[lwip-devel] [bug #33952] PUSH flag in incoming packet is lost when packet is aggregated and sent to application

[Klaus Hansen]

URL:
  <http://savannah.nongnu.org/bugs/?33952>

                 Summary: PUSH flag in incoming packet is lost when packet is
aggregated and sent to application
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: foss_eswp
            Submitted on: Fri Aug  5 10:37:32 2011
                Category: TCP
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 
            lwIP version: 1.3.0

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Details:

I noticed this faulty behaviour when I was trying to "put" a file from a
Windows FTP client to a FTP server using version 1.3.0 of the lwIP stack.
Sometimes the last bytes in the file were missing.

The FTP server would call lwip_recvfrom which again would call netconn_recv.
In this case the file to be transferred was 52228 bytes long and the tcp
packets eache contained 536 bytes with the last one containing only 236 bytes
(97 * 536 + 236).
By instrumenting the lwip_recvfrom code I could see that most of the calls to
netconn_recv returned just 536 bytes from one tcp packet. And when this one
tcp packet contained a PUSH flag (I had WireShark running), the packet
returned from the netconn_recv call also had the PUSH flag set.
But some of the calls to netconn_recv returned an aggregate of several tcp
packets - I suspect because the receiving application had been busy doing
other things. And if one of the tcp packets had the PUSH flag set, the flag
was not set in the aggregate packet.
In this situation, the last 7 packets were aggregated (6 * 536 + 236). The
last 256 byte packet had the PUSH flag set, but the aggregate packet returned
from the netconn_recv call had no PUSH flag. So the lwip_recvfrom function
would loop and call netconn_recv again, this time finding out that the
connection had been closed, and returning 0 bytes.

I found out that the aggregate packet was a result of the last 7 packets being
taken out of the out-of-sequence queue in the tcp_receive function. When a
packet is inserted into the out-of-sequence queue, the tcp flags are
remembered. But when the packets are taken out of the queue, the PUSH flag is
forgotten.

I fixed the bug by adding a TF_GOT_PSH flag (tcp.h) which the tcp_receive
function would set in the global recv_flags, if one of the packets in the
out-of-sequence queue has the PUSH flag set.

In the tcp_input function, where the received packets (simple or aggregate)
are returned from the tcp_process call, I added a check for the TF_GOT_PSH
flag in the global recv_flags, and set the PUSH flag in the recv_data->flags
accordingly before returning recv_data to the application.

I checked version 1.4.0 to see if this bug had already been fixed, but it
didn't seem so.

I have attached sockets.c, tcp_in.c and tcp.h with my debug code and bug fix.
The bug is named TD#10887.





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File Attachments:


-------------------------------------------------------
Date: Fri Aug  5 10:37:32 2011  Name: sockets.c  Size: 58kB   By: foss_eswp

<http://savannah.nongnu.org/bugs/download.php?file_id=23744>
-------------------------------------------------------
Date: Fri Aug  5 10:37:32 2011  Name: tcp.h  Size: 25kB   By: foss_eswp

<http://savannah.nongnu.org/bugs/download.php?file_id=23745>
-------------------------------------------------------
Date: Fri Aug  5 10:37:32 2011  Name: tcp_in.c  Size: 55kB   By: foss_eswp

<http://savannah.nongnu.org/bugs/download.php?file_id=23746>

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