[docs] Added buffer configuration guide

docs/API/configure-buffers.md

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+# Configuration Guidelines
+
+## Receiver Buffer Size
+
+The receiver buffer can be configured with `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](https://github.com/Haivision/srt/issues/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 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`.
+Setting `SRTO_RCVBUF` option value using `srt_setsockopt(..)` function converts internally the provided size in bytes to size in packets
+using the configured value of `SRTO_MSS` option to estimate the maximum possible payload of a packet.
+
+The following function returns the buffer size in packets:
+
+```c++
+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 in pkts = (rbuf_size / (SRTO_MSS - UDPHDR_SIZE));
+
+    return min(pkts, SRTO_FC);
+}
+```
+
+If `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  
+
+pktsRBufSize = bps / 8 × (RTTsec + latency_sec) / bytePayloadSize,  
+
+where
+
+- `bps` - is the payload bitrate of the stream in bits per second;
+- `RTTsec` - RTT of the network connection in seconds;
+
+- `bytePayloadSize` - expected size of the payload of SRT data packet.
+
+If the whole remainder of the MTU is expected to be used,  
+
+bytePayloadSize = MSS - 44
+
+- 44 - size of **IPv4** headers: 
+   - 20 bytes **IPv4** 
+   - 8 bytes of UDP
+   - 16 bytes of SRT packet header).
+
+- `MSS` - 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`.
+
+
+### To Summ up
+
+
+```c++
+
+auto CalculateTargetRBufSize(int msRTT, int bpsRate, int bytesPayloadSize, int msLatency, int SRTO_MSS, int SRTO_FC)
+{
+    const int UDPHDR_SIZE = 28;
+    const int targetPayloadBytes = (msLatency + msRTT / 2) * bpsRate / 1000 / 8;
+    const int targetNumPackets   = targetPayloadBytes / bytesPayloadSize;
+    const int targetSizeValue    = targetNumPackets * (SRTO_MSS - UDPHDR_SIZE);
+    return {targetNumPackets, targetSizeValue};
+}
+
+// Configuring
+
+const auto [fc, rcvbuf] = CalculateTargetRBufSize(msRTT, bpsRate, bytesPayloadSize, SRTO_RCVLATENCY, SRTO_MSS, SRTO_FC);
+
+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);
+
+
+```
+
+
+
+
+