rfc802.txt

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         RFC 802: The ARPANET 1822L Host Access Protocol








                         Andrew G. Malis
                     Netmail: malis@bbn-unix








                  Bolt Beranek and Newman Inc.








                          November 1981

















RFC 802                                           Andrew G. Malis



                        Table of Contents




1   INTRODUCTION.......................................... 1
2   THE ARPANET 1822L HOST ACCESS PROTOCOL................ 4
2.1   Addresses and Names................................. 6
2.2   Name Authorization and Effectiveness................ 8
2.3   Uncontrolled Messages.............................. 14
2.4   The Short-Blocking Feature......................... 15
2.4.1   Host Blocking.................................... 16
2.4.2   Reasons for Host Blockage........................ 19
2.5   Establishing Host-IMP Communications............... 22
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































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RFC 802                                           Andrew G. Malis



                             FIGURES




1822 Address Format....................................... 6
1822L Name Format......................................... 7
1822L Address Format...................................... 7
Communications between different host types.............. 13
Host-to-IMP 1822L Leader Format.......................... 27
NDM Message Format....................................... 30
IMP-to-Host 1822L Leader Format.......................... 35





































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RFC 802                                           Andrew G. Malis



1  INTRODUCTION


This document proposes two major changes to the  current  ARPANET

host  access  protocol.  The first change will allow hosts to use

logical addressing (i.e., host addresses that are independent  of

their  physical location on the ARPANET) to communicate with each

other, and the second will allow a host to shorten the amount  of

time  that  it  may  be  blocked  by  its IMP after it presents a

message to the network (currently,  the  IMP  can  block  further

input from a host for up to 15 seconds).


The new host access protocol is known as the ARPANET  1822L  (for

Logical)  Host  Access Protocol, and it represents an addition 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,  hosts  using  either  protocol can readily communicate

with each other (the IMPs handle the translation automatically).


The new option for shortening the host blocking timeout is called

the short-blocking feature, and it replaces the non-blocking host

interface described in section 3.7 of Report 1822.  This  feature

will  be  available  to  all  hosts  on  C/30  IMPs (see the next

paragraph), regardless of whether they  use  the  1822  or  1822L

protocol.



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RFC 802                                           Andrew G. Malis



There is one major restriction  to  the  new  capabilities  being

described.   Both  the  1822L  protocol  and  the  short-blocking

feature will be implemented on C/30 IMPs only, and will therefore

only be useable by hosts connected to C/30 IMPs, as the 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 address a host on  a  non-C/30  IMP.

However, the ARPANET will shortly be completely converted to C/30

IMPs, and at that time this  restriction  will  no  longer  be  a

problem.


I will try to keep my terminology consistent with  that  used  in

Report  1822, and will define new terms when they are first used.

Of course, familiarity with Report 1822 (section 3 in particular)

is assumed.


This document  makes  many  references  to  Report  1822.   As  a

convenient  abbreviation,  I  will  use  "see 1822(x)" instead of

"please refer to Report 1822, section x, for further details".


This document is a proposal, not a description of an  implemented

system.   Thus,  described  features  are subject to change based

upon responses to this  document  and  restrictions  that  become

evident  during  implementation.   However,  any such changes are

expected to be minor.  A new RFC will be made available once  the



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RFC 802                                           Andrew G. Malis



implementation  is  complete containing the actual as-implemented

description.


Finally, I would like to thank Dr. Eric C. Rosen, who wrote  most

of section 2.4, and James G. Herman, Dr. Paul J. Santos Jr., John

F.  Haverty, and Robert M. Hinden, all of  BBN,  who  contributed

many of the ideas found herein.





































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RFC 802                                           Andrew G. Malis



2  THE ARPANET 1822L HOST ACCESS PROTOCOL


The ARPANET  1822L  Host  Access  Protocol,  which  replaces  the

ARPANET  1822  Host  Access  Protocol  described  in Report 1822,

sections 3.3 and 3.4, 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 And 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




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RFC 802                                           Andrew G. Malis



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

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

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 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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RFC 802                                           Andrew G. Malis



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

                 Figure 1. 1822 Address Format



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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RFC 802                                           Andrew G. Malis






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

                  Figure 2. 1822L Name Format



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

                 Figure 3. 1822L Address Format








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RFC 802                                           Andrew G. Malis



This format gives 1822L hosts the  ability  to  directly  address

hosts  0-59  at  IMPs 1-255 (IMP 0 does not exist).  Host numbers

60-63 are reserved for addressing the four  fake  hosts  at  each

IMP.




2.2  Name Authorization and Effectiveness


Every host on a C/30 IMP, regardless of whether it is  using  the

1822 or 1822L protocol to access the network, will be assigned at

least one 1822L name (logical address).  Other 1822L  hosts  will

use  this name to address the host, wherever it may be physically

located.  Because of the implementation constraints mentioned  in

the introduction, hosts on non-C/30 IMPs cannot be assigned 1822L

names.  To circumvent this restriction, however, 1822L hosts  can

use  1822L addresses to access all other hosts on the network, no

matter where they reside.


At this point, several questions  arise:   How  are  these  names

assigned,  how  do  they  become  known  to  the  IMPs  (so  that

translations to physical addresses can be made), and how  do  the

IMPs know which host is currently using a shared port?  To answer

each question in order:







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RFC 802                                           Andrew G. Malis



Names are assigned by a central network administrator.  When each

name  is  created, it is assigned to a host (or a group of hosts)

at one or more specific host ports.  The host(s) are  allowed  to

reside at those specific host ports, and nowhere else.  If a host

moves, it will keep the same name, but the administrator  has  to

update  the  central  database  to  reflect  the  new  host port.

Changes to this database are  distributed  to  the  IMPs  by  the

Network  Operations  Center  (NOC) at BBN.  For a while, the host

may be allowed to reside at either of (or both) the new  and  old

ports.   Once  the  correspondence between a name and one or more

hosts ports where it may be used has been made  official  by  the

administrator,   that  name  is  said  to  be  authorized.  1822L

addresses, which actually  refer  to  physical  host  ports,  are