embedded web server (EWS) & persistent HTTP 1.1 connection

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Mischa Oliver Altmann 21 years ago

Hello,

I'm trying to build a minimal embedded web server, EWS (possibly using PIC and later slow ARMs) as a proof of concept allowing the remote monitoring of various automated measurements. There seems to be a wide variety of available (open source, commercial and hobby) systems on various platforms. One thing I haven't found too often is the use of HTTP 1.1's "persistent connections"[1] which could cut down the amount of admin work needed for TCP/IP connections. While I realise that it might not be significant for "big" cores, it is for minimal configurations such as micro-controllers (eg. PICs). Any ideas as to why that is? (maybe i should post this in TCP/IP news groups instead?)

thanks, Mischa

[1]

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P.S.: Sorry, JB, I only thought of using newsgroups after I wrote you the email.

Mischa Oliver Altmann http://www.altmann.at/~mischa (in terrible state)

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Richard H. 21 years ago

[...]

A few reasons you might not find it widely implemented:

1) It's optional, requires lots more resources, and benefit isn't obvious to the user. Embedded systems operate on tight resources, and optional features (and many non-essential "required" features) are omitted to conserve resources. 2) A common application is an admin page, which is often a dynamic HTML page with static (cache-able) graphics, so only the single HTML object is being fetched in many cases. (i.e., a persistent connection would not be of benefit.) 3) TCBs = RAM, which is usually precious with embedded systems, especially when each TCB easily requires 512+ bytes. Many have resources for only 1-2 connections at once, so making them last for seconds instead of milliseconds can have a huge impact on memory requirements. (IIRC, persistent connections are often 10+ seconds, though I exect this is tunable.)

Practically, it would seem straightforward to implement: jump back to the TCB input handler instead of closing the session; timeout idle connections after X msecs. (Probably can't be

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Wolfgang Mües 21 years ago

Have you checked

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best regards Wolfgang Muees

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Mischa Oliver Altmann 21 years ago

no, excellent resouce. thanks!

-- Mischa Oliver Altmann

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(in terrible state)

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Mischa Oliver Altmann 21 years ago

hm, interesting thought. I assumed that using persistent connections would make TCP/IP stack coding easier as less connections have to be built up and closed down.

you definately have a point here. For my purposes it would not be needed (I am only doing a proof of concept EWS) but i thought it would be a useful "exercise".

agreed -- see 2)

[snip]

see 2)

thanks for your indepth explanation - it's very much appreciated.

I'm in Leeds, are you nearby ;-) or on the other side of the globe?

thanks, Mischa

-- Mischa Oliver Altmann

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(in terrible state)

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Richard H. 21 years ago

The quantity might be lower, but the TCP behavior would necessarily work identically. Probably a touch more code to detect / activate / timeout persistence for HTTP.

Exercises are always good if you have the time - practicality need not apply. :-)

Happy to!

In the Arizona desert, USA (which this week is cold and rainy).

Though I remember visiting Leeds many years back - enjoyed the castle enough that we missed the last train back to London. :-)

Have fun! Richard

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Richard H. 21 years ago

Also...

I highly recommend Jeremy Bentham's book, TCP/IP Lean. It's the best text so far about implementing TCP on a microcontroller. And the example is based on a PIC.

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Even if you aren't interested in the included code, he's got some clever techniques for dynamic HTML objects like slide bar graphics, and another for handling the TCP checksum with HTML. Well worth the money as a backgrounder to the issues and some ideas for handling them.

Enjoy!

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Mischa Oliver Altmann 21 years ago

hehe -- that's exactly where I was coming from! It doesn't even mention persistent connections and I was wondering why. It's pretty clear to me now, but I might just write the code to show it's possible.

thanks again, Mischa

P.S.: I am not aware of a castle in Leeds (though there are some ruins of monasteries/churches). You might mean Richmond which isn't "that" far form Leeds and also in the "north".

-- Mischa Oliver Altmann

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(in terrible state)

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Richard H. 21 years ago

Aha!

I've been thinking about a different technique than his for the checksum handling. It makes more sense to write a null checksum, then go back and update the NIC buffer with the right checksum value. And it works for protocols other than HTTP.

I suspect the technique of guessing a checksum value & stuffing bytes at the end to make it right was all due to the original implementation that was SLIP-based, not Ethernet-based, and there was no buffer to go back and update - it'd already been serialized out the port. Very clever, considering the resources, but unnecessary using Ethernet NIC buffers, I think.

What? They moved it?! ;-)

Probably a bad assumption on my part that Leeds Castle was in Leeds. :-)

Cheers, Richard

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embedded web server (EWS) & persistent HTTP 1.1 connection

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