TCP/UDP receive error in serial_lte_modem

RE: TCP/UDP receive error in serial_lte_modem

Siyou — Thu, 02 Apr 2020 09:29:35 GMT

Hi Håkon,

Thanks for your update, that fix works.

RE: TCP/UDP receive error in serial_lte_modem

Håkon Alseth — Mon, 02 Mar 2020 07:45:48 GMT

Hi,

Proposed fixes are in this pull-request:

https://github.com/NordicPlayground/fw-nrfconnect-nrf/pull/1971

Kind regards,

Håkon

RE: TCP/UDP receive error in serial_lte_modem

Siyou — Wed, 26 Feb 2020 10:30:00 GMT

Hi Håkon,

Thanks for you info, understand and wait for your update.

RE: TCP/UDP receive error in serial_lte_modem

Håkon Alseth — Wed, 26 Feb 2020 10:02:13 GMT

Hi,

[quote user="Siyou"]1, For TCP case, we understood the design when the recv() returned 0, but we could not understand it when recv() was timeout(return was -60), we think the socket should keep as open after client recv() from the server which did not send out any data;[/quote][quote user="Siyou"]2, For UDP case, due to it is not connection based, client is no need to re-establish connection when recvfrom() was timeout, so we think the socket should be kept as open. But we found the UDP recvfrom()/sendto() were not available after the recvfrom() timeout(return also was -60).[/quote]

Thank you for clarifying both scenarios, and my apologies for misunderstanding.

This looks like a bug from our side. A timeout doesn't necessarily mean that the connection is gone, it should rather return with the error without closing the socket. The same needs to be handled for errno EAGAIN. I'll report this back to the developers.

Kind regards,

Håkon

RE: TCP/UDP receive error in serial_lte_modem

Siyou — Wed, 26 Feb 2020 02:37:36 GMT

Hi Håkon,

Let me separate it to below two questions:
1, For TCP case, we understood the design when the recv() returned 0, but we could not understand it when recv() was timeout(return was -60), we think the socket should keep as open after client recv() from the server which did not send out any data;
2, For UDP case, due to it is not connection based, client is no need to re-establish connection when recvfrom() was timeout, so we think the socket should be kept as open. But we found the UDP recvfrom()/sendto() were not available after the recvfrom() timeout(return also was -60).

PS: the timeout was produced by below steps:
1), Create TCP or UDP socket and bind to TCP or UDP server;
2), invoke recv() or recvfrom() with timeout;
3), before and within the timeout, the TCP or UDP server does not send out any data to client.

RE: TCP/UDP receive error in serial_lte_modem

Håkon Alseth — Tue, 25 Feb 2020 12:43:08 GMT

Hi,

If you are receiving 0 bytes, it means that the remote side is trying to gracefully shut down the connection, as also discussed here:

https://stackoverflow.com/questions/38021659/can-a-c-socket-recv-0-bytes-without-the-client-shutting-the-connection

Is there a specific reason why you want to keep the socket alive after this point? The connection would be lost at this point and you would need to re-establish a new connection.

Kind regards,

Håkon