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Re: [lwip-users] buffering outgoing packets

From: Kieran Mansley
Subject: Re: [lwip-users] buffering outgoing packets
Date: Fri, 30 Apr 2010 17:09:08 +0100

On Fri, 2010-04-30 at 11:54 -0400, Jeff Barber wrote:
> Hmm... I seem to have not been paying attention when this discussion
> went by.  Now wondering what (if anything) I need to do.  I am using
> zero-copy on both transmit and receive, and relying on the ability to
> increase the pbuf reference count to keep the pbuf from being freed
> while it's being transmitted.  (None of the issues mentioned in task
> 7896 apply to me so I think I'm okay on receives.)
> 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.

> 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.


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