Spolehlivost TCP/IP prenosu po internetu

[Jindrich Kubec]

At 18:45 18.9.2006, ck@cbox.cz wrote:
>TCP/IP paket je chranen CRC32 zabezpecenim. Vlastnosti tohoto zabezpeceni 
>jsou zavisle na delce vysilaneho paketu a urovni zaruseni prenosu. 
>Pravdepodobnost neodhaleni chyby je mensi u kratsiho paketu, ne vsak 
>vzdycky - bohuzel se pro nektere delky vyrazne zvysuje (treba 40x).
>Pocitali jsme grafy pravdepodobnosti chyb pro ruzne CRC polynomy, ale pro 
>jeden to trva dny az tydny. Nema tu nekdo pristup k superpocitaci, nebo k 
>nejakemu koprocesoru z hradlovych poli?
>
>Odhaduju pravdepodobnost nedetekovani chyby na TCP/IP tak 10^-9.
The 16-bit <http://en.wikipedia.org/wiki/Checksum>checksum field is used 
for error-checking of the header and data. The method used to compute the 
checksum is defined in <http://tools.ietf.org/html/rfc793>RFC 793:
The checksum field is the 16 bit one's complement of the one's complement 
sum of all 16-bit words in the header and text. If a segment contains an 
odd number of header and text octets to be checksummed, the last octet is 
padded on the right with zeros to form a 16-bit word for checksum purposes. 
The pad is not transmitted as part of the segment. While computing the 
checksum, the checksum field itself is replaced with zeros.
Ta pravdepodobnost bude tim padem asi vyssi...


Jindroush (kubecj@avast.com)
http://www.kostky.org - Pro fany stavebnic LEGO.