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[lwip-devel] [bug #46467] ip_frag() shouldn't modify pbuf in case of a r

From: Zach Smith
Subject: [lwip-devel] [bug #46467] ip_frag() shouldn't modify pbuf in case of a retransmission
Date: Thu, 13 Oct 2016 21:26:13 +0000 (UTC)
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36

Follow-up Comment #11, bug #46467 (project lwip):

Simon, I wanted to let you know I am trying to test this but I am having
trouble. I have downloaded the newest source code today and updated. But I
can't seem to get ip fragmentation to work as before. So, I haven't been able
to get to testing the re-transmission of an ip fragment.

As noted in the repro. info below my application sends ~1000 byte packets.
With the old code (from Aug 16) I just set the netif mtu to 700 in
low_level_init (netif->mtu = 700;) and then defined the following in

#define LWIP_ND6_ALLOW_RA_UPDATES 0 (so mtu doesn't get changed on me)
#define TCP_CALCULATE_EFF_SEND_MSS 0 (so MSS stays larger than MTU which
causes ip fragmentation)

Then I just start running my application and I can see lwip sending the data
in fragments.

With this new code lwip seems to send the fragment just like before but the
fragmented packet is not ACKed by the other side (PC). I can't see why it
doesn't get ACKed because the fragmented packet looks the same compared to the
fragments generated by the older lwip code.

I am attaching some wireshark captures showing the difference between the two
attempts (one with older code and one with newer code) in case someone has
time to look at it.

I don't know what I am doing wrong yet.

(file #38724, file #38725)

Additional Item Attachment:

File name: ip_frag_lwip_20100816.pcapng   Size:383 KB
File name: ip_frag_lwip_20101013.pcapng   Size:55 KB


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