2019-04-05 06:59:04 +03:00
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/*
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NAME
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parse.go
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DESCRIPTION
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parse.go provides functionality for parsing RTP packets.
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AUTHOR
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Saxon A. Nelson-Milton <saxon@ausocean.org>
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LICENSE
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Copyright (C) 2019 the Australian Ocean Lab (AusOcean)
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This is free software: you can redistribute it and/or modify them
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under the terms of the GNU General Public License as published by the
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Free Software Foundation, either version 3 of the License, or (at your
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option) any later version.
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It is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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for more details.
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You should have received a copy of the GNU General Public License
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2019-04-05 07:03:47 +03:00
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in gpl.txt. If not, see [GNU licenses](http://www.gnu.org/licenses).
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2019-04-05 06:59:04 +03:00
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*/
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package rtp
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import (
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"encoding/binary"
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"errors"
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)
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const badVer = "incompatible RTP version"
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// Payload returns the payload from an RTP packet provided the version is
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// compatible, otherwise an error is returned.
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func Payload(d []byte) ([]byte, error) {
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if version(d) != rtpVer {
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return nil, errors.New(badVer)
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}
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extLen := 0
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if hasExt(d) {
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extLen = 4 + 4*extHeaderLen(d)
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}
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payloadIdx := optionalFieldIdx + 4*csrcCount(d) + extLen
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return d[payloadIdx:], nil
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}
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// extHeaderLen returns the extension header length. The RTP packet must have
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// a compatible version, and must have an extension field, otherwise we panic.
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func extHeaderLen(d []byte) int {
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if version(d) != rtpVer {
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panic(badVer)
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}
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if !hasExt(d) {
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panic("RTP packet does not have extension")
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}
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extIdx := optionalFieldIdx + 4*csrcCount(d)
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return int(binary.BigEndian.Uint16(d[extIdx+2 : extIdx+4]))
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}
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// hasExt returns true if an extension is present in the RTP packet. The version
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// must be compatible otherwise we panic.
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func hasExt(d []byte) bool {
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if version(d) != rtpVer {
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panic(badVer)
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}
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if (d[0] & 0x10 >> 4) == 1 {
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return true
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}
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return false
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}
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// csrcCount returns the number of CSRC fields. If the version is not compatible
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// we panic.
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func csrcCount(d []byte) int {
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if version(d) != rtpVer {
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panic(badVer)
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}
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return int(d[0] & 0x0f)
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}
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// version returns the version of the RTP packet.
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func version(d []byte) int {
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return int(d[0] & 0xb0 >> 4)
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}
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