[Click] SetUDPChecksum dropping packets

[Eddie Kohler]

Hi Robert,

If you're using DecIPTTL, that element already updates the IP checksum 
incrementally.

Eddie


Robert Ross wrote:
> We're modifying the TTL on outgoing packets, so I assumed we had to 
> modify the checksums.  Now that I think about it, we probably only need 
> to set the IP checksum and not the UDP or TCP checksum.  I'll try that 
> out...
> 
> 
> -----Original Message-----
> From: Eddie Kohler [mailto:kohler at cs.ucla.edu]
> Sent: Tue 5/1/2007 4:03 PM
> To: Robert Ross
> Cc: click at amsterdam.lcs.mit.edu
> Subject: Re: [Click] SetUDPChecksum dropping packets
> 
> Robert,
> 
> Yes, it is impossible to set a UDP packet's checksum without the whole 
> packet.
>   Fragments won't work: not all the data is available.  I've just 
> checked in a
> change so that IP fragments, or too-short packets, are emitted on the 2nd
> output, or dropped if there is no 2nd output.
> 
> I am surprised you need to fix TCP and UDP checksums on input packets.
> Usually that is not necessary for input packets; and in fact fixing the
> checksums is a bad idea, because it can hide errors elsewhere.  
> SetUDPChecksum
> is generally used when you're generating packets from scratch.
> 
> Eddie
> 
> 
> Robert Ross wrote:
>  > We have found what appears to be a bug (or undocumented feature) in 
> SetUDPChecksum where packets sent into this element are somehow 
> discarded.  In the attached config, you will see an ElementClass called 
> FixChecksumsDebug which counts packets into each checksum fixer and 
> counts packets out the other side.  We have found that, under specific 
> circumstances, packets sent into SetUDPChecksum never come out the other 
> side.  This is true of the latest version from SVN.
>  >
>  > As to the particular situations for this problem, it seems to happen 
> only with fragmented packets.  In a set of packet fragments, the first 
> portion (offset +0) always seems to be discarded when going through 
> SetUDPChecksum.  There is no indication that I have found as to the 
> reason for the discard.
>  >
>  > Has anyone else seen this issue?  Does SetUDPChecksum have a second 
> output that is undocumented?  Any suggestions?
>  >
>  > Thanks,
>  >
>  > Robert Ross
>  >
>  >
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