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2003/10/17 packet

© October 2003 Tony Lawrence

In networking, a packet is the collection of information that carries your data to its destination. If you are, for example, sending a large email to a friend, it does not travel in a contiguous flow of bytes. Instead, it is broken up into chunks, and each chunk is wrapped up in other information: where it came from, where its going to, how big this chunk is, etc. The size of the packet can vary depending upon the media and technology used, which means that gateways where (for example) you are switched from dialup ppp to ethernet and later to frame relay have to repackage data in different size packets. Your packets intermingle with other people's, but the encapsulating header information ensures that they will eventually arrive and the payload will be extracted and reasssembled.

Packet radio is digitized radio. Same concept as above, but a different payload.

A "packet storm" is network packets flooding your hardware. This could be from a deliberate DOS (denial of service) attack, but it's more apt to be used when the storm was caused by some misconfiguration or misbehaving software: two machines chattering back at each other with infinitely increasing fervor. Also called a "broadcast storm", though that should specifically refer to broadcast packets.

The general idea of breaking data into discrete chunks is attractive for all sorts of problems. There are packet i/o buses on higher-end hardware.

CD and DVD writers also use "packet" to refer to a method of writing data. When these things first came out, they supported two general methods: disk at once, which meant that you prepared everything you wanted to put on the media and wrote it all in one swoop, and track at once, which let you write a smaller amount, thereby (before rewritables) leaving you the ability to add more later. Packet writing (which came later) is just the ability to write a smaller amount, and that can let software treat the media much like a floppy or disk drive. I don't know if that's packets in the sense that header information identifies the source, or they are just using it in the sense of smaller units of data.

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More Articles by © Tony Lawrence

"CD and DVD writers also use 'packet' to refer to a method of writing data."

A bit of technical arcanity: SCSI (small computer system interface) is a "packet" interface. Each SCSI transaction is initiated by the host (initiator) sending a command descriptor block (CDB) to the target device (e.g., hard disk, CD-ROM, etc.). The CDB is always a fixed size of 6, 10 or 12 bytes, depending on the command being issued and the capabilities of the target device. Returning data, such as read from the device's medium, comes in fixed size blocks (typically 512 bytes for disks and tapes, 2048 bytes for CD's). You can never, for example, ask for and receive 511 or 513 bytes from a 512 byte per block device.

The packet mode of driving CD/DVD writers comes from the SCSI method of writing to any SCSI device. In fact, when CD's were adapted to PC's for data storage, a new communications method had to be implemented: ATAPI (AT attachment packet interface).



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