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.:: TCP/IP: A Tutorial Part 2 of 2 ::.

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Current issue : #34 | Release date : 1991-10-13 | Editor : Dispater
Introduction to Phrack 34Dispater & Crimson Death
Phrack LoopbackPhrack Staff
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An Introduction to MILNETBrigadier General Swipe
TCP/IP: A Tutorial Part 2 of 2The Not
Advanced Modem-Oriented BBS SecurityDead Cow & Laughing Gas
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

   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

                          A      B      C
                          |      |      |
                        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
                                   |     |      |      |
                                  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

     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, 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  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:     alpha     beta     gamma     delta     epsilon     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

   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:     devnetrouter    delta     facnetrouter     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

   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

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    |
                        ---------         ---------
                             |               |
                      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  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   |
                                  |     --------
                                  |        |
                                    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

  |network      direct/indirect flag  router         interface number|
  |223.1.2      direct                <blank>        1               |
  |223.1.3      indirect          1               |
  |223.1.4      indirect          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
   (  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

   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

 |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

   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

   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.


   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

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

13.  Authors' Addresses

   Theodore John Socolofsky

   Claudia Jeanne Kale

   Note:  This info taken from RFC-1180.
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