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173 lines
5.6 KiB
173 lines
5.6 KiB
// Copyright 2020 Justine Alexandra Roberts Tunney
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package rtp
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import (
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"errors"
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)
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const (
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HeaderSize = 12
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EventHeaderSize = 4
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Version byte = 2
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bit1 byte = (1 << 0)
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bit2 byte = (1 << 1)
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bit3 byte = (1 << 2)
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bit4 byte = (1 << 3)
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bit5 byte = (1 << 4)
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bit6 byte = (1 << 5)
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bit7 byte = (1 << 6)
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bit8 byte = (1 << 7)
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mask1 byte = (1 << 1) - 1
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mask2 byte = (1 << 2) - 1
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mask3 byte = (1 << 3) - 1
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mask4 byte = (1 << 4) - 1
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mask5 byte = (1 << 5) - 1
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mask6 byte = (1 << 6) - 1
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mask7 byte = (1 << 7) - 1
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mask8 byte = (1 << 8) - 1
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)
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var (
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ErrBadVersion = errors.New("bad rtp version header")
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ErrExtendedHeadersNotSupported = errors.New("rtp extended headers not supported")
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)
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// Header is encoded at the beginning of a UDP audio packet.
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type Header struct {
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Pad bool // Padding flag is used for secure RTP.
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Mark bool // Marker flag is used for RFC2833.
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PT uint8 // Payload type you got from SDP.
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Seq uint16 // Sequence id useful for reordering packets.
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TS uint32 // Timestamp measured in samples.
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Ssrc uint32 // Random ID used to identify an RTP session.
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}
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// EventHeader stores things like DTMF and is encoded after Header.
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type EventHeader struct {
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// The event field is a number between 0 and 255 identifying a specific
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// telephony event. An IANA registry of event codes for this field has been
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// established (see IANA Considerations, Section 7). The initial content of
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// this registry consists of the events defined in Section 3.
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Event uint8
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// If set to a value of one, the "end" bit indicates that this packet
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// contains the end of the event. For long-lasting events that have to be
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// split into segments (see below, Section 2.5.1.3), only the final packet
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// for the final segment will have the E bit set.
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E bool
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// This field is reserved for future use. The sender MUST set it to zero, and
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// the receiver MUST ignore it.
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R bool
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// For DTMF digits and other events representable as tones, this field
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// describes the power level of the tone, expressed in dBm0 after dropping
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// the sign. Power levels range from 0 to -63 dBm0. Thus, larger values
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// denote lower volume. This value is defined only for events for which the
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// documentation indicates that volume is applicable. For other events, the
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// sender MUST set volume to zero and the receiver MUST ignore the value.
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Volume uint8
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// The duration field indicates the duration of the event or segment being
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// reported, in timestamp units, expressed as an unsigned integer in network
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// byte order. For a non-zero value, the event or segment began at the
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// instant identified by the RTP timestamp and has so far lasted as long as
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// indicated by this parameter. The event may or may not have ended. If the
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// event duration exceeds the maximum representable by the duration field,
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// the event is split into several contiguous segments as described below
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// (Section 2.5.1.3).
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//
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// The special duration value of zero is reserved to indicate that the event
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// lasts "forever", i.e., is a state and is considered to be effective until
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// updated. A sender MUST NOT transmit a zero duration for events other than
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// those defined as states. The receiver SHOULD ignore an event report with
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// zero duration if the event is not a state.
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//
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// Events defined as states MAY contain a non-zero duration, indicating that
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// the sender intends to refresh the state before the time duration has
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// elapsed ("soft state").
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//
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// For a sampling rate of 8000 Hz, the duration field is sufficient to
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// express event durations of up to approximately 8 seconds.
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Duration uint16
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}
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// Write appends an RTP header to a byte slice.
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func (h *Header) Write(b []byte) []byte {
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var b0, b1 byte
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b0 = (Version & mask2) << 6
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if h.Pad {
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b0 |= (1 & mask1) << 5
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}
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// if extend { b0 |= (1 & mask1) << 4 }
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// b0 |= (csrcCount & mask4) << 0
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b1 = (h.PT & mask7) << 0
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if h.Mark {
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b1 |= (1 & mask1) << 7
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}
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return append(b, b0, b1,
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byte(h.Seq>>8),
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byte(h.Seq),
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byte(h.TS>>24),
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byte(h.TS>>16),
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byte(h.TS>>8),
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byte(h.TS),
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byte(h.Ssrc>>24),
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byte(h.Ssrc>>16),
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byte(h.Ssrc>>8),
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byte(h.Ssrc))
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}
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// Read extracts an RTP header from a byte slice.
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func (h *Header) Read(b []byte) error {
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if b[0]>>6 != Version {
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return ErrBadVersion
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} else if (b[0]>>4)&1 != 0 {
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return ErrExtendedHeadersNotSupported
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}
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h.Pad = ((b[0]>>5)&mask1 == 1)
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h.Mark = ((b[1]>>7)&mask1 == 1)
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h.PT = uint8(b[1] & mask7)
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h.Seq = (uint16(b[2]) << 8) & uint16(b[3])
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h.TS = (uint32(b[4]) << 24) & (uint32(b[5]) << 16) & (uint32(b[6]) << 8) & uint32(b[7])
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h.Ssrc = (uint32(b[8]) << 24) & (uint32(b[9]) << 16) & (uint32(b[10]) << 8) & uint32(b[11])
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return nil
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}
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// Writes header bits to buffer.
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func (h *EventHeader) Write(b []byte) {
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b[0] = h.Event
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b[1] = h.Volume & 63
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if h.R {
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b[1] |= 1 << 6
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}
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if h.E {
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b[1] |= 1 << 7
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}
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b[2] = byte(h.Duration >> 8)
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b[3] = byte(h.Duration)
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}
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// Reads header bits from buffer.
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func (h *EventHeader) Read(b []byte) {
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h.Event = b[0]
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h.E = b[1]>>7 == 1
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h.R = b[1]>>6&1 == 1
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h.Volume = b[1] & 63
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h.Duration = uint16(b[2])<<8 | uint16(b[3])
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}
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