nRF9160 packet data receive buffer size

[Marcus_S]

What is the receive buffer size for IP packet data, i.e. how many bytes of data can the modem receive (in one or several UDP packets) before I call recv() or recvfrom() on the UDP socket? I cannot find any Information on that in the product specs. Are things like this documented somewhere and where can I get such information?

[Didrik Rokhaug]

Hi.

The maximum size of an IP packet (that we support) is defined in bsd_limits.h: 4096

https://github.com/NordicPlayground/nrfxlib/blob/master/bsdlib/include/bsd_limits.h#L28

In addition, you might be interested in the MTU size of your network. You can find it with the +GGDCONTRDP AT command: https://infocenter.nordicsemi.com/topic/ref_at_commands/REF/at_commands/packet_domain/cgcontrdp_set.html (note that you have to subtract any protocol (from the IP layer and up) overhead to find the largest payload that can be sent in one packet)

Best regards,

Didrik

[Marcus_S]

OK, thanks. Actually I am interested in the cumulative size of packets that can be buffered by the modem. The maximum IP packet size is not necessarily the same as the modem's receive buffer size. I do not expect packets larger than the MTU size anyway, because I don't want to handle arbitrarily fragmented packets. But I expect to receive more than one packet within short time.

So for example, if the modem receives a burst of 512-byte packets, how many of them can the modem store before packets are dropped or I call recv()? Is it 8 packets? Of course I could try to determine this limit experimentally, but I prefer solid data ;-)

[Didrik Rokhaug]

Hi, and sorry for the long wait.

Turns out your question was not straight forward to answer, and due to vacations it took some time to find someone that could answer.

The answer I got from the developers is that there is no fixed buffer. Instead, the buffer is allocated dynamically. We can therefore not say how much that can be buffered at a given time. It will depend on the overall memory allocation status in the application.

Best regards,

Didrik

[Marcus_S]

OK, thank you for your answer. So I will just assume the max IP packet size as a safe lower limit.