rfc851.txt

RFC 的详细文档！

     Request for Comments: 851
     Obsoletes RFC: 802






                  The ARPANET 1822L Host Access Protocol



                                  RFC 851





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





                       Bolt Beranek and Newman Inc.
                              50 Moulton St.
                           Cambridge, MA  02238





                                April 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.  The RFC is also
     being  presented  as  a  solicitation  of  comments   on   1822L,
     especially   from   host   network   software   implementers  and
     maintainers.










     1822L Host Access Protocol                             April 1983
     RFC 851



                             Table of Contents




     1   INTRODUCTION.......................................... 1
     2   THE ARPANET 1822L HOST ACCESS PROTOCOL................ 4
     2.1   Addresses and Names................................. 6
     2.2   Name Translations................................... 8
     2.2.1   Authorization and Effectiveness................... 8
     2.2.2   Translation Policies............................. 11
     2.2.3   Reporting Destination Host Downs................. 13
     2.2.4   1822L and 1822 Interoperability.................. 16
     2.3   Uncontrolled Packets............................... 18
     2.4   Establishing Host-IMP Communications............... 20
     2.5   Counting RFMS When Using 1822L..................... 22
     2.6   1822L Name Server.................................. 24
     3   1822L LEADER FORMATS................................. 27
     3.1   Host-to-IMP 1822L Leader Format.................... 28
     3.2   IMP-to-Host 1822L Leader Format.................... 35
     4   REFERENCES........................................... 43





























                                   - i -



     1822L Host Access Protocol                             April 1983
     RFC 851



                                  FIGURES




     1822 Address Format....................................... 6
     1822L Name Format......................................... 7
     1822L Address Format...................................... 7
     Communications between different host types.............. 17
     Host-to-IMP 1822L Leader Format.......................... 28
     NDM Message Format....................................... 31
     IMP-to-Host 1822L Leader Format.......................... 35
     Name Server Reply Format................................. 39





































                                  - ii -



     1822L Host Access Protocol                             April 1983
     RFC 851



     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).


     There is one major restriction to the  new  1822L  protocol:   it

     will be implemented in C/30 IMPs only, and will therefore only be

     usable by hosts connected to C/30 IMPs, as Honeywell and Pluribus

     IMPs  do  not have sufficient memory to hold the new programs and

     tables.  This restriction  also  means  that  logical  addressing

     cannot  be used to identify a host on a non-C/30 IMP.  While this

     is not a problem on the ARPANET, which only has  C/30  IMPs,  the

     restriction  will  apply  if  logical  addressing  is used on any

     network that mixes C/30 and non-C/30 IMPs.





                                   - 1 -



     1822L Host Access Protocol                             April 1983
     RFC 851



     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 802.  The changes from  that

     RFC include:


     o The Short Blocking Feature, which had also  been  described  in

       RFC 802, now has its own RFC, RFC 852 [2].  It was moved to its

       own  RFC,  since  it  is  completely  independent  of   logical

       addressing.


     o In section 2.2, descriptions of  the  three  address  selection

       policies and of host error handling have been added.


     o In section 2.3, the IMP's uncontrolled packet service has  been

       further  improved.  This applies to hosts using 1822 as well as

       1822L.


     o Pointers on using RFNM counting with 1822L have been  added  as

       section 2.5.






                                   - 2 -



     1822L Host Access Protocol                             April 1983
     RFC 851



     o Section 2.6 describes the new "1822L name server" in  the  IMP,

       which  makes use of two new Host-to-IMP messages to allow hosts

       to do their own name-to-address mapping.


     o In section 3.2, the subtypes for the type  15  (1822L  Name  or

       Address Error) IMP-to-Host message have been changed.








































                                   - 3 -



     1822L Host Access Protocol                             April 1983
     RFC 851



     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) [3] 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



                                   - 4 -



     1822L Host Access Protocol                             April 1983
     RFC 851



     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.












                                   - 5 -



     1822L Host Access Protocol                             April 1983
     RFC 851



     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            |
           |                |                                 |
           +----------------+---------------------------------+