doc: add design-rtpcollision.txt that explains when GstRTPCollision is created

It also talks about "BYE only the corresponding source, not the whole session."

diff --git a/docs/design/Makefile.am b/docs/design/Makefile.am
new file mode 100644 (file)
index 0000000..69c452b
--- /dev/null
+++ b/docs/design/Makefile.am
@@ -0,0 +1,6 @@
+SUBDIRS = 
+
+
+EXTRA_DIST = \
+       design-rtpcollision.txt
+

diff --git a/docs/design/design-rtpcollision.txt b/docs/design/design-rtpcollision.txt
new file mode 100644 (file)
index 0000000..816802c
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+++ b/docs/design/design-rtpcollision.txt
@@ -0,0 +1,43 @@
+RTP collision design
+
+
+GstRTPCollision
+---------------
+
+Custon upstream event which contains the ssrc marked as collided.
+
+This event is generated on both pipeline sender and receiver side by
+the gstrtpsession element when it detects a conflict between ssrc.
+(same session id and same ssrc)
+
+It's an upstream event so that means this event is for now only
+useful on pipeline sender side. Because rtppayloader and rtpaux elements
+are placed upstream from the gstrtpsession.
+
+On pipeline receiver side, gstrtpsession is the most upstream element
+compared to other rtp elements like rtpauxreceive, ssrcdemux, rtpjitterbuffer
+and rtpdepayloader.
+
+rtppayloader
+------------
+
+When handling a GstRTPCollision event, the rtppayloader has to choose another
+ssrc. Actually this is the gstrtpsession that suggests him a newer ssrc through
+the caps.
+
+BYE only the corresponding source, not the whole session.
+---------------------------------------------------------
+
+When a collision happens for the given ssrc, the associated source is marked
+bye. But we make sure that the whole session is not itself set bye.
+Because internally, gstrtpsession can manages several sources and all have
+their own distinct ssrc.
+
+For example when using rtprtxreceive, it uses one session which contains
+2 internal rtpsources. One for the master stream, and one for the rtx stream.
+Actually in this case there are always 3 sources because the session always
+has an other internal one, maybe for rtcp but I'm still nore sure about that.
+Then gstrtpsession make sure to only bye the rtx stream if this is the one
+which collided.
+Then gstrtpsession make sure to only bye the master stream if this is the one
+which collided.