9 Apr 1995

Transport (TCP/IP)


You like this so much you decide to send me a letter. Inside Yale, you can send it by Campus Mail. From the outside, you can mail to "Box 2112 Yale Station." Overnight Express companies cannot deliver to a Post Office, so you use "175 Whitney Avenue." You can fax it to "203-432-6165". Each choice represents a different speed, cost, address, and method of delivery. In computer networks, this is the problem of "Transport." Transport is simple on a LAN, but becomes more complex when a message has to cross several continents using a variety of phone links.

Ethernet establishes some rules for messages that flow between two computers on the same LAN. Each message must be smaller than 1500 characters. The messages are addressed by a 6 byte number configured on the receiver's workstation. Since the messages go directly between the client and the server machines, Transport on a LAN simply involves breaking large messages up into 1500 byte chunks and reassembling them at the other end.

However, if a message is going from New Haven to Los Angeles, it will flow through Yale on a LAN, then by phone line to Nearnet in Boston. Then Nearnet forwards it to the NSF Network. They in turn deliver it to Los Angeles where local networks send it to its destination.

The Internet manages messages using protocols called TCP and IP. IP is responsible for routing messages from network to network. TCP is responsible for detecting when a message is lost or damaged and getting it retransmitted.

PCs may participate in other networks that use other Transport protocols. Novell uses "IPX." Microsoft and IBM file servers use "NETBEUI." Macs and Laserwriters use "Appletalk." Mainframes and AS/400s use "SNA." A PC user can add software supporting any of these protocols, subject to the limits of DOS memory.

Each protocol has advantages and disadvantages. IPX is fast, but depends on having a central Novell server to control things. NETBEUI is a simple LAN protocol that can be used by peer desktop machines (no server is required) but it has no long distance routing. Appletalk works well on a Macintosh, but the PC software is poorly supported. SNA has worked well to manage large corporate networks, but it is too complicated for ordinary PC use.

These commercial networks were designed to handle the problems of ordinary business communications. A phone line stops working. Power is lost at a branch office. A computer crashes. TCP/IP, in contrast, was developed by the Department of Defense. They need to communicate when people are shooting at the computers.TCP/IP was designed to recover from network problems by instantly rerouting messages through alternate paths. Since routing has to be decided message by message, this places a substantial burden on the network equipment. In the early days, routing was done by minicomputers costing $150,000. With today's RISC microprocessors, much higher traffic can today be handled by a device that costs $2000.

Domain Names

Internet messages, like phone calls, are routed by use of a number. Numbers are difficult to remember, so the Internet has an automated directory service much like calling information. To call the Yale Computer Center, first check the phone book to discover that the area code for Connecticut is "203". Then dial 203- 555-1212 to get information. The operator asks for the town, and the correct reply is "New Haven". Then they ask for the party, and the response is "Yale Computer Center". The Phone Information Name is
"Yale Computer Center" in "New Haven" in "203"

The domain name for the PCLT server is "pclt.cis.yale.edu". It operates much the same way. The ".edu" suffix means that it is located at a University. Nationally, there is a machine maintained at by the Internet administration that converts each university name into the IP address of a machine that provides directory services for that university. The "yale.edu" part tells the central server to find the machine (which happens to be that handles names at Yale. Other name groups include ".com" for commercial companies (ibm.com, novell.com, microsoft.com), ".gov" for government, and ".org" for (generally non- profit) organizations (npr.org).

An organization generally maintains several name server machines, each providing backup. The Yale name server ( will convert "pclt.cis.yale.edu" into the IP address of the PCLT server ( This address can then be used to browse the files.

Network administrators usually find it convenient if the domain names closely relate to the IP addresses and the physical location of the machine. However, the architecture does not require this. Suppose someone wanted to offer me a ton of money to sell the commercial rights to PCLT. Everyone has a price. It would be possible to move the server to some other network in some other state. Yet it would still be possible to update the Yale name server so that the name "pclt.cis.yale.edu" pointed to the new IP address of the server in its new location, even though that network and machine no longer had anything to do with Yale.

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Copyright 1995 PC Lube and Tune -- Windows on the World -- H. Gilbert