rfc1005.txt

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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
   "[e-mail]".






























Khanna & Malis                                                  [Page 1]

RFC 1005                                                        May 1987


                             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












Khanna & Malis                                                  [Page 2]

RFC 1005                                                        May 1987


                                  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







































Khanna & Malis                                                  [Page 3]

RFC 1005                                                        May 1987


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".



Khanna & Malis                                                  [Page 4]

RFC 1005                                                        May 1987


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.












































Khanna & Malis                                                  [Page 5]

RFC 1005                                                        May 1987


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.





Khanna & Malis                                                  [Page 6]

RFC 1005                                                        May 1987


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