Re: [ECOS] ecos: tcp/ip stack implementation error

[Martin Buck <martin dot buck at ascom dot ch>]

? ? wrote:
> Hi,
>    i am willing to show a implementation error in ecos tcp/ip stack:
>    as for tcp urgent pointer,when urp >65496(not 65535),urp should be 
> 65535.because 65536-20(iphdr)-20(tcphdr)=65496.
>  following is ecos code:
>             u_int32_t urp = tp->snd_up - tp->snd_nxt;
> 		if (urp > IP_MAXPACKET)//65535 
> 			urp = IP_MAXPACKET;
> 
> it should like this:
>                   if(urp > IP_MAXPACKET-sizeof(struct tcpip))
>                            urp = IP_MAXPACKET;

Most likely it shouldn't. The urgent pointer is a 16 bit field and the
check for IP_MAXPACKET just seems to be there to limit the value of the
urgent pointer to values representable with 16 bits. The reasoning is
probably that it's better to send a saturated value than a completely
bogus value (which you'd get if you just sent the lower 16 bits of a 32
bit value).

BTW, the FreeBSD code doesn't contain the check for IP_MAXPACKET at all,
so it's guaranteed to send bogus urgent pointers if you have a
sufficiently large and filled send buffer. But then, nobody cares about
urgent pointers anyway these days...

BTW#2, even if would make sense to take the size of the IP+TCP headers
into account here, your fix would be wrong beause it assumes that their
size is constant.

Martin


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