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Re: Resolved!, was Re: Uthernet receive error: bytes swapped

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David Schmenk wrote:
> On Thursday, 5 March 2015 16:31:09 UTC-8, D Finnigan  wrote:
>> gids.rs wrote:
>> >> What's incredible is how the checksum routine still managed to give
>> >> correct
>> >> results in so many cases. That's definitely one of those tricky bugs.
>> >> 
>> > 
>> > 
>> > Maybe not so incredible.  Wouldn't 10 bytes of code followed by 10
>> > bytes
>> > of
>> > 0's give the same checksum?
>> 
>> I don't know. This IP checksum routine was supposed to only cover the 20
>> byte IP header. If it was running twice as long, it was either
>> checksumming
>> a bit of the data payload, or something else. I'm not really interested
>> in
>> looking into exactly what it was doing, just surprised that it was giving
>> correct results in its broken state.
>> 
> 
> Are you sure it was giving correct results?

Yes, I had my application print a + sign if the checksum came out to be 0,
and an @ if not.

I also have a test suite. Granted, it involves me manually entering the test
cases and verifying the result, but I trust it.

This problem came when I unrolled the loop. See, I had this checksum routine
verified correct by my test suite, but then I decided to unroll the loop,
and that's when I started getting these occasional errors.


> Most TCP/IP headers can get
> away with a checksum of 0 (no checksum) so you don't need to spend time
> calculating them. If you are on a local network and not putting your Apple
> II on the internetz, this is probably a safe thing to do.

I appreciate the advice, but I don't want to take any shortcuts in this
area.

I just completed and debugged the UDP checksum routine last week. Both
outgoing and incoming UDP datagrams have their checksum verified or computed
as necessary.

-- 
]DF$
Apple II Book: http://macgui.com/newa2guide/
Usenet: http://macgui.com/usenet/  <-- get posts by email!
Apple II Web & Blog hosting: http://a2hq.com/

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Thread

Uthernet receive error: bytes swapped D Finnigan <dog_cow@macgui.com> - 2015-02-28 20:23 +0000
  Re: Uthernet receive error: bytes swapped David Schmenk <dschmenk@gmail.com> - 2015-02-28 12:58 -0800
  Re: Uthernet receive error: bytes swapped a2retrosystems@gmail.com - 2015-02-28 16:20 -0800
    Re: Uthernet receive error: bytes swapped D Finnigan <dog_cow@macgui.com> - 2015-03-01 00:34 +0000
      Re: Uthernet receive error: bytes swapped a2retrosystems@gmail.com - 2015-02-28 17:17 -0800
      Re: Uthernet receive error: bytes swapped David Schmidt <schmidtd@my-deja.com> - 2015-02-28 21:58 -0500
  Re: Uthernet receive error: bytes swapped ol.sc@web.de (Oliver Schmidt) - 2015-03-01 14:33 +0000
    Re: Uthernet receive error: bytes swapped David Schmenk <dschmenk@gmail.com> - 2015-03-01 12:15 -0800
  Resolved!, was Re: Uthernet receive error: bytes swapped D Finnigan <dog_cow@macgui.com> - 2015-03-02 20:47 +0000
    Re: Resolved!, was Re: Uthernet receive error: bytes swapped gids.rs@sasktel.net - 2015-03-02 16:14 -0800
      Re: Resolved!, was Re: Uthernet receive error: bytes swapped D Finnigan <dog_cow@macgui.com> - 2015-03-06 00:30 +0000
        Re: Resolved!, was Re: Uthernet receive error: bytes swapped David Schmidt <schmidtd@my-deja.com> - 2015-03-05 20:53 -0500
          Re: Resolved!, was Re: Uthernet receive error: bytes swapped D Finnigan <dog_cow@macgui.com> - 2015-03-07 17:37 +0000
        Re: Resolved!, was Re: Uthernet receive error: bytes swapped David Schmenk <dschmenk@gmail.com> - 2015-03-06 07:21 -0800
          Re: Resolved!, was Re: Uthernet receive error: bytes swapped D Finnigan <dog_cow@macgui.com> - 2015-03-07 17:31 +0000

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