docs: improve (#5277)

docs/2-usage/26-decrease-packet-loss.md

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+# Decrease packet loss
+
+MediaMTX is meant for routing live streams, and makes use of a series of protocols and techniques which try to preserve the real-time aspect of streams and minimize latency at cost of losing packets in transmit, in particular:
+
+- most protocols are built on UDP, which is an "unreliable transport", specifically picked because it allows to drop late packets in case of network congestions.
+- there's a circular buffer that stores outgoing packets and drops packets if full.
+
+Packet losses are usually detected and printed in _MediaMTX_ logs.
+
+If you need to improve the stream reliability and decrease packet losses, the first thing to do is to check whether the physical network between the _MediaMTX_ instance and the intended publishers and readers has sufficient bandwidth for transmitting the media stream. Most of the times, packet losses are caused by a network which is not fit for this scope. This limitation can be overcome by either recompressing the stream with a lower bitrate, or by upgrading the network infrastructure (routers, cables, Wi-Fi, firewalls, topology, etc).
+
+There are however some parameters that can be tuned to improve the situation, at cost of increasing RAM consumption:
+
+- When publishing a stream with a UDP-based protocol (currently RTSP, MPEG-TS, RTP, SRT, WebRTC), packets might get discarded by the server because the read buffer size of UDP sockets is too small. It can be increased with this parameter:
+
+  ```yml
+  udpReadBufferSize: 1000000
+  ```
+
+  The `udpReadBufferSize` parameter requires the `net.core.rmem_max` system parameter to be equal or greater than it. It can be set with this command:
+
+  ```sh
+  sudo sysctl net.core.rmem_max=100000000
+  ```
+
+- When reading a stream, packets might get discarded because the write queue is too small. This can be noticed in logs through the "reader is too slow" message. Try increasing the write queue:
+
+  ```yml
+  writeQueueSize: 1024
+  ```
+
+- When publishing or reading a stream with RTSP, it's possible to switch from the UDP transport protocol to the TCP transport protocol, which is less performant but has a packet retransmission mechanism:
+
+  ```yml
+  rtspTransports: [tcp]
+  ```
+
+  In case the source is a camera:
+
+  ```yml
+  paths:
+    test:
+      source: rtsp://..
+      rtspTransport: tcp
+  ```