rfc791.txt

RFC 相关的技术文档

                                                                [Page 3]


                                                          September 1981
Internet Protocol






















































[Page 4]                                                                


September 1981                                                          
                                                       Internet Protocol



                              2.  OVERVIEW

2.1.  Relation to Other Protocols

  The following diagram illustrates the place of the internet protocol
  in the protocol hierarchy:

                                    
                 +------+ +-----+ +-----+     +-----+  
                 |Telnet| | FTP | | TFTP| ... | ... |  
                 +------+ +-----+ +-----+     +-----+  
                       |   |         |           |     
                      +-----+     +-----+     +-----+  
                      | TCP |     | UDP | ... | ... |  
                      +-----+     +-----+     +-----+  
                         |           |           |     
                      +--------------------------+----+
                      |    Internet Protocol & ICMP   |
                      +--------------------------+----+
                                     |                 
                        +---------------------------+  
                        |   Local Network Protocol  |  
                        +---------------------------+  

                         Protocol Relationships

                               Figure 1.

  Internet protocol interfaces on one side to the higher level
  host-to-host protocols and on the other side to the local network
  protocol.  In this context a "local network" may be a small network in
  a building or a large network such as the ARPANET.

2.2.  Model of Operation

  The  model of operation for transmitting a datagram from one
  application program to another is illustrated by the following
  scenario:

    We suppose that this transmission will involve one intermediate
    gateway.

    The sending application program prepares its data and calls on its
    local internet module to send that data as a datagram and passes the
    destination address and other parameters as arguments of the call.

    The internet module prepares a datagram header and attaches the data
    to it.  The internet module determines a local network address for
    this internet address, in this case it is the address of a gateway.


                                                                [Page 5]


                                                          September 1981
Internet Protocol
Overview



    It sends this datagram and the local network address to the local
    network interface.

    The local network interface creates a local network header, and
    attaches the datagram to it, then sends the result via the local
    network.

    The datagram arrives at a gateway host wrapped in the local network
    header, the local network interface strips off this header, and
    turns the datagram over to the internet module.  The internet module
    determines from the internet address that the datagram is to be
    forwarded to another host in a second network.  The internet module
    determines a local net address for the destination host.  It calls
    on the local network interface for that network to send the
    datagram.

    This local network interface creates a local network header and
    attaches the datagram sending the result to the destination host.

    At this destination host the datagram is stripped of the local net
    header by the local network interface and handed to the internet
    module.

    The internet module determines that the datagram is for an
    application program in this host.  It passes the data to the
    application program in response to a system call, passing the source
    address and other parameters as results of the call.

                                    
   Application                                           Application
   Program                                                   Program
         \                                                   /      
       Internet Module      Internet Module      Internet Module    
             \                 /       \                /           
             LNI-1          LNI-1      LNI-2         LNI-2          
                \           /             \          /              
               Local Network 1           Local Network 2            



                            Transmission Path

                                Figure 2







[Page 6]                                                                


September 1981                                                          
                                                       Internet Protocol
                                                                Overview



2.3.  Function Description

  The function or purpose of Internet Protocol is to move datagrams
  through an interconnected set of networks.  This is done by passing
  the datagrams from one internet module to another until the
  destination is reached.  The internet modules reside in hosts and
  gateways in the internet system.  The datagrams are routed from one
  internet module to another through individual networks based on the
  interpretation of an internet address.  Thus, one important mechanism
  of the internet protocol is the internet address.

  In the routing of messages from one internet module to another,
  datagrams may need to traverse a network whose maximum packet size is
  smaller than the size of the datagram.  To overcome this difficulty, a
  fragmentation mechanism is provided in the internet protocol.

  Addressing

    A distinction is made between names, addresses, and routes [4].   A
    name indicates what we seek.  An address indicates where it is.  A
    route indicates how to get there.  The internet protocol deals
    primarily with addresses.  It is the task of higher level (i.e.,
    host-to-host or application) protocols to make the mapping from
    names to addresses.   The internet module maps internet addresses to
    local net addresses.  It is the task of lower level (i.e., local net
    or gateways) procedures to make the mapping from local net addresses
    to routes.

    Addresses are fixed length of four octets (32 bits).  An address
    begins with a network number, followed by local address (called the
    "rest" field).  There are three formats or classes of internet
    addresses:  in class a, the high order bit is zero, the next 7 bits
    are the network, and the last 24 bits are the local address; in
    class b, the high order two bits are one-zero, the next 14 bits are
    the network and the last 16 bits are the local address; in class c,
    the high order three bits are one-one-zero, the next 21 bits are the
    network and the last 8 bits are the local address.

    Care must be taken in mapping internet addresses to local net
    addresses; a single physical host must be able to act as if it were
    several distinct hosts to the extent of using several distinct
    internet addresses.  Some hosts will also have several physical
    interfaces (multi-homing).

    That is, provision must be made for a host to have several physical
    interfaces to the network with each having several logical internet
    addresses.



                                                                [Page 7]


                                                          September 1981
Internet Protocol
Overview



    Examples of address mappings may be found in "Address Mappings" [5].

  Fragmentation

    Fragmentation of an internet datagram is necessary when it
    originates in a local net that allows a large packet size and must
    traverse a local net that limits packets to a smaller size to reach
    its destination.

    An internet datagram can be marked "don't fragment."  Any internet
    datagram so marked is not to be internet fragmented under any
    circumstances.  If internet datagram marked don't fragment cannot be
    delivered to its destination without fragmenting it, it is to be
    discarded instead.

    Fragmentation, transmission and reassembly across a local network
    which is invisible to the internet protocol module is called
    intranet fragmentation and may be used [6].

    The internet fragmentation and reassembly procedure needs to be able
    to break a datagram into an almost arbitrary number of pieces that
    can be later reassembled.  The receiver of the fragments uses the
    identification field to ensure that fragments of different datagrams
    are not mixed.  The fragment offset field tells the receiver the
    position of a fragment in the original datagram.  The fragment
    offset and length determine the portion of the original datagram
    covered by this fragment.  The more-fragments flag indicates (by
    being reset) the last fragment.  These fields provide sufficient
    information to reassemble datagrams.

    The identification field is used to distinguish the fragments of one
    datagram from those of another.  The originating protocol module of
    an internet datagram sets the identification field to a value that
    must be unique for that source-destination pair and protocol for the
    time the datagram will be active in the internet system.  The
    originating protocol module of a complete datagram sets the
    more-fragments flag to zero and the fragment offset to zero.

    To fragment a long internet datagram, an internet protocol module
    (for example, in a gateway), creates two new internet datagrams and
    copies the contents of the internet header fields from the long
    datagram into both new internet headers.  The data of the long
    datagram is divided into two portions on a 8 octet (64 bit) boundary
    (the second portion might not be an integral multiple of 8 octets,
    but the first must be).  Call the number of 8 octet blocks in the
    first portion NFB (for Number of Fragment Blocks).  The first
    portion of the data is placed in the first new internet datagram,
    and the total length field is set to the length of the first


[Page 8] 


September 1981 
                                                       Internet Protocol
                                                                Overview



    datagram.  The more-fragments flag is set to one.  The second
    portion of the data is placed in the second new internet datagram,
    and the total length field is set to the length of the second
    datagram.  The more-fragments flag carries the same value as the
    long datagram.  The fragment offset field of the second new internet
    datagram is set to the value of that field in the long datagram plus
    NFB.

    This procedure can be generalized for an n-way split, rather than
    the two-way split described.

    To assemble the fragments of an internet datagram, an internet
    protocol module (for example at a destination host) combines
    internet datagrams that all have the same value for the four fields:
    identification, source, destination, and protocol.  The combination
    is done by placing the data portion of each fragment in the relative
    position indicated by the fragment offset in that fragment's
    internet header.  The first fragment will have the fragment offset
    zero, and the last fragment will have the more-fragments flag reset
    to zero.

2.4.  Gateways

  Gateways implement internet protocol to forward datagrams between
  networks.  Gateways also implement the Gateway to Gateway Protocol
  (GGP) [7] to coordinate routing and other internet control
  information.

  In a gateway the higher level protocols need not be implemented and
  the GGP functions are added to the IP module.

                                    
                   +-------------------------------+   
                   | Internet Protocol & ICMP & GGP|   
                   +-------------------------------+   
                           |                 |         
                 +---------------+   +---------------+ 
                 |   Local Net   |   |   Local Net   | 
                 +---------------+   +---------------+ 

                           Gateway Protocols

                               Figure 3.

  





                                                                [Page 9]


                                                          September 1981
Internet Protocol