--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/doc/RTP-analysis	Sun Dec 11 22:20:36 2022 +0000
@@ -0,0 +1,20 @@
+The present package includes a utility named rtp-gsmfr-extr; this utility
+extracts a single RTP stream in either FR1 or EFR codec format from a pcap file,
+presumably captured with tcpdump on an IP network serving either an IP-based BTS
+or a gateway from an E1-based BTS to RTP - the intent is to extract a GSM call
+uplink that has been rendered into an RTP stream by a BTS.  The RTP stream being
+extracted must be fully continuous without any gaps, using Themyscira
+RTP-BFI-extension BFI marker packets in those 20 ms windows where no good
+traffic frame has been received.  rtp-gsmfr-extr verifies continuity of the RTP
+stream being extracted: any detected discontinuity (either a sequence number
+jump indicating packet loss or a timestamp jump indicating an intentional gap
+generated at the source) will be reported, and the extraction will stop there.
+
+To run rtp-gsmfr-extr, you need to have a pcap file (obviously), and you need to
+identify the RTP stream to be extracted by either source or destination IP:port.
+rtp-gsmfr-extr will look at every UDP packet that matches this src-IP-port or
+dest-IP-port filter, and then check for valid RTP, verify the expected increment
+in sequence and timestamp numbers, check for either FR1 or EFR payload (or a
+Themyscira BFI marker for FR & EFR), and finally write the extracted frame
+stream to a gsmx file.  This gsmx output can then be analyzed with gsmrec-dump,
+or decoded to playable WAV with gsmfr-decode or gsmefr-decode.