Re: [FFmpeg-devel] [PATCH v2 5/6] avformat/whip: implement NACK and RTX suppport

[Steven Liu <lingjiujianke-at-gmail.com@ffmpeg.org>]

Jack Lau <jacklau1222gm-at-gmail.com@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@gmail.com>
> Signed-off-by: Jack Lau <jacklau1222@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)
> +                    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.

> +                        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);
> +                }
> +            }
> +        }
>      } 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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