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RFC 0802

Pages: 45
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ARPANET 1822L Host Access Protocol

Obsoleted by:    0851

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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

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                        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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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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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


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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


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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implementation  is  complete containing the actual as-implemented


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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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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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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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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                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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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


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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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

always authorized in this sense.

Once a host has been assigned one or more names, it  has  to  let

the  IMPs  know  where it is and what name(s) it is using.  There

are two cases to consider, one for 1822L hosts  and  another  for

1822  hosts.   The following discussion only pertains to hosts on

C/30 IMPs.

When an IMP sees an 1822L host come up on a host  port,  the  IMP

has  no way of knowing which host has just come up (several hosts

may share the same port, or one host may prefer to  be  known  by

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different  names  at different times).  This requires the host to

let the IMP know what is happening before it  can  actually  send

and  receive messages.  This function is performed by a new host-

to-IMP message, the Name Declaration Message (NDM),  which  lists

the  names  that  the  host  would  like to be known by.  The IMP

checks its tables to see if each of the names is authorized,  and

sends an NDM Reply to the host saying which names in the list can

be used for sending and receiving messages (i.e., which names are

effective). A host can also use an NDM message to change its list

of effective addresses (it can add to and delete from  the  list)

at  any  time.  The only constraint on the host is that any names

it  wishes  to  use  can  become  effective  only  if  they   are


In the second case, if a host comes up on a C/30  IMP  using  the

1822 protocol, the IMP automatically makes the first name the IMP

finds in its tables for that host become effective.   Thus,  even

though  the host is using the 1822 protocol, it can still receive

messages from 1822L hosts via its 1822L name.  Of course, it  can

also receive messages from an 1822L host via its 1822L address as

well.   (Remember,  the  distinction  between  1822L  names   and

addresses  is that the addresses correspond to physical locations

on  the  network,  while   the   names   are   strictly   logical

identifiers).   The  IMPs translate between the different leaders

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and send the proper leader in each case (more on this below).

The third question above has by now already been answered.   When

an  1822L  host comes up, it uses the NDM message to tell the IMP

which host it is (which names it is known by).  Even if this is a

shared port, the IMP knows which host is currently connected.

Whenever a host goes down, its names  automatically  become  non-

effective.   When it comes back up, it has to make them effective


Several hosts can share the same 1822L name.  If more than one of

these  hosts  is  up  at the same time, any messages sent to that

1822L name will be delivered to just one  of  the  hosts  sharing

that  name,  and  a RFNM will be returned as usual.  However, the

sending host will  not  receive  any  indication  of  which  host

received  the  message,  and subsequent messages to that name are

not guaranteed to be sent to the  same  host.   Typically,  hosts

providing  exactly  the  same  service could share the same 1822L

name in this manner.

Similarly, when a host is multi-homed, the same  1822L  name  may

refer  to  more  than  one  host  port (all connected to the same

host).  If the host is up on only one of those ports,  that  port

will  be  used for all messages addressed to it.  However, if the

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host were up  on  more  than  one  port,  the  message  would  be

delivered  over  just  one  of  those ports, and the subnet would

choose which port to use.  This port selection could change  from

message  to  message.   If  a  host wanted to insure that certain

messages were delivered to it on specific ports,  these  messages

could  use  either  the  port's 1822L address or a specific 1822L

name that referred to that port alone.

Some further details are required on communications between  1822

and  1822L  hosts.   Obviously, when 1822 hosts converse, or when

1822L hosts converse, no conversions between leaders and  address

formats  are  required.   However,  this becomes more complicated

when 1822 and 1822L hosts converse with each other.

The   following   figure   illustrates   how   these   addressing

combinations  are  handled,  showing  how  each  type of host can

access every other type of host.  There are three types of hosts:

"1822  on  C/30"  signifies  an  1822 host that is on a C/30 IMP,

"1822L" signifies an 1822L host (on a C/30  IMP),  and  "1822  on

non-C/30"  signifies  a  host  on  an  non-C/30 IMP (which cannot

support the 1822L protocol).  The table entry shows the  protocol

and  host address format(s) that the source host can use to reach

the destination host.

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                            Destination Host
  Host    | 1822 on C/30   | 1822L          | 1822 on non-C/30
          |                |                |
  1822 on | 1822           | 1822           | 1822
  C/30    |                | (note 1)       |
          |                |                |
          |                |                |
          | 1822L, using   | 1822L, using   | 1822L, using
  1822L   | 1822L name or  | 1822L name or  | 1822L address
          |address (note 2)| address        | only (note 2)
          |                |                |
          |                |                |
  1822 on | 1822           | 1822           | 1822
  non-C/30|                | (note 1)       |
          |                |                |

  Note 1: The message is presented  to  the  destination  host
          with  an 1822L leader containing the 1822L addresses
          of the source  and  destination  hosts.   If  either
          address  cannot be encoded as an 1822L address, then
          the message is not delivered and and  error  message
          is sent to the source host.

  Note 2: The message is presented  to  the  destination  host
          with  an  1822 leader containing the 1822 address of
          the source host.

     Figure 4. Communications between different host types

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2.3  Uncontrolled Messages

Uncontrolled messages (see 1822(3.6)) present  a  unique  problem

for  the  1822L  protocol.  Uncontrolled messages use none of the

normal ordering and error-control mechanisms in the IMP,  and  do

not  use  the  normal  subnetwork  connection  facilities.   As a

result, uncontrolled messages need to carry all of their overhead

with  them, including source and destination addresses.  If 1822L

addresses  are  used  when  sending  an   uncontrolled   message,

additional information is now required by the subnetwork when the

message is transferred to the destination IMP.  This  means  that

less  host-to-host  data  can be contained in the message than is

possible between 1822 hosts.

Uncontrolled messages  that  are  sent  between  1822  hosts  may

contain  not  more  than 991 bits of data.  Uncontrolled messages

that are sent to and/or from 1822L hosts are limited to  32  bits

less,  or  not  more  than  959  bits.  Messages that exceed this

length will result in an error indication to the  host,  and  the

message  will  not  be sent.  This error indication represents an

enhancement to the previous level of service provided by the IMP,

which  would  simply  discard an overly long uncontrolled message

without notification.

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Other enhancements that are  provided  for  uncontrolled  message

service  are  a  notification  to the host of any message-related

errors that are detected by the host's IMP when it  receives  the

message.   A  host  will  be  notified if an uncontrolled message

contains an error in the 1822L name specification,  such  as  the

name  not being authorized or effective, or if the remote host is

unreachable (which is  indicated  by  none  of  its  names  being

effective),  or  if  network  congestion  control  throttled  the

message before it left the source IMP.   The  host  will  not  be

notified  if  the  uncontrolled  message was lost for some reason

once it was transmitted by the source IMP.

2.4  The Short-Blocking Feature

The short-blocking feature of the 1822  and  1822L  protocols  is

designed  to  allow a host to present messages to the IMP without

causing the IMP to not accept further messages from the host  for

long amounts of time (up to 15 seconds).  It is a replacement for

the non-blocking host interface described in 1822(3.7), and  that

description should be ignored.

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2.4.1  Host Blocking

Most commonly, when a source host submits a message  to  an  IMP,

the  IMP  immediately  processes that message and sends it on its

way to its destination host.  Sometimes, however, the IMP is  not

able  to  process  the message immediately.  Processing a message

requires a significant number of resources, and when the  network

is heavily loaded, there can sometimes be a long delay before the

necessary resources become available.  In  such  cases,  the  IMP

must  make  a decision as to what to do while it is attempting to

gather the resources.

One possibility is for the IMP to stop  accepting  messages  from

the  source  host  until  it has gathered the resources needed to

process the message just submitted.  This strategy  is  known  as

blocking  the  host,  and is basically the strategy that has been

used in the ARPANET up to the present.  When  a  host  submits  a

message  to  an  IMP, all further transmissions from that host to

that IMP are blocked until the message can be processed.

It is important to note, however, that not all  messages  require

the  same  set  of resources in order to be processed by the IMP.

The particular set of resources needed will depend on the message

type, the message length, and the destination host of the message

(see below).  Therefore, although it might take a  long  time  to

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gather  the  resources needed to process some particular message,

it might take only a short time to gather the resources needed to

process  some  other  message.   This  fact exposes a significant

disadvantage in the strategy of blocking the host.  A host  which

is  blocked may have many other messages to submit which, if only

they could be submitted, could be processed immediately.   It  is

"unfair"  for  the IMP to refuse to accept these message until it

has gathered the resources for  some  other,  unrelated  message.

Why  should messages for which the IMP has plenty of resources be

delayed for an arbitrarily long amount of time just  because  the

IMP lacks the resources needed for some other message?

A simple way to alleviate the problem would be to place  a  limit

on  the  amount of time during which a host can be blocked.  This

amount  of  time  should  be  long  enough  so  that,   in   most

circumstances,  the  IMP  will  be  able  to gather the resources

needed to process the message within the given time period.   If,

however, the resources cannot be gathered in this period of time,

the IMP will flush the message, sending a  reply  to  the  source

host   indicating   that  the  message  was  not  processed,  and

specifying the reason that it could not be  processed.   However,

the  resource gathering process would continue.  The intention is

that the host  resubmit  the  message  in  a  short  time,  when,

hopefully,   the   resource   gathering   process  has  concluded

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successfully.   In  the  meantime,  the  host  can  submit  other

messages,  which may be processed sooner.  This strategy does not

eliminate the phenomenon of host blocking, but  only  limits  the

time  during  which  a  host is blocked.  This shorter time limit

will generally fall somewhere in the range of 100 milliseconds to

2  seconds,  with  its value possibly depending on the reason for

the blocking.

Note, however, that there  is  a  disadvantage  to  having  short

blocking  times.  Let us say that the IMP accepts a message if it

has all the resources needed to process it.  The ARPANET provides

a  sequential  delivery  service,  whereby messages with the same

priority, source host, and destination host are delivered to  the

destination  host in the same order as they are accepted from the

source host.  With short blocking times, however,  the  order  in

which  the  IMP accepts messages from the source host need not be

the same as  the  order  in  which  the  source  host  originally

submitted  the messages.  Since the two data streams (one in each

direction) between the host and the IMP are not synchronized, the

host  may  not  receive the reply to a rejected message before it

submits subsequent messages of the same  priority  for  the  same

destination host.  If a subsequent message is accepted, the order

of acceptance differs from the order of original submission,  and

the ARPANET will not provide the same type of sequential delivery

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that it has in the past.

Up to now, type 0 (regular)  messages  have  only  had  sub-types

available  to  request the standard blocking timeout.  The short-

blocking feature makes available new  sub-types  that  allow  the

host  to  request  messages to be short-blocking, i.e. only cause

the host to be blocked for a short amount of time if the  message

cannot be immediately processed.   See section 3.1 for a complete

list of the available sub-types.

If sequential delivery by the subnet is a strict requirement,  as

would  be  the  case  for  messages  produced  by NCP, the short-

blocking feature cannot be used.  For messages produced  by  TCP,

however,  the  use  of  the short-blocking feature is allowed and


2.4.2  Reasons for Host Blockage

There are a number of reasons why a message could  cause  a  long

blockage  in  the  IMP,  which would result in the rejection of a

short-blocking message.  The IMP  signals  this  rejection  of  a

short-blocking message by using the Incomplete Transmission (Type

9) message, using the sub-type field to  indicate  which  of  the

above  reasons  caused the rejection of the message.  See section

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3.2 for a summary of the Incomplete Transmission  message  and  a

complete  list of its sub-types.  The sub-types that apply to the

short-blocking feature are:

6.  Connection setup-delay: Although the IMP  presents  a  simple

    message-at-a-time  interface  to  the  host,  it  provides an

    internal  connection-oriented  (virtual   circuit)   service,

    except  in  the  case  of  uncontrolled messages (see section

    2.3).   Two  messages  are  considered  to  be  on  the  same

    connection  if they have the same source host (i.e., they are

    submitted to the same IMP over the same host interface),  the

    same priority, and the same destination host name or address.

    The subnet maintains internal connection set-up and tear-down

    procedures.   Connections  are set up as needed, and are torn

    down  only  after  a  period  of  inactivity.   Occasionally,

    network  congestion or resource shortage will cause a lengthy

    delay in connection set-up.  During this period, no  messages

    for  that  connection can be accepted, but other messages can

    be accepted.

7.  End-to-end flow  control:  For  every  message  that  a  host

    submits  to  an  IMP  (except  uncontrolled messages) the IMP

    eventually  returns  a  reply  to  the  host  indicating  the

    disposition  of  the  message.   Between  the  time  that the

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    message is submitted and  the  time  the  host  receives  the

    reply,  the  message  is  said to be outstanding. The ARPANET

    allows  only  eight  outstanding  messages   on   any   given

    connection.   If  there  are  eight outstanding messages on a

    given connection, and a ninth is  submitted,  it  cannot  the

    accepted.  If  a message is refused because its connection is

    blocked due to flow control, messages  on  other  connections

    can still be accepted.

    End-to-end flow control is the  most  common  cause  of  host

    blocking in the ARPANET at present.

8.  Destination IMP buffer space shortage: If the host submits  a

    message  of  more  than  1008  bits  (exclusive of the 96-bit

    leader), buffer space at the destination IMP must be reserved

    before  the  message  can  be  accepted.  Buffer space at the

    destination IMP is always reserved on a per-connection basis.

    If  the  destination  IMP  is  heavily loaded, there may be a

    lengthy wait for the buffer space;  this  is  another  common

    cause  of  blocking  in  the  present  ARPANET.  Messages are

    rejected  for  this  reason  based  on   their   length   and

    connection;  messages  of  1008 or fewer bits or messages for

    other connections may still be acceptable.

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9.  Congestion control: A message may be refused for  reasons  of

    congestion  control if the path via the intermediate IMPs and

    lines to the destination IMP is too heavily loaded to  handle

    additional  traffic.   Messages  to other destinations may be

    acceptable, however.

10.  Local resource shortage: Sometimes the source IMP itself  is

    short  of buffer space, table entries, or some other resource

    that it needs to accept a message.  Unlike the other  reasons

    for message rejection, this resource shortage will affect all

    messages equally,  except  for  uncontrolled  messages.   The

    message's size or connection is not relevant.

The short-blocking feature is available  to  all  hosts  on  C/30

IMPs,  whether they are using the 1822 or 1822L protocol, through

the use of Type 0, sub-type 1 and 2 messages.  A host using these

sub-types  should  be  prepared  to  correctly  handle Incomplete

Transmission messages from the IMP.

2.5  Establishing Host-IMP Communications

When a host comes up on an IMP, or after there has been  a  break

in   the  communications  between  the  host  and  its  IMP  (see

1822(3.2)), the orderly flow of messages between the host and the

Top       Page 26 
IMP  needs  to  be properly (re)established.  This allows the IMP

and host to recover from most any failure  in  the  other  or  in

their communications path, including a break in mid-message.

The first messages that a host should send to its IMP  are  three

NOP  messages.   Three  messages  are  required to insure that at

least one message will be properly read by the IMP (the first NOP

could be concatenated to a previous message if communications had

been broken in mid-stream, and the third provides redundancy  for

the   second).    These   NOPs   serve  several  functions:  they

synchronize the IMP with the host, they tell  the  IMP  how  much

padding  the  host  requires  between  the message leader and its

body, and they also tell the IMP whether the host will  be  using

1822 or 1822L leaders.

Similarly, the IMP will send three  NOPs  to  the  host  when  it

detects  that  the host has come up.  Actually, the IMP will send

six NOPs, alternating three 1822  NOPs  with  three  1822L  NOPs.

Thus, the host will see three NOPs no matter which protocol it is

using.   The  NOPs  will  be  followed  by  two  Interface  Reset

messages,  one of each style.  If the IMP receives a NOP from the

host while the above sequence is occurring,  the  IMP  will  only

send  the  remainder  of  the NOPs and the Interface Reset in the

proper style.  The 1822 NOPs will contain the 1822 address of the

Top       Page 27 
host interface, and the 1822L NOPs will contain the corresponding

1822L address.

Once the IMP  and  the  host  have  sent  each  other  the  above

messages, regular communications can commence.  See 1822(3.2) for

further details concerning the ready line,  host  tardiness,  and

other issues.

Top       Page 28 

The following sections describe the formats of the  leaders  that

precede  messages  between  an 1822L host and its IMP.  They were

designed to be as compatible with the 1822 leaders  as  possible.

The  second,  fifth,  and  sixth  words  are identical in the two

leaders, and all  of  the  existing  functionality  of  the  1822

leaders has been retained.  The first difference one will note is

in the first word.  The 1822 New Format Flag is now also used  to

identify  the  two  types of 1822L leaders, and the Handling Type

has been moved to the second byte.  The third  and  fourth  words

contain the Source and Destination 1822L Name, respectively.

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3.1  Host-to-IMP 1822L Leader Format

               1      4 5      8 9             16
              |        |  1822L |                |
              | Unused |  H2I   | Handling Type  |
              |        |  Flag  |                |
               17    20 21 22 24 25            32
              |        |T|Leader|                |
              | Unused |R|Flags |  Message Type  |
              |        |C|      |                |
               33                              48
              |                                  |
              |           Source Host            |
              |                                  |
               49                              64
              |                                  |
              |         Destination Host         |
              |                                  |
               65                     76 77    80
              |                         |        |
              |       Message ID        |Sub-type|
              |                         |        |
               81                              96
              |                                  |
              |              Unused              |
              |                                  |

           Figure 5. Host-to-IMP 1822L Leader Format

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Bits 1-4: Unused, must be set to zero.

Bits 5-8: 1822L Host-to-IMP Flag:

     This field is set to decimal 13 (1101 in binary).

Bits 9-16: Handling Type:

     This  field  is  bit-coded  to  indicate  the   transmission

     characteristics  of  the connection desired by the host. See


     Bit 9: Priority Bit:

          Messages with this bit on will be treated  as  priority


     Bits 10-16: Unused, must be zero.

Bits 17-20: Unused, must be zero.

Bit 21: Trace Bit:

     If equal to one, this message is designated for  tracing  as

     it proceeds through the network.  See 1822(5.5).

Bits 22-24: Leader Flags:

     Bit 22: A flag available for use by  the  destination  host.

          See 1822(3.3) for a description of its use by the IMP's

          TTY fake host.

     Bits 23-24: Reserved for future use, must be zero.

Top       Page 31 
Bits 25-32: Message Type:

     Type 0: Regular Message  -  All  host-to-host  communication

          occurs  via  regular  messages, which have several sub-

          types, found in bits 77-80.  These sub-types are:

          0: Standard - The IMP uses its full message  and  error

               control facilities, and host blocking (see section

               2.4) may occur.

          1: Standard, short-blocking - See section 2.4.

          2: Uncontrolled, short-blocking - See section 2.4.

          3: Uncontrolled - The  IMP  will  perform  no  message-

               control  functions  for  this type of message, and

               network flow and congestion control  (see  section

               2.4)  may  cause  loss  of  the message.  Also see

               1822(3.6) and section 2.3.

          4-15: Unassigned.

     Type 1: Error Without Message ID - See 1822(3.3).

     Type 2: Host Going Down - see 1822(3.3).

     Type 3: Name Declaration Message (NDM)  -  This  message  is

          used by the host to declare which of its 1822L names is

          or is not effective (see section 2.2), or to  make  all

          of  its  names non-effective.  The first 16 bits of the

          data portion of the NDM message, following  the  leader

          and  any  padding,  contains  the  number of 1822L name

Top       Page 32 
          entries contained in the message.  This is followed  by

          the 1822L name entries, each 32 bits long, of which the

          first 16 bits is a 1822L name and the  second  16  bits

          contains  either  of  the  integers  zero or one.  Zero

          indicates that the name should not  be  effective,  and

          one  indicates  that the name should be effective.  The

          IMP will reply with a NDM Reply  message  (see  section

          3.2)  indicating  which  of the names are now effective

          and which are not.  Pictorially, a NDM message has  the

          following   format  (including  the  leader,  which  is

          printed in hexadecimal):

Top       Page 33 
            1             16 17            32 33            48
           |                |                |                |
           |      0D00      |      0003      |      0000      |
           |                |                |                |
            49            64 65            80 81            96
           |                |                |                |
           |      0000      |      0000      |      0000      |
           |                |                |                |
            97           112 113          128 129          144
           |                |                |                |
           |  # of entries  |  1822L name #1 |     0 or 1     |
           |                |                |                |
           145           160 161          176
           |                |                |
           |  1822L name #2 |     0 or 1     |       etc.
           |                |                |

                  Figure 6. NDM Message Format

          An  NDM  with  zero  entries  will  cause  all  current

          effective names for the host to become non-effective.

     Type 4: NOP - This allows the IMP to  know  which  style  of

          leader  the  host wishes to use.  A 1822L NOP signifies

          that the host wishes to use 1822L leaders, and an  1822

          NOP signifies that the host wishes to use 1822 leaders.

          All of the other remarks concerning the NOP message  in

Top       Page 34 
          1822(3.3)  still  hold.   The  host should always issue

          NOPs in groups of three to insure proper  reception  by

          the IMP.  Also see section 2.5 for a further discussion

          on the use of the NOP message.

     Type 8: Error with Message ID - see 1822(3.3).

     Types 5-7,9-255: Unassigned.

Bits 33-48: Source Host:

     This field contains one of the  source  host's  1822L  names

     (or,  alternatively,  the 1822L address of the host port the

     message  is  being  sent   over).    This   field   is   not

     automatically filled in by the IMP, as in the 1822 protocol,

     because the host may be known by several names and may  wish

     to use a particular name as the source of this message.  All

     messages from the same host need not use the  same  name  in

     this  field.   Each  source  name, when used, is checked for

     authorization, effectiveness, and actually belonging to this

     host.  Messages using names that do not satisfy all of these

     requirements will not be delivered, and will instead  result

     in  an  error  message being sent back into the source host.

     If the host places its 1822L  Address  in  this  field,  the

     address is checked to insure that it actually represents the

     host port where the message originated.  If the  message  is

     destined for an 1822 host on a non-C/30 IMP, this field MUST

Top       Page 35 
     contain the source host's 1822L address  (see  Figure  4  in

     section 2.2).

Bits 49-64: Destination Host:

     This field  contains  the  1822L  name  or  address  of  the

     destination  host.   If it contains a name, the name will be

     checked for effectiveness, with an error message returned to

     the  source  host  if  the  name  is  not effective.  If the

     message is destined for an 1822 host on a non-C/30 IMP, this

     field MUST contain the destination host's 1822L address (see

     Figure 4 in section 2.2).

Bits 65-76: Message ID:

     This is a host-specified identification used in all  type  0

     and  type  8  messages, and is also used in type 2 messages.

     When used in type 0 messages, bits 65-72 are also  known  as

     the  Link  Field,  and  should  contain  values specified in

     Assigned  Numbers  [3]  appropriate  for  the   host-to-host

     protocol being used.

Bits 77-80: Sub-type:

     This field is used as a modifier by message types 0,  2,  4,

     and 8.

Top       Page 36 
Bits 81-96: Unused, must be zero.

Top       Page 37 
3.2  IMP-to-Host 1822L Leader Format

               1      4 5      8 9             16
              |        |  1822L |                |
              | Unused |  I2H   | Handling Type  |
              |        |  Flag  |                |
               17    20 21 22 24 25            32
              |        |T|Leader|                |
              | Unused |R|Flags |  Message Type  |
              |        |C|      |                |
               33                              48
              |                                  |
              |           Source Host            |
              |                                  |
               49                              64
              |                                  |
              |         Destination Host         |
              |                                  |
               65                     76 77    80
              |                         |        |
              |       Message ID        |Sub-type|
              |                         |        |
               81                              96
              |                                  |
              |          Message Length          |
              |                                  |

           Figure 7. IMP-to-Host 1822L Leader Format

Top       Page 38 
Bits 1-4: Unused and set to zero.

Bits 5-8: 1822L IMP-to-Host Flag:

     This field is set to decimal 14 (1110 in binary).

Bits 9-16: Handling Type:

     This has the value assigned by the source host (see  section

     3.1).   This  field is only used in message types 0, 5-9, 11

     and 15.

Bits 17-20: Unused and set to zero.

Bit 21: Trace Bit:

     If equal to one, the source host designated this message for

     tracing as it proceeds through the network.  See 1822(5.5).

Bits 22-24: Leader Flags:

     Bit 22: Available as a destination host flag.

     Bits 23-24: Reserved for future use, set to zero.

Bits 25-32: Message Type:

     Type 0: Regular Message  -  All  host-to-host  communication

          occurs  via  regular  messages, which have several sub-

          types.  The sub-type field (bits 77-80) is the same  as

          sent in the host-to-IMP leader (see section 3.1).

     Type 1: Error in Leader - See 1822(3.4).

Top       Page 39 
     Type 2: IMP Going Down - See 1822(3.4).

     Type 3: NDM Reply - This is a reply to the  NDM  host-to-IMP

          message  (see  section  3.1).   It  will  have the same

          number of entries as the  NDM  message  that  is  being

          replying  to,  and  each  listed  1822L  name  will  be

          accompanied by a zero or a one.  A zero signifies  that

          the  name  is  not  effective, and a one means that the

          name is now effective.

     Type 4: NOP - The host should discard this message.   It  is

          used    during    initialization    of   the   IMP/host

          communication.  The Destination Host field will contain

          the  1822L  Address of the host port over which the NOP

          is being sent.  All other fields are unused.

     Type 5: Ready for Next Message (RFNM) - See 1822(3.4).

     Type 6: Dead Host Status - See 1822(3.4).

     Type 7: Destination Host or IMP Dead  (or  unknown)  -  This

          message  is  sent  in  response  to  a  message  for  a

          destination which the IMP cannot reach.  The message to

          the "dead" destination is discarded.  See 1822(3.4) for

          a complete list of the applicable sub-types.   If  this

          message  is in response to a standard (type 0, sub-type

          0 or 1) message, it will be followed  by  a  Dead  Host

          Status  message,  which gives further information about

Top       Page 40 
          the status of the dead host.  If  this  message  is  in

          response  to  an uncontrolled (type 0, sub-type 2 or 3)

          message, only sub-type 1 (The destination host  is  not

          up) will be used, and it will not be followed by a Dead

          Host Status message.

     Type 8: Error in Data - See 1822(3.4).

     Type 9: Incomplete Transmission - The  transmission  of  the

          named  message  was  incomplete  for  some  reason.  An

          incomplete transmission message is similar to  a  RFNM,

          but  is  a  failure  indication  rather  than a success

          indication.  This message is also used  by  the  short-

          blocking feature to indicate that the named message was

          rejected because it would have caused to IMP  to  block

          the  host  for  a long amount of time.  See section 2.4

          for more details concerning the short-blocking feature.

          The message's sub-types are:

          0: The destination host  did  not  accept  the  message

               quickly enough.

          1: The message was too long.

          2: The host took more than 15 seconds to  transmit  the

               message  to  the  IMP.  This time is measured from

               the last bit of the leader through the last bit of

               the message.

Top       Page 41 
          3: The message was lost in the network due  to  IMP  or

               circuit failures.

          4: The IMP could not accept the entire  message  within

               15 seconds because of unavailable resources.  This

               sub-type is only used in  response  to  non-short-

               blocking  messages.   If  a short-blocking message

               timed out, it will be responded to with one of the

               sub-types 6-10.

          5: Source IMP I/O failure occurred  during  receipt  of

               this message.

          Sub-types 6-10 are all issued in response to  a  short-

          blocking  message that timed out (would have caused the

          host to become blocked for a long amount of time).  The

          sub-types are designed to give the host some indication

          of why it timed out and what other messages would  also

          time  out.   See  section  2.4.2  for  further  details

          concerning each of these sub-types.

          6: The message timed out because of  connection  set-up

               delay.   Further  messages to the same host (if on

               the same connection) may also be affected.

          7: The message timed out  because  of  end-to-end  flow

               control.  Further messages to the same host on the

               same connection will also be affected.

Top       Page 42 
          8: Destination IMP buffer shortage caused  the  message

               to  time  out.  This affects multi-packet standard

               messages  to  the  specified  host,  but   shorter

               messages  or  messages  to hosts on other IMPs may

               not be affected.

          9: Network congestion control caused the message to  be

               rejected.  Messages to hosts on other IMPs may not

               be affected, however.

          10: Local resource shortage kept  the  IMP  from  being

               able  to  accept  the  message  within  the short-

               blocking timeout period.

          11-15: Unassigned.

     Type 10: Interface Reset - See 1822(3.4).

     Type 15: 1822L Name or Address Error - This message is  sent

          in  response  to  a  type  0  message  from a host that

          contained an erroneous Source Host or Destination  Host

          field.  Its sub-types are:

          0: The Source Host 1822L name is not authorized or  not


          1: The Source Host 1822L address  does  not  match  the

               host port used to send the message.

          2: The Destination Host 1822L name is not authorized.

          3: The Destination Host 1822L name  is  authorized  but

Top       Page 43 
               not  effective,  even though the named host is up.

               If the host were actually down, a type  7  message

               would be returned, not a type 15.

          4: The Source or  Destination  Host  field  contains  a

               1822L  name,  but the host being addressed is on a

               non-C/30 IMP (see Figure 4 in section 2.2).

          5-15: Unassigned.

     Types 11-14,16-255: Unassigned.

Bits 33-48: Source Host:

     For type 0 messages, this field contains the 1822L  name  or

     address  of  the  host  that  originated  the  message.  All

     replies to the message should be sent to the host  specified

     herein.   For  message  types  5-9,  11  and  15, this field

     contains the source host field used in  a  previous  type  0

     message sent by this host.

Bits 49-64: Destination Host:

     For type 0 messages, this field contains the 1822L  name  or

     address  that  the  message  was  sent  to.  This allows the

     destination host to detect  how  it  was  specified  by  the

     source  host.   For message types 5-9, 11 and 15, this field

     contains the destination host field used in a previous  type

     0 message sent by this host.

Top       Page 44 
Bits 65-76: Message ID:

     For message types 0, 5, 7-9, 11 and 15, this  is  the  value

     assigned  by  the  source  host to identify the message (see

     section 3.1).  This field is also used by  message  types  2

     and 6.

Bits 77-80: Sub-type:

     This field is used as a modifier by message types 0-2,  4-7,

     9, 11 and 15.

Bits 81-96: Message Length:

     This field is contained in type 0 and type 3 messages  only,

     and  is  the actual length in bits of the message (exclusive

     of leader, leader padding, and hardware padding) as computed

     by the IMP.

Top       Page 45 

[1]  Specifications for the Interconnection of a Host and an IMP,

     BBN Report 1822, May 1978 Revision.

[2]  E. C. Rosen et. al., ARPANET Routing Algorithm Improvements,

     IEN  183 (also published as BBN Report 4473, Vol. 1), August

     1980, pp. 55-107.

[3]  J. Postel, Assigned Numbers, RFC 790, September 1981, p. 10.

Top       Page 46 

1822...................................................... 4
1822 address.............................................. 6
1822 host................................................. 5
1822L..................................................... 4
1822L address............................................. 7
1822L host................................................ 5
1822L name................................................ 6
authorized................................................ 9
blocking................................................. 16
congestion control................................... 22, 39
connection........................................... 20, 38
destination host..................................... 32, 40
effective................................................ 10
flow control......................................... 20, 38
handing type......................................... 27, 35
incomplete transmission message...................... 19, 37
leader flags......................................... 27, 35
link field............................................... 32
logical addressing........................................ 4
message ID........................................... 32, 41
message length........................................... 41
message type......................................... 28, 35
multi-homing.............................................. 4
NDM.................................................. 10, 28
NDM reply............................................ 10, 36
NOC....................................................... 9
NOP........................................... 5, 22, 30, 36
outstanding.............................................. 21
priority bit............................................. 27
regular message...................................... 28, 35
RFNM..................................................... 36
short-blocking feature................................... 15
short-blocking message............................... 19, 28
source host.......................................... 31, 40
standard message......................................... 28
sub-type............................................. 32, 41
symmetric................................................. 5
trace bit............................................ 27, 35
uncontrolled message................................. 14, 28