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Current issue : #34 | Release date : 1991-10-13 | Editor : Dispater
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Title : TCP/IP: A Tutorial Part 2 of 2
Author : The Not
==Phrack Inc.==
Volume Three, Issue Thirty-Four, File #8 of 11
A TCP/IP Tutorial : Behind The Internet
Part Two of Two
October 4th, 1991
Presented by The Not
5. Internet Protocol
The IP module is central to internet technology and the essence of IP
is its route table. IP uses this in-memory table to make all
decisions about routing an IP packet. The content of the route table
is defined by the network administrator. Mistakes block
communication.
To understand how a route table is used is to understand
internetworking. This understanding is necessary for the successful
administration and maintenance of an IP network.
The route table is best understood by first having an overview of
routing, then learing about IP network addresses, and then looking
at the details.
5.1 Direct Routing
The figure below is of a tiny internet with 3 computers: A, B, and C.
Each computer has the same TCP/IP protocol stack as in Figure 1.
Each computer's Ethernet interface has its own Ethernet address.
Each computer has an IP address assigned to the IP interface by the
network manager, who also has assigned an IP network number to the
Ethernet.
A B C
| | |
--o------o------o--
Ethernet 1
IP network "development"
Figure 6. One IP Network
When A sends an IP packet to B, the IP header contains A's IP address
as the source IP address, and the Ethernet header contains A's
Ethernet address as the source Ethernet address. Also, the IP header
contains B's IP address as the destination IP address and the
Ethernet header contains B's Ethernet address as the des
----------------------------------------
|address source destination|
----------------------------------------
|IP header A B |
|Ethernet header A B |
----------------------------------------
TABLE 5. Addresses in an Ethernet frame for an IP packet
from A to B
For this simple case, IP is overhead because the IP adds little to
the service offered by Ethernet. However, IP does add cost: the
extra CPU processing and network bandwidth to generate, transmit, and
parse the IP header.
When B's IP module receives the IP packet from A, it checks the
destination IP address against its own, looking for a match, then it
passes the datagram to the upper-level protocol.
This communication between A and B uses direct routing.
5.2 Indirect Routing
The figure below is a more realistic view of an internet. It is
composed of 3 Ethernets and 3 IP networks connected by an IP-router
called computer D. Each IP network has 4 computers; each computer
has its own IP address and Ethernet address.
A B C ----D---- E F G
| | | | | | | | |
--o------o------o------o- | -o------o------o------o--
Ethernet 1 | Ethernet 2
IP network "development" | IP network "accounting"
|
|
| H I J
| | | |
--o-----o------o------o--
Ethernet 3
IP network "factory"
Figure 7. Three IP Networks; One internet
Except for computer D, each computer has a TCP/IP protocol stack like
that in Figure 1. Computer D is the IP-router; it is connected to
all 3 networks and therefore has 3 IP addresses and 3 Ethernet
addresses. Computer D has a TCP/IP protocol stack similar to that in
Figure 3, except that it has 3 ARP modules and 3 Ethernet drivers
instead of 2. Please note that computer D has only one IP module.
The network manager has assigned a unique number, called an IP
network number, to each of the Ethernets. The IP network numbers are
not shown in this diagram, just the network names.
When computer A sends an IP packet to computer B, the process is
identical to the single network example above. Any communication
between computers located on a single IP network matches the direct
routing example discussed previously.
When computer D and A communicate, it is direct communication. When
computer D and E communicate, it is direct communication. When
computer D and H communicate, it is direct communication. This is
because each of these pairs of computers is on the same IP network.
However, when computer A communicates with a computer on the far side
of the IP-router, communication is no longer direct. A must use D to
forward the IP packet to the next IP network. This communication is
called "indirect".
This routing of IP packets is done by IP modules and happens
transparently to TCP, UDP, and the network applications.
If A sends an IP packet to E, the source IP address and the source
Ethernet address are A's. The destination IP address is E's, but
because A's IP module sends the IP packet to D for forwarding, the
destination Ethernet address is D's.
----------------------------------------
|address source destination|
----------------------------------------
|IP header A E |
|Ethernet header A D |
----------------------------------------
TABLE 6. Addresses in an Ethernet frame for an IP packet
from A to E (before D)
D's IP module receives the IP packet and upon examining the
destination IP address, says "This is not my IP address," and sends
the IP packet directly to E.
----------------------------------------
|address source destination|
----------------------------------------
|IP header A E |
|Ethernet header D E |
----------------------------------------
TABLE 7. Addresses in an Ethernet frame for an IP packet
from A to E (after D)
In summary, for direct communication, both the source IP address and
the source Ethernet address is the sender's, and the destination IP
address and the destination Ethernet addrss is the recipient's. For
indirect communication, the IP address and Ethernet addresses do not
pair up in this way.
This example internet is a very simple one. Real networks are often
complicated by many factors, resulting in multiple IP-routers and
several types of physical networks. This example internet might have
come about because the network manager wanted to split a large
Ethernet in order to localize Ethernet broadcast traffic.
5.3 IP Module Routing Rules
This overview of routing has shown what happens, but not how it
happens. Now let's examine the rules, or algorithm, used by the IP
module.
For an outgoing IP packet, entering IP from an upper layer, IP must
decide whether to send the IP packet directly or indirectly, and IP
must choose a lower network interface. These choices are made by
consulting the route table.
For an incoming IP packet, entering IP from a lower interface, IP
must decide whether to forward the IP packet or pass it to an upper
layer. If the IP packet is being forwarded, it is treated as an
outgoing IP packet.
When an incoming IP packet arrives it is never forwarded back out
through the same network interface.
These decisions are made before the IP packet is handed to the lower
interface and before the ARP table is consulted.
5.4 IP Address
The network manager assigns IP addresses to computers according to
the IP network to which the computer is attached. One part of a 4-
byte IP address is the IP network number, the other part is the IP
computer number (or host number). For the computer in table 1, with
an IP address of 223.1.2.1, the network number is 223.1.2 and the
host number is number 1.
The portion of the address that is used for network number and for
host number is defined by the upper bits in the 4-byte address. All
example IP addresses in this tutorial are of type class C, meaning
that the upper 3 bits indicate that 21 bits are the network number
and 8 bits are the host number. This allows 2,097,152 class C
networks up to 254 hosts on each network.
The IP address space is administered by the NIC (Network Information
Center). All internets that are connected to the single world-wide
Internet must use network numbers assigned by the NIC. If you are
setting up your own internet and you are not intending to connect it
to the Internet, you should still obtain your network numbers from
the NIC. If you pick your own number, you run the risk of confusion
and chaos in the eventuality that your internet is connected to
another internet.
5.5 Names
People refer to computers by names, not numbers. A computer called
alpha might have the IP address of 223.1.2.1. For small networks,
this name-to-address translation data is often kept on each computer
in the "hosts" file. For larger networks, this translation data file
is stored on a server and accessed across the network when needed. A
few lines from that file might look like this:
223.1.2.1 alpha
223.1.2.2 beta
223.1.2.3 gamma
223.1.2.4 delta
223.1.3.2 epsilon
223.1.4.2 iota
The IP address is the first column and the computer name is the
second column.
In most cases, you can install identical "hosts" files on all
computers. You may notice that "delta" has only one entry in this
file even though it has 3 IP addresses. Delta can be reached with
any of its IP addresses; it does not matter which one is used. When
delta receives an IP packet and looks at the destination address, it
will recognize any of its own IP addresses.
IP networks are also given names. If you have 3 IP networks, your
"networks" file for documenting these names might look something like
this:
223.1.2 development
223.1.3 accounting
223.1.4 factory
The IP network number is in the first column and its name is in the
second column.
From this example you can see that alpha is computer number 1 on the
development network, beta is computer number 2 on the development
network and so on. You might also say that alpha is development.1,
Beta is development.2, and so on.
The above hosts file is adequate for the users, but the network
manager will probably replace the line for delta with:
223.1.2.4 devnetrouter delta
223.1.3.1 facnetrouter
223.1.4.1 accnetrouter
These three new lines for the hosts file give each of delta's IP
addresses a meaningful name. In fact, the first IP address listed
has 2 names; "delta" and "devnetrouter" are synonyms. In practice
"delta" is the general-purpose name of the computer and the other 3
names are only used when administering the IP route table.
These files are used by network administration commands and network
applications to provide meaningful names. They are not required for
operation of an internet, but they do make it easier for us.
5.6 IP Route Table
How does IP know which lower network interface to use when sending
out a IP packet? IP looks it up in the route table using a search
key of the IP network number extracted from the IP destination
address.
The route table contains one row for each route. The primary columns
in the route table are: IP network number, direct/indirect flag,
router IP address, and interface number. This table is referred to
by IP for each outgoing IP packet.
On most computers the route table can be modified with the "route"
command. The content of the route table is defined by the network
manager, because the network manager assigns the IP addresses to the
computers.
5.7 Direct Routing Details
To explain how it is used, let us visit in detail the routing
situations we have reviewed previously.
--------- ---------
| alpha | | beta |
| 1 | | 1 |
--------- ---------
| |
--------o---------------o-
Ethernet 1
IP network "development"
Figure 8. Close-up View of One IP Network
The route table inside alpha looks like this:
--------------------------------------------------------------
|network direct/indirect flag router interface number|
--------------------------------------------------------------
|development direct <blank> 1 |
--------------------------------------------------------------
TABLE 8. Example Simple Route Table
This view can be seen on some UNIX systems with the "netstat -r"
command. With this simple network, all computers have identical
routing tables.
For discussion, the table is printed again without the network number
translated to its network name.
--------------------------------------------------------------
|network direct/indirect flag router interface number|
--------------------------------------------------------------
|223.1.2 direct <blank> 1 |
--------------------------------------------------------------
TABLE 9. Example Simple Route Table with Numbers
5.8 Direct Scenario
Alpha is sending an IP packet to beta. The IP packet is in alpha's
IP module and the destination IP address is beta or 223.1.2.2. IP
extracts the network portion of this IP address and scans the first
column of the table looking for a match. With this network a match
is found on the first entry.
The other information in this entry indicates that computers on this
network can be reached directly through interface number 1. An ARP
table translation is done on beta's IP address then the Ethernet
frame is sent directly to beta via interface number 1.
If an application tries to send data to an IP address that is not on
the development network, IP will be unable to find a match in the
route table. IP then discards the IP packet. Some computers provide
a "Network not reachable" error message.
5.9 Indirect Routing Details
Now, let's take a closer look at the more complicated routing
scenario that we examined previously.
--------- --------- ---------
| alpha | | delta | |epsilon|
| 1 | |1 2 3| | 1 |
--------- --------- ---------
| | | | |
--------o---------------o- | -o----------------o--------
Ethernet 1 | Ethernet 2
IP network "Development" | IP network "accounting"
|
| --------
| | iota |
| | 1 |
| --------
| |
--o--------o--------
Ethernet 3
IP network "factory"
Figure 9. Close-up View of Three IP Networks
The route table inside alpha looks like this:
---------------------------------------------------------------------
|network direct/indirect flag router interface number|
---------------------------------------------------------------------
|development direct <blank> 1 |
|accounting indirect devnetrouter 1 |
|factory indirect devnetrouter 1 |
--------------------------------------------------------------------
TABLE 10. Alpha Route Table
For discussion the table is printed again using numbers instead of
names.
--------------------------------------------------------------------
|network direct/indirect flag router interface number|
--------------------------------------------------------------------
|223.1.2 direct <blank> 1 |
|223.1.3 indirect 223.1.2.4 1 |
|223.1.4 indirect 223.1.2.4 1 |
--------------------------------------------------------------------
TABLE 11. Alpha Route Table with Numbers
The router in Alpha's route table is the IP address of delta's
connection to the development network.
5.10 Indirect Scenario
Alpha is sending an IP packet to epsilon. The IP packet is in
alpha's IP module and the destination IP address is epsilon
(223.1.3.2). IP extracts th network portion of this IP address
(223.1.3) and scans the first column of the table looking for a
match. A match is found on the second entry.
This entry indicates that computers on the 223.1.3 network can be
reached through the IP-router devnetrouter. Alpha's IP module then
does an ARP table translation for devnetrouter's IP address and sends
the IP packet directly to devnetrouter through Alpha's interface
number 1. The IP packet still contains the destination address of
epsilon.
The IP packet arrives at delta's development network interface and is
passed up to delta's IP module. The destination IP address is
examined and because it does not match any of delta's own IP
addresses, delta decides to forward the IP packet.
Delta's IP module extracts the network portion of the destination IP
address (223.1.3) and scans its route table for a matching network
field. Delta's route table looks like this:
----------------------------------------------------------------------
|network direct/indirect flag router interface number|
----------------------------------------------------------------------
|development direct <blank> 1 |
|factory direct <blank> 3 |
|accounting direct <blank> 2 |
----------------------------------------------------------------------
TABLE 12. Delta's Route Table
Below is delta's table printed again, without the translation to
names.
----------------------------------------------------------------------
|network direct/indirect flag router interface number|
----------------------------------------------------------------------
|223.1.2 direct <blank> 1 |
|223.1.3 direct <blank> 3 |
|223.1.4 direct <blank> 2 |
----------------------------------------------------------------------
TABLE 13. Delta's Route Table with Numbers
The match is found on the second entry. IP then sends the IP packet
directly to epsilon through interface number 3. The IP packet
contains the IP destination address of epsilon and the Ethernet
destination address of epsilon.
The IP packet arrives at epsilon and is passed up to epsilon's IP
module. The destination IP address is examined and found to match
with epsilon's IP address, so the IP packet is passed to the upper
protocol layer.
5.11 Routing Summary
When a IP packet travels through a large internet it may go through
many IP-routers before it reaches its destination. The path it takes
is not determined by a central source but is a result of consulting
each of the routing tables used in the journey. Each computer
defines only the next hop in the journey and relies on that computer
to send the IP packet on its way.
5.12 Managing the Routes
Maintaining correct routing tables on all computers in a large
internet is a difficult task; network configuration is being modified
constantly by the network managers to meet changing needs. Mistakes
in routing tables can block communication in ways that are
excruciatingly tedious to diagnose.
Keeping a simple network configuration goes a long way towards making
a reliable internet. For instance, the most straightforward method
of assigning IP networks to Ethernet is to assign a single IP network
number to each Ethernet.
Help is also available from certain protocols and network
applications. ICMP (Internet Control Message Protocol) can report
some routing problems. For small networks the route table is filled
manually on each computer by the network administrator. For larger
networks the network administrator automates this manual operation
with a routing protocol to distribute routes throughout a network.
When a computer is moved from one IP network to another, its IP
address must change. When a computer is removed from an IP network
its old address becomes invalid. These changes require frequent
updates to the "hosts" file. This flat file can become difficult to
maintain for even medium-size networks. The Domain Name System helps
solve these problems.
6. User Datagram Protocol
UDP is one of the two main protocols to reside on top of IP. It
offers service to the user's network applications. Example network
applications that use UDP are: Network File System (NFS) and Simple
Network Management Protocol (SNMP). The service is little more than
an interface to IP.
UDP is a connectionless datagram delivery service that does not
guarantee delivery. UDP does not maintain an end-to-end connection
with the remote UDP module; it merely pushes the datagram out on the
net and accepts incoming datagrams off the net.
UDP adds two values to what is provided by IP. One is the
multiplexing of information between applications based on port
number. The other is a checksum to check the integrity of the data.
6.1 Ports
How does a client on one computer reach the server on another?
The path of communication between an application and UDP is through
UDP ports. These ports are numbered, beginning with zero. An
application that is offering service (the server) waits for messages
to come in on a specific port dedicated to that service. The server
waits patiently for any client to request service.
For instance, the SNMP server, called an SNMP agent, always waits on
port 161. There can be only one SNMP agent per computer because
there is only one UDP port number 161. This port number is well
known; it is a fixed number, an internet assigned number. If an SNMP
client wants service, it sends its request to port number 161 of UDP
on the destination computer.
When an application sends data out through UDP it arrives at the far
end as a single unit. For example, if an application does 5 writes
to the UDP port, the application at the far end will do 5 reads from
the UDP port. Also, the size of each write matches the size of each
read.
UDP preserves the message boundary defined by the application. It
never joins two application messages together, or divides a single
application message into parts.
6.2 Checksum
An incoming IP packet with an IP header type field indicating "UDP"
is passed up to the UDP module by IP. When the UDP module receives
the UDP datagram from IP it examines the UDP checksum. If the
checksum is zero, it means that checksum was not calculated by the
sender and can be ignored. Thus the sending computer's UDP module
may or may not generate checksums. If Ethernet is the only network
between the 2 UDP modules communicating, then you may not need
checksumming. However, it is recommended that checksum generation
always be enabled because at some point in the future a route table
change may send the data across less reliable media.
If the checksum is valid (or zero), the destination port number is
examined and if an application is bound to that port, an application
message is queued for the application to read. Otherwise the UDP
datagram is discarded. If the incoming UDP datagrams arrive faster
than the application can read them and if the queue fills to a
maximum value, UDP datagrams are discarded by UDP. UDP will continue
to discard UDP datagrams until there is space in the queue.
7. Transmission Control Protocol
TCP provides a different service than UDP. TCP offers a connection-
oriented byte stream, instead of a connectionless datagram delivery
service. TCP guarantees delivery, whereas UDP does not.
TCP is used by network applications that require guaranteed delivery
and cannot be bothered with doing time-outs and retransmissions. The
two most typical network applications that use TCP are File Transfer
Protocol (FTP) and the TELNET. Other popular TCP network
applications include X-Window System, rcp (remote copy), and the r-
series commands. TCP's greater capability is not without cost: it
requires more CPU and network bandwidth. The internals of the TCP
module are much more complicated than those in a UDP module.
Similar to UDP, network applications connect to TCP ports. Well-
defined port numbers are dedicated to specific applications. For
instance, the TELNET server uses port number 23. The TELNET client
can find the server simply by connecting to port 23 of TCP on the
specified computer.
When the application first starts using TCP, the TCP module on the
client's computer and the TCP module on the server's computer start
communicating with each other. These two end-point TCP modules
contain state information that defines a virtual circuit. This
virtual circuit consumes resources in both TCP end-points. The
virtual circuit is full duplex; data can go in both directions
simultaneously. The application writes data to the TCP port, the
data traverses the network and is read by the application at the far
end.
As with all sliding window protocols, the protocol has a window size.
The window size determines the amount of data that can be transmitted
before an acknowledgement is required. For TCP, this amount is not a
number of TCP segments but a number of bytes.
8. Network Appliations
Why do both TCP and UDP exist, instead of just one or the other?
They supply different services. Most applications are implemented to
use only one or the other. You, the programmer, choose the protocol
that best meets your needs. If you need a reliable stream delivery
service, TCP might be best. If you need a datagram service, UDP
might be best. If you need efficiency over long-haul circuits, TCP
might be best. If you need efficiency over fast networks with short
latency, UDP might be best. If your needs do not fall nicely into
these categories, then the "best" choice is unclear. However,
applications can make up for deficiencies in the choice. For
instance if you choose UDP and you need reliability, then the
application must provide reliability. If you choose TCP and you need
a record oriented service, then the application must insert markers
in the byte stream to delimit records.
What network aplications are available?
There are far too many to list. The number is growing continually.
Some of the applications have existed since the beginning of internet
technology: TELNET and FTP. Others are relatively new: X-Windows and
SNMP. The following is a brief description of the applications
mentioned in this tutorial.
8.1 TELNET
TELNET provides a remote login capability on TCP. The operation and
appearance is similar to keyboard dialing through a telephone switch.
On the command line the user types "telnet delta" and receives a
login prompt from the computer called "delta".
TELNET works well; it is an old application and has widespread
interoperability. Implementations of TELNET usually work between
different operating systems. For instance, a TELNET client may be on
VAX/VMS and the server on UNIX System V.
8.2 FTP
File Transfer Protocol (FTP), as old as TELNET, also uses TCP and has
widespread interoperability. The operation and appearance is as if
you TELNETed to the remote computer. But instead of typing your
usual commands, you have to make do with a short list of commands for
directory listings and the like. FTP commands allow you to copy
files between computers.
8.3 rsh
Remote shell (rsh or remsh) is one of an entire family of remote UNIX
style commands. The UNIX copy command, cp, becomes rcp. The UNIX
"who is logged in" command, who, becomes rwho. The list continues
and is referred to collectively to as the "r" series commands or the
"r*" (r star) commands.
The r* commands mainly work between UNIX systems and are designed for
interaction between trusted hosts. Little consideration is given to
security, but they provide a convenient user environment.
To execute the "cc file.c" command on a remote computer called delta,
type "rsh delta cc file.c". To copy the "file.c" file to delta, type
"rcp file.c delta:". To login to delta, type "rlogin delta", and if
you administered the computers in a certain wa, you will not be
challenged with a password prompt.
8.4 NFS
Network File System, first developed by Sun Microsystems Inc, uses
UDP and is excellent for mounting UNIX file systems on multiple
computers. A diskless workstation can access its server's hard disk
as if the disk were local to the workstation. A single disk copy of
a database on mainframe "alpha" can also be used by mainframe "beta"
if the database's file system is NFS mounted commands to
use the NFS mounted disk as if it were local disk.
8.5 SNMP
Simple Network Management Protocol (SNMP) uses UDP and is designed
for use by central network management stations. It is a well known
fact that if given enough data, a network manager can detect and
diagnose network problems. The central station uses SNMP to collect
this data from other computers on the network. SNMP defines the
format for the data; it is left to the central station or network
manager to interpret the data.
8.6 X-Window
The X Window System uses the X Window protocol on TCP to draw windows
on a workstation's bitmap display. X Window is much more than a
utility for drawing windows; it is entire philosophy for designing a
user interface.
9. Other Information
Much information about internet technology was not included in this
tutorial. This section lists information that is considered the next
level of detail for the reader who wishes to learn more.
o administration commands: arp, route, and netstat
o ARP: permanent entry, publish entry, time-out entry, spoofing
o IP route table: host entry, default gateway, subnets
o IP: time-to-live counter, fragmentation, ICMP
o RIP, routing loops
o Domain Name System
10. References
[1] Comer, D., "Internetworking with TCP/IP Principles, Protocols,
and Architecture", Prentice Hall, Englewood Cliffs, New Jersey,
U.S.A., 1988.
[2] Feinler, E., et al, DDN Protocol Handbook, Volume 2 and 3, DDN
Network Information Center, SRI International, 333 Ravenswood
Avenue, Room EJ291, Menlow Park, California, U.S.A., 1985.
[3] Spider Systems, Ltd., "Packets and Protocols", Spider Systems
Ltd., Stanwell Street, Edinburgh, U.K. EH6 5NG, 1990.
11. Relation to other RFCs
This RFC is a tutorial and it does not UPDATE or OBSOLETE any other
RFC.
12. Security Considerations
There are security considerations within the TCP/IP protocol suite.
To some people these considerations are serious problems, to others
they are not; it depends on the user requirements.
This tutorial does not discuss these issues, but if you want to learn
more you should start with the topic of ARP-spoofing, then use the
"Security Considerations" section of RFC 1122 to lead you to more
information.
13. Authors' Addresses
Theodore John Socolofsky
EMail: TEDS@SPIDER.CO.UK
Claudia Jeanne Kale
EMail: CLAUDIAK@SPIDER.CO.UK
Note: This info taken from RFC-1180.
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