rfc1180.txt

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                          /           \
                       data           data
                      comes in         goes out
                     here               here

            Figure 5.  Example of IP Forwarding a IP Packet

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

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

2.5  IP Creates a Single Logical Network

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



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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.






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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...".




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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



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

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

   4.3  ARP Request/Response Pair

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

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

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

     2. The outgoing IP packet is queued.

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

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

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









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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.







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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.