epdg: Add README.md

Change-Id: I6326b6c35f66d30787758d83e96e8a070375b0c0
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A epdg/README.md
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git pull ssh://gerrit.osmocom.org:29418/osmo-ttcn3-hacks refs/changes/37/36637/1

diff --git a/epdg/README.md b/epdg/README.md
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+# EPDG_Tests.ttcn
+
+* external interfaces
+    * CEAI: GSUP (emulates strongswan)
+    * SWx: Diameter (emulates HSS)
+    * S6b: Diameter (emulates SMF)
+    * S2b: GTPv2C (emulates SMF)
+    * Emulates userplane IPsec decapsulated IP traffic at strongswan
+    * Emulates userplane GTPv1U encapsulated IP traffic at UPF
+
+{% dot epdg_tests.svg
+digraph G {
+  rankdir=LR;
+  ATS [label="ATS\nGEPDG_Tests.ttcn"];
+  EPDG [label="EPDG\nosmo-epdg",shape="box", color=red];
+  GTP_KERN [label="gtp\nkernel-module", shape="box"];
+
+  ATS -> EPDG [label="CEAI (GSUP)"];
+  EPDG -> ATS [label="S6b (Diameter)"];
+  ATS -> EPDG [label="SWx (Diameter)"]
+  ATS -> EPDG [label="S2b (GTPv2C)"];
+  ATS -> EPDG [label="IP traffic (to/from strongswan ipsec)", style=dashed];
+  EPDG -> GTP_KERN [label="netlink"]
+  GTP_KERN -> ATS [label="IP traffic (GTPv1U to/from UPF)", style=dashed]
+}
+%}

