krtp/lib/Packet.ts

Node implementation of rtp protocol. RFC 3550

/*
 * +===============================================
 * | Author:        Parham Alvani (parham.alvani@gmail.com)
 * |
 * | Creation Date: 01-06-2017
 * |
 * | File Name:     packet.ts
 * +===============================================
 */
/**
 */

export class Packet {
  public readonly payload: Buffer;
  public readonly sequenceNumber: number;
  public readonly ssrc: number;
  public readonly timestamp: number;
  public readonly payloadType: number;

  private csrc: number[] = [];

  constructor(
    payload: Buffer,
    sequenceNumber: number,
    ssrc: number,
    timestamp: number = 0,
    payloadType: number = 95
  ) {
    this.payload = payload;
    this.sequenceNumber = sequenceNumber;
    this.ssrc = ssrc;
    this.timestamp = timestamp;
    this.payloadType = payloadType;
  }

  public addCSRC(csrc: number): void {
    /* TODO: csrc must be a 32 bit integer */
    this.csrc.push(csrc);
  }

  public serialize(): Buffer {
    const buff: Buffer = Buffer.alloc(
      12 + 4 * this.csrc.length + this.payload.length
    );

    /* buff[0] = (V << 6 | P << 5 | X << 4 | CC) */
    buff[0] = (2 << 6) | (0 << 5) | (0 << 4) | this.csrc.length;
    /* buff[1] = (M << 7 | PT) */
    buff[1] = (0 << 7) | this.payloadType;
    /* buff[2, 3] = SN */
    buff.writeUInt16BE(this.sequenceNumber, 2);
    /* buff[4, 5, 6, 7] = TS */
    buff.writeUInt32BE(this.timestamp, 4);
    /* buff[8, 9, 10, 11] = SSRC */
    buff.writeUInt32BE(this.ssrc, 8);

    /* CSRC section */
    for (let i = 0; i < this.csrc.length; i++) {
      const csrc = this.csrc[i];
      if (csrc === undefined) {
        continue;
      }
      buff.writeUInt32BE(csrc, 12 + i * 4);
    }

    this.payload.copy(buff, 12 + this.csrc.length * 4, 0);

    return buff;
  }

  static deserialize(buff: Buffer): Packet {
    if (buff.length < 12) {
      throw new Error("invalid rtp packet");
    }
    const csrc: Array<number> = [];

    /* buff[0] = (V << 6 | P << 5 | X << 4 | CC) */
    if ((buff[0]! & 0xc0) >> 6 !== 2) {
      throw new Error(`invalid rtp packet ${buff[0]! & 0xc0}`);
    }
    const cc = buff[0]! & 0x0f;
    if (buff.length < 12 + cc * 4) {
      throw new Error("invalid rtp packet");
    }
    /* buff[1] = (M << 7 | PT) */
    const payloadType = buff[1]! & 0x7f;
    /* buff[2, 3] = SN */
    const sequenceNumber = buff.readUInt16BE(2);
    /* buff[4, 5, 6, 7] = TS */
    const timestamp = buff.readUInt32BE(4);
    /* buff[8, 9, 10, 11] = SSRC */
    const ssrc = buff.readUInt32BE(8);

    /* CSRC section */
    for (let i = 0; i < cc; i++) {
      csrc.push(buff.readUInt32BE(12 + i * 4));
    }

    const payload: Buffer = Buffer.from(buff.slice(12 + csrc.length * 4));

    const packet: Packet = new Packet(
      payload,
      sequenceNumber,
      ssrc,
      timestamp,
      payloadType
    );

    for (const s of csrc) {
      packet.addCSRC(s);
    }

    return packet;
  }
}