[En-Nut-Discussion] TCP/IP problems with httpd

[Paolo Simoncelli]

Hi all,
shall the problem under investigation involve also
the very first packet sent over UDP ?

----- from an old post of mine -----------


... if you throw out an UDP packet
very early (seems about before 500 ms) after opening the socket
it gets lost into the TCP guts of NutOs  :(  ...

(workaround: added NutSleep(500); before testing incoming reply)

-------------------------------------------

Cheers,
Paolo



Ulrich Prinz wrote:
...
> Hi!
> 
> So we are now with three people checking the stack. Thanks to my 
> colleague Daniel, we found a thing. We don't have a soluting, but I'll 
> try to describe exactly how it comes to long time unanswered pings if 
> not any other TCP/UDP/FTP access is happening from/to the same host.
> 
> If icmpin.c NutIcmpReflect() is requested to echo a packet, it issues 
> this echo. The called routine checks if there is an ARP cache entry for 
> the target and if so, the echo packet takes its way.
> If there is no ARP entry for the target, cause it is the very first 
> packet ever sent to the host, or cause the ARP cache entry aged out, the 
> called function from icmpin issues an ARP request.
> 
> This ARP request is handled by icmpout and is sent through the line, and 
> the host answers the packet. Unfortunately the icmpin routine is waiting 
> for the initial echo packet to be sent and the ARP request answer is 
> staying in the input buffer, never retrieved by icmpin.
> 
> There are timeout mechanisms working in the background, as this blocking 
> situation sometimes recoveres after some minutes.
> 
> The described bug doesn't appear very often as most times a system is 
> busy with transferring anything useful but icmp. Other protocols check 
> the ARP cache too and request a new address if the desired is missing, 
> but while their stacks are blocked until the response comes in, icmp is 
> free and working.
> The problem was difficult to find as you have to meet the situation 
> where 1st, the ARP cache entry aged, and an icmp echo was answered right 
> before a TCP response was sent.
> The higher the icmp traffic is, the higher the chance to hit the bug.
> 
> Best regards
> Ulrich
> 

 
 
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