rfc878.txt

RFC 相关的技术文档

     Request for Comments: 878
     Obsoletes RFCs: 851, 802








                  The ARPANET 1822L Host Access Protocol



                                  RFC 878





                              Andrew G. Malis
                       ARPANET Mail: malis@bbn-unix





                         BBN Communications Corp.
                              50 Moulton St.
                           Cambridge, MA  02238





                               December 1983





     This RFC specifies the ARPANET 1822L Host Access Protocol,  which
     is  a successor to the existing 1822 Host Access Protocol.  1822L
     allows ARPANET hosts to use  logical  names  as  well  as  1822's
     physical port locations to address each other.












     1822L Host Access Protocol                          December 1983
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                             Table of Contents





     1   INTRODUCTION.......................................... 1

     2   THE ARPANET 1822L HOST ACCESS PROTOCOL................ 3
     2.1   Addresses and Names................................. 5
     2.2   Name Translations................................... 7
     2.2.1   Authorization and Effectiveness................... 7
     2.2.2   Translation Policies............................. 11
     2.2.3   Reporting Destination Host Downs................. 13
     2.2.4   1822L and 1822 Interoperability.................. 15
     2.3   Uncontrolled Packets............................... 16
     2.4   Establishing Host-IMP Communications............... 19
     2.5   Counting RFNMs When Using 1822L.................... 20
     2.6   1822L Name Server.................................. 23

     3   1822L LEADER FORMATS................................. 25
     3.1   Host-to-IMP 1822L Leader Format.................... 26
     3.2   IMP-to-Host 1822L Leader Format.................... 34

     4   REFERENCES........................................... 42

     A   1822L-IP ADDRESS MAPPINGS............................ 43























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                                  FIGURES




     2.1  1822 Address Format.................................. 5
     2.2  1822L Name Format.................................... 6
     2.3  1822L Address Format................................. 6
     3.1  Host-to-IMP 1822L Leader Format..................... 27
     3.2  NDM Message Format.................................. 30
     3.3  IMP-to-Host 1822L Leader Format..................... 35
     3.4  Name Server Reply Format............................ 38
     A.1  1822 Class A Mapping................................ 44
     A.2  1822L Class A Mapping............................... 44
     A.3  1822L Class B Mapping............................... 45
     A.4  1822L Class C Mapping............................... 46


































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     1  INTRODUCTION


     This RFC specifies the ARPANET 1822L Host Access Protocol,  which

     will allow hosts to use logical addressing (i.e., host names that

     are independent of their physical location  on  the  ARPANET)  to

     communicate  with  each  other.  This new host access protocol is

     known as the ARPANET 1822L (for Logical)  Host  Access  Protocol,

     and  is  a  successor  to  the  current  ARPANET 1822 Host Access

     Protocol, which is described in  sections  3.3  and  3.4  of  BBN

     Report  1822  [1].   Although  the  1822L protocol uses different

     Host-IMP leaders than the 1822 protocol, the IMPs  will  continue

     to support the 1822 protocol, and hosts using either protocol can

     readily communicate with each other (the  IMPs  will  handle  the

     translation automatically).


     The RFC's terminology is consistent  with  that  used  in  Report

     1822, and any new terms will be defined when they are first used.

     Familiarity  with  Report  1822  (section  3  in  particular)  is

     assumed.   As could be expected, the RFC makes many references to

     Report 1822.  As a result, it uses, as a convenient abbreviation,

     "see 1822(x)" instead of "please refer to Report 1822, section x,

     for further details".


     This RFC updates, and obsoletes, RFC 851.  The changes from  that

     RFC are:




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     o Section 2.2.4 was rewritten for clarity.


     o Section 2.5 was expanded to  further  discuss  the  effects  of

       using 1822L names on host-to-host virtual circuits.


     o In section 3.2, the type 1  IMP-to-host  message  has  two  new

       subtypes,  the type 9 message has one new subtype, and the type

       15, subtype 4 message is no longer defined.


     o An appendix describing the  mapping  between  1822L  names  and

       internet (IP) addresses has been added.


     All of these changes to RFC 851 are marked by revision  bars  (as  |

     shown here) in the right margin.                                   |



























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     2  THE ARPANET 1822L HOST ACCESS PROTOCOL


     The ARPANET 1822L Host Access  Protocol  allows  a  host  to  use

     logical  addressing  to  communicate  with  other  hosts  on  the

     ARPANET.  Basically, logical addressing allows hosts to refer  to

     each  other  using  an  1822L  name  (see  section  2.1) which is

     independent of a host's physical location in  the  network.   IEN

     183  (also  published  as  BBN  Report 4473) [2] gives the use of

     logical  addressing  considerable   justification.    Among   the

     advantages it cites are:


     o The ability to refer to each host on  the  network  by  a  name

       independent of its location on the network.


     o Allowing different hosts to share  the  same  host  port  on  a

       time-division basis.


     o Allowing a host to use multi-homing (where a single  host  uses

       more than one port to communicate with the network).


     o Allowing several hosts that provide the same service  to  share

       the same name.


     The main differences between the 1822 and 1822L protocols are the

     format of the leaders that are used to introduce messages between

     a host and an IMP, and the specification in those leaders of  the

     source  and/or  destination  host(s).   Hosts  have the choice of



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     using the 1822 or the 1822L protocol.  When a host comes up on an

     IMP,  it  declares  itself to be an 1822 host or an 1822L host by

     the type of NOP message (see section  3.1)  it  uses.   Once  up,

     hosts  can  switch  from  one protocol to the other by issuing an

     appropriate NOP.  Hosts that do not use the 1822L  protocol  will

     still  be  addressable by and can communicate with hosts that do,

     and vice-versa.


     Another difference between the two protocols  is  that  the  1822

     leaders are symmetric, while the 1822L leaders are not.  The term

     symmetric means that in the 1822 protocol, the exact same  leader

     format  is used for messages in both directions between the hosts

     and IMPs.  For example, a leader sent from a host  over  a  cable

     that  was  looped  back onto itself (via a looping plug or faulty

     hardware) would arrive back at the host and appear to be a  legal

     message  from  a  real host (the destination host of the original

     message).  In contrast, the 1822L headers are not symmetric,  and

     a  host  can  detect  if  the  connection to its IMP is looped by

     receiving a message with the wrong leader  format.   This  allows

     the host to take appropriate action upon detection of the loop.












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     2.1  Addresses and Names


     The 1822 protocol defines one form of host specification, and the

     1822L  protocol  defines  two additional ways to identify network

     hosts.  These three forms are 1822 addresses,  1822L  names,  and

     1822L addresses.


     1822 addresses are  the  24-bit  host  addresses  found  in  1822

     leaders.  They have the following format:


            1              8 9                              24
           +----------------+---------------------------------+
           |                |                                 |
           |  Host number   |           IMP number            |
           |                |                                 |
           +----------------+---------------------------------+


                            1822 Address Format
                                Figure 2.1



     These fields are quite large, and the ARPANET will never use more

     than  a  fraction of the available address space.  1822 addresses

     are used in 1822 leaders only.


     1822L names are 16-bit unsigned numbers that serve as  a  logical

     identifier  for  one  or  more  hosts.   1822L  names have a much

     simpler format:








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                     1                             16
                    +--------------------------------+
                    |                                |
                    |           1822L name           |
                    |                                |
                    +--------------------------------+


                             1822L Name Format
                                Figure 2.2



     The 1822L names are just 16-bit  unsigned  numbers,  except  that

     bits  1  and  2 are not both zeros (see below).  This allows over

     49,000 hosts to be specified.


     1822 addresses cannot be used in 1822L leaders, but there may  be

     a  requirement for an 1822L host to be able to address a specific

     physical host port or IMP fake host.  1822L  addresses  are  used

     for  this  function.   1822L addresses form a subset of the 1822L

     name space, and have both bits 1 and 2 off.


                    1   2  3          8 9             16
                  +---+---+------------+----------------+
                  |   |   |            |                |
                  | 0 | 0 |   host #   |   IMP number   |
                  |   |   |            |                |
                  +---+---+------------+----------------+