Maximum Segment Size (MSS)

[butaikis]

Hello. Is it possible to increase the size of the data segment when receiving a large amount? As I found out, the size depends on the configuration in the SDK - Maximum Segment Size (MSS) 1440. But when using a standard web server from the espressif sdk, for example, the file downloads much faster.

Is it possible to increase the segment size for ESPAsyncWebServer?

[mathieucarbou]

The MSS is set by esp-idf framework (CONFIG_LWIP_TCP_MSS if I am not mistaken), but I do not know if other things have to be set also. @me-no-dev might know more.

To change a esp-idf setting and still use arduino framework, you need to you pioarduino and hybrid compilation.

There is an example in the project in pioarduino_examples/IncreaseMaxSockets which shows how to increase the socket count (which is 16 by default). It would work exactly the same way with CONFIG_LWIP_TCP_MSS:

custom_sdkconfig = CONFIG_LWIP_MAX_ACTIVE_TCP=32
                   CONFIG_LWIP_TCP_MSS=1440

[butaikis]

@mathieucarbou thanks for the answer, but I mean how to increase this size if the maximum is 1440 in the config and it doesn't allow you to do more, maybe there are workarounds?

[mathieucarbou]

I think you didn’t get my answer.
pioarduino hybrid compilation lets you redefine esp-idf config flags. So yes, you can increase and set a new value.

[mathieucarbou]

Look at the example i pointed to you and try to understand how it works…

[butaikis]

yes, it can be overridden and I can do it, but the problem is that the maximum value that can be set is 1460

[mathieucarbou]

Yes that’s normal: tcpip v4 max mss is 1460 and max mss for ipv6 is 1440. The numbers are good as set by default and this is not what causes the perf difference you see

[butaikis]

I got it, thank you for your answer. Second question, is there any chance that https connection support will be added?

[mathieucarbou]

No - not on short term at least.

[butaikis]

Got it, thanks