[docs] Added a note on UDP buffer size in srt-live-transmit

docs/srt-live-transmit.md

@@ -293,3 +293,46 @@ uses the opposite connection mode. Typically you use the **SRT listener** on the
 since it is easier to configure in terms of firewall/router setup. It also makes sense to leave the 
 Sender in listener mode when trying to connect from various end points with possibly 
 unknown IP addresses.
+
+## UDP Performance
+
+Performance issues concerning reading from UDP medium were reported
+in [#933](https://github.com/Haivision/srt/issues/933) and
+[#762](https://github.com/Haivision/srt/issues/762).
+
+The dedicated research showed that at high and bursty data rates (~60 Mbps)
+the `epoll_wait(udp_socket)` is not fast enough to signal about the possibility
+to read from a socket. It results in losing data when the input bitrate is very high (above 20 Mbps).
+
+Waiting on a UDP socket with `::select(...)` works perfect,
+but it can't be used in the current implementation of the `srt-live-transmit`.
+
+PR [#1152](https://github.com/Haivision/srt/pull/1152) (v1.5.0 and above) adds a possibility
+to set the buffer size of the UDP socket in `srt-live-transmit`.
+Having a bigger buffer of UDP socket to store incomming data, `srt-live-transmit` handles high bitrates.
+
+The following steps have to be performed to use the bigger UDP buffer size.
+
+### Increase the system-default max rcv buffer size
+
+```
+$ cat /proc/sys/net/core/rmem_max
+212992
+$ sudo sysctl -w net.core.rmem_max=26214400
+net.core.rmem_max = 26214400
+$ cat /proc/sys/net/core/rmem_max
+26214400
+```
+
+### Specify the size of the UDP socket buffer via the URI
+
+Example URI:
+```
+"udp://:4200?rcvbuf=67108864"
+```
+
+Example full URI:
+```
+./srt-live-transmit "udp://:4200?rcvbuf=67108864" srt://192.168.0.10:4200 -v
+```
+