Number 225 - February 2002

9-11
by Burton Shane (bshane39@yahoo.com) ACGNJ News October 2001
A nation attacked, injured, shocked, frightened, and angered spent the day watching horrific scenes of the fruits of cowardly terrorism on their televisions. One must consider what would have been done by a frantic population had the terrorists also disabled the television networks and phones that provided the information that emergency services were heroically coping with the tragedy, and that our government was beginning its efforts to mitigate and retaliate for this horrendous attack on innocent civilians. We now have the Internet to provide the communication and dissemination of this vital information. Stricken by the graphic scenes on the television, my wife called my daughter to satisfy her motherly instinct for the safety of her child. My cousins from New Jersey left their Loop motel before the attack, and called us from Ohio. My wife tried to phone her brother in Germany and contact her sister who was visiting him. She could not get on his military base but we were able to verify her well-being by e-mail.     A 1969 U.S. Department of Defense study led to the deployment of an experimental packet-switched network (the ARPANET) that eventually evolved into the Internet. The military theorized that a distributed data network would be more fault-tolerant than a telephone network, which could be disabled simply by attacking its central office. The ARPANET was created to test this theory.     From the tiny seed of ARPANET, the Internet flowered as universities and corporations across the country embraced the enhanced communication now available. So valuable has this mode of communication become that large-scale privatization has occurred. Reliability     The TCP (Transmission Control Protocol) must recover data that is damaged, lost, duplicated, or delivered out of order by the Internet communication system. This is achieved by assigning a sequence number to each octet transmitted, and requiring a positive acknowledgment (ACK) from the receiving TCP. If the ACK is not received within a timeout interval, the data is retransmitted. At the receiver,	sequence numbers are used to correctly order segments that may be received out of order and to eliminate duplicates. Damage is handled by adding a checksum to each segment transmitted, checking it at the receiver, and discarding damaged segments.     As long as the TCPs continue to function properly and the Internet system does not become completely partitioned, no transmission errors will affect the correct delivery of data. TCP recovers from Internet communication system errors. Unreliable Delivery Model     One of the Internet's earliest design decisions was that the fundamental transport protocol (IP Protocol) would be based on an assumption of unreliable delivery. This means that an IP packet can be legally discarded at any time, without notification to the sender or receiver. No guarantee is made that any particular packet will be delivered. Instead, network reachability takes a statistical form - there is a pretty good chance that anyone packet will be delivered, and if a group of packets are transmitted, most of them should arrive at their destinations.     The main function of the TCP Protocol is to use sequence numbers and timeouts to detect data loss, and then retransmit lost data until it is received and acknowledged.     Why go through all this? Well, for one thing, if our network fails briefly at any point, in any way, there should be no serious problems. If a switching node becomes overloaded with traffic, it can simply discard some of the excess. If a link fails while a packet is being transferred, there's no need for an elaborate recovery procedure. The assumption of unreliable delivery, and the consequent demand that software be able to handle intermittent failures, significantly reduces demands on hardware and low-level software design. Sporadic network outages might slow the tempo, but the show will go on.     So will America.     Click here for a more complete source on operation of the Internet. http://www.freesoft.org/ CIE/ index.htm
Number 225 - February 2002