# Known Issues in EMQX 6.3

## 6.3.1

| Since version | Issue                                                        | Workaround                                                   | Status |
| ------------- | ------------------------------------------------------------ | ------------------------------------------------------------ | ------ |
| 6.0.0         | **Throttled inbound traffic on TCP listeners with `tcp_backend = socket` and a small `tcp_options.recbuf`**<br />A TCP listener configured with `tcp_backend = socket` (the default on Linux and macOS since 6.3.0) together with a `tcp_options.recbuf` below the operating system's default receive buffer (for example `4KB`) receives all inbound traffic very slowly, whatever the message size: with `recbuf = 4KB` each connection receives about 20 KB/s, so a 256 KB message takes several seconds to arrive. Listeners with `tcp_backend = gen_tcp`, and `socket` listeners that do not set `recbuf`, are not affected. | Remove `tcp_options.recbuf` from listeners that use `tcp_backend = socket`, or switch the listener to `tcp_backend = gen_tcp`. | Fix scheduled for 6.3.2 ([emqx/emqx#19190](https://github.com/emqx/emqx/pull/19190)) |
| 6.3.0         | **TCP listeners with `tcp_backend = socket` may stay at full CPU and deliver slowly after a subscriber falls behind**<br />`tcp_backend = socket` is the default for TCP listeners on Linux and macOS since 6.3.0. When a subscriber on such a listener reads slower than messages arrive, for example during a large burst of QoS 1 messages, undelivered messages queue up in its connection. The connection then spends most of its time in garbage collection and delivers fewer messages than it receives, so the backlog does not shrink. CPU stays at the limit and delivery drops to tens of messages per second, even after publishing stops. TCP listeners with `tcp_backend = gen_tcp` and SSL, WebSocket and QUIC listeners are not affected. | Set `tcp_backend = gen_tcp` on TCP listeners, for example `listeners.tcp.default.tcp_backend = gen_tcp`. | Fix in progress ([emqx/emqx#19201](https://github.com/emqx/emqx/issues/19201)) |