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Re: [lwip-users] Issues receiving streaming data

From: JM
Subject: Re: [lwip-users] Issues receiving streaming data
Date: Thu, 30 Jul 2009 09:57:32 -0700 (PDT)

That's a great point that I did not realize.  All this time I was going off the assumption the pbufs were holding only data for the application.  I can see now how 2 or 3 would not work.

I was using uIP before this, and configuration was much easier.  But, I believe that stack was hopeless as far as suitability for what I'm trying to do.

--- On Thu, 7/30/09, address@hidden <address@hidden> wrote:

From: address@hidden <address@hidden>
Subject: Re: [lwip-users] Issues receiving streaming data
To: "Mailing list for lwIP users" <address@hidden>
Date: Thursday, July 30, 2009, 11:25 AM

Setting PBUF_POOL_SIZE *much* lower than 16 is also not a good idea as this limits the number of RX-packets being processed simultaneously. If you are low on memory, it's a better idea to decrease the size of each pbuf in the pool so that, for a big packet, multiple pbufs are used, while smaller packets (e.g. ACKs) consume less memory.

Also, if you're on a slow system, chances are high the place were a buffer overrun occurs is the ethernet MAC and lwIP won't even notice an overrun (if the MAC driver doesn't record).


JM wrote:
> Thank you for the quick reply Kieran.  After looking at these settings again, and finding an old post where you give a rough estimate for MEMP_NUM_TCP_SEG, I believe I have this set way too low.  I will try your suggestions later and see what happens.  Some of the defaults, such as PBUF_POOL_SIZE = 16, is so high it will consume too much RAM.  My guess is I can get by with much less.
> Oops....for the TCP_WND discrepancy, that's easy to explain.  I changed it after taking the capture, but it the Wireshark results were basically the same.
> I will enable Stats and I can view them with my debugger after it runs for awhile.  That is something I haven't thought of before and should give good information.  I'll set these numbers high, and try shrinking them until things stop working.
> --- On *Thu, 7/30/09, Kieran Mansley /<address@hidden>/* wrote:
>     From: Kieran Mansley <address@hidden>
>     Subject: Re: [lwip-users] Issues receiving streaming data
>     To: "Mailing list for lwIP users" <address@hidden>
>     Date: Thursday, July 30, 2009, 10:05 AM
>     On Thu, 2009-07-30 at 05:26 -0700, JM wrote:
>     >
>     > I've included two Wireshark captures in an attached .zip file
>     (please
>     > add .zip to filename).  The one labeled "lan" is when the server was
>     > my computer on my LAN, and the one labeled "wan" is a streaming
>     audio
>     > server on the Internet.  I've verified these both work fine when a
>     > computer is is playing the stream.  On the "lan" trace, there is a
>     > perfectly repeated Dup ACK and retransmission that should not be
>     > happening.  On the "wan" trace, the connection just seems to break
>     > down before I close it.  This is similar behavior to all other
>     > Internet streaming audio servers, so it's not just this one.
>     The retransmissions are happening because there are packets being
>     dropped.  They are repetitive because after packet loss the sender is
>     very cautious about the sending rate.  As soon as the sending rate
>     increases, you see packet loss.  This suggests strongly that your lwIP
>     device can't handle > 1 packet arriving at a time, and so you're
>     likely
>     running out of buffers.  You could confirm this by using the lwIP
>     stats,
>     and seeing which of the buffer types are reporting errors (if you can
>     get a print out of these somehow).
>     Looking at your settings, I think it would be good to change:
>     MEMP_NUM_PBUF: you'll need more than one of these, I'm pretty sure.
>     PBUF_POOL_SIZE: 3 seems rather small.  What's the default here - see
>     src/include/lwip/opt.h
>     MEMP_NUM_TCP_SEG: only having 2 TCP segments is going to cause you
>     problems
>     LWIP_UDPLITE: you can probably set this to zero
>     TCP_SNDBUF: you could set this larger (smallest sane size is two MSS)
>     without using any extra memory, but probably not the source of your
>     problems as you're not sending much.
>     Something that looks odd to me is that you're setting TCP_WND to 2920,
>     but in the packet captures lwIP is giving out a larger receive
>     window of
>     4380.
>     Kieran
>     _______________________________________________
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