MadSci Network: Computer Science

RE: Bad Packets

Area: Computer Science
Posted By: Keith Little, Systems Analyst/Programmer
Date: Thu Nov 21 20:50:48 1996


When downloading a file, the sending computer breaks it into chunks and transmits them one at a time to your computer. These Packets have a checksum (an 8, 16 or 32 bit number calculated from the data using an algebraic formula) stored in them, and are also assigned a sequence number (usually starting with 0 or 1).

When a packet is received, the checksum is recalculated and compared against the one that just arrived. If they don't match, your computer will request a retransmission. The sequence number of the packet is also verified to make sure that no other packets have been lost in transmission, or received more than once. For instance, if a packet number skips from 1 to 3, packet 2 has been lost, and your computer will request a retransmission. If any packet is received twice, your computer simply throws it away and continues on.

In any event, when your computer indicates a "Packet Error" (and the file transfer completes successfully), it's simply telling you that corrections have been made, but the file is OK.

Unfortunately, transmission errors are sometimes unrecoverable, and the transfer protocol (at one end or the other) will give up, in which case you should try transmitting the file again. Occasionally there are temporary glitches (caused bad phone lines, power line drops, people picking up the extension, etc) that won't necessarily occur the second time around. Be persistant!

Hope this helps,

Keith Little

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