squeezelite-esp32/components/raop/rtp.c

/*
 * HairTunes - RAOP packet handler and slave-clocked replay engine
 * Copyright (c) James Laird 2011
 * All rights reserved.
 *
 * Modularisation: philippe_44@outlook.com, 2019
 *
 * Permission is hereby granted, free of charge, to any person
 * obtaining a copy of this software and associated documentation
 * files (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use,
 * copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
#include <sys/types.h>
#include <pthread.h>
#include <math.h>
#include <errno.h>
#include <sys/stat.h>
#include <stdint.h>
#include <fcntl.h>
#include <assert.h>

#include "platform.h"
#include "rtp.h"
#include "raop_sink.h"
#include "log_util.h"
#include "util.h"

#ifdef WIN32
#include <openssl/aes.h>
#include "alac_wrapper.h"
#else
#include "esp_pthread.h"
#include "esp_system.h"
#include <mbedtls/version.h>
#include <mbedtls/aes.h>
#include "alac_wrapper.h"
#endif

#define NTP2MS(ntp) ((((ntp) >> 10) * 1000L) >> 22)
#define MS2NTP(ms) (((((u64_t) (ms)) << 22) / 1000) << 10)
#define NTP2TS(ntp, rate) ((((ntp) >> 16) * (rate)) >> 16)
#define TS2NTP(ts, rate)  (((((u64_t) (ts)) << 16) / (rate)) << 16)
#define MS2TS(ms, rate) ((((u64_t) (ms)) * (rate)) / 1000)
#define TS2MS(ts, rate) NTP2MS(TS2NTP(ts,rate))


extern log_level 	raop_loglevel;

static log_level 	*loglevel = &raop_loglevel;

//#define __RTP_STORE

// default buffer size
#define BUFFER_FRAMES 	( (150 * RAOP_SAMPLE_RATE * 2) / (352 * 100) )
#define MAX_PACKET       1408
#define MIN_LATENCY		11025
#define MAX_LATENCY   	( (120 * RAOP_SAMPLE_RATE * 2) / 100 )

#define RTP_STACK_SIZE	(4*1024)

#define RTP_SYNC	(0x01)
#define NTP_SYNC	(0x02)

#define RESEND_TO	200

enum { DATA = 0, CONTROL, TIMING };

static const u8_t silence_frame[MAX_PACKET] = { 0 };

typedef u16_t seq_t;
typedef struct audio_buffer_entry {   // decoded audio packets
	int ready;
	u32_t rtptime, last_resend;
	s16_t *data;
	int len;
} abuf_t;

typedef struct rtp_s {
#ifdef __RTP_STORE
	FILE *rtpIN, *rtpOUT;
#endif
	bool running;
	unsigned char aesiv[16];
#ifdef WIN32
	AES_KEY aes;
#else
	mbedtls_aes_context aes;
#endif
	bool decrypt;
	u8_t *decrypt_buf;
	u32_t frame_size, frame_duration;
	u32_t in_frames, out_frames;
	struct in_addr host;
	struct sockaddr_in rtp_host;
	struct {
		unsigned short rport, lport;
		int sock;
	} rtp_sockets[3]; 					 // data, control, timing
	struct timing_s {
		u64_t local, remote;
	} timing;
	struct {
		u32_t 	rtp, time;
		u8_t  	status;
	} synchro;
	struct {
		u32_t time;
		seq_t seqno;
		u32_t rtptime;
	} record;
	int latency;			// rtp hold depth in samples
	u32_t resent_req, resent_rec;	// total resent + recovered frames
	u32_t silent_frames;	// total silence frames
	u32_t discarded;
	abuf_t audio_buffer[BUFFER_FRAMES];
	seq_t ab_read, ab_write;
	pthread_mutex_t ab_mutex;
#ifdef WIN32
	pthread_t thread;
#else
	TaskHandle_t thread, joiner;
	StaticTask_t *xTaskBuffer;
    StackType_t *xStack;
#endif

	struct alac_codec_s *alac_codec;
	int flush_seqno;
	bool playing;
	raop_data_cb_t data_cb;
	raop_cmd_cb_t cmd_cb;
} rtp_t;


#define BUFIDX(seqno) ((seq_t)(seqno) % BUFFER_FRAMES)
static void 	buffer_alloc(abuf_t *audio_buffer, int size);
static void 	buffer_release(abuf_t *audio_buffer);
static void 	buffer_reset(abuf_t *audio_buffer);
static void 	buffer_push_packet(rtp_t *ctx);
static bool 	rtp_request_resend(rtp_t *ctx, seq_t first, seq_t last);
static bool 	rtp_request_timing(rtp_t *ctx);
static void*	rtp_thread_func(void *arg);
static int	  	seq_order(seq_t a, seq_t b);

/*---------------------------------------------------------------------------*/
static struct alac_codec_s* alac_init(int fmtp[32]) {
	struct alac_codec_s *alac;
	unsigned sample_rate;
	unsigned char sample_size, channels;
	struct {
		uint32_t	frameLength;
		uint8_t		compatibleVersion;
		uint8_t		bitDepth;
		uint8_t		pb;
		uint8_t		mb;
		uint8_t		kb;
		uint8_t		numChannels;
		uint16_t	maxRun;
		uint32_t	maxFrameBytes;
		uint32_t	avgBitRate;
		uint32_t	sampleRate;
	} config;

	config.frameLength = htonl(fmtp[1]);
	config.compatibleVersion = fmtp[2];
	config.bitDepth = fmtp[3];
	config.pb = fmtp[4];
	config.mb = fmtp[5];
	config.kb = fmtp[6];
	config.numChannels = fmtp[7];
	config.maxRun = htons(fmtp[8]);
	config.maxFrameBytes = htonl(fmtp[9]);
	config.avgBitRate = htonl(fmtp[10]);
	config.sampleRate = htonl(fmtp[11]);

	alac = alac_create_decoder(sizeof(config), (unsigned char*) &config, &sample_size, &sample_rate, &channels);
	if (!alac) {
		LOG_ERROR("cannot create alac codec", NULL);
		return NULL;
	}

	return alac;
}

/*---------------------------------------------------------------------------*/
rtp_resp_t rtp_init(struct in_addr host, int latency, char *aeskey, char *aesiv, char *fmtpstr,
								short unsigned pCtrlPort, short unsigned pTimingPort,
								raop_cmd_cb_t cmd_cb, raop_data_cb_t data_cb)
{
	int i = 0;
	char *arg;
	int fmtp[12];
	bool rc = true;
	rtp_t *ctx = calloc(1, sizeof(rtp_t));
	rtp_resp_t resp = { 0, 0, 0, NULL };

	if (!ctx) return resp;
	
	ctx->host = host;
	ctx->decrypt = false;
	ctx->cmd_cb = cmd_cb;
	ctx->data_cb = data_cb;
	ctx->rtp_host.sin_family = AF_INET;
	ctx->rtp_host.sin_addr.s_addr = INADDR_ANY;
	pthread_mutex_init(&ctx->ab_mutex, 0);
	ctx->flush_seqno = -1;
	ctx->latency = latency;
	ctx->ab_read = ctx->ab_write;

#ifdef __RTP_STORE
	ctx->rtpIN = fopen("airplay.rtpin", "wb");
	ctx->rtpOUT = fopen("airplay.rtpout", "wb");
#endif

	ctx->rtp_sockets[CONTROL].rport = pCtrlPort;
	ctx->rtp_sockets[TIMING].rport = pTimingPort;

	if (aesiv && aeskey) {
		memcpy(ctx->aesiv, aesiv, 16);
#ifdef WIN32
		AES_set_decrypt_key((unsigned char*) aeskey, 128, &ctx->aes);
#else
		memset(&ctx->aes, 0, sizeof(mbedtls_aes_context));
		mbedtls_aes_setkey_dec(&ctx->aes, (unsigned char*) aeskey, 128);
#endif
		ctx->decrypt = true;
		ctx->decrypt_buf = malloc(MAX_PACKET);
	}

	memset(fmtp, 0, sizeof(fmtp));
	while ((arg = strsep(&fmtpstr, " \t")) != NULL) fmtp[i++] = atoi(arg);

	ctx->frame_size = fmtp[1];
	ctx->frame_duration = (ctx->frame_size * 1000) / RAOP_SAMPLE_RATE;

	// alac decoder
	ctx->alac_codec = alac_init(fmtp);
	rc &= ctx->alac_codec != NULL;

	buffer_alloc(ctx->audio_buffer, ctx->frame_size*4);

	// create rtp ports
	for (i = 0; i < 3; i++) {
		ctx->rtp_sockets[i].sock = bind_socket(&ctx->rtp_sockets[i].lport, SOCK_DGRAM);
		rc &= ctx->rtp_sockets[i].sock > 0;
	}

	// create http port and start listening
	resp.cport = ctx->rtp_sockets[CONTROL].lport;
	resp.tport = ctx->rtp_sockets[TIMING].lport;
	resp.aport = ctx->rtp_sockets[DATA].lport;
	
	if (rc) {
		ctx->running = true;
#ifdef WIN32
		pthread_create(&ctx->thread, NULL, rtp_thread_func, (void *) ctx);
#else
		// xTaskCreate((TaskFunction_t) rtp_thread_func, "RTP_thread", RTP_TASK_SIZE, ctx,  CONFIG_ESP32_PTHREAD_TASK_PRIO_DEFAULT + 1 , &ctx->thread);
		ctx->xTaskBuffer = (StaticTask_t*) heap_caps_malloc(sizeof(StaticTask_t), MALLOC_CAP_INTERNAL | MALLOC_CAP_8BIT);
		ctx->xStack = (StackType_t*) malloc(RTP_STACK_SIZE);
		ctx->thread = xTaskCreateStatic( (TaskFunction_t) rtp_thread_func, "RTP_thread", RTP_STACK_SIZE, ctx, 
										 CONFIG_ESP32_PTHREAD_TASK_PRIO_DEFAULT + 1, ctx->xStack, ctx->xTaskBuffer );
#endif
	} else {
		rtp_end(ctx);
		ctx = NULL;
	}

	resp.ctx = ctx;

	return resp;
}

/*---------------------------------------------------------------------------*/
void rtp_end(rtp_t *ctx)
{
	int i;

	if (!ctx) return;

	if (ctx->running) {
#if !defined WIN32		
		ctx->joiner = xTaskGetCurrentTaskHandle();
#endif
		ctx->running = false;
#ifdef WIN32
		pthread_join(ctx->thread, NULL);
#else
		xTaskNotifyWait(0, 0, NULL, portMAX_DELAY);
		free(ctx->xStack);
		heap_caps_free(ctx->xTaskBuffer);
#endif
	}
	
	for (i = 0; i < 3; i++) closesocket(ctx->rtp_sockets[i].sock);

	if (ctx->alac_codec) alac_delete_decoder(ctx->alac_codec);
	if (ctx->decrypt_buf) free(ctx->decrypt_buf);
	
	pthread_mutex_destroy(&ctx->ab_mutex);
	buffer_release(ctx->audio_buffer);
	
	free(ctx);

#ifdef __RTP_STORE
	fclose(ctx->rtpIN);
	fclose(ctx->rtpOUT);
#endif
}

/*---------------------------------------------------------------------------*/
bool rtp_flush(rtp_t *ctx, unsigned short seqno, unsigned int rtptime)
{
	bool rc = true;
	u32_t now = gettime_ms();

	if (now < ctx->record.time + 250 || (ctx->record.seqno == seqno && ctx->record.rtptime == rtptime)) {
		rc = false;
		LOG_ERROR("[%p]: FLUSH ignored as same as RECORD (%hu - %u)", ctx, seqno, rtptime);
	} else {
		pthread_mutex_lock(&ctx->ab_mutex);
		buffer_reset(ctx->audio_buffer);
		ctx->playing = false;
		ctx->flush_seqno = seqno;
		pthread_mutex_unlock(&ctx->ab_mutex);
	}

	LOG_INFO("[%p]: flush %hu %u", ctx, seqno, rtptime);

	return rc;
}

/*---------------------------------------------------------------------------*/
void rtp_record(rtp_t *ctx, unsigned short seqno, unsigned rtptime)
{
	ctx->record.seqno = seqno;
	ctx->record.rtptime = rtptime;
	ctx->record.time = gettime_ms();

	LOG_INFO("[%p]: record %hu %u", ctx, seqno, rtptime);
}

/*---------------------------------------------------------------------------*/
static void buffer_alloc(abuf_t *audio_buffer, int size) {
	int i;
	for (i = 0; i < BUFFER_FRAMES; i++) {
		audio_buffer[i].data = malloc(size);
		audio_buffer[i].ready = 0;
	}
}

/*---------------------------------------------------------------------------*/
static void buffer_release(abuf_t *audio_buffer) {
	int i;
	for (i = 0; i < BUFFER_FRAMES; i++) {
		free(audio_buffer[i].data);
	}
}

/*---------------------------------------------------------------------------*/
static void buffer_reset(abuf_t *audio_buffer) {
	int i;
	for (i = 0; i < BUFFER_FRAMES; i++) audio_buffer[i].ready = 0;
}

/*---------------------------------------------------------------------------*/
// the sequence numbers will wrap pretty often.
// this returns true if the second arg is after the first
static int seq_order(seq_t a, seq_t b) {
	s16_t d = b - a;
	return d > 0;
}

/*---------------------------------------------------------------------------*/
static void alac_decode(rtp_t *ctx, s16_t *dest, char *buf, int len, int *outsize) {
	unsigned char iv[16];
	int aeslen;
	assert(len<=MAX_PACKET);

	if (ctx->decrypt) {
		aeslen = len & ~0xf;
		memcpy(iv, ctx->aesiv, sizeof(iv));
#ifdef WIN32
		AES_cbc_encrypt((unsigned char*)buf, ctx->decrypt_buf, aeslen, &ctx->aes, iv, AES_DECRYPT);
#else
		mbedtls_aes_crypt_cbc(&ctx->aes, MBEDTLS_AES_DECRYPT, aeslen, iv, (unsigned char*) buf, ctx->decrypt_buf);
#endif
		memcpy(ctx->decrypt_buf+aeslen, buf+aeslen, len-aeslen);
		alac_to_pcm(ctx->alac_codec, (unsigned char*) ctx->decrypt_buf, (unsigned char*) dest, 2, (unsigned int*) outsize);
	} else {
		alac_to_pcm(ctx->alac_codec, (unsigned char*) buf, (unsigned char*) dest, 2, (unsigned int*) outsize);
	}	
	
	*outsize *= 4;
}


/*---------------------------------------------------------------------------*/
static void buffer_put_packet(rtp_t *ctx, seq_t seqno, unsigned rtptime, bool first, char *data, int len) {
	abuf_t *abuf = NULL;
	u32_t playtime;

	pthread_mutex_lock(&ctx->ab_mutex);

	if (!ctx->playing) {
		if ((ctx->flush_seqno == -1 || seq_order(ctx->flush_seqno, seqno)) &&
		   (ctx->synchro.status & RTP_SYNC) && (ctx->synchro.status & NTP_SYNC)) {
			ctx->ab_write = seqno-1;
			ctx->ab_read = seqno;
			ctx->flush_seqno = -1;
			ctx->playing = true;
			ctx->resent_req = ctx->resent_rec = ctx->silent_frames = ctx->discarded = 0;
			playtime = ctx->synchro.time + (((s32_t)(rtptime - ctx->synchro.rtp)) * 1000) / RAOP_SAMPLE_RATE;
			ctx->cmd_cb(RAOP_PLAY, &playtime);
		} else {
			pthread_mutex_unlock(&ctx->ab_mutex);
			return;
		}
	}

	if (seqno == (u16_t) (ctx->ab_write+1)) {
		// expected packet
		abuf = ctx->audio_buffer + BUFIDX(seqno);
		ctx->ab_write = seqno;
		LOG_SDEBUG("packet expected seqno:%hu rtptime:%u (W:%hu R:%hu)", seqno, rtptime, ctx->ab_write, ctx->ab_read);

	} else if (seq_order(ctx->ab_write, seqno)) {
		// newer than expected
		if (ctx->latency && seq_order(ctx->latency / ctx->frame_size, seqno - ctx->ab_write - 1)) {
			// only get rtp latency-1 frames back (last one is seqno)
			LOG_WARN("[%p] too many missing frames %hu seq: %hu, (W:%hu R:%hu)", ctx, seqno - ctx->ab_write - 1, seqno, ctx->ab_write, ctx->ab_read);
			ctx->ab_write = seqno - ctx->latency / ctx->frame_size;
		}
		if (ctx->latency && seq_order(ctx->latency / ctx->frame_size, seqno - ctx->ab_read)) {
			// if ab_read is lagging more than http latency, advance it
			LOG_WARN("[%p] on hold for too long %hu (W:%hu R:%hu)", ctx, seqno - ctx->ab_read + 1, ctx->ab_write, ctx->ab_read);
			ctx->ab_read = seqno - ctx->latency / ctx->frame_size + 1;
		}
		if (rtp_request_resend(ctx, ctx->ab_write + 1, seqno-1)) {
			seq_t i;
			u32_t now = gettime_ms();
			for (i = ctx->ab_write + 1; seq_order(i, seqno); i++) {
				ctx->audio_buffer[BUFIDX(i)].rtptime = rtptime - (seqno-i)*ctx->frame_size;
				ctx->audio_buffer[BUFIDX(i)].last_resend = now;
			}
		}
		LOG_DEBUG("[%p]: packet newer seqno:%hu rtptime:%u (W:%hu R:%hu)", ctx, seqno, rtptime, ctx->ab_write, ctx->ab_read);
		abuf = ctx->audio_buffer + BUFIDX(seqno);
		ctx->ab_write = seqno;
	} else if (seq_order(ctx->ab_read, seqno + 1)) {
		// recovered packet, not yet sent
		abuf = ctx->audio_buffer + BUFIDX(seqno);
		ctx->resent_rec++;
		LOG_DEBUG("[%p]: packet recovered seqno:%hu rtptime:%u (W:%hu R:%hu)", ctx, seqno, rtptime, ctx->ab_write, ctx->ab_read);
	} else {
		// too late
		LOG_DEBUG("[%p]: packet too late seqno:%hu rtptime:%u (W:%hu R:%hu)", ctx, seqno, rtptime, ctx->ab_write, ctx->ab_read);
	}

	if (ctx->in_frames++ > 1000) {
		LOG_INFO("[%p]: fill [level:%hu rec:%u] [W:%hu R:%hu]", ctx, ctx->ab_write - ctx->ab_read, ctx->resent_rec, ctx->ab_write, ctx->ab_read);
		ctx->in_frames = 0;
	}

	if (abuf) {
		alac_decode(ctx, abuf->data, data, len, &abuf->len);
		abuf->ready = 1;
		// this is the local rtptime when this frame is expected to play
		abuf->rtptime = rtptime;
		buffer_push_packet(ctx);

#ifdef __RTP_STORE
		fwrite(data, len, 1, ctx->rtpIN);
		fwrite(abuf->data, abuf->len, 1, ctx->rtpOUT);
#endif
	}

	pthread_mutex_unlock(&ctx->ab_mutex);
}

/*---------------------------------------------------------------------------*/
// push as many frames as possible through callback
static void buffer_push_packet(rtp_t *ctx) {
	abuf_t *curframe = NULL;
	u32_t now, playtime, hold = max((ctx->latency * 1000) / (8 * RAOP_SAMPLE_RATE), 100);
	int i;

	// not ready to play yet
	if (!ctx->playing ||  ctx->synchro.status != (RTP_SYNC | NTP_SYNC)) return;

	// maybe re-evaluate time in loop in case data callback blocks ...
	now = gettime_ms();

	// there is always at least one frame in the buffer
	do {

		curframe = ctx->audio_buffer + BUFIDX(ctx->ab_read);
		playtime = ctx->synchro.time + (((s32_t)(curframe->rtptime - ctx->synchro.rtp)) * 1000) / RAOP_SAMPLE_RATE;

		if (now > playtime) {
			LOG_DEBUG("[%p]: discarded frame now:%u missed by:%d (W:%hu R:%hu)", ctx, now, now - playtime, ctx->ab_write, ctx->ab_read);
			ctx->discarded++;
			curframe->ready = 0;
		} else if (curframe->ready) {
			ctx->data_cb((const u8_t*) curframe->data, curframe->len, playtime);
			curframe->ready = 0;
		} else if (playtime - now <= hold) {
			LOG_DEBUG("[%p]: created zero frame (W:%hu R:%hu)", ctx, ctx->ab_write, ctx->ab_read);
			ctx->data_cb(silence_frame, ctx->frame_size * 4, playtime);
			ctx->silent_frames++;
		} else break;

		ctx->ab_read++;
		ctx->out_frames++;

	} while (seq_order(ctx->ab_read, ctx->ab_write));

	if (ctx->out_frames > 1000) {
		LOG_INFO("[%p]: drain [level:%hd head:%d ms] [W:%hu R:%hu] [req:%u sil:%u dis:%u]",
				ctx, ctx->ab_write - ctx->ab_read, playtime - now, ctx->ab_write, ctx->ab_read,
				ctx->resent_req, ctx->silent_frames, ctx->discarded);
		ctx->out_frames = 0;
	}

	LOG_SDEBUG("playtime %u %d [W:%hu R:%hu] %d", playtime, playtime - now, ctx->ab_write, ctx->ab_read, curframe->ready);

	// each missing packet will be requested up to (latency_frames / 16) times
	for (i = 0; seq_order(ctx->ab_read + i, ctx->ab_write); i += 16) {
		abuf_t *frame = ctx->audio_buffer + BUFIDX(ctx->ab_read + i);
		if (!frame->ready && now - frame->last_resend > RESEND_TO) {
			rtp_request_resend(ctx, ctx->ab_read + i, ctx->ab_read + i);
			frame->last_resend = now;
		}
	}

}


/*---------------------------------------------------------------------------*/
static void *rtp_thread_func(void *arg) {
	fd_set fds;
	int i, sock = -1;
	int count = 0;
	bool ntp_sent;
	char *packet = malloc(MAX_PACKET);
	rtp_t *ctx = (rtp_t*) arg;

	for (i = 0; i < 3; i++) {
		if (ctx->rtp_sockets[i].sock > sock) sock = ctx->rtp_sockets[i].sock;
		// send synchro requets 3 times
		ntp_sent = rtp_request_timing(ctx);
	}

	while (ctx->running) {
		ssize_t plen;
		char type;
		socklen_t rtp_client_len = sizeof(struct sockaddr_storage);
		int idx = 0;
		char *pktp = packet;
		struct timeval timeout = {0, 100*1000};

		FD_ZERO(&fds);
		for (i = 0; i < 3; i++)	{ FD_SET(ctx->rtp_sockets[i].sock, &fds); }

		if (select(sock + 1, &fds, NULL, NULL, &timeout) <= 0) continue;

		for (i = 0; i < 3; i++)
			if (FD_ISSET(ctx->rtp_sockets[i].sock, &fds)) idx = i;

		plen = recvfrom(ctx->rtp_sockets[idx].sock, packet, MAX_PACKET, 0, (struct sockaddr*) &ctx->rtp_host, &rtp_client_len);

		if (!ntp_sent) {
			LOG_WARN("[%p]: NTP request not send yet", ctx);
			ntp_sent = rtp_request_timing(ctx);
		}

		if (plen < 0) continue;
		assert(plen <= MAX_PACKET);

		type = packet[1] & ~0x80;
		pktp = packet;

		switch (type) {
			seq_t seqno;
			unsigned rtptime;

			// re-sent packet
			case 0x56: {
				pktp += 4;
				plen -= 4;
			}

			// data packet
			case 0x60: {
				seqno = ntohs(*(u16_t*)(pktp+2));
				rtptime = ntohl(*(u32_t*)(pktp+4));

				// adjust pointer and length
				pktp += 12;
				plen -= 12;

				LOG_SDEBUG("[%p]: seqno:%hu rtp:%u (type: %x, first: %u)", ctx, seqno, rtptime, type, packet[1] & 0x80);

				// check if packet contains enough content to be reasonable
				if (plen < 16) break;

				if ((packet[1] & 0x80) && (type != 0x56)) {
					LOG_INFO("[%p]: 1st audio packet received", ctx);
				}

				buffer_put_packet(ctx, seqno, rtptime, packet[1] & 0x80, pktp, plen);

				break;
			}

			// sync packet
			case 0x54: {
				u32_t rtp_now_latency = ntohl(*(u32_t*)(pktp+4));
				u64_t remote = (((u64_t) ntohl(*(u32_t*)(pktp+8))) << 32) + ntohl(*(u32_t*)(pktp+12));
				u32_t rtp_now = ntohl(*(u32_t*)(pktp+16));
				u16_t flags = ntohs(*(u16_t*)(pktp+2));

				pthread_mutex_lock(&ctx->ab_mutex);

				// re-align timestamp and expected local playback time (and magic 11025 latency)
				ctx->latency = rtp_now - rtp_now_latency;
				if (flags == 7 || flags == 4) ctx->latency += 11025;
				if (ctx->latency < MIN_LATENCY) ctx->latency = MIN_LATENCY;
				else if (ctx->latency > MAX_LATENCY) ctx->latency = MAX_LATENCY;
				ctx->synchro.rtp = rtp_now - ctx->latency;
				ctx->synchro.time = ctx->timing.local + (u32_t) NTP2MS(remote - ctx->timing.remote);

				// now we are synced on RTP frames
				ctx->synchro.status |= RTP_SYNC;

				// 1st sync packet received (signals a restart of playback)
				if (packet[0] & 0x10) {
					LOG_INFO("[%p]: 1st sync packet received", ctx);
				}

				pthread_mutex_unlock(&ctx->ab_mutex);

				LOG_DEBUG("[%p]: sync packet latency:%d rtp_latency:%u rtp:%u remote ntp:%llx, local time:%u local rtp:%u (now:%u)",
						  ctx, ctx->latency, rtp_now_latency, rtp_now, remote, ctx->synchro.time, ctx->synchro.rtp, gettime_ms());

				if (!count--) {
					rtp_request_timing(ctx);
					count = 3;
				}

				if ((ctx->synchro.status & RTP_SYNC) && (ctx->synchro.status & NTP_SYNC)) ctx->cmd_cb(RAOP_TIMING, NULL);

				break;
			}

			// NTP timing packet
			case 0x53: {
				u64_t expected;
				u32_t reference   = ntohl(*(u32_t*)(pktp+12)); // only low 32 bits in our case
				u64_t remote 	  =(((u64_t) ntohl(*(u32_t*)(pktp+16))) << 32) + ntohl(*(u32_t*)(pktp+20));
				u32_t roundtrip   = gettime_ms() - reference;

				// better discard sync packets when roundtrip is suspicious
				if (roundtrip > 100) {
					LOG_WARN("[%p]: discarding NTP roundtrip of %u ms", ctx, roundtrip);
					break;
				}

				/*
				  The expected elapsed remote time should be exactly the same as
				  elapsed local time between the two request, corrected by the
				  drifting
				*/
				expected = ctx->timing.remote + MS2NTP(reference - ctx->timing.local);

				ctx->timing.remote = remote;
				ctx->timing.local = reference;

				// now we are synced on NTP (mutex not needed)
				ctx->synchro.status |= NTP_SYNC;

				LOG_DEBUG("[%p]: Timing references local:%llu, remote:%llx (delta:%lld, sum:%lld, adjust:%lld, gaps:%d)",
						  ctx, ctx->timing.local, ctx->timing.remote);

				break;
			}
		}
	}

	free(packet);
	LOG_INFO("[%p]: terminating", ctx);

#ifndef WIN32
	xTaskNotify(ctx->joiner, 0, eNoAction);
	vTaskDelete(NULL);
#endif

	return NULL;
}

/*---------------------------------------------------------------------------*/
static bool rtp_request_timing(rtp_t *ctx) {
	unsigned char req[32];
	u32_t now = gettime_ms();
	int i;
	struct sockaddr_in host;

	LOG_DEBUG("[%p]: timing request now:%u (port: %hu)", ctx, now, ctx->rtp_sockets[TIMING].rport);

	req[0] = 0x80;
	req[1] = 0x52|0x80;
	*(u16_t*)(req+2) = htons(7);
	*(u32_t*)(req+4) = htonl(0);  // dummy
	for (i = 0; i < 16; i++) req[i+8] = 0;
	*(u32_t*)(req+24) = 0;
	*(u32_t*)(req+28) = htonl(now); // this is not a real NTP, but a 32 ms counter in the low part of the NTP

	if (ctx->host.s_addr != INADDR_ANY) {
		host.sin_family = AF_INET;
		host.sin_addr =	ctx->host;
	} else host = ctx->rtp_host;

	// no address from sender, need to wait for 1st packet to be received
	if (host.sin_addr.s_addr == INADDR_ANY) return false;

	host.sin_port = htons(ctx->rtp_sockets[TIMING].rport);

	if (sizeof(req) != sendto(ctx->rtp_sockets[TIMING].sock, req, sizeof(req), 0, (struct sockaddr*) &host, sizeof(host))) {
		LOG_WARN("[%p]: SENDTO failed (%s)", ctx, strerror(errno));
	}

	return true;
}

/*---------------------------------------------------------------------------*/
static bool rtp_request_resend(rtp_t *ctx, seq_t first, seq_t last) {
	unsigned char req[8];    // *not* a standard RTCP NACK

	// do not request silly ranges (happens in case of network large blackouts)
	if (seq_order(last, first) || last - first > BUFFER_FRAMES / 2) return false;
	
	ctx->resent_req += last - first + 1;

	LOG_DEBUG("resend request [W:%hu R:%hu first=%hu last=%hu]", ctx->ab_write, ctx->ab_read, first, last);

	req[0] = 0x80;
	req[1] = 0x55|0x80;  // Apple 'resend'
	*(u16_t*)(req+2) = htons(1);  // our seqnum
	*(u16_t*)(req+4) = htons(first);  // missed seqnum
	*(u16_t*)(req+6) = htons(last-first+1);  // count

	ctx->rtp_host.sin_port = htons(ctx->rtp_sockets[CONTROL].rport);

	if (sizeof(req) != sendto(ctx->rtp_sockets[CONTROL].sock, req, sizeof(req), 0, (struct sockaddr*) &ctx->rtp_host, sizeof(ctx->rtp_host))) {
		LOG_WARN("[%p]: SENDTO failed (%s)", ctx, strerror(errno));
	}

	return true;
}