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rfc: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.

Socolofsky & Kale [Page 1] RFC 1180 A TCP/IP Tutorial January 1991

 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:
  1. —————————

| 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

Socolofsky & Kale [Page 2] RFC 1180 A TCP/IP Tutorial January 1991

 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.

Socolofsky & Kale [Page 3] RFC 1180 A TCP/IP Tutorial January 1991

       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.

Socolofsky & Kale [Page 4] RFC 1180 A TCP/IP Tutorial January 1991

 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.
  1. —————————

| 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.

Socolofsky & Kale [Page 5] RFC 1180 A TCP/IP Tutorial January 1991

       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

Socolofsky & Kale [Page 6] RFC 1180 A TCP/IP Tutorial January 1991

 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.

Socolofsky & Kale [Page 7] RFC 1180 A TCP/IP Tutorial January 1991

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...".

Socolofsky & Kale [Page 8] RFC 1180 A TCP/IP Tutorial January 1991

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:
  1. ———————————–

|IP address Ethernet address |

  1. ———————————–

|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

Socolofsky & Kale [Page 9] RFC 1180 A TCP/IP Tutorial January 1991

 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:
  1. ————————————–

|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:

Socolofsky & Kale [Page 10] RFC 1180 A TCP/IP Tutorial January 1991

  1. ————————————–

|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:
  1. ———————————

|IP address Ethernet address |

  1. ———————————

|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.

Socolofsky & Kale [Page 11] RFC 1180 A TCP/IP Tutorial January 1991

   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.

Socolofsky & Kale [Page 12] RFC 1180 A TCP/IP Tutorial January 1991

                        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.
  1. —————————————

|address source destination|

  1. —————————————

|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.

Socolofsky & Kale [Page 13] RFC 1180 A TCP/IP Tutorial January 1991

        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.

Socolofsky & Kale [Page 14] RFC 1180 A TCP/IP Tutorial January 1991

  1. —————————————

|address source destination|

  1. —————————————

|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.
  1. —————————————

|address source destination|

  1. —————————————

|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

Socolofsky & Kale [Page 15] RFC 1180 A TCP/IP Tutorial January 1991

   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

Socolofsky & Kale [Page 16] RFC 1180 A TCP/IP Tutorial January 1991

 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

Socolofsky & Kale [Page 17] RFC 1180 A TCP/IP Tutorial January 1991

 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.
  1. ——– ———

| 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:
  1. ————————————————————-

|network direct/indirect flag router interface number|

  1. ————————————————————-

|development direct <blank> 1 |

  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.

Socolofsky & Kale [Page 18] RFC 1180 A TCP/IP Tutorial January 1991

  1. ————————————————————-

|network direct/indirect flag router interface number|

  1. ————————————————————-

|223.1.2 direct <blank> 1 |

  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.

Socolofsky & Kale [Page 19] RFC 1180 A TCP/IP Tutorial January 1991

  1. ——– ——— ———

| 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.
  1. ——————————————————————-

|network direct/indirect flag router interface number|

  1. ——————————————————————-

|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.

Socolofsky & Kale [Page 20] RFC 1180 A TCP/IP Tutorial January 1991

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

Socolofsky & Kale [Page 21] RFC 1180 A TCP/IP Tutorial January 1991

 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

Socolofsky & Kale [Page 22] RFC 1180 A TCP/IP Tutorial January 1991

 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

Socolofsky & Kale [Page 23] RFC 1180 A TCP/IP Tutorial January 1991

 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

Socolofsky & Kale [Page 24] RFC 1180 A TCP/IP Tutorial January 1991

 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

Socolofsky & Kale [Page 25] RFC 1180 A TCP/IP Tutorial January 1991

 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

Socolofsky & Kale [Page 26] RFC 1180 A TCP/IP Tutorial January 1991

 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.

Socolofsky & Kale [Page 27] RFC 1180 A TCP/IP Tutorial January 1991

 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: TEDS@SPIDER.CO.UK
 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: CLAUDIAK@SPIDER.CO.UK

Socolofsky & Kale [Page 28]

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