF code.
+The default eBPF code calculates RSS hash based on Toeplitz algorithm for
+a fixed RSS key. It is calculated on fixed packet offsets. For IPv4 and IPv6 it
+is calculated over src/dst addresses (8 or 32 bytes for IPv4 or IPv6
+respectively) and src/dst TCP/UDP ports (4 bytes).
+
+The RSS algorithm is written in file ``tap_bpf_program.c`` which
+does not take part in TAP PMD compilation. Instead this file is compiled
+in advance to eBPF object file. The eBPF object file is then parsed and
+translated into eBPF byte code in the format of C arrays of eBPF
+instructions. The C array of eBPF instructions is part of TAP PMD tree and
+is taking part in TAP PMD compilation. At run time the C arrays are uploaded to
+the kernel via BPF system calls and the RSS hash is calculated by the
+kernel.
+
+It is possible to support different RSS hash algorithms by updating file
+``tap_bpf_program.c`` In order to add a new RSS hash algorithm follow these
+steps:
+
+1. Write the new RSS implementation in file ``tap_bpf_program.c``
+
+BPF programs which are uploaded to the kernel correspond to
+C functions under different ELF sections.
+
+2. Install ``LLVM`` library and ``clang`` compiler versions 3.7 and above
+
+3. Compile ``tap_bpf_program.c`` via ``LLVM`` into an object file::
+
+ clang -O2 -emit-llvm -c tap_bpf_program.c -o - | llc -march=bpf \
+ -filetype=obj -o <tap_bpf_program.o>
+
+
+4. Use a tool that receives two parameters: an eBPF object file and a section
+name, and prints out the section as a C array of eBPF instructions.
+Embed the C array in your TAP PMD tree.
+
+The C arrays are uploaded to the kernel using BPF system calls.
+
+``tc`` (traffic control) is a well known user space utility program used to
+configure the Linux kernel packet scheduler. It is usually packaged as
+part of the ``iproute2`` package.
+Since commit 11c39b5e9 ("tc: add eBPF support to f_bpf") ``tc`` can be used
+to uploads eBPF code to the kernel and can be patched in order to print the
+C arrays of eBPF instructions just before calling the BPF system call.
+Please refer to ``iproute2`` package file ``lib/bpf.c`` function
+``bpf_prog_load()``.
+
+An example utility for eBPF instruction generation in the format of C arrays will
+be added in next releases
+
+TAP reports on supported RSS functions as part of dev_infos_get callback:
+``ETH_RSS_IP``, ``ETH_RSS_UDP`` and ``ETH_RSS_TCP``.
+**Known limitation:** TAP supports all of the above hash functions together
+and not in partial combinations.
+
+Systems supporting flow API
+---------------------------
+
+- "tc flower" classifier requires linux kernel above 4.2
+- eBPF/RSS requires linux kernel above 4.9
+
++--------------------+-----------------------+
+| RH7.3 | No flow rule support |
++--------------------+-----------------------+
+| RH7.4 | No RSS action support |
++--------------------+-----------------------+
+| RH7.5 | No RSS action support |
++--------------------+-----------------------+
+| SLES 15, | No limitation |
+| kernel 4.12 | |
++--------------------+-----------------------+
+| Azure Ubuntu 16.04,| No limitation |
+| kernel 4.13 | |
++--------------------+-----------------------+