-and SUT2. For N=1 DUT forwards packets between vhost/memif and physical
-interfaces. For N>1 DUT a logical service chain forwarding topology is
-created on DUT by applying L2 or IPv4/IPv6 configuration depending on
-the test suite. DUT test topology with N VM/Ctr instances is shown in
-the figure below including applicable packet flow thru the DUTs and
-VMs/Ctrs (marked in the figure with ``***``).::
-
- +-------------------------+ +-------------------------+
- | +---------+ +---------+ | | +---------+ +---------+ |
- | |VM/Ctr[1]| |VM/Ctr[N]| | | |VM/Ctr[1]| |VM/Ctr[N]| |
- | | ***** | | ***** | | | | ***** | | ***** | |
- | +--^---^--+ +--^---^--+ | | +--^---^--+ +--^---^--+ |
- | *| |* *| |* | | *| |* *| |* |
- | +--v---v-------v---v--+ | | +--v---v-------v---v--+ |
- | | * * * * |*|***********|*| * * * * | |
- | | * ********* ***<-|-----------|->*** ********* * | |
- | | * DUT1 | | | | DUT2 * | |
- | +--^------------------+ | | +------------------^--+ |
- | *| | | |* |
- | *| SUT1 | | SUT2 |* |
- +-------------------------+ +-------------------------+
- *| |*
- *| |*
- *| +-----------+ |*
- *| | | |*
- *+--------------------> TG <--------------------+*
- **********************| |**********************
- +-----------+
-
-For VM/Ctr tests, packets are switched by DUT multiple times: twice for
-a single VM/Ctr, three times for two VMs/Ctrs, N+1 times for N VMs/Ctrs.
-Hence the external throughput rates measured by TG and listed in this
-report must be multiplied by (N+1) to represent the actual DUT aggregate
-packet forwarding rate.
+and SUT2. Three types of service chain topologies are tested in CSIT |release|:
+
+#. "Parallel" topology with packets flowing from NIC via DUT (VPP) to
+ VM/Container and back to VPP and NIC;
+
+#. "Chained" topology (a.k.a. "Snake") with packets flowing via DUT (VPP) to
+ VM/Container, back to DUT, then to the next VM/Container, back to DUT and
+ so on until the last VM/Container in a chain, then back to DUT and NIC;
+
+#. "Horizontal" topology with packets flowing via DUT (VPP) to Container,
+ then via "horizontal" memif to the next Container, and so on until the
+ last Container, then back to DUT and NIC. "Horizontal" topology is not
+ supported for VMs;
+
+For each of the above topologies, DUT (VPP) is tested in a range of L2
+or IPv4/IPv6 configurations depending on the test suite. A sample DUT
+"Chained" service topology with N of VM/Ctr instances is shown in the
+figure below. Packet flow thru the DUTs and VMs/Ctrs is marked with
+``***``::
+
+ +-------------------------+ +-------------------------+
+ | +---------+ +---------+ | | +---------+ +---------+ |
+ | |VM/Ctr[1]| |VM/Ctr[N]| | | |VM/Ctr[1]| |VM/Ctr[N]| |
+ | | ***** | | ***** | | | | ***** | | ***** | |
+ | +--^---^--+ +--^---^--+ | | +--^---^--+ +--^---^--+ |
+ | *| |* *| |* | | *| |* *| |* |
+ | +--v---v-------v---v--+ | | +--v---v-------v---v--+ |
+ | | * * * * |*|***********|*| * * * * | |
+ | | * ********* ***<-|-----------|->*** ********* * | |
+ | | * DUT1 | | | | DUT2 * | |
+ | +--^------------------+ | | +------------------^--+ |
+ | *| | | |* |
+ | *| SUT1 | | SUT2 |* |
+ +-------------------------+ +-------------------------+
+ *| |*
+ *| |*
+ *| +-----------+ |*
+ *| | | |*
+ *+--------------------> TG <--------------------+*
+ **********************| |**********************
+ +-----------+
+
+In above "Chained" topology, packets are switched by DUT multiple times:
+twice for a single VM/Ctr, three times for two VMs/Ctrs, N+1 times for N
+VMs/Ctrs. Hence the external throughput rates measured by TG and listed
+in this report must be multiplied by (N+1) to represent the actual DUT
+aggregate packet forwarding rate.
+
+For a "Parallel" and "Horizontal" service topologies packets are always
+switched by DUT twice per service chain.