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packet.go
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packet.go
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package rtp
import (
"encoding/binary"
"github.com/pkg/errors"
)
// Packet represents an RTP Packet
// RTP is a network protocol for delivering audio and video over IP networks.
type Packet struct {
Raw []byte
Version uint8
Padding bool
Extension bool
Marker bool
PayloadOffset int
PayloadType uint8
SequenceNumber uint16
Timestamp uint32
SSRC uint32
CSRC []uint32
ExtensionProfile uint16
ExtensionPayload []byte
Payload []byte
}
const (
headerLength = 4
versionShift = 6
versionMask = 0x3
paddingShift = 5
paddingMask = 0x1
extensionShift = 4
extensionMask = 0x1
ccMask = 0xF
markerShift = 7
markerMask = 0x1
ptMask = 0x7F
seqNumOffset = 2
seqNumLength = 2
timestampOffset = 4
timestampLength = 4
ssrcOffset = 8
ssrcLength = 4
csrcOffset = 12
csrcLength = 4
)
// Unmarshal parses the passed byte slice and stores the result in the Packet this method is called upon
func (p *Packet) Unmarshal(rawPacket []byte) error {
if len(rawPacket) < headerLength {
return errors.Errorf("RTP header size insufficient; %d < %d", len(rawPacket), headerLength)
}
/*
* 0 1 2 3
* 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* |V=2|P|X| CC |M| PT | sequence number |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* | timestamp |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* | synchronization source (SSRC) identifier |
* +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
* | contributing source (CSRC) identifiers |
* | .... |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
*/
p.Version = rawPacket[0] >> versionShift & versionMask
p.Padding = (rawPacket[0] >> paddingShift & paddingMask) > 0
p.Extension = (rawPacket[0] >> extensionShift & extensionMask) > 0
p.CSRC = make([]uint32, rawPacket[0]&ccMask)
p.Marker = (rawPacket[1] >> markerShift & markerMask) > 0
p.PayloadType = rawPacket[1] & ptMask
p.SequenceNumber = binary.BigEndian.Uint16(rawPacket[seqNumOffset : seqNumOffset+seqNumLength])
p.Timestamp = binary.BigEndian.Uint32(rawPacket[timestampOffset : timestampOffset+timestampLength])
p.SSRC = binary.BigEndian.Uint32(rawPacket[ssrcOffset : ssrcOffset+ssrcLength])
currOffset := csrcOffset + (len(p.CSRC) * csrcLength)
if len(rawPacket) < currOffset {
return errors.Errorf("RTP header size insufficient; %d < %d", len(rawPacket), currOffset)
}
for i := range p.CSRC {
offset := csrcOffset + (i * csrcLength)
p.CSRC[i] = binary.BigEndian.Uint32(rawPacket[offset:offset])
}
if p.Extension {
p.ExtensionProfile = binary.BigEndian.Uint16(rawPacket[currOffset:])
currOffset += 2
extensionLength := binary.BigEndian.Uint16(rawPacket[currOffset:])
currOffset += 2
p.ExtensionPayload = rawPacket[currOffset : currOffset+int(extensionLength)]
currOffset += len(p.ExtensionPayload) * 4
}
p.Payload = rawPacket[currOffset:]
p.PayloadOffset = currOffset
p.Raw = rawPacket
return nil
}
// Marshal returns a raw RTP packet for the instance it is called upon
func (p *Packet) Marshal() ([]byte, error) {
/*
* 0 1 2 3
* 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* |V=2|P|X| CC |M| PT | sequence number |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* | timestamp |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* | synchronization source (SSRC) identifier |
* +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
* | contributing source (CSRC) identifiers |
* | .... |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
*/
rawPacketLength := 12 + (len(p.CSRC) * csrcLength)
if p.Extension {
rawPacketLength += 4 + len(p.ExtensionPayload)
}
rawPacket := make([]byte, rawPacketLength)
rawPacket[0] |= p.Version << versionShift
if p.Padding {
rawPacket[0] |= 1 << paddingShift
}
if p.Extension {
rawPacket[0] |= 1 << extensionShift
}
rawPacket[0] |= uint8(len(p.CSRC))
if p.Marker {
rawPacket[1] |= 1 << markerShift
}
rawPacket[1] |= p.PayloadType
binary.BigEndian.PutUint16(rawPacket[seqNumOffset:], p.SequenceNumber)
binary.BigEndian.PutUint32(rawPacket[timestampOffset:], p.Timestamp)
binary.BigEndian.PutUint32(rawPacket[ssrcOffset:], p.SSRC)
for i, csrc := range p.CSRC {
binary.BigEndian.PutUint32(rawPacket[csrcOffset+(i*csrcLength):], csrc)
}
currOffset := csrcOffset + (len(p.CSRC) * csrcLength)
for i := range p.CSRC {
offset := csrcOffset + (i * csrcLength)
p.CSRC[i] = binary.BigEndian.Uint32(rawPacket[offset:offset])
}
if p.Extension {
binary.BigEndian.PutUint16(rawPacket[currOffset:], p.ExtensionProfile)
currOffset += 2
binary.BigEndian.PutUint16(rawPacket[currOffset:], uint16(len(p.ExtensionPayload))/4)
currOffset += 2
copy(rawPacket[currOffset:], p.ExtensionPayload)
}
p.PayloadOffset = csrcOffset + (len(p.CSRC) * csrcLength)
rawPacket = append(rawPacket, p.Payload...)
p.Raw = rawPacket
return rawPacket, nil
}