[FFmpeg-devel] [PATCH 6/6] avformat/whip: implement NACK and RTX suppport
Jack Lau
jacklau1222gm at gmail.com
Wed Jul 2 07:25:05 EEST 2025
RTP retransmission described in RFC4588 (RTX) is an effective packet
loss recovery technique for real-time applications with relaxed delay bounds.
This patch provides a minimal implementation for RTX and RTCP NACK (RFC3940)
and its associated SDP signaling and negotiation.
Co-authored-by: Sergio Garcia Murillo <sergio.garcia.murillo at gmail.com>
Signed-off-by: Jack Lau <jacklau1222 at qq.com>
---
libavformat/whip.c | 198 ++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 195 insertions(+), 3 deletions(-)
diff --git a/libavformat/whip.c b/libavformat/whip.c
index 15d1de691e..a32177e0b4 100644
--- a/libavformat/whip.c
+++ b/libavformat/whip.c
@@ -114,6 +114,7 @@
/* Referring to Chrome's definition of RTP payload types. */
#define WHIP_RTP_PAYLOAD_TYPE_H264 106
#define WHIP_RTP_PAYLOAD_TYPE_OPUS 111
+#define WHIP_RTP_PAYLOAD_TYPE_RTX 105
/**
* The STUN message header, which is 20 bytes long, comprises the
@@ -150,6 +151,11 @@
#define WHIP_SDP_SESSION_ID "4489045141692799359"
#define WHIP_SDP_CREATOR_IP "127.0.0.1"
+/**
+ * Retransmission / NACK support
+*/
+#define HISTORY_SIZE_DEFAULT 512
+
/* Calculate the elapsed time from starttime to endtime in milliseconds. */
#define ELAPSED(starttime, endtime) ((int)(endtime - starttime) / 1000)
@@ -194,9 +200,19 @@ enum WHIPState {
};
typedef enum WHIPFlags {
- WHIP_FLAG_IGNORE_IPV6 = (1 << 0) // Ignore ipv6 candidate
+ WHIP_FLAG_IGNORE_IPV6 = (1 << 0), // Ignore ipv6 candidate
+ WHIP_FLAG_DISABLE_RTX = (1 << 1) // Enable NACK and RTX
} WHIPFlags;
+typedef struct RtpHistoryItem {
+ /* original RTP seq */
+ uint16_t seq;
+ /* length in bytes */
+ int size;
+ /* malloc-ed copy */
+ uint8_t* pkt;
+} RtpHistoryItem;
+
typedef struct WHIPContext {
AVClass *av_class;
@@ -285,6 +301,7 @@ typedef struct WHIPContext {
/* The SRTP send context, to encrypt outgoing packets. */
SRTPContext srtp_audio_send;
SRTPContext srtp_video_send;
+ SRTPContext srtp_video_rtx_send;
SRTPContext srtp_rtcp_send;
/* The SRTP receive context, to decrypt incoming packets. */
SRTPContext srtp_recv;
@@ -309,6 +326,14 @@ typedef struct WHIPContext {
/* The certificate and private key used for DTLS handshake. */
char* cert_file;
char* key_file;
+
+ /* RTX / NACK */
+ uint8_t rtx_payload_type;
+ uint32_t video_rtx_ssrc;
+ uint16_t rtx_seq;
+ int history_size;
+ RtpHistoryItem *history; /* ring buffer */
+ int hist_head;
} WHIPContext;
/**
@@ -606,6 +631,16 @@ static int generate_sdp_offer(AVFormatContext *s)
whip->audio_payload_type = WHIP_RTP_PAYLOAD_TYPE_OPUS;
whip->video_payload_type = WHIP_RTP_PAYLOAD_TYPE_H264;
+ /* RTX and NACK init */
+ whip->rtx_payload_type = WHIP_RTP_PAYLOAD_TYPE_RTX;
+ whip->video_rtx_ssrc = av_lfg_get(&whip->rnd);
+ whip->rtx_seq = 0;
+ whip->hist_head = 0;
+ whip->history_size = FFMAX(64, whip->history_size);
+ whip->history = av_calloc(whip->history_size, sizeof(*whip->history));
+ if (!whip->history)
+ return AVERROR(ENOMEM);
+
av_bprintf(&bp, ""
"v=0\r\n"
"o=FFmpeg %s 2 IN IP4 %s\r\n"
@@ -656,7 +691,7 @@ static int generate_sdp_offer(AVFormatContext *s)
}
av_bprintf(&bp, ""
- "m=video 9 UDP/TLS/RTP/SAVPF %u\r\n"
+ "m=video 9 UDP/TLS/RTP/SAVPF %u %u\r\n"
"c=IN IP4 0.0.0.0\r\n"
"a=ice-ufrag:%s\r\n"
"a=ice-pwd:%s\r\n"
@@ -669,9 +704,16 @@ static int generate_sdp_offer(AVFormatContext *s)
"a=rtcp-rsize\r\n"
"a=rtpmap:%u %s/90000\r\n"
"a=fmtp:%u level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=%02x%02x%02x\r\n"
+ "a=rtcp-fb:%u nack\r\n"
+ "a=rtpmap:%u rtx/90000\r\n"
+ "a=fmtp:%u apt=%u\r\n"
+ "a=ssrc-group:FID %u %u\r\n"
+ "a=ssrc:%u cname:FFmpeg\r\n"
+ "a=ssrc:%u msid:FFmpeg video\r\n"
"a=ssrc:%u cname:FFmpeg\r\n"
"a=ssrc:%u msid:FFmpeg video\r\n",
whip->video_payload_type,
+ whip->rtx_payload_type,
whip->ice_ufrag_local,
whip->ice_pwd_local,
whip->dtls_fingerprint,
@@ -681,8 +723,16 @@ static int generate_sdp_offer(AVFormatContext *s)
profile,
whip->constraint_set_flags,
level,
+ whip->video_payload_type,
+ whip->rtx_payload_type,
+ whip->rtx_payload_type,
+ whip->video_payload_type,
+ whip->video_ssrc,
+ whip->video_rtx_ssrc,
whip->video_ssrc,
- whip->video_ssrc);
+ whip->video_ssrc,
+ whip->video_rtx_ssrc,
+ whip->video_rtx_ssrc);
}
if (!av_bprint_is_complete(&bp)) {
@@ -1396,6 +1446,12 @@ static int setup_srtp(AVFormatContext *s)
goto end;
}
+ ret = ff_srtp_set_crypto(&whip->srtp_video_rtx_send, suite, buf);
+ if (ret < 0) {
+ av_log(whip, AV_LOG_ERROR, "WHIP: Failed to set crypto for video rtx send\n");
+ goto end;
+ }
+
ret = ff_srtp_set_crypto(&whip->srtp_rtcp_send, suite, buf);
if (ret < 0) {
av_log(whip, AV_LOG_ERROR, "Failed to set crypto for rtcp send\n");
@@ -1425,6 +1481,37 @@ end:
return ret;
}
+
+/**
+ * RTX history helpers
+ */
+ static void rtp_history_store(WHIPContext *whip, const uint8_t *pkt, int size)
+{
+ int pos = whip->hist_head % whip->history_size;
+ RtpHistoryItem *it = &whip->history[pos];
+ /* free older entry */
+ av_free(it->pkt);
+ it->pkt = av_malloc(size);
+ if (!it->pkt)
+ return;
+
+ memcpy(it->pkt, pkt, size);
+ it->size = size;
+ it->seq = AV_RB16(pkt + 2);
+
+ whip->hist_head = ++pos;
+}
+
+static const RtpHistoryItem *rtp_history_find(const WHIPContext *whip, uint16_t seq)
+{
+ for (int i = 0; i < whip->history_size; i++) {
+ const RtpHistoryItem *it = &whip->history[i];
+ if (it->pkt && it->seq == seq)
+ return it;
+ }
+ return NULL;
+}
+
/**
* Callback triggered by the RTP muxer when it creates and sends out an RTP packet.
*
@@ -1461,6 +1548,10 @@ static int on_rtp_write_packet(void *opaque, const uint8_t *buf, int buf_size)
return 0;
}
+ /* Store only ORIGINAL video packets (non-RTX, non-RTCP) */
+ if (!is_rtcp && is_video)
+ rtp_history_store(whip, buf, buf_size);
+
ret = ffurl_write(whip->udp, whip->buf, cipher_size);
if (ret < 0) {
av_log(whip, AV_LOG_ERROR, "Failed to write packet=%dB, ret=%d\n", cipher_size, ret);
@@ -1469,6 +1560,45 @@ static int on_rtp_write_packet(void *opaque, const uint8_t *buf, int buf_size)
return ret;
}
+/**
+ * See https://datatracker.ietf.org/doc/html/rfc4588
+ * Build and send a single RTX packet
+ */
+static int send_rtx_packet(AVFormatContext *s, const uint8_t *orig_pkt, int orig_size)
+{
+ WHIPContext *whip = s->priv_data;
+ int new_size, cipher_size;
+ if (whip->flags & WHIP_FLAG_DISABLE_RTX)
+ return 0;
+
+ /* allocate new buffer: header + 2 + payload */
+ if (orig_size + 2 > sizeof(whip->buf))
+ return 0;
+
+ memcpy(whip->buf, orig_pkt, orig_size);
+
+ uint8_t *hdr = whip->buf;
+ uint16_t orig_seq = AV_RB16(hdr + 2);
+
+ /* rewrite header */
+ hdr[1] = (hdr[1] & 0x80) | whip->rtx_payload_type; /* keep M bit */
+ AV_WB16(hdr + 2, whip->rtx_seq++);
+ AV_WB32(hdr + 8, whip->video_rtx_ssrc);
+
+ /* shift payload 2 bytes */
+ memmove(hdr + 12 + 2, hdr + 12, orig_size - 12);
+ AV_WB16(hdr + 12, orig_seq);
+
+ new_size = orig_size + 2;
+
+ /* Encrypt by SRTP and send out. */
+ cipher_size = ff_srtp_encrypt(&whip->srtp_video_rtx_send, whip->buf, new_size, whip->buf, sizeof(whip->buf));
+ if (cipher_size <= 0 || cipher_size < new_size) {
+ av_log(whip, AV_LOG_WARNING, "WHIP: Failed to encrypt packet=%dB, cipher=%dB\n", new_size, cipher_size);
+ return 0;
+ }
+ return ffurl_write(whip->udp, whip->buf, cipher_size);
+}
/**
* Creates dedicated RTP muxers for each stream in the AVFormatContext to build RTP
@@ -1791,6 +1921,66 @@ static int whip_write_packet(AVFormatContext *s, AVPacket *pkt)
goto end;
}
}
+ /**
+ * Handle RTCP NACK
+ * Refer to RFC 4585, Section 6.2.1
+ * The Generic NACK message is identified by PT=RTPFB and FMT=1.
+ * TODO: disable retransmisstion when "-tune zerolatency"
+ */
+ if (media_is_rtcp(whip->buf, ret)) {
+ int ptr = 0;
+ uint8_t pt = whip->buf[ptr + 1];
+ uint8_t fmt = (whip->buf[ptr] & 0x1f);
+ if (ptr + 4 <= ret && pt == 205 && fmt == 1) {
+ /**
+ * Refer to RFC 3550, Section 6.4.1.
+ * The length of this RTCP packet in 32-bit words minus one,
+ * including the header and any padding.
+ */
+ int rtcp_len = (AV_RB16(&whip->buf[ptr + 2]) + 1) * 4;
+ /* SRTCP index(4 bytes) + HMAC (SRTP_AES128_CM_SHA1_80 10bytes) */
+ int srtcp_len = rtcp_len + 4 + 10;
+ if (srtcp_len == ret && rtcp_len >= 12) {
+ int i = 0;
+ uint8_t *pkt = av_malloc(srtcp_len);
+ memcpy(pkt, whip->buf, srtcp_len);
+ int ret = ff_srtp_decrypt(&whip->srtp_recv, pkt, &srtcp_len);
+ if (ret < 0)
+ av_log(whip, AV_LOG_ERROR, "WHIP: SRTCP decrypt failed: %d\n", ret);
+ while (12 + i < rtcp_len && ret == 0) {
+ /**
+ * See https://datatracker.ietf.org/doc/html/rfc4585#section-6.1
+ * Handle multi NACKs in bundled packet.
+ */
+ uint16_t pid = AV_RB16(&pkt[ptr + 12 + i]);
+ uint16_t blp = AV_RB16(&pkt[ptr + 14 + i]);
+
+ /* retransmit pid + any bit set in blp */
+ for (int bit = -1; bit < 16; bit++) {
+ uint16_t seq = (bit < 0) ? pid : pid + bit + 1;
+ if (bit >= 0 && !(blp & (1 << bit)))
+ continue;
+
+ const RtpHistoryItem *it = rtp_history_find(whip, seq);
+ if (it) {
+ av_log(whip, AV_LOG_VERBOSE,
+ "WHIP: NACK, packet found: size: %d, seq=%d, rtx size=%d, lateset stored packet seq:%d\n",
+ it->size, seq, ret, whip->history[whip->hist_head-1].seq);
+ ret = send_rtx_packet(s, it->pkt, it->size);
+ if (ret <= 0 && !(whip->flags & WHIP_FLAG_DISABLE_RTX))
+ av_log(whip, AV_LOG_ERROR, "WHIP: Failed to send RTX packet\n");
+ } else {
+ av_log(whip, AV_LOG_VERBOSE,
+ "WHIP: NACK, packet not found, seq=%d, latest stored packet seq: %d, latest rtx seq: %d\n",
+ seq, whip->history[whip->hist_head-1].seq, whip->rtx_seq);
+ }
+ }
+ i = i + 4;
+ }
+ av_free(pkt);
+ }
+ }
+ }
} else if (ret != AVERROR(EAGAIN)) {
av_log(whip, AV_LOG_ERROR, "Failed to read from UDP socket\n");
goto end;
@@ -1903,6 +2093,8 @@ static const AVOption options[] = {
AV_OPT_TYPE_FLAGS, { .i64 = 0 }, 0, UINT_MAX, ENC, .unit = "flags" },
{ "ignore_ipv6", "Ignore any IPv6 ICE candidate", 0,
AV_OPT_TYPE_CONST, { .i64 = WHIP_FLAG_IGNORE_IPV6 }, 0, UINT_MAX, ENC, .unit = "flags" },
+ { "disable_rtx", "Disable RFC 4588 RTX", 0,
+ AV_OPT_TYPE_CONST, { .i64 = WHIP_FLAG_DISABLE_RTX }, 0, UINT_MAX, ENC, .unit = "flags" },
{ NULL },
};
--
2.49.0
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