From: Subject: Introduction to TCP/IP Date: Tue, 21 Sep 2004 18:22:02 +0100 MIME-Version: 1.0 Content-Type: multipart/related; boundary="----=_NextPart_000_0000_01C4A007.E405D820"; type="text/html" X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1441 This is a multi-part message in MIME format. ------=_NextPart_000_0000_01C4A007.E405D820 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.yale.edu/pclt/COMM/TCPIP.HTM Introduction to TCP/IP

2 Feb 1995

Introduction to TCP/IP

Summary: TCP and IP were developed by a Department of Defense (DOD) = research=20 project to connect a number different networks designed by different = vendors=20 into a network of networks (the "Internet"). It was initially successful = because=20 it delivered a few basic services that everyone needs (file transfer, = electronic=20 mail, remote logon) across a very large number of client and server = systems.=20 Several computers in a small department can use TCP/IP (along with other = protocols) on a single LAN. The IP component provides routing from the=20 department to the enterprise network, then to regional networks, and = finally to=20 the global Internet. On the battlefield a communications network will = sustain=20 damage, so the DOD designed TCP/IP to be robust and automatically = recover from=20 any node or phone line failure. This design allows the construction of = very=20 large networks with less central management. However, because of the = automatic=20 recovery, network problems can go undiagnosed and uncorrected for long = periods=20 of time.

As with all other communications protocol, TCP/IP is composed of = layers:

Network of Lowest Bidders =

The Army puts out a bid on a computer and DEC wins the bid. The Air = Force=20 puts out a bid and IBM wins. The Navy bid is won by Unisys. Then the = President=20 decides to invade Grenada and the armed forces discover that their = computers=20 cannot talk to each other. The DOD must build a "network" out of systems = each of=20 which, by law, was delivered by the lowest bidder on a single contract. =

3Dipdept.gif

The Internet Protocol was developed to create a Network of Networks = (the=20 "Internet"). Individual machines are first connected to a LAN (Ethernet = or Token=20 Ring). TCP/IP shares the LAN with other uses (a Novell file server, = Windows for=20 Workgroups peer systems). One device provides the TCP/IP connection = between the=20 LAN and the rest of the world.

To insure that all types of systems from all vendors can communicate, = TCP/IP=20 is absolutely standardized on the LAN. However, larger networks based on = long=20 distances and phone lines are more volatile. In the US, many large = corporations=20 would wish to reuse large internal networks based on IBM's SNA. In = Europe, the=20 national phone companies traditionally standardize on X.25. However, the = sudden=20 explosion of high speed microprocessors, fiber optics, and digital phone = systems=20 has created a burst of new options: ISDN, frame relay, FDDI, = Asynchronous=20 Transfer Mode (ATM). New technologies arise and become obsolete within a = few=20 years. With cable TV and phone companies competing to build the National = Information Superhighway, no single standard can govern citywide, = nationwide, or=20 worldwide communications.

The original design of TCP/IP as a Network of Networks fits nicely = within the=20 current technological uncertainty. TCP/IP data can be sent across a LAN, = or it=20 can be carried within an internal corporate SNA network, or it can = piggyback on=20 the cable TV service. Furthermore, machines connected to any of these = networks=20 can communicate to any other network through gateways supplied by the = network=20 vendor.

Addresses

Each technology has its own convention for transmitting messages = between two=20 machines within the same network. On a LAN, messages are sent between = machines=20 by supplying the six byte unique identifier (the "MAC" address). In an = SNA=20 network, every machine has Logical Units with their own network address. = DECNET,=20 Appletalk, and Novell IPX all have a scheme for assigning numbers to = each local=20 network and to each workstation attached to the network.

On top of these local or vendor specific network addresses, TCP/IP = assigns a=20 unique number to every workstation in the world. This "IP number" is a = four byte=20 value that, by convention, is expressed by converting each byte into a = decimal=20 number (0 to 255) and separating the bytes with a period. For example, = the PC=20 Lube and Tune server is 130.132.59.234.

An organization begins by sending electronic mail to = Hostmaster@INTERNIC.NET=20 requesting assignment of a network number. It is still possible for = almost=20 anyone to get assignment of a number for a small "Class C" network in = which the=20 first three bytes identify the network and the last byte identifies the=20 individual computer. The author followed this procedure and was assigned = the=20 numbers 192.35.91.* for a network of computers at his house. Larger=20 organizations can get a "Class B" network where the first two bytes = identify the=20 network and the last two bytes identify each of up to 64 thousand = individual=20 workstations. Yale's Class B network is 130.132, so all computers with = IP=20 address 130.132.*.* are connected through Yale.

The organization then connects to the Internet through one of a dozen = regional or specialized network suppliers. The network vendor is given = the=20 subscriber network number and adds it to the routing configuration in = its own=20 machines and those of the other major network suppliers.

There is no mathematical formula that translates the numbers = 192.35.91 or=20 130.132 into "Yale University" or "New Haven, CT." The machines that = manage=20 large regional networks or the central Internet routers managed by the = National=20 Science Foundation can only locate these networks by looking each = network number=20 up in a table. There are potentially thousands of Class B networks, and = millions=20 of Class C networks, but computer memory costs are low, so the tables = are=20 reasonable. Customers that connect to the Internet, even customers as = large as=20 IBM, do not need to maintain any information on other networks. They = send all=20 external data to the regional carrier to which they subscribe, and the = regional=20 carrier maintains the tables and does the appropriate routing.

New Haven is in a border state, split 50-50 between the Yankees and = the Red=20 Sox. In this spirit, Yale recently switched its connection from the = Middle=20 Atlantic regional network to the New England carrier. When the switch = occurred,=20 tables in the other regional areas and in the national spine had to be = updated,=20 so that traffic for 130.132 was routed through Boston instead of New = Jersey. The=20 large network carriers handle the paperwork and can perform such a = switch given=20 sufficient notice. During a conversion period, the university was = connected to=20 both networks so that messages could arrive through either path.

Subnets

Although the individual subscribers do not need to tabulate network = numbers=20 or provide explicit routing, it is convenient for most Class B networks = to be=20 internally managed as a much smaller and simpler version of the larger = network=20 organizations. It is common to subdivide the two bytes available for = internal=20 assignment into a one byte department number and a one byte workstation = ID.

3Dclassb.gif

The enterprise network is built using commercially available TCP/IP = router=20 boxes. Each router has small tables with 255 entries to translate the = one byte=20 department number into selection of a destination Ethernet connected to = one of=20 the routers. Messages to the PC Lube and Tune server (130.132.59.234) = are sent=20 through the national and New England regional networks based on the = 130.132 part=20 of the number. Arriving at Yale, the 59 department ID selects an = Ethernet=20 connector in the C& IS building. The 234 selects a particular = workstation on=20 that LAN. The Yale network must be updated as new Ethernets and = departments are=20 added, but it is not effected by changes outside the university or the = movement=20 of machines within the department.

A Uncertain Path

Every time a message arrives at an IP router, it makes an individual = decision=20 about where to send it next. There is concept of a session with a = preselected=20 path for all traffic. Consider a company with facilities in New York, = Los=20 Angeles, Chicago and Atlanta. It could build a network from four phone = lines=20 forming a loop (NY to Chicago to LA to Atlanta to NY). A message = arriving at the=20 NY router could go to LA via either Chicago or Atlanta. The reply could = come=20 back the other way.

How does the router make a decision between routes? There is no = correct=20 answer. Traffic could be routed by the "clockwise" algorithm (go NY to = Atlanta,=20 LA to Chicago). The routers could alternate, sending one message to = Atlanta and=20 the next to Chicago. More sophisticated routing measures traffic = patterns and=20 sends data through the least busy link.

If one phone line in this network breaks down, traffic can still = reach its=20 destination through a roundabout path. After losing the NY to Chicago = line, data=20 can be sent NY to Atlanta to LA to Chicago. This provides continued = service=20 though with degraded performance. This kind of recovery is the primary = design=20 feature of IP. The loss of the line is immediately detected by the = routers in NY=20 and Chicago, but somehow this information must be sent to the other = nodes.=20 Otherwise, LA could continue to send NY messages through Chicago, where = they=20 arrive at a "dead end." Each network adopts some Router Protocol which=20 periodically updates the routing tables throughout the network with = information=20 about changes in route status.

If the size of the network grows, then the complexity of the routing = updates=20 will increase as will the cost of transmitting them. Building a single = network=20 that covers the entire US would be unreasonably complicated. = Fortunately, the=20 Internet is designed as a Network of Networks. This means that loops and = redundancy are built into each regional carrier. The regional network = handles=20 its own problems and reroutes messages internally. Its Router Protocol = updates=20 the tables in its own routers, but no routing updates need to propagate = from a=20 regional carrier to the NSF spine or to the other regions (unless, of = course, a=20 subscriber switches permanently from one region to another).

Undiagnosed Problems

IBM designs its SNA networks to be centrally managed. If any error = occurs, it=20 is reported to the network authorities. By design, any error is a = problem that=20 should be corrected or repaired. IP networks, however, were designed to = be=20 robust. In battlefield conditions, the loss of a node or line is a = normal=20 circumstance. Casualties can be sorted out later on, but the network = must stay=20 up. So IP networks are robust. They automatically (and silently) = reconfigure=20 themselves when something goes wrong. If there is enough redundancy = built into=20 the system, then communication is maintained.

In 1975 when SNA was designed, such redundancy would be prohibitively = expensive, or it might have been argued that only the Defense Department = could=20 afford it. Today, however, simple routers cost no more than a PC. = However, the=20 TCP/IP design that, "Errors are normal and can be largely ignored," = produces=20 problems of its own.

Data traffic is frequently organized around "hubs," much like airline = traffic. One could imagine an IP router in Atlanta routing messages for = smaller=20 cities throughout the Southeast. The problem is that data arrives = without a=20 reservation. Airline companies experience the problem around major = events, like=20 the Super Bowl. Just before the game, everyone wants to fly into the = city. After=20 the game, everyone wants to fly out. Imbalance occurs on the network = when=20 something new gets advertised. Adam Curry announced the server at = "mtv.com" and=20 his regional carrier was swamped with traffic the next day. The problem = is that=20 messages come in from the entire world over high speed lines, but they = go out to=20 mtv.com over what was then a slow speed phone line.

Occasionally a snow storm cancels flights and airports fill up with = stranded=20 passengers. Many go off to hotels in town. When data arrives at a = congested=20 router, there is no place to send the overflow. Excess packets are = simply=20 discarded. It becomes the responsibility of the sender to retry the data = a few=20 seconds later and to persist until it finally gets through. This = recovery is=20 provided by the TCP component of the Internet protocol.

TCP was designed to recover from node or line failures where the = network=20 propagates routing table changes to all router nodes. Since the update = takes=20 some time, TCP is slow to initiate recovery. The TCP algorithms are not = tuned to=20 optimally handle packet loss due to traffic congestion. Instead, the = traditional=20 Internet response to traffic problems has been to increase the speed of = lines=20 and equipment in order to say ahead of growth in demand.

TCP treats the data as a stream of bytes. It logically assigns a = sequence=20 number to each byte. The TCP packet has a header that says, in effect, = "This=20 packet starts with byte 379642 and contains 200 bytes of data." The = receiver can=20 detect missing or incorrectly sequenced packets. TCP acknowledges data = that has=20 been received and retransmits data that has been lost. The TCP design = means that=20 error recovery is done end-to-end between the Client and Server machine. = There=20 is no formal standard for tracking problems in the middle of the = network, though=20 each network has adopted some ad hoc tools.

Need to Know

There are three levels of TCP/IP knowledge. Those who administer a = regional=20 or national network must design a system of long distance phone lines, = dedicated=20 routing devices, and very large configuration files. They must know the = IP=20 numbers and physical locations of thousands of subscriber networks. They = must=20 also have a formal network monitor strategy to detect problems and = respond=20 quickly.

Each large company or university that subscribes to the Internet must = have an=20 intermediate level of network organization and expertise. A half dozen = routers=20 might be configured to connect several dozen departmental LANs in = several=20 buildings. All traffic outside the organization would typically be = routed to a=20 single connection to a regional network provider.

However, the end user can install TCP/IP on a personal computer = without any=20 knowledge of either the corporate or regional network. Three pieces of=20 information are required:

  1. The IP address assigned to this personal computer=20
  2. The part of the IP address (the subnet mask) that distinguishes = other=20 machines on the same LAN (messages can be sent to them directly) from = machines=20 in other departments or elsewhere in the world (which are sent to a = router=20 machine)=20
  3. The IP address of the router machine that connects this LAN to the = rest of=20 the world.

In the case of the PCLT server, the IP address is 130.132.59.234. = Since the=20 first three bytes designate this department, a "subnet mask" is defined = as=20 255.255.255.0 (255 is the largest byte value and represents the number = with all=20 bits turned on). It is a Yale convention (which we recommend to = everyone) that=20 the router for each department have station number 1 within the = department=20 network. Thus the PCLT router is 130.132.59.1. Thus the PCLT server is=20 configured with the values:

The subnet mask tells the server that any other machine with an IP = address=20 beginning 130.132.59.* is on the same department LAN, so messages are = sent to it=20 directly. Any IP address beginning with a different value is accessed = indirectly=20 by sending the message through the router at 130.132.59.1 (which is on = the=20 departmental LAN).

Additional information is available in self-study courses from SRA=20 (1-800-SRA-1277)

Copyright 1995 = PCLT --=20 Introduction to TCP/IP -- H. Gilbert

This document generated by SpHyDir = another=20 fine product of PC = Lube and=20 Tune.

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