protocol/rtp: added parse.go file with parsing functionality

Added parse.go. This file contains functionality to obtain the Payload from an RTP packet.
Unexported functions to help with this extraction have been added; extHeaderLen, hasExt,
csrcCount and version.

protocol/rtp/parse.go

@@ -0,0 +1,88 @@
+/*
+NAME
+  parse.go
+
+DESCRIPTION
+  parse.go provides functionality for parsing RTP packets.
+
+AUTHOR
+  Saxon A. Nelson-Milton <saxon@ausocean.org>
+
+LICENSE
+  Copyright (C) 2019 the Australian Ocean Lab (AusOcean)
+
+  This is free software: you can redistribute it and/or modify them
+  under the terms of the GNU General Public License as published by the
+  Free Software Foundation, either version 3 of the License, or (at your
+  option) any later version.
+
+  It is distributed in the hope that it will be useful, but WITHOUT
+  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+ for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with revid in gpl.txt.  If not, see [GNU licenses](http://www.gnu.org/licenses).
+*/
+
+package rtp
+
+import (
+	"encoding/binary"
+	"errors"
+)
+
+const badVer = "incompatible RTP version"
+
+// Payload returns the payload from an RTP packet provided the version is
+// compatible, otherwise an error is returned.
+func Payload(d []byte) ([]byte, error) {
+	if version(d) != rtpVer {
+		return nil, errors.New(badVer)
+	}
+	extLen := 0
+	if hasExt(d) {
+		extLen = 4 + 4*extHeaderLen(d)
+	}
+	payloadIdx := optionalFieldIdx + 4*csrcCount(d) + extLen
+	return d[payloadIdx:], nil
+}
+
+// extHeaderLen returns the extension header length. The RTP packet must have
+// a compatible version, and must have an extension field, otherwise we panic.
+func extHeaderLen(d []byte) int {
+	if version(d) != rtpVer {
+		panic(badVer)
+	}
+	if !hasExt(d) {
+		panic("RTP packet does not have extension")
+	}
+	extIdx := optionalFieldIdx + 4*csrcCount(d)
+	return int(binary.BigEndian.Uint16(d[extIdx+2 : extIdx+4]))
+}
+
+// hasExt returns true if an extension is present in the RTP packet. The version
+// must be compatible otherwise we panic.
+func hasExt(d []byte) bool {
+	if version(d) != rtpVer {
+		panic(badVer)
+	}
+	if (d[0] & 0x10 >> 4) == 1 {
+		return true
+	}
+	return false
+}
+
+// csrcCount returns the number of CSRC fields. If the version is not compatible
+// we panic.
+func csrcCount(d []byte) int {
+	if version(d) != rtpVer {
+		panic(badVer)
+	}
+	return int(d[0] & 0x0f)
+}
+
+// version returns the version of the RTP packet.
+func version(d []byte) int {
+	return int(d[0] & 0xb0 >> 4)
+}

protocol/rtp/rtp.go

@@ -41,6 +41,7 @@ const (
 	defaultHeadSize  = 3 * 4                            // Header size of an rtp packet.
 	defPayloadSize   = sendSize                         // Default payload size for the rtp packet.
 	defPktSize       = defaultHeadSize + defPayloadSize // Default packet size is header size + payload size.
+	optionalFieldIdx = 12                               // This is the idx of optional fields including CSRC and extension header in an RTP packet.
 )
 
 // Pkt provides fields consistent with RFC3550 definition of an rtp packet