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2 Feb 1995
Summary: TCP and IP were developed by a Department of
Defense (DOD) research project to connect a number
different networks designed by different vendors into a
network of networks (the "Internet"). It was
initially successful because it delivered a few basic
services that everyone needs (file transfer, electronic
mail, remote logon) across a very large number of client
and server systems. 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 department to the enterprise network, then to
regional networks, and finally to the global Internet.
On the battlefield a communications network will sustain
damage, so the DOD designed TCP/IP to be robust and
automatically recover from any node or phone line
failure. This design allows the construction of very
large networks with less central management. However,
because of the automatic recovery, network problems can
go undiagnosed and uncorrected for long periods of time.
As with all other communications protocol, TCP/IP is
composed of layers:
- IP - is responsible for moving packet of
data from node to node. IP forwards each packet
based on a four byte destination address (the IP
number). The Internet authorities assign ranges of
numbers to different organizations. The
organizations assign groups of their numbers to
departments. IP operates on gateway machines that
move data from department to organization to region
and then around the world.
- TCP - is responsible for verifying the
correct delivery of data from client to server. Data
can be lost in the intermediate network. TCP adds
support to detect errors or lost data and to trigger
retransmission until the data is correctly and
completely received.
- Sockets - is a name given to the package of
subroutines that provide access to TCP/IP on most
systems.
The Army puts out a bid on a computer and DEC wins
the bid. The Air Force puts out a bid and IBM wins. The
Navy bid is won by Unisys. Then the President decides to
invade Grenada and the armed forces discover that their
computers cannot talk to each other. The DOD must build
a "network" out of systems each of which, by
law, was delivered by the lowest bidder on a single
contract.
The Internet Protocol was developed to create a
Network of Networks (the "Internet").
Individual machines are first connected to a LAN
(Ethernet or Token Ring). TCP/IP shares the LAN with
other uses (a Novell file server, Windows for Workgroups
peer systems). One device provides the TCP/IP connection
between the LAN and the rest of the world.
To insure that all types of systems from all vendors
can communicate, TCP/IP is absolutely standardized on
the LAN. However, larger networks based on long
distances and phone lines are more volatile. In the US,
many large corporations would wish to reuse large
internal networks based on IBM's SNA. In Europe, the
national phone companies traditionally standardize on
X.25. However, the sudden explosion of high speed
microprocessors, fiber optics, and digital phone systems
has created a burst of new options: ISDN, frame relay,
FDDI, Asynchronous Transfer Mode (ATM). New technologies
arise and become obsolete within a few years. With cable
TV and phone companies competing to build the National
Information Superhighway, no single standard can govern
citywide, nationwide, or worldwide communications.
The original design of TCP/IP as a Network of
Networks fits nicely within the current technological
uncertainty. TCP/IP data can be sent across a LAN, or it
can be carried within an internal corporate SNA network,
or it can piggyback on the cable TV service.
Furthermore, machines connected to any of these networks
can communicate to any other network through gateways
supplied by the network vendor.
Each technology has its own convention for
transmitting messages between two machines within the
same network. On a LAN, messages are sent between
machines by supplying the six byte unique identifier
(the "MAC" address). In an SNA network, every
machine has Logical Units with their own network
address. DECNET, Appletalk, and Novell IPX all have a
scheme for assigning numbers to each local network and
to each workstation attached to the network.
On top of these local or vendor specific network
addresses, TCP/IP assigns a unique number to every
workstation in the world. This "IP number" is
a four byte value that, by convention, is expressed by
converting each byte into a decimal number (0 to 255)
and separating the bytes with a period. For example, the
PC Lube and Tune server is 130.132.59.234.
An organization begins by sending electronic mail to
Hostmaster@INTERNIC.NET requesting assignment of a
network number. It is still possible for almost anyone
to get assignment of a number for a small "Class
C" network in which the first three bytes identify
the network and the last byte identifies the individual
computer. The author followed this procedure and was
assigned the numbers 192.35.91.* for a network of
computers at his house. Larger organizations can get a
"Class B" network where the first two bytes
identify the network and the last two bytes identify
each of up to 64 thousand individual workstations.
Yale's Class B network is 130.132, so all computers with
IP 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 subscriber
network number and adds it to the routing configuration
in its own machines and those of the other major network
suppliers.
There is no mathematical formula that translates the
numbers 192.35.91 or 130.132 into "Yale
University" or "New Haven, CT." The
machines that manage large regional networks or the
central Internet routers managed by the National Science
Foundation can only locate these networks by looking
each network number up in a table. There are potentially
thousands of Class B networks, and millions of Class C
networks, but computer memory costs are low, so the
tables are reasonable. Customers that connect to the
Internet, even customers as large as IBM, do not need to
maintain any information on other networks. They send
all external data to the regional carrier to which they
subscribe, and the regional 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 Sox. In this spirit, Yale
recently switched its connection from the Middle
Atlantic regional network to the New England carrier.
When the switch occurred, tables in the other regional
areas and in the national spine had to be updated, so
that traffic for 130.132 was routed through Boston
instead of New Jersey. The large network carriers handle
the paperwork and can perform such a switch given
sufficient notice. During a conversion period, the
university was connected to both networks so that
messages could arrive through either path.
Although the individual subscribers do not need to
tabulate network numbers or provide explicit routing, it
is convenient for most Class B networks to be internally
managed as a much smaller and simpler version of the
larger network organizations. It is common to subdivide
the two bytes available for internal assignment into a
one byte department number and a one byte workstation
ID.
The enterprise network is built using commercially
available TCP/IP router boxes. Each router has small
tables with 255 entries to translate the one byte
department number into selection of a destination
Ethernet connected to one of the routers. Messages to
the PC Lube and Tune server (130.132.59.234) are sent
through the national and New England regional networks
based on the 130.132 part of the number. Arriving at
Yale, the 59 department ID selects an Ethernet connector
in the C& IS building. The 234 selects a particular
workstation on that LAN. The Yale network must be
updated as new Ethernets and departments are added, but
it is not effected by changes outside the university or
the movement of machines within the department.
Every time a message arrives at an IP router, it
makes an individual decision about where to send it
next. There is concept of a session with a preselected
path for all traffic. Consider a company with facilities
in New York, Los Angeles, Chicago and Atlanta. It could
build a network from four phone lines forming a loop (NY
to Chicago to LA to Atlanta to NY). A message arriving
at the NY router could go to LA via either Chicago or
Atlanta. The reply could come back the other way.
How does the router make a decision between routes?
There is no correct answer. Traffic could be routed by
the "clockwise" algorithm (go NY to Atlanta,
LA to Chicago). The routers could alternate, sending one
message to Atlanta and the next to Chicago. More
sophisticated routing measures traffic patterns and
sends data through the least busy link.
If one phone line in this network breaks down,
traffic can still reach its destination through a
roundabout path. After losing the NY to Chicago line,
data can be sent NY to Atlanta to LA to Chicago. This
provides continued service though with degraded
performance. This kind of recovery is the primary design
feature of IP. The loss of the line is immediately
detected by the routers in NY and Chicago, but somehow
this information must be sent to the other nodes.
Otherwise, LA could continue to send NY messages through
Chicago, where they arrive at a "dead end."
Each network adopts some Router Protocol which
periodically updates the routing tables throughout the
network with information about changes in route status.
If the size of the network grows, then the complexity
of the routing updates will increase as will the cost of
transmitting them. Building a single network that covers
the entire US would be unreasonably complicated.
Fortunately, the Internet is designed as a Network of
Networks. This means that loops and redundancy are built
into each regional carrier. The regional network handles
its own problems and reroutes messages internally. Its
Router Protocol updates the tables in its own routers,
but no routing updates need to propagate from a regional
carrier to the NSF spine or to the other regions
(unless, of course, a subscriber switches permanently
from one region to another).
IBM designs its SNA networks to be centrally managed.
If any error occurs, it is reported to the network
authorities. By design, any error is a problem that
should be corrected or repaired. IP networks, however,
were designed to be robust. In battlefield conditions,
the loss of a node or line is a normal circumstance.
Casualties can be sorted out later on, but the network
must stay up. So IP networks are robust. They
automatically (and silently) reconfigure themselves when
something goes wrong. If there is enough redundancy
built into 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 afford it. Today,
however, simple routers cost no more than a PC. However,
the TCP/IP design that, "Errors are normal and can
be largely ignored," produces 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 cities throughout the Southeast. The problem is
that data arrives without a reservation. Airline
companies experience the problem around major events,
like the Super Bowl. Just before the game, everyone
wants to fly into the city. After the game, everyone
wants to fly out. Imbalance occurs on the network when
something new gets advertised. Adam Curry announced the
server at "mtv.com" and his regional carrier
was swamped with traffic the next day. The problem is
that messages come in from the entire world over high
speed lines, but they go out to mtv.com over what was
then a slow speed phone line.
Occasionally a snow storm cancels flights and
airports fill up with stranded passengers. Many go off
to hotels in town. When data arrives at a congested
router, there is no place to send the overflow. Excess
packets are simply discarded. It becomes the
responsibility of the sender to retry the data a few
seconds later and to persist until it finally gets
through. This recovery is provided by the TCP component
of the Internet protocol.
TCP was designed to recover from node or line
failures where the network propagates routing table
changes to all router nodes. Since the update takes some
time, TCP is slow to initiate recovery. The TCP
algorithms are not tuned to optimally handle packet loss
due to traffic congestion. Instead, the traditional
Internet response to traffic problems has been to
increase the speed of lines 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 number to each byte. The
TCP packet has a header that says, in effect, "This
packet starts with byte 379642 and contains 200 bytes of
data." The receiver can detect missing or
incorrectly sequenced packets. TCP acknowledges data
that has been received and retransmits data that has
been lost. The TCP design means that error recovery is
done end-to-end between the Client and Server machine.
There is no formal standard for tracking problems in the
middle of the network, though each network has adopted
some ad hoc tools.
There are three levels of TCP/IP knowledge. Those who
administer a regional or national network must design a
system of long distance phone lines, dedicated routing
devices, and very large configuration files. They must
know the IP numbers and physical locations of thousands
of subscriber networks. They must also have a formal
network monitor strategy to detect problems and respond
quickly.
Each large company or university that subscribes to
the Internet must have an intermediate level of network
organization and expertise. A half dozen routers might
be configured to connect several dozen departmental LANs
in several buildings. All traffic outside the
organization would typically be routed to a single
connection to a regional network provider.
However, the end user can install TCP/IP on a
personal computer without any knowledge of either the
corporate or regional network. Three pieces of
information are required:
- The IP address assigned to this personal computer
- The part of the IP address (the subnet mask) that
distinguishes other machines on the same LAN
(messages can be sent to them directly) from
machines in other departments or elsewhere in the
world (which are sent to a router machine)
- The IP address of the router machine that connects
this LAN to the rest of the world.
In the case of the PCLT server, the IP address is
130.132.59.234. Since the first three bytes designate
this department, a "subnet mask" is defined as
255.255.255.0 (255 is the largest byte value and
represents the number with all bits turned on). It is a
Yale convention (which we recommend to everyone) that
the router for each department have station number 1
within the department network. Thus the PCLT router is
130.132.59.1. Thus the PCLT server is configured with
the values:
- My IP address: 130.132.59.234
- Subnet mask: 255.255.255.0
- Default router: 130.132.59.1
The subnet mask tells the server that any other
machine with an IP address beginning 130.132.59.* is on
the same department LAN, so messages are sent to it
directly. Any IP address beginning with a different
value is accessed indirectly by sending the message
through the router at 130.132.59.1 (which is on the
departmental LAN).
Additional information is available in self-study
courses from SRA (1-800-SRA-1277)
Copyright
1995 PCLT -- Introduction to TCP/IP -- H.
Gilbert
This document generated by SpHyDir
another fine product of PC
Lube and Tune.
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