Re: [lwip-users] Understanding retransmission

[Jan Menzel]

Hi Simon!

On 19.01.2015 16:19, Simon Goldschmidt wrote:
> Jan Menzel wrote:
>> In line 21 the shark records a duplicated ACK, which
>> I've seen a few times already (there is one in line 7 for example.).
> 
> I don't know what the reason for the Dup ACK in frame #7 is, but for frame 
> #21,
> the reason seems to be a corrupted TCP checksum of frame #14.
> To see what the reason is here, try to:
> - pass TCP_WRITE_FLAG_COPY to tcp_write() (when using the raw api)
> - set LWIP_CHECKSUM_ON_COPY to 0 (or LWIP_CHECKSUM_ON_COPY to 1)
> 
> and see if that helps (and which of the two).
> 
> Also, I've commited a possible bugfix for LWIP_CHECKSUM_ON_COPY some days 
> ago, maybe that fixes your problem?
> 
Your patch is already part of my code. I just disabled
LWIP_CHECKSUM_ON_COPY and have not seen a bad packet since then.
        Many thanks!

        Jan