RFC1180

Network Working Group T. Socolofsky Request for Comments: 1180 C. Kale

                                                 Spider Systems Limited
                                                           January 1991

                          A TCP/IP Tutorial

Status of this Memo

  This RFC is a tutorial on the TCP/IP protocol suite, focusing
  particularly on the steps in forwarding an IP datagram from source
  host to destination host through a router.  It does not specify an
  Internet standard.  Distribution of this memo is unlimited.

Table of Contents

   1.  Introduction................................................   1
   2.  TCP/IP Overview.............................................   2
   3.  Ethernet....................................................   8
   4.  ARP.........................................................   9
   5.  Internet Protocol...........................................  12
   6.  User Datagram Protocol......................................  22
   7.  Transmission Control Protocol...............................  24
   8.  Network Applications........................................  25
   9.  Other Information...........................................  27
  10.  References..................................................  27
  11.  Relation to other RFCs......................................  27
  12.  Security Considerations.....................................  27
  13.  Authors' Addresses..........................................  28

1. Introduction

  This tutorial contains only one view of the salient points of TCP/IP,
  and therefore it is the "bare bones" of TCP/IP technology.  It omits
  the history of development and funding, the business case for its
  use, and its future as compared to ISO OSI.  Indeed, a great deal of
  technical information is also omitted.  What remains is a minimum of
  information that must be understood by the professional working in a
  TCP/IP environment.  These professionals include the systems
  administrator, the systems programmer, and the network manager.

  This tutorial uses examples from the UNIX TCP/IP environment, however
  the main points apply across all implementations of TCP/IP.

  Note that the purpose of this memo is explanation, not definition.
  If any question arises about the correct specification of a protocol,
  please refer to the actual standards defining RFC.

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  The next section is an overview of TCP/IP, followed by detailed
  descriptions of individual components.

2. TCP/IP Overview

  The generic term "TCP/IP" usually means anything and everything
  related to the specific protocols of TCP and IP.  It can include
  other protocols, applications, and even the network medium.  A sample
  of these protocols are: UDP, ARP, and ICMP.  A sample of these
  applications are: TELNET, FTP, and rcp.  A more accurate term is
  "internet technology".  A network that uses internet technology is
  called an "internet".

2.1 Basic Structure

  To understand this technology you must first understand the following
  logical structure:

                    ----------------------------
                    |    network applications  |
                    |                          |
                    |...  \ | /  ..  \ | /  ...|
                    |     -----      -----     |
                    |     |TCP|      |UDP|     |
                    |     -----      -----     |
                    |         \      /         |
                    |         --------         |
                    |         |  IP  |         |
                    |  -----  -*------         |
                    |  |ARP|   |               |
                    |  -----   |               |
                    |      \   |               |
                    |      ------              |
                    |      |ENET|              |
                    |      ---@--              |
                    ----------|-----------------
                              |
        ----------------------o---------
            Ethernet Cable

                 Figure 1.  Basic TCP/IP Network Node

  This is the logical structure of the layered protocols inside a
  computer on an internet.  Each computer that can communicate using
  internet technology has such a logical structure.  It is this logical
  structure that determines the behavior of the computer on the
  internet.  The boxes represent processing of the data as it passes
  through the computer, and the lines connecting boxes show the path of

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  data.  The horizontal line at the bottom represents the Ethernet
  cable; the "o" is the transceiver.  The "*" is the IP address and the
  "@" is the Ethernet address.  Understanding this logical structure is
  essential to understanding internet technology; it is referred to
  throughout this tutorial.

2.2 Terminology

  The name of a unit of data that flows through an internet is
  dependent upon where it exists in the protocol stack.  In summary: if
  it is on an Ethernet it is called an Ethernet frame; if it is between
  the Ethernet driver and the IP module it is called a IP packet; if it
  is between the IP module and the UDP module it is called a UDP
  datagram; if it is between the IP module and the TCP module it is
  called a TCP segment (more generally, a transport message); and if it
  is in a network application it is called a application message.

  These definitions are imperfect.  Actual definitions vary from one
  publication to the next.  More specific definitions can be found in
  RFC 1122, section 1.3.3.

  A driver is software that communicates directly with the network
  interface hardware.  A module is software that communicates with a
  driver, with network applications, or with another module.

  The terms driver, module, Ethernet frame, IP packet, UDP datagram,
  TCP message, and application message are used where appropriate
  throughout this tutorial.

2.3 Flow of Data

  Let's follow the data as it flows down through the protocol stack
  shown in Figure 1.  For an application that uses TCP (Transmission
  Control Protocol), data passes between the application and the TCP
  module.  For applications that use UDP (User Datagram Protocol), data
  passes between the application and the UDP module.  FTP (File
  Transfer Protocol) is a typical application that uses TCP.  Its
  protocol stack in this example is FTP/TCP/IP/ENET.  SNMP (Simple
  Network Management Protocol) is an application that uses UDP.  Its
  protocol stack in this example is SNMP/UDP/IP/ENET.

  The TCP module, UDP module, and the Ethernet driver are n-to-1
  multiplexers.  As multiplexers they switch many inputs to one output.
  They are also 1-to-n de-multiplexers.  As de-multiplexers they switch
  one input to many outputs according to the type field in the protocol
  header.

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        1   2 3 ...   n                   1   2 3 ...   n
         \  |      /      |               \  | |      /       ^
          \ | |   /       |                \ | |     /        |
        -------------   flow              ----------------   flow
        |multiplexer|    of               |de-multiplexer|    of
        -------------   data              ----------------   data
             |            |                     |              |
             |            v                     |              |
             1                                  1

       Figure 2.  n-to-1 multiplexer and 1-to-n de-multiplexer

  If an Ethernet frame comes up into the Ethernet driver off the
  network, the packet can be passed upwards to either the ARP (Address
  Resolution Protocol) module or to the IP (Internet Protocol) module.
  The value of the type field in the Ethernet frame determines whether
  the Ethernet frame is passed to the ARP or the IP module.

  If an IP packet comes up into IP, the unit of data is passed upwards
  to either TCP or UDP, as determined by the value of the protocol
  field in the IP header.

  If the UDP datagram comes up into UDP, the application message is
  passed upwards to the network application based on the value of the
  port field in the UDP header.  If the TCP message comes up into TCP,
  the application message is passed upwards to the network application
  based on the value of the port field in the TCP header.

  The downwards multiplexing is simple to perform because from each
  starting point there is only the one downward path; each protocol
  module adds its header information so the packet can be de-
  multiplexed at the destination computer.

  Data passing out from the applications through either TCP or UDP
  converges on the IP module and is sent downwards through the lower
  network interface driver.

  Although internet technology supports many different network media,
  Ethernet is used for all examples in this tutorial because it is the
  most common physical network used under IP.  The computer in Figure 1
  has a single Ethernet connection.  The 6-byte Ethernet address is
  unique for each interface on an Ethernet and is located at the lower
  interface of the Ethernet driver.

  The computer also has a 4-byte IP address.  This address is located
  at the lower interface to the IP module.  The IP address must be
  unique for an internet.

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  A running computer always knows its own IP address and Ethernet
  address.

2.4 Two Network Interfaces

  If a computer is connected to 2 separate Ethernets it is as in Figure
  3.

               ----------------------------
               |    network applications  |
               |                          |
               |...  \ | /  ..  \ | /  ...|
               |     -----      -----     |
               |     |TCP|      |UDP|     |
               |     -----      -----     |
               |         \      /         |
               |         --------         |
               |         |  IP  |         |
               |  -----  -*----*-  -----  |
               |  |ARP|   |    |   |ARP|  |
               |  -----   |    |   -----  |
               |      \   |    |   /      |
               |      ------  ------      |
               |      |ENET|  |ENET|      |
               |      ---@--  ---@--      |
               ----------|-------|---------
                         |       |
                         |    ---o---------------------------
                         |             Ethernet Cable 2
          ---------------o----------
            Ethernet Cable 1

            Figure 3.  TCP/IP Network Node on 2 Ethernets

  Please note that this computer has 2 Ethernet addresses and 2 IP
  addresses.

  It is seen from this structure that for computers with more than one
  physical network interface, the IP module is both a n-to-m
  multiplexer and an m-to-n de-multiplexer.

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        1   2 3 ...   n                   1   2 3 ...   n
         \  | |      /    |                \  | |      /       ^
          \ | |     /     |                 \ | |     /        |
        -------------   flow              ----------------   flow
        |multiplexer|    of               |de-multiplexer|    of
        -------------   data              ----------------   data
          / | |     \     |                 / | |     \        |
         /  | |      \    v                /  | |      \       |
        1   2 3 ...   m                   1   2 3 ...   m

       Figure 4.  n-to-m multiplexer and m-to-n de-multiplexer

  It performs this multiplexing in either direction to accommodate
  incoming and outgoing data.  An IP module with more than 1 network
  interface is more complex than our original example in that it can
  forward data onto the next network.  Data can arrive on any network
  interface and be sent out on any other.

                          TCP      UDP
                            \      /
                             \    /
                         --------------
                         |     IP     |
                         |            |
                         |    ---     |
                         |   /   \    |
                         |  /     v   |
                         --------------
                          /         \
                         /           \
                      data           data
                     comes in         goes out
                    here               here

           Figure 5.  Example of IP Forwarding a IP Packet

  The process of sending an IP packet out onto another network is
  called "forwarding" an IP packet.  A computer that has been dedicated
  to the task of forwarding IP packets is called an "IP-router".

  As you can see from the figure, the forwarded IP packet never touches
  the TCP and UDP modules on the IP-router.  Some IP-router
  implementations do not have a TCP or UDP module.

2.5 IP Creates a Single Logical Network

  The IP module is central to the success of internet technology.  Each
  module or driver adds its header to the message as the message passes

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  down through the protocol stack.  Each module or driver strips the
  corresponding header from the message as the message climbs the
  protocol stack up towards the application.  The IP header contains
  the IP address, which builds a single logical network from multiple
  physical networks.  This interconnection of physical networks is the
  source of the name: internet.  A set of interconnected physical
  networks that limit the range of an IP packet is called an
  "internet".

2.6 Physical Network Independence

  IP hides the underlying network hardware from the network
  applications.  If you invent a new physical network, you can put it
  into service by implementing a new driver that connects to the
  internet underneath IP.  Thus, the network applications remain intact
  and are not vulnerable to changes in hardware technology.

2.7 Interoperability

  If two computers on an internet can communicate, they are said to
  "interoperate"; if an implementation of internet technology is good,
  it is said to have "interoperability".  Users of general-purpose
  computers benefit from the installation of an internet because of the
  interoperability in computers on the market.  Generally, when you buy
  a computer, it will interoperate.  If the computer does not have
  interoperability, and interoperability can not be added, it occupies
  a rare and special niche in the market.

2.8 After the Overview

  With the background set, we will answer the following questions:

  When sending out an IP packet, how is the destination Ethernet
  address determined?

  How does IP know which of multiple lower network interfaces to use
  when sending out an IP packet?

  How does a client on one computer reach the server on another?

  Why do both TCP and UDP exist, instead of just one or the other?

  What network applications are available?

  These will be explained, in turn, after an Ethernet refresher.

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3. Ethernet

  This section is a short review of Ethernet technology.

  An Ethernet frame contains the destination address, source address,
  type field, and data.

  An Ethernet address is 6 bytes.  Every device has its own Ethernet
  address and listens for Ethernet frames with that destination
  address.  All devices also listen for Ethernet frames with a wild-
  card destination address of "FF-FF-FF-FF-FF-FF" (in hexadecimal),
  called a "broadcast" address.

  Ethernet uses CSMA/CD (Carrier Sense and Multiple Access with
  Collision Detection).  CSMA/CD means that all devices communicate on
  a single medium, that only one can transmit at a time, and that they
  can all receive simultaneously.  If 2 devices try to transmit at the
  same instant, the transmit collision is detected, and both devices
  wait a random (but short) period before trying to transmit again.

3.1 A Human Analogy

  A good analogy of Ethernet technology is a group of people talking in
  a small, completely dark room.  In this analogy, the physical network
  medium is sound waves on air in the room instead of electrical
  signals on a coaxial cable.

  Each person can hear the words when another is talking (Carrier
  Sense).  Everyone in the room has equal capability to talk (Multiple
  Access), but none of them give lengthy speeches because they are
  polite.  If a person is impolite, he is asked to leave the room
  (i.e., thrown off the net).

  No one talks while another is speaking.  But if two people start
  speaking at the same instant, each of them know this because each
  hears something they haven't said (Collision Detection).  When these
  two people notice this condition, they wait for a moment, then one
  begins talking.  The other hears the talking and waits for the first
  to finish before beginning his own speech.

  Each person has an unique name (unique Ethernet address) to avoid
  confusion.  Every time one of them talks, he prefaces the message
  with the name of the person he is talking to and with his own name
  (Ethernet destination and source address, respectively), i.e., "Hello
  Jane, this is Jack, ..blah blah blah...".  If the sender wants to
  talk to everyone he might say "everyone" (broadcast address), i.e.,
  "Hello Everyone, this is Jack, ..blah blah blah...".

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4. ARP

  When sending out an IP packet, how is the destination Ethernet
  address determined?

  ARP (Address Resolution Protocol) is used to translate IP addresses
  to Ethernet addresses.  The translation is done only for outgoing IP
  packets, because this is when the IP header and the Ethernet header
  are created.

4.1 ARP Table for Address Translation

  The translation is performed with a table look-up.  The table, called
  the ARP table, is stored in memory and contains a row for each
  computer.  There is a column for IP address and a column for Ethernet
  address.  When translating an IP address to an Ethernet address, the
  table is searched for a matching IP address.  The following is a
  simplified ARP table:

                 ------------------------------------
                 |IP address       Ethernet address |
                 ------------------------------------
                 |223.1.2.1        08-00-39-00-2F-C3|
                 |223.1.2.3        08-00-5A-21-A7-22|
                 |223.1.2.4        08-00-10-99-AC-54|
                 ------------------------------------
                     TABLE 1.  Example ARP Table

  The human convention when writing out the 4-byte IP address is each
  byte in decimal and separating bytes with a period.  When writing out
  the 6-byte Ethernet address, the conventions are each byte in
  hexadecimal and separating bytes with either a minus sign or a colon.

  The ARP table is necessary because the IP address and Ethernet
  address are selected independently; you can not use an algorithm to
  translate IP address to Ethernet address.  The IP address is selected
  by the network manager based on the location of the computer on the
  internet.  When the computer is moved to a different part of an
  internet, its IP address must be changed.  The Ethernet address is
  selected by the manufacturer based on the Ethernet address space
  licensed by the manufacturer.  When the Ethernet hardware interface
  board changes, the Ethernet address changes.

4.2 Typical Translation Scenario

  During normal operation a network application, such as TELNET, sends
  an application message to TCP, then TCP sends the corresponding TCP
  message to the IP module.  The destination IP address is known by the

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  application, the TCP module, and the IP module.  At this point the IP
  packet has been constructed and is ready to be given to the Ethernet
  driver, but first the destination Ethernet address must be
  determined.

  The ARP table is used to look-up the destination Ethernet address.

  4.3  ARP Request/Response Pair

  But how does the ARP table get filled in the first place?  The answer
  is that it is filled automatically by ARP on an "as-needed" basis.

  Two things happen when the ARP table can not be used to translate an
  address:

    1. An ARP request packet with a broadcast Ethernet address is sent
       out on the network to every computer.

    2. The outgoing IP packet is queued.

  Every computer's Ethernet interface receives the broadcast Ethernet
  frame.  Each Ethernet driver examines the Type field in the Ethernet
  frame and passes the ARP packet to the ARP module.  The ARP request
  packet says "If your IP address matches this target IP address, then
  please tell me your Ethernet address".  An ARP request packet looks
  something like this:

               ---------------------------------------
               |Sender IP Address   223.1.2.1        |
               |Sender Enet Address 08-00-39-00-2F-C3|
               ---------------------------------------
               |Target IP Address   223.1.2.2        |
               |Target Enet Address <blank>          |
               ---------------------------------------
                    TABLE 2.  Example ARP Request

  Each ARP module examines the IP address and if the Target IP address
  matches its own IP address, it sends a response directly to the
  source Ethernet address.  The ARP response packet says "Yes, that
  target IP address is mine, let me give you my Ethernet address".  An
  ARP response packet has the sender/target field contents swapped as
  compared to the request.  It looks something like this:

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               ---------------------------------------
               |Sender IP Address   223.1.2.2        |
               |Sender Enet Address 08-00-28-00-38-A9|
               ---------------------------------------
               |Target IP Address   223.1.2.1        |
               |Target Enet Address 08-00-39-00-2F-C3|
               ---------------------------------------
                    TABLE 3.  Example ARP Response

  The response is received by the original sender computer.  The
  Ethernet driver looks at the Type field in the Ethernet frame then
  passes the ARP packet to the ARP module.  The ARP module examines the
  ARP packet and adds the sender's IP and Ethernet addresses to its ARP
  table.

  The updated table now looks like this:

                  ----------------------------------
                  |IP address     Ethernet address |
                  ----------------------------------
                  |223.1.2.1      08-00-39-00-2F-C3|
                  |223.1.2.2      08-00-28-00-38-A9|
                  |223.1.2.3      08-00-5A-21-A7-22|
                  |223.1.2.4      08-00-10-99-AC-54|
                  ----------------------------------
                  TABLE 4.  ARP Table after Response

4.4 Scenario Continued

  The new translation has now been installed automatically in the
  table, just milli-seconds after it was needed.  As you remember from
  step 2 above, the outgoing IP packet was queued.  Next, the IP
  address to Ethernet address translation is performed by look-up in
  the ARP table then the Ethernet frame is transmitted on the Ethernet.
  Therefore, with the new steps 3, 4, and 5, the scenario for the
  sender computer is:

    1. An ARP request packet with a broadcast Ethernet address is sent
       out on the network to every computer.

    2. The outgoing IP packet is queued.

    3. The ARP response arrives with the IP-to-Ethernet address
       translation for the ARP table.

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    4. For the queued IP packet, the ARP table is used to translate the
       IP address to the Ethernet address.

    5. The Ethernet frame is transmitted on the Ethernet.

  In summary, when the translation is missing from the ARP table, one
  IP packet is queued.  The translation data is quickly filled in with
  ARP request/response and the queued IP packet is transmitted.

  Each computer has a separate ARP table for each of its Ethernet
  interfaces.  If the target computer does not exist, there will be no
  ARP response and no entry in the ARP table.  IP will discard outgoing
  IP packets sent to that address.  The upper layer protocols can't
  tell the difference between a broken Ethernet and the absence of a
  computer with the target IP address.

  Some implementations of IP and ARP don't queue the IP packet while
  waiting for the ARP response.  Instead the IP packet is discarded and
  the recovery from the IP packet loss is left to the TCP module or the
  UDP network application.  This recovery is performed by time-out and
  retransmission.  The retransmitted message is successfully sent out
  onto the network because the first copy of the message has already
  caused the ARP table to be filled.

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 learning 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.

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                         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 destination
  Ethernet address.

               ----------------------------------------
               |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.

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         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.

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               ----------------------------------------
               |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 address 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

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    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

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  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

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  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.

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    --------------------------------------------------------------
    |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.

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         ---------           ---------           ---------
         | 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.

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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 the 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

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  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

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  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

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  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.

  TCP packetizes the byte stream at will; it does not retain the
  boundaries between writes.  For example, if an application does 5
  writes to the TCP port, the application at the far end might do 10
  reads to get all the data.  Or it might get all the data with a
  single read.  There is no correlation between the number and size of

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  writes at one end to the number and size of reads at the other end.

  TCP is a sliding window protocol with time-out and retransmits.
  Outgoing data must be acknowledged by the far-end TCP.
  Acknowledgements can be piggybacked on data.  Both receiving ends can
  flow control the far end, thus preventing a buffer overrun.

  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 Applications

  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 applications 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

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  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 way, 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 on "beta".

  NFS adds significant load to a network and has poor utility across
  slow links, but the benefits are strong.  The NFS client is
  implemented in the kernel, allowing all applications and 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

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  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.

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  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
  Spider Systems Limited
  Spider Park
  Stanwell Street
  Edinburgh EH6 5NG
  United Kingdom

  Phone:
    from UK        031-554-9424
    from USA 011-44-31-554-9424
  Fax:
    from UK        031-554-0649
    from USA 011-44-31-554-0649

  EMail: [email protected]

  Claudia Jeanne Kale
  12 Gosford Place
  Edinburgh EH6 4BJ
  United Kingdom

  Phone:
    from UK        031-554-7432
    from USA 011-44-31-554-7432

  EMail: [email protected]

Socolofsky & Kale