Spolehlivost TCP/IP prenosu po internetu

[Ondrej Puzman]

Zdravim,
situace je o neco slozitejsi. Ethernetovy ramec je chranen CRC-32,
nicmene TCP/IP samozrejme nemusi byt prenaseno po ethernetu.
A TCP/IP paket (ktery muze a nemusi byt uvnitr ETH ramce) je chraneny
pouze 16-ti bitovym checksumem. Da se tedy rict, ze TCP/IP castecne spoleha
na zabezpeceni na linkove vrstve.
S pozdravem,
     Ondrej Puzman


> 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...