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+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:
+
+                     ----------------------------
+                     |    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.
+
+                ----------------------------
+                |    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:
+
+                  ------------------------------------
+                  |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
+
+
+
+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:
+
+                ---------------------------------------
+                |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
+
+
+                ---------------------------------------
+                |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.
+
+
+
+
+
+
+
+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.
+
+                ----------------------------------------
+                |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.
+
+
+
+
+
+
+
+
+
+
+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
+
+
+                ----------------------------------------
+                |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
+
+
+
+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.
+
+                        ---------         ---------
+                        | 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.
+
+
+
+
+
+
+
+
+Socolofsky & Kale                                              [Page 18]
+
+RFC 1180                   A TCP/IP Tutorial                January 1991
+
+
+     --------------------------------------------------------------
+     |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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Socolofsky & Kale                                              [Page 19]
+
+RFC 1180                   A TCP/IP Tutorial                January 1991
+
+
+          ---------           ---------           ---------
+          | 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.
+
+
+
+
+
+
+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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