sequencer.go

// SPDX-FileCopyrightText: 2023 The Pion community <https://pion.ly>
// SPDX-License-Identifier: MIT

package rtp

import (
	"sync"
)

// Sequencer generates sequential sequence numbers for building RTP packets
type Sequencer interface {
	NextSequenceNumber() uint16
	RollOverCount() uint64
}

// maxInitialRandomSequenceNumber is the maximum value used for the initial sequence
// number when using NewRandomSequencer().
// This uses only half the potential sequence number space to avoid issues decrypting
// SRTP when the sequence number starts near the rollover and there is packet loss.
// See https://webrtc-review.googlesource.com/c/src/+/358360
const maxInitialRandomSequenceNumber = 1<<15 - 1

// NewRandomSequencer returns a new sequencer starting from a random sequence
// number
func NewRandomSequencer() Sequencer {
	return &sequencer{
		sequenceNumber: uint16(globalMathRandomGenerator.Intn(maxInitialRandomSequenceNumber)),
	}
}

// NewFixedSequencer returns a new sequencer starting from a specific
// sequence number
func NewFixedSequencer(s uint16) Sequencer {
	return &sequencer{
		sequenceNumber: s - 1, // -1 because the first sequence number prepends 1
	}
}

type sequencer struct {
	sequenceNumber uint16
	rollOverCount  uint64
	mutex          sync.Mutex
}

// NextSequenceNumber increment and returns a new sequence number for
// building RTP packets
func (s *sequencer) NextSequenceNumber() uint16 {
	s.mutex.Lock()
	defer s.mutex.Unlock()

	s.sequenceNumber++
	if s.sequenceNumber == 0 {
		s.rollOverCount++
	}

	return s.sequenceNumber
}

// RollOverCount returns the amount of times the 16bit sequence number
// has wrapped
func (s *sequencer) RollOverCount() uint64 {
	s.mutex.Lock()
	defer s.mutex.Unlock()

	return s.rollOverCount
}