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configuration-guidelines.md

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Configuration Guidelines

Receiver Buffer Size

The receiver buffer can be configured with the SRTO_RCVBUF socket option. Buffer size in bytes is expected to be passed in the optval argument of the srt_setsockopt(..) function. However, internally the value will be converted into the number of packets stored in the receiver buffer.

The allowed value of SRTO_RCVBUF is also limited by the value of the flow control window size SRTO_FC socket option. See issue #700.

The default flow control window size is 25600 packets. It is approximately:

  • 270 Mbits of payload in the default live streaming configuration with an SRT payload size of 1316 bytes;
  • 300 Mbits of payload in the default file transfer configuration with an SRT payload size of 1456 bytes.

The default receiver buffer size is 8192 packets. It is approximately:

  • 86 Mbits of payload with the effective SRT payload size of 1316 bytes.

Setting Receiver Buffer Size

When the SRTO_RCVBUF option value is set using the srt_setsockopt(..) function, the provided size in bytes is internally converted to the corresponding size in packets. The size of a cell for a single packet in the buffer is defined by the SRTO_MSS option, which is 1500 by default. This value, decreased by 28 in the case of IPv4 (20 bytes for the IPv4 header and 8 bytes for the UDP header), gives 1472 bytes per packet to be allocated. The actual memory occupied by the receiver buffer will be a multiple of that value. For the default 8192 packets it will be 11776 kB (11.5 MB).

Note that every cell has 16 bytes for the SRT header. The remaining space is for the payload.

As already mentioned, the maximum allowed size of the receiver buffer is limited by the value of SRTO_FC.

The following function returns the configured buffer size in packets depending on the SRTO_RCVBUF, SRTO_MSS and SRTO_FC values set:

int getRbufSizePkts(int SRTO_RCVBUF, int SRTO_MSS, int SRTO_FC)
{
    // UDP/IPv4 header size is assumed to be 28 bytes
    // 20 bytes IPv4 + 8 bytes of UDP
    const int UDP_IPv4_HDR= 28;
    const int pkts = (rbuf_size / (SRTO_MSS - UDP_IPv4_HDR));

    return min(pkts, SRTO_FC);
}

If the value of SRTO_RCVBUF in packets exceeds SRTO_FC, then it is silently set to the value in bytes corresponding to SRTO_FC. Therefore, to set higher values of SRTO_RCVBUF the value of SRTO_FC must be increased first.

Calculating Target Size in Packets

The minimum size of the receiver buffer in packets can be calculated as follows:

pktsRBufSize = bps / 8 × (RTTsec + latency_sec) / bytePayloadSize

where

  • bps is the payload bitrate of the stream in bits per second;

  • RTTsec is the RTT of the network connection in seconds;

  • bytePayloadSize is the expected size of the payload of the SRT data packet.

If the whole remainder of the MTU is expected to be used, payload size is calculated as follows:

bytePayloadSize = MSS - UDP_IPv4_HDR - SRT_HDR

where

  • MSS: Maximum Segment Size (size of the MTU); see SRTO_MSS (default: 1500)
  • UDP_IPv4_HDR: 20 bytes for IPv4 + 8 bytes for UDP
  • SRT_HDR: 16 bytes of SRT header (belonging to the user space)

Calculating Target Size to Set

To determine the value to pass in srt_setsockopt(..) with SRTO_RCVBUF the size in packets pktsRBufSize must be converted to the size in bytes assuming the internal conversion of the srt_setsockopt(..) function.

The target size of the payload stored by the receiver buffer would be:

SRTO_RCVBUF = pktsRBufSize × (SRTO_MSS - UDPHDR_SIZE)

where

  • UDPHDR_SIZE = 28 (20 bytes IPv4, 8 bytes of UDP)
  • SRTO_MSS is the corresponding socket option value at the moment of setting SRTO_RCVBUF.

Summing Up

auto CalculateTargetRBufSize(int msRTT, int bpsRate, int bytesPayloadSize, int msLatency, int SRTO_MSS)
{
    const int UDPHDR_SIZE = 28;
    const long long targetPayloadBytes = static_cast<long long>(msLatency + msRTT / 2) * bpsRate / 1000 / 8;
    const long long targetNumPackets   = targetPayloadBytes / bytesPayloadSize;
    const long long targetSizeValue    = targetNumPackets * (SRTO_MSS - UDPHDR_SIZE);
    return {targetNumPackets, targetSizeValue};
}

// Configuring

const auto [fc, rcvbuf] = CalculateTargetRBufSize(msRTT, bpsRate, bytesPayloadSize, SRTO_RCVLATENCY, SRTO_MSS);

int optval = fc;
int optlen = sizeof optval;
srt_setsockopt(sock, 0, SRTO_FC, (void*) &optval, optlen);

optval = rcvbuf;
srt_setsockopt(sock, 0, SRTO_RCVBUF, (void*) &optval, optlen);