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Re: [lwip-users] buffering outgoing packets

From: Jeff Barber
Subject: Re: [lwip-users] buffering outgoing packets
Date: Fri, 30 Apr 2010 12:32:39 -0400

On Fri, Apr 30, 2010 at 12:09 PM, Kieran Mansley <address@hidden> wrote:
> On Fri, 2010-04-30 at 11:54 -0400, Jeff Barber wrote:
>> Looking at the 29361 bugtracker entry, it appears that zero-copy
>> transmit is only an issue if there is IP fragmentation.  Is that
>> correct?  If so, then is it true that I would not have problems if I
>> have a known MTU and never send a "PDU" larger than the MTU?  Or in
>> other words, if I never send a UDP datagram that doesn't fit in the
>> MTU, and ensure that the TCP maximum segment size fits within the MTU?
> MTU is a property of the local network.  IP fragmentation could happen
> at any point in the path between the communicating devices, so I don't
> think that would help.

I don't think that's a problem for me because I'm only communicating
across a backplane ethernet to a single host and I can control
settings on both ends.

>> In fact, it sounds like maybe I should just disable support for IP
>> fragmentation and reassembly altogether.
> I don't fully understand the problem, but Simon mentions a workaround in
> the bug tracker.  Disabling IP fragmentation sounds like it would work
> but perhaps not necessary to go that far.

I currently have fragmentation and a bunch of other stuff enabled
simply because I didn't know my way around lwIP or TCP when I started
and I wanted as "full-featured" a stack as I could get and was not
memory constrained (as many other lwip users seem to be).  However, I
should probably revisit my settings.

Thanks for the input.

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