rfc1180.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                      T. Socolofsky
Request for Comments:  1180                                      C. Kale
                                                  Spider Systems Limited
                                                            January 1991


                           A TCP/IP Tutorial

Status of this Memo

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

Table of Contents

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

1.  Introduction

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

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

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



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

2.  TCP/IP Overview

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

2.1  Basic Structure

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

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

                  Figure 1.  Basic TCP/IP Network Node

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



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

2.2  Terminology

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

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

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

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

2.3  Flow of Data

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

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





Socolofsky & Kale                                               [Page 3]

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         1   2 3 ...   n                   1   2 3 ...   n
          \  |      /      |               \  | |      /       ^
           \ | |   /       |                \ | |     /        |
         -------------   flow              ----------------   flow
         |multiplexer|    of               |de-multiplexer|    of
         -------------   data              ----------------   data
              |            |                     |              |
              |            v                     |              |
              1                                  1

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

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

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

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

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

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

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

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




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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.











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

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

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

                           TCP      UDP
                             \      /
                              \    /
                          --------------
                          |     IP     |
                          |            |
                          |    ---     |
                          |   /   \    |
                          |  /     v   |
                          --------------
                           /         \