doc/protocols: document "srtp" protocol

Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/protocols.texi

@@ -716,6 +716,32 @@ If set to any value, listen for an incoming connection. Outgoing connection is d
 Set the maximum number of streams. By default no limit is set.
 @end table
 
+@section srtp
+
+Secure Real-time Transport Protocol.
+
+The accepted options are:
+@table @option
+@item srtp_in_suite
+@item srtp_out_suite
+Select input and output encoding suites.
+
+Supported values:
+@table @samp
+@item AES_CM_128_HMAC_SHA1_80
+@item SRTP_AES128_CM_HMAC_SHA1_80
+@item AES_CM_128_HMAC_SHA1_32
+@item SRTP_AES128_CM_HMAC_SHA1_32
+@end table
+
+@item srtp_in_params
+@item srtp_out_params
+Set input and output encoding parameters, which are expressed by a
+base64-encoded representation of a binary block. The first 16 bytes of
+this binary block are used as master key, the following 14 bytes are
+used as master salt.
+@end table
+
 @section tcp
 
 Trasmission Control Protocol.