> This is rather weird. It seems to be based on
> several completely
> incorrect assumptions. First, there is the notion
> that that UDP must
> always do a checksum. That isn't true. If the sender
> sets the checksum
> to zero, it means, in the existing spec, that you
> aren't to do a
> checksum. Of course, none of this is useful anyway.
> In practice, all
> our link layers discard damaged packets long before
> they get to you.

Setting the checksum to zero means not performing any
checksum i.e. not even the header. But, not
checksumming the header is totally absurd because you
might end up processing a packet meant for someone
else. Therefore, UDP Lite does mandatory checksumming
on the header but checksum on the payload is optional.
This allows application layer (error-resilient)
protocols to utilize partially damaged packets.

And UDP Lite is always used in combination with a
variation of the Link Layer Protocol. This variation
is called "PPP Lite" and it allows damaged packets to
go through thus making transport layer responsible for
the checksum.

> The rest of what I read seemed to indicate other
> misunderstandings on
> the part of the authors about what UDP can already
> do.

Please elaborate on these "misunderstandings".
 
> Anyway, I doubt any of this code will ever be
> mainlined in NetBSD. It
> doesn't seem to have much of a purpose.

UDP Lite has shown a lot of promise for real-time
multimedia applications on wireless networks.
Comparison with traditional UDP is provided in the
following papers:

  (1) Larzon, L-Å, Degermark, M., Pink, S., "Efficient
use of Wireless Bandwidth for Multimedia Applications"
, Proc. 6th IEEE International Workshop on Mobile
Multimedia Communications (MOMUC), Nov. 1999.
(http://www.cs.arizona.edu/classes/cs652/f00/articles.html)

  (2) Amoolya Singh, Almudena Konrad, Anthony D.
Joseph, "Performance Evaluation of UDP Lite for
Cellular Video", Proc. 11th International Workshop on
Network and Operating Systems Support for Digital
Audio and Video (NOSSDAV 2001).
(http://www.cs.berkeley.edu/~almudena/research.html)

So I recommend that you look into it more seriously if
you are targeting anything wireless.

Thanks for your insight and please keep the
constructive discussion going.


  -- Ali

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