[lwip-users] Re: bug fix

[Bill Knight]

I just finished a port from lwIP-0.7.2 to the current code in CVS.
The processor is an LPC2292 with a CS8900 ethernet chip.  The LPC is
an ARM7TDMI which operates in LITTLE_ENDIAN mode.  The problem shows
up for me when using Adam's minimal http server.  I have isolated it
to the TCP_SEQ_BETWEEN in tcp_in.c at line 822.  When I use the
previous macros, the code serves pages just fine.  When I use the new
macro, it does not.  The code fragment the works is:

    while (pcb->unsent != NULL &&
           TCP_SEQ_LEQ(ntohl(pcb->unsent->tcphdr->seqno) + 
TCP_TCPLEN(pcb->unsent), ackno) &&
           TCP_SEQ_LEQ(ackno, pcb->snd_max)
           /*TCP_SEQ_BETWEEN(ackno, ntohl(pcb->unsent->tcphdr->seqno) + 
TCP_TCPLEN(pcb->unsent), pcb->snd_max)*/

So far I have been unable to determine why in this instance the macro
does not work.  Maybe something to do with LITTLE_ENDIAN.  Maybe
another set of eyes can spot it.

Regards
-Bill Knight
theARMPatch

PS - the compiler is arm-elf-gcc version 3.4.0


On 12 Sep 2004 17:03:22 +0100, K.J. Mansley wrote:

>On Fri, 2004-09-10 at 17:14, Brett Jones wrote:
>> The problem is that tcp protocol uses a unsigned 32 bit value for a
>> sequence number and acknowlegement. As each packet is sent the
>> sequence number is increased by the amount of data that is sent and
>> normal comparisons work fine until the sequence number becomes more
>> than a 32 bit value will hold ( 4,294,967,295 ) the sequence number
>> rolls over and even though you have added to the sequence number the
>> sequence number becomes less than it was before. with a signed number
>> the problem is even worse because it rolls over at 2147483647. when
>> you add 2 to this number it becomes -1
>> then lwip comparisons dont work correctly and does not think that its
>> packets have been acknowleged (out of window) and the que becomes
>> full. to fix this you check for rollover and if a rollover has occured
>> you reverse the comparison logic :)
>> This fix has made it possible for me to transfer large amounts of data
>> whereas a tcp connection was shutdown every 2 or so gigabytes of data
>> transfered before.

>OK.  I've had a look at how linux does things, and its macros are the
>same as ares (and the comment says it copes with wrap-around) except
>they had a TCP_SEQ_BETWEEN macro to determine if b <= a <= c, and it's
>conceivable that compound use of the simpler "TCP_SEQ_LTEQ" macro to
>achieve the same effect doesn't work for wrap-around, but I'm not sure.

>To be on the safe side I've added this "BETWEEN" macro to lwip, and
>replaced any logically equivalent statements with this.  I've left the
>old statements in place just to be clear what the real intention is, in
>case I've translated them wrongly.

>Would be grateful for another pair of eyes to look these changes over
>and see if this:
>(a) fixes the problem
>(b) makes things worse

>Any volunteers?

>Thanks!

>Kieran



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