add ReadMe.

ReadMe.md

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+# SigmaDouble6
+---
+
+## Usage:
+
+```sh
+usage: sigmadouble.py [-h] [-v] [-f filename] [-i mapped_ipv4]
+                      [-p mapped_port_base]
+                      [-s mapping_stored_filename | -l mapping_loaded_filename]
+
+CLI argument parsing
+
+optional arguments:
+  -h, --help            show this help message and exit
+  -v, --verbose         verbosity
+  -f filename, --filename filename
+                        filename of UCC log
+  -i mapped_ipv4, --ip mapped_ipv4
+                        mapped IPv4 address; only for mapping stored filename
+  -p mapped_port_base, --port mapped_port_base
+                        mapped TCP listening port base; only for mapping
+                        stored filename
+  -s mapping_stored_filename, --store mapping_stored_filename
+                        mapping stored filename, from UCC log; YAML formatted
+  -l mapping_loaded_filename, --load mapping_loaded_filename
+                        mapping loaded filename, to UCC log; YAML formatted
+```
+
+## Description:
+A test double to **emulate** Sigma handshaking based on existing log.
+This design is for the condition that Sigma instances such as DUT and/or sniffer are unavailable.
+
+A Sigma instance follows the **WTS CAPI specification** to receive and send formatted strings over a TCP socket; UCC core sends CAPI request to a Sigma instance, and the Sigma instance sends CAPI response to UCC core accordingly.
+
+This test double is designed to replace Sigma instances by a few socket servers; the socket client, i.e. UCC core, switches its connection with those socket servers (instead of actual Sigma instances). All the CAPI request/response patterns are retrieved from existing UCC log.
+This test double would:
+> parse the specified UCC log,
+> gather every Sigma instances,
+> gather request/response from each Sigma instances, and
+> spawn TCP socket servers with request/response collection accordingly.
+
+The UCC core must alter:
+> initialization configuration, i.e. AllInitConfig_\*.txt file in cmds/WTS-\* directory, with the IP and port setting from this test double.
+> > Notice here, when DUT is one of testbeds, the handle (IP-port pair) of the testbed is recommended to be commented/removed.
+
+Thus, when UCC core sends request to test double, then, test double will send corresponding response back; **just like replaying the tapes of music**.