manual: some tweaks in overview

Change-Id: I5a672d24eb12bd29d8684117b2658ad4cd89d682
---
M doc/manuals/chapters/overview.adoc
1 file changed, 13 insertions(+), 10 deletions(-)


git pull ssh://gerrit.osmocom.org:29418/osmo-upf refs/changes/36/30836/1

diff --git a/doc/manuals/chapters/overview.adoc b/doc/manuals/chapters/overview.adoc
index fa01939..80d5982 100644
--- a/doc/manuals/chapters/overview.adoc
+++ b/doc/manuals/chapters/overview.adoc
@@ -117,11 +117,14 @@
     Header Creation for GTP.
   - A second PDR+FAR pair like above, with Access and Core swapped.
 
+Access and Core must be indicated by the Source Interface IE (PDR) and
+Destination Interface IE (FAR) in PFCP.
+
 Any set of rules only partially or not at all matching the above PDR and FAR
 rules will not result in any actions on the GTP user plane, but will still
 return a successful outcome in the PFCP messages.
 
-For example, a rule set using an interface other than "Access" or "Core" results
+For example, a rule set using a Source Interface other than "Access" or "Core" results
 in a PFCP no-op, returning PFCP responses with successful outcome, but not
 providing any GTP-U service.
 
@@ -132,11 +135,11 @@
 - OsmoUPF using Linux kernel features for the GTP user plane, where there is
   either a full bidirectional GTP tunnel in place or none at all.
 
-For example, a typical CPF will first establish a PFCP session with the UPF
-before passing on a data service request from Access to Core, because the UPF
-must first choose the GTP TEID that needs to be forwarded towards Core. When the
-Core side has responded with its GTP details, the UPF is updated, to form a
-complete PFCP rule set.
+For example, for `tunmap`, a typical CPF will establish a PFCP session in two
+steps: first request a local F-TEID from the UPF before passing on a data
+service request from Access to Core. When the Core side has responded with its
+GTP details, the PFCP session at the UPF is updated (Session Modifification),
+to form a complete PFCP rule set.
 
 .Typical sequence of establishing a GTP-U tunnel relay
 ["mscgen"]
@@ -156,16 +159,16 @@
 
 	|||;
 
-	sgwc <= sgwu [label="PFCP Session Establishment Response\n\n2x Created PDR\nwith chosen F-TEID"];
+	sgwc <= sgwu [label="PFCP Session Establishment Response\n\n2x Created PDR\nwith chosen local F-TEID"];
 
 	|||;
 
-	sgwc => pgwc [label="GTP Create Session Request"];
-	sgwc <= pgwc [label="GTP Create Session Response"];
+	sgwc => pgwc [label="GTP Create Session Request\nwith chosen local F-TEID towards Core"];
+	sgwc <= pgwc [label="GTP Create Session Response\nwith remote F-TEID at Core"];
 
 	|||;
 
-	sgwc => sgwu [label="PFCP Session Modification Request\n\nUpdate FAR\nwith F-TEID from Core"];
+	sgwc => sgwu [label="PFCP Session Modification Request\n\nUpdate FAR\nwith remote F-TEID at Core"];
 
 	|||;
 

