Issues with Multiple TLS Azure Connections

[TS]

Hello,

I am working on a project that uses Azure as a cloud IoT provider and the NRF9160 as a gateway-like device which is supposed to make simultaneous connections to up to 5 end devices. I am using an older version of the SDK before the Azure library / example was merged in.

This gateway functionality is working well with up to 2 simultaneous TLS / MQTT connections, but if more than 2 connections is attempted, mqtt_connect returns -45 which is "EOPNOTSUPP 45 /* Operation not supported on socket *". I have seen some other posts that claim the -45 error is because of credential problems with TLS but I am confident this isn't the case here. 

I have taken a look at the new Azure library and have found a define that is a little suspicious to me and may indicate there is some hard limit of 2 simultaneous connections. This define is TLS_MAX_CONTEXTS and it is set to 2 in the new library. 

My question is: Is there a hard limit for the amount of TLS connections that can be made at the same time? From my testing it seems that limit is 2. I have attached a debug trace here which shows the first 2 successful connections and the error on the 3rd connection attempt. Thanks in advance

00> [00:00:26.356,018] <dbg> net_mqtt.mqtt_subscribe: (0x20021fcc): [CID 0x200200fc]:[State 0x06]: << result 0x00000000

00> [00:00:26.673,248] <dbg> net_mqtt.mqtt_input: (0x20021fcc): state:0x00000006

00> [00:00:26.673,278] <dbg> net_mqtt_dec.unpack_uint8: (0x20021fcc): >> cur:0x20025bd0, end:0x20025bd2

00> [00:00:26.673,278] <dbg> net_mqtt_dec.unpack_uint8: (0x20021fcc): << val:90

00> [00:00:26.673,309] <dbg> net_mqtt_dec.packet_length_decode: (0x20021fcc): length:0x00000003

00> [00:00:26.673,339] <dbg> net_mqtt_rx.mqtt_handle_packet: (0x20021fcc): [CID 0x200200fc]: Received MQTT_PKT_TYPE_SUBACK!

00> [00:00:26.673,370] <dbg> net_mqtt_dec.unpack_uint16: (0x20021fcc): >> cur:0x20025bd2, end:0x20025bd5

00> [00:00:26.673,370] <dbg> net_mqtt_dec.unpack_uint16: (0x20021fcc): << val:04d2

00> [00:00:26.673,370] <dbg> net_mqtt_dec.unpack_data: (0x20021fcc): >> cur:0x20025bd4, end:0x20025bd5

00> [00:00:26.673,400] <dbg> net_mqtt_dec.unpack_data: (0x20021fcc): << bin len:00000001

00> Subscribed3!

00> 

00> Connecting on Client4[00:00:34.244,201] <dbg> net_mqtt_sock_tls.mqtt_client_tls_connect: (0x20021fcc): Created socket 5

00> [00:00:36.293,395] <dbg> net_mqtt_sock_tls.mqtt_client_tls_connect: (0x20021fcc): error5

00> mqtt_connect4 -45

00> 

00> [00:00:40.487,915] <dbg> net_mqtt_sock_tls.mqtt_client_tls_connect: (0x20021fcc): Created socket 5

00> [00:00:42.531,188] <dbg> net_mqtt_sock_tls.mqtt_client_tls_connect: (0x20021fcc): error5

00> mqtt_connect4 -45

00> 

00> [00:00:47.039,398] <dbg> net_mqtt_sock_tls.mqtt_client_tls_connect: (0x20021fcc): Created socket 5

00> [00:00:48.841,461] <dbg> net_mqtt_sock_tls.mqtt_client_tls_connect: (0x20021fcc): error5

00> mqtt_connect4 -45
00> 

[TS]

Hi Carl!

Thanks for the answer here and that seems to explain it the error on socket 5. The modem firmware relesase notes was one place I wasn't looking for the explanation. I was confused by this post in the devzone where it says the maximum number of sockets is 8: 

https://devzone.nordicsemi.com/f/nordic-q-a/49614/can-i-have-multiple-af_lte-sockets-on-nrf9160#:~:text=From%20the%20documentation%3A,open%20at%20the%20same%20time.%22&text=That's%20great%20news!,-Well%20done%20Nordic

As far as why it is failing on the third connection.. Could it be because I am using an older modem firmware version or SDK version? From the debug trace, the first mqtt connection always starts on socket 3 for some reason. I can do more investigation into that.

[Carl Richard]

Hi, TS!

Glad to help! It can be a bit difficult to navigate all the different docs, so that's understandable! 

What modem firmware version are you using? I do not believe that to be the issue however, as the 3 socket limit has been in place as long as I can remember. It could be related to the SDK, so if you have the opportunity to try out a newer one (1.4.0 release candidate for example) that could narrow down the issue. 

If you want I could take a look at your code and test it out here? 

Best regards,
Carl Richard

[Carl Richard]

Hello again!

I asked some of the developers regarding this. They say that while up to three simultaneous TLS/DTLS connections are possible it's not guaranteed that it will work with for example three TLS connections. The reason for this is that TLS consumes a lot of internal memory. However, if you could share a modem trace with us we can have a look at exactly what's happening. You can follow our guide on modem tracing.

Best regards,
Carl Richard