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RFC 1005 - ARPANET AHIP-E Host Access Protocol (enhanced AHIP)


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Network Working Group                            Atul Khanna, Andy Malis
Request for Comments: 1005                      BBN Communications Corp.
                                                                May 1987

        The ARPANET AHIP-E Host Access Protocol (Enhanced AHIP)

1. Status of this Memo

   This RFC is a proposed specification for the encoding of Class A
   IP addresses for use on ARPANET-style networks such as the Milnet
   and Arpanet, and for enhancements to the ARPANET AHIP Host Access
   Protocol (AHIP; formerly known as 1822).  These enhancements
   increase the size of the PSN field, allow ARPANET hosts to use
   logical names to address each other, allow for the communication
   of type-of-service information from the host to the PSN and
   enable the PSN to provide congestion feedback to the host on a
   connection basis. Distribution of this memo is unlimited.
   Comments on this RFC should be sent to the netmail address
   "ahipe@bbn.com".

                             Table of Contents

        1   INTRODUCTION.......................................... 4

        2   IP ISSUES............................................. 6
        2.1   Current Interpretation of Class A IP Address
              Fields
               ................................................... 6
        2.2   Requirements and Constraints Affecting New
              Class A Mapping
               ................................................... 7
        2.3   New Interpretation of IP Address Fields............. 8
        2.4   Discussion of the New Mapping.......................10
        2.5   Interoperability between Current AHIP and
              AHIP-E
               ...................................................11

        3   LOGICAL ADDRESSING................................... 13
        3.1   Addresses and Names................................ 13
        3.2   Name Translations.................................. 14
        3.2.1   Authorization and Effectiveness.................. 15
        3.2.2   Translation Policies............................. 16
        3.2.3   Reporting Destination Host Downs................. 17
        3.3   Establishing Host-PSN Communications............... 18
        3.4   Name Server........................................ 19

        4   OTHER CHANGES........................................ 20
        4.1   Type-of-Service Specification...................... 20
        4.2   Subnet Congestion Feedback......................... 21
        4.3   Precedence Level Information....................... 21

        5   FORMATS FOR NEW AHIP-E MESSAGES...................... 23
        5.1   Host-to-PSN AHIP-E Leader Format................... 23
        5.2   PSN-to-Host AHIP-E Leader Format................... 27

        6   AHIP-E VERSIONS...................................... 33

        7   REFERENCES........................................... 34

                                  FIGURES

        2.1  IP Class A Mapping................................... 6
        2.2  New Class A IP Address Interpretation................ 8
        2.3  AHIP-E Address and Name.............................. 9
        3.1  Current AHIP Address Format......................... 13
        3.2  AHIP-E Address Format............................... 14
        3.3  Logical Name Format................................. 14
        5.1  Host-to-PSN AHIP-E Leader Format.................... 23
        5.2  NDM Message Format.................................. 25
        5.3  PSN-to-Host AHIP-E Leader Format.................... 27
        5.4  Name Server Reply Format............................ 30

1  INTRODUCTION

This RFC is a proposed specification for the encoding of Class A
IP addresses for use on ARPANET-style networks such as the Milnet
and Arpanet, and for enhancements to the AHIP Protocol (AHIP is the
preferred term for what has previously been known as the 1822
protocol).  These enhancements and modifications are partially
motivated by a need to overcome the current address limitation
of 256 PSNs per network and by a desire to allow hosts to take
advantage of logical addressing with minimal change to their AHIP
software. This enhanced AHIP protocol will be referred to as
"AHIP-E".  These enhancements will:

    1.   Increase the size of the PSN field to 10 bits.

    2.   Allow hosts to use logical names (i.e., host names that are
         independent of physical location on the network) in addition to
         physical port addresses to communicate with each other.

    3.   Enable the host to specify a type-of-service to the PSN.

    4.   Provide a mechanism for the PSN to communicate subnetwork
         congestion information to the host on a destination host basis.
         This will give the host an opportunity to selectively reduce
         its congesting flows, thus preventing all of its flows from
         being blocked b y the network.  Currently, a host has no way of
         knowing which of its flows is experiencing congestion;
         consequently, it is possible that one congesting flow can
         result in the blocking of all the host's flows .

    5.   Enable the PSN to inform the host about changes in precedence
         cutoff levels and about precedence level violations.

A host can take advantage of the extended and logical addressing
capabilities without making substantial changes to its AHIP
implementation.  In particular, the specification provides three
versions of AHIP-E: version 0 is current AHIP with no changes; version 1
allows use of logical and extended addressing with minimal change to
code; version 2 constitutes full-fledged AHIP-E.  This is described in
further detail in chapter 6.

This RFC's terminology is consistent with that used in BBN Report 1822
[1], and any new terms are 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".

The rest of this RFC is organized as follows.  Chapter 2 describes the
new mapping between IP class A addresses and subnetwork hosts.  Chapter
3 discusses logical addressing.  Chapter 4 describes the enhancements
related to type-of-service and reliability specification and to
congestion and precedence feedback.  Chapter 5 includes a specification
of the new message types and their formats.  Finally, chapter 6
describes the AHIP-E version numbering scheme.

2  IP ISSUES

This section discusses the changes to the mapping between Class A IP
addresses [5] and subnet addresses.  These changes are made necessary
by:

      1.  The introduction of logical names.

      2.  The expansion of the PSN-number field.

Note that this RFC does not affect Class B and C mappings [5].

2.1  Current Interpretation of Class A IP Address Fields

Class A IP addresses are 32 bits in length, with 8 bits devoted to
network number and 24 to the local address.  In particular, they are of
the form n.h.l.i, where n,h,l and i are decimal integers less than 256.
AHIP addresses are 24 bits in length.  The current ARPANET-style class A
mapping is as follows (from RFC 796):

             0       7 8     15 16   23 24     31
             +--------+--------+-------+---------+
             | net #  |  HOST  |   LH  | PSN     |   IP Address
             +--------+--------+-------+---------+
                 8        8         8      8

                      8          8        8
              +--------+--------+--------+
              |  HOST  |  ZERO  | PSN    |  AHIP Physical Address
              +--------+--------+--------+
              41     48 49    56 57    64
            (bit positions in the AHIP leader)

                            IP Class A Mapping
                                Figure 2.1

The LH (logical host) field is used by the hosts only and is not passed
to the network.

2.2  Requirements and Constraints Affecting New Class A Mapping

This section discusses some of the requirements and constraints that
were considered significant in determining the new address mapping.

     1.   Address Mapping Stability Requirement:

          Any current IP physical address with l (logical host) = 0
          should remain unchanged under the new design.  For example,
          the binary string corresponding to 10.0.0.51 should continue
          to refer to sri-nic.arpa (assuming, of course, that sri-nic
          continues to reside on psn 51, port 0).  This requirement is
          motivated by a desire to avoid a network-wide address
          switchover.

     2.   Existing implementation compatibility:

          Existing compliant implementations of AHIP should continue to
          function for destinations with addresses fitting the
          restrictions in 1.  In other words, such addresses should
          continue to refer to their original destinations, not only
          with the AHIP-E implementation (which is the condition in 1),
          but also with current ones.

     3.   Compatibility between X.25's IP address to subnet host mapping
          and AHIP's IP address to subnet host mapping:

          The AHIP-E IP to host mapping should be able to co-exist in
          some sense with the IP to host mapping specified by the DDN
          X.25 Specification [6].  In particular, restricted use of the
          revised IP to DDN host mapping should produce addresses that
          are consistent with the current X.25 mapping.  In other words,
          there should be a set that includes "sufficiently many"
          logical names and physical addresses, with the property that
          each address/name in the set maps onto the same host under
          both the AHIP and X.25 mappings.

     4.   Maximum number of PSNs that can be supported:

          The new design should support a maximum of more than 256 PSNs
          per network.

2.3  New Interpretation of IP Address Fields

The following is the new interpretation of the IP address field, in the
context of ARPANET-style networks:

            Proposed IP Address Interpretation

              8        8     1   5       10
          +--------+--------+-+-----+----------+
          | net #  |  HOST  |0|XXXXX|   PSN    |  Physical Address
          +--------+--------+-+-----+----------+
          0       7 8     15  17  21 22      31

              8        8      2   6       8
          +--------+--------+--+------+--------+
          | net #  | UPPER  |11|XXXXXX| LOWER  |  Logical Name
          +--------+--------+--+------+--------+
          0       7 8     15   18   23 24    31

                   16         2       14
          +-----------------+--+---------------+
          |                 |10|               |  Reserved Format
          +-----------------+--+---------------+
          0               15    18            31

                      (X = don't care)

                 New Class A IP Address Interpretation
                               Figure 2.2

The fields have the following meanings:

          HOST = host-number

          PSN = 10 bit PSN-number field

          UPPER = upper 8 bits of the 16-bit logical name

          LOWER = lower 8 bits of the 16-bit logical name

AHIP-E physical addresses and logical names have the following formats:

               8     1   5       10
          +--------+-+-----+----------+
          |  HOST  |0|XXXXX|   PSN    |  Physical Address
          +--------+-+-----+----------+
          41     48         55      64
          (bit positions in the AHIP leader)
                (X = don't care)

              8      2   6       8
          +--------+--+------+--------+
          | UPPER  |11|XXXXXX| LOWER  |  Logical Name
          +--------+--+------+--------+
          41     48           57     64
               (bit positions in the AHIP leader)

          +--------+--+---------------+
          |        |10|               |  Reserved Address Format
          +--------+--+---------------+
          41     48    51            64
               (bit positions in the AHIP leader)

                        AHIP-E Address and Name
                               Figure 2.3

The reserved address format is currently undefined and will be rejected
by the PSN, which will return an error message (message type 6, subtype
3) to the host.

    -----------------------------------------------------------------
   |This design does not require the AHIP-E host to do any processing|
   |of the address -- the host need only copy bits 8-31 of the IP    |
   |address into bits 41-64 of the AHIP leader.  The host no longer  |
   |needs to zero out bits 49-56 of the AHIP leader.  The PSN will   |
   |take care of the AHIP to subnet address conversion.  In other    |
   |words, bits 8-31 of the IP address field should be passed        |
   |unchanged to the PSN, which interprets them exactly as shown in  |
   |figure 2.3.                                                      |
    -----------------------------------------------------------------

2.4  Discussion of the New Mapping

This section presents an evaluation of the design in terms of the
requirements in section 2.2

     1.   Address mapping stability requirement:

          Current physical IP addresses will not have to be changed, as
          long as they have been following the convention of setting LH
          = 0.  This ensures that bit 16 is set to 0, indicating that
          the address is physical, and that the PSN number comes out
          right.

     2.   Existing implementation compatibility:

          The design meets this requirement, as the address that gets to
          the PSN has its second octet = 0, which results in its correct
          interpretation as a physical address.

     3.   Compatibility with the current X.25 IP address to DDN host
          mapping:

          The current X.25 IP to HOST mapping [6] is as follows: If h <
          64, the address is considered physical, i.e., it refers to
          host h on PSN i.  If h >= 64, the address is considered
          logical, i.e., it refers to the host whose logical name is h
          concatenated with i.

          The design is compatible in a limited sense with the current
          X.25 logical addressing implementation, as long as logical
          names are assigned such that host-number > 63 (also PSN-number
          < 256 which is automatic, given the 16-bit size of the logical
          name field) and physical addresses are in the range host-
          number < 64 and PSN- number < 256, with the appropriate
          setting of bits 16 and 17 of the IP address field.  This works
          because the X.25 mapping ignores the value of the l field,
          i.e., the third IP address octet.

          Given the desire to be able to address more than 64 hosts
          physically and for PSN numbers > 255, this address assignment
          restriction should not be considered permanent, but rather as
          an interim compromise until the hosts' X.25 implementations
          are revised to incorporate the new mapping between IP and DDN
          addresses.

     4.   Maximum number of PSNs that can be supported:

          The design allows addressing of up to 1024 PSNs per network.

2.5  Interoperability between Current AHIP and AHIP-E

This section discusses the interoperability between hosts using current
AHIP and AHIP-E.  It also discusses the general issue of current AHIP
host operation in the AHIP-E addressing environment.

The proposed modifications to AHIP have been designed with backward
compatibility in mind.  However, note that bits 41-64 of the PSN-to-host
leader (see 1822(3.4)) will always contain the physical address of the
source host.  This means that an error could occur when a host on a PSN
numbered greater than 255 attempts to send a message to a host running a
current AHIP implementation, which interprets the address of the source
host as one with PSN-number < 256.

There are other possibilities for errors, caused by incorrect address
translation between IP and current AHIP:

   1.  A host running current AHIP cannot physically address
       any host on a PSN numbered greater than 255 (see Figure
       3.1).  Consequently, an error will result if the host
       attempts to use an address from the NIC host table that
       has PSN-number > 255.

   2.  If a host running current AHIP attempts to use a
       logical name that it might have in its host table, an
       error will occur.  This is because the logical name flag
       bits 16 and 17 of the IP address, bits 49 and 50 of the
       AHIP leader.  Recal that bits 49 - 56 of the AHIP
       leader get set to zero with current AHIP (see figure
       2.1).

Since these errors cannot be detected by the subnetwork, it is essential
that all hosts implement at least version 1 AHIP-E (see chapter 6)
before PSN numbers over 255 and logical names are assigned.

Another aspect of interoperability has to do with the IP LH field, which
is currently used by a handful of Arpanet hosts to demultiplex a single
host port.  The 5 don't-care bits of the physical IP address (bits 17-
21) and the 6 don't-care bits of the IP logical name (bits 18-23) can be
used for this purpose -- in particular, the use of these bits is divided
between the network and external devices, based on administrative
agreement.  At the very least, the IP addresses of such hosts will have
to change to reflect the changed position of the LH field.  However, the
preferred way to demultiplex a single host port is via the mechanism of
logical names.  The only change this involves is to get the port
expander implementation to look at the entire IP address, rather than
just the LH field.

3  LOGICAL ADDRESSING

The modifications to AHIP allow a host to use logical addressing to
communicate with other hosts on the network.  Basically, logical
addressing allows hosts to refer to each other using a logical name (see
section 3.1) 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 in the network (especially
  important if the host has to move to another physical port).

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.

o Allowing a host to provide services that have their own unique
  names.

3.1  Addresses and Names

The AHIP-E protocol allows two forms of host specification.  The first
is a slightly modified version of the form used by the current AHIP
protocol, the physical address.  The second form is the logical name
(the terms "name", "logical name" and "logical address" are used
interchangeably in this document).

Current AHIP addresses are the 24-bit host addresses found in AHIP
leaders.  They have the following format:

                       8          8        8
               +-------------+--------+------------+
               | host-number |00000000| PSN-number |
               +-------------+--------+------------+
                41         48 49    56 57        64
                (bit positions in the AHIP leader)

                      Current AHIP Address Format
                               Figure 3.1

AHIP-E addresses have the following format:

                  8     1   5       10
             +--------+-+-----+----------+
             |  HOST  |0|XXXXX|   PSN    |  Physical Address
             +--------+-+-----+----------+
             41     48         55      64
             (bit positions in the AHIP leader)
           (X = don't care)

                         AHIP-E Address Format
                               Figure 3.2

Logical names are 16-bit unsigned numbers that serve as a logical
identifier for one or more hosts.  A logical name is the concatenation
of two separate octets in the AHIP leader, bits 41-48 (Upper 8) and 57-
64 (Lower 8) in particular.

                         8      2   6       8
                     +--------+--+------+--------+
                     | UPPER  |11|XXXXXX| LOWER  |
                     +--------+--+------+--------+
                     41     48           57     64
                    (bit positions in the AHIP leader)
                           (X = don't care)

                          Logical Name Format
                               Figure 3.3

3.2  Name Translations

There are a number of factors that determine how a logical name is
translated by the PSN into a physical address on the network.  These
factors include which translations are legal; in what order different
translations for the same name should be attempted; and which legal
translations should not be attempted because a particular host port is
down.  These issues are discussed in the following sections.

3.2.1  Authorization and Effectiveness

Every host on a PSN, regardless of whether it is using the AHIP or
AHIP-E protocol to access the network, can have one or more logical
names.  Hosts using AHIP-E can then use these names to address the hosts
in the network independent of their physical locations.

At this point, several questions arise:  How are these names assigned,
how do they become known to the PSNs (so that translations to physical
addresses can be made), and how do the PSNs know which host is currently
using a shared port?  To answer each question in order:

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 PSNs by the Monitoring Center (MC).  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.  Physical addresses, which actually refer to
physical host ports, are always authorized in this sense.

When the PSN detects that a host has come up on one of its ports, it
makes effective the default name(s), if any, for that host.  This
default action is specified in the configuration table for that host,
and can be one of the following: Enable All Names, Enable No Names,
Enable One Particular Name.  In the case of an AHIP-E host, the default
name might not be the one that the host desires to be known as (recall
that several hosts may share the same port, or one host may prefer to be
known by different names at different times).  This requires that an
AHIP-E host be able to declare its name to the PSN.  This function is
performed by a new host-to-PSN message, the Name Declaration Message
(NDM), which lists the names that the host would like to be known by.
The PSN checks its tables to see if each of the names is authorized, and
sends an NDM Reply to the host saying which names were actually
authorized and can now 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 names (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 authorized.

If a host is using the current AHIP protocol, it can still receive
messages from hosts via its logical name.  Of course, it can also
receive messages from a current AHIP host via its physical address as
well.  (Remember, the distinction between logical names and physical

addresses is that the addresses correspond to physical locations on the
network, while the names are strictly logical identifiers).

The third question above has by now already been answered. An AHIP-E
host can use the NDM message to tell the PSN which host it is (which
names it is known by).  Thus, even if this is a shared port, the PSN
knows which host is currently connected.

WHENEVER A HOST GOES DOWN, ITS NAMES AUTOMATICALLY BECOME NON-
EFFECTIVE.  When it comes back up, the default action (from the host's
configuration) is taken.  If the host wishes to be known by a name other
than the default, it will have to issue a NDM.  It will also have to do
this upon receipt of reset NOPS from the PSN.

3.2.2  Translation Policies

Several hosts can share the same logical name.  If more than one of
these hosts is up at the same time, any messages sent to that logical
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 logical name in this manner.

Similarly, when a host is multi-homed, the same logical 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 the host.  However, if the 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 physical address or a
specific logical name that referred to that port alone.

Three different address selection policies are available for the name
mapping process.  When translated, each name uses one of the three
policies (the policy is administratively pre-determined on a per-name
basis).  The three policies are:

o  Attempt each translation in the order in which the physical
   addresses are listed in the PSN's translation tables, to find
   the first reachable physical host address.  This list is
   always searched from the top whenever a new virtual circuit
   connection has to be created.  This is the most commonly used
   policy.

o  Selection of the closest physical address, which uses the
   PSN's internal routing tables to find the translation to the
   destination PSN with the least cost path for the particular
   type-of-service whenever a new virtual circuit connection has
   to be created.

o  Use load leveling.  This is similar to the first policy, but
   differs in that searching the address list for a valid
   translation starts at the address following where the
   previous translation search ended whenever a new virtual
   circuit connection has to be created. This attempts to
   spread out the load from any one PSN's hosts to the various
   host ports associated with a particular name.  Note that
   this is NOT network-wide load leveling, which would require
   knowledge about flows throughout the network.

3.2.3  Reporting Destination Host Downs

As is explained in Report 1822, whenever regular messages are sent by a
host, the PSN opens a virtual circuit connection to each destination
host from the source host.  A new connection is opened for each new
source-address/destination-name (or address, as the case might
be)/handling-type/type-of-service combination.  A connection will stay
open at least as long as there are any outstanding (un-RFNMed) messages
using it and both the source and destination hosts stay up.  Connections
are also closed after a period of inactivity.

However, the destination host may go down for some reason during the
lifetime of a connection.  If the host goes down while there are no
outstanding messages to it in the network, then the connection is closed
and no other action is taken until the source host submits the next
message for that destination.  At that time, ONE of the following events
will occur:

A1.  If a physical address is being used to specify the
     destination host, then the source host will receive a type
     7, subtype 0 (Destination Host Dead) message from the PSN.

A2.  If a logical name is being used to specify the
     destination host, and the name maps to only one authorized
     host port,then a type 7, subtype 0 message will be sent to
     the source host.

A3.  If a logical name is being used to specify the destination
     host, and the name maps to more than one authorized host
     port, then the PSN attempts to open a connection to another
     authorized and effective host port for that name.  If no
     such connection can be made, the host will receive a type

     15 (AHIP Name or Address Error), subtype 5 (no effective
     translations) message (see section 5.2).  Note that a type
     7 message cannot be returned to the source host, since type
     7 messages refer to a particular destination host port, and
     the name maps to more than one destination port.  However,
     in the case of a version 0 or 1 host, a type 7, subtype 0
     message will be returned for each outstanding message.  See
     chapter 6 for further details on version numbers.

Things get a bit more complicated if there are any outstanding messages
on the connection when the destination host goes down.  The connection
will be closed, and one of the following will occur:

B1.  If a physical address is being used to specify the
     destination host, then the source host will receive a type
     7 message for each outstanding message.

B2.  If a logical name is being used to specify the
     destination host, then the source host will receive a type
     9 (Incomplete Transmission), subtype 6 (message lost due to
     logically addressed host going down) message for each
     outstanding  message.  The next time the source host
     submits another message for that same destination name,
     the previous algorithm will be used (either step A2 or
     step A3). However,in the case of a version 0 or 1 host, a
     type 7,subtype 0 message will be returned for each
     outstanding message. See chapter 6 for further details
     on version numbers.

3.3  Establishing Host-PSN Communications

When a host comes up on a PSN, or after there has been a break in the
communications between the host and its PSN (see 1822 (3.2)),the orderly
flow of messages between the host and the PSN needs to be properly (re-
)established.  This allows the PSN and host to recover from almost 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 PSN are three NOPs.
Three messages are required to ensure that at least one message will be
properly read by the PSN (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 to
synchronize the PSN with the host, to inform the PSN about how much
padding the host requires between the message leader and its body and to
specify the host's AHIP-E version number to the PSN (see chapter 6).

Similarly, the PSN will send three NOPs to the host when it detects that

the host has come up.  The NOPs will be followed by an Interface Reset
message.  These NOPs will contain the physical address of the host
interface.

Once the PSN 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.

3.4  Name Server

There may be times when a host wants to perform its own translations, or
might need the full list of physical addresses to which a particular
name maps.  For example, a connection- based host-to-host protocol may
require that the same physical host port on a multi-homed host be used
for all messages using that host-to-host connection, and the host does
not wish to trust the PSN to always deliver messages using a destination
name to the same host port.

In these cases, the host can submit a type 11 (Name Server Request)
message to the PSN, which requests the PSN to translate the destination
name and return a list of the addresses to which it maps.  The PSN will
respond with a type 11 (Name Server Reply) message, which contains the
selection policy in use for that name, the number of addresses to which
the name maps, the addresses themselves, and for each address, whether
it is effective and its routing distance (for the particular type-of-
service specified in the Name Server Request message) from the PSN.  See
section 5.2 for a complete description of these messages' contents.

Using this information, the source host could make an informed decision
on which of the physical host ports corresponding to a logical name to
use and then send the messages to that port, rather than to the name.

The PSN also supports a different type of name service.  A host needs to
issue a Name Declaration Message to the PSN in order to change its
effective names, but it may not wish to keep its names in some table or
file in the host.  In this case, it can ask the PSN to tell it which
names it is authorized to use.

In this case, the host submits a type 12 (Port List Request) message to
the PSN, and the PSN replies with a type 12 (Port List Reply) message.
It contains, for the host port over which the PSN received the request
and sent the reply, the number of names that map to the port, the list
of names, and whether or not each name is effective.  The host can then
use this information in order to issue the Name Declaration Message.
Section 5.2 contains a complete description of the reply's contents.

4  OTHER CHANGES

This section describes the enhancements to the AHIP protocol involving
type-of-service specification, subnet congestion feedback and network
precedence level feedback.  Note that only version 2 hosts will receive
the congestion and precedence messages described in this section.

4.1  Type-of-Service Specification

Bits 9 and 10 of the AHIP leader, currently unused, will be used by the
host to specify desired delay and throughput characteristics to the PSN.
Bit 11, also currently unused, will be used to specify reliability.  The
bits have the following meaning:

Bit 9:    delay bit

            0 -- normal delay
            1 -- low delay

Bit 10:   throughput bit

            0 -- normal throughput
            1 -- high throughput

Bit 11:   reliability bit

            0 -- normal reliability
            1 -- high reliability

The values of these bits are consistent with those of IP, and bits 11,
12 and 13 of the IP header can be copied directly into bits 9, 10 and 11
of the AHIP leader.

The type-of-service bits should be considered as extensions of the
"Handling Type" field (bits 33-40 of the AHIP leader -- see 1822 (3.3)).
Messages from host A to host B using the same destination name and of
the same handling type and type-of- service will use the same
connection, while those that differ in either type-of-service,
destination name or handling type will use separate connections.  In
other words, for a given source host and destination name pair, a new
connection will be established whenever a message with a new handling-
type/type-of- service combination is received.

4.2  Subnet Congestion Feedback

This section describes the new messages that are part of the mechanism
used by the PSN to communicate subnetwork congestion information to the
host. Note that a host will be blocked by the PSN when its share of
buffers in the PSN is used up.  Thus, this information, which is
communicated on a connection basis, will give the host an opportunity to
selectively reduce its congesting flows, thus preventing all of its
flows from getting blocked. Currently, a host has no way of knowing
which of its flows is experiencing congestion; consequently, it is
possible that one congesting flow can result in the blocking of all the
host's flows.

Three new PSN-to-host messages have been created. These messages are:

      1.  STOP: Blocking Imminent -- Stop Sending on this
          Connection (Message type 13)

      2.  SLOW: Subnet Congestion -- Send at Slow Rate on this
          Connection (Message type 14) -- Maintain Window Size of
          1, i.e., do not send a new message to this destination
          host with this type-of-service and handling type until
          all previous messages have been acknowledged by RFNMs.

      3.  GO: Congestion Subsided -- Send at Regular Rate on this
          Connection (Message type 16) -- Maintain Window Size of
          8

These messages may be sent in any order and correspond to states, not
transitions.  A participating host should support three states with
effective windows of 8, 1 and 0.  The format of these messages can be
found in section 5.2.

4.3  Precedence Level Information

Two new messages have been created:

     1.  Network Not Accepting Messages at this Precedence Level
         (Message type 9, subtype 7).

     2.  Network Precedence Level Cutoff Change (Message type
         17).

The first message will be generated whenever the host attempts to send a
message at a precedence level lower than the cutoff.  The cutoff
represents a precedence level below which no traffic may be submitted

into the subnetwork; note that a cutoff set to the lowest possible
precedence level implies that no precedence restrictions are in effect.
If the host has chosen not to receive the new AHIP-E messages, then the
PSN will send a type 7, sub-type 3 message (communication with the
destination host is administratively prohibited) instead.  The second
message will be generated whenever the network precedence level cutoff
changes.  Both messages contain the network precedence cutoff value.
The format of these messages can be found in section 5.2.

5  FORMATS FOR NEW AHIP-E MESSAGES

The following sections describe the formats of the leaders that precede
messages between an AHIP-E host and its PSN.  The formats are almost
identical to those of AHIP (see 1822(3.3) and 1822(3.4)). New message
types are marked by margin bars (as shown | here).

5.1  Host-to-PSN AHIP-E Leader Format

 1      4 5      8         13  16 17    20 21 22 24 25            32
+---------+--------+-+-+-+-+------+--------+-+------+----------------+
|         | FORMAT |D|T|R|U|      |        |T|LEADER|                |
| UNUSED  |  FLAG  |E|H|E|N| VERS | UNUSED |R|FLAGS |  MESSAGE TYPE  |
|         |  (15)  |L|R|L|U|      |        |C|      |                |
+---------+--------+-+-+-+-+------+--------+-+------+----------------+

 33                  40 41                                          64
+----------------------+----------------------------------------------+
|                      |                                              |
|    HANDLING TYPE     |                DESTINATION HOST              |
|                      |                                              |
+----------------------+----------------------------------------------+

 65                     76 77    80 81                              96
+-------------------------+--------+----------------------------------+
|                         |        |                                  |
|       MESSAGE ID        |SUB-TYPE|            UNUSED                |
|                         |        |                                  |
+-------------------------+--------+----------------------------------+

                    Host-to-PSN AHIP-E Leader Format
                               Figure 5.1

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

Bits 5-8: Format Flag
     This field is set to decimal 15 (1111 in binary).

Bits 9-11: Type-of-Service

     Bit 9: Delay Bit:
               0 -- normal delay
               1 -- low delay
     Bit 10: Throughput Bit:
               0 -- normal throughput
               1 -- high throughput
     Bit 11: Reliability Bit:

               0 -- normal reliability
               1 -- high reliability

Bit 12: Unused, must be set to zero.

Bits 13-16: AHIP-E Version number
     Ignored by the PSN except in the case of a NOP -- see
     chapter 6.

Bits 17-20: Unused, must be set to 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 AHIP(3.3) for a description of its use by the
          PSN's TTY Fake Host.
     Bits 23-24: Reserved for future use, must be zero.

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 PSN uses its full message and error
             control facilities, and host blocking may occur.
          3: Uncontrolled Packet - The PSN will perform no
             message-control functions for this type of
             message, and network flow and congestion control
             may cause loss of the packet.  Also see
             1822(3.6). 1-2,4-15: 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 logical names   |
             is or is not effective (see section 3.2.1), 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 leader padding, contains the number of    |
             logical names contained in the message.  This is         |
             followed by the logical name entries, each 32 bits       |

             long, of which the first 16 bits is a logical 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.  Note that only the names explicitly in the   |
             NDM will remain enabled after the NDM is processed       |
             (assuming that they are authorized).  The PSN will       |
             reply with a NDM Reply message (see section 5.2)         |
             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, and without any leader padding):         |

                 1             16 17            32 33            48
                +----------------+----------------+----------------+
                |                |                |                |
                |      0F00      |      0003      |      0000      |
                |                |                |                |
                +----------------+----------------+----------------+
                 49            64 65            80 81            96
                +----------------+----------------+----------------+
                |                |                |                |
                |      0000      |      0000      |      0000      |
                |                |                |                |
                +----------------+----------------+----------------+
                 97           112 113          128 129          144
                +----------------+----------------+----------------+
                |                |                |                |
                |  # of entries  |     name #1    |     0 or 1     |
                |                |                |                |
                +----------------+----------------+----------------+
                145           160 161          176
                +----------------+----------------+
                |                |                |
                |   name #2      |     0 or 1     |       etc.
                |                |                |
                +----------------+----------------+

                           NDM Message Format
                               Figure 5.2

               An NDM with zero entries will cause all current
               effective names for the host to become non-effective.

          Type 4: NOP -- see 1822(3.3).  Bits 13-16 of the NOP leader   |
               are used to determine the host's AHIP-E version -- see   |
               chapter 6.                                               |

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

          Type 11: Name Server Request - This allows the host to use    |
               the PSN's logical addressing tables as a name server.    |
               The destination name in the AHIP-E leader is             |
               translated, and the PSN replies with a Name Server       |
               Reply message, which lists the physical host addresses   |
               to which the destination name maps.  The type-of-        |
               service bits (bits 9-11) should be set correctly by      |
               the host, as the Name Server Reply message contains      |
               information about characteristics of the subnetwork      |
               route(s) to that destination, which will depend on the   |
               type-of-service.                                         |

          Type 12: Port List Request - This allows the physical host    |
               to request the list of names that map to the host port   |
               over which this request was received by the PSN.  The    |
               PSN replies with a Port List Reply message, which        |
               lists the names that map to the port.                    |

          Types 5-7,9-10,13-255: Unassigned.

     Bits 33-40: Handling Type:
          The top two bits (33 and 34) specify the precedence of the
          connection.  There are 4 precedence levels, level 3 being
          the highest and level 0 the lowest.  Bits 35-40 are used to
          specify up to 64 separate connections at a particular
          precedence level and type-of-service.

     Bits 41-64: Destination Host:
          This field contains the name or address of the destination
          host, as described in figures 3.3 and 3.2 respectively.  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.

     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.

     Bits 81-96: Unused

5.2  PSN-to-Host AHIP-E Leader Format

 1      4 5      8       12    16 17    20 21 22 24 25            32
+--------+--------+-+-+-+--------+--------+-+------+----------------+
|        | FORMAT |D|T|R|        |        |T|LEADER|                |
| UNUSED |  FLAG  |E|H|E| UNUSED | UNUSED |R|FLAGS |  MESSAGE TYPE  |
|        |  (15)  |L|R|L|        |        |C|      |                |
+--------+--------+-+-+-+--------+--------+-+------+----------------+

 33                  40 41                                          64
+----------------------+----------------------------------------------+
|                      |                                              |
|    HANDLING TYPE     |                SOURCE HOST                   |
|                      |                                              |
+----------------------+----------------------------------------------+

 65                     76 77    80 81                              96
+-------------------------+--------+----------------------------------+
|                         |        |                                  |
|       MESSAGE ID        |SUB-TYPE|        MESSAGE LENGTH            |
|                         |        |                                  |
+-------------------------+--------+----------------------------------+

                    PSN-to-Host AHIP-E Leader Format
                               Figure 5.3

Bits 1-4: Unused and set to zero.

Bits 5-8: Format Flag
     This field is set to decimal 15 (1111 in binary).

Bits 9-11: Type-of-Service
     Specified by the source host (see section 5.1).

Bits 12-20: Unused, must be set to zero.

Bit 21: Trace Bit:

     If equal to one, the source host has 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
          that sent in the host-to-PSN leader (see section 5.1).

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

     Type 2: PSN Going Down - See 1822(3.4).

     Type 3: NDM Reply - This is a reply to the NDM host-to-PSN    |
          message (see section 5.1).  It has the same number of    |
          entries as the NDM message to which it replies, and      |
          each listed name is accompanied by a zero or a one       |
          (see figure 5.2).  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 PSN/host
          communication.  The Destination Host field will
          contain the physical 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 PSN Dead (or unknown) - See
          1822(3.4).

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

     Type 9: Incomplete Transmission - See 1822(3.4).  In
          addition to its already defined sub-types, this
          message has two new sub-types:
          6: Logically Addressed Host Went Down - A logically      |

               addressed message was lost in the network because   |
               the destination host to which it was being          |
               delivered went down.  The message should be         |
               resubmitted by the source host, since there may     |
               be another effective host port to which the         |
               message could be delivered (see section 2.2.3).     |
          7: Network Not Accepting Messages at this Precedence     |
               Level - bits 33 and 34 encode the minimum           |
               precedence level currently being accepted by the    |
               network.  See section 4.3.

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

     Type 11: Name Server Reply - This reply to the Name Server    |
          Request host-to-PSN message contains, following the      |
          leader and any leader padding, a word with the           |
          selection policy and the number of physical addresses    |
          to which the destination name maps, followed by five     |
          octets per physical address: the first three octets      |
          contain an AHIP-E address, and the last two contain a    |
          bit signifying whether or not that particular            |
          translation is effective and the routing distance        |
          (expected network transmission delay, in 6.4 ms units)   |
          to the address's PSN for the type-of-service specified   |
          in the Name Server Request being replied to.  This       |
          type-of-service will be included in the Name Server      |
          Reply leader.  In figure 5.4, which includes the         |
          leader without any leader padding and has type-of        |
           -service set  to 000, EFF is 1 for effective and 0      |
          for non-effective, the destination name is in the format |
          of figure 3.3, and POL is a two-bit number indicating    |
          the selection policy for the name (see section 3.2.2):   |

          0: First reachable.
          1: Closest physical address.                             |
          2: Load leveling.                                        |
          3: Unused.

                 1             16 17            32 33     40
                +----------------+----------------+--------+
                |                |                |        |
                |      0F00      |      000B      |   00   |
                |                |                |        |
                +----------------+----------------+--------+

                 41                    64 65            80
                +------------------------+-----------------+
                |                        |                 |
                |  Destination name      |      0000       |
                |                        |                 |
                +------------------------+-----------------+
                     81            96 97            112
                    +----------------+-+--------------+
                    |                |P|              |
                    |      0000      |O|  # of addrs  |
                    |                |L|              |
                    +----------------+-+--------------+

                 113                    136 137          152
                +--------------------------+-+-------------+
                |                          |E|             |
                |    AHIP-E addr #1        |F| routing dist|
                |                          |F|             |
                +--------------------------+-+-------------+

                 153                    176 177          192
                +--------------------------+-+-------------+
                |                          |E|             |
                |    AHIP-E addr #2        |F| routing dist|   etc.
                |                          |F|             |
                +--------------------------+-+-------------+

                        Name Server Reply Format
                               Figure 5.4

          Type 12: Port List Reply - This is the reply to the Port      |
               List Request host-to-PSN message.  It contains the       |
               number of names that map to this physical host port,     |
               followed by two words per name: the first word           |
               contains a logical name that maps to this port, and      |
               the second contains either a zero or a one,              |
               signifying whether or not that particular translation    |
               is effective.  The format is identical to the type 3     |
               NDM Reply message(see figure 5.2).                       |

          Type 13: STOP -- Stop Sending on this Connection.  See        |

               section 4.2.                                             |

          Type 14: SLOW -- maintain window size of 1 on this            |
               connection.  See section 4.2.                            |

          Type 15: Name or Address Error - This message is sent in      |
               response to a type 0 message from a host that            |
               contained an erroneous Destination Host field.  Its      |
               sub-types are:                                           |
               2: The Destination Host name is not authorized.          |
               3: The physical host to which this singly-homed          |
                    Destination Host name translated is authorized      |
                    and up, but not effective.  If the host was         |
                    actually down, a type 7 message would be            |
                    returned, not a type 15.                            |
               5: The multi-homed Destination Host name is              |
                  authorized  but has no available effective            |
                  translations.                                         |
               6: A logically-addressed uncontrolled packet was sent    |
                    to a dead or non-effective host port.  However,     |
                    if it is resubmitted, there may be another          |
                    effective host port to which the PSN may be able    |
                    to attempt to send the packet.                      |
               7: Logical addressing is not in use.                     |
                    The PSN has no table of mappings from logical       |
                    addresses to physical host ports.                   |
               0, 1, 4, 8-15: Unassigned                                |

          Type 16: GO -- maintain window size of 8 on this              |
                connection. See section 4.2.                            |

          Type 17: Network Precedence Level Cutoff Change -- bits 33    |
               and 34 encode the minimum precedence level currently     |
               being accepted by the network.  See section 4.3.

          Types 18-255: Unassigned.

Bits 33-40: Handling Type:
     This has the value assigned by the source host (see
     1822(3.1)).  This field is only used in message types 0, 5-
     9, and 13-16.

Bits 41-64: Source Host:
     See 1882(3.4).  For type 0 messages this contains the
     physical address of the source host, in the format detailed
     in figure 3.2.  For type 4 messages, this contains the
     physical address of the local host.  For messages of type
     5-9, 11 and 13-16 which are responses to messages from the

     local host, this contains the destination name as specified
     in the message from the local host.

Bits 65-76: Message ID:
     For message types 0, 5, 7-9, and 15, this is the value
     assigned by the source host to identify the message (see
     section 5.1).  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, 5-7,
     9, and 15.

Bits 81-96: Message Length:
     This field is contained in type 0 messages only, and is the
     actual length in bits of the message (exclusive of leader,
     leader padding, and hardware padding) as computed by the
     PSN.

6  AHIP-E VERSIONS

This specification provides three versions of AHIP-E and allows a host
to specify its version in bits 13-16 of the leader of the NOP.  The PSN
will set the version of a host based on the value contained in the most
recent NOP that it has received from the host.  Thus, a host can change
the PSN's idea of its version by issuing a NOP containing a different
version value.  Note that the version field in all other host-to-PSN
messages will be ignored by the PSN.

Version 0:

A host that doesn't change its current AHIP implementation will
presumably have the version bits in the AHIP leader set to zero.
Version 0, thus, is nothing but current AHIP.

A version 0 host will not receive any of the new AHIP-E messages from
the PSN, nor will the PSN expect any of the new host-to-PSN message
types from the host.  The type-of-service bits will always be set to
zero in the PSN-to-host leader.

Version 1:

A version 1 host will be able to use logical names to address other
hosts, will be able to use the 10-bit PSN field, will be able to specify
desired type-of-service to the PSN, but will not receive any of the new
AHIP-E messages from the PSN.  The PSN will not expect any of the new
host-to-PSN message types from the host either.

To implement version 1, a host need only make the following changes to
its AHIP implementation:

     1. Set the version number field to 1 when sending type 4
        messages (NOPs).

     2. When sending type 0 messages, copy IP address bits 8-31
        into bits 41-64 of the AHIP leader.

     3. When sending type 0 messages, copy IP header bits 11-13
        to AHIP leader bits 9-11.

Version 2:

A version 2 host is one that is fully compliant with the AHIP-E protocol
as described in this document.  In addition to being able to take
advantage of the features described under version 1 above, it should be
able to send and receive all the new AHIP-E messages described in this
document.

7  REFERENCES

   [1]  "Specifications for the Interconnection of a Host and an
        PSN", BBN Report 1822, as found in "DDN Protocol Handbook",
        December 1985, vol. 3, section 3.10.

   [2]  E. C. Rosen et. al., "ARPANET Routing Algorithm
        Improvements", Internet Experimenter's Note 183 (also
        published as BBN Report 4473, Vol. 1), August 1980, pp. 55-
        107.

   [3]  J. Reynolds and J. Postel, "Assigned Numbers", Request For
        Comments 990, November 1986.

   [4]  J. Postel, ed., "Internet Protocol -- DARPA Internet
        Program Protocol Specification", Request for Comments 791,
        September 1981.

   [5]  J. Postel, "Address Mappings", Request for Comments 796,
        September 1981, as found in "DDN Protocol Handbook", vol.
        3, section 3.4.

   [6]  "Defense Data Network X.25 Host Interface Specification",
        pp. 497-498, DDN protocol handbook, vol. 1, December 1985.

 

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