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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 11:54:49 -0400

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?

In fact, it sounds like maybe I should just disable support for IP
fragmentation and reassembly altogether.


On Fri, Apr 30, 2010 at 2:10 AM, Simon Goldschmidt <address@hidden> wrote:
> Tyrel Newton wrote:
>> I'm wondering if there is any truth to this statement. Do outgoing
>> packets that are queued at the network interface driver level
>> really need to be copied verbatim into a separate buffer? This seems
>> counter-intuitive to lwIP's low-memory usage design goals.
> That's a topic we've already discussing about and which we wanted to fix with 
> 1.4.0 (as it might involve API- or doc-changes). You can always try to not 
> copy the packets (which is of course desirable), but you *might* run into 
> changes if you are using the RAW API and are re-using pbufs passed to one of 
> the output/send/write functions *or* if you are using IP-fragmentation 
> (IP_FRAG==1).
> See http://savannah.nongnu.org/task/?7896 and 
> https://savannah.nongnu.org/bugs/?29361 for bugtracker entries about this 
> issue.
> Simon
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