configuration-guidelines.md

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

As already mentioned, the maximum allowed size of the receiver buffer is limited by the value of SRTO_FC. 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 using the configured value of the SRTO_MSS option to estimate the maximum possible payload of a packet.

The following function returns the buffer size in packets:

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

    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 - 44

where

• 44 is the size in bytes of an IPv4 header:

  • 20 bytes IPv4
  • 8 bytes of UDP
  • 16 bytes of SRT packet header.

• MSS is the Maximum Segment Size (aka MTU); see SRTO_MSS.

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);