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From: Stas Degteff (2:5080/102.1)
To: All
Date: Sat, 11.02.12 22:47
binkp checksum protected file transfer
Hello Andre!

10 Feb 12 18:54, you wrote to me:


>> AG> I agree that we cannot just rely on the link to be error free.
>> Look an Binkp preambule, please: "a protocol to handle a session
>> between two Fidonet Technology systems over a *reliable* connection"
>> "

AG> I know this. Why do we have the NR mode then? We've got a reliable
AG> link -- connections never break. Do they?

No.

Reliable link may be breaked, but guarantees delivery. If the datagram isn't
delivered, the sender (application level OSI) receives the notification about
nondelivery or (transport level OSI) doesn't receive the notice on delivery at
the timeout.

AG> Reliable may have different meanings.

Webster speaks: "Suitable or fit to be relied on; worthy of dependance or
reliance; trustworthy."

>> AG> We should introduce a checksum.
>> Each TCP packet contain CRC16. This is adequately for 1.5 kb of
>> packet length.

AG> Indeed, CRC16 can catch a few bit errors.

Probability of an error passed by CRC16 is very low: 2^-16. For not great 1450
bytes of the data in the TCP datagram, the probability of an passed error is
one bit in the one of 5-billion broken packet. In the real world file length
(average) 300 kb CRC32 may detect an error in the 14316th file, that is one
errored bit in 4 billion bytes. It means that CRC32 on a file detects less
errors than CRC16 on a TCP packet. You offer a monkey job Smile


Stas
Jabber-ID: grumbler@grumbler.org
GPG key 0x72186DB9 (keyserver: hkp://wwwkeys.eu.pgp.net)

... Golded+, Husky & RNTrack maintainer, Binkd developer&webmaster
--- GoldED+/LNX 1.1.5-b20110302
* Origin: Grumbler at home (2:5080/102.1)

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