[FFmpeg-devel] [PATCH v2 5/6] avformat/whip: implement NACK and RTX suppport
Jack Lau
jacklau1222gm at gmail.com
Wed Jul 2 17:10:30 EEST 2025
Hi
> On Jul 2, 2025, at 21:46, Steven Liu <lingjiujianke-at-gmail.com at ffmpeg.org> wrote:
>
> Jack Lau <jacklau1222gm-at-gmail.com at ffmpeg.org <mailto:jacklau1222gm-at-gmail.com at ffmpeg.org>> 于2025年7月2日周三 20:09写道：
>>
>> 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 e287a3062f..fa6e3855d3 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; /* original RTP seq */
>> + /* length in bytes */
>> + int size; /* length in bytes */
>> + /* malloc-ed copy */
> move the comments.
>> + uint8_t* buf; /* malloc-ed copy */
>> +} RtpHistoryItem;
>
> for example:
>
>
> 171 typedef struct mkv_cuepoint {
> 172 uint64_t pts;
> 173 int stream_idx;
> 174 int64_t cluster_pos; ///< offset of the
> cluster containing the block relative to the segment
> 175 int64_t relative_pos; ///< relative offset from
> the position of the cluster containing the block
> 176 int64_t duration; ///< duration of the
> block according to time base
> 177 } mkv_cuepoint;
>
>
>
>> +
>> 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 and 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)) {
>> @@ -1398,6 +1448,12 @@ 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");
> Since av_log(whip, has been set, the "WHIP:" prefix might be
> unnecessary. Same comment to bellow.
>
>> + 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");
>> @@ -1427,6 +1483,37 @@ end:
>> return ret;
>> }
>>
>> +
>> +/**
>> + * RTX history helpers
>> + */
>> + static void rtp_history_store(WHIPContext *whip, const uint8_t *buf, int size)
> return value.
>> +{
>> + int pos = whip->hist_head % whip->history_size;
>> + RtpHistoryItem *it = &whip->history[pos];
>> + /* free older entry */
>> + av_free(it->buf);
>> + it->buf = av_malloc(size);
>> + if (!it->buf)
>> + return;
> return AVERROR(ENOMEM)
>> +
>> + memcpy(it->buf, buf, size);
>> + it->size = size;
>> + it->seq = AV_RB16(buf + 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->buf && it->seq == seq)
>> + return it;
>> + }
>> + return NULL;
>> +}
>> +
>> /**
>> * Callback triggered by the RTP muxer when it creates and sends out an RTP packet.
>> *
>> @@ -1463,6 +1550,10 @@ 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 = rtp_history_store(whip, buf, buf_size);
> if (ret < 0)
> process failed;
>
>> +
>> 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);
>> @@ -1471,6 +1562,45 @@ 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_buf, 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_buf, 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
>> @@ -1793,6 +1923,66 @@ 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 *buf = av_malloc(srtcp_len);
> if (!buf)
> process AVEROR(ENOMEM)
Thanks for your detailed reviews! I totally agree the top of ideas.
>> + memcpy(buf, whip->buf, srtcp_len);
>> + int ret = ff_srtp_decrypt(&whip->srtp_recv, buf, &srtcp_len);
>> + if (ret < 0)
> if error here, should free buf, otherwise will memleak.
But I think this is not an issue,
>
>> + av_log(whip, AV_LOG_ERROR, "WHIP: SRTCP decrypt failed: %d\n", ret);
>> + while (12 + i < rtcp_len && !ret) {
>> + /**
>> + * See https://datatracker.ietf.org/doc/html/rfc4585#section-6.1
>> + * Handle multi NACKs in bundled packet.
>> + */
>> + uint16_t pid = AV_RB16(&buf[ptr + 12 + i]);
>> + uint16_t blp = AV_RB16(&buf[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->buf, 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(buf);
Because we have this free function after the loop so there’s no memory leak.
>> + }
>> + }
>> + }
>> } else if (ret != AVERROR(EAGAIN)) {
>> av_log(whip, AV_LOG_ERROR, "Failed to read from UDP socket\n");
>> goto end;
>> @@ -1898,6 +2088,8 @@ static const AVOption options[] = {
>> { "key_file", "Optional private key file path for DTLS", OFFSET(key_file), AV_OPT_TYPE_STRING, { .str = NULL }, 0, 0, ENC },
>> { "whip_flags", "Set flags affecting WHIP connection behavior", OFFSET(flags), 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" },
>> + { "rtx_history_size", "Packet history size", OFFSET(history_size), AV_OPT_TYPE_INT, { .i64 = HISTORY_SIZE_DEFAULT }, 64, 2048, ENC },
>> { NULL },
>> };
>>
>> --
>> 2.49.0
>>
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I’ll submit new version of this patch later.
Thanks
Jack
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