Getting a reply (ECHO)

By default the server does not reply, so your device only sends (uplink). To also test receiving (downlink), start your payload with the 4 characters ECHO (upper case). The server then sends the complete message back unchanged, over the same protocol, to the address and port it came from.

Destination IP	Port	Payload	Reply
`164.30.75.205`	`41300` (UDP)	`ECHO hello`	`ECHO hello`
`164.30.75.205`	`12400` (TCP)	`ECHO hello`	`ECHO hello`
`164.30.75.205`	`41300` or `12400`	`hello`	none

UDP: checked for every datagram. Each datagram starting with ECHO is sent back.
TCP: checked on the first 4 bytes of the connection. If they are ECHO, everything you send on that connection is sent back.
CoAP: checked on the payload of the request (for Block1 uploads: the complete payload of all blocks). If it starts with ECHO, the response is 2.05 Content with the first 512 bytes of your payload. Longer payloads are not echoed completely. Without ECHO the response is 2.04 Changed without payload.
HTTP: checked on the request body. If it starts with ECHO, the response is 200 OK with the complete body and the same Content-Type. Without ECHO the response is 204 No Content.
Anything can follow ECHO, also binary data. echo in lower case gets no reply.

In the live traffic, echoed messages are marked ECHO ✓. If an echo could not be sent, the mark is ECHO ✗ and the details show why. ECHO works the same inside DTLS and TLS: the reply is encrypted.