Explaining TCP/IP by Analogy

Damian Gordon

Question

• Did anyone see the program about the Pony Express on the History Channel the other night?

• They hosted an hour long show about how the postal service has changed through the years with most of the show emphasizing the Pony Express.

The Pony Express• With the move West in the 1840’s and 50’s, a need was

seen by a gentleman named William Russell to move mail to and from those western fortune seekers.

• Russell established the Pony Express in 1860 to most efficiently (and cost effectively) transport mail. The Pony Express was made up of eighty riders (less than 120 pounds each), their steeds, four hundred men to run the way stations, and 2000 miles of land between St. Joseph, Missouri and Sacramento, California.

• Each horse carried twenty-five pounds of equipment (a saddle, four mail pouches, a rifle, and a revolver), up to twenty pounds of mail, and the rider for a maximum of 165 pounds.

Lies Begin Here

• Trouble occurred early in the history of the Express - a patron mailed a dozen twenty pound packages from St. Joseph. Of course, twelve riders took the twelve packages onto the trail.

• The problem was there were only twelve horses and riders available. When other mail came to the St. Joseph station, the customers were told there would be a delay of up to five days before the mail could leave town.

Lies Continue

• The customers, as one can imagine, were incensed. This scenario caused Mr. Russell to restrict all letters and packages ‘to a maximum of ten ounces (about twenty pages). This restriction ensured that no one letter or package would take up the entire twenty pound maximum load, keeping other equally important mail from being transported.

Lies Continue

• After the ten ounce restriction was put into place, a businessman devised a plan to make money on the new restriction.

• He found an empty building near the Pony Express station in each town that the Express served. Anyone who wanted to ship a book of greater than twenty pages would call upon his business to split the book apart, address each twenty page portion to a corresponding business in the recipient’s town, and mail the portions.

• Upon arrival at the other end, the business would reassemble the book and deliver it to the intended recipient.

Lies Continue

• Reassembly was not, always a simple task. Since the book was split ,into many packages, they did not necessarily arrive on the same horse, on the same day, or in the same order that they were originally intended.

• Fortunately, the originating business numbered the packages for easy and correct reassembly.

Lies Finish

• Furthermore, this business knew the risks of the Pony Express, so it provided a special guaranteed service (for a fee of course). The business made exact copies of the mail to provide this guaranteed service. Upon receiving a packet, an acknowledgement of receipt was returned to the sender. If no receipt acknowledgement was received within ten days for a particular packet, it was assumed lost and a copy of the packet was resent.

Analogy

• This analogy illustrates the TCP/IP protocols in the Internet. The Internet Protocol (IP) restricts the packet size to ensure that no user monopolizes the resources (as in the Pony Express): IP does not specify how things must be transformed into the 64K size it is capable of transporting.

• Also, IP does not concern itself with ensuring that the packets arrive in the correct order, or that they arrive at all. Thus, the Pony Express closely resembles the Internet Protocol.

Analogy

• The Transmission Control Protocol (TCP) works closely with IP, and fixes the things which the business fixed for the Pony Express. TCP breaks the data (which may be larger than 64k characters) into packets of a size acceptable by IP. Since TCP is responsible for breaking the data into packets, it is also responsible for reassembling the data before handing it to the intended recipient.

• As packets arrive TCP gathers the packets into the correct order and returns an acknowledgement. If, after some period of time, an expected acknowledgement does not arrive, the TCP sender resends the missing packet

Analogy

PONY EXPRESSTrail

Horse & Rider

Breaking book

Taping book

Acknowledge receipt

Ten day wait for acknowledgement

TCP/IPNetwork

IP

TCP breaks packets

TCP reassembly

TCP ACK

TCP timeout before resending